NaV6O15 microflowers as a stable cathode material for high-performance aqueous zinc-ion batteries

Reversible aqueous zinc-ion batteries (ZIBs) have great potential for large-scale energy storage owing to their low cost and safety. However, the lack of long-lifetime positive materials severely restricts the development of ZIBs. Herein, we report NaV₆O₁₅ microflowers as a cathode material for ZIBs with excellent electrochemical performance, including a high specific capacity of ∼300 mA h g⁻¹ at 100 mA g⁻¹ and 141 mA h g⁻¹ maintained after 2000 cycles at 5 A g⁻¹ with a capacity retention of ∼107%. The high diffusion coefficient and stable tunneled structure of NaV₆O₁₅ facilitate Zn²⁺ intercalation/extraction and long-term cycle stability.

NaV₆O₁₅ microflowers were synthesized as a stable cathode material for aqueous zinc ion batteries, which show a high specific capacity and excellent long-term cycling performance.     

A hybrid composite of H2V3O8 and graphene for aqueous lithium-ion batteries with enhanced electrochemical performance

Aqueous rechargeable lithium-ion batteries (ARLBs) are regarded as a competitive challenger for large-scale energy storage systems because of their high safety, modest cost, and green nature. A kind of modified composite material composed of H₂V₃O₈ nanorods and graphene sheets (HVO/G) has been effectively made by a one-step hydrothermal method and following calcination at 523 K. XRD, SEM, TEM, and TG are used to determine the phase structures and morphologies of the composite materials. Owing to the advantage of the layered structure of H₂V₃O₈ nanorods, the excellent conductivity of the graphene sheets, and the 3D network structure of the modified composite, the ARLBs with HVO/G can deliver an adequate specific capacity of 271 mA h g⁻¹ at 200 mA g⁻¹ and have a retention rate of 73.4% after 50 cycles. The average discharge capacity of ARLB with HVO/G as anode has a considerable improvement over that of HVO/CNTs and HVO, whatever the current rate used. Moreover, we find that the diffusion coefficient of lithium-ion increases by an order of magnitude through the theoretical calculation for HVO/G ARLB. The new ARLB with HVO/G electrode is a potential energy storage system with great advantages, such as simple preparation, easy assembly process, excellent safety and low-cost environmental protection.

Aqueous rechargeable lithium-ion batteries (ARLBs) are regarded as a competitive challenger for large-scale energy storage systems because of their high safety, modest cost, and green nature.     

La2O3-coated Li2ZnTi3O8@C as a high performance anode for lithium-ion batteries

Li₂ZnTi₃O₈C@La₂O₃ (LZTO@C@La₂O₃) coated with composite protective layers is successfully fabricated via a facile solid-state route. The co-coating strategy greatly improves the electrochemical performance of LZTO. 89.8%, 77.2% and 76.7% of the discharge specific capacities for the 2nd cycle can be retained at the 200th cycle at 1, 2 and 3 A g⁻¹, respectively. At 4 and 5 A g⁻¹, 174.3 and 166.1 are still retained for the 100th cycle, respectively. Even at a high temperature of 55 °C, LZTO@C@La₂O₃ still has good cycling performance. The excellent electrochemical performance is due to the stable surface structure between LZTO and the electrolyte, a good conductive network, small particle size, and large specific surface area as well as pore volume.

LZTO@C@La₂O₃ coated with composite protective layers with excellent electrochemical performance has been synthesized using a simple solid-state method.     

K-doped Li2ZnTi3O8/C as an efficient anode material with high performance for Li-ion batteries

Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were successfully synthesized using the sol–gel method. Doping K apparently yielded a wider tunnel, helpful for increasing the rate of transport of lithium ions, and furthermore yielded excellent electrochemical properties. The first discharge capacity for Li_1.9K_0.1ZnTi₃O₈/C was 352.9 mA h g⁻¹ at a current density of 200 mA g⁻¹. Li_1.9K_0.1ZnTi₃O₈/C also performed stably, retaining a capacity of 323.7 mA h g⁻¹ at the 100th cycle, indicative of its excellent cycling properties. In the rate performance test, Li_1.9K_0.1ZnTi₃O₈/C showed at the first cycle a high discharge capacity of 379.5 mA h g⁻¹ for a current density of 50 mA g⁻¹ and a capacity of 258.9 mA h g⁻¹ at 1000 mA g⁻¹. The results indicated that K-doping should be considered a useful method for improving electrochemical performances.

