Recent Advances of the Electrode Materials for Sodium⁃Ion Capacitors

Qi Qi; Peizhu Xu; Zhidong Tian; Wei Sun; Yangjie Liu; Xiang Hu

doi:10.7536/PC220121

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 2051-2062 DOI: 10.7536/PC220121

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Recent Advances of the Electrode Materials for Sodium⁃Ion Capacitors

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Abstract

Sodium ion hybrid capacitors (SIHCs) have been considered to be one of the most promising electrochemical energy storage devices because of the abundant and low cost of sodium resources, and their similar physical and chemical properties to that of the lithium. SIHCs are usually assembled with high⁃energy anode and high⁃power cathode, which can bridge the energy and power gaps between sodium⁃ion batteries and supercapacitors. However, their large⁃scale application is strongly impeded by the kinetics and capacity imbalance between capacitive⁃type cathode and battery⁃type anode. In this paper, the working principle of SIHCs and the research progress of various anode and cathode materials are summarized. The development trend of SIHCs is reviewed emphatically from the aspects of controlled preparation and modification of material structure. The main challenges encountered in the development of SIHCs are discussed and the future research direction of the electrode materials in this field is prospected.

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sodium ion hybrid capacitors / capacitive⁃type cathode / battery⁃type anode / kinetics / capacity

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Qi Qi, Peizhu Xu, Zhidong Tian, Wei Sun, Yangjie Liu, Xiang Hu. Recent Advances of the Electrode Materials for Sodium⁃Ion Capacitors. Progress in Chemistry, 2022, 34(9): 2051-2062 DOI:10.7536/PC220121

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1 Introduction

2 Operating principle

3 Cathode materials

3.1 Activated carbon

3.2 Carbon nanotube

3.3 Graphene

3.4 MXene

4 Anode materials

4.1 Intercalation materials

4.2 Conversion materials

4.3 Alloying materials

5 Conclusion and outlook

1 Introduction

In recent years, with the increasingly prominent environmental pollution and the growing demand for sustainable energy, the development of high-performance energy storage devices has become the focus of attention. Rechargeable secondary battery has been considered as the most promising energy storage system because of its high energy density, and lithium-ion battery has become the mainstream energy storage device in the current market^[1⇓~3]. However, lithium-ion batteries rely on the Faraday reaction mechanism for energy storage during cycling, resulting in slow ionic reaction kinetics. At the same time, the insertion/extraction of lithium ions in the electrode material will inevitably produce a large volume change, which will cause the whole electrode structure to crack, resulting in a thick insulating interface layer, and ultimately lead to poor power density and short cycle life^[4]. Secondly, electrochemical capacitors capable of ultra-fast charge and discharge show extremely high power density and long cycle life of tens of thousands of times, but their energy density is often low due to the limitation of double layer energy storage, which is far inferior to that of secondary batteries^[5]. Alkali metal ion hybrid capacitors are usually composed of a battery type negative electrode with high energy density and a capacitor type positive electrode with high power density, which can significantly broaden the working voltage window of the device.It can effectively improve energy density, high power density and prolong cycle life, and integrates the respective advantages of secondary batteries and electrochemical capacitors, which has attracted wide attention in recent years.

Since Amatucci et al. First reported a lithium-ion hybrid capacitor (LIHCs) with Li₄Ti₅O₁₂ as the negative electrode and activated carbon as the positive electrode in 2001, there has been a boom in the field of new electrochemical energy storage. However, the scarcity, high price and uneven distribution of lithium resources have brought great obstacles to the large-scale application of lithium-based electrochemical systems^[6]. The alkali metal sodium and lithium have similar physical and chemical properties. Sodium is more abundant in nature, and its cost is lower than that of lithium. Therefore, the hybrid ionic capacitor using Na⁺ instead of Li⁺ has more potential for large-scale application in the future^[7]. On the other hand, although the research on zinc-potassium hybrid capacitors is in full swing at present, the voltage window of zinc-potassium hybrid capacitors is too narrow compared with SIHCs, resulting in its energy density is significantly lower than that of SIHCs. Although the potassium ion hybrid capacitor has similar internal composition and electrochemical reaction mechanism with SIHCs, it still has the following disadvantages compared with SIHCs: (1) the ion diffusivity of potassium ion in the electrode material is low, resulting in poor potassium ion reaction kinetics in the whole system; (2) The larger particle size of potassium ions leads to a large volume change of electrode materials in the process of potassium intercalation/deintercalation, which seriously affects their cycle life; (3) K is more active than Na, and its dendritic dendrite and potential safety hazard are more serious; (4) The relative atomic mass of K is higher than that of Na, resulting in a more limited energy density/power density of the potassium-ion hybrid capacitor^[8]. Therefore, among the above types of metal-ion hybrid capacitors, SIHCs will be the best choice to replace lithium-ion energy storage systems in the future.

