Ion–solvent chemistry in lithium battery electrolytes: From mono-solvent to multi-solvent complexes

Solvation Effect: The Cornerstone of High-Performance Battery Design for Commercialization-Driven Sodium Batteries

Sodium batteries (SBs) emerge as a potential candidate for large-scale energy storage and have become a hot topic in the past few decades. In the previous researches on electrolyte, designing electrolytes with the solvation theory has been the most promising direction is to improve the electrochemical performance of batteries through solvation theory. In general, the four essential factors for the commercial application of SBs, which are cost, low temperature performance, fast charge performance and safety. The solvent structure has significant impact on commercial applications. But so far, the solvation design of electrolyte and the practical application of sodium batteries have not been comprehensively summarized. This review first clarifies the process of Na+ solvation and the strategies for adjusting Na+ solvation. It is worth noting that the relationship between solvation theory and interface theory is pointed out. The cost, low temperature, fast charging, and safety issues of solvation are systematically summarized. The importance of the de-solvation step in low temperature and fast charging application is emphasized to help select better electrolytes for specific applications. Finally, new insights and potential solutions for electrolytes solvation related to SBs are proposed to stimulate revolutionary electrolyte chemistry for next generation SBs.

Reaction Center Shifting in Partially Fluorinated Electrolytes for Robust Lithium Metal Battery

The strategic formulation of a compatible electrolyte plays a pivotal role in extending the longevity of lithium-metal batteries (LMBs). Here, we present findings on a partially fluorinated electrolyte distinguished by a subdued solvation affinity towards Li+ ions and a concentrated anion presence within the primary solvation layer. This distinctive solvation arrangement redirects the focal points of reactions from solvent molecules to anions, facilitating the predominant involvement of anions in the creation of a LiF-enriched solid-electrolyte interphase (SEI). Electrochemical assessments showcase effective Li+ transport kinetics, diminished overpotential polarization for Li nucleation (28 mV), and prolonged cycling durability in Li||Li cells employing the partially fluorinated electrolyte. When tested in Li||NCM811 cells, the designed electrolyte delivers a capacity retention of 89.30 % and exhibits a high average Coulombic efficiency of 99.80 % over 100 cycles with a charge-potential cut-off of 4.6 V vs. Li/Li+ under the current density of 0.4C. Furthermore, even at a current density of 1C, the cells maintain 81.90 % capacity retention and a high average Coulombic efficiency of 99.40 % after 180 cycles. This work underscores the significance of weak-solvation interaction in partially fluorinated electrolytes and highlights the crucial role of solvent structure in enabling the long-term stability and high-energy density of LMBs.

Competitive Coordination of Sodium Ions for High-Voltage Sodium Metal Batteries with Fast Reaction Speed

The unstable electrode-electrolyte interface and the narrow electrochemical window of normal electrolytes hinder the potential application of high-voltage sodium metal batteries. These problems are actually related to the solvation structure of the electrolyte, which is determined by the competition between cations coordinated with anions or solvent molecules. Herein, we design an electrolyte incorporating ethyl (2,2,2-trifluoroethyl) carbonate and fluoroethylene carbonate, which facilitates a pronounced level of cation-anion coordination within the solvation sheath by enthalpy changes to reduce the overall coordination of cation-solvents and increase sensitivity to salt concentration. Such an electrolyte regulated by competitive coordination leads to highly reversible sodium plating/stripping with extended cycle life and a high Coulombic efficiency of 98.0%, which is the highest reported so far in Na||Cu cells with ester-based electrolytes. Moreover, 4.5 V high-voltage Na||Na3V2(PO4)2F3 cells exhibit a high rate capability up to 20 C and an impressive cycling stability with an 87.1% capacity retention after 250 cycles with limited Na. The proposed strategy of solvation structure modification by regulating the competitive coordination of the cation provides a new direction to achieve stable sodium metal batteries with high energy density and can be further extended to other battery systems by controlling enthalpy changes of the solvation structure.

Crosslinked Gel Polymer Electrolyte from Trimethylolpropane Triglycidyl Ether by In Situ Polymerization for Lithium-Ion Batteries

Electrolytes play a critical role in battery performance. They are associated with an increased risk of safety issues. The main challenge faced by many researchers is how to balance the physical and electrical properties of electrolytes. Gel polymer electrolytes (GPEs) have received increasing attention due to their satisfactory properties of ionic conductivity, mechanical stability, and safety. Herein, we develop a gel network polymer electrolyte (GNPE) to address the challenge mentioned earlier. This GNPE was formed by tri-epoxide monomer and bis(fluorosulfonyl)imide lithium salt (LiFSI) via an in situ cationic polymerization under mild thermal conditions. The obtained GNPE exhibited a relatively high ionic conductivity (σ) of 2.63 × 10-4 S cm-1, lithium transference number (tLi+, 0.58) at room temperature (RT), and intimate electrode compatibility with LiFePO4 and graphite. The LiFePO4/GNPE/graphite battery also showed a promising cyclic performance at RT, e.g., a suitable discharge specific capacity of 127 mAh g-1 and a high Coulombic efficiency (>97%) after 100 cycles at 0.2 C. Moreover, electrolyte films showed good mechanical stability and formed the SEI layer on the graphite anode. This study provides a facile method for preparing epoxy-based electrolytes for high-performance lithium-ion batteries (LIBs).

