The reserves of lithium on earth are limited and the price is rising. With the further expansion of the application of lithium-ion batteries, lithium resources will inevitably be in short supply. Rechargeable sodium-ion batteries, as the most promising candidate for lithium-ion batteries, have attracted great attention from researchers. The abundant reserves and low price of sodium have greatly reduced the cost of sodium-ion batteries. In addition, cheap Al does not react with Na and can be used as the current collector of the anode and cathode of sodium-ion batteries, which further reduces the cost of sodium-ion batteries.
Sodium-ion batteries started developing almost at the same time as lithium-ion batteries, but due to their relatively low energy density and cycle life, their development level is relatively lagging. Initially, researchers simply applied the electrode materials successfully applied to lithium-ion batteries directly to sodium-ion batteries, but did not consider the different requirements of sodium ions with larger ionic radius on the lattice of electrode materials, and did not achieve the desired effect. Later, researchers designed electrode materials based on the characteristics of sodium-ion batteries and achieved many good results. With further research on the mechanism of sodium-ion batteries, such as computational research on the voltage, stability and diffusion barrier of sodium-ion battery electrode materials, it was found that the sodium-ion battery system is highly competitive.
Electrolyte is one of the decisive components of sodium-ion battery performance, and it plays a vital role in balancing and transferring ions/charges. Therefore, appropriate electrolyte components must be designed to improve the chemical properties and safety of sodium-ion batteries. The high thermal stability, electrochemical stability and low volatility of ionic liquids make them potential as electrolytes for sodium-ion batteries.
1 Overview of sodium-ion batteries
Sodium and lithium belong to the first main group of the periodic table, with similar electronic structures and only one electron in the outermost layer. They have similar physical and chemical properties and show strong reactivity. Table 1 compares some physical properties of the two alkali metal elements. It can be seen that relative to the standard hydrogen electrode, the electrode potentials of metallic sodium and lithium are ENa+/Na= -2.71V and ELi+/Li= -3.04V, respectively, with a difference of 330mV. Therefore, in most cases, the electrode potential of the negative electrode material of sodium ion batteries is always higher than that of the negative electrode material of lithium ion batteries. At the same time, the sodium element has a larger atomic weight, which makes the sodium ion battery show a lower reversible specific capacity than the lithium ion battery, and the atomic radius of the sodium element (0.102nm) is larger, which makes most electrode materials produce obvious volume expansion during the sodium ion embedding process, reducing the cycle stability of the electrode. Although these problems have limited the development of sodium ion batteries to a certain extent, their abundant resources and low cost make sodium ion batteries show great development potential in the field of large-scale energy storage with low energy density requirements.
1.1 Working Principle
The advantages of sodium-ion batteries are not only reflected in their abundant reserves and low costs, but also in their excellent electrochemical properties:
(1) Although sodium-ion batteries have a lower theoretical specific capacity than lithium-ion batteries, they have a significant capacity advantage over metal ion batteries such as potassium, magnesium, aluminum, and zinc.
(2) Although the chemical equivalent of Na/Na is more than three times that of Lit/Li, compound electrodes are usually used instead of metal negative electrodes in practical applications. The gap between the mass theoretical specific capacity of compounds that can deintercalate ions and lithium-ion batteries has been significantly narrowed. For example, the mass theoretical specific capacities of layered oxides LiCoO2 and NaCoO2 with similar crystal structures are 274mAh·g-1 and 235mAh·gA-1, respectively, and the reversible capacity differs by only 14%.
(3) Due to the large difference in molar volume between metallic lithium (0.0213 nm3) and metallic sodium (0.0393 nm3) (AV=0.018 nm3), the volumetric capacity of metallic sodium is much lower than that of metallic lithium, which significantly narrows the gap in volumetric theoretical capacity of compound electrodes. For example, the difference in molar volume between LiCoO (0.0323 nm3) and NaCoO2 (0.0373 nm3) is very small (△V=0.005 nm3).
(4) The gap between sodium-ion batteries and lithium-ion batteries is also reflected in the voltage difference. This technical problem can be solved by developing new electrode materials or modifying existing electrode materials.
Although the larger ionic radius of sodium ions limits the improvement of the energy density of sodium ion batteries to a certain extent, the larger ionic radius shows lower solvation energy in polar solutions. This conclusion has been confirmed by systematic studies on Li+, Na+, and Mg2+ in different aprotic polar solvents. The solvation energy has a great influence on the kinetic factors of alkali metal ions embedded in electrode materials at the electrolyte interface. Lower solvation energy is the key to designing high-power electrochemical energy storage devices. Compared with sodium ions, lithium ions with smaller ionic radius have higher charge density and need to accept more electrons from solvated polar molecules to maintain energy stability. In the process of lithium ions in the solution being embedded in electrode materials, a desolvation process is required to remove the solvent molecules around lithium ions. Compared with the desolvation of sodium ions, lithium ions require a relatively large energy. At the same time, the larger solvation energy of lithium ions also affects their diffusion behavior in the electrolyte. Compared with lithium ion battery electrolytes, sodium ion battery electrolytes can have higher ionic conductivity. NaClO4 and LiC1O4 are often used as electrolyte salts. NaC1O4 solutions in aprotic solvents exhibit lower viscosity and their conductivity is 10%~20% higher than that of LiC1O4.
The composition and working principle of sodium-ion batteries are similar to those of lithium-ion batteries. During the charge and discharge process of the battery, sodium ions are repeatedly embedded/extracted between the lattices of the positive and negative electrode materials, which can be figuratively called a “rocking chair-style battery”. As the redox reaction proceeds, the metal valence or the number of ions changes inside the electrode material, and the electrons in the external circuit are transferred between the positive and negative electrodes. The schematic diagram of the battery working process is shown in Figure 1. During the charging process, the transition metal elements at the cathode (positive electrode) undergo an oxidation reaction to provide electrons to the external circuit. In order to maintain the electrical neutrality of the electrode, sodium ions are extracted from the cathode into the electrolyte, and the sodium ions in the electrolyte are embedded in the anode. The anode (negative electrode) simultaneously accepts electrons from the external circuit to undergo a reduction reaction. During the discharge process, the process proceeds in opposite directions between the positive and negative electrodes. The redox reactions that occur during the charge and discharge of sodium-ion batteries are all carried out in a closed system. During the oxidation process, for every electron produced, an electron is consumed in the reduction reaction at the other electrode.
