Lithium-ion battery charge and discharge process, the battery pole inside the existence of lithium ions and electron transport, in which lithium ions through the electrode pores filled with electrolyte transmission, and electrons mainly through solid particles, especially conductive agent composed of three-dimensional network conduction to The effect of electron transport properties on the performance of the battery is mainly affected by the efficiency of the battery, while the main factors affecting the conductivity of the battery pole include the bonding interface between the foil substrate and the coating The state of the conductive agent, the contact state between the particles, etc. The electrical conductivity of the electrode pole piece can judge the uniformity of the microstructure in the pole piece and predict the performance of the battery.Based on its experience and literature, The conductivity test method is briefly summarized, and some factors influencing the conductivity of the pole piece are listed.
1, the battery pole conductivity test method
(1) Method 1: four probe membrane impedance test method
Four probe test method shown in Figure 1, the infinite number of uniform samples on the four samples of the distance between the S arranged in a straight line from the constant current source to the outside of the two probes 1, 4 into the small current I , Measure the potential difference between the two probes 2, 3 U, then the value of U, I, S according to the formula (1) obtained by the sample resistivity ρ.
According to the four-probe membrane impedance test principle, in the field of lithium-ion battery, this method is often used to test the plasma membrane impedance, through the resistivity quantitative analysis of the distribution of conductive agent in the slurry to determine the dispersion effect of the slurry The test procedure was as follows: the coating was uniformly coated on the insulating film with an applicator, and then dried and dried. After drying, the thickness of the coating was measured and the size of the sample was cut to meet the infinite requirement (more than four times the probe pitch) Finally, four electrodes were used to measure the electrode film impedance, and the resistivity was calculated from the thickness.
The four-probe membrane resistance test method avoids the contact resistance between the probe and the sample, and the parallel direction of the test current and the coating also avoid the base diversion, so the method can accurately measure the absolute resistance of the battery pole coating. The method can only characterize the resistance of the thin layer on the surface of the coating. It can not fully characterize the pole piece resistance value for the thicker and present component gradient. In addition, it can not test the contact between the coating and the substrate in the real pole piece resistance.
(2) Method two: two probe pole piece overall resistivity direct measurement method
ArnoKwade et al. Used the two-probe method to directly measure the overall resistivity of the pole piece, as shown in Fig. 2, where the measured resistance includes the resistance of the probe itself, the probe, The contact resistance of the coating with the coating, the resistance of the coating, the contact resistance between the coating and the current collector, and the current resistance of the current collector can be expressed by the formula (5).
A detailed measurement device is shown in Figure 3. The test probe is mounted on a material mechanical performance test device. The measurement of the pole piece resistance is very easy. The main parameters include the loading current and the probe application pressure. The measured resistivity ρ is calculated by equation (6):
In the formula, R is the measured resistance value, A is the contact area, U is the detection voltage, I is the loading current, δ is the thickness of the pole piece, and Δδ is the change value of the thickness of the pole after pressing.
Since the method includes the resistance of the probe, the probe and the coating, the absolute value of the pole piece resistivity can not be measured, but it contains some advantages: the electronic conduction path of the test process is substantially the same as that of the actual battery, and a total test value The effects of the test parameters on the results are shown in Figures 4 and 5. For LFP poles with high resistance, the load current is small, and the effect of the test parameters on the results is shown in Fig. Stable results, and for the low resistance of the graphite electrode, the load current is relatively high to get a stable result, we can see from Figure 4, LFP, NMC, graphite electrode, load current 10mA or more can be relatively stable measurement results. High, graphite electrode resistivity decreased to reach more than 350kPa, the test results have nothing to do with the pressure.
2, pole piece conductivity of the influencing factors examples
(1) the effect of the content of the conductive agent on the conductivity of the pole piece
In the negative electrode, the proportion of conductive material is very small, and the conductivity of preserved eggs is mainly the contribution of graphite particles, while the positive electrode, the active material of the electronic conductivity is very small, negligible, pole piece conductivity is mainly conductive agent network contribution. , For the positive pole piece, the conductive agent content must be able to increase the conductivity of the pole piece, as shown in Figure 6. Conductive agent and binder ratio remains unchanged, the conductive agent content increased, the proportion of living material decreased pole pole conductivity When the binder ratio is increased, the conductivity of the pole piece is also increased.
(2) Effect of dry powder agitation on the conductivity of pole piece
The intensity of the high-speed dispersion can be characterized by the Froude-toolnumber, defined as the ratio of the centrifugal force to the gravity acting on the particles, which can be described by equation (1). When the radius of the rotor remains constant, Fr depends on the rotor The speed of the ω, the higher the rotor speed, the greater the number of Froude, indicating that the greater the intensity of high-speed dispersion.
(3) the effect of compaction density on the conductivity of the pole piece
As shown in Figure 8, the resistivity of the poles continues to decrease as the compaction density increases, and the effective conductance characteristics of the lithium ions and electrons are contradictory. As the compaction density The porosity decreases, and the volume fraction of the conductive agent increases, and the electron effective conductivity increases, but the effective conductivity of the lithium ion decreases, and it is also critical to balance the two in the electrode design.
There are many factors influencing the conductivity of the battery pole, such as the distribution of the conductive agent, the combination of the current collector and the coating, and so on.
