Lithium ion batteries are mainly composed of positive and negative active materials, current collectors, binders, and conductive agents. The current collector mainly collects the electrons in the active material to the external circuit, while the binder will the active material, conductive The agent adheres to the current collector. The positive and negative active materials will have a significant volume change during the charging and discharging process. Therefore, higher requirements are placed on the adhesion between the active material and the current collector. In addition to the influence of the binder selection on the adhesion between the electrode active material and the current collector, the surface state of the current collector will also have a significant effect on the adhesion.
Recently, TingtingXia (first author) and Tongxiang Liang (corresponding author), Jun Chen (corresponding author) and Shengwen Zhong (corresponding author) of Jiangxi University of Science and Technology obtained copper foil with nano-sized grains through the ultrasonic surface treatment process This copper foil has a rougher surface and a larger surface area, so it can provide better adhesion.
The diameter of the equipment used in the preparation of the nano-grain copper foil is 80 mm and the thickness is 8 mm. In the preparation process, a carbon steel ball with a diameter of 14 mm was used to process the copper foil at a pressure of 0.2 MPa and a speed of 2 m/min at 28 kHz and an amplitude of 12 um.
After ultrasonic pressing, the grains on the surface of the copper foil were integrated into coaxial and elongated nano-grain bodies with a size of about 70 nm, but ultrasonic pressing did not significantly change the crystal structure of the copper foil.
In order to analyze the effect of ultrasonic stick pressing on the surface roughness of copper foil, the author observed the surface of the copper foil using an atomic force microscope. The average roughness of the surface of the copper foil before the treatment was 31 nm, and the surface of the copper foil after ultrasonic stick pressing The average roughness is increased to 53.5nm.
The contour curve of the interface of the copper foil before and after the treatment can be seen that the surface of the copper foil after the ultrasonic pressure treatment is uneven and has large fluctuations. The rough surface can help the active material and the copper foil bond better. Effectively improve the peel strength of the electrode.
The SEM images of the original copper foil and the copper foil after ultrasonic stick pressing show that the grain diameter of the untreated copper foil is about 100-200um, while the grain diameter of the copper foil after ultrasonic stick pressing is reduced to the nanometer level. The polarization curve of copper foil, the corrosion current of untreated copper foil is 4.148 uA cm-2, and the copper foil after ultrasonic stick pressing is 2.77uAcm-2, which shows the corrosion resistance of copper foil after ultrasonic stick pressing There has been a significant improvement. The surface state of the ordinary copper foil and the nano-grain copper foil after being corroded by the electrolyte can be seen that a large amount of corrosion products are produced on the surface of the ordinary copper foil, while the surface of the nano-grain copper foil hardly shows significant corrosion, which shows that Nano-grain copper foil has better anti-corrosion properties, mainly because after ultrasonic stick pressing, a large number of nano-grains are formed on the surface of the copper foil, which increases more active points. In the process, a layer of relatively uniform and thin decomposition products was formed on the surface to avoid further corrosion of the copper foil, while the grain size of the ordinary copper foil is larger, and the corrosion mainly occurs at the grain boundary Location, so it is impossible to produce a uniform protective layer on the surface of the copper foil, so that effective protection cannot be formed, which intensifies the corrosion of the copper foil.
Cross-sectional views of negative electrodes using ordinary copper foil and nano-grain copper foil, respectively, we can see that there is a large gap between the electrode active material layer using ordinary copper foil and the copper foil, and there is a separation between the active material and the current collector Layer phenomenon. However, the active material layer of the electrode using nano-grain copper foil is in close contact with the copper foil without obvious delamination, which indicates that the nano-grain copper foil and the active material have better adhesion.
shows the charge and discharge curves of graphite anodes using two types of copper foils. The first discharge capacity of ordinary copper foils is 394mAh/g, while the first discharge capacity of anodes using nano-grain copper foils reaches 521mAh/g, corresponding to The first efficiencies were 71% and 64%, respectively. Using the test results of the rate performance of the negative electrode using two types of copper foil, it can be seen that the rate performance of the copper foil after ultrasonic stick pressing is significantly better than that of ordinary copper foil. The 0.1C cycle performance curve of the negative electrode using two types of copper foils can be seen from the figure. After 100 cycles, the capacity retention rate of the negative electrode of the copper foil after ultrasonic pressure treatment is 89%, while that of the ordinary copper foil The capacity retention rate of the negative electrode is only 79%. It can be seen from the figure d below that at a cycle rate of 0.2C, the nano-grain copper foil also exhibits good cycle performance. After 100 cycles, the capacity retention rate is about 91%, while the negative electrode of ordinary copper foil The capacity retention rate is only 61%. Not only that, in terms of the capacity of the negative electrode active material, the nano-grain copper foil is significantly higher than the ordinary copper foil material. This is mainly because the surface roughness of the nano-grain copper foil increases significantly after ultrasonic stick pressing. Therefore, the bonding strength between the active material and the copper foil is significantly improved, and the conductivity of the electrode is improved, which is beneficial to the rate performance and cycle performance of the electrode.
The AC impedance spectrum of the negative electrode using two kinds of copper foils, the charge exchange impedance of the negative electrode of the nano-grain copper foil is 21.8Ω, and the charge exchange impedance of the negative electrode using the ordinary copper foil is 44.13Ω. The impedance should be significantly smaller than ordinary copper foil.
Tingting Xia, etc. converted the ordinary copper foil into a copper foil with a nano-grain structure by ultrasonic stick pressing, which greatly increased the roughness of the copper foil surface and improved the adhesion between the graphite negative electrode and the copper foil current collector Strength, improve the cycle performance and rate performance of graphite anode.