Silicon Anode Drives Calendar Aging in Lithium-Ion Cells, Study Finds

Lithium-ion batteries containing silicon anodes lose capacity even when they simply sit on the shelf, and a new study pinpoints where that degradation comes from. Researchers at Argonne National Laboratory, the University of Wisconsin-Madison, the National Laboratory of the Rockies and the University of Chicago Pritzker School of Molecular Engineering stored three-electrode cells built with lithium nickel manganese cobalt oxide (NMC811) cathodes and milled silicon anodes for 11 months, tracking how each electrode changed during open-circuit storage. Their conclusion: calendar aging in these cells is driven mainly by interfacial reactions and the resulting loss of cyclable lithium, not by extensive loss of active electrode material.

Calendar aging matters because parasitic reactions continue inside a battery even when no current flows. These side reactions consume cyclable lithium and alter electrode behavior, gradually eroding the energy a cell can deliver. To separate the contributions of the two electrodes, the team used capacity measurements, voltage-relaxation analysis, impedance evaluation and post-test characterization, monitoring the individual electrodes throughout the storage period.

The silicon anode emerged as the dominant player. It governed the full cell’s voltage relaxation and its discharge impedance throughout aging. As parasitic reactions consumed lithium inventory, the operating windows of both electrodes shifted continuously, forcing the NMC811 cathode into increasingly higher states of delithiation. In other words, the cathode had to work harder to compensate for lithium lost at the silicon interface.

Voltage-relaxation analysis proved to be the study’s key diagnostic tool. When a cell is allowed to rest after cycling or storage, its voltage does not settle instantly; it drifts as the electrodes re-equilibrate internally. The analysis revealed two kinetically distinct equilibration processes in these cells. The longer-timescale process evolved systematically within each month-long storage interval and partially reset after the cells underwent reference performance test (RPT) cycling, indicating a strong dependence on the electrochemical history of the silicon electrode.

Post-test characterization added a crucial nuance. The damage from storage, the results suggest, accumulates at interfaces rather than through bulk degradation of the electrode materials themselves.

The harvested NMC811 and Si electrodes showed only minor capacity loss, but the increase in anode impedance pointed to evolution of the SEI and the silicon interfacial environment.

The study also highlights the value of voltage-relaxation analysis as a non-destructive diagnostic. The authors note that the approach can distinguish the two equilibration processes and follow how the slower one changes with storage time and cycling history.

The work, published as an original research article in Frontiers in Batteries and Electrochemistry, was led by Hrishikesh Tupkar with correspondence from Daniel P. Abraham, and included contributions from Pavan Badami, Ankit Verma, Avi Gargye, Marco-Tulio Fonseca Rodrigues, Andrew Colclasure, Matthew Gebbie and Ilya Shkrob. For designers of silicon-containing lithium-ion cells, the message is that the silent chemistry occurring during storage, concentrated at the silicon interface and expressed as lithium inventory loss, is what ultimately limits how well these batteries hold their capacity over time.

Subject of Research: Calendar aging mechanisms in three-electrode lithium-ion cells with NMC811 cathodes and silicon anodes

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

Article References: Tupkar, H., Badami, P., Verma, A., Gargye, A., Rodrigues, M.-T. F., Colclasure, A., Gebbie, M. A., Shkrob, I., & Abraham, D. P. (2026). Mechanisms governing calendar aging in three-electrode lithium-ion cells with layered oxide cathode and milled silicon anode. Frontiers in Batteries and Electrochemistry, 5, Article 1995764. https://doi.org/10.3389/fbael.2026.1995764

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1995764

Keywords: calendar aging, lithium-ion batteries, silicon anode, NMC811 cathode, loss of lithium inventory, SEI, voltage relaxation, impedance, three-electrode cells, battery degradation, Mechanisms, governing