Lithium iron phosphate LiFePO₄ has been extensively studied as a promising electrode material for lithium ion batteries, owing to its low cost and high safety [1]. It has been shown that it mostly undergoes a biphasic reaction during the charge-discharge processes and various analyses have been applied to elucidate the phase transition behavior. Since the phase transition in typical nanoparticles is fast, particle-to-particle (not concurrent) transitions are expected and thus macroscopic reaction inhomogeneity can occur. For example, ex-situ XAFS analysis has shown such reaction inhomogeneity in the LiFePO₄ electrode, particularly in the cross-sectional direction [2,3]. However, laboratory observation of reaction distribution during battery operation (operando) is not easy; XRD has too low spatial resolutions and Raman too low time resolution.

Optical analysis has widely been used to capture macroscopic changes in the electrode and phase transitions can also be analyzed when color changes occur as shown in the graphite electrode [4]. Here we report phase transition behavior of LiFePO₄ electrodes analyzed by color confocal optical system. Reaction distribution in the cross-section of the electrode was observed in operando. We also detected a metastable intermediate "LxFP", which is stabilized at low temperatures [5].

A layer of the LiFePO₄ composite electrode, separator and lithium foil was cut and the cross-section was observed by color confocal optical system (ECCS B320, Lasertec Corporation) after the electrolyte was immersed. The reflective images of the electrode were recorded as a color video and the time changes of the whole (averaged) or local RGB brightness were compared with charge-discharge data. The blue brightness change (centered at 436 nm) can reasonably be correlated to the state of charge, with FePO₄ having higher reflection intensity as shown in Fig. 1(a). At 0.1C at room temperature, the change occurred faster than the linear correlation, probably because the observed surface was filled with electrolyte and thus the reaction proceeded fast in the observed area under ion-transfer limitation conditions.

The inhomogeneity in the cross-sectional direction of the electrode was clearly observed in operando using the optical system. As shown in Fig. 1(b), the separator side reacted faster than the current collector side, in good agreement with the result analyzed by XAFS [3].

At a low temperature (-10 deg.), the sample in the fully charged state (FePO₄) was able to be discharged only ca. 75%, which implies the slow kinetics of “LxFP” to form LiFePO₄ [5]. The reflection intensity of “LxFP” was much lower than that at room temperature. When the temperature was increased to room temperature, the intensity was recovered to the originally observed value, suggesting the disproportionation of metastable “LxFP” to LiFePO₄ and FePO₄ [5]. These results clearly indicate that “LxFP” is optically captured as well as LiFePO₄ and FePO_4.

References

[1] K. Padhi et al., J. Electrochem. Soc., 144, 1188 (1997).

[2] H. Tanida et al., J. Phys. Chem. C, 120, 4739 (2016).

[3] Y. Orikasa et al., Sci. Rep., 6, 26382 (2016).

[4] P. Maire et al., J. Electrochem. Soc., 155, A862 (2008).

[5] Y. Koyama et al., Chem. Mater., 29, 2855 (2017).