Lithium-ion batteries (LIBs) do not only dominate the small format battery market for portable electronic devices but have also been successfully implemented as the technology of choice for electric vehicles. However, for successful consumer acceptance and broad market penetration of electric vehicles, further improvements of LIBs in terms of energy density and cost along are required. The practically usable energy density of LIB cells is reduced by parasitic side reactions including electrolyte decomposition and formation of the “solid-electrolyte interphase” (SEI) at the surface of the anode, as this process is related to the consumption of active lithium. Especially high-capacity silicon-based anodes suffer from ongoing lithium loss and rapid capacity fading.
Pre-lithiation is considered as a highly appealing technique to compensate for active lithium losses and, therefore, to increase the practical energy density. A critical parameter for a pre-lithiation strategy is to establish a technique for achieving lithiation of the active storage material at the most uniform lateral and in-depth distributions possible. Despite extensive exploration of various pre-lithiation techniques, controlling the lithium amount precisely while keeping an even lithium distribution remains challenging.
Here, we report the thermal evaporation of lithium metal as a novel pre-lithiation technique for silicon anodes that allows both, i.e., precise control of the degree of pre-lithiation and a homogeneous deposition at the electrode surface. Silicon | Lithium metal interface and SEI characterization show that this approach enables the preserved Li to be consumed for the formation of a pre-formed SEI and creates moderate mechanical cracks by volume changes. The highly uniform Li on thin-film Si is beneficial for increasing capacity retention and significantly suppressing active lithium losses. The phase changes are evaluated and the terms dry-state and wet-state pre-lithiation (without/with electrolyte) are revisited. Finally, a series of electrochemical methods are also validated to allow a direct correlation of active lithium losses with the pre-formed SEI formation and improved electrochemical performance of silicon anodes.