Advanced Electrolyte Design for High‐Energy‐Density Li‐Metal Batteries under Practical Conditions

Ether-Based High-Voltage Lithium Metal Batteries: The Road to Commercialization

Ether-based high-voltage lithium metal batteries (HV-LMBs) are drawing growing interest due to their high compatibility with the Li metal anode. However, the commercialization of ether-based HV-LMBs still faces many challenges, including short cycle life, limited safety, and complex failure mechanisms. In this Review, we discuss recent progress achieved in ether-based electrolytes for HV-LMBs and propose a systematic design principle for the electrolyte based on three important parameters: electrochemical performance, safety, and industrial scalability. Finally, we summarize the challenges for the commercial application of ether-based HV-LMBs and suggest a roadmap for future development.

Wide Temperature Electrolytes for Lithium Batteries: Solvation Chemistry and Interfacial Reactions

Improving the wide-temperature operation of rechargeable batteries is crucial for boosting the adoption of electric vehicles and further advancing their application scope in harsh environments like deep ocean and space probes. Herein, recent advances in electrolyte solvation chemistry are critically summarized, aiming to address the long-standing challenge of notable energy diminution at sub-zero temperatures and rapid capacity degradation at elevated temperatures (>45°C). This review provides an in-depth analysis of the fundamental mechanisms governing the Li-ion transport process, illustrating how these insights have been effectively harnessed to synergize with high-capacity, high-rate electrodes. Another critical part highlights the interplay between solvation chemistry and interfacial reactions, as well as the stability of the resultant interphases, particularly in batteries employing ultrahigh-nickel layered oxides as cathodes and high-capacity Li/Si materials as anodes. The detailed examination reveals how these factors are pivotal in mitigating the rapid capacity fade, thereby ensuring a long cycle life, superior rate capability, and consistent high-/low-temperature performance. In the latter part, a comprehensive summary of in situ/operational analysis is presented. This holistic approach, encompassing innovative electrolyte design, interphase regulation, and advanced characterization, offers a comprehensive roadmap for advancing battery technology in extreme environmental conditions.

Optimizing Li-ion Solvation in Gel Polymer Electrolytes to Stabilize Li-Metal Anode

Gel polymer electrolytes (GPEs) have potential as substitutes for liquid electrolytes in lithium-metal batteries (LMBs). Their semi-solid state also makes GPEs suitable for various applications, including wearables and flexible electronics. Here, we report the initiation of ring-opening polymerization of 1,3-dioxolane (DOL) by Lewis acid and the introduction of diluent 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to regulate electrolyte structure for a more stable interface. This diluent-blended GPE exhibits enhanced electrochemical stability and ion transport properties compared to a blank version without it. FTIR and NMR proved the effectiveness of monomer polymerization and further determined the molecular weight distribution of polymerization by gel permeation chromatography (GPC). Experimental and simulation results show that the addition of TTE enhances ion association and tends to distribute on the anode surface to construct a robust and low-impedance SEI. Thus, the polymer battery achieves 5 C charge-discharge at room temperature and 200 cycles at low temperature -20 °C. The study presents an effective approach for regulating solvation structures in GPEs, promoting advancements in the future design of GPE-based LMBs.

Prospects of spectroscopic neutron imaging: optimizing experimental setups in battery electrolyte research

Spectral neutron imaging methods provide valuable insights into the characterization of hydrogenous materials, including battery electrolytes. However, their application is constrained by sample geometry, setup parameters, and material chemistries, especially when studying physico-chemical changes in battery electrolytes. To address these limitations, we present a framework for simulating and optimizing the investigation of hydrogenous materials. Our approach combines quantitative modeling with experimental data to predict and optimize the contrast achievable in wavelength-resolved neutron imaging methods, thereby maximizing the information obtained in specific neutron imaging setups. While initially demonstrated at the BOA beamline of the Paul Scherrer Institute, this framework is applicable to any continuous source with spectral neutron imaging capabilities with a chopper disk. This work establishes a pathway for accurate studies of hydrogenous materials and their physico-chemical behavior, paving the way for advancements in the field of material characterization with wavelength-resolved neutron imaging.