In 2012, Chen et al. Reported the first sodium-ion hybrid capacitor (SIHCs) with V₂O₅ nanowires as the negative electrode and activated carbon as the positive electrode. Because the radius of sodium ion is larger than that of lithium ion, there is still a certain gap between the energy density and cycle life of SIHCs and that of lithium ion energy storage devices^[9]. As the main component of energy storage devices, electrode materials determine their comprehensive electrochemical performance to a certain extent. After this work, a series of electrode materials, such as metal elements, metal oxides, sulfides and selenides, have been studied in SIHCs^[10]^[11]^[12]^[13]. Although researchers have made great progress in applying electrode materials with unique composition and structure to SIHCs, the kinetic and capacity mismatch between the battery anode based on faradaic reaction mechanism and the capacitor cathode based on surface adsorption and desorption mechanism has become the main bottleneck for their large-scale practical application. Therefore, it is very important to design functional electrode materials to accelerate the reaction kinetics of battery-type anode and enhance the specific capacity of capacitive-type anode, so as to improve the comprehensive electrochemical performance of SIHCs devices.

2 Principle of operation

SIHCs are composed of a capacitor type cathode material, a battery type anode material, a current collector, an electrolyte, and a separator, as shown in Figure 1. The working principle is as follows: during charging, the Na⁺ migrates into the negative electrode material under the action of an applied voltage, and a corresponding oxidation-reduction reaction occurs; meanwhile, the anions in the electrolyte are adsorbed to the surface of the positive electrode, and electrons of the same substance amount flow from the positive electrode to the negative electrode in an external circuit; During discharge, sodium ions are desorbed from the negative electrode, and with the desorption of anions, electrons in the external circuit flow from the negative electrode to the positive electrode to maintain charge balance^[14]. In SIHCs, the anion storage/release in the capacitive positive electrode has high reversibility and shows fast adsorption-desorption reaction kinetics during the reaction, while the battery negative electrode shows Faraday reaction with slow kinetics, which leads to the kinetic imbalance between the positive and negative electrodes. In addition, there is a large volume change in the process of sodium ion intercalation/deintercalation, which greatly affects the structural stability of anode materials and the cycle life of the whole system, which is the main problem faced by SIHCs at present^[15]. Studies have shown that in order to improve the above problems, we can start with the design of electrode materials, such as controlling the micro-morphology of electrode materials, designing porous structure and modifying the surface of materials with heterogeneous atoms^[8]. Nano-sized materials can not only shorten the diffusion path of ions, but also alleviate the volume expansion problem of materials during cycling. The porous structure can increase the specific surface area of the electrode material, facilitate the penetration of the electrolyte and provide abundant reaction active sites for the active material; Heteroatom doping on the surface of electrode materials can further control the electronic structure of the surface of materials, and at the same time, heterogeneous atoms can also be used as reactive sites to synergistically improve the specific capacity of the positive electrode and the reaction kinetics of the negative electrode.

3 Positive electrode material

The cathode materials of SIHCs are usually composed of capacitive carbon-based nanomaterials with large specific surface area, which rely on the rapid adsorption and desorption reaction of anions on the surface of electrode materials to store energy.Therefore, it has fast reaction kinetics, but its capacity is much lower than that of battery anode materials for Faraday energy storage, which greatly drags down the overall energy density output of SIHCs. To this end, researchers have done a lot of research work to improve the specific capacity of capacitive cathode materials, mainly including optimizing the pore structure design of materials and adding additional surface functional groups to provide more convenient diffusion channels and more redox active sites for ion adsorption process^[16]. At present, activated carbon, carbon nanotubes, graphene, MXenes and some polymers are the main cathode materials.

3.1 Activated carbon

Activated carbon is one of the most potential capacitive electrode materials because of its abundant resources, low processing and manufacturing costs, non-toxic and pollution-free, good chemical stability, low thermal expansion coefficient and wide temperature range^[16]. However, most activated carbons have a single microporous structure (< 2 nm), which makes the migration of sodium ions in the micropores of activated carbons difficult to form electric double layer capacitance quickly, thus hindering the effective exertion of their capacitive performance. Therefore, the key to improve the electrochemical performance of SIHCs is to rationally construct the pore structure of activated carbon-based materials.

According to the pore size, the pore structure can be divided into micropore (< 2 nm), mesopore (2 ~ 50 nm) and macropore (> 50 nm). In general, micropores can provide larger specific surface area and more energy storage sites for carbon materials. The mesopore can provide a smoother channel for the transmission of electrolyte ions, so that the transmission speed is faster; The macropores have a large open space, which can be used as a buffer layer to shorten the diffusion path from the electrolyte to the surface of the electrode material^[18]. The results show that the electrochemical performance of electrode materials can be effectively improved by designing carbon nanomaterials into three-dimensional porous structures with different scales and interconnection, or doping heterogeneous atoms on their surfaces.

