Mechanisms governing calendar aging in three-electrode lithium-ion cells with layered oxide cathode and milled silicon anode

Calendar aging is a major degradation pathway for silicon-containing lithium-ion batteries because parasitic reactions continue during open-circuit storage, consuming cyclable lithium and altering electrode behavior. Here, we investigate the calendar aging of three-electrode cells, with LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) and silicon (Si) electrodes, over an 11-month period to identify electrode-specific degradation mechanisms. Capacity measurements, voltage-relaxation analysis, impedance evaluation and post-test characterization were used to monitor evolution of the individual electrodes. The Si anode dominates full-cell voltage relaxation and discharge impedance throughout aging, while progressive loss of lithium inventory continuously shifts operating windows of both electrodes, forcing the NMC811 cathode to increasingly higher states of delithiation. Voltage-relaxation analysis reveals two kinetically distinct equilibration processes, with the longer-timescale process evolving systematically within each month-long storage interval and partially resetting following the reference performance test (RPT) cycling indicating a strong dependence on the electrochemical history of the Si electrode. Although the harvested NMC811 and Si electrodes exhibit only minor capacity loss, the increase in anode impedance demonstrates evolution of the SEI and the Si interfacial environment. These results indicate that calendar-aging-induced capacity loss is governed primarily by interfacial reactions and associated loss of lithium inventory rather than extensive loss of active material. More broadly, the results demonstrate the utility of voltage-relaxation analysis as a non-destructive diagnostic for resolving electrode-specific degradation mechanisms in Si-containing lithium-ion cells.