Figure 1 Schematic diagram of the working principle of sodium ion battery
Sodium ion batteries are generally composed of six parts: electrode materials, conductive additives, binders, current collectors, diaphragms, and electrolytes, as shown in Figure 2. Electrode materials are the most important part of the battery composition, and determine the electrochemical properties of sodium ion batteries, such as battery capacity, cycle performance, rate performance, operating voltage, etc. The development of high-performance electrode materials is an important research direction for the development of sodium ion batteries. At present, the more mature positive electrode materials are mainly transition metal oxides, polyanionic compounds, and organic compounds represented by Prussian blue materials. For negative electrode materials, metallic sodium as the negative electrode of sodium ion batteries shows the highest energy density. However, in practical applications, metallic sodium negative electrodes show extremely high chemical activity and are flammable and explosive. During the battery charging and discharging process, sodium dendrites are easily formed at the negative electrode, and even puncture the diaphragm to cause a battery short circuit. From a safety perspective, using materials with lower redox potential instead of metallic sodium as the negative electrode material of sodium ion batteries is an ideal choice for the development of sodium ion batteries.
The electrolyte is the contact medium between the cathode and the anode, which promotes the transport of ions through the porous separator and participates in the electrode reaction in the electrode. The electrolyte must contain enough ions for the charge transfer reaction while ensuring electronic insulation. The speed of ion movement is affected by the degree of solvation of the ions and the number of opposite ions.
The separator of the battery is usually made of glass fiber. The choice of current collector for sodium ion batteries is usually the same as that of lithium ion batteries. The positive electrode generally uses aluminum foil as the current collector and the negative electrode uses copper foil. Current research shows that metal aluminum does not undergo alloying reaction with sodium, so aluminum foil can also be used as the current collector for the negative electrode of sodium ion batteries to further save costs.
1.2 Commonly used electrolytes in sodium-ion batteries
In sodium-ion batteries, electrolytes play a role in balancing and transferring charges between the two electrodes in the form of ions. The frontier molecular orbital energies of each component can determine the electrochemical platform of the battery, and their electronic structure properties also affect the thermal stability of the battery. The stability and ionic conductivity of the electrolyte material are the key to determining battery performance. The interaction between the electrolyte and the electrode surface greatly affects the state of the solid electrolyte interface (SEID film and the internal structure of the active material; the compatibility between the electrolyte and the electrode affects the overall electrochemical performance of the battery. At present, the electrolytes used in sodium-ion batteries can be divided into liquid electrolytes (non-aqueous or aqueous electrolytes), solid electrolytes, ionic liquid electrolytes and gel electrolytes according to their phase state.
1 Non-aqueous electrolytes
The non-aqueous electrolyte of sodium ion batteries should meet the following requirements:
(1) Wide temperature range for battery operation. It is very important for the electrolyte to remain liquid within a wide temperature range (-30°C to 60°C). Therefore, the melting point and boiling point of the electrolyte should be much lower and higher than the operating temperature, respectively.
(2) High ionic conductivity. In order to ensure the rapid transport of Na+ during battery operation, high ionic conductivity is required. In addition, ionic conductivity over a wide temperature range is required. Therefore, an excellent electrolyte should show a suitable viscosity and dielectric constant. A high ionic conductivity is also required within a wide range. To support conventional battery operation, the conductivity should be at least 5~10mS·cm-1.
(3) Electrochemical and chemical stability. Electrochemical stability is referred to as the (electrochemical window) voltage range between the oxidation/reduction reaction limits. The electrochemical window is closely related to the energy gap (Eg) between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the electrolyte. At the same time, the electrolyte should be able to form a stable intermediate phase with high Na+ conductivity at the anode and cathode. In addition, chemical stability requires that the electrolyte remain stable to other battery components.
(4) To ensure sustainable development, the electrolyte should be environmentally friendly.
Low cost, simple preparation and environmental friendliness are the main reasons why non-aqueous liquid electrolytes are widely studied. They are not only suitable for commercial lithium-ion batteries, but also for emerging sodium-ion batteries.
The application of conventional organic electrolytes in sodium-ion batteries is mainly concentrated on ester-based and ether-based organic solvents. Generally, esters, especially carbonates, are solvents with (C—O)-group characteristics [carbonates contain O(C—O) groups], including ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Among them, EC and PC solvents have the advantages of wide electrochemical window, large intercalation constant and high chemical stability, and are currently very attractive organic solvents in sodium-ion battery electrolytes. Ethers are solvents with O-group characteristics, including dimethoxyethane (DME), 1,3-dioxolane (DOL), diethylene glycol dimethyl ether (DEGDME) and tetraethylene glycol dimethyl ether (TEGDME). Compared with ester solvents, ethers have a lower oxidation window (generally less than 4V) and are volatile, with poor thermodynamic stability, but they have good compatibility with graphite electrodes, which ester solvents do not have.
Komaba et al. investigated the electrochemical performance of hard carbon|INaNi0.5Mn0.5O2 sodium-ion batteries in EC, PC, butyl carbonate (BC) and mixed solvents EC:DMC (1:1), EC:EMC (1:1) and EC:DEC (1:1) with NaCIO4 as electrolyte salt. The study found that at room temperature, with PC, EC and EC:DEC as electrolyte solvents, hard carbon|INaNi0.5Mn0.5O2 sodium-ion batteries showed stable cycle performance. At low temperatures, sodium-ion batteries with PC as electrolyte solvent showed better electrochemical performance. As shown in Figure 3, the two types of electrolytes, 1 mol·L-1 NaCIO4/PC and 1 mol·L-1INaCIO4/EC+DEC, made the battery show excellent electrochemical performance. However, the 1 molL-1NaCIO4/PC electrolyte made the performance of the hard carbon||NaNiosMno5Oz battery unsatisfactory. Using NaTFSI/NaPF6 instead of NaClO4 can achieve more stable cycle performance.
