Solid-state prealkylation of electrode architectures to tune solid electrolyte interphase composition

Efficient electrochemical cycling of certain Si anodes is limited by irreversible Li consumption to form and continually reform the solid electrolyte interface (SEI) due to Si expansion/contraction and fracture. Prelithiation can compensate for these losses; however, the starting open circuit potential (V_OC) becomes highly reducing and, therefore, the electrolyte reduction chemistry that influences the SEI composition can change. Herein, we compare SEI formation for electrodes prelithiated using Solid State Prealkylation of Electrode Architectures (SPEAR) versus traditional electrochemically lithiated architectures (ECLAR), focusing on SEI compositional changes as a function of stoichiometry (0.28 ≤ x ≤ 1.38 in Li_xSi). Increasing SPEAR prelithiation decreased the initial V_OC of Si anodes vs. Li/Li⁺ from ∼3 V (Li_0.28Si) to < 0.5 V for Li_1.38Si, enabling simultaneous competitive reduction of EC, EMC, and LiPF₆ at low potentials. Ex situ⁷Li and ²⁹Si cross-polarization NMR and XPS reveal that SPEAR drives thicker SEI formation with substantially increased P/F contributions and a predominantly inorganic insoluble SEI (71.4% inorganic for Li_1.38Si), consistent with accelerated LiPF₆-derived PO_x/LiPF_x/LiF formation relative to ECLAR analogs which exhibit carbonate-rich organic SEI compositions. Symmetric-cell EIS further indicates SPEAR-specific impedance features consistent with pore reduction (filling) during Li_xSi formation. In full cells, SPEAR prelithiation increases the initial coulombic efficiency (ICE) and accelerates SEI formation and stabilization with Li_1.38Si reaching 99.4% coulombic efficiency (CE) by cycle 2.