Lithium-rich layered solid solutions of Li₂MnO₃ and LiMO₂, where M denotes at least one 3d transition metal, has attracted much attention as a cathode material of lithium ion batteries because they offer high capacities of around 250 mAh g^-1 within the potential range of 2.0 to 4.8 V (versus Li/Li⁺). However, most of the works on the material have reported the electrochemical performance at low current rates. Arranging the crystal structures and morphologies of the materials is particularly important for improving the cell performance.

  Spray pyrolysis is a simple one-step synthesis technique to obtain various kinds of functional oxide powders. The particles prepared by spray pyrolysis have some advantages such as small particle size distribution, high purity, and easy control in composition and morphology of multi-component metal oxides. In addition, powder morphology can be controlled by acid addition to the starting solution of spray pyrolysis. We have found that a small amount of SiO₂ addition remarkably suppressed the capacity fade of Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ prepared by spray pyrolysis during charge and discharge cycling. In this study, we investigated the effect of acid addition to the starting solutions of unmodified and the SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ of spray pyrolysis. 

  Stoichiometric amounts of lithium nitrate, manganese nitrate hexahydrate, nickel nitrate hexahydrate, cobalt nitrate hexahydrate were dissolved in deionized water with the addition of citric acid or boric acid. SiO₂ aerosol was added to the solution. The reaction furnace consisted of four independent heating zones. Cycling tests were performed galvanostatically at a 0.1 C rate (1 C = 314 mAh g^-1) between 2.0 and 4.8 V at 30ºC. The rate tests were performed by charging and discharging the electrode at 0.1 C for 5 cycles, 0.2 C for 5 cycles, 0.5 C for 5 cycles, and 1 C for 5 cycles. Prior to the cycling, a stepwise pre-cycling treatment was carried out by increasing the upper potential limit by 0.1 V from 4.5 V every two cycles to 4.8 V [1].

  We investigated the capacity retention for 0, 2 and 5 wt.% SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ samples at 0.1C in 1 M LiPF₆/EC+DMC (1:2) after the stepwise pre-cycling treatment. The capacity fade on the cycling was successfully suppressed for all SiO₂-modified samples. The highest capacity retention was obtained for the 2 wt.% SiO₂-modified sample. SEM images showed that the as-synthesized particles were not agglomerated and had spherical shapes. After heat-treatment, the particles retained their shapes. The spherical shapes were partially collapsed by SiO₂ modification. The interior primary particle size was more grown than those of the unmodified particles. The specific surface area slightly decreased with SiO₂ content to 2 wt.%. The specific surface area increased at a SiO₂ content of 5 wt.% and this may be due to the collapsed shape compared with other particles.

  We synthesized the SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ using the starting solutions containing various amounts of boric acid. SEM images showed that boric acid increased the densities of the materials by acting as a sintering agent. With more than 0.5 wt.% boric acid addition, Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ powders displayed aggregated structures and large sizes. Improvement of the densities of the materials means the decrease of the isolated particles observed in the case of the SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂, which may lead to the enhancing the electrochemical performance. Moreover, we synthesized the unmodified and SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ using the starting solutions containing various amounts of citric acid. SEM images showed that most of the particles had spherical shapes but some particles were collapsed. The specific surface area of the synthesized Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ from the starting solutions containing citric acid is much higher than that of the unmodified and SiO₂-modified Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂. For example, the specific surface area of the as-synthesized Li₂MnO₃-LiMn_1/3Ni_1/3Co_1/3O₂ using the solution containing 0.93 mol l^-1 of citric acid is 28 m² g^-1. Higher specific surface area is expected to favor the high rate capability. It can be found from the above results that boric acid and citric acid are effective in controlling the powder morphology. We will present the electrochemical properties including rate capability.

[1] A. Ito, D. Li, Y. Ohsawa, Y. Sato, J. Power Sources 183 (2008) 344-346.