Local Structure and First Cycle Redox Mechanism of Layered Li[sub 1.2]Cr[sub 0.4]Mn[sub 0.4]O[sub 2] Cathode Material

Abstract

Electrochemical, X-ray diffraction, and K and L edge X-ray absorption data are reported for the layered cathode material Li 1.2 Mn 0.4 Cr 0.4 O 2 . The structural data show that this material can be understood as a solid solution of the layered phases Li 2 MnO 3 and LiCrO 2 , comprising tretravalent Mn and trivalent Cr, with approximately 0.2 lithium incorporated in the transition metal layers. According to the analysis of the K edge extended X-ray absorption fine structure, lithium ions in the transition metal layers are clustered around Mn ions. L edge X-ray absorption near edge spectra show that in the first charge-discharge cycle chromium is the electrochemically active species, cycling between Cr 3ϩ and Cr 6ϩ . Manganese remains as Mn 4ϩ throughout charge and discharge.Layered lithium manganese oxides are of interest as cathodes for rechargeable lithium batteries due to the safety, low cost, and low toxicity of manganese-based materials. However, basic problems such as the collapse of the layer structure toward the spinel structure have not yet been solved. This collapse usually leads to poor rate performance and to evolution of a two-plateau voltage profile, both of which are undesirable for practical applications. 1 Recently the development of a novel layered oxide cathode material, Li 1.2 Cr 0.4 Mn 0.4 O 2 , was reported, showing high capacity and good cycling stability in lithium-ion cells. 2 The material belongs to the solid solution series Li 2ϩx Cr y Mn 2Ϫy O 4ϩ␦ first reported by Davidson et al.,3,4 corresponding to the formulation Li 3 CrMnO 5 using the notation given by Davidson et al. ͑in the present work we prefer to use the notation Li 1.2 Cr 0.4 Mn 0.4 O 2 because it relates better to the rock salt crystal structure, as discussed below͒. Davidson et al. have evaluated the solid solution range in Li 2ϩx Cr y Mn 2Ϫy O 4ϩ␦ from y ϭ 0.49 to y ϭ 1.46, and found that discharge capacity tends to increase with higher Cr/Mn ratio, up to 230 mAh/g. 3 Although lower Cr/Mn ratios gave lower capacities, high reversible capacities of up to 200 mAh/g were found in examples with a Cr/Mn ratio of around 1.0.The objective of the work reported here is to examine in greater detail the structure and electrochemistry of the Li 1.2 Cr 0.4 Mn 0.4 O 2 material. We are interested in this phase because the composition has shown high capacity and yet contains 50% of its transition metal content as Mn. During the first charge of this material, up to 270 mAh/g ͑corresponding to nearly 1 Li in Li 1.2 Cr 0.4 Mn 0.4 O 2 ͒ can be extracted. This is only possible if either side reactions occur or the average transition metal valence state in the charged cathode is higher than the expected tetravalent state. The work reported here uses a combination of X-ray diffraction, and X-ray absorption at the transition metal K and L edges, to investigate the crystal and electronic structure of Li 1.2 Cr 0.4 Mn 0.4 O 2 , as prepared and during the first charge-discharge cycle. X-ray diffraction ͑XRD͒ gives information about the long range o...

Preparation and Cyclic Performance of Li1.2(Fe0.16Mn0.32Ni0.32)O2Layered Cathode Material by the Mixed Hydroxide Method

“…It has been reported that with increasing c/a ratio beyond 4.90, the ordering of the layered structure increases and the crystal structure becomes more hexagonal. 15,16 In this study, the c/a values of all the samples were is an effective indicator of the disorder of the cations in the lattice, with higher ratios representing lower degrees of cation mixing. This intensity ratio linearly increased with increasing calcination temperature, implying that increased calcination temperatures facilitated better layering of the structures and reduced cation mixing.…”

“…Therefore, X-ray absorption near edge structure (XANES) analyses were carried out to compare changes of local structure around the Mn, Fe, and Ni atoms in Li 1.2 (Fe 0. 16 Mn 0.32 Ni 0.32 )O 2 samples. Figure 4 shows the Mn, Fe and Ni K-edge XANES spectra of Li 1.2 -(Fe 0.…”

“…Figure 4 shows the Mn, Fe and Ni K-edge XANES spectra of Li 1.2 -(Fe 0. 16 Mn 0.32 Ni 0.32 )O 2 together with those of some reference compounds which have known oxidation states. All spectra display two important pre-edge and main absorption features, labeled A and B in Figure 4.…”

Adipic Acid Assisted Sol-Gel Synthesis of Li1+x(Mn0.4Ni0.4Fe0.2)1-xO2(0 > x > 0.3) as Cathode Materials for Lithium Ion Batteries

“…It is well known that with increasing c/a ratio beyond 4.90, the ordering of the layered structure increases and the crystal structure becomes more hexagonal. 4 However, it is well known that the c/a value and cation mixing amount showed change with increase in Ni content. The higher Ni content resulted in worse hexagonal ordering along with a higher cation mixing.…”

“…[2][3][4] The olivine type LiFePO 4 is a promising cathode material for LIB due to its low cost and stable cyclic performance. But, low electronic conductivity and low cell voltage (~3 V) of LiFePO 4 confined its possibility to utilize in HEVs.…”