According to the requirements of group friends, talk about the understanding of lithium battery quick charging:

The picture

Use this diagram to illustrate the process of battery charging. The abscissa is time and the ordinate is voltage. At the initial charging stage of lithium battery, there will be a small current pre-charge process, namely CC pre-charge, which aims to stabilize the anode and cathode materials. After that, the battery can be adjusted to Charge with high current, namely CC Fast Charge, after the battery is stable. Finally, it enters the constant voltage charging mode (CV). For lithium battery, the system starts constant voltage charging mode when the voltage reaches 4.2V, and the charging current gradually decreases until the charging ends when the voltage is lower than a certain value.

During the whole process, there are different standard charging currents for different batteries. For example, for 3C products, the standard charging current is generally 0.1C-0.5C, while for high-power power batteries, the standard charging is generally 1C. The low charging current is also considered for the safety of the battery. So, say at ordinary times fast charge, it is to point to several times higher than standard charge current to tens of times.

Some people say that charging lithium batteries is like pouring beer, fast and filling beer fast, but with a lot of foam. It’s slow, it’s slow, but it’s a lot of beer, it’s solid. Fast charging not only saves charging time, but also damages the battery itself. Due to the polarization phenomenon in the battery, the maximum charging current it can accept will decrease with the increase of the charge and discharge cycle. When the continuous charging and the charging current is large, the ion concentration at the electrode increases and the polarization intensifies, and the battery terminal voltage cannot directly correspond to the charge/energy in a linear proportion. At the same time, high current charging, the increase of internal resistance will lead to intensified Joule heating effect (Q=I2Rt), bringing side reactions, such as the reaction decomposition of electrolyte, gas production and a series of problems, the risk factor increases suddenly, has an impact on battery safety, the life of non-power battery will be greatly shortened.

01

The anode material

The rapid charging process of lithium battery is the rapid migration and embedding of Li+ in the anode material. The particle size of the cathode material can affect the response time and diffusion path of ions in the electrochemical process of the battery. According to studies, the diffusion coefficient of lithium ions increases with the decrease of the grain size of the material. However, with the decrease of material particle size, there will be serious agglomeration of particles in the production of pulping, resulting in uneven dispersion. At the same time, nanoparticles will reduce the compaction density of the electrode sheet, and increase the contact area with the electrolyte in the process of charge and discharge side reaction, affecting the performance of the battery.

The more reliable method is to modify the positive electrode material by coating. For example, the conductivity of LFP itself is not very good. Coating the surface of LFP with carbon material or other materials can improve its conductivity, which is conducive to improving the quick charging performance of the battery.

02

Anode materials

Fast charging of lithium battery means that lithium ions can quickly come out and “swim” to the negative electrode, which requires the cathode material to have the ability of fast embedding lithium. The anode materials used for rapid charge of lithium battery include carbon material, lithium titanate and some other new materials.

For carbon materials, lithium ions are preferentially embedded into graphite under the condition of conventional charging because the potential of lithium embedding is similar to that of lithium precipitation. However, under the condition of fast charging or low temperature, lithium ions may precipitate on the surface and form dendrite lithium. When dendrite lithium punctured SEI, Li+ secondary loss was caused and battery capacity was reduced. When the lithium metal reaches a certain level, it will grow from the negative electrode to the diaphragm, causing the risk of battery short circuit.

As for LTO, it belongs to the “zero strain” oxygen-containing anode material, which does not produce SEI during battery operation, and has stronger binding ability with lithium ion, which can meet the requirements of fast charge and release. At the same time, because SEI cannot be formed, the anode material will directly contact with the electrolyte, which promotes the occurrence of side reactions. The problem of LTO battery gas generation cannot be solved, and can only be alleviated by surface modification.

