In the field of electrochemical energy storage and home energy storage, lithium-ion batteries occupy a dominant position, and China's installed capacity accounts for as high as 91%. However, with the continuous expansion of the lithium battery market, the contradiction of lithium resource shortage has gradually emerged.
The sodium-ion battery has attracted much attention due to its advantages of abundant resources, low price, and high safety, and is an important supplement and strategic reserve for lithium-ion batteries.
1. Technical features and advantages of sodium ion batteries
Due to the rich content of sodium in nature, the upstream raw materials of sodium-ion batteries are relatively cheap, which can effectively supplement lithium-ion battery technology. At the cell level, the constituent elements of the cathode material for sodium-ion batteries are mainly Na, Cu, Fe, and Mn, all of which are relatively cheap and come from a wide range of sources. Compared lithium vs sodium battery, with the constituent elements Li, Ni, Co, etc. of lithium-ion battery cathode materials, the cost advantage of sodium battery is obvious.
The anode materials are mostly carbon-based materials, which are usually obtained by high-temperature carbonization using anthracite biomass, phenolic resin, etc. as precursors. The raw materials have a wide range of sources and low prices, and the carbonization process is simpler than the graphitization process of graphite anode. In terms of current collectors, cheaper aluminum foil can be used instead of copper foil, further reducing the cost of sodium-ion batteries.
In terms of working principle, sodium-ion batteries are similar to lithium-ion batteries. During charging, Na+ is extracted from the cathode material, passes through the electrolyte, passes through the separator, and is embedded in the anode material.
During the discharge process, Na+ is released from the anode material, and returns to the cathode material through the electrolyte and separator again. At the same time, during the charging and discharging process, the same number of electrons will be transferred in the external circuit to maintain the charge balance of the battery system.
Therefore, combined with the characteristics of the sodium-ion battery itself, it has the following characteristics and advantages:
● The production equipment of lithium-ion batteries can be used for the production of sodium-ion batteries after simple improvement, with less equipment and process investment, providing hardware support for the transformation from lithium batteries to sodium batteries;
● Compared with lithium ions, sodium ions have lower solvation energy, stronger interfacial ion diffusion ability, and higher ionic conductivity of the electrolyte. As a result, the rate performance of sodium-ion batteries is better, and high power input and output can be achieved;
● Sodium-ion batteries have better high and low temperature performance, and can work safely under wide temperature (-40°C~80°C) conditions;
● Sodium-ion batteries show good safety performance in the safety project test.
2. What are the challenges in the current development of sodium-ion batteries?
At present, the industrialization of sodium-ion batteries in top 10 sodium-ion battery companies in the world is developing rapidly, but there are still some challenges to be overcome, mainly including the following aspects:
● The cycle life needs to be further improved. Most of the current commercialized sodium-ion batteries have less than 1000 cycles, which is far below the expected level;
● There is still a certain gap between the actual capacity of the electrode material and the theoretical capacity, especially the first cycle Coulombic efficiency and capacity of the anode material, which leads to a large gap between the actual energy density and the theoretical energy density;
● Further take advantage of the low solvation energy of Na+ to achieve fast charging and fast discharging at the level of several minutes (above 6C);
● Construct sodium-ion battery aging, failure and thermal runaway models to further improve the safety performance of sodium-ion batteries;
● Optimize the production and assembly process of each component of the sodium ion battery, give full play to the low cost advantage of the raw material of the sodium ion battery, and then realize the large-scale application.
The structure of oxide materials is unstable, and there are many side reactions on the surface. Compared with lithium-ion batteries, sodium-ion batteries are less stable in air due to the increased alkalinity, hydrophilicity and solubility of sodium;
Prussian blue materials have many structural defects and high lattice water content. In the preparation process of Prussian blue materials, the simple chemical precipitation method is mostly used, which will lead to the hydration of the Prussian blue material lattice and produce various adverse effects;
Polyanionic materials have low intrinsic electronic conductivity. Its crystal structure is a three-dimensional framework structure formed by the interconnection of octahedrons and tetrahedrons, which leads to poor kinetics of electron migration within it;
Carbon-based materials have low Coulombic efficiency and unclear electrochemical mechanisms. Carbon-based materials are currently the most widely used anode materials for sodium-ion batteries, and their electrochemical performance has been greatly improved. However, the coulombic efficiency of carbon-based anode materials is low due to the occurrence of side reactions or irreversible intercalation reactions during the charge and discharge process, and there is still a certain distance from the commercial standard.
Electrolyte, separator and battery cell
At present, most sodium ion electrolytes use organic solvents as carriers, and a certain concentration of sodium salt is added to them. Under the condition of wide working temperature range, the volatility, instability and coagulation of organic solvents require further exploration of the mechanism of action and compatibility between the components of the solvent or sodium salt in the electrolyte.
Currently commercialized battery separators mainly include polyethylene and polypropylene separators, which have excellent mechanical properties, chemical stability and low price. However, due to inherent disadvantages, such as poor thermal stability and poor wettability to the electrolyte of sodium-ion batteries, it is not suitable for sodium-ion batteries.
Therefore, it is particularly important to find new separators that can match the sodium-ion battery system. The development of low-cost, high-safety, and mass-producible Na-ion battery separators is worth exploring.
The core problem of sodium-ion battery devices is the aging and failure mechanism of batteries. Compared with lithium-ion batteries with mature technology, commercial sodium-ion batteries are still in their infancy, and the aging and failure mechanisms of their cells are still unclear, especially in large-scale sodium-ion battery energy storage power stations.
3. Key development directions of sodium-ion batteries
There are two key development directions for sodium-ion batteries, including the development of key technologies for ultra-long cycle life sodium-ion batteries and further improvement of the energy density of sodium-ion batteries.
Focus on the development of key technologies for ultra-long cycle life sodium-ion batteries, mainly including: electrode material crystal structure regulation, microstructure design, and preparation method optimization. Thus, a high-performance electrode material with stable structure, good uniformity, simple preparation process, and environmental protection is obtained.
Another key development direction is to further increase the energy density of sodium-ion batteries (>200Wh/kg). The aim is to develop high-voltage cathode materials and phosphorus-based anodes or non-anode technologies to achieve the adaptation between high-voltage cathodes and low-voltage, high-capacity anode materials.
Solid-state sodium-ion batteries replace traditional electrolytes and separators, and on the basis of reducing the quality of sodium-ion batteries, strive to use metal-containing sodium composite negative electrodes to build high-voltage sodium-ion batteries. Bipolar battery and moduleless battery pack technology can effectively reduce the total mass of the sodium-ion battery energy storage system and increase the total energy density of the battery pack.