Unexpectedly Large Contribution of Oxygen to Charge Compensation Triggered by Structural Disordering: Detailed Experimental and Theoretical Study on a Li3NbO4-NiO Binary System.

Dependence on lithium-ion batteries for automobile applications is rapidly increasing. The emerging use of anionic redox can boost the energy density of batteries, but the fundamental origin of anionic redox is still under debate. Moreover, to realize anionic redox, many reported electrode materials rely on manganese ions through π-type interactions with oxygen. Here, through a systematic experimental and theoretical study on a binary system of Li3NbO4-NiO, we demonstrate for the first time the unexpectedly large contribution of oxygen to charge compensation for electrochemical oxidation in Ni-based materials. In general, for Ni-based materials, e.g., LiNiO2, charge compensation is achieved mainly by Ni oxidation, with a lower contribution from oxygen. In contrast, for Li3NbO4-NiO, oxygen-based charge compensation is triggered by structural disordering and σ-type interactions with nickel ions, which are associated with a unique environment for oxygen, i.e., a linear Ni-O-Ni configuration in the disordered system. Reversible anionic redox with a small hysteretic behavior was achieved for LiNi2/3Nb1/3O2 with a cation-disordered Li/Ni arrangement. Further Li enrichment in the structure destabilizes anionic redox and leads to irreversible oxygen loss due to the disappearance of the linear Ni-O-Ni configuration and the formation of unstable Ni ions with high oxidation states. On the basis of these results, we discuss the possibility of using σ-type interactions for anionic redox to design advanced electrode materials for high-energy lithium-ion batteries.

Elucidating Influence of Mg- and Cu-Doping on Electrochemical Properties of O3-Nax [Fe,Mn]O2 for Na-Ion Batteries.

Although O3-NaFe1/2 Mn1/2 O2 delivers a large capacity of over 150 mAh g-1 in an aprotic Na cell, its moist-air stability and cycle stability are unsatisfactory for practical use. Slightly Na-deficient O3-Na5/6 Fe1/2 Mn1/2 O2 (O3-Na5/6 FeMn) and O3-Na5/6 Fe1/3 Mn1/2 Me1/6 O2 (Me = Mg or Cu, O3-FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist-air stability. O3-Na5/6 FeMn, O3-FeMnCu, and O3-FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g-1 , respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3-FeMnMg participate in the redox reaction more apparently than O3-Na5/6 FeMn. X-ray diffraction studies unveil the formation of a P-O intergrowth phase during charging up to >4.0 V.

Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li1.2Ni0.2Mn0.6O2 during charge-discharge cycles.

Lithium- and manganese-rich layered oxides (LMRs) are promising positive electrode materials for next-generation rechargeable lithium-ion batteries. Herein, the structural evolution of Li1.2Ni0.2Mn0.6O2 during the initial charge-discharge cycle was examined using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy to elucidate the unique delithiation behavior. The pristine material contained a composite layered structure composed of Ni-free and Ni-doped Li2MnO3 and LiMO2 (M = Ni, Mn) nanoscale domains, and Li ions were sequentially and inhomogeneously extracted from the composite structure. Delithiation from the LiMO2 domain was observed in the potential slope region associated with the Ni2+/Ni4+ redox couple. Li ions were then extracted from the Li2MnO3 domain during the potential plateau and remained mostly in the Ni-doped Li2MnO3 domain at 4.8 V. In addition, structural transformation into a spinel-like phase was partly observed, which is associated with oxygen loss and cation migration within the Li2MnO3 domain. During Li intercalation, cation remigration and mixing resulted in a domainless layered structure with a chemical composition similar to that of LiNi0.25Mn0.75O2. After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.

Nanosize Cation-Disordered Rocksalt Oxides: Na2 TiO3 -NaMnO2 Binary System.

To realize the development of rechargeable sodium batteries, new positive electrode materials without less abundant elements are explored. Enrichment of sodium contents in host structures is required to increase the theoretical capacity as electrode materials, and therefore Na-excess compounds are systematically examined in a binary system of Na2 TiO3 -NaMnO2 . After several trials, synthesis of Na-excess compounds with a cation disordered rocksalt structure is successful by adapting a mechanical milling method. Among the tested electrode materials, Na1.14 Mn0.57 Ti0.29 O2 in this binary system delivers a large reversible capacity of ≈200 mA h g-1 , originating from reversible redox reactions of cationic Mn3+ /Mn4+ and anionic O2- /On - redox confirmed by X-ray absorption spectroscopy. Holes in oxygen 2p orbitals, which are formed by electrochemical oxidation, are energetically stabilized by electron donation from Mn ions. Moreover, reversibility of anionic redox is significantly improved compared with a former study on a binary system of Na3 NbO3 -NaMnO2 tested as model electrode materials.

Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance.

Conversion-type iron trifluoride (FeF3) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF3 operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF3 electrode material. Bulk analyses of FeF3 upon cycling reveal incomplete oxidation to Fe3+ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF3 reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF3 shows better performance in an electrolyte comprising LiBF4 solute in cyclic carbonate solvents than in chain carbonate-containing LiPF6 as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF3 electrode.

Influence of FCP-COMPLEX on bond strength and the adhesive-artificial caries-affected dentin interface.

FCP-COMPLEX is a newly developed solution containing fluoride, calcium, and phosphoric acid that has the potential to reinforce caries-affected dentin. This study evaluated the effect of FCP-COMPLEX on micro-tensile bond strength (µTBS) and acid-challenge at the dentin-adhesive interface. FCP-COMPLEX, 2% NaF, and distilled water were applied to artificial caries-affected dentin (ACAD) and the effect on acid-induced damage after resin composite restoration was observed. Scanning electron microscopy and X-ray absorption fine structure (XAFS) were used to evaluate tooth morphology. The µTBS test revealed no effect of FCP-COMPLEX either immediately or after 3 months' storage. The area of acid damage in caries-affected dentin was reduced by FCP-COMPLEX. XAFS analysis revealed that absorbed fluorine on the surface would form CaF2. In conclusion, FCP-COMPLEX significantly reduced the damage of acidic attack at the ACAD-adhesive interface, while the µTBS value was maintained after storage.

Nanostructuring one-dimensional and amorphous lithium peroxide for high round-trip efficiency in lithium-oxygen batteries.

The major challenge facing lithium-oxygen batteries is the insulating and bulk lithium peroxide discharge product, which causes sluggish decomposition and increasing overpotential during recharge. Here, we demonstrate an improved round-trip efficiency of ~80% by means of a mesoporous carbon electrode, which directs the growth of one-dimensional and amorphous lithium peroxide. Morphologically, the one-dimensional nanostructures with small volume and high surface show improved charge transport and promote delithiation (lithium ion dissolution) during recharge and thus plays a critical role in the facile decomposition of lithium peroxide. Thermodynamically, density functional calculations reveal that disordered geometric arrangements of the surface atoms in the amorphous structure lead to weaker binding of the key reaction intermediate lithium superoxide, yielding smaller oxygen reduction and evolution overpotentials compared to the crystalline surface. This study suggests a strategy to enhance the decomposition rate of lithium peroxide by exploiting the size and shape of one-dimensional nanostructured lithium peroxide.

Heterogeneous catalase-like activity of gold(i)-cobalt(iii) metallosupramolecular ionic crystals.

Unique heterogeneous catalase-like activity was observed for metallosupramolecular ionic crystals [AuI4CoIII2(dppe)2(d-pen)4]X n ([1]X n ; dppe = 1,2-bis(diphenylphosphino)ethane; d-pen = d-penicillaminate; X n = (Cl-)2, (ClO4-)2, (NO3-)2 or SO42-) consisting of AuI4CoIII2 complex cations, [1]2+, and inorganic anions, X- or X2-. Treatment of the ionic crystals with an aqueous H2O2 solution led to considerable O2 evolution with a high turnover frequency of 1.4 × 105 h-1 for the heterogeneous cobalt complexes, which was dependent on their size and shape as well as the arrangement of cationic and anionic species. These dependencies were rationalized by the presence of cobalt(ii) centers on the crystal surface and their efficient exposure on the (111) plane rather than the (100) plane based on morphological and theoretical studies.

Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries.

Further increase in energy density of lithium batteries is needed for zero emission vehicles. However, energy density is restricted by unavoidable theoretical limits for positive electrodes used in commercial applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extraction results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic conductivity. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g-1 based on solid-state redox reaction of oxide ions.

Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with CeO2 Nanoparticles in Li-O2 Batteries.

