Carbon-Coated Mesoporous TiO2 Nanocrystals Grown on Graphene for Lithium-Ion Batteries

TiO2 as an Anode of High-Performance Lithium-Ion Batteries: A Comprehensive Review towards Practical Application

Lithium-ion batteries (LIBs) are undeniably the most promising system for storing electric energy for both portable and stationary devices. A wide range of materials for anodes is being investigated to mitigate the issues with conventional graphite anodes. Among them, TiO2 has attracted extensive focus as an anode candidate due to its green technology, low volume fluctuations (<4%), safety, and durability. In this review, the fabrication of different TiO2 nanostructures along with their electrochemical performance are presented. Different nanostructured TiO2 materials including 0D, 1D, 2D, and 3D are thoroughly discussed as well. More precisely, the breakthroughs and recent developments in different anodic oxidation processes have been explored to identify in detail the effects of anodization parameters on nanostructure morphology. Clear guidelines on the interconnected nature of electrochemical behaviors, nanostructure morphology, and tunable anodic constraints are provided in this review.

Synthesis of Metal Oxide/Carbon Nanofibers via Biostructure Confinement as High-Capacity Anode Materials

For applications in energy storage and conversion, many metal oxide (MO)/C composite fibers have been synthesized using cellulose as the template. However, MO particles in carbon fibers usually experience anomalous growth to a size of >200 nm, which is detrimental to the overall performance of the composite. In this paper, we report the successful development of a generic approach to synthesize a fiber composite with highly dispersed MO nanoparticles (10-80 nm) via simple swelling, nitrogen doping, and carbonization of the cellulose microfibril. The growth of the MO nanoparticles is confined by the structure of the microfibrils. Density functional theory calculation further reveals that the doped N atoms supply ample nucleation sites for size confinement of the nanoparticles. The encapsulation structure of small MO nanoparticles in the conductive carbon matrix improves their electrochemical performance. For example, the formed SnO_x/carbon nanocomposite exhibits high specific capacities of 1011.0 mA h g^-1 at 0.5 A g^-1 and 581.8 mA h g^-1 at 5 A g^-1. Moreover, the fiber-like nanocomposite can be combined with carbon nanotubes to form a flexible binder-free electrode with a capacity of ∼10 mA h cm^-2, far beyond the commercial level. The process developed in this study offers an alternative approach to sophisticated electrospinning for the synthesis of other fiber-like MO/carbon nanocomposites for versatile applications.

Ti-Based Oxide Anode Materials for Advanced Electrochemical Energy Storage: Lithium/Sodium Ion Batteries and Hybrid Pseudocapacitors

Titanium-based oxides including TiO₂ and M-Ti-O compounds (M = Li, Nb, Na, etc.) family, exhibit advantageous structural dynamics (2D ion diffusion path, open and stable structure for ion accommodations) for practical applications in energy storage systems, such as lithium-ion batteries, sodium-ion batteries, and hybrid pseudocapacitors. Further, Ti-based oxides show high operating voltage relative to the deposition of alkali metal, ensuring full safety by avoiding the formation of lithium and sodium dendrites. On the other hand, high working potential prevents the decomposition of electrolyte, delivering excellent rate capability through the unique pseudocapacitive kinetics. Nevertheless, the intrinsic poor electrical conductivity and reaction dynamics limit further applications in energy storage devices. Recently, various work and in-depth understanding on the morphologies control, surface engineering, bulk-phase doping of Ti-based oxides, have been promoted to overcome these issues. Inspired by that, in this review, the authors summarize the fundamental issues, challenges and advances of Ti-based oxides in the applications of advanced electrochemical energy storage. Particularly, the authors focus on the progresses on the working mechanism and device applications from lithium-ion batteries to sodium-ion batteries, and then the hybrid pseudocapacitors. In addition, future perspectives for fundamental research and practical applications are discussed.

