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Understanding and preventing electron transport through the solid electrolyte interphase (SEI) films in Li-ion batteries, is critical to battery capacity drop and power loss. By studying the point defect formation and diffusion in main SEI components such as crystalline Li₂CO₃, via density functional theory (DFT) based thermodynamics, a possible mechanism is proposed to explain electron leakage through an SEI film into the electrolyte. It's found that electron or polaron hopping is facilitated by the diffusion of neutral Li interstitials, whose concentration increases with lower voltage. This mechanism can occur even when the SEI thickness is beyond the electron tunneling length. Ab initio molecular dynamics of SEI/electrolyte interface simulations further demonstrated that when a neutral Li interstitial diffuse into the electrolyte, ethylene carbonate (EC) reduction is observed immediately. Thus, neutral  Li interstitials diffusion is also responsible for self-discharge of Li-ion batteries.