Cation-disordered Rocksalt Research Articles

Origin of the Order-Disorder Transition and the Associated Anomalous Change of Thermopower in AgBiS2 Nanocrystals: A Combined Experimental and Theoretical Study.

Bulk AgBiS2 crystallizes in a trigonal crystal structure (space group, P3̅m1) at room temperature, which transforms to a cation disordered rock salt structure (space group, Fm3̅m) at ∼473 K. Surprisingly, at room temperature, a solution-grown nanocrystal of AgBiS2 crystallizes in a metastable Ag/Bi ordered cubic structure, which transforms to a thermodynamically stable disorded cubic structure at 610 K. Moreover, the order-disorder transition in nanocrystalline AgBiS2 is associated with an unusual change in thermopower. Here, we shed light on the origin of a order-disorder phase transition and the associated anomalous change of thermopower in AgBiS2 nanocrystals by using a combined experimental, density functional theory based first-principles calculation and ab initio molecular dynamics simulations. Positron-annilation spectroscopy indicates the presence of higher numbers of Ag vacancies in the nanocrystal compared to that of the bulk cubic counterpart at room temperature. Furthermore, temperature-dependent two-detector coincidence Doppler broadening spectroscopy and Doppler broadening of the annihilation radiation (S parameter) indicate that the Ag vacancy concentration increases abruptly during the order-disorder transition in nanocrystalline AgBiS2. At high temperature, a Ag atom shuttles between the vacancy and interstitial sites to form a locally disordered cation sublattice in the nanocrystal, which is facilitated by the formation of more Ag vacancies during the phase transition. This process increases the entropy of the system at higher vacancy concentration, which, in turn, results in the unusual rise in thermopower.

Spinel-rock salt transformation in LiCoMnO4-δ.

The transformation on heating LiCoMnO4, with a spinel structure, to LiCoMnO3, with a cation-disordered rock salt structure, accompanied by loss of 25% of the oxygen, has been followed using a combination of diffraction, microscopy and spectroscopy techniques. The transformation does not proceed by a topotactic mechanism, even though the spinel and rock salt phases have a similar, cubic close-packed oxygen sublattice. Instead, the transformation passes through two stages involving, first, precipitation of Li2MnO3, leaving behind a Li-deficient, Co-rich non-stoichiometric spinel and, second, rehomogenization of the two-phase assemblage, accompanied by additional oxygen loss, to give the homogeneous rock salt final product; a combination of electron energy loss spectroscopy and X-ray absorption near edge structure analyses showed oxidation states of Co2+ and Mn3+ in LiCoMnO3. Subsolidus phase diagram determination of the Li2O-CoOx-MnOy system has established the compositional extent of spinel solid solutions at approximately 500°C.

Synthesis and electrochemical properties of Li(1.3)Nb(0.3)V(0.4)O2 as a positive electrode material for rechargeable lithium batteries.

The binary system, xLi3NbO4-(1 - x)LiVO2, was first examined as an electrode material for rechargeable lithium batteries. The sample (x = 0.43) crystallizes into a cation-disordered rocksalt structure and delivers a reversible capacity of ca. 230 mA h g(-1), which originates from V(3+)/V(5+) redox with electrochemically inactive niobium ions.

High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure

Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for electric vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy density of lithium batteries. In the past decade, lithium-excess compounds, Li2MeO3 (Me = Mn(4+), Ru(4+), etc.), have been extensively studied as high-capacity positive electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co(3+), Ni(3+), etc.). Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examined as the host structure of a new series of high-capacity positive electrode materials for rechargeable lithium batteries. Approximately 300 mAh ⋅ g(-1) of high-reversible capacity at 50 °C is experimentally observed, which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochemically inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.

Cation disordered rock salt phase Li2CoTiO4 as a potential cathode material for Li-ion batteries

Li2CoTiO4 as a new titanate cathode material for Li-ion batteries was successfully synthesized by a sol–gel process. It had a cation disordered rock salt structure and showed an excellent cycle stability at room temperature. After coating with carbon, the Li2CoTiO4 possessed a high discharge capacity of 144.3 mAh g−1. The poor conductivity and lithium ion diffusion coefficient of pure Li2CoTiO4 are the main reasons for the limited electrochemical performance of the lithium batteries. The irreversible capacity loss during the first cycle was assigned to the irreversibility of the structural changes.

Formation of disordered and partially ordered LixCo1−xO

Analysis of XRD, TG, XANES and density measurement data show that a new, partially-ordered, rock salt-like phase, W, exists for LixCo1−xO, 0.075 ≤ x ≤ 0.24(2)–0.31(2), depending on temperature, with doping mechanism: 2Co2+ ↔ Li+ + Co3+. Phase W forms as ordered domains within a cation-disordered rock salt solid solution based on CoO; domain size depends on temperature and composition. Binary phase diagrams of LixCo1−xO and phase W solid solutions in air and N2 are presented.

Oxygen Nonstoichiometry and Phase Transitions in LiMn1.5Ni0.5O4 − δ

Crystal structure and oxygen stoichiometry in , a potential lithium-battery cathode, vary with temperature, as observed in samples quenched from different temperatures and by in situ diffraction and thermogravimetry techniques. When prepared in high pressure, this cation-ordered spinel is oxygen-stoichiometric, , space group . Upon heating between 650 and , increasing oxygen deficiency occurs exclusively in octahedra and bonds, which induces a volume increase of the 12d octahedra, a reduction of Mn as shown by X-ray absorption near-edge structure, equalization of and bond lengths, and disordering of Mn, Ni on octahedral sites. Hence, the transformation to space group , shown by Rietveld refinement of variable-temperature neutron diffraction data, is a direct consequence of oxygen loss from the structure. On further oxygen loss, a second phase transformation occurs to give a cation-deficient cubic rock salt phase, , at , which loses more oxygen at higher temperatures until, at , the material is essentially a stoichiometric, single-phase cation-disordered rock salt, space group . A second spinel phase persists in small amounts from 950 to . Differences in electrochemical behavior depend on sample preparation and correlate with the oxygen content of when used as a cathode in Li test cells.

Li 2MTiO 4 ( M=Mn, Fe, Co, Ni): New cation-disordered rocksalt oxides exhibiting oxidative deintercalation of lithium. Synthesis of an ordered Li 2NiTiO 4

We describe the synthesis and characterization of a new series of oxides, Li 2 MTiO 4 ( M=Mn, Fe, Co, Ni) that crystallize in the rocksalt structure. For M=Ni, we have also obtained a low-temperature modification that adopts a Li 2SnO 3-type structure. All the phases, excepting M=Ni, undergo oxidative deinsertion of lithium in air/O 2 at elevated temperatures (>150°C), yielding Li MTiO 4 ( M=Mn, Fe) spinels and a spinel-like Li 1+ x CoTiO 4 as final products.

Reversible spinel to rock salt transition in LiCoMnOδ by oxygen (de) intercalation

LiCoMnO4 with the spinel structure loses oxygen on heating in air above ∼570 °C and by ∼1050 °C reaches the approximate stoichiometry ‘LiCoMnO3’ which has the cation-disordered rock salt structure. On cooling, the transformation is fully reversible.