We have taken advantage of the element specific nature of X-ray absorption spectroscopy to elucidate the chemical and structural details of a surface treatment intended for the protection of high-capacity cathode materials. Electrochemical data have shown that surface treatments of 0.5Li 2 MnO 3 •0.5LiCoO 2 (Li 1.2 Mn 0.4 Co 0.4 O 2 ) with an acidic solution of lithium–nickel-phosphate significantly improves electrode capacity, rate, and cycling stability. XAS data reveal that the surface treatment results in a modification of the composite structure itself, where Ni 2+ cations, intended to be present in a lithium–nickel-phosphate coating, have instead displaced lithium in the transition metal layers of Li 2 MnO 3 -like domains within the 0.5Li 2 MnO 3 •0.5LiCoO 2 structure. X-ray diffraction data show the presence of Li 3 PO 4, suggesting that phosphate ions from the acidic solution are responsible for lithium extraction and nickel insertion with the formation of vacancies and/or manganese reduction for charge compensation. Furthermore, we show that the above effects are not limited to lithium–nickel-phosphate treatments. The studies described are consistent with a novel approach for synthesizing and tailoring the structures of high-capacity cathode materials whereby a Li 2 MnO 3 framework is used as a precursor for synthesizing a wide variety of composite metal oxide insertion electrodes for Li-ion battery applications.
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Croy et al. (2011) studied this question.