The Enhancement of the Interface Reaction in LiFePO4 Two-Phase System
Authors: Kato Eri , Yoshinari Takahiro , Yamamoto Kentaro , Mori Takuya , Orikasa Yuki , Nakanishi Koji , Uchimoto Yoshiharu 
Published: February 2020

LiFePO4 is considered as a high rate capability cathode material for lithium-ion battery. During charging and discharging, the material undergoes a two-phase reaction between Li-rich and Li-poor phases1. The rate-determining step in the phase transition of LixFePO4 is expected to be the nucleation reaction2. Recently, it is reported that surface modification of LiFePO4 with nitrogen improves the rate capability3. However, the origin of the improvement in rate performance still has not been understood well. This is because it is difficult to analyze the interfacial reaction using composite electrodes, which have complex structures of mixture with active material, carbon, and binder. To discuss correlation between the surface modification and reaction kinetics of LiFePO4, we made surface modified LiFePO4 thin-film electrode with nitrogen, and investigated the surface electronic structure of the electrode by total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS).

LiFePO4 thin-film (Bare-LFP) 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. Surface modified LiFePO4 thin-film with nitrogen (N-LFP) was prepared by heating the Bare-LFP under NH3 atmosphere at 923K for 10 min. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4 in propylene carbonate as a electrolyte were prepared. Bare-LFP was characterized by cyclic voltammetry and transmission electron microscope (TEM). N-LFP was characterized by N-K edge XAS. The rate capability of Bare-LFP and N-LFP estimated charge/discharge measurements from 1C to 100C.

We also discuss the reaction kinetics of LiFePO4 with anion surface modification to enhance rate performance. The reaction kinetics of total process included charge transfer was calculated by cyclic voltammetry (CV) and of phase transition by potentiostatic intermittent titration technique (PITT). The result of CV indicated better kinetics by nitrogen surface modification than by undoped and of PITT corresponded with them.The results support improvement of nucleation kinetics.

Here we report a characterization of surfaces of LiFePO4 thin-film model electrodes by TRF-XAS that shows the slightly higher oxidation state of iron by nitrogen surface modification than by undoped. This result indicates that the existence of Fe(2+δ)+at the surface decreases the nucleation energy as well as moderates the lattice strain, leading to better battery performance. The facilitation of phase transition, rather than particle size minimization or intimate carbon contact reforming bulk structures, is a novel approach to enhance reaction kinetics.

1. Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc., 144, 1188,(1997).

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

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

This work was partially supported by KAKENHI Grant Number 25249141.