Quasi-Solid Sulfur Conversion for Energetic All-Solid-State Na-S Battery

The high theoretical energy density (1274 Wh kg-1) and high safety enable the all-solid-state Na-S batteries with great promise for stationary energy storage system. However, the uncontrollable solid-liquid-solid multiphase conversion and its associated sluggish polysulfides redox kinetics pose a great challenge in tunning the sulfur speciation pathway for practical Na-S electrochemistry. Herein, we propose a new design methodology for matrix featuring separated bi-catalytic sites that control the multi-step polysulfide transformation in tandem and direct quasi-solid reversible sulfur conversion during battery cycling. It is revealed that the N, P heteroatom hotspots are more favorable for catalyzing the long-chain polysulfides reduction, while PtNi nanocrystals manipulate the direct and full Na2S4 to Na2S low-kinetic conversion during discharging. The electrodeposited Na2S on strongly coupled PtNi and N, P-codoped carbon host is extremely electroreactive and can be readily recovered back to S8 without passivation of active species during battery recharging, which delivers a true tandem electrocatalytic quasi-solid sulfur conversion mechanism. Accordingly, stable cycling of the all-solid-state soft-package Na-S pouch cells with an attractive specific capacity of 876 mAh gS -1 and a high energy of 608 Wh kgcathode -1 (172 Wh kg-1, based on the total mass of cathode and anode) at 60 °C are demonstrated.

External Pressure in Polymer-Based Lithium Metal Batteries: An Often-Neglected Criterion When Evaluating Cycling Performance?

Solid-state batteries based on lithium metal anodes, solid electrolytes, and composite cathodes constitute a promising battery concept for achieving high energy density. Charge carrier transport within the cells is governed by solid-solid contacts, emphasizing the importance of well-designed interfaces. A key parameter for enhancing the interfacial contacts among electrode active materials and electrolytes comprises externally applied pressure onto the cell stack, particularly in the case of ceramic electrolytes. Reports exploring the impact of external pressure on polymer-based cells are, however, scarce due to overall better wetting behavior. In this work, the consequences of externally applied pressure in view of key performance indicators, including cell longevity, rate capability, and limiting current density in single-layer pouch-type NMC622||Li cells, are evaluated employing cross-linked poly(ethylene oxide), xPEO, and cross-linked cyclodextrin grafted poly(caprolactone), xGCD-PCL. Notably, externally applied pressure substantially changes the cell's electrochemical cycling performance, strongly depending on the mechanical properties of the considered polymers. Higher external pressure potentially enhances electrode-electrolyte interfaces, thereby boosting the rate capability of pouch-type cells, despite the fact that the cell longevity may be reduced upon plastic deformation of the polymer electrolytes when passing beyond intrinsic thresholds of compressive stress. For the softer xGCD-PCL membrane, cycling of cells is only feasible in the absence of external pressure, whereas in the case of xPEO, a trade-off between enhanced rate capability and minimal membrane deformation is achieved at cell pressures of ≤0.43 MPa, which is considerably lower and more practical compared to cells employing ceramic electrolytes with ≥5 MPa external pressure.

A New Method for In-Situ Characterization of Solid-State Batteries Based on Optical Coherence Tomography

With the in-depth study of solid-state batteries (SSBs), various in situ and ex situ characterization technologies have been widely used to study them. The performance and reliability of SSBs are limited by the formation and evolution of lithium dendrites at the interfaces between solid electrodes and solid electrolytes. We propose a new method based on optical coherence tomography (OCT) for in situ characterization of the internal state of solid-state batteries. OCT is a low-loss, high-resolution, non-invasive imaging technique that can provide real-time monitoring of cross-sectional images of internal structures of SSBs. The morphology, growth, and evolution of lithium dendrites at different stages of cycling under various conditions can be visualized and quantified by OCT. Furthermore, we validate and correlate the OCT results with scanning electron microscopy (SEM) and XPS, proving the accuracy and effectiveness of the OCT characterization method. We reveal the interfacial phenomena and challenges in SSBs and demonstrate the feasibility and advantages of OCT as a powerful tool for in situ and operando imaging of battery interfaces. This study provides new insights into the mechanisms and factors that affect SSB performance, safety, and lifetime, and suggests possible solutions for improvement and application in the field of applied energy.

In situ microscopy techniques for understanding Li plating and stripping in solid-state batteries

Solid-state batteries have potential to realize a rechargeable Li-metal anode. However, several challenges persist in the charging and discharging processes of the Li-metal anode, which require a fundamental understanding of Li plating and stripping across the interface of solid-state electrolytes (SEs) to address. This review overviews studies on Li-metal anodes in solid-state batteries using in situ observation techniques with an emphasis on Li electrodeposition and dissolution using scanning electron microscopy and SEs such as lithium phosphorus oxynitride and garnet-type compounds such as Li7La3Zr2O12. The previous research is categorized into three topics: (i) Li nucleation, growth and dissolution at the anode-free interface, (ii) electrochemical reduction of SE and (iii) short-circuit phenomena in SE. The current trends of each topic are summarized.

Boosting the Interfacial Stability of the Li6PS5Cl Electrolyte with a Li Anode via In Situ Formation of a LiF-Rich SEI Layer and a Ductile Sulfide Composite Solid Electrolyte

Due to its good mechanical properties and high ionic conductivity, the sulfide-type solid electrolyte (SE) can potentially realize all-solid-state batteries (ASSBs). Nevertheless, challenges, including limited electrochemical stability, insufficient solid-solid contact with the electrode, and reactivity with lithium, must be addressed. These challenges contribute to dendrite growth and electrolyte reduction. Herein, a straightforward and solvent-free method was devised to generate a robust artificial interphase between lithium metal and a SE. It is achieved through the incorporation of a composite electrolyte composed of Li6PS5Cl (LPSC), polyethylene glycol (PEG), and lithium bis(fluorosulfonyl)imide (LiFSI), resulting in the in situ creation of a LiF-rich interfacial layer. This interphase effectively mitigates electrolyte reduction and promotes lithium-ion diffusion. Interestingly, including PEG as an additive increases mechanical strength by enhancing adhesion between sulfide particles and improves the physical contact between the LPSC SE and the lithium anode by enhancing the ductility of the LPSC SE. Moreover, it acts as a protective barrier, preventing direct contact between the SE and the Li anode, thereby inhibiting electrolyte decomposition and reducing the electronic conductivity of the composite SE, thus mitigating the dendrite growth. The Li|Li symmetric cells demonstrated remarkable cycling stability, maintaining consistent performance for over 3000 h at a current density of 0.1 mA cm-2, and the critical current density of the composite solid electrolyte (CSE) reaches 4.75 mA cm-2. Moreover, the all-solid-state lithium metal battery (ASSLMB) cell with the CSEs exhibits remarkable cycling stability and rate performance. This study highlights the synergistic combination of the in-situ-generated artificial SE interphase layer and CSEs, enabling high-performance ASSLMBs.

Benchmarking of Coatings for Cathode Active Materials in Solid-State Batteries Using Surface Analysis and Reference Electrodes

Fast and reliable evaluation of degradation and performance of cathode active materials (CAMs) for solid-state batteries (SSBs) is crucial to help better understand these systems and enable the synthesis of well-performing CAMs. However, there is a lack of well-thought-out procedures to reliably evaluate CAMs in SSBs. Current approaches often rely on X-ray photoelectron spectroscopy (XPS) for the evaluation of degradation. Unfortunately, XPS sensitivity is not very high, and minor but relevant degradation products may not be detected and distinguished. Furthermore, degradation caused by the current collector (CC) itself is usually not distinguished from CAM-induced degradation. This study uses a modified CC, which allows us to separate electrochemical degradation caused by the CC from degradation at the CAM itself. Using this CC, we present an approach using time-of-flight secondary ions mass spectrometry (ToF-SIMS) that offers high sensitivity and reliability. Principal component analysis (PCA) is applied to differentiate secondary ions as well as identify those mass fragments that correlate with degradation products. This approach also enables distinguishing between different pathways of degradation. To evaluate the kinetic performance of the samples, three-electrode rate tests are performed. Electrochemical characterization evaluates the kinetic performance of the samples under investigation. The samples are finally rated with a score that allows a reliable comparison between the different materials and offers a complete picture of the materials' characteristics in terms of electrochemical performance and degradation.

