What are the reasons for the self-discharge of polymer lithium-ion batteries for power tools?

The polymer lithium-ion battery of lithium-powered electric tools does not need to be discharged first (because the polymer lithium-ion battery of lithium-powered electric tools has no memory), which brings great convenience to use and greatly saves energy. . The polymer lithium-ion battery for lithium-powered electric tools also has the advantage of low self-discharge. It is stored in a non-use state, and there is almost no chemical reaction inside, which is quite stable. The self-discharge rate of polymer lithium-ion batteries for lithium-powered electric tools is only 5% to 10%.

The self-discharge of the polymer lithium ion battery of lithium-powered electric tools refers to the automatic discharge phenomenon of the polymer lithium-ion battery of the lithium-powered electric tool when the polymer lithium-ion battery is left in an open circuit. After being placed for a period of time at a temperature of 0, a part of the capacity will be lost, which is self-discharge. The self-discharge of the polymer lithium-ion battery of lithium-powered electric tools will directly reduce the output power of the polymer lithium-ion battery of the lithium-powered electric tool, and reduce the capacity of the polymer lithium-ion battery of the lithium-powered electric tool. The self-discharge size of the polymer lithium-ion battery of lithium-powered electric tools can be expressed in three forms:

1) Measured by how many mV the terminal voltage of the lithium-ion battery of lithium-powered electric tools has dropped per day, that is, mV/day, the voltage drop of a qualified lithium-powered electric tool's polymer lithium-ion battery should not exceed 2mV per day.

2) The K value is used to indicate how much the voltage drop of the polymer lithium-ion battery of lithium-powered electric tools per unit time, that is, mV/h, that is, how much mV the terminal voltage of the lithium-powered electric tool polymer lithium-ion battery drops in one hour , The K value of a qualified lithium-powered power tool polymer lithium-ion battery is generally within 0.08mV/h. The K value of a lithium-powered power tool polymer lithium-ion battery is expressed as follows:

K=V1-V2/△T

Where: V1 is the terminal voltage of the polymer lithium-ion battery of the lithium-powered electric tool one hour ago, and V2 is the terminal voltage of the polymer lithium-ion battery of the lithium-powered electric tool one hour later.

3) Expressed by self-discharge rate, that is, the percentage decrease in the capacity of the polymer lithium-ion battery of lithium-powered electric tools within the prescribed time:

Where: Y% is the self-discharge rate; C1 is the capacity of the lithium-powered power tool polymer lithium-ion battery before shelving; C2 is the capacity of the lithium-powered power tool polymer lithium-ion battery after shelving; T is the lithium-powered power tool polymer The shelf time of lithium-ion batteries is generally expressed in days, weeks, months or years.

Lithium-powered electric tools polymer lithium-ion batteries are often subject to voltage drops during use or storage due to electrolyte compatibility, graphite negative characteristics, and inconsistent assembly. A large part of the voltage drop is caused by the self-discharge of the polymer lithium-ion battery cells of lithium-powered electric tools.

2. Causes of self-discharge of polymer lithium-ion batteries for lithium-powered electric tools

The important reason for the self-discharge of the polymer lithium ion battery of lithium power tools is that the electrodes are in a thermodynamically unstable state in the electrolyte. Lithium is the result of the oxidation-reduction reaction of the two electrodes of the polymer lithium ion battery of power tools. . In the two electrodes of the polymer lithium-ion battery of lithium-powered electric tools, the self-discharge of the negative electrode is important. The occurrence of self-discharge causes the active material to be consumed and transformed into unusable heat energy.、

The self-discharge rate of polymer lithium-ion batteries for lithium-powered electric tools is determined by dynamic factors, and it depends on the nature of the electrode material, the surface state, the composition and concentration of the electrolyte, and the impurity content. It also depends on the nature of the electrode material, the surface state, and the impurity content. Environmental conditions, such as temperature and humidity.

