The Importance of the N/P Ratio of Lithium Batteries

Usually, we think that the N/P ratio of lithium battery is too large, that is, the negative electrode is too large, which will cause shallow charge and discharge of the negative electrode of the lithium battery, and deep charge and discharge of the positive electrode of the lithium battery (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, and LTO materials will not precipitate lithium), which is safer, but the increase in the oxidation state of the positive electrode of lithium batteries increases the safety hazard. Since the first effect of the negative electrode of the lithium battery remains unchanged, more parts need to be reacted. At the same time, due to the influence of kinetics, the gram capacity of the positive electrode of the lithium battery will be low, but when the N/P of the lithium battery is insufficient to a certain extent, the lithium battery The positive electrode of the battery cannot be fully utilized, which will also affect the performance of the gram capacity of the lithium battery.

Next, the lithium battery manufacturer Winshinepower will give you a detailed introduction to the N/P of various lithium batteries and the specific impact of the N/P ratio on lithium batteries.

All kinds of lithium batteriesN/P

The N/P of graphite-negative lithium batteries 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 should be considered in the design, such as the coating deviation of lithium batteries. However, when the N/P ratio is too large, the irreversible capacity of the lithium battery will be lost, resulting in a low capacity of the lithium battery and a decrease in the energy density of the battery.

For the lithium titanate negative electrode, the positive electrode excess design is adopted, and the lithium battery capacity is determined by the capacity of the lithium titanate negative electrode. The excess design of the positive electrode is beneficial to improve the high-temperature performance of the lithium 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 the lithium titanate.

Influence of N/P Ratio on Lithium Batteries

Influence of N/P Ratio of Lithium Batteries on Cathode

If the N/P ratio is too high, the oxidation state of the cathode material 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 an excess N/P ratio, perform a hot box (130°C/150°C) or high-temperature storage experiment in a fully charged state, and disassemble the battery cell, it is usually found that the positive electrode powder is separated from the foil, and the diaphragm turns yellow.

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 [1] that the addition of easily oxidizable protective additives such as PS to the electrolyte can alleviate the oxidation of the diaphragm.

It has been reported in the literature that among the negative electrode MCMB materials, 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. Lithium salt deposition exists at the current collector contact interface, but it is not observed for graphite-based materials. However, there are few studies on the positive electrode SEI film. Since the contact position between the positive electrode powder and the current collector is at a high potential and has high oxidizing property, it is assumed that a layer of positive electrode lithium salt deposit will be formed (the reaction is accelerated at high temperature) ), hindering the contact between the positive electrode powder and the current collector, 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.

Cathode peeling increases internal resistance and directly leads to cycle failure under high-temperature use conditions.

Influence of N/P Ratio on Negative Electrode in Lithium Batteries

The released excess Li will provide a source of Li for the deposition of lithium salts on the negative electrode surface [2], 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 positive electrode is used here, and the N/P ratio is different by adjusting the amount of the negative electrode. At the end of the discharge, the voltage of the positive and negative electrodes with a low N/P ratio is low, the positive electrode is deeply discharged, and the negative electrode is shallowly discharged. At the charging end, the voltages of the positive and negative electrodes with low N/P ratio are also low, the negative electrode is deeply charged, and the positive electrode is shallowly charged.

Since the ratio of the negative electrode to be reacted in the first effect of the lithium battery is the same, and the total amount of negative electrodes are different, the charge-discharge curve of the negative electrode with the more negative electrodes and the negative electrode with less negative electrode produces a phase difference corresponding to the positive charge-discharge curve of the same lithium battery. Since the positive electrode potential gradually decreases with the increase of lithium intercalation (discharge process), during the process of 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 of the lithium battery with the more negative electrode and the less negative electrode is different. Yes, the positive electrode voltage corresponding to the negative electrode discharge end of the lithium battery with fewer negative electrodes is lower. In order to achieve the same full battery voltage, the negative electrode voltage of the lithium battery with few negative electrodes rises lower, which also avoids the excessive degree of Li removal from the negative electrode of the lithium battery. Excessive removal of Li from the negative electrode of lithium batteries will cause damage and reformation of the SEI film, resulting in cycle failure. This analysis method can also be applied to the charging end, and it can be concluded that in the case of the excess positive electrode, the positive electrode is in a shallow charge and the negative electrode is in a deep charge.

Summary: In a lithium battery with a small N/P ratio, that is, a lithium battery with excess and insufficient negative electrode, the positive electrode can reach the state of shallow charge and deep discharge in the cycle, and the state of the negative electrode is deep charge and shallow discharge. vice versa.

The above is the whole content of lithium battery N/P brought to you by the lithium battery manufacturer Microcharge. I believe that after reading the whole text, everyone has an understanding of the impact of the N/P ratio on lithium batteries. For more information on lithium batteries, please refer to Our past articles:

Lithium battery energy density improvement method

Can Lithium Batteries Really Replace Oil?