High Performance LiNi0.5Mn1.5O4 Cathode Material for Lithium Ion Batteries
Authors: Chong Jin , Wu Kai 
Published: February 2020

As applications of Lithium ion batteries increasingly expand into the plug hybrid electric vehicles (PHEV), or even electric vehicles (EV), the existing battery technology established for portable electronics has faced challenges to meet the criteria for automotive application, especially in terms of energy density. One strategy of increase energy density of batteries is to increase the voltage. Thus, spinel LiNi0.5Mn1.5O4 has received tremendous research efforts due to its high discharge voltage (~ 4.7 V), low cost and environmentally benign properties. However the high operational potential will easily lead to the decomposition of electrolyte and salt, forming a thick solid-electrolyte interface (SEI) layer on anode side, which not only increases the cell impedance but also traps cyclable Li resulting in the capacity fade. Another challenge of LiNi0.5Mn1.5O4 is that Mn dissolution can be accelerated by HF arising from the reaction between trace of water with electrolyte. In addition, many studies have shown that the Mn is more readily dissolved in electrolyte at high potential leading to the loss of active materials. The generated Mn cations can be easily reduced to deposit on anode side trapping more Li and increasing the cell impedance during cycling. The combination of these factors leads to the dramatic capacity decay of the cells based on LiNi0.5Mn1.5O4, then limiting its practical application. Thus, development of LiNi0.5Mn1.5O4 materials with stable long-term cyclablity is crucial but challenging.
It is known that in lithium batteries, most of the electrochemical reactions happen on the solid-liquid interface followed a mass transport. For LiNi0.5Mn1.5O4, electrolyte decomposition and Mn dissolution most likely initialize on its surface, especially at high potential. Thus, the interface property between LiNi0.5Mn1.5O4 and electrolyte is believed to be a key factor affecting the electrochemical performance. To establish a more stable interface to minimize the unwanted reactions, coating a protective layer on LiNi0.5Mn1.5O4 surface has been demonstrated as an efficient approach to improve its electrochemical performance. In this study, we demonstrate that cyclability of LiNi0.5Mn1.5O4 can be significantly enhanced by applying a Li3PO4 coating layer on LiNi0.5Mn1.5O4 particles. For half cell performance, Li3PO4/LiNi0.5Mn1.5O4 can retain 80 % of initial capacity in 650 cycles. On the contrary, non-coated LiNi0.5Mn1.5O4 lost most of the capacity in 345 cycles. Li3PO4/LiNi0.5Mn1.5O4 materials also exhibit a superior cycling stability in full cells without obvious fading in 250 cycles. The substantial improvement of cycling performance is attributed to Li3PO4 coating, which can significantly minimize the electrolyte decomposition on active material surface by separating the active material from the electrolyte and preventing the Ni2+/Ni3+ or Ni3+/Ni4+ redox couple from catalyzing electrolyte decomposition. Thus, by demonstrating that Li3PO4 coating layer can enable the cycling of high voltage LiNi0.5Mn1.5O4 materials with regular electrolyte, we believe that Li3PO4 coating can be applied to other functional materials for lithium-ion batteries with the desire of greater stability and cyclability.

Figure 1