Improvement of Lithium Metal Electrode Performance with Shot Peening and Sputtering By Controlling Stacking Pressure of All Solid State Battery
Authors: Fukusumi Hito , Kodama Manabu , Hirai Shuichiro 
Published: December 2024

All-solid-state lithium metal batteries (ASSLiMBs) are expected to load into a next-generation electric vehicle by its high safety and energy density. However, in high current density charging over critical current density (CCD), the lithium dendrite is generated and growth into the solid electrolyte (SE) layer with crack generation and propagation into SE by inhomogeneous deposition, and causing an internal short circuit. It is suggested that if the crack generation and propagation can be suppressed in the SE, the growth of lithium dendrites can be suppressed, and high CCD would be achieved. Some methods have been proposed to suppress dendrite formation.
In the author’s research group, shot peening (S.P.) to the SE surface is proposed to increase the CCD by suppressing the crack generation and propagation into the SE with increasing the fracture toughness of the SE [1]. S.P. is a processing method with impacting media onto the target and increase the fracture toughness by mechanical processing. In our previous research, The CCD was successfully increased x7 by the S.P.. The CCD can be increased with combine use of S.P. and conventional sputtering method. By the combination use of these, the CCD was successfully increased x20 [1].
The stacking pressure of the ASSLiMB is important operation factor for high CCD, because it controlees the contactivity between Li metal and SE. It is reported that if the stacking pressure is higher than the critical pressure, the lithium dendrite generation is suppressed by high contactivity of Li to SE [2].
Synthesizing the findings from above previous research, it can be inferred that ASSLiMB with S.P. sputtering also has a dependency of CCD on stacking pressure. Moreover, the S.P. can change the mechanical characteristics of SE (surface shape, fracture toughness), sputtering can change the interfacial electrochemical resistance. Thus, the influence of stacking pressure on CCD would be different from w/o those processing and it would give us insight for more high performance (high CCD) ASSLiMB. Therefore, in this study, with the aim of improving the performance of the battery, CCD tests under various surface pressures were conducted.
Pellets of oxide SE (Li6.25Ga0.25La3Zr2O12) were synthesized via the solid phase reaction method, followed by shot peening using abrasive grain of Al2O3 powder. Subsequently, Au sputtering was processed on the surface of the pellets after shot peening. The experiment carried out to assess the performance of synthesized oxide SE by passing current through it in a lithium symmetric cell sandwiched the SE between lithium metal foils. Subsequently, the acquired data underwent Gaussian regression analysis to predict the true data while accounting for variability by SmartUQ (KEISOKU ENGINEERING SYSTEM CO., LTD.).
Fig. 1 shows the pressure dependence of CCD under various processing conditions. In either processing condition, CCD remained consistently low at low pressure, increased as pressure increased at middle pressure, and decreased at high pressure. It became apparent that applying the appropriate pressure during charging and discharging led to an improvement in performance under all processing condition. This is because at low pressure, it is not possible to maintain sufficient contact between SE and Li, while conversely, at high pressure, SE is speculated to be mechanically damaged. At the appropriate pressure, it is presumed that the pressure compresses the surface voids generated by lithium dissolution, thus ensuring consistently good contact. Furthermore, at low pressure, CCD remains unchanged regardless of whether S.P. is performed or not, but CCD changes depending on whether Au sputtering is applied or not. It was found that while performance improvement was observed at all pressure levels with Au sputtering, S.P. resulted in performance enhancement only within a limited range of pressure. This is because Au sputtering improves electrochemical contact, allowing for consistent good contact regardless of pressure. On the other hand, for the roughness imparted by S.P. processing to sufficiently embed, a pressure exceeding the creep pressure of lithium is necessary.
This study shows that to improve the performance of ASSLiMBs using S.P. and Au sputtering, it is necessary to apply optimum stacking pressure during charging and discharging.
Acknowledgement
This work was partially supported by JSPS KAKENHI Grant Number 22K14761, the Kurata Grants, and JKA foundation. Gaussian regression process analysis conducted by SmartUQ was carried out with the cooperation of KEISOKU ENGINEERING SYSTEM CO., LTD.
References
[1] Kodama, M., et al. Journal of Power Sources 537, 231556. https://doi.org/10.1016/j.jpowsour.2022.231556
[2] J. Wang, et al, Joule 3, 2165-2178 , https://doi.org/10.1016/j.joule.2019.06.017

Figure 1