To increase the energy density of LIB, it is important to keep the capacity balance of the cathode and anode. Even if the capacity of only one electrode increased, it is not possible to increase the energy density of LIB. Such a capacity balance of electrodes is calculated as the battery on the basis of a silicon-based anode and a sulfur-based cathode. It is important as the reactive silicon anode to overcome the problem that the large volume change during charging and discharging to encourage the cyclability, thus we developed a Si-O-C composite anode, which is prepared by the reduction of silicon source and organic solvent. The Si-O-C composite anode was composed of SiOx (0<x<2) and C from organic compounds, resulting in buffering the volume change of silicon. The Si-O-C composite anode delivered high discharge capacity of ~800 mAh/g of Si even after 7000 cycles. [2] The Si-O-C composite anode was evaluated as a full cell by combining with Li2S cathode developed by Professor Bruno Scrosati group. At that time, this battery is one of the rare examples of lithium-metal-free sulfur battery. [3] As for the sulfur-based cathode, it is the key issue to overcome the problem of lithium polysulfide dissolution, which result in capacity fading and lowering of the charge-discharge efficiency. We demonstrated one of the solutions as the coating sulfur electrode with a PPy film, which is prepared by oxidative electro-polymerization.[4] The simple structure prepared by directly coating the PPy film on the sulfer cathode, enabled the suppression of polysulfide dissolution into the electrolyte, thereby inhibiting the shuttle phenomenon. Therefore, it is one of the attractive methods to work sulfur cathode, since this allows the flexibility of choosing electrolytes according to the choice of the anode.
Electrochemical impedance spectroscopy (EIS) is superior method for analysis of the battery health while in use. EIS has been utilized to characterize each factor of batteries, because it enables us to analyze dynamics of each elemental step sensitively and separately without destruction of the cell. In addition, un-destructive EIS is expected to utilize for premonitory diagnosis of batteries on board in electric vehicles. In our previous study, the equivalent circuit to express each elemental step in a commercial LIB by EIS has been carefully investigated. We proposed the designed equivalent circuit, and on the equivalent circuit, the impedance responses of LIB during capacity fading were analyzed with continuous charge-discharge cycling. EIS using conventional FRA – potentiostat systems is not easy to measure the impedance of the large-scale LIB because of its low internal resistance. Moreover, FRA – potentiostat system for conventional EIS measurement could not be mounted on the vehicle. Thus, impedance measurement system is needed without using FRA – potentiostat systems. In our study, application of square wave potential for input signals of EIS was investigated in simple electrochemical reaction to verify a new technique called “Square-potential/current electrochemical impedance spectroscopy (SP-EIS, SC-EIS)” which is a method for EIS without using the FRA systems. We applied SC-EIS to evaluate a state of a commercial LIB. Introduction of SC-EIS to diagnosis technology of laminated LIBs and the LIB module was discussed. [5]
References:
- S. Kakuda, T. Momma, T. Osaka, G. B. Appetecchi, B. Scrosati, J. Electrochem. Soc., 142, (1995) 1.
- T. Osaka, H. Nara, T. Momma, T. Yokoshima, J. Mater. Chem. A, 2 (2014) 883.
- M. Agostini, J. Hassoun, J. Liu, M. Jeong, H. Nara, T. Momma, T. Osaka, Y. Sun, B. Scrosati, ACS Appl. Mater. Interfaces, 6 (2014) 10924.
- N. Nakamura, T. Yokoshima, H. Nara, T. Momma, T. Osaka, J. Power Sources, 274 (2015) 1263.
- T. Osaka, D. Mukoyama, H. Nara, J. Electrochem. Soc., 162 (2015) A2529.