(A and B) CV curves of (A) Li₂ZnTi₃O₈/C and (B) Li_1.9K_0.1ZnTi₃O₈/C, (C) Initial charge–discharge curves and (D) cycling capacity and (E) rate performances of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C. (F) The composition of the CR2032 cell.     

A stable tunnel-type NaGe3/2Mn1/2O4 anode for Na-ion batteries

Tunnel-type NaGe_3/2Mn_1/2O₄ was fabricated for anode of sodium ion batteries, delivering a discharge capacity of 200.32 mAh g⁻¹ and an ultra-low potential platform compared with that of pure Na₄Ge₉O₂₀ (NGO). The results of X-ray photoelectron spectroscopy (XPS) demonstrate that Ge redox occurs, and partial substitution of Mn effectively improves the Na-storage properties compared to those of NGO.

We investigated tunnel-type NaGe_3/2Mn_1/2O₄; the main structure is Na₄Ge₉O₂₀. NaGe_3/2Mn_1/2O₄ electrodes as anodes for sodium ions batteries deliver a discharge capacity of 200.32 mAh g⁻¹ and satisfactory capacity retention after 50 cycles.

In terms of the high abundance and ready availability of sodium, sodium-ion batteries (SIBs) have been generally regarded as a better alternative to lithium-ion batteries for power stations.^1–4 Hard carbon is widely recognized as one of the most attractive and ideal anode materials for SIBs.^5,6 However, the potential required for sodium ions to insert into hard carbon is very close to that for sodium plating, resulting in sodium dendrites, which raise safety concerns.^7,8 Moreover, the reaction of electrode materials with sodium through alloying or conversion mechanisms always results in serious volume changes in the process of sodium insertion and extraction.⁹ Therefore, insertion-type transition-metal oxides as anodes have attracted much attention owing to their suitable operating potentials and minor volume expansion.^10,11 Recently, embedded titanium/vanadium/molybdenum based oxides with layered structures have been studied as anode materials for SIBs,¹² such as layered Na₂Ti₃O₇,¹³ tunnel Na₂Ti₆O₁₃ (ref. 14) and spinel Li₄Ti₅O₁₂.¹⁵ In addition, post-spinel structured materials have been proposed, which show ultra-stable cycle performances via highly reversible sodium-ion insertion/desertion through large-size tunnels. Recently, in Zhou''s group, NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄ (ref. 17) have been prepared and they have been shown to possess robust cycle lifetimes (more than 10 000 cycles) and discharge plateaus of 0.84 V and 0.7 V, respectively. Meanwhile, in our group, Na_0.76Mn_0.48Ti_0.44O₂ has been developed, which holds an initial discharge capacity of 103.4 mAh g⁻¹, shows a superb rate capability and retains 74.9% capacity after 600 cycles.¹⁸ The large radius of the redox active metal center could optimize the tunnel size and thus boosting the electrochemical performance. It is also a big challenge to find further suitable active centers for insertion-type transition metal oxides as anodes of SIBs. Besides, a host of published reports have said that germanium-based materials can be used as alloy anodes for SIBs with highly reversible sodium storage properties and satisfactory ionic/electronic conductivity.¹⁹ However, it is unclear to us whether Ge could act as an active center in a transition-metal oxide anodes.In this work, we fabricated a tunnel-type NaGe_3/2Mn_1/2O₄ (NGMO) material. When used as the anode of SIBs, it delivers a sustained discharge capacity of 200.32 mAh g⁻¹. Compared with NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄,¹⁷ NGMO delivers a lower safety voltage of 0.36 V. Pure Na₄Ge₉O₂₀ (NGO) as a comparative sample, only exhibits a capacity of 24.8 mAh g⁻¹, which is far inferior to that of NGMO. During discharge and charge process, reversible redox reactions around Ge center occur, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The introduction of Mn in the NGMO improves the reversibility of the Ge redox performance.The structure of NGMO was carefully characterized by XRD, and Rietveld refinement was performed as depicted in Fig. 1. The main Bragg peaks of NGMO could be assigned to space groups of P1(2) and I4₁/a(88), which were fitted to give lattice parameters of a = 10.56/15.04 Å, b = 11.18/15.04 Å, and c = 9.22/7.39 Å, and a volume of 811.2/1672.2 ų, respectively. Na₄Ge₉O₂₀ has a typical tunnel structure, which consists of polymerized Ge/MnO₄ tetrahedra connected with Ge/MnO₆ octahedra. Four Ge/MnO₆ octahedra are connected together by sharing edges to form a tetrameric (Ge/Mn)₄O₁₆ cluster. Each cluster is connected to six GeO₄ tetrahedra, and adjacent clusters are connected by GeO₄ tetrahedra. Na atoms are located in the channels and have elongated Na–O bonds.¹⁹ This highly stable crystal structure can effectively accelerate the migration of sodium ions.²⁰Fig. 2 shows the low and high magnification scanning electron microscopy (SEM) images of NGMO, which is composed of particles of different sizes from 1 to 3 μm; the larger particles are the result of sintering at high temperature. SEM images of NGO with different magnifications are given in Fig. S1,^† and show that the average particle size of NGO is 1 μm.Open in a separate windowFig. 1The Rietveld refinement spectra of NGMO.Open in a separate windowFig. 2(a) and (b) SEM images of NGMO at different magnifications.The morphology and fine structure were studied by transmission electron microscopy (TEM). Fig. 3a and b show the low magnification TEM images. It can be seen from the images that NGMO has an irregular sheet-like morphology with particle sizes from 250 nm to 2 μm. As shown in Fig. 3c, the lattice spacing of the (200) plane is 4.55 Å. In the SAED pattern of Fig. 3d, the red line corresponds to the (020) plane in NGMO, and the lattice spacing is 13.100 Å. These results clearly demonstrate that NGMO exhibits good crystallinity. The corresponding energy dispersive X-ray spectroscopy (EDS) results, and Raman and infrared spectra (IR) are provided in Table S1 and Fig. S2.