Figure 3 Performance of hard carbon electrodes in (a) single solvent or (b) binary solvent electrolyte containing 1 mol·L-1 NaCIO4
Ponrouch et al. systematically analyzed the electrochemical performance of Na丨丨hard carbon batteries in a series of different electrolytes, including the viscosity, ionic conductivity, thermal stability, electrochemical stability and other properties of the electrolytes of three different sodium salts NaCIO4, NaTFSI and NaPF6 in different organic solvents PC, EC, DMC, dimethyl ether (DME), diethyl carbonate (DEC), tetrahydrofuran (THF), triethylene glycol dimethyl ether (Triglyme) and mixed solvents EC: DMC, EC: DME, EC: PC, EC: Triglyme. The study found that the two electrolyte salts NaPF6 and NaCIO4 dissolved in different solvents had little effect on the electrochemical performance of Nall hard carbon batteries. NaPF6/(EC: PC) is the most suitable electrolyte for Nall hard carbon batteries. Its advantage is that it can form a stable SEI film on the hard carbon negative electrode, which greatly improves the cycle performance of sodium ion batteries, making its reversible capacity reach 200 mAh·g-!, and can be stably cycled for 180 weeks. As shown in Figure 4(a), the electrolytes of 1 mol·L-1NaClOa, NaPF6 and NaTFSI in PC show similar ionic conductivity and viscosity. As shown in Figure 4(b), large changes in ionic conductivity and viscosity can be observed in 1 mol·L-1NaClO4 electrolytes in single solvents or binary solvent mixtures (mass ratio of 50:50). Obviously, the electrolyte ionic conductivity in binary solvent mixtures is higher than that in single solvent electrolytes (the highest ionic conductivity of 1 mol·L-‘NaClO4/EC-DME is 12.55mS·cm-1). The electrochemical window of the electrolyte solution is shown in Figure 4(c). As shown in Figures 4(d) and (e), the hard carbon|I sodium-type battery based on EC/PC solvent electrolyte exhibits the highest capacity of 185mAh·g-1 in 70 cycles, while the battery in PC or EC+X (DMC, DEC or DME) type electrolytes shows faster capacity decay.
Kim et al. found that when the ether organic solvent DEGDME is matched with the graphite negative electrode in the sodium ion battery, the battery has good cycle stability at 0.5C, with a discharge capacity of 125mAh·g-1 in the first week, and the discharge capacity can still reach 80% of the first week discharge capacity after 2500 cycles, which shows that in the DEGDME electrolyte, Na+ can undergo a reversible deintercalation reaction in graphite.
Li et al. also demonstrated that high Coulombic efficiency and reversible capacity were achieved after coupling TiO2 and other anode materials in DEGDMEbased electrolytes, which can be attributed to the fact that DEGDMEbased electrolytes formed a thinner and more uniform SEI than EC/DEC-based electrolytes, thereby shortening the Nat diffusion distance and acting as a fast transporter of Na+, facilitating the rapid transport of Na+.
Carbonate electrolytes with NaPF6 or NaClO4 as electrolyte salts are currently the most commonly used electrolytes for sodium-ion batteries. They have the advantages of wide electrochemical window, large dielectric constant and high chemical stability, but their high flammability poses a great safety hazard. Under conditions of battery abuse, it may cause the electrolyte to burn, or the electrolyte volatilization may cause high internal pressure in the battery, making the battery dangerous such as fire or explosion. Therefore, the development of organic electrolytes with high safety, good film-forming properties and stable electrochemical properties is of great significance to the development of sodium-ion batteries.
Figure 4 (a) Conductivity and viscosity of 1 mol·L-1 NaTFSI, NaClO4 and NaPF6 in PC; (b) Conductivity and viscosity of 1 mol·L-1 NaClO4 in various solvents; (c) Electrochemical window and thermal stability of various electrolyte solutions; (d) First week discharge curve (top) and cycle performance diagram (bottom) of hard carbon|sodium type battery at 0.05C in different electrolytes containing 1 mol·L-1NaClO4
2 Aqueous Electrolytes
Aqueous electrolytes have many advantages, such as low cost, high safety, and environmental friendliness. Due to the abundant sodium resources, aqueous electrolytes make sodium-ion batteries more attractive than other battery systems for large-scale energy storage. Aqueous solutions, as an important component of aqueous sodium-ion batteries (ASIBs), are usually the key to improving the performance of ASIBs. Compared with organic solvents [such as ethylene carbonate (EC)], water is an excellent solvent for aqueous electrolytes due to its high dielectric constant, low viscosity, high ionic conductivity, and low vapor pressure, especially its inherent safety.
In many studies of aqueous sodium-ion batteries, Na2SO4 is the most widely used electrolyte. In 2012, Whitacre et al. assembled an 80V, 2.4kWh battery pack in 1mol·L-1 Na2SO4 aqueous solution (neutral pH), with λ-MnO2 as cathode and activated carbon as anode. Zhang et al. studied a complete aqueous sodium-ion battery consisting of Na0.58MnO2·0.48H2O as cathode, NaTiz(PO2)4 as anode and 1mol·L-1Na2SO4/H2O as electrolyte. The battery showed a capacity of 39mAh·g-1 at 10C, a capacity retention rate of 94% after 1000 cycles, and excellent cycle stability.
The pH of aqueous electrolytes affects the oxygen evolution potential at the cathode and the hydrogen evolution potential at the anode. By adjusting the electrolyte pH, the Na+ insertion and deinsertion potentials of certain materials in water can be controlled. Wang et al. reported a promising cathode material, Na0.66[Mn0.66Ti0.34]O2, and assembled it into a complete battery. With NaTi2(PO4)3/C as the anode, the battery exhibited excellent rate performance in 1 mol·L-1Na2SO4 aqueous electrolyte, with a specific capacity of about 54 mAh·g-1 at 10C, and excellent cycle stability and 89% capacity retention after 300 cycles at 2C (Figure 5).
