Regulating the Hidden Solvation‐Ion‐Exchange in Concentrated Electrolytes for Stable and Safe Lithium Metal Batteries

Lithium–sulfur batteries are attractive for automobile and grid applications due to their high theoretical energy density and the abundance of sulfur. Despite the significant progress in cathode development, lithium metal degradation and the polysulfide shuttle remain two critical challenges in the practical application of Li–S batteries. Development of advanced electrolytes has become a promising strategy to simultaneously suppress lithium dendrite formation and prevent polysulfide dissolution. Here, a new class of concentrated siloxane‐based electrolytes, demonstrating significantly improved performance over the widely investigated ether‐based electrolytes are reported in terms of stabilizing the sulfur cathode and Li metal anode as well as minimizing flammability. Through a combination of experimental and computational investigation, it is found that siloxane solvents can effectively regulate a hidden solvation‐ion‐exchange process in the concentrated electrolytes that results from the interactions between cations/anions (e.g., Li⁺, TFSI^?, and S^2?) and solvents. As a result, it could invoke a quasi‐solid‐solid lithiation and enable reversible Li plating/stripping and robust solid‐electrolyte interphase chemistries. The solvation‐ion‐exchange process in the concentrated electrolytes is a key factor in understanding and designing electrolytes for other high‐energy lithium metal batteries.     

Lithium Difluorophosphate‐Based Dual‐Salt Low Concentration Electrolytes for Lithium Metal Batteries

The safety hazards and low Coulombic efficiency originating from the growth of lithium dendrites and decomposition of the electrolyte restrict the practical application of Li metal batteries (LMBs). Inspired by the low cost of low concentration electrolytes (LCEs) in industrial applications, dual‐salt LCEs employing 0.1 m Li difluorophosphate (LiDFP) and 0.4 m LiBOB/LiFSI/LiTFSI are proposed to construct a robust and conductive interphase on a Li metal anode. Compared with the conventional electrolyte using 1 m LiPF₆, the ionic conductivity of LCEs is reduced but the conductivity decrement of the separator immersed in LCEs is moderate, especially for the LiDFP–LiFSI and LiDFP–LiTFSI electrolytes. The accurate Coulombic efficiency (CE) of the Li||Cu cells increases from 83.3% (electrolyte using 1 m LiPF₆) to 97.6%, 94.5%, and 93.6% for LiDFP–LiBOB, LiDFP–LiFSI, and LiDFP–LiTFSI electrolytes, respectively. The capacity retention of Li||LiFePO₄ cells using the LiDFP–LiBOB electrolyte reaches 95.4% along with a CE over 99.8% after 300 cycles at a current density of 2.0 mA cm^?2 and the capacity reaches 103.7 mAh g^?1 at a current density of up to 16.0 mA cm^?2. This work provides a dual‐salt LCE for practical LMBs and presents a new perspective for the design of electrolytes for LMBs.     

Advanced Electrolytes for Fast‐Charging High‐Voltage Lithium‐Ion Batteries in Wide‐Temperature Range

LiNi_xMn_yCo_1?x?yO₂ (NMC) cathode materials with Ni ≥ 0.8 have attracted great interest for high energy‐density lithium‐ion batteries (LIBs) but their practical applications under high charge voltages (e.g., 4.4 V and above) still face significant challenges due to severe capacity fading by the unstable cathode/electrolyte interface. Here, an advanced electrolyte is developed that has a high oxidation potential over 4.9 V and enables NMC811‐based LIBs to achieve excellent cycling stability in 2.5–4.4 V at room temperature and 60 °C, good rate capabilities under fast charging and discharging up to 3C rate (1C = 2.8 mA cm^?2), and superior low‐temperature discharge performance down to ?30 °C with a capacity retention of 85.6% at C/5 rate. It is also demonstrated that the electrode/electrolyte interfaces, not the electrolyte conductivity and viscosity, govern the LIB performance. This work sheds light on a very promising strategy to develop new electrolytes for fast‐charging high‐energy LIBs in a wide‐temperature range.     

