Technical introduction: the past and present of sodium electricity
Basic concepts and historical background: the “twin brother” of lithium batteries
In recent years, the development of clean energy has become the consensus of most countries in the world. my country has even put forward the grand goal of “carbon peaking and carbon neutrality”. Clean energy power generation technologies such as solar energy, wind energy and tidal energy have been developed rapidly. It has the characteristics of intermittent, random, and strong geographical dependence. In order to solve the time and space limitations of new energy power generation and improve the utilization rate of new energy, the importance of energy storage technology has become increasingly prominent. According to the conversion and storage methods of electrical energy, energy storage technologies are divided into physical energy storage, chemical energy storage and electrochemical energy storage. Among them, electrochemical energy storage includes secondary battery technology and supercapacitors, which have the characteristics of high energy conversion efficiency and fast response speed. Especially the secondary battery technology also has the advantages of high energy density and easy modularization.
Secondary battery, also known as rechargeable battery or accumulator, is a device that utilizes reversible chemical reactions and can be repeatedly charged and discharged to convert electrical energy and chemical energy into each other to achieve energy storage. The ability of a secondary battery to store energy is expressed by energy density (also called specific energy), that is, the total energy that can be output by the battery per unit mass or volume, which is the product of the specific capacity and the average discharge voltage. The specific capacity is theoretically determined by the molar mass of the substances participating in the electrode reaction and the number of gained and lost electrons. Therefore, the greater the charge-to-mass ratio of the charge carrier, the greater the theoretical specific capacity of the battery. Theoretically, the discharge voltage is mainly determined by the potential difference and internal resistance of the positive and negative materials. Therefore, the higher the positive electrode potential, the lower the negative electrode potential, and the smaller the battery internal resistance, the greater the discharge voltage. Second, the charge carrier must have good transport capacity and kinetic activity, which directly affects the rate capability and power density of the battery. Finally, factors such as the reversibility of electrode reactions and side reactions determine the cycle performance and lifetime of secondary batteries. Alkali metals represented by lithium have the lowest redox electrode potential, large ion charge-to-mass ratio and low desolvation energy, so they have been tried as anode materials for secondary batteries as early as the 1960s. Early lithium-ion batteries used metal lithium or lithium alloy as the negative electrode, and transition metal halides (such as AgCl, CuCl, NiF2, etc.) as the positive electrode, but such positive electrode materials have poor conductivity, easy to dissolve, and the volume of charge and discharge changes drastically, and it is difficult to solve. In the late 1960s, transition metal-chalcogenide compounds represented by TiS2 were found to have interlayer intercalation and deintercalation capabilities, which can be used as cathode materials for lithium-ion batteries, and have high electrical conductivity and electrochemical reactivity. 2.2 V, with practical value. However, the high activity of metal lithium makes the battery accident frequent, forcing people to use lithium intercalation compounds (such as lithium intercalation graphite) as the negative electrode. This is the concept of “rocking chair battery”: using low intercalation potential compounds as negative electrodes, high The intercalation potential compound acts as the positive electrode, avoiding the problem of alkali metal dendrites. Since the potential of the negative electrode of the lithium intercalation compound is higher than that of the metal lithium, the overall voltage and energy density of the battery are reduced, which forces people to search for new positive electrode materials, and has successively discovered positive electrode materials such as lithium cobaltate, lithium manganate and lithium iron phosphate. .
The cost and rate capability of sodium ion battery has advantages over lithium ion battery. Sodium and lithium are in the same family in the periodic table, have the same number of valence electrons, and have more active chemical properties. Because the atomic mass and radius of sodium are much larger than those of lithium, the energy density of sodium ion battery is obviously difficult to compare with lithium ion batteries, but the natural abundance of sodium is abundant. The degree of density is more than a thousand times that of lithium, and the desolvation energy of sodium ions is much lower than that of lithium ions. The sodium ion battery came out in the 1970s almost at the same time as the lithium ion battery, but the research process of the two is slightly different. The sodium secondary battery that first appeared at that time was a sodium-sulfur battery, with elemental sulfur and metallic sodium as the positive and negative electrodes, β-alumina fast ion conductor as the solid electrolyte, and the working temperature was 300~350 °C. This high-temperature sodium-sulfur battery has a high energy density (150~240Wh/kg) and a cycle life of 2500 times, while the similar lithium-sulfur battery has a cycle life of less than 10 times. In order to improve the safety of sodium secondary batteries, room temperature sodium ion batteries have been developed, using a similar idea to lithium ion batteries, the cathode material has undergone layered transition metal sulfide (TiS2) to layered oxide (NaxCoO2) ) and phosphate (Na3M2(PO4)3, M is a transition metal). But in the late 1980s, the research on sodium ion battery was cold, and related research almost stopped. There are three reasons for this: first, it is difficult to find a suitable anode material (graphite that can store lithium efficiently in ester solvents is difficult to store sodium); second, the research conditions are limited (the water and oxygen content of the system is high, and it is difficult to use metal Sodium was used as the benchmark electrode for material evaluation experiments); third, lithium-ion batteries came out on top (a large number of researchers anchored the direction on lithium-ion batteries).