(1) Physical micro short circuit

The physical micro-short circuit is the direct cause of the drop in the terminal voltage of the polymer lithium-ion battery of lithium-powered power tools. The direct manifestation is that the polymer lithium-ion battery of the lithium-powered power tool has been stored at room temperature and high temperature for a period of time. The ion battery voltage is lower than the normal cut-off voltage. Compared with self-discharge caused by chemical reaction, self-discharge caused by physical micro-short circuit will not cause irreversible loss of capacity of polymer lithium-ion battery for lithium-powered electric tools

By observing and measuring the number, morphology, size, element composition, etc. of the black spots on the polymer lithium-ion battery separator of the disassembled lithium-powered electric tool, the size and physical self-discharge of the lithium-ion battery of the lithium-powered electric tool Possible reasons: Under normal circumstances, the greater the physical self-discharge, the greater the number of black spots and the deeper the morphology (especially penetrating to the other side of the diaphragm); the polymerization of lithium-powered electric tools is judged based on the metal element composition of the black spots Metal impurities that may be contained in lithium-ion batteries. There are many reasons for physical micro-short circuit, which can be divided into the following categories:

1) Dust. Disassemble the slightly short-circuited polymer lithium-ion battery of lithium-powered electric tools, and black spots will appear on the diaphragm of the lithium-powered electric tool's polymer lithium-ion battery. If the position of the black spot is in the middle of the diaphragm, it is likely to be penetrated by dust. In the production process of lithium-powered electric tools, polymer lithium-ion batteries are inevitably mixed with dust and impurities. These impurities have complex properties. Some impurities can cause slight conduction of the positive and negative electrodes, neutralizing the charge and impairing the power.

When the lithium-powered battery is made, the irreversible reaction caused by the micro-short circuit caused by the impurities is the most important reason for the excessive self-discharge of the polymer lithium-ion battery of individual lithium-powered power tools. Dust in the air or metal powder on the pole piece or diaphragm when it is made can cause internal micro short circuits. Absolute dust-free production is impossible. When the dust is not enough to penetrate the diaphragm and make the positive and negative electrodes short-circuited, it will not have a big impact on the polymer lithium-ion battery of lithium-powered electric tools.

But when the dust is severe enough to pierce the diaphragm, the impact on the lithium-powered battery will be very obvious. Due to the existence of whether the diaphragm is pierced, when testing the self-discharge rate of a large number of lithium-powered batteries, it is often found that the self-discharge rate of most lithium-powered batteries is concentrated in a small range, and only a small part of lithium The self-discharge of the power electricity is obviously high and the distribution is discrete. These should be the lithium power electricity that the diaphragm is pierced.

2) Glitch. Disassemble the micro-short-circuit lithium-powered polymer lithium-ion battery for power tools. When it is found that the black spots appearing on the diaphragm of the lithium-powered power tool polymer lithium-ion battery are mostly at the edge, they appear during the pole piece cutting process. Caused by glitches. In the early stage of life of lithium-powered electric tool polymer lithium-ion battery cells, they only show higher self-discharge, and the longer the time, the greater the possibility of causing large-scale short circuit between the positive and negative electrodes. It is lithium-powered electric tool polymer lithium An important cause of thermal runaway of ion batteries.

3) Positive metal impurities. The metal impurities in the positive electrode also break through the separator after the charging reaction, forming black spots on the separator, causing physical micro-short circuits. Generally speaking, as long as it is a metal impurity, it will have a greater impact on the self-discharge of the polymer lithium-ion battery of lithium-powered power tools. Generally, the metal element has the greatest impact. According to some documents, the order of influence is as follows: Cu>Zn>Fe>Fe2O3. For example, many positive iron-lithium materials will face the problem of excessive self-discharge, which is caused by excessive iron impurities.

