Soft chemistry synthesis and structure-property relationships of lithium-ion battery cathodes

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Choi, Seungdon

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Lithium-ion batteries have become attractive for portable electronic devices due to their higher energy density. While the commercial lithium-ion cells presently use the layered LiCoO2 as the cathode, there is enormous interest to develop alternative inexpensive and environmentally benign cathodes for next generation cells. In this regard, design of novel synthesis procedures to obtain metastable phases that are otherwise inaccessible by conventional methods and a fundamental understanding of the factors that control the electrochemical properties – cell voltage, capacity, and cyclability – play a key role. This dissertation explores the use of soft chemistry procedures to obtain new electrode materials and investigates the structureproperty relationship of some lithium-ion battery cathodes. Oxidation reactions of transition metal ions in solutions are used to synthesize oxide cathodes based on Mn, Fe, Co, Ni, and Cu. For example, the metastable spinel Li2Mn4O9-d and the layered LiNi1-xCoxO2 synthesized by such an approach show capacities of, respectively, 130 and 165 mAh/g with good cyclability. On the other hand, nanocrystalline LixCu1-yFeyOz synthesized, for the first time, shows a high initial capacity of 340 mAh/g, but declining to 220 mAh/g after 40 cycles. This system is attractive as both Fe and Cu are inexpensive and environmentally benign. Nanocrystalline LixCu1-yFeyOz as well as some amorphous manganese oxides having high capacities are also investigated for use in polymer electrolyte cells. An investigation of the influence of synthesis conditions on the phase relationships of the system LixMn3-xO4+d indicates that the lithium-rich spinel phases with x > 1 are more stable at intermediate firing temperatures T » 600 oC compared to LiMn2O4. A systematic investigation of the layered to spinellike phase transition in chemically synthesized Li0.5MO2 (M = Mn, Co and Ni) reveals that the ease of transformation decreases in the order Mn > Ni > Co. Presence of mixed cations in the transition metal ion plane is found to be effective to suppress such a transition and impart better structural stability. A closer look at the origin of the high voltage (> 4.5 V) capacity in some of the spinel series of materials suggests that the high voltage capacity is due to the oxidation of primarily the oxide ions.



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