Revealing Surfactant Effect of Trifluoromethylbenzene in Medium-Concentrated PC Electrolyte for Advanced Lithium-Ion Batteries

4-Fluorobenzyl cyanide, a sterically-hindered solvent expediting interfacial kinetics in lithium-ion batteries

The electrochemical performance of lithium-ion batteries (LIBs) is plagued by sluggish interfacial kinetics. Fortunately, the Li+ solvation structure bridges the bulk electrolyte and interfacial chemistry, providing a pathway for promoting electrochemical kinetics in LIBs. Herein, we improve the interfacial kinetics by tuning the Li+ coordination chemistry based on solvent molecular engineering. Specifically, 4-fluorobenzyl cyanide (FBCN), featuring steric hindrance and a weak Lewis basic center, is designed to construct a bulky coordination structure with Li+, weakening ion-dipole interaction (Li+-solvents) but promoting coulombic attraction (Li+-anions) at a normal Li salt concentration. This sterically-controlled solvation chemistry reduces the interfacial barrier and thus contributes to improved rate performance, as demonstrated practically in LiFePO4//graphite pouch cells. This study provides fresh insights into solvent steric control and coordination chemistry engineering, opening a new avenue for enhancing electrochemical kinetics in LIBs.

Two-Dimensional Electrolyte Design: Broadening the Horizons of Functional Electrolytes in Lithium Batteries

ConspectusSince their commercialization in the 1990s, lithium-ion batteries (LIBs) have been increasingly used in applications such as portable electronics, electric vehicles, and large-scale energy storage. The increasing use of LIBs in modern society has necessitated superior-performance LIB development, including electrochemical reversibility, interfacial stability, efficient kinetics, environmental adaptability, and intrinsic safety, which is difficult to simultaneously achieve in commercialized electrolytes. Current electrolyte systems contain a solution with Li salts (e.g., LiPF6) and solvents (e.g., ethylene carbonate and dimethyl carbonate), in which the latter dissolves Li salts and strongly interacts with Li+ (lithiophilic feature). Only lithiophilic agents can be functionally modified (e.g., additives and solvents), altering the bulk and interfacial behaviors of Li+ solvates. However, such approaches alter pristine Li+ solvation and electrochemical processes, making it difficult to strike a balance between the electrochemical performance and other desired electrolyte functions. This common electrolyte design in lithiophilic solvents shows strong coupling among formulation, coordination, electrochemistry, and electrolyte function. The invention of lithiophobic cosolvents (e.g., multifluorinated ether and fluoroaromatic hydrocarbons) has expanded the electrolyte design space to lithiophilic (interacts with Li+) and lithiophobic (interacts with solvents but not with Li+) dimensions. Functional modifications switch to lithiophobic cosolvents, affording superior properties (carried by lithiophobic cosolvents) with little impact on primary Li+ solvation (dictated by lithiophilic solvents). This electrolyte engineering technique based on lithiophobic cosolvents is the 2D electrolyte (TDE) principle, which decouples formulation, coordination, electrochemistry, and function. The molecular-scale understanding of TDEs is expected to accelerate electrolyte innovations in next-generation LIBs.This Account provides insights into recent advancements in electrolytes for superior LIBs from the perspective of lithiophobic agents (i.e., lithiophobic additives and cosolvents), establishing a generalized TDE principle for functional electrolyte design. In bulk electrolytes, a microsolvating competition emerges because of cosolvent-induced dipole-dipole and ion-dipole interactions, forming a loose solvation shell and a kinetically favorable electrolyte. At the electrode/electrolyte interface, the lithiophobic cosolvent affords reliable passivation and efficient desolvation, with interfacial compatibility and electrochemical reversibility even under harsh conditions. Based on this unique coordination chemistry, functional electrolytes are formulated without significantly sacrificing their electrochemical performance. First, lithiophobic cosolvents are used to tune Li+-solvent affinity and anion mobility, promoting Li+ diffusion and electrochemical kinetics of the electrolyte to benefit high-rate and low-temperature applications. Second, the lithiophobic cosolvent undergoes less thermally induced decomposition and constructs a thermally stable interphase in TDEs, affording electrolytes with high-temperature adaptability and cycling stability. Third, the lithiophobic cosolvent modifies the local Li+-solvent-anion topography, controlling electrolyte electrochemical reversibility to afford numerous promising solvents that cannot be used in common electrolyte design. Finally, the lithiophobic cosolvent mitigates detrimental crosstalk between flame retardants and carbonate solvents, improving the intrinsic electrolyte safety without compromising electrochemical performance, which broadens the horizons of electrolyte design by optimizing versatile cosolvents and solvents, inspiring new ideas in liquid electrochemistry in other battery systems.

