Graphene/metal oxide nanocomposites have been widely used as the anode materials for Li ion batteries, which exhibit much higher Li storage capacity beyond their theoretical capacity. In order to make clear the Li storage mechanism in graphene/metal oxide, we systematically investigated the interface and (B, N, O, S) doping effects on Li ion storage behavior in graphene/Li2O using first-principles total energy calculations. It is revealed that the doping elements increase the van der Waals interface interaction of graphene/Li2O by changing the electronic structure of graphene through different mechanisms. The Li storage at the graphene/Li2O interface exhibits the synergistic effect resulting from the enhanced interface interaction by the Li insertion at the interface. The p-type and n-type doping induced by B and N dopants in graphene enhance and reduce the Li storage capability of graphene/Li2O, respectively. O and S doping result in the localization of the electronic states in graphene which benefits the Li adsorption at the interface. The localization of electronic states combined with the appropriate dopant electronegativity can enhance the Li atoms adsorption and diffusion simultaneously. Thereby, the highest interfacial lithium storage (0.330 mhA/m2) is obtained for the O-doped system, while the S-doped system possesses the good balance between interfacial Li storage (0.220 mhA/m2) and diffusion energy barrier (0.27 eV). The results open a new insight for the design of graphene/metal oxide composites as energy storage materials.

Interconnected 3D graphene foams with a large number of controllable micro–mesoporous structures are promising electrode materials for supercapacitors due to their high electrical conductivity and large effective surface area. Here we successfully obtained a flexible high-quality nitrogen and oxygen co-doped 3D nanoporous duct-like graphene@carbon nano-cage film by chemical vapor deposition using nanoporous copper (NPC) as the substrate and further modifying with HNO3. The bicontinuous mesoporous architecture catalyzed by NPC retains the 2D coherent electronic properties of graphene. Carbon nano-cages grown on the surface of the 3D nanoporous graphene combined with the microporous structure caused by heteroatom-doping greatly increase the effective specific surface area. Moreover, heteroatom-doping and oxygen-containing groups on the surface of graphene can obtain extra redox capacitance and achieve good wettability between the electrode and the electrolyte. The superior structure means that NO-3DG@CNC-based multi-style supercapacitors, such as aqueous-system, ionic-system, and lithium-ion capacitors etc., all show high energy density, high power density and excellent cycling stability.

Sluggish surface reaction kinetics hinders the power density of Li-ion battery. Thus, various surface modification techniques have been applied to enhance the electronic/ionic transfer kinetics. However, it is challenging to obtain a continuous and uniform surface modification layer on the prime particles with structure integration at the interface. Instead of classic physical-adsorption/deposition techniques, we propose a novel chemical-adsorption strategy to synthesize double-shell modified lithium-rich layered cathodes with enhanced mass transfer kinetics. On the basis of experimental measurement and first-principles calculation, MoO2S2 ions are proved to joint the layered phase via chemical bonding. Specifically, the Mo–O or Mo–S bonds can flexibly rotate to bond with the cations in the layered phase, leading to the good compatibility between the thiomolybdate adsorption layer and layered cathode. Followed by annealing treatment, the lithium-excess-spinel inner shell forms under the thiomolybdate adsorption layer and functions as favorable pathways for lithium and electron. Meanwhile, the nanothick MoO3–x(SO4)x outer shell protects the transition metal from dissolution and restrains electrolyte decomposition. The double-shell modified sample delivers an enhanced discharge capacity almost twice as much as that of the unmodified one at 1 A g–1 after 100 cycles, demonstrating the superiority of the surface modification based on chemical adsorption.Keywords: chemical-adsorption; DFT calculations; double shell; lithium rich layered cathode; surface modification

