Phosphate Framework Electrode Materials for Sodium Ion Batteries.

Sodium ion batteries (SIBs) have been considered as a promising alternative for the next generation of electric storage systems due to their similar electrochemistry to Li-ion batteries and the low cost of sodium resources. Exploring appropriate electrode materials with decent electrochemical performance is the key issue for development of sodium ion batteries. Due to the high structural stability, facile reaction mechanism and rich structural diversity, phosphate framework materials have attracted increasing attention as promising electrode materials for sodium ion batteries. Herein, we review the latest advances and progresses in the exploration of phosphate framework materials especially related to single-phosphates, pyrophosphates and mixed-phosphates. We provide the detailed and comprehensive understanding of structure-composition-performance relationship of materials and try to show the advantages and disadvantages of the materials for use in SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed. Phosphate framework materials will be a competitive and attractive choice for use as electrodes in the next-generation of energy storage devices.

Introduction

Rapid growth of renewable electricity in global energy markets has continuously propelled the development of effective and affordable energy storage technologies for constructing a future energy internet (Figure ).1 Though battery technologies have been developed over a hundred of years, few of them can meet the needs for ever‐increasing electric energy storage applications. For example, Lithium ion batteries (LIBs) are now considered as a promising candidate for a number of electric storage applications, there exists a great concern about the widespread availability and rising price of lithium resources.2 Therefore, room temperature Na‐ion batteries (SIBs) appear to be a good choice as a viable technology for large scale electric storage applications due to their low cost, widespread abundance of sodium resources, cheap Al anode collector used and chemical similarity with Li‐ion batteries.

Figure 1

Schematic illustration of the “energy internet”.

In the past few years, tremendous efforts have been made to exploring suitable Na‐host materials with high reversible capacity, rapid Na ion insertion/extraction and cycling stability.3, 4, 5 A large variety of compounds, such as transition metal oxides,6, 7, 8, 9 phosphates,10, 11, 12 ferrocyanide,13, 14, 15 hard carbon,16, 17, 18, 19 metal alloys,20, 21, 22 and organic materials,23, 24, 25 have demonstrated considerable Na‐storage capacities for SIBs. However, most of them suffer from structural instability during Na‐insertion/extraction reactions. For example, the Na ion insertion/extraction in the layered metal oxides often results in very complicated multiphase transitions, leading to rapid structural degradation of the hosts during cycling.7 In addition, they are very sensitive to atmosphere and water, making the large scale application of these materials a severe problem.5 For the metal alloy anodes, they possess high reversible capacity and huge volumetric expansion (for example, a 420% volumetric expansion for Na3.75Sn to Sn) during Na insertion, resulting in exfoliation and inactivation of the active materials.26

From the viewpoint of structural stability, open framework favors to accommodate the large‐sized Na ions. For example, the Prussian blue analogues have a cubic framework capable of reversible extraction of two Na per formula unit at high rates. However, these compounds from conventional synthesis always contain a large amount of lattice defects and coordinated water, which cause a huge loss of the active sites for Na ion storage.14, 27 Additionally, the thermal unstable structure also arise concerns about the safety issues of the materials for application. In this respect, transition metal compounds containing polyanions such as PO4 3− are intensively investigated because the strong P‐O covalent bonds can stabilize the lattice oxygen even at highly charged state. And the phosphate framework materials show very low thermal expansion (the coefficient of thermal expansion is around 10–6 °C–1),28 indicating high structural stability at high temperature.

Phosphate framework materials have attracted increasing attention due to their excellent electrochemical performance and versatile structure,29 and are considered as promising Na‐storage electrodes due to the following considerations: (i) phosphate frameworks have high structural stability due to the very stable P‐O frameworks, thus ensuring long‐term cycling and safety of SIBs; the thermal properties of phosphate materials are linked directly to the stability of the phosphate‐metal bonds, which greatly reduce the likelihood of oxygen liberation from the structure; (ii) the 3D framework possesses many roomy interstices, thus leading to lower volumetric expansion and less phase transition during Na ion insertion/extraction, which benefits the structural stability; (iii) phosphate or other substituent groups exhibit inductive effect on the redox couple, thus to give rise to higher redox potential values vs Na/Na+. However, the big size and intrinsic isolating nature of the PO4 3– groups lead to a moderate capacity and low electron conductivity. In this context, constructing elaborate structure with highly conductive matrix is an effective approach to improve high performance of the phosphate materials.

Phosphate framework materials are full of variety, exhibiting versatile and adjustable structure and electrochemical performance, such as phosphates, pyrophosphates, mixed‐anions, and diverse optional redox centers (Fe, V, Mn, Ni, Co, et al.). From the viewpoint of electrochemistry, most of the reactions are attributed to phase transition mechanism, and some belong to solid solution, surface or interface charging mechanisms.

In this review, recent research progress on the use of phosphate framework materials for SIBs is summarized, concentrating especially on single‐phosphates, pyrophosphates and mixed‐phosphates. We provide a detailed and comprehensive understanding of structure–composition–performance relationship of the materials and try to reveal the advantages and disadvantages of the materials for SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed.

Single‐Phosphate Materials for Na Storage

Single‐phosphate materials are the first to be investigated as electrode materials for SIBs. A large variety of phases used in Li ion intercalation chemistry have been investigated as drop‐in replacements for Na ion. Among the phosphates materials, olivine structure, NASICON‐type materials and other kind of materials have attracted much more attention due to their decent electrochemical performance.

Olivine and Maricite Structure

Olivine Structure

As LiFePO4 is successfully commercialized as a cathode material for lithium ion battery applications, its sodium analogue, olivine NaFePO4, has attracted interest as a Na ions host material due to its high theoretical specific capacity (154 mAh g–1) and decent voltage (≈2.8 V). However, direct high‐temperature synthesis cannot produce pure olivine‐phased NaFePO4 but usually lead to a thermodynamically favored maricite phase, which has poor electrochemical activity because its one‐dimensional, edge‐sharing FeO6 octahedrons form frustrated pathways for Na‐ion migration (Figure a).30, 31, 32

Figure 2

(a) Schematic presentation of orthorhombic structured triphylite NaFePO4 (left) and maricite NaFePO4 (right) polymorphs; (b) STEM image of Na≈2/3FePO4. The arrow line highlights the orientation of the Na−vacancies (black dots); (c) Synthetic scheme of the aqueous electrochemical displacement process from olivine LiFePO4 to isostructural NaFePO4; (d) The voltage‐capacity profiles of the NaFePO4/C electrode at different charge/discharge rates from 0.05 to 2 C; (e) Cyclic voltammograms of NaFePO4/C electrode in 1 mol L–1 NaPF6/EC: DEC (1:1 in vol.) solution at various scan rates; (f) Galvanostatic curves of maricite NaFePO4 over 200 cycles at C/20 in a Na cell, inset: discharge curves of maricite NaFePO4 as a function of the C rate from C/20 to 3 C (charging under CCCV mode (C/20 rate and 5 hour holding at 4.5 V)), during the first charge of CV, 20 mAh g–1 of capacity was recovered. (a) Reproduced with permission.55 Copyright 2013, American Chemical Society. (b) Reproduced with permission.38 Copyright 2014, American Chemical Society. (c, d, e) Reproduced with permission.44 Copyright 2015, American Chemical Society. (f) Reproduced with permission.57 Copyright 2015, Royal Society of Chemistry.