For example, Yang et al. Obtained three-dimensional macroporous framework carbon nanosheets (3DFCs) with specific surface area up to 1094 m²·g^-1 by pyrolysis of sodium citrate followed by KOH activation treatment^[19]. When the capacitive performance was tested at a voltage window of 2.5 – 4.2 V, the specific capacitance was 151 F·g^-1 at a current density of 1 A·g^-1, while the specific capacitance of the carbon nanosheets without activation treatment was only 44.5 F·g^-1 under the same conditions, indicating that the large specific surface area is more conducive to the adsorption and desorption behavior of carbon materials for ions. Li et al. Used zinc ions to coordinate with 2-methylimidazole to self-assemble a metal-organic framework material (ZIF8), and the synthesized ZIF8 was carbonized and then activated by KOH to obtain nanoporous carbon (ZDPC) with higher specific surface area and pore volume, and the measured values were 3738 m²·g^-1 and 2.58 cm³·g^-1, respectively, which greatly enriched the adsorption and desorption reaction sites of ions^[20]. When tested as a capacitive material, a high capacitance of 218.4 F·g^-1 can be achieved at 0.1 A·g^-1, and the specific capacitance can still obtain 138.4 F·g^-1 even at a high current density of 10 A·g^-1. When assembled into SIHCs with the anode material obtained by high temperature carbonization of ZIF 8 and KOH activation, it can produce an energy density up to 142.7 Wh·kg^-1, which can still maintain an energy density of 61.8 Wh·kg^-1 at an ultra-high power output of 25 kW·kg^-1, and can still retain more than 90% of the capacity after 10 000 cycles. Liu et al. Used aniline and pyrrole as carbon sources and F127 as a template to prepare nitrogen-doped hierarchical porous hollow carbon nanospheres (HMCSs) by one-step carbonization method, which were further activated by KOH to obtain a large number of microporous structures.The combination of heterogeneous atoms and porosity provides abundant active sites for the adsorption and desorption of ions, which can produce high energy density of 140.1 Wh·kg^-1 when used as the cathode of SIHCs^[21]. In addition to heterogeneous atomic nitrogen doping, a variety of atomic co-doping can also play a role in improving the capacitance capacity. For example, Hu et al. Obtained ultrathin porous carbon nanosheet-supported metal Mn single-atom materials (MnSAs/NF CNs) co-doped with fluorine and nitrogen after uniform mixing and pyrolysis of manganese acetate, melamine and polytetrafluoroethylene^[22]. After the metal single atom and the non-metal atom are co-doped, more abundant reactive sites and an open porous structure are also provided for ion adsorption and desorption, and the metal single atom is used as a cathode material of SIHCs for the first time to show higher capacitance and longer cycle life than commercial activated carbon, thereby opening up a new application field of the single atom material.

The above studies show that the reasonably designed three-dimensional porous structure can accelerate the migration rate of ions by using the synergistic effect of its hierarchical voids. In addition, the introduction of heterogeneous atom doping can further improve the electrochemical performance of activated carbon, so the modified activated carbon is widely used as cathode materials for hybrid capacitors.

3.2 Carbon nanotube

Nanomaterials can increase the number of electrochemical active sites, transfer charge and control the transport of ions and electrons on a small diffusion scale, so nanomaterials can be used as ideal materials to improve ion adsorption and desorption reactions. Among them, carbon nanotubes have unique hollow porous structure, high surface area (120~500 m²·g^-1), excellent mechanical properties and thermal stability, and their crystallinity and conductivity are better than those of activated carbon.And the adsorption and transport properties of electrons and electrolyte ions on its network structure are better, but due to the influence of low specific surface area, the specific capacitance of carbon nanotubes as electrode materials is generally low, so some measures need to be taken to increase its specific capacitance.

Studies have shown that increasing the specific surface area or preparing dense carbon nanotubes, or doping boron (B), nitrogen (N), oxygen (O), sulfur (S) and other heteroatoms in the materials can further improve the active sites and conductivity of carbon materials, thereby improving the electrochemical performance of electrode materials, which is conducive to the development of electrode materials with better performance^[23]. In 2006, Futaba et al. Used the liquid induction effect to obtain single-walled carbon nanotube blocks (SWNTs) by soaking and evaporating the nanotube bundle aggregates standing on the conductive substrate, in which the carbon nanotubes were stacked together neatly and densely, maintaining the original characteristics of single carbon nanotube, such as high surface area and flexibility^[24]. SWNTs have high specific surface area and exhibit higher specific capacitance and rate capability than commercial activated carbon when used as capacitor electrode materials. Carbon nanotubes (CNTs) with an average diameter of 6 nm were synthesized on carbon paper by water-assisted chemical vapor deposition using aluminum/iron as a catalyst by Kim and his team, and about 70% of them were double-walled CNTs^[25]. The effective arrangement and integration of CNTs are beneficial to the rapid diffusion of ions in the electrode, but also improve the degree of graphitization and conductivity, and the specific capacitance is still as high as 200 F·g^-1 at the ultra-high current density of 20 A·g^-1 when used as the positive electrode.

In order to make carbon nanotubes show more excellent capacitive properties, researchers have done a lot of modification work, mainly based on non-metallic heterogeneous atom doping treatment. Li et al. prepared molybdenum-dopamine-doped nanotubes from molybdenum trioxide nanorods, ammonium molybdate, and dopamine hydrochloride, and then prepared hierarchical active nitrogen-doped carbon nanotubes (ANCN) by calcination and etching, as shown in Fig. 2a and B^[26]. The high specific surface area (1406 m²·g^-1) and open layered structure of ANCN provide more active sites for the adsorption and desorption of ions, in addition, the bending degree of ANCN is very low, which greatly improves the electrolyte wettability. The electrochemical performance test results of solid-state SIHCs with ANCN as the cathode material of SIHCs showed that the energy density was 100.6 Wh·kg^-1 at a power density of 117.5W·kg^-1, and the cycle life was as long as 1000 cycles. Cui et al. Used the template method to obtain sulfur and nitrogen co-doped hollow carbon nanotubes (N/S HCNs) as the positive electrode and tin foil as the negative electrode to form SIHCs, as shown in Figure 2C ~ f, the capacity almost did not decay after 10,000 cycles at 5 A·g^-1 and high current density. The theoretical calculation showed that the co-doping of heterogeneous atoms S and N could significantly increase the adsorption capacity of ions, thus showing higher capacitance performance^[27].

