New synthesis strategies to improve Co-Free LiNi0.5Mn0.5O2 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 0.5 Mn 0.5 O 2 (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 0.495 Mn 0.495 Mo 0.01 O 2 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 0.5 Mn 0.5 O 2 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 Mn 2 P 2 O 7 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. Image 1 • Synthesis of high capacity cobalt-free Ni–Mn cathodes with d0 dopants Mo and Ti. • Lower cation mixing on Mo doping from x-ray diffraction and electron microscopy. • Small Mo and Ti doping improve capacity retention and rate performance. • Mn 2 P 2 O 7 coating on surface provides higher cycling stability at high voltage. [ABSTRACT FROM AUTHOR]