Understanding the capacity fade mechanisms of spinel manganese oxide cathodes and improving their performance in lithium ion batteries
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Lithium ion batteries have been successful in portable electronics market due to their high energy density, adopting the layered LiCoO₂ as the cathode material in commercial lithium ion cells. However, increasing interest in lithium ion batteries for electric vehicle and hybrid electric vehicle applications requires alternative cathode materials due to the high cost, toxicity, and limited power capability of the layered LiCoO₂ cathode. In this regard, spinel LiMn₂O₄ has become appealing as manganese is inexpensive and environmentally benign, but LiMn₂O₄ is plagued by severe capacity fade at elevated temperatures. This dissertation explores the factors that control and limit the electrochemical performance of spinel LiMn₂O₄ cathodes and focuses on improving the performance parameters such as the capacity, cyclability, and rate capability of various spinel cathodes derived from LiMn₂O₄. From a systematic investigation of a number of cationic and anionic (fluorine) substituted spinel oxide compositions, the improvements in electrochemical properties and performances are found to be due to the reduced manganese dissolution and suppressed lattice parameter difference between the two cubic phases formed during the charge-discharge process. Investigations focused on fluorine substitution reveal that spinel LiMn[subscript 2-yz]LiyZnzO[subscript 4-eta]F[subscript eta] oxyfluoride cathodes synthesized by solid-state reactions at 800 °C employing ZnF₂ as a raw material and spinel LiMn[subscript 2-y-z]Li[subscript y]Ni[subscript z]O[subscript 4-eta]F[subscript eta] oxyfluoride cathodes synthesized by firing the cation-substituted LiMn[subscript 2-y-z]LiyNi[subscript z]O₄ oxides with NH₄HF₂ at a moderate temperature of 450 °C show superior cyclability, increased capacity, reduced Mn dissolution, and excellent storage performance compared to the corresponding oxide analogs and the conventional LiMn₂O₄. Spinel-layered composite cathodes are found to exhibit better electrochemical performance with graphite anode when charged to 4.7 V in the first cycle followed by cycling at 4.3-3.5 V compared to the normal cycling at 4.3 - 3.5 V. The improved performance is explained to be due to the trapping of trace amounts of protons that may be present in the electrolyte within the layered oxide lattice during the first charge to 4.7 V and the consequent reduction in Mn dissolution. Electrochemical performances of 3 V spinel Li₄Mn₅O₁₂ cathodes are also improved by fluorine substitution due to the suppression of the disproportionation of Li4Mn5O12 during synthesis and the formation of the Li₂MnO₃ phase.