Research progress of room temperature sodium ion battery
Among many electrochemical energy storage systems, lithium (Li) ion (secondary) batteries have been used on a large scale due to their high energy density and long cycle life. However, due to the wide application of lithium resources in the industry, the total amount available for electrochemical energy storage is limited. It is reported that up to a quarter of the global lithium resources will be used in the electric vehicle field by 2050. Secondary battery systems using sodium (Na) as a guest ion have been extensively studied for the development of more economical and environmentally friendly energy storage systems. This is based on the similarity of the reversible storage and migration mechanism of Na and Li electrodes, and the Na element ranks sixth in the abundance of the earth's crust (about 2.6%), while there are massive Na resources in the seawater. Since Na has a large ionic radius (Na+: 1.02, Li+: 0.76) and a low standard electrode potential (about 2.71V vs. Na+/Na, about 3.04V vs. Li+/Li), this leads to sodium ion batteries. The energy density is generally lower than that of a lithium ion battery. Nevertheless, cost and resources are the primary considerations in large-scale energy storage systems. Preliminary calculations indicate that the cost of future sodium-ion batteries is about 0.37 yuan/Wh, which is lower than the cost of lithium-ion batteries of 0.47 yuan/Wh. Continue to reduce the space of
The working principle of sodium ion battery is similar to that of lithium ion battery. It is also based on the mechanism of “rocking battery” which reversibly inserts/extracts Na+ in positive and negative electrodes: during charging, Na+ is removed from the positive electrode in the inner circuit and inserted into the negative electrode through electrolyte. While the electrons move from the positive electrode to the negative electrode in the external circuit; the discharge process is just the opposite. The typical lithium ion battery and sodium ion battery structure are shown in Fig. 1. The key materials of sodium ion batteries include positive and negative materials, electrolyte materials and membrane materials. This paper mainly discusses the current research results of major positive and negative materials and organic non-aqueous electrolytes and the whole battery system based on this.
I. Types and progress of electrode materials
The cathode materials of sodium ion batteries mainly include transition metal oxide materials, polyanionic compound materials, Prussian blue compounds and organic positive electrodes with typical layered structure and tunnel structure. Materials; anode materials mainly include carbon anode materials, metal oxide materials, alloy materials and organic anode materials.
1. Cathode Materials
Transition Metal Oxide (TMO2) mainly consists of layered and tunnel structure materials. The classification of layered transition metal oxides mainly follows the structural classification method proposed by Delmas et al. , which is mainly divided into O3, P2 and P3 phases according to the stacking order of O (O3: ABCABC stack; P2: ABBA stack; P3: ABBCCA stack). The sodium ions diffuse in the interlayer spacing of the triangular prism (P phase) or the octahedron (O phase), as shown in FIG.
As early as 1988, Shacklette et al.  studied the electrochemical properties of NaxCoO2 in O3, O'3, P3 and P2 phases and found that P2-NaxCoO2 exhibits higher energy density and longer cycle life. This makes P2-NaxCoO2 arousing extensive research interest. Subsequently, Delmas et al  studied the deintercalation mechanism of P2-NaxCoO2 by in-situ XRD and other electrochemical tests. It was found that 9 different phases appeared on the constant current cycle curve with the change of Na content. Change point, mainly due to the generation of different Na vacancies. Considering the toxicity and cost of cobalt (Co), positive electrode materials based on other transition metal elements, particularly manganese, have been widely concerned. P2-NaxMnO2 (0.45 ≤ x ≤ 0.85) has a high electrochemical activity. Caballero et al  studied P2-Na0.6MnO2 and showed that the initial reversible capacity of the material can reach 140mAh/g under the voltage range of 2.0～3.8V, but its structural stability is poor and the cycle life is short. However, the O3 phase α-NaMnO2 has no obvious structural changes during the cycle, and accordingly, it exhibits good electrochemical performance: in the voltage range of 2.0 to 3.8 V, the first week discharge specific capacity is as high as 185 mAh/g, cycle 20 After week, the specific capacity was 132 mAh/g. In addition, the O3-phase O3-NaNiO2 and NaCrO2 also have corresponding electrochemical activities. Doping an inert metal such as magnesium (Mg), manganese (Mn), iron (Fe) or the like in the layered transition metal oxide can enhance structural stability, suppress phase transformation, and obtain better electrochemical reversibility. The cathode material P2-NaxFe0.5Mn0.5O2 (reversible interval is 0.13≤x≤0.86) is obtained by doping Mn element into the α-NaFeO2 framework structure, and the voltage range is 1.5-4.3V (vs.Na+/Na.). When the electrode is reversible . Komaba et al  showed that NaNi0.5Mn0.5O2 has a reversible capacity of 105-125 mAh/g at a current density of 4.8 mA/g and a voltage range of 2.2-3.8 V (vs. Na+/Na).