Hierarchical C/SiO x /TiO2 ultrathin nanobelts as anode materials for advanced lithium ion batteries

TiO₂-based nanomaterials are demonstrated to be a promising candidate for next generation lithium ion batteries due to their stable performance and easy preparation. However, their inherent low capacity impedes their wide application compared to commercial carbon nanomaterials. Here we present a unique in situ grafting-graphitization method to achieve a ternary nanocomposite of C/SiO _x /TiO₂ ultrathin nanobelts with a core-shell heterostructure. The obtained ternary nanocomposite integrates the merits of high specific capacity of SiO _x , the excellent mechanical stability of graphite-like carbon and the high reactivity of TiO₂. Cyclic voltammetric curves and cycling performance manifest the optimal ternary nanocomposite and deliver a very high initial specific capacity of ∼1196 mA h g^-1 with both good rate capability (∼200 mA h g^-1 up to 10 C) and especially enhanced cycle stability. Our work demonstrates that building hierarchical core-shell heterostructures is an effective strategy to improve capacity and cycling performance in other composite anodes for electrochemical energy storage materials.

Density Functional Theory and Atomic Force Microscopy Study of Oleate Functioned on Siderite Surface

Efficiently discovering the interaction of the collector oleate and siderite is of great significance for understanding the inherent function of siderite weakening hematite reverse flotation. For this purpose, investigation of the adsorption behavior of oleate on siderite surface was performed by density functional theory (DFT) calculations associating with atomic force microscopy (AFM) imaging. The siderite crystal geometry was computationally optimized via convergence tests. Calculated results of the interaction energy and the Mulliken population verified that the collector oleate adsorbed on siderite surface and the covalent bond was established as a result of electrons transferring from O1 atoms (in oleate molecule) to Fe1 atoms (in siderite lattice). Therefore, valence-electrons’ configurations of Fe1 and O1 changed into 3d6.514s0.37 and 2s1.832p4.73 from 3d6.214s0.31 and 2s1.83p4.88 correspondingly. Siderite surfaces with or without oleate functioned were examined with the aid of AFM imaging in PeakForce Tapping mode, and the functioned siderite surface was found to be covered by vesicular membrane matters with the average roughness of 16.4 nm assuring the oleate adsorption. These results contributed to comprehending the interaction of oleate and siderite.

Superior shuttling of lithium and sodium ions in manganese-doped titania @ functionalized multiwall carbon nanotube anodes

In order to improve the electrochemical kinetics of anatase titania (TiO₂), Mn-doped TiO₂ incorporated with functionalized multiwall carbon nanotubes (MWCNTs) has been prepared by a modified hydrothermal method and tested for both lithium (LIB) and sodium-ion battery (SIB) anodes. The size of the TiO₂ particles is controlled to ∼35-40 nm, with almost even distribution on the MWCNTs surface. The nanostructuring and appropriate doping of cost-effective manganese into the TiO₂ host improved the electrochemical performance in terms of high rate capability and specific capacity for both the rechargeable battery systems. For the LIBs, the charge capacity of the 5% Mn-TiO₂/MWCNT anode is 226.3 mA h g^-1 in the first cycle, and is retained at 176.4 mA h g^-1 after 80 cycles as compared with the SIBs, in which the charge capacity is 152.1 mA h g^-1 in the first cycle, and is retained at 121.4 mA h g^-1 after 80 cycles. After testing the electrodes at a high current rate of 20C, the nanocomposite electrode can still demonstrate charge capacities of 131.2 and 117.2 mA h g^-1 at a 0.1C rate for LIBs and SIBs, respectively. The incorporation of Mn-ions (2+, 4+) is found to play a crucial role in terms of defects and vacancy creation, increasing conduction band electrons and lattice expansion to facilitate alkali metal ion diffusion for superior electrochemical performance. The combination of heteroatom doping and use of a highly conductive additive in the form of MWCNTs has resulted in excellent electrode integrity, high ion accessibility, and fast electron transport. Its outstanding cycling stability and remarkable rate performance make the 5% Mn-TiO₂/MWCNT a promising anode material for high-performance LIBs and SIBs.