Figure 5 (a) Rate performance of Na0.66[Mn0.66Ti0.34O2/NaTi2(PO4)3/C full cell at various current densities in the voltage range of 0.3~1.7V: (b) Cycling performance (red) and Coulombic efficiency (blue) of Na0.66[Mn0.66Ti0.34]O2|NaTi2(PO4)3/C full cell at 2C rate
Capacity decay at low temperatures has always restricted the application of aqueous sodium-ion batteries. Compared with flammable and toxic organic antifreeze, adding common cheap inorganic inert additives to improve low-temperature performance has important scientific significance. Zhu et al. added low-cost CaCl2 to 1 mol·-1NaClO4 aqueous electrolyte. The strong interaction between CaCl2 and water molecules reduced the freezing point of the electrolyte to -50℃ and showed an ionic conductivity of 7.13mS·cm-1. The sodium-ion battery had no obvious capacity decay after 6000 cycles at -30℃ and 10C rate, showing extremely high cycle stability.
With the continuous deepening of research on aqueous sodium-ion battery systems, the comprehensive performance of aqueous sodium-ion batteries will be continuously improved, and with their unique resource and environmental advantages, they will promote the rapid development of clean energy technology.
3 Solid Electrolyte
Traditional liquid electrolytes have safety hazards such as leakage, combustion and corrosion. All-solid-state sodium-ion batteries have excellent safety, reliability and stability, and are strong candidates for the next generation of energy storage batteries. Solid electrolytes are the key components that give all-solid-state sodium-ion batteries high electrochemical performance. Solid electrolytes are light in size, low in cost, can effectively avoid electrolyte leakage, and can alleviate the volume expansion of electrode materials during charging and discharging to a certain extent, so they are increasingly valued. Solid electrolytes are divided into solid polymer electrolytes and glass ceramic electrolytes.
1) Solid polymer electrolyte
Solid polymer electrolyte (SPE) is one of the most studied electrolytes in sodium ion batteries. SPE has the advantages of light weight, low leakage, low cost, and good flexibility. SPE is usually composed of an organic polymer matrix and a salt dissolved in the polymer matrix. In order to improve the performance of SPE, it may also contain additives such as inorganic functional materials. At present, the research methods and mechanisms of SPE for sodium ion batteries are based on the research methods of SPE for lithium ion batteries. Commonly used polymer matrices include polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyethylene oxide (PEO), polyethylene oxide (PEO), etc.
Nimah et al. reported a solid polymer electrolyte TiO2: polyethylene oxide (PEO): NaCIO4 for sodium ion batteries. The highest ionic conductivity of PEO-based SPE with EO/Na=20 (EO is ethylene oxide unit) at moderate temperature (60°C) was 1.34×10-5S·cm-1 (Figure 6). No significant increase in the thickness of the ion conductive membrane (nCPE) was observed at an EO/Na ratio of 30. Compared with pure PEO and PEO-based SPE, the ionic conductivity of 5% (mass fraction) TiO2 mixed nCPE with EO/Na=20 at 60°C increased to 2.62×10-4 S·cm-1. The increase in ionic conductivity is due to the formation of more amorphous regions in the polymer-based electrolyte and the improved mobility of the PEO chains. At 60°C, the first-week discharge capacity of the half-cell Na2/3Co2/3Mn1/3O2//nCPE//Na based on 5%-TiO2 blended nCPE was only 49.2mAh·g-1at a current density of 0.1C. The low discharge capacity of Na2/3Co2/3Mn1/3O2//nCPE//Na is mainly attributed to the large polarization caused by the excessive thickness of the nCPE electrolyte membrane (~0.18mm).
Figure 6. (a) PEO and PEO/NaCIO4 polymer electrolyte membranes with various concentrations and (b) nCPE mixed with PEO, PEO-NaCIO4 and TiO2 as a function of temperature.
Boschin et al. first proposed a SPE based on NaFSI and polyethylene oxide (PEO). NaFSI(PEO)n materials were prepared using different molar ratios n of ether oxygen to sodium (ONa). The assembled Nal|Na0.67Ni0.33Mn0.67O2 and Nal[Na3V2(PO3)4 half-cells showed good cycling performance at a high temperature of 80°C and showed high stability to Na. In addition, the studied NalSPE|Na symmetric battery was able to stably cycle for 1000h at a current density of 0.1 mA·cm-2. Afterwards, the effects of different anion types (NaFSI and NaTFSI) and (O:Na) molar ratio n on conductivity were further analyzed. When n was 9 and the temperature was below 40°C, NaTFSI(PEO)n showed higher conductivity. Although the TFSI- anion is large in size, it is relatively flexible in movement inside the electrolyte, effectively preventing the crystallization of the electrolyte. Compared with TFSI-, FSI- is more tightly bound to Nat, which makes NaFSI-based electrolytes exhibit low conductivity. Oi et al. prepared a SPE composed of NaFSI and polyethylene oxide (PEO) by solution casting. The mixed polymer electrolyte of PEO-NaFSL (EO/Na molar ratio = 20) exhibits a lower glass transition temperature (i.e., -37.9°C), higher ionic conductivity (41mS·m-1 at 80°C), and high electrochemical and thermal stability. Most importantly, the NaFSI/PEO mixed polymer electrolyte exhibits excellent interfacial stability in a half-cell with metallic sodium as the counter electrode, and also exhibits good cycling performance in a full cell with Na0.67Ni0.33Mn0.67O2 as the cathode material. Figure 7 shows the first-week charge and discharge performance and cycling performance of the all-solid-state battery with PEO/NaFSI (EO/Na=20) as the electrolyte at 80°C and 0.2C.
Figure 7 (a) Charge and discharge curves of NNM (0.2C) in SPE-based batteries; (b) Charge and discharge curves of NVP@C (0.2C); (c) Cycling performance of NNM (0.2C) at 80°C; (d) Cycling performance of NVP@C (0.2C) at 80°C
2) Glass ceramic electrolyte
Compared with polymer electrolytes, glass and ceramic electrolytes have higher ionic conductivity. Sulfide-based glass-ceramic electrolytes have attracted much attention due to their high conductivity and excellent mechanical properties. It usually has a conductivity higher than 10-4S·cm-1, is dense even when cold pressed, and has a low elastic modulus.
