Highly Cyclable Deposition-type Li Metal Anode For All-Solid-State Li Batteries

Lithium-ion batteries (LIBs) are currently used to power a wide range of portable devices and, in particular, a number of pure electric and plug-in hybrid vehicles. As the capacity of a graphite anode conventionally used in LIBs is approaching its theoretical limit, the use of Li metal anode is receiving serious attention.<br/>Currently there are two major difficulties in using Li metal anode in a conventional lithium-ion battery: dendrite formation and low coulombic efficiency (CE). Although several approaches have been reported to mitigate Li dendrite formation, the phenomenon has not been sufficiently avoided so far. The low CE is induced by not only the dendritic growth of the Li metal, but also the subsequent destruction and rebuild of the SEI layer according to the cycle. The currently reported CE of the Li metal anode is ~99% at most, and it requires a large amount of Li reservoir, leading to the decrease of the energy density and the consumption of the electrolyte.<br/>To overcome these difficulties, we use a deposition-type Li metal anode in an all-solid-state battery (ASSB). Since no SEI forms on the anode in an ASSB, high CE is anticipated. The existing challenge is that the short circuit cannot be prevented even by using a solid electrolyte (SE), because Li-metal is known to grow on the surface of the SE grains and propagates through the grain boundaries.<br/>In this study, we found that such a short circuit phenomenon is largely inhibited by using a metal foil coated with carbon black (CB) as a substrate for the Li deposition. The CB electrochemically reacts with Li, but we set the capacity of the CB layer much smaller than that of the cathode, and hence a large part of the Li ions are to precipitate as Li metal on the anode during the charge, and works as a Li metal anode.<br/>We fabricated the ASSB in the following way. First, we made a slurry of carbon black by mixing it with a PVdF binder in NMP, and coated it on a Ni foil. The SE layer was made from Argyrodite-type Li₆PS₅Cl powders spread over non-woven cloth, and the cathode sheet was made from the mixture of NCM, Argyrodite SE, conductive agent and PTFE binder. They were stacked and enclosed in a pouch cell, comprising an all-solid-state battery.<br/>This CB-based anode effectively suppressed short circuit of the cell, and an ASSB with this anode showed an excellent cycle property of over 150 cycles at 0.5 mA/cm² (0.1C) with good capacity retention. From measurements involving cross-sectional scanning electron microscopy, deposited Li metal layer was observed at the CB/Ni interface.<br/>It was also found that the addition of metal particles in the CB-based anode drastically improves cell performance by extending the cycle life. An ASSB with an Ag/CB-based anode is operated over 700 cycles at 3 mA/cm² current density (0.5 C) with a capacity retention of ~86% after 700 cycles.