Impact of O3/P3 phase transition on the performance of the NaxTi1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 cathode material for Na-ion batteries

The results of research on the structural, transport and electrochemical properties of the NaxTi1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 high entropy oxide cathode material for Na-ion batteries are presented. Pristine high entropy oxide (HEO) O3-NaTi1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 exhibits semiconducting behaviour and shows only one reversible O3/P3 phase transition. Electrochemical tests carried out in the voltage range of 1.5-3.9 V indicate a high decrease in capacity. Additional electrochemical tests performed for the limited voltage range of 3.2-3.9 V, for which only the P3 phase is present, show very high cycling stability. Experimental findings are discussed in view of electronic structure calculations performed using the KKR-CPA method in order to account for different models of chemical disorder in O3 and P3 phases of the NaTi1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 system. It was determined that specific atomic ordering of transition metal atoms in the O3 phase may lead to a semiconducting state, while a fully random distribution of these atoms tends to result in a metallic-like one. Conversely, a finite density of states was systematically found in the case of the P3 phase, irrespective of the considered model of transition metal atom arrangement. Hence, the extra ordering of transition metals in the O3 phase is a possible explanation for its poor transport properties, whereas the much better transport properties of the P3 phase are reflected in the very high efficiency during the cycling of the battery in the limited voltage range. Furthermore, XAS total electron yield (TEY) measurements showed that only manganese, cobalt and nickel are electrochemically active during charging and discharging of the battery in the 1.5-3.9 V range and that Co is the most effective element in the charge compensation mechanism of the NaxTi1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 oxide.