The conversion from a maricite phase to an olivine phase by cation exchange seems to be a feasible way to obtain pure olivine NaFePO4. Le Poul et al. first studied the Na ion insertion behavior of olivine FePO4 and found that 0.65 sodium ion can be inserted to form Na0.65FePO4 at 0.1 C rate.33 After that, tremendous efforts have been focused on the synthesis and characterization of olivine NaFePO4 material for SIBs. Moreau et al. synthesized the NaxFePO4 phases (x = 0.7 and 1) via electrochemical method and provided a detailed structural analysis of olivine NaFePO4,31 By examining Na intercalation behaviors in the olivine FePO4, they unraveled a stable immediate Na2/3FePO4 phase with a superstructure (Figure 2b).34, 35 The olivine NaFePO4 electrode exhibits two charge plateaus separated by a voltage drop corresponding to the intermediate Na2/3FePO4 phase, and one discharge plateau. Oh et al. have also studied the electrochemical performance of NaFePO4 obtained by an electrochemical exchange from LiFePO4, and obtained a stable capacity of 125 mAh g–1 with a cycle life of 50 cycles.36 This work demonstrates the possibility to achieve good Na storage performance for olivine NaFePO4 material.

Casas‐Cabanas's group has done exhaustive investigation on the Na insertion/deinsertion behaviors of olivine NaFePO4. They found that the intermediate Na2/3FePO4 phase forms in both charge and discharge process and characterized the intermediate phase with Na/vacancies ordering.37, 38 The Na insertion and deinsertion occur through different mechanisms due to the huge volumetric mismatch between FePO4 and NaFePO4 (17.58% difference in unit volume).39 A Na5/6FePO4 intermediate phase was also revealed by DFT calculations and high resolution synchrotron X‐ray diffraction.40 Yamada's group determined the composition‐temperature phase diagram of the FePO4/NaFePO4 system and found that NaxFePO4 (0 < x < 2/3) appeared in a two‐phase region, while NaxFePO4 (2/3 < x < 1) is in solid‐solution region.41 Polaron mobility and disordering of the sodium sublattice and variation of local magnetic environments of NaxFePO4 have also been settled.42, 43

High‐performance olivine NaFePO4 electrodes were also reported. Fang et al. synthesized olivine NaFePO4 microsphere through aqueous electrochemical displacement method from LiFePO4 and showed excellent cycling stability of the material with 90% capacity retention over 240 cycles. They reported for the first time a Na2/3FePO4 intermediate phase during discharge process through conventional electrochemical techniques (Figure 2c–e).44 Tang et al. promoted the research by similar synthesis route and the staged evolution of phases during sodiation/desodiation were also studied.45 Ali et al. have modified the NaFePO4 with polythiophene and demonstrated that the discharge capacity and cycle life of NaFePO4 electrode can be greatly improved by polythiophene coating.46 Ionic liquid electrolytes with various sodium solutes were tested for olivine NaFePO4 by Chang's group. At elevated temperature (50 and 75 °C), the ionic liquid electrolytes assist NaFePO4 in delivering higher capacity with stable cycle life.47, 48

Structural and electrochemical features between olivine NaFePO4 and LiFePO4 have also been well investigated. Zhu et al. compared the electrochemical performances of olivine NaFePO4 and olivine LiFePO4 and attributed the more sluggish Na storage behavior of NaFePO4 to the lower Na ion diffusion coefficient and higher charge transfer resistance in NaFePO4 electrodes.49 Whiteside et al. computed the surface structures and equilibrium morphology of olivine NaFePO4 and compared with those of LiFePO4. They found that NaFePO4 differs from LiFePO4 in the detail of its surface structures and their relative energies, such that the equilibrium morphology is thinner in the b‐axis direction, which is important for the rate performance.50 Density functional studies of Li and Na diffusion in LiFePO4 and NaFePO4 were also conducted by Nakayama and Major's groups,51, 52 and they found that electronic and Li or Na ionic migration in the bulk did not show large difference between LiFePO4 and NaFePO4.The changes of phase structure indicated a two‐phase reaction mechanism for the Li1–xFePO4 electrode and a solid‐solution reaction mechanism for the Na1–xFePO4 electrode.51, 53

Maricite and Alluaudite Structure

As discussed above, it is well accepted that the maricit NaFePO4 is electrochemically inactive due to the reversed phosphate framework compared to olivine structure, where the M1 and M2 sites are occupied by Fe2+ and Na+, respectively, resulting in no free channels for Na ion diffusion in the closed framework (Figure 2a).30, 31, 32, 54 The magnetic structures of maricite and olivine NaFePO4 were well studied through experimental test and theoretical computation.55, 56 However, maricite NaFePO4 was recently proved to enable excellent Na storage performance and all Na ions can be extracted from the nano‐sized maricite NaFePO4 with simultaneous transformation of the maricite structure to amorphous FePO4.57 The maricite NaFePO4 electrode can deliver a capacity of 142 mAh g–1 at 1/20 C with sloping charge/discharge curves and stable cycle life over 200 cycles (Figure 2f).

Huang et al. have synthesized an alluaudite Na0.67FePO4/CNT (Na2Fe3(PO4)3/CNT) material, which delivered a discharge capacity of 143 mAh g–1 with stable cyclability. However, the first charge capacity is a quite low (≈40 mAh g–1), which restricts its battery application.58 Alluaudite NaMnFe2(PO4)3 has also been investigated but its electrochemical performance needs to be improved.59

NASICON‐type NaxM2(PO4)3 (M = V, Ti, 1 ≤ x ≤ 3)

To obtain electrodes with high‐performance and stable cyclability, unique crystal structure with roomy interstices and large tunnels, such as the 3D framework compounds that possess fast Na ions transport routes, are welcomed to accommodate these large sodium ions during a lattice perturbation‐free intercalation/deintercalation process.60 In this regard, NASICON (Na Super Ionic Conductor) type materials are well known for their high Na ionic conductivity and stable 3D framework and have been widely investigated as battery materials (electrodes, solid electrolytes, and membranes).61 The NASICON structure can be described as a covalent skeleton [M2P3O12]– consisting of MO6 octahedra and PO4 tetrahedra, which form 3D interconnected tunnels and two types of interstitial positions (M1 and M2) where Na ions are distributed (Figure ).62, 63, 64 Generally, the compounds crystallize with a thermally stable rhombohedral structure, where the Na ions move from one site to another through bottlenecks.63 But members of A3M2(PO4)3 family (where A = Li, Na and M = Cr, Fe, Zr) crystallize in monoclinic modification and show reversible structural phase transitions at high temperatures.65, 66 As electrode materials, the NASICON NaxM2(PO4)3 (M = V, Fe, Ti, 0 ≤ x ≤ 3) have been widely studied, based on the consideration that the NASICON‐type lattices ensures long term cycle life as well as high rate capability.

Figure 3

A view of the NaxM2(PO4)3 NASICON structure along the a (a) and c (b) axis, respectively.

NASICON‐type Na3V2(PO4)3

Na3V2(PO4)3, first reported by Porter et al.,67 has a 3D framework of VO6 octahedra sharing all of its corners with PO4 tetrahedra and one Na+ ion occupies the M1 sites with sixfold coordination and the other two Na+ ions occupy the M2 sites forming eightfold coordination. Only Na ions residing at M2 sites can be extracted for electrochemical reaction due to the weak bonding to surrounding oxygen atoms (Figure 3), giving a theoretical capacity of 117 mAh g–1 and a flat plateau at around 3.3 V (vs Na/Na+).68 When discharge to a low potential (<1.5 V), an additional Na+ can be inserted at M2 sites, achieving a fully occupied Na4V2(PO4)3 state. Therefore, the Na3V2(PO4)3 can either be used as cathode or anode materials.69 The Na3V2(PO4)3 material can also maintain good thermal stability up to 450 °C even in the desodiated state.69 The crystal structure of the NASICON Na3V2(PO4)3 phase has been investigated as a function of temperature by Chotard's group, who demonstrated that the Na3V2(PO4)3 displayed four distinct crystal structures in the range of –30 °C to 225 °C, named the α‐, β‐, β′‐, and γ‐NVP.70