•Li adsorption capability of layered metal sulfide surface is enhanced due to presence of graphene.•Li storage at Li2S/G interface contributes to the extra storage capacity of graphene/metal sulfide.•Li diffusion barriers at the graphene/metal sulfide interfaces maintain relatively small values.•The synergistic effect of graphene and metal sulfide is revealed through detailed electronic structure analysis.The layered graphene/metal sulfide composites exhibit excellent electrochemical properties as anode materials for lithium ion battery, due to the synergistic effect between metal sulfide and graphene which still needs to be further understood. In this study, Li adsorption and diffusion on MoS2 and SnS2 monolayers and Li2S surface, as well as at their interfaces with graphene, are systematically investigated through first-principles calculations. The analysis of charge density difference, Bader charge, and density of states indicates that the adsorbed Li atoms interact with both the S atoms at metal sulfide surfaces and C atoms in graphene, resulting in larger Li adsorption energies at the interfaces compared with that on the corresponding surfaces, but with almost no enhancement of the energy barriers for Li atom diffusion. The enhanced Li adsorption capability at Li2S/G interface contributes to the extra storage capacity of graphene/metal sulfide composites. Furthermore, the synergistic mechanism between metal sulfide and graphene is revealed. Moreover, band structure analysis shows the electronic conductivity is enhanced with the incorporation of graphene. The results corroborate the interfacial pseudocapacity-like Li atom storage mechanism, and are helpful for the design of layered graphene/metal sulfide composites as anode materials for lithium ion batteries.Li adsorption at Li2S/G interface is enhanced with almost no increase of the energy barriers for Li atom diffusion.

•The mechanism for the growth of MoS2 onto the surface of TiO2 is first demonstrated.•Both rich defects and intimate interfaces contribute to excellent performance.•The synergistic effect between rich defects and interfaces is first revealed.•The UT-TiO2/C@DR-MoS2 anodes enable brilliant cycling stability and rate capacity.Poor cycling performance and rate capability are the major barriers to the application of layered transition-metal sulfide as the next generation anodes materials for lithium-ion batteries (LIBs). In this paper, a smart composite consisting of defect-rich MoS2/carbon-coated ultrathin TiO2/defect-rich MoS2 sandwich-like nanosheets has been constructed to enhance the cycling and rate performances of MoS2. In this uniform sandwich-like structure, carbon-coated ultrathin TiO2 is conformably embedded by defect-rich MoS2 shells via intimate interfacial contacts, while the carbon coats TiO2via Ti–O–C bonds. It is first revealed that the synergistic effect between rich defect in few-layer MoS2 nanosheets and abundant interfaces in the composites, as well as the high structure stability of TiO2 during discharge and charge process, results in excellent performance of the composites as LIBs anode materials. These LIBs anodes achieve the best rate capability (785.9, 507.6 and 792.3 mA h g−1 at 0.1, 2 and 0.1 A g−1, respectively) and cycling performance (805.3 mA h g−1 at 0.1 A g−1 after 100 cycles) among the TiO2 supported MoS2 based composites reported previous. The synergistic strategy can be expected to benefit the rational design of other anode materials for high-performance LIBs.Smart composites composed of defect-rich MoS2 shell, in which the carbon coats TiO2via Ti–O–C bonds and connects with the defect-rich MoS2 through intimate interfacial contact. The new-found synergistic effect between rich defect in few-layer MoS2 nanosheets and abundant interfaces in the composites results in high-performance of lithium-ion battery anode materials.

Surface properties of olivine phosphate LiFePO4, as cathodes in lithium ion batteries, are of importance for overall performance as the nanoscale of particles has become indispensable. Using the first-principles total energy calculations, the effects of graphene or Mn dopant on the electrochemical properties of graphene/LiFePO4 have been comprehensively investigated. It is revealed that the interfacial binding between graphene and LiFePO4 in parallel orientation is stable and improved in the process of doping. The Li adsorption energy at different sites elucidates the core–shell model in Li extraction/insertion process and indicates the anomalous Li storage in the interface between graphene and LiFePO4. Mechanisms underlying influences of adsorption site, Mn dopant, and graphene modification on the Li adsorption energy are discussed through edge effect, doping stability, and interfacial binding strength, respectively. The surface conductivity is improved in the presence of graphene or Mn dopant with respect to the bandlike electron transport.