^† The results indicate that the atomic ratio of Na : Ge : Mn is close to 1 : 1.5 : 0.5 and that there is little sodium loss. The Raman and IR peaks in the high frequency region are attributed to stretching vibrations of Ge–O–Ge and the peaks between 600 and 400 cm⁻¹ are attributed to the bending vibrations of Ge–O–Ge in NGMO.Open in a separate windowFig. 3(a) and (b) Low resolution TEM images, (c) a HRTEM image and (d) a SAED image of NGMO.Galvanostatic electrochemical measurements were evaluated in a voltage range of 0.05–2.0 V, with the current density of 20 mA g⁻¹. Fig. 4a and b show the discharge and charge profiles of NGMO and NGO, respectively. Because of the formation of a solid electrolyte interface (SEI) layer in the initial cycle, the electrochemical behaviour tends stabilize in the second cycle, so voltage profiles are given from the second cycle; the first cycles of the discharge–charge curves of NGMO and NGO materials are given in Fig. S3.^† It can be seen intuitively that both NGMO and NGO have low voltage platforms, while NGMO has the smaller polarization. In Fig. 4a, we notice a reversible voltage profile in the second cycle with discharge capacity of 200.32 mAh g⁻¹ for NGMO. Only NGMO has a flat potential platform and delivers an ultra-low plateau potential. It can be seen from Fig. 4b that NGO shows a capacity of 24.8 mAh g⁻¹, obvious polarization at the 20^th cycle and increased capacity due to the surface side-effect. Fig. 4c indicates that the capacity retention of the NGMO electrode after 50 cycles is 86.2%, which is superior to that of NGO; the coulombic efficiency of NGMO is also provided in Fig. S4.^† To further understand the redox reactions along with the discharge/charge process in NGMO, Fig. 4d displays the differential capacity versus voltage (dQ/dV) curve. The clear anodic peak at 0.33 V and cathodic peak at 0.81 V correspond well with the redox reactions of NGMO.Open in a separate windowFig. 4Electrochemical performance: (a) and (b) voltage profiles of NGMO and NGO, respectively; (c) cycling performance; (d) dQ/dV profile of NGMO. The current density was controlled at 20 mA g⁻¹ over a voltage range of 0.05–2.0 V.The electrochemical impedance spectra (EIS) of fresh and cycled electrodes of NGMO and NGO, with a frequency range of 0.01 Hz to 100 kHz, are shown in Fig. 5. From Fig. 5a, it is obvious that the charge-transfer resistance of the fresh NGMO electrode is lower than that of NGO. This indicates that the migration of charges in the NGMO material occurs more easily than in NGO, which also facilitates the shifting of ions on the surface and inside of the electrodes of NGMO. In Fig. 5b, NGMO electrode in its 10^th cycle exhibits a smaller charge-transfer resistance than both NGO and the NGMO fresh electrode, indicating that the surface of NGMO more readily forms a stable SEI film.¹⁸ The EIS results were fitted by the model shown in Fig. S5.^† The fitting results are provided in Table S2.^† The resistances of the fresh and cycled electrodes of NGMO and NGO are composed of an internal resistance (R_s), the resistance of the surface film (SEI) (R_f; a small semicircle in the high frequency region), the resistance of the charge transfer (R_ct; another opposite semicircle in the middle frequency region), and the Warburg resistance (W; an oblique line in the low frequency region).²¹ Both the fresh and cycled electrodes of NGMO deliver lower charge transfer resistance than NGO. Meanwhile, the transfer resistance of the cycled NGMO electrode is lower than that of the fresh electrode and its slope at low frequency is higher than that of the fresh one (Table S2^†). All these results show that NGMO has lower resistance and better electronic/ionic conductivity than NGO.Open in a separate windowFig. 5Nyquist plots of (a) fresh NGMO and NGO electrodes, and (b) NGMO and NGO electrodes after ten cycles.The evolution of the chemical valence states of the 150-times discharged electrodes were observed by XPS and SEM as provided in Fig. S6.^† It is generally clear that the electrode surface was covered with a thick SEI layer after discharging. Ar plasma etching was used to obtain the internal information. The Ge 3d core-level of the discharged NGMO electrode with and without etching is shown in Fig. S7.^† Before etching, the peaks of the Ge 3d core-level could be fitted to Ge¹⁺ and Ge²⁺.²² After etching, (Fig. S7c^†), the peaks at 30.8 eV and 30.2 eV were also associated with Ge¹⁺ and Ge²⁺, respectively. This result indicates that the valence of Ge decreases as a whole and that there is no obvious difference between the etched and non-etched samples. The reversible redox reactions of Ge remain stable even after cycling. Meanwhile, the Mn 2p core-level spectra are shown in Fig. S7d–f.^† For the Mn 2p core level, owing to the spin orbit coupling, the valence states of Mn comprise two couples including Mn³⁺ and Mn²⁺ (Fig. S7e and f^†). The binding energies of Mn³⁺ are 642.37 eV and 654.04 eV, and the binding energies of Mn²⁺ are 640.71 eV and 652.18 eV. Similarly, after discharging, the binding energies of Mn³⁺ are 642.40 eV and 654.06 eV, and those for Mn²⁺ are 640.69 eV and 652.20 eV, indicating that there are no changes in Mn binding energies before and after etching. This is in good agreement with results in previous reports.^23,24 All results also show that a thin SEI layer has been formed, favoring ions transfer on the repeatedly cycled electrode. It can be seen from the refined XRD results that NGMO consists of Ge⁴⁺, Mn²⁺ and Mn³⁺, and combined with XPS analysis, the results show that the valence states of Mn does not change for the discharged NGMO electrode. Ge displays electrochemical activity in NGMO, and Mn exhibits good chemical stability in the framework.     