Yamaki group first studied the Na storage performance of Na3V2(PO4)3 and obtained a reversible capacity of 140 mAh g–1 in the voltage range of 1.2–3.5 V.71 They constructed a symmetric cell using Na3V2(PO4)3 as both cathode and anode materials and ionic liquid as electrolyte, which delivered a reversible capacity of 64 mAh g–1.72 Due to the intrinsic low conductivity of the phosphate framework, the cycling stability and reversible capacity of the Na3V2(PO4)3 material appeared unsatisfactory. Hu's group realized the importance of improving conductivity of this material and used the carbon coating to obtain a Na3V2(PO4)3/C material.73 The Na3V2(PO4)3/C electrode delivered a reversible capacity of 93 mAh g–1 with improved cycling performance. They also optimized the carbon content and electrolyte system and significantly improved the Na storage performance, such as capacity, coulombic efficiency and cyclability (Figure a).74 After that, they continued to investigate the Na+ ion positions at atomic resolution and the kinetics by using Rietveld refined‐XRD, ABF‐STEM (Figure 4c), and NMR (Figure 4d). The experiments exhibited that there were two kinds of Na sites in Na3V2(PO4)3 with different coordination environments, and only Na at the M2 sites can be extracted during electrochemical/chemical oxidation at room temperature, suggesting a M2‐M2 conduction pathway.75

Figure 4

(a) In situ XRD patterns of the Na3V2(PO4)3/Na cell cycled between 3.7 and 2.7 V at a current rate of C/10, ♦ Na3V2(PO4)3, ♣ NaV2(PO4)3; (b) Schematic depiction of pathways 1, 2 and 3. The black‐green lines that connect Na at the Na1 site and the Na vacancy represent pathway 3; (c) STEM HAADF images of (i) Na3V2(PO4)3 and (ii) NaV2(PO4)3 along the projection; (d) The 23Na MAS spectra for Na3V2(PO4)3 and NaV2(PO4)3 recorded at the spinning rate of 14 kHz at 298 K; (e) Ex‐situ XPS studies of NaV2(PO4)3/C electrodes; (f) Selected Fourier transform spectra of k3‐weighted V EXAFS spectra of the NaV2(PO4)3 sample during the first cycle of charge and discharge. Experiment–solid line; EXAFS model–dashed line. (a) Reproduced with permission.74 Copyright 2012, Wiley‐VCH. (b) Reproduced with permission.77 Copyright 2015, Royal Society of Chemistry. (c,d) Reproduced with permission.75 Copyright 2014, Wiley‐VCH. (e) Reproduced with permission.60 Copyright 2012, Wiley‐VCH. (f) Reproduced with permission.79 Copyright 2012, Elsevier.

The Na storage mechanism of Na3V2(PO4)3 has been well investigated by various spectroscopic, electrochemical and computing techniques. Song et al. used first principle calculations to explore the Na ion migration pathways and occupations. According to their study, two pathways along the x and y directions and one possible curved route for migration were favored with a 3D transport characteristics, and the ion occupation of 0.75 for all Na sites was suitable for the configuration of [Na3V2(PO4)3]2.68, 76 The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON‐type Na3V2(PO4)3 have been investigated based on the hybrid density functional Heyd–Scuseria–Ernzerhof (HSE06) by Ohno's group. Three diffusion pathways, bound polaron behavior and activation barriers are revealed (Figure 4b).77 Yamada's group has measured the reaction entropy of the biphasic reaction of Na1+2xV2(PO4)3 using the potentiometric method and found that the reaction entropy is almost constant for 0.1 ≤ x ≤ 0.9.78 X‐ray absorption and electron paramagnetic resonance were also introduced to determine the local environment of the Na3V2(PO4)3 material at different charge states (Figure 4f).79, 80

Overall, the promising results above‐mentioned have ignited tremendous efforts to improve the electrochemical performance of the Na3V2(PO4)3 electrode, including metal ion doping, carbon coating and particle downsizing. Metal ion doping was widely conducted to improve the structural stability of the Na3V2(PO4)3. K‐ions with larger ionic radius were incorporated as functional pillar ions into the Na3V2(PO4)3 structure by Kim et al. These results indicate that the K‐ions play an important role in enlarging the Na‐ion diffusion pathway and elongating the c‐axis thus to increase the lattice volume.81 Na3V2− Mg(PO4)3/C composites with various Mg2+ doping contents were investigated by Li et al.82 The doped Mg was substituted for the vanadium site and did not alter the structure. The ionic and electronic conductivities of the Na3V2− Mg(PO4)3 are significantly improved after Mg doping, resulting in a enhancement of the rate and cycle performances (Figure b).82, 83 Lavela's group has done extensive work about various metal ions substitution.84, 85, 86, 87 Iron substitution (both Fe2+ and Fe3+) was also found to effectively activate a V4+/5+ redox couple. In addition, the cell volume was increased after the larger Fe3+ substitution, resulting in the distortion of M1 octahedra, thus Na ions residing at M1 sites can be extracted for electrochemical reaction (Figure 5c).84, 88, 89 Similar phenomenon was also found by chromium, manganese and aluminum substitution (Figure 5d), respectively, by the same group.85, 86, 87

Figure 5

(a) Rate capabilities (0.1–5 C) of Na3− KV2(PO4)3/C; (b) Rate capability of Na3V2–xMgx(PO4)3/C (x = 0, 0.01, 0.03, 0.05, 0.07 and 0.1) at different current densities; (c) Cyclic voltammograms of the Na3V2–xFex(PO4)3 series recorded at a scan rate of 0.5 mV s–1; (d) Galvanostatic charge and discharge curves of Na3V2–xAlx(PO4)3 samples performed at several C rates. (a) Reproduced with permission.81 Copyright 2014, Royal Society of Chemistry. (b) Reproduced with permission.82 Copyright 2015, Royal Society of Chemistry. (c) Reproduced with permission.84 Copyright 2015, The Electrochemical Society. (d) Reproduced with permission.87 Copyright 2015, Elsevier.

Carbon decoration has been confirmed to be an effect strategy to improve the electrochemical performance of the Na3V2(PO4)3 electrode and many research groups employed different carbon matrixes to prepare high‐performance electrodes.90, 91, 92, 93, 94, 95, 96, 97, 98 Duan et al. synthesized Na3V2(PO4)3@C core‐shell nanocomposite by hydrothermal assisted sol‐gel method and obtained an initial capacity of 104.3 mAh g−1 at 0.5 C and 94.9 mAh g−1 at 5 C with a remarkable capacity retention of 96.1% after 700 cycles.99 Mechanically ball milling methods, including low temperature pre‐reduction and high temperature carbon thermal reduction, were reported to construct carbon coated Na3V2(PO4)3 electrodes with improved performance.100, 101, 102 Nitrogen and Boron doped carbon were also used to coat the Na3V2(PO4)3, respectively.103, 104, 105 The Boron doping can increase numerous extrinsic defects and active sites in the carbon coated layer, which could significantly accelerate Na+ transport in the carbon layer thus to greatly improve the rate performance and cycling stability of the Na3V2(PO4)3/C+B electrode.104 Encapsulating Na3V2(PO4)3 nanoparticles in one‐dimensional carbon sheath were widely reported through an electrospinning method by different groups.106, 107, 108 The one‐dimensional sodium ion transport pathway and the highly conductive network can further improve the electrochemical performance greatly. Due to its remarkably high conductivity and mechanical properties, graphene was also introduced to improve the performance of the Na3V2(PO4)3 electrode,12, 83, 105, 109, 110, 111, 112, 113 for example, Jung and Tao et al. have reported a graphene‐supported Na3V2(PO4)3 with a rate capability of 30 C and a stable cycling performance over 300 cycles.109, 110

Recently, the use of hierarchical carbon to decorate the Na3V2(PO4)3 electrode has been widely adopted to make high‐performance Na3V2(PO4)3 electrodes.11, 12, 60, 105, 111, 112, 113, 114, 115, 116 Zhu et al. reported a carbon‐coated Na3V2(PO4)3 embedded in porous carbon with remarkable rate capability of 44 mAh g–1 at a current rate of 200 C (Figure d).114 Xu et al. have also synthesized a layer‐by‐layer Na3V2(PO4)3@rGO material with ≈3% rGO and 0.5% amorphous carbon, which shows excellent rate capability (41 mAh g–1 at a current rate of 200 C) and cyclability (70.0% capacity retention after 15 000 cycles at 50 C).112 Coated carbon formed from pyrolysis of organic precursor is usually in an amorphous state with low electric conductivity. To further improve the electron conductivity of Na3V2(PO4)3, Fang et al. reported a facile chemical vapor deposition (CVD) method to build highly conductive coated‐carbon with graphical structure. The hierarchical carbon framework‐wrapped Na3V2(PO4)3 exhibited an unprecedented electrochemical performance of both ultra‐high rate capability (38 mA h g–1 at 500 C) and ultra‐long cycling stability (54% capacity retention over 20 000 cycles at 30 C rate) (Figure 6f,g).11 The highly conductive carbon framework is strongly effective in promoting ultra‐fast electronic transport and assisting in buffering volume change during Na ion insertion/deinsertion.

