Olivine-type LiFePO₄ is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO₄ and FePO₄. During the phase transition between LiFePO₄ and FePO₄, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.¹ To improve the rate capability of LiFePO₄, Park et al performed the surface modification by nitrogen on LiFePO₄ particles.² However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.³ In this study, we prepared LiFePO₄thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy.

LiFePO₄ thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO₄in propylene carbonate liquid electrolyte were prepared.

Annealing of ammonia gas introduces nitrogen into LiFePO₄ surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO₄ thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO₄. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO₄ and FePO₄, which is the origin of surface modification effect.

References

[1] Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F., Nat. Mater. 2008, 7(8), 665.

[2] Park, K. S.; Xiao, P. H.; Kim, S. Y.; Dylla, A.; Choi, Y. M.; Henkelman, G.; Stevenson, K. J.; Goodenough, J. B., Chem. Mater. 2012, 24(16), 3212.

[3] Takamatsu, D.; Koyama, Y.; Orikasa, Y.; Mori, S.; Nakatsutsumi, T.; Hirano, T.; Tanida, H.; Arai, H.; Uchimoto, Y.; Ogumi, Z., Angew. Chem. Int. Edit. 2012, 51(46), 11597.

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number 25249141.