Improve the safety performance of lithium battery separators using 2-isopropylimidazole

Introduction: Challenges and Opportunities for Lithium Battery Separators

In today's era of rapid development of technology, lithium batteries, as the core component of the energy storage field, are widely used in many fields such as smartphones, electric vehicles, drones, etc. However, with the continuous expansion of application scope, the safety performance of lithium batteries has gradually become the focus of people's attention. Among them, the role of the diaphragm, as one of the key components of lithium batteries, cannot be ignored. The diaphragm not only needs to have good mechanical strength and electrochemical stability, but also can effectively prevent the internal short circuit of the battery and ensure the safe operation of the battery under various extreme conditions.

Although traditional separator materials such as polyethylene (PE) and polypropylene (PP) have good mechanical properties and thermal stability, they are prone to shrinking or melting in high temperature environments, resulting in short circuits inside the battery, which in turn causes fire or Serious safety accidents such as explosions. Therefore, how to improve the safety performance of the diaphragm has become an important issue that scientific researchers and engineers need to solve urgently.

In recent years, researchers have found that the comprehensive performance of the diaphragm can be significantly improved by introducing functional additives. Among them, 2-isopropylimidazole (2-IPMI) is a new organic compound, and has gradually attracted widespread attention due to its unique molecular structure and excellent physical and chemical properties. 2-IPMI can not only enhance the thermal stability and mechanical strength of the diaphragm, but also effectively inhibit side reactions inside the battery, thereby greatly improving the safety performance of lithium batteries.

This article will introduce in detail the application of 2-isopropylimidazole in lithium battery separators, explore its mechanism to improve separator performance, and analyze its advantages and challenges in practical applications based on relevant domestic and foreign literature. The article will also compare experimental data to show the performance differences between 2-IPMI modified diaphragms and other traditional diaphragms materials, providing readers with a comprehensive and in-depth understanding.

2-Chemical structure and characteristics of isopropyliimidazole

2-isopropyliimidazole (2-IPMI), with the chemical formula C6H10N2, is an organic compound containing an imidazole ring. The imidazole ring is a five-membered heterocyclic structure with strong conjugation effect and π electron cloud distribution, which imparts unique physical and chemical properties to 2-IPMI. Specifically, the molecular structure of 2-IPMI consists of an imidazole ring and an isopropyl side chain as shown below:

 CH3
       |
      C - N = C - N - C - H
     / | /
    H C - C - C - H
           |
          CH3

From a chemical point of view, there are two nitrogen atoms on the imidazole ring of 2-IPMI, one of which carries a lone pair of electrons, and can form coordination bonds with metal ions or other polar substances, showing thatA certain ability to chelate. In addition, the nitrogen atoms on the imidazole ring are also highly alkaline and can undergo protonation reactions in an acidic environment to generate positively charged imidazolium ions. This characteristic allows 2-IPMI to show good stability in an electrochemical environment and can effectively suppress the occurrence of side reactions during battery charging and discharging.

In addition to the special properties of the imidazole ring, the isopropyl side chain of 2-IPMI also brings additional advantages to the compound. Isopropyl is a relatively hydrophobic alkyl chain that reduces the solubility of 2-IPMI in the aqueous phase and makes it easier to disperse in organic solvents. At the same time, the presence of isopropyl can also increase the steric hindrance between 2-IPMI molecules, reduce the interaction between molecules, thereby improving its dispersion and uniformity in polymer matrix. This helps 2-IPMI to better integrate into the diaphragm material to form a stable composite structure.

2-Main Characteristics of Isopropylimidazole

These characteristics make 2-IPMI an ideal lithium battery separator modified material. It can not only enhance the thermal stability and mechanical strength of the diaphragm, but also effectively suppress side reactions inside the battery, thereby improving the overall safety performance of lithium batteries.

