Investigation of Phase Transition Behavior of the Olivine-Type LiFePO₄ at Low Temperature Condition By Operando Time-Resolved X-Ray Diffraction

Introduction LiFePO4 is a promising cathode material for lithium-ion batteries1 as it exhibits high rate performance.2 The origin of the high rate performance exemplified in LiFePO4 should provide design principles for further development of high rate cathode materials. The (dis)charge reaction of LiFePO4 proceeds through a two phase behavior between Li-rich Li1-αFePO4 (LFP) and Li-poor LiβFePO4 (FP).3 Under high rate cycling, we revealed the formation of a metastable phase of Li x FePO4 (x = 0.6–0.75) (L x FP) which acts as a buffer layer between LFP and FP.4 However, the detailed reaction between LFP and FP during high rate cycling has not fully been understood due to short lifetime of metastable L x FP phase. To investigate the phase transition mechanism, with respect to L x FP phase, cycling at low temperatures was performed because metastable L x FP phase might have long lifetime to fully capture its spatial-temporal formation. Phase transition mechanism is analyzed by operando time-resolved X-ray diffraction (TR-XRD) measurements at low temperatures. Experimental Three-electrode aluminum-laminated pouch-type cells were fabricated for operando TR-XRD measurements. LiFePO4/C particles were synthesized by hydrothermal methods.4 The working electrode comprised a composite mixture of LiFePO4/C, carbon black (Denka), and polyvinylidene fluoride (Kureha) at a ratio of 75:15:10 (wt%) and coated on aluminum foil current collectors. Lithium foil was used as counter and reference electrodes. The electrolyte was 1 mol dm–3 of LiPF6 dissolved in a mixture of ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio, Battery Grade, Kishida). operando TR-XRD measurements were performed in transmission mode at the beam line BL28XU at SPring-8 (Japan). The wavelength of X-ray was 0.9998Å. The cells were set at fixed temperatures (–5, 5 and 25 °C) using temperature-controlled jackets,5 and were charged and discharged at constant rates of 1C (170 mA g–1), 5C (850 mA g–1) and 10C (1700 mA g–1) between 2.0 and 4.3 V versus Li/Li+ for 5 cycles. Results and Discussions In the charge-discharge cycle at 25ºC and rate of 1C, operando TR-XRD pattern looks typical of the two-phase reaction between the LFP and FP phases. LFP 211 and 020 diffractions disappeared and the FP 211 and 020 diffractions simultaneously appeared, as the cell was charged. Conversely, the FP diffractions disappeared and the LFP diffractions appeared, as the cell was discharged. On the other hand, the behavior of the operando TR-XRD patterns in the first charging at –5°C is different from that at 25°C. The intensity bridge between LFP and FP 211 was observed. This indicates the formation of the LxFP phase even in the first charging. The diffraction peaks of FP phase disappeared and the peak of the LxFP phase at 19.4° appeared as the cells were discharged. These results suggest that the first charging process is different from the first discharging process except for the condition of 1C rate and 25°C. Moreover, at 10C rates and –5°C, the fact that the discharging finishes at the single LxFP phase suggests that the phase transition between the LxFP and LFP phases is slow. The difference of phase transition kinetics between LFP/LxFP and LxFP/FP cause the initial irreversible capacity at high rate, and/or low temperature conditions. References (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc., 1997, 144, 1188. (2) Meethong, N.; Kao, Y.-H.; Carter, W. C.; Chiang, Y.-M. Chem. Mater., 2010, 22, 1088. (3) Yamada, A.; Koizumi, H.; Nishimura, S. I.; Sonoyama, N.; Kanno, R.; Yonemura, M.; Nakamura, T.; Kobayashi, Y. Nat. Mater., 2006, 5, 357. (4) Orikasa, Y.; Maeda, T.; Koyama, Y.; Murayama, H.; Fukuda, K.; Tanida, H.; Arai, H.; Matsubara, E.; Uchimoto, Y.; Ogumi, Z. J. Am. Chem. Soc., 2013, 135, 5497. (5) Takahashi, I.; Murayama, H.; Sato, K.; Naka, T.; Kitada, K.; Fukuda, K.; Koyama, Y.; Arai, H.; Matsubara, E.; Uchimoto, Y.; Ogumi, Z. J. Mater. Chem. A, 2014, 2, 15414. Acknowledgement This study was partially supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) Project under the auspices of New Energy and Industrial Technology Development Organization (NEDO), Japan.