New synthesis strategies to improve Co-Free LiNi_0.5Mn_0.5O₂ cathodes: Early transition metal d⁰ dopants and manganese pyrophosphate coating

In this work, we report solution-based doping and coating strategies to improve the electrochemical performance of the Co-free layered oxide cathode LiNi_0.5Mn_0.5O₂ (NM-50/50). Small amounts of d⁰ dopants (e.g., Mo⁶⁺and Ti⁴⁺, 0.5–1 at. %) increase the cathode's specific capacity, cycling stability, and rate capability. For example, a Mo-doped cathode with the nominal composition LiNi_0.495Mn_0.495Mo_0.01O₂ exhibits a high reversible capacity of 180 mA h/g at 20 mA/g compared to only 156 mA h/g for undoped NM-50/50. Effects of 1 at.% Mo dopant on the cathode structure were studied using a suite of characterization tools including X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy. These measurements demonstrate that Mo⁶⁺ dopant is enriched near the particle surface and improves the electrochemical performance of LiNi_0.5Mn_0.5O₂ by: (i) reducing Li⁺/Ni²⁺ cation mixing which facilitates Li⁺ transport, (ii) mitigating undesirable phase transformations near the cathode surface, and (iii) altering the cathode/electrolyte interfacial chemistry. This work also reports the use of an inorganic Mn₂P₂O₇ coating which enhances the cycling stability of Mo-doped NM-50/50, presumably through formation of a stable cathode electrolyte interphase (CEI) layer. Overall, the synthesis approaches reported herein are quite general and can potentially be expanded to other high voltage Li-ion battery cathodes.