Hayashi’s group first reported cubic Na3PS4crystal phase as sodium ion battery electrolyte in 2012. Glass ceramics containing cubic Na3PS4 were formed by heating at 270°C, and their conductivity at room temperature was 2×10-4S·cm-1 with an activation energy of 27kJ·mol-1. Under the same conditions, the conductivity of Na3PS4 glass particles was 6×10-6S·cm-1and the activation energy was 47kJ·mol-1. Na3PS4 glass ceramic electrolyte exhibits a wide voltage window of 5V (Figure 8) and is electrochemically stable to metallic Na.
Figure 8 Cyclic voltammogram of Na3PS4 glass-ceramic electrolyte.
Compared with sulfide-based glass-ceramic electrolytes, electrolytes based on selenide have two advantages. The first is that the radius of the Se atom is larger than that of S, which indicates that its lattice can be expanded. Second, the high polarizability of Se2- can reduce the hindrance between the mobile ions (Nat) and the anion framework. In 2015, Zhang et al. proposed the use of Na3PS4 as a solid electrolyte for sodium-ion batteries. The cyclic voltammogram of the material with an ionic conductivity of 1.16×10-3S·cm-1 at room temperature.
Inorganic solid electrolytes are expected to avoid the safety hazards of organic electrolytes and are an important direction for the development of electrolytes. Currently, inorganic solid composite electrolytes with NASICON structure are widely concerned. They are generally composed of five elements such as sodium, zirconium, silicon, phosphorus, and oxygen, and their molecular formula is AM2(XO4)3 (A=alkali metal, M=transition metal, X=Si4+, p5+, S6+, etc.).
The NASICON structured Na1+xZr2SixP3-xO12 (0<x<3) is the earliest proposed solid electrolyte for sodium ion batteries, and its room temperature ionic conductivity reaches 27 μS·cm-1. Guin et al. studied the ionic conductivity of different types of NASICON electrolytes. Among more than 150 electrolytes, the ionic conductivity of Na3Zr2Si2PO12 reached 0.67mS·cm-1 at room temperature.
The high interfacial resistance between the solid electrolyte and the electrode limits the electrical and chemical performance of inorganic ceramic solid electrolyte batteries. The interfacial contact resistance between the electrode and the NASICON electrolyte can be effectively reduced by introducing materials with good interfacial properties for interfacial modification. The composite materials reported so far include flexible polymer electrolytes, ionic liquid electrolytes, and sulfide solid electrolytes. Combining the advantages of polymer electrolytes (flexible) and inorganic electrolytes to form a hybrid solid electrolyte (HSE) is an effective composite method. The HSE based on the PVDF-HFP matrix can be prepared by a mixed dispersion process. The matrix contains NASICON ceramic powder (Na3Zr2Si2PO12) and 1 mol·L-1 sodium trifluoroformate (NaCF3SO3)/TEGDME. The mixed preparation process is shown in Figure 9.
Figure 9 HSE mixing preparation process
In solid electrolytes, the diffusion of ions is relatively difficult, resulting in low conductivity, which limits its application in sodium ion batteries. Therefore, improving conductivity and reducing interface contact resistance are important future development directions for solid electrolytes.
4. Gel polymer electrolyte
Compared with solid polymer electrolytes, except for the polymer matrix and the added sodium salt, GPE with added plasticizer can be regarded as an intermediate state between solid polymer electrolytes and liquid electrolytes. It perfectly combines the diffusion properties of liquids with the mechanical properties of solids, effectively avoiding the problems of easy leakage and insecurity of liquid electrolytes.
Yang et al. prepared polyvinylidene fluoride-co-hexafluoropropylene by a simple phase separation method, and impregnated it in 1 mol·L-1 NaCIO2/(EC:DEC:DMC) to synthesize a sodium ion battery gel polymer electrolyte. At room temperature, the conductivity of the electrolyte reached 0.6mS·cm-1, which is higher than the conductivity of the commercial separator Celgard 2730 (0.16 mS·cm-1). At the same time, the sodium ion migration number of this GPE reached 0.3, and it showed good electrochemical stability and mechanical properties.
Kumer et al. added dispersed SiO2 nanoparticles to the polymethyl methacrylate (PMMA) matrix, and the prepared GPE showed good mechanical properties, thermal stability and electrochemical properties. The addition of an appropriate amount of SiO2 nanoparticles (about 4%, mass fraction) promoted the aggregation and decomposition of undissociated salts/ions in the gel polymer matrix into free ions, making the GPE show an electrical conductivity of about 3.4×10-3 S·cm. Tan et al. proposed a method for in-situ synthesis of SiO2particles in PEO. The ionic conductivity of the in-situ synthesized nanocomposite polymer electrolyte at 30°C was about 1.1×10-4S·cm-1.
Gao et al. prepared a composite gel PVDF-HFP polymer electrolyte based on low-cost glass fiber paper (GF) and polydopamine (PDA) coating. The gel-polymer composite electrolyte (GF/PVDF-HFP/PDA) exhibited good mechanical properties with a tensile strength of 20.9 MPa and maintained thermal stability at temperatures up to 200°C. The electrochemical window of the GF/PVDF-HFP/PDA composite electrolyte was 4.8 V measured by linear sweep voltammetry. The ionic conductivities of GF/PVDF-HFP and GF/PVDF-HEP/PDA electrolytes at 25°C were 4.6 mS·cm-1 and 5.4 mS·cm-1, respectively. The improvement in ionic conductivity may be attributed to the hydrophilic coating of polydopamine, which accelerates the transport of Nat in the gel polymer electrolyte. The rate capacity, cycling stability, and coulombic efficiency of the sodium ion battery Na2MnFe(CN)6/Na using PVDF-HEP/PDA as the electrolyte were significantly improved, as shown in Figure 10.
