Li Plating/Stripping Behaviors in Li+/Na+ Mixed Li+ Conductor
Authors: Sakai Mizuki , Miyazaki Reona , Hihara Takehiko 
Published: December 2024

Background
Lithium-ion batteries (LIBs) are widely used as portable devices because of their high energy density. However, Li dendrite, which is the reason for the internal short-circuit is generated during charge-discharge cycling1. In addition, because reduction voltage of Li ion is much lower than onset potential of H2 release, flammable organic solvents are used as the electrolyte. Conventional LIBs have the safety problems because of these reasons. All solid-state batteries (ASSBs) which use nonflammable solid electrolytes are featured as next generation LIBs compatible with the safety and energy density. Because ceramic electrolytes are the solid, it has been considered that Li dendrites growth in such a “rigid” solid electrolyte is not likely. Nevertheless, Li penetrations are known as one of the most severe issues in ASSBs2. This short-circuit caused by Li penetration through the electrolyte is the common problem in Li/Na mixed solid electrolyte; 9(15NaI･NaBH4)･LiI. In this study, we prepare the Li/Li symmetry cells with 9(15NaI･NaBH4)･LiI and conduct the Li plating/stripping cycles. Furthermore, voltage profiles in the solid electrolyte are in-situ monitored during cycling.
Experimental
9(15NaI･NaBH4)･LiI was prepared by ball-milling. First, NaI and NaBH4 were mixed in a molar ratio of 15:1 and ball-milled for 5 hours (15NaI･NaBH4). Subsequently, 10 mol% of LiI was mixed with 15NaI･NaBH4 and further ball-milled for 5 hours (9(15NaI･NaBH4)･LiI). 9(15NaI･NaBH4)･LiI was uniaxially pressed at 270 MPa to obtain the pellet with ca. 1 mm thickness and 10 mm in diameter. Li metals were used as both side of the electrodes. This Li/Li symmetric cell was sealed in HS cell. Li plating/stripping cycles were performed at stepwise increasing the current density between 0.1 and 0.5 mA/cm2. After charge-discharge cycles, the cell was deconstructed in the glove box. The Li foil was detached from the pellet and in-plane fracture surface of the solid electrolyte was observed by SEM-EDS. To monitor the voltage profile inside the solid electrolyte, the drills with the diameter of 0.5 mm were embedded at the interval of 5 mm at lateral side of the pellet, which served as the electrodes. Li plating/stripping cycles were performed under the constant current density. During the measurement, the voltage of both ends of the cell was measured by the potentiostat (SP-150, Biologic Co.) while the voltage between Li electrode and embedded drills were simultaneously monitored.
Results and Discussions
The results of the Li plating/stripping cycles are shown in Fig.1(a). Red and black lines show the current density and cell voltage, respectively. The cell voltage was increased by increasing the current density. The cell voltage was rapidly decreased when the current density exceeded 0.2 mA/cm2, indicating the short-circuit of the cell. The results of the SEM-EDS after charge-discharge are shown in Fig.1(b)~(g). As shown in Fig.1(b)~(d), we can see some convex tissues, where neither Na nor I were detected. Hence, these convex forms can be considered as the Li pilar penetrated through the electrolyte. As shown in Fig.1(e)~(g), some plain form Na metals were observed. The dominant conduction ion in 9(15NaI･NaBH4)･LiI has been confirmed to be Li+ with the slight contribution of Na+ conduction3. The deposition of Na metal requires the continuous transport of Na+ in the solid electrolyte coupled with the continuous supply of Na+ from the counter electrode. However, Li metal was used in this experiment, indicating that there was no Na+ supply from the counter electrode. Nevertheless, the deposition of Na metal was observed. Deposition of Na metal is explained by Li → Li+ + e- at the counter electrode and Na+ + e- → Na at the working electrode, coupled with Li+ conduction in the electrolyte. This asymmetric process can cause the Na+ deficit region around the Na metal because Na+ conduction in 9(15NaI･NaBH4)･LiI is minor. Indeed, from the EDS mapping figure, Na was not detected around the plain Na metal (Fig.1(f)). The voltage profile inside the solid electrolytes will be presented in the poster.
Acknowledgement
This work was supported by JSPS KAKENHI Grant Numbers 23K23447 and the NAGAI Foundation for Science and Technology.
(1) Xiao, J. SCIENCE
2019, 366 (6464).
(2) Liu, X.et al. Nature Materials
2021, 20, 1485-1490.
(3) R. Miyazaki., et al. Journal of Solid State Electrochemistry
2021, 25 (6), 1927-1936.

Figure 1