Until the 21st century, the sodium ion battery ushered in a turning point. In 2000, it was found that the hard carbon material obtained by the pyrolysis of glucose has a specific sodium storage capacity as high as 300 mA h/g, which provides a crucial anode material for sodium ion battery. In 2007, the polyanion cathode material Na2FePO4F was found, and the volume deformation rate of this material was only 3.7%, with almost no strain. From 2000 to 2010, the research speed of sodium ion battery was relatively slow, mainly concentrated in a few experimental teams. After 2010, the research on sodium ion battery has entered the spring, and new material systems have been emerging, and they are gradually trying to industrialize.
Working principle and materials: similar to lithium batteries
The working principle of sodium ion battery is exactly the same as that of lithium ion battery, that is, under a certain potential condition, the reversible desorption and intercalation of guest alkali metal ions in the host material, in which the higher intercalation potential is used as the positive electrode, and the lower intercalation potential is used as the positive electrode. The negative electrode, the charging and discharging cycle process of the whole battery is the round-trip directional migration process of alkali metal ions between the positive and negative electrodes. The battery with this working mechanism is the “rocking chair battery” proposed by M. Armand. The composition structure of sodium ion battery is exactly the same as that of lithium ion, mainly including positive electrode, negative electrode, electrolyte, separator and current collector. According to whether the material host directly participates in the electrochemical reaction process, they can be divided into active materials and inactive materials.
Active materials: positive electrode, negative electrode, electrolyte
The active materials of sodium ion battery include positive electrode material, negative electrode material and electrolyte material, which directly participate in the electrochemical reaction and thus determine the intrinsic characteristics of the battery. Since the radius and electronic structure of sodium ions are quite different from those of lithium ions, the thermodynamics and kinetic behaviors of the reactions are quite different, so the research and development of active materials for sodium ion batteries cannot fully imitate lithium ion batteries.
(1) Cathode material: oxide, Prussian blue, polyanion three main lines
The positive electrode material undergoes an oxidation reaction during charging and a reduction reaction during discharge, and generally has a high reduction potential. The ideal cathode material should meet the requirements of high reduction potential (but must be lower than the oxidation potential of the electrolyte), large reversible capacity, stable cycle performance, high electronic and ionic conductivity, stable structure and not afraid of air, high safety, and low price. For sodium ion batteries, the theoretical specific capacity of the existing cathode materials is relatively low, so it becomes one of the main determinants of the overall capacity of the battery. At present, the cathode materials of sodium ion batteries are mainly divided into five types: oxides, polyanions, Prussian blue, fluorides, and organic compounds. The first three types have the highest maturity and have entered the early stage of industrialization. .
Oxides: the most mature technology, high specific capacity, and the most abundant types
Oxide-based cathode materials are generally transition metal oxides, mainly including layered oxides and tunnel oxides. The research on layered oxides is the earliest and most extensive. Compared with lithium-ion batteries, layered oxide cathodes with only three elements, Mn, Co, and Ni, have reversible electrochemical activity. Na-ion batteries have a wider selection range. The fourth cycle The transition metals from Ti to Ni have high activity, and their working mechanisms are more complicated, often accompanied by multiple phase transition behaviors. The general formula of layered oxides is NaxMO2, where M is a transition metal. The common structural types mainly include O3 type and P2 type. The former has higher specific capacity but poor rate and cycle performance; the latter has better rate and cycle performance. , but the actual specific capacity is slightly lower. In addition, layered oxides tend to be susceptible to hygroscopic hydrolysis in air. At this stage, layered oxides have high energy density and mature preparation technology, and are expected to take the lead in industrialization, especially P2-type layered oxides with better stability. Tunnel-type oxides have a three-dimensional pore structure and are often found in oxides with low sodium content. They have excellent cycling and rate performance and are stable to water and oxygen, but their specific capacity is too low. In the future, tunnel oxides may have potential competitive advantages in the research and development of sodium-rich cathodes and aqueous sodium ion batteries.
Prussian blue: low material cost, high specific capacity, high technical barriers
Prussian blue cathode materials are transition metal cyanide coordination polymers with the general formula AxM1 [M2 (CN)6]1-y·□y·nH2O, where A is an alkali metal ion, M1 and M2 are transition metal ions (coordinated to N and C, respectively), □ is a [M2(CN)6] vacancy defect. Due to the unique electronic structure of cyanide double coordination and the open three-dimensional space, the material has the advantages of stable structure, fast intercalation and deintercalation rate, and large specific capacity. In addition, the core transition metals of such materials are mainly cheap metals such as Fe and Mn, and the raw materials are readily available and low in cost. However, in practical applications, the lattice water content (including crystal water and adsorbed water) and vacancy defect density of the material will seriously restrict the battery performance, resulting in the reduction of its capacity utilization, energy efficiency and cycle life. It is worth mentioning that recently researchers found that the sodium ion battery using Prussian blue cathode material will release highly toxic hydrogen cyanide and cyanide gas under thermal runaway conditions, and the thermal decomposition mechanism is closely related to lattice water and vacancy defects. Closely related, it can be seen that this technology has higher requirements on material quality. In addition, the preparation of this material involves highly toxic sodium cyanide, which requires special qualifications in production and supply.
