Average, Local and Electronic Structures of LiMn2-XAlxO4(x=0, 0.2) in Charge-Discharge Process by Neutron and Synchrotron X-ray

INTRODUCTION

 As Li ion battery cathode materials, stratified formation materials such as LiCoO₂ in which the Co are used mainly now. The Co-free spinel compound LiMn₂O₄ has been studied as Li-ion battery cathode materials. Simple composition and low cost of Mn is preferable for the battery material. However, there are the problems such as elution of the Mn with the cycle and the Jahn-Teller distortion by the Mn³⁺.^1,2) Characteristic remarkable deterioration in this way occurs for a cycle. As solution to this problem, characteristic improvement by the substitution of the metallic element (Al, Co, Cr, Li, Mg, Ni, Ti, Zn) was reported.³⁾

 It is intended to examine a crystal structure change during charge and discharge process of the Al substituted spinel using the average and local structure analysis with ex-situ method. The average structures were calculated by Rietveld analysis and electron density by Maximum Entropy Method (MEM) using the neutron and synchrotron X-ray diffractions. In addition, we examined the local structure by XAFS and the PDF analysis using synchrotron X-ray total scattering.

EXPERIMENTAL

 LiMn_2-xAl_xO₄ (x=0, 0.2) were synthesized by the solid-phase method. The samples were determined for the valence state of Mn by potentiometric titration, and also characterized by XRD and ICP measurements. Charge-discharge test was performed using the HS cell (3.5 ~ 4.3V vs Li/Li⁺, the negative electrode: Li metal, separator: polypropylene, electrolyte: 1M LiPF₆/EC:DMC (1:2)).

 Measured sample was obtained by the electrode after two cycles of charge and discharge process. These samples were measured at 4.04V, 4.15V, 4.3V in the charge process, and 4.12V, 4.02V, 3.5V in the discharge process. These samples were performed by neutron diffraction (BL20, BL09, J-PARC) and synchrotron X-ray diffraction (BL02B2, SPring-8). We attempted to Rietveld analysis, PDF and XAFS analysis using the measured data, and examined the electronic structure and the distortion during the charge discharge process.

RESULTS AND DISCUSSION

 At first we performed the Rietveld analysis using the neutron diffractions about each sample of Al substituted material at charge process after 2 cycles. As a result, the occupancy of Li at 8a site changed with charge discharge process, and the occupancy of Li at 16d site was constant. Thus it was thought that only the Li at 8asite participates in electrochemical insertion and extraction. In addition, distortion parameter λ in the distance calculated from each analysis showed few changes. As for the Al　substituted spinel, it was suggested that the host structure was stable in  2nd cycles during charge discharge process.

 We performed Rietveld analysis by the synchrotron X-ray diffraction measurement and calculated electron density in MEM and demanded a line profile of M(Mn,Al)-O. The line profile of charge discharge process of LiMn_1.8Al_0.2O₄showed Fig. 1. It was demonstrated that the M(Mn, Al)-O bond of the Al substituted sample was more stable than unsubstituted one in charge discharge process. 

 We examined XANES and EXAFS. The EXAFS spectrum of LiMn₂O₄ and LiMn_1.8Al_0.2O₄were shown Fig. 2. From EXAFS spectrum, the relaxation of the Jahn-Teller distortion of the Mn was suggested, because peak intensity at charge state increased. Furthermore, we examined the PDF by synchrotron X-ray total scattering. In unsubstituted sample at discharge, the decrease in peak was confirmed in comparison with before charge. It was thought that this decrease was attributed to the elution of Mn. This study reported in detail on that day. It was performed by help of NEDO (RISING) and shows thanks to the members concerned.

References

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