Reinforcing the Electrode/Electrolyte Interphases of Lithium Metal Batteries Employing Locally Concentrated Ionic Liquid Electrolytes

Lithium metal batteries (LMBs) with nickel-rich cathodes are promising candidates for next-generation high-energy-density batteries, but the lack of sufficiently protective electrode/electrolyte interphases (EEIs) limits their cyclability. Herein, trifluoromethoxybenzene is proposed as a cosolvent for locally concentrated ionic liquid electrolytes (LCILEs) to reinforce the EEIs. With a comparative study of a neat ionic liquid electrolyte (ILE) and three LCILEs employing fluorobenzene, trifluoromethylbenzene, or trifluoromethoxybenzene as cosolvents, it is revealed that the fluorinated groups tethered to the benzene ring of the cosolvents not only affect the electrolytes' ionic conductivity and fluidity, but also the EEIs' composition via adjusting the contribution of the 1-ethyl-3-methylimidazolium cation (Emim+ ) and bis(fluorosulfonyl)imide anion. Trifluoromethoxybenzene, as the optimal cosolvent, leads to a stable cycling of LMBs employing 5 mAh cm-2 lithium metal anodes (LMAs), 21 mg cm-2 LiNi0.8 Co0.15 Al0.05 (NCA) cathodes, and 4.2 µL mAh-1 electrolytes for 150 cycles with a remarkable capacity retention of 71%, thanks to a solid electrolyte interphase rich in inorganic species on LMAs and, particularly, a uniform cathode/electrolyte interphase rich in Emim+ -derived species on NCA cathodes. By contrast, the capacity retention under the same condition is only 16%, 46%, and 18% for the neat ILE and the LCILEs based on fluorobenzene and benzotrifluoride, respectively.

Bidirectional Interphase Modulation of Phosphate Electrolyte Enables Intrinsic Safety and Superior Stability for High-Voltage Lithium-Metal Batteries

Developing advanced high voltage lithium-metal batteries (LMBs) with superior stability and intrinsic safety is of great significance for practical applications. However, the easy flammability of conventional carbonate solvents and inferior compatibility of commercial electrolytes for both highly reactive Li anodes and high-voltage cathodes severely hinder the implementation process. Hence, we rationally designed an intrinsically nonflammable and low-cost phosphate electrolyte toward stable high-voltage LMBs by bidirectionally modulating the interphases. Benefiting from the synergistic regulation of LiNO3 and DME dual-additives in the 1.5 M LiTFSI/Triethyl phosphate electrolyte, thin, dense and robust electrodes/electrolyte interphases were well constructed simultaneously on the surfaces of Li anode and Ni-rich cathode, dramatically improving the stability and compatibility between electrodes and electrolyte. Consequently, boosted kinetic and high Coulombic efficiency of 98.6% for Li metal plating/stripping over 400 cycles and superior cycling stability of exceeding 4,000 h in Li symmetric cell is achieved. More importantly, the Li∥LiNi0.6Mn0.2Co0.2O2 cell assembled with a thin Li anode and high mass-loading cathode at a high cutoff voltage of 4.6 V retains a 98.4% capacity retention after 500 cycles at 1C. This work affords a promising strategy to develop nonflammable electrolytes enabling the high safety, good cyclability, and low cost of high-energy LMBs.

Stable Operation Induced by Plastic Crystal Electrolyte Used in Ni-Rich NMC811 Cathodes for Li-Ion Batteries

The LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material has been of significant consideration owing to its high energy density for Li-ion batteries. However, the poor cycling stability in a carbonate electrolyte limits its further development. In this work, we report the excellent electrochemical performance of the NMC811 cathode using a rational electrolyte based on organic ionic plastic crystal N-ethyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide C2mpyr[FSI], with the addition of (1:1 mol) LiFSI salt. This plastic crystal electrolyte (PC) is a thick viscous liquid with an ionic conductivity of 2.3 × 10-3 S cm-1 and a high Li+ transference number of 0.4 at ambient temperature. The NMC811@PC cathode delivers a discharge capacity of 188 mA h g-1 at a rate of 0.2 C with a capacity retention of 94.5% after 200 cycles, much higher than that of using a carbonate electrolyte (54.3%). Moreover, the NMC811@PC cathode also exhibits a superior high-rate capability with a discharge capacity of 111.0 mA h g-1 at the 10 C rate. The significantly improved cycle performance of the NMC811@PC cathode can be attributed to the high Li+ conductivity of the PC electrolyte, the stable Li+ conductive CEI film, and the maintaining of particle integrity during long-term cycling. The admirable electrochemical performance of the NMC811|C2mpyr[FSI]:[LiFSI] system exhibits a promising application of the plastic crystal electrolyte for high voltage layered oxide cathode materials in advanced lithium-ion batteries.

1,3,5-Trifluorobenzene, an electrolyte additive with high thermal stability and superior film-forming properties for lithium-ion batteries

The introduction of 1,3,5-trifluorobenzene (F3B) as an additive for lithium-ion battery electrolytes can produce a LiF-rich solid electrolyte interface (SEI). Meanwhile, F3B has superior thermal stability compared with traditional fluorinated additives and is less likely to produce hydrogen fluoride to damage the cathode.

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.