A Balance-of-Charges Descriptor for Electron-Delocalized Solvents Enables Calendar-Stable and Thermally Tolerant...

Silicon-based anodes provide a practical route to lithium-ion batteries with cell-level energy densities beyond 350 Wh kg^{- 1}, but their calendar life is limited by parasitic reductive reactions at the anode-electrolyte interface. Existing single-parameter electrolyte descriptors, such as lowest unoccupied molecular orbital energy and donor number, describe only part of solvent reductive stability and often fail to distinguish structurally similar solvents. Here, we introduce a balance-of-charges (BOC) descriptor to quantify intramolecular electron delocalization from gas-phase electrostatic potentials. BOC correlates with reductive stability across eight solvent classes, with low-BOC solvents favoring compact, inorganic-rich solid-electrolyte interphases (SEI). Based on this criterion, tetraethyl orthosilicate is selected and incorporated into a localized high-concentration electrolyte with lithium bis(fluorosulfonyl)imide, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, fluoroethylene carbonate, and lithium difluorophosphate, denoted TLFE. The resulting LiF-reinforced SEI suppresses parasitic leakage and remains compositionally stable after three days at 60°C. In 1 Ah µ-SiO_x)/graphite‖LiNi₀.₈Co₀.₁Mn₀.₁O₂ pouch cells, TLFE shows 25% lower exchange-capacity loss, 88.1% capacity retention after 250 cycles at 60°C, and a 75°C higher thermal-runaway onset than the baseline. BOC offers a simple descriptor for low-reactivity electrolyte design.