Cryogenic engineering of solid polymer electrolytes for room temperature and 4 V-class all-solid-state lithium batteries

Solid polymer electrolytes (SPEs) are promising candidates for all-solid-state lithium batteries (ASSLBs) due to their advantages of good interfacial adhesion and shape flexibility. However, the low ionic conductivity at room temperature (RT) and inferior electrochemical stability at high voltages limit the practical applications of SPEs. In this work, we demonstrate a cryogenic engineering to improve PEO-based SPEs for ASSLBs operated at RT. The rapid in situ cooling process will lead to the uniform formation of PEO crystal nuclei, which can limit the PEO crystal growth in SPEs. The novel crystallization structure could promote the ionic conductivity of SPEs effectively. Such improved cryogenic SPEs display a superior ionic conductivity of 2.17 × 10⁻⁵ S cm⁻¹ and a superior electrochemical stability at RT. A discharge capacity of 154.9 mAh g⁻¹ can be achieved at 0.1 C and RT when LiFePO₄ (LFP) is used as the cathode. The discharge capacity can remain at ~98%, even after 100 cycles performed at 0.1 C. Notably, cryogenic SPEs can be utilized in 4 V-class ASSLBs. A high discharge capacity of 118 mAh g⁻¹ can be achieved at 0.2 C and RT with the LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) as the cathode and can retain a 94.1% capacity even after 100 cycles. These extraordinary performances of cryogenic SPEs break new ground for the fabrication of ASSLBs operated at RT and high voltages.