Exploiting Lithium‐Depleted Cathode Materials for Solid‐State Li Metal Batteries

Solid‐state lithium metal batteries (SSLMBs) may become one of the high‐energy density storage devices for the next generation of electric vehicles. High safety and energy density can be achieved by utilizing solid electrolytes and Li metal anodes. Therefore, developing cathode materials which can match with Li metal anode efficiently is indispensable. In SSLMBs, Li metal anodes can afford the majority of active lithium ions, then lithium‐depleted cathode materials can be a competitive candidate to achieve high gravimetric energy density as well as save lithium resources. Li_0.33MnO₂ lithium‐depleted material is chosen, which also has the advantages of low synthesis temperature and low cost (cobalt‐free). Notably, solid‐state electrolyte can greatly alleviate the problem of manganese dissolution in the electrolyte, which is beneficial to improve the cycling stability of the battery. Thus, SSLMBs enable practical applications of lithium‐depleted cathode materials.     

Transition Metal Oxide Anodes for Electrochemical Energy Storage in Lithium‐ and Sodium‐Ion Batteries

Lithium‐ion batteries (LIBs) with outstanding energy and power density have been extensively investigated in recent years, rendering them the most suitable energy storage technology for application in emerging markets such as electric vehicles and stationary storage. More recently, sodium, one of the most abundant elements on earth, exhibiting similar physicochemical properties as lithium, has been gaining increasing attention for the development of sodium‐ion batteries (SIBs) in order to address the concern about Li availability and cost—especially with regard to stationary applications for which size and volume of the battery are of less importance. Compared with traditional intercalation reactions, conversion reaction‐based transition metal oxides (TMOs) are prospective anode materials for rechargeable batteries thanks to their low cost and high gravimetric specific capacities. In this review, the recent progress and remaining challenges of conversion reactions for LIBs and SIBs are discussed, covering an overview about the different synthesis methods, morphological characteristics, as well as their electrochemical performance. Potential future research directions and a perspective toward the practical application of TMOs for electrochemical energy storage are also provided.     

Recent Progress in High Donor Electrolytes for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries continue to be considered promising post‐lithium‐ion batteries owing to their high theoretical energy density. In pursuit of a Li–S cell with long‐term cyclability, most studies thus far have relied on using ether‐based electrolytes. However, their limited ability to dissolve polysulfides requires a high electrolyte‐to‐sulfur ratio, which impairs the achievable specific energy. Recently, the battery community found high donor electrolytes to be a potential solution to this shortcoming because their high solubility toward polysulfides enables a cell to operate under lean electrolyte conditions. Despite the increasing number of promising outcomes with high donor electrolytes, a critical hurdle related to stability of the lithium‐metal counter electrode needs to be overcome. This review provides an overview of recent efforts pertaining to high donor electrolytes in Li–S batteries and is intended to raise interest from within the community. Furthermore, based on analogous efforts in the lithium‐air battery field, strategies for protecting the lithium metal electrode are proposed. It is predicted that high donor electrolytes will be elevated to a higher status in the field of Li–S batteries, with the hope that either existing or upcoming strategies will, to a fair extent, mitigate the degradation of the lithium–metal interface.     

Coated Lithium Powder (CLiP) Electrodes for Lithium‐Metal Batteries

Safety issues caused by the metallic lithium inside a battery represent one of the main reasons for the lack of commercial availability of rechargeable lithium‐metal batteries. The advantage of anodes based on coated lithium powder (CLiP), compared to plain lithium foil, include the suppression of dendrite formation, as the local current density during stripping/plating is reduced due to the higher surface area. Another performance and safety advantage of lithium powder is the precisely controlled mass loading of the lithium anode during electrode preparation, giving the opportunity to avoid Li excess in the cell. As an additional benefit, the coating makes electrode manufacturing safer and eases handling. Here, electrodes based on coated lithium powder electrodes (CLiP) are introduced for application in lithium‐metal batteries. These electrodes are compared to lithium foil electrodes with respect to cycling stability, coulombic efficiency of lithium stripping/plating, overpotential, and morphology changes during cycling.     

Hybrid and Aqueous Lithium‐Air Batteries

Lithium‐air (Li‐air) batteries have become attractive because of their extremely high theoretical energy density. However, conventional Li‐air cells operating with non‐aqueous electrolytes suffer from poor cycle life and low practical energy density due to the clogging of the porous air cathode by insoluble discharge products, contamination of the organic electrolyte and lithium metal anode by moist air, and decomposition of the electrolyte during cycling. These difficulties may be overcome by adopting a cell configuration that consists of a lithium‐metal anode protected from air by a Li⁺‐ion solid electrolyte and an air electrode in an aqueous catholyte. In this type of configuration, a Li⁺‐ion conducting “buffer” layer between the lithium‐metal anode and the solid electrolyte is often necessary due to the instability of many solid electrolytes in contact with lithium metal. Based on the type of buffer layer, two different battery configurations are possible: “hybrid” Li‐air batteries and “aqueous” Li‐air batteries. The hybrid and aqueous Li‐air batteries utilize the same battery chemistry and face similar challenges that limit the cell performance. Here, an overview of recent developments in hybrid and aqueous Li‐air batteries is provided and the factors that influence their performance and impede their practical applications, followed by future directions are discussed.     