Advances in Subcritical Hydro-/Solvothermal Processing of Graphene Materials

Many promising graphene-based materials are kept away from mainstream applications due to problems of scalability and environmental concerns in their processing. Hydro-/solvothermal techniques overwhelmingly satisfy both the aforementioned criteria, and have matured as alternatives to wet-chemical methods with advances made over the past few decades. The insolubility of graphene in many solvents poses considerable difficulties in their processing. In this context hydro-/solvothermal techniques present an ideal opportunity for processing of graphenic materials with their versatility in manipulating the physical and thermodynamic properties of the solvent. The flexibility in hydro-/solvothermal techniques for manipulation of solvent composition, temperature and pressure provides numerous handles to manipulate graphene-based materials during synthesis. This review provides a comprehensive look at the subcritical hydro-/solvothermal synthesis of graphene-based functional materials and their applications. Several key synthetic strategies governing the morphology and properties of the products such as temperature, pressure, and solvent effects are elaborated. Advances in the synthesis, doping, and functionalization of graphene in hydro-/solvothermal media are highlighted together with our perspectives in the field.

Nb2O5 microstructures: a high-performance anode for lithium ion batteries

We report the synthesis of three-dimensional (3D) urchin-like Nb₂O₅ microstructures by a facile hydrothermal approach with subsequent annealing treatment. As anode materials for lithium-ion batteries, the 3D urchin-like Nb₂O₅ microstructures exhibit superior electrochemical performance with excellent rate capability as well as long-term cycling stability. The electrode delivers high capacity of 131 mA h g^-1 after 1000 cycles at a high current density of 1 A g^-1. The excellent electrochemical performance suggests the 3D urchin-like Nb₂O₅ microstructures may be a promising anode candidate for high-power lithium ion batteries.

Oxygen vacancy induced fast lithium storage and efficient organics photodegradation over ultrathin TiO2 nanolayers grafted graphene sheets

In this work we have developed a unique structure of ultrathin (5nm) TiO2 nanolayers grafted graphene nanosheets (TiO2/G) and integrated oxygen vacancy (VO) into TiO2 to enhance its lithium storage and photocatalytic performances. The defective TiO2/G was synthesized by a solvothermal and subsequent thermal treatment method. When treated in a H2 atmosphere, the resulting TiO2-x/G(H2) has lower crystallinity, smaller crystal size, richer surface VO, higher surface area, larger pore volume, and lower charge transfer resistance than that reduced by NaBH4 solid, i.e., TiO2-x/G(NaBH4). More importantly, the surface VO in the TiO2-x/G(H2) could remarkably inhibit the recombination of photogenerated electron-hole pairs compared with the bulk Vo in the TiO2-x/G(NaBH4). As a result, the combination of all the factors contributed to the superiority of TiO2-x/G(H2), which demonstrated not only 70% higher specific capacity, longer cycling performance (1000 cycles) and better rate capability for lithium-ion battery, but also higher photocatalytic activity and 1.5 times faster degradation rate for organic pollutants removal than TiO2-x/G(NaBH4). The findings in this work will benefit the fundamental understanding of TiO2/G surface chemistry and advance the design and preparation of functional materials for energy storage and water treatment.

Nanoscale Engineering of Heterostructured Anode Materials for Boosting Lithium-Ion Storage

Rechargeable lithium-ion batteries (LIBs), as one of the most important electrochemical energy-storage devices, currently provide the dominant power source for a range of devices, including portable electronic devices and electric vehicles, due to their high energy and power densities. The interest in exploring new electrode materials for LIBs has been drastically increasing due to the surging demands for clean energy. However, the challenging issues essential to the development of electrode materials are their low lithium capacity, poor rate ability, and low cycling stability, which strongly limit their practical applications. Recent remarkable advances in material science and nanotechnology enable rational design of heterostructured nanomaterials with optimized composition and fine nanostructure, providing new opportunities for enhancing electrochemical performance. Here, the progress as to how to design new types of heterostructured anode materials for enhancing LIBs is reviewed, in the terms of capacity, rate ability, and cycling stability: i) carbon-nanomaterials-supported heterostructured anode materials; ii) conducting-polymer-coated electrode materials; iii) inorganic transition-metal compounds with core@shell structures; and iv) combined strategies to novel heterostructures. By applying different strategies, nanoscale heterostructured anode materials with reduced size, large surfaces area, enhanced electronic conductivity, structural stability, and fast electron and ion transport, are explored for boosting LIBs in terms of high capacity, long cycling lifespan, and high rate durability. Finally, the challenges and perspectives of future materials design for high-performance LIB anodes are considered. The strategies discussed here not only provide promising electrode materials for energy storage, but also offer opportunities in being extended for making a variety of novel heterostructured nanomaterials for practical renewable energy applications.

