Improving rate capability and cyclability of Co-free and high-voltage LiNi۰.۵Mn۱.۵O۴ cathode material via morphology modification by the co-precipitation method

LiNi۰.۵Mn۱.۵O۴ (LNMO) Cobalt-free spinel is a promising cathode material for high-voltage lithium-ion batteries, offering an impressive discharging voltage of ۴.۷ (V) besides remarkable energy density. The co-precipitation method is innovatively employed to synthesize high-performance LNMO, showcasing its potential for advanced energy storage applications. The FE-SEM images show that the morphology of LNMO particles strictly depends on the morphology of the precursor particles that are affected by the synthesis parameters and conditions, such as the molar ratio of Mn۲+ and Ni۲+ in the initial solution, reaction temperature, pH value, and concentration of the chelating agent. Among these, the chelating agent has a key role in achieving a uniform and semi-spherical precursor particle.In this work, a co-precipitation method using an appropriate amount of NH۴OH as a chelating agent was used to prepare semi-spherical Ni۰.۲۵Mn۰.۷۵(OH)۲ as a precursor for LNMO in order to reach non-agglomerate particles and also improve rate capability and cyclability, as shown in Figure ۱. LNMO is generally reported to crystallize in the spinel structure within two different space groups (Fd-۳m and P۴۳۳۲) depending on the oxygen stoichiometry and ordering of the Ni/Mn cations. From the enlarged region of XRD patterns, it is evident that the peaks at ≈۳۶.۵° and ≈۳۸.۲° are shifted to lower angles, meaning an increase of lattice parameter. Therefore, the presence of Mn۳+ and disordered spinel is probable. A more detailed analysis is performed using the dQ/dV versus V plots as shown. The two dQ/dV peaks around ۴.۷ V indicate the two-step oxidation or reduction for the Ni۲+/Ni۴+ redox couple. The separation between these two peaks indicates the disordered/ordered phases, where a separation >۵۰ mV is considered predominantly disordered. As indicated in the following equations, in the disordered spinel structure, two electron hopping paths can be provided due to the randomness of the positions of Ni and Mn; in contrast to the single path of the ordered spinel