An article to understand the important technological progress of CR2032 battery separators

Today, lithium batteries have become the most commonly used energy devices for 3C products (computers, communications and consumer electronics). High capacity, stable charge and discharge performance and long enough service life have always been the pursuit of engineers for lithium batteries, and consumers' expectations for lithium batteries. And the separator material is the key to these pursuits and expectations.

1. Importance of separators

Lithium batteries are mainly composed of five parts: positive electrode materials, negative electrode materials, electrolytes, separators, and packaging materials.

The separator plays the role of electronic insulation between the positive and negative electrodes and provides microporous channels for ion migration. It is a key material to ensure the safety of the battery system and affect the battery performance. Although the separator does not directly participate in the electrode reaction, it affects the battery kinetics process and determines the battery's charge and discharge, cycle life, rate and other performance.

In recent years, researchers and related companies have a strong interest in the research and development of separator materials and breakthroughs in industrial technology. According to the online patent analysis system of the Chinese Academy of Sciences, with the Chinese keywords "CR2032 battery, diaphragm", a total of 2,106 patent applications were retrieved (as of September 2015), of which 51.19% were authorized patents and 1,078 were valid patents. With the Chinese keywords "polyethylene, diaphragm", "polypropylene, diaphragm", "ceramics, diaphragm", and "modification, diaphragm", 419, 415, 390, and 272 patent applications were retrieved, with authorization rates of 44.4%, 42.4%, 32.0%, and 33.1%, respectively, and 186, 176, 125, and 90 valid patents, respectively. According to statistical analysis, the hot words in the field of lithium diaphragm research and development and technology in recent years are: high safety, new materials, ceramics, coating, and improved wettability. At the same time, in the past decade, especially in the past five years, patent applications involving buried diaphragms have shown an accelerating upward trend. 2. Functions of CR2032 battery separators

The functions of separators in lithium-ion batteries are important in two aspects:

First, it provides safety guarantee for batteries. The separator material must first have good insulation to prevent short circuits caused by contact between the positive and negative electrodes or short circuits caused by burrs, particles, and dendrites. Therefore, the separator must have a certain tensile and puncture strength, not easy to tear, and basically maintain dimensional stability under sudden high temperature conditions, and will not shrink and cause large-area short circuits and thermal runaway of the battery.

Second, it provides lithium-ion batteries with microporous channels to realize charging and discharging functions and rate performance. Therefore, the separator must be a film with high porosity and uniform micropore distribution. The characteristics of the material itself and the pore characteristics after film formation restrict the migration of lithium ions in the battery, which is reflected in the performance parameters as ionic conductivity.

3. Influencing factors of CR2032 battery separators

Providing safety guarantee for batteries is reflected in the basic properties of separator manufacturing materials. Safety requirements determine that the separator must have outstanding insulation, mechanical strength, chemical stability, electrochemical stability and thermal stability. Therefore, the materials for making the diaphragm can only be selected from polymers and their composite materials with good insulation, good film-forming properties, mechanical properties and easy processing.

The mainstream materials that have been commercialized are polypropylene microporous membranes and polyethylene microporous membranes, and the materials under development include non-woven ceramic particle composite membranes, and the materials under research and development include polyimide (PI).

The lithium ion conduction function of the battery is achieved through the structure and microporous structure characteristics of the diaphragm. There are also some inherent properties of the material itself that affect this performance. The requirement for lithium ion conduction determines that the diaphragm must have good wettability to the electrolyte, because only by absorbing and retaining an appropriate amount of electrolyte in the pore structure of the diaphragm can the ion migration and normal operation be realized to prevent the occurrence of electrode polarization. The microstructure of the diaphragm, such as pore size and its distribution, porosity, air permeability (Gurley value), dimensional stability and other factors are related to ionic conductivity and significantly affect the performance of the battery.

As the industry continues to pay more attention to battery safety, battery companies' requirements and expectations for diaphragm safety are also continuously increasing. In the application of some special battery models, the requirement for the thermal shrinkage ratio of diaphragm materials has been raised to less than 2% after heating at 180°C for 60 minutes, and some foreign battery companies even seek diaphragms that can maintain stable dimensions in the 250~300"C temperature range.

The thickness of the diaphragm is of course as thin as possible while ensuring safety. For wound batteries, the thinner the diaphragm thickness, the smaller the internal resistance of the battery, which can leave more space for electrode materials and reduce the misalignment of the pole piece during winding. However, if the emphasis is only on thinning the thickness, the mechanical properties will be affected, and it will be more likely to be pierced by large particles, pole piece burrs and dendrites, resulting in battery safety. The coefficient is reduced. The laminated battery has fewer burrs and the thickness requirement is not high.

With the increasing diversification of CR2032 battery material systems, uses, capacities, and shapes, the requirements for diaphragm performance and technical indicators have gradually become more detailed, and production companies have a deeper understanding of diaphragms. However, no diaphragm is currently excellent in all technical parameters.

Therefore, when selecting a diaphragm for a battery, you should focus on which performance to highlight, safety, power performance, or cycle life? Depending on the battery design and application field, the type of diaphragm application should also be different. There have been relevant reports on the specific analysis of various technical parameters of the diaphragm.