Figure 6

(a) Schematic illustration of the synthesis of hierarchically carbon coated Na3V2(PO4)3; (b) Facile softchemistry‐based double carbon‐embedding approach for (C@NVP)@pC; (c) Schematic illustration for the synthesis of NVP‐P/GO and NVP@rGO composites; (d) Galvanostatic charging−discharging profiles of (C@NVP)@pC at different current rates; (e) Rate performance of NVP@C@rGO, T‐NVP@C@rGO, NVP@C, and NVP@rGO cathodes; (f) Rate capability of the NVP and HCF‐NVP electrodes; (g) Long‐term cycling performance of the HCF‐NVP electrode at a high current rate of 30 C over 20 000 cycles. (a,f,g) Reproduced with permission.11 Copyright 2015, Wiley‐VCH. (b,d) Reproduced with permission.114 Copyright 2014, American Chemical Society. (e) Reproduced with permission.12 Copyright 2015, Wiley‐VCH.

Apart from being used as cathode material, the Na3V2(PO4)3 was also widely investigated as anode materials due to the lower potential of V2+/3+ an V1+/2+ redox couple.117, 118, 119 The Na3V2(PO4)3 anode could reversibly insert/extract three sodium ions between 3.0 and 0.01 V, corresponding to a reversible capacity of about 170 mAh g–1, and exhibited two voltage plateaus at 1.57 and 0.28 V.117, 118 Based on the low potential of Na3V2(PO4)3, many symmetric cells constructed of Na3V2(PO4)3 electrodes were reported.72, 90, 113, 120, 121 For example, Zhang et al. have assembled a symmetric full cell by using self‐supporting Na3V2(PO4)3 electrodes, the full cell exhibited an output voltage plateau of 1.8 V with a capacity of 90.2 mAh g–1 and a 81% capacity retention over 280 cycles.120

NASICON‐type NaTi2(PO4)3

NaTi2(PO4)3 was first reported by Hagman et al.122 This compound possesses a NASICON crystalline structure similar to Na3V2(PO4)3 with a rhombohedral structure in the R‐3c space group and two different Na sites, where M2 sites can reversibly insert two Na+, corresponding a theoretical capacity of 133 mAh g–1. Delmas et al. first studied the Na storage behavior of NaTi2(PO4)3, and found that two Na ions could reversibly insert in NaTi2(PO4)3 at room temperature either chemically and electrochemically.123 The NaTi2(PO4)3 electrode exhibited a discharge plateau at 2.1 V (vs. Na/Na+), associated with a two‐phase reaction. The moderate voltage range ensures that the NaTi2(PO4)3 can be used as anode and cathode depending on the counter electrodes. When used as anodes, the NaTi2(PO4)3 exhibits a higher coulombic efficiency with much higher safety due to the reduction of the possibility of forming a solid electrolyte interface (SEI) and avoidance of sodium deposition. The moderate voltage range and stable structure also make it an idea candidate as anode materials in aqueous solution, and the studies of NaTi2(PO4)3 in aqueous solution will be discussed in section 5.1.

Due to the intrinsic low electronic conductivity of the phosphate framework, improving the electron conductivity of the NaTi2(PO4)3 material by carbon matrix becomes an important route to obtain electrodes with high‐performance.124, 125, 126, 127, 128, 129, 130, 131, 132, 133 NaTi2(PO4)3 nanoparticles were synthesized by many groups through solvothermal strategy with the aim at promoting the electrode performance.124, 125, 126, 127, 128, 129, 130 For example, Wu and Wang et al. reported grapheme‐ and carbon nanotube‐ decorated NaTi2(PO4)3 nanoparticles, respectively, both of which had a high rate capability of 50 C rate (Figure a) and long cycle life of 1000 cycles.127, 130 Yang et al. have synthesized porous NaTi2(PO4)3 nanocubes with controllable size and the as‐prepared products have shown outstanding high‐rate capability of 100 C with long cycling life of 10 000 cycles (Figure 7e).129 Carbon‐ and rutile TiO2‐coated NaTi2(PO4)3 nanocubes were reported by Yang et al., the composite also exhibited high cycle stability with capacity retention of 89.3% over 10 000 cycles.126 Hierarchical carbon matrixes were also reported to effectively improve the electrochemical performance of the NaTi2(PO4)3 electrode. Jiang et al. have synthesized thinner carbon shell and interconnected carbon network decorated NaTi2(PO4)3, which showed high reversible capacity of 108 mAh g–1 at 100 C and a long cycle life of 83 mAh g–1 at 50 C after 6000 cycles.134 Fang et al. also reported a spray‐drying method to prepare hierarchical graphene supported NaTi2(PO4)3, where graphene coated nanosized NaTi2(PO4)3 and 3D graphene network could be achieved simultaneously. The electrode exhibited high reversible capacity of 130 mAh g–1 at 0.1 C and an ultra‐high rate capability of 38 mAh g–1 at 200 C (Figure 7b). Simultaneously, an all NASICON‐type NaTi2(PO4)3//Na3V2(PO4)3 full cell was also assembled with superior capacity and high‐power performance (Figure 7c and d).135 The structural elucidation during electrochemical reaction was studied by density functional theory on a full‐titanium‐base symmetric cell.136 The Na storage at low operation voltages was also found to associate with an extra working plateau at around 0.4 V,137, 138 which was attributed to the further reduction of Ti3+ to Ti2+. The electrode could also give a huge improved reversible capacity of ≈210 mAh g–1 at 0.1 C, a rate capability of 50 mAh g–1 at 100 C and long‐term cycling life with ≈68% capacity retention over 10 000 cycles.137

Figure 7

(a) Galvanostatic discharge–charge profiles of the NTP‐GN electrode at various current rates; (b) Rate capability of the NTP@rGO electrode; (c) Rate capability of the NTP@rGO//Na3V2(PO4)3/C sodium ion battery; (d) Ragone plots of the NTP@rGO//Na3V2(PO4)3/C sodium ion battery based on the cathode and anode mass; (e) Cycling performances of the NTP‐NBA electrodes obtained at 10 C rate. (a) Reproduced with permission.130 Copyright 2015, American Chemical Society. (b,c,d) Reproduced with permission.135 Copyright 2016, Wiley‐VCH. (e) Reproduced with permission.129 Copyright 2015, Royal Society of Chemistry.

Iron‐substituted NaTi2(PO4)3 was investigated by a number of groups.139, 140, 141 Differing from NaTi2(PO4)3, the Fe‐substituted sample, Na1.5Fe0.5Ti1.5(PO4)3, exhibited more complex mechanisms than the two‐phase and one‐phase mechanisms observed for NaTi2(PO4)3 and Na3Fe2(PO4)3, respectively, and iron was an electrochemically active center at 2.2 V with the reversible Fe3+/Fe2+ transformation.140, 141

Amorphous Phosphate Structure

Amorphous materials lacks the long‐range ordering of a crystal but has certain short‐range ordering at atomic length scale due to their favorable chemical bonding.142, 143 Due to the lack of three‐dimensional long‐range order, amorphous solids do not constructively diffract X‐rays, as do crystalline solids. Therefore, broad, diffuse haloes are observed in X‐ray powder diffraction patterns instead of well‐defined peaks.143 Amorphous solids are supposed as potential electrodes considering less lattice pressure during electrochemical reaction. However, it is not easy to make an amorphous structure for most of the crystalline electrode materials with satisfying electrochemical performance because of the metastable state of amorphous materials.

