Sodium-ion Battery Introduction
Sodium-ion battery is a secondary battery that mainly relies on the movement of sodium ions between the positive and negative electrodes to work. It has a similar working principle to lithium-ion batteries, and both are called "rocking chair" batteries.
The main components of sodium-ion batteries are positive electrode, negative electrode, separator, electrolyte and current collector, among which the structure and performance of the positive and negative electrode materials determine the sodium storage performance of the entire battery. The positive and negative electrodes are separated by a separator to prevent short circuits. The electrolyte infiltrates the positive and negative electrodes as a medium for ion flow, and the current collector plays the role of collecting and transmitting electrons. During charging, Na+ escapes from the positive electrode, passes through the separator and embeds into the negative electrode, so that the positive electrode is in a high potential sodium-poor state and the negative electrode is in a low potential sodium-rich state. The discharge process is the opposite. Na+ escapes from the negative electrode and passes through the separator and re-embeds into the positive electrode material, restoring the positive electrode to a sodium-rich state. In order to maintain charge balance, the same number of electrons are transferred through the external circuit during the charge and discharge process, and migrate between the positive and negative electrodes together with Na+, causing oxidation and reduction reactions at the positive and negative electrodes.
History
Sodium-ion battery development took place in the 1970s and early 1980s. However, by the 1990s, lithium-ion batteries had demonstrated more commercial promise, causing interest in sodium-ion batteries to decline. In the early 2010s, sodium-ion batteries experienced a resurgence, driven largely by the increasing cost of lithium-ion battery raw materials.
Materials
Cathodes
Oxides
Layered oxide NaxMO2 (M is a transition metal element, such as Mn, Ni, Cr, Fe, Ti and V and their composite materials) has high specific capacity and is easy to process and mass produce. It can be divided into single metal oxides, binary metal oxides, ternary metal oxides and multi-metal oxides, and has similarities with lithium batteries in synthesis and battery manufacturing. Among them, the single-layer metal oxide is studied with reference to the lithium battery LiCoO2, but the structure is unstable, while the binary or ternary metal oxides doped with multiple elements can have a higher reversible capacity and better cycle life, but at the same time also increase the cost. It has been widely used in industrial practice. The British Faradion uses Mn-Ni-Ti-Mg quaternary layered oxide positive electrode material. The battery energy density exceeds 160Wh/kg and the cycle life is more than 3000 times. There is space for further improvement in the future.
Prussian blue and analogues
Prussian blue is a transition metal hexacyanoferrate NaxMa[Mb(CN)6] (Ma is Fe, Mn or Ni. Mb is Fe or Mn), with an open framework structure, which is conducive to the rapid migration of sodium ions, has more redox active sites, has a higher theoretical capacity, and has strong structural stability. However, on the other hand, there are many vacancies and a large amount of crystal water in the crystal framework, which will affect the specific capacity and coulomb efficiency of the material, and affect the stability and cycle performance. These shortcomings need to be compensated by technological research and development. The main methods currently include the use of nanostructures, surface coatings, metal element doping, and improved synthesis processes to reduce coordinated water and vacancies.
Oxoanions
The polyanion NaxMy[(XOm)n-]z (M is a metal ion with variable valence such as Fe, V, etc., and X is an element such as P, S, etc.) has a three-dimensional network structure and good structural stability. It also has the advantages of high operating voltage and good cycle performance, but low specific capacity and low conductivity. Currently, it is mainly improved by carbon material coating, fluorination, doping, mixing of different anion groups, sizing nano-sizing and forming a porous structure.
Anodes
The negative electrode material is the carrier of ions and electrons during the charge and discharge process of sodium-ion batteries, and determines the energy storage and release. Carbon-based materials are preferred. Due to the different radii of sodium ions and lithium ions, sodium ions cannot be effectively embedded and released in graphite. If graphite is to be used, the distance between graphite layers must be expanded, which is difficult to achieve with ordinary graphite materials. At present, the negative electrode materials that can be applied to sodium-ion batteries include amorphous carbon, metal compounds and alloy materials. Since most alloy materials have large volume changes and poor cycle performance, and metal compounds have low capacity, amorphous carbon is currently the most mainstream negative electrode material, with a specific capacity of up to 200--450mAh/g. It is divided into hard carbon and soft carbon, and is mainly composed of randomly distributed graphite-like microcrystals, without the long-range ordered and stacked ordered structure of graphite. Soft carbon can be completely graphitized at high temperatures and has excellent conductive properties. The advantages of hard carbon are high sodium storage capacity, low sodium insertion potential, high specific capacity and easy synthesis. Its capacity in sodium batteries (200-450mAh/g) is comparable to the capacity of graphite in lithium batteries (375mAh/g), and it is more widely used.
Electrolytes & Separators
The electrolyte of sodium-ion batteries is very similar to that of lithium-ion batteries. Sodium salts are used instead of lithium salts, such as sodium perchlorate, which has a lower cost than lithium salts. Solvents are divided into aqueous and non-aqueous systems, and most of them use ester organic solvents used in lithium batteries. There is almost no difference in additives compared with lithium-ion batteries. The separator is used to separate the positive and negative electrodes on the one hand, and to form a charge and discharge circuit for ions to pass through on the other hand. The technology of sodium-ion batteries and lithium-ion batteries is similar in terms of separators. The PP/PE separators widely used in lithium batteries can be reused, but sodium-ion batteries use more glass fiber separators, which are lower in cost.
Current Collector
The current collector of sodium-ion batteries uses aluminum foil, which is lower in cost than lithium-ion batteries. The current collector is used to connect the powdered active material, collect and output the current generated by the active material, and input the electrode current to the active material. In graphite-based lithium-ion batteries, because lithium reacts with aluminum to produce alloys, copper foil must be used as the current collector for the negative electrode. In sodium-ion batteries, sodium and aluminum do not react to produce alloys, so aluminum foil can be used for both the positive and negative current collectors.
Sodium-ion Battery Advantages
The abundance of sodium in the earth's crust is 2.3%, ranking sixth among all elements, significantly higher than 0.0017% of lithium. Sodium is widely found on land and in the ocean in the form of salt and is easy to obtain. Currently, the world's proven lithium resource reserves that can be exploited can only satisfy 1.48 billion electric vehicles. As global electrification accelerates, the pressure of lithium resource shortage is further reflected.
After the sodium-ion battery has undergone short-circuit, nail, crush and other tests, there is no fire or explosion. Lithium-ion batteries have the problem of over-discharge, which will cause the copper foil current collector to dissolve, resulting in irreversible loss of battery capacity; while sodium-ion batteries have no over-discharge, and the positive electrode can be discharged to 0V without affecting performance, making the battery safer during storage and transportation. At the same time, sodium batteries are prone to passivation during thermal runaway, so they perform better in safety tests.
The high conductivity of sodium ions requires a lower concentration of electrolyte. The viscosity of the electrolyte at low temperatures is lower than that of lithium-ion batteries, and the overall performance of the battery is better. The normal operating temperature range of sodium-ion batteries is -40℃-60℃, and the capacity retention rate at -20℃ can reach more than 85%, which is significantly better than the 60-70% capacity retention rate of lithium iron phosphate.