4) Metal impurities in the negative electrode. Due to the formation of the galvanic battery, the negative electrode metal impurities will be released and deposited on the diaphragm to cause the diaphragm to conduct and form a physical micro-short circuit. Some low-end negative electrode materials often encounter this situation. The metal impurities in the negative electrode slurry have less influence on the self-discharge than the metal impurities in the positive electrode, and Cu and Zn have a greater influence on the self-discharge.

5) Metal impurities in auxiliary materials. For example, CMC, metal impurities in tape. As time increases, metal dendrites caused by metal impurities continue to grow, and finally penetrate the diaphragm, resulting in micro-short circuits between the positive and negative electrodes, continuously consuming power, and reducing the terminal voltage of the polymer lithium-ion battery of lithium-powered power tools.

(2) Side reactions of electrochemical materials

1) Cathode materials, importantly various types of lithium compounds, always have a slight reaction with the electrolyte, and the intensity of the reaction varies with different environmental conditions. The positive electrode material reacts with the electrolyte to generate insoluble products, making the reaction irreversible. The positive electrode material involved in the reaction loses its original structure, and the polymer lithium-ion battery for lithium-powered electric tools loses the corresponding power and permanent capacity.

The irreversible reaction between the positive electrode and the electrolyte mainly occurs in two materials that are prone to structural defects, lithium manganate and lithium nickelate, such as the reaction between lithium manganate positive electrode and lithium ions in the electrolyte:

LiyMn2O4+xLi++xe-→Liy+xMn2O4

2) Anode material, graphite anode originally has the ability to react with electrolyte. During the formation process, the reaction product SEI film adheres to the surface of the electrode, which stops the intense reaction between the electrode and the electrolyte. If the SEI film is defective, this reaction has also been carried out in small amounts. The reaction between the electrolyte and the negative electrode consumes lithium ions and the negative electrode material in the electrolyte at the same time. While the reaction brings about the loss of power, it also brings about the loss of the maximum usable capacity of the polymer lithium-ion battery of lithium-powered electric tools.

The irreversible reaction between the negative electrode material and the electrolyte. The SEI film formed during formation is to protect the negative electrode from the corrosion of the electrolyte. The possible reactions between the negative electrode and the electrolyte are:

LiyC6→Liy-xC6+xLi++xe-

3) Electrolyte. In addition to reacting with the positive and negative electrodes, the electrolyte also reacts with impurities in its own materials and reacts with impurities in the positive and negative materials. These reactions will generate irreversible products, reducing the total amount of lithium ions. The reason for the loss of the maximum usable capacity of the polymer lithium-ion battery for lithium-powered power tools. The irreversible reactions caused by impurities in the electrolyte are:

①The possible reaction of CO2 in the solvent:

2CO2+2e-+2Li+→Li2CO3+CO

②The reaction of O2 in the solvent:

1/2O2+2e+2Li+→Li2O

A similar irreversible reaction consumes lithium ions in the electrolyte, which in turn loses the capacity of the polymer lithium-ion battery of lithium-powered electric tools.

4) Moisture. The moisture causes the electrolyte to decompose and release a large amount of electrons, which are then embedded in the oxidation structure of the positive electrode, which causes the potential of the positive electrode to drop, and causes the voltage of the polymer lithium ion battery of the lithium-powered electric tool to drop. In addition, when there is H2O in the polymer lithium ion battery of lithium power tools, it will react with LiPF6 to produce corrosive gases such as HF; at the same time, it will react with solvents and other gases such as CO2 to cause the lithium power tool polymer lithium ion battery Swelling; HF will react with many substances in the battery such as important components of SEI to destroy the SEI film; generate CO2 and H2O, etc.; CO2 will cause the expansion of lithium-powered power tool polymer lithium-ion batteries, and the regenerated H2O will participate in the reaction of LiPF6, solvents, etc. Form a malignant chain reaction.