Polyanions: the best safety, too low specific capacity, high material cost
Polyanion-based cathode materials refer to sodium-containing double salts whose crystal framework is constructed by a series of tetrahedral and polyhedral anion units, with the general formula NaxMy(XaOb)zZw. , where M is a cation such as transition metal or alkaline earth metal, X is a highly electronegative element such as phosphorus or sulfur, and Z is an anion such as fluorine or hydroxide. The anionic polyhedral units of this type of material have strong covalent bonding, so the crystal structure is very stable, and its chemical stability, thermal stability and electrochemical stability are high, so it has good cycle life and safety. , and its voltage platform tends to be wide. Secondly, the valence electrons of transition metal ions have a high degree of localization, and this electronic structure can easily utilize the inductive effect of strongly electronegative elements to improve the working voltage of the material. However, due to its wide-bandgap characteristic, the intrinsic electronic conductivity is very low, which greatly limits its rate capability and must be modified by adding conductive agents or nanoscale. In addition, the specific capacity of this material is generally low. At present, the most typical polyanionic materials are mainly phosphates, represented by olivine-type NaFePO4 and NASICON-type Na3V2(PO4)3. The structure of NaFePO4 is the same as that of lithium iron phosphate, but the synthesis must rely on a complex ion exchange method, and the cost is high. Na3V2(PO4)3 has excellent rate performance and cycle life, but the specific capacity is lower than that of olivine-type materials. In addition, new polyanionic materials such as pyrophosphate, sulfate, and molybdate are also under study. These systems have improved operating voltage and rate performance, but there are still many problems such as low actual specific capacity and poor cycle reversibility. defect.
Fluorides: relatively cheap materials, difficult to practical
Transition metal fluorides have high reduction potentials similar to oxides, and the intercalation and deintercalation of sodium ions can be achieved through the valence conversion of transition metal ions, so they are also potential cathode materials. The biggest problem of this type of material is that the resistivity is too high, which seriously affects its rate performance, and the actual specific capacity is generally low. So far, the fluoride materials with larger specific capacity are iron-based fluorides, typically represented by NaFeF3 (actual 128mAh/g, theoretical 197mAh/g). In addition, some hydrated iron fluoride materials have high specific capacity, such as Fe2F5 H2O (initial 251 mAh/g), but the cycle performance is still poor.
Organic compounds: not dependent on mineral resources, still in the research stage
Certain organic compounds with abundant conjugated systems and lone pairs of electrons can undergo reversible redox reactions, so they can also be used to develop cathode materials. The advantages of this type of material are that it does not need to rely on transition metal resources, and its structure and properties are easy to design and control, so it has certain potential. However, there are still significant defects at this stage: the conductivity is generally low, and it is prone to dissolution. At present, there are mainly conjugated system conductive polymers (such as modified polyaniline, polypyrrole, etc.), conjugated carbonyl compounds (such as aromatic derivatives of sodium phenate, sodium carboxylate) and the like.
(2) Anode materials: carbon-based materials are the most mature and are expected to take the lead in industrialization
The negative electrode material undergoes a reduction reaction during charging and an oxidation reaction during discharge, and generally has a lower reduction potential. The ideal cathode material should meet the requirements of low reduction potential (but must be higher than the deposition potential of metallic sodium), large reversible capacity, stable cycle performance, high electronic and ionic conductivity, stable structure and not afraid of air, high safety, and low price. For a sodium ion battery, the negative electrode material plays an important role in loading and releasing sodium ions, which directly affects the overall dynamic performance of the battery, such as rate performance, power density, etc. At present, the anode materials of sodium ion battery are mainly divided into five types: carbon-based materials, titanium-based materials, alloy materials, organic compounds, and other systems. Among them, carbon-based materials have the highest technological maturity and are rich in resources, and are expected to take the lead in realizing industrialization. change.
Carbon-based materials: soft carbon and hard carbon have their own merits, and graphite negative electrodes are still under study
According to the microstructure of carbon atoms, carbon-based anode materials are divided into graphite-based materials, amorphous carbon materials, and nano-carbon materials. Different from other alkali metal ions, it is difficult for sodium ions to effectively intercalate between graphite layers in carbonate solvents, which is mainly due to the ΔG>0 of the sodium ion-graphite intercalation reaction. Therefore, the graphite anode, which is widely used in lithium-ion batteries, is difficult to use in sodium-ion batteries with carbonate as a solvent. In fact, in ether solvents, graphite can also effectively insert and remove sodium ions, but the stability of the electrolyte is weakened, and it is easy to react with the positive electrode, which needs further study. Amorphous carbon materials have high specific sodium storage capacity and are also the anode materials that are closest to industrialization at present. According to the difficulty of heat treatment graphitization, it is divided into soft carbon and hard carbon. Soft carbon can be completely graphitized at temperatures above 2800 °C, and hard carbon is also difficult to graphitize at high temperatures. The difference between soft and hard carbon lies in the cross-linking interactions of the carbon layers in the microstructure, which is fundamentally dependent on the structure and shape of the carbonization precursor used. Generally speaking, thermoplastic precursors (petrochemical raw materials and by-products) tend to form soft carbon, while thermosetting precursors (biomass, resin polymers, etc.) tend to form hard carbon. Relatively speaking, the manufacturing cost of soft carbon is lower, the process is easy to control, but the specific capacity is not as good as that of hard carbon; the specific capacity of hard carbon is higher, but the efficiency of the first cycle is often lower, and its performance depends on the precursor used and the treatment. process, the carbon yield is low. It is worth mentioning that the sodium storage mechanism of hard carbon materials is still not completely understood, and there is still much room for improvement. Carbon nanomaterials mainly include graphene and carbon nanotubes, and sodium ions are mainly stored on its surface and defects by adsorption. The theoretical specific capacity of these materials is large, but the first week Coulombic efficiency is low, the reaction potential is high, and the price is high. expensive.
