Function and technical requirements of lithium-ion power battery separators
Function and technical requirements of lithium-ion power battery separators
Due to high financial investment risks and high technical thresholds, lithium battery separators have not had the ability to attain domestic automation, which has actually become a traffic jam limiting the growth of my nation's lithium battery market.
As the main power source of current new energy vehicles, lithium-ion power batteries have the advantages of relatively high specific energy and voltage, wide operating temperature range, no memory effect, and relatively long cycle life. From a structural perspective, lithium-ion power batteries are mainly composed of five parts: positive electrode material, negative electrode material, electrolyte, battery separator, and packaging material. Now let’s talk about the preparation and technical requirements of lithium-ion power battery separators.
Functional and technical requirements of diaphragms
Battery separators are porous films that, after absorbing battery electrolyte, isolate the positive and negative electrodes to prevent short circuits. At the same time, it provides microporous channels for lithium-ion batteries to realize charge and discharge functions and rate performance, and realizes the conduction of lithium ions. When the battery is overcharged or the temperature changes greatly, the separator blocks current conduction through closed pores to prevent explosion. Therefore, the main functions of lithium-ion power batteries include charging and discharging and safety assurance.
Compared with consumer lithium-ion batteries, lithium-ion power batteries need to provide higher voltage, greater power and more electricity when used in new energy vehicles. The separator is one of the main components of the battery. The advantages of the separator performance determine the user interface framework and internal resistance of the battery, which in turn affects the battery's capacity, cycle performance, charge and discharge current density and other key characteristics.
Factors affecting membranes include thickness, air permeability, wettability, pore structure and porosity, thermal stability and consistency, etc. Some effects of separators on battery performance are shown in the table below:
Function and technical requirements of lithium-ion power battery separators
Excellent performance separators play an important role in improving the overall performance of power lithium batteries. Therefore, higher requirements are put forward for the use of lithium-ion power battery separators:
1) It must have good insulation to prevent the positive and negative electrodes from contacting short circuits or being punctured by burrs, particles, and dendrites;
2) It has sufficient puncture strength, tensile strength, etc., and basically maintains dimensional stability under sudden high temperature conditions, and will not shrink and cause large-area short circuits and thermal runaway of the battery;
3) The separator needs to maintain high uniformity in terms of thickness, breathability, pore size distribution, etc.;
4) Able to withstand electrolyte corrosion and have sufficient chemical and electrochemical stability;
5) The migration of lithium ions is affected by the separator material and pores, so the separator needs to have high porosity and uniform micropore distribution.
Type of diaphragm
According to the structural characteristics of lithium-ion battery separators, they can be divided into polyolefin microporous membranes, non-woven membranes and coated composite membranes:
1) Polyolefin microporous membrane: Polyolefin separator is currently the most mainstream power lithium-ion battery separator, mainly polyethylene (PE), polypropylene (PP), etc.
Function and technical requirements of lithium-ion power battery separators:
Although polyethylene and polypropylene microporous membranes have higher porosity, lower resistance, higher tear resistance, better acid and alkali resistance, good elasticity and retention of aprotic solvents. However, its heat resistance, electrolyte absorption performance and electrochemical oxidation resistance are relatively poor, and it cannot meet the requirements of the development of power lithium-ion battery technology.
They are generally modified to obtain high-performance separators. One of the methods is to add or compound special materials with lyophilic properties and high temperature resistance to obtain a composite separator with better performance.
2) Non-woven fabric-based separators and coated composite membranes: Non-woven fabric-based separators are divided into PP non-woven fabrics, PET non-woven fabrics, cellulose non-woven fabrics, PI non-woven fabrics, etc. according to their materials. The advantages of this type of diaphragm are high temperature resistance, ability to maintain output at low temperatures, long cycle life, and moderate mechanical properties.
Compared with polyolefin-type separators, it is characterized by a three-dimensional pore structure. Some studies believe that this structure can effectively avoid short circuits caused by pinholes and effectively improve the moisture retention rate. However, the shortcomings of non-woven fabric-based separators such as large pore size and uneven distribution limit further applications. Therefore, they are usually modified. Commonly used methods include nanoparticle-modified non-woven separators, microporous coating-coated modified non-woven separators, and electrospinning separators.
Both polyolefin separators and non-woven fabric-based separators have certain shortcomings, so transfer coating or dipping is usually used to make coated composite separators to change and improve the performance of the separator. Coating composite membranes can be divided into three types according to the composition of the coating: organic coating composite membranes, inorganic coating composite membranes, and organic/inorganic hybrid coating composite membranes.
Preparation
1) Dry method:
In the dry method, polyolefin resin is melted, extruded, and blown into a crystalline polymer film. After crystallization heat treatment and annealing, a highly oriented multilayer structure is obtained. It is further stretched at high temperature to peel off the crystallized cross section to form porous structure. Dry methods can be mainly divided into two categories: melt extrusion/stretching/heat setting method and nucleating agent added co-extrusion/stretching/heat setting method.
The dry method is a commonly used preparation process. The method is simple and pollution-free, but the pore size and porosity are hard to control, the stretch ratio is small, and the separator cannot be too thin. The main problem with dry-process lithium-ion battery separators is that the separators tend to shrink or even melt when the temperature rises, seriously threatening the safety of power lithium-ion batteries.
2) Wet method:
The wet method, also known as the phase separation method or the thermally induced phase separation method, is a commonly used method for preparing microporous membranes that has been developed in recent years. The thermally induced phase separation method uses thermoplastic and crystalline polymers and certain high-boiling point small molecule compounds (diluents) to form a homogeneous solution at a higher temperature.
Solid-liquid or liquid-liquid separation occurs when the temperature decreases, and a polymer porous membrane is formed after the diluent is removed. The wet method can better control the pore size and porosity, but the equipment required for wet preparation requires high precision, high investment, and the production cost and difficulty are higher than that of general thin film preparation technology.
3) New preparation method
The radiation method means that the polymer film is irradiated by electron beams, gamma rays, etc., and the surrounding atoms are ionized and excited due to the high-density energy deposition on the path where the ions pass, causing the long chains of the polymer molecules to break, rearrange, and generate freedom. The material in this area has a high chemical reaction capacity and can be etched with chemical reagents to form holes.
In addition, the high-energy ion battery separator system based on the electrospinning method mainly includes a spinneret, a high-voltage generator, an infusion system, and a wire connection system. In the high-voltage electrostatic field formed by the interaction between the wire connection system and the spinneret, polymers The flow stream of the solution is divided into several thin streams, thereby volatilizing the solvent and forming a fiber membrane in the joint system, which has good porosity and mechanical strength.
Summary
In the field of technological development, diaphragms have developed from a single polyolefin material to a variety of materials and composite materials, and from simple structures to complex structures. The research goal of separators is to improve the safety of batteries and ensure the full use of battery power performance. As lithium batteries develop towards higher specific energy systems, opportunities for the development of new separators will further emerge. In 2016, global sales of lithium battery separators will reach 1.8 billion square meters, and the proportion of separators used in power batteries will increase significantly.
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