Analysis of Structural Changes in Practical Batteries during Overcharging Using Synchrotron X-Ray CT Imaging

Abstract

Lithium-ion rechargeable batteries are desired for applications such as power sources for electric vehicles, which require increased energy density to extend cruising range. In the cathode, increasing the nickel content makes it possible to increase the energy density, but this is problematic due to a decrease in safety. To solve this problem, it is necessary to clarify the phenomena leading to structural breakdown by operando observation[1]. We have developed a system for operando non-destructive observation of the internal structure of all batteries under transient conditions using 100 keV collimated light of SPring-8 beamline. In this study, the difference in the behavior of LiCoO2 (LCO), LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) during overcharging was discussed in combination with the measurement of the electronic structure of the cathode active materials from the surface to the bulk.A 40-layer laminated battery (1 Ah) was used, with LCO, NMC622 and NMC811 as the cathodes and natural graphite as the anode. After charging to 100% SOC at 1C (1C=160 mAh/g) rate at room temperature, the overcharge process performed under 3C and/or 6C rate from 100% SOC while measuring voltage and temperature. Overcharge behavior inside the battery was observed simultaneously by operando X-ray CT during overcharge process.During the overcharge test at 6C rate, the maximum temperature of the LCO laminated battery was 390 ℃, while NMC811 laminated battery reached over 600 ℃ (Fig. 1). Point 1 in Fig. 1 is the point where changes in the electrode structure begin to be observed by the operando X-ray CT measurements. The point 1 of NMC811 occurred earlier (203 s) than LCO (542 s). From the O-K XAS result (Fig. 1d), the pre-edge (located at 519 ev) of NMC811 was much higher than LCO and NMC622 when overcharging to 200% SOC, meanwhile the O-Ni bond in NMC is more unstable than O in LCO in the overcharged state. The unstable electronic structure of oxygen on the surface of NMC811 is the starting point for the rapid structural change of the battery during overcharging. Reference s : [1] Donal P. Finegan; Mario Scheel; James. Robinson; Bernhard T.; Marco Di M.; Gareth H. Phys.Chem.Chem.Phys. (2016), 18, 30912. Figure 1