Suppression of Dendritic Growth on Li Negative Electrode for All-Solid-State Rechargeable Battery

Abstract

Introduction All-Solid-State Lithium Battery (ASSLiB) is expected as one of the next-generation batteries to achieve high power and energy density and high safety. Applying the Li metal negative electrode with a high mass capacity and low standard electrode potential to the ASSB system is desirable; however, there is a critical problem of the Li dendritic growth which induces internal short-circuits. The Li dendritic growth 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, high mechanical stress due to volume expansion is applied to the electrode/SE interface during charging. For conventional planer current collectors, Li metal easily penetrates into SE through voids for the relaxation of mechanical stress (Fig. 1a). In contrast, for porous current collectors, the mechanical stress will be reduced because the deposited Li-metal can fill pores. In this study, we investigated the usefulness of an Au-coated membrane filter with micro-size pores as a porous current collector for suppressing the Li dendritic growth (Fig. 1b).1 Experimental A porous current collector was prepared by cutting a commercial membrane filter (Advantec, 16080004) with 8 µm-diameter pores and 7 µm-thickness into a circular shape. Gold or nickel was coated with a sputter coater to give electric conductivity. The gold-coated porous current collector is named as Au-MF, and nickel-coated one is named as Ni-MF.A laboratory-made cell was assembled in an Ar-filled grove box. Li3PS4-glass electrolyte as SE was prepared by mechanical-milling according to ref. 1, and pelletized in a polycarbonate tube. Gold was coated on the negative electrode side of the SE pellet. The SE was put between the porous current collector and Li-In alloy as the counter electrode, followed by pressing to obtain an ASSB cell.All charge/discharge measurements were conducted by galvanostatic technique at 30 ℃. 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. Li deposition behavior in the porous current collectors was investigated by scanning electron microscope (SEM). Results and Discussion Fig. 2(a) shows an SEM image of the Au-MF as prepared. Circular pores with 8 µm in diameter are vertically and sparsely located. After charging at 0.010 mA cm-2 for 10 h, the surface morphology of Au-MF changed like moss (Fig. 2(b)), which means Li was deposited on the surface of Au-MF. When the Ni-MF, in which Ni layer does not form any alloys with Li, was used to charge at the current density of 0.010 mA cm-2 for 10 h, Li deposits were observed only in the pores of Ni-MF and surface morphology did not change (Fig. 2(c)). These results suggest that the Li deposition on Au-MF is due to the Li diffusion in the Au layer on the MF, and the mechanical stress on the Au-MF/SE interface is relaxed by the Li deposition in pores.Fig. 3 shows Coulombic efficiencies of Au-MF, Ni-MF, planer Ni foil and Au-coated Ni-foil as a function of cycle number. The Coulombic efficiency reached to the maximum at the 4th cycle in each case. The Au-MF showed the highest coulombic efficiency of 82 %, while other electrodes showed lower than 80 % values. The charge/discharge curve for the Ni foil and Ni-MF showed the short-circuit at the 8th cycle; however, the Au-MF and the Au-coated Ni foil did not make a short-circuit for 20 cycles. These results indicate Au-MF is superior in coulombic efficiency and the suppression of Li dendritic growth to the other current collectors.We assume the synergetic effect of Au-coating and pores on the current collector. During charging, the electrochemically prepared Li metal diffuses to the pores through the Au layer and fills the pores. This effective filling of the pores with Li metal is essential for suppressing the Li dendritic growth. The Coulombic efficiency depends on the amount of the dead Li, which is formed by interruption of conduction during the dissolution of lithium deposited in dendrite form. Preventing the dendritic growth of Li metal also leads to the increase in the Coulombic efficiency.In conclusion, the Au-coated porous current collector can effectively work for inhibiting the Li dendritic growth. Acknowledgement This work was partially supported by JST ALCA-SPRING. References Shinzo et. al., ACS Appl. Mater. Interfaces (2020), 12, 20, 22798 Figure 1