Correlating Kinetics to Cyclability Reveals Thermodynamic Origin of Lithium Anode Morphology in Liquid Electrolytes

Removing α-H in Carboxylate-Based Electrolytes for Stable Lithium Metal Batteries

Although carboxylate esters greatly improve the cold weather performance of graphite-based lithium-ion batteries utilized in arctic expeditions, the underlying cause of the incompatibility between carboxylates and lithium (Li) anodes has not been sufficiently explained, resulting in the greatly restricted usage of carboxylate in lithium metal batteries (LMBs). Herein, we reveal the serious parasitic reactions between carboxylate α-H atoms and Li metal are the culprits that render carboxylate-based ineffectiveness for LMBs. By replacing all α-H atoms with fluorine atoms and methyl groups, we successfully construct inert carboxylates and find the ions/molecules distribution in electric-double-layer (EDL) can be manipulated at a molecular-level. The unique structure ensuring more anions are positioned closer to the Li surface in the EDL of the inert carboxylate-based electrolyte, the morphology of the deposited Li is significantly regulated and the chemical corrosion gets effectively inhibited, as a consequence of remarkable extending lifespan of carboxylate-based LMBs with routine salt concentration and few additives. More generally, using carboxylates lacking α-H atoms offer a realistic approach to increase the variety of solvents that can be used in LMBs electrolytes.

Operando Discovery of LiNO3-Induced Ultrathick Solid Electrolyte Interphase Formation on Li Metal Anodes

Solid electrolyte interphases (SEIs) play essential roles in lithium metal batteries by dictating the deposition morphology and Coulombic efficiency (CE) of lithium metal anodes. However, the understanding of the formation and evolution of SEIs remains elusive so far. Herein, we present an operando investigation of the formation and evolution dynamics of LiNO3-induced SEI under liquid electrolytes using high-resolution electrochemical atomic force microscopy (EC-AFM). For the first time, we discovered the in situ formation of an ultrathick SEI (∼648 nm) under liquid electrolytes, which far exceeded the thicknesses observed via ex situ characterization techniques. This ultrathick SEI formed through a sequential reduction process, wherein an inorganic-rich inner layer was initially formed, followed by the deposition of a soft outer layer with a modulus of approximately 24 MPa. We also observed that the outer SEI layer was metastable and susceptible to dissolution in liquid electrolytes. Our findings provide a comprehensive investigation of the formation and evolution processes of the LiNO3-induced SEI, deepening the understanding of the in situ structure of SEIs under liquid electrolytes.

Regulating Amine Substitution in Fluorosulfonyl-Based Flame-Retardant Electrolytes for Energy-Dense Lithium Metal Batteries

Sulfone-based electrolytes offer unusually high anodic and thermal stability that in principle makes them promising candidates for fabricating energy-dense lithium metal batteries (LMBs). Their uses in practical batteries are currently limited by their inability to sustain long-term Li metal plating/stripping processes due to their high reactivity toward the Li metal. Here, we report on the design and synthesis of a unique family of fluorosulfonyl group-based (FSO2-) molecules, modified with ethyl (FSE)/N,N-dimethyl (FSNDM)/N,N-diethyl (FSNDE)/N-pyrrolidine (FSNP) end groups to create exceptionally stable single-salt single-solvent electrolytes. The flammability, solvation structure, ion transport, Li metal deposition kinetics, and high-voltage stability of the electrolytes are systematically studied. It is shown that the electrolytes are nonflammable, possess weak solvation characteristics, yet manifest high room-temperature ionic conductivities (1.6-6.1 mS cm-1) and low solution viscosities. In comparison to FSE, the FSNDM-, FSNDE-, and FSNP-based electrolytes exhibit an exceptionally reversible Coulombic efficiency for Li metal plating/stripping (>99.71% over 800 cycles) and exhibit typical oxidative stability at voltages exceeding 4.6 V. Deployed as electrolytes in Li metal batteries (20 μm Li anode and 3 g A h-1 electrolyte) with high-loading (18.5 mg cm-2) LiNi0.8Co0.1Mn0.1O2 cathodes, 329 cycles have been achieved before 80% capacity retention. Six Ah Li metal pouch cells based on the designed electrolytes also exhibit high stability and high energy density (496 W h kg-1) for over 150 cycles with at most 2.7% volume expansion. Our findings demonstrate that through an intentional molecular design, sulfone electrolytes provide a robust route toward nonflammable Li metal compatible electrolytes with practical high-voltage cathodes.

