All-solid-state batteries with nonflammable inorganic solid electrolytes, an alternative to conventional inflammable organic liquid electrolytes, are widely studied as next generation batteries with low risk of leakage and explosion. Bulk-type batteries use composite electrodes of active materials and solid electrolytes. Solid electrolytes play a role of delivering Li ions to active materials. Bulk-type batteries are capable of having high energy density by adding large amounts of active materials into composite electrodes. We have investigated the electrochemical performance of bulk-type all-solid-state cells using a LiCoO₂ composite positive electrode and a Li₂S-P₂S₅ solid electrolyte [1]. Composite electrodes can be fabricated by mixing LiCoO₂ particles and solid electrolyte particles. There are many solid-solid interfaces in composite electrodes. It is important that electrochemical reactions at LiCoO₂-electrolyte solid-solid interfaces are clarified to improve the cell performance. Raman microscopy is one of the useful methods for investigating the reactions because of its feature of high spatial resolution and surface sensitivity. Raman spectral changes of composite electrodes are closely related to the structural changes of active material during charge-discharge cycling. In this study, Raman spectroscopy was carried out for LiCoO₂ composite positive electrodes in all-solid-state cells before and after the charge-discharge process. Moreover, Raman mapping was conducted to evaluate state-of-charge (SOC) distributions of each active material in the electrodes.

   The 75Li₂S·25P₂S₅ (mol%) glass and indium foil were used as solid electrolyte and a negative electrode, respectively. A composite positive electrode was prepared by mixing LiCoO₂ particles and 75Li₂S·25P₂S₅ glass particles (80:20 wt.%). All-solid-state cells were charged and discharged with a cut-off voltage of 2.6-4.2 V (vs. Li⁺/Li) at 25 ^oC under a current density of 0.064 mA cm^-2. Raman spectra were obtained for the surface part of composite positive electrodes prepared by an Ar ion-milling technique.

   There are two strong Raman peaks at 486 and 596 cm^-1, originating from the E_g and A_1g modes of oxygen vibration in LiCoO₂, respectively. Those peaks shifted to the lower wavenumber side and their intensity decreased after the initial charging process. After the discharging process, those peaks returned to the original positions. Mapping images showed charge-discharge reactions did not proceed uniformly at the areas of insufficient contacts between LiCoO₂ particles and solid electrolyte particles [2]. Our approaches to fabricate composite positive electrodes having uniform charge-discharge reactions will be demonstrated.

Acknowledgement

This research was financially supported by JST, ALCA-SPRING.

References

[1] T. Minami et al., Solid State Ionics, 177 (2006) 2715-2720.

[2] M. Otoyama et al., J. Power Sources, 302 (2016) 419-425.