Generally speaking, we can improve the rate performance of lithium ion batteries by adjusting the choice of positive and negative materials and formula adjustment, such as selecting ternary materials and NCA materials with higher ionic conductivity and electronic conductivity, and selecting small particles for the negative electrode. Graphite material, or Li4Ti5O12 with a larger Li+ diffusion coefficient, reduces the impedance and polarization of the electrode by increasing the amount of conductive agent of carbon black, and improves the rate performance of the lithium ion battery. In fact, we pay attention to the selection of less current collectors. Will have a certain impact on the rate performance of lithium-ion batteries.
Generally speaking, the positive electrode of the lithium ion battery uses Al foil, and the negative electrode uses Cu foil as the current collector. The main function of the current collector is to conduct electrons from the positive and negative active materials. The common Al foil is mechanically compacted with pure Al. The method is compacted to 10-30um, the surface is relatively smooth, so the contact area between the active material and the Al foil is small, and the electron conduction between the Al foil and the active material may become a limiting link during large-rate discharge. Recently, Chang Uk Jeong (first author) and Kuk Young Cho (corresponding author) of Gongju National University of Korea have made the original smooth Al foil into a rugged surface by electro-corrosion method, adding LCO material and current collector. The contact area between the two increases the adhesion, reduces the polarization and contact resistance during charging and discharging, and greatly improves the rate performance and cycle life of the lithium ion battery.
The processing technology adopted by Chang Uk Jeong is as shown in the above figure. First, a 30V constant voltage power supply Al foil is used for electrolytic oxidation treatment for 10 minutes. A thick layer of Al2O3 is formed on the surface of the Al foil, and then etched with chromium oxide CrO3 and H3PO4 for 18 hours. , remove the surface of Al2O3, and form a rugged surface on the surface of the Al foil.
Al foils usually exhibit a certain degree of strength reduction after oxidation-corrosion treatment. In Figure a, Chang UkJeong compares the tensile strength of untreated Al foil, 30V oxidation for 5min and 30V oxidation for 10min. The tensile strength of the untreated Al foil reached 245 MPa. After 5 minutes of oxidation treatment and corrosion, the strength of the Al foil was reduced to 235 MPa, further increasing the oxidation treatment event to 10 min, the tensile strength of the Al foil was reduced to 227 MPa, and the tensile strength of the Al foil. The decrease is mainly due to the oxidation-corrosion treatment process destroying part of Al. This can be seen from the change in the thickness of the Al foil. It can be seen from the following figure b that the thickness of the untreated Al foil is 11 um, after 5 min of treatment. It decreased to 10.4um and decreased to 9.9um after 10min. Although the strength of the treated Al foil was reduced, the tensile strength of 227-235MPa can fully meet the requirements of Al foil strength during coating and winding process. .
The XRD analysis of the surface of the Al foil (as shown in the figure below) indicates that the surface of the Al foil is mainly composed of a cubic Al metal, and no diffraction peak of alumina is found, which indicates that the Al oxide formed during the first anodizing process It has been completely removed during the subsequent corrosion treatment of CrO3 and H3PO4. This ensures good conductivity of the Al foil surface, which is of great significance for reducing the contact resistance between the active material and the current collector and reducing the polarization of the electrode.
The surface of the Al foil after oxidation-corrosion treatment exhibits a honeycomb structure, which is mainly formed by the anodized layer on the surface of the Al foil being etched away, and the diameter of these honeycomb structures is anodized Voltage and time are closely related. Increasing the anodization voltage and processing time will increase the diameter of these honeycomb structures. At the same time, we have observed the contact angle of water with Al foil as the anodizing time increases and the anodizing voltage increases. Will show a decreasing trend, indicating that the hydrophilicity of Al foil increases.
Atomic force microscopy is more sensitive to the roughness of the material surface. Chang Uk Jeong observed the ordinary Al foil and the treated Al foil by atomic force microscopy. The results are shown in the figure below. The surface of ordinary Al foil is smooth and the roughness is 3.41. The roughness of the Al foil after 5 min treatment increased to 4.078, and the surface roughness of the Al foil after 10 min treatment was further increased to 4.98. At the same time, we can see from the figure that the surface of the Al foil showed a mountain-like protrusion after treatment. As observed in the previous SEM.
The changes in surface topography and roughness will eventually be reflected in the effect on the rate performance of lithium-ion batteries. The authors used LCO materials to test the electrochemical properties of the treated Al foil. From the following figure a, ordinary Al foil can be seen. The capacity of the Al foil treated for 5 min and treated for 10 min during the first charge was 184 mAh/g, 183 mAh/g and 189 mAh/g, respectively, and the subsequent discharge capacities were 176, 175 and 180 mAh/g, respectively. After the treatment, the polarization of the Al foil is reduced. For example, during the discharge process, the voltage platforms of the three are 3.82V, 3.84V and 3.86V respectively. In the cycle test, the difference between the three is small, but in the magnification test. A significant difference can be observed. It can be seen from the figure that when the current density is increased to 450 mA/g, the treated Al foil shows a very obvious advantage. When the current density is further increased to 750 mA/g, ordinary Al The discharge capacity of the foil electrode is only about 20mAh/g, and the Al foil after 10min treatment shows the best rate performance, and the capacity still reaches 145mAh/g.
The improved Al foil rate performance after treatment is mainly due to the decrease in contact resistance, which causes the polarization of the battery to decrease during discharge. The following figure compares the polarization resistance and depth of discharge of the battery in the 2nd and 20th cycles. The relationship between the two can be seen from the figure, the polarization resistance of the common Al foil is the largest, the polarization resistance of the Al foil after 5 min treatment is slightly lower, and the polarization resistance of the Al foil after 10 min treatment is the lowest, which is compared with the previous magnification test. The results are consistent.
Chang Uk Jeong prepared an Al foil with a rugged surface structure through a relatively simple anodizing-corrosion treatment process. This process has little effect on the tensile strength of Al foil, but can significantly enhance the active material and Al foil. The adhesion between the two, reduce the contact impedance, reduce the polarization, and effectively improve the rate performance of the lithium-ion battery.