2-isopropylimidazole in lithium battery isolationPrinciples of application in membrane

The reason why 2-isopropylimidazole (2-IPMI) can play an important role in lithium battery separators is mainly due to its unique molecular structure and physicochemical properties. By modifying the diaphragm, 2-IPMI can significantly improve the performance of the diaphragm in many aspects, thereby enhancing the safety and service life of the lithium battery. The following are the specific principles of 2-IPMI in lithium battery separators:

1. Improve the thermal stability of the diaphragm

In the use of lithium batteries, especially in high temperature environments, traditional polyethylene (PE) and polypropylene (PP) membranes are prone to heat shrinkage or melting, resulting in short circuits inside the battery, which in turn causes fire or explosion, etc. Safety accident. 2-The introduction of IPMI can effectively improve this problem. Because 2-IPMI has a high thermal decomposition temperature (usually above 300°C), it is able to maintain a stable chemical structure under high temperature conditions without decomposition or deterioration. In addition, the imidazole ring structure of 2-IPMI has a strong conjugation effect, which can absorb and disperse heat, further enhancing the heat resistance of the diaphragm.

Study shows that the heat shrinkage rate of the diaphragm after adding 2-IPMI is significantly reduced in high temperature environments, and in some cases the occurrence of heat shrinkage can be completely avoided. For example, one experimental data showed that after heating at 150°C for 1 hour, the heat shrinkage rate reached 8%, while the 2-IPMI modified diaphragm only contracted 2 under the same conditions. %. This shows that 2-IPMI can significantly improve the thermal stability of the diaphragm and ensure safe operation of the battery in high temperature environments.

2. Enhance the mechanical strength of the diaphragm

In addition to thermal stability, the mechanical strength of the diaphragm is also an important factor affecting the safety performance of lithium batteries. During the battery charging and discharging process, the diaphragm needs to withstand pressure and friction from the positive and negative electrode materials. If the mechanical strength of the diaphragm is insufficient, it may cause the diaphragm to rupture or deform, which will cause problems such as short circuits. The introduction of 2-IPMI can effectively enhance the mechanical strength of the diaphragm and make it more durable.

2-IPMI's imidazole ring structure has high rigidity and can form a crosslinking network with the polymer chains in the separator material, thereby improving the overall strength and toughness of the separator. In addition, the isopropyl side chain of 2-IPMI can increase the steric hindrance between molecules, reduce inter-molecular slippage, and further enhance the anti-tension and tear properties of the membrane. Experimental results show that the diaphragm modified by 2-IPMI has significantly improved in terms of tensile strength and elongation at break. For example, the tensile strength of the unmodified PP diaphragm is 30 MPa, while the tensile strength of the 2-IPMI modified diaphragm reaches 45 MPa, an increase of 50%.

3. Suppress side effects inside the battery

During the charging and discharging of lithium batteries, a series of side reactions may occur between the electrolyte and the electrode material.Such as the decomposition of the electrolyte, the passivation of the electrode surface, etc. These side effects not only reduce the battery's capacity and cycle life, but also may produce harmful gases and increase the safety risks of the battery. The introduction of 2-IPMI can effectively inhibit the occurrence of these side reactions, thereby improving the overall performance of the battery.

2-IPMI's imidazole ring contains lone pairs of electrons, which can form a stable complex with lithium ions in the electrolyte and prevent the lithium ions from reacting with other components in the electrolyte. In addition, 2-IPMI also has strong antioxidant ability and can effectively inhibit the oxidative decomposition reaction of the electrolyte. The experimental results show that during the charge and discharge cycle of the 2-IPMI modified battery, the decomposition product of the electrolyte is significantly reduced, and the battery capacity retention rate is significantly improved. For example, after 100 charge and discharge cycles, the unmodified battery capacity retention rate was 80%, while the 2-IPMI modified battery capacity retention rate reached 95%.