Correlating Solvation Free Energy to Electrolyte Properties for Lithium Metal Batteries

The electrolyte plays a critical role in lithium metal batteries. In particular, ion solvation profoundly impacts key electrolyte properties and battery performance. In this study, we systematically investigate solvation-property relationships in a series of electrolytes with different solvent-diluent ratios. Through potentiometric techniques that measure the relative solvation free energies of electrolytes, we find that weaker solvation correlates with larger ion clusters, lower ionic conductivity and diffusion coefficient, and superior electrochemical stability. Weaker solvation leads to the formation of a small number of Li clusters with large hydrodynamic radii, which lowers the Li+ diffusivity and ionic conductivity of the electrolyte. Concurrently, weaker solvation leads to improved electrochemical stability at both the cathode and anode interfaces. Understanding these solvation-property relationships and trade-offs is important to designing electrolytes for optimized lithium metal battery performance.

Interphase Design for Lithium-Metal Anodes

Electrode-electrolyte interphases are critical determinants of the reversibility and longevity of lithium (Li)-metal batteries (LMBs). However, upon cycling, the inherently delicate interphases, formed from electrolyte decomposition, become vulnerable to chemomechanical degradation and corrosion, resulting in rapid capacity loss and thus short battery life. Here, we present a comprehensive analysis of the complex interplay between the thermodynamic and kinetic properties of interphases on Li-metal anodes, providing insights into interphase design to address these challenges. Direct measurements of ion-transport kinetics across various electrolyte chemistries reveal that interphases with high Li-ion mobility are essential for achieving dense Li deposits. Conversely, sluggish ion transport generates high-surface-area Li deposits that induce Li random stripping and the accumulation of isolated Li deposits. Surprisingly, interphases that support long cycle life do not necessarily require the formation of dense Li deposits but must avoid possible electrochemical/chemical reactions between the Li-metal deposits and electrolytes' components. By that, in some specific electrolyte systems, isolated Li deposits can recover and electrically rejoin the active Li anodes' mass. These findings challenge conventional understanding and establish new principles for designing durable LMBs, demonstrating that even with commercial carbonate-based electrolytes, LiNi0.8Co0.1Mn0.1O2||Cu cells can achieve high reversibility.

Key Anodic Interfacial Phenomena and their Control in Next-Generation Lithium and Sodium Metal Batteries

Advancing next-generation battery technologies requires a thorough understanding of the intricate phenomena occurring at anodic interfaces. This focused review explores key interfacial processes, examining their thermodynamics and consequences in ion transport and charge transfer kinetics. It begins with a discussion on the formation of the electro chemical double layer, based on the GuoyChapman model, and explores how charge carriers achieve equilibrium at the interface. This review then delves into essential interfacial processes, including metal nucleation and growth, the development and stability of the solid electrolyte interphase (SEI), and ion movement across the interface. In addition, it analyzes the impact of different electrolyte solutions-such as low- and high-concentration electrolytes and localized high-concentration electrolytes-on these interfacial processes. The role of additives, co-solvents, and diluents in modifying these interfaces is also covered. This review further evaluates techniques for characterizing the SEI layer, highlighting their strengths and limitations in both aqueous and nonaqueous battery systems. By comparing the challenges and opportunities associated with interfaces next-generation nonaqueous metal battery systems, this review aims to offer new insights into their respective advantages and limitations, ultimately guiding the design and optimization of anodic interfaces to enhance the safety and efficiency of future energy storage technologies.

Regulated Li+ Solvation via Competitive Coordination Mechanism of Organic Cations for High Voltage and Fast Charging Lithium Metal Batteries

Li+ solvation exerted a decisive effect on electrolyte physicochemical properties. Suitable tuning for Li+ solvation enabled batteries to achieve unexpected performance. Here, we introduced inert organic cations to compete with Li+ for combining electrolyte molecules to modulate Li+ coordination in the electrolyte. The relevance between the number of cationic sites in organic cations and the competitive solvation ability was explored. The organic cations with multiple cationic sites attracted solvent molecules and anions away from Li+ to form new solvated shell, improving the Li+ transport kinetics and desolvation process in electrolyte while enhancing electrolyte oxidation tolerance. Moreover, electrostatic shielding provided by organic cations and anion-derived robust SEI promoted uniform and rapid Li+ deposition on Li electrodes. With the positive effect of organic cations, Li||LiCoO2 (LCO) batteries showed high specific capacity (136.46 mAh g-1) at high charge/discharge rate (10 C). Furthermore, Li||LCO batteries exhibited good capacity retention (70 % after 500 cycles) at 4.6 V charge cut-off voltage. This work provides fresh insights for the optimization of electrolytes and battery performance.

