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Layered P2-type Na2/3Ni1/3Mn2/3O2: conductivity and electrochemical characteristics
The cathode material of Na2/3Ni1/3Mn2/3O2 composition (NNMO) was obtained by the carbonate coprecipitation followed by solid-phase reaction and characterized by XRD analysis, ICP-MS, electron and impedance spectroscopy. NNMO crystallizes in a P2-type layered hexagonal structure (sp. gr. P63/mmc), consists of spherical agglomerates of ~1 - 3 microns in size, formed from the plate-like primary grains. The particles have the smallest size in the direction of the crystallographic "c" axis. The NNMO ionic conductivity value at room temperature was 1.8*10−4 and 1.3*10−4 S cm−1 when measured perpendicularly and parallel to the compression axis, respectively. The activation energy determined from impedance spectroscopy data was equal to the value of ~21 kJ mol−1. The estimated in dc mode values of electronic conductivity were by 2-3 orders of magnitude less than ionic conductivity. The obtained materials were tested as cathodes in sodium-ion battery cells versus sodium metal. The discharge capacity of NNMO was 160 mAh g−1 and 86 mAh g−1 in the potential range of 1.5-4.0 V and 2.3-4.0 V, respectively (20 mA g−1). NNMO was shown to be stable under cycling in the potential range of 2.3-4.0 V.