Li2ZnTi3O8/graphene nanocomposite as a high-performance anode material for lithium-ion batteries

An Li₂ZnTi₃O₈/graphene (LZTO/G) anode is successfully synthesized by a two-step reaction. The results show that LZTO particles can be well dispersed into the graphene conductive network. The conductive structure greatly improves the electrochemical performance of LZTO/G. When cycled for 400 cycles, 76.4% of the capacity for the 2nd cycle is maintained at 1 A g⁻¹. Also, 174.8 and 156.5 mA h g⁻¹ are still delivered at the 100th cycle for 5 and 6 A g⁻¹, respectively. The excellent cyclic performance and the large specific capacities at high current densities are due to the good conductive network of the LZTO active particles, large pore volume, small particle size, low charge-transfer resistance and high lithium diffusion coefficient.

The structure of G modifying LZTO is beneficial for the diffusion of Li⁺ ions and transportation of electrons.     

Role of polyvinylpyrrolidone in the electrochemical performance of Li2MnO3 cathode for lithium-ion batteries

While Li₂MnO₃ as an over-lithiated layered oxide (OLO) shows a significantly high reversible capacity of 250 mA h g⁻¹ in lithium-ion batteries (LIBs), it has critical issues of poor cycling performance and deteriorated high rate performance. In this study, modified OLO cathode materials for improved LIB performance were obtained by heating the as-prepared OLO at different temperatures (400, 500, and 600 °C) in the presence of polyvinylpyrrolidone (PVP) under an N₂ atmosphere. Compared to the as-prepared OLO, the OLO sample heated at 500 °C with PVP exhibited a high initial discharge capacity of 206 mA h g⁻¹ and high rate capability of 111 mA h g⁻¹ at 100 mA g⁻¹. The superior performance of the OLO sample heated at 500 °C with PVP is attributed to an improved electronic conductivity and Li⁺ ionic motion, resulting from the formation of the graphitic carbon structure and increased Mn³⁺ ratio during the decomposition of PVP.