In the above series of work, carbon materials were designed into nanotube morphology and doped with heterogeneous atoms, which significantly improved the adsorption and desorption ability of carbon materials for ions and showed excellent capacitance capacity. However, at present, there are few studies on single-walled carbon nanotubes and multi-walled carbon nanotubes doped with heterogeneous atoms and applied to SIHCs.The development potential of carbon nanotubes in SIHCs will be further enhanced by reasonable control and design of the surface properties and internal pore size distribution of carbon nanotubes, as well as their excellent electronic conduction ability.

3.3 Graphene

Graphene is a six-membered ring lattice structure composed of carbon atoms, which has a large theoretical specific surface area (>2600 m²·g^-1) and high conductivity, and the thickness of a single layer of atoms is about 0.34 nm. Its unique pore structure is conducive to the diffusion of ions, which to some extent makes up for the shortcomings of activated carbon and carbon nanotubes as electrode materials^[28,29]. Graphene also has high chemical stability, good mechanical and thermal properties, and has become one of the most commonly used materials for developing capacitive electrode materials. However, the existence of π-π-conjugated bonds on the surface of graphene has a strong force, which leads to severe agglomeration in the process of preparing graphene, and the re-stacking of graphene nanosheets will sharply reduce the active surface area of the two-dimensional structure, which greatly limits its performance.

It has been shown that oxygen-containing functional groups (especially carbonyl and carboxyl groups) can increase the specific capacitance through surface redox reactions with cations. Dong et al. Used natural graphite as raw material to prepare graphene oxide by modified Hummers method, and then obtained oxygen-rich pleated graphene (OCG)^[30]^[31]. In the process of preparing OCG, ZnO nanosheets were grown in situ as a blocking agent, and then hydrochloric acid was used to remove ZnO after synthesis, which could prevent the accumulation of graphene layers. The capacity retention rate is 91% after 3000 cycles at 1 A·g^-1, which fully reflects the excellent specific capacity and cycle stability of the capacitive cathode. When they were used as both positive and negative electrodes to assemble SIHCs, a high energy density of 121.3 Wh·kg^-1 could be obtained at a power density of 300 W·kg^-1, and they also showed good cycle stability after 2500 cycles.

In addition, the three-dimensional porous crosslinked structure can effectively prevent the secondary stacking of graphene nanosheets, and the three-dimensional graphene-based materials have high specific surface area and more open active sites when used as capacitive electrode materials.The path of ions is shorter and faster, and it also has good conductivity, flexibility and mechanical strength, which can effectively improve the kinetic and cycle stability of graphene materials. However, the traditional method of preparing three-dimensional graphene materials is derived from chemically modified graphene (CMG), which will damage the layered structure of graphene, reduce the conductivity and stability of three-dimensional graphene materials, and has high cost and pollution. Wang and his research team used microwave-assisted electrochemical exfoliation and hard template method in aqueous solution to prepare three-dimensional macroporous graphene (MG) at a very low cost^[32]. When it is used as a capacitive electrode material, the open pore structure not only effectively improves the dense packing phenomenon, but also increases the electrochemical reaction interface, and provides abundant active sites for ion adsorption and desorption, resulting in high specific capacitance. The SIHCs with MG as the cathode exhibited the highest energy density of 168 Wh·kg^-1, and the capacity could still maintain 85% of the initial capacity after 1200 cycles, showing excellent cycle life.

The above research work shows that by changing the oxygen-containing functional groups of graphene materials or designing the three-dimensional porous graphene structure, the secondary stacking of graphene nanosheets can be effectively prevented, and the specific surface area and active sites of the materials can be increased, thus showing excellent capacitive capacity.

3.4 MXene

MXene is a two-dimensional layered transition metal carbide and nitride with multiple functional groups. The "MX" in the name means that it comes from the MAX phase ceramic material. MAX phase is a nano-layered compound with hexagonal crystal structure. A is mainly the main group elements of Ⅲa and Ⅳa. "ene" means that its microstructure is similar to the graphene sheet structure^[33]. The chemical formula of MXene can be abbreviated as M_n₊₁X_nT_x(n=1,2 or 3), M represents transition metal elements (Ti, V, Cr, Zr, etc.), X represents carbon or nitrogen, and T_x represents surface functional groups such as OH, O, F, etc. MXene with different structures can be obtained by selectively removing the A layer in the MAX phase using an etchant (such as HF, HCl, etc.)^[34]. The special elemental composition and surface functional groups of MXene make it show excellent properties in terms of mechanical strength and chemical stability, as well as extremely high volumetric capacitance and reversibility.

