First-principles study of the structural and electrochemical properties of NaxTi2O4 (0 ≤x≤ 1) with tunnel structure for anode applications in alkali-ion batteries.

Due to their low cost and easy synthesis method, several kinds of sodium titanates have been explored as anode materials for sodium ion batteries (SIBs). However, some of them have not yet been considered as electrode materials for SIBs, and here we have carried out a first-principles study on NaxTi2O4 compounds with two different tunnel structures, denoted as single and double phases, to demonstrate their structural and electrochemical properties upon Na or Li insertion. Our calculation results reveal that these compounds exhibit structural stability during sodiation/desodiation and a moderate electrode voltage of ∼0.82 V vs. Na+/Na with a specific capacity of ∼150 mA h g-1. In particular, the activation energy of Na+ ion migration in the double phase is estimated to be as low as 0.28 eV, which is the lowest value among the SIB electrodes developed so far, and this can be attributed to the wide tunnel structure. In addition, we verify their potentiality for use as anode materials in lithium ion batteries (LIBs) by exploring their properties upon Li insertion. Since these compounds are predicted to be promising anode materials for SIBs or LIBs by our calculations, we believe that our findings will promote further experimental studies.

Interface Engineering in Hybrid Iodide CH3NH3PbI3 Perovskites Using Lewis Base and Graphene toward High-Performance Solar Cells.

Photovoltaic solar cells based on organic-inorganic hybrid halide perovskites have achieved a substantial breakthrough via advanced interface engineering. Reports have emphasized that combining the hybrid perovskites with the Lewis base and/or graphene can definitely improve the performance through surface trap passivation and band alignment alteration; the underlying mechanisms are not yet fully understood. Here, using density functional theory calculations, we show that upon the formation of CH3NH3PbI3 interfaces with three different Lewis base molecules and graphene, the binding strength with S-donors thiocarbamide and thioacetamide is higher than with O-donor dimethyl sulfoxide, while the interface dipole and work function reduction tend to increase from S-donors to O-donor. We provide evidences of deep trap state elimination in the S-donor perovskite interfaces through the analysis of defect formation on the CH3NH3PbI3(110) surface and of stability enhancement by estimation of activation barriers for vacancy-mediated iodine atom migrations. These theoretical predictions are in line with the experimental observation of performance enhancement in the perovskites prepared using thiocarbamide.

Two-Dimensional Hybrid Composites of SnS2 with Graphene and Graphene Oxide for Improving Sodium Storage: A First-Principles Study.

Among the recent achievements of sodium-ion battery (SIB) electrode materials, hybridization of two-dimensional (2D) materials is one of the most interesting appointments. In this work, we propose to use the 2D hybrid composites of SnS2 with graphene or graphene oxide (GO) layers as the SIB anode, based on the first-principles calculations of their atomic structures, sodium intercalation energetics, and electronic properties. The calculations reveal that a graphene or GO film can effectively support not only the stable formation of a heterointerface with the SnS2 layer but also the easy intercalation of a sodium atom with low migration energy and acceptable low volume change. The electronic charge-density differences and the local density of states indicate that the electrons are transferred from the graphene or GO layer to the SnS2 layer, facilitating the formation of a heterointerface and improving the electronic conductance of the semiconducting SnS2 layer. These 2D hybrid composites of SnS2/G or GO are concluded to be more promising candidates for SIB anodes compared with the individual monolayers.

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.