Layered LixMn1-yCoyO2 intercalation electrodes-influence of ion exchange on capacity and structure upon cycling

Layered LixMn1-yCoyO2 with the O3 (alpha NaFeO2) structure has been prepare d from the analogous P3 sodium phase by ion exchange using LiBr in either e thanol at 80 degreesC or hexanol at 160 degreesC. The former preserves, to some extent, vacancies present on the transitional metal sites of the sodiu m phase, whereas the latter eliminates the vacancies. Materials with vacanc ies exhibit better performance as cathodes in rechargeable lithium batterie s. The 2.5% Co doped material prepared in ethanol exhibits capacities of 20 0 mAhg(-1) when cycled at C/8 between 2.4 and 4.6 V at 30 degreesC and with a fade of only 0.08% per cycle. A capacity of 180 mA h g(-1) can be obtain ed at C/2 and 200 mAhg(-1) at C rate and 55 degreesC. Importantly, this per formance is obtained despite the fact that the materials convert to spinel- like phases on cycling. The spinel-like phases that form are nanostructured , with each crystallite being composed of a mosaic of nanodomains. The reli ef of strain at the domain wall boundaries accompanying the cubic-tetragona l phase transition may explain, at least in part, the facile cycling of the se materials over a wide composition range (including the 3 V plateau) comp ared with high-temperature spinel which does not possess such nanodomains. Furthermore, vacancies present in the ethanol materials appear to migrate t o the domain walls on cycling, rendering even more facile the Jahn-Teller-d riven phase transformation on cycling these materials compared with those p repared in hexanol.