Designing high-capacity, lithium-ion cathodes using x-ray absorption spectroscopy

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 ₂MnO ₃·0.5LiCoO ₂ (Li _1.2Mn _0.4Co _0.4O 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 ₂MnO ₃-like domains within the 0.5Li ₂MnO ₃·0. 5LiCoO ₂ structure. X-ray diffraction data show the presence of Li ₃PO 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 ₂MnO ₃ framework is used as a precursor for synthesizing a wide variety of composite metal oxide insertion electrodes for Li-ion battery applications.