Research on the technology of long-life electric vehicle battery plates

China's electric bicycle batteries have gone through a process of research, development, improvement and improvement based on the technology of small sealed batteries and starting batteries. As an emerging industry, it is now in a period of rapid development. The understanding of DZM batteries by electric bicycle assembly plants and users has also gone through a process of gradual improvement from shallow to deep. At first, they only focused on capacity (actually initial capacity). The battery with the largest capacity and the longest running distance was used. Now the focus is on long life, resistance to overcharge and overdischarge, and good high current discharge performance. However, most users do not have professional knowledge. After each trip, the charging time is long. Some even charge for a long time after riding for several thousand meters. Most of them are overcharged. The phrase "batteries are not broken, but charged" highly summarizes this phenomenon.

Another situation is that the user forces the battery to "work" when driving against the wind, uphill, accelerating, or driving a long distance. Even when the battery is displayed as dead, the voltage rises again after a period of idling, and the user continues to use the "rising voltage", causing the battery to over-discharge. Overcharging and over-discharging of batteries often occur during the use of electric power-assisted bicycles. The severe reality forces battery manufacturers to improve battery performance, especially in terms of battery resistance to overcharging and over-discharging, and extend the battery life.

1. Measures to improve battery resistance to overcharge and overdischarge

According to existing theories and research results, the harm of overcharging to batteries is mainly caused to the positive electrode. After multiple overcharging, the structure of the positive electrode active material will change, the connection between the particles will become loose, and the electrochemical activity will be lost. The excess oxygen generated will not be able to completely recombine at the negative electrode, resulting in water loss, and ultimately causing the battery to fail.

Overdischarge is the most harmful to the negative electrode. Repeated overdischarge causes the generated PbSO4 crystals to swell, which not only makes it difficult to reduce itself and lose its function, but also blocks the pores, making the inner active material unable to react electrochemically with H2SO4 and lose its function, resulting in a decrease in capacity and ultimately causing the battery to fail.

Improving the battery's ability to withstand overcharge and over-discharge is a systematic problem and there is no simple way to achieve it.

Since the theory of lead-acid batteries is imperfect and many problems have not been clarified, improvement requires a lot of practice. First, we need to determine and find suitable additives, and then combine them reasonably so that they can give full play to their respective roles and compensate for each other's shortcomings; there must be clear technical requirements for the raw materials used, and correct processes and inspection standards must be formulated for each process and manufacturing link, and the entire process must be carried out under controlled conditions.

1.1 Selection of active substance additives

1.1.1 Cathode additives

Formula materials and content of positive electrode additives: mixed graphite (high purity graphite + Z1) 0.3%~1.0%; phosphoric acid 0.2%~1.0%; macromolecular organic matter E2 0.05%~2.0%; PTFE emulsion 0.1%~0.5%; short fiber 0.1%~0.2%

The role of mixed graphite is to change the PbO2 crystal into a pine-like shape, increase the porosity of the active material, and improve the conductivity. At the same time, it increases the oxygen evolution overpotential and reduces the oxygen evolution rate. Some people believe that the added carbon disappears after 100 cycles and has no effect on the total life cycle. This view needs to be discussed. The role of additives in the cycle process is divided into long-term, medium-term and short-term. This depends on the role of the additives used and the purpose to be achieved. Carbon plays a role in the entire life cycle. In the first 100 cycles, it played a role in increasing battery capacity, reducing oxygen evolution rate and high current discharge. After that, gaps are left and the specific surface area is increased. The above performance of batteries without carbon is much worse.

Phosphoric acid is added to reduce the relaxation and shedding of active substances and extend the life. PO42- has the function of balancing the electric field of Pb4+ in the positive electrode. PTFE emulsion and short fibers are stable under high electric fields and are not damaged during the entire cycle of the battery. The two together form a strong network and enhance the strength of the active substances. As early as 1980, Mr. Wu Shousong discussed the application of PTFE emulsion. The amount of emulsion should be moderate. Too much addition will easily become a rubber plaster and is not easy to apply to the board. Some people use PTFE powder added to the positive electrode paste, but it turns out that it doesn't work. The reason is that the powder cannot form a network in the active substance, which is not good for overcoming the looseness of graphite. Experiments have shown that the plate is resistant to overcharge and over-discharge.

Organic matter has never been added to the positive electrode. The purpose of adding macromolecular organic matter E is to make it completely oxidized and disappear at the high potential of the positive electrode during the formation charging and battery activation process. The functions are: leaving larger pores to facilitate mass transfer; secondly, preventing the organic matter in the negative electrode from being decomposed at the positive electrode, and maintaining the effective amount of organic matter in the negative electrode. More than 10 years ago, this method was used in the production of starting batteries and forklift batteries, and the effect was remarkable.

1.1.2 Anode additives

Negative electrode additive formula materials and their content: ultrafine barium sulfate 0.3%~1.0%; high-purity humic acid 0.3%~1.0%; new lignin 0.1%~0.5%; acetylene black 0.2%~0.8%; short fiber 0.05%~0.2%

Note: The total amount of ultrafine barium sulfate, high-purity humic acid, neolignin and acetylene black is 1.6%~2%.

The reasonable combination and ratio of the above inorganic additives and organic additives are very important. Reasonable combination and ratio can effectively improve the output capacity, extend the cycle life and inhibit the generation of oxygen. Many literatures have detailed descriptions of their functions. It should be emphasized that in DZM batteries, the amount of lignin cannot be too little due to poor control of the paste process. A sufficient amount of lignin can ensure that it works throughout the life cycle of the battery. Ultrafine barium sulfate BaSO4 cannot be used too much because it is not conductive. The main components of new lignin are oak powder, tannin, lignin, etc. Its performance is comparable to that of imported ones, and its price is less than half of that of imported lignin.