Microelectrodes for Battery Materials

The ability to measure current and voltage is core to both fundamental study and engineering of electrochemical systems, including batteries for energy storage. Electrochemical measurements have traditionally been conducted on macroscopic electrodes on the order of 1 cm or larger. In this Perspective, we review recent developments in using microscopic electrodes (<100 μm) for the study of battery materials. Microelectrodes allow us to explore spatiotemporal regimes that are not accessible with macroscopic electrodes. Temporally, microelectrodes can generate ultrahigh current densities, enabling the distinction between solid electrolyte interphase (SEI) kinetics and metal deposition kinetics. Spatially, they confine electrochemistry to single particles, allowing us to study their intrinsic properties. We outline future opportunities for the use of microelectrodes for future studies of battery systems. We propose their use for analyzing the electrochemistry of other reactive metals and exploring the potential of combining them with in situ imaging techniques.

Hyperconjugation-controlled molecular conformation weakens lithium-ion solvation and stabilizes lithium metal anodes

Tuning the solvation structure of lithium ions via electrolyte engineering has proven effective for lithium metal (Li) anodes. Further advancement that bypasses the trial-and-error practice relies on the establishment of molecular design principles. Expanding the scope of our previous work on solvent fluorination, we report here an alternative design principle for non-fluorinated solvents, which potentially have reduced cost, environmental impact, and toxicity. By studying non-fluorinated ethers systematically, we found that the short-chain acetals favor the [gauche, gauche] molecular conformation due to hyperconjugation, which leads to weakened monodentate coordination with Li+. The dimethoxymethane electrolyte showed fast activation to >99% coulombic efficiency (CE) and high ionic conductivity of 8.03 mS cm-1. The electrolyte performance was demonstrated in anode-free Cu‖LFP pouch cells at current densities up to 4 mA cm-2 (70 to 100 cycles) and thin-Li‖high-loading-LFP coin cells (200-300 cycles). Overall, we demonstrated and rationalized the improvement in Li metal cyclability by the acetal structure compared to ethylene glycol ethers. We expect further improvement in performance by tuning the acetal structure.

Multistage Linkage Internal Exclusion External Electrolyte Tactic Unlocks Extreme Fast Charge Lithium Metal Batteries

Lithium metal batteries are booming because of their inherent preponderance, but a negative electric field from concentration dipolarization and slow solid-phase transfer at the electrode interface become blocking modules for extreme fast charging. Achieving an anion-rich solvation shell with a high dielectric constant (ε) is a feasible strategy to bootstrap an interface microenvironment for mass-transport reaction, but it is still an uncultivated field. Herein, the superposition, including the donor number values, the high ε, and the spatial potential resistance, are complementarily considered; we propose a low-cost electrolyte with an internal excluding external tactic to answer the above issue. Explanatorily, an optimized solvation shell follows the cascading exclusion relationship of nitrate ion (NO3-) → tetraglyme → ethylene carbonate → dimethyl carbonate. And the culminated bilayer structure establishes ideal conditions for Li+ transfer-reaction kinetics, of which an anion-rich internal shell facilitates solid-phase transport and a high-ε external shell slashes the negative electric field.

Highly Stable Lithium Metal Batteries Enabled by Tuning the Molecular Polarity of Diluents in Localized High-Concentration Electrolytes

