Synthesis, thermal behavior, and dehydrogenation kinetics study of lithiated ethylenediamine.

The lithiation of ethylenediamine by LiH is a stepwise process to form the partially lithiated intermediates LiN(H)CH2 CH2 NH2 and [LiN(H)CH2 CH2 NH2 ][LiN(H)CH2 CH2 N(H)Li]2 prior to the formation of dilithiated ethylenediamine LiN(H)CH2 CH2 N(H)Li. A reversible phase transformation between the partial and dilithiated species was observed. One dimensional {Lin Nn } ladders and three-dimensional network structures were found in the crystal structures of LiN(H)CH2 CH2 NH2 and LiN(H)CH2 CH2 N(H)Li, respectively. LiN(H)CH2 CH2 N(H)Li undergoes dehydrogenation with an activation energy of 181±8 kJ mol(-1) , whereas the partially lithiated ethylenediamine compounds were polymerized and released ammonia at elevated temperatures. The dynamical dehydrogenation mechanism of the dilithiated ethylenediamine compounds was investigated by using the Johnson-Mehl-Avrami equation.

Lithiated primary amine--a new material for hydrogen storage.

A facile method for synthesizing crystalline lithiated amines by ball milling primary amines with LiH was developed. The lithiated amines exhibit an unprecedented endothermic dehydrogenation feature in the temperature range of 150-250 °C, which shows potential as a new type of hydrogen storage material. Structural analysis and mechanistic studies on lithiated ethylenediamine (Li2EDA) indicates that Li may mediate the dehydrogenation through an α,ß-LiH elimination mechanism, creating a more energy favorable pathway for the selective H2 release.

Structural Evolution of Lithium-Exchanged Na3(VO)2(PO4)2F Cathode under Operation in Sodium Ion Batteries.

Na superionic conductor (NASICON)-type Na3(VO)2(PO4)2F (NVOPF) exhibits excellent cycling stability for high-voltage sodium ion batteries. Various strategies have been developed to form ion-exchanged NVOPF which can enhance the ionic and electronic conductivity. However, the underlying ion transport mechanism and complex structural transitions during battery operation remained uninvestigated. In this work, we prepared lithium-exchanged NVOPF (namely NLVOPF) which shows improved ionic conductivity and increased capacity at high discharging rates. Solid-state nuclear magnetic resonance (SSNMR) revealed the distinctive presence of two kinds of Li-exchanged sites in the NLVOPF, which are attributed to the occupied lithium ions at the Na1 and Na2 sites (namely Li1 and Li2, respectively). The Li1 site was metastably replaced in the first cycle, yet the Li2 site participated in ion insertion/extraction in the subsequent cycles. Our characterizations show that the dynamic doping of lithium in NLVOPF could contribute to the improved cycling stability and capacity retention.

Alkali and alkaline-earth metal borohydride hydrazinates: synthesis, structures and dehydrogenation.

Four new borohydride hydrazinates, including NaBH4·NH2NH2, LiBH4·1/2NH2NH2, LiBH4·1/3NH2NH2 and Mg(BH4)2·3NH2NH2, were synthesized. NaBH4·NH2NH2 and Mg(BH4)2·3NH2NH2 possess monoclinic and trigonal structures, respectively, while LiBH4·1/2NH2NH2 and LiBH4·1/3NH2NH2 exhibit orthorhombic and monoclinic structures. The effects of composition on the dehydrogenation of hydrazinates were investigated. It is demonstrated that cations with high Pauling electronegativity hold hydrazine strongly in the vicinity of borohydride and result in direct dehydrogenation at elevated temperatures. Specifically, Mg(BH4)2 hydrazinates can directly generate hydrogen upon heating it under a flow of Ar; on the other hand, the Li and Na counterparts lost part or all of the hydrazine components under the same condition. In addition, reducing NH2NH2 content in the complexes leads to improved dehydrogenation properties. Mechanistic investigation of Mg(BH4)2 hydrazinates using isotopic labelling indicates that hydrogen desorption is via homogeneous dissociation of N-N bond of NH2NH2 followed by the establishment of B-N bond and combination of H(δ+) (N) and H(δ-) (B).

The chemical evolution of solid electrolyte interface in sodium metal batteries.

The solid electrolyte interface (SEI) formed on the anode is one of the key factors that determine the life span of sodium metal batteries (SMBs). However, the continuous evolution of SEI during charging/discharging processes complicates the fundamental understanding of its chemistry and structure. In this work, we studied the underlying mechanisms of the protection effect offered by the SEI derived from sodium difluoro(oxalato)borate (NaDFOB). In situ nuclear magnetic resonance (NMR) shows that the prior reduction of DFOB anion contributes to the SEI formation, and it suppresses the decomposition of carbonate solvents. Depth-profiling x-ray photoelectron spectroscopy and high-resolution solid-state NMR reveal that the DFOB anion is gradually turned into borate and fluoride-rich SEI with cycling. The protection effect of SEI reaches the optimum at 50 cycles, which triples the life span of SMB. The detailed investigations provide valuable guidelines for the SEI engineering.

Metal oxide and sulfide hollow spheres: layer-by-layer synthesis and their application in lithium-ion battery.

A novel layer-by-layer approach has been developed to synthesize polycrystalline SnO(2) hollow spheres with tunable shell thickness and size using SiO(2) spheres as a template. The surface of the SiO(2) spheres has been first modified by the polyelectrolyte, and subsequently, the compact SnO(2) layer has deposited on the surface of the SiO(2) spheres through a redox reaction because of the electrostatic attraction between the charged species. After HF etching treatment, the uniform SnO(2) hollow spheres have been obtained. The approach presented herein has been extended to synthesize other metal oxide and sulfide hollow spheres such as In(2)O(3) and ZnS. Moreover, the as-synthesized SnO(2) hollow spheres have been applied in lithium-ion battery and show improved performance compared with SnO(2) nanoparticles. The high surface area and stable hollow structure of the SnO(2) hollow spheres may be responsible for the improved performance.

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid-State NMR.

Surface and interfacial chemistry is of fundamental importance in functional nanomaterials applied in catalysis, energy storage and conversion, medicine, and other nanotechnologies. It has been a perpetual challenge for the scientific community to get an accurate and comprehensive picture of the structures, dynamics, and interactions at interfaces. Here, some recent examples in the major disciplines of nanomaterials are selected (e.g., nanoporous materials, battery materials, nanocrystals and quantum dots, supramolecular assemblies, drug-delivery systems, ionomers, and graphite oxides) and it is shown how interfacial chemistry can be addressed through the perspective of solid-state NMR characterization techniques.

Sodium-difluoro(oxalato)borate (NaDFOB): a new electrolyte salt for Na-ion batteries.

A new electrolyte salt, sodium-difluoro(oxalato)borate (NaDFOB), was synthesized and studied, which enables excellent reversible capacity and high rate capability when used in Na/Na0.44MnO2 half cells. NaDFOB has excellent compatibility with various common solvents used in Na-ion batteries, in strong contrast to the solvent dependent performances of NaClO4 and NaPF6. In addition, NaDFOB possesses good stability and generates no toxic or dangerous products when exposed to air and water. All these properties demonstrate that NaDFOB could be used to prepare high performance electrolytes for emerging Na-ion batteries.