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What is the effect of N/P ratio on 1s lipo battery?

A few days ago, CNHL introduced the N/P ratio design of 1s lipo battery , today we continue to study 1s lipo battery N /P ratio, this article will introduce the impact of N/P ratio on 1s lipo battery in detail. Interested partners should go and see.

1. What is the effect of N/P ratio on 1s lipo battery

Usually we think that the N/P ratio is too large, that is, the negative electrode is too large, which will cause the shallow charge and discharge of the negative electrode of the 1s lipo battery, and the deep charge and discharge of the positive electrode (and vice versa, of course, this is only a very general statement). The fully charged negative electrode is not easy to precipitate lithium (some materials, such as soft and hard carbon, LTO materials will not precipitate lithium), which is safer, but the increase in the oxidation state of the positive electrode of 1s lipo battery increases the safety hazard.

Since the first effect of the negative electrode of the 1s lipo battery remains the same, more parts need to be reacted. At the same time, due to the influence of kinetics, the gram capacity of the positive electrode will be low, but when the N/P is insufficient to a certain extent, the positive electrode cannot be completely Utilization will also affect the performance of the gram capacity. To sum up, it is very important to find a suitable N/P ratio.

The N/P of 1s lipo battery of graphite negative electrode should be greater than 1.0, generally 1.04~1.20. This is mainly for safety design, mainly to prevent lithium precipitation in the negative electrode, and the process capability, such as coating deviation, should be considered in the design. However, when the N/P is too large, the irreversible capacity loss of the 1s lipo battery will result in a low capacity of the 1s lipo battery, and the energy density of the 1s lipo battery will also decrease.
This article about the attenuation of lipo battery capacity has a detailed introduction:
Explain in detail the reasons for the capacity decay of 2s 5600 lipo battery

For the lithium titanate negative electrode, the positive electrode excess design is adopted, and the capacity of the 1s lipo battery is determined by the capacity of the lithium titanate negative electrode. The excessive design of the positive electrode is beneficial to improve the high temperature performance of the 1s lipo battery: the high temperature gas mainly comes from the negative electrode. When the positive electrode is excessively designed, the negative electrode potential is lower, and it is easier to form an SEI film on the surface of lithium titanate.

2. The effect of N/P ratio on the positive electrode of 1s lipo battery

If the N/P ratio is too high, the oxidation state of the positive electrode material of the 1s lipo battery will increase. In addition to safety problems, what are the hidden dangers? Here only ternary/graphite materials are used as an example.
For a battery with excess N/P ratio, perform a hot box (130°C/150°C) or high-temperature storage experiment in a fully charged state, disassemble the battery, and usually find that the positive powder of the 1s lipo battery is separated from the foil, and Diaphragm yellows.
First define two concepts:
Concept 1: First of all, it is necessary to clarify the different positions of the pole piece, even if the reaction of different positions of the particles is not uniform, which involves the problem of potential difference in the direction of the thickness of a pole piece.
Concept 2: Ni3+/4+ and Co3+/4+ have overlapping energy bands with O, and O will be extracted from the lattice in the form of free radicals, which is extremely oxidizing.

The yellowing of the diaphragm is caused by oxidation, and the mechanism has been very clear. It has been reported in the literature that the addition of easily oxidizable protective additives such as PS to the electrolyte of 1s lipo battery can alleviate the oxidation of the diaphragm.
It has been reported in the literature that in the negative electrode MCMB material of the 1s lipo battery, since the interface potential between the negative electrode powder and the current collector is the most negative, the deposition of lithium salt first occurs at the contact position between the negative electrode powder and the current collector, and the cross section of the MCMB material is clearly observed. Lithium salt deposition exists at the contact interface between the anode material and the current collector, but it is not observed for graphite-based materials.

