Solvent Reorganization and Additives Synergistically Enable High-Performance Na-Ion Batteries

Abstract

Electrolytes with high and localized salt concentrations have been successfully developed to mitigate parasitic reactions in rechargeable batteries, but at the expense of high cost, high viscosity, and slow kinetics. An alternative electrolyte design concept beyond the current strategies is crucial to reaching practical applications. From salt to solvent, herein, we demonstrate solvent reorganization to develop highly stable dilute phosphate electrolytes (0.16–0.85 M) for Na-ion batteries. This solvent reorganization is the result of optimal intermolecular interactions between the main solvent phosphate and charge-asymmetric trifluorotoluene (PhCF3) as a solvent coordinator. Appropriate solvent reorganization induced by PhCF3 can reinforce the solvation network of electrolytes without employing high concentrations of expensive salts, which enables a highly stable dilute phosphate electrolyte in combination with synergic additives. With the designed dilute electrolyte, NaCu1/9Ni2/9­Fe1/3Mn1/3O2 (CNFM)||HC Na-ion full cells demonstrate stable cycling for 400 cycles and a high Coulombic efficiency of >99.95% at 0.2 C. An Ah-level pouch cell is evidenced with highly safe cycling of over 100 cycles with 90.1% capacity retention and without inflation at 60 °C. This work opens an alternative avenue in designing electrolytes by manipulating solvent interactions instead of focusing on local solvation structure and high salt concentrations.

Nonaqueous Liquid Electrolytes for Sodium‐Ion Batteries: Fundamentals, Progress and Perspectives

“…developed a solvent reorganization and additive synergistic strategy for NaCu 1/9 Ni 2/9 Fe 1/3 Mn 1/3 O 2 (CNFM) ||HC sodium‐ion full cells. [ 84 ] Trifluorotoluene (PhCF 3 ) showed a conjugated benzene ring and a strong negative −CF 3 group, which reinforced the solvation network of electrolytes. With MD simulations, the authors identified some TEP−PhCF 3 aggregates and n ‐(TEP−PhCF 3 ) nanoclusters (Figure 12e), which reduced free TEP molecules and apparently altered the Na + solvation structure.…”

“…The functional additives are often sacrificial and intended for modification of SEI/CEI layers during the initial formation cycles. [83,84] They usually exhibit a lower LUMO or a higher HOMO energy level than that of electrolyte components for preferential reduction or oxidation to regulate the formation of electrode/electrolyte interphases (Figure 12a). [85] The most popular additive in SIBs is fluoroethylene carbonate (FEC), which is preferentially reduced to form a stable and robust SEI layer.…”

“…f) Linear sweep voltammetry curves for different electrolytes. Reproduced with permission [84]. Copyright 2023, American Chemical Society.…”

Towards stable electrode–electrolyte interphases: Regulating solvation structures in electrolytes for rechargeable batteries

“…Recently, Ma et al have developed a high‐performance flame‐retardant phosphate‐based electrolyte with an ultralow concentration (0.16–0.85 M). [ 67 ] By optimizing the ratio of strong and weak solvents, they achieved optimal intermolecular interactions, inducing a stable “solvation cluster” reorganization structure (Figure 6B). This leads to a strengthened solvation network and enhances the electrochemical stability window of the electrolytes on both the negative and positive voltage sides (Figure 6C).…”

“…Reproduced with permission. [ 67 ] Copyright 2022, American Chemical Society. CEI, cathode electrolyte interphase; DME, dimethyl ether; HC, hard carbon; LHCE, localized high‐concentration electrolytes; LiFSI, lithium bis(fluorosulfonyl)imide; LMB, Li‐metal battery; NaFSI, sodium bis(fluorosulfonyl)imide; SEI, solid electrolyte interphase; VC, vinylene carbonate.…”