Revealing the formation and electrochemical properties of bis(trifluoromethanesulfonyl)imide intercalated graphite with first-principles calculations.

Graphite has been reported to have anion, as well as cation, intercalation capacities as both a cathode and an anode host material for dual ion batteries. In this work, we study the intercalation of bis(trifluoromethanesulfonyl)imide (TFSI) anions from an ionic liquid electrolyte into graphite with first-principles calculations. We build models for TFSI-Cn compounds with systematically increasing graphene sheet unit cell sizes and investigate their stabilities by calculating the formation energy, resulting in the linear decrease of and arrival at the limit of stability. With unit cell sizes identified for stable compound formation, we reveal that the interlayer distance and relative volume expansion ratio of TFSI-Cn increases as we increase the concentration of the TFSI intercalate during the charge process. The electrode voltage is determined to range from 3.8 V to 3.0 V at a specific capacity ranging from 30 mA h g-1 to 54 mA h g-1, in agreement with experiment. Moreover, a very low activation barrier of under 50 meV for TFSI migration, as well as a good electronic conductivity, provide evidence for using these compounds as a promising cathode. Through the analysis of the charge transfer, we clarify the mechanism of TFSI-Cn formation, and reveal new prospects for developing graphite based cathodes.

Ab initio thermodynamic study of the SnO2(110) surface in an O2 and NO environment: a fundamental understanding of the gas sensing mechanism for NO and NO2.

For the purpose of elucidating the gas sensing mechanism of SnO2 for NO and NO2 gases, we determine the phase diagram of the SnO2(110) surface in contact with an O2 and NO gas environment by means of an ab initio thermodynamic method. Firstly we build a range of surface slab models of oxygen pre-adsorbed SnO2(110) surfaces using (1 × 1) and (2 × 1) surface unit cells and calculate their Gibbs free energies considering only oxygen chemical potential. The fully reduced surface containing the bridging and in-plane oxygen vacancies under oxygen-poor conditions, while the fully oxidized surface containing the bridging oxygen atom and the oxygen dimer under oxygen-rich conditions, and the stoichiometric surface in between, was proved to be most stable. Using the selected plausible NO-adsorbed surfaces, we then determine the surface phase diagram of SnO2(110) surfaces in (ΔµO, ΔµNO) space. Under NO-rich conditions, the most stable surfaces were those formed by NO adsorption on the most stable surfaces in contact with only oxygen gas. Through the analysis of electronic charge transfer and density of states during NOx adsorption on the surface, we provide a meaningful understanding about the gas sensing mechanism.