Lithium-ion battery bulging is a common problem, especially for large aluminum and soft-pack batteries. Lithium battery bulging is divided into two categories, one is the swelling caused by the thickness variation of the battery pole piece; the second is due to the oxidative decomposition of the electrolyte. The bulging caused by gas production. Battery bulging, on the one hand, changes in battery thickness and stress may cause changes in battery performance, adversely affecting battery life and reliability. On the other hand, it also restricts the battery group design.
The internal gas production of the battery is an important cause of battery bulging. Whether the battery is in normal temperature circulation, high temperature circulation, high temperature, it will produce different degrees of bulging gas production. According to the current research results, the essence of the battery core is flat. It is caused by decomposition of the electrolyte. There are two cases of electrolyte decomposition. One is that the electrolyte has impurities, such as moisture and metal impurities to decompose the electrolyte, and the other is that the electrochemical window of the electrolyte is too low, causing charging. In the process of decomposition, EC, DEC and other solvents in the electrolyte will generate free radicals after the electrons are obtained. The direct consequence of the free radical reaction is the production of low boiling hydrocarbons, esters, ethers and CO2.
There are several changes in the thickness of the battery pole piece:
(1) After the pole piece is rolled, the thickness rebounds when it is left, and the larger the compaction density, the larger the rebound; under the same stress, the larger the elastic modulus of the adhesive, the smaller the rebound of the pole piece, and the drying will also result in The pole piece rebounded.
(2) The pole piece absorbs the swelling of the electrolyte and increases the thickness of the pole piece.
(3) Lithium insertion causes electrode expansion caused by changes in lattice parameters during charge and discharge.
2. This paper mainly introduces the lithium ionization and the pole piece expansion process of lithium ion battery graphite anode.
The graphite button half-cell lithiation and the pole piece expansion process are shown in Fig. 1. During the first discharge lithiation, as the lithium ions are intercalated between the graphite layers, the electrode potential gradually decreases, and the pole piece thickness expansion rate gradually increases. It can be divided into multiple stages a~e. As the lithium content embedded in the graphite layer increases (x increases gradually), LixC6 has several different phases. Table 1 lists the characteristics of these phases, and x represents the compound LixC6. The molar content of lithium in the medium, d is the lattice parameter graphite layer spacing. As the lithium intercalation increases, the graphite changes from the 2H phase. When the SOC is 50%, it changes to LiC12, and after complete lithiation, it becomes LiC6, and the theoretical capacity is 372mAh/ g. During this transition, the layer spacing d is gradually increased, resulting in an increase in the thickness of the pole piece.
In Figure 1, the lithiation and expansion processes at each stage are as follows:
(1) f+e interval: When the graphite is first lithiated, in the voltage range of 800mV-200mV, mainly the SEI film formation process, the particle rearrangement process in the pole piece, and the process of 2H =>1L, the overall pole piece expansion rate About 1.5%.
(2) d+c interval: In the voltage range of 200mV-100mV, the main conversion process is 1L=>4L=>3L, and the pole piece expansion rate is also about 1.5%.
(3) b interval: On the 100mV voltage platform, the main process occurs 3L=>2. In this process, the pole piece hardly expands.
(4) a range: In the 70mV voltage platform, the main 2=>1 process occurs. In this process, the pole piece expansion rate is about 1.2%.
Subsequent delithiation process, except for the formation of SEI film, the various stages are almost irreversible. The process of delithiation voltage changes through a, b, c, d, e process in turn, and the corresponding pole piece expansion process is A, B, C, D, E. It can be seen from the figure that in the B interval, the pole piece hardly expands, and the slope of the expansion curve is almost 0. This stage mainly occurs 3L=>2 process. We can explain this from the variation of the layer spacing during the transformation process. Phenomenon. The slope D of the expansion curve can be derived from the following equation. According to the variation of the layer spacing and the change of lithium content in each stage, it can be seen from the following calculation results that the slope of 3L=>2 is much smaller than other processes, so the expansion hardly occurs.
Fig.1 Electrochemical expansion process of graphite electrode charging and discharging electrode
Figure 2 shows the evolution of the XRD pattern measured on-line during graphite deintercalation of lithium. It can visually see the evolution of various phases in the process of graphite delithiation.
Figure 2 Online XRD pattern of graphite charge and discharge process
Figure 3 is the expansion curve of the NMC-graphite full battery. The expansion ratio is the second derivative of the capacity. The two inflection points of the expansion curve are obtained, which correspond to x=0.23 and x=0.5 respectively. It can be seen from Table 1 that these two points correspond to Table 1. In the 3L and 2 phases, between the two points, the battery hardly expands, and the slope of the expansion curve is small, which is consistent with the expansion curve of the graphite electrode, corresponding to the 3L=>2 transition process, and its slope is much smaller than other processes. Therefore, the expansion process of the whole battery mainly depends on the expansion of the graphite electrode.
