Improved cyclic stability by octahedral Co³⁺ substitution in spinel lithium manganese oxide thin-film cathode for rechargeable microbattery

Highlights: • Sol-gel-prepared thin-film LiCo_xMn_2-xO₄ cathodes exhibit improved capacity retention after long-term charge-discharge cycles (>200) compared to pristine LiMn₂O₄ cathode. • LiCo_xMn_2-xO₄ cathodes have octahedral Co³⁺ ions mostly and lattice constant decreasing with increasing Co composition (x). • LiCo_xMn_2-xO₄ cathodes exhibit rate capability better than pristine LiMn₂O₄ cathode at high discharge rates (>10C). - Abstract: Co-doped lithium manganese oxide (LiCo_xMn_2-xO₄) cathodes were prepared as thin films via a sol-gel process to investigate the effects of the octahedral Co substitution on electrochemical properties. X-ray diffraction and scanning electron microscopy results showed that the polycrystalline films (x ≤ 0.3) were composed of phase-pure grains with the sizes distributed mainly in 100–200 nm range. The cubic lattice parameters of the spinel LiCo_xMn_2-xO₄ samples decreased gradually with increasing Co composition. X-ray photoelectron spectroscopy data indicated that cobalt ions in LiCo_xMn_2-xO₄ exist as Co³⁺ mostly. The electrochemical investigations of LiCo_xMn_2-xO₄ cathodes include cyclic voltammetry, cycling performance, charge-discharge profile, and rate capability in the 3.5–4.3 V potential range. The x = 0.05 cathode exhibited initial discharge capacity close to that of LiMn₂O₄ cathode and less capacity fading than LiMn₂O₄ after extended charge-discharge cycles (>200). The x = 0.1 cathode exhibited a reduction in initial discharge capacity by ∼10% but the capacity was retained up to ∼300 cycles. The improved capacity retention of LiCo_xMn_2-xO₄ cathodes is attributable to the improved structural stability by octahedral Co³⁺ substitution for suppressing Mn³⁺-related Jahn-Teller distortion and cathode dissolution into electrolyte.