Polymer-Inorganic Nanocomposite Coating with High Ionic Conductivity and Transference Number for Stable Lithium Metal Anode

Abstract

Lithium metal is an ideal candidate for battery anodes due to its high specific capacity (3860 mAh g–1) and low electrochemical potential (–3.04 V). However, non-uniform lithium electrodeposition causes low Coulombic efficiency, poor cycling stability, potential safety issues, and has hindered the commercialization of lithium metal batteries at scale. The parasitic reaction of lithium metal with liquid electrolytes results in the formation of a solid electrolyte interphase (SEI). During cycling, the SEI tends to break continuously, exposing unreacted lithium metal to the electrolyte. This not only promotes non-uniform lithium plating and stripping, but also leads to further SEI formation, which consumes the liquid electrolyte. In addition, the detachment of dendritic lithium from the bulk lithium electrode produces electrically isolated 'dead' lithium during lithium stripping, which no longer participates in the cell reaction. Dendritic lithium and 'dead' lithium formed during cycling largely increase the surface area of the lithium metal anode and consequently accelerates electrolyte consumption. These mechanisms lead to rapid capacity fading and cell failure due to either the complete consumption of the electrolyte or the complete conversion of the lithium metal anode to 'dead' lithium. The formation of lithium metal dendrites can also causes abrupt cell short circuit, which results in thermal runaway. Therefore, a stable interface on lithium metal electrode that enables uniform lithium electrodeposition is the key for the development of lithium metal batteries.Here, we report a nanocomposite coating for the lithium metal protection, which consists of nano-sized LiF particles embedded into a PEO polymer matrix as shown in the inset of the following figure. The nano-sized LiF particles ensure high interface area, which provides lithium-ion pathway throughout the composite. The LiF/PEO nanocomposite coating offers exceptionally high ionic conductivity of 0.97 mS cm–1 and high lithium-ion transference number of 0.77 at room temperature in contact with a carbonate-based liquid electrolyte. Both properties are key to realize uniform lithium plating and stripping and prevent lithium dendrite formation. Lithium metal electrodes coated with this protective coating enable stable lithium plating and stripping at 1 mA cm–2 for over 1000 h. As shown in the following figure, a full cell with the coated lithium metal anode and a NMC622 cathode with an areal capacity of 1 mAh cm–2 shows much improved specific capacity and cycling stability. The cell exhibits a high initial capacity of 135 mAh g–1 and a capacity retention of 83% after 500 cycles at 1 mA cm–2. Even at a high current density of 3 mA cm–2, the full cell shows an initial capacity of 125 mAh g–1 and a capacity retention of 74% after 300 cycles. Figure 1