Nanocrystalline Li x Mn2 − y O 4 Cathodes for Solid‐State Thin‐Film Rechargeable Lithium Batteries

Thin-film cathodes of lithium manganese oxide, 0.3-3 μm thick, were deposited by rf magnetron sputtering a LiMn2O4 ceramic target onto unheated substrates. The resulting films were dense, approximately 4.2 g/cm3, with a approximately 50 angstroms nanocrystalline spinel structure. The film composition was typically LixMn2-yO4 with y to approximately 0.3 and 1.2 < x < 2.2. When cycled in a thin-film rechargeable lithium battery, specific cathode capacities of 145±23 and approximately 270 mAh/g were realized for discharge from 4.5 V to either 2.5 or 1.5 V, respectively. The discharge and charge current densities were limited by the resistivity of lithium transport into and through the cathode. After thousands of cycles at 25 °C, there was a small increase in cell resistance. After several hundred cycles at 100 °C, the discharge curves developed a stable knee at approximately 4 V characteristic of crystalline LiMn2O4 cathodes. The polarization of the discharge/charge cycles were interpreted in terms of free energy of mixing curves.