Currently commercial lithium-ion battery separators are mainly polyolefin-based microporous membranes, such as polyethylene (PE) and polypropylene (PP) separators. Although this type of separator has good electrochemical stability and mechanical strength, due to its inherent non-polar surface and low melting temperature, polyolefin separators have poor electrolyte wettability and poor thermal stability. The problem affects the electrochemical performance and safety performance of the battery, which also restricts the development of high-performance lithium-ion batteries. Therefore, the preparation of high-performance battery separators with high electrolyte absorption rate, high thermal stability, and excellent electrochemical performance is extremely important for the development of lithium-ion batteries.
Application advantages of boehmite in diaphragm
Boehmite is used in the preparation of lithium ion battery separators because of its unique advantages:
1) Excellent insulation, expands when the battery is working, and effectively blocks the current;
2) Sufficient chemical and electrochemical stability, will not be corroded by electrolyte;
3) Excellent heat resistance, which can effectively improve the thermal stability of the lithium battery when it is working;
4) As a coating material, it can effectively reduce the thickness of the coating and reduce the wear and tear of the machine, and the cost is low and the price is high.
Brief description of some research
A microporous PE membrane coated and modified with boehmite as a coating material is reported. Compared with the commonly used Al₂O₃ coating material, the γ-AlOOH/PE membrane is thinner and lower in cost. In addition, the film has excellent thermal stability (the shrinkage rate is less than 3% even at 180°C, while the unmodified PE film is severely deformed and the shrinkage rate exceeds 85%), and excellent wettability (in contact with electrolyte Angle is 0°), high ion conductivity (6.56mS/cm) and excellent overcharge protection function. Xu et al. studied the influence of different specifications of boehmite on the modified PP film coated with boehmite. Experiments show that when the particle size of boehmite is 0.78 μm, the boehmite modified PP film has the highest thermal stability, liquid absorption and ionic conductivity.
Boehmite/polyacrylonitrile composite nanofiber diaphragm
Chen Shilin prepared a stable boehmite sol by the sol-gel method using aluminum alkoxide salt as the precursor, and then compounded it with the polyacrylonitrile solution to form a homogeneous spinning solution, and prepared the boehmite by electrospinning. Polyacrylonitrile (BM/PAN) composite nanofiber diaphragm.
Preparation process of BM/PAN composite nanofiber diaphragm
The preparation of diaphragms by electrospinning is currently one of the hot topics of diaphragm researchers. Electrospinning is a basic method for preparing ultra-fine (sub-micron or nano-scale) continuous polymer fibers. A typical electrospinning device is mainly composed of a high-voltage power supply, an injector, a jet needle and a receiving device (the core technology is in a high-voltage electric field). The nanofiber membrane prepared by the electrospinning method has a unique three-dimensional network structure, and has the characteristics of large specific surface area, high porosity, and high liquid absorption rate. Using it as a battery separator can increase ion conductivity and improve battery cycle and rate performance.
Schematic diagram of electrospinning device
Through scanning electron microscope (SEM), Fourier infrared tester (FTIR), X-ray diffractometer (XRD), differential scanning calorimeter (DSC), thermogravimetric analyzer (TG), contact angle test, stretch Researchers have explored the effects of different boehmite content on the morphology, structural composition, thermal stability, wettability, and mechanical properties of the boehmite/polyacrylonitrile composite nanofiber membrane.
The above composite diaphragms were assembled into button batteries, and their electrochemical properties such as ionic conductivity, electrochemical stability window, cycle and rate were tested through electrochemical workstation and high-performance battery detection system. The experimental results show that the boehmite/polyacrylonitrile (BM/PAN) composite nanofiber separator has higher porosity, greater liquid absorption, and better thermal stability than PP membranes, and it has better performance when assembled in batteries. A more excellent electrochemical performance.
Among them, 30 wt% BM/PAN composite nanofiber membrane has the best overall performance. Assembled in the battery, the battery has the largest ionic conductivity (2.85mS/cm), the highest electrochemical stability window (5.5 V vs Li+/Li), the smallest interface resistance (84Ω), and the highest first discharge The specific capacity (162mAh/g) and the maximum discharge specific capacity retention rate (90.7%, 100 cycles of charge and discharge at 0.5C).
Therefore, the researchers believe that the boehmite/polyacrylonitrile composite nanofiber separator can be used as a potential substitute for commercial separators in lithium-ion batteries.