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Noticeable pseudo-capacitance behavior out of charge storage mechanism (CSM) has attracted intensive studies because it can provide both high energy density and large output power. Although cyclic voltammetry is recognized as the feasible electrochemical technique to determine it quantitatively in the previous works, the results are inferior due to uncertainty in the definitions and application conditions. Herein, three successive treatments, including de-polarization, de-residual and de-background, as well as a non-linear fitting algorithm are employed for the first time to calibrate the different CSM contribution of three typical cathode materials, LiFePO4 , LiMn2 O4 and Na4 Fe3 (PO4 )2 P2 O7 , and achieve well-separated physical capacitance, pseudo-capacitance and diffusive contributions to the total capacity. This work can eliminate misunderstanding concepts and correct ambiguous results of the pseudo-capacitance contribution and recognize the essence of CSM in electrode materials.

Sodium-ion battery is regarded as a promising power source for large-scale energy storage systems. However, the development of sodium-ion batteries is hindered by the lack of applicable cathode materials with low cost and long cycle life. Here, we report a successful synthesis of Na4Fe3(PO4)2P2O7/C nanospheres with tunable particle size and carbon coating thickness by a template approach. The as-prepared Na4Fe3(PO4)2P2O7/C nanospheres deliver a high discharge capacity of 128.5 mAh g−1 (near to the theoretical capacity: 129 mAh g−1) at 0.2C, with capacity retention of 63.5% at 10 C after 4000 cycles. Particularly, a high reversible capacity of 79 mAh g−1 is exhibited at an ultrahigh current rate of 100 C (charge/discharge in 36s). The excellent performances result from the shortened Na+ ion diffusion length within the nanospheres (∼30 nm) and highly conductive pathways for electrons in the carbon coating layers (∼3 nm). Owing to their low cost, long lifespan and outstanding rate capability, we believe that the Na4Fe3(PO4)2P2O7/C nanospheres are considerable competitive to other cathode materials for application in stationary sodium-ion batteries.

Vanadium-based compounds hold great promise as high capacity cathode candidate for future lithium rechargeable batteries. However, developing highly stable vanadium-based cathode materials with long cycle life remains a great challenge. Herein, we report a novel layered sodium vanadium oxide, NaVO3, as a promising cathode electrode contender. This material is capable of delivering a capacity of 224.8 mAh g−1 at the current density of 150 mA g−1, and a high rate capability of 85 mAh g−1 even at a high current density of 3 A g−1. Moreover, outstanding capacity retention of 77% after 1000 cycles is achieved. Ex situ characterizations verify that the excellent electrochemical performance of NaVO3 is attributed to superior structural stability and electrochemical reversibility upon long-term cycling. Furthermore, the lithium ion de/intercalation mechanism for NaVO3 is also revealed involving one electron transfer reaction between V5+ and V4+ redox couple. Considering the low cost and material sustainability as well as the outstanding electrochemical performances, we believe that NaVO3 is a highly promising cathode material for lithium rechargeable batteries and our findings may help to pave the way for developing vanadium-based layered structure materials for high-performance alkali and alkaline-earth ion batteries.

Sodium-ion battery has emerged as one of most promising technologies for large-scale energy storage system, and hence has stimulated extensive exploration of applicable electrode materials with low cost and superb electrochemical properties. Herein, 3D graphene decorated Na4Fe3(PO4)2(P2O7) microspheres as a low-cost and environmentally friendly cathode material are synthesized by using a facile spray-drying method. The as-prepared NFPP@rGO composite exhibits a high reversible capacity of 128 mAh g−1 at 0.1 C, a superior rate capability (35 mAh g−1 at 200 C), and a long cycling life (62.3% capacity retention over 6000 cycles at 10 C). The excellent electrochemical performance is attributed to combined advantages of graphene coating on the surface of nanoparticles and the flexible 3D graphene network, which not only improve the electronic conductivity, but also accommodate the structural stress of the material during charging and discharging. Therefore, the NFPP@rGO microsphere with superior electrochemical performances, low-cost raw materials, simple synthetic route and high thermal stability is considered as a very attractive cathode electrode for sodium ion battery.