The modified OLO cathode materials for improved LIB performance were obtained by heating the as-prepared OLO in the presence of polyvinylpyrrolidone (PVP) under an N₂ atmosphere.     

Rod-like anhydrous V2O5 assembled by tiny nanosheets as a high-performance cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion batteries offer a low-cost and high-safety alternative for next-generation electrochemical energy storage, whereas suitable cathode materials remain to be explored. Herein, rod-like anhydrous V₂O₅ derived from a vanadium-based metal–organic framework is investigated. Interestingly, this material is assembled by tiny nanosheets with a large surface area of 218 m² g⁻¹ and high pore volume of 0.96 cm³ g⁻¹. Benefiting from morphological and structural merits, this material exhibits excellent performances, such as high reversible capacity (449.8 mA h g⁻¹ at 0.1 A g⁻¹), good rate capability (314.3 mA h g⁻¹ at 2 A g⁻¹), and great long-term cyclability (86.8% capacity retention after 2000 cycles at 2 A g⁻¹), which are significantly superior to the control sample. Such great performances are found to derive from high Zn²⁺ ion diffusion coefficient, large contribution of intercalation pseudocapacitance, and fast electrochemical kinetics. The ex situ measurements unveil that the intercalation of Zn²⁺ ion is accompanied by the reversible V⁵⁺ reduction and H₂O incorporation. This work discloses a direction for designing and fabricating high-performance cathode materials for zinc-ion batteries and other advanced energy storage systems.

V₂O₅ with intriguing micro/nano-hierarchical structure is fabricated via the pyrolysis of MIL-47 (a MOF material) and displays great performances as the cathode material for aqueous zinc-ion batteries.     

ZrO2 coated Li1.9K0.1ZnTi3O8 as an anode material for high-performance lithium-ion batteries

The Li_1.9K_0.1ZnTi₃O₈@ZrO₂ (1 wt%, 3 wt%, and 5 wt%) anode material was synthesized by doping Li₂ZnTi₃O₈ with potassium and coating ZrO₂, where the ZrO₂ coating layer was prepared by citric acid and zirconium acetate, and the potassium source was KCl. When the added ZrO₂ amount is 3%, the material has the most uniform size, reduced polarization, and reduced charge transfer resistance, and the specific capacity of LKZTO@ZrO₂ (3 w%) was 361.5 mA h g⁻¹ at 200 mA g⁻¹ at the 100th cycle, which is higher than that of LKZTO, of 311.3 mA h g⁻¹. The specific capacities of LKZTO@ZrO₂ (3 w%) at 50, 100, 200, 500, and 1000 mA g⁻¹ after 10 cycles were 424.9, 410.7, 394.1, 337.6 and 270.6 mA h g⁻¹, indicating that LKZTO@ZrO₂ (3 w%) has excellent electrochemical performance.

The Li_1.9K_0.1ZnTi₃O₈@ZrO₂ (1 wt%, 3 wt%, and 5 wt%) anode material was successfully synthesized, where the ZrO₂ coating layer was prepared by citric acid and zirconium acetate, and the potassium source was KCl.     

High performance composites of spinel LiMn2O4/3DG for lithium ion batteries

A highly crystalline nanosized spinel LiMn₂O₄/3DG composite cathode material for high rate lithium ion batteries was successfully prepared by mixing spinel LiMn₂O₄ particles with reduced graphene oxide (3DG). Spinel LiMn₂O₄ and reduced three-dimensional graphene oxide were synthesized using a hydrothermal method and freeze-drying technology, respectively. The structure, morphology and electrochemical performance of the synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge techniques. The results showed that the LiMn₂O₄/3DG composites exhibited excellent rate capability and stable cycling performance. The discharge capacity was 131 mA h g⁻¹ and the capacity remains at 89.3% after 100 cycles at a 0.5 C rate, while the discharge capacity was 90 mA h g⁻¹ at 10 C. Compared with spinel LiMn₂O₄ materials, the LiMn₂O₄/3DG composites showed obvious improvement in electrochemical performance.

3DG/LiMn₂O₄ composites exhibit a high specific capacity and excellent rate performance.     

