Potassium-ion batteries (KIBs) are receiving increasing interest in grid-scale energy storage owing to the earth abundant and low cost of potassium resources. However, their development still stays at the infancy stage due to the lack of suitable electrode materials with reversible depotassiation/potassiation behavior, resulting in poor rate performance, low capacity, and cycling stability. Herein, the first example of synthesizing single-crystalline metallic graphene-like VSe nanosheets for greatly boosting the performance of KIBs in term of capacity, rate capability, and cycling stability is reported. Benefiting from the unique 2D nanostructure, high electron/K -ion conductivity, and outstanding pseudocapacitance effects, ultrathin VSe nanosheets show a very high reversible capacity of 366 mAh g at 100 mA g , a high rate capability of 169 mAh g at 2000 mA g , and a very low decay of 0.025% per cycle over 500 cycles, which are the best in all the reported anode materials in KIBs. The first-principles calculations reveal that VSe nanosheets have large adsorption energy and low diffusion barriers for the intercalation of K -ion. Ex situ X-ray diffraction analysis indicates that VSe nanosheets undertake a reversible phase evolution by initially proceeding with the K -ion insertion within VSe layers, followed by the conversion reaction mechanism.

Ru x Ti 1Àx Nb 2 O 7 (x ¼ 0 and 0.01) materials have been synthesized via a solid-state reaction method. X-ray diffraction combined with Rietveld refinements demonstrates that both samples have a Wadsley-Roth shear structure with a C2/m space group without any impurities, and that the unit cell volume increases after the trace Ru 4+ doping. Scanning electron microscopy and specific surface area tests reveal that the Ru 4+ doping decreases the average particle size. The Li + ion diffusion coefficient and electronic conductivity of Ru 0.01 Ti 0.99 Nb 2 O 7 are respectively 64% and at least two orders of magnitude larger than those of the pristine TiNb 2 O 7 . First-principles calculations show that the increased electronic conductivity can result from the formation of impurity bands after the Ru 4+ doping. Ru 0.01 Ti 0.99 Nb 2 O 7 exhibits a large initial discharge capacity of 351 mA h g À1 at 0.1 C between 3.0 and 0.8 V vs. Li/Li + , approaching its theoretical capacity (388 mA h g À1 ). At 5 C, unlike the pristine TiNb 2 O 7 with a small charge capacity of 115 mA h g À1 , Ru 0.01 Ti 0.99 Nb 2 O 7 delivers a large value of 181 mA h g À1 , even exceeding the theoretical capacity of the popular spinel Li 4 Ti 5 O 12 (175 mA h g À1 ). After 100 cycles, Ru 0.01 Ti 0.99 Nb 2 O 7 shows a large capacity retention of 90.1%. These outstanding electrochemical performances can be attributed to its improved Li + ionic and electronic conductivity as well as smaller particle size. Electronic supplementary information (ESI) available: Crystal structure of TiNb 2 O 7 showing the m  n  N (m ¼ n ¼ 3) ReO 3 -type blocks (Fig. S1); Nyquist plots of the Li 4 Ti 5 O 12 /Li cell and Li + ion diffusion coefficient of Li 4 Ti 5 O 12 (Fig. S2); Coulombic efficiency of the Ru 0.01 Ti 0.99 Nb 2 O 7 /Li cell at 5 C (Fig. S3); ex situ XRD patterns of TiNb 2 O 7 electrodes (Fig. S4); SEM image and EDX mapping of Ru 0.01 Ti 0.99 Nb 2 O 7 (Fig. S5). See

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TiNbO is explored as a new anode material for lithium-ion batteries. Microsized TiNbO particles (M-TiNbO) are fabricated through a simple solid-state reaction method and porous TiNbO microspheres (P-TiNbO) are synthesized through a facile solvothermal method for the first time. TiNbO exhibits a Wadsley-Roth shear structure with a structural unit composed of a 3 × 4 octahedron-block and a 0.5 tetrahedron at the block-corner. P-TiNbO with an average sphere size of ∼2 μm is constructed by nanoparticles with an average size of ∼100 nm, forming inter-particle pores with a size of ∼8 nm and inter-sphere pores with a size of ∼55 nm. Such desirable porous microspheres are an ideal architecture for enhancing the electrochemical performances by shortening the transport distance of electrons/Li-ions and increasing the reaction area. Consequently, P-TiNbO presents outstanding electrochemical performances in terms of specific capacity, rate capability and cyclic stability. The reversible capacities of P-TiNbO are, respectively, as large as 296, 277, 261, 245, 222, 202 and 181 mA h g at 0.1, 0.5, 1, 2, 5, 10 and 20 C, which are obviously larger than those of M-TiNbO (258, 226, 210, 191, 166, 147 and 121 mA h g). At 10 C, the capacity of P-TiNbO still remains at 183 mA h g over 500 cycles with a decay of only 0.02% per cycle, whereas the corresponding values of M-TiNbO are 119 mA h g and 0.04%. These impressive results indicate that P-TiNbO can be a promising anode material for lithium-ion batteries of electric vehicles.

Intercalation-type MoNb12O33 with a porous-microspherical nanoarchitecture is explored as the first molybdenum niobium oxide anode material for Li+ storage.

Intercalation-type TiNbO (x = 2, 5, and 24) anode materials have recently become more interesting for lithium-ion batteries (LIBs) due to their large theoretical capacities of 388-402 mAh g. However, the Ti/Nb ions in TiNbO with empty 3d/4d orbitals usually lead to extremely low electronic conductivity of <10 S cm, greatly restricting their practical capacity and rate capability. Herein, we report a class of highly conductive CrNbO nanowires as an intercalation-type anode material for high-performance LIBs. The as-made CrNbO nanowires show an open shear ReO crystal structure (C2 space group) with 4% tetrahedra and a conducting characteristic with ultrahigh electronic conductivity of 3.6 × 10 S cm and a large Li-ion diffusion coefficient of 2.19 × 10 cm s. These important characteristics make them deliver outstanding electrochemical properties in term of the largest reversible capacity (344 mAh g at 0.1 C) in all the known niobium- and titanium-based anode materials, safe working potential (∼1.65 V vs Li/Li), high first-cycle Coulombic efficiency (90.8%), superior rate capability (209 mAh g at 30 C), and excellent cycling stability, making them among the best for LIBs in niobium- and titanium-based anode materials.

The research progress on Ti2Nb2xO4+5x is presented with emphases on the research history, structures, characteristics, working mechanisms, modifications and perspectives.

We report a new class of Si/SiOx@void@nitrogen-doped carbon double-shelled hollow superstructure electrodes that are capable of accommodating huge volume changes without pulverization during cycling.