Designing the 3D Porous Anode Based on Pore Size Dependent Li Deposition Behavior for Reversible Li Metal-Free Solid-State-Batteries

Abstract

Li metal-based all-solid-state batteries (ASSBs) can potentially combine the high energy of Li metal anodes and the safety of ASSBs. Among Li metal-based ASSBs, lithium-free or anodeless ASSBs are considered optimal battery configurations because of their higher energy density and economic advantages attributed to the absence of Li metal during the battery assembly process. Despite the extensive interest in Li-free ASSBs, they continue to suffer from low Coulombic efficiency and poor cycle performance. One reason for this inferior performance is the unstable interface between the current collector and solid electrolyte (SE), which can eventually lead to inhomogeneous Li deposition, dendritic Li growth, and internal short circuits. Various approaches including 3D porous anodes have been proposed to control the Li deposition behavior and improve the reversibility of anodeless ASSBs; however, there is no clarity on the mechanism and conditions for determining the Li deposition behavior in this emerging system.In this study, we systematically investigate the Li deposition behavior depending on the pore size of 3D anode and successfully demonstrate the strategy to obtain a highly reversible 3D porous anode for Li-free ASSBs. We found that more Li deposits could be accommodated within the pores of the anode with a smaller pore size using stacked Ni particles as the Li-hosting porous anode; this implies that the Li movement into the anode occurs via diffusional Coble creep. We proposed the modification of the Ni surface with carbon coating and Ag nanoparticle decoration (Ni_C_Ag particles) to further improve the Li storage capacity of the Ni-based 3D anode and, thereby, secure the interfacial contact between the 3D Ni anode and SE. The resulting Ni_C_Ag 3D anode successfully accommodated the entire Li deposit of 2 mAh cm−2 within the porous architecture without the separation of the anode/SE interface. We clarified the improved Li storage capacity of the Ni_C_Ag anode as follows.(1) C and especially Ag electrochemically react with Li ions above 0 V; thus, Li ions can be transported to 3D Ni_C_Ag porous anode before Li deposition at the SE/anode interface at < 0 V. Further, the high Li ion diffusion coefficient of lithiated carbon and Li-Ag alloy can further reduce Li ions within the pores of the 3D anode; therefore, Li deposition can occur within the porous 3D Ni anode.(2) Lithiophilic C and Ag facilitated the movement of Li via diffusional Coble creep. In particular, Ag with solid solubility in Li (Li(Ag)) can significantly enhance Li adatom mobility because of the identical structure of Li(Ag) with pure Li.(3) Li(Ag) is widely known to lower the energy barrier for Li nucleation.With the significantly reduced nucleation overpotential and interfacial resistance, the Ni_C_Ag anode showed high reversibility in Li deposition and stripping. The Ni_C_Ag anode could be cycled for more than 60 and 100 cycles with Li3PS4 and Li6PS5Cl0.5Br0.5 SE in half cells with a capacity limit of 2 mAh cm−2 and a current density of 0.5 mA cm− 2maintaining the CE of 97.9% and 96.9 %, respectively. Further, the synergistic effects of the stable anode/SE interface and reduced nucleation energy barrier enable stable NCM full-cell cycling at a room temperature of 30 °C. The NCM811 cathode/Ni_C_Ag anode full cell in the Li-free configuration showed an initial areal discharge capacity of 2 mAh cm−2, and it operated stably with a CE of 99.47% for 100 cycles. Figure 1