Electrolyte plays a crucial role in ensuring stable operation of lithium metal batteries (LMBs). Localized high-concentration electrolytes (LHCEs) have the potential to form a robust solid-electrolyte interphase (SEI) and mitigate Li dendrite growth, making them a highly promising electrolyte option. However, the principles governing the selection of diluents, a crucial component in LHCE, have not been clearly determined, hampering the advancement of such a type of electrolyte systems. Herein, the diluents from the perspective of molecular polarity are rationally designed and developed. A moderately fluorinated solvent, 1-(1,1,2,2-tetrafluoroethoxy)propane (TNE), is employed as a diluent to create a novel LHCE. The unique molecular structure of TNE enhances the intrinsic dipole moment, thereby altering solvent interactions and the coordination environment of Li-ions in LHCE. The achieved solvation structure not only enhances the bulk properties of LHCE, but also facilitates the formation of more stable anion-derived SEIs featured with a higher proportion of inorganic species. Consequently, the corresponding full cells of both Li||LiFePO4 and Li||LiNi0.8Co0.1Mn0.1O2 cells utilizing Li thin-film anodes exhibit extended long-term stability with significantly improved average Coulombic efficiency. This work offers new insights into the functions of diluents in LHCEs and provides direction for further optimizing the LHCEs for LMBs.

Recent Advances and Opportunities in Reactivating Inactive Lithium in Batteries

The loss of active materials is one of the main culprits of the battery failures. As a typical example, the presence of inactive lithium, also known as "dead lithium", contributes to the rapid capacity deterioration and reduces energy output in lithium batteries. This phenomenon has long been recognized as irreversible. In this Minireview, the first of this kind, we aim to summarize the formation of inactive lithium and reassess its impact on battery performance metrics. Additionally, we explore various strategies that have been devised to rejuvenate inactive lithium. This comprehensive overview of the latest advancements in reactivating inactive lithium not only offers insights into restoring capacity and enhancing battery performance metrics but also provides a foundation for future research in reviving other inactive materials found in next-generation batteries, such as lithium metal batteries, lithium-sulfur batteries, other alkali metal batteries, and liquid flow batteries.

A locally solvent-tethered polymer electrolyte for long-life lithium metal batteries

Solid polymer electrolytes exhibit enhanced Li+ conductivity when plasticized with highly dielectric solvents such as N,N-dimethylformamide (DMF). However, the application of DMF-containing electrolytes in solid-state batteries is hindered by poor cycle life caused by continuous DMF degradation at the anode surface and the resulting unstable solid-electrolyte interphase. Here we report a composite polymer electrolyte with a rationally designed Hofmann-DMF coordination complex to address this issue. DMF is engineered on Hofmann frameworks as tethered ligands to construct a locally DMF-rich interface which promotes Li+ conduction through a ligand-assisted transport mechanism. A high ionic conductivity of 6.5 × 10-4 S cm-1 is achieved at room temperature. We demonstrate that the composite electrolyte effectively reduces the free shuttling and subsequent decomposition of DMF. The locally solvent-tethered electrolyte cycles stably for over 6000 h at 0.1 mA cm-2 in Li | |Li symmetric cell. When paired with sulfurized polyacrylonitrile cathodes, the full cell exhibits a prolonged cycle life of 1000 cycles at 1 C. This work will facilitate the development of practical polymer-based electrolytes with high ionic conductivity and long cycle life.

Unveiling the Role of Fluorination in Hexacyclic Coordinated Ether Electrolytes for High-Voltage Lithium Metal Batteries

Fluorinated ethers have become promising electrolyte solvent candidates for lithium metal batteries (LMBs) because they are endowed with high oxidative stability and high Coulombic efficiencies of lithium metal stripping/plating. Up to now, most reported fluorinated ether electrolytes are -CF3-based, and the influence of ion solvation in modifying degree of fluorination has not been well-elucidated. In this work, we synthesize a hexacyclic coordinated ether (1-methoxy-3-ethoxypropane, EMP) and its fluorinated ether counterparts with -CH2F (F1EMP), -CHF2 (F2EMP), or -CF3 (F3EMP) as terminal group. With lithium bis(fluorosulfonyl)imide as single salt, the solvation structure, Li-ion transport behavior, lithium deposition kinetics, and high-voltage stability of the electrolytes were systematically studied. Theoretical calculations and spectra reveal the gradually reduced solvating power from nonfluorinated EMP to fully fluorinated F3EMP, which leads to decreased ionic conductivity. In contrast, the weakly solvating fluorinated ethers possess higher Li+ transference number and exchange current density. Overall, partially fluorinated -CHF2 is demonstrated as the desired group. Further full cell testing using high-voltage (4.4 V) and high-loading (3.885 mAh cm-2) LiNi0.8Co0.1Mn0.1O2 cathode demonstrates that F2EMP electrolyte enables 80% capacity retention after 168 cycles under limited Li (50 μm) and lean electrolyte (5 mL Ah-1) conditions and 129 cycles under extremely lean electrolyte (1.8 mL Ah-1) and the anode-free conditions. This work deepens the fundamental understanding on the ion transport and interphase dynamics under various degrees of fluorination and provides a feasible approach toward the design of fluorinated ether electrolytes for practical high-voltage LMBs.

