Influence of Lithium Battery Aging System on Battery Performance

The production process of lithium batteries can be divided into three stages: front pole piece manufacturing, middle cell packaging, and back battery activation. The purpose of the battery activation stage is to fully activate the active substances and electrolytes in the battery to achieve electrochemical performance. Stable performance. The activation stage includes stages such as pre-charging, formation, aging, and constant volume. The purpose of pre-charging and formation is to activate the positive and negative materials for the first few times of charge and discharge, so that the materials are in the best state of use. There are several main purposes of aging: first, to make the infiltration of the electrolyte better, which is conducive to the stability of battery performance; second, after the active substances in the positive and negative materials are aged, they can accelerate some side effects, such as gas production, electrolysis Liquid decomposition, etc., make the electrochemical performance of lithium batteries quickly stabilize; the third is to conduct consistency screening of lithium batteries after aging for a period of time. After the formation, the voltage of the cell is unstable, and the measured value will deviate from the actual value. The voltage and internal resistance of the cell after aging are more stable, which is convenient for screening batteries with high consistency.

There are two main factors that affect the performance of lithium batteries by the aging system, namely aging temperature and aging time. In addition, it is also more important whether the battery is in a sealed or open state during aging. For open-ended formation, if the plant can control the humidity, it can be aged and then sealed. If high temperature aging is used, aging after sealing is better. For different battery systems, ternary positive electrode/graphite negative electrode lithium battery, lithium iron phosphate positive electrode/graphite negative electrode lithium battery or lithium titanate negative electrode battery, it is necessary to conduct targeted tests according to the material characteristics and lithium battery characteristics. In the experimental design, the optimal aging system can be determined by the capacity difference, internal resistance difference, and voltage drop characteristics of lithium batteries.

1. Ternary or lithium iron phosphate positive electrode/graphite negative electrode lithium battery

For lithium batteries with ternary as the positive electrode material and graphite as the negative electrode material, a solid electrolyte film (SEI) will be formed on the surface of the graphite negative electrode during the pre-charging stage of the lithium ion battery. The formation potential of this film is about 0.8 Around V, the SEI allows ions to penetrate but does not allow electrons to pass through, thus inhibiting the further decomposition of the electrolyte after forming a certain thickness, which can prevent the degradation of battery performance caused by the decomposition of the electrolyte. However, the SEI film formed after chemical formation has a compact structure and small pores. Re-aging the battery will help to reorganize the SEI structure and form a loose and porous film, thereby improving the performance of lithium batteries. The aging of ternary/graphite lithium batteries generally chooses normal temperature aging for 7 days to 28 days, but some factories use a high temperature aging system, and the aging time is 1-3 days. The so-called high temperature is generally between 38°C and 50°C. High-temperature aging is only to shorten the entire production cycle. The purpose is the same as normal temperature aging, which is to make the positive and negative electrodes, diaphragms, electrolytes, etc. fully carry out chemical reactions to achieve a balance, so that the lithium battery can reach a more stable state.

2. Lithium titanate negative electrode lithium battery

Commonly known as lithium titanate battery, the negative electrode uses lithium titanate battery, and the positive electrode material is mainly ternary, lithium cobalt oxide and other materials. The difference between lithium titanate battery and graphite anode battery is that the lithium insertion potential of lithium titanate is 1.55V (relative to lithium metal), which is higher than 0.8V formed by SEI, so a solid electrolyte film will not be formed during the charging and discharging process ( SEI) also does not form dendrites of lithium, resulting in higher safety. This means that during the charging process of lithium titanate, electrons constantly react with the electrolyte to generate by-products and generate gases such as hydrogen, CO, CH4, C2H4, etc., which will cause the battery to bulge. The bulging problem of lithium titanate mainly depends on the change of material properties to alleviate, such as coating the material surface, changing the particle size distribution, and finding a suitable electrolyte. In addition, the bulging phenomenon of lithium titanate can also be appropriately alleviated by optimizing the system of precharge, formation, and aging. The aging system of lithium titanate battery is generally preferred high temperature aging system. The aging temperature is 40℃-55℃, and the aging time is generally 1-3 days. Negative pressure exhaust is required after aging. After multiple high-temperature aging, the moisture inside the battery can fully react, and the gas can be effectively suppressed after the gas is discharged, which can effectively suppress the flatulence problem of the lithium titanate battery and improve its cycle life.