Figure 10 Electrochemical performance of GF/PVDF-HFP and GF/PVDF-HFP/PDA in sodium ion batteries with Na2MnFe(CN)6as cathode
(a) First charge and discharge curve at 0.2C; (b) Coulomb efficiency at different current densities; (c) Charge and discharge curves of the battery using GF/PVDF-HFP/PDA gel-polymer composite electrolyte at different current densities; (d) Discharge capacity of the battery at different current densities
At present, the main problems of gel polymer electrolytes for sodium ion batteries are that the room temperature conductivity is lower than that of organic electrolytes, and the mechanical strength is not high enough. Through some modification methods, such as cross-linking, copolymerization, and adding fillers, it is expected to improve the above problems of gel polymer electrolytes.
2 Application of ionic liquids in sodium ion batteries
2.1 Ionic Liquid Electrolytes
Ionic liquid (ILs) electrolytes with advantages such as wide electrochemical window, low volatility and high thermal stability have shown great potential in lithium batteries, which has also aroused the interest of researchers in applying them to sodium-ion batteries. In this section, we will discuss the research and application of ILs in sodium-ion batteries. The ILs commonly studied are mainly composed of imidazoles, pyrrolidines, amines and anions and cations such as TFSI–, FSI–, BF–4, and PF–6, as shown in Figure 11.
Figure 11 Schematic diagram of cations and anions of common ILs in sodium-ion batteries
The ideal electrolyte needs to have good viscosity/conductivity, high thermal stability and good SEI film formation ability, but this is difficult to achieve in a single ILs. By mastering the properties of different ionic liquids, the shortcomings of a single ionic liquid can be overcome through the combination of different ionic liquids. For example, among cations, imidazolium ionic liquids have excellent ionic conductivity and good chemical stability; pyrrolidine ionic liquids show excellent electrochemical stability up to 6V; the asymmetric core structure and ether substituents of the electrolyte molten salt can significantly reduce the viscosity of ILs and improve their conductivity; ILs with ethyl connectors have higher thermal stability than ILs with methyl connectors; ester groups, nitrile groups, allyl groups, etc. will promote the formation of SEI films to improve battery performance. The position of alkyl substitution and the length of the spacer chain also seriously affect the thermal stability of ILs; among anions, FSI- shows lower viscosity and higher SEI film formation ability than TFSI-, while TESI- shows better thermal and electrochemical properties, high stability and lower price. Na concentration and operating temperature also have a significant impact on the viscosity and conductivity of ILs electrolytes and the reversible capacity and rate performance of electrode materials.
ILs that are better than imidazolide, such as 1-ethyl-3-methylimidazole ([Emim]–) and 1-butyl-3-methylimidazole ([Bmim]–), have attracted widespread attention due to their relatively high ionic conductivity, low viscosity and good chemical stability. In early 2010, Plashnitsa et al. reported 0.4 mol·L-1[NaBF4/TEmim]BF4 electrolyte for Na3Vz(PO4)3|lNa3V2(PO4) 3batteries, which showed high thermal stability at temperatures up to 400°C. In 2014, Monti et al. also studied the characteristics of ILs based on [Emim]TFSI and [Bmim]TFSI.
Wu et al. studied NaBF4/TEmim]BF4 electrolyte in 2016 and found that the electrolyte has good thermal stability and a wide electrochemical window. At 20°C, the ionic conductivity of pure [Emim]BF4 is 11.1 mS·cm-1‘, while the conductivity of 0.1 mol·L-1‘NaBF4/[Emim]BF4 electrolyte is 9.833 mS·cm-1 Wu et al. demonstrated that the NaPF6/[Bmim]TFSI electrolyte of the Nal|Na3V2(PO4)3 battery exhibits low flammability and high thermal stability. Compared with batteries based on commercial carbonate electrolytes, 0.25 mol·L-1NaPF6 /[Bmim]TFSI electrolyte gives the battery a higher initial capacity and better cycling stability at 0.1C.
In 2018, Hwang et al. reported on the study of Nal|Na3V2(PO4)3 batteries using [Emim]FSI electrolytes with different NaFSI concentrations. Batteries using NaFSI/[Emim]FSI electrolytes showed excellent cycle performance and rate performance over a wide temperature range (-30~90°C). After 300 cycles at 1C, the capacity retention rate was 99% and the coulombic efficiency was close to 100%. Even after 5000 cycles at 363K and 20C rate, the battery still had a capacity retention rate of 89.2%.
Sun et al. reported a new type of chloroaluminate-based ILs electrolyte, which is a mixed electrolyte of NaCI-buffered AlCI3 and [Emim]CI, and used it in sodium-ion batteries [Figure 12(a)]. This ILs electrolyte shows an ionic conductivity of 9.2 mS·cm-1 at 25°C, good thermal stability at temperatures up to 400°C [Figure 12(b), (c)1. And a wide electrochemical window. The Na||Na3V2(PO4)3 battery using this ILs electrolyte exhibits stable cycling performance (about 96% of the capacity is retained after 460 cycles at 150mA·g-1). And good rate performance (reversible capacity of 70mAh·g-1 at a current density of 500mA·g-1) [Figure 12(d)]. Similarly, excellent electrochemical performance can also be achieved in NallNa3V2(PO4)2F3 batteries.
In 2014, Moreno et al. studied a pyrrole ionic liquid electrolyte N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI). The electrolyte mixed with NaTFSI and Pyr14TFSI has a conductivity of 1mS·cm-1 at room temperature, low flammability and low volatility, and its freezing point reaches 30℃, which is expected to be applied at low temperatures. Wang et al. used 1 mol·L-1NaCIO4/bis(trifluoromethylsulfonyl)-1-butyl-1-methylpyrrole (BMPTFSI) ionic liquid electrolyte to match the positive electrode material Na0.44aMnO2. Since the electrolyte has low solid-liquid interface resistance and charge transfer resistance at the Na electrode and Na0.44aMnO2 electrode, the battery exhibits good charge and discharge performance. In addition, the ILs electrolyte has high thermal stability and is very suitable for high temperature applications. At 75°C and 0.05C, the battery discharge capacity reaches 115mAh·g-1, close to the theoretical capacity of 121mAh·g-1, and at 1C the capacity can still reach 85% of its theoretical capacity.
