Definition of N/P ratio
In battery capacity design, an important criterion is that the negative electrode must have a larger reversible capacity than the positive electrode. Although the negative electrode capacity is smaller, the battery may have some advantages, such as large battery capacity, but lithium may appear during charging. Deposition of the surface of the negative electrode produces dendrites leading to safety problems. The so-called N/P ratio actually has another term called CB (cell balance).
The calculation method is N/P = negative electrode capacity per unit area / positive electrode capacity per unit area.
Figure 1 Schematic diagram of lithium dendrites
Factors affecting the N/P ratio
Under normal circumstances, the N/P ratio is determined by the following conditions:
1, the first efficiency of the active material
2, coating accuracy
3, the decay rate of the positive and negative cycles
According to the coating accuracy, the ideal coating accuracy can be 100%, and the positive efficiency of the positive electrode is greater than the first efficiency of the negative electrode. In this case, the theoretical value of CB can be close to 1, for example.
A positive electrode material, lithium cobalt oxide, has a design capacity of 140 mAh/g (first efficiency is 95%) and a positive surface density of 300 g/m.2
Anode material Artificial graphite, design capacity is 340mAh / g (90% artificial graphite), negative surface density 200g / m2
Coating accuracy Assume coating accuracy deviation is 2.5%
According to a reasonable calculation, the positive electrode negative electrode NP ratio is generally between 1.1 and 1.5, and the specific values are considered according to the design of the material system.
Effect of N/P ratio on rate performance
In order to study the effect of positive and negative electrode matching and electrolyte and battery rate performance, keep the positive active material unchanged, select the negative electrode / positive electrode N / P = 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 to assemble the battery, as shown in the figure below According to the current discharge of 0.2C, the positive and negative pole capacities are 421mAh, 444mAh, 454mAh, 479mAh, 502mAh, obviously when N/P=0.8, the capacity is the lowest, when N/P=1.8, the capacity is the highest. But N/P=0.8 When, the discharge voltage platform is the highest.
Figure 2 0.2C battery capacity discharge curve
Continue to follow the experimental design, gradually increase the discharge rate, and perform 1C, 3C and 5C discharge on the battery.
Figure 3 1C battery capacity discharge curve
When the magnification is 1C, it is obvious that when N/P=0.8, the capacity is the lowest, and when N/P=1.6, the capacity is the highest. However, N/P=0.8, the discharge voltage platform is the highest.
Figure 4 3C battery capacity discharge curve
When the magnification is 3C, it is obvious that when N/P=0.8, the capacity is the lowest, and when N/P=1.6, the capacity is the highest. However, N/P=1.2, the discharge voltage platform is the highest.
Figure 5 5C battery capacity discharge curve
When the magnification is 5C, it is obvious that when N/P=0.8, the capacity is the lowest, and when N/P=1.6, the capacity is the highest. But when N/P=1.2, the battery also has a higher capacity, and the discharge voltage platform is the highest.
In summary, as the discharge rate increases, the electrochemical polarization becomes larger and larger, and the discharge voltage platform decreases. At a higher discharge rate, the voltage platform is the highest at the N/P ratio = 1.2, and also has a higher High capacity.
Effect of N/P ratio on battery cycle performance
The same is also selected different N / P ratio = 1.0, 1.2, 1.4, 1.6, 1.8 After 125 times 1C / 1C cycle, the capacity retention rate of the battery is as shown in the following figure, obviously when N / P = 1.0 capacity retention rate is the lowest, when N /P=1.8 The capacity retention rate is the highest, and the capacity retention rate increases as the N/P ratio increases.
Figure 6 battery cycle curve
Effect of N/P ratio on battery impedance
According to the experiment, when the N/P ratio is 0.8, 1.2, 1.6, and 1.8, the battery SOC=50%, and the EIS of the assembled battery test battery is as follows.
Figure 7 EIS map test curve
Be seen, when the N / P ratio = 0.8, the maximum radius is small semicircle, N / P ratio = 1.6 times the minimum radius of a small semicircle, in ascending order of sequence: R0.8> R1.0> R1.8> R1.2> R1.6 indicates that the SEI film impedance decreases first and then increases with the increase of the negative electrode capacity while maintaining the positive electrode capacity. When the N/P ratio is 1.2 and 1.6, the R is minimum. .
Conclusion
The author thinks that the NP ratio is the most realistic battery design parameter from the actual production, battery rate, battery cycle, battery safety, and battery impedance. The NP ratio is between 1.1 and 1.5, but the battery material system is more complicated. With a variety of complex ways of use, combined with theoretical and practical experience to design the best Chinese core!
