Preventing Li Dendritic Growth in All-Solid-State Batteries: Effect of Current Collector with High Aperture Ratio

Abstract

Introduction All-Solid-State Battery (ASSB) is expected as one of the next-generation batteries to achieve high power and energy density and high safety. A Li metal shows high mass capacity and low standard electrode potential, so it is a promising negative electrode active material. However, the Li metal has a critical problem such as the Li dendritic growth during charging, which induces internal short-circuits. The ASSB was expected to prevent the Li dendritic growth; however, currently it is known to proceed even in the voids and cracks of solid electrolytes (SEs). Moreover, because the Li metal electrode is sandwiched between a current collector and SE, there aren’t any spaces for Li deposition in the cell. This cell configuration enhances the mechanical pressure in Li and accelerates the Li dendritic growth in charging. We previously demonstrated that the current collector with micro-sized pores has the ability for absorbing the volumetric expansion of Li and relaxation of mechanical pressure in Li1. However, the suppression effect for the internal short-circuiting was limited because of low aperture ratio of the current collector.In this study, we used a mesh-like current collector with high aperture ratio named as a porous current collector (PCC) to introduce regularly arranged small rooms for Li metal deposition in a cell. Li metal was preferentially deposited in the pores of the PCC. We also investigated the effect of Au-coating on the PCC on the Li metal plating/stripping performance. The improved ASSB cell achieved a record-high value of critical current density (at which a short-circuit occurs) over 6 mA cm-2. Moreover, the Coulombic efficiency was 98.7 % for 300 cycles, which demonstrated highly reversible Li deposition/dissolution cycles. Experimental The PCC was prepared by cutting the commercial electroformed sieve (ASONE, S11H30) into a circular shape. Au-coating on the PCC was performed with a sputter coater.An electrochemical cell was assembled in an Ar-filled glove box. Li3PS4-glass (LPS) or 54Li3PS4-46LiI-glass (LPSLI) electrolytes were prepared by mechanical-milling, and pressed into a pellet at 360 MPa in a polycarbonate cylinder. Au thin film was coated on the negative electrode side surface of the SE pellet with a sputter coater. Then, The SE pellet was put between the PCC and Li-In alloy as the positive electrode, followed by pressing to obtain an ASSB cell.Li deposition/dissolution performances were evaluated by galvanostatic cycle tests at 30 °C. The critical current density was evaluated by increasing the current density by 0.1 mA cm-2 per cycle until the short-circuit occurs. The deposition capacity for each charge process was kept at 0.1 mAh cm-2. The charge/discharge current density and deposition capacity for evaluating coulombic efficiency were 0.1 mA cm-2 and 0.1 mAh cm-2, respectively. Results and Discussion Fig. 1(a) shows the SEM images of a pristine PCC. Square pores with a diameter of 11 µm are vertically and densely aligned. After charging at 0.064 mA cm-2, pores were filled with deposited Li (Fig. 1(b)). After peeling off the PCC from the SE, cubic shaped Li metal remained on the SE (Fig. 1(c)). After discharging at 0.064 mA cm-2, cubic shaped Li metal disappeared (Fig. 1(d)). These results suggest that the Li deposits in pores of the PCC reversibly dissolved.Fig. 2 shows the critical current density for different cell configurations. Increased critical current density means increased resistance to the short-circuit. The PCC exhibited higher critical current density than the planer Ni foil. Moreover, the LPSLI, which showed higher tolerance to the reductive decomposition with the Li metal than LPS2, also improved the critical current density. The Au-coating on the PCC greatly increased the critical current density up to 6.0 mA cm-2. Our previous result about the other PCC exhibited that the Li deposition behavior depended on the coating material, and the Li diffusion in the Au thin layer on the PCC would cause the suppression of the Li dendritic growth1.Fig. 3 shows Coulombic efficiencies for different current collectors with LPSLI as a function of cycle number. The drop of the Coulombic efficiency for the Ni foil suggests the short-circuit. The cycle number at which short-circuiting occurred increased for the PCC and Au-coated PCC. Notably, the Au-coated PCC achieved high Coulombic efficiency of 98.7 % even at 300th cycle. Such the significant improvement clearly demonstrates that the Au-coated PCC is effective for reversible deposition/dissolution of Li. Acknowledgement This work was partially supported by JST ALCA-SPRING. Reference Shinzo et al., ACS Appl. Mater. Interfaces (2020), 12, 20, 22798Suyama et al., Electrochimica Acta, 286, 158 (2018). Figure 1