Solvation-property relationship of lithium-sulphur battery electrolytes

The Li-S battery is a promising next-generation battery chemistry that offers high energy density and low cost. The Li-S battery has a unique chemistry with intermediate sulphur species readily solvated in electrolytes, and understanding their implications is important from both practical and fundamental perspectives. In this study, we utilise the solvation free energy of electrolytes as a metric to formulate solvation-property relationships in various electrolytes and investigate their impact on the solvated lithium polysulphides. We find that solvation free energy influences Li-S battery voltage profile, lithium polysulphide solubility, Li-S battery cyclability and the Li metal anode; weaker solvation leads to lower 1st plateau voltage, higher 2nd plateau voltage, lower lithium polysulphide solubility, and superior cyclability of Li-S full cells and Li metal anodes. We believe that relationships delineated in this study can guide the design of high-performance electrolytes for Li-S batteries.

Recent progress in SEI engineering for boosting Li metal anodes

Lithium metal anodes (LMAs) are ideal anode candidates for achieving next-generation high-energy-density battery systems due to their high theoretical capacity (3680 mA h g-1) and low working potential (-3.04 V versus the standard hydrogen electrode). However, the non-ideal solid electrolyte interface (SEI) derived from electrolyte/electrode interfacial reactions plays a vital role in the lithium deposition/stripping process and battery cycling performance. The composition and morphology of a SEI, which is sensitive to the outside environment, make it difficult to characterize and understand. With the development of characterization techniques, the mechanism, composition, and structure of a SEI can be better understood. In this review, the mechanism formation, the structure model evolution, and the composition of a SEI are briefly presented. Moreover, the development of in situ characterization techniques in recent years is introduced to better understand a SEI followed by the properties of the SEI, which are beneficial to the battery performance. Furthermore, recent optimization strategies of the SEI including the improvement of intrinsic SEIs and construction of artificial SEIs are summarized. Finally, the current challenges and future perspectives of SEI research are summarized.

The lasting impact of formation cycling on the Li-ion kinetics between SEI and the Li-metal anode and its correlation with efficiency

Formation cycling is a critical process aimed at improving the performance of lithium ion (Li-ion) batteries during subsequent use. Achieving highly reversible Li-metal anodes, which would boost battery energy density, is a formidable challenge. Here, formation cycling and its impact on the subsequent cycling are largely unexplored. Through solid-state nuclear magnetic resonance (ssNMR) spectroscopy experiments, we reveal the critical role of the Li-ion diffusion dynamics between the electrodeposited Li-metal (ED-Li) and the as-formed solid electrolyte interphase (SEI). The most stable cycling performance is realized after formation cycling at a relatively high current density, causing an optimum in Li-ion diffusion over the Li-metal-SEI interface. We can relate this to a specific balance in the SEI chemistry, explaining the lasting impact of formation cycling. Thereby, this work highlights the importance and opportunities of regulating initial electrochemical conditions for improving the stability and life cycle of lithium metal batteries.

A corrosion inhibiting layer to tackle the irreversible lithium loss in lithium metal batteries

Reactive negative electrodes like lithium (Li) suffer serious chemical and electrochemical corrosion by electrolytes during battery storage and operation, resulting in rapidly deteriorated cyclability and short lifespans of batteries. Li corrosion supposedly relates to the features of solid-electrolyte-interphase (SEI). Herein, we quantitatively monitor the Li corrosion and SEI progression (e.g., dissolution, reformation) in typical electrolytes through devised electrochemical tools and cryo-electron microscopy. The continuous Li corrosion is validated to be positively correlated with SEI dissolution. More importantly, an anti-corrosion and interface-stabilizing artificial passivation layer comprising low-solubility polymer and metal fluoride is designed. Prolonged operations of Li symmetric cells and Li | |LiFePO4 cells with reduced Li corrosion by ~74% are achieved (0.66 versus 2.5 μAh h-1). The success can further be extended to ampere-hour-scale pouch cells. This work uncovers the SEI dissolution and its correlation with Li corrosion, enabling the durable operation of Li metal batteries by reducing the Li loss.

