On the electrochemical properties of the Fe–Ti doped LNMO material LiNi0.5Mn1.37Fe0.1Ti0.03O3.95

Abstract

LiNi_0.5Mn_1.5O₄ (LNMO) based spinel cathode materials for lithium-ion batteries are promising alternatives to widely used mixed transition-metal layered Li(Ni,Co,Mn)O₂ (NCM) oxides. LNMO is cobalt free and thus cost efficient, while providing a high operating voltage of 4.7 V (vs. Li/Li⁺) and remarkable energy density of ∼650 W h kg⁻¹. Commercialization and large-scale application however are still hindered, as short cycle life remains a main issue. To help overcome this problem, we present a comprehensive investigation into Fe–Ti doped LNMO materials with the formal composition LiNi_0.5Mn_1.37Fe_0.1Ti_0.03O_3.95 (LNMFTO). Within this study, samples were calcined at temperatures between 460 °C and 940 °C and were cooled down to room temperatures rapidly or slowly. Small changes in the crystal structures were tracked by using a high-precision powder X-ray diffraction (PXRD) setup, while changes in cation order were investigated with Raman spectroscopy. It is shown that carefully elaborated calcination programs allow to maintain the optimized morphological features such as particle size, shape and specific surface, while crystallographic properties, such as the amounts of Mn(III) or (partial) cation order, can be adjusted independently. We provide experimental evidence that calcination at high temperatures leads to nickel loss in the spinel phase, but not to the formation of additional oxygen defects. LNMFTO samples show good cycling stabilities and over 98% capacity retention after 100 cycles with capacities larger than 90 mA h g⁻¹ at discharge rates of 10C when cycled vs. lithium metal. These results are almost fully transferable to cathodes with high active material loadings cycled vs. graphite anodes. A capacity retention of > 89% for 500 cycles and residual capacities of >100 mA h g⁻¹ are observed, which makes LNMFTO a suitable candidate for industrial applications.