The tunnel structure Na0.44MnO [2, as shown in Fig. 3(a)] belongs to the orthorhombic system, and the space group is Pbam. As early as 1994, Doeff et al  reported the electrochemical activity of the tunnel structure Na0.44MnO2 for the first time. Sauvage et al.  studied the electrochemical performance of Na0.44MnO2 in a non-aqueous electrolyte. The results show that a reversible capacity of up to 140 mAh/g can be obtained in the voltage range of 2 to 3.8 V (vs. Na+/Na). At the same time, the electrode process experienced six two-phase transformations. In a full-battery system, a general battery system composed of a common anode material and Na0.44MnO2 cannot provide sufficient sodium, and it is difficult to obtain a high specific capacity comparable to that of a half-battery. Therefore, it is necessary to design a tunnel type having a higher sodium content. Sodium storage material.
Similar to LiFePO4, a widely used cathode material in lithium-ion batteries, polyanionic compounds have an open framework structure, strong induction effect and strong covalent bond of XO [X=phosphorus (P), sulfur (S), silicon (Si ), boron (B)], therefore it has the advantages of fast ion transport, high operating voltage and stable structure as a positive electrode material for sodium ion batteries.
The molecular formula of NASICON (NA Super Ionic CONductor) is AxMM'(XO4)3, which is a three-dimensional network of MO6 and XO4 polyhedrons co-angled, as shown in Fig. 3(b). Uebou et al  reported the electrochemical performance of Na3V2(PO4)3 in sodium ion batteries for the first time, and then a large number of researchers have improved the electrochemical performance of Na3V2(PO4)3. Olivine NaFePO4 has a one-dimensional Na+ transport channel, as shown in Fig. 3(c), which has a theoretical capacity of 154 mAh/g as a positive electrode material for sodium ion batteries. Fluorinated phosphate materials have attracted the attention of researchers due to their special sodium storage structure and high sodium storage potential. Na2FePO4 and NaVPO4F have been extensively studied for their exbatteryent kinetic properties.
The molecular formula of Prussian blue (PBAs) is KMIIFeIII(CN)6 (M= Mn, Fe, Co, Ni, Zn, etc.). The common complexes of such materials belong to the cubic system, and the space group is Fm3m. There are a large number of alkali ion channels in the structure to facilitate Na+ rapid de-embedding without structural distortion, as shown in Figure 3(d). In 2012, Lu et al.  reported the first study of KMIIFeIII(CN)6 as a cathode material for non-aqueous sodium ion batteries. The results show that the reversible capacity of KFe2(CN)6 is about 100 mAh/g, and the two sodium storage potentials are 3.5V. And 2.6V, respectively, correspond to changes in the high-spin state Fe3+/Fe2+ pair paired with N and the low-spin state Fe3+/Fe2+ pair paired with C. In order to improve the electrochemical performance of AFe2(CN)6 (A=KorNa; 0≤A≤1), a large number of studies have been carried out to optimize carbon coating, nanocrystallization and crystallinity. In addition, in order to develop a green sustainable energy storage system, organic materials with sufficient source, environmental friendliness and high theoretical capacity, such as disodium salt of palmitic acid (Na2C6O6), perylenetetracarboxylic dianhydride (PTCDA) and phthalimide (PTCDI) ), has become a new alternative to traditional inorganic cathode materials.
