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Vanadium oxides with various compositions have been reported as electrodes for potassium-ion batteries, but the poor conductivity is the main obstacle to fulfilling the energy storage requirements. In this study, we synthesize carbon-coated V2O3 hollow spheres (HS-V2O3@C) by a facile solvothermal reaction. The HS-V2O3@C electrode exhibits promising cycling stability and rate performances. It retains a high reversible capacity of 330 mA h g−1 at 100 mA g−1 after 500 cycles. Its capacity reaches 79 mA h g−1 at a high current density of 5000 mA g−1. The superior performances are mainly due to the homogeneous carbon coating layer that greatly improves the electronic conductivity and the hollow structure that buffers the volume change during cycling. By means of ex situ X-ray diffraction and cyclic voltammetry, the mechanism of potassium ion storage in this HS-V2O3@C electrode is determined as the combination of intercalation and pseudocapacitive effects. [ABSTRACT FROM AUTHOR]

Potassium-ion batteries (KIBs) are considered a favorable candidate for large-scale electric energy storage system applications owing to the abundance and low-cost of potassium resources. Herein, a series of three-dimensional pomegranate-like SbxBi1−x@C composites are fabricated by a modified Pechini sol–gel method. As an anode for KIBs, the Sb0.5Bi0.5@C composite exhibits the optimal performance (382 mA h g−1 at 0.05 A g−1). More importantly, the synergistic effects in the binary alloy bring a promising long-term cycle life (83% capacity retention after 400 cycles) for Sb0.5Bi0.5@C. The ex situ X-ray diffraction results reveal the K storage mechanism of this material, which is Sb0.5Bi0.5 ↔ KSb0.5Bi0.5 ↔ K3Sb0.5Bi0.5 during discharge and charge processes. In addition, a full cell constructed by coupling Sb0.5Bi0.5@C with a Prussian blue cathode delivers outstanding electrochemical performance. [ABSTRACT FROM AUTHOR]

The influence of partial substitution of manganese by nickel or magnesium in Na0.44MnO2on cathode performance in sodium ion batteries has been investigated. Partial substitution changes the structure of parent Na0.44MnO2 from tunnel into layered P2-type, or a mixture of P2- and P3-type materials. Substitution smoothes the charge/discharge curves and may significantly improve capacity, albeit with a lower capacity retention relative to pristine Na0.44MnO2. In particular, high discharge capacities are found in the voltage range 2.0–4.2 V at 0.1 C rate for Na0.44Mn0.89Ni0.11O2 (193 mAh g−1) and Na0.44Mn0.89Mg0.11O2 (188 mAh g−1), with a capacity retention of 74% and 81%, respectively, after 70 cycles. [ABSTRACT FROM AUTHOR]

A series of Na3V2−xGax(PO4)3 (x = 0, 0.1, 0.2, 0.4 and 0.6) with in situ catalytic formation of graphene-like graphitic layer decoration are synthesized via a solid-state reaction process. It is shown for the first time that the substitution of gallium for vanadium in Na3V2−xGax(PO4)3 (x = 0.1, 0.2, 0.4 and 0.6) enhances its energy density by about 4.4% (from 355.5 W h kg−1 for Na3V2(PO4)3 to 371 W h kg−1 for Na3V1.6Ga0.4(PO4)3) and particularly increases significantly its power density to up to 11 075 W kg−1 at 50C for Na3V1.6Ga0.4(PO4)3 compared to 5060 W kg−1 for Na3V2(PO4)3. The optimal composition Na3V1.6Ga0.4(PO4)3 is able to deliver a reversible capacity of 96.3 mA h g−1 at 30C and 80.6 mA h g−1 at 50C. After 1000 cycles at 5C, the reversible capacity of the Na3V1.6Ga0.4(PO4)3 electrode can still reach 101.1 mA h g−1 with a capacity retention of 97.3% (a very slow capacity decay of 0.0027% per cycle). These excellent electrochemical performances can be attributed largely to the highly graphitic carbon coating. Combining the analyses of transmission electron microscopy and Raman spectroscopy, it can be concluded that the Ga-doping in this series of electrode materials increases the proportion of sp2-type carbon. The development of these Na3V2−xGax(PO4)3 powders provides a promising cathode material for high power sodium-ion batteries. [ABSTRACT FROM AUTHOR]

Nanosized porous carbon-coated NaTi2(PO4)3 (NTP) particles with a high yield (more than 95%) and superior rate properties are synthesized by a reflux method in ethylene glycol (EG). We report the controllable synthesis of three novel NTP nanocubes. The as-synthesized NTP-EG shows excellent high-rate performances with charge capacities of 99.8 and 96.1 mA h g−1 at 30C and 50C. After 1000 cycles at 5C, the reversible capacity of NTP-EG can still reach 101.7 mA h g−1 with a capacity retention of 94.2%. These superb electrochemical performances can be attributed to both the unique nano-architecture and the suitable carbon layers. Their structures and morphologies are analyzed by X-ray diffraction, scanning/transmission electron microscopy, Raman spectroscopy and N2 adsorption/desorption measurements. Such a low-cost, high-yield and environment friendly synthesis strategy may be extended to other electrode materials. [ABSTRACT FROM AUTHOR]