Efficient Ion Percolating Network for High-Performance All-Solid-State Cathodes

All-solid-state lithium batteries (ASSLBs) face critical challenges of low cathode loading and poor rate performances, which handicaps their energy/power densities. The widely-accepted aim of high ionic conductivity and low interfacial resistance seems insufficient to overcome these challenges. Here, it is revealed that an efficient ion percolating network in the cathode exerts a more critical influence on the electrochemical performance of ASSLBs. By constructing vertical alignment of Li0.35 La0.55 TiO3 nanowires (LLTO NWs) in solid-state cathode through magnetic manipulation, the ionic conductivity of the cathode increases twice compared with the cathode consisted of randomly distributed LLTO NWs. The all-solid-state LiFePO4 /Li cells using poly(ethylene oxide) as the electrolyte is able to deliver high capacities of 151 mAh g-1 (2 C) and 100 mAh g-1 (5 C) at 60 °C, and a room-temperature capacity of 108 mAh g-1 can be achieved at a charging rate of 2 C. Furthermore, the cell can reach a high areal capacity of 3 mAh cm-2 even with a practical LFP loading of 20 mg cm-2 . The universality of this strategy is also presented showing the demonstration in LiNi0.8 Co0.1 Mn0.1 O2 cathodes. This work offers new pathways for designing ASSLBs with improved energy/power densities.

Electrochemical Activation of Fe-LiF Conversion Cathodes in Thin-Film Solid-State Batteries

Transition metal fluoride (TMF) conversion-type cathodes promise up to 4 times higher gravimetric energy densities compared to those of common intercalation-type cathodes. However, TMF cathodes demonstrate sluggish kinetics, poor efficiencies, and incompatibility with many liquid electrolytes. In this work, coevaporated heterostructured iron and lithium fluoride (Fe-LiF) cathodes are investigated in thin-film solid-state batteries with a LiPON electrolyte and a lithium metal anode. The cells were cycled 2000 times at a cycling rate of 6C. They show a gradual improvement in voltaic efficiency (37-53%) and specific capacity (146-216 mAh/g) during cycling. After 2000 cycles, the cathode capacity reaches 480 mAh/g at a cycling rate of C/3.6, close to its theoretical capacity of 498 mAh/g, at room temperature conditions. This capacity gain is correlated with an observed electrochemically activated nanorestructuring of the cathode, characterized by cycling-induced coarsening (from 2.8 to 4.2 nm) of the metallic iron phase and its accumulation near the current collector interface, as well as lithium fluoride phase accumulation near the LiPON interface.

Recent Progress on the Air-Stable Battery Materials for Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) offer numerous advantages in terms of safety and theoretical specific energy density. However, their main components namely lithium metal anode, solid-state electrolyte, and cathode, show chemical instability when exposed to humid air, which results in low capacities and poor cycling stability. Recent studies have shown that bioinspired hydrophobic materials with low specific surface energies can protect battery components from corrosion caused by humid air. Air-stable inorganic materials that densely cover the surface of battery components can also provide protection, which improves the storage stability of the battery components, broadens their processing conditions, and ultimately decreases their processing costs while enhancing their safety. In this review, the mechanism behind the surface structural degradation of battery components and the resulting consequences are discussed. Subsequently, recent strategies are reviewed to address this issue from the perspectives of lithium metal anodes, solid-state electrolytes, and cathodes. Finally, a brief conclusion is provided on the current strategies and fabrication suggestions for future safe air-stable SSLMBs.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Achieving Dendrite-Free Solid-State Lithium-Metal Batteries via In Situ Construction of Li3P/LiCl Interfacial Layers

Hybrid solid electrolyte (HSE) exhibits potential as a solid electrolyte due to its satisfactory Li+ conductivity, superior flexibility, and optimal interface compatibility. However, the inadequate wettability of the Li/HSE interface leads to significant contact impedance, thus fostering the formation of Li dendrites and limiting their practical applicability. Here, a straightforward strategy to enhance the interfacial wettability between Li and HSE and promote the uniform migration of Li+ by in situ construction of a multifunctional interface consisting of Li3P/LiCl (PCl@Li) was created. The Li3P component acts as a Li+ channel, banishing Li+ diffusion obstacles within the interface layer, while the electronically insulating LiCl component acts as an electron-blocking shield at the Li/HSE interface, promoting uniform Li+ deposition and preventing the formation of Li dendrites. The interface impedance of the symmetric PCl@Li|HSE|PCl@Li battery decreases markedly from 230.2 to 47.4 Ω cm-2. Additionally, the battery demonstrates superb cycling stability for over 1300 h at 0.1 mA cm-2 and maintains a minimal overpotential of 32 mV at 30 °C. The PCl@Li|HSE|LiFePO4 battery shows an initial discharge-specific capacity of 135.6 mA h g-1 at 1 C, with a notable capacity retention of 87.0% (118.0 mA h g-1) after 500 cycles. This work provides a new facile strategy for all-solid-state batteries to address interface issues between Li electrodes and HSE.

Influence of Au, Pt, and C Seed Layers on Lithium Nucleation Dynamics for Anode-Free Solid-State Batteries

In the concept of anode-free lithium-ion batteries, cells are manufactured with a bare anode current collector where the lithium metal anode is electrochemically formed from the lithium-containing cathode during the first charge cycle. While this concept has many attractive aspects from a manufacturing and energy density standpoint, stable plating and stripping remain challenging. We have investigated gold, platinum, and amorphous carbon as seed layers placed between the copper current collector and the lithium phosphorus oxynitride thin-film solid electrolyte. These layers guide lithium nucleation and improve the plating and stripping dynamics. All seed layers facilitate reversible lithium plating and stripping even at high current densities up to 8 mA cm-2. Of particular note is the amorphous carbon seed layer, which allowed a significant reduction in plating potential from 300 mV to as low as 50 mV. These results underscore the critical role of seed layers in improving the efficiency of anode-free solid-state batteries and open the door to simplified manufacturing of anode-free battery designs.

Polycarbonate-Based Solid-Polymer Electrolytes for Solid-State Sodium Batteries

Solid-polymer electrolytes comprised of polypropylene carbonate (PPC) and varied sodium bis(fluorosulfonyl)imide (NaFSI) salt concentrations are investigated for implementation as a conductive solid polymer electrolyte into solid-state cathode composites utilizing a sodium-layered oxide active material. The ionic conductivity generally increases with NaFSI salt content, reaching ≈1 mS cm-1 at 80 °C at the highest salt concentration (PPC:NaFSI = 0.5:1). Through an all-in-one slurry casting method, Na2/3 Ni1/3 Mn2/3 O2 cathode composites are fabricated in which the dispersed PPC electrolyte acts as the primary binder. Enabled by a bilayer polymer electrolyte system, cycling performance with the PPC cathode electrolyte is optimized with respect to salt concentration and anode material. The best cyclability is achieved with a moderate salt concentration electrolyte (PPC:NaFSI = 5:1), showcasing an initial capacity of 83 mA h g-1 with a remarkable 80% capacity retention after 150 cycles at C/5 rate and 60 °C. The superior performance of the lower salt concentration electrolyte is attributed to better electrochemical stability, as confirmed by linear sweep voltammetry and online electrochemical mass spectrometry measurements. These results underscore the potential of carbonate-based polymer electrolytes and the importance of balancing electrolyte conductivity and stability in cell design.

