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AIST researchers synthesize new class of high-voltage, high-capacity cathode materials for Li-ion batteries

Researchers at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) have developed a new class of contenders for high-voltage and high-capacity Li-ion cathode materials with the composition NaxLi0.7-xNi1-yMnyO2 (0.03 < x 0.25, 0.5 y 0.8).

One of the compositions—Na0.093Li0.57Ni0.33Mn0.67O2—exhibited a maximum discharge capacity of 261 mAh g-1 at an average voltage of 3.36 V at 25 ˚C (between 2.0 and 4.8 V), which translates to an energy density of 943 Wh kg-1. A paper on their work is published in the Journal of Power Sources.

Since the commercialization of Li-ion batteries by Sony in 1991, lithium cobalt oxide (LiCoO2) has been extensively studied as a positive electrode material in rechargeable lithium batteries. However, LiCoO2 is not suitable for electric vehicle applications because of limitations in the natural resources of Co; hence, it is not cost-effective. To address this issue, LiFePO4 has emerged as a promising positive electrode material, owing to the abundance of iron resources. Nevertheless, the energy density of LiFePO4 is approximately 500 Wh kg-1, which does not meet the specifications for battery devices in electric vehicles. Hence, LiNi0.80Co0.15Al0.05O2, LiNi0.33Co0.33Mn0.33O2 and LiNi0.5Mn0.5O2 with layered structures have been investigated, and some of these materials have been assessed for use as electrode materials for practical applications. However, so far, no definitive electrode material has been utilized commercially.

Recently, a lithium-deficient nickel manganese oxide Li0.7Ni0.33Mn0.67O2 with a layered structure (O2 and O3-type) has extensively been investigated as a positive electrode material for use in rechargeable lithium batteries. … O3-Li0.7Ni0.33Mn0.67O2 exhibits a discharge capacity of approximately 200 mAh g-1 between 4.7 and 2.5 V, which is higher than that of the O2-type compound. However, O3-Li0.7Ni0.33Mn0.67O2 exhibits poor capacity retention on subsequent cycling, due to phase transformation from layered to spinel structure.

… Recently, we reported that a thermal treatment after the ion-exchange reaction using a molten salt improved the charge- discharge performance of O3-Li0.7Ni0.33Mn0.67O2. … Inspired by the preliminary studies, in this study we sought to synthesize NaxLi0.7-xNi1-yMnyO2 (0.03 < x ≤ 0.25, 0.5 ≤ y ≤ 0.8) by thermal treatment. The residual Na content x in ion exchange and the Mn content y are utilized to clarify the impact of the charge and discharge voltage-capacity curves for thermally treated NaxLi0.7-xNi1-yMnyO2 (0.03 < x ≤ 0.25, 0.5 ≤ y ≤ 0.8) and the results obtained can be utilized for the design of high-capacity materials.

—Chiba et al.

The team attributed the performance of the new composition to the predominant retention of the layered rock-salt structure over the spinel phase. Thus, they posited, varying the residual Na and Mn content in the compositions can curtail the spinel-phase transformation. This in turn demonstrated the feasibility of modifying voltage-capacity characteristics through judicious control of the constituent Na and Mn contents in related compositions.

The team is currently working to optimize the electrochemical performance of the composition, as well as investigating the underlying reaction mechanism.

Resources

  • Kazuki Chiba, Noboru Taguchi, Masahiro Shikano, Hikari Sakaebe (2016) “NaxLi0.7−xNi1−yMnyO2 as a new positive electrode material for lithium-ion batteries,” Journal of Power Sources, Volume 311, Pages 103-110, doi: 10.1016/j.jpowsour.2016.02.008

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