Artificial Solid Electrolyte Interphases for Lithium Metal Electrodes by Wet Processing: The Role of Metal Salt Concentration and Solvent Choice

Customized Solvation Structures for Long-Term Stable Lithium Metal Batteries

Lithium metal batteries (LMBs) suffer from severe lithium dendrite growth and side reactions in conventional carbonate electrolytes, which are characterized by low coulombic efficiency and poor cycling stability, and electrolyte engineering is an effective method for increasing the reversibility of lithium anodes. Herein, the solubility of lithium nitrate (LiNO3), which is almost insoluble in carbonate electrolyte, is improved by adding zinc trifluoroacetate (Zn(TFA)2), and a competitive solvation structure is constructed, forming an anion-enriched Li+ solvation structure, which is conducive to the formation of stable SEI and effectively inhibits adverse side reactions. The lithium metal anode exhibits uniform lithium deposition and extended cycle life, with high reversibility over plating/stripping for 640 h. Furthermore, the Li||LFP full cell with the upgraded carbonate electrolyte can operate steadily for over 300 cycles at 1 C, and the compatibility of the lithium anode with the high-voltage NCM811 cathode are also significantly improved. This work provides a feasible strategy for dependable interfacial chemistry of lithium metal anodes.

Interfacial Engineering of Polymer Solid-State Lithium Battery Electrolytes and Li-Metal Anode: Current Status and Future Directions

A combination of material innovations, advanced manufacturing, battery management systems, and regulatory standards is necessary to improve the energy density and safety of lithium (Li) batteries. High-energy-density solid-state Li-batteries have the potential to revolutionize industries and technologies, making them a research priority. The combination of improved safety and compatibility with high-capacity electrode materials makes solid-stateLi-batteries with polymer solid-electrolytes an attractive option for applications where energy density and safety are critical. While polymer-based solid-state Li-batteries hold enormous promise, there are still several challenges that must be addressed, particularly regarding interface between polymer solid-electrolyte and Lianode. There are significant advancements in improving the performance of solid-state Li batteries, and researchers continue to explore new methods to address these challenges. These improvements are critical for enabling the widespread adoption of solid-state Li-batteries invariety of applications, from electrical vehicles to portable electronics. Here, common polymer solid-electrolyte and its interface challenges with Lianode are first introduced, highlighting the trend in polymer solid-state-electrolyte research toward enhancing stability, safety, and performance of solid-state Li-batteries. This includes developing novel polymer materials with improved properties, exploring advanced fabrication techniques, and integrating these electrolytes into battery designs that optimize both safety and energy density.

In Situ Formation of an Artificial Lithium Oxalate-Rich Solid Electrolyte Interphase on 3D Ni Host for Highly Stable Lithium Metal Batteries

Li metal, with a high theoretical capacity, is considered the most promising anode for next-generation high-energy-density batteries. However, the commercialization of the Li metal anode is limited owing to its high reactivity, significant volume expansion, continuous solid electrolyte interphase (SEI) layer degradation caused by undesirable Li deposition, and uncontrollable dendrite growth. This study demonstrates the in situ construction of a Li2C2O4-enriched SEI layer from NiC2O4 nanowires on three-dimensional Ni foam. The lithiophilic Li2C2O4-enriched SEI layer provides a uniform distribution of the electrical field and sufficient nucleation and deposition sites for Li without dendrite formation. Consequently, the stable Li2C2O4-enriched SEI layer successfully inhibits the formation of lithium dendrites, resulting in reversible Li stripping/plating behavior, maintained over an extended period of 5000 h with a deposition capacity of 1 mAh cm-2 at 1 mA cm-2. Additionally, a high cycling stability is observed in the full cell test with ∼70% capacity retention after 1300 cycles at 3 C. This approach offers a large-scale and facile synthesis process via the in situ precipitation growth of NiC2O4 followed by lithiation to form Li2C2O4. Furthermore, the significant stability of the Li2C2O4-enriched SEI layer aids the design of in situ-constructed SEI layers for highly stable Li metal batteries.