2. Anode material
As a negative electrode of sodium ion battery, carbon material mainly includes graphite, soft carbon and hard carbon. As a negative electrode material for lithium ion batteries, graphite materials have good lithium storage performance and high reversible capacity (the theoretical capacity can reach 372 mAh/g). However, the direct use as a negative electrode material for sodium ion batteries is not satisfactory. Jache et al.  demonstrated for the first time that Na can be embedded in a graphite layer by solvent co-intercalation. In this system, the reversible capacity of graphite anode material is close to 100 mAh/g, and there is still a fairly high capacity retention after 1 000 cycles. . Stevensa et al.  obtained a hard carbon material from glucose decomposition and obtained a reversible capacity of up to 300 mAh/g. The in-situ XRD and potential-capacity plots were used to analyze the sodium storage mechanism of hard carbon. They believed that the high-pressure slope region corresponds to the intercalation process of Na in parallel (or nearly parallel) layers of hard carbon, and the low-pressure platform region corresponds to Na in the interlayer. An embedding process similar to adsorption in micropores. A schematic diagram of the charge and discharge curve and the sodium storage structure of the hard carbon material is shown in FIG.
Metal oxides can achieve sodium storage by intercalation reaction and conversion reaction, so it has attracted extensive attention as a potential negative electrode material for sodium ion batteries. Xiong et al  reported the electrochemical performance of amorphous titanium dioxide (TiO2) nanotubes (TiO2NT) in sodium ion batteries for the first time. They found that only TiO2NTs with diameters larger than 80 nm exhibit electrochemical activity and specific capacity The number of cycles is gradually increasing. The study on the mechanism of sodium storage of commercial TiO2 nanoparticles shows that the anatase XRD diffraction peak shifts slightly to the low angle direction and then disappears with the first discharge process, and does not reappear during the subsequent charging process; The embedding causes Ti4+ to be reduced to Ti3+ while irreversibly generating a portion of Ti0 and NaO2. Through the study of Li4Ti5O12 (LTO), Hu Yongsheng's research group reported its sodium storage performance for the first time : the highest reversible capacity of spinel LTO can reach 155mAh/g, corresponding to the sodium storage potential of about 0.9V. Senguttuvan et al.  reported for the first time the performance of Na2Ti3O7 and superconducting carbon black mixed powder in sodium ion batteries. By analyzing the voltage-composition curve, it is considered that the potential at 0.7V (vs. Na+/Na) corresponds to charcoal. For the reaction of the black additive, the potential at 0.3 V (vs. Na+/Na) corresponds to the deintercalation of 2 Na+ (reversible capacity up to 200 mAh/g). Other metal oxides, such as: Fe3O4, Fe2O3, Co3O4, CuO, and SnO2, can be stored by conversion reaction. These compounds have the advantages of theoretical high capacity, high rate performance, and stable cycle performance as a negative electrode material for sodium ion batteries.
Metal and metalloid materials (such as tin (Sn), bismuth (Sb), P, etc.) as sodium anode materials can achieve sodium storage by forming Na-Me alloy materials, and have a high theoretical capacity (370 ~ 2000 mAh/g) and lower sodium storage potential (less than 1 V). However, the volume of the material in the electrode process is greatly deformed, which is not conducive to cycle stability. Therefore, the research on metal and metalloid negative electrode materials is mainly focused on enhancing the cycle stability and the reaction mechanism of Na-Me alloying. In addition, a large number of studies on carboxylate organic anode materials show that these materials have low reversible capacity and sodium storage potential. For this reason, various improved methods including molecular design, surface coating, and polymer polymerization. Has been studied successively.
Second, the development of electrolyte materials and full battery
Electrolyte as a key material of the battery, in the battery plays a role of conduction charge transport current. A good electrolyte should have the following characteristics: large ion conductivity; wide electrochemical window; good thermal stability; good chemical stability; A sodium ion battery based on an organic liquid electrolyte is the main research type. Ponrouch et al.  systematically studied the ionic conductivity, viscosity, thermal stability, electrochemical window and other properties of a series of organic electrolytes. The results show that the properties of the mixed solvent are better than the single solvent, and the most prominent is EC:PC (1:1), while the NaClO4/NaPF6-EC:PC (1:1) electrolyte system shows in all tests. Exbatteryent performance. In further electrochemical properties studies, it was found that the battery of the above electrolyte system has a high reversible capacity (about 200 mAh/g) and exbatteryent cycle performance (cycle for 180 weeks). Subsequently, the research team continued to study the performance of ternary electrolytes on the basis of NaClO4/ NaPF6-EC:PC (1:1). The results show that the electrochemical performance of EC0.45:PC0.45:DMC0.1 is best. A specific capacity of 97 mAh/g and stable cycle performance are obtained in a Na3V2(PO4)2F3/hard carbon full battery . As an effective lithium ion battery electrolyte additive, fluoroethylene carbonate (FEC) has also been proven to be an additive to sodium ion battery non-aqueous electrolytes to effectively achieve improved battery electrochemical performance.