Titanium-based materials: unique potential advantages, difficult to commercialize in the short term
The reduction potential of tetravalent titanium is generally low, its compounds are air-stable, and titanium compounds with different crystal structures have different sodium storage potentials, so they are used to develop anode materials. At present, titanium-based materials are mainly some titanium oxides and polyanionic compounds. Oxides include layered Na2Ti3O7, Na0.6[Cr0.6Ti0.4]O2 and spinel-type Li4Ti5O12 (also used in lithium-ion battery negative electrodes), etc. Polyanionic compounds include orthogonal NaTiOPO4, NASICON type of NaTi2(PO4)3. The specific capacity of these materials is generally not high, but they have many unique advantages. For example, Li4Ti5O12 is a strain-free material, Na0.6[Cr0.6Ti0.4]O2 can act as both positive and negative materials, and NaTi2(PO4)3 can For water-based sodium ion battery.
Alloy materials: huge theoretical specific capacity, technical difficulties to be overcome
Metal sodium can form alloys with Sn, Sb, In and other metals, and can be used as the negative electrode of sodium ion battery, which is similar to the silicon-based negative electrode of lithium ion battery. The advantages of this type of material are high theoretical specific capacity and low reaction potential, so it is expected to manufacture sodium ion batteries with high energy density and high voltage. However, the reaction kinetics of these materials are poor, and the volume change before and after sodium de-intercalation can reach several times. With the huge stress, the active material is easy to fall off the surface of the current collector, and the specific capacity decays rapidly.
Organic compounds: mild synthesis conditions, still in the research stage
The advantages and disadvantages of organic anode materials are similar to those of organic cathode materials. The current types mainly include carbonyl compounds, Schiff base compounds, organic radical compounds and organic sulfides, which are still in the laboratory research stage.
Other systems: V and VI compounds, mostly transition metals, are still in the research stage
Some transition metal oxides, sulfides, selenides, nitrides, and phosphides also have electrochemical activity for reversible sodium storage. Such materials are often accompanied by conversion reactions and alloying reactions, so their theoretical specific capacity can exceed the corresponding Alloy anode materials, but also more technical problems.
(3) Electrolyte material: mainly liquid electrolyte, the form is the same as that of lithium battery
Electrolyte is a bridge for material transfer between positive and negative electrodes. It is used to transport ions to form a closed loop. It is an important guarantee for maintaining electrochemical reactions. It not only directly affects the rate, cycle life, self-discharge and other performance of the battery, but also determines the stability and safety of the battery. one of the core elements of sexuality. According to the physical form, the electrolyte of sodium ion battery can be divided into liquid electrolyte and solid electrolyte.
Liquid electrolyte: similar to lithium batteries, lithium salts become sodium salts
Liquid electrolytes are often referred to as electrolytes and generally consist of solvents, solutes and additives. Since the upper limit of the electrochemical window of water does not exceed 2V, the solvent is some polar aprotic organic solvent, which can not only dissolve a large amount of sodium salts, but also cannot release proton hydrogen, and also has a certain anti-oxidation-reduction ability. Has a lower viscosity. Therefore, carbonates with high dielectric constant and high viscosity are generally used in combination with ethers with low dielectric constant and low viscosity, so the electrolyte is highly flammable. Solutes are mainly sodium salts with large radius anions, which are divided into inorganic sodium salts and organic sodium salts. Sodium salts, etc. In general, organic sodium salts are more stable, while inorganic sodium salts are less expensive. Currently expected to achieve industrial application is mainly sodium hexafluorophosphate, which has relatively best conductivity, but is highly sensitive to water. The content of additives in the electrolyte is less than 5%, mainly some compounds such as sodium salts, esters, nitriles, ethers, etc., which play a role in assisting the formation of SEI film and CEI film, overcharge protection, and flame retardant.
Solid-state electrolyte: for solid-state sodium electricity, still in the research stage
Solid-state electrolyte materials mainly include three types: inorganic solid-state electrolytes, polymer solid-state electrolytes, and composite solid-state electrolytes. Due to the avoidance of flammable and explosive organic solvents, the safety of the battery has been substantially improved, and the electrochemical window has been greatly broadened, making it possible to use high-potential cathode materials and metal sodium anodes, thereby greatly improving the energy density of the whole battery. . In addition, due to the rigid solid electrolyte barrier between the positive and negative electrodes, a separate separator is no longer required, and with the bipolar electrode process, the system energy density of the battery can be further improved. Such materials are currently facing problems such as low room temperature conductivity and high interface impedance, and their industrialization will take time.
