Lithium battery fast charge

Borrow this figure to illustrate the battery charging process, the abscissa is the time and the ordinate is the voltage. There will be a small current pre-charge process at the beginning of lithium battery charging, that is, CC Pre-charge, the purpose is to stabilize the positive and negative materials. After that, after the battery state is stable, it can be adjusted to high current charging, that is, CC Fast Charge. Finally, enter the constant voltage charging mode (CV). For lithium batteries, the system starts the constant voltage charging mode after detecting that the voltage reaches 4.2V, the charging current is gradually reduced, and the charging ends when it is less than a certain value.

In the whole process, different batteries have different standard charging currents. For example, the battery standard for 3C products is generally 0.1C-0.5C, and for high-power power batteries, the standard charging is generally 1C. Choosing a lower charging current also considers the safety of the battery. Therefore, the usual fast charging refers to the charging current that is several times to tens of times higher than the standard charging current.

Some people say that charging a lithium battery is like pouring beer. It's fast and fills up with beer, but it has a lot of foam. The pour is slow and slow, but there is a lot of beer, which is very real. While fast charging saves charging time, it will also cause greater damage to the battery itself. Due to the polarization phenomenon in the battery, the maximum charge current that it can accept will decrease with the increase of the charge and discharge cycle. When the charge is continued and the charge current is large, the ion concentration at the electrode will increase, and the polarization will increase. The terminal voltage cannot be directly linearly proportional to the charged power/energy. At the same time, when charging with high current, the increase of internal resistance will increase the Joule heating effect (Q=I2Rt), and cause side reactions, such as the reaction decomposition of electrolyte, gas production and a series of problems. The risk factor suddenly increases, which will affect the safety of the battery. Influence, the life of non-power batteries will inevitably be greatly shortened.

01 Cathode material

The process of fast charging of lithium batteries is the process of rapid migration of Li+ in the positive electrode material into the negative electrode. The particle size of the positive electrode material can affect the response time of the battery electrochemical process, the diffusion path of ions, etc. According to research, as the grain size of the material decreases, the diffusion coefficient of lithium ions increases. However, as the particle size of the material decreases, there will be serious particle agglomeration in the production of pulp, resulting in uneven dispersion. At the same time, the nanoparticles will reduce the compaction density of the pole pieces and contact the electrolyte during the charge and discharge process. The area increases and side reactions affect the performance of the battery.

       A more reliable method is to coat and modify the positive electrode material. For example, the conductivity of LFP itself is not very good. After the surface is coated with carbon material or other materials, its conductivity can be improved, which is beneficial to improve the fast charging of the battery. performance.

02 Anode material

Fast charging of lithium batteries means that lithium ions are quickly extracted and "swimming" to the negative electrode. At this time, the negative electrode material needs to have the ability to quickly insert lithium. The anode materials used for fast charging of lithium batteries include carbon materials, lithium titanate and other new materials.

       For carbon materials, since the potential of lithium insertion is similar to the potential of lithium precipitation, in the case of conventional charging, lithium ions are generally preferentially inserted into graphite, but under fast charging or low temperature conditions, lithium ions may precipitate on the surface to form dendritic lithium . Dendrite lithium pierces the SEI, causing secondary loss of Li+ and reducing battery capacity. When the lithium metal reaches a certain amount, it will grow from the negative electrode to the separator, causing the risk of short-circuiting the battery.

       For LTO, it is a "zero strain" oxygen-containing negative electrode material, which does not produce SEI when the battery is working, and has a stronger binding capacity with lithium ions, which can meet the requirements of fast charge and fast discharge. At the same time, it is precisely because the SEI cannot be formed, the negative electrode material will directly contact the electrolyte, which promotes the occurrence of side reactions. The gas production problem of LTO batteries cannot be solved for a long time and can only be alleviated by surface modification.

