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 Li-y(Me1Me2)(2-y)O-2 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-x)O2-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 degrees C. Stabilisation of single-phase solid solutions Co Cu-0.2 ( 0.2) Mg Ni-0.2 (0.2) Zn (0.2) O and Li x (Co 0.2 Cu 0.2 Mg Ni-0.2 Zn-0.2 ( 0.2) ) O2- x F2- x (x) (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 (Cr- 0.2 Fe- 0.2 Mn (0.2) Co (0.2) Ni (0.2) ) (3) O (4 )obtained by solid-state synthesis at T = 500-1000 degrees 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 center dot h)/g and decreases to 329.9 (mA center dot 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) Li-y(MeMn3+)(2-y)O-2, where Me = Ti4+, Nb5+; 2) Li (1+ x) (MeMn (3+ )) O1- x (2- y) F (y) , where Me = Ti4+, Nb5+; 3) Li (x) (Co- 0.2 Cu- 0.2 Mg- 0.2 Ni (0.2) Zn (0.2 )) (2- x) O (1- x) F (x) ; 4) (Cr,Fe,Mn,Co,Ni)(3)O-4.