1.2.2. Inactive materials: diaphragms, current collectors, conductive agents, binders
The inactive materials in the sodium ion battery mainly include diaphragms, current collectors, conductive agents, binders, etc. They do not directly participate in the electrochemical reaction, but are essential auxiliary materials, and their compatibility with active materials and other factors will have a significant impact on battery performance.
(1) Diaphragm: common to lithium-ion batteries
The function of the separator is to physically separate the positive and negative electrodes to avoid direct contact and reaction between the two, and at the same time to ensure the infiltration and penetration of solvent molecules, allowing the rapid passage of solvated sodium ions. The ideal separator material should have good electronic insulation and ionic conductivity, high mechanical strength and as thin as possible, high chemical inertness (neither react with electrolyte, nor react with positive and negative electrodes), and good thermal stability sex. Polyolefin polymer separators, such as PE, PP, and composite films, are widely used in lithium-ion batteries, and these separator materials can be directly transplanted into the sodium ion battery system. In the future, in the all-solid-state sodium ion battery system, the diaphragm material will no longer be required.
(2) Current collector: both positive and negative electrodes are made of aluminum foil
The current collector is the base member to which the positive and negative active materials are attached, accounting for about 10-13% of the weight of the battery, and is used to collect the current generated by the electrode material and release conduction to the outside. Although the current collector does not participate in the electrode reaction, it is the fundamental guarantee for the performance of the electrode material, and its purity, thickness, stress and other parameters indirectly affect the actual working performance of the electrode. Materials used as current collectors must have excellent electrical conductivity, low contact resistance with active materials, high chemical inertness (not to react with electrolyte and positive and negative electrodes), good processability and stable mechanical properties. In lithium-ion batteries, the positive electrode current collector is aluminum foil, and the negative electrode current collector is copper foil to avoid the alloying of aluminum and lithium under low potential conditions. In sodium-ion batteries, since sodium and aluminum do not undergo an alloying reaction, aluminum foils can be used for both positive and negative current collectors, avoiding relatively expensive copper foils.
(3) Conductive agent: same as lithium ion battery
When the electrode material is actually used, it is also necessary to add a conductive agent, which has three main functions: reducing the self-polarization of the electrode material, reducing the contact resistance between the active material particles and between the current collector, adsorbing the electrolyte and improving the infiltration of the electrode Effect. Commonly used conductive agents are carbon materials with large specific surface area and good conductivity, such as carbon black, graphite powder, carbon nanotubes, and graphene.
(4) Binder: same as Li-ion battery
The function of the binder is to combine the electrode material, the conductive agent and the current collector to make a complete pole piece that can be used. The material used as a binder must have good stability, be easy to process, and be low in cost. Commonly used binders for sodium ion batteries are similar to lithium ion batteries, mostly strong polar polymers, such as polyvinylidene fluoride (PVDF), sodium alginate (SA), polyacrylic acid (PAA), sodium carboxymethyl cellulose ( CMC), polytetrafluoroethylene (PTFE), etc.
Manufacturing process and route: in the same vein as lithium batteries
1.3.1. Electrode material synthesis: only Prussian blue is special
The synthesis method of sodium ion battery cathode material should be determined according to the specific material category, which is mainly divided into solid-phase reaction method and liquid-phase synthesis method. Oxide and polyanion materials can be synthesized by either solid-phase reaction method or liquid-phase synthesis method. The synthesis process is basically the same as that of the corresponding materials for lithium-ion batteries, so the production line can be compatible to a certain extent. At present, the solid-phase reaction method is the most widely used in industry. The uniformity of the product prepared by this method has certain limitations, but the operation is simple and the technological process is short, which is suitable for large-scale production. The liquid phase synthesis method has high product uniformity, but is relatively expensive, requires high equipment, and has a lot of waste water. In addition, there are technologies such as sol-gel method, microwave synthesis method, spray drying method, ion exchange method, etc., which generally have high cost and are not suitable for industrial production for the time being.
1.3.2. Batteries are assembled into groups: the assembly process and appearance classification are the same as those of lithium-ion batteries
Similar to lithium-ion batteries, the production of sodium ion batteries also undergoes processes such as pulping, coating, assembly, liquid injection, and chemical formation. Among them, the assembly process is mainly to combine the completed positive and negative plates through the diaphragm interlayer to establish the sodium ion path inside the battery, and isolate the positive and negative electrodes to prevent internal short circuits. The assembly process follows the lithium-ion battery technology and is divided into winding and lamination processes. The former is further divided into cylindrical winding and square winding. In addition, the structural design and packaging process of sodium ion battery products basically follow the lithium-ion battery, and the appearance is roughly divided into three categories: cylindrical, soft pack and square hard shell, each with its own advantages and disadvantages.