Correlating Electrode–Electrolyte Interface and Battery Performance in Hybrid Solid Polymer Electrolyte‐Based Lithium Metal Batteries

Solid polymer electrolytes (SPEs) are desirable in lithium metal batteries (LMBs) since they are nonflammable and show excellent lithium dendrite growth resistance. However, fabricating high performance polymer LMBs is still a grand challenge because of the complex battery system. In this work, a series of tailor‐designed hybrid SPEs are used to prepare LMBs with a LiFePO₄‐based cathode. High performance LMBs with both excellent rate capability and long cycle life are obtained at 60 and 90 °C. The well‐controlled network structure in this series of hybrid SPEs offers a model system to study the relationship between the SPE properties and the LMB performance. It is shown that the cycle life of the polymer LMBs is closely correlated with the SPE–Li interface ionic conductivity, underscoring the importance of the solid electrolyte interface in LMB operation. LMB performance is further correlated with the molecular network structure. It is anticipated that results from this study will shed light on designing SPEs for high performance LMB applications.     

Ethylene Carbonate‐Free Electrolytes for High‐Nickel Layered Oxide Cathodes in Lithium‐Ion Batteries

Layered lithium nickel oxide (LiNiO₂) can provide very high energy density among intercalation cathode materials for lithium‐ion batteries, but suffers from poor cycle life and thermal‐abuse tolerance with large lithium utilization. In addition to stabilization of the active cathode material, a concurrent development of electrolyte systems of better compatibility is critical to overcome these limitations for practical applications. Here, with nonaqueous electrolytes based on exclusively aprotic acyclic carbonates free of ethylene carbonate (EC), superior electrochemical and thermal characteristics are obtained with an ultrahigh‐nickel cathode (LiNi_0.94Co_0.06O₂), capable of reaching a 235 mA h g^?1 specific capacity. Pouch‐type graphite|LiNi_0.94Co_0.06O₂ cells in EC‐free electrolytes withstand several hundred charge–discharge cycles with minor degradation at both ambient and elevated temperatures. In thermal‐abuse tests, the cathode at full charge, while reacting aggressively with EC‐based electrolytes below 200 °C, shows suppressed self‐heating without EC. Through 3D chemical and structural analyses, the intriguing impact of EC is visualized in aggravating unwanted surface parasitic reactions and irreversible bulk structural degradation of the cathode at high voltages. These results provide important insights in designing high‐energy electrodes for long‐lasting and reliable lithium‐ion batteries.     

Nonpolar Alkanes Modify Lithium‐Ion Solvation for Improved Lithium Deposition and Stripping

Lithium metal batteries have been plagued by the high reactivity of lithium. Reactive additives that can passivate the lithium metal surface and limit electrolyte accessibility to a fresh lithium surface have been widely explored, but can have limited utility with continuous consumption of the additive. In this work, an alternative strategy is explored. The use of nonreactive cosolvents such as nonpolar alkanes is studied and its is shown that hexane and cyclohexane addition to ether solvents (1,3‐dioxolane and 1,2‐dimethoxyethane) halves the nucleation and growth overpotentials for lithium deposition, increases the cell coulombic efficiency, improves the lithium deposition morphology, increases the electrolyte oxidative stability (>0.2 V), and doubles the cycle life—even when compared to a widely used fluorinated ether. The nonpolar alkanes modify the lithium‐ion solvation environment and reduce the solvation free energy; hence reducing the reaction barrier for lithium deposition. Exploration of nonpolar alkanes as part of the electrolyte mixture is a promising strategy for controlling metal deposition.     