Uniform transition metal sulfide deposition on a smooth TiO2 surface to form a coating structure is a well-known challenge, caused mainly due to their poor affinities. Herein, we report a facile strategy for fabricating mesoporous 3D few-layered (<4 layers) MoS2 coated TiO2 nanosheet core–shell nanocomposites (denoted as 3D FL-MoS2@TiO2) by a novel two-step method using a smooth TiO2 nanosheet as a template and glucose as a binder. The core–shell structure has been systematically examined and corroborated by transmission electron microscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy analyses. It is found that the resultant 3D FL-MoS2@TiO2 as a lithium-ion battery anode delivers an outstanding high-rate capability with an excellent cycling performance, relating to the unique structure of 3D FL-MoS2@TiO2. The 3D uniform coverage of few-layered (<4 layers) MoS2 onto the TiO2 can remarkably enhance the structure stability and effectively shortens the transfer paths of both lithium ions and electrons, while the strong synergistic effect between MoS2 and TiO2 can significantly facilitate the transport of ions and electrons across the interfaces, especially in the high-rate charge–discharge process. Moreover, the facile fabrication strategy can be easily extended to design other oxide/carbon–sulfide/oxide core–shell materials for extensive applications.

Poor rate capability and cycling performance are the major barriers to the application of lithium rich layered oxides (LLOs) as the next generation cathodes materials for lithium-ion batteries. In this paper, a novel surface double phase network modification has been applied to enhance the rate property of Li1.2Co0.13Ni0.13Mn0.54O2 (LR) via flexible electrostatic heterocoagulation and thermal treatment. The template action of multiwalled carbon nanotubes (MWCNTs) network on LR clusters results in the spinel phase network formation at the interface between the LR and MWCNTs. The phase transformation process from layered component toward spinel phase is identified through the detailed investigation of the interface using high-resolution transmission electron microscopy, fast Fourier transformation, and the detailed analysis on the transformation of simulated diffraction patterns. The double phases stretch two sets of networks with both fine Li ion and electron conductivity onto and within the clusters of LR, lowering the surface resistance, reducing the electrochemical polarization, and as a result, significantly enhancing the rate capability of LR. The double phase network modification, combining MWCNT coagulation and spinel phase modification, has profound potential in accelerating kinetics for LLOs.Keywords: double phase network; lithium rich layered oxides; rate capability; surface modification

Ti-doping effects on the structure stability, Li+ diffusion, and working voltage of LiCo0.5Ni0.5O2 have been studied based on first-principle total energy calculations. It is revealed that due to Ti-doping, the structure stability of LiCo0.5Ni0.5O2 is enhanced because of the decrease of both the formation energy and the lattice distortion during lithium deintercalation. The diffusion of Li+ gets improved because the introduction of Ti weakens the electrostatic attraction effect of the host on Li+. Furthermore, after Ti-doping, the relaxation energy during lithium deintercalation decreases owing to the alleviation of Jahn–Teller effects, while the contribution of oxygen atoms to the redox process increases, resulting in the increase of the working voltage. Our research gives an effective method to investigate the cathode materials’ doping effect, which benefits for the design of electrode materials in lithium-ion battery.