With the deepening of research on key materials of sodium ion batteries, the sodium ion full battery system has gradually been studied. The positive and negative materials in the symmetrical full battery are identical, which makes it advantageous in terms of mass production, cost and safety. The inorganic material Na2.55V6O16·0.6H2O (NVO) symmetric full battery has a specific capacity of about 140 mAh/g and an energy density of about 140 Wh/kg, but the cycle performance and rate performance still need to be improved . The first week reversible capacity of the organic material 2,5-dihydroxyterephthalic acid (Na4DHTPA or Na4C8H2O6) symmetric whole battery is close to 200 mAh/g, and the capacity retention rate after 76 cycles is 76% . In addition, the research on embedded positive/embedded negative asymmetric full-battery batteries has been extensive, but such full-battery batteries often face problems such as low efficiency and poor cycle performance in the first week. Recently, Hu Yongsheng's research group  prepared O3-NaCu1/9Ni2/9Fe1/3Mn1/3O2 (CNFM) cathode material, and the whole battery assembled with hard carbon sphere anode material has exbatteryent electrochemical performance: after 400 cycles, There is still a reversible capacity above 200 mAh/g with a retention of 71% and a theoretical energy density of 248 Wh/kg.
Due to the urgent demand for multi-dimensional, new-type and economical large-scale energy storage devices, room temperature sodium ion batteries have gradually become a research hotspot The number of published SCI documents has rapidly increased to more than 300 per year. The performance of sodium ion batteries is closely related to cathode materials, anode materials, electrolyte materials, etc. Therefore, the development of electrode materials with high specific capacity and long life is the first consideration. For the positive electrode material, the polyanionic material exhibits exbatteryent cycle stability, while the Prussian blue material has an outstanding cost advantage. In contrast, the transition metal oxide gives the highest theoretical specific capacity, so each material attracts Many researchers. However, for the negative electrode material, although many work has examined the electrochemical properties of the metal oxide material and the alloy material, at present, only the hard carbon material has the reversible specific capacity and cycle life satisfying the industrialization requirements. On the other hand, research based on electrode materials is not enough to ensure the electrochemical performance of the whole battery. The development of a stable non-aqueous electrolyte is also the key to achieving high-performance full-battery construction, which determines the cycle life and safety of the whole battery. Several studies have shown that the diffusion of sodium ions in the electrode material of sodium ion battery may be superior to that of lithium ion due to the weaker action of carriers and guest. It provides a practical performance of sodium ion battery system in the future. Life expectancy is better than lithium-ion batteries, which may provide a core competitive advantage for future sodium-ion batteries in addition to low material costs.
Compared with lithium-ion batteries, firstly, the positive electrode material of sodium ion battery utilizes more abundant metal elements such as sodium, iron and manganese while avoiding the consumption of expensive lithium. Secondly, sodium Both the positive and negative materials of the ion battery can use only relatively inexpensive aluminum foil as the current collector, so the sodium ion battery has unique advantages in terms of resources and cost. However, current sodium ion batteries, especially those based on organic electrolytes, have the same basic structure as conventional non-aqueous electrolyte lithium-ion batteries, and do not stand out in terms of separators, current collectors, binders, and battery structures. In the case of advantages, in the overall cost of the battery, it is unrealistic to surpass the lithium-ion battery supported by the mature industry chain in the short term. But on the other hand, the author noticed that the cost of lithium-ion batteries in the early industrialization in the 1990s was extremely high. However, with the development of battery technology, the performance of lithium-ion batteries has been increasing and the cost has been decreasing. Current high-performance secondary batteries have been able to support electric vehicles for years to more than a decade. The current mature lithium-ion battery industry chain can provide a lot of ready-made materials to support the development of sodium-ion batteries. Under the premise of strengthening the exploration of new electrode materials and systems involving sodium ions, sodium-ion battery technology has achieved rapid development. Obtaining a mature product can be expected.
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