Li4MeWO6 (Me=Ni, Mn, Co) As Positive Electrode Materials for Li-Ion Batteries

Li-rich layered oxides, Li₂MO₃ (M = a variety of tetravalent metal(s), typically Mn), have been widely studied as next generation cathode materials for Li ion batteries. Recently, Li₄NiTeO₆ was reported as a cathode material with 1.5 Li reversible reaction.[1] It has honeycomb structure with ordered arrangement of Te and Ni similar to that of Mn and Li in Li₂MO₃. The hexavalent Te stabilizes divalent state of Ni ions in the structure. A reversible electrochemical performance based on the redox reaction of Ni²⁺/Ni⁴⁺ was observed. Here, we tried to replace Te for W in Li₄NiTeO₆. Tungsten is often used as a dopant for positive electrode materials to improve rate performance of Li ion batteries etc. but not popular as a main element building framework because of its heaviness, cost and electrochemical inactivity. In order to understand the influence on the structure and electrochemical properties, we synthesized mainly tungsten-containing materials, Li₄MeWO₆ (Me=Ni, Mn, Co) expecting hexavalent W⁶⁺ ions to stabilize the crystal structure with divalent ions and to improve the diffusion of Li ions during charge and discharge.

Li₄MeWO₆ was obtained by a conventional solid state reaction with heating and grinding several times. Me used was Ni, Mn, and Co to prepare Li₄NiWO₆, Li₄MnWO₆ and Li₄Ni_0.6Mn_0.2Co_0.2O₆, respectively. XRD pattern of the Li₄MeWO₆ samples showed a single phase and clear diffraction peaks attributed to honeycomb ordering of Li, Me and W ions in the slab. The electrochemical properties for the Li₄MeWO₆ materials with different Me composition, were examined using coin-type lithium cells with electrolyte solution of 1M LiPF₆/ EC:DMC (1:1).

Figure 1 shows charge/discharge curves of Li₄NiWO₆, measured with different voltage window at a constant current of 13 mA g^-1. In the voltage range of 4.1 - 3.0 V, initial discharge capacity of 83 mAh g^-1, corresponding to 1.1 Li extraction from Li₄NiWO₆ is delivered. Reversible discharge curve and good cycle stability with low polarization are observed in the voltage range, probably on the basis of Ni redox reaction. On the other hand, charging up to 4.9 V results in more than 250 mAh g^-1 of charge capacity equal to 3.4 Li extraction suggesting more than 2 electron reaction of Ni^2/4+couple. In addition, the polarization is getting increased by increasing upper cut-off charge voltage.

The detailed structural change during charge/discharge and the correlation between Me composition and the electrochemical performance of Li₄MeWO₆as positive electrode materials in Li cells will be presented.

Reference

[1] M. Sathiya, J. M. Tarascon et al., Chem. Commun, 49, 11376 (2013).