Kai Feng

Synthesis and electrochemical properties of Li3V2(P1−xBxO4)3/C cathode materials

B-doped compounds Li3V2(P1−xBxO4)3/C (x = 0, 0.01, 0.03, and 0.07) are prepared by a sol–gel method. The crystal structure, morphology and electrochemical properties of B-doped Li3V2(PO4)3 are investigated. X-ray diffraction (XRD) analysis indicates that a B atom enters the crystal structure of Li3V2(PO4)3 but does not change the monoclinic structure. Cycle stability and rate performance measurements reveal that moderate B doping improves the electrochemical properties of Li3V2(PO4)3. Among all the B-doped samples, Li3V2(P0.97B0.03O4)3/C shows the largest initial discharge capacity, best cycle stability and rate performances. In the potential range of 3.0–4.3 V, Li3V2(P0.97B0.03O4)3/C delivers a discharge capacity of 127.5 mA h g−1 at 0.2C rate, while at 20C the discharge capacity remains above 100 mA h g−1. After 100 cycles, the discharge capacity retention is 98%. Moreover, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves indicate that B doping not only decreases the charge transfer resistance but also increases the Li-ion diffusion rate. The excellent electrochemical performance of Li3V2(P0.97B0.03O4)3/C can be attributed to its larger Li ion diffusion, smaller particle size, and higher structural stability and electronic conductivity induced by B doping.

Rational design and synthesis of LiTi2(PO4)3−xFx anode materials for high-performance aqueous lithium ion batteries

Aqueous lithium ion batteries have shown advantages of high safety and low cost, because of the use of nontoxic and nonflammable aqueous electrolytes. LiTi2(PO4)3/LiMn2O4 is considered as one of the most promising aqueous lithium ion battery systems for its moderate working voltage, and high electrochemical stability in aqueous electrolytes. However, the critical issue of hydrogen evolution during the charge process hinders its development. In this paper, F− was first introduced in LiTi2(PO4)3 to raise the ion intercalation potential and further to solve the hydrogen evolution problem. Besides, F− doping can decrease the band gap and further increase the intrinsic electronic conductivity. Additionally, the diffusion coefficient of Li+ increased by one order of magnitude after F− doping. Combined with the elevated potential and high conductivity, F-doped LiTi2(PO4)3 exhibited excellent cycling ability and rate performance. As a result, the power density and energy density of LiTi2(PO4)2.88F0.12/LiMn2O4 full cells reach 2794 W kg−1 and 43.7 W h kg−1 respectively, which are among the highest values ever reported for aqueous lithium or sodium ion batteries. The F− doping strategy was demonstrated to be a facile and effective method to fabricate anode materials for high-performance aqueous lithium ion batteries.

Low Cost Room Temperature Synthesis of NaV3O8.1.69H2O Nanobelts for Mg Batteries

Potentially safe and economically feasible magnesium batteries (MBs) have attracted tremendous research attention as an alternative to high-cost and unsafe lithium ion batteries. In the current work, for the first time, we report a novel room-temperature approach to dope the atomic species sodium between the vanadium oxide crystal lattice to obtain NaV3O8·1.69H2O (NVO) nanobelts. The synthesized NVO nanobelts are used as electrode materials for MBs. The MB cells demonstrate stable discharge specific capacity of 110 mA h g-1 at a current density of 10 mA g-1 and a high cyclic stability, that is 80% capacity retention after 100 cycles, at a current density of 50 mA g-1. Moreover, the effects of cutoff voltages (ranging from 2 to 2.6 V) on their electrochemical performance were investigated. The reason for the limited specific capacity of MBs is attributed to the trapping of Mg ions inside the NVO lattices. This work opens up a new pathway to explore different electrode materials for MBs with improved electrochemical performance.

Facile construction of nanoscale laminated Na3V2(PO4)3 for a high-performance sodium ion battery cathode

A novel facile construction of nanoscale laminated Na3V2(PO4)3 for a high-performance sodium ion battery (SIB) cathode is proposed. In the synthesis process, a crystallized intermediate precursor with low-cost raw materials is prepared by introducing a high temperature molten-state NH3 thermal-reduction process, which acts as a reaction template to control the crystal growth and the final morphology of Na3V2(PO4)3. The as-synthesized nanoscale laminated Na3V2(PO4)3 possesses continuous Na+/electron pathways, large electrode/electrolyte contact area and sufficient carbon coating, resulting in fast Na+ extraction/insertion and electron transport during the electrochemical reaction process, which is shown to achieve excellent rate capability and decent cycling stability. At a low rate of 0.5C, the discharge specific capacity is approximately 117 mA h g−1, which is very close to its theoretical specific capacity, and there is only a very minor capacity fade after continuous 250 cycles at 2C. Even at 50C, the discharge specific capacity is as high as 80 mA h g−1 and the reversible capacity retention after 3000 cycles remains more than 78%. In addition, the X-NVP cathode shows stable cycling performance and acceptable rate performance with a reversible capacity of 110 mA h g−1 at 0.2C at a low temperature of −20 °C, which has rarely been reported previously in the SIB field. The intermediate precursor prepared by the high temperature molten-state thermal-reduction method, acting as the reaction template of the final product, provides a facile and economic solution for the synthesis of high-performance SIB cathode materials.

One-pot synthesis of 3D hierarchical porous Li3V2(PO4)3/C nanocomposites for high-rate and long-life lithium ion batteries

Li3V2(PO4)3 (abbreviated as LVP) is considered as a prospective cathode material for next-generation Li ion batteries due to its high specific capacity and high operating potential. However, its low electronic conductivity and the difficulty in morphology control restrict its widespread application. Carbon coating has been proved to be an effective method for solving these problems. However, too thick a carbon layer will act as a barrier for Li+ diffusion. 3D hierarchical porous materials with unique electronic and structural properties have exhibited outstanding advantages. However, most methods employed for fabricating the hierarchical porous materials need templates and need acid or alkali to remove the template afterwards. This is detrimental to the LVP material because the LVP reacts with both acid and alkali. Here we present a rational design for a hierarchical porous LVP/C nanocomposite via a one-pot process, in which F127 molecules and the LVP colloids induce self-assembly. After high-temperature annealing, the mesoporous structure was developed due to the decomposition of the F127 and then the LVP/C clusters piled up to form stacked macropores. As a cathode for Li ion batteries, the LVP/C nanocomposite exhibited excellent cycle stability (96% capacity retention over 800 cycles) and enhanced high-rate performance (117 mA h g−1 at 20C). This method provides a new approach for synthesizing high-performance 3D hierarchical porous cathode materials used in other energy storage applications.

A novel facile and fast hydrothermal-assisted method to synthesize sulfur/carbon composites for high-performance lithium–sulfur batteries

Hydrothermal-assisted sulfur impregnation method was first proposed to prepare sulfur/carbon (S/C) composites for lithium–sulfur (Li–S) battery applications. Comparing with the currently existing sulfur impregnation method, this facile one-pot method is proved to be energy-saving and time-saving, to have a sulfur content that is exactly controllable and to be environment-friendly. In the hydrothermal environment, sulfur would selectively diffuse into the pores of carbon hosts due to its high mobility, homogeneous dispersibility, hydrophobicity and carbon affinity. As a result, the S/C composite obtained from hydrothermal-assisted method under a low temperature of 120 °C and a short time of 2 hours exhibits a comparable battery performance to that obtained from the traditional melting method under 155 °C for 20 hours, which reached 1239 mA h g−1 at 0.2C and 796 mA h g−1 at even 1C between 1.85–2.8 V, being rather suitable for large-scale manufacture and commercial development.