High-Temperature Cycling Performance of Cathode with DLC Protective Film

One of the most important issues is related to the long cycling performance in the secondary batteries, considering their application to hybrid electric vehicle, electric vehicle and stationary storage system. Especially, the improvement of high-temperature cycling performance is the most challenging target. Under such an inhospitable condition, several reasons may be taken for the battery degradation. Among them, the side reaction at the electrode/electrolyte interface is taken place, irrespective to the kind of the electrode material. It can yield high-resistive interfacial layer, giving the kinetic limitation, and dissolution of transition metals, leading to the reductive precipitation on the anode surface. We have tried to suppress the side reaction with help of a diamond-like carbon (DLC) protective film and presented our experimental results.

     The sub-nm DLC films were deposited by a hybrid process of plasma-based ion implantation and deposition (PBIID), which enables us to obtain uniform and thin coating at low processing temperature. This process is applicable to the cathode films as well as the electrode particles. In this study, the LiCo_1/3Ni_1/3Mn_1/3O₂ cathode film was treated as a typical electrode. An aspect of the DLC protective layer was observed by a cross-sectional TEM. It was found that the sub-nm DLC film was uniformly coated along the form of cathode surface without the damage to the cathode. The electrochemical cycling of the cathode film was evaluated with Li coin cell at the range from 2.5 to 4.5 V under the CC mode at 55°C. The DLC film coating had no influence on the initial charge-discharge properties but a significant positive effect on the cycling stability. For the cathode with DLC protective film, the discharge capacity hardly decreased even after 100th cycles. The cycling stability was also studied with the AC impedance spectra; it was found that the interfacial impedance growth during the cycling was suppressed by the presence of the DLC film. At the conference, the results about the surface analysis of the high-temperature cycled cathode will be presented and discussed.

     This work was financially supported by Advanced and Seamless Technology Transfer Program through target-driven R&D, JST.