Influencing Factors on Li-ion Conductivity and Interfacial Stability of Solid Polymer Electrolytes, Exampled by Polycarbonates, Polyoxalates and Polymalonates

The application of solid polymer electrolytes (SPEs) in all-solid-state(ASS) batteries is hindered by lower Li⁺ -conductivity and narrower electrochemical window. Here, three families of ester-based F-modified SPEs of poly-carbonate (PCE), poly-oxalate (POE) and poly-malonate (PME) were investigated. The Li⁺ -conductivity of these SPEs prepared from pentanediol are all higher than the counterparts made of butanediol, owing to the enhanced asymmetry and flexibility. Because of stronger chelating coordination with Li⁺ , the Li⁺ -conductivity of PME and POE is around 10 and 5 times of PCE. The trifluoroacetyl-units are observed more effective than -O-CH₂ -CF₂ -CF₂ -CH₂ -O- during the in situ passivation of Li-metal. Using trifluoroacetyl terminated POE and PCE as SPE, the interfaces with Li-metal and high-voltage-cathode are stabilized simultaneously, endowing stable cycling of ASS Li/LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622) cells. Owing to an enol isomerization of malonate, the cycling stability of Li/PME/NCM622 is deteriorated, which is recovered with the introduce of dimethyl-group in malonate and the suppression of enol isomerization. The coordinating capability with Li⁺ , molecular asymmetry and existing modes of elemental F, are all critical for the molecular design of SPEs.

Stable Operation of Lithium Metal Batteries with Aggressive Cathode Chemistries at 4.9 V

High-voltage lithium metal batteries (LMBs) pose severe challenges for the matching of electrolytes with aggressive electrodes, especially at low temperatures. Here, we report a rational modification of the Li⁺ solvation structure to extend the voltage and temperature operating ranges of conventional electrolytes. Ion-ion and ion-dipole interactions as well as the electrochemical window of solvents were tailored to improve oxidation stability and de-solvation kinetics of the electrolyte. Meanwhile, robust and elastic B and F-rich interphases are formed on both electrodes. Such optimization enables Li||LiNi_0.5 Mn_1.5 O₄ cells (90.2 % retention after 400 cycles) and Li||LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622) cells (74.0 % retention after 200 cycles) to cycle stably at an ultra-high voltage of 4.9 V. Moreover, NCM622 cells deliver a considerable capacity of 143.5 mAh g^-1 at -20 °C, showing great potential for practical uses. The proposed strategy sheds light on further optimization for high-voltage LMBs.

The Anionic Chemistry in Regulating the Reductive Stability of Electrolytes for Lithium Metal Batteries

Advanced electrolyte design is essential for building high-energy-density lithium (Li) batteries, and introducing anions into the Li⁺ solvation sheaths has been widely demonstrated as a promising strategy. However, a fundamental understanding of the critical role of anions in such electrolytes is very lacking. Herein, the anionic chemistry in regulating the electrolyte structure and stability is probed by combining computational and experimental approaches. Based on a comprehensive analysis of the lowest unoccupied molecular orbitals, the solvents and anions in Li⁺ solvation sheaths exhibit enhanced and decreased reductive stability compared with free counterparts, respectively, which agrees with both calculated and experimental results of reduction potentials. Accordingly, new strategies are proposed to build stable electrolytes based on the established anionic chemistry. This work unveils the mysterious anionic chemistry in regulating the structure-function relationship of electrolytes and contributes to a rational design of advanced electrolytes for practical Li metal batteries.