Among the phosphate compounds, iron‐based phosphates are easy to form amorphous phase. Amorphous FePO4 has been widely reported as cathodes for lithium ion batteries with high reversible capacity (175 mAh g–1) and stable cyclability, and was investigated as drop‐in replacements for SIBs. Shiratsuchi et al. firstly compared the Li and Na storage performance of amorphous and crystalline FePO4 and found that both amorphous and crystalline FePO4 showed similarly reversible capacities not only for Li but also for Na ion batteries. The amorphous FePO4 for Na ion storage delivered an optimal capacity of 146 mAh g–1 at a current rate of 0.1 mA cm–2.144 Mathew et al. have studied the amorphous FePO4 as host for various charge carrier ions (mono‐/di‐/tri‐valent ions) (Figure a), which delivered a capacity of 179 mAh g–1 for SIBs, based on a reversible amorphous‐to‐crystalline transition during electrochemical reactions.145 Zhao et al. used a solvent extraction route to obtain monodisperse amorphous FePO4·2H2O nanospheres, which exhibited a reversible Na ion storage capacity of 108 mAh g–1 at 3.5 mA g–1.146

Figure 8

(a) Schematic representation of alkali‐ions (Li/Na/K) insertion in crystalline and amorphous FePO4 electrode hosts; (b,c) Galvanostatic discharging/charging profiles of the FePO4/C cathode performed at a current density of 20 mA g–1 and the corresponding cycling performance; (d,e) Galvanostatic discharging‐charging profiles performed at a current density of 0.1 C and rate capability of amorphous NaFePO4 nanospheres. (a) Reproduced with permission.145 Copyright 2014, Nature Publishing Group. (b,c) Reproduced with permission.10 Copyright 2014, American Chemical Society. (d,e) Reproduced with permission.156 Copyright 2015, Royal Society of Chemistry.

Because high temperature calcination can lead to the crystallization of FePO4, carbon decoration of this material has to be conducted at a relative low temperature. Various carbon matrixes have been introduced to improve the Na storage performance of the amorphous FePO4. Liu et al. reported single‐wall carbon nanotubes wired FePO4 with reversible capacity of 120 mAh g–1.147 Fang et al. also reported a mesoporous amorphous FePO4 embedded in carbon matrix. The obtained FePO4/C exhibited a high initial discharge capacity of 151 mAh g–1 at 20 mA g–1 with stable cyclability (94% capacity retention ratio over 160 cycles) (Figure 8b,c) as well as high rate capability (44 mAh g–1 at 1000 mA g–1), and the electrode kept amorphous structure at different states of charge.10 Multi‐walled carbon nanotubes, graphene and carbonized polyaniline decorated FePO4 were also synthesized with well‐improved electrochemical performances.148, 149, 150, 151 The Na storage behavior of amorphous and crystalline FePO4 were compared by Wang and Liu et al.152, 153 The amorphous FePO4 have been testified to exhibit better reversible capacity and cycling stability over the crystalline FePO4.152, 153 During sodiation process, the amorphous FePO4 transformed into NaFePO4 with amorphous and triphylite phase simultaneously, while trigonal FePO4 partly transformed into the maricite NaFePO4.153, 154 Recently, 2D amorphous iron phosphate nanosheets were also reported to have a high initial discharge capacity of 168.9 mA h g−1 at 0.1 C and a stable cycle life with 92.3% capacity retention over 1000 cycles, showing a highest reversible capacity among phosphate framework materials.155

The above discussed FePO4 are Na‐vacant, which is not convenient for practical battery applications as cathode materials. Recently, Li et al. reported Na‐riched, amorphous NaFePO4 nanospheres with a high initial discharge capacity of 152.1 mAh g–1 (Figure 8d), high rate capability (67.4 mAh g–1 at 10 C) (Figure 8e) and stable cyclability (95% capacity retention over 300 cycles).156 More Na‐rich amorphous materials are needed to explore for enriching the amorphous cathode systems.

Pyrophosphate Materials for Na Storage

In parallel to the massive effort on phosphate materials, their analogue, pyrophosphate materials, have also attracted wide interest. The sodium metal pyrophosphate compounds generally consist of transition metal octahedral MO6 and P2O7 units connected to form a robust framework.157

Komatsu and Yamada's group first reported the pyrophosphate‐class material for SIBs. They first tested a Na2FeP2O7 material to obtain a reversible capacity of 82 mAh g–1 with the Fe3+/Fe2+ redox potential at around 3 V and 2.5 V (Figure a).158, 159 It is found that the Na2FeP2O7 phase and desodiated NaFeP2O7 phase delivered excellent thermal stability up to 600 °C with no thermal decomposition and/or oxygen evolution due to the stable pyrophosphate (P2O7)4– anion.160 Atomistic simulation indicated that the Na2FeP2O7 exhibited a 3D Na+ diffusion behavior.161 Kim et al. also conducted combined experimental and theoretical study to investigate the structure, electrochemical and thermal properties of Na2FeP2O7 material. Both quasi‐equilibrium measurements and first‐principles calculations consistently indicated that Na2FeP2O7 underwent two kinds of reactions: a single‐phase reaction around 2.5 V and a series of two‐phase reactions in the voltage range of 3.0–3.25 V.162 After that, some efforts were made to optimize the electrochemical performance of Na2FeP2O7 electrode for SIBs.163, 164, 165, 166, 167, 168 For example, carbon nanotubes decorated Na2FeP2O7 was demonstrated to have a high rate capability at 20 C rate,166 and can work well in inorganic ionic liquid.167, 168 Due to the higher electrochemically active Mn3+/Mn2+ redox potential, Manganese substitution were also employed to raise the average redox potential up to 3.2 V.169, 170 Ex situ XRD and CV analyses indicated that Na2Fe0.5Mn0.5P2O7 underwent a single phase reaction rather than a biphasic reaction due to different Na coordination environment and different Na sites occupancy.169 Recently, off‐stoichiometric iron‐based pyrophosphate, named Na2–xFe1+x/2P2O7, were reported To have similar structure and charge/discharge plateaus with Na2FeP2O7, but higher reversible capacity (114 mAh g–1) and better cyclability exceeding 3000 cycles in ionic liquid electrolyte.171, 172, 173, 174, 175

Figure 9

(a) Galvanostatic voltage‐composition curve of Na2FeP2O7 at a rate of C/20; (b) Galvanostatic charge‐discharge curves of Na2MnP2O7; (c) A charge/discharge profile of Na7V3(P2O7)4 with the calculated voltage (inset: dQ/dV of Na7V3(P2O7)4); (d) Galvanostatic cycles of the Co rose (RN), Fe rose, and Co blue polymorphs; (e) Voltage‐capacity charge‐discharge profile of Na2(VO)P2O7 cathode at a rate of C/20. (a) Reproduced with permission.158 Copyright 2012, Elsevier. (b) Reproduced with permission.176 Copyright 2013, American Chemical Society. (c) Reproduced with permission.184 Copyright 2016, Wiley‐VCH. (d) Reproduced with permission.180 Copyright 2016, Wiley‐VCH. (e) Reproduced with permission.183 Copyright 2014, Wiley‐VCH.