Targeting the Right Metrics for an Efficient Solvent-Free Formulation of PEO:LiTFSI:Li6PS5Cl Hybrid Solid Electrolyte

Hybrid solid electrolytes (HSEs) aim to combine the superior ionic conductivity of inorganic fillers with the scalable process of polymer electrolytes in a unique material for solid-state batteries. Pursuing the goal of optimizing the key metrics (σion ≥ 10-4 S·cm-1 at 25 °C and self-standing property), we successfully developed an HSE based on a modified poly(ethylene oxide):LiTFSI organic matrix, which binds together a high loading (75 wt %) of Li6PS5Cl particles, following a solvent-free route. A rational study of available formulation parameters has enabled us to understand the role of each component in conductivity, mixing, and mechanical cohesion. Especially, the type of activation mechanism (Arrhenius or Vogel-Fulcher-Tammann (VFT)) and its associated energy are proposed as a new metric to unravel the ionic pathway inside the HSE. We showed that a polymer-in-ceramic approach is mandatory to obtain enhanced conduction through the HSE ceramic network, as well as superior mechanical properties, revealed by the tensile test. Probing the compatibility of phases, using electrochemical impedance spectroscopy (EIS) alongside 7Li nuclear magnetic resonance (NMR), reveals the formation of an interphase, the quantity and resistivity of which grow with time and temperature. Finally, electrochemical performances are evaluated by assembling an HSE-based battery, which displays comparable stability as pure ceramic ones but still suffers from higher polarization and thus lower capacity. Altogether, we hope these findings provide valuable knowledge to develop a successful HSE, by placing the optimization of the right metrics at the core of the formulation.

Reference Electrode Reveals Insights on Sodium Metal/Solid Electrolyte Interface Cycling and Voiding Behaviors at High Current Densities and Areal Capacities

Plating and stripping processes at solid metal electrode/solid electrolyte interfaces are of great significance for high-energy, solid-state batteries. Here, we introduce a Na metal reference electrode to a symmetric Na metal/sodium β″ alumina/Na metal cell and study both cycling and unidirectional protocols with a focus on high current density and areal capacity. For example, in a current ramp test at 5 mAh cm-2 we find a shift from stable to unstable interfacial polarization during stripping at ≳3 mA cm-2, and at 7.5 mA cm-2 we measure 100s of mV of voltage magnitude rise at the stripping electrode and 10s of mV of voltage changes at the plating electrode. In unidirectional testing (i.e., passing current in a single direction until cell failure), at 1.2 mA cm-2 we find only ∼40% of the initial Na foil could be transferred through the solid electrolyte and again observe 100s of mV (and larger) voltage magnitude rise at the stripping electrode and 10s of mV of voltage change at the plating electrode. This test also shows that the 100s of mV of interfacial polarization can be sustained for hours (at 1.2 mA cm-2) to tens of hours (in a test at 0.3 mA cm-2). Hence, across several test protocols we find a Na metal reference electrode provides quantitative insights on electrochemical interfacial behavior that are not revealed in two-electrode testing. We also built a two-dimensional model of our three-electrode symmetric cell to quantify the link between the measured interfacial potentials in our testing and changes in electrochemically active interfacial contact and find that 100s of mV of interfacial potential rise indicates loss of electrochemically active contact area of >80%. Our work provides a promising approach to clarify the coupled interfacial electrochemical and contact mechanics processes at solid metal electrode/solid electrolyte interfaces.

Influence of Microstructure on the Material Properties of LLZO Ceramics Derived by Impedance Spectroscopy and Brick Layer Model Analysis

Variants of garnet-type Li7La3Zr2O12 are being intensively studied as separator materials in solid-state battery research. The material-specific transport properties, such as bulk and grain boundary conductivity, are of prime interest and are mostly investigated by impedance spectroscopy. Data evaluation is usually based on the one-dimensional (1D) brick layer model, which assumes a homogeneous microstructure of identical grains. Real samples show microstructural inhomogeneities in grain size and porosity due to the complex behavior of grain growth in garnets that is very sensitive to the sintering protocol. However, the true microstructure is often omitted in impedance data analysis, hindering the interlaboratory reproducibility and comparability of results reported in the literature. Here, we use a combinatorial approach of structural analysis and three-dimensional (3D) transport modeling to explore the effects of microstructure on the derived material-specific properties of garnet-type ceramics. For this purpose, Al-doped Li7La3Zr2O12 pellets with different microstructures are fabricated and electrochemically characterized. A machine learning-assisted image segmentation approach is used for statistical analysis and quantification of the microstructural changes during sintering. A detailed analysis of transport through statistically modeled twin microstructures demonstrates that the transport parameters derived from a 1D brick layer model approach show uncertainties up to 150%, only due to variations in grain size. These uncertainties can be even larger in the presence of porosity. This study helps to better understand the role of the microstructure of polycrystalline electroceramics and its influence on experimental results.

Silicon-Based Solid-State Batteries: Electrochemistry and Mechanics to Guide Design and Operation

Solid-state batteries (SSBs) are promising alternatives to the incumbent lithium-ion technology; however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids causing diminished cycle life due to fracture and delamination. This modeling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimize the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si, which leads to localized strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure is desirable. Alternatively, the mechanical properties of the SE could be tailored to optimize cell performance. To reduce Si stress, a SE with a moderate Young's modulus similar to that of lithium phosphorous oxynitride (∼77 GPa) with a low yield strength comparable to sulfides (∼0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young's modulus (∼29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasizes the need for SE material selection and the consideration of other cell components in order to optimize the performance of thin film solid-state batteries.

The Role of Lithium-Ion Batteries in the Growing Trend of Electric Vehicles

Within the automotive field, there has been an increasing amount of global attention toward the usability of combustion-independent electric vehicles (EVs). Once considered an overly ambitious and costly venture, the popularity and practicality of EVs have been gradually increasing due to the usage of Li-ion batteries (LIBs). Although the topic of LIBs has been extensively covered, there has not yet been a review that covers the current advancements of LIBs from economic, industrial, and technical perspectives. Specific overviews on aspects such as international policy changes, the implementation of cloud-based systems with deep learning capabilities, and advanced EV-based LIB electrode materials are discussed. Recommendations to address the current challenges in the EV-based LIB market are discussed. Furthermore, suggestions for short-term, medium-term, and long-term goals that the LIB-EV industry should follow are provided to ensure its success in the near future. Based on this literature review, it can be suggested that EV-based LIBs will continue to be a hot topic in the years to come and that there is still a large amount of room for their overall advancement.

Temperature and Pressure Effects on Unrecoverable Voids in Li Metal Solid-State Batteries

All-solid-state batteries (ASSB) can potentially achieve high gravimetric and volumetric energy densities (900 Wh/L) if paired with a lithium metal anode and solid electrolyte. However, there is a lack in critical understanding about how to operate lithium metal cells at high capacities and minimize unwanted degradation mechanisms such as dendrites and voids. Herein, we investigate how pressure and temperature influence the formation and annihilation of unrecoverable voids in lithium metal upon stripping. Stack pressure and temperature are effective means to initiate creep-induced void filling and decrease charge transfer resistances. Applying stack pressure enables lithium to deform and creep below the yield stress during stripping at high current densities. Lithium creep is not sufficient to prevent cell shorting during plating. Three-electrode experiments were employed to probe the kinetic and morphological limitations that occur at the anode-solid electrolyte during high-capacity stripping (5 mAh/cm2). The role of cathode-LLZO interface, which dictates cyclability and capacity retention in full cells, was also studied. This work elucidates the important role that temperature (external or in situ generated) has on reversible operation of solid-state batteries.