Figure 12 (a) Schematic diagram of battery configuration and (b) TG test of mixed electrolyte based on NaCl buffered AlCl3 and [Emim]C1; (c) flammability test of ILs electrolyte based on chloroaluminate and 1mol·L-1NaC1O4/EC-DEC electrolyte;
(d) Cyclic diagram and Coulombic efficiency diagram of Na3V2(PO4)3||Na at different current densities in ILs electrolyte
Manohar et al. prepared a composite material of sodium vanadium phosphate and carbon (NVP@C) using ionic liquid, which showed stable cycling performance and could be stably cycled for 5000 cycles at a rate of 10C while maintaining a Coulombic efficiency of 99%. Then, 1 mol·L-1 NaFSI was dissolved in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C3C1PvrrFSI) to prepare an electrolyte. The full battery made of NVP@C as the cathode and hard carbon as the anode had an energy density of 368 Wh·kg-1 and a capacity retention rate of 75% after 100 cycles, which was significantly better than the NVP@CII hard carbon full battery based on organic electrolyte.
Ferdousi et al. found that adding water (~1000ppm) to the C3C1PyrrFSI superconcentrated liquid electrolyte [50% (molar fraction) NaFSI) can promote the formation of a solid electrolyte interface (SEI) film and improve the cycle stability of the battery. The addition of water can promote the formation of a new uniformly distributed complex (Na2[SO3-N-SO2F]·nH2O(n=0~2)] between FSI anions, water molecules and sodium ions, thereby improving the cycle performance of the battery. The capacity retention rate of the battery with 1000 ppm water after 500 cycles at 1.0 mA·cm-2 reached 99%, which is higher than that of the battery using anhydrous electrolyte (98%).
Ding et al. reported NaFSI[C3C1Pyrr]FSI electrolyte for sodium ion batteries with a wide temperature range. The effects of Na salt concentration [0~60% (molar fraction) NaFSI range] and operating temperature (20~90℃) on the viscosity, ionic conductivity and electrochemical performance of NallNaCrO2, batteries were studied. As the concentration of NaFSI increased, the viscosity of NaFSI/[C3C1Pyrr]FSI increased while the ionic conductivity decreased. In addition, the ionic conductivity of NaFSI/[C3C1PyrrlFSI increased with increasing temperature. For NaFSI/[C3CPyrr]FSI containing 20% NaFSI (1 mol·L-1), the ionic conductivity at 25℃ and 95℃ was 3.6 mS·cm-1 and 21 mS·cm-1, respectively. Using 40% NaFSI enables the battery to achieve the best rate performance at 90℃.
Wang et al. applied 1 mol·L-1INaFSI/[C3C1Pyrr]FSI to hard carbon||Na0.44MnO2 full cell. The charge transfer resistance of hard carbon, NaFePO4 and Na0.44MnO2 electrodes was significantly lower than that of electrodes in conventional organic electrolytes, making the discharge capacity of the full cell at 0.1 C (25°C) 117 mAh·g-1, and still maintaining 97% of the initial capacity after 100 cycles.
Kim et al. studied an ILs electrolyte consisting of 5% (mass fraction) EC and 0.1 mol·L NaFSI in a mixture of 0.6 mol·L-1 [C3C1Pyrr]FSI-0.3 mol·L-1[C3C1Pyrr] TFSI. The ionic conductivity of the electrolyte was 5 mS·cm-1 at 20°C and 15.8 mS·cm-1 at 60°C. With the EC additive, the Nall hard carbon battery had an initial charge capacity of 331 mAh·g at 0.1C (ICE=59.4%) and maintained a capacity of 312 mAh·g-1 after 100 cycles. Even at a 5C rate, the battery showed a capacity of 82 mAh·g-1.
Brutti et al. prepared a safer electrolyte for sodium-ion batteries by mixing Pyr14TFSI ionic liquid with NaTFSI salt. Preliminary constant current cycling tests were performed on Na/NaMnO3 batteries at room temperature. The NaTFSI-Pyr14TFSI electrolyte prepared in this study has high purity. At 10C, its ionic conductivity is about 10-1S·cm-1, and it still has excellent thermal stability at temperatures as high as 250°C, showing an electrochemical stability window of more than 4V even in carbon-rich working electrodes.
Yang et al. systematically studied the thermal stability and electrochemical properties of Na[FTA]-[C3C1Pyrr][FTA] applied to sodium secondary batteries. The results showed that the asymmetric FTA structure hindered the effective filling of grains during crystallization, resulting in a periodic structure, which gave it a wide liquid temperature range. As shown in Figure 13(a), in the range of 0~0.4 mole fraction of Na[FTA], its glass transition temperature is between 170~209K. The ionic conductivity and viscosity of pure C3C1Pyrr][FTA] follow the Vogel-Tammann-Fulcher equation, as shown in Figure 13(b). In the temperature range of 298~368K, the ionic conductivity is between 6.64~30.9 mS·cm-1. The half-cell is composed of hard carbon and metallic sodium, and the anode potential limit is above 5V. As shown in Figure 13(c), when the Na[FTA] mole fraction is 0.3, the discharge capacities of the hard carbon electrode are 260 mAh g-1 and 236 mAh g-1 at a temperature of 363 K and current densities of 20 mA·g-1 and 200 mA·g-1, respectively. As shown in Figure 13(d), when the Na[FTA] mole fraction is 0.3, it has the best cycle performance and satisfactory rate performance and coulombic efficiency. At a current density of 20 mA·g-1, the capacity retention rate is 97% after 400 cycles.
Figure 13 (a) Glass transition temperature Tg and melting point Tm of NaIFTA1; (b) Temperature curve of conductivity of Na[FTA] with mole fraction of 0~0.4: (c) Rate performance at current density of 20~2000mA·g-1; (d) Cyclic performance at current density of 20mA·g-1 when x(Na[FTA])=0.3
Compared with carbonate organic solvent electrolytes, ionic liquid-based electrolytes have the advantages of wide electrochemical window, non-flammability, and non-volatility. They can effectively solve safety issues in sodium ion batteries. In addition to being used as materials for surface modification and electrolyte materials, ionic liquids can also be used as precursors of electrode materials for induction. However, due to their high cost, they cannot be applied on a large scale.