03

Electrode liquid

As mentioned above, in the process of fast charging, due to the inconsistency of lithium ion migration rate and electron transfer rate, the battery will have a large polarization. So in order to minimize the negative reaction caused by battery polarization, the following three points are needed to develop the electrolyte: 1, high dissociation electrolyte salt; 2, solvent composite – lower viscosity; 3, interface control – lower membrane impedance.

04

The relationship between production technology and fast filling

Before, the requirements and influences of fast filling were analyzed from three key materials, such as positive and negative electrode materials and electrode liquid. The following is the process design that has a relatively large impact. The technological parameters of battery production directly affect the migration resistance of lithium ions in each part of the battery before and after battery activation, so the technological parameters of battery preparation have an important influence on the performance of lithium ion battery.

(1) slurry

For the properties of slurry, on the one hand, it is necessary to keep the conductive agent evenly dispersed. Because the conductive agent is evenly distributed among the particles of the active substance, a more uniform conductive network can be formed between the active substance and the active substance and the collector fluid, which has the function of collecting micro current, reducing the contact resistance, and can improve the movement rate of electrons. On the other hand is to prevent the over-dispersion of conductive agent. In the charging and discharging process, the crystal structure of anode and cathode materials will change, which may cause the peeling off of conductive agent, increase the internal resistance of the battery, and affect the performance.

(2) Extremely partial density

In theory, multiplier batteries and high-capacity batteries are incompatible. When the polarization density of the positive and negative electrodes is low, the diffusion velocity of lithium ions can be increased, and the ion and electron migration resistance can be reduced. The lower the surface density is, the thinner the electrode is, and the change of the electrode structure caused by the continuous insertion and release of lithium ions in charge and discharge is also smaller. However, if the surface density is too low, the energy density of the battery will be reduced and the cost will increase. Therefore, the surface density should be considered comprehensively. The following figure is an example of lithium cobalate battery charging at 6C and discharging at 1C.

The picture

(3) Polar piece coating consistency

Before, a friend asked, will extremely partial density inconsistency have an impact on the battery? Here by the way, for fast charging performance, the main is the consistency of the anode plate. If the negative surface density is not uniform, the internal porosity of the living material will vary greatly after rolling. The difference of porosity will lead to the difference of internal current distribution, which will affect the formation and performance of SEI in the formation stage of battery, and ultimately affect the fast charging performance of battery.

(4) Compaction density of pole sheet

Why do poles need to be compacted? One is to improve the specific energy of the battery, the other is to improve the performance of the battery. The optimum compaction density varies with the electrode material. With the increase of compaction density, the smaller the porosity of electrode sheet, the closer the connection between particles, and the smaller the thickness of electrode sheet under the same surface density, so the migration path of Lithium ions can be reduced. When the compaction density is too large, the infiltration effect of electrolyte is not good, which may destroy the material structure and the distribution of conductive agent, and the later winding problem will occur. Similarly, lithium cobalate battery is charged at 6C and discharged at 1C, and the influence of compaction density on discharge specific capacity is shown as follows:

The picture

05

Formation aging and others

For carbon negative battery, formation – aging is the key process of lithium battery, which will affect the quality of SEI. The thickness of SEI is not uniform or the structure is unstable, which will affect the quick charging capacity and cycle life of the battery.

In addition to the above several important factors, the production of cell, charge and discharge system will have a great impact on the performance of lithium battery. With the extension of service time, the battery charging rate should be moderately reduced, otherwise the polarization will be aggravated.

conclusion

The essence of fast charging and discharging of lithium batteries is that lithium ions can be rapidly de-embedded between anode and cathode materials. The material properties, process design and charging and discharging system of batteries all affect the performance of high current charging. The structural stability of anode and anode materials is conducive to the rapid delithium process without causing structural collapse, lithium ions in the material diffusion rate is faster, in order to withstand high current charging. Due to the mismatch between ion migration speed and electron transfer rate, polarization will occur in the charging and discharging process, so polarization should be minimized to prevent the precipitation of lithium metal and reduce the capacity to affect the life.