Revealing the effect of Nb or V doping on anode performance in Na2Ti3O7 for sodium-ion batteries: a first-principles study.

Sodium titanate Na2Ti3O7 (NTO) has superior electrochemical properties as an anode material in sodium-ion batteries (SIBs), and Nb or V doping is suggested to enhance the electrode performance. In this work, we carry out systematic first-principles calculations of the structural, electronic and electrochemical properties of NTO and Na2Ti2.75M0.25O7 (M = Nb, V), using supercells to reveal the effect of Nb or V NTO-doping on its anode performance. It is found that Nb doping gives rise to the expansion of cell volume but V doping induces the shrinkage of cell volume due to the larger and smaller ionic radius of the Nb and V ions, respectively, compared to that of the Ti ion. We perform structural optimization of the intermediate phases of Na2+xM3O7 with increasing Na content x from 0 to 2, revealing that the overall relative volume expansion rate is slightly increased by Nb and V doping but remains lower than 3%. Our calculations demonstrate that the electrode potential of NTO is slightly raised and the specific capacity is reduced, but the electronic and ionic conductivities are improved by Nb or V doping. With the revealed understanding and mechanisms, our work will contribute to the search for advanced electrode materials for SIBs.

Insight into the structural and electrochemical properties of the interface between a Na6SOI2 solid electrolyte and a metallic Na anode.

Solid electrolytes (SE) have attracted a great deal of interest as they can not only mitigate the safety issues related to currently used liquid organic electrolytes but also enable the introduction of a metallic Na anode with extreme energy density in sodium-ion batteries. For such application, SE should exhibit high interfacial stability against metallic Na as well as high ionic conductivity, and Na6SOI2 with a Na-rich double anti-perovskite structure was recently identified as a promising SE candidate. In this work, we performed first principles calculations to investigate the structural and electrochemical properties of the interface between Na6SOI2 and a metallic Na anode. Our calculations revealed that interfaces could be formed safely, keeping the ultra-fast ionic conductivity of the bulk phase near the interface. Through the electronic structure analysis of the interface models, we found the change of upward valence band bending at the surface to downward band bending at the interface, being accompanied by electronic charge transfer from a metallic Na anode to Na6SOI2 SE at the interface. This work provides valuable atomistic insight into the formation and properties of the interface between SE and alkali metal for enhancing battery performance.

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.

Effect of vacancy concentration on the lattice thermal conductivity of CH3NH3PbI3: a molecular dynamics study.

Hybrid halide perovskites are drawing great interest for photovoltaic and thermoelectric applications, but the relationship of thermal conductivities with vacancy defects remains unresolved. Here, we present a systematic investigation of the thermal conductivity of perfect and defective CH3NH3PbI3, performed using classical molecular dynamics with an ab initio-derived force field. We calculate the lattice thermal conductivity of perfect CH3NH3PbI3 as the temperature increases from 300 K to 420 K, confirming a good agreement with the previous theoretical and experimental data. Our calculations reveal that the thermal conductivities of defective systems at 330 K, containing vacancy defects such as VMA, VPb and VI, decrease overall with some slight rises, as the vacancy concentration increases from 0 to 1%. We show that such vacancies act as phonon scattering centers, thereby reducing the thermal conductivity. Moreover, we determine the elastic moduli and sound velocities of the defective systems, revealing that their slower sound speed is responsible for the lower thermal conductivity. These results could be useful for developing hybrid halide perovskite-based solar cells and thermoelectric devices with high performance.

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.

First-principles study of NaxTiO2 with trigonal bipyramid structures: an insight into sodium-ion battery anode applications.

Developing efficient anode materials with low electrode voltage, high specific capacity and superior rate capability is urgently required on the road to commercially viable sodium-ion batteries (SIBs). Aiming at finding a new SIB anode material, we investigate the electrochemical properties of NaxTiO2 compounds with unprecedented penta-oxygen-coordinated trigonal bipyramid (TB) structures by using first-principles calculations. Identifying the four different TB phases, we perform the optimization of their crystal structures and calculate their energetics such as sodium binding energy, formation energy, electrode potential and activation energy for Na ion migration. The computations reveal that the TB-I phase is the best choice among the four TB phases for a SIB anode material due to a relatively low volume change of under 4% upon Na insertion, low electrode voltage under 1.0 V with a possibility of realizing the highest specific capacity of ∼335 mA h g-1 from full sodiation at x = 1, and reasonably low activation barriers under 0.35 eV at the Na content from x = 0.125 to x = 0.5. Through the analysis of electronic density of states and charge density difference upon sodiation, we find that the NaxTiO2 compounds in TB phases change from electron insulating to electron conducting materials due to the electron transfer from Na atoms to Ti ions, offering the Ti4+/Ti3+ redox couple for SIB operation.

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.