Structural Investigation of Layered Li[sub 1−δ]Mn[sub x]Cr[sub 1−x]O[sub 2] by XANES and In Situ XRD Measurements

Abstract

A wide solid solution of lithium manganese chromium oxides, LiMn x Cr 1Ϫx O 2 (0 р x р 0.6), having the layered ␣-NaFeO 2 structure, was synthesized by employing the emulsion drying method, and the Mn and Cr oxidation states of the prepared powders were identified by combination of the Rietveld analysis of the X-ray diffraction ͑XRD͒ data and X-ray absorption near edge spectroscopic ͑XANES͒ analysis. A reversible structural change of LiMn 0.6 Cr 0.4 O 2 was observed using the in situ XRD technique after the first charge. With increasing Mn contents, the reversible capacity increased due to the redox couples of Mn 3ϩ/4ϩ and Cr 3ϩ/6ϩ , as observed by the XANES spectra measured as a function of the Li contents during the first cycle. The rechargeable capacity for LiMn 0.6 Cr 0.4 O 2 was about 140 mAh g Ϫ1 between 4.3 and 2.7 V vs. Li for 30 cycles. The measured chemical diffusion coefficient for LiMn 0.6 Cr 0.4 O 2 was much higher than those for LiAl x Mn 2Ϫx O 4 and LiAl x Co 1Ϫx O 2 . This emulsion drying synthesis is an excellent powder preparation alternative method of high capacity cathode materials to be used in a Li-ion secondary battery.The Li-Mn-Cr-O system has been studied extensively by several research groups. 1-7 Earlier studies have shown that the partial substitution of the Mn sites of the tetragonal spinel Li 2 Mn 2 O 4 by Cr produced a continuous solid solution of Li 2 Cr x Mn 2Ϫx O 4 . 1-3 The final products of this system depend on the Cr doping amount and calcination conditions; different Cr doping amounts resulted in different crystal systems, such as tetragonal (I4 1 /amd), distorted hexagonal (C2/m), and hexagonal (R3 m). 2,3 The corresponding electrochemical properties of the prepared powders were also quite different. For example, a lower Cr doping level in the tetragonal Li 2 Cr x Mn 2Ϫx O 4 showed a two-step Li ϩ deintercalation/intercalation during cycling with two potential plateaus at 3 and 4 V, which were mainly caused by the effect of the Jahn-Teller ion, Mn 3ϩ . A dilution of the effect was achieved by higher level Cr 3ϩ doping in Li 2 Cr x Mn 2Ϫx O 4 where crystallization occurred to produce a distorted hexagonal system. A much higher level doping led to increased symmetry from distorted hexagonal (C2/m) to hexagonal (R3 m) where the cooperative Jahn-Teller distortion was absent. Obviously, the layered compounds showed simple S-shaped charge/ discharge curves and stable cycling behavior. According to the obtained capacity, one is able to postulate that both the Mn 3ϩ and Cr 3ϩ elements were electrochemically active. Unfortunately, the pure hexagonal structured compound, LiCr 0.75 Mn 0.25 O 2 , delivered a capacity of about 100 mAh g Ϫ1 ͑current density: 3.6 mA g Ϫ1 at 25°C͒.Li 3 MnCrO 5 ͑can also be written as Li 1.2 Cr 0.4 Mn 0.4 O 2 ) is a solid solution series, Li 2 MnO 3 -LiCrO 2 . 4,5,7 Obviously, both compounds are famous as electrochemically inactive materials. Nonetheless, once Li 1.2 Cr 0.4 Mn 0.4 O 2 is formed, the hexagonal structured layer compound provides excellent cap...

Ion Diffusion in Solids Probed by Muon-Spin Spectroscopy

“…7(c) For LiCrO 2 , on the other hand, ν is found to be almost T -independent from 275 K to the highest T measured (475 K). This demonstrates that both µ + s and Li + ions are static in the LiCrO 2 lattice at least until 475 K. This is very consistent with the electrochemical behavior of LiCrO 2 [29,30]. Moreover, since E a for LiCrO 2 (19 meV) is very small compared with those for the other compounds (70 meV-115 meV), the small increase in ν with T for LiCrO 2 is likely caused by another mechanism, such as, an electron contribution.…”

“…Furthermore, there is no information on the T dependence of a reaction surface of the Li x CoO 2 electrode in a liquid electrolyte. Therefore, we compare the µ + SR result only with the prediction at 300 K. Figure 6 shows the ZF-and longitudinal field (LF-) µ + SR spectrum for the Li 0.98 Ni 1.02 O 2 sample obtained at 300 K, 350 K, and 450 K, together with that for LiCrO 2 at 450 K. Here, LiCrO 2 poses the same R3m crystal structure as LiNiO 2 , but is electrochemically inactive [29,30], i.e. Li + ions are not deintercalated from the lattice by a usual electrochemical reaction.…”

Electrochemical behavior of current collectors for lithium batteries in non-aqueous alkyl carbonate solution and surface analysis by ToF-SIMS

“…These results would reflect the dissolution of hexavalent Cr from the formed passive layer. Myung et al [27] proved that trivalent Cr could be oxidized to hexavalent Cr at such a higher potential, as confirmed by X-ray absorption spectroscopy. As analyzed by ToF-SIMS in Fig.…”

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…[69][70][71] When the ionic liquid electrolyte NaFSA-KFSA (FSA represents the anion bis(fl uorosulfonyl)amide) was adopted as a thermally stable electrolyte, the NaCrO 2 electrode exhibited a stable discharge capacity of 113 mA h g −1 at a current density of 125 mA h g −1 and 90 °C with a 98.5% capacity retention. [ 69 ] A recent report by Yu et al showed that carbon-coated NaCrO 2 exhibits excellent capacity retention over 50 cycles with a reversible capacity of 120 mA h g −1 at a current density of 20 mA g −1 .…”