LiNi0.80Co0.15Al0.05O2 (NCA) is explored to be applied in a hybrid Li+/Na+ battery for the first time. The cell is constructed with NCA as the positive electrode, sodium metal as the negative electrode, and 1 M NaClO4 solution as the electrolyte. It is found that during electrochemical cycling both Na+ and Li+ ions are reversibly intercalated into/de-intercalated from NCA crystal lattice. The detailed electrochemical process is systematically investigated by inductively coupled plasma-optical emission spectrometry, ex situ X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The NCA cathode can deliver initially a high capacity up to 174 mAh g−1 and 95% coulombic efficiency under 0.1 C (1 C = 120 mA g−1) current rate between 1.5-4.1 V. It also shows excellent rate capability that reaches 92 mAh g−1 at 10 C. Furthermore, this hybrid battery displays superior long-term cycle life with a capacity retention of 81% after 300 cycles in the voltage range from 2.0 to 4.0 V, offering a promising application in energy storage. [ABSTRACT FROM AUTHOR]

Layer-structured O3 type cathode materials Na1−xCr1−xTixO2 (x=0, 0.03, 0.05) are fabricated by a thermo-polymerization method. The structures and morphologies are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. It has been found that the appropriate Ti doping effectively leads to the formation of uniform morphology. As a cathode, the x=0.03 sample delivers a quite high discharge capacity of 110 mAh/g at 32 C in the voltage range from 2.0 V to 3.6 V (vs. Na/Na+) and with a capacity retention of 96% after 100 cycles at 0.2 C. The Na//Na0.97Cr0.97Ti0.03O2 cell exhibits very high coulombic efficiency (above 96%). All these results suggest that Na0.97Cr0.97Ti0.03O2 is very promising for high-rate sodium ion batteries. [ABSTRACT FROM AUTHOR]

We report, for the first time, that a nanocrystalline layered TiP2O7 as a cathode material for Li-, Na- and K-alkali metal batteries is prepared by a solvothermal process followed by a heat treatment at 600 °C. For comparison, a crystalline cubic TiP2O7 is also synthesized at 800 °C. It is clarified that the layered TiP2O7 is slightly poorer for lithium storage but much better for sodium and potassium storages than the cubic phase. The detailed electrochemical processes are investigated by means of ex situ XRD analyses. Different from the two-phase co-existence mechanism for the cubic phase, a solid-solution mechanism is identified for the layered phase to follow in the Li/Na/K intercalation reactions based on electrochemical and structural studies. To overcome the poor electronic conductivity limit for high rate applications, carbon coating is employed on the TiP2O7 samples. The carbon-coated layered phase TiP2O7 (600 °C) displays fairly reversible capacity close to 100 mA h g−1 in both Li and Na rechargeable alkali metal batteries. It is a promising electrode material for large energy storage systems. [ABSTRACT FROM AUTHOR]

A solid-state reaction process with poly(vinyl alcohol) as the carbon source is developed to synthesize LiFePO4-based active powders with or without modification assistance of a small amount of Li3V2(PO4)3. The samples are analyzed by X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy. It is found that, in addition to the minor effect of a lattice doping in LiFePO4 by substituting a tiny fraction of Fe2+ ions with V3+ ions, the change in the form of carbon coating on the surface of LiFePO4 plays a more important role to improve the electrochemical properties. The carbon changes partially from sp3 to sp2 hybridization and thus causes the significant rise in electronic conductivity in the Li3V2(PO4)3-modified LiFePO4 samples. Compared with the carbon-coated baseline LiFePO4, the composite material 0.9LiFePO4·0.1Li3V2(PO4)3 shows totally different carbon morphology and much better electrochemical properties. It delivers specific capacities of 143.6 mAh g−1 at 10 C rate and 119.2 mAh g−1 at 20 C rate, respectively. Even at the low temperature of −20 °C, it delivers a specific capacity of 118.4 mAh g−1 at 0.2 C. [ABSTRACT FROM AUTHOR]

Graphene-decorated Na3V2(PO4)3 (NVP@G) material is synthesized through an in situ catalytic process using the intermediate product component VOx as a catalyst and polyvinyl alcohol as the graphene source. NVP@G shows a superb rate performance and an ultralong cycle life. [ABSTRACT FROM AUTHOR]

Cr 2 O 3 is generally considered as an impurity phase with negative effects on the electrochemical performance of NaCrO 2 because it may cause a certain degree of capacity loss. In this study, however, we have found the bright side of Cr 2 O 3 as a protective coating material, which greatly improves the Na + storage capability, especially the cycling stability, of NaCrO 2 . After 1000 cycles at 10C, a capacity of 100.4 mAh g -1 with a high capacity retention of 84.8% can be achieved for a Cr 2 O 3 -coated NaCrO 2 sample. The optimal sample exhibits a rate performance with 108.0 mAh g -1 at a high rate of 60C. Cyclic voltammetry analysis indicates that such an in situ-formed inactive Cr 2 O 3 layer has little influence on Na + diffusion in NaCrO 2 electrodes, but it prevents the direct contact between the active material and the electrolyte, suppressing the side reactions effectively.

Surface coating and lattice doping are widely used to enhance the interfacial and structural stabilities of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (LNCM). In this paper, KF is used to modify LNCM for the first time. A Li + /K + exchange in the Li slabs is realized via a high-temperature treatment. Consequently, subsurface K + gradient doping and surface K 1- x Li x F gradient coating are obtained simultaneously on LNCM. Such an Li + /K + exchange mechanism and double-gradient modification are clarified by X-ray diffraction, energy-dispersive spectrometry line scans, and high-resolution transmission electron microscopy analyses. As a result, the optimal 0.5 wt % KF-modified LNCM material shows markedly alleviated voltage degradation (0.0031 V@1 cycle), improved cycling stability (88%@100 cycles@0.5 C), and rate capability (108 mA h g -1 @10 C), revealing large application potential in high-energy materials.