Na2MnP2O7 was also introduced as electrode materials for SIBs,176, 177 exhibiting good electrochemical activity at ≈3.8 V with reversible capacity of 90 mAh g–1 (Figure 9b). First‐principles calculations indicated that the enhanced electrochemical performance was mainly resulted from the small extent of atomic rearrangements, which lower the barriers for electron conduction and phase boundary migration.176

In addition to Na2MnP2O7, Na2CoP2O7 was also investigated as a high voltage cathode.178, 179, 180 Yamada's group reported an orthorhombic structure Na2CoP2O7 with the space group of Pna21, which offered a two‐dimensional Na‐diffusion pathway and could deliver a reversible discharge capacity of 80 mAh g–1 at an average potential of 3 V.178 Kim et al. have employed a strategy by controlling the Na deficiencies to successfully obtain a triclinic Na2CoP2O7, which showed great improvement of energy density. The optimized material showed an average voltage of 4.3 V with reversible capacity of 80 mAh g–1 (Figure 9d).180 This work may provide a concept of developing new materials via nonstoichiometry‐driven control of polymorphism.

Vanadium‐based pyrophosphates exhibit structural diversity with different compositions, such as NaVP2O7, Na7V3(P2O7)4, and t‐Na2(VO)P2O7.181, 182, 183, 184 The t‐Na2(VO)P2O7 delivered a capacity of 80 mAh g–1 through V5+/V4+ redox reaction at the potential of 3.8 V (Figure 9e),183 while Na7V3(P2O7)4 showed a capacity of 80 mAh g–1 with average potential of 4 V (Figure 9c).184 The high‐voltage vanadium‐based pyrophosphates paved the way for further exploration of Na2MP2O7 family.

Mixed‐Anion Materials for Na Storage

A series of compounds with new classes of host structures and compositions can be obtained by using mixed‐anion approach. By introducing new anion into the phosphate structure, some of the mix‐anion compounds can show improved electrochemical performance. For example, the F‐substituted sample exhibited higher potential due to the strong inductive effect of the F− anion. In the phosphate materials with mixed‐anion structure, F‐substituted samples and mixed multivalent anions (CO3 2–, P2O7 4–) substituted samples showed better electrochemical performance.

F‐substituted Materials for Na Storage

The combination of F anions with phosphates will lead to a variety of compounds with enhanced operating voltage due to the higher iconicity of the M—F bond.185 Among these materials, the Fe‐based and V‐based materials emerged to have excellent Na storage performance.

Fe‐based F‐substituted Materials

Na2FePO4F was first employed as cathode for lithium ion batteries by Nazar's group. This material has a layered structure with a Pbcn orthrorhombic space group, in which face‐sharing FeO4F2 octahedra are connected via bridging F atoms to form chains and are joined by PO4 tetrahedra to form [FePO4F] infinite layers with Na cations located in the interlayer space (Figure a).186, 187 Tarascon's group first tested Na2FePO4F for SIBs, which exhibited two well‐defined discharge voltage plateaus at 3.1 and 2.9 V with a reversible capacity of 0.8 Na per unit formula.188 Enhanced electrochemical performance has been reported by many groups through carbon coating technology and different synthesis routes.189, 190, 191, 192 For example, Law et al. have synthesized Na2FePO4F via a soft template method to deliver impressive capacity of 116 mAh g–1 (Figure 10d), high rate capability of 21 mAh g–1 at 10 C rate and 80% capacity retention over 200 cycles.192 Atomistic simulation method was introduced to study the Na ion migration property of Na2FePO4F. Na ion conduction in Na2FePO4F was predicted to be two‐dimensional (2D) in the interlayer plane with a low activation energy, indicating high Na mobility through a 2D network in the ac plane.193 Manganese substitution was also reported, due to the Mn2+/Mn3+ redox reaction.188, 194 The manganese substituted sample exhibited higher average operating voltage, which was sufficient to trigger a 2D–3D structural transition.188 The reported Na2MnPO4F possesses a three‐dimensional P21/n structure (Figure 10b) with sloping charge/discharge curves (Figure 10e). First principles calculations indicated that extracting the second Na ion from Na2MnPO4F required a much higher voltage (≈4.67 V vs. Na/Na+).195, 196, 197 The Na2CoPO4F also has a two‐dimensional layered structure (Figure 10c), which exhibited high discharge voltage of 4.3 V with reversible capacity of ≈ 100 mAh g–1. However, this material showed a low coulombic efficiency with degenerated capacity (Figure 10f).198, 199

Figure 10

View of the crystal structure of (a) Na2FePO4F with Pbcn space group, (b) Na2MnPO4F with P21/n space group, and (c) Na2CoPO4F with Pbcn space group; Galvanostatic charge‐discharge curves of the (d) Na2FePO4F pristine and ball milled samples, (e) Na2MnPO4F sample, and (f) Na2CoPO4F/C sample. (a,d) Reproduced with permission.192 Copyright 2015, Royal Society of Chemistry. (b) Reproduced with permission.195 Copyright 2012, Royal Society of Chemistry. (c,f) Reproduced with permission.199 Copyright 2015, The Electrochemical Society. (e) Reproduced with permission.196 Copyright 2014, Royal Society of Chemistry.

V‐based F‐substituted Materials

F‐substitution can enrich largely the family of V‐based materials and introduce new structure with varying electrochemical properties. The inductive effect of F anions can also elevate the operating voltage. With respect to sodium‐vanadium fluorophosphates, three phases widely investigated are NaVPO4F, Na3V2(PO4)2F3 and Oxygen substituted Na3(VO1− PO4)2F1+2 (0 ≤ x < 1).

NaVPO4F was first reported by Barker et al.200 They assembled a hard carbon//NaVPO4F full cell, which exhibited an average working potential of 3.6 V with reversible capacity of about 80 mAh g–1. The effect of carbon and graphene coating on NaVPO4F was also reported.201, 202 Cr and Al substitution were reported to be effective for the cycle stability.203, 204 However, it's worth noting that there are no structural data about the NaVPO4F phase. The XRD data shown in these works match well with the NASICON compound, so that the existence of the NaVPO4F phase has been questioned by some authors.205, 206

The similar structure, Na3V2(PO4)2F3, is attracting strong interest as cathodes for SIBs due to its high capacity, rate capability and long‐term cycling stability. Meins et al. first reported the crystal structure of Na3V2(PO4)2F3 with a tetragonal structure (P4 space group) (Figure a–e),207 featuring a strongly covalent 3D framework with large interstitial spaces for ion diffusion. Not only the specially constructed [PO4]3– network can help to stabilize the crystal structure of the material, but the oxygen atoms fixed in the [PO4]3– formation may also decrease the likelihood of oxygen liberation, leading to better thermal stability.208 Barker's group first studied lithium storage performance of Na3V2(PO4)2F3.209, 210 Shakoor et al. studied the Na storage performance of Na3V2(PO4)2F3 through combined computation and experiments. This material exhibited two plateaus with average voltages of about 3.7 and 4.2 V. Structural evaluation indicated that the reversible sodiation/desodiation occured through one‐phase reaction.211 High‐performance Na3V2(PO4)2F3 electrodes were also reported.212, 213 For example, Liu et al. have synthesized carbon coated Na3V2(PO4)2F3, which delivered a high reversible capacity of 130 mAh g−1 with high rate capabitlity (57 mAh g–1 at 30 C rate) (Figure 11f,g) and long cycle life (50% capacity retention over 3000 cycles).213 The Na ion (de)intercalation mechanism was well studied by some groups.208, 214, 215, 216 Bianchini et al. have performed high angular resolution synchrotron radiation diffraction measurement to carefully reveal the phase diagram. It was found that four intermediate phases existed during the Na extraction reaction and only one of these phases underwent a solid solution reaction (Figure 11h,i).214 Liu et al. studied the structural and dynamical changes of Na3V2(PO4)2F3 during charge process and found distinct changes in Na‐ion and electronic mobility and V electronic configurations: the Na ions were removed non‐selectively from the two distinct Na sites, while Na mobility increased steadily with increased more Na vacancies in the structure on charging.216

