Li Electroplating/Stripping on the Metal Substrate Coated with Carbon Black
Authors: Suzuki Naoki , Yashiro Nobuyoshi , Aihara Yuichi 
Published: February 2020

The history of the secondary battery with Li metal anode, having a high theoretical specific capacity of 3860mAh/g and the most negative electrochemical potential among anode materials, is rather old, and the first portable phone equipped with a Li metal secondary battery was commercially released in 1988. But it soon faded out from the market because of the safety issue. Instead of the Li metal anode, various carboneous anodes (so-called lithium ion batteries) emerged and took over the market, and since then they have been used to power a wide range of portable devices and, in particular, a number of pure electric and plug-in hybrid vehicles.

The capacity of a conventional carboneous anode, however, is approaching its theoretical limit, and the use of Li metal anode is regaining serious attention for secondary batteries. Currently there are two major difficulties in using Li metal anode: dendrite formation and low coulombic efficiency (CE). In a typical operating condition of a lithium-ion battery, Li metal easily grows dendritically on the substrate, leading to a poor coulombic efficiency and sometime a short circuit with penetrating through the separator and reaching to the cathode. Although several approaches have reduced Li dendrite formation, the phenomenon has not been avoided adequately 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 exhaustion of the electrolyte.

To overcome these difficulties, we use Li anode in an all-solid-state battery (ASSB). Since no SEI forms on anode, high CE is anticipated. But it is known that the short circuit is not prevented even by using solid electrolyte (SE), because Li grows on the surface of the SE grains and propagates through the grain boundaries.

In this study, we found that such 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.

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 are stacked and enclosed in a pouch cell to form an all-solid-state battery.

The cycle test was performed at 0.1C for charge and at 0.1C (1st cycle) and 0.33C (later cycles) for discharge. The result is shown in fig. 1. The cell showed an excellent cycle performance with no short circuit over 150 cycles. The initial capacity was 166 mAh/g-NCM (~5mAh/cm2), and the averaged cycle retention was ~ 99.91%/cycle between 2nd and 150th cycle. We also fabricated an ASSB whose carbon layer on the anode is substituted with graphite and performed the same test, but it was terminated by a short circuit at the 2nd cycle.

Figure 1