Design and Optimization of Composite Cathodes for Solid-State Batteries Using Hybrid Carbon Networks with Facile Electronic and Ionic Percolation Pathways

Solid-state batteries (SSBs) have emerged as a promising alternative to conventional liquid electrolyte batteries due to their potential for higher energy density and improved safety. However, achieving high performance in SSBs is difficult because of inadequate contact and interfacial reactions that generate high interfacial resistance, as well as inadequate solid-solid contact between electrodes. These chronic issues are associated with inhomogeneous ion and electron transport networks owing to imperfect solid-solid interfacial contact. This study developed an optimal interfacial engineering strategy to facilitate an ion-electron transport network by designing an active material (NCM622) uniformly filled with a thin layer of a solid electrolyte (garnet-type Li_6.25Ga_0.25La₃Zr₂O₁₂) and conductive additives. The optimal composite electrode architecture enhanced the high capacity, high rate capability, and long-term cycle stability, even at room temperature, owing to the percolating network for facile ionic conduction that assured a homogeneous reaction. In addition to mitigating the mechanical degradation of the cathode electrode, it also reduced the crosstalk effects on the anode-solid electrolyte interface. Effectively optimizing the selection and use of conductive additives in composite electrodes offers a promising approach to addressing key performance-limiting factors in SSBs, including interfacial resistance and solid-solid contact issues. This study underscores the critical importance of cathode architecture design for achieving high-performance SSBs by ensuring that the interfaces are intact with solid electrolytes at both the cathode and anode interfaces while promoting uniform reactions. This study provides valuable insights into the development of SSBs with improved performance, which could have significant implications for a wide range of applications.

Ferroelectric Nanorods as a Polymer Interface Additive for High-Performance Garnet-Based Solid-State Batteries

Sandwiching polymer interlayers between the electrode and solid electrolyte is considered promising in solving the interfacial issues arising from solid-solid contact in garnet-based solid-state batteries, but drawbacks including low ionic conductivity, inferior Li⁺ transference number, and unsatisfying mechanical property of the polymer hindered the practical application of such strategy. To solve the mentioned shortcomings of the polymer interlayer simultaneously, we introduce the ferroelectric material, BaTi₂O₅ (BT) nanorods, into the polymer matrix in this work. By taking full advantage of the plasticization effect and intrinsic spontaneous polarization of the introduced ferroelectric, the polymer's ionic conductivity and Li⁺ transference number have been significantly enhanced. The built-in electric field BT introduced also benefits the modulation of CEI components formed on the cathode particles, further enhancing the battery performance by decreasing cathode degradation. Besides, the BT nanorods' particular high aspect ratio also helps increase the mechanical property of the obtained polymer film, making it more resistant to lithium dendrite growth across the interface. Benefitting from the merits mentioned above, the assembled lithium symmetric cells using garnet SE with the BT-modified polymer interlayer exhibit stable cycling performance (no short circuit after 1000 h under RT) with low polarization voltage. The full battery employing LiFePO₄ as a cathode also presents superior capacity retentions (94.6% after 200 cycles at 0.1 C and 93.4% after 400 cycles at 0.2 C). This work highlights the importance of ferroelectric materials with specific morphology in enhancing the electrochemical performance of polymer-based electrolytes, promoting the practical application of solid-state batteries.

Solid Hybrid Electrolyte Featuring a Vertically Aligned Li6.4La3.0Zr1.4Ta0.6O12 Membrane for All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) with enhanced safety are considered one of the most promising substitutes for liquid electrolyte-based Li-ion batteries. However, many properties of solid electrolytes, such as ionic conductivity, film formability, and electrochemical, mechanical, thermal, and interfacial stability need to be improved for their practical application. In this study, a vertically aligned Li_6.4La_3.0Zr_1.4Ta_0.6O₁₂ (LLZO) membrane with finger-like microvoids was prepared using processes involving phase inversion and sintering. A hybrid electrolyte was then obtained by infusing the LLZO membrane with a solid polymer electrolyte based on poly(ε-caprolactone). The solid hybrid electrolyte (SHE) was a flexible thin film with high ionic conductivity, superior electrochemical stability, high Li⁺ transference number, enhanced thermal stability, and improved Li metal electrode-solid electrolyte interfacial stability. A solid-state Li/LiNi_0.78Co_0.10Mn_0.12O₂ cell assembled with the hybrid electrolyte exhibited good cycling performance, in terms of discharge capacity, cycling stability, and rate capability. Accordingly, the SHE using a vertically aligned LLZO membrane is a promising solid electrolyte for realizing safe, high-performance ASSLBs.

Amorphous Phase Induced Lithium Dendrite Suppression in Glass-Ceramic Garnet-Type Solid Electrolytes

Lithium metal-based solid-state batteries (SSBs) have attracted much attention due to their potentially higher energy densities and improved safety compared with lithium-ion batteries. One of the most promising solid electrolytes, garnet-type Li₇La₃Zr₂O₁₂ (LLZO), has been investigated intensively in recent years. It enables the use of a lithium metal anode, but its application is still challenging because of lithium dendrites that grow at voids, cracks, and grain boundaries of sintered bodies during cycling of the battery cell. In this work, glass-ceramic Ta-doped LLZO produced in a unique melting process was investigated. Upon cooling, an amorphous phase is generated intrinsically, whose composition and fraction are adjusted during the process. Herein, it was set to about 4 wt % containing Li₂O and a Li₂O-SiO₂ phase. During sintering, it was shown to segregate into the grain boundaries and decrease porosity via liquid phase sintering. Sintering temperature and sintering time were found to be reduced compared with the LLZO fabricated by a solid-state reaction while maintaining high density and ionic conductivity. The glass-ceramic sintered at 1130 °C for 0.5 h showed a room-temperature ionic conductivity of 0.64 mS cm^-1. Most importantly, the evenly distributed amorphous phase along the grain boundaries effectively hinders lithium dendrite growth. Besides mechanically blocking pores and voids, it helps to prevent inhomogeneous distribution of current density. The critical current density (CCD) of the Li|LLZTO|Li symmetric cell was determined as 1.15 mA cm^-2, and in situ lithium plating experiments in a scanning electron microscope revealed superior dendrite stability properties. Therefore, this work provides a promising strategy to prepare a dense and dendrite-suppressing solid electrolyte for future implementation in SSBs.

Pressure Dependence of Solid Electrolyte Ionic Conductivity: A Particle Dynamics Study

The search for safe, reliable, and compact high-capacity energy storage devices has led to increased interest in all-solid-state battery research. The use of solid electrolytes provides enhanced safety and durability due to their reduced flammability and increased mechanical strength compared to organic liquid electrolytes. Still, the use of solid electrolytes remains challenging. A significant issue is their generally low Li-ion conductivity, which depends on the lattice diffusion of Li ions through the solid phase, as well as on the limited contact area between the electrolyte particles. While the lattice diffusion can be addressed through the chemistry of the solid electrolyte material, the contact area is a mechanical and structural problem of packing and compression of the electrolyte particles depending on their size and shape. This work studies the effect of pressurization on the electrolyte conductivity exploring cases of low as well as high grain boundary (GB) conductivity, compared to the bulk conductivity. Scaling dependence, σ ∼ P^η, of the conductivity σ with pressure P is revealed. For an idealized electrolyte represented as spheres in hexagonal closely packed configuration, η = 2/3 and η = 1/3 have been theoretically calculated for the two cases of low and high GB conductivity, respectively. For randomly packed spheres, the equivalent exponent values were numerically estimated to be approximately 3/4 and 1/2, respectively, which are higher than the closed packed values due to the additional decrease of porosity with the increase in pressure. As demonstrated in the study, experimental measurement of η can indicate which type of bulk or GB conductivity is dominant in a particular electrolyte powder and could be used in addition to electrochemical impedance spectroscopy measurements.

