Effect of Local Structural Changes on Rate Capability of LiNi0.5Mn1.5O4-d Cathode Material for Lithium Ion Batteries
Authors: Yoon Jaesang , Kim Donghwi , Um Ji Hyun , Jeong Mihee , Son Suhan , Yoon Won-Sub 
Published: February 2020

Due to its high working voltage (4.7 V vs. Li/Li+ for Ni2+/Ni4+ redox couple), high theoretical capacity (147 mAh/g), and inherent cyclic stability (no Jahn-Teller distortion from Mn3+), LiNi0.5Mn1.5O4 is a promising cathode material for the practical application in high energy density lithium ion batteries [1-3]. The cation ordering, the degree of presence of Mn3+ in the spinel structure is regarded as an important factor influencing the electrochemical properties of LiNi0.5Mn1.5O4. Especially, at high current densities, the better electrochemical performance was obtained in the disordered spinel, resulted from the higher electronic conductivity and lithium ion diffusivity [4,5].

With a simply strategy of cooling rate, the degree of ordering in disordered spinel LiNi0.5Mn1.5O4-δ structure is controlled at 0.5 oC/min for slow cooled spinel and at 5 oC/min for fast cooled spinel. From XRD and FTIR analyses and charge/discharge profiles, it is confirmed that the fast cooled spinel develops more disordered structure than the slow cooled spinel, retaining more oxygen vacancies and more abundant Mn3+. Moreover, through in situ XRD analysis, the more disordered spinel follows the extended solid-solution reaction in favor of Li+ transport, which results in an improved rate capability [6,7]. But, the local structure information of interesting atoms without a destructive interference which can arise from long-range structural order is highly related to the electrochemical performance. Therefore, the examination in local structural variations of interesting atoms is essential for understanding their electrochemical properties. X-ray absorption spectroscopy (XAS) is generally applied to examine the valence and local structure variations of an interesting atom within short-range order.

In this work, we investigate local structural changes in the vicinity of Mn and Ni cations at various C-rates by using ex situ XAS analysis and quantify the structural changes by fitting a theoretical model to the extended X-ray absorption fine structure data. This finding will propose that a change of structural distortion can be another platform to understand the enhanced Li+ transport in the disordered spinel and offer valuable guidance for developing LiNi0.5Mn1.5O4, a promising cathode material, for the practical application in high energy density lithium ion batteries. More detailed discussion will be presented at the PRIME 2016 meeting.

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