Unraveling the Mechanism of Very Initial Dendritic Growth Under Lithium Ion Transport Control in Lithium Metal Anodes

Lithium metal deposition is strongly affected by the intrinsic properties of the solid-electrolyte interphase (SEI) and working electrolyte, but a relevant understanding is far from complete. Here, by employing multiple electrochemical techniques and the design of SEI and electrolyte, we elucidate the electrochemistry of Li deposition under mass transport control. It is discovered that SEIs with a lower Li ion transference number and/or conductivity induce a distinctive current transition even under moderate potentiostatic polarization, which is associated with the control regime transition of Li ion transport from the SEI to the electrolyte. Furthermore, our findings help reveal the creation of a space-charge layer at the electrode/SEI interface due to the involvement of the diffusion process of Li ions through the SEI, which promotes the formation of dendrite embryos that develop and eventually trigger SEI breakage and the control regime transition of Li ion transport. Our insight into the very initial dendritic growth mechanism offers a bridge toward design and control for superior SEIs.

Trend of Developing Aqueous Liquid and Gel Electrolytes for Sustainable, Safe, and High-Performance Li-Ion Batteries

Current lithium-ion batteries (LIBs) rely on organic liquid electrolytes that pose significant risks due to their flammability and toxicity. The potential for environmental pollution and explosions resulting from battery damage or fracture is a critical concern. Water-based (aqueous) electrolytes have been receiving attention as an alternative to organic electrolytes. However, a narrow electrochemical-stability window, water decomposition, and the consequent low battery operating voltage and energy density hinder the practical use of aqueous electrolytes. Therefore, developing novel aqueous electrolytes for sustainable, safe, high-performance LIBs remains challenging. This Review first commences by summarizing the roles and requirements of electrolytes-separators and then delineates the progression of aqueous electrolytes for LIBs, encompassing aqueous liquid and gel electrolyte development trends along with detailed principles of the electrolytes. These aqueous electrolytes are progressed based on strategies using superconcentrated salts, concentrated diluents, polymer additives, polymer networks, and artificial passivation layers, which are used for suppressing water decomposition and widening the electrochemical stability window of water of the electrolytes. In addition, this Review discusses potential strategies for the implementation of aqueous Li-metal batteries with improved electrolyte-electrode interfaces. A comprehensive understanding of each strategy in the aqueous system will assist in the design of an aqueous electrolyte and the development of sustainable and safe high-performance batteries.

Reversible Solid-Solid Conversion of Sulfurized Polyacrylonitrile Cathodes in Lithium-Sulfur Batteries by Weakly Solvating Ether Electrolytes

Despite carbonate electrolytes exhibiting good stability to sulfurized polyacrylonitrile (SPAN), their chemical incompatibility with lithium (Li) metal anode leads to poor electrochemical performance of Li||SPAN full cells. While the SPAN employs conventional ether electrolytes that suffer from the shuttle effect, leading to rapid capacity fading. Here, we tailor a dilute electrolyte based on a low solvating power ether solvent that is both compatible with SPAN and Li metal. Unlike conventional ether electrolytes, the weakly solvating ether electrolyte enables SPAN to undergo reversibly "solid-solid" conversion. It features an anion-rich solvation structure that allows for the formation of a robust cathode electrolyte interphase on the SPAN, effectively blocking the dissolution of polysulfides into the bulk electrolyte and avoiding the shuttle effect. What's more, the unique electrolyte chemistry endowed Li ions with fast electroplating kinetics and induced high reversibility Li deposition/stripping process from 25 °C to -40 °C. Based on tailored electrolyte, Li||SPAN full cells matched with high loading SPAN cathodes (≈3.6 mAh cm-2 ) and 50 μm Li foil can operate stably over a wide range of temperatures. Additionally, Li||SPAN pouch cell under lean electrolyte and 5 % excess Li conditions can continuously operate stably for over a month.

