Exploring the Promise of Multifunctional “Zintl-Phase-Forming” Electrolytes for Si-Based Full Cells

Li–M–Si ternary Zintl phases have gained attention recently due to their high structural stability, which can improve the cycling stability compared to a bulk Si electrode. Adding multivalent cation salts (such as Mg²⁺ and Ca²⁺) in the electrolyte was proven to be a simple way to form Li–M–Si ternary phases in situ in Si-based Li-ion cells. To explore the promise of Zintl-phase-forming electrolytes, we systematically investigated their application in pouch cells via electrochemical and multiscale postmortem analysis. The introduction of multivalent cations, such as Mg²⁺, during charging can form Li_xM_ySi ternary phases. They can stabilize Si anions and reduce side reactions with electrolyte, improving the bulk stability. More importantly, Mg²⁺ and Ca²⁺ incorporate into interfacial side reactions and generate inorganic-rich solid–electrolyte interphase, thus enhancing the interfacial stability. Therefore, the full cells with Zintl-phase-forming electrolytes achieve higher capacity retentions at the C/3 rate after 100 cycles, compared to a baseline electrolyte. Additionally, strategies for mitigating the electrode-level fractures of Si were evaluated to make the best use of Zintl-phase-forming electrolytes. This work highlights the significance of synergistic impact of multifunctional additives to stabilize both bulk and interface chemistry in high-energy Si anode materials for Li-ion batteries.