Guerman Popov

The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium–oxygen batteries

Na0.44MnO2 nanowires were acid leached in nitric acid, and dehydrated by heat treatment to induce controllable defect formation as monitored by high resolution TEM studies. The charge–discharge tests using these materials as catalysts (or “promoters”) in rechargeable lithium–oxygen batteries (in non-carbonate electrolytes) showed that a high defect concentration results in a doubling of the reversible energy storage capacity up to 11 000 mA h g−1, and lowered overpotentials for oxygen evolution. The role of the defects/vacancies in determining oxygen reduction behavior is highlighted.

Lithium metal fluorosulfate polymorphs as positive electrodes for Li-ion batteries: synthetic strategies and effect of cation ordering

Transition-metal fluorosulfates are currently being extensively explored for their use as cathodes in Li-ion batteries. Several new polymorphs of LiMSO4F (M = Fe, Mn, Zn) crystallizing in the tavorite, triplite and sillimanite structures have captured much recent interest, but synthetic access is limited and the underlying phase stability and ion transport in these materials are poorly understood. Here we report that solvothermal routes to LiMSO4F (M = Fe, Mn, Zn) offer significant advantage over both exotic ionothermal methods and solid state synthesis by enabling greater control of the chemistry. We show new limits for the onset of triplite crystallization, and report new phases in the Li[Fe,Zn]SO4F system that enable a fuller understanding of the complex chemistry and thermodynamics underlying these fascinating materials. The transformation of LiFeSO4F from the tavorite to the triplite polymorph is triggered in the absence of any substituents, proving that tavorite is an intermediate in the reaction pathway. As a result of structural changes between tavorite and triplite, their Li+ transport paths are quite different. Combined X-ray/neutron diffraction studies of the triplites suggest that distinct inter-site zig-zag paths must be involved, owing to complete cation disorder that impacts the electrochemical behavior.

Ultra-rapid microwave synthesis of triplite LiFeSO4F

Quick, effective synthesis of the 4 V Li-ion battery cathode material, triplite LiFeSO4F, takes place via facile conversion of the defect-peppered nanocrystalline tavorite precursor that forms on ultra-rapid microwave heating (10 min) of FeSO4·H2O/LiF. We propose a mechanism for its unique phase transformation to the triplite that occurs as a consequence of the disorder and hydroxyl defects induced by the fast nucleation. The electrochemical properties of the resultant triplite exhibits a doubling of its practical gravimetric capacity compared to the material prepared by conventional methods.

Crystallographic and magnetic structure of the Sr2MnReO6 double perovskite

The crystal and magnetic structure of the ordered double perovskite Sr2MnReO6 was investigated by neutron diffraction. Monoclinic (space group P 21/n) distortion of the parent cubic double perovskite structure was revealed, and the refined Mn?O?Re bond angles deviate significantly from 180?. The monoclinic distortion produces antisymmetric exchange interactions, leading to a canted magnetic structure, for which a possible model is proposed and refined. No structural transitions were observed upon cooling or in an external magnetic field. Complementary x-ray synchrotron diffraction data support the neutron diffraction findings.

Effects of vacancy concentration on the magnetic and transport properties of (La1−xPbx)1−y□yMnO3

Abstract (La1−xPbx)1−y□yMnO3 with x=0.05–0.5 and y=0, 0.05, 0.1 (where □ is a vacancy) was studied to evaluate the effects of A-site vacancies on the physical properties. In this system manganese perovskites form with tolerance factors close to 1 and low A-site cation size mismatch due to similarities in the effective ionic radii of La3+ and Pb2+. Increasing vacancy concentration indicates no significant effect on the lattice parameters or volume. However, the vacancies introduce a greater A-site cation size mismatch, which leads to a lowering of the ferromagnetic and metal–insulator transition temperatures, although the transitions are not broadened with increasing vacancy content. Due to the vacancies a distribution of Mn–O–Mn angles and Mn–O distances are created, and long range order in (La1−xPbx)1−y□yMnO3 appears to be determined by Mn–O–Mn angles and Mn–O distances which most distort from 180° and are the longest, respectively, in the structure.