Hierarchical TiO2/C nanocomposite monoliths with a robust scaffolding architecture, mesopore-macropore network and TiO2-C heterostructure for high-performance lithium ion batteries

Engineering hierarchical structures of electrode materials is a powerful strategy for optimizing the electrochemical performance of an anode material for lithium-ion batteries (LIBs). Herein, we report the fabrication of hierarchical TiO2/C nanocomposite monoliths by mediated mineralization and carbonization using bacterial cellulose (BC) as a scaffolding template as well as a carbon source. TiO2/C has a robust scaffolding architecture, a mesopore-macropore network and TiO2-C heterostructure. TiO2/C-500, obtained by calcination at 500 °C in nitrogen, contains an anatase TiO2-C heterostructure with a specific surface area of 66.5 m(2) g(-1). When evaluated as an anode material at 0.5 C, TiO2/C-500 exhibits a high and reversible lithium storage capacity of 188 mA h g(-1), an excellent initial capacity of 283 mA h g(-1), a long cycle life with a 94% coulombic efficiency preserved after 200 cycles, and a very low charge transfer resistance. The superior electrochemical performance of TiO2/C-500 is attributed to the synergistic effect of high electrical conductivity, anatase TiO2-C heterostructure, mesopore-macropore network and robust scaffolding architecture. The current material strategy affords a general approach for the design of complex inorganic nanocomposites with structural stability, and tunable and interconnected hierarchical porosity that may lead to the next generation of electrochemical supercapacitors with high energy efficiency and superior power density.

Simple size control of TiO2 nanoparticles and their electrochemical performance: emphasizing the contribution of the surface area to lithium storage at high-rates

The particle size effects of TiO2 nanoparticles (TNPs), which are composed of small crystallites, on Li ion storage are a very fundamental and important subject. However, size control of TNPs under 200 nm using a sol-gel method has been limited due to the highly reactive precursor, titanium alkoxide. In this study, TNPs with various sizes even under 100 nm are obtained by controlling the reactant concentrations in a mixed solvent of ethanol and acetonitrile. Among them, three different sizes of TNPs are prepared to compare the Li ion storage capacity, and 60 nm TNPs are found to have the best reversible capacity of 182 mA h g(-1) after 50 cycles at 1 C and a remarkable rate performance of 120 mA h g(-1) at 10 C. Capacity increase upon cycling is observed in the size-controlled TNPs, and the explanation of this phenomenon is proposed to the lattice volume expansion of TiO2 upon intercalation for enabling further penetration of the electrolyte into the particles' interior. Moreover, the capacity at high rates is more closely related to the surface area from Hg porosimetry analysis than from typical N2 adsorption/desorption analysis.

Nanoconfined nitrogen-doped carbon-coated MnO nanoparticles in graphene enabling high performance for lithium-ion batteries and oxygen reduction reaction

We prepared N-doped double carbon coated MnO composites and explored their applications in lithium ion batteries and oxygen reduction reaction.

To tackle the issues of inferior cycling stability and low conductivity for MnO as an anode material for lithium ion batteries (LIBs) and as a catalyst for oxygen reduction reaction (ORR), a facile and effective strategy is explored to confine N-doped carbon-coated MnO nanoparticles in a conductive graphene matrix. The synthesis of the GMNCs involves the two-step coating of Mn₃O₄ nanocrystals with polydopamine and graphene, followed by heat treatment to form the GNS@MnO@N-doped carbon composites (GMNCs). When evaluated as anode materials for LIBs, the as-prepared GMNCs exhibit an improved cycling stability (754.3 mA h g^–1 after 350 cycles at 0.1 A g^–1) compared to carbon-coated MnO and pure Mn₃O₄ due to the double carbon coating design. When evaluated as catalysts for ORR, the as-prepared GMNCs exhibit higher electrocatalytic activity than that of pure Mn₃O₄ and MnO catalysts, and superior stability to a commercial Pt/C catalyst due to the synergetic effect between the MnO and N-doped double carbon coating. The optimum design of the unique nanostructures with the synergetic effect provides a new route to design advanced materials as electrode/catalysts for energy conversion and storage.

Seeing is believing: atomic force microscopy imaging for nanomaterial research

Atomic force microscopy can image nanomaterial properties such as the topography, elasticity, adhesion, friction, electrical properties, and magnetism.