CR2032 battery diaphragm performance requirements and performance parameters of several commercial membranes

IV. Important technical progress of CR2032 battery diaphragms

1. Polyolefins Surface modification

Adding or compounding materials with hydrophilic properties and high temperature resistance to a single-layer polyolefin diaphragm to obtain a composite diaphragm with better performance is a major research direction for preparing high-performance diaphragms.

Currently, the commonly used processes include coating, dipping, spraying, and compounding. Studies have shown that polyarylate materials are coated on PE diaphragms to form a composite diaphragm of porous polymer precipitates. Since polyarylate has good heat resistance, the melting temperature of the composite diaphragm is increased to more than 180°C.

Dopamine is coated on the PE diaphragm by dip coating, and the modified diaphragm obtained has a higher performance of adsorbing electrolyte, which effectively improves the high-rate cycle performance of the diaphragm.

Using a mixture of PVDF/SiO2 to modify the polyolefin diaphragm, the composite The diaphragm has both the electrolyte affinity of PVDF and the high temperature resistance of SiO2. The charge and discharge efficiency of the prepared Zhong ion battery reaches 94% at a 2C discharge rate.

2. Polyolefin-ceramic composite diaphragm

Polyolefin organic diaphragms have good mechanical properties and low cost, but they are insufficient in thermal stability and lyophilicity. Therefore, as battery diaphragms, their safety performance needs to be improved. Therefore, the process of coating inorganic ceramic particles on polyolefin organic diaphragms to prepare composite membranes came into being.

Although the impact of ceramic coating on battery performance still requires further research and evaluation to draw a final conclusion, this technology has been imitated by many diaphragm companies and battery companies and has been rapidly promoted.

In polymer ceramic composite membranes, polyolefin organic microporous membrane materials provide flexibility to meet the needs of battery assembly processes. Inorganic ceramic particles form a rigid skeleton in the composite membrane to prevent the diaphragm from shrinking or even melting under high temperature conditions, so as to improve battery safety performance. The adhesive has an important influence on the surface properties, pore structure, mechanical strength and other properties of the ceramic composite membrane.

The polymer-ceramic composite membrane improves the thermal stability and electrolyte wettability of the polyolefin diaphragm to a certain extent, but the biggest problem of this composite technology is that the ceramic phase and the organic bonding force are weak, and the ceramic is prone to fall off (powdering). This phenomenon can be alleviated to a certain extent by reasonably controlling the amount of diluent, using in-situ composite technology to pre-disperse inorganic ceramic particles in the film-forming solution, and using wet biaxial stretching technology or electrospinning to make the diaphragm.

The composite diaphragm product with polyolefin diaphragm as the substrate maintains the processability of polyolefin diaphragm to be easy to stretch and pore, while improving the safety, lyophilicity and other properties of the diaphragm. Before the replacement products of the diaphragm are commercialized, it will still occupy an important market share.

3 , New material system

According to the materials used, battery separators are divided into polyolefin modified separators and new material system separators. Among them, the new material system mainly includes fluorinated polymer separators, cellulose separators, polyimide (PI) separators, polyester (PET) separators and other polymer ceramic composite separators.

(1) Fluorinated polymer separators mainly refer to PVDF separator materials. From the material point of view, they can be divided into three categories: single polymer, multi-polymer and organic-inorganic composite. The most commonly used single polymers include PVDF, P (VDF-HFP) (polyvinylidene fluoride-hexafluoropropylene) and P (VDF-TrFE) (polyvinylidene fluoride-trifluoroethylene).

Compared with polyolefin separator materials, fluorinated polymer separators have stronger polarity and more The high dielectric constant greatly improves the lyophilicity of the diaphragm and helps the ionization of lithium salts.

In addition, there are various molding methods for this type of material, such as casting, electrospinning, hot pressing, etc., which are conducive to regulating porosity.

(2) The battery performance of cellulose diaphragms is comparable to that of polyolefin diaphragms, but its resources are abundant and renewable. At the same time, the initial decomposition temperature of cellulose materials is relatively high (>270"C), and the thermal stability is significantly better than that of polyolefin materials.

The cellulose materials used in the early days have excellent fast charging and discharging performance, but there is a self-discharge phenomenon, the cycle performance is not stable enough, and the voltage resistance is not enough. Some researchers use non-woven cellulose as the substrate and P (VDF-HFP) as the coating to prepare a cellulose/PVDF composite diaphragm, which is similar to the traditional PP membrane. Compared with the conventional method, the liquid affinity is significantly enhanced and the thermal stability is greatly improved.

(3) New process methods

There are two core contents in the research and development of diaphragms: one is the new material system, and the other is the process method that can realize industrial production. Without an efficient process method, no matter how good the material is, it cannot become a widely accepted commodity.

The conventional methods for preparing polyolefin diaphragms are dry and wet methods. However, developing polyolefin diaphragms in a thinner direction to meet the performance requirements of 3C lithium batteries is a key entry point for improving diaphragm performance.

Relatively speaking, in terms of preparation methods, the coating process and equipment of polyolefin modified diaphragms are very mature. The coating modification of polyolefin diaphragms is more feasible and can improve the heat resistance of polyolefin diaphragms and their wettability to electrolytes. At present, many research institutions and manufacturers at home and abroad have key research and development projects on ceramic coated diaphragms.

In short, with the increasing abundance of diaphragm materials and the increasing maturity of preparation processes, it is believed that in the near future, new diaphragms with high safety and strong heat resistance that meet consumer needs will be successfully launched, which will have a profound impact on our lives.

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