What does this equation mean?

Vetter and coauthors’ widely cited review of lithium-ion aging mechanisms catalogs why cells fade: solid-electrolyte-interphase growth on the anode consumes cyclable lithium and increases impedance; lithium plating at low temperature or high charge rate can occur when the anode’s insertion kinetics cannot keep pace with the applied current, creating a safety hazard distinct from ordinary capacity fade; cathode particle cracking and transition-metal dissolution degrade the cathode independently of anode-side mechanisms [ 1 ] . A single capacity-versus-cycle-number curve cannot distinguish these mechanisms from each other; that requires complementary measurements — incremental capacity analysis, electrochemical impedance spectroscopy, and, at end of test, post-mortem structural characterization of the same XRD and TEM kind described above, now applied to a harvested electrode rather than a pristine one. A coarse but common summary statistic is coulombic efficiency, the ratio of discharge to charge capacity within one cycle, $\mathrm{CE} = \frac{Q_{\mathrm{discharge}}}{Q_{\mathrm{charge}}}$. whose slow, sustained departure from unity across cycling is itself diagnostic of a parasitic side reaction consuming charge without returning usable capacity — a small departure per cycle, summed over a thousand cycles, is where most of the calendar-relevant fade actually accumulates, and is a more sensitive early warning than watching total discharge capacity alone.