Regulation of the cathode inner Helmholtz plane in dilute ether electrolytes using an electric-field-responsive solvent for high-voltage lithium metal batteries†

Abstract

Raising the cut-off voltage to 4.6 V can boost the energy density of lithium metal batteries (LMBs) with the LiCoO₂ cathode. However, the unfavorable cathode electrolyte interphase (CEI) formed on the high-voltage cathode when using Li metal-compatible ether electrolytes leads to a short lifespan of batteries. Here, we harness the battery intrinsic electric field to manipulate the cathode inner Helmholtz plane (IHP) during the battery's charge by simply introducing an electric-field-responsive solvent into an ether electrolyte. Specifically, 2,2-difluoroethyl trifluoromethanesulfonate (DTS) as a weak solvated co-solvent preferentially adsorbs on the cathode surface under electric field activation. Owing to the majority of “high-voltage fragile” 1,2-dimethoxyethane ether molecules squeezed out from the IHP by DTS, a LiF-rich and organics-less stable CEI film is formed, which minimizes cathode cracking and structure degradation under high-voltage. The DTS-containing electrolyte shows superior cycling stability of LMBs under harsh conditions of 4.6 V, 4.2 mA h cm⁻² cathode loading and 15 μL less electrolyte. Furthermore, the feasibility of this concept is validated in practical A h-level pouch full cells. This work highlights the significance of the IHP modulation and interfacial chemistry, providing an alternative pathway towards ether-based electrolyte engineering for high-voltage LMBs.