•The effects of alloying elements on the interface between CNT and Al are revealed.•The chemical bonding is detected between CNT and Al substrate doped with Sc and Ti.•Bonding character is not fully reflected by the interfacial binding energy (Eb).•Interaction potential energy and restructuring energy competitively affect the Eb.The effects of alloying elements (Mg, Zn, Cu, Sc and Ti) on the interfacial bonding between a carbon nanotube (CNT) and an Al(111) surface are studied systematically using first-principles total energy calculations. The redistribution of the charge density of the Al(111) surfaces induced by the five alloying elements, the projected densities of state, and the crystal orbital Hamilton population analysis of the atomic pairs in the vicinity of the interface are analyzed in detail. Thereby, it is revealed that the bonding characteristic between the CNT and the Al(111) surfaces are modified when alloying with the early transition metals Sc and Ti that occupy subsurface positions. Also for the other alloying elements a weak interaction between CNT and the substrates is found although these elements occupy surface positions in the Al(111) surface. Through a detailed study of the interaction potential energy and the restructuring energy, we demonstrate that different mechanisms are responsible for the substrate–CNT interactions.

The lithium-rich layered oxide materials (LLOs) have attracted much attention as candidates for the next generation of LIBs because of their high voltage and high capacity, which are still poorly understood. In this study, the origin of high voltage and high capacity of LLOs has been comprehensively investigated through first-principles calculations. It is revealed that due to the asymmetric oxidation behavior of Li2MnO3/LiMO2 interface, the transition-metal–oxygen (TMO) layer of Li2MnO3 phase in Li-rich materials gains more electrons from Li layer than that in pure Li2MnO3, which results in the stronger hybrid between Mn-3d and O-2p states enhancing the activity of Mn in Li2MnO3. Moreover, the deintercalated Li-rich models possess smaller spacing than pure LiMO2, which reflects stronger electrostatic interaction between TMO and Li layers. The two factors are both beneficial to the high voltage of the Li-rich materials. However, the asymmetric interface also results in the increase of electronic states of transition metal atoms near the Femi level, which changes the oxidized sequence of Ni2+/Ni4+ and Co3+/Co4+, and reduces the participation of oxygen in the redox process. As a result, the voltage and reversible capacity of Li-rich materials are significantly enhanced compared with that of pure LiMO2.

Graphene/metal-oxide nanocomposites have been widely studied as anode materials for lithium ion batteries and exhibit much higher lithium storage capacity beyond their theoretical capacity through mechanisms that are still poorly understood. In this research, we present a comprehensive understanding in microscale of the discharge process of graphene/TiO2 containing surface, bulk, and interfacial lithium storage based on the first-principles total energy calculations. It is revealed that interfacial oxygen atoms play an important role on the interfacial lithium storage. The additional capacity originating from surface and interfacial lithium storage via an electrostatic capacitive mechanism contributes significantly to the electrode capacity. The research demonstrates that for nanocomposites used in energy storage materials, electrode and capacitor behavior could be optimized to develop high-performance electrode materials with the balance of storage capacity and rate.Keywords: discharge process; first-principles calculations; interfacial lithium storage; interfacial oxygen atoms; pseudocapacity-like storage mechanism

Surface decoration with metal atoms is an efficient way to improve the hydrogen storage capacity of graphene. The hydrogen adsorption behavior on Al-decorated graphene is studied based on first-principles calculations. The dissociation of hydrogen molecule adsorbed on Al-decorated graphene is revealed, and its effect on the hydrogen storage capacity is investigated. Moreover, the effects of B-dopant on the dispersion of Al atoms on graphene and the dissociation behavior of hydrogen molecules are also studied. Our results indicate that hydrogen dissociation behavior has a significant effect on the hydrogen storage capacity of metal-decorated graphene and must be considered in the investigation of hydrogen storage behavior of metal-decorated carbon nanomaterials. Furthermore, through a comparison study on the hydrogen adsorption behavior on Li-, Ca-, and Al-decorated graphene, the factors relating to the hydrogen molecule dissociation are revealed.