High Rate, Dendrite Free Lithium Metal Batteries of Extended Cyclability via a Scalable Separator Modification Approach

Owing to their great promise of high energy density, the development of safer lithium metal batteries (LMBs) has become the necessity of the hour. Herein, a scalable method based on conventional Celgard membrane (CM) separator modification is adopted to develop high-rate (10 mA cm‒2) dendrite-free LMBs of extended cyclability (>1000 hours, >1500 cycles with 3 mA cm‒2, the bare fails within 50 cycles) with low over potential losses. The CM modification method entails the deposition of thin coatings of (≈6.6 µm) graphitic fluorocarbon (FG) via a large area electrophoretic deposition, where it helps for the formation of a stable LiF and carbon rich solid electrolyte interface (SEI) aiding a uniform lithium deposition even in higher fluxes. The FG@CM delivers a high transport number for Li ion (0.74) in comparison to the bare CM (0.31), indicating a facile Li ion transport through the membrane. A mechanistic insight into the role of artificial SEI formation with such membrane modification is provided here with a series of electrochemical as well as spectroscopic in situ/ex situ and postmortem analyses. The simplicity and scalability of the technique make this approach unique among other reported ones towards the advancement of safer LMBs of high energy and power density.

Lightweight 3D Lithiophilic Graphene Aerogel Current Collectors for Lithium Metal Anodes

Constructing three-dimensional (3D) current collectors is an effective strategy to solve the hindrance of the development of lithium metal anodes (LMAs). However, the excessive mass of the metallic scaffold structure leads to a decrease in energy density. Herein, lithiophilic graphene aerogels comprising reduced graphene oxide aerogels and silver nanowires (rGO-AgNW) are synthesized through chemical reduction and freeze-drying techniques. The rGO aerogels with large specific surface areas effectively mitigate local current density and delay the formation of lithium dendrites, and the lithiophilic silver nanowires can provide sites for the uniform deposition of lithium. The rGO-AgNW/Li symmetric cell presents a stable cycle of about 2000 h at 1 mA cm-2. When coupled with the LiFePO4 cathode, the assembled full cells exhibit outstanding cycle stability and rate performance. Lightweight rGO-AgNW aerogels, as the host for lithium metal, can significantly improve the energy density of lithium metal anodes.

Artificial Interphase Design Employing Inorganic-Organic Components for High-Energy Lithium-Metal Batteries

To increase the energy density of today's lithium batteries, it is necessary to develop an anode with higher energy density than graphite or carbon/silicon composites. Hence, research on metallic lithium has gained a steadily increasing momentum. However, the severe safety issues and poor Coulombic efficiency of this highly reactive metal hinder its practical application in lithium-metal batteries (LMBs). Herein, the development of an artificial interphase is reported to enhance the reversibility of the lithium stripping/plating process and suppress the parasitic reactions with the liquid organic carbonate-based electrolyte. This artificial interphase is spontaneously formed by an alloying reaction-based coating, forming a stable inorganic/organic hybrid interphase. The accordingly modified lithium-metal electrodes provide substantially improved cycle life to symmetric Li||Li cells and high-energy Li||LiNi_0.8Co_0.1Mn_0.1O₂ cells. For these LMBs, 7 μm thick lithium-metal electrodes have been employed while applying a current density of 1.0 mA cm^-2, thus highlighting the great potential of this tailored interphase.

In-Situ Constructing A Heterogeneous Layer on Lithium Metal Anodes for Dendrite-Free Lithium Deposition and High Li-ion Flux

Constructing efficient artificial solid electrolyte interface (SEI) film is extremely vital for the practical application of lithium metal batteries. Herein, a dense artificial SEI film, in which lithiophilic Zn/Li_x Zn_y are uniformly but nonconsecutively dispersed in the consecutive Li⁺ -conductors of Li_x SiO_y , Li₂ O and LiOH, is constructed via the in situ reaction of layered zinc silicate nanosheets and Li. The consecutive Li⁺ -conductors can promote the desolvation process of solvated-Li⁺ and regulate the transfer of lithium ions. The nonconsecutive lithiophilic metals are polarized by the internal electric field to boost the transfer of lithium ions, and lower the nucleation barrier. Therefore, a low polarization of ≈50 mV for 750 h at 2.0 mA cm^-2 in symmetric cells, and a high capacity retention of 99.2 % in full cells with a high lithium iron phosphate areal loading of ≈13 mg cm^-2 are achieved. This work offers new sights to develop advanced alkali metal anodes for efficient energy storage.

