Impact of Coin Cell Preparation and Testing Protocols on Coulombic Efficiency Measurements

Abstract

The lithium (Li) metal anode holds great promise for high energy-density rechargeable batteries due to its high gravimetric capacity (3860 mAh/g), an order of magnitude larger than graphite at a similar electrochemical potential (-3.040 V vs SHE for Li). However, limited Coulombic efficiency (CE), caused by the thermodynamic instability of conventional Li-ion electrolyte solvents and salts with Li that leads to unstable solid-electrolyte interphase (SEI) during repeated plating and stripping, has hindered the commercialization of Li metal batteries. Research efforts have focused on tuning the electrolyte composition to promote a stable SEI that simultaneously blocks continuous side reactions and electronic conductivity while facilitating Li+ transport [1], with coin cells being by far the most preferred form factor for evaluating these electrolyte designs.Coin cells enable accessible and rapid testing of diverse electrolyte compositions. Their preparation requires no specialized equipment beyond an inert chemical environment, a crimping mechanism to seal the cell, and modest quantities of Li metal (~50 - 100 μm thickness) and electrolyte (~50 - 100 μL volume) [2]. Despite these attractive features, the procedures for preparing and techniques for measuring CE in coin cells are not fully standardized. In this presentation, we examine the impact on measured CE from preparation and testing procedures, including the internal coin cell stack consisting of components such as the electrodes, current collectors / spacers, and separators, and the cycling protocol. While the cycling protocol and cycling capacity are known to affect capacity loss mechanisms in Li metal batteries [3], we show that variances in CE can additionally be attributed to specific details of cell preparation. In particular, the working electrode area and stack thickness affect the pressure magnitude and uniformity inside the cell. We demonstrate that standardizing the procedures for preparing coin cells reduces variation due to nonuniformities and edge effects and hence improves the reproducibility of CE measurements.[1] Hobold, G. M., et al. Nature Energy 2021 6 (10), 951-960.[2] Xiao, J., et al. Nature Energy 2020 5 (8), 561-568.[3] Adams, B. D., et al. Advanced Energy Materials 2018 8 (7), 1702097. Figure 1