Horizontal comparison: sodium battery vs lithium battery, liquid flow, lead acid
As the industrialization of sodium ion battery advances, it is bound to have varying degrees of impact on other secondary battery technologies. The first to bear the brunt is lithium-ion batteries, as well as flow batteries and lead-acid batteries that have long been widely used in the market. In this section, we briefly predict the future competitive landscape of sodium ion battery through the horizontal comparison between sodium ion battery and the above three battery technologies.
Sodium battery vs lithium battery: performance comparable to lithium iron phosphate, comprehensive cost performance or higher
Sodium ion battery is a supplement and extension of lithium ion battery, not a complete replacement relationship. First of all, in terms of performance, the existing lithium-ion battery system is not perfect: ternary cathode batteries have high energy density, but poor cycle life; lithium iron phosphate cathode batteries have high cycle life, but low energy density; lithium manganate cathode batteries The working voltage is high, but the energy density and cycle life are poor. In addition, lithium-ion batteries are prone to severe capacity fading at low temperatures, requiring a temperature control system, which consumes at least 5% of the energy of the battery system and increases the manufacturing cost. In contrast, the energy density of the existing sodium-ion battery system has approached that of lithium iron phosphate; although the cycle life is not as good as that of lithium iron phosphate, it is significantly better than ternary materials and lithium manganate.
Secondly, from the perspective of safety, since the initial temperature of thermal runaway of the sodium ion battery is slightly higher than that of the lithium ion battery, the safety at the cell level has been improved, but both batteries need to use highly flammable organic electrolytes , there is a risk of deflagration under thermal runaway conditions. From the current destructive experiments such as cell puncture, the actual safety of sodium ion battery may be similar to that of lithium iron phosphate battery.
Finally, from the perspective of cost, sodium ion battery can effectively reduce the cost of raw materials. First, the lithium compounds in the active materials (cathode, electrolyte) are replaced by sodium compounds as a whole, and cheap metals such as iron and manganese have largely replaced the more expensive metals such as cobalt and nickel in the cathode; second, metal sodium does not form with metal aluminum. Eutectic alloy, both positive and negative current collectors can be made of cheap aluminum foil, replacing the more expensive copper negative electrode current collectors in the original lithium-ion battery; thirdly, because the Stokes radius of sodium ions is smaller than that of lithium ions, so The amount of solute in the electrolyte can be greatly reduced. In the future, sodium ion battery is likely to form a strong competitive relationship with lithium iron phosphate batteries, especially in alpine regions; lithium ion batteries will continue to develop in the direction of high energy density and high working voltage, and gradually develop into all-solid-state batteries, etc. New technology iterations.
Sodium electricity vs liquid flow: the advantages and disadvantages are highly complementary, or stand side by side in the energy storage market
Sodium ion battery and flow battery have strong complementarity, the former is suitable for small and flexible energy storage, and the latter is suitable for large and medium-scale energy storage. A flow battery is a liquid-phase (mainly water-phase system) electrochemical energy storage device, which is characterized in that the active working material is dissolved in the electrolyte, and the energy storage and release are realized by changing the oxidation valence state of the active material. Representatives include all-vanadium flow batteries, iron-chromium flow batteries, and zinc-bromine flow batteries. The biggest advantage of the flow battery lies in the intrinsic safety of its water-phase system and its ultra-long cycle life, which is especially suitable for medium and large electrochemical energy storage facilities, but the disadvantages are low energy density and narrow operating temperature range, so It is difficult to miniaturize or apply to alpine regions. In contrast, the energy density of a sodium ion battery is about three times that of a flow battery, and it can withstand a low temperature of -40 °C, but its intrinsic safety and cycle life are not as good as a flow battery. In the future, sodium ion battery and flow battery are expected to complement each other in the field of energy storage. For example, household and mobile small energy storage devices have higher requirements on energy density and are suitable for the use of sodium ion batteries; large and medium-sized electrochemical energy storage power stations have higher requirements for safety and are suitable for the use of flow batteries.
Sodium battery vs lead acid: gradually replace traditional lead acid, forcing the latter to iteratively upgrade
Sodium ion battery is expected to gradually replace traditional lead-acid batteries, forcing the development of new technologies such as lead-carbon batteries. The industrial application of lead-acid batteries has been more than a century and a half, and its industrial closed loop of “production-consumption-recycling” has been highly complete. The advantages are low cost, easy recycling, and good safety. The disadvantages are low energy density, short cycle life, Charging takes a long time. At present, lead-acid batteries are still being continuously developed and upgraded. The most representative one is the “lead-carbon battery” that integrates supercapacitor technology. Its cycle life is as high as 3,000 times, it has fast charging capability, and retains the characteristics of the original lead-acid battery. Safety and other advantages, but the energy density is further reduced, and the manufacturing cost is also increased accordingly. In contrast, most of the performance of sodium-ion batteries is better than that of traditional lead-acid batteries. In the future, as the cost is further reduced, it is expected to gradually replace traditional lead-acid batteries. At the same time, the rise of sodium ion battery may indirectly accelerate the process of upgrading and iterating from traditional lead-acid batteries to lead-carbon batteries. In the future, lead-acid batteries may be reborn in the form of lead-carbon batteries, rather than completely withdraw from the historical stage. (Report source: Report Institute)
Sodium battery Industry status
At present, there are nearly 30 companies involved in the sodium ion battery industry worldwide. Since the pros and cons of technical routes are still inconclusive, and there is no unified standard, the competition of different enterprises is essentially the competition of different technical routes. Although the research history of sodium ion battery lasts for half a century, its real rapid development has been in the last decade, thanks to important breakthroughs in the research and development of electrode materials. We believe that the industry will still be in the transition stage from the introduction period to the growth period in the next 3 years.
