Lithium ion batteries (LIBs) have attracted much attention as large-scale power sources for hybrid electric vehicles (HEVs), plug-in HEVs, and electric vehicles (EVs) due to their high energy density, long cycle life, and environmental friendliness. Lithium-rich layered manganese oxides containing nickel, cobalt, and iron represented by the chemical formula xLi₂MnO₃·(1-x)LiMO₂ (M= Co, Ni, and Fe) are promising candidate as an cathode material having the high operating voltage and large capacity. In particular, Fe- and Ni-substituted lithium-rich layered oxide Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂ (0.6Li₂MnO₃·0.2LiFeO₂·0.2LiNiO₂) with about 250 mAh/g would be a good material to make a low-cost high-capacity cathode because Mn and Fe are abundant mineral resources [1]. The large capacity has been achieved by not only the well-known redox process of transition metal but also the reversible anionic redox process between O₂^2- and O^2- [2]. At the present, the major problem to be solved is the capacity fades and gas generations during cycles. In this study, we succeeded in the improvement of them by using oxide-coated Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂. We also investigated the deterioration mechanism of cells during cycles.

 Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂ was synthesized using the coprecipitation-calcination method. A mixed solutions containing Fe, Ni, and Mn ions were co-precipitated continuously into the reactor and they was oxidized by air-bubbling for 24 h. After that ,the co-precipitated precursor was mixed with Li₂CO₃ and calcined at 700°C for 5 h in air and then at 850 °C for 5 h in N₂ atmosphere. The obtained sample was mixed with Sm(NO₃)₃·6H₂O and calcined at 400°C for 2 h in air (Sm/ Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂). Al-laminated film-packed cells were assembled using a cathode, a graphite anode, and a polyolefin porous separator containing the electrolyte solution (1M LiPF₆ethylene carbonate EC/DEC =3/7 (v/v)).

 The stepwise activation process was carried out in the range of 1.5 to 4.5 V in the thermostatic chamber kept at 45ºC [3]. The cycle test was carried out by repeating cycles of charge and discharge processes at 40 mA/g to the lower limit voltage of 1.5 V. The positive electrodes after charge-discharge were characterized by synchrotron X-ray absorption fine structure (XAFS) and hard X-ray photoemission spectroscopy (HAXPES) measurements in the beam line BL08B2 and BL46XU at SPring-8.

 The initial discharge capacity of the cells with Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂ and Sm/Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂electrodes showed 265 and 258 mAh/g, respectively, which was almost same. The capacity retention and gas generation quantities during 50 cycles became 3% higher and 20% smaller, respectively, by the Sm oxide coating.

 XAFS results indicated that the irreversible reactions of Fe ions during cycles were suppressed by Sm oxide coating although the Mn and Ni were not affected practically. The HAXPES measurements, which can obtain the information of valence states and/or surface deposits below about 50 nm, demonstrated that the formation of the SEI (Solid Electrolyte Interphase) on cathode and irreversibility of Fe, Ni, and oxide ions were suppressed by Sm oxide coating during cycles. From the above results, we found that the oxide film on cathode materials prevented the reaction between cathode materials and electrolytes. We believe that the improvement of the cell properties with Sm/Li_1.23Mn_0.46Fe_0.15Ni_0.15O₂electrodes accelerates in progress of the development of next generation storage devices.

Acknowledgments

This work was in part performed under the management of Research and Development of High Energy Density Lithium-Ion Battery utilizing High Capacity and Low-Cost Oxide Cathodes project supported by the New Energy and Industrial Technology Department Organization (NEDO) in Japan.

References

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[2] R. Yuge, A. Toda, S. Kuroshima, H. Sato, T. Miyazaki, N. Tamura, M. Tabuchi, K. Nakahara, Electrochim. Acta, 189 (2016) 166.

[3] K. Nakahara, M. Tabuchi, S. Kuroshima, A. Toda, K. Tanimoto, K. Nakano, J. Electrochem. Soc. 161 (2012) A1398.