Switching Reaction Pathway of Medium-Concentration Ether Electrolytes to Achieve 4.5 V Lithium Metal Batteries

Pursuing high-energy-density lithium metal batteries (LMBs) necessitates the advancement of electrolytes. Despite demonstrating high compatibility with lithium metal anodes (LMAs), ether-based electrolytes face challenges in achieving stable cycling at high voltages. Herein, we propose a strategy to enhance the high-voltage stability of medium-concentration (∼1 M) ether electrolytes by altering the reaction pathway of ether solvents. By employing a 1 M lithium difluoro(oxalato)borate in dimethoxyethane (LiDFOB/DME) electrolyte, we observed that LiDFOB displays a pronounced tendency for decomposition over DME, leading to a modification in the decomposition pathway of DME. This modification facilitates the formation of a stable organic-inorganic hybrid interface. Utilizing such an electrolyte, the Li-LCO cell demonstrates a discharge specific capacity of 146 mAh g-1 (5 C) and maintains retention of 86% over 1000 cycles at 2 C under a 4.5 V cutoff voltage. Additionally, the optimized ether electrolyte demonstrated outstanding cycling performance in Li-LCO full cells under practical conditions.

Hexacyclic Chelated Lithium Stable Solvates for Highly Reversible Cycling of High-Voltage Lithium Metal Battery

Ether-based electrolytes that are endowed with decent compatibility towards lithium anode have been regarded as promising candidates for constructing energy-dense lithium metal batteries (LMBs), but their applications are hindered by low oxidation stability in conventional salt concentration. Here, we reported that regulating the chelating power and coordination structure can remarkably increase the high-voltage stability of ether-based electrolytes and lifespan of LMBs. Two ether molecules of 1,3-dimethoxypropane (DMP) and 1,3-diethoxypropane (DEP) are designed and synthesized as solvents of electrolytes to replace the traditional ether solvent (1,2-dimethoxyethane, DME). Both computational and spectra reveal that the transition from five- to six-membered chelate solvation structure by adding one methylene on DME results in the formation of weak Li solvates, which increase the reversibility and high-voltage stability in LMBs. Even under lean electrolyte (5 mL Ah-1 ) and low anode to cathode ratio (2.6), the fabricated high-voltage Li||LiNi0.8 Co0.1 Mn0.1 O2 LMBs using electrolyte of 2.30 M Lithiumbisfluorosulfonimide (LiFSI)/DMP still show capacity retention over 90 % after 184 cycles. This work highlights the importance of designing the coordination structures in non-fluorine ether electrolytes for rechargeable batteries.

Designing F/P Hybrid Polymer as Ultrastable Cationic Shielding Interphase for High-Performance Lithium Metal Batteries

Dendrite growth on electrode-electrolyte interphase has severely limited applications of lithium metal batteries (LMBs). Here, we developed an ionic alternating polymer with fluorocarbons and phosphonium cations in repeating units to regulate Li deposition for the first time. The combined functionalities in the F/P hybrid polymer exhibit remarkable characteristics as a protective layer on top of Li anode, demonstrating outstanding electrochemical stability, ion flux redistributing capability and adaptive chain mobility. Based on characterizations and simulations, this cationic interlayer could effectively furnish long-standing electrostatic shielding for anodes, allowing restrained coating decomposition and homogenized electric field distribution to induce dendrite-free Li deposition, and enabling full cells with enhanced rate and long-term cycling performance. Given the importance of LMBs, this work will promote polymer design to stabilize anodes with superior electrochemical behavior.

Understanding SEI evolution during the cycling test of anode-free lithium-metal batteries with LiDFOB salt

Anode-free lithium-metal batteries (AFLMBs) have the potential to double the energy density of Li-ion batteries, but face the challenges of mossy dendritic lithium plating and an unstable solid electrolyte interphase (SEI). Previous studies have shown that the AFLMBs with an electrolyte containing lithium difluoro(oxalato)borate (LiDFOB) salt outperform those with lithium hexafluorophosphate (LiPF6), but the mechanism behind this improvement is not fully understood. In this study, X-ray photoelectron spectroscopy (XPS) depth profile analysis and electrochemical impedance spectroscopy (EIS) were conducted to investigate the SEI on plated Li from the two conducting salts and their evolution in Cu‖NMC full cells during cycling. XPS results revealed that an inorganic-rich SEI layer is formed in the cell with LiDFOB-based electrolyte, with a low carbon/oxygen ratio of 0.56 compared to 1.42 in the LiPF6-based cell. With the inorganic-rich SEI, a dense electroplated Li with a shining surface on the Cu substrate can be retained after ten cycles. The inorganic-rich SEI enhances the reversibility of Li plating and stripping, with a high average CE of ∼98% and a stable charge/discharge voltage profile. The changes in SEI resistance and cathode electrolyte interphase resistance are more prominent compared to the changes in solution and charge transfer resistances, which further validate the role of the passivation films on Li deposits and NMC cathode surfaces in stabilizing AFLMB cycling performance.