Hybrid Lithium‐Sulfur Batteries with an Advanced Gel Cathode and Stabilized Lithium‐Metal Anode

The insulating nature of sulfur, polysulfide shuttle effect, and lithium‐metal deterioration cause a decrease in practical energy density and fast capacity fade in lithium‐sulfur (Li‐S) batteries. This study presents an integrated strategy for the development of hybrid Li‐S batteries based on a gel sulfur cathode, a solid electrolyte, and a protective anolyte composed of a highly concentrated salt electrolyte containing mixed additives. The dense solid electrolyte completely blocks polysulfide diffusion, and also makes it possible to investigate the cathode and anode independently. This gel cathode effectively traps the polysulfide active material while maintaining a low electrolyte to sulfur ratio of 5.2 mL g^?1. The anolyte effectively protects the Li metal and suppresses the consumption of liquid electrolyte, enabling stable long‐term cycling for over 700 h in Li symmetric cells. This advanced design can simultaneously suppress the polysulfide shuttle, protect Li metal, and reduce the liquid electrolyte usage. The assembled hybrid batteries exhibit remarkably stable cycling performance over 300 cycles with high capacity. Finally, surface‐sensitive techniques are carried out to directly visualize and probe the interphase formed on the surface of the Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) pellet, which may help stabilize the solid–liquid interface.     

Ionogel Electrolytes for High‐Performance Lithium Batteries: A Review

Ionic liquids (ILs) are important electrolytes for applications in electrochemical devices. An emerging trend in ILs research is their hybridization with solid matrices, named ionogels. These ionogels can not only overcome the fluidity of ILs but also exhibit high mechanical strength of the solid matrix. Therefore, they show promise for applications in building lithium batteries. In this review, various types of solid matrices for confining ILs are summarized, including nonmetallic oxides, metal oxides, IL‐tethered nanoparticles, functionalized SiO₂, metal–organic frameworks, and other structural materials. The synthetic strategies for ionogels are first documented, focusing on physical confinement and covalent grafting. Then, the structure, ionic conductivity, thermal stability, and electrochemical stability of ionogels are addressed in detail. Furthermore, the authors highlight the potential applications of state‐of‐art ionogels in lithium batteries. The authors conclude this review by outlining the remaining challenges as well as personal perspectives on this hot area of research.     

Incorporating Solvate and Solid Electrolytes for All‐Solid‐State Li2S Batteries with High Capacity and Long Cycle Life

The development of all‐solid‐state lithium–sulfur batteries is hindered by the poor interfacial properties at solid electrolyte (SE)/electrode interfaces. The interface is modified by employing the highly concentrated solvate electrolyte, (MeCN)₂?LiTFSI:TTE, as an interlayer material at the electrolyte/electrode interfaces. The incorporation of an interlayer significantly improves the cycling performance of solid‐state Li₂S batteries compared to the bare counterpart, exhibiting a specific capacity of 760 mAh g^?1 at cycle 100 (330 mAh g^?1 for the bare cell). Electrochemical impedance spectroscopy shows that the interfacial resistance of the interlayer‐modified cell gradually decreases as a function of cycle number, while the impedance of the bare cell remains almost constant. Cross‐section scanning electron microscopy (SEM)/ energy dispersive X‐ray spectroscopy (EDS) measurements on the interlayer‐modified cell confirm the permeation of solvate into the cathode and the SE with electrochemical cycling, which is related to the decrease in cell impedance. In order to mimic the full permeation of the solvate across the entire cell, the solvate is directly mixed with the SE to form a “solvSEM” electrolyte. The hybrid Li₂S cell using a solvSEM electrolyte exhibits superior cycling performance compared to the solid‐state cells, in terms of Li₂S loading, Li₂S utilization, and cycling stability. The improved performance is due to the favorable ionic contact at the battery interfaces.     

Nanoporous Hybrid Electrolytes for High‐Energy Batteries Based on Reactive Metal Anodes

Successful strategies for stabilizing electrodeposition of reactive metals, including lithium, sodium, and aluminum are a requirement for safe, high‐energy electrochemical storage technologies that utilize these metals as anodes. Unstable deposition produces high‐surface area dendritic structures at the anode/electrolyte interface, which causes premature cell failure by complex physical and chemical processes that have presented formidable barriers to progress. Here, it is reported that hybrid electrolytes created by infusing conventional liquid electrolytes into nanoporous membranes provide exceptional ability to stabilize Li. Electrochemical cells based on γ‐Al₂O₃ ceramics with pore diameters below a cut‐off value above 200 nm exhibit long‐term stability even at a current density of 3 mA cm^?2. The effect is not limited to ceramics; similar large enhancements in stability are observed for polypropylene membranes with less monodisperse pores below 450 nm. These findings are critically assessed using theories for ion rectification and electrodeposition reactions in porous solids and show that the source of stable electrodeposition in nanoporous electrolytes is fundamental.