Optical Study of the Surface Film Formed during Li-Metal Deposition and Dissolution Investigated by Surface Plasmon Resonance Spectroscopy

The features of the electrode surface film during Li-metal deposition and dissolution cycles are essential for understanding the mechanism of the negative electrode reaction in Li-metal battery cells. The physical and chemical property changes of the interface during the initial stages of the reaction should be investigated under operando conditions. In this study, we focused on the changes in the optical properties of the electrode surface film of the negative electrode of a Li-metal battery. Cu-based electrochemical surface plasmon resonance spectroscopy (EC-SPR) was applied because of its high sensitivity to optical phenomena on the electrode surface and its stability against Li-metal deposition. The feature of SPR reflectance dip depends on the optical properties of the electrode surface; namely, the wavelength and depth of the reflectance dip directly connected the refractive index and extinction coefficient (color of electrode surface film), which was confirmed by reflectance simulation. In the operando EC-SPR experiment, various changes in optical properties were clearly observed during the cycles. In particular, the change in the extinction coefficient was more remarkable at the second process than the first process of Li-metal deposition. By electrochemical quartz-crystal microbalance (EQCM) measurements, surface film formation was confirmed during the first Li-metal deposition process. The remarkable change in the extinction coefficient is based on the color change of the surface film, which is caused by the chemical condition change during Li-metal deposition cycles.

Regulating the Li Nucleation/Growth Behavior via Cu2O Nanowire Array and Artificial Solid Electrolyte Interphase toward Highly Stable Li Metal Anode

Lithium (Li) metal has been considered to be the most promising anode material for next-generation rechargeable batteries. Unfortunately, the hazards induced by dendrite growth and volume fluctuation hinder its commercialized application. Here, a three-dimensional (3D) current collector composed of a vertically aligned Cu2O nanowire that is tightly coated with a polydopamine protective layer is developed to solve the encountered issues of lithium metal batteries (LMBs). The Cu2O nanowire arrays (Cu2O NWAs) provide abundant lithiophilic sites for inducing Li nucleation selectively to form a thin Li layer around the nanowires and direct subsequent Li deposition. The well-defined nanochannel works well in confining the Li growth spatially and buffering the volume change during the repeated cycling. The PDA coatings adhered onto the outline of the Cu2O NWAs serve as the artificial solid electrolyte interface to isolate the electrode and electrolyte and retain the interfacial stability. Moreover, the increased specific area of copper foam (CF) can dissipate the local current density and further suppress the growth of Li dendrites. As a result, CF@Cu2O NWAs@PDA realizes a dendrite-free morphology and the assembled symmetrical batteries can work stably for over 1000 h at 3 mA cm-2. When CF@Cu2O NWAs@PDA is coupled with a LiFePO4 cathode, the full cells exhibit improved cycle stability and rate performance.

Confined Lithium Deposition Triggered by an Integrated Gradient Scaffold for a Lithium-Metal Anode

Constructing a composite lithium anode with a rational structure has been considered as an effective approach to regulate and relieve the tough problems of a sparkling Li anode. However, the potential short circuits risk that Li deposition at the surface of the framework has not yet been resolved. Here, we present a simple regulating-deposition strategy to guide the preferentially bottom-up deposition/growth of Li. The triple-gradient structure of modified porous copper with electrical passivation (top) and chemical activation (bottom) shows significant improvements in the morphological stability and electrochemical performance. Meanwhile, the in situ generation of Li₂Se can as an advanced artificial SEI layer be devoted to homogeneous Li plating/stripping. As a result, the composite anode exhibits a long-term cycling over 250 cycles with a high average CE of 98.2% at 1 mA cm^-2. Furthermore, a capacity retention of 94.4% in full cells can be achieved when pairing with LiFePO₄ as the cathode. These results ensure a bright direction for developing high-performance Li metal anodes.

Effect of the supergravity on the formation and cycle life of non-aqueous lithium metal batteries

Extra-terrestrial explorations require electrochemical energy storage devices able to operate in gravity conditions different from those of planet earth. In this context, lithium (Li)-based batteries have not been fully investigated, especially cell formation and cycling performances under supergravity (i.e., gravity > 9.8 m s-2) conditions. To shed some light on these aspects, here, we investigate the behavior of non-aqueous Li metal cells under supergravity conditions. The physicochemical and electrochemical characterizations reveal that, distinctly from earth gravity conditions, smooth and dense Li metal depositions are obtained under supergravity during Li metal deposition on a Cu substrate. Moreover, supergravity allows the formation of an inorganic-rich solid electrolyte interphase (SEI) due to the strong interactions between Li+ and salt anions, which promote significant decomposition of the anions on the negative electrode surface. Tests in full Li metal pouch cell configuration (using LiNi0.8Co0.1Mn0.1O2-based positive electrode and LiFSI-based electrolyte solution) also demonstrate the favorable effect of the supergravity in terms of deposition morphology and SEI composition and ability to carry out 200 cycles at 2 C (400 mA g-1) rate with a capacity retention of 96%.

