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We address the reactivity of lithiated graphite–anode material for Li-ion batteries with standard organic solvents used in batteries (ethylene carbonate and dimethyl carbonate) by following changes in neutron scattering signals. The reaction produces a nanosized layer, the solid-electrolyte interphase (SEI), on the graphite particles. We probe the structure and chemistry of the SEI using small-angle neutron scattering (SANS) and inelastic neutron scattering. The SANS results show that the SEI fills 20–30 nm sized pores, and inelastic scattering experiments with H/D substitution show that this “chemical” SEI is primarily organic in nature; that is, it contains a large amount of hydrogen. The graphite–SEI particles show surface fractal scattering characteristic of a rough particle–void interface and are interconnected. The observed changes in the SEI structure and composition provide new insight into SEI formation. The …