Calendar life of lithium metal batteries: Accelerated aging and failure analysis

Lithium-metal batteries (LMBs) are prime candidates for next-generation energy storage devices. Despite the critical need to understand calendar aging in LMBs; cycle life and calendar life have received inconsistent attention. For acceptance into an application, especially electric vehicles, batteries are required to have sufficient calendar life which is defined as periods of low or intermittent use. In this study, an in-depth exploration into the calendar aging of LMB (Li||Li[Ni_0.8Mn_0.1Co_0.1]O₂ in pouch cell format) is conducted across multiple states-of-charge, temperatures, and pressures. The work identified the key limiting factors in calendar life as electrolyte depletion and increased cell impedance. Consumption of lithium did occur but due to cell design the losses were masked by the excess lithium in the cell design. Application of pressure extends calendar life. Moderate aging condition (i.e., OCV, 70 % SOC, 25 °C, and 10 psi) leads to <1 % reduction in capacity over 18 months. For this condition, the calendar life is conservatively projected at 31 months with an optimistic projection of 13.6 years. Additionally, this work contributes to the development of accelerated aging methods which can include elevated temperature (45 °C) and extended voltage holds which lead to intermittent impacts to cell passivation. The findings of this work strongly suggest that electrode mechanical aspects in addition to the chemical and electrochemical reactivities are important for long LMB calendar life.