(Invited) Exploring the Impact of Mechanical Pressure on the Performance of Anode-Free Lithium Metal Cells

Research on lithium metal batteries has made a resurgence in the quest to surpass the energy density of conventional lithium ion batteries. Many reports in the literature utilize Li-metal cells with significant excess lithium, thereby artificially inflating the cycling efficiency and cycle life of such cells since lost lithium inventory can be replaced from a vast reservoir. However, utilizing significant excess lithium can drastically reduce the energy density of Li-metal cells even below that of conventional Li-ion as shown in Figure 1; at 200% excess lithium, the theoretical volumetric capacity of Li of 2060 mAh/L becomes 687 mAh/L, lower than the volumetric capacity of graphite of 719 mAh/L. Clearly cells with significant excess lithium are not poised to succeed conventional lithium ion cells in delivering superior energy density. In contrast, anode-free cells do not utilize excess lithium and thus can deliver the maximum theoretical capacity of lithium metal. In an anode-free cell, the lithium ions stored in the positive electrode plate on a bare current collector during the first charge to form a lithium metal anode in-situ. 1,2 As such, lithium metal is not required during the construction of anode-free cells, resulting in a cheaper, safer and more practical Li-metal cell. In this work, we evaluate anode-free Li-metal pouch cells (NMC532||Cu) with operando pressure measurements constrained to different stack pressures between 75-2205 kPa with two different electrolytes, 1M LiPF 6 fluoroethylene carbonate: diethyl carbonate (FEC:DEC 1:2) and 1M LiPF6 fluoroethylene carbonate:bis(2,2,2-trifluoroethyl) carbonate (FEC:TFEC 1:2). Increasing the initial average pressure from 75 to 2205 kPa was found to generally improve cycling performance, with the most significant benefits achieved up to 1205 kPa. Cells containing FEC:TFEC electrolyte exhibited a superior initial plating efficiency than FEC:DEC cells. The benefit of constraining cells containing FEC:TFEC electrolyte is shown in Figure 1. Although generally beneficial, we found that the effect of increased pressure on the performance of cells with different solvent systems was not equal, particularly at high pressures, indicating that the physical properties of the electrolyte play an important role in cells constrained to higher pressures between 1205 and 2205 kPa. This work presents anode-free cells which deliver a larger energy density than conventional Li-ion cells for 50 cycles. Figure 1

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