Revamping Lithium-Sulfur Batteries for High Cell-Level Energy Density by Synergistic Utilization of Polysulfide Additives and Artificial Solid-Electrolyte Interphase Layers

Despite the high theoretical capacity of lithium-sulfur (Li-S) batteries, a high cell-level energy density and a long cycling life are barely achieved, mainly due to the large electrolyte-to-sulfur ratio, polysulfide (PS) shuttle causing the loss of active sulfur, and the formation of passivation layers on the Li anode. To raise the energy density, holding PS in the cathode has been the most popular approach. Still, it has failed, particularly, when the sulfur loading is high enough to have energy densities similar to those of commercial Li-ion batteries. Here, a practical approach of achieving high "cell-level" energy densities is attempted using lithium PS (LPS)-containing electrolytes instead of a pure electrolyte, reducing the electrolyte-to-sulfur ratio and PS diffusion out of the cathode due to concentration differences. Meanwhile, the persistent problems including PS passivation and Li dendrites are suppressed using Li₂ S-phobic artificial solid-electrolyte interphase (A-SEI) layers on Li metal. The synergistic effects from the LPS additives and A-SEI result in a superior cell-level volumetric energy density of 650 Wh L^-1 as well as large cumulative energy densities considering cycling life. This approach provides an important stepping stone to realize commercial Li-S batteries rivaling the current Li-ion batteries.

Interfacial processes in electrochemical energy systems

Electrochemical energy systems such as batteries, water electrolyzers, and fuel cells are considered as promising and sustainable energy storage and conversion devices due to their high energy densities and zero or negative carbon dioxide emission. However, their widespread applications are hindered by many technical challenges, such as the low efficiency and poor long-term cyclability, which are mostly affected by the changes at the reactant/electrode/electrolyte interfaces. These interfacial processes involve ion/electron transfer, molecular/ion adsorption/desorption, and complex interface restructuring, which lead to irreversible modifications to the electrodes and the electrolyte. The understanding of these interfacial processes is thus crucial to provide strategies for solving those problems. In this review, we will discuss different interfacial processes at three representative interfaces, namely, solid-gas, solid-liquid, and solid-solid, in various electrochemical energy systems, and how they could influence the performance of electrochemical systems.

Effective Solid Electrolyte Interphase Formation on Lithium Metal Anodes by Mechanochemical Modification

Lithium metal batteries are gaining increasing attention due to their potential for significantly higher theoretical energy density than conventional lithium ion batteries. Here, we present a novel mechanochemical modification method for lithium metal anodes, involving roll-pressing the lithium metal foil in contact with ionic liquid-based solutions, enabling the formation of an artificial solid electrolyte interphase with favorable properties such as an improved lithium ion transport and, most importantly, the suppression of dendrite growth, allowing homogeneous electrodeposition/-dissolution using conventional and highly conductive room temperature alkyl carbonate-based electrolytes. As a result, stable cycling in symmetrical Li∥Li cells is achieved even at a high current density of 10 mA cm^-2. Furthermore, the rate capability and the capacity retention in NMC∥Li cells are significantly improved.

Effect of Electrolyte Additives on the Cycling Performance of Li Metal and the Kinetic Mechanism Analysis

Lithium metal secondary batteries (LMBs) have extremely high energy densities and are considered the most promising energy storage and conversion systems in the future. We start with the formation and growth process of the Li metal deposited layer to reveal and clarify the reasons for the apparent comprehensive performance of the Li metal anode. Specifically, under the conditions of ether electrolyte and typical additives, the apparent Coulombic efficiency, micromorphology of the deposition layer, SEI information, and the kinetic mechanism of the Li plating/stripping process under a series of current density conditions are studied. The results show that in the electrolyte containing LiNO₃, Li metal exhibits excellent cycling performance, the Li plating layer is denser, and the particles in the plating layer are smooth and uniform. In the electrolyte containing FEC, the performance of Li metal is also improved to some extent. Then, we use microelectrode technology to obtain the kinetic parameters of elementary steps in the deposition process of Li metal and find that the stability of the kinetic parameters of mass transfer, interface, and surface steps and their good matching degree are conducive to the good cycling stability of the Li metal anode. This study reveals the kinetic relationship among the apparent comprehensive performances of Li metal, the electrolyte composition, and operating conditions, which provides a reliable dynamic reference for screening and optimizing electrolytes.