Industrial structure: similar to lithium-ion batteries
The sodium ion battery industry chain is similar to lithium-ion batteries, including upstream, midstream and downstream. Upstream: supply of raw materials and synthesis of electrode materials, the main raw materials include soda ash, aluminum foil, manganese ore, etc., as well as various auxiliary materials, involving basic chemicals and non-ferrous metals and other industries. Midstream: cell packaging, battery system construction and integration, etc., involving various consumables and electronic components. Downstream: End-use markets, mainly including energy storage and low-speed electric vehicles.
Major companies: There are more than 20 companies in the world, and Chinese companies dominate
Sodium ion battery-related companies at home and abroad were established (or entered the field) after 2010. At present, there are more than 20 related companies in the world, mainly located in China, the United States, Europe and Japan. Most of them are start-up companies. Technology research and development and strategic layout are the main ones, and the scale has not yet been formed.
3.2.1. Domestic: Zhongke Haina has accumulated a lot, and the Ningde era has taken the lead
my country’s sodium ion battery research and industrialization lead the world in the world. Domestic sodium ion battery enterprises can be divided into two categories: one is a start-up enterprise created by the self-developed technology of scientific research institutes, represented by Zhongke Haina; the other is a Mature lithium-ion battery companies have entered the sodium ion battery track to participate in the competition, represented by the Ningde era.
Founded in 2017, Zhongke Haina is the first high-tech enterprise in China that focuses on the development and manufacture of sodium ion batteries. It was incubated by the Institute of Physics, Chinese Academy of Sciences and has a R&D team led by Academician Chen Liquan and Researcher Hu Yongsheng. , is one of the few battery companies with core patented technologies in all fields of sodium ion battery, has launched a number of demonstration projects, and started the construction of the first 1GWh scale production line. As a technology enterprise incubated by the Institute of Physics, Chinese Academy of Sciences, Zhongke Haina has strong innovation capabilities, and has mastered all fields from basic research and development of active materials to scaled production, from materials to batteries, from single cells to battery modules, and from battery components to applications. technology.
Founded in 2017, Ben’an Energy is a high-tech multinational enterprise mainly engaged in the research and development and production of sodium ion batteries for water systems. The company has global R&D centers in Singapore, China and the United States to carry out research and development of materials, cells and structures for water-based sodium ion batteries; regional companies in China, the United States and Australia are responsible for battery material manufacturing and regional market business. The company focuses on the application field of stationary energy storage. The products have the characteristics of high intrinsic safety, environmental protection and non-toxicity. They are especially suitable for energy storage power stations in densely populated urban areas, and are also suitable for indoor environment layout; they are also suitable for long-term floating operation. , can be widely used in industrial backup power system.
3.2.2. Abroad: Most of them are start-ups, with small scale and strong forward-looking
Foreign sodium ion battery enterprises are mainly located in developed countries such as Europe, America and Japan. Various material systems and technical routes are adopted by companies. Most of these enterprises have a relatively short establishment time and limited production scale, but their technology is very forward-looking.
The British company Faradion mainly pushes layered oxide cathodes. Founded in 2011, the company is the world’s first commercialized sodium ion battery enterprise, and currently holds 31 sodium ion battery patents, covering battery materials, battery infrastructure, battery safety and transportation, etc. The company places great emphasis on product cost and energy density, with the ultimate goal of delivering lithium-ion performance at a lead-acid price. At the end of 2021, the company was acquired by India’s Reliance New Energy Solar Limited (RNESL) for £100 million, which will also invest £25 million as growth capital to accelerate the commercial rollout of the sodium ion battery.
The Natron Energy company in the United States mainly pushes the sodium ion battery of the water system. Founded in 2012, the company is an enterprise that develops and produces water-based sodium ion batteries, mainly promoting Prussian blue cathode materials. The company attaches great importance to the safety of sodium ion batteries, and does not use organic solvent electrolytes. Its products are extremely safe and have a long cycle life. The power density is only slightly lower than that of lithium ion batteries, but the energy density is only comparable to that of lead batteries, mainly for static electricity. Energy storage applications (fast charging stations for data centers, forklifts and electric vehicles). Currently, its sodium ion battery has been initially commercialized, with a pilot production line operating in Santa Clara, California. The next goal is to expand production and form a sodium ion battery industry chain.
