Electrochemical performances of Li4Mn5O12 prepared by spraycoating assisted Sol-Gel reaction

There is an intense interest in the battery industry to identify ways to improve the cathodes used in terms of energy, power, safety, life, and cost. Li4Mn5O12 has attracted great interest as a potential 3V cathode material for rechargeable Li batteries in recent years, due to its high specific capacity of 163 mAh/g in the 3V region. Li4Mn5O12 is conventionally synthesized via solid-state reaction where the high temperature needed makes it difficult to obtain stoichiometric Li4Mn5O12 and the solid-state reaction gives a solid that needs to be mixed with a carbon source and a binder for electrochemical characterization and application. The fabrication of thin film rechargeable lithium batteries by a Sol-Gel method combined with spray-coating technique is expected to achieve both simplification and cost reduction of fabrication process by direct deposition of the active material on the electrode during its synthesis. A precursor sol of LiCH3COO.2H2O:Mn(CH3COO)2.4H2O (4:5) and L-lysine (total metal ions:L-lysine 50:1) was sprayed onto steel coins repeatedly, dried at 280 °C by heat gun and converted to Li4Mn5O12 thin film by heating at 400 °C. This process was repeated till target mass was deposited. For a target battery capacity of 0.65 mAh, a deposit of 4 mg is necessary. The charge-discharge test of the Li/Li4Mn5O12 coin cell battery was carried out at different rates, 0.1 C to 2 C (1C = 163 mA/g), between 1.8 and 3.6V. It is important that the total capacity is close to theoretical value and stable with cycling. The table below on the evolution of the capacity with the cycles at different rates shows that even at a medium rate of C/2, the capacity decreases by 10 % after 70 cycles, result which is better than those observed in the literature [1-3] but not good enough for applications.
Doping of the sol with TiO2 before spray coating is necessary to improve electrochemical performances. The figure below shows the charge-discharge curves of the TiO2-doped Li4Mn5O12 at different rates. During the successive cycles, the discharge and charge decrease only slightly at a given rate, demonstrating the high-degree stable discharge plateau at 2.8 V. Very good capacities are obtained for rates ≤ 1C but for higher rates, kinetic problems appear and part of the lithium does not take part to the insertion-extraction process, even with TiO2 doping. In the potential range of 3.6-1.8 V, TiO2-doped Li4Mn5O12 still display very good electrochemical properties in respect of the capacity (high capacity values), capacity retention (stability) and rate capability (high capacity and stability at medium rates) as shown in the table below.