Lithium-ion batteries for automotive applications have required to enhance power density, low-temperature performance, life, energy density, and safety. All-solid-state batteries using lithium-ion conducting solid electrolytes have been much interest as candidates to respond to these demands. However, for practical applications, all-solid-state batteries have some problems to solve low charge-discharge performance under high rate and low temperature operations because of low ionic conductivities and large resistances of interface between the solid electrolyte and the active electrode material. We have focused on hybrid electrolytes consisting of lithium-ion conducting solid particles (LCSP) and a gel polymer electrolyte leading to a decrease of cell resistance, especially at low temperatures for LTO cells [1,2]. In this study, we have investigated effects of　LCSP with liquid electrolytes in composite electrodes on electrochemical properties and electrode performance in order to develop high-power lithium-ion batteries using the solid electrolytes.

NASICON-type Li-Al-Ti-P-O (LATP) particles with a particle size of submicron were used as LCSP. The composite electrodes consisted of LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) as cathode active material, carbon filler, polyvinylidene difluoride binder, and LATP particles, which were coated on aluminum foil as current corrector. Electrochemical measurements of the composite electrodes containing LCSP were performed using a three-electrode glass cell with a lithium metal foil counter electrode, a lithium metal chip reference electrode, a glass filter separator, and LiPF₆-based liquid electrolytes.

Figure 1 shows typical AC-impedance spectra of the composite electrodes with and without LATP particles at -20°C. The impedance spectra of both electrodes exhibited two depressed semicircles in the high- and low-frequency range, which can be interpreted as resulting from the passivating film formed on the NCM523 active material and the charge transfer process of lithium insertion, respectively. The ohmic resistances at 100 kHz such as the ionic resistance of electrolyte and the resistance of passivating film in the spectra showed no difference between the electrodes with and without LATP.

On the other hand, it was noted that the charge transfer resistance of electrode with LATP was significantly smaller than that of electrode without one. Furthermore, the activation energy for the charge transfer of the electrodes with LATP is estimated into 55.7 kJ/mol, which is smaller than that of the electrode without LCSP as shown in Fig. 2. These results indicate that LCSP and liquid electrolytes in the electrodes plays an important role of reducing the charge transfer resistance rather than enhancing the lithium ion conductivity in the electrolytes. Therefore, we consider that LCSP in the electrode enhances to provide lithium ions in the liquid electrolyte to the surface of the active materials, leading to maintain high concentration of lithium ions at the surface of active material. The detail of the mechanism for reducing the charge transfer resistance by containing LCSP to the electrode will be discussed in this presentation.

[1] K. Yoshima, Y. Harada and N. Takami, J. Power Sources, 302,283-290(2016).

[2] N. Takami, K. Yoshima and Y. Harada, J. Electrochem. Soc., 164,A6254(2017).