Correlating Nanoscale Structures with Electrochemical Properties of Solid Electrolyte Interphases in Solid-State Battery Electrodes

Here, we investigate the nonlinear relationship between the content of solid electrolytes in composite electrodes and the irreversible capacity via the degree of nanoscale uniformity of the surface morphology and chemical composition of the solid electrolyte interphase (SEI) layer. Using electrochemical strain microscopy (ESM) and X-ray photoelectron spectroscopy (XPS), changes of the chemical composition and morphology (Li and F distribution) in SEI layers on the electrodes as a function of solid electrolyte contents are analyzed. As a result, we find that the solid electrolyte content affects the variation of the SEI layer thickness and chemical distributions of Li and F ions in the SEI layer, which, in turn, influence the Coulombic efficiency. This correlation determines the composition of the composite electrode surface that can maximize the physical and chemical uniformity of the solid electrolyte on the electrode, which is a key parameter to increase electrochemical performance in solid-state batteries.

Phase-Changeable Dynamic Conformal Electrode/electrolyte Interlayer enabling Pressure-Independent Solid-State Lithium Metal Batteries

Lithium-metal-based solid-state batteries (Li-SSBs) are one of the most promising energy storage devices due to their high energy densities. However, under insufficient pressure constraints (<MPa-level), Li-SSBs usually exhibit poor electrochemical performances owing to the continuous interfacial degradation between the solid-state electrolyte (SSE) and electrodes. Herein, a phase-changeable interlayer is developed to construct the self-adhesive and dynamic conformal electrode/SSE contact in Li-SSBs. The strong adhesive and cohesive strengths of the phase-changeable interlayer enable Li-SSBs to resist up to 250 N pulling force (=1.9 MPa), affording Li-SSBs ideal interfacial integrality even without extra stack pressure. Remarkably, this interlayer exhibits a high ionic conductivity of 1.3 × 10^-3 S cm^-1 , attributed to the shortened steric solvation hindrance and optimized Li⁺ coordination structure. Furthermore, the changeable phase property of the interlayer endows Li-SSBs with a healable Li/SSE interface, accommodating the stress-strain evolution of the lithium metal and constructing the dynamic conformal interface. Consequently, the contact impedance of the modified solid symmetric cell exhibits a pressure-independent manner and does not increase over 700 h (0.2 MPa). The LiFePO₄ pouch cell with the phase-changeable interlayer shows 85% capacity retention after 400 cycles at a low pressure of 0.1 MPa.

Scalable Glass-Fiber-Polymer Composite Solid Electrolytes for Solid-State Sodium-Metal Batteries

In this work, we report a method for producing a thin (<50 μm), mechanically robust, sodium-ion conducting composite solid electrolyte (CSE) by infiltrating the monomers of polyethylene glycol diacrylate (PEGDA) and polyethylene glycol (PEG) and either NaClO₄ or NaFSI salt into a silica-based glass-fiber matrix, followed by an UV-initiated in situ polymerization. The glass fiber matrix provided mechanical strength to the CSE and enabled a robust, self-supporting separator. This strategy enabled the development of CSEs with high loadings of PEG as a plasticizer to enhance the ionic conductivity. The fabrication of these CSEs was done under ambient conditions, which was highly scalable and can be easily implemented in roll-to-roll processing. While NaClO₄ was found to be unstable with the sodium-metal anode, the use of a NaFSI salt was found to promote stable stripping and plating in a symmetric cell, reaching current densities of as high as 0.67 mA cm^-2 at 60 °C. The PEGDA + PEG + NaFSI separators were then used to form solid-state full cells with a cobalt-free, low-nickel layered Na_2/3Ni_1/3Mn_2/3O₂ cathode and a sodium-metal anode, achieving a full capacity utilization exhibiting 70% capacity retention after 50 cycles at a cycling rate of C/5 at 60 °C.

LiNi0.6Co0.2Mn0.2O2 Cathode-Solid Electrolyte Interfacial Behavior Characterization Using Novel Method Adopting Microcavity Electrode

Due to the limitations of organic liquid electrolytes, current development is towards high performance all-solid-state lithium batteries (ASSLBs). For high performance ASSLBs, the most crucial is the high ion-conducting solid electrolyte (SE), with a focus on interface analysis between SE and active materials. In the current study, we successfully synthesized the high ion-conductive argyrodite-type (Li₆PS₅Cl) solid electrolyte, which has 4.8 mS cm^-1 conductivity at room temperature. Additionally, the present study suggests the quantitative analysis of interfaces in ASSLBs. The measured initial discharge capacity of a single particle confined in a microcavity electrode was 1.05 nAh for LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622)-Li₆PS₅Cl solid electrolyte materials. The initial cycle result shows the irreversible nature of active material due to the formation of the solid electrolyte interphase (SEI) layer on the surface of the active particle; further second and third cycles demonstrate high reversibility and good stability. Furthermore, the electrochemical kinetic parameters were calculated through the Tafel plot analysis. From the Tafel plot, it is seen that asymmetry increases gradually at high discharge currents and depths, which rise asymmetricity due to the increasing of the conduction barrier. However, the electrochemical parameters confirm the increasing conduction barrier with increased charge transfer resistance.

Covalent Organic Framework with Multi-Cationic Molecular Chains for Gate Mechanism Controlled Superionic Conduction in All-Solid-State Batteries

Although solid-state batteries (SSBs) are high potential in achieving better safety and higher energy density, current solid-state electrolytes (SSEs) cannot fully satisfy the complicated requirements of SSBs. Herein, a covalent organic framework (COF) with multi-cationic molecular chains (COF-MCMC) was developed as an efficient SSE. The MCMCs chemically anchored on COF channels were generated by nano-confined copolymerization of cationic ionic liquid monomers, which can function as Li+ selective gates. The coulombic interaction between MCMCs and anions leads to easier dissociation of Li+ from coordinated states, and thus Li+ transport is accelerated. While the movement of anions is restrained due to the charge interaction, resulting in a high Li+ conductivity of 4.9×10-4 S cm-1and Li+ transference number of 0.71 at 30 °C. The SSBs with COF-MCMC demonstrate an excellent specific energy density of 403.4 Wh kg-1 with high cathode loading and limited Li metal source.

Suppressing Unfavorable Interfacial Reactions Using Polyanionic Oxides as Efficient Buffer Layers: Low-Cost Li3PO4 Coatings for Sulfide-Electrolyte-Based All-Solid-State Batteries

The poor electrochemical performance of all solid-state batteries (ASSBs) that use sulfide electrolytes can be attributed to undesirable side reactions at the cathode/sulfide-electrolyte interface; this issue can be addressed via surface coating. Ternary oxides such as LiNbO₃ and Li₂ZrO₃ are generally used as coating materials because of their high chemical stabilities and ionic conductivities. However, their relative high cost discourages their use in mass production. In this study, Li₃PO₄ was introduced as a coating material for ASSBs, because phosphates possess good chemical stabilities and ionic conductivities. Phosphates also prevent the exchange of S^2- and O^2- in the electrolyte and cathode and, thus, inhibit interfacial side reactions caused by ionic exchange, because they contain the same anion (O^2-) and cation (P⁵⁺) species as those present in the cathode and sulfide electrolyte, respectively. Furthermore, the Li₃PO₄ coatings can be prepared using low-cost source materials such as polyphosphoric acid and lithium acetate. We investigated the electrochemical performance of the Li₃PO₄-coated cathodes and found that the Li₃PO₄ coating significantly improved the discharge capacities, rate capabilities, and cyclic performances of the all-solid-state cell. While the discharge capacity of the pristine cathode was ∼181 mAh·g^-1, that of 0.15 wt % Li₃PO₄-coated cathode was ∼194-195 mAh·g^-1. And the capacity retention of the Li₃PO₄-coated cathode over 50 cycles was much superior (∼84-85%) to that of the pristine sample (∼72%). Simultaneously, the Li₃PO₄ coating reduced the side reactions and interdiffusion at the cathode/sulfide-electrolyte interfaces. The results of this study demonstrate the potential of low-cost polyanionic oxides, such as Li₃PO₄, as commercial coating materials for ASSBs.