The effects of discharge cutoff voltages on the structural evolution and electrochemical performance of the LiVO3 cathode upon cycling are investigated by electrochemical measurements, electrochemical impedance spectroscopy, ex situ X-ray diffraction, Raman spectra, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. It is found that a lower cutoff voltage causes formation and accumulation of unstable V4+ ions on the surface of the electrode, which easily leads to severe structural deterioration and capacity fading. A limited cutoff voltage between 3.5 and 1.5 V can effectively enhance the structural stability and consequently the electrode demonstrates 75.9% capacity retention and neglectable working voltage decay over 400 cycles. The result that the operation voltage range strongly affects the structural stability of cycled LiVO3 provides a new insight into exploring feasible approaches to achieve highly stable LiVO3 and other vanadium-based electrodes for lithium-ion batteries.

We report for the first time a zero-strain cathode, Na4Fe7(PO4)6, for sodium–ion batteries (SIBs). This new iron-based polyanionic cathode delivers a reversible capacity of 66.5 mA h g−1 at 5 mA g−1 with almost 100% capacity retention over 1000 cycles under 200 mA g−1, and the outstanding performance benefits from single-phase-transition processes with a tiny volume change of only ∼0.24%.

Owing to the advantages of high safety, low cost, high theoretical volumetric capacities, and environmental friendliness, magnesium-ion batteries (MIBs) have more feasibility for large-scale energy storage compared to lithium-ion batteries. However, lack of suitable cathode materials due to sluggish kinetics of magnesium ion is one of the biggest challenges. Herein, water-pillared sodium vanadium bronze nanowires (Na2 V6 O16 ·1.63H2 O) are reported as cathode material for MIBs, which display high performance in magnesium storage. The hydrated sodium ions provide excellent structural stability. The charge shielding effect of lattice water enables fast Mg2+ diffusion. It exhibits high specific capacity of 175 mAh g-1 , long cycle life (450 cycles), and high coulombic efficiency (≈100%). At high current density of 200 mA g-1 , the capacity retention is up to 71% even after 450 cycles (compared to the highest capacity), demonstrating excellent long-term cycling performance. The nature of charge storage kinetics is explored. Furthermore, a highly reversible structure change during the electrochemical process is proved by comprehensive electrochemical analysis. The remarkable electrochemical performance makes Na2 V6 O16 ·1.63H2 O a promising cathode material for low-cost and safe MIBs.

Li2MnSiO4 is considered as a promising cathode candidate for high energy density Lithium-ion batteries (LIBs), due to its high theoretical capacity, favorable working potential as well as the low-cost and non-toxic nature of manganese element. However, it suffers from low capacity utilization and inferior cycling performance, due to the structural rearrangement upon cycling, phase impurities and manganese dissolution. To address these issues, nanostructure construction and carbon coating proves to be effective approach. In this work, we introduce an in situ template synthesis of a phase-pure Li2MnSiO4@C (MP-LMS@C) composite, which exhibits unique mesoporous morphology with nanosized Li2MnSiO4 granules homogenously encapsulated in the interconnected carbon networks. Such unique Li2MnSiO4@C structure not only facilitates electron conduction and Li+ transport, but also suppresses the manganese dissolution, as a result, the mesoporous Li2MnSiO4@C cathode exhibits greatly improved electrochemical performance, with a high specific capacity of 230 mAh g−1, good rate capability and superior capacity retention of 81% over 100 cycles.

A rechargeable aqueous Zn/Mn battery is a promising device for large-scale energy storage because of its abundant resources, low cost, and high safety. However, its application is plagued by a poor life cycle because of the electrochemical instability of MnO2 in aqueous electrolytes. Here, an alkaline Zn-Na0.44MnO2 dual-ion battery (denoted AZMDIB) is developed for the first time using Na0.44MnO2 as the cathode, a zinc metal sheet as the anode, and a 6 M NaOH aqueous solution as the electrolyte. When the discharge cutoff voltage is lowered to 0.3 V (vs Zn/Zn2+), the Na0.44MnO2 cathode delivers a high capacity of 345.5 mA h g-1 but with a poor cycling performance. The charge-discharge mechanism and structural evolution of the Na0.44MnO2 cathode in an extended potential window (1.95-0.3 V) are also explored. The Na0.44MnO2 electrode experiences two different electrochemical processes: Na+ ions insert/extract reversibly in the potential range of 1.95-1.1 V, and a phase transition occurs from Na0.559MnO2 to Mn(OH)2 below 1.1 V. The latter irreversible reaction is probably due to proton insertion, leading to a severe capacity fade. Nevertheless, in a narrower voltage range (2.0-1.1 V), the AZMDIB full cell exhibits a high reversible capacity (80.2 mA h g-1 at 0.5 C), high rate capability (32 mA h g-1 at 50 C), and excellent cycling stability (73% capacity retention over 1000 cycles at 10 C). Benefiting from the merits of environmental friendliness, cost-effectiveness, and high electrochemical performance, the rechargeable AZMDIB is a promising contender for grid-scale energy storage applications.