Electrolytes for Multivalent Metal-Ion Batteries: Current Status and Future Prospect

Electrochemical energy storage has experienced unprecedented advancements in recent years and extensive discussions and reviews on the progress of multivalent metal-ion batteries have been made mainly from the aspect of electrode materials, but relatively little work comprehensively discusses and provides an outlook on the development of electrolytes in these systems. Under this circumstance, this Review will initially introduce different types of electrolytes in current multivalent metal-ion batteries and explain the basic ion conduction mechanisms, preparation methods, and pros and cons. On this basis, we will discuss in detail the research and development of electrolytes for multivalent metal-ion batteries in recent years, and finally, critical challenges and prospects for the application of electrolytes in multivalent metal-ion batteries will be put forward.

A Competitive Solvation of Ternary Eutectic Electrolytes Tailoring the Electrode/Electrolyte Interphase for Lithium Metal Batteries

The development of electrolytes with high safety, high ionic conductivity, and the ability to inhibit lithium dendrites growth is crucial for the fabrication of high-energy-density lithium metal batteries. In this study, a ternary eutectic electrolyte is designed with LiTFSI (TFSI = bis(trifluoromethanesulfonyl)imide), butyrolactam (BL), and succinonitrile (SN). This electrolyte exhibits a high ion conductivity, nonflammability, and a wide electrochemical window. The competitive solvation effect among SN, BL, and Li⁺ reduces the viscosity and improves the stability of the eutectic electrolyte. The preferential coordination of BL toward Li⁺ facilitates the formation of stable solid electrolyte interphase films, leading to homogeneous and dendrite-free Li plating. As expected, the LiFePO₄/Li cell with this ternary eutectic electrolyte delivers a high capacity retention of 90% after 500 cycles at 2 C and an average Coulombic efficiency of 99.8%. Moreover, Ni-rich LiNi_0.8Co_0.1Al_0.1O₂/Li and LiNi_0.8Co_0.1Mn_0.1O₂/Li cells based on the modified ternary eutectic electrolyte achieve an outstanding cycling performance. This study provides insights for understanding and designing better electrolytes for lithium metal batteries and analogous sodium/potassium metal batteries.

Alloy-Type Anodes for High-Performance Rechargeable Batteries

Alloy-type anodes are one of the most promising classes of next-generation anode materials due to their ultrahigh theoretical capacity (2-10 times that of graphite). However, current alloy-type anodes have several limitations: huge volume expansion, high tendency to fracture and disintegrate, an unstable solid-electrolyte interphase (SEI) layer, and low Coulombic efficiency. Efforts to overcome these challenges are ongoing. This Review details recent progress in the research of batteries based on alloy-type anodes and discusses the direction of their future development. We conclude that improvements in structural design, the introduction of a protective interface, and the selection of suitable electrolytes are the most effective ways to improve the performance of alloy-type anodes. Furthermore, future studies should direct more attention toward analyzing their synergistic promoting effect.

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.

Concentrated Electrolyte for High-Performance Ca-Ion Battery Based on Organic Anode and Graphite Cathode

Due to the large abundance, low redox potential, and multivalent properties of calcium (Ca), Ca-ion batteries (CIBs) show promising prospects for energy storage applications. However, current research on CIBs faces the challenges of unsatisfactory cycling stability and capacity, mainly restricted by the lack of suitable electrolytes and electrode materials. Herein, we firstly developed a 3.5 m concentrated electrolyte with a calcium bis(fluorosulfonyl)imide (Ca(FSI)₂ ) salt dissolved in carbonate solvents. This electrolyte significantly improved the intercalation capacity for anions in the graphite cathode and contributed to the reversible insertion of Ca²⁺ in the organic anode. By combining this concentrated electrolyte with the low-cost and environmentally friendly graphite cathode and organic anode, the assembled Ca-based dual-ion battery (Ca-DIB) exhibits 75.4 mAh g^-1 specific discharge capacity at 100 mA g^-1 and 84.7 % capacity retention over 350 cycles, among the best results known for CIBs.