Analysis for the Electronic Structure at Cathode/Electrolyte Interface in Lithium-Ion Battery Via In Situ Total Reflection Fluorescence X-ray Absorption Spectroscopy

Several research groups have reported that the cathode/electrolyte interface plays an essential role for the deterioration of lithium ion batteries [1, 2]. To further enhance the cycle time of lithium-ion batteries, it is important to understand the reactions at the cathode/electrolyte interface. However, the mechanisms of such interface reactions have not been fully understood due to the difficulty of their direct observation at the nanometer scale. Recently, we have developed a new technique named in-situ total-reflection fluorescence X-ray absorption spectroscopy (in-situ TRF-XAS) to analyze directly the electronic structure at the cathode/electrolyte interface under operating condition [3, 4]. By using in-situ TRF-XAS, we investigate the electronic structure of LiCoO₂ and/or LiFePO₄/electrolyte interface under battery operation condition and discuss the correlation between the electronic structure at the interface and durability of the active materials. LiCoO₂ is one of the most popular active materials whose capacity easily fades with the long charge/discharge cycling [5]. On the other hand, LiFePO₄ exhibits a more stable capacity with cycling compared to LiCoO₂ [6].

LiCoO₂ and LiFePO₄ thin-films were prepared on Pt and Au polycrystalline substrate by pulsed layer deposition (PLD), respectively. In-situ TRF-XAS measurements were performed at the BL01B1, BL28XU and BL37XU at SPring-8 (Japan) using a home-made cell with a counter electrode of Li foil (Fig. 1). The separator (CELGARD^®3501) was immersed with 1 mol dm^-3 LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The TRF-XAS measurements were conducted under setting the X-ray incident angle at 0.17-0.20° and the generated fluorescence X-ray was detected with a solid state detector.

In-situ TRF XAS spectra corresponding to the information at cathode/electrolyte interface of the LiCoO₂ and LiFePO₄ thin films were analyzed before and after electrolyte immersion. The XANES spectrum of the LiCoO₂ surface was shifted to lower energy upon electrolyte immersion, meaning the reduction of the Co ions at the surface. In contrast to LiCoO₂, the XANES spectrum of the LiFePO₄ surface did not show any shift upon electrolyte immersion, meaning that no reduction of the Fe ions at the surface occurred. At the electrode/electrolyte interface, the electrochemical potential gap between the two phases should be compensated. The potential change at the electrode side and the electrolyte side forms space charge layer and electrical double layer, respectively. For LiCoO₂, the reduction of the surface means the formation of the space charge layer via electron transfer form the electrolyte and therefore the potential gap is compensated by both the space charge layer and the electrical double layer (Fig. 2 (a)). As for LiFePO₄, the space charge layer hardly forms at the surface because the surface was not reduced via electron transfer from the electrolyte. Therefore, the electrical double layer contributes locally to the potential compensation for LiFePO₄ (Fig. 2(b)). The durability of the electrode materials depends on the formation of the space charge layer.

Acknowledgments

This work was partially supported by New Energy and Industrial Technology Department Organization (NEDO), Japan, for Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project.

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