Thermal Decomposition Assisted Construction of Nano-Li3 N Sites Interface Layer Enabling Homogeneous Li Deposition

Lithium (Li) metal is a highly desirable anode for all-solid-state lithium-ion batteries (ASSLBs) due to its high theoretical capacity and being well matched with solid-state electrolytes. However, the practical applications of Li metal anode are hindered by the uneven Li metal plating/stripping behavior and poor contact between electrolyte and Li anode. Herein, a convenient and efficient strategy to construct the Li₃ N-based interlayer between solid poly(ethylene oxide) (PEO) electrolyte and Li anode is proposed by in situ thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN) additive. The evolved Li₃ N nanoparticles are capable of combining LiF, cyano derivatives and PEO electrolyte to form a buffer layer of about 0.9 μm during the cell cycle, which can buffer Li⁺ concentration and homogenize Li deposition. The Li||Li symmetric cells with Li₃ N-based interlayer show excellent cycle stability at 0.2 mA cm^-2 , which is at least 4 times longer cycle life than that of PEO electrolytes without Li₃ N layer. This work provides a convenient strategy for designing interface engineering between solid-state polymer electrolyte and Li anode.

Single-Ion Conducting Polymeric Protective Interlayer for Stable Solid Lithium-Metal Batteries

With many reported attempts on fabricating single-ion conducting polymer electrolytes, they still suffer from low ionic conductivity, narrow voltage window, and high cost. Herein, we report an unprecedented approach on improving the cationic transport number (t_Li⁺) of the polymer electrolyte, i.e., single-ion conducting polymeric protective interlayer (SIPPI), which is designed between the conventional polymer electrolyte (PVEC) and Li-metal electrode. Satisfied ionic conductivity (1 mS cm^-1, 30 °C), high t_Li⁺ (0.79), and wide-area voltage stability are realized by coupling the SIPPI with the PVEC electrolyte. Benefiting from this unique design, the Li symmetrical cell with the SIPPI shows stable cycling over 6000 h at 3 mA cm^-2, and the full cell with the SIPPI exhibits stable cycling performance with a capacity retention of 86% over 1000 cycles at 1 C and 25 °C. This incorporated SIPPI on the Li anode presents an alternative strategy for enabling high-energy density, long cycling lifetime, and safe and cost-effective solid-state batteries.

Single- to Few-Layer Nanoparticle Cathode Coating for Thiophosphate-Based All-Solid-State Batteries

Bulk-type solid-state batteries (SSBs) composed of lithium thiophosphate superionic solid electrolytes (SEs) and high-capacity cathode active materials (CAMs) have recently attracted much attention for their potential application in next-generation electrochemical energy storage. However, compatibility issues between the key components in this kind of battery system are difficult to overcome. Here, we report on a protective cathode coating that strongly reduces the prevalence of detrimental side reactions between CAM and SE during battery operation. This is demonstrated using preformed HfO₂ nanoparticles as a secondary particle coating for a layered Ni-rich oxide CAM, LiNi_0.85Co_0.1Mn_0.05O₂ (NCM85). The preparation of a stable dispersion of the HfO₂ nanoparticles enabled the deposition of a uniform coating of thickness ≤11 nm. When incorporated into Li₆PS₅Cl-based, pellet-stack SSBs, the coated NCM85 showed superior performance in terms of reversibility, cell capacity, longevity, and rate capability over its uncoated counterpart. The effectiveness of the protective coating in mitigating electro-chemo-mechanical degradation was investigated using a suite of physical and electrochemical characterization techniques. In addition, the adaptability to wet processing of the coated NCM85 is demonstrated in slurry-cast SSBs and liquid-electrolyte-based Li-ion cells.

Li2Se: A High Ionic Conductivity Interface to Inhibit the Growth of Lithium Dendrites in Garnet Solid Electrolytes

All-solid-state Li metal batteries (ASSLBs) are currently regarded as one of the most promising next-generation energy storage technologies because of their great potential in realizing both high energy density and safety. However, the development of high performance ASSLBs is still restricted by the large interfacial resistance and Li dendrite propagation within solid electrolytes. Herein, a simple and efficient interfacial modification strategy is proposed to improve the interfacial contact between Li and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) by introducing a uniform and thin Li₂Se buffer layer. The Li₂Se buffer layer formed by an in situ conversion reaction can not only enhance the wettability of lithium metal toward LLZTO electrolyte but also facilitate uniform lithium plating/stripping. As a result, the interfacial resistance of Li/LLZTO decreased from 270.5 to 5.1 Ω cm², and the lithium symmetric cell can cycle stably for 350 h at a current density of 0.5 mA cm^-2. Meanwhile, the Li|LLZTO-Li₂Se|LiNi_0.8Co_0.1Mn_0.1O₂ full cells exhibit a high initial capacity of 162.3 mAh g^-1 and good cycling stability with a capacity retention of 84.3% after 100 cycles at 0.2 C. These results prove the effectiveness of this modification method and provide new design strategies for the development of high performance ASSLBs.

Fabrication of Oxide-Based All-Solid-State Batteries by a Sintering Process Based on Function Sharing of Solid Electrolytes

Garnet-type Li7La3Zr2O12 (LLZ) has advantages of stability with Li metal and high Li+ ionic conductivity, achieving 1 × 10-3 S cm-1, but it is prone to react with electrode active materials during the sintering process. LISICON-type Li3.5Ge0.5V0.5O4 (LGVO) has the advantage of less reactivity with the electrode active material during the sintering process, but its ionic conductivity is on the order of 10-5 S cm-1. In this study, these two solid electrolytes are combined as a multilayer solid electrolyte sheet, where 2 μm thick LGVO films are coated on LLZ sheets to utilize the advantages of these two solid electrolytes. These two solid electrolytes adhere well through Ge diffusion without significant interfacial resistance. The LLZ-LGVO multilayer is combined with a LiCoO2 positive electrode and a lithium metal anode through annealing at 700 °C. The resultant all-solid-state battery can undergo repeated charge-discharge reactions for over 100 cycles at 25 or 60 °C. The LGVO coating suppresses the increases in the resistance from the solid electrolyte and interfacial resistance induced by annealing by ca. 1/40. As with sulfide-based all-solid-state batteries, function sharing of solid electrolytes will be a promising method for developing advanced oxide-based all-solid-state batteries through a sintering process.

Combating Li metal deposits in all-solid-state battery via the piezoelectric and ferroelectric effects

All-solid-state Li-metal batteries (ASSLBs) are highly desirable, due to their inherent safety and high energy density; however, the irregular and uncontrolled growth of Li filaments is detrimental to interfacial stability and safety. Herein, we report on the incorporation of piezo-/ferroelectric BaTiO₃ (BTO) nanofibers into solid electrolytes and determination of electric-field distribution due to BTO inclusion that effectively regulates the nucleation and growth of Li dendrites. Theoretical simulations predict that the piezoelectric effect of BTO embedded in solid electrolyte reduces the driving force of dendrite growth at high curvatures, while its ferroelectricity reduces the overpotential, which helps to regularize Li deposition and Li⁺ flux. Polarization reversal of soft solid electrolytes was identified, confirming a regular deposition and morphology alteration of Li. As expected, the ASSLBs operating with LiFePO₄/Li and poly(ethylene oxide) (PEO)/garnet solid electrolyte containing 10% BTO additive showed a steady and long cycle life with a reversible capacity of 103.2 mAh g^-1 over 500 cycles at 1 C. Furthermore, the comparable cyclability and flexibility of the scalable pouch cells prepared and the successful validation in the sulfide electrolytes, demonstrating its universal and promising application for the integration of Li metal anodes in solid-state batteries.

3D Impedance Modeling of Metal Anodes in Solid-State Batteries-Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models

A non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a geometric constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.

