Shortchanged by the realization of the consequences of carbon emissions, a global trend towards hybridized and electric vehicles is rapidly gaining popularity. Electric vehicles mainly use lithium-ion batteries (LIBs). This application demands that the LIBs have a high energy density and high-power density. To meet such demands, the operating voltage of LIBs must be increased and their internal resistance must be decreased. For the former, lithium metal anodes and high-voltage cathodes have been proposed while as for the latter, technical hinderances have been encountered. To be specific, the electrical conductivity of transition metal oxides for cathodes has proven insufficient, compared with that of carbon materials for anodes. This can however be resolved through the addition of electroconductive materials in the cathode layer. Electrochemical impedance spectroscopy (EIS) has been a powerful tool for evaluating the decrease in the interfacial resistance between the electrode layer and the current collector. Generally, cathode impedance, which is measured by EIS, comprises a surface film resistance on an active material, a charge transfer resistance, and a diffusion impedance. However, depending on the preparation conditions of the cathode, the cause of characteristic high frequency is occasionally different.
Recently, Waseda University researchers led by Professor Tetsuya Osaka from the Graduate School of Advanced Science and Engineering reported a study in which they investigated the interfacial resistance between the cathode layer and the current collector observed at high frequencies, which generally is attributed to a resistance of surface film like solid electrolyte interphase. They focused on using EIS to study LIBs with different cathode densities but with the same active material loading, and LIBs with or without an interlayer between the cathode and the current collector. Their work is currently published in the research journal, Journal of Power Sources.
To investigate the interfacial resistance systematically, different interfaces between the cathode layer and the current collector were prepared by controlling the press rate for the cathode preparation, or by introducing a carbon under-coating layer, followed by electrochemical impedance spectroscopy.
The authors observed that the interfacial resistance between the cathode layer and the current collector prepared with an insufficient press rate or without a carbon under-coating layer was extremely high for the entire cathode. In addition, from the cathode cross-sectional observation, they noted that the high interfacial resistance was caused by low contact rate at the interface. Using a pouch-type symmetric cell, EIS revealed that the interfacial resistance could be attributed to electric resistance, that is, contact resistance at the interface. Also, the other resistances were attributed to be the ionic resistance of the electrolyte and pores in the cathode, and the charge transfer resistance of the cathode.
In summary, the Hiroki Nara and colleagues looked carefully at the impedance observed at high frequencies, which is generally attributed to a surface film resistance. Generally, the effectiveness of the carbon under-coating layer was shown to decrease the cathode impedance. The effectiveness of carbon coating on the current collector was thus demonstrated. Altogether, this is the first study to report the discussion of the activation energy of the interlayer resistance for LIBs.
Hiroki Nara, Daikichi Mukoyama, Ryo Shimizu, Toshiyuki Momma, Tetsuya Osaka. Systematic analysis of interfacial resistance between the cathode layer and the current collector in lithium-ion batteries by electrochemical impedance spectroscopy. Journal of Power Sources, volume 409 (2019) page 139–147.Go To Journal of Power Source