Data-Driven Insight into the Reductive Stability of Ion-Solvent Complexes in Lithium Battery Electrolytes

Lithium (Li) metal batteries (LMBs) are regarded as one of the most promising energy storage systems due to their ultrahigh theoretical energy density. However, the high reactivity of the Li anodes leads to the decomposition of the electrolytes, presenting a huge impediment to the practical application of LMBs. The routine trial-and-error methods are inefficient in designing highly stable solvent molecules for the Li metal anode. Herein, a data-driven approach is proposed to probe the origin of the reductive stability of solvents and accelerate the molecular design for advanced electrolytes. A large database of potential solvent molecules is first constructed using a graph theory-based algorithm and then comprehensively investigated by both first-principles calculations and machine learning (ML) methods. The reductive stability of 99% of the electrolytes decreases under the dominance of ion-solvent complexes, according to the analysis of the lowest unoccupied molecular orbital (LUMO). The LUMO energy level is related to the binding energy, bond length, and orbital ratio factors. An interpretable ML method based on Shapley additive explanations identifies the dipole moment and molecular radius as the most critical descriptors affecting the reductive stability of coordinated solvents. This work not only affords fruitful data-driven insight into the ion-solvent chemistry but also unveils the critical molecular descriptors in regulating the solvent's reductive stability, which accelerates the rational design of advanced electrolyte molecules for next-generation Li batteries.

A review on nickel-rich nickel-cobalt-manganese ternary cathode materials LiNi0.6Co0.2Mn0.2O2 for lithium-ion batteries: performance enhancement by modification

The new energy era has put forward higher requirements for lithium-ion batteries, and the cathode material plays a major role in the determination of electrochemical performance. Due to the advantages of low cost, environmental friendliness, and reversible capacity, high-nickel ternary materials are considered to be one of ideal candidates for power batteries now and in the future. At present, the main design idea of ternary materials is to fully consider the structural stability and safety performance of batteries while maintaining high energy density. Ternary materials currently face problems such as low lithium-ion diffusion rate and irreversible collapse of the structure, although the battery performance can be improved utilizing coating, ion doping, etc., the actual demand requires a more effective modification method based on the intrinsic properties of the material. Based on the summary of the current research status of the ternary material LiNi0.6Co0.2Mn0.2O2 (NCM622), a comparative study of the modification paths of the material was conducted from the level of molecular action mechanism. Finally, the major problems of ternary cathode materials and the future development direction are pointed out to stimulate more innovative insights and facilitate their practical applications.

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.

Nanoarchitecture factors of solid electrolyte interphase formation via 3D nano-rheology microscopy and surface force-distance spectroscopy

The solid electrolyte interphase in rechargeable Li-ion batteries, its dynamics and, significantly, its nanoscale structure and composition, hold clues to high-performing and safe energy storage. Unfortunately, knowledge of solid electrolyte interphase formation is limited due to the lack of in situ nano-characterization tools for probing solid-liquid interfaces. Here, we link electrochemical atomic force microscopy, three-dimensional nano-rheology microscopy and surface force-distance spectroscopy, to study, in situ and operando, the dynamic formation of the solid electrolyte interphase starting from a few 0.1 nm thick electrical double layer to the full three-dimensional nanostructured solid electrolyte interphase on the typical graphite basal and edge planes in a Li-ion battery negative electrode. By probing the arrangement of solvent molecules and ions within the electric double layer and quantifying the three-dimensional mechanical property distribution of organic and inorganic components in the as-formed solid electrolyte interphase layer, we reveal the nanoarchitecture factors and atomistic picture of initial solid electrolyte interphase formation on graphite-based negative electrodes in strongly and weakly solvating electrolytes.

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.