Grain Boundary Engineering in Ta-Doped Garnet-Type Electrolyte for Lithium Dendrite Suppression

Solid-state lithium batteries (SSLBs) based on Ta-doped Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) suffer from lithium dendrite growth, which hinders their practical application. Herein, first principles simulations indicate that the Ta element prefers to segregate along grain boundaries in the form of Ta₂O₅ precipitates due to a high energy difference induced by Ta doping. Grain boundary engineering is employed to regulate the distribution of the Ta element and enhance the density of LLZTO by introducing the La₂O₃ additive. The sufficient La₂O₃ additive reacts with the Ta₂O₅ precipitates, while the residual La₂O₃ nanoparticles fill up void defects, promoting the homogeneous distribution of the Ta element and improving the relative density to ∼98%. Critical current density of the symmetric Li battery reaches 2.12 mA·cm^-2 at room temperature with the solid-state electrolyte (LLZTO + 5 wt % La₂O₃), which increases by 41% compared to pure LLZTO. SSLBs with the LiFePO₄ cathode achieve a stable cycling performance with a discharge capacity of 138.6 mA·h·g^-1 after 400 cycles at 0.2 C. This work provides theoretical insights into the distribution of Ta-doped LLZTO and inhibits lithium dendrite growth through grain boundary engineering.

Ionically and Electronically Conductive Phases in a Composite Anode for High-Rate and Stable Lithium Stripping and Plating for Solid-State Lithium Batteries

Intensive efforts have been taken to decrease the over-potentials of solid-state lithium batteries. Lowering the anode-electrolyte interface resistance is an effective method. Compared to simply improving the interface contact, constructing both ionically and electronically conductive phases within the anode demonstrates superior improvement in reducing the interface resistance and promoting electrochemical stability. However, complex preparation procedures are usually involved in the construction of the conductive phases and the loading of metallic lithium. Herein, a composite anode containing metallic lithium and well-dispersed ionically conductive Li₃N and electronically conductive components (Fe, Fe₃C, and amorphous carbon) shows an effective decrease in lithium stripping/plating over-potentials at high current densities of up to 3 mA cm^-2. The unique dual ionically and electronically conductive phases exhibit good cycling stability for 3000 h. A full battery with the composite anode and a LiFePO₄ cathode also demonstrates decent performance. This work confirms the importance of constructing dual conductive phases that are electrochemically stable to Li and will not be consumed during the electrochemical reaction and provides a facile preparation method. The new knowledge discovered and the new methods developed in this work would inspire the future development of new Li-containing composite anodes.

Interplay of Dynamic Constriction and Interface Morphology between Reversible Metal Anode and Solid Electrolyte in Solid State Batteries

In an all-solid-state battery, the electrical contact between its individual components is of key relevance in addition to the electrochemical stability of its interfaces. Impedance spectroscopy is particularly suited for the non-destructive investigation of interfaces and of their stability under load. Establishing a valid correlation between microscopic processes and the macroscopic impedance signal, however, is challenging and prone to errors. Here, we use a 3D electric network model to systematically investigate the effect of various electrode/sample interface morphologies on the impedance spectrum. It is demonstrated that the interface impedance generally results from a charge transfer step and a geometric constriction contribution. The weights of both signals depend strongly on the material parameters as well as on the interface morphology. Dynamic constriction results from a non-ideal local contact, e.g., from pores or voids, which reduce the electrochemical active surface area only in a certain frequency range. Constriction effects dominate the interface behavior for systems with small charge transfer resistance like garnet-type solid electrolytes in contact with a lithium metal electrode. An in-depth analysis of the origin and the characteristics of the constriction phenomenon and their dependence on the interface morphology is conducted. The discussion of the constriction effect provides further insight into the processes at the microscopic level, which are, e.g., relevant in the case of reversible metal anodes.

Effects of Molecular Weight on the Electrochemical Properties of Poly(vinylidene difluoride)-Based Polymer Electrolytes

Polymer-based electrolytes have attracted ever-increasing attention for solid-state batteries due to their excellent flexibility and processability. Among them, poly(vinylidene difluoride) (PVDF)-based electrolytes with high ionic conductivity, wide electrochemical stability window, and good mechanical properties show great potential and have been widely investigated by using different Li salts, solvents, and inorganic fillers. Here, we report the influence of the molecular weight of PVDF itself on the electrochemical properties of the electrolytes by using two kinds of common PVDF polymers, i.e., PVDF 761 and 5130. Our results demonstrate that the electrolyte with a larger molecular weight (PVDF 5130) has a denser structure and lower crystallinity, and thus much better electrochemical performance, than one with a smaller molecular weight (PVDF 761). With PVDF 5130, the LiFePO₄-based solid-state cells present a steady cycling performance with a capacity retention of 85% after 1000 cycles at 1 C and 30 °C. The cycle life of the LiCoO₂-based solid-state cells is also extended by using PVDF 5130.

Redox Catalysis Promoted Activation of Sulfur Redox Chemistry for Energy-Dense Flexible Solid-State Zn-S Battery

In aqueous Zn-ion batteries, the intercalation chemistry often foil attempts at the realization of high energy density. Unlocking the full potential of zinc-sulfur redox chemistry requires the manipulation of the feedbacks between kinetic response and the cathode's composition. The cell degradation mechanism also should be tracked simultaneously. Herein, we design a high-energy Zn-S system where the high-capacity cathode was fabricated by in situ interfacial polymerization of Fe(CN)₆^4--doped polyaniline within the sulfur nanoparticle. Compared with sulfur, the Fe^II/III(CN)₆^4/3- redox mediators exhibit substantially faster cation (de)insertion kinetics. The higher cathodic potential (Fe^II(CN)₆^4-/Fe^III(CN)₆^3- ∼ 0.8 V vs S/S^2- ∼ 0.4 V) spontaneously catalyzes the full reduction of sulfur during battery discharge (S₈ + Zn₂Fe^II(CN)₆ ↔ ZnS + Zn_1.5Fe^III(CN)₆, ΔG = -24.7 kJ mol^-1). The open iron redox species render a lower energy barrier to ZnS activation during the reverse charging process, and the facile Zn²⁺ intercalative transport facilitates highly reversible conversion between S and ZnS. The yolk-shell structured cathode with 70 wt % sulfur delivers a reversible capacity of 1205 mAh g^-1 with a flat operation voltage of 0.58 V, a fade rate over 200 cycles of 0.23%/cycle, and an energy density of 720 Wh kg_sulfur^-1. A range of ex situ investigations reveal the degradation nature of Zn-S cells: aggregation of inactive ZnS nanocrystals rather than the depletion of Zn anode. Impressively, the flexible solid-state Zn battery employing the composite cathode was assembled, realizing an energy density of 375 Wh kg_sulfur^-1. The proposed redox electrocatalysis effect provides reliable insights into the tunable Zn-S chemistry.

A Two-Parameter Space to Tune Solid Electrolytes for Lithium Dendrite Constriction

Li dendrite penetration, and associated microcrack propagation, at high current densities is one main challenge to the stable cycling of solid-state batteries. The interfacial decomposition reaction between Li dendrite and a solid electrolyte was recently used to suppress Li dendrite penetration through a novel effect of "dynamic stability". Here we use a two-parameter space to classify electrolytes and propose that the effect may require the electrolyte to occupy a certain region in the space, with the principle of delicately balancing the two property metrics of a sufficient decomposition energy with the Li metal and a low critical mechanical modulus. Furthermore, in our computational prediction prepared using a combination of high-throughput computation and machine learning, we show that the positions of electrolytes in such a space can be controlled by the chemical composition of the electrolyte; the compositions can also be attained by experimental synthesis using core-shell microstructures. The designed electrolytes following this principle further demonstrate stable long cycling from 10 000 to 20 000 cycles at high current densities of 8.6-30 mA/cm² in solid-state batteries, while in contrast the control electrolyte with a nonideal position in the two-parameter space showed a capacity decay that was faster by at least an order of magnitude due to Li dendrite penetration.