•TiO2 nanocrystalline film with through-hole cracks is fabricated by gel chapping.•The CNTs/TiO2 nanocomposites fabrication method guarantees the contact of CNTs with current collector.•The conversion efficiency is 3 times superior to that of TiO2 nanocrystalline film.TiO2 nanoparticles are widely used in dye-sensitized solar cells (DSSCs). The combination of one-dimensional materials such as carbon nanotubes (CNTs) with TiO2 nanocrystals by mechanical mixing has been expected to improve the electron transport of anode. However, these methods cannot ensure the connection between one-dimensional materials and collector, which is the key for exerting the fast electron transport property. In this study, a novel synthesis method named gel chapping for CNTs/TiO2 nanocomposites preparation is presented, in which porous nanocrystalline TiO2 film with through-hole cracks is fabricated. By using the porous films with cracks as templates, CNTs are deposited in pores and well connected with the template and current collector which favors the transportation of electrons. The current–voltage curve test reveals that the conversion efficiency of CNTs/TiO2 nanocrystalline composite structures is 2.6 times superior to TiO2 nanocrystalline film.

In this work, first-principles total energy calculations were performed in order to study the structure and hydrogen storage behavior on Ca-decorated graphene. On the stable structure of Ca-decorated graphene with 3×3−300 reconstruction, the first hydrogen molecule adsorbed is dissociative, with the energy barrier of only 0.05 eV. The electrons of H atoms saturate the electronic states of Ca around the Fermi level and enhance the system stability. Further adsorption of hydrogen molecules on Ca-adsorbed graphene is weak, which indicates Ca-adsorbed graphene does not suit for the hydrogen storage via physical adsorption of hydrogen molecules. On the other hand, hydrogen spillover mechanism could exist on Ca-decorated graphene. On the graphene with one Ca dimer adsorbed, one of the four H atoms adsorbed on the Ca dimer adsorbes chemically on C in graphene more stably by 0.37 eV than on the Ca dimer. With the number of hydrogen atoms adsorbed on Ca-decorated graphene increases, the binding energy of hydrogen atoms tends to increase. Thus, the spillover process is energetically favorable. The hydrogen storage capacity via the spillover mechanism in Ca-adsorbed graphene depends on the Ca content and could approach 7.7 wt.%.Highlights► Dissociation of hydrogen molecules on Ca-decorated graphene is discovered. ► Adsorption of hydrogen molecules on Ca-decorated graphene is weakened. ► The spillover process is energetically favorable on Ca-decorated graphene. ► Ca dimers act as nucleation positions of hydrogen adsorption on graphene.

•P25@carbon supported MoS2 composite was prepared by a one-step process.•The distribution and interaction of C, MoS2 and TiO2 are systematically examined.•The enjoyable features of the three components are complementarily integrated.•The smart ternary electrode exhibits excellent cycling stability and rate capability.Ternary anodes have attracted more and more attention due to the characteristic advantages resulting from the effect integration of three different materials on the lithium storage mechanism with functional interfaces interaction. However, clarifying the distribution and interaction of carbon, MoS2 and TiO2 in the MoS2/C/TiO2 composite, which is helpful for the understanding of the formation and lithium storage mechanism of the ternary anodes, is a well-known challenge. Herein, a novel pore core-double shell nanostructure of P25@carbon network supported few-layer MoS2 nanosheet (P25@C@FL-MoS2) is successfully synthesized by a one-pot hydrothermal approach. The distribution and interaction of the carbon, MoS2 and TiO2 in the obtained P25@C@FL-MoS2 hybrid are systematically characterized by transmission electron microscopy, Raman spectra and X-ray photoelectron spectroscopy analysis et al. It is found that the carbon serves as binder, which supports few-layer MoS2 shell and coats the P25 core via TiOC bonds at the same time. Such multi-functional integration with smart structure and strong interfacial contact generates favorable structure stability and interfacial pseudocapacity-like storage mechanism. As a consequence, superior cycling and rate capacity of the muti-functional integration ternary P25@C@FL-MoS2 anode are achieved.Figure optionsDownload full-size imageDownload high-quality image (203 K)Download as PowerPoint slide