Preparation and electrochemical performance of VO2(A) hollow spheres as a cathode for aqueous zinc ion batteries

Rechargeable aqueous zinc ion batteries (ZIBs), owing to their low-cost zinc metal, high safety and nontoxic aqueous electrolyte, have the potential to accelerate the development of large-scale energy storage applications. However, the desired development is significantly restricted by cathode materials, which are hampered by the intense charge repulsion of bivalent Zn²⁺. Herein, the as-prepared VO₂(A) hollow spheres via a feasible hydrothermal reaction exhibit superior zinc ion storage performance, large reversible capacity of 357 mA h g⁻¹ at 0.1 A g⁻¹, high rate capability of 165 mA h g⁻¹ at 10 A g⁻¹ and good cycling stability with a capacity retention of 76% over 500 cycles at 5 A g⁻¹. Our study not only provides the possibility of the practical application of ZIBs, but also brings a new prospect of designing high-performance cathode materials.

VO₂(A) hollow spheres exhibit superior zinc ion storage performance, large reversible capacity of 357 mA h g⁻¹ at 0.1 A g⁻¹, and good cycling stability with a capacity retention of 76% over 500 cycles at 5 A g⁻¹     

Hierarchical structured Mn2O3 nanomaterials with excellent electrochemical properties for lithium ion batteries

A series of Mn₂O₃ nanomaterials with hierarchical porous structures was synthesized using three types of leaves as templates. In addition to their different morphologies, different porous nanostructures were achieved by choosing different leaves. The Mn₂O₃ nanomaterial prepared by using gingko leaves as a template provides a larger pore volume and a higher Brunauer–Emmett–Teller (BET) surface area. At the same time, this material also displays excellent electrochemical performance, that is, the specific capacities are 1274.6 mA h g⁻¹ after 300 cycles and 381.5 mA h g⁻¹ at current densities of 300 and 3000 mA g⁻¹, respectively.

A series of Mn₂O₃ nanomaterials with hierarchical porous structures was synthesized using three types of leaves as templates.     

The electrochemical performance of fluorinated ketjenblack as a cathode for lithium/fluorinated carbon batteries

The inferior rate capacity of lithium/fluorinated carbon (Li/CF_x) batteries limits their application in the field, requiring large discharge current and high power density. Herein, we report a novel type of fluorinated carbon with superior performance through gas-phase fluorination of ketjenblack. The investigation shows that the F/C ratio of the fluorinated ketjenblack (FKB) increases with the fluorination temperature, whereas the discharge voltage decreases due to the lowered content of semi-ionic C–F bonds. Accordingly, a suitable fluorination temperature of 520 °C was selected, under which the product exhibits the largest specific capacity of 924.6 mA h g⁻¹ with discharge potential exceeding 3.1 V (vs. Li/Li⁺) and the highest energy density of 2544 W h kg⁻¹ with power density of 27 493 W kg⁻¹. This energy density is higher than the theoretical energy density of commercial fluorinated graphite (2180 W h kg⁻¹). In addition, the sample delivers good rate capability demonstrated by a specific capacity retention ratio of 79.5% even at a current density of 20C. Therefore, the FKB material may have very promising practical applications in lithium primary batteries.

Fluorinated kejtenblack as the cathode of Li/CF_x batteries exhibits excellent energy density and power density with high rate capability.     

CuFeO2–NiFe2O4 hybrid electrode for lithium-ion batteries with ultra-stable electrochemical performance

Stable electrode materials with guaranteed long-term cyclability are indispensable for advanced lithium-ion batteries. Recently, delafossite CuFeO₂ has received considerable attention, due to its relative structural integrity and cycling stability. Nevertheless, the low conductivity of delafossite and its relatively low theoretical capacity prevent its use as feasible electrodes for next-generation batteries that require higher reversible capacities. In this work, we suggest a simple and straightforward approach to prepare CuFeO₂–NiFe₂O₄ by introducing Ni precursor into Cu and Fe precursor to form NiFe₂O₄, which exhibits higher capacity but suffers from capacity fading, through sol–gel process and subsequent heat treatments. The presence of both NiFe₂O₄ and CuFeO₂ is apparent, and the heterostructure arising from the formation of NiFe₂O₄ within CuFeO₂ renders some synergistic effects between the two active materials. As a result, the CuFeO₂–NiFe₂O₄ hybrid sample exhibits excellent cycling stability and improved rate capability, and can deliver stable electrochemical performance for 800 cycles at a current density of 5.0 A g⁻¹. This work is an early report on introducing a foreign element into the sol–gel process to fabricate heterostructures as electrodes for batteries, which open up various research opportunities in the near future.

Novel NiFe₂O₄–CuFeO₂ heterostructures were synthesized by sol–gel process and subsequent heat treatments, which exhibit excellent long-term high-rate cyclability.     

