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

• Sol-gel-prepared thin-film LiCoxMn2-xO4 cathodes exhibit improved capacity retention after long-term charge-discharge cycles (>200) compared to pristine LiMn2O4 cathode.
• LiCoxMn2-xO4 cathodes have octahedral Co3+ ions mostly and lattice constant decreasing with increasing Co composition (x).
• LiCoxMn2-xO4 cathodes exhibit rate capability better than pristine LiMn2O4 cathode at high discharge rates (>10C).

Co-doped lithium manganese oxide (LiCoxMn2-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 LiCoxMn2-xO4 samples decreased gradually with increasing Co composition. X-ray photoelectron spectroscopy data indicated that cobalt ions in LiCoxMn2-xO4 exist as Co3+ mostly. The electrochemical investigations of LiCoxMn2-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 LiCoxMn2-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.