Studies have shown that factors such as specific surface area, elemental composition and surface functional groups play a decisive role in the number of active sites of MXene materials, so researchers can improve the electrochemical performance of MXene materials by surface modification, exfoliation and other preparation methods. For example, Zhu et Al. Etched the Al layer of MAX phase materials with Ti₃AlC₂ as raw materials, and replaced the Al layer with — OH, = O or — F functional groups to present unique layered structure Ti₃C₂T_x materials^[35]. Electrochemical tests showed high energy density (up to 113 Wh·kg^-1), specific capacitance (current density of 150 F·g^-1,200 mA·g^-1) and stable cycling performance (capacity retention of 95.9% after 450 cycles). Like traditional two-dimensional materials, MXene nanosheets are also prone to serious stacking phenomena, which greatly limits the electrochemical performance. In order to make full use of the advantages of MXene materials, Li and his team used Ti₃C₂T_x and acidified activated carbon (AAC) particles as raw materials to prepare a new type of three-dimensional conductive network MXene composite material through mixing, ultrasonic and vacuum filtration processes.Wherein the AAC nanoparticles are wrapped by the MXene sheet, which not only effectively avoids the secondary accumulation of the MXene sheet, but also exhibits high mechanical strength, which can provide a more favorable diffusion path for ions during the rapid charge-discharge process.In addition, the acidified activated carbon has high porosity, shows more excellent adsorption performance, greatly improves the reaction kinetics and cycle stability of the capacitor, and the test result shows that the specific capacitance of the activated carbon is 378 F·g^-1 at a current density of 0.5 A·g^-1,The capacitance retention rate can reach 88.9% at 30 A·g^-1, and the capacity retention rate is as high as 97.4% after 10 000 cycles at a current density of 5 A·g^-1, which shows excellent long life characteristics compared with other cathode materials, as shown in Table 1^[36].

However, the energy density of MXene is still low when it is used as a supercapacitor, so Dall 'Agnese et al. Used MXene in SIHCs to improve the energy density of the whole energy storage device^[37]. The team used V₂AlC as raw materials and HF-selective etching of Al layer to obtain layered material V₂CT_x, whose preparation process is shown in Figure 3. The SIHCs composed of the cathode and hard carbon (HC) showed good rate performance and cycle performance, with a capacity of 40% of the initial capacity at a high current density of 20 C and a capacity retention of 70% of the initial capacity after 300 cycles at 1A·g^-1. It can be seen that the surface transition metal and functional groups of MXene can form redox active sites, which has good electrochemical performance and excellent energy storage performance.

4 Negative electrode material

Since the study of SIHCs, a large number of research results have shown that their comprehensive electrochemical performance mainly depends on the negative electrode. In order to make SIHCs have higher energy density, power density and excellent cycle performance, the anode material should have the following characteristics: (1) low working voltage, which makes SIHCs have higher energy and power density^[38]; (2) fast reaction kinetics, which can be used at high current densities and can be matched with high-power anodes; (3) high specific capacity and longer discharge time; (4) Excellent structural stability, resulting in a longer cycle life. Among them, sodium metal anode has an ultra-high theoretical specific capacity (1166 mAh·g^-1) and a low electrode potential (− 2.71 V vs standard hydrogen electrode), but as an anode material for SIHCs, the specific capacity will be seriously mismatched with that of the positive electrode, and at the same time, serious sodium dendrite phenomenon will occur, resulting in uncontrollable safety risks, so there is little research in the field of SIHCs^[39⇓~41]. Anode materials can be divided into three types according to their sodium storage mechanism: intercalation type, conversion type and alloy type. Carbon materials, titanium dioxide, transition metal sulfides, selenides and other materials have been widely studied.

4.1 Intercalation material

Intercalation anode materials mainly include carbon materials and titanium-based oxides, etc. During the charge-discharge process, the Na⁺ is intercalated/deintercalated between the layers of the materials to store capacity^[41]. Carbon materials are one of the most promising anode materials for SIHCs due to their low cost, good chemical stability and low reaction potential. The energy density and cycle life of SIHCs are seriously restricted by the slow diffusion rate of Na⁺ with large particle size in active materials, and the serious polarization effect and volume expansion problems. Therefore, the rational design of the structure of carbon anode materials is essential to enhance their ion storage capacity.

The sodium storage mechanisms of carbon-based materials can be divided into three types: (1) sodium adsorption on the surface and defects; (2) storage of sodium in micropores; (3) intercalation reaction of sodium ion between carbon layers. Carbon materials exist in various forms, mainly including soft carbon, hard carbon, graphite materials and so on^[18]. Cao et al. Showed through theoretical calculation that only carbon materials with an interlayer spacing greater than 0.37 nm are electrochemically active for the insertion of Na⁺, while graphite with an interlayer spacing of only 0.34 nm is difficult to store Na⁺, and also exhibits poor electrochemical performance when applied to SIHCs negative electrodes, so it is necessary to expand the interlayer spacing of carbon to improve its energy storage performance^[42]. Compared with graphite, hard carbon and soft carbon have wider interplanar spacing and stronger compatibility in electrolyte, so they have more reactive sites, and can obtain higher specific capacity and longer cycle life when used as negative electrodes of SIHCs. Yang et al. Proposed a three-dimensional framework carbon formed by the pyrolysis of sodium citrate, which not only has a large interlayer spacing and high conductivity, but also shows a hierarchical porous structure combining macropores, mesopores and micropores.The electrochemical test showed that it exhibited a 111 Wh·kg^-1 energy density at a power density of 200 W·kg^-1, and the capacity could still maintain 75.6% of the initial capacity after 15 000 cycles at a current density of 2 A·g^-1, exhibiting an ultra-long cycle life^[19]. In addition to broadening the interlayer spacing of carbon materials, designing them into three-dimensional porous structures is also crucial for improving the reaction kinetics of sodium ions. Ding et al. Obtained three-dimensional porous carbon nanosheets (PSNC) by pyrolysis of peanut shell, and the discharge plateau voltage as the negative electrode of sodium-ion battery was basically below 0.1 V. Such a flat charge-discharge plateau and low voltage is an ideal negative electrode material, which can maximize the energy density of SIHCs, showing an energy density as high as 201 Wh·kg^-1 at a power density of 285 W·kg^-1^[43]. Wang et al. Also carbonized peanut shells, and then further activated them with KOH to obtain carbon nanosheets with larger specific surface area. When they were used as both positive and negative electrodes of SIHCs, the capacity retention rate could still reach 85% after 3000 cycles at a high current density of 5 A·g^-1, showing excellent cycling stability^[44].

