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.

The advent of a Li+ or Na+ glass electrolyte with a cation conductivity σi > 10−2 S cm−1 at 25 °C and a motional enthalpy ΔHm = 0.06 eV that is wet by a metallic lithium or sodium anode is used to develop a new strategy for an all-solid-state, rechargeable, metal-plating battery.

The application of flexible, robust, and low-cost solid polymer electrolytes in next-generation all-solid-state lithium metal batteries has been hindered by the low room-temperature ionic conductivity of these electrolytes and the small critical current density of the batteries. Both issues stem from the low mobility of Li+ ions in the polymer and the fast lithium dendrite growth at the Li metal/electrolyte interface. Herein, Mg­(ClO4)2 is demonstrated to be an effective additive in the poly­(ethylene oxide) (PEO)-based composite electrolyte to regulate Li+ ion transport and manipulate the Li metal/electrolyte interfacial performance. By combining experimental and computational studies, we show that Mg2+ ions are immobile in a PEO host due to coordination with ether oxygen and anions of lithium salts, which enhances the mobility of Li+ ions; more importantly, an in-situ formed Li+-conducting Li2MgCl4/LiF interfacial layer homogenizes the Li+ flux during plating and increases the critical current density up to a record 2 mA cm–2. Each of these factors contributes to the assembly of competitive all-solid-state Li/Li, LiFePO4/Li, and LiNi0.8Mn0.1Co0.1O2/Li cells, demonstrating the importance of surface chemistry and interfacial engineering in the design of all-solid-state Li metal batteries for high-current-density applications.

A garnet-type solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was modified using dopamine to improve the wettability of LLZTO with PEO, allowing 80 wt% LLZTO to be uniformly dispersed in 20 wt% PEO/LiTFSI polymer electrolyte.