Suppressing Ionic-to-Electronic Conduction Transition on Cathode Interface Enables 4.4 V Poly(ethylene oxide)-Based All-Solid-State Batteries

The implementation of energy-dense poly(ethylene oxide) (PEO)-based all-solid-state lithium batteries is impeded by the limited working voltage and underexplored cathode interfacial reaction mechanism. Here, through analyzing interfacial resistances using the Wagner model, the change of the interfacial reaction parameter (k) is proposed to unveil the ionic-to-electronic conduction transition and kinetic formation mechanism of the cathode-electrolyte-interphase (CEI) under voltage ≥4.2 V, thereby constructing ionic conductor-dominated CEIs to enable 4.4 V batteries. With the open-circuit voltage ≥4.2 V, k1 and k2 are derived; k2 is smaller than k1, caused by the enhanced electronic conduction and indicating the ionic-to-electronic conduction transition of the CEI. Moreover, by introducing LiPO2F2 in high-concentration solid electrolytes, ionic conductors Li3PO4 and LixPOFy dominate the CEI, overcoming the ionic-to-electronic conduction transition; the resulting 4.4 V cell bears a discharge capacity of 130 mAh/g with a retention of 90% after 100 cycles, about 2 times that of the normal PEO-based cell.