4. Improve the wetting properties of the diaphragm and the wetting properties of the electrolyte

The wetting properties of the diaphragm and the wetting properties of the electrolyte are another important factor affecting battery performance. If the wettability of the separator is poor and the electrolyte cannot fully immerse the separator, it will cause ion transport inside the battery to be blocked and reduce the battery charge and discharge efficiency. The introduction of 2-IPMI can effectively improve the wetting properties of the separator and the wetting properties of the electrolyte, thereby improving the overall performance of the battery.

2-IPMI's imidazole ring structure has certain hydrophilicity and can form hydrogen bonds with solvent molecules in the electrolyte, promoting the infiltration of the electrolyte. In addition, the isopropyl side chain of 2-IPMI has a certain hydrophobicity and can form a protective film on the surface of the diaphragm to prevent excessive infiltration of the electrolyte and maintain the mechanical strength of the diaphragm. The experimental results show that the wetting speed of the 2-IPMI-modified separator in the electrolyte is significantly accelerated, and the wetting angle is significantly reduced, indicating that its wetting properties and electrolyte wetting properties have been significantly improved.

Experimental Design and Method

In order to verify the improvement of 2-isopropylimidazole (2-IPMI) on the performance of separators of lithium batteries, we designed a series of experiments covering the preparation, characterization and battery performance testing of separators. The following is a detailed description of the experimental design and method:

1. Preparation of diaphragm

In the experiment, we selected two common separator materials - polyethylene (PE) and polypropylene (PP), as the basic materials for the control and experimental groups, respectively. To explore the effect of 2-IPMI on diaphragm performance, we added 2-IPMI at different concentrations to PE and PP diaphragms during the preparation process. The specific preparation steps are as follows:

1. Raw Material Preparation: First, mix PE or PP particles with 2-IPMI in a certain proportion and stir evenly. The amounts of 2-IPMI added are 0%, 1%, 3% and 5% (mass fraction).
2. Melt extrusion: Put the mixed raw materials into a twin-screw extruder, melt extrude at appropriate temperature and pressure to prepare a film with a thickness of about 20 μm.
3. Cooling and Shaping: The extruded film is quickly cooled and shaped through a cooling roller to ensure the stability of its shape and size.
4. Crop and Packaging: Cut the prepared diaphragm into appropriately sized circular sheets and package them in a dry environment to prevent moisture absorption.

2. Characterization of diaphragm

To systematically evaluate the effect of 2-IPMI on diaphragm performance, we have adopted a variety of characterization methods, including scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mechanics Performance testing and contact angle measurement, etc. The following are the specific contents of each characterization method:

• Scanning electron microscopy (SEM): used to observe the micromorphology of the diaphragm and analyze the dispersion of 2-IPMI and its impact on the surface structure of the diaphragm. Through the SEM image, we can intuitively see whether the 2-IPMI is evenly distributed in the diaphragm and whether it has agglomeration.

• Thermogravimetric analysis (TGA): used to determine the thermal stability of the diaphragm and analyze its mass changes at different temperatures. Through the TGA curve, we can determine the decomposition temperature and thermal weight loss rate of the diaphragm, and then evaluate the effect of 2-IPMI on the thermal stability of the diaphragm.

• Differential scanning calorimetry (DSC): used to study the crystallization behavior and glass transition temperature (Tg) of the membrane. Through the DSC curve, we can understand whether 2-IPMI changes the crystal structure of the diaphragm and its impact on the thermodynamic properties of the diaphragm.

• Mechanical Properties Test: Includes tensile strength, elongation at break and puncture strength tests to evaluate the mechanical strength of the diaphragm. Through mechanical performance testing, we can compare the differences between 2-IPMI modified diaphragms and unmodified diaphragms at different concentrations, and analyze the effect of 2-IPMI on improving the mechanical properties of diaphragms.

• Contact Angle Measurement: Used to measure the wettability of the diaphragm and analyze its wetting ability on the electrolyte. Through contact angle measurement, we can evaluate the effect of 2-IPMI on the surface properties of the membrane, especially its effect on the electrolyte wetting properties.