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

Abstract

© The Author(s) 2019. In the pursuit of surpassing the energy density of conventional lithium ion cells, significant efforts have been made to develop lithium metal cells. However, many reports in the literature utilize Li-metal cells with significant excess lithium, resulting in a dramatically reduced practical energy density. In contrast, anode-free cells do not utilize excess lithium; instead, a lithium metal anode is formed in-situ from the stored lithium within the positive electrode during the first charge. Here, we evaluate anode-free lithium metal pouch cells (NMC532||Cu) with operando pressure measurements constrained to different stack pressures between 75–2205 kPa with two different electrolytes, 1M LiPF6 FEC:DEC (1:2) and 1M LiPF6 FEC:TFEC (1:2). Increasing the initial average pressure from 75–2200 kPa was found to generally improve cycle life, with the most significant benefits achieved up to 1200 kPa. Cells containing FEC:TFEC electrolyte exhibited a superior initial performance compared to FEC:DEC cells, as evidenced by cycling data and SEM analysis of the lithium morphology. Although generally beneficial, we found that the effect of increased pressure on the performance of cells with different solvent systems was not equal, indicating that the physical properties of electrolyte play an important roll in cells constrained to higher pressures between 1200–2200 kPa.