Mechanical Impacts from Cycling on Silicon Calendar Aging Measurements

Abstract

Much of the silicon anodes for lithium ion batteries literature concentrates on the volume expansion of silicon leading to poor cycle life. Recently, the poor calendar life of silicon has become more of a focus. Calendar aging is typically measured by long periods of open circuit voltage (OCV) that are intermittently interrupted with a reference performance test (RPT) to quantify performance and capacity fade. The United States Advanced Battery Consortium LLC protocol calls for an RPT once a month with daily voltage pulses to keep the state of charge the same during the rest. If the solid electrolyte interphase (SEI) is unstable and there is substantial volumetric changes during cycling, as is the case with silicon, the frequency and amount of cycling interruption may impact how calendar life is quantified. It was hypothesized that an SEI that equilibrates during a rest may be disrupted enough during RPT cycling that the SEI passivation is decreased and the start of the next rest will result in greater irreversible lithium inventory consumption to rebuild the SEI that was lost during cycling. To test this, a specially designed variable OCV-RPT protocol was applied to silicon – NMC622 full cells. It was found that for all rest durations, time since assembly, rather than cycling, dominated capacity fade, indicating that calendar aging is likely primarily governed by chemical SEI degradation during rest rather than mechanical SEI degradation during RPTs.This research was supported by the U.S. Department of Energy’s Vehicle Technologies Office under the Silicon Consortium Project, directed by Brian Cunningham, and managed by Anthony Burrell. This work was conducted in part by the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Sandia National Laboratories is a multimission Laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.