Impacts of Cr Substitution on Ion Distributions and Electrochemical Performance of LiNi_0.5-xCr_2xMn_1.5-xO₄ Cathode Materials

Abstract

A spinel-structured LiNi0.5-xCr2xMn1.5-xO4(x=0, 0.05) were prepared via co-precipitation routes. The experiment results from XRD, SEM and electrochemical analyses show that the substitutions of Ni and Cr for Mn in LiMn2O4can not prevent Mn2+from being oxidized into Mn3+in solution process, however,Mn2+oxidation does not change their final crystal structures of spinel phase, but do influence the Li/Ni2+cation mixing and Mn3+occurrence in lattice. After substitution the first charge and discharge capacities decrease but its cycleability is improved significantly, especially for the Ni and Cr co-substitution

Molten salt synthesis of LiMn 1 . 2 Ni 0 . 3 Cr 0 . 1 Co 0

“…Several researchers report that the trend of thermodynamic stability observed during the substitution of Mn by other transition metal ions stabilizes the spinel structure and that such stabilization increases with decreasing Mn ion radius and spinel lattice parameters. This tendency to stabilization may be due to a decrease in the deformation energy that results from the size difference between Mn 3+ and Mn 4+ ions, when Mn 3+ ions are substituted by a smaller cation in the case of a single doping element 31‐33 . But in our work, a multicationic doping was performed, based on the difference between the ionic radius of Mn 3+ and the doping elements (MNi 3+ (0.69 nm), Co 3+ (0.61 nm), Al 3+ (0.67 nm), La 3+ (1.06 nm) and Cr 3+ (0.61 nm)) and the oxidation state of the elements.…”

“…This tendency to stabilization may be due to a decrease in the deformation energy that results from the size difference between Mn 3+ and Mn 4+ ions, when Mn 3+ ions are substituted by a smaller cation in the case of a single doping element. [31][32][33] But in our work, a multicationic doping was performed, based on the difference between the ionic radius of Mn 3+ and the doping elements Δr Mn 3þ ÀM (M Ni 3+ (0.69 nm), Co 3+ (0.61 nm), Al 3+ (0.67 nm), La 3+ (1.06 nm) and Cr 3+ (0.61 nm)) and the oxidation state of the elements. It is obvious that the lattice parameter needs to be increased, which agrees well with our result.…”

“…The presence of Mn 3+ and Mn 4+ in the matrix is due to the charge neutrality of the spinel matrix. 31 The ionic radius of Mn 4+ is 0.53 Å, but the ionic radius of Mn 3+ depends on the spin state. In the high spin state (HS), the ionic radius is 0.645 Å but in the low spin state (LS), the ion radius is 0.58 Å.…”

“…LiMn 2 O 4 adopts a cubic spinel structure with Li + ions occupying the 8a tetrahedral sites and Mn 3+ and Mn 4+ ions occupying the 16d octahedral sites, resulting in a cationic distribution of Li[Mn 4+ Mn 3+ ]O 4 . The presence of Mn 3+ and Mn 4+ in the matrix is due to the charge neutrality of the spinel matrix 31 . The ionic radius of Mn 4+ is 0.53 Å, but the ionic radius of Mn 3+ depends on the spin state.…”