Lithium metal rechargeable batteries (LRBs) have the potential to meet the 500 Wh·kg^-1 energy density goal set by the US Department of Energy for high energy density batteries [1]. However, two issues need to be addressed in these batteries before their widespread commercialization: (1) safety concerns arising from dendritic growth and (2) limited cycle life due to low Coulombic efficiency and loss of active Li. Since organic solvents are thermodynamically unstable in the presence of highly-reducing Li, significant side reactions take place, the products of which form a solid electrolyte interphase (SEI) on the Li surface.

During the production process, Li-ion batteries undergo formation cycling to form a stable SEI and to evaluate the condition of the cell. However, the advantages of early cycle conditioning or formation are not well understood for LRBs, because new SEI forms on the Li surface during each cycle. The present study was undertaken to better understand how early cycling impacts interfacial processes in LRBs and ultimately how variation in the initial cycles impacts the life of the battery.

Cycling conditions, which include both charging (deposition) and discharging (stripping), affects the Li anode morphology [2, 3]. Build-up of a stable SEI layer and formation of a uniform Li surface during initial cycling of LRBs helps limit dendritic growth, and cell impedance in subsequent cycling. A stable initial SEI is also necessary to prevent corrosion of the Li metal electrode and to prolong the cycle and calendar life of the cells by limiting side reactions with the new Li surface. Using a high-Ni NMC cathode and a Li metal anode, an assortment of different cycling conditions which impact the dynamics of the Li-electrolyte interface were investigated. Performance was characterized with electrochemical diagnostic tools and surface morphology analysis. Both the deposition and stripping rate affected the impedance of the cell, surface kinetics, and interfacial mass transport, all of which impact the cycle life of LRBs.

References:

[1] Battery500 Consortium to Spark EV Innovations: Pacific Northwest National Laboratory-led, 5-year $50M effort seeks to almost triple energy stored in electric car batteries, in Office of Technology Transitions Energy.gov (2016).

[2] Hongfa Xiang, Pengcheng Shi, Priyanka Bhattacharya, Xilin Chen, Donghai Mei, Mark E. Bowden, Jianming Zheng, Ji-Guang Zhang, and Wu Xu, “Enhanced charging capability of lithium metal batteries based on Lithium bis(trifluoromethanesulfonyl)imide-lithium bis(oxalato) borate dual-salt electrolytes,” J Power Sources, 318, 170 (2016).

[3] Jiangfeng Qian, Wesley A. Henderson, Wu Xu, Priyanka Bhattacharya, Mark Engelhard, Oleg Borodin, and Ji-Guang Zhang, “High rate and stable cycling of lithium metal anode,” Nat Comm, 6, 6362 (2015).