Figure 11

Schematic representation of a refined Na3V2(PO4)2F3 structure projected along (a) the a axis and (b) the c axis, (c) all of the possible Na sites and (d) most stable configuration of Na ions in Na3V2(PO4)2F3 and (e) Na2V2(PO4)2F3 from the first principles calculations. Na1 indicates fully occupied Na sites and Na2 indicates half occupied Na sites. Red arrows of (d) represent a shift of Na ions off the centers of prismatic sites. (f) Galvanostatic charging‐discharging profiles of the NVPF@C nanocomposite at various current rates; (g) Rate capability at various current rates of the NVPF@C nanocomposite. Different angular domains observed during the extraction of 1 Na+ from Na3VPF (black curve). Several single‐phase compositions are formed through biphasic domains, namely, Na2.4VPF (blue), Na2.2VPF (orange), and Na2VPF (red). (h) 17°–17.7° (left) and 24.1°–24.9° (right) angular domains. Peaks are indexed for the Na3VPF phase. (i) 2.7°–3.3° and 13.5°–14.9° angular domains (weak peaks related to sodium ordering). (a–e) Reproduced with permission.211 Copyright 2012, Royal Society of Chemistry. (f,g) Reproduced with permission.213 Copyright 2015, Royal Society of Chemistry. (h,i) Reproduced with permission.214 Copyright 2015, American Chemical Society.

A family of oxygen substituted samples, namely Na3(VO1− PO4)2F1+2 (0 ≤ x < 1), have attracted much attention due to the high energy density and good cycle life.205, 217, 218, 219, 220, 221 In all cases, the samples have similar X‐ray diffraction characteristics and charge/discharge curves with two voltage plateaus at the same voltages, suggesting that all the materials belong to the same family of compounds, where the fluorine content is modulated by the presence of V3+ and VO2+ (V4+) and the redox mechanism varies depending on the compositions.205 An example of structural comparison of Na3V2(PO4)2F3 and Na3(VO)2(PO4)2F phases is shown in Figure a–b. Both of them present the same framework, where one of the fluorine atoms in Na3V2(PO4)2F3 is replaced by an oxygen in Na3(VO)2(PO4)2F.205 Teófilo Rojo's group have done much work to study the Na storage mechanism of the Na3(VO1− PO4)2F1+2 materials.220, 222, 223 The reaction mechanisms during charge/discharge process include combinations of solid solution and two‐phase reaction behavior, but the structural motif was maintained throughout these reactions.223 The relationship among V3+/V4+/V5+ redox reactions, Na+−Na+ ordering, and Na+ intercalation mechanisms of Na3(VO1− PO4)2F1+2 in SIBs were investigated by Park et al. through a combined theoretical and experimental approach.218, 224 They found that the redox mechanism and phase reactions varied with fluorine content. High performance Na3(VO1− PO4)2F1+2 electrodes were also reported. Qi et al. have synthesized a series of Na3(VO1− PO4)2F1+2 (0 ≤ x ≤1) materials by a solvothermal strategy. Among them, the Na3(VOPO4)2F sample exhibited the best Na‐storage performance with both high rate capability and long cycle life.219 Peng et al. reported a RuO2‐coated Na3V2O2(PO4)2F nanowires with enhanced electrochemical performance of high reversible capacity of 120 mAh g–1, high rate capability (95 mAh g–1 at 20 C rate) (Figure 12c–d) and long cycle life of 1000 cycles.225

Figure 12

Structural comparison of (a) Na3(VO)2(PO4)2F and (b) Na3V2(PO4)2F3; (c) Charge and discharge profiles of RuO2‐coated Na3V2O2(PO4)2F nanowires at a current density of 0.1 C; (d) Rate capacity of RuO2‐coated Na3V2O2(PO4)2F nanowires and uncoated Na3V2O2(PO4)2F nanowires; Evidence of reaction mechanism evolution of the cathode during charge and discharge. (e) Selected 2θ regions of in situ synchrotron XRD data of Na3V2O2(PO4)2F highlighting the evolution of the 220, 113 (left) and 311, 222 (right) reflections by a color scale and the potential profile. Selected temporal region (f) and snapshots (g) of the in situ data of the 220 and 113 reflections with the phases P, P′, and P″ shown. (h) Selected temporal region of in situ data showing the two‐phase region at higher charge. (a,b) Reproduced with permission.205 Copyright 2012, Royal Society of Chemistry. (c,d) Reproduced with permission.225 Copyright 2015, Wiley‐VCH. (e–h) Reproduced with permission.223 Copyright 2014, American Chemical Society.

Mixed Multivalent Anion Materials for Na Storage

Combining different multivalent anions together, we can get a series of materials with novel structure and electrochemical activity. The compounds containing (PO4)(P2O7) and (PO4)(CO3) are in the spotlight due to the low‐volume changes upon cycling, indicative of long‐life operation.

(PO4)(P2O7)‐based Materials

Na4M3(PO4)2(P2O7) (M = Fe, Mn, Co, Ni) materials have been structurally characterized at early years and tested as electrochemically active materials for SIBs recently.226, 227, 228 The materials show double chains built up from PO4 tetrahedron and MO6 octahedra sharing corners with interlayer linkages via P–O–P bridges of the pyrophosphate groups in such a way that large tunnels extending along the [010] and [001] directions occur between two neighboring sheets.226 Kang's group first studied the Li and Na storage performance of Na4Fe3(PO4)2(P2O7) through a combined first principles calculations and experiments and reported a reversible capacity of 129 mAh g–1 and average potential of 3 V for the Na‐ion cell (Figure a).229 The Na storage mechanism of Na4Fe3(PO4)2(P2O7) was testified to be a one‐phase reaction accompanying an exceptionally small volumetric change of less than 4%.230 They also extended the study to a Mn‐based material, Na4Mn3(PO4)2(P2O7), as cathodes for SIBs. The Na4Mn3(PO4)2(P2O7) material exhibited a largest Mn2+/Mn3+ redox potential of 3.84 V yet reported for a manganese‐based cathode for SIBs, and a reversible capacity of 109 mAh g–1. First‐principles calculations and experimental study showed that three‐dimensional Na diffusion pathways with low activation energy ensured the high rate capability of the material (20 C rate) (Figure 13b), and it was worth to be noted that the structural distortion induced by Jahn‐Teller distortion could open up sodium diffusion channels thus to increase the sodium ion mobility.231 Na4Co3(PO4)2P2O7 material has been employed as cathode for SIBs by Nose et al, which showed multi redox couples in the high potential region between 4.1 and 4.7 V with reversible capacity of 97 mAh g–1 (Figure 13c).232 Density functional theory calculations indicated that the removal of Na down to NaCo3(PO4)2P2O7 was found to be accompanied by oxidation of Co2+ to Co3+, and further removal of Na to give Co3(PO4)2P2O7 requires oxidation of oxygen 2p orbitals in the P2O7 polyhedra instead of Co3+ being oxidized to Co4+.233 They also reported a Na4Co2.4Mn0.3Ni0.3(PO4)2P2O7 material with two redox couples around 4.2 and 4.6 V with a capacity of 110 mAh g–1.234

Figure 13

(a) Galvanostatic charge/discharge profiles of Na4Fe3(PO4)2(P2O7) in a Na‐ion cell at the C/20 rate; (b) The discharge profiles of Na4Mn3(PO4)2(P2O7) at various rates of C/20 to 20 C at 25 °C; (c) Galvanostatic charge–discharge curves at 1st, 10th and 50th cycles of Na4Co3(PO4)2P2O7; (d) The charge/discharge curves of the Na7V4(P2O7)4(PO4) nanorods at different current density. (a) Reproduced with permission.230 Copyright 2013, American Chemical Society. (b) Reproduced with permission.231 Copyright 2015, Royal Society of Chemistry. (c) Reproduced with permission.232 Copyright 2013, Elsevier. (d) Reproduced with permission.236 Copyright 2014, American Chemical Society.