Solid-State Lithium Metal Battery of Low Capacity Fade Enabled by a Composite Electrolyte with Sulfur-Containing Oligomers

A solid-state lithium metal battery of low capacity fade is acquired using the electrolyte membrane of a polyurethane-acrylate-thiocarbonate (PUAT) oligomer, macromolecules, lithium salt, and an oxide additive. Two types of composite electrolytes have been prepared: the free-standing electrolyte (PUAT-FS) and the electrode-coated electrolyte (PUAT-EC). Featuring a less PUAT content and a finer granular size, PUAT-FS is less ion-conductive than PUAT-EC; 0.44 mS cm^-1 in contrast to 0.51 mS cm^-1 at room temperature. Nonetheless, the lithium iron phosphate battery of PUAT-FS is far superior to that of PUAT-EC in terms of cycling stability. When cycled at 0.1C and room temperature, the PUAT-FS battery reaches a maximum discharge capacity of 169.7 mAh g^-1 at its 20th cycle and decreases to 141.0 mAh g^-1 at the 500th cycle, 83.1% retention. The capacity fading rate of the PUAT-FS battery is 0.034% per cycle at 0.1C, significantly less than that of the PUAT-EC battery, 0.138% per cycle. Other maximum capacities and fading rates of the PUAT-FS battery are 152.5 mAh g^-1 and 0.050% per cycle at 0.2C in 800 cycles and 126.1 mAh g^-1 and 0.051% per cycle at 0.5C in 1000 cycles. These features of a low fading rate and high capacity are attributed to a balanced ratio of oligomer to macromolecule (1:1 w/w) in the free-standing electrolyte and the sulfur-containing oligomer.

High-Power Bipolar Solid-State Batteries Enabled by In-Situ-Formed Ionogels for Vehicle Applications

Employing solid electrolytes (SEs) for lithium-ion batteries can boost the battery tolerance under abusive conditions and enable the implementation of bipolar cell stacking, leading to higher cell energy and power density as well as simplified thermal management. In this context, a bipolar solid-state battery (SSB) has received ever-increasing attention in recent years. However, poor solid-solid interfacial contact within the bipolar SSB deteriorates the battery power capability, representing a technical challenge for vehicle applications. In this work, a bipolar SSB pouch cell with two cell units connected in series is demonstrated without any short circuit or current leakage. With the assistance of an in-situ-formed nonflammable ionogel at particle-to-particle interfaces, the constructed bipolar cell manifests superior power capability and can meet the engineering cold crank requirements in 0, -10, and -18 °C environments. Furthermore, the excellent tolerance of the ionogel-introduced bipolar SSB under abusive conditions was proved by folding, cutting, and burning the cells. The above salient features suggested that the developed strategy herein holds promise to advance the next-generation high-performance SSBs.

Actualizing a High-Energy Bipolar-Stacked Solid-State Battery with Low-Cost Mechanically Robust Nylon Mesh-Reinforced Composite Polymer Electrolyte Membranes

To meet the rapidly growing and diversified demand for energy storage, advanced rechargeable batteries with high-performance materials and efficient battery configuration are widely being exploited and developed. Bipolar-stacked electrode coupling with solid-state electrolytes enables achieving batteries with high output voltage, high energy density, and simple components. Here, a polymer electrolyte membrane is designed with polyethylene oxide containing bis(trifluoromethanesulfonyl)-imide as the electrolyte, succinonitrile as the plasticizer, and nylon mesh as a reinforcement for the bipolar-stacked battery. The as-prepared nylon mesh-reinforced polymer electrolyte membrane shows advantageous features, that is, excellent ionic conductivity (3.38 × 10^-4 S cm^-1) at room temperature, low interface impedance, and good tolerance against the expansion caused by the plating/stripping of the Li anode and the electrode upon cycling. When used as a polymer electrolyte membrane in the bipolar-stacked battery, the LiFePO₄(LFP)-Li₄Ti₅O₁₂(LTO) cell with three cells connected in series delivers a higher discharge voltage (5.4 V) and a volumetric energy density (0.328 mW h cm^-3), nearly 3 times as much as that of the LFP-LTO battery. In addition, LiFePO₄-Li pouch cells using the polymer electrolyte membrane can sustain the abuse tests including bending, cutting, and nail penetration well. These results pave a new avenue to develop high-performance polymer electrolyte membranes and allow for the design of high-voltage and volumetric energy density bipolar-stacked batteries.

Impacts of vacancy-induced polarization and distortion on diffusion in solid electrolyte Li3OCl

Lithium-rich oxychloride antiperovskites are promising solid electrolytes for enabling next-generation batteries. Here, we report a comprehensive study varying Li⁺ concentrations in [Formula: see text] using ab initio molecular dynamics simulations. The simulations accurately capture the complex interactions between Li⁺ vacancies ([Formula: see text]), the dominant mobile species in [Formula: see text]. The [Formula: see text] polarize and distort the host lattice, inducing additional non-vacancy-mediated diffusion mechanisms and correlated diffusion events that reduce the activation energy barrier at concentrations as low as 1.5% [Formula: see text]. Our analyses of discretized diffusion events in both space and time illustrate the critical interplay between correlated dynamics, polarization and local distortion in promoting ionic conductivity in [Formula: see text]. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Development and Progression of Polymer Electrolytes for Batteries: Influence of Structure and Chemistry

Polymer electrolytes continue to offer the opportunity for safer, high-performing next-generation battery technology. The benefits of a polymeric electrolyte system lie in its ease of processing and flexibility, while ion transport and mechanical strength have been highlighted for improvement. This report discusses how factors, specifically the chemistry and structure of the polymers, have driven the progression of these materials from the early days of PEO. The introduction of ionic polymers has led to advances in ionic conductivity while the use of block copolymers has also increased the mechanical properties and provided more flexibility in solid polymer electrolyte development. The combination of these two, ionic block copolymer materials, are still in their early stages but offer exciting possibilities for the future of this field.

All-Solid-State Thin Film μ-Batteries for Microelectronics

Continuous advances in microelectronics and micro/nanoelectromechanical systems enable the use of microsized energy storage devices, namely solid-state thin-film μ-batteries. Different from the current button batteries, the μ-battery can directly be integrated on microchips forming a very compact "system on chip" since no liquid electrolyte is used in the μ-battery. The all-solid-state battery (ASSB) that uses solid-state electrolyte has become a research trend because of its high safety and increased capacity. The solid-state thin-film μ-battery belongs to the family of ASSB but in a small format. However, a lot of scientific and technical issues and challenges are to be resolved before its real application, including the ionic conductivity of the solid-state electrolyte, the electrical conductivity of the electrode, integration technologies, electrochemical-induced strain, etc. To achieve this goal, understanding the processing of thin films and fundamentals of ion transfer in the solid-state electrolytes and hence in the μ-batteries becomes utmost important. This review therefore focuses on solid-state ionics and provides inside of ion transportation in the solid state and effects of chemistry on electrochemical behaviors and proposes key technology for processing of the μ-battery.

Stable Interfaces in a Sodium Metal-Free, Solid-State Sodium-Ion Battery with Gradient Composite Electrolyte

Composite electrolytes (CE) combining a ceramic filler and a polymer matrix is an effective way to enhance battery safety. But the increased ceramic filler mass fraction decreases the flexibility, which increases the interfacial resistance. To alleviate interfacial resistance further, a gradient composite electrolyte (GCE) using a Sc, Ge-doped Na₃Zr₂Si₂PO₁₂ (NZSP) as the ceramic filler and poly(ethylene oxide) (PEO) as the polymer matrix is proposed. The outer layer contains a low concentration of ceramic filler to improve interfacial contact, and the central layer contains a high concentration of ceramic filler to inhibit dendrite penetration. This GCE possesses an enhanced conductivity (4.0 × 10^-5 S cm^-1 at 30 °C) and a reduced interfacial resistance. Furthermore, the safety was boosted using Sn₄P₃@CNT/C as the high-capacity anode active material and Na₃V₂(PO₄)₃ (NVP) as the cathode active material. This ultrasafe sodium metal-free, solid-state sodium-ion battery (SSSIB) displays an impressive cycling performance.