No matter what kind of battery system, aging is an essential process. Although the aging of lithium batteries is understood as the loss and destruction of lithium batteries, it is actually an effective way to screen batteries with high consistency and eliminate defective products. Only by aging, can lithium batteries suitable for packaging be selected to improve the service life of power tools.

Analysis of common problems in battery processing of lithium iron phosphate materials.
Due to the low diffusion coefficient of lithium ions and poor electrical conductivity of lithium iron phosphate, the current practice is to make its particles small, or even make it into nanometers, and improve its charge and discharge speed by shortening the migration path of LI+ and electrons (Theoretically, the migration time is inversely proportional to the square of the migration path). But this brings a series of problems to the battery processing.

Material dispersion problem

Pulping is one of the most critical processes in the battery production process. Its core task is to uniformly mix active materials, conductive agents, binders and other materials so that the material properties can be better played. To mix, it must first be able to disperse. As the particles decrease, the corresponding specific surface area increases, the surface energy increases, and the tendency for aggregation between particles increases. The greater the energy required to overcome the surface energy dispersion. At present, mechanical stirring is generally used, and the energy distribution of mechanical stirring is uneven. Only in a certain area, the shear strength is large enough and the energy is high enough to separate the aggregated particles. To improve the dispersing ability, one is to optimize the structure of the stirring equipment to increase the space ratio of the effective dispersion area without changing the maximum shear rate; the other is to increase the stirring power (increase the stirring speed) and increase the shearing speed. The effective dispersion space will also increase. The former belongs to the problem of equipment, how much room for improvement, and coating online does not make comments. In the latter case, the space for improvement is limited, because the shearing speed is raised to a certain limit, which will cause damage to the material and lead to particle breakage.

A more effective method is to use ultrasonic dispersion technology. It’s just that the price of ultrasonic equipment is relatively high. The price of the one I contacted some time ago is comparable to that of imported Japanese mechanical mixers. The ultrasonic dispersion process time is short, the overall energy consumption is reduced, the slurry dispersion effect is good, the polymerization of the material particles is effectively delayed, and the stability is greatly improved.

In addition, the dispersion effect can be improved by using a dispersant.

Difficulty drying

Due to the large specific surface of lithium iron phosphate and the large amount of binder, the amount of solvent required to prepare the slurry is also large, and it is difficult to dry after coating. How to control the volatilization rate of the solvent is a problem worthy of attention. High temperature, large air volume, and fast drying speed will result in large voids. At the same time, it may also drive the migration of colloids, resulting in uneven distribution of materials in the coating. If the colloids accumulate on the surface layer, it will hinder the conduction of charged particles. , increasing the impedance. Low temperature, low air volume, slow solvent escape, long drying time and low production capacity.

Poor adhesion

The particles of lithium iron phosphate material are small, and the specific surface ratio is much larger than that of lithium cobaltate and lithium manganate, and more binders are needed. However, if too much binder is used, if the content of active material is reduced, the energy density will be reduced, so if possible, the amount of binder will be reduced as much as possible in the battery production process. In order to improve the bonding effect, the current general practice of lithium iron phosphate processing is to increase the molecular weight of the binder (high molecular weight, the bonding ability is improved, but the more difficult the dispersion, the higher the impedance), on the one hand, is to increase the amount of binder. So far the results seem to be unsatisfactory.

Poor flexibility

At present, when the lithium iron phosphate pole piece is processed, it is generally felt that the pole piece is hard and brittle, which may not have a slight impact on the lamination, but it is very unfavorable when it is wound. The flexibility of the pole piece is not good, and it is easy to drop powder and break when winding and bending, resulting in short circuit and other defects. The mechanism explanation in this regard is not clear, the guess is that the particles are small and the elastic space of the coating is small. Lowering the compaction density can improve, but then the volumetric energy density is also reduced. Originally, the compaction density of lithium iron phosphate is relatively low, and reducing the compaction density is the only way to take it.

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