Synthesis of manganese-based complex as cathode material for aqueous rechargeable batteries

A low-cost and eco-friendly system based on a manganese-based complex cathode and zinc anode was demonstrated. The cathode is able to reversibly (de-)insert Zn²⁺ ions, providing a high capacity of 248 mA h g⁻¹ at 0.1 A g⁻¹. Ex situ TEM and XRD were utilized to determine the electrochemical mechanism of this high capacity cathode. Moreover, the contribution of pre-added Mn²⁺ in electrolyte to the capacity was revealed, and nearly 18.9% of the capacity is ascribed to the contribution of pre-added Mn²⁺. With the help of additive, this aqueous rechargeable battery shows outstanding electrochemical property. Its cycling performance is good with 6% capacity loss after 2000 cycles at 4.0 A g⁻¹, highlighting it as a promising system for aqueous rechargeable battery applications.

Manganese-based complex with high energy density shows a capacity retention of 94% over 2000 cycles.     

Preparation and study of characteristics of LiCoO2/Fe3O4/Li2B2O4 nanocomposites as ideal active materials for electrochemical hydrogen storage

The nanocomposites of LiCoO₂/Fe₃O₄/Li₂B₂O₄ were designed by the Pechini route using different fuels for the optimization of their morphology and structure. Compared to other fuels, citric acid can act as both an ideal fuel and a capping agent. The ratio of the EG : H₂O mixture is another parameter, which was studied in terms of its effects on the structural characterization. The optimized sample with a rod shape was selected to compare with the bulk sample through electrochemical hydrogen storage capacity. The discharge capacity for rod-shaped nanocomposites measured was 1284 mA h g⁻¹. However, the discharge capacity for the bulk morphology was calculated to be about 694 mA h g⁻¹. The magnetic, electrochemical and structural analyses were performed to investigate the properties of LiCoO₂/Fe₃O₄/Li₂B₂O₄ nanocomposites.

To the best of our knowledge, for the first time, the effects of LiCoO₂/Fe₃O₄/Li₂B₂O₄ nanocomposites as a favorable hydrogen capacitor were investigated to enhance hydrogen storage performance.     

Rheological phase reaction synthesis and electrochemical performance of rufigallol anode for lithium ion batteries

1,2,3,5,6,7-hexahydroxy-anthraquinone (rufigallol) and its metal–organic complex (rufigallol-Li/Ni, R-LN) were both synthesized. The electrochemical performance investigation of rufigallol and R-LN as anodes for lithium ion batteries indicates that pure rufigallol delivers high initial capacity but poor cycling stability, by contrast, the R-LN complex exhibits high initial capacity and excellent cycling stability.

Rufigallol was synthesized by rheological phase method with high yield and investigated as anode for lithium ion batteries.

Lithium ion batteries (LIBs) have proven to be clean and efficient energy-storage technologies to meet the growing demand for green and sustainable electric power storage. Considering the sustainability, low cost, abundant natural sources, structural design with tolerance for variable functional groups, fast reaction kinetics and high power density, organic electrode materials have been the most competitive alternative to traditional inorganic materials.^1–6 Quinone organic materials with carbonyl functional group have received great concern for their high theoretical capacities and reaction reversibility. While, the dissolution of small molecule quinones in the organic electrolyte and the resulting capacity attenuation have restricted their application. Therefore, it is urgently demanded to design new organic electrode materials with high energy efficiency and good cycling stability. Among small molecule quinones, 1,2,3,5,6,7-hexahydroxy-anthraquinone (rufigallol) as a critical component has been widely used in dye industry, drug synthesis, and organic materials.^7–10 Rufigallol was firstly synthesized in 1836.¹¹ Since then, very little efficient method for the preparation of rufigallol has been reported. Recently, Bisoyi and Kumar''s research indicates that the rufigallol can be achieved by self-condensation of gallic acid in the presence of sulfuric acid under microwave-assisted, and the yield reached 86%.¹² Also, as far as we know, there is no correlative literature exists about rufigallol being used as electrode materials for lithium ion batteries.It is well-known that the dissolution of organic molecule in the electrolyte can be effectively suppressed via salt formation. Therefore, constructing the metal–organic complex is an efficient way to obtain a stable and flexible framework as well as a better cycling stability.^13–19 In our previous study,^20,21 we have successfully realized the improvement of cycling stability for 3,4,9,10-perylene-tetracarboxylic acid-dianhydride (NTCDA) through the introducing of Li/Ni or Co/Mn to the matrix material. The obtained metal–organic complex, namely, Li/Ni-1,4,5,8-naphthalenetetracarboxylate or Co/Mn-1,4,5,8-naphthalenetetracarboxylate, showed a high specific capacity and a good cycling stability.Herein, we developed a simple, economical and effective rheological phase method²² to synthesize the rufigallol with high yield and investigated its electrochemical application in lithium ion batteries. In view of the easily connecting of hydroxyl groups in aromatic carbonyl compound for rufigallol with metal ions, in order to modify the cycling stability of pure rufigallol, we synthesized rufigallol-Li/Ni complex (R-LN) through the introducing of lithium and nickel by a hydrothermal method. When used as the LIBs anode, rufigallol shows initial discharge and charge capacities of 977 mA h g⁻¹ and 460 mA h g⁻¹, respectively, which is far higher than that of traditional graphite anode. The R-LN complex delivers initial a charge capacity of 560 mA h g⁻¹ and still remain at about 500 mA h g⁻¹ after 100 cycles, indicating a good electrochemical performance. Our work presents a new quinone-based organic materials with high capacity and competitive cycling stability, which enriches the organic electrode materials.     

