Investigation of LiMn2O4-Li2NiF4 Solid Solutions As Electrode Materials for Lithium-Ion Batteries

Lithium ion batteries play a very important role in consumer electronics as the component where energy stored. However, today’s lithium ion batteries are not suitable for widespread application in electric vehicles due to the high cost required to achieve desired levels of energy density and life.¹ Improvements in energy density (cell voltage and/or storage capacity) are still sought for these applications. Given the high capacity of graphite-based negative electrodes, the focus is placed on increasing performance at the positive electrode. Today, the most popular positive electrode materials are layered LiMO₂ (M = Co, Mn, Ni), spinel LiM₂O₄ (M = Mn, Ni), and olivine LiMPO₄ (M = Fe, Co, Mn).^{2, 3} LiM₂O₄ and its derivatives are widely used in current electric vehicles mainly because of their robust framework with fast ionic diffusion and low cost. In this work, we explore the possibility of raising their voltage of operation by incorporating Li₂NiF₄ into the LiMn₂O₄ lattice to form a solid solution of LiMn₂O₄-Li₂NiF₄ by taking the advantage of their structural similarity. The reaction between the two components is complex. Apart from single phase samples, core-shell structures could also be obtained by varying the ratio of two components as well as reaction duration. An example of this competition is provided by LiMn₂O₄-20% Li₂NiF₄, as shown by the different elemental distributions from energy dispersive spectroscopy (EDS) elemental maps in Figure 1, indicating solid solution and core-shell structures, respectively. The local chemical environment change of oxygen, fluorine and transition metals has been investigated by X-ray absorption spectroscopy. The electrochemical performance of the phase-pure LiMn₂O₄-Li₂NiF₄solid solutions will also be presented. 

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