For lithium-ion batteries, the uniformity of the electrode coating for its electrical properties have an important impact, in order to ensure the uniformity of the electrode, in general, we have to smash many times before coating, try to ensure uniform electrode But in fact the battery slurry in the use of active substances will occur sedimentation, stratification and viscosity changes and other issues, so the process of coating the state of the electrode will continue to change, in order to ensure that the coating process In the uniformity, we also need some online detection tools, the current common online detection tool is mainly used for different substances for the different shading rate of the electrode to the amount of electrode coating on-line detection, such as common g-ray, X-ray, etc. , These online testing tools not only expensive, but also allow operators to varying degrees of ionizing radiation, but also the existence of radiation source regulation of the problem, so from the perspective of lithium-ion battery production, we expect to use a safe , Efficient online detection means.
Recently, Przemyslaw Rupnowski of the National Renewable Energy Laboratory and Oak Ridge National Laboratory have developed an on-line method for the detection of electrodes using infrared imaging technology, which not only detects the amount of coating (surface density) The porosity of the electrode is measured in real time.The principle of this method is that the electrode is heated for a short time, and then the temperature of the electrode is detected by infrared camera.The finite element analysis shows that the electrode temperature is increased by the electrode porosity and coating The thickness of the electrode (thickness) of the double impact, through the electrode temperature rise of the reverse derivation of the parameters, with real-time electrode thickness measurement, we can get the electrode porosity and other parameters.
Thermal imaging technology is a relatively mature technology, and recent thermal imaging techniques have also been used to detect defects in lithium ion electrodes by applying a pulsed heating (e.g., flash, infrared laser, etc.) to the electrodes and then using the infrared camera to record the thermal response of the electrodes , The temperature of the electrode can be obtained by analyzing the uniformity of the electrode temperature distribution. It is shown that the temperature feedback of the electrode is affected by the porosity and thickness of the electrode. Therefore, the production of the electrode can be detected by infrared imaging.
Infrared on-line detection tool as shown below, including a heat source and an infrared camera, the electrode under the action of the heat source temperature will rise, leaving the heat source after the electrode temperature will drop, and ultimately return to normal temperature, the entire process by the infrared camera , For subsequent analysis.
The positive electrode active material is NMC532, the negative electrode active material is graphite, and the auxiliary components such as carbon black and PVDF are also used as well as copper foil, aluminum foil, and the like. Etc., the thermal conductivity of several different materials and other parameters listed in Table 2.
In order to predict the reaction of the electrode to the heating, Przemyslaw Rupnowski first established a model for the electrode (the electrode contains two solid particles), as shown in the following figure.
The thermal properties of the electrode are mainly affected by two parameters - the specific heat capacity cp and the thermal conductivity K, the two parameters of the electrode by the thermal properties of components, as well as their volume fraction, weight fraction, so the specific heat capacity of the electrode Can be calculated from the following formula
The parameters such as the thermal conductivity and specific heat capacity of the two positive and two negative electrodes calculated from the above model are shown in the following table.
According to the above model PrzemyslawRupnowski further use the finite element analysis tool to simulate and simulate the temperature feedback signal of the battery under the action of heat source. The simulation model is shown in the following figure.
PrzemyslawRupnowski uses the above finite element simulation model to simulate the two states of the electrode, namely 'static electrode' and 'fixed speed moving electrode'.
Static electrode
The following figure shows the simulation results and experimental results of the temperature rise curve of the static electrode under the action of the heat source. In Fig. A, we can see that the simulation results are in good agreement with the experimental results. Very close to the trend.When we note that both in the experimental results or in the simulation results in the C1 electrode temperature changes are significantly faster than C2, which is mainly because the C1 electrode is thinner, porosity is higher, which will affect the electrode Thermal conductivity and specific heat capacity of the thermal characteristics.
It is interesting to note that although the porosity and thickness of the two electrodes are significantly different in Figure b, the temperature curve has a highly uniform curve, which is not only reflected in the simulation results. The experimental results also show the same The curve.
Moving electrode
The following figure shows the heat source for continuous heat input, the electrode at 0.15m / min speed movement of the electrode temperature along the direction of movement of the electrode from the figure c we can note that the experimental results and simulation results in line with the very good The temperature distribution of the two positive poles is very different, and C1 has a higher porosity and a thinner thickness, so the maximum temperature is higher, and we note that the temperature drop on the left side of C1 is also faster , Which is also in line with our prediction of its characteristics.From the figure we note that the two most sensitive to the change in electrode parameters are mainly the 'highest temperature' of the temperature profile and the 'slope' of the left curve.
The temperature profile of the two negative poritudes is almost identical to that of the two different porosities and thicknesses, because there is no representation of the change in the two parameters for the change in the two parameters compared to the positive porosity for parameters such as porosity and thickness.
The above experiments show that we can derive the porosity and thickness parameters of the electrode by the temperature feedback of the heat source of the positive electrode.To derive the relationship, Przemyslaw Rupnowski has carried out two experiments, the first experiment is the fixed electrode Thickness of 60um, change the porosity of the electrode, the second experiment is fixed electrode porosity 61%, change the thickness of the electrode 35-185um, observe the heat source heating temperature feedback, the results shown below.
Figure 1 is the maximum temperature of the electrode with the electrode porosity of the curve, Figure b for the electrode maximum temperature with the electrode thickness of the curve, from the figure we can note that in a wide range, the maximum temperature of the electrode and electrode pores The thickness of the electrode and the electrode is almost linearly changed, which also provides us with this data for the battery porosity on-line detection provides the feasibility.
From the above figure, we note that the sensitivity of the negative feedback is higher than that of the positive pole, but the feedback between the two different negative electrodes on the temperature in the previous experiment is almost the same, which seems contradictory between the two. From Table 1 we note that the increase in the thickness of the positive electrode leads to a decrease in the porosity of the electrode, but the negative electrode is opposite to the electrode thickness, but instead increases the porosity of the electrode. From the above picture, we can note that the increase in porosity The increase in thickness will weaken the temperature feedback of the electrode, so the thickness of the negative electrode increases, the porosity increases, both of which affect the thermal properties of the electrode offset each other, resulting in two kinds of negative thermal characteristics are almost the same.
PrzemyslawRupnowski proposed the use of electrode porosity and thickness for its thermal properties, combined with the current more mature on-line thickness measurement technology (such as the use of the infrared imaging detection method, for our online detection of electrode porosity provides a very favorable tool. Laser thickness measurement, etc.), can achieve on-line detection of electrode porosity, is conducive to improving the quality of the electrode to improve the electrical performance of lithium-ion batteries.