However, there are few studies on the positive electrode SEI film of 1s lipo battery. Since the contact position between the positive electrode powder and the current collector is at a high potential and has high oxidation, it is assumed that a layer of positive electrode lithium salt deposit will be formed (high temperature accelerates this process). The reaction progresses), which hinders the contact between the positive electrode powder and the current collector of the 1s lipo battery, resulting in the peeling between the positive electrode powder and the current collector. The specific characterization experiments were not carried out, which is also the point of controversy in this paper. The peeling of the positive electrode of the 1s lipo battery increases the internal resistance and directly leads to the failure of the cycle under high temperature use conditions.

3. The influence of N/P ratio on the negative electrode of 1s lipo battery

The released excess Li will provide a Li source for the deposition of lithium salts on the negative electrode surface, and the continuous deposition of lithium salts leads to the failure of the cycle. Therefore, an N/P ratio that is too low will increase this risk.
But here we discuss what might happen in another dimension, what happens if the N/P ratio is too high?

The same 1s lipo battery positive electrode is used here, and the N/P ratio is different by adjusting the amount of negative electrode. 1s lipo battery is at the end of the discharge, the voltage of the positive and negative poles with a low N/P ratio is low, the positive pole is deep, and the negative pole is shallow. The 1s lipo battery is at the end of the charging, and the voltage of the positive and negative poles with a low N/P ratio is also low, the negative pole is deeply charged, and the positive pole is shallowly charged.

You must read this article about Lipo battery charging and discharging. The article introduces the charging and discharging principle of Lipo battery in detail:
Lipo battery 4s charging and discharging principle, be sure to store it well!
It should be noted:
1. A potential curve represents the two processes of charging and discharging of 1s lipo battery, which can be considered as the potential of the equilibrium state.
2. The capacity decay caused by the first effect of the positive electrode of the 1s lipo battery is ignored here. Even after the first effect loss, the negative electrodes with different N/P ratios correspond to the same positive curve. It is believed that the first effect loss of the positive electrode of the 1s lipo battery is only caused at the beginning of charging, and the film formation caused by oxidation at the charging end is ignored here. The actual situation is that only with the progress of the cycle, the oxidation film formation will affect the capacity.

3. The ratio of the first effect of the negative electrode is considered to be independent of the N/P ratio. It is a constant. There are many negative electrodes, and the 1s lipo battery loses a lot of capacity through the first effect. The stage where the reaction occurs is also at the beginning of charging.
4. The positive and negative potentials are free, and the only limitation is the voltage of the full cell. The voltages of the two full cells at the discharge end and the charge end are respectively equal.
Since the ratio of the negative electrode to be reacted in the first effect of the 1s lipo battery is the same, and the total amount of negative electrodes is different, the charge-discharge curve of the negative electrode with more negative electrodes and the negative electrode with less negative electrode produce a phase difference corresponding to the same positive electrode charge-discharge curve.

Since the positive electrode potential gradually decreases with the increase of lithium intercalation (discharge process), in the process of 1s lipo battery negative electrode de-Li/negative electrode voltage rise, the use position of the positive electrode discharge curve corresponding to the end of the negative electrode discharge curve with more negative electrode and less negative electrode is: Differently, the positive voltage of the 1s lipo battery corresponding to the negative discharge end with less negative electrode is lower.

In order to achieve the same full battery voltage, the voltage of the negative electrode with less negative electrode rises lower, which also avoids the excessive degree of Li removal from the negative electrode. Excessive removal of Li from the negative electrode will damage and reform the SEI film of the 1s lipo battery, resulting in cycle failure. This analysis method can also be applied to the charging end, and it is concluded that when the positive electrode of the 1s lipo battery is excessive, the positive electrode is in a shallow charge and the negative electrode is in a deep charge.


Summary

For a 1s lipo battery with a small N/P ratio, that is, a 1s lipo battery with an excess of negative negative electrodes, the positive electrode can reach the state of shallow charging and deep discharging in the cycle, and the condition of the negative electrode is deep charging and shallow discharging. vice versa.
Well, the above is the whole content of today. I hope that through this article, everyone can understand the N/P ratio of 1s lipo battery, and the impact of lipo battery N/P ratio on the battery. More lipo battery Information can be read below:
Explain in detail the reasons for the capacity decay of 2s 5600 lipo battery

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