Lim et al. have reported a V‐based mixed polyanion material, Na7V4(P2O7)4(PO4) with a tetragonal structure (a space group of P 21 c). The Na7V4(P2O7)4(PO4) holds exceptional electrochemical properties represented by well‐defined high voltage profiles at 3.88 V (Figure 13d) and substantial capacity retention over 1000 cycles.235 The material showed a V3+/V4+ redox reaction with Na5V3.5+ 4(P2O7)4(PO4) as intermediate phase, resulting in two plateaus in charge/discharge curves.236 Through elaborate structure design and conductive carbon coating, the Na7V4(P2O7)4(PO4) material showed a high rate capability of 30 C with excellent cycling stability of 94% capacity retention over 800 cycles.237

(PO4)(CO3)‐based Materials

PO4 and CO3 can also be combined together to construct novel structure of carbonophosphates. Ceder's group reported a Na3MnPO4CO3 material with a reversible capacity of 125 mAh g−1 and average potential of 3.3 V (Figure b). In situ X‐ray diffraction measurement suggested that the sidorenkite Na3MnPO4CO3 underwent a solid solution type reversible topotactic structural evolution upon electrochemical cycling.238 After that, Na3MnPO4CO3 electrodes with improved reversible capacity were reported.239, 240 Hassanzadeh et al. revealed a Na3MnCO3PO4/rGO hybrid with a high reversible capacity of 156 mAh g–1.240 However, the Na3MnCO3PO4 electrode exhibited high charge/discharge polarization with fast capacity degradation. Huang et al. reported another carbonophosphate, Na3FeCO3PO4, with a reversible capacity of 120 mAh g–1 and average working potential of 2.6 V (Figure 14d). In situ experiments indicated that both Fe2+/Fe3+ and Fe3+/Fe4+ redox couples were electrochemically active.241

Figure 14

(a) The structure of Na3MnPO4CO3 viewed along [001]; (b) The voltage curves of sidorenkite Na3MnPO4CO3 at the first, second, and 10th cycles with a C/100 rate; (c) Geometrical model of the Na3FePO4CO3 along the a axis; (d) Galvanostatic charge/discharge profiles of Na3FePO4CO3 nanoplates at 10 mA/g for different cycles. (a,b) Reproduced with permission.238 Copyright 2013, American Chemical Society. (c,d) Reproduced with permission.241 Copyright 2014, Nature Publishing Group.

New Perspectives of Phosphate Framework Materials

Aqueous Sodium Ion Batteries

Batteries using aqueous electrolyte can reduce the cost and be safer. Due to the very stable structure of phosphate materials, many of the materials have been shown excellent electrochemical performance in aqueous batteries.

Considering the proper redox potential and highly stability, NaTi2(PO4)3 has been widely accepted as an ideal candidate as anodes for aqueous SIBs.242 It exhibited a moderately high reversible capacity of 124 mAh g–1 and the plateau voltage was –0.8 V vs. Ag/AgCl reference electrode. By conductive carbon decoration, the rate capability and cycle stability of the NaTi2(PO4)3 electrodes in aqueous SIBs can be largely enhanced.243, 244, 245, 246, 247 For example, Li et al. reported a graphene decorated NaTi2(PO4)3, which exhibited high rate capability of 20 C and a long cycle life with a capacity retention of 71% over 2000 cycles.245 A number of aqueous SIBs have also been constructed by using NaTi2(PO4)3 anodes, such as the NaTi2(PO4)3//Na2NiFe(CN)6,248 NaTi2(PO4)3//Na0.44MnO2,249, 250, 251, 252 NaTi2(PO4)3//Na3V2(PO4)3,253 NaTi2(PO4)3//Na2FeP2O7,254 NaTi2(PO4)3//NaFePO4,255 and NaTi2(PO4)3//Na3V2O2x(PO4)2F3‐2x couples.256

Other phosphate materials have been investigated as electrode materials for aqueous SIBs, for example, the Na2FeP2O7,257 Na3V2(PO4)3,258, 259 Na2VTi(PO4)3,260 Na7V4(P2O7)4(PO4),261 Na3V2O2(PO4)2F.262 However, due to the partial dissolution of electrode materials and inappropriate redox potential (close to the potential of hydrogen or oxygen evolution), these materials cannot be widely accepted as electrodes for large scale application. More efforts should be made to explore novel materials or elaborated structure design to gain electrodes with high performance in aqueous SIBs.

Ab Initio Computations

Novel materials are the key to the development of electrodes for energy storage system, however, conventional discovery of compounds with new structure and electrochemical activity requires large amount of repetitive experiments, considering the uncertain synthetic conditions and parameters. Ab initio computations in the density functional theory could be used to provide insight into the fundamental properties of electrode materials of lithium ion batteries.263, 264 Ab initio computations are accurate enough to understand and even predict electrode properties (eg., voltage, lithium diffusion, stability, and safety). Using a computational high‐throughput approach of computing properties on thousands of materials, the high scalability of computing can offer the possibility to discover new electrode materials.265, 266

Phosphate materials have been evaluated as electrode materials for lithium ion batteries by using high‐throughput ab initio computations. The limits and opportunities for the phosphate chemistry in terms of voltage, capacity (gravimetric and volumetric), specific energy, energy density, and safety were analyzed and discussed.265, 267 Using the same model and database, phosphate materials, even other kind of materials, can be evaluated and analyzed, which can help the experimental process of exploring new electrode materials for SIBs.268, 269, 270

Conclusions and Outlook

Sodium ion batteries have attracted increasing attention due to the wide availability and low cost of sodium resources. Exploring electrodes with higher structural and thermal stability is the key to promote electrochemical performance (long‐term cyclability and rate capability) of the SIBs for large scale energy storage application. From the structural point of view, phosphate materials possess the robust framework and exhibit low structure expansion or distortion, the potentially high operating voltages due to the inductive effect of phosphate groups or fluorophosphates groups, and high rate capability and long cycling life for high‐performance SIBs. The phosphates with proper reduction reaction potential should be the promising candidate for future energy storage application.

Phosphate framework materials are full of variety, exhibiting versatile and adjustable structure and electrochemical performance. Such as the phosphates, pyrophosphates, mixed‐anions, and optional redox centers (Fe, Mn, Co, V, Ni, et al.). The voltages and capacities (practical and theoretical) of representative phosphate framework materials for SIBs are summarized in Figure . It's worth noting that some of the materials have shown high energy density (vs. metal Na anode) around 500 Wh kg–1, namely the Na1.5VPO4.8F0.7 and Na3V2O2x(PO4)2F3–2x, which exceed that of Li/LiMn2O4 (429 Wh kg–1) and are very close to those of Li/LiFePO4 (510 Wh kg–1) and Li/LiCoO2 (530 Wh kg–1), showing potential application for high energy density SIBs. Additionally, it can be seen that the exhibited capacities of some materials have been very close to their theoretical values, for example the NaTi2(PO4)3, Na3V2(PO4)3, and Na3V2O2x(PO4)2F3–2x. Therefore, most of the materials deliver capacities far lower than the theoretical capacities, for example the olivine NaFePO4, NaVOPO4, Na3MnPO4CO3, Na4Mn3(PO4)2(P2O7) and so on. Particularly, some systems such as Na/Na3MnPO4CO3 and Na/Na4Mn3(PO4)2(P2O7) might reach the theoretical energy density of 600 Wh kg–1. Thus, there will be a lot of work to do to improve the capacities of these materials in the future.

Figure 15

Phosphate framework materials and corresponding electrochemical data in the SIB technologies.

On the other hand, due to the intrinsic poor electron conductivity of phosphate materials, it is important and effective to improve the electrochemical performance of phosphate materials by carbon decoration. Enhancing the potential and capacity of electrode materials is also the important strategy to obtain SIBs with a high energy density. In addition, full cells based on phosphate materials should also be paid attention to accelerate the application of SIBs.