Reduced Graphene Oxide Decorated Na3V2(PO4)3 Microspheres as Cathode Material With Advanced Sodium Storage Performance

Preparation of Nb5+ Doped Na3V2(PO4)3 Cathode Material for Sodium Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the abundance and low cost of sodium resources. Cathode material plays a crucial role in the performance of sodium ion batteries determining the capacity, cycling stability, and rate capability. Na3V2(PO4)3 (NVP) is a promising cathode material due to its stable three-dimensional NASICON structure, but its discharge capacity is low and its decay is serious with the increase of cycle period. We focused on modifying NVP cathode material by coating carbon and doping Nb5+ ions for synergistic electrochemical properties of carbon-coated NVP@C as a cathode material. X-ray diffraction analysis was performed to confirm the phase purity and crystal structure of the Nb5+ doped NVP material, which exhibited characteristic diffraction peaks that matched well with the NASICON structure. Nb5+-doped NVP@C@Nbx materials were prepared using the sol-gel method and characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Brunauer -Emmett-Teller (BET) analysis. First-principles calculations were performed based on density functional theory. VASP and PAW methods were chosen for these calculations. GGA in the PBE framework served as the exchange-correlation functional. The results showed the NVP unit cell consisted of six NVP structural motifs, each containing octahedral VO6 and tetrahedral PO4 groups to form a polyanionomer [V2(PO4)3] along with the c-axis direction by PO4 groups, which had Na1(6b) and Na2(18e) sites. And PDOS revealed that after Nb doping, the d orbitals of the Nb atoms also contributed electrons that were concentrated near the Fermi surface. Additionally, the decrease in the effective mass after Nb doping indicated that the electrons could move more freely through the material, implying an enhancement of the electron mobility. The electrochemical properties of the Nb5+ doped NVP@C@Nb cathode material were evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectric spectroscopy (XPS). The results showed that NVP@C@Nb0.15 achieved an initial discharge capacity as high as 114.27 mAhg-1, with a discharge capacity of 106.38 mAhg-1 maintained after 500 cycles at 0.5C, and the retention rate of the NVP@C@Nb0.15 composite reached an impressive 90.22%. NVP@C@Nb0.15 exhibited low resistance and high capacity, enabling it to create more vacancies and modulate crystal structure, ultimately enhancing the electrochemical properties of NVP. The outstanding performance can be attributed to the Nb5+-doped carbon layer, which not only improves electronic conductivity but also shortens the diffusion length of Na+ ions and electrons, as well as reduces volume changes in electrode materials. These preliminary results suggested that the as-obtained NVP@C@Nb0.15 composite was a promising novel cathode electrode material for efficient sodium energy storage.

Na+-Activation Engineering in the Na3V2(PO4)3 Cathode with Boosting Kinetics for Fast-Charging Na-Ion Batteries

Na superionic conductor-structured phosphates have attracted wide interest due to their high working voltage and fast Na⁺ migration facilitated by the robust 3D open framework. However, they usually suffer from low-rate capability and inferior cycling stability due to the low intrinsic electronic conductivity and limited activated Na⁺ ions. Herein, a doping protocol with Na⁺ in the V³⁺ site is developed to activate extra electrochemical Na⁺ ions and expand the migration path of Na⁺, leading to the improvement of the electronic conductivity and diffusion kinetics. It is also disclosed that the generated stronger Na-O bonds with high ionicity significantly conduce to the enhanced structural stability in the Na⁺-substituted Na_3.05V_1.975Na_0.025(PO₄)₃/C cathode. The obtained composite can deliver an excellent rate capacity of 83.8 mA h g^-1 at 20 C and a moderate cycling persistence of 91.3% over 1500 cycles at 10 C with great fast-charging properties. The reversible structure evolution is confirmed by the ex situ XRD, XPS, and ICP characterization. This work sheds light on awakening electroactive Na⁺ ions and designing phosphates with superior electrochemical stability for practical Na-ion batteries.

Synthesis and Characterizations of Na4MnCr(PO4)3/rGO as NASICON-Type Cathode Materials for Sodium-ion Batteries

NASICON-type Na₄MnCr(PO₄)₃ (NMCP) wrapped with reduced graphene oxide (rGO) was synthesized via a simple sol-gel method as composite cathode material Na₄MnCr(PO₄)₃/rGO (NMCP/rGO) for Na ion batteries. The surface morphology, crystal structure and pore size distribution of pristine NMCP and as-synthesized NMCP/rGO composite cathode are identified by X-ray diffraction (XRD), field emission-scanning electron microscopy (SEM), transmission electron microscope (TEM), the Brunauer–Emmett–Teller (BET) method and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of composition-optimized NMCP/rGO composite cathode presents stable capacity retention and rate capability. The capacity retention of as-synthesized NMCP/rGO composite is 63.8%, and average coulombic efficiency maintains over 98.7% for 200 cycles. The reversible capacity of as-synthesized NMCP/rGO composite cathode still retained 45 mAh/g and 38 mAh/g under a current density of 0.5 A/g and 1.0 A/g, respectively, which was better than that of pristine NMCP, with only 6 mAh/g and 4 mAh/g. The redox reactions of pristine NMCP and as-synthesized NMCP/rGO composite are studied via cyclic voltammetry. The improved electronic conductivity and structure stability of bare NMCP is attributed to the contribution of the rGO coating.

