4.2V polymer all-solid-state lithium batteries enabled by high-concentration PEO solid electrolytes

Polyethylene oxide (PEO) solid electrolytes (SEs) are practicable in all-solid-state lithium batteries (ASSLBs) with high safety for driving electric vehicles. However, the low oxidative decomposition potential (below 4 V) of normal PEO SEs rules out high-voltage (≥4.2 V) cathodes in PEO-based ASSLBs with sacrificed energy densities. Herein, high-concentration PEO SEs (EO:Li⁺ ≤ 6:1) possessing high oxidation potentials (>5 V vs. Li/Li⁺) are designed based on concentrated-salt chemistry with oxidation potential surging incessantly with increasing the degree of coordinated EO. Thereby, double-layered SEs with PEO(EO:Li=4:1) on the cathode side and PEO(EO:Li=16:1) on the anode side are designed to resist oxidation and bate interfacial impedance. Coupled with 4.2 V-class LiCoO₂ and LiNi_0.6Co_0.2Mn_0.2O₂, the ASSLBs using SEs exhibit enhanced stable cycling performances when charged to 4.2 V and 4.4 V at 60 °C. As revealed by the Wagner-type model and Raman spectra, high-concentration PEO SE could suppress the interfacial degradation kinetics, the production of electronic conduction in the cathode electrolyte interphase (CEI) and the irreversible phase-change of LiCoO₂ to Co₃O₄. All these contribute to the improved electrochemical performance of PEO/LiCoO₂ system with high-volage, offering a potential pathway toward high-voltage stable polymer electrolytes for high-energy-density lithium batteries.