Raman Imaging for LiCoO2 Composite Positive Electrodes in All-Solid-State Lithium Batteries to Investigate State-of-Charge Distributions

Abstract

All-solid-state batteries show higher safety with low risk of leakage and explosion because of nonflammable inorganic solid electrolytes, which are alternative to conventional organic liquid electrolytes. Bulk-type all-solid-state batteries are constructed by pressing positive and negative electrode layers and a solid electrolyte layer at room temperature. The batteries are capable of having high energy density by adding large amounts of active materials into composite electrodes, which are composed of solid electrolyte particles and active material particles. We have investigated electrochemical performances of bulk-type all-solid-state cells using a LiCoO2 composite positive electrode and a Li2S-P2S5 solid electrolyte.1 Composite positive electrodes have many solid-solid interfaces, and electrochemical reactions at the interfaces have not been studied well. Inhomogeneous reaction may occur in the composite electrodes when solid-solid contact areas are insufficient. To improve cell performance, investigation of state-of-charge (SOC) distributions in the electrodes and fabrication of electrodes showing uniform reaction distributions are important. Raman spectroscopy is a suitable technique for investigating SOC of active materials because Raman spectra are sensitive to the structural changes of active materials during charge-discharge tests. Moreover, Raman mapping technique enables us to obtain SOC distribution maps of composite electrodes. In this study, Raman mapping was carried out for charged and discharged LiCoO2 composite positive electrodes in all-solid-state cells to investigate SOC distributions. A 75Li2S·25P2S5 (mol%) glass and indium foil were used as a solid electrolyte and a negative electrode, respectively. A composite positive electrode was prepared by mixing LiCoO2 particles and 75Li2S·25P2S5 glass particles with 80:20 in weight ratio. All-solid-state cells were charged and discharged with a cut-off voltage of 2.6-4.2 V (vs. Li+/Li) at 25oC under a current density of 0.064 mA cm-2. Raman mapping was conducted for surface parts of composite positive electrodes prepared by an Ar ion-milling technique. There are two strong Raman bands at 486 and 596 cm-1, originating from the E g and A 1g modes corresponding to O-Co-O bending and Co-O stretching, respectively.2 Those peaks of E g and A 1g modes shifted to 470 cm-1 and 582 cm-1 respectively, when the cell was charged to 4.2 V (vs. Li+/Li) and returned to original peak positions after the discharging process. To evaluate SOC of the composite positive electrode, A 1g peak positions were investigated in detail. The mapping image showed that charge-discharge reactions did not proceed uniformly, and inhomogeneous reaction distributions existed at the areas of insufficient contacts between LiCoO2 particles and solid electrolyte particles.3 To achieve uniform electrochemical reactions, a composite positive electrode having sufficient interfaces between LiCoO2 and solid electrolyte particles should be prepared. To increase contact points between them, a composite positive electrode with solid electrolytes with a smaller particle size was fabricated. Raman mapping images of the charged and discharged electrodes suggest that uniform electrochemical reactions are achieved. It is noteworthy that the use of smaller solid electrolyte particles is effective way to obtain uniform reaction distributions for composite positive electrodes in all-solid-state lithium batteries. Acknowledgement This research was financially supported by JST, ALCA-SPRING.