Suppressing Dissolution of Vanadium from Cation-Disordered Li_2–xVO₂F via a Concentrated Electrolyte Approach

Abstract

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Improving Cycling Stability of Vanadium Sulfide (VS4) as a Li Battery Cathode Material Using a Localized High-Concentration Carbonate-Based Electrolyte

“…To elucidate the mechanism responsible for the improved cycling stability of VS 4 in the high-concentration and localized high-concentration electrolytes, we investigated the dissolution of V from the VS 4 cathode during cycling. Several studies have shown that the capacity fading of transition metal-based cathodes is associated with the dissolution of the transition metal from the cathode material, which leads to the loss and degradation of the active material, along with an increase in the internal resistance. − For V-based cathodes, such as V 2 O 5 and Li 2– x VO 2 F, the dissolution of V is one of the primary reasons for their poor cycling stability. − We determined that the color of the separators changed after the discharge/charge cycling in the VS 4 |Li half-cells with the 1.2 M LiFSI/EC:PC electrolyte. The separators turned reddish after 100 cycles in the 1.2 M LiFSI/EC:PC electrolyte; however, the color of the cycled separators did not change in 3.2 M LiFSI/EC:PC and 1.9 M LiFSI/EC:PC:HFE electrolytes (Figure a).…”

“…Studies have shown that the reasons for capacity fading should be the degradation of VS 4 with the accumulation of insulating Li 2 S ,, and the formation of a resistive film on the VS 4 surface owing to electrolyte decomposition . The poor cycling stability of VS 4 may also be associated with dissolution of V from the active material, which results in the degradation of VS 4 during cycling, as has been observed in the cathodes of other V-based materials such as V 2 O 5 , and Li 2– x VO 2 F …”

“…High-concentration electrolytes with lithium salt concentrations of more than 3 mol dm –3 (M) have recently attracted considerable attention owing to their unique physicochemical and electrochemical properties . Their unique coordination structure, which consists of anion–cation–solvent networks, suppresses the dissolution of the transition metal from lithium transition metal oxide cathodes, thus improving their cycling stability. ,− Additionally, the solution structure leads to a downward shift of the anion’s lowest unoccupied molecular orbital (LUMO) energy level. Hence, the anions are reduced more preferentially than the solvents to form a stable solid electrolyte interphase (SEI). ,− In particular, high-concentration electrolytes of lithium bis­(fluorosulfonyl)­imide (LiFSI) allow for the highly stable cycling of Li metal anodes owing to the formation of the FSI-derived SEI. − However, high-concentration electrolytes are highly viscous and thus show poor wettability to conventional polyolefin separators and electrodes, which hinders their practical applicability.…”

Vanadium-Based Materials: Next Generation Electrodes Powering the Battery Revolution?

“…The deficiency of being less electronically reactive poses a significant barrier for V-compounds to fully claim their theoretical energy and power density. Another major issue of V-compounds is associated with the amphiprotic nature of vanadium, where deleterious interfacial reaction between the electrolyte (strong Lewis acid) and vanadium through acid–base interaction exacerbates its dissolution problem . As a result, the catalytic decomposition of the electrolyte can trigger rapid capacity fading due to the loss of active materials.…”

“…Another major issue of V-compounds is associated with the amphiprotic nature of vanadium, where deleterious interfacial reaction between the electrolyte (strong Lewis acid) and vanadium through acid−base interaction exacerbates its dissolution problem. 5 As a result, the catalytic decomposition of the electrolyte can trigger rapid capacity fading due to the loss of active materials. The moderate toxicity of vanadium is one of the concerns that should not be overlooked.…”