Development of in situ high resolution NMR: Proof-of-principle for a new (spinning) cylindrical mini-pellet approach applied to a Lithium ion battery.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful technique for characterizing the local structure and dynamics of battery and other materials. It has been widely used to investigate bulk electrode compounds, electrolytes, and interfaces. Beside common ex situ investigations, in situ and operando techniques have gained considerable importance for understanding the reaction mechanisms and cell degradation of electrochemical cells. Herein, we present the recent development of in situ magic angle spinning (MAS) NMR methodologies to study batteries with high spectral resolution, setting into context possible advances on this topic. A mini cylindrical cell type insert for 4 mm MAS rotors is introduced here, being demonstrated on a Li/VO2F electrochemical system, allowing the acquisition of high-resolution 7Li MAS NMR spectra, spinning the electrochemical cell up to 15 kHz.

Fluorine-Substituted Halide Solid Electrolytes with Enhanced Stability toward the Lithium Metal.

The high ionic conductivity and good oxidation stability of halide-based solid electrolytes evoke strong interest in this class of materials. Nonetheless, the superior oxidative stability compared to sulfides comes at the expense of limited stability toward reduction and instability against metallic lithium anodes, which hinders their practical use. In this context, the gradual fluorination of Li2ZrCl6-xFx (0 ≤ x ≤ 1.2) is proposed to enhance the stability toward lithium-metal anodes. The mechanochemically synthesized fluorine-substituted compounds show the expected distorted local structure (M2-M3 site disorder) and significant change in the overall Li-ion migration barrier. Theoretical calculations reveal an approximate minimum energy path for Li2ZrCl6-xFx (x = 0 and 0.5) with an increase in the Li+ migration energy barrier for Li2ZrCl5.5F0.5 in comparison to Li2ZrCl6. However, it is found that the fluorine-substituted compound exhibits substantially lower polarization after 800 h of lithium stripping and plating owing to enhanced interfacial stability against the lithium metal, as revealed by density functional theory and ex situ X-ray photoelectron spectroscopy, thanks to the formation of a fluorine-rich passivating interphase.

Design and Tuning of the Electrochemical Properties of Vanadium-Based Cation-Disordered Rock-Salt Oxide Positive Electrode Material for Lithium-Ion Batteries.

Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM2+0.5V0.54+O2 stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM2+0.5V4+0.5O2 (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g-1, respectively. In comparison, for the disordered rock-salt Li2VO3, an average discharge voltage of ∼2.2 V with V5+/4+ redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

Interface in Solid-State Lithium Battery: Challenges, Progress, and Outlook.

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes promise improved safety, higher energy density, longer cycle life, and lower cost than conventional Li-ion batteries. However, their practical application is hampered by the high resistance arising at the solid-solid electrode-electrolyte interface. Although the exact mechanism of this interface resistance has not been fully understood, various chemical, electrochemical, and chemo-mechanical processes govern the charge transfer phenomenon at the interface. This paper reports the interfacial behavior of the lithium and the cathode in oxide and sulfide inorganic solid-electrolytes and how that affects the overall battery performance. An overview of the recent reports dealing with high resistance at the anodic and cathodic interfaces is presented and the scientific and engineering aspects of the approaches adopted to solve the issue are summarized.

Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems.

Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3 ̅m LiNi0.5M0.5O2 positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiM+3O2 with metals active for +3/+4 redox couples is extended to ternary oxides with LiA0.5B0.5O2 with A = Ni2+ and B = Ti4+, Zr4+, and V+4 to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3̅ m cation-disordered rock-salt positive electrode materials is presented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg-1 corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V4+/V5+. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration.

Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries.

Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment because of their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize phase-pure vanadium oxyfluoride (VO2F). The structure of VO2F was identified by Rietveld refinement of the powder X-ray diffraction (XRD) pattern. It crystallizes in a perovskite-type structure with disorder of the oxide and fluoride ions. The as-synthesized VO2F was tested as a cathode material for lithium ion batteries after being surface-coated with few-layer graphene. The VO2F delivered a first discharge capacity of 254 mA h g(-1) and a reversible capacity of 208 mA h g(-1) at a rate of C/20 for the first 20 cycles with an average discharge voltage of 2.84 V, yielding an energy density of 591 W h kg(-1). Improved rate capability that outperforms the previous report has been achieved, showing a discharge capacity of 150 mA h g(-1) for 1 C. The structural changes during lithium insertion and extraction were monitored by ex-situ XRD analysis of the electrodes discharged and charged to various stages. Lithium insertion results in an irreversible structural change of the anion lattice from (3)/4 cubic close packing to hexagonal close packing to accommodate the inserted lithium ions while keeping the overall space-group symmetry. For the first time we have revealed a structural change for the ReO3-type structure of as-prepared VO2F to the RhF3 structure after lithiation/delithiation, with structural changes that have not been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds.

Performance study of magnesium-sulfur battery using a graphene based sulfur composite cathode electrode and a non-nucleophilic Mg electrolyte.

Here we report for the first time the development of a Mg rechargeable battery using a graphene-sulfur nanocomposite as the cathode, a Mg-carbon composite as the anode and a non-nucleophilic Mg based complex in tetraglyme solvent as the electrolyte. The graphene-sulfur nanocomposites are prepared through a new pathway by the combination of thermal and chemical precipitation methods. The Mg/S cell delivers a higher reversible capacity (448 mA h g(-1)), a longer cyclability (236 mA h g(-1) at the end of the 50(th) cycle) and a better rate capability than previously described cells. The dissolution of Mg polysulfides to the anode side was studied by X-ray photoelectron spectroscopy. The use of a graphene-sulfur composite cathode electrode, with the properties of a high surface area, a porous morphology, a very good electronic conductivity and the presence of oxygen functional groups, along with a non-nucleophilic Mg electrolyte gives an improved battery performance.

Controlled synthesis of linear and branched Au@ZnO hybrid nanocrystals and their photocatalytic properties.

Colloidal Au@ZnO hybrid nanocrystals with linear and branched shape were synthesized. The number of ZnO domains on the Au seeds can be controlled by the solvent mixture. Imidazole-functionalized Au@ZnO hybrid nanocrystals were soluble in water and exhibited a greatly enhanced photocatalytic activity compared to ZnO nanocrystals. The pristine heterodimeric NPs were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-Vis spectroscopy.