Future development: give full play to the resource endowment and comparative advantage of sodium electricity
Current problems: poor materials, high costs, undetermined standards
4.1.1. Materials research needs to be furthered: hard carbon mechanism, performance improvement, safety assessment
At present, there are still many controversies about the sodium storage mechanism of hard carbon in academia, and it has not been fully elucidated. In order to improve the defects of the existing hard carbon anode, such as the low first cycle efficiency, it is necessary to deeply understand the kinetic mechanism of its sodium storage and provide the most fundamental theoretical guidance for technology research and development. There is still much room for improvement in the material properties of the existing sodium ion battery. In general, the energy density of the current sodium ion battery is far from the theoretical value, and its cycle performance also needs to be further improved. On the one hand, continuous improvement of active materials is required. On the other hand, its overall system design and integrated management also need to be considered. The actual operational safety of the sodium ion battery requires careful evaluation. At present, the safety test experiment of sodium ion battery is at the cell level. The results show that although the safety is high, the safety after actual operation needs to be observed urgently, and it is not advisable to be blindly optimistic. In particular, the Prussian blue positive electrode will release highly toxic gases such as hydrocyanic acid and cyanide in the event of thermal runaway.
4.1.2. The cost advantage remains to be realized: technological R&D and economies of scale are indispensable
The cost reduction of sodium ion battery depends on the reduction of variable costs through continuous technology iteration and the dilution of fixed costs through mass production. In theory, sodium ion battery does have a great material cost advantage, but the actual total cost of the current product is more than 1 yuan/Wh, which is higher than that of lithium iron phosphate. scale effect. On the one hand, the types of electrode materials and manufacturing processes are not standardized, and the precursors also lack a stable and reliable supply chain, which leads to low yield and consistency of electrode materials and high actual costs. Technological exploration improvements. On the other hand, the price of production equipment is high and the depreciation loss is large, accounting for about 20~30% of the manufacturing cost, which can only be diluted by mass production.
4.1.3. Technical standards to be formulated: standardize market order and promote healthy development
The sodium ion battery industry needs to establish a scientific unified standard to regulate the production activities of enterprises and promote the healthy and orderly development of the industry. At this stage, the technical routes of manufacturers engaged in the R&D and production of sodium ion batteries are different, and there is great controversy over which one is better. At present, manufacturers mainly refer to lithium-ion batteries, combined with the characteristics of sodium ion batteries and industrial development, to formulate standards or product specifications suitable for their respective companies, and use this to guide product design and manufacturing processes to ensure product yield and consistency. As a result, the product performance and technical level of different enterprises are uneven. The unified formulation of industry technical standards can play a better role in leading the industry and is a necessary guarantee for achieving economies of scale. In particular, safety standards are an important basis for constraining product quality and an important means to regulate market order and promote the healthy and sustainable development of the industry.
Technology Outlook: Enhanced Safety and Higher Specific Energy
4.2.1. Water-based sodium ion battery: intrinsically safe sodium ion battery
Replacing organic electrolytes with aqueous electrolytes can fundamentally improve the safety of sodium ion batteries. The current sodium ion battery continues the organic electrolyte system of the lithium-ion battery, so it is impossible to fundamentally avoid the risk of deflagration. If it is replaced with an aqueous solution, it will not only greatly improve the safety, but also simplify the production process, while reducing the production process. environmental pollution. At present, a large number of water-based sodium ion battery system solutions have been reported, among which the Prussian blue system has the best cycle performance, and industrialization attempts have been started. Representative companies include Natron Energy and Ben’an Energy. In the long run, water-based sodium ion battery is a very promising direction, especially for energy storage.
4.2.2. Solid state sodium ion battery: high energy density sodium ion battery
Replacing liquid organic electrolytes with solid electrolyte materials can produce solid-state sodium ion batteries. Due to the avoidance of flammable and explosive organic solvents, the safety of the battery has been substantially improved, and the electrochemical window has been greatly broadened, making it possible to use high-potential cathode materials and metal sodium anodes, thereby greatly improving the energy density of the whole battery. . In addition, due to the rigid solid electrolyte barrier between the positive and negative electrodes, it is no longer necessary to set up a separate separator, and with the bipolar electrode process, the system energy density of the battery can be further improved. Such materials are currently facing problems such as low room temperature conductivity and high interface impedance, and their industrialization will take time.
4.2.3. Multi-guest co-intercalation anode: graphite as a universal anode
The graphite anode can also achieve effective intercalation and deintercalation of complexed sodium ions in the “multi-guest co-intercalation reaction”. Since ΔG>0 of the sodium ion-graphite intercalation reaction, it is difficult for sodium ions to effectively intercalate between graphite layers in carbonate solvents, so it is difficult to use graphite negative electrodes. In fact, in ether solvents, sodium ions form coordination bonds with ether oxygen atoms, which can be co-inserted between the graphite layers in the form of coordination ions. This “multi-guest co-insertion reaction” has important enlightening significance. On the one hand, this means that the graphite negative electrode may also be used as the negative electrode of the sodium ion battery, thus sharing the material production line with the lithium ion battery, which is conducive to large-scale cost reduction. On the other hand, this opens up the possibility to design a new generation of multi-charged ion batteries. However, ether electrolytes have weak stability and are prone to react with the positive electrode, which requires further in-depth research