In addition, the introduction of heterogeneous atoms on the surface of carbon materials can not only broaden the interlayer spacing of carbon layers, but also introduce additional defect sites, thus further improving the reaction kinetics of carbon anodes. Li et al. Prepared nitrogen-doped hollow carbon nanowires (NHCNWs) with high specific surface area, which showed 108 Wh·kg^-1 high energy density and 2000 cycle life when combined with activated carbon cathode^[45]. Zou et al. Synthesized nitrogen-doped hierarchically porous carbon (NHPC) by pyrolysis, and further obtained nitrogen-doped hierarchically porous carbon (NHPAC) by KOH activation^[46]. Due to the large interlayer spacing and hierarchical pore structure, the SIHCs constructed with NHPC as the negative electrode and NHPAC as the positive electrode exhibited excellent electrochemical performance, with an energy density up to 115Wh·kg^-1 at a power density of 200W·kg^-1. Controlling the structure of carbon lattice by doping heterogeneous atoms can improve the electrochemical performance of materials to a certain extent, but there are still some problems to be solved, such as the content and uniformity of doped heterogeneous atoms need to be further improved.

In recent years, titanium-based materials have attracted more and more attention because of their advantages such as suitable working voltage, environmental friendliness, and small strain expansion^[47]. In general, most titanium-based oxide anode materials for sodium storage are based on Ti⁴⁺/Ti³⁺ redox, among which titanium dioxide is considered to be a promising battery anode material due to its open framework, which can quickly insert/extract Na⁺ during electrochemical cycling.

In 2013, Xu et al. First studied that anatase TiO₂ nanocrystals were used to store Na⁺, but their capacity was low^[48]. To improve their electrochemical performance, Que et al. Proposed a SIHCs with flexible oxygen-deficient TiO_2-_x/CNT as the negative electrode and activated carbon/carbon nanotubes as the positive electrode, which showed 109Wh·kg^-1 energy density at a power density of 250 W·kg^-1^[49]. In addition, Huang et al. Proposed oxygen vacancy-rich TiO₂/ carbon nanofibers (TiO₂@PBC), with its three-dimensional cross-linked conductive network, open ion transport channels and a large number of oxygen vacancies, the TiO₂@PBC electrode showed excellent sodium storage characteristics, and the constructed sodium-ion hybrid capacitor showed energy density 91.0 Wh·kg^-1 at a power density of 165 W·kg^-1 (Figure 4A, B)^[50]. Recently, Zhu et al. Used TiO₂@CNT@C nanorods as the negative electrode and biomass-derived carbon (BAC) with high surface area as the positive electrode to form SIHCs, which can produce up to 81.2 Wh·kg^-1 energy density and 12 400 W·kg^-1 power density, shorten the distance between lithium-ion batteries and supercapacitors, and achieve 85.3% capacity retention after 5000 cycles at 1 A·g^-1 current density (Figure 4C, E)^[51].

4.2 Conversion material

Conversion anode materials mainly include transition metal oxides, sulfides and selenides. The typical metal oxide, SnO₂, has been widely studied as the anode of lithium-ion batteries due to its low resistivity and high specific capacitance, metal semi-conductivity, etc., but the reaction kinetics in the process of sodium storage is slow, resulting in a short cycle life. In order to improve this problem, a large number of researchers have focused on shortening the particle size of SnO₂ to alleviate the volume expansion problem in the cycle process. For example, Zhang et al. ^[52 uniformly dispersed nanocrystals less than 10 nm on graphene to effectively improve the rate capability of sodium ions. Niu et al. Uniformly dispersed SnO₂ sub-nanoclusters with an average diameter of 0.93 nm on N, O co-doped porous carbon nanosheets, which produced obvious pseudocapacitive characteristics when used as the negative electrode of sodium-ion batteries, greatly improved the kinetics of sodium-ion reaction, and simultaneously improved the cycle life of SIHCs, and the capacity retention rate could maintain 72% of the initial capacity after 5000 cycles at a high current density of 5 A·g^-1^[53].

