Abstract:

High-energy rechargeable lithium metal batteries (LMBs) have attracted significant attention recently. These batteries can be bulit using high areal-capacity (> 4 mAh∙cm⁻²) layered oxide cathodes and thin lithium (Li) metal anodes (< 50 μm in thickness), whose cycle performance are severely limited by the unregulated growth of Li particles having high surface areas, including dendrites and mossy Li. To improve the cycle performance of LMBs, many approaches have been developed in recent years to promote dendrite-free and dense Li electrodeposition, such as electrolyte engineering (for liquid cells), Li anode surface modification, three-dimensional current collector design, and using solid-state electrolytes. In addition to these heavily researched chemical-based approaches, applying external pressure to LMBs can also strongly impact the morphology of the electrochemically deposited Li particles due to the malleable nature of metallic Li and has been shown to improve the cycle performance. However, the relationship between the applied pressure, morphological evolution of the Li anode and the cycle performance has not been fully understood, especially in coin cells, which are widely used for LMB research. Here we report a custom-designed pressure applying/measurement device based on thin-film pressure sensors to realize real-time tracking of the pressure evolution in LMB coin cells. Our results show that moderate pressure is conducive to dense Li deposition and increases the cycle life, whereas excessive pressure causes Li inward-growth and the deformation of Li anode, which will impare the electrochemical performance of LMBs. Although these observations are made in coin cells, they could have important implications for pouch cells and solid-state batteries, both of which are commonly tested under pressure. The cycle performance of LMBs is significantly improved in both coin cells (under actual relevant conditions) and large pouch cells. A 5 Ah pouch-type LMB with a high energy density exceeding 380 Wh∙kg⁻¹ could achieve stable cycling over 50 cycles under a stack pressure of ~1.2 MPa. It was also confirmed that the cell holders or clamps commonly used for coin cells can only exert a small amount of pressure, which is unlikely to exaggerate the cycle performance of the LMB coin cells. However, we do suggest that the electrochemical performance of LMBs should be reported along with the information on the applied pressure. This research practice will improve the consistency and quality of the reported data in the LMB research community and help unite the efforts to further improve the high energy density LMBs.

Key words: High energy-density battery, In situ pressure measurement, Lithium metal anode, Electrodeposition, Pressure control