Synthesis of LnMn2O5 (Ln = Nd, Pr) crystals using fused salt electrolysis

Abstract A simple method of synthesis of LnMn 2 O 5 (Ln = Nd, Pr) is presented. Mixtures of sodium molybdate, molybdenum(VI) oxide, MnCO 3 , and Nd 2 O 3 or Pr 6 O 11 were electrolyzed for 3–24 h at 930–1020°C with platinum electrodes. Single crystals of LnMn 2 O 5 (Ln = Nd, Pr) with an edge up to 1.5 mm formed at the anode. The crystal structures of NdMn 2 O 5 and PrMn 2 O 5 were refined by Rietveld profile analysis in space group Pbam (Z = 4); the lattice parameters are a = 7.4970(2) A, b = 8.6086(2) A, c = 5.6963(1) A and a = 7.5384(2) A, b = 8.6319(2) A, c = 5.7022(1) A, respectively. The magnetic susceptibility measurements show an antiferromagnetic transition at 25 K for PrMn 2 O 5 . Both the NdMn 2 O 5 and PrMn 2 O 5 crystals are insulating.

Micromagnetic and magnetoresistance studies of ferromagnetic La0.83Sr0.13MnO2.98 crystals

Scanning probe microscopy was used to investigate the surface topography and micromagnetic structure of La 0 . 8 3 Sr 0 . 1 3 MnO 2 . 9 8 single crystals. The crystals were grown by fused-salt electrolysis and characterizedby chemical analysis, x-ray-diffraction, magnetic and transport measurements, Surface topography of as-grown crystals exhibits well-developed surface corrugations due to extensive twinning. Magnetic force microscopy (MFM) images show that magnetic-domain boundaries are pinned to the crystallographic twins; a small number of unpinned boundaries are observed, The statistical analysis of domain boundary angle distribution is consistent with cubic magnetocrystalline anisotropy for this material. Unusual magnetization behavior in the vicinity of topological defects on the surface is reported. MFM contrast was found to disappear above the ferromagnetic Curie temperature; formation of magnetic structure comprised of Bloch walls of opposite chiralities was observed after quenching.

Comparison of Young's modulus and specific heat anomalies at the magnetic transition in α′-NaV2O5

Abstract We have measured Youngs modulus (using a vibrating reed technique) and the specific heat (using ac calorimetry) on the same crystals of α′-NaV 2 O 5 at its T c =34 K magnetic phase transition. Both properties exhibit large, unsymmetrical, and sample-dependent anomalies. While the specific heat results suggest tricritical behavior of the transition, large fluctuation effects are observed in the modulus above T c . Fits of the modulus in terms of the specific heat, entropy, and free energy suggest that fluctuations are strongly stress- and sample-dependent.

Stabilization of Lithium Transition Metal Silicates in the Olivine Structure

While olivine LiFePO4 shows amongst the best electrochemical properties of Li-ion positive electrodes with respect to rate behavior owing to facile Li+ migration pathways in the framework, replacing the [PO4]3- polyanion with a silicate [SiO4]4- moiety in olivine is desirable. This could allow additional alkali content and hence electron transfer, and increase the capacity. Herein we explore the possibility of a strategy toward new cathode materials and demonstrate the first stabilization of a lithium transition metal silicate (as a pure silicate) in the olivine structure type. Using LiInSiO4 and LiScSiO4 as the parent materials, transition metal (Mn, Fe, Co) substitutions on the In/Sc site were investigated by computational modeling via atomic scale simulation. Transition metal substitution was found to be only favorable for Co, a finding confirmed by the successful solid state synthesis of olivine LixInyCo2-x-ySiO4. Stabilization of the structure was achieved by entropy provided by cation disorder.