low specific resistance graphite electrodes
The lithiation and expansion process of low specific resistance graphite electrodes
Swelling of lithium-ion batteries with low specific resistance graphite electrodes in Ulanqab is a common problem, especially for large aluminum shells and soft-pack batteries. The causes of swelling of lithium batteries are divided into two categories. One is caused by the change in the thickness of the battery pole pieces. Swelling; the second is the swelling caused by the oxidation and decomposition of electrolyte to produce gas. Battery swelling, on the one hand, changes in battery thickness and stress may cause changes in battery performance, which will adversely affect battery life and reliability. On the other hand, it also restricts the group design of the battery. Gas generated inside the battery is an important cause of battery swelling. Whether the battery is stored at room temperature, high temperature cycle, or high temperature, it will produce different degrees of swelling and gas production. According to the current research results, the essence of the cell bloating is caused by the decomposition of the electrolyte.
There are two cases of electrolyte decomposition,
That there are impurities in the electrolyte, such as moisture and metal impurities that cause the electrolyte to decompose and produce gas, and the other is that the electrochemical window of the electrolyte is too low, causing decomposition during the charging process, and the solvents such as EC and DEC in the electrolyte are getting After electrons, free radicals are generated. The direct consequence of free radical reactions is the production of low-boiling hydrocarbons, esters, ethers and CO2.
The thickness changes of battery pole pieces have the following situations:
(1) After the pole piece is rolled, the thickness of the pole piece rebounds when it is placed. The greater the compaction density, the greater the rebound; under the same stress, the greater the elastic modulus of the adhesive, the smaller the pole piece placed on hold, and the drying will also cause The pole piece rebounds.
(2) The pole piece absorbs the electrolyte and swells, and the thickness of the pole piece increases.
(3) During the charging and discharging process, the low specific resistance graphite electrodes swelling caused by the change of the lattice parameter caused by the lithium insertion. This article mainly introduces the lithiation of the graphite negative electrode and the expansion process of the pole piece of the lithium ion battery. Graphite button half-cell lithiation and pole piece expansion process are shown in Figure 1. During the first discharge lithiation, as lithium ions are inserted between the graphite layers, the potential of the low specific resistance graphite electrodes gradually decreases, and the pole piece thickness expansion rate gradually Increase.
The whole process can be divided into multiple stages from a to e. As the amount of lithium intercalated between graphite layers increases (x gradually increases), LixC6 has several different phases. Table 1 lists the characteristics of these phases, and x represents The molar content of lithium in compound LixC6, d is the lattice parameter graphite interlayer spacing. As the amount of lithium insertion increases, the graphite changes sequentially from 2H phase. When the SOC is 50%, it changes to LiC12, and becomes LiC6 after complete lithiation. The theoretical capacity is 372 mAh/g. During this transition, the layer spacing d gradually increases, which leads to an increase in the thickness of the pole piece. As shown in the figure, the lithiation and expansion process of each stage is as follows:
(1) f+e interval: during the first lithiation of graphite, in the 800 mV-200 mV voltage interval, it is mainly the SEI film formation process, the particle rearrangement process in the pole piece, and the process of 2 H => 1 L. The expansion rate of the pole piece is about 1.5%.
(2) d+c interval: In the voltage interval of 200 mV-100 mV, the transformation process of 1 L=>4 L=>3 L mainly occurs, and the expansion rate of the pole piece is also about 1.5%.
(3) Interval b: In the 100 mV voltage platform, the 3 L => 2 process mainly occurs, during which the pole piece hardly expands.
(4) Interval a: In the 70 mV voltage platform, the 2 => 1 process mainly occurs. In this process, the expansion rate of the pole piece is about 1.2%. The subsequent delithiation process, except for the formation of the SEI film, is almost irreversible at all stages. The change process of delithiation voltage goes through the process of a, b, c, d, e in sequence, and the corresponding pole piece expansion process is A, B, C, D, E in sequence. 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. At this stage, the 3 L => 2 process mainly occurs. We can explain this phenomenon from the change of the layer spacing during the transition process. The slope D of the expansion curve can be derived from the following formula, which is calculated according to the changes in the interlayer spacing and the change in the lithium content in each stage. According to the calculation results below, the transition slope of 3 L => 2 is much smaller than that of other processes. Therefore, the expansion hardly occurs.