Structural, electronic, and optical properties of three types Ca3N2 from first-principles study.

CONTEXT: To find the potential value of Ca3N2 in the field of optoelectronics, the physical properties of Ca3N2 will be analyzed. It can be concluded from the electronic properties that the Ca-N bonds of α-Ca3N2 are more stable than those of δ-Ca3N2 and ε-Ca3N2. The dielectric function, reflectivity function, and absorption function of three types of Ca3N2 were accurately calculated, and it was concluded that α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 have greater transmittance for visible light and exhibit optical transparency in the near-infrared frequency domain. Combined with the high hardness, strong bonding, high melting point, and wear resistance of Ca3N2, Ca3N2 can be used as a new generation of window heat-resistant materials. The α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 are indirect, direct, and indirect narrow bandgap compounds, respectively, that is, δ-Ca3N2 is more suitable for luminescent materials than α-Ca3N2 and ε-Ca3N2. α-Ca3N2 and δ-Ca3N2 have high reflective properties in the ultraviolet region and can be used as UV protective coatings. All three Ca3N2 materials can be used industrially to synthesize photovoltaic devices that operate in the ultraviolet region. METHODS: Based on the first-principles of density functional theory calculations, the structures, electronic properties, and optical properties of α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 were calculated. The calculation results show that although the α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 have similar electronic structures, some phases have better properties in some aspects.

Immobilization of Anions on Polymer Matrices for Gel Electrolytes with High Conductivity and Stability in Lithium Ion Batteries.

This study reports on a high ionic-conductivity gel polymer electrolyte (GPE), which is supported by a TiO2 nanoparticle-decorated polymer framework comprising poly(acrylonitrile-co-vinyl acetate) blended with poly(methyl methacrylate), i.e. , PAVM: TiO2. High conductivity GPE-PAVM: TiO2 is achieved by causing the PAVM:TiO2 polymer framework to swell in 1 M LiPF6 in carbonate solvent. Raman analysis results demonstrate that the poly(acrylonitrile) (PAN) segments and TiO2 nanoparticles strongly adsorb PF6(-) anions, thereby generating 3D percolative space-charge pathways surrounding the polymer framework for Li(+)-ion transport. The ionic conductivity of GPE-PAVM: TiO2 is nearly 1 order of magnitude higher than that of commercial separator-supported liquid electrolyte (SLE). GPE-PAVM: TiO2 has a high Li(+) transference number (0.7), indicating that most of the PF6(-) anions are stationary, which suppresses PF6(-) decomposition and substantially enlarges the voltage that can be applied to GPE-PAVM: TiO2 (to 6.5 V vs Li/Li(+)). Immobilization of PF6(-) anions also leads to the formation of stable solid-electrolyte interface (SEI) layers in a full-cell graphite|electrolyte|LiFePO4 battery, which exhibits low SEI and overall resistances. The graphite|electrolyte|LiFePO4 battery delivers high capacity of 84 mAh g(-1) even at 20 C and presents 90% and 71% capacity retention after 100 and 1000 charge-discharge cycles, respectively. This study demonstrates a GPE architecture comprising 3D space charge pathways for Li(+) ions and suppresses anion decomposition to improve the stability and lifespan of the resulting LIBs.