Structural Changes (Degradation) of Oxysulfide LiAl[sub 0.24]Mn[sub 1.76]O[sub 3.98]S[sub 0.02] Spinel on High-Temperature Cycling

Abstract

The structural integrity of the oxysulfide material, LiAl 0.24 Mn 1.76 O 3.98 S 0.02 before and after charge-discharge cycling at high temperature was studied by X-ray diffraction and high-resolution transmission electron microscopy. The rock salt phase Li 2 MnO 3 which is associated with capacity loss has been detected at the surface of the oxysulfide particles in the electrode cycled in the 4 V region at 80°C. The small structural degradation of the new oxysulfide material may be responsible for the excellent cyclability of the oxysulfide spinel at high temperature over the 4 V region. LiMn 2 O 4 spinels have generated great interest as the most promising cathode materials positive electrodes for lithium secondary batteries, due to their high energy density, low cost, abundance, and nontoxicity. Li x Mn 2 O 4 (x 1) has a cubic spinel structure with space group symmetry Fd3m in which the Li and Mn 3/4 ions are located on the 8a tetrahedral sites and the 16d octahedral sites of the structure, respectively. 1,2 In the 4 V region, it appears that the cubic structure of the material is maintained during the extraction and insertion of Li ions, but the capacity of the spinel electrode slowly loses during cycling. Recently, Tarascon and Guyomard 3 reported that lithium-rich spinel (Li 1x Mn 2 O 4) showed excellent cy-clability at room temperature. However, a poor performance of the material at high temperature prevents its wider use as cathode material for lithium secondary batteries. For commercial use, the high temperature performance of LiMn 2 O 4 must be improved. To improve the cyclability of spinel LiMn 2 O 4 electrodes at high temperature , many research groups have studied cation substitution for Mn, and anion F for O, and surface passivation treatment of LiMn 2 O 4. 4-6 Surface passivation treatments of Li 1.05 Mn 1.95 O 4 particles coated with lithium borate glass and acetylacetone improved the performance of the material at 55°C to some degree owing to minimizing the LiMn 2 O 4 /electrolyte interface. Amatucci et al. reported that the cyclability of fluorine-doped spinel LiMn 2 O 4x F x at 55°C was improved to some extent because of a decreased Mn dissolution. 6 The main possible reason for such poor performance has been attributed largely to the solubility of LiMn 2 O 4 spinel due to the formation of HF resulting from the reaction of fluorinated anions with residual H 2 O. 7,8 Liu et al. reported that a significant amount of tetragonal Li 2 Mn 2 O 4 in Li x Mn 2 O 4 electrode after 80 cycles at the 4 V region appeared when charged and discharged at a high rate between 3.5 and 4.5 V and its existence was the result of kinetic limitations at the electrode surface during fast intercalation. 9 It was subsequently shown by Thackeray and co-workers that evidence of tetragonal Li 2 Mn 2 O 4 phase on the LiMn 2 O 4 particles surface at the end of discharge above 3 V was detected by transmission electron microscopy TEM imaging after a few high rate cycles. 10 The presence of Li 2 MnO 3 and spinel compositions other than LiMn 2 O 4 in cycled Li x Mn 2 O 4 electrodes has been reported previously by Rob-ertson and co-workers. 11 Cho and Thackeray suggested that a capacity fade for Li/LiMn 2 O 4 cycled hundreds of times in the 4 V region at room temperature is ascribed to the formation of Li 2 MnO 3 at the particle surface which is attributed to the dissolution of MnO from Li 2 Mn 2 O 4. 12 It is generally accepted that the capacity loss for the LiMn 2 O 4 electrode in the 4 V region could be attributed largely to a slow dissolution of MnO at the spinel particle into electrolyte. However , there is no explicit experimental data for the structural degradation at the spinel particle surface after cycling at room temperature as well as high temperature to confirm many researchers' observation and suggestion that is associated with capacity loss. Recently, we reported that a new sulfur-doped spinel, LiAl 0.24 Mn 1.76 O 3.98 S 0.02 , could eliminate the effect of Jahn-Teller distortion and showed excellent cyclability in the 3, 4 V, and both the 3 and 4 V regions at room temperature. 13,14 In this paper, we study the high-temperature 50 and 80°C performance of LiAl 0.24 Mn 1.76 O 3.98 S 0.02 in the 4 V region combined with the structural degradation at the spinel particle surface. X-ray diffraction XRD and high-resolution transmission electron micros-copy HRTEM were used to investigate the cycling-induced phase transformation of individual spinel particles at high temperature. Experimental LiAl 0.24 Mn 1.76 O 3.98 S 0.02 powders were prepared by a sol-gel method as reported in our previous work. 13 Li 1.05 Al 0.24 Mn 1.75 O 3.85 S 0.15 was a starting composition of LiAl 0.24 Mn 1.76 O 3.98 S 0.02. The slight excess of Li was introduced to compensate for losses during calcination. The Li/LiAl 0.24 Mn 1.76 O 3.98 S 0.02 cell discharged to 3.0 V was allowed to equilibrate for 5 h at each operating temperature. After cooling the cell to room temperature, the LiAl 0.24 Mn 1.76 O 3.98 S 0.02 electrode was removed from the cell and then dried for one day. Powder XRD Rigaku, Rint-2000 using Cu K radiation was used to identify the crystalline phase of as-prepared powders and cycled electrodes at various temperatures. Rietveld refinement was then performed using the XRD data to measure lattice constants. The chemical composition was determined by chemical analysis using an inductively coupled plasma ICP. The chemical analysis was conducted three times for each sample and the accuracy of the measured data was less than 3%. To evaluate the solubility of spinel, 0.4 g of powders was loaded into a Teflon container bag, followed by immersing into 4 mL 1 M LiPF 6 in ethylene carbonate/ dimethyl carbonate EC/DMC 1:2 by volume electrolyte, which was kept at room temperature, 50 and 80°C, respectively, for one week. The electrolyte separated from the powders was analyzed for the measurement of Mn content. The particle morphology of LiAl 0.24 Mn 1.76 O 3.98 S 0.02 powders before and after cycling at 80°C was observed using a field emission scanning electron microscope FESEM, Hitachi Co., S-4100. Mi-crostructures of individual oxide particles of the samples were also