(3) Diaphragm defect

The original function of the diaphragm is to separate the positive and negative electrodes. If the quality of the diaphragm is defective, the purpose of the diaphragm cannot be brought into full play. A slight defect in the diaphragm will also have a significant impact on the self-discharge rate of the polymer lithium-ion battery of lithium-powered electric tools. Consequences of SEI membrane damage:

1) The solvent enters the graphite layer and reacts with LixC6, causing irreversible capacity loss.

2) The repair of damaged SEI will consume Li+ and solvents, which will further cause irreversible capacity loss.

With the continuous increase in the recycling of lithium-powered electric tools and polymer lithium-ion batteries, the uniformity and compactness of the SEI film will change. The aging SEI film has gradually leaked the protection of the negative electrode, making the negative electrode and the electrolyte more and more contact, and new side reactions. For the same reason, SEI films of different quality will also bring different self-discharge rates in the early life of lithium-powered electric tool polymer lithium-ion batteries. Therefore, taking the self-discharge rate as a characterization of the quality of the SEI film is often used in production; one of the means to improve the self-discharge rate is to add additives to improve the quality of the SEI film.

3. Acquired factors affecting the self-discharge rate of polymer lithium-ion batteries for lithium-powered electric tools

In different use environments, application states and life stages, the self-discharge rate of polymer lithium-ion batteries for lithium-powered power tools will vary.

1) Temperature. The higher the ambient temperature, the higher the activity of the electrochemical materials of the polymer lithium-ion battery for lithium-powered power tools, and the more intense the side reactions involved in the positive electrode material, negative electrode material, and electrolyte of the polymer lithium-ion battery for lithium-powered power tools. , In the same time period, causing more capacity loss. The chemical self-discharge of the polymer lithium-ion battery of lithium-powered electric tools is more significant at high temperatures. It is more effective to use high-temperature storage to judge the self-discharge of the polymer lithium-ion battery of lithium-powered electric tools.

2) External short circuit. For open-circuit lithium-powered electric tool polymer lithium-ion batteries, the external short circuit is mainly affected by the degree of air pollution and air humidity. When conducting self-discharge characteristic test experiments for lithium-powered electric tools polymer lithium-ion batteries, the laboratory environment and humidity range are strictly required for this reason. High air humidity will lead to an increase in conductivity, and air pollution importantly refers to the possibility of conductive particles in the pollutants, which will increase the conductivity of the air.

3) The amount of charge. By comparing the influence of the charge of the polymer lithium-ion battery of lithium-powered power tools on the self-discharge rate, the general trend is that the higher the charge of the polymer lithium-ion battery of lithium-powered power tools, the higher the self-discharge rate. That is, the higher the charge of the polymer lithium-ion battery of lithium-powered electric tools, the higher the positive electrode potential and the lower the negative electrode potential. In this way, the stronger the oxidation of the positive electrode and the stronger the reduction of the negative electrode, the more intense the side reaction.

4) Time. Under the same power and capacity loss efficiency of lithium-powered electric tools, the longer the time, the more power and capacity will be lost. But the self-discharge performance is generally used as an indicator for comparing different lithium-powered electric tools polymer lithium-ion battery cells, that is, under the same preconditions and at the same time, the comparison is made, so the purpose of time can only be said to affect self-discharge. the amount. The physical micro-short circuit of the polymer lithium-ion battery of lithium-powered electric tools has an obvious relationship with time, and the judgment of physical self-discharge is more effective for long-term storage.

5) Loop. The cycle will cause the internal micro-short-circuit melting of the polymer lithium-ion battery of lithium-powered power tools, thereby reducing the physical self-discharge. Therefore: if the self-discharge of the lithium-powered power tool polymer lithium-ion battery is mainly physical self-discharge, the self-discharge after the cycle The discharge is significantly reduced; if the self-discharge of the polymer lithium-ion battery of the lithium-powered power tool is mainly chemical self-discharge, there is no obvious change in the self-discharge after the cycle.