Architecting Phase‐Selective Interlayers for Stable SiO x Lithium Storage via Dual‐Function Design

Silicon suboxide (SiO x ) anode offer higher capacities than graphite; however, their practical use is hindered by unstable interfaces and poorly controlled formation of lithium silicates during initial lithiation. Insufficient or nonuniform lithiation results in oxygen‐rich lithium silicates that block deep silicon (Si) alloying, while uncontrolled interfacial reaction consumes active lithium, lowers initial Coulombic efficiency, and destabilizes the solid‐electrolyte interphase (SEI). In this study, we address a specific, falsifiable question: what factors dictate which lithium silicate phase forms in the subsurface, and can this process be manipulated to improve both deep Si alloying and SEI stability? We define the Phase‐Selective Conductive Interfacial Interlayer (PSCII) as a Li 4 SiO 4 ‐rich and ion‐accessible subsurface formed through coupled control of bulk oxygen stoichiometry and local electron supply. A stepwise control series from commercial SiO to layered SiO x (LS‐SiO x ), deoxygenated layered SiO x (DLS‐SiO x ), and single‐walled carbon nanotubes (SWCNTs)‐wired DLS‐SiO x @CNT separates the contributions of layered buffering, bulk deoxygenation, and conductive wiring. As a result, DLS‐SiO x @CNT delivers 1650 mAh g −1 , retains >820 mAh g −1 after 700 cycles at 1 A g −1 , and enables stable LCO full‐cell cycling. These results identify subsurface lithium silicate phase selection as a practical design strategy for durable high‐capacity SiO x anodes.