(Invited) High Electrochemical Performance of High Voltage LiNi0.5Mn1.5O4 By Decoupling the Ni/Mn Disordering from the Presence of Mn3+ ions

High energy density of a lithium ion battery (LIB) is an essential requirement for novel applications such as plug-in hybrid electric vehicles and electric vehicles. Increasing the redox potential of cathode materials is an effective way to achieve high energy density in the cell. For this purpose, high-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode material for LIBs because it has a high redox potential of ~ 4.7 V, which makes its energy density (~650 W∙h/kg) 20% higher than that of conventional LiCoO₂. However, the electrochemical properties of LNMO spinel depend on several factors such as its structure, the quantity of Mn³⁺ ions, the particle size, and the morphology of particles.

  Especially, the structure of the spinel has a critical influence on its electrochemical performance. LNMO structure is dictated by the ordering of Ni and Mn at two octahedral sites, which take two forms: disordered and ordered spinel. However, under typical experimental conditions, disordered spinel shows the disordering of Ni/Mn with the presence of Mn³⁺ ions, whereas ordered spinel shows the ordering of Ni/Mn without Mn³⁺ ions. As a result, electrochemical properties of the disordered spinel always depend on both the disordering of Ni/Mn and Mn³⁺ ions. For example, disordered spinel shows an extended solid-solution reaction during phase transformation whereas ordered spinel shows two distinct two-phases reactions. Furthermore, the presence of Mn³⁺ ions in the disordered spinel increases its electronic conductivity, and thereby improves its electrochemical performance. Therefore, disordered LNMO spinel can have better electrochemical performance than ordered spinel due to high electronic conductivity and facile phase transformation. The origin of the different electrochemical properties in the disordered spinel is not clear because of the coupling of the Ni/Mn disordering with Mn³⁺ ions. Understanding of this origin could guide improvement of LMNO’s electrochemical properties. However, experimentally understanding the effect of disordering or the effect of Mn³⁺ ions on the electrochemical properties is not easy because the Ni/Mn disordering is always coupled with the presence of Mn³⁺ ions.

  In this presentation, we will discuss the effect of the Ni/Mn disordering without the presence of Mn³⁺ on the electrochemical performance. At first, the Ni/Mn disordering was decoupled from the Mn³⁺ ions in this study. By using this decoupling, we will show how the Ni/Mn disordering affects electrochemical properties and the phase transformation behavior. Furthermore, we will discuss about key factors affecting superior rate capability in high voltage spinel by decoupling of the disordering from Mn³⁺ ions.