Li+‐conducting oxides are considered better ceramic fillers than Li+‐insulating oxides for improving Li+ conductivity in composite polymer electrolytes owing to their ability to conduct Li+ through the ceramic oxide as well as across the oxide/polymer interface. Here we use two Li+‐insulating oxides (fluorite Gd0.1Ce0.9O1.95 and perovskite La0.8Sr0.2Ga0.8Mg0.2O2.55) with a high concentration of oxygen vacancies to demonstrate two oxide/poly(ethylene oxide) (PEO)‐based polymer composite electrolytes, each with a Li+ conductivity above 10−4 S cm−1 at 30 °C. Li solid‐state NMR results show an increase in Li+ ions (>10 %) occupying the more mobile A2 environment in the composite electrolytes. This increase in A2‐site occupancy originates from the strong interaction between the O2− of Li‐salt anion and the surface oxygen vacancies of each oxide and contributes to the more facile Li+ transport. All‐solid‐state Li‐metal cells with these composite electrolytes demonstrate a small interfacial resistance with good cycling performance at 35 °C.

The unclear Li + local environment and Li + conduction mechanism in solid polymer electrolytes, especially in a ceramic/polymer composite electrolyte, hinder the design and development of a new composite electrolyte. Moreover, both the low room-temperature Li + conductivity and large interfacial resistance with a metallic lithium anode of a polymer membrane limit its application below a relatively high temperature. Here we have identified the Li + distribution and Li + transport mechanism in a composite polymer electrolyte by investigating a new solid poly(ethylene oxide) (PEO)-based NASICON−LiZr 2 (PO 4 ) 3 composite with 7 Li relaxation time and 6 Li → 7 Li trace-exchange NMR measurements. The Li + population of the two local environments in the composite electrolytes depends on the Li-salt concentration and the amount of ceramic filler. A composite electrolyte with a [EO]/[Li + ] ratio n = 10 and 25 wt % LZP filler has a high Li + conductivity of 1.2 × 10 −4 S cm −1 at 30 °C and a low activation energy owing to the additional Li + in the mobile A2 environment. Moreover, an in situ formed solid electrolyte interphase layer from the reaction between LiZr 2 (PO 4 ) 3 and a metallic lithium anode stabilized the Li/composite-electrolyte interface and reduced the interfacial resistance, which provided a symmetric Li/Li cell and all-solid-state Li/LiFePO 4 and Li/LiNi 0.8 Co 0.1 Mn 0.1 O 2 cells a good cycling performance at 40 °C.

Flexible and low-cost poly(ethylene oxide) (PEO)-based electrolytes are promising for all-solid-state Li-metal batteries because of their compatibility with a metallic lithium anode. However, the low room-temperature Li-ion conductivity of PEO solid electrolytes and severe lithium-dendrite growth limit their application in high-energy Li-metal batteries. Here we prepared a PEO/perovskite Li3/8Sr7/16Ta3/4Zr1/4O3 composite electrolyte with a Li-ion conductivity of 5.4 × 10−5 and 3.5 × 10−4 S cm−1 at 25 and 45 °C, respectively; the strong interaction between the F− of TFSI− (bis-trifluoromethanesulfonimide) and the surface Ta5+ of the perovskite improves the Li-ion transport at the PEO/perovskite interface. A symmetric Li/composite electrolyte/Li cell shows an excellent cyclability at a high current density up to 0.6 mA cm−2. A solid electrolyte interphase layer formed in situ between the metallic lithium anode and the composite electrolyte suppresses lithium-dendrite formation and growth. All-solid-state Li|LiFePO4 and high-voltage Li|LiNi0.8Mn0.1Co0.1O2 batteries with the composite electrolyte have an impressive performance with high Coulombic efficiencies, small overpotentials, and good cycling stability.