Morphology, Structure, and Electrochemistry of Solution-Derived LiMn[sub 0.5−x]Cr[sub 2x]Ni[sub 0.5−x]O[sub 2] for Lithium-Ion Cells

Abstract

In recent years LiMn 0.5 Ni 0.5 O 2 has emerged as a promising alternative to LiCoO 2 because of its lower cost, better high voltage stability, and improved thermal abuse characteristics. In this paper we report on Li͓Mn 0.5−x Cr 2x Ni 0.5−x ͔O 2 ͑0 ഛ 2x ഛ 0.2͒ compounds, which is formed by the partial substitution of nickel and manganese in LiMn 0.5 Ni 0.5 O 2 by chromium. The Li͓Mn 0.5−x Cr 2x Ni 0.5−x ͔O 2 particles, prepared by a chemical solution route, displayed a faceted morphology, with particle size increasing with increasing chromium content. Rietveld refinement of X-ray diffraction data from the oxides showed that the c lattice parameter of the layered ␣-NaFeO 2 -type structure increased, and lithium/nickel intermixing decreased, with increasing chromium content. Electrochemical cycling in the 3-4.3 V voltage window indicated that the discharge capacity was highest for the 2x = 0.05 oxide composition. Charge and discharge capacities decreased with increasing chromium content for electrodes cycled in the 3-4.8 V range, and cells containing the higher chromium content oxides showed significant polarization. Peaks corresponding to oxygen loss were observed in dQ/dV plots of cells containing the 2x = 0, 0.05, and 0.1 oxides, but not for the 2x = 0.2 oxide. The first cycle "irreversible" capacity observed for cells cycled up to 145 mAh/g capacity was recovered by deep-discharging the cells to voltages below 1.5 V. Oxidation state and local structural analysis of X-ray absorption spectroscopy data from the deep-discharged and as-prepared samples showed remarkable similarity, which indicated that the deep-discharge allowed the sample to almost regain its original state. Significant capacity loss was observed during electrochemical cycling, especially for the high chromium containing oxides, which appears to result from rate limitations induced by the coupling of lithium diffusion and chromium migration to and from the tetrahedral sites during the charge-discharge cycles.The use of rechargeable lithium-ion batteries for consumer electronic applications has increased dramatically ever since Sony Corp. introduced the first commercial cell in 1990. The search for alternative positive-electrode materials that are cheaper, safer, and more environmentally benign than the currently used LiCoO 2 is an active area of research in the rechargeable lithium-ion battery community. Among the various manganese-based materials being considered, a substantial amount of research has been done on manganese-based spinel, LiMn 2 O 4 , systems. 1,2 Although this material has excellent structural stability upon lithium cycling at ambient temperatures, its moderate capacity ͑ϳ120 mAh/g͒ and severe capacity fade at elevated temperatures ͑ϳ50°C͒ make it less appealing for practical applications. LiMnO 2 has also been examined as a potential alternative to LiCoO 2 . However, the thermodynamically stable phase with the composition LiMnO 2 crystallizes with an orthorhombic structure ͑space group Pmmn͒ and not with the rhombohedral structur...

Role of Cr3+/Cr6+ redox in chromium-substituted Li2MnO3·LiNi1/2Mn1/2O2 layered composite cathodes: electrochemistry and voltage fade

“…24 The overall increase in the particle size as a result of Cr substitutions is consistent with previous reports. 21,23 In Fig. 2a, all of the as-synthesized samples show a typical XRD pattern for LMR-NMC materials.…”

“…The Cr 3+ /Cr 6+ redox reaction can deliver three electrons per cation, which is higher than the twoelectron delivery of the Ni 2+ /Ni 4+ redox. 20,21,30 Therefore, more Li ions can be deintercalated from the Cr-substituted electrodes before the activation of Li 2 MnO 3 domains. Observing the higher charge compensation from the Cr 3+ / Cr 6+ redox reaction, one may expect higher total initial charge capacity for the Cr substituted samples assuming the same amount of Li 2 MnO 3 activation for all samples.…”

“…Furthermore, the three-electron transfer process of Cr 3+ /Cr 6+ redox could provide the charge compensation required for high capacity cycling and eliminate the necessity for the Li 2 MnO 3 activation, which triggers structural instability and voltage fade. Although the electrochemical performance of the Crcontaining layered-layered composite materials has been reported by several groups, [20][21][22][23] their voltage fade behavior has never been systematically evaluated, to the best of our knowledge. Therefore, we synthesized a series of Li 1.2 Ni 0.2Àx/2 Mn 0.6Àx/2 Cr x O 2 samples (0.5Li 2 MnO 3 $0.5LiNi 0.5Ày/2 Mn 0.5Ày/2 Cr y O 2 in composite notation) by the sol-gel method and tested the effect of Cr substitution on the electrochemical performance, particularly with respect to the voltage fade.…”

Review of the U.S. Department of Energy’s “Deep Dive” Effort to Understand Voltage Fade in Li- and Mn-Rich Cathodes

“…This process, related to the "re-entrant" environments described by NMR, may also explain why there is a minimum in the dashed curve of Figure 12 where discharging below ∼3.2 V may result in more reversibility due to higher driving forces. 23 (2) The cations are forced further along their migration path and come to occupy new sites (e.g., Li-layer octahedral sites). This changes the local structure and lowers the associated Li site energies with respect to further extraction/ insertion, thus contributing to voltage fade.…”

First-charge instabilities of layered-layered lithium-ion-battery materials

“…The migration of transition element ions to the lithium layer during delithiation in the layered systems LiMO 2 and Li 2 M 0 O 3 presents an obstacle to the design of materials that cycle without irreversible structural transformation. Some elements, such as Ga and Cr, 44,45 tend to migrate even in the absence of oxygen vacancies, with only minimal delithiation. Trivalent Mn in Li 1Àx MnO 2 has a tendency to migrate to the lithium layer, assisted by disproportionation.…”