In addition, the M — S and M — Se bonds in metal sulfides and selenides are easier to break than the M — O bonds in metal oxides, and have better conductivity, mechanical properties and thermal stability, so they are more widely studied in SIHCs^[54,55]. However, in the practical application process, the conversion reaction is similar to that of metal oxides, and it is easy to produce a large volume expansion problem, which leads to the rapid degradation of the electrochemical performance of electrode materials, and seriously hinders the large-scale application of SIHCs. In order to improve the above scientific problems, researchers have done a lot of modification research on transformation materials. Generally, the following two measures are mainly adopted: (1) the electrode material is designed into a nanostructure, which can significantly shorten the diffusion path of electrons and ions and show more abundant active sites^[56]; (2) Compounding the active material with the carbon material can further improve the electron transfer capacity, provide a protective layer for the volume expansion of the active material during cycling, and weaken the stress and strain of the material during charging and discharging, thereby improving the structural stability of the material.

Hu et al. Used a template method to obtain FeS quantum dots confined in a nitrogen-doped three-dimensional ordered macroporous carbon framework (3D IO FeS QDs @ NC), as shown in Figure 5A and Figure B. Electrochemical tests showed that the size of the quantum dots combined with the three-dimensional ordered porous structure could significantly improve the structural stability of the entire electrode material and prolong the cycle life.It promotes the rapid transport of electrons and ions, thus effectively accelerating the reaction kinetics of sodium ions. As a sodium storage anode, the discharge plateau is around 0. 7 V, and the first coulombic efficiency is as high as 78%^[57]. When combined with activated carbon (AC), the SHICs can achieve the maximum energy density of 151.8 Wh·kg^-1 and the maximum power density of 9280 W·kg^-1, and the capacity retention rate can still maintain 91% of the initial capacity after 5000 cycles under the condition of 1 A·g^-1. Liu et al. Synthesized a mixed bimetallic sulfide (SnS₂/ZnS⁃RGO) anode material embedded on reduced graphene oxide by hydrothermal method. Although its first coulombic efficiency is low, only 53.25%, the SIHCs can show a high energy density of 145.6 Wh·kg^-1 at 2250 W·kg^-1 by virtue of its rich heterostructure and phase boundary characteristics^[58]. Cui et al. Grew SnS₂ nanosheets on graphene and carbon nanotube gels to obtain three-dimensional porous composites (SnS₂/GCA), as shown in Figure 5C, d, whose high specific surface area provided abundant reactive sites for sodium ions and produced higher rate performance.When used as the negative electrode of sodium-ion battery, the discharge plateau is in the range of 0. 7 ~ 0.9 V, the initial coulombic efficiency is 64. 5%, and the specific capacity of 304.8 mAh·g^-1 can still be obtained at a high current density of 10 A·g^-1, showing excellent sodium-ion reaction kinetics^[59]. Zhao et al. Grew upright MoSe₂ on graphene in situ to obtain MoSe₂/G composites (as shown in Fig. 5 e, f), which significantly improved the intrinsic conductivity of MoSe₂ and obtained the first coulombic efficiency as high as 89%^[13]. The combination of it as a negative electrode and activated carbon positive electrode into SIHCs exhibited excellent rate performance, and a high energy density of 82 Wh·kg^-1 was still obtained at an ultra-high power density of 10752W·kg^-1, and a capacity retention rate of 81% was still achieved after 5000 cycles at a current density of 5 A·g^-1. Li et al. Used room temperature in-situ polymerization and subsequent heat treatment process to obtain nitrogen-doped carbon matrix coupled WS₂ composite (WS₂@NCNs), and the nitrogen-doped carbon matrix not only effectively improved the electron conduction ability of WS₂, but also alleviated its volume expansion during cycling, thus effectively improving the structural stability of the whole electrode^[60]. At the same time, the nitrogen-doped carbon matrix (NCNs) shows more reactive sites than commercial activated carbon, which greatly improves the specific capacity of anion adsorption and desorption. The NCNs cathode exhibited excellent comprehensive electrochemical performance when combined with the WS₂@NCNs anode to construct SIHCs, with a high energy density of 134.7 Wh·kg^-1 at a power density of 117.5 W·kg^-1 and a capacity retention of up to 90% after 2000 cycles at 200 mA·g^-1.

In addition, the built-in electric field effect generated by the heterojunction of multiple metal sulfides can be used to enhance the Na⁺ reaction kinetics and achieve the synergistic effect. For example, Tang et al. Used carbon nanofibers penetrating through SnS₂ and CoS₂ composites to form a three-dimensional network structure, which combined the high specific surface area, low density and adjustable pore structure of metal organic framework, and could still obtain a specific capacity of 327.6 mAh·g^-1 at an ultra-high current density of 20 A·g^-1^[61]^[62]. Wu et al. Used a similar method to penetrate carbon nanofibers through the FeS₂/WS₂ cube composite, which showed significant pseudo-capacitive sodium storage characteristics when used as a sodium storage anode, greatly improving the reaction kinetics of sodium ions^[63].

4.3 Alloy type material

Alloying materials exhibit large theoretical capacity through multi-electron transfer alloying/dealloying reactions with sodium, which has attracted wide attention of researchers. In the actual use process, there are serious volume expansion and pulverization phenomena in the alloying reaction, which leads to poor electrochemical performance when used as SIHCs anode, so there are few studies on alloying materials in SIHCs. Few studies have shown that the three-dimensional porous network structure can effectively shorten the diffusion distance of ions and slow down the volume change, which is conducive to improving the reaction kinetics. For example, Yuan et al. Used NaBi with a three-dimensional porous structure as the negative electrode, paired with activated carbon positive electrode to form SIHCs, which has a high energy density of 106.3 Wh·kg^-1 at a power density of 105 W·kg^-1, and the energy density can still obtain 58.2 Wh·kg^-1 even at a high power density of 11100 W·kg^-1, and the capacity retention rate is still 98.6% after 1000 cycles under the condition of 2 A·g^-1^[10].