03 Electrode solution

As mentioned earlier, due to the inconsistency of lithium ion migration speed and electron transmission rate during fast charging, the battery will have a greater polarization. So in order to minimize the negative reaction caused by battery polarization, the following three points will be the direction of electrolyte research and development: 1. High dissociation degree electrolyte salt; 2. Solvent recombination-lower viscosity; 3. Interface control-membrane impedance Lower.

04 The relationship between production technology and fast charging

Previously, the requirements and effects of fast charging were analyzed from three key materials, such as positive and negative electrode materials and electrode liquid. The following is a process design that has a relatively large impact. The battery manufacturing process parameters directly affect the migration resistance of lithium ions in various parts of the battery before and after the battery is activated, so the battery manufacturing process parameters have an important impact on the performance of the lithium ion battery.

(1) Slurry

       Regarding the properties of the slurry, one aspect is to maintain a uniform dispersion of the conductive agent. Because the conductive agent is evenly distributed among the active material particles, a relatively uniform conductive network can be formed between the active materials and between the active material and the current collector, which has the function of collecting micro currents, reducing contact resistance, and increasing the speed of electrons. . The other aspect is to prevent the over-dispersion of the conductive agent. During the charge and discharge process, the crystal structure of the positive and negative materials will change, which may cause the conductive agent to peel off, increase the internal resistance of the battery, and affect the performance.

(2) Pole piece area density

       In theory, rate-type batteries and high-capacity batteries cannot have both. When the surface density of the positive and negative pole pieces is low, the diffusion rate of lithium ions can be increased, and the resistance of ion and electron migration can be reduced. The lower the areal density, the thinner the pole piece, and the smaller the change in the structure of the pole piece caused by the continuous insertion and extraction of lithium ions during charging and discharging. However, if the areal density is too low, the energy density of the battery will be reduced and the cost will increase, so a comprehensive consideration of the areal density is required. The following figure is an example of 6C charging and 1C discharging of lithium cobalt oxide battery, you can see:

(3) Consistency of pole piece coating

       A friend asked before, will the inconsistent surface density of the poles affect the battery? By the way, for fast charging performance, it is mainly the consistency of the negative pole piece. If the surface density of the negative electrode is inconsistent, after rolling, the internal porosity of the active material will be quite different. The difference in porosity will cause the difference in internal current distribution, affect the formation and performance of SEI in the battery formation stage, and ultimately affect the fast charging performance of the battery.

(4) Compaction density of pole piece

       Why should the pole piece be compacted? One is to increase the specific energy of the battery, and the other is to improve the battery performance. Different electrode materials have different optimal compaction density. Increasing the compaction density, the smaller the porosity of the electrode pole piece, the tighter the connection between the particles, and the smaller the thickness of the pole piece under the same areal density, thus reducing the migration path of lithium ions. When the compaction density is too high, the electrolyte infiltration effect is not good, which may damage the material structure and conductive agent distribution, and winding problems will occur later. The same is the 6C charge of lithium cobalt oxide battery 1C discharge, the impact of compaction density on discharge specific capacity is as follows:

05 Chemical aging and others

For carbon anode batteries, formation-aging is a key process for lithium batteries, and this process will affect the quality of SEI. The uneven thickness or unstable structure of the SEI will affect the fast charging capacity and cycle life of the battery.

        In addition to the above several important factors, cell production and charging and discharging systems will have a greater impact on the performance of lithium batteries. As the use time is prolonged, the battery charging rate should be moderately reduced, otherwise it will increase polarization.

Conclusion

The essence of rapid charging and discharging of lithium batteries is that lithium ions can be quickly extracted between the positive and negative materials. Battery material properties, process design, and charging and discharging system will all have an impact on high current charging performance. The structural stability of the positive and negative electrode materials will not cause structural collapse during the rapid lithium removal process, and the lithium ions diffuse faster in the material to withstand high current charging. Due to the mismatch between ion migration speed and electron transmission rate, polarization will occur during charging and discharging. It is necessary to reduce polarization as much as possible to prevent the precipitation of lithium metal and reduce capacity to affect life.