Synthesis and electrochemical performances of Na3V2(PO4)2F3/C composites as cathode materials for sodium ion batteries

Na₃V₂(PO₄)₂F₃ (NVPF) with NASCION (Na superionic conductor) is recognized as a potential cathode material owing to its high theoretical capacity. However, the electronic conductivity of NVPF is much lower than its ionic conductivity, which seriously affects the properties of this material. The carbon layer can be used as the conductive medium to enhance the conductivity of NVPF. In this study, we propose a single-step solid-state reaction method based on mechanical activation with pitch as the carbon source to synthesize NVPF/C composites. The crystallographic structure and morphology of all as-prepared samples were investigated by XRD, Raman spectroscopy, BET measurement, thermal analysis, SEM and TEM. Furthermore, the electrochemical performance and kinetic properties were analyzed by CV, galvanostatic charge–discharge and EIS tests. These tests outcomes demonstrated that the NVPF/C-2 composite with a carbon content of 12.14 wt% showed an excellent rate performance and cycle stability. It presented reversible capacities of 103 and 95 mA h g⁻¹ at 0.2 and 10C, respectively, and an outstanding retention of 91.9% after 500 cycles at 5C. These excellent properties of the NVPF/C-2 composite are attributed to its high ion diffusion coefficient and small charge transfer impedance.

Na₃V₂(PO₄)₂F₃/C composites were synthesized by a solid-state reaction method using pitch as the carbon source, the as-prepared sample with the carbon content of 12.14% possesses an excellent rate performance and cycle stability.     

Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties,solubilities and battery performance

Water-soluble 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have been frequently utilized as catholytes for aqueous redox flow batteries to achieve cost-effective renewable energy storage. However, fundamental knowledge of TEMPO derivatives is still largely underdeveloped. Herein, a comprehensive study on the properties of TEMPO derivatives has been conducted in aqueous electrolytes. The results confirm that the redox potential, diffusion coefficient, electron transfer rate constant and solubility are clearly influenced by functional groups of TEMPO derivatives and supporting electrolytes. The charge–discharge cycling performance is evaluated using a symmetric redox flow battery configuration. The capacity decay for TEMPO-based catholytes is mainly derived from the crossover of the oxidized state. The presented study not only advances an in-depth understanding of TEMPO-based RFB applications, but also highlights the challenge of crossover of redox-active TEMPO derivative molecules applied in aqueous RFBs.

The effects of substituent groups of TEMPO-based catholytes and supporting electrolytes on electrochemical properties, solubility and battery performance were examined systematically for aqueous redox flow batteries.     

Improved cycling performance and rate stability of ITO-compounded Li2MnSiO4 for lithium-ion batteries

Li₂MnSiO₄ compounded with indium tin oxide (ITO) was successfully synthesized through a sol–gel method. The structure and morphology characterization of Li₂MnSiO₄/ITO nanocomposite are demonstrated by XRD, SEM, TEM, EDS and XPS. Galvanostatic charge–discharge tests, EIS and CV are employed to examine the electrochemical performance of the composite. From those results, it could be observed that the electrochemical performance of Li₂MnSiO₄ cathode material has been significantly improved due to the introducing of indium tin oxide. The 3 wt% ITO-compounded sample displayed a discharge specific capacity around 141 mA h g⁻¹ at 0.05C, 134.4 mA h g⁻¹ at 0.1C, 132.9 mA h g⁻¹ at 0.2C and 127.4 mA h g⁻¹ at 0.5C in the first cycle, which is much higher than the pristine sample.

Li₂MnSiO₄ compounded with indium tin oxide (ITO) was successfully synthesized through a sol–gel method.