In lithium-oxygen (Li-O2) batteries, it is believed that lithium peroxide (Li2O2) electrochemically forms thin films with thicknesses less than 10 nm resulting in capacity restrictions due to limitations in charge transport. Here we show unexpected Li2O2 film growth with thicknesses of ∼60 nm on a three-dimensional carbon nanotube (CNT) electrode incorporated with cerium dioxide (ceria) nanoparticles (CeO2 NPs). The CeO2 NPs favor Li2O2 surface nucleation owing to their strong binding toward reactive oxygen species (e.g., O2 and LiO2). The subsequent film growth results in thicknesses of ∼40 nm (at cutoff potential of 2.2 V vs Li/Li(+)), which further increases up to ∼60 nm with the addition of trace amounts of H2O that enhances the solution free energy. This suggests the involvement of solvated superoxide species (LiO2(sol)) that precipitates on the existing Li2O2 films to form thicker films via disproportionation. By comparing toroidal Li2O2 formed solely from LiO2(sol), the thick Li2O2 films formed from surface-mediated nucleation/thin-film growth following by LiO2(sol) deposition provides the benefits of higher reversibility and rapid surface decomposition during recharge.

A perfluorinated moiety-grafted carbon nanotube electrode for the non-aqueous lithium-oxygen battery.

A perfluorinated alkyl chain grafted to carbon nanotubes is employed in the lithium-oxygen (Li-O2) cell. We demonstrate a highly localized Li-O2 electrochemical reaction in close proximity to the perfluorinated moiety owing to its high O2 affinity. This hydrophobic modification can provide an enhancement in capacity for a very thin microbattery system.

A structured three-dimensional polymer electrolyte with enlarged active reaction zone for Li-O2 batteries.

The application of conventional solid polymer electrolyte (SPE) to lithium-oxygen (Li-O2) batteries has suffered from a limited active reaction zone due to thick SPE and subsequent lack of O2 gas diffusion route in the positive electrode. Here we present a new design for a three-dimensional (3-D) SPE structure, incorporating a carbon nanotube (CNT) electrode, adapted for a gas-based energy storage system. The void spaces in the porous CNT/SPE film allow an increased depth of diffusion of O2 gas, providing an enlarged active reaction zone where Li(+) ions, O2 gas, and electrons can interact. Furthermore, the thin SPE layer along the CNT, forming the core/shell nanostructure, aids in the smooth electron transfer when O2 gas approaches the CNT surface. Therefore, the 3-D CNT/SPE electrode structure enhances the capacity in the SPE-based Li-O2 cell. However, intrinsic instability of poly(ethylene oxide) (PEO) of the SPE matrix to superoxide (O2(·-)) and high voltage gives rise to severe side reactions, convincing us of the need for development of a more stable electrolyte for use in this CNT/SPE design.

Promoting formation of noncrystalline Li2O2 in the Li-O2 battery with RuO2 nanoparticles.

Low electrical efficiency for the lithium-oxygen (Li-O2) electrochemical reaction is one of the most significant challenges in current nonaqueous Li-O2 batteries. Here we present ruthenium oxide nanoparticles (RuO2 NPs) dispersed on multiwalled carbon nanotubes (CNTs) as a cathode, which dramatically increase the electrical efficiency up to 73%. We demonstrate that the RuO2 NPs contribute to the formation of poorly crystalline lithium peroxide (Li2O2) that is coated over the CNT with large contact area during oxygen reduction reaction (ORR). This unique Li2O2 structure can be smoothly decomposed at low potential upon oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the more typical Li2O2 structure with a large size, small CNT contact area, and insulating crystals.

Phylogeography of Japanese horse chestnut (Aesculus turbinata) in the Japanese Archipelago based on chloroplast DNA haplotypes.

Japanese horse chestnut (Aesculus turbinata: Hippocastanaceae) is one of the typical woody plants that grow in temperate riparian forests in the Japanese Archipelago. To analyze the phylogeography of this plant in the Japanese Archipelago, we determined cpDNA haplotypes for 337 samples from 55 populations covering the entire distribution range. Based on 1,313 bp of two spacers, we determined ten haplotypes that are distinguished from adjacent haplotypes by one or two steps. Most of the populations had a single haplotype, suggesting low diversity. Spatial analysis of molecular variance suggested three obvious phylogeographic structures in western Japan, where Japanese horse chestnut is scattered and isolated in mountainous areas. Conversely, no clear phylogeographic structure was observed from the northern to the southern limit of this species, including eastern Japan, where this plant is more common. Rare and private haplotypes were also found in southwestern Japan, where Japanese horse chestnuts are distributed sparsely. These findings imply that western Japan might have maintained a relatively large habitat for A. turbinata during the Quaternary climatic oscillations, while northerly regions could not.