3-(2,2,2-Trifluoroethoxy)propionitrile-based electrolytes for high energy density lithium metal batteries

In this study, 3-(2,2,2-trifluoroethoxy)-propionitrile (FEON), a fluorinated nitrile compound with high oxidative stability, low volatility and non-flammability, is introduced as an electrolyte solvent for high-energy density Li|NCM batteries. After optimization of the electrolyte as (0.8 M LiTFSI + 0.2 M LiODFB)/FEC : FEON (1 : 3, by vol., abbreviated as FF13), the FEON-based electrolyte exhibits better cycling performance for both the lithium metal anode and 4.4 V high-voltage NCM cathode, compared with those of a commercial carbonate electrolyte of 1 M LiPF₆/EC : EMC : DMC (1 : 1 : 1, by vol.). As for the FF13 electrolyte, the maximum coordination number of 3 for FEON molecules in the solvation structure is disclosed through molecular dynamics simulation combined with Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy measurements. Furthermore, the solid electrolyte interphase on the lithium metal anode is enriched with organic components and LiF, which is proposed from FEON decomposition based on density functional theory calculations and X-ray photoelectron spectroscopy analysis. All the above results demonstrate that fluorinated nitrile electrolytes constitute a promising platform for high energy density Li|NCM batteries.

Modulating SEI formation via tuning the solvation sheath for lithium metal batteries

The solvation sheath of Li⁺-glyme was modulated to enhance Li⁺-TFSI^- association by adopting a highly polar solvent, especially water molecules, which affects the solid electrolyte interface (SEI) layer composition. By the Li⁺-TFSI^- association, a TFSI^- anion-derived SEI layer is formed on the Li metal anode, resulting in higher Li metal anode efficiency.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

Modification of Nitrate Ion Enables Stable Solid Electrolyte Interphase in Lithium Metal Batteries

The lifespan of high-energy-density lithium metal batteries (LMBs) is hindered by heterogeneous solid electrolyte interphase (SEI). The rational design of electrolytes is strongly considered to obtain uniform SEI in working batteries. Herein, a modification of nitrate ion (NO₃ ^- ) is proposed and validated to improve the homogeneity of the SEI in practical LMBs. NO₃ ^- is connected to an ether-based moiety to form isosorbide dinitrate (ISDN) to break the resonance structure of NO₃ ^- and improve the reducibility. The decomposition of non-resonant -NO₃ in ISDN enriches SEI with abundant LiN_x O_y and induces uniform lithium deposition. Lithium-sulfur batteries with ISDN additives deliver a capacity retention of 83.7 % for 100 cycles compared with rapid decay with LiNO₃ after 55 cycles. Moreover, lithium-sulfur pouch cells with ISDN additives provide a specific energy of 319 Wh kg^-1 and undergo 20 cycles. This work provides a realistic reference in designing additives to modify the SEI for stabilizing LMBs.

Molecular simulation of glycerol-derived triether podands for lithium ion solvation

Solvate ionic liquids (ILs) are promising candidates for several applications due to their stability, high coulombic efficiency, and low volatility. In this work, we investigate the solvation of lithium-bistriflimide by different glycerol-derived triether solvents, using molecular dynamics simulations. Very strong interactions between Li⁺ and the solvent oxygen sites are found, leading to significant conformational changes in the solvent. By comparing the conformation of the neat solvents with their IL mixtures at different concentrations and temperatures, we find that the presence of Li⁺ induces a distinct crown-like structure in the solvent molecules. The Li⁺ cations and the surrounding solvent form a podand complex, which is stable even at elevated temperatures. These glycerol-derived solvents exhibit distinct interactions with Li⁺ cations which may be exploited in electrolytic applications or lithium recovery processes.

Deciphering the Role of Fluoroethylene Carbonate towards Highly Reversible Sodium Metal Anodes

Sodium metal anodes (SMAs) suffer from extremely low reversibility (<20%) in carbonate-based electrolytes—this piece of knowledge gained from previous studies has ruled out the application of carbonate solvents for sodium metal batteries. Here, we overturn this conclusion by incorporating fluoroethylene carbonate (FEC) as cosolvent that renders a Na plating/stripping efficiency of >95% with conventional NaPF₆ salt at a regular concentration (1.0 M). The peculiar role of FEC is firstly unraveled via its involvement into the solvation structure, where a threshold FEC concentration with a coordination number>1.2 is needed in guaranteeing high Na reversibility over the long-term. Specifically, by incorporating an average number of 1.2 FEC molecules into the primary Na⁺ solvation sheath, lowest unoccupied molecular orbital (LUMO) levels of such Na⁺-FEC solvates undergo further decrease, with spin electrons residing either on the O=CO(O) moiety of FEC or sharing between Na⁺ and its C=O bond, which ensures a prior FEC decomposition in passivating the Na surface against other carbonate molecules. Further, by adopting cryogenic transmission electron microscopy (cryo-TEM), we found that the Na filaments grow into substantially larger diameter from ~400 nm to >1 μm with addition of FEC upon the threshold value. A highly crystalline and much thinner (~40 nm) solid-electrolyte interphase (SEI) is consequently observed to uniformly wrap the Na surface, in contrast to the severely corroded Na as retrieved from the blank electrolyte. The potence of FEC is further demonstrated in a series of “corrosive solvents” such as ethyl acetate (EA), trimethyl phosphate (TMP), and acetonitrile (AN), enabling highly reversible SMAs in the otherwise unusable solvent systems.