Studies of layered lithium nickel manganese oxide as positive electrode materials for lithium-ion secondary batteries : Studiesoflayeredlithium nickel manganeseoxideaspositive electrodematerialsforlithium-ion secondarybatteries
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Polyaniline is coated on Li[Li0.2Ni0.2Mn0.6]O2 synthesized via co-precipitation. X-ray diffraction patterns exhibit that the polyaniline coating does not affect structural change of the Li[Li0.2Ni0.2Mn0.6]O2, and the resulting transmission electron microscopic images show the presence of coating layers on the surface of Li[Li0.2Ni0.2Mn0.6]O2. Electrochemical tests using coin type cells confirm that the surface modification by polyaniline is substantially effective in maintaining capacity and retention upon cycling. The conducting coating character also assists improvement in rate capability. The polyaniline layer forms F-doped polyaniline during cycling, as is proved by time-of-flight secondary ion mass spectroscopy. Therefore, the presence of the polyaniline layers plays a role in lowering HF levels via scavenging F– from HF in the electrolyte, and this F–doped polyaniline layer also assists in protecting the Li[Li0.2Ni0.2Mn0.6]O2 from HF attack upon cycling, resulting in improved electrochemical properties. In order to confirm reasons that deteriorate cathode performances, Ni-rich Li[Ni0.7Mn0.3]O2 is modified by lithium isopropoxide to artificially provide lithium excess environment by forming Li2O on the surface of active materials. X-ray diffraction patterns indicate that the lithium oxide coating does not affect structural change comparing to the bare material. Scanning electron microscopy and transmission electron microscopy data show the presence of coating layers on the surface of Li[Ni0.7Mn0.3]O2. Electrochemical tests demonstrate that the Li2O-coated Li[Ni0.7Mn0.3]O2 exhibits a greater irreversible capacity with a small capacity because of the presence of insulating layers composed of lithium compounds on the active materials since these layers delay facile Li+ diffusion. Also, the Li2O layer forms by products such as Li2CO3, LiOH, and LiF, as are proved by x-ray photo electron spectroscopy and time-of flight secondary ion mass spectrometry.The presence of residual lithium tends to bond with hydrocarbons generated by decomposition of electrolytic salt during electrochemical reactions, and the reaction accelerates the decomposition of electrolytic salt that produces the byproducts cause the formation of passive layers on the surface of active material. As a result, the new layers consequently impede diffusion of lithium ions that deteriorate electrochemical properties.