Synthesis of SiO2-Modified Li2MnO3-LiMn1/3Ni1/3Co1/3O2 by Spray Pyrolysis with Acid Addition as Cathode Materials for Lithium Ion Batteries

Abstract

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-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2, which may lead to the enhancing the electrochemical performance. Moreover, we synthesized the unmodified and SiO2-modified Li2MnO3-LiMn1/3Ni1/3Co1/3O2 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-LiMn1/3Ni1/3Co1/3O2 from the starting solutions containing citric acid is much higher than that of the unmodified and SiO2-modified Li2MnO3-LiMn1/3Ni1/3Co1/3O2. For example, the specific surface area of the as-synthesized Li2MnO3-LiMn1/3Ni1/3Co1/3O2 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.