Spinel LiMn2O4 nanoparticles fabricated by the flexible soft template/Pichini method as cathode materials for aqueous lithium-ion capacitors with high energy and power density

Spinel LiMn2O4 (LMO) with a three-dimensional structure has become one of the cathode materials that has gained the most interest due to its safety, low price and abundant resources. However, the lithium ion transmission is limited by large particle size and particle agglomeration of LMO. Thus, reducing the particle size and agglomeration of LMO can effectively improve its lithium ion transmission. Here, we synthesized a LMO cathode material with a nanoscale crystal size using the flexible expanded graphite (EG) soft template and Pichini method. EG-controlled particle size and particle agglomeration of LMO is conducive to charge transfer and diffusion of lithium ions between LMO and the electrolyte, meanwhile, there are more redox sites on the nanosized LMO particles, which makes the redox reaction of LMO more thorough during the charge and discharge process, resulting in high capacitance performance. In order to obtain the considerably required lithium-ion capacitors (LICs) with high energy density and power density, we assembled aqueous LMO//activated carbon (AC) LICs with 5 M LiNO3 as the aqueous electrolytes, which are environmentally friendly, safe, low cost and have higher electrical conductivity than organic electrolytes. The optimal LIC has an energy density of 32.63 W h kg-1 at a power density of 500 W kg-1 and an energy density of 8.06 W h kg-1 at a power density of 10 000 W kg-1, which is higher than most of the LMO-based LICs in previous reports. After 2000 cycles, the specific capacitance retention rate was 75.9% at a current density of 3 A g-1. Therefore, our aqueous LMO//AC LICs synthesized by the soft template/Pichini method have wide prospects and are suitable for low-cost, high-safety and high-power applications.

Heterostructured SnO2-SnS2@C Embedded in Nitrogen-Doped Graphene as a Robust Anode Material for Lithium-Ion Batteries

Tin-based anode materials with high capacity attract wide attention of researchers and become a strong competitor for the next generation of lithium-ion battery anode materials. However, the poor electrical conductivity and severe volume expansion retard the commercialization of tin-based anode materials. Here, SnO₂-SnS₂@C nanoparticles with heterostructure embedded in a carbon matrix of nitrogen-doped graphene (SnO₂-SnS₂@C/NG) is ingeniously designed in this work. The composite was synthesized by a two-step method. Firstly, the SnO₂@C/rGO with a nano-layer structure was synthesized by hydrothermal method as the precursor, and then the SnO₂-SnS₂@C/NG composite was obtained by further vulcanizing the above precursor. It should be noted that a carbon matrix with nitrogen-doped graphene can inhibit the volume expansion of SnO₂-SnS₂ nanoparticles and promote the transport of lithium ions during continuous cycling. Benefiting from the synergistic effect between nanoparticles and carbon matrix with nitrogen-doped graphene, the heterostructured SnO₂-SnS₂@C/NG further fundamentally confer improved structural stability and reaction kinetics for lithium storage. As expected, the SnO₂-SnS₂@C/NG composite exhibited high reversible capacity (1201.2 mA h g⁻¹ at the current rate of 0.1 A g⁻¹), superior rate capability and exceptional long-life stability (944.3 mAh g⁻¹ after 950 cycles at the current rate of 1.0 A g⁻¹). The results demonstrate that the SnO₂-SnS₂@C/NG composite is a highly competitive anode material for LIBs.

Na3V2(PO4)3: an advanced cathode for sodium-ion batteries

Sodium-ion batteries (SIBs) are considered to be the most promising electrochemical energy storage devices for large-scale grid and electric vehicle applications due to the advantages of resource abundance and cost-effectiveness. The electrochemical performance of SIBs largely relies on the intrinsic chemical properties of the cathodic materials. Among the various cathodes, rhombohedral Na3V2(PO4)3 (NVP), a typical sodium super ionic conductor (NASICON) compound, is very popular owing to its high Na+ mobility and firm structural stability. However, the relatively low electronic conductivity makes the theoretical capacity of NVP cathodes unviable even at low rates, not to mention the high rate of charging/discharging. This is a major drawback of NVPs, limiting their future large-scale applications. Herein, a comprehensive review of the recent progresses made in NVP fabrication has been presented, mainly including the strategies of developing NVP/carbon hybrid materials and elemental doping to improve the electronic conductivity of NVP cathodes and designing 3D porous architectures to enhance Na-ion transportation. Moreover, the application of NVP cathodic materials in Na-ion full batteries is summarized, too. Finally, some remarks are made on the challenges and perspectives for the future development of NVP cathodes.

In-situ Grown SnS2 Nanosheets on rGO as an Advanced Anode Material for Lithium and Sodium Ion Batteries

SnS₂ nanosheets/reduced graphene oxide (rGO) composite was prepared by reflux condensation and hydrothermal methods. In this composite, SnS₂ nanosheets in-situ grew on the surface of rGO nanosheets. The SnS₂/rGO composite as anode material was investigated both in lithium ion battery (LIB) and sodium ion battery (SIB) systems. The capacity of SnS₂/rGO electrode in LIB achieved 514 mAh g^-1 at 1.2 A g^-1 after 300 cycles. Moreover, the SnS₂/rGO electrode in SIB delivered a discharge capacity of 645 mAh g^-1 at 0.05 A g^-1; after 100 cycles at 0.25 A g^-1, the capacity retention still keep 81.2% relative to the capacity of the 6th cycle. Due to the introduction of rGO in the composite, the charge-transfer resistance became much smaller. Compared with SnS₂/C electrode, SnS₂/rGO electrode had higher discharge capacity and much better cycling performance.