Metal/LiF/Li₂O Nanocomposite for Battery Cathode Prelithiation: Trade-off between Capacity and Stability

Lithium-ion batteries (LIBs) are currently dominating the portable electronics market and supplying power for electric vehicles and grid-level storage. However, lithium loss in the formation cycle at the anode side reduces the energy density of state-of-the-art LIBs with carbon anode materials. This situation will be even more severe for future LIBs using high-capacity Si-based anode materials. In this study, a transition metal-based nanocomposite with built-in lithium source was synthesized, featuring Fe nanodomains with a size of ∼5 nm uniformly dispersed in a hybrid Li₂O and LiF matrix with intimate contact between them. The Fe/LiF/Li₂O nanocomposite released a high Li-ion capacity of 550 mA h/g based on a multielectron inverse conversion reaction during the first-cycle charge process and exhibited better ambient stability than the counterpart with a pure Li₂O matrix and also a lower lithium-extraction voltage and faster reaction kinetics than the counterpart with a pure LiF matrix. Serving as an additive to various cathodes (e.g., LiCoO₂, LiFePO₄, and LiNi_1-CoMnO₂), the Fe/LiF/Li₂O nanocomposite showed excellent lithium compensation effect. Using 4.8 wt % Fe/LiF/Li₂O additive based on the total mass of the electrodes, a LiNi_0.8Co_0.1Mn_0.1O₂|SiO-graphite full cell with a high cathode mass loading of 20 mg/cm² exhibited a high reversible capacity of 2.9 mA h/cm² at 0.5 C after 100 cycles which is a 15% increase in comparison to the counterpart without the prelithiation additive. After the Fe/LiF/Li₂O nanocomposite was immersed into the electrolyte and rested for 72 h, the content of iron metal in the electrolyte was negligible, indicating that this prelithiation additive was stable in the electrolyte and would not cause any side reactions, such as the shuttle of iron ions during cycling. The high "donor" Li-ion capacity, good ambient stability, and its compatibility with existing cathode materials and battery fabrication processes make the Fe/LiF/Li₂O nanocomposite a promising cathode prelithiation additive to offset the initial lithium loss and improve the energy density of LIBs.