Stable cycling and fast charging of high-voltage lithium metal batteries enabled by functional solvation chemistry

Dimethyl ether (DME)-based localized high-concentration electrolytes (LHCEs) with Li+: DME ratio ∼ 1: 1.5 could ensure stable cycling of LMBs with high Coulombic efficiency but suffer from oxidation decomposition at high voltages and poor charging ability due to its low conductivity. Herein, we design an electrolyte consisting of lithium bis(fluorosulfonyl) imide (LiFSI, 1.0 M) along with lithium hexafluorophosphate (LiPF6, 0.1 M) in DME-hydrofluoroether (TTE) (with Li+: DME = 1: 4), which could both enhance the Li+ ion transportation kinetics of electrolyte and the interfacial stability of both Li metal and LiNi0.9Mn0.05Co0.05O2 (NMC90) cathode. Additive amounts of PF6− anions are found to reduce the probability of FSI− anions entering into the first solvation sheath and simultaneously strengthen the interaction between Li+ and DME, hence stabilizing the DME solvent against reduction/oxidation. Meanwhile, the polymerization of free DME is initiated by PF5, forming a flexible SEI layer on the Li metal anode. On the other hand, PF6− anions facilitate stabilizing NMC90 cathode by forming a LiF-rich interfacial layer. These enable the Li||NMC90 (40 µm Li, 20 mg cm−2 NMC90, 12 μL electrolytes) battery to achieve outstanding capacity retention of 93.7% after 250 cycles at a high charging current density of 4.0 mA cm−2 with a charge cut-off voltage of 4.3 V. These fundamental understandings emphasize the importance in the rational design of electrolyte solvation structures and chemistry to promote the interfacial stability and kinetics, and thus promoting the practical application of high-energy LMBs.