When the external power supply charges the lithium ion battery, the electron e on the positive electrode runs to the negative electrode through the external circuit, and the lithium ion Li+ 'jumps into the electrolyte from the inside of the positive electrode active material particle, 'crawling over the small diameter of the diaphragm. The pores, 'swim' reach the negative pole, and combine with the electrons that have already ran, entering the inside of the negative active material particles. If the negative electrode does not accept the position of lithium ions, lithium ions will precipitate on the surface of the negative electrode, forming lithium dendrites, piercing The diaphragm, causing a short circuit inside the battery, causes thermal runaway. Therefore, in the design of a lithium battery, the negative electrode often needs to be over-designed to avoid such a situation, including two aspects:
(1) N/P design, that is, the ratio of the negative electrode capacity per unit area to the positive electrode capacity, the NP ratio is generally between 1.1 and 1.5, ensuring that the negative electrode has a certain excess to avoid lithium dendrite precipitation, and the NP ratio is not required. System design considerations.
(2) Overhang design, Overhang refers to the part of the negative pole piece that has more length and width than the positive and negative pole pieces.
The design of the above two aspects of negative electrode excess needs to consider the battery manufacturing engineering ability, such as coating surface density precision, pole piece size accuracy, cell assembly precision, etc., in the production accuracy range, it is necessary to ensure the excess of the negative electrode. From the battery energy density and In terms of cost, the excess of the anode should be as low as possible. However, the actual situation is particularly complicated. N/P design and Overhang design need to consider various factors.
Then, what impact does the Overhang design have on the performance of lithium-ion batteries? Tim Daggera of the University of Münster in Germany did a special experiment to study this problem.
Figure 1 different Overhang design schematic
Figure 1 shows the different Overhang designs. Then, according to the procedure in Table 1, the above batteries are cycle tested. Then, the ICP test is performed on the pole pieces at different stages to study the distribution of the lithium concentration of the negative electrode sheets. The SD in Table 1 indicates that the CCCV is charged. Stand for 120h for battery self-discharge experiment, dcv means constant current discharge and then do 0.05C constant voltage discharge test.
Table 1 battery cycle test procedure
Figure 2 shows the effect of the Overhang design on the battery's first effect and capacity. As the excess area of the negative electrode increases, the battery's first effect decreases, and the battery capacity gradually decreases. During the charging process, part of the lithium ions diffuse into the negative electrode excess region. As a result, the first effect and capacity are reduced. After the 7th charge, the self-discharge is allowed to stand for 120 hours, the battery capacity is further reduced, and as the excess area of the negative electrode increases, the self-discharge capacity loss increases. However, the subsequent charge and discharge cycle, partial capacity Can be restored again, when the excess area of the negative electrode is relatively large, the number of cycles of capacity recovery is more, as shown in Figure 3.
Figure 2 Effect of Overhang design on battery efficiency and capacity
Figure 3 The impact of SD and dcv on different Overhang designs
The above process is accompanied by the self-diffusion of lithium ions. As shown in Fig. 4, the self-discharge static experiment after charging, the lithium ion of the negative electrode piece is self-diffused, and the uniform distribution is in the entire area of the negative electrode piece, which also includes the overhang area. Part of the lithium ion diffuses from the overlap region of the positive electrode to the overhang region, and the lithium ion in the overhang region remains in the negative electrode after discharge, which reduces the discharge capacity. In the subsequent cycle, some residual lithium ions in the overhang region diffuse to the overlap region with the positive electrode. Function, capacity recovery, as shown in Figure 3, the discharge capacity after the 8th cycle is higher than the charge capacity.
Figure 4: Lithium concentration distribution of the negative electrode sheet: (a) schematic diagram of the pole piece, (b) state of charge after the 7th cycle (after the self-discharge experiment), (c) discharge state after the 7th cycle, (d) subsequent cycle discharge status
In order to accelerate the diffusion of residual lithium ions in the overhang region to the overlapping region, a small current constant voltage discharge is added after 20 cycles of discharge. Under the action of the electric field, the lithium ions in the overhang region accelerate into the overlapping region, as shown in FIG. After that, the capacity recovery is more obvious, and the larger the area of the overhang area, the more capacity recovery.
Table 2 Lithium concentration in overhang region under different states
In order to confirm the above conclusions, the authors performed an ICP test to test the lithium concentration in the overhang region of the negative electrode, as shown in Table 2. The lithium concentration after the cc discharge was 0.81 mg, if it was allowed to stand for 120 h after charging, then the cc was placed in the overhang region. It is 0.98mg, indicating that lithium diffuses from the overlap region to the overhang region, and remains in this region after discharge. If the lithium concentration in the overhang region is reduced after the discharge, the lithium diffusion back to the overlap region plays a role, and the lithium concentration is detailed. 5 is shown.
Figure 5: Lithium concentration distribution of the negative electrode sheet: (a) not cycled, (b) discharge state after the sixth cycle (no self-discharge), (c) discharge state after the seventh cycle (after self-discharge experiment), (d The discharge state after the 20th cycle (after the constant voltage discharge experiment)
Conclusion: Overhang will affect the electrochemical performance of the battery. The positive and negative electrodes are completely overlapped. The battery without overhang design has the best performance. However, due to the engineering accuracy, the battery is prone to lithium. Overhang will cause lithium ions to diffuse. Residual capacity loss, especially if stored for a long time in the state of charge is more obvious. After the discharge, a small current and constant voltage discharge can make the residual lithium ions in the overhang area diffuse back to the overlapping area.
