| An enormous growth in portable electronic devices as cellular phones and laptop computers has led to increasing demand for compact lightweight batteries with high energy density and power capacity. Lithium ion batteries have satisfied this demand to a greater degree than other rechargeable battery systems. The presently commercialized lithium batteries use layered-structured LiCoO2 cathodes. Because of the high cost and toxicity of cobalt, an intensive search for new cathode materials has been underway in recent years. Tremendous efforts are underway on the spinel LiMn2O4 and its derivatives, which are considered to be attractive alternatives in term of cost, abundance and nontoxicity. The significant capacity loss of the spinel LiMn2O4 during cycling hereby prevents its wider use as cathode materials for lithium secondary batteries.There are two main factors resulting in fading capacity and poor cyclability, one is the Jahn-Teller distortion of Mn3+, and the other is Mn dissolution to electrolyte. The main objects of this paper are to resolved these key problems effecting on the performance of batteries.During the research of suppressing Jahn-Teller distortion, low valence cation doped method are usually adopted. This method can improve the cyclability of batteries, but it will also lead to the decreasing of initial capacity. In this paper, it is studied for the effects of F-Co or S-Co co-doped on the capacity and cyclability of cathode materials. The results indicated that F-Co co-doped could improve cyclability with the high initial capacity. Because F-doped increases the content of Mn3+, which is the active ion at oxido-reduction, so F-doped can increase the initial capacity, but the increased Mn + intensified the Jahn-Teller distortion and the dissolution of Mn2+ , both of which deteriorated the cyclability. In contrast, F-Co co-doped combined the high initial capacity of F-doped and the superior cyclability of Co-doped. Subsequently, the cathode materials with desired electrochemical properties was obtained. The sample LiCo0.1Mn1.9O3.5F0.5 showed the highest initial capacity (125mAh/g) and the best cyclability (the reservation ratio of capacity to 93.6% after 30 cycles), so it was the optimum composition.S-doped couldn't improve the initial capacity and the cycleability in 4V region, but the capacity in 3V region increased instead of fading within 30 cycles. In addition, the peaks of Li2Mn2O4 couldn't be found in XRD patterns of LiMn2O4-xSx after 30 cycles, both the results indicated S-doping suppressed the Jahn-Teller effects. Similar to F-Co co-doped, S-Co co-doped combined the stable cyclability of Co-doped in 4V region and S-doped in 3V region. Thus, the S-Co co-doping LiMn2O4 spinel can be operated in a wide voltage range 2.4-4.3V. Accordingly, the capacity was elevated intensively with good cycleability.The capacity fading during cycling or with storage at high temperature is primarily the result of Mn dissolution in electrolyte. With LiCoO2 as the coating materials, a uniform coating of LiCoO2 is applied to a spinel surface to reduce the contact between the spinel and the electrolyte, and thereby reduce the propensity of spinel dissolution in the electrolyte. The LiCoO2 coating is deposited by sol-gel process, which enhanced the uniformity of the surface coverage.LiMn2O4 spinel was coated with LiCoO2 using sol-gel process. XRD and EPMA studies have confirmed the presence of a Co-rich spinel LiMn2-xCoxO4 on the surface of LiMn2O4 particle and the content of Co decreased from surface to the core. The Co-rich phase on thesurface prevented the direct contacting between the core materials and the electrolyte. Furthermore, the amount of Mn3+ in the surface phase was less than in the core, so the possibility of dissolution decreased. The surface treatment was founded to be effective in reducing the rate of Mn dissolution into electrolyte and improving cyclability of spinel. After bulk Ni-doped spinel LiNi0.1Mn1.9O4 was coated with LiCoO2, the cyclability of material is better than that...
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