The Exceptional Challenges of Si-Based Anodes and the Eternal Hunt for Electrolyte Additives: A Perspective on the...

Si-based anodes (e.g., SiO _x ) can increase the specific energy of Li-ion batteries but decrease cycle life due to volume change-reasoned material stress, which requires continuous reformation of the solid electrolyte interphase (SEI) at the cost of active Li loss (ALL). Even in small ratios within a binary anode with graphite, Si can be exposed to a high state-of-charge as it can be stressed by transfer lithiation from lithiated graphite. SEI engineering via rational design of electrolyte additives can be an eligible measure, but is particularly challenging, as the SEI must effectively passivate and prevent detrimental SiO _x |LiPF₆ contact in order to suppress failure cascades caused by generated HF (e.g., electrode crosstalk), while efficiently minimizing ALL. Also, theoretically, the recovery of the "SEI waste" stemming from the continuous SEI reformation can be advantageous and, for example, be realized via a designed and controlled electrolyte solubility, where the respective diffusion toward the cathode would compensate ALL by oxidation reactions. Here, an easily modifiable B-based compound (dioxazaborole) is synthesized and exemplarily investigated. While it forms a partly effective SEI and can be suitable for "SEI-waste" recovery, the remaining challenges of gassing and resistance require further design optimization, consequently highlighting the difficulty of an "allrounder" additive.