What is the capacity grading principle of lithium ion battery? What about formation and capacity grading?

What are formation and capacity grading?

Formation: make the first-charging battery tend to be stable by a series of processing measures, including small current charging and discharging, idling under constant temperature, etc.

Capacity grading: capacity separating, performance grading

The application of lithium ion battery

Lithium ion battery owns the advantages of high energy density, environmentally friendly, no memory effect, long cycle life, less self-discharging volume and so on. It is not only the ideal light-weight small power supply of the devices, such as mobile phone, camera, laptop, portable measuring instrument, etc., but also the ideal military light-weight high energy power source. The manufacturing process of lithium ion battery is complicated, and it belongs to capital intensive industry. Now let’s try to know more about the formation and capacity grading of lithium ion battery.

The formation of lithium ion battery

During the first charging and discharging process of lithium ion battery, the cathode aprotic solvent reacts inevitably on the surface between cathode and electrolyte, and generates passivated film on the cathode, which called dielectric film or solid electrolyte boundary film (SEI film). It is reported that the passivated film is made of Li2O, LiF, LiCl, Li2CO3, LiCO2-R, alkoxide and no-conducting polymer. It has multilayered structure, the layer closed to electrolyte is porous, while the layer closed to electrode is pyknotic. The formation of SEI film plays an important role on the performance of electrode material. On one hand, the formation of SEI film runs out part of the Li ion, increasing irreversible capacity of first charging and discharging and decreasing the charging and discharging efficiency of electrode material. On the other hand, SEI film has insolubility of organic solvent, can exist stably in organic electrolyte solution, avoiding damage the electrode material by the molecule of the solvent, so as to greatly improve the cycle performance a cycle life of the electrode. The quality of formation has great effect on SEI film and the electrochemical performance of cycle life, stability, self-discharging performance, security, etc. Therefore, the battery enterprise will summarize valuable process and parameter of formation and capacity grading based on actual manufacturing, so as to make sure maximum capacity, better stability and longer cycle life. It takes a long time to charging and discharging for formation. It is not only necessary to control the voltage of charging and discharging, or current difference, but also voltage of charging and discharging, and impulse waveform of the current, preventing the SEI film impedance to increase, affecting discharging rate and improving production efficiency during formation process. For example, when the charging voltage is over 3.7V, it may damage the crystal structure of lithium iron phosphate and affect the cycle performance of the battery.

The capacity grading of lithium ion battery

The capacity grading needs a formation & capacity grading container (due to the similar basic principles of formation and capacity grading, both of them can realize in the same container. This container is named formation & capacity grading container) It is something like a charger, and it can even charge a battery with large capacity at the same time. Analyze the capacity and internal resistance data by the test data from the computer management system, and then define the quality grade of the battery. This process is called capacity grading.

After the first capacity grading, idle the battery for no less than 15 days. During these days, some inner quality issues will break out, such as over-discharge, internal resistance, etc. After idling, recheck the capacity grading, take away the battery with quality issue.

The other purposes of the capacity are classifying the battery, and combine the single battery with the same internal resistance and capacity into one group. Only the battery with similar performance can make up a battery pack. For example, in order to meet the requirement of electric vehicle, the power battery pack needs dozens of even thousands of batteries to constitute. Considered the effect of the complicated system the performance of the battery pack is not simply addition or subtraction to the single battery. Ideally, every single battery of the battery pack needs to be totally the same. Actually, even the single battery from the same batch has a little difference on performance (including capacity, internal resistance, etc.) although selecting before dividing into battery pack, we cannot make sure they are 100% the same. Besides, affected by the volume of the battery pack, there are many differences in different heat dissipation performance, so the battery pack has large temperature gradient. All issues above make different rate of decay during application. On one hand, it will decrease the valuable capacity of the battery pack (limited by the minimum capacity of single battery in series). On the other hand, it may lessen the safety performance. It is said by the research that the cycle life if a single battery reaches over 1000 times, but may less than 200 times without protection of equalizing equipment. All in all, the consistence of the single battery is a very important parameter.

The precision requirement of formation and capacity grading

Strictly control the consistency of the lithium ion battery or access the battery grade accurately, so as to upgrade the electrochemistry performance of cycle life, stability, self-discharging performance, safety performance, etc. It has high requirement on the measurement accuracy of the current and voltage of formation and capacity grading devices. Limited by acquisition accuracy of the sensor, the measurement accuracy of formation and capacity grading devices in recent market is about one thousandth. As for formation, the higher control accuracy of current and voltage, the better quality of lithium ion battery. It is an ideal choice for accuracy of current voltage sensor over one ten-thousandth. Yinhe Electric can provide many choices of current & voltage sensor with accuracy over one ten-thousandth.