One main challenge for phosphate cathodes in sodium-ion batteries (SIBs) is to increase the working voltage and energy density to promote its practicability. Herein, an advanced Na3V2(PO4)2F3@C cathode is prepared successfully for sodium-ion full cells. It is revealed that, carbon coating can not only enhance the electronic conductivity and electrode kinetics of Na3V2(PO4)2F3@C and inhibit the growth of particles (i.e., shorten the Na+-migration path), but also unexpectedly for the first time adjust the dis-/charging plateaux at different voltage ranges to increase the mean voltage (from 3.59 to 3.71 V) and energy density (from 336.0 to 428.5 Wh kg−1) of phosphate cathode material. As a result, when used as cathode for SIBs, the prepared Na3V2(PO4)2F3@C delivers much improved electrochemical properties in terms of larger specifc capacity (115.9 vs. 93.5 mAh g−1), more outstanding high-rate capability (e.g., 87.3 vs. 60.5 mAh g−1 at 10 C), higher energy density, and better cycling performance, compared to pristine Na3V2(PO4)2F3. Reasons for the enhanced electrochemical properties include ionicity enhancement of lattice induced by carbon coating, improved electrode kinetics and electronic conductivity, and high stability of lattice, which is elucidated clearly through the contrastive characterization and electrochemical studies. Moreover, excellent energy-storage performance in sodium-ion full cells further demonstrate the extremely high possibility of Na3V2(PO4)2F3@C cathode for practical applications.

Carbon-coating-increased working voltage and energy density towards an advanced Na3V2(PO4)2F3@C cathode in sodium-ion batteries

Recently, due to the excessive consumption of fossil energy and the intermittent nature of clean energy resources, electricity storage has attracted great attention from both academia and industry. Lithium ion batteries (LIBs) with high energy density have been widely used for portable energy storage systems. However, limited lithium resources cannot meet the demands for large-scale applications, especially the grid-scale energy storage. Owing to the abundant natural resources of sodium and the low price, sodium ion batteries (SIBs) have become potential substitutes for LIBs. Cathode materials are key components that determine the overall performance and entire cost of SIBs. However, the unstable structure and limited electrochemical performance make them difficult to meet the requirement of practical applications. This review summarizes the recent development of the cathode materials used for SIBs, and the modified strategies applied to improve their electrochemical performance. Finally, critical challenges and perspectives to promote the development of SIBs are also presented.

Recent progress of emerging cathode materials for sodium ion batteries

ConspectusAs the world transitions away from fossil fuels, energy storage, especially rechargeable batteries, could have a big role to play. Though rechargeable batteries have dramatically changed ...

Vanadium-Based Materials: Next Generation Electrodes Powering the Battery Revolution?

Sodium-vanadium fluorophosphate (Na3 V2 O2 x (PO4 )2 F3-2 x , NVPF, 0 ≤ x ≤ 1) is considered to be a promising Na-storage cathode material due to its high operation potentials (3.6-4 V) and minor volume variation (1.8%) during Na+ -intercalation. Research about NVPF is mainly focused on powder-type samples, while its ordered array architecture is rarely reported. In this work, large-area and uniform Na3 (VO)2 (PO4 )2 F cuboid arrays are vertically grown on carbon nanofiber (CNF) substrates for the first time. Owing to faster electron/ion transport and larger electrolyte-electrode contact area, the as-prepared NVPF array electrode exhibits much improved Na-storage properties compared to its powder counterpart. Importantly, a quasi-solid-state sodium-ion hybrid capacitor (SIHC) is constructed based on the NVPF array as an intercalative battery cathode and porous CNF as a capacitive supercapacitor anode together with the P(VDF-HFP)-based polymer electrolyte. This novel hybrid system delivers an attractive energy density of ≈227 W h kg-1 (based on total mass of two electrodes), and still remains as high as 107 Wh kg-1 at a high specific power of 4936 W kg-1 , which pushes the energy output of sodium hybrid capacitors toward a new limit. In addition, the growth mechanism of NVPF arrays is investigated in detail.

Large-Area, Uniform, Aligned Arrays of Na 3 (VO) 2 (PO 4 ) 2 F on Carbon Nanofiber for Quasi-Solid-State Sodium-Ion Hybrid Capacitors

Vanadium-based polyanionic compounds as cathode materials for sodium-ion batteries: Toward high-energy and high-power applications

Developing high power-high energy electrochemical energy storage systems is an ultimate goal in the energy storage field, which is even more difficult but significant for low-cost sodium ion batteries. Here, fluoride is successfully prepared by the electrostatic spray deposition (ESD) technique, which greatly expands the application scope of ESD. A two-step strategy (solvothermal plus ESD method) is proposed to construct a bicontinuous ordered network of 3D porous Na3 (VO)2 (PO4 )2 F/reduced graphene oxide (NVOPF/rGO). This two-step strategy makes sure that NVOPF can be prepared by ESD, since it avoids the loss of F element during synthesis. The obtained NVOPF particles are as small as 15 nm, and the carbon content is only 3.5% in the final nanocomposite. Such a bicontinuous ordered network and small size of electroactive particles lead to the significant contribution of the pseudocapacitance effect to sodium storage, resulting in real high power-high energy sodium cathodes. The cathode exhibits excellent rate capability and cycling stability, whose rate performance is one of the best ever reported in both half cells and full cells. Moreover, this work provides a general and promising strategy for developing high power-high energy electrode materials for various electrochemical energy storage systems.

Toward High Power-High Energy Sodium Cathodes: A Case Study of Bicontinuous Ordered Network of 3D Porous Na3(VO)2(PO4)2F/rGO with Pseudocapacitance Effect

A Micron-Size Carbon-Free K3V2O2(PO4)2F Cathode with High-Rate Performance for Potassium-Ion Batteries

The dramatic growth of the sodium-ion battery market evokes a high demand for high-performance cathodes. In this work, a nanosized amorphous FePO4@rGO composite is developed using coprecipitation combined with low-temperature hydrothermal synthesis, which registered a surface area of 179.43 m2 g-1. The composites maintain three-dimensional mesoporous morphology with a pore size in the range of 3-4 nm. Uniform distribution of amorphous FePO4 allows a reversible capacity of 175.4 mA h g-1 at 50 mA g-1 while maintaining a stable cycle life of 500 cycles at 200 mA g-1. The amorphous FePO4@rGO, obtained by energy-efficient synthesis, significantly improved the rate performance compared to the crystalline material prepared at high temperatures. Cyclic voltammetry tests reveal that the fast reaction kinetics can be attributed to the pseudocapacitive behavior of the electrode. In addition, we demonstrated the promise of FePO4@rGO cathodes for low-temperature sodium-ion batteries.

Kunyao Peng

Papers

Toward High Power-High Energy Sodium Cathodes: A Case Study of Bicontinuous Ordered Network of 3D Porous Na3(VO)2(PO4)2F/rGO with Pseudocapacitance Effect

A Micron-Size Carbon-Free K3V2O2(PO4)2F Cathode with High-Rate Performance for Potassium-Ion Batteries

Low-Temperature Synthesis of Amorphous FePO4@rGO Composites for Cost-Effective Sodium-Ion Batteries

Dual-Conductive Li Alloy Composite Anode Constructed by a Synergetic Conversion-Alloying Reaction with LiMgPO4