(Invited) Importance of Electrolyte Amount for Lithium Metal Batteries

Abstract

Lithium metal anode has been identified as one of the most promising anode materials for the next generation battery technologies, whether it is used in a liquid, a semi-solid or an all-solid state cell configuration. The Li metal anode can be paired with intercalation-type cathodes such as LiMeO2, and conversion-type cathodes such as S8 and O2, whose energy densities can surpass than that (300 Whkg-1) of state-of-the-art lithium-ion batteries (LIBs).1) Numerous studies relating to these lithium metal batteries (LMBs) have been published over the last decade, however, most researches focus on battery materials level without cell design of practical batteries. The information on the following three key technological parameters is essential to evaluate the battery performance.2) Cathode loading The weight fraction of an active material is mainly determined by its mass loading (mgcm-2) for a given current collector, and the areal capacity (Cs, mAhcm-2) of cathode becomes one of the most important parameters. The areal capacity of cathodes in advanced LIBs is about 4 mAhcm-2, so that the higher loading level than this value is necessary for LMBs, but most literature has adapted much lower loading level, which will lead to apparent high reversible capacity, long cycle life and high rate performance. Anode/cathode pairing (N/P ratio) Another important factor is the capacity ratio of negative to positive electrodes called as N/P ratio. Because the Li metal anode suffers from low Coulombic efficiency during cycling, excess amount of Li is required, which can constantly offset Li loss during cycling and then mask poor cycling performance. However, too much excess of Li will lose the advantage of energy density over graphite anodes in LIBs. The N/P ratio for LMBs is recommended to set to about 2.5, which is equivalent to 50 mm thick Li foil for the cathode with 4 mAhcm-2. Electrolyte amount (E/C ratio) Many researchers are only interested in the kind of electrolyte materials, and unconcerned about the amount of the electrolyte. However, a small excess amount of the electrolyte to fill the pores in the electrodes and the separator is adequate to maximize the energy density. The electrolyte amount is empirically represented by the ratio of electrolyte weight to cell capacity (E/C, gAh-1). The E/C ratio for the advanced LIBs is about 1.5 gAh-1. The electrolyte amount used in a common coin type cell can be as high as 100 gAh-1, but such a large amount of electrolyte sacrifices the energy density, and much lower amount, for example 2.5 gAh-1, is required for practical batteries. Energy density (Wg) Using a pouch cell having a capacity of 1 Ah, a size of 70 x 41.5 mm, and the similar architecture used for LIBs,2) the energy densities of three LMBs, (a) Li/NCM811, (b) Li/S, and (c) Li/O2, were calculated by changing the electrolyte amount (E/C) under the parameters of Cs = 4 mAhcm-2 and N/P = 2.5 as shown in Fig. 1. The weight fractions of cathodes (including a current collector) in Li/S and Li/O2 systems are less than half of that of Li/NCM811 at E/C = 2.5 gAh-1, however, the weight fraction of electrolytes is higher instead. The energy densities of Li/S and Li/O2 systems are very dependent on the electrolyte amount compared with Li/NCM811 or Gr/NCM811 (LIB). Therefore, it is a very challenging subject to establish electrode architecture to generate and accommodate Li2S or Li2O2 reversibly under smaller amount of electrolytes (E/C ≤ 2.5 gAh-1) to achieve a high energy density beyond LIBs. M. Ue and K. Uosaki, Curr. Opin. Electrochem., 17, 106 (2019).M. Ue, K. Sakaushi, and K. Uosaki, Mater. Horiz., DOI: 10.1039/D0MH00067A (2020).S. Chen, C. Niu, H. Lee, Q. Li, L. Yu, W. Xu, J.-G. Zhang, E. J. Dufek, M. S. Whittingham, S. Meng, J. Xiao, and J. Liu, Joule, 3, 1 (2019). Figure 1