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The combination of Li-metal anode and high-voltage cathode is regarded as a solution for the next-generation high-energy-density secondary batteries. However, a traditional electrolyte is either incompatible with the Li-metal anode or vulnerable to high voltage. This work reports a 1 M dual-salts Localized-High-Concentration-Electrolyte with Fluoroethylene carbonate (FEC) additive. It enables stable cycling of Li||LiNi0.8Co0.1Mn0.1O2 (NMC811) battery, which shows 81.5% capacity retention after 300 cycles with a charge/discharge current density of 1 C and a voltage range of 2.7–4.4 V. Scanning electron microscopy (SEM) images show that this electrolyte not only largely reduced Li dendrites and ‘dead’ Li on anode surface but also well protected the microstructure of NMC811 cathode. Possible components of both solid-electrolyte interlayer (SEI) and cathode-electrolyte interlayer (CEI) were characterized by energy-dispersive X-ray spectroscopy (EDX). The result illustrates that FEC protected Li salts from decomposition on the anode side and suppressed the decomposition of solvents on the cathode side.

All-solid-state batteries (ASSBs) are among the remarkable next-generation energy storage technologies for a broad range of applications, including (implantable) medical devices, portable electronic devices, (hybrid) electric vehicles, and even large-scale grid storage. All-solid-state thin film Li-ion batteries (TFLIBs) with an extended cycle life, broad temperature operation range, and minimal self-discharge rate are superior to bulk-type ASSBs and have attracted considerable attention. Compared with conventional batteries, stacking dense thin films reduces the Li-ion diffusion length, thereby improving the rate capability. It is vital to develop TFLIBs with higher energy density and stability. However, multiple challenges, such as interfacial instability, low volumetric energy density, and high manufacturing cost, still hinder the widespread application of TFLIBs. At present, many approaches, such as materials optimization and novel architecture design, have been explored to enhance the stability and energy density of TFLIBs. An overview of these discoveries and developments in TFLIBs is presented in this review, together with new insights into the intrinsic mechanisms of operation; this is of great value to the batteries research community and facilitates further improvements in batteries in the near future.