Batteries and electrochemical capacitors play very important roles in the portable electronic devices and electric vehicles and have shown promising potential for large-scale energy storage applications. However, batteries or capacitors alone cannot meet the energy and power density requirements because rechargeable batteries have a poor power property, whereas supercapacitors offer limited capacity. Here, a novel symmetric sodium-ion capacitor (NIC) is developed based on low-cost Na0.44MnO2 nanorods. The Na0.44MnO2 with unique nanoarchitectures and iso-oriented feature offers shortened diffusion path lengths for both electronic and Na+ transport and reduces the stress associated with Na+ insertion and extraction. Benefiting from these merits, the symmetric device achieves a high power density of 2432.7 W kg–1, an improved energy density of 27.9 Wh kg–1, and a capacitance retention of 85.2% over 5000 cycles. Particularly, the symmetric NIC based on Na0.44MnO2 permits repeatedly reverse-polarity characteri...

Na4Fe3(PO4)2(P2O7) (NFPP), as a typical cathode material of sodium ion battery, has great application prospect because of its low-cost, non-toxicity and appropriate working voltage and theoretical capacity. However, its poor electron and ion conductivities associated by non-erasable NaFePO4 impurity generated in all the synthesis methods limits the capacity utilization of NFPP. Herein, we report a novel pure-phase Na4Fe2.91(PO4)2(P2O7) cathode material prepared simply by introducing a small amount of Fe defects in the lattice. The first-principles calculations reveal that Fe defects in the NFPP materials result in a lower band gap and migration energy barriers, thereby leading to a higher electron and Na+ ion conductivity. As a result, the pure-phase Na4Fe2.91(PO4)2(P2O7) cathode exhibits a high discharge capacity (110.9 mA h g−1 at 0.2 C), excellent rate performance (~52 mA h g−1 at 100 C) and outstanding long cycle stability over 10,000 cycles without discernible capacity decay. The pouch cell assembled with Na4Fe2.91(PO4)2(P2O7) cathode and hard carbon anode, shows high capacity retention rate of 87.4% over 1000 cycles. These results suggest a feasible application of the simple defect regulation strategy to synthesize high-quality and pure-phase Na4Fe2.91(PO4)2(P2O7) materials for low-cost sodium-ion batteries.

Lithium iron silicate has caught tremendous attentions as an appealing cathode for future lithium-ion batteries due to high capacity, low cost, and environmental friendliness, and its drawback of extremely low conductivity can be overcome efficiently through nanoarchitecture building. However, the construction of nanostructures always involves with expensive surfactants and complicated synthetic processes, which restrict these methods from large-scale production. In this paper, we develop a simple synthetic route to prepare hierarchical porous Li2FeSiO4/C. XRD, SEM, TEM, Raman, and N2 adsorption-desorption are employed to investigate its physical properties. Electrochemical tests reveal that the composite delivers a high specific capacity of 243.5 mAh g−1, superior rate capability, and excellent cycling performance with capacity retention of 95.2% after 200 cycles. The excellent electrochemical performance should be attributed to the unique structure in which hierarchical pores provides fast transport channels for lithium ions and interconnected carbon coating builds up conductive networks to enhance the conductivity of Li2FeSiO4. In addition, electrochemical impedance spectroscopy, ex situ SEM, and TEM are conducted to demonstrate its structural stability upon long-term cycling. In addition, the route described in this work is facile, cheap, and easily scaled-up, which allows for extension to the fabrication of other energy storage materials.