2.2 Ionic liquid gel electrolytes are confined and dispersed in
Ion gels, also known as ionic liquid gels, are a new class of hybrid materials in which ILs are present throughout or permeate the entire continuous solid phase. Inorganic hybrids are usually prepared by five methods: sol-gel, solvent casting, swelling of the matrix in ionic liquids, hot pressing, and in situ polymerization. Ionic liquids act as both plasticizers and charge carriers, so the conductivity of ion gels is significantly improved (up to 10-2 S·cm-1 at 25°C) compared to all-solid electrolytes (10-8~10-5S·cm-1). In addition, ion gel electrolytes exhibit higher mechanical flexibility and better electrode/electrolyte interfacial contact.
Compared with traditional gel electrolytes containing volatile organic solvents (such as alkyl carbonates), ion gels have higher safety and maintain the ultra-low vapor pressure, non-flammability and good thermal stability of ILs, so they can be used at high temperatures. In addition, since Na ions can diffuse through the gel channels, ion gels are less affected by the high viscosity of ILs than pure ILs electrolytes, and their flexibility provides more possibilities for electro-hydraulic design. Compared with the limited types of ILs electrolytes studied [the ionic liquid electrolytes that have been successfully applied are usually composed of fluoride-based anions (such as [TFSI]–, [BF4]– and [TfO]–) and imidazolium/pyrrolidinium-based cations, the sources of ionic liquid gel matrices are very wide, including inorganic oxides, linear polymers, cross-linked polymers, and inorganic-organic hybrid networks. By controlling the composition and structure, the conductivity of ion gels can be close to or even higher than that of ILs electrolytes.
Inorganic matrices generally have excellent thermal stability, safety, and mechanical strength. Currently, silica forests are the most commonly used inorganic matrices for confining ILs. During the synthesis process, the morphology of the matrix (such as pore size, orientation, and shape) can be easily adjusted, and silica-based ion gels can be easily obtained by physically mixing fumed silica nanoparticles with ILs via an in situ sol-gel synthesis method.
Most ion gels reported in recent years are based on polymer matrices. Parveen et al. fixed ILs containing complex cations [Na-glyme and anions C1O4 in PVDF-HFP to prepare a flexible gel polymer electrolyte film with an ion conductivity of 2.54×10-3S·cm-1. The sodium ion transmission number is 0.73. The GPE was applied to a sodium ion half-cell with V2O5 as the positive electrode, and the first-week capacity of the battery at 0.05C can reach 111mAh·g-1.
Harshlata et al. used PVDF-HFP/TiO2 as a porous membrane, fixed a nanocomposite gel polymer electrolyte of NaCF3SO3in 1-butyl-3-methylimidazolium-sodium trifluoromethanesulfonate ([Bmim]CF3SO3) ionic liquid, and prepared a porous gel polymer electrolyte membrane by phase inversion method. The high dielectric constant of PVDF-HFP of 8.4 ensures a high degree of dissociation of salts in the electrolyte. The ionic liquid [BmimlCF3SO3; used in the study is a non-proton neutral ionic liquid, which is reported to have a thermal stability of about 370°C and does not expand with temperature. The addition of TiO2 nanoparticles can interact with the anions (CF3SO–3) of the liquid electrolyte and enhance the amorphous state of the polymer network. The membrane formed by 0.50% (mass fraction) of TiO2nanoparticles dispersed in PVDF-HFP has the largest porosity and absorptivity, up to 72% and 270%, respectively. The dispersed TiO2 nanoparticles increased the cation (TNa+) transport number of the polymer gel electrolyte membrane from 0.16 to 0.27, the total transport ion number (Tion) reached 0.99, the ionic conductivity reached 0.4mS·cm-1, and the electrochemical stability window was about 4.3V.
Vélez et al. reported a new ionic liquid gel polymer electrolyte for sodium ion batteries, which contains 0.2 mol·L-1 NaN(CF3SO2)2 solution and a pVDF-HFP copolymer matrix fixed in a mono-/di-imidazole ionic liquid. The membrane has an ionic conductivity of 2.2×10-4S·cm-1 at room temperature, a thermal stability of 150°C, and begins to decompose at around 400°C. The electrochemical stability window is 1.5~5.0V, showing excellent thermal and electrochemical stability.
Song et al. reported a hybrid polymer/oxide/ILs (PEO20/SiO2/(Emim][FSI]-NaClO4) solid electrolyte by a mechanochemical method. PEO20-NaC1O4-5%SiO2-70%[Emim][FSI] has an ionic conductivity of 1.3×10-3S·cm-1 at room temperature, good mechanical properties, a wide electrochemical window (4.2V) and a high Nat+ migration number (0.61). The electrolyte was used in solid Na metal ion batteries and had a stable capacity of 62.7 mAh·g-1 after 100 cycles. Composite solid electrolytes (CSEs) with high ceramic content have the combined advantages of ceramic electrolytes and flexible polymer components, such as excellent thermal stability and mechanical properties. Shen et al. [110] coupled trace amounts of N-propyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide ([Pv1a][NT61) to In CSE, a NASICON-type CSE containing 80% (mass fraction) Na3.4Zr1.9Zn0.1Si2.2P0.8O12 in polyethylene oxide was prepared, achieving an ionic conductivity of 1.48×10-4S·cm-1. The assembled Na3Vz(PO4)3//Na solid-state sodium battery showed excellent rate performance and cycle stability, with a capacity retention rate of 90.0% after 150 cycles at 0.5C and 60°C.
Compared with traditional liquid electrolyte systems, gel-state ionic liquids exhibit higher stability and safety. Compared with traditional all-solid electrolyte systems, ionic liquid gels are simpler to prepare, with lower costs and energy consumption, and the materials are easier to recycle. The combination of different gels and a variety of ionic liquids provides a wealth of options for the research and application of sodium-ion battery electrolytes. So far, the focus of research has been mainly on breaking through the limitations of specific ionic liquid functions, for example, incorporating active substances such as homogeneous catalysts such as enzymes, metal nanoparticles, molecules, or polymers into ionic liquid gels. This constructible ionic liquid gel system has shown great potential in the fields of catalysis and preparation of energetic materials.
For more information about capacitors, please click: https://capacitorsfilm.com/
