In the recent years, significant progress have been made on the long-term cycle life of lithium (Li) metal batteries (LMBs). However, very limited research have been done on the calendar life of LMBs. Although the analysis of low voltage LMBs developed in 70s have revealed its ultra-long calendar life,1 recent report on the initial capacity fade of LMBs also reveals their potential instability.2 In this work, we will analyze various factors that affect both short-term (a few days) instability of Li metal anode (LMA) and long-term (a few years) calendar life of LMBs. Approaches to eliminate the short-term instability of LMA will be discussed. The results on a systematic investigation on the long-term (>18 month) calendar life of high voltage LMBs will be reported for the first time. A highly stable, fluorinated orthoformate based localized high concentration electrolytes were used in Li||LiNi0.8M0.1Co0.1 O2 (NMC811) cells studied in this work. The cells demonstrate highly stable calendar life as revealed by the periodic measurement of self-discharge and capacity recovery at different time of storage during 18-month period. Fully charged cells can still retain 53% of the original capacity after 9-month-storage. Fully discharged cells can still recover ~90% capacity after 18-months-storage. These results have demonstrated great potential on the long-term calendar life of high voltage LMBs. It also points to several new approaches to further extend the calendar life of LMBs required for large scale practical applications.References Pereira, N.; Amatucci, G. G.; Whittingham, M. S.; Hamlen, R., Lithium–titanium disulfide rechargeable cell performance after 35 years of storage. Journal of Power Sources 2015, 280, 18-22.Boyle, D. T.; Huang, W.; Wang, H.; Li, Y.; Chen, H.; Yu, Z.; Zhang, W.; Bao, Z.; Cui, Y., Corrosion of lithium metal anodes during calendar ageing and its microscopic origins. Nature Energy 2021.