One-pot low-temperature synthesis of TiO2 nanowire/rGO composites with enhanced photocatalytic activity

In situ growth of TiO₂ nanowires on graphene oxide was achieved at 80 °C in an open atmosphere. The optimized TiO₂/rGO hybrid exhibited a reaction rate constant 5.5 times that of TiO₂ nanowires towards photodegradations of rhodamine B in water under the UV light illumination.

Probing the pseudo-1-D ion diffusion in lithium titanium niobate anode for Li-ion battery

Comprehensive understanding of the charge transport mechanism in the intrinsic structure of an electrode material is essential in accounting for its electrochemical performance.

Growth of 3D hierarchical porous NiO@carbon nanoflakes on graphene sheets for high-performance lithium-ion batteries

Nickelocene was used as the precursor for both NiO and carbon to construct a 3D hierarchical graphene based nanocomposite.

A Biomineralization Strategy for "Net"-Like Interconnected TiO2 Nanoparticles Conformably Covering Reduced Graphene Oxide with Reversible Interfacial Lithium Storage

A green and simple biomineralization-inspired method to create "net"-like interconnected TiO₂ nanoparticles conformably covering reduced graphene oxide (RGO) with high loading density is reported. This method uses polyamine as both the biomineralization agent and linker to manipulate the nucleation, growth, and crystallization of TiO₂ nanoparticles on the surface of graphene oxide. The obtained TiO₂/RGO composites demonstrate sub-10-nm TiO₂ nanoparticles with (001) facets, ultrathin thickness (10-12 nm), and a high surface area of 172 m² g^-1. When used as anode material for lithium ion batteries, the material displayed excellent rate capability and long cycle life; a capacity of 155 mAh g^-1 is obtained after 50 cycles at the rate of 5C (1C = 168 mA g^-1) and a specific capacity of 115 mAh g^-1 is retained after 2000 cycles at the rate of 25C, which is much higher than that of mechanically mixed TiO₂/graphene composites. Detailed discharge curve analysis reveals that the high rate and cycle performance are partly a result of the reversible interfacial lithium storage of materials, which might be attributed to the pores in the TiO₂ nets on the RGO and may provide a sufficient number of interfaces for accepting both electrons and lithium ions.

Phase Tuning of Nanostructured Gallium Oxide via Hybridization with Reduced Graphene Oxide for Superior Anode Performance in Li-Ion Battery: An Experimental and Theoretical Study

The crystal phase of nanostructured metal oxide can be effectively controlled by the hybridization of gallium oxide with reduced graphene oxide (rGO) at variable concentrations. The change of the ratio of Ga2O3/rGO is quite effective in tailoring the crystal structure and morphology of nanostructured gallium oxide hybridized with rGO. This is the first example of the phase control of metal oxide through a change of the content of rGO hybridized. The calculations based on density functional theory (DFT) clearly demonstrate that the different surface formation energy and Ga local symmetry of Ga2O3 phases are responsible for the phase transition induced by the change of rGO content. The resulting Ga2O3-rGO nanocomposites show promising electrode performance for lithium ion batteries. The intermediate Li-Ga alloy phases formed during the electrochemical cycling are identified with the DFT calculations. Among the present Ga2O3-rGO nanocomposites, the material with mixed α-Ga2O3/β-Ga2O3/γ-Ga2O3 phase can deliver the largest discharge capacity with the best cyclability and rate characteristics, highlighting the importance of the control of Ga2O3/rGO ratio in optimizing the electrode activity of the composite materials. The present study underscores the usefulness of the phase-control of nanostructured metal oxides achieved by the change of rGO content in exploring novel functional nanocomposite materials.

Fe-doped anatase TiO2/carbon composite as an anode with superior reversible capacity for lithium storage

This paper reports an economic and effective wet chemistry process to prepare Fe-doped anatase TiO₂/carbon composite with excellent performance in lithium-ion batteries.

Crystalline TiO2@C nanosheet anode with enhanced rate capability for lithium-ion batteries

TiO₂@C nanosheets have been obtained by a facile solvothermal method using titanate butoxide and hydrofluoric acid as precursors, followed by our novel carbon coating technique using oleic acid as the carbon source.