CATION-DISORDERED AND HIGH-ENTROPY OXIDES/OXYFLUORIDES AS ELECTRODE MATERIALS FOR LITHIUM-ION BATTERIES

In recent years, new classes of lithium-excess cathode and anode materials with disordered rock-salt structure (DRX), in which any channels for Li+ ion diffusion are absent, have attracted increased interest. In DRX cathode materials of Liy(Me1Me2)2-yO2 composition, Li+ and transition metal ions (Me) are equally likely to occupy the same octahedral positions in the lattice, and the diffusion of Li+ ions occurs by hopping from one octahedron to another through an intermediate tetrahedron (o-t-o diffusion). The presence of fluorine in oxyfluorides DRX-F (Li1+x(MeMn3+)1-xO2-yFy, where Me = Ti4+, Nb5+) affects the local ordering of Mn3+ ions, the stability of the redox couple O2-/O- and its contribution into specific capacity. On the other hand, high-entropy oxides (HEO), which are single-phase oxide systems containing five and more cations, were synthesised by mixing five oxides CoO, CuO, MgO, NiO and ZnO in equimolar ratios, followed by thermal treatment at 1000 °C. Stabilisation of single-phase solid solutions Co0.2Cu0.2Mg0.2Ni0.2Zn0.2O and Lix(Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)2-xO2-xFx (0 ≤ x ≤ 1) with a rock-salt crystal structure turned out to be possible due to the large contribution of the entropy of mixing to the Gibbs free energy. Another group of anode materials are high-entropy oxides with a spinel structure (HES), for example (Cr0.2Fe0.2Mn0.2Co0.2Ni0.2)3O4 obtained by solid-state synthesis at T = 500-1000 °C in air. After ball grinding, the particle size is reduced to 20 nm. The initial specific capacity during cycling in the 0.01-3.0 V range is 1333.6 (mA•h)/g and decreases to 329.9 (mA•h)/g after 20 cycles. A comparative investigation of synthesis conditions, crystal structure, morphology and electrochemical characteristics has been carried out for the four classes of high-entropy oxides: 1) Liy(MeMn3+)2-yO2, where Me = Ti4+, Nb5+; 2) Li1+x(MeMn3+)1-xO2-yFy, where Me = Ti4+, Nb5+; 3) Lix(Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)2-xO1-xFx; 4) (Cr,Fe,Mn,Co,Ni)3O4.