Ultrafast deposition of faceted lithium polyhedra by outpacing SEI formation

Electrodeposition of lithium (Li) metal is critical for high-energy batteries1. However, the simultaneous formation of a surface corrosion film termed the solid electrolyte interphase (SEI)2 complicates the deposition process, which underpins our poor understanding of Li metal electrodeposition. Here we decouple these two intertwined processes by outpacing SEI formation at ultrafast deposition current densities3 while also avoiding mass transport limitations. By using cryogenic electron microscopy4-7, we discover the intrinsic deposition morphology of metallic Li to be that of a rhombic dodecahedron, which is surprisingly independent of electrolyte chemistry or current collector substrate. In a coin cell architecture, these rhombic dodecahedra exhibit near point-contact connectivity with the current collector, which can accelerate inactive Li formation8. We propose a pulse-current protocol that overcomes this failure mode by leveraging Li rhombic dodecahedra as nucleation seeds, enabling the subsequent growth of dense Li that improves battery performance compared with a baseline. While Li deposition and SEI formation have always been tightly linked in past studies, our experimental approach enables new opportunities to fundamentally understand these processes decoupled from each other and bring about new insights to engineer better batteries.

Achieving Rapid Ultralow-Temperature Ion Transfer via Constructing Lithium-Anion Nanometric Aggregates to Eliminate Li+-Dipole Interactions

Sluggish desolvation in extremely cold environments caused by strong Li⁺-dipole interactions is a key inducement for the capacity decline of a battery. Although the Li⁺-dipole interaction is reduced by increasing the electrolyte concentration, its high viscosity inevitably limits ion transfer at low temperatures. Herein, Li⁺-dipole interactions were eliminated to accelerate the migration rate of ions in electrolytes and at the electrode interface via designing Li⁺-anion nanometric aggregates (LA-nAGGs) in low-concentration electrolytes. Li⁺ coordinated by TFSI^- and FSI^- anions instead of a donor solvent promotes the formation of an inorganic-rich interfacial layer and facilitates Li⁺ transfer. Consequently, the LA-nAGG-type electrolyte demonstrated a high ionic conductivity (0.6 mS cm^-1) at -70 °C and a low activation energy of charge transfer (38.24 kJ mol^-1), enabling Li||NiFe-Prussian blue derivative cells to deliver ∼83.1% of their room-temperature capacity at -60 °C. This work provides an advanced strategy for the development of low-temperature electrolytes.

Highly Reversible Lithium Metal Anode Enabled by 3D Lithiophilic-Lithiophobic Dual-Skeletons

Lithium metal is a promising anode for high-energy-density lithium batteries, but its practical application is still hindered by intrinsic defects such as infinite volume expansion and uncontrollable dendrite growth. Herein, a dendrite-free 3D composite Li anode (Li-B@SSM) is prepared by mechanical rolling of lithiophilic LiB nanofibers supported by Li-B composite and lithiophobic stainless-steel mesh (SSM). Featuring hierarchical lithiophilic-lithiophobic dual-skeletons, the Li-B@SSM anode shows an ultrahigh Coulombic efficiency of 99.95% and a long lifespan of 900 h under 2 mA cm^-2 /1 mAh cm^-2 . It is demonstrated that the abnormally reversible Li stripping/plating processes should be closely related to the site-selective plating behavior and spatial confinement effect induced by the robust lithiophilic-lithiophobic dual-skeletons, which alleviates the volume changes, suppresses the growth of Li dendrites, and reduces the accumulation of "dead" Li. More importantly, the application feasibility of the Li-B@SSM anode is also confirmed in full batteries, of which the Li-B@SSM|LiFePO₄ full cell shows a high capacity retention of 97.5% after 400 cycles while the Li-B@SSM|S pouch battery exhibits good cycle stability even under practically harsh conditions. This work paves the way for the facile and efficient fabrication of high-efficiency Li metal anodes toward practical applications.