In addition, how to adjust the nanostructure of the alloy to improve its cycle stability and redox kinetics is crucial for high-performance SIHCs. Gao et al. Dispersed Bi nanoparticles uniformly in the flexible graphene fiber fabric, which effectively enhanced the electron/ion transfer ability of Bi in the process of sodium storage, greatly improved the reaction kinetics of sodium ions and the structural stability of the whole electrode, and constructed flexible solid-state SIHCs with maximum energy density of 16.2 mWh·cm^-3 and maximum power density of 81.8 W·cm^-3^[64]. Zheng et al. Assembled Bi nanospheres into micron flower spheres, which could effectively alleviate the problem of volume expansion during the redox process of sodium ions, when the Bi NAMPS//B, N co-doped C nanosheet SIHCs composed of carbon nanosheets co-doped with B, N could obtain the maximum energy density of 93 Wh·kg^-1 and the maximum power density of 33000 W·kg^-1^[65]. Fig. 6 is the energy and power density comparison Ragone curve of different SIHCs, and Table 2 summarizes the electrochemical performance of sodium-ion hybrid capacitors composed of different negative/positive electrodes. The results show that although the volume expansion of the alloy negative electrode material Bi NAMPS during cycling is more severe than that of some intercalation and conversion negative electrodes,However, by adjusting the particle size, structure and composition of the material, the structural stability and sodium ion reaction kinetics of the whole electrode can be improved to a certain extent, thus making up for the energy density and power density difference of the original alloy material^{[13,19,22,45,51,53,57,60,65]}.

To sum up, the reaction kinetics of anode materials is slow due to the Faradaic reaction mechanism, and it is difficult to match the cathode materials with fast adsorption-desorption reaction mechanism, so improving the reaction kinetics of anode materials is the key to improve the comprehensive electrochemical performance of the whole SIHCs system.In recent years, researchers have taken the following measures to accelerate the anode reaction kinetics: (1) designing active materials into nanostructures can effectively shorten the diffusion path of Na⁺ and increase the active sites of Na⁺ reaction; (2) the active material is compounded with the carbon material with good conductivity, thereby promoting the rapid migration of electrons and ions in the negative electrode and increasing the structural stability of the electrode material; (3) The incorporation of heterogeneous atoms in the active material can not only broaden the interlayer spacing of the material, but also produce abundant defect sites, so that the sodium-ion hybrid capacitor can show more excellent rate performance and long cycle life.

5 Conclusion and prospect

In this paper, the recent research progress of SIHCs is systematically reviewed. Firstly, the development process, working principle, classification and composition of SIHCs are introduced. The current status, current challenges, development and applications of electrode materials are introduced in detail. The electrochemical properties of electrode materials can be improved by increasing the specific surface area of the material, selecting the appropriate processing technology, and introducing heterogeneous atoms. SIHCs can integrate the complementary advantages of capacitors and batteries. It is worth noting that the advantages of high energy density, high power density, long life and low raw material cost will make SIHCs promising for electric vehicles and urban rail transit. Its wide application prospects and huge potential commercial value have also attracted wide attention from governments and scientific research institutions. Although researchers have made some progress in the field of SIHCs, it is still in the initial stage. The biggest challenge to realize high-performance SIHCs devices is mainly the kinetic and capacity imbalance between the capacitor-type cathode materials and battery-type anode materials. Therefore, the following aspects need to be the focus of research in the future.

(1) Develop materials with large lattice space and interlayer spacing, so that sodium ions can be better stored and quickly migrated, and anions can be quickly adsorbed and desorbed on the electrode, thus improving the energy density, power density and cycle performance of SIHCs.

(2) Develop three-dimensional self-supporting nanomaterials without using binders or additives, rationally design the spatial structure and pore distribution of three-dimensional nanomaterials, and provide high specific surface area and sufficient active sites.

And (3) selecting appropriate heterogeneous atoms or functional groups to change the physical and chemical characteristics of the material and enrich the reaction active sites of the material so as to improve the electrochemical performance of the electrode material.

(4) Carbon-based materials have the advantages of good conductivity, porous structure, high specific surface area and low manufacturing cost, which are the first choice for the preparation of high-performance and low-cost SIHCs. At the same time, the use of carbon materials can also reuse waste, reduce environmental pollution, and achieve the goal of carbon neutralization/carbon peak as soon as possible, but the low first coulombic efficiency of carbon materials for sodium ion storage is the main bottleneck, so it is necessary to further modify the structural characteristics of carbon materials and the selection of electrolytes.

In conclusion, SIHCs can play an important role in the field of energy storage due to their high energy density, high power density, long cycle life and abundant sodium reserves. At present, the development of SIHCs is still in its infancy, and further research and exploration are needed. The engineering design of battery-type electrodes and capacitive electrodes also has a significant impact on the energy/power characteristics and cycle stability of SIHCs.

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