How to improve the fast charge performance of lithium batteries?

The essence of rapid charge and discharge of lithium batteries is that lithium ions can be quickly deintercalated between positive and negative materials. Battery material properties, process design, and charge and discharge systems all have an impact on high current charging performance.

Use this diagram to illustrate the process of charging the battery. The abscissa is the time ordinate and the voltage is used. At the beginning of charging the lithium battery, there will be a small current pre-charging process, namely CCPre-charge, in order to stabilize the positive and negative materials. Thereafter, after the battery state is stable, it can be adjusted to high current charging, that is, CCFastCharge.

Finally, enter the constant voltage charging mode (CV). For the lithium battery, the system starts the constant voltage charging mode after the voltage reaches 4.2V, the charging current is gradually reduced, and finally the charging is over after the value is less than a certain value.

Throughout the process, different standard charging currents are available for different batteries. For example, for 3C product battery standards, 0.1C-0.5C is generally selected, while for high-power power batteries, standard charging is generally 1C. Choosing a lower charging current also takes into account the safety of the battery. Therefore, the fast charge that is usually said means that it is several times to several tens of times higher than the standard charging current.

Some people say that lithium battery charging is like pouring beer. It is fast and full of beer, but it has a lot of foam slow down, slow, but lots of beer, very real. Fast charging saves the charging time and also causes greater damage to the battery itself.

Due to the polarization phenomenon in the battery, the maximum charging current that can be accepted decreases with the increase of the charge and discharge cycle. When the charging is continued and the charging current is large, the ion concentration at the electrode increases, and the polarization is intensified. The terminal voltage cannot be directly proportional to the charged amount/energy. At the same time, large current charging, the increase of internal resistance will lead to aggravation of Joule heating effect (Q=I2Rt), bringing side reactions, such as electrolyte reaction decomposition, gas production and other problems, the risk factor suddenly increases, resulting in battery safety. The impact, non-power battery life will inevitably be significantly reduced.

01 Cathode material

The process of fast charging of lithium battery is the process of rapid migration of Li+ into the negative electrode in the positive electrode material. The particle size of the positive electrode material can affect the response time and ion diffusion path in the electrochemical process of the battery. It is studied that as the grain size of the material decreases, the diffusion coefficient of lithium ions increases. However, as the particle size of the material decreases, severe particle agglomeration occurs during production, resulting in uneven dispersion, while the nanoparticles reduce the compaction density of the pole piece and contact with the electrolyte during charge and discharge. The area is increased by side reactions, which affect the performance of the battery.

The more reliable method is to modify the coating of the positive electrode material. For example, the conductivity of LFP itself is not so good. After coating the carbon material or other materials on the surface, the conductivity can be improved, which is beneficial to improve the rapid charging of the battery performance.

02 Anode material.

Lithium battery fast charging means that lithium ions quickly escape and "swipe" to the negative electrode, at this time, the negative electrode material needs to have a fast lithium insertion capability, anode materials for fast charging of lithium batteries include carbon materials, lithium titanate, and other novel materials.

For carbon materials, since the potential of lithium intercalation is similar to that of lithium, in the case of conventional charging, lithium ions are preferentially embedded in graphite, but under fast charge or low temperature, lithium ions may precipitate on the surface to form dendrites. . Dendritic lithium pierces 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, which may cause a short circuit of the battery.

For LTO, it is a zero-strain oxygen-containing anode material, which does not produce SEI when the battery is working, and its ability to combine with lithium ions is stronger, which can meet the requirements of fast charge and fast release. 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 problem of gas production of the LTO battery cannot be solved, and can only be alleviated by the surface modification method.

03 Electrode liquid

As mentioned earlier, during the fast charge process, due to the inconsistent lithium ion migration rate and electron transfer rate, the battery will have a large polarization. In order to minimize the negative reaction caused by battery polarization, the following three points will be the development direction of the electrolyte: 1. High dissociation electrolyte salt; 2. Solvent compound-viscosity is lower; 3. Interface control-membrane impedance Lower.

04 Relationship between production process and fast charge

In the past, three key materials, such as positive and negative materials and electrode liquids, were analyzed to analyze the requirements and effects of fast charge. The following is a relatively large process design. The battery manufacturing process parameters directly affect the migration resistance of lithium ions in various parts of the battery before and after the activation of the battery. Therefore, the battery preparation process parameters have an important impact on the performance of the lithium ion battery.

(1) Slurry

For the nature of the slurry, on the one hand it is necessary to maintain a uniform dispersion of the conductive agent. Because the conductive agent is evenly distributed between 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, reduces contact resistance, and can improve the moving rate of electrons. . Another aspect is to prevent excessive dispersion of the conductive agent. During the charging and discharging process, the crystal structure of the positive and negative materials will change, which may cause the peeling and peeling off of the conductive agent, which will increase the internal resistance of the battery and affect the performance.

(2) Pole density

In theory, the rate type battery and the high capacity battery cannot be combined. When the surface density of the positive and negative pole pieces is low, the diffusion speed of lithium ions can be increased, and the ion and electron migration resistance 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 due to the continuous insertion and ejection of lithium ions during charge and discharge.

However, if the areal density is too low, the energy density of the battery will be lowered and the cost will increase. Therefore, it is necessary to comprehensively consider the surface density. The following figure is an example of a 6C charging 1C discharge of a lithium cobalt oxide battery. You can see:

(3) Pole piece coating consistency

A friend asked before, does the inconsistent density of the poles affect the battery? By the way, for the fast charge performance, it is mainly the consistency of the negative pole piece. If the density of the negative surface is inconsistent, there is a large difference in the internal porosity of the active material after rolling. The difference in porosity causes a difference in internal current distribution, which affects the formation and performance of SEI during the battery formation stage, and ultimately affects the fast charge performance of the battery.

(4) Pole piece compaction density

Why is the pole piece compacted? One is to increase the battery specific energy, and the other is to improve battery performance. The optimum compaction density is different for different electrode materials. To increase 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 at the same areal density, thereby reducing the migration path of lithium ions.

When the compaction density is too large, the electrolyte infiltration effect is not good, and the material structure and the distribution of the conductive agent may be destroyed, and the winding problem may occur later. The same is the lithium cobalt oxide battery 6C charging 1C discharge the effect of compaction density on the discharge specific capacity is as follows:

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05 into aging and other

For carbon negative batteries, chemical-aging is a key process for lithium batteries, and this process affects the quality of SEI. SEI thickness is not uniform or structurally unstable, which will affect the battery's fast charge capacity and cycle life.

In addition to the above important factors, battery manufacturing, charging and discharging systems will have a greater impact on the performance of lithium batteries. As the use time is extended, the battery charge rate should be moderately reduced otherwise the polarization will be aggravated.

Conclusion

The essence of rapid charge and discharge of lithium batteries is that lithium ions can be quickly deintercalated between positive and negative materials. Battery material properties, process design, and charge and discharge systems all have an impact on high current charging performance. The structural stability of the positive and negative materials facilitates the collapse of the structure during rapid delithiation, and the lithium ions diffuse faster in the material to withstand high current charging. Since the ion migration speed and the electron transport rate do not match, polarization occurs during charging and discharging, and polarization should be minimized to prevent lithium metal from being precipitated and the capacity to be affected.

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