Introduction of battery separator materials for lithium battery energy storage system
In recent years, the lithium battery energy storage power station has developed rapidly, taking the leading position. However, in the case of mechanical, thermal and electrochemical abuse, lithium batteries using flammable and volatile organic electrolytes sometimes suffer from thermal runaway or even explosion due to external impact puncture, local hot spots, internal and external short circuits and other factors, which limits their large-scale application.
How to ensure the battery storage safety and stable operation of the power plant has become the primary problem under the condition of the continuous expansion of the newly added installed capacity.
This article will discuss the work on smart battery separator in recent years from the point of view of cell materials and the existing problems of commercial lithium battery separator, and emphasize the importance of developing high safety battery separator materials.
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The function of battery separator
Although the battery separator belongs to the inert component of lithium battery cell materials, it plays an indispensable role. On the one hand, under normal working conditions, the porous battery separator can be fully saturated by organic electrolyte to ensure the smooth transmission of ions without additional bulk impedance;
On the other hand, the battery separator can strictly block the positive and negative electrode materials on both sides to avoid their contact crosstalk, especially in the case of external impact and battery overheating.
However, in the process of thermal runaway, as the internal temperature of the battery system continues to rise, the size of the battery separator will seriously shrink, resulting in a short circuit in the battery.
In addition, most commercial battery separators are made of polyethylene and polypropylene, with low mechanical modulus, unstable to heat, and extremely flammable, which cannot ensure the safe and reliable operation of the battery.
Therefore, it is imperative to develop intelligent battery separator with high strength, high temperature resistance and flame retardancy from the point of view of core materials under the condition of huge annual increment of installed capacity of energy storage power plants.
Especially in the direction of introducing intelligent materials into the battery system, whether adding thermal response materials or realizing automatic detection function through structural design.
If the battery separator can respond in time before or at the early stage of the thermal runaway event and inhibit the positive feedback process of battery temperature rise, the battery life and safety will be greatly improved.
Modification of commercial separator materials of existing lithium batteries
Polyolefin separator and heat resistant material composite
The main problem of industrial and commercial polyolefin separators is poor thermal stability. The melting point of polyethylene is 135 ℃, and that of polypropylene is 165 ℃.
When the temperature is close to the melting point of the polymer, the battery separator will shrink and collapse at multiple scales, and the coverage area will shrink, which cannot strictly block the positive and negative poles, resulting in short circuit in the battery.
At present, the common means of battery separator optimization is to improve the thermal stability of traditional polyolefin separators by coating or grafting some heat-resistant materials on their surfaces, such as:
● Silicon dioxide (SiO2)
● Titanium dioxide (TiO2)
● Aluminum oxide (Al2O3)
● Polybenzimidazole (PBI)
● Polyether ether ketone (PEEK)
● Polyimide (PI)
Regularized ion channel
During the cycle, lithium ions shuttle back and forth between the cathode and anode poles through the electrolyte and the battery separator in lithium ion battery, and the pore structure of the separator determines the distribution of electrolyte and ion transmission. Because commercial polyolefin separators are made by dry or wet stretching, the surface pore structure is often poor uniformity.
The lithium ions in the electrolyte will gather near the pore structure during the shuttle process, which results in heterogeneous nucleation and deposition at the negative side, inducing the generation and extension of dendrites.
In order to solve this problem, it is an effective method to regularize the lithium ion transport channel by lithiophilizing the battery separator.
New intelligent battery separator material and structure design
Slow release SEI additive
Compared with commercial graphite (with a specific capacity of 372mA · h/g), lithium metal anode has attracted much attention due to its ultra-high specific capacity (3860mA · h/g).
However, it is difficult to predict the properties of the interface between lithium metal and electrolyte, especially the chemical heterogeneity and mechanical instability of the interface passivation layer SEI, which will lead to uneven ion flux, the formation of lithium dendrites, which will induce short circuit in the battery and threaten battery safety.
Extensive research has been devoted to regulating the surface reactivity of lithium metal, among which regulating the electrolyte composition is the most direct way.
For example, the lithium nitrate (LiNO3) additive with good performance in the ether based electrolyte of lithium sulfur battery has poor solubility in commercial carbonate electrolyte, and cannot play its role in improving the SEI interface.
Taking battery separator as the starting point, increasing the content of lithium nitrate in carbonate based electrolyte can also greatly improve battery performance and safety.
Thermal response switch
Commercialized PP/PE/PP three-layer structure battery separator melts in the middle layer under the condition of overheating, blocking the channel and cutting off the passage. The outer layer has a relatively high melting point, so as to ensure strong mechanical support and protect itself.
However, because the melting points of the two polymers in the battery separator materials are too close to each other, the effect is limited, so expanding the difference of melting points between different layers of the battery separator is the key.
Paraffin wax (melting point 65 ℃) or low molecular weight polyethylene microspheres (melting point 110 ℃) have been used as protective layer materials for lithium battery separator. The poor thermal stability of polyolefin separators makes it impossible to maintain structural integrity at high temperatures.
Flame retardant modification
Replacing the existing flammable polyolefin battery separator with other electrochemically stable and flame retardant polymer battery separator can improve the intrinsic safety of batteries, especially materials containing chlorine (Cl), bromine (Br) and other flame retardant halogen elements.
There are many kinds of flame retardants, but the basic principles can be summarized as follows: reducing the generation of combustible gas, hindering the combustion chain reaction, absorbing the heat of combustibles, diluting and isolating oxygen, etc.
The limiting oxygen index is used to evaluate the combustion behavior of a material, which refers to the volume fraction of oxygen when the polymer just supports its combustion in the mixed gas of oxygen and nitrogen.
In order to truly achieve the effect of flame retardancy, a large amount of flame retardants must be added to the electrolyte, but this often leads to a decrease in bulk ionic conductivity and a significant decline in battery performance.
Automatic detection and identification of dendrites
Although researchers generally believe that graphite anode has higher safety than lithium metal anode, the influence of dendrite on the safety of graphite anode cannot be completely ignored.
During the battery cycle, dendritic crystals will be generated due to local uneven deposition, which will bring many potential safety hazards, such as continuous reaction between highly active surface and electrolyte, continuous accumulation and thickening of passivation layer, and needle dendritic crystals piercing the battery separator.
If the health status inside the battery can be detected in real time through the structural design of the battery separator, the dangerous working conditions can be checked early in the experiment.
First of all, the battery intelligence and intrinsic safety of commercial polyolefin battery separator have been continuously improved in the optimization process of various modification methods. However, there are still some problems that need further study.
For example, coating inorganic filler and filling flame retardant on the commercial polyolefin battery separator can improve its mechanical properties, thermal stability and flame retardancy, but it is necessary to strictly control the particle dispersion and coating thickness, otherwise it will damage the electrochemical performance of the battery.
High temperature resistant special plastic battery separator has excellent performance in multi-dimensional indicators, but its complex preparation process and high manufacturing cost also limit its further application.
Secondly, under the circumstance that the lithium battery based on liquid electrolyte will continue to dominate the market, how to make the traditional battery separator multifunctional, especially the “low-cost” intelligent modification, is a very important way to solve the battery safety problem.
In addition, all solid state battery is considered as the next generation of high-energy and safe lithium battery. If it can be commercialized successfully, it will replace the traditional battery separator.
Among them, polymer solid electrolyte has good flexibility and interface adhesion, while inorganic ceramic solid electrolyte has excellent mechanical properties and inherent flame retardancy.
In energy storage industry, taking reasonable strategies to combine the advantages of the two and overcome the disadvantages will greatly improve the safety of future lithium battery energy storage power stations.