DZM batteries are charged when leaving the factory. In addition to selectively using antioxidants in the impregnation of the negative plate after washing, there is no need to add antioxidants in the paste formula to avoid adversely affecting the battery's ability to resist overcharge, overdischarge and charge acceptance. Slight surface oxidation during the production process can be reduced during the charging activation process. 1.2 Electrolyte Additives

According to the type of active substance added, select matching and suitable electrolytic additives, the materials and contents of the electrolyte formula: H3PO4 0.1%~1%; NmSiO23 0.1%~0.5%; KHSO4 0.5%~2%

H3PO4 is added to the electrolyte to compensate for the decrease in oxygen overpotential caused by the addition of acetylene black to the negative electrode. NmSiO2 has the function of retaining moisture and adsorbing antimony. The radius of K+ in KHSO4 is larger than that of Na+, which can better prevent dendrite formation and micro-short circuits. K+ also helps to solve the problem of early capacity loss. In the later stage of discharge, in an alkaline environment lacking H+, HSO4 will ionize and provide a portion of H+. The combination of the above three has a "colloid-like" effect, which improves the performance of lean VRLA.

1.3 Ratio of positive and negative plates and positive and negative active materials

VRLA design is a positive limit capacity design. Generally, in DZM batteries, the ratio of positive and negative active materials is 1:1.05. In order to improve the ability of DZM batteries to withstand overcharge and overdischarge, it should be appropriately increased to 1:(1.06~1.10) so that the oxygen cycle will not terminate in the overcharge state. The density of active materials should be appropriately increased, and the density of negative lead paste should be controlled at 4.50~4.65g/cm3. Higher density is conducive to deep cycle life. The thickness ratio of positive and negative grids is 1.40~1.75. Thin plates are used to improve the battery's resistance to overcharge, overdischarge and high current, with more sheets and smaller internal resistance.

1.4 Premixing of active substance additives

The concept of premixing active material additives was proposed in China by Mr. Wu Shousong, which is reported in the literature. In fact, this method is based on the preparation of batch materials in the glass industry with a history of more than 2,000 years. In the glass industry, some additives only account for a few ten-thousandths of the total raw materials. How to mix them evenly is the method of premixing additives. The use of a horizontal closed spiral mixer can avoid the problem of acetylene black chain breaking when mixing in a ball mill. The raw materials roll up and down in the machine, and move left and right to the middle. It overcomes the stratification of various raw materials due to the influence of gravity due to different specific gravitational forces, and is very uniform. In order to make the additives and the main materials more evenly distributed, a certain weight of the main material is mixed with the mixed additives (Mr. Wu Shousong called it "capacity expansion"), and then used in the production batching. This method has been used for 10 years, and we have a lot of experience, and the consistency of the plates is very good.

Here we can talk about an anecdote from World War II. At that time, the Soviet Union's military optical glass was not up to standard, so it sent representatives to Britain and the United States for inspection. As a result, it spent 300,000 US dollars to buy back a process with four words: "mix evenly", which is worth a thousand gold. This also shows the importance of uniform mixing of raw materials in the chemical production of multiple materials. The poor consistency of batteries is caused by uneven mixing of raw materials. Many companies have carried out strict process control from the grid to the battery off-line and storage, but there are still many returns of batteries due to inconsistency, and they are unable to find the reason. The main reason is that the uniformity of the paste (consistency of the lead paste) is ignored. The lead paste is unevenly mixed, the chemical composition is uneven, and the chemical and electrochemical properties of the plates are very different. Even if the size and weight of the plates are strictly controlled, it cannot change the fact of poor consistency. Therefore, in the production process control, we should not only pay attention to the consistency of physical properties, but also pay attention to the consistency of chemical composition.

Here, the question of how to detect whether the paste is uniform or not naturally arises. The method is to take three points at the top, middle and bottom of the lead paste after dry mixing, and use one of the raw materials (or elements) as the benchmark to determine its percentage content. The raw material (or element) contained in the three points is qualified if the error does not exceed 5%. In mass production, it is impossible to measure every cylinder of material and every day (limited by existing conditions and testing methods). It is through this method that the relevant operating parameters of the paste process are found and controlled to ensure the uniformity and consistency of the chemical composition of the paste. 1.5 Main process characteristics

1.5.1 and paste

The paste is made by stepless speed change. After adding PTFE emulsion to the positive lead paste, the viscosity of the lead paste increases, and the lead paste often clumps and is pushed away by the paddles of the paste machine, and cannot be dispersed. When making the paste, the machine speed is controlled at 45r/min. After adding the mixture of PTFE and pure water, continue to stir for 3~5min, and then add acid. When the amount of acid is added to about 60%, increase the machine speed to 55~60r/min. After adding the acid, continue to stir for 7~10min. The apparent density of the paste is qualified and it is easy to apply to the board.

The negative lead paste contains lignin. If the process is not accurate and strict, it is easy to become thin and the coated board will have serious pits. The method of foreign companies is to add lead powder when it becomes thin, and add it in 3 times, with a total amount of 3% of the paste lead powder. The method is: dry mix the lead powder and the pre-mixed additives for 5~6 minutes, let it be fully pounded and evenly mixed (this is very important, because when wet pounding, the additives can no longer be evenly dispersed in the paste), and then add water and acid according to the process regulations. When the acid is added to 60%, increase the machine speed to 55~60r/min. After adding the acid, continue to pound for 2~3 minutes. The time should not be too long, otherwise it will become thin. Measure the apparent density and meet the requirements. If the lead paste is slightly hard (the apparent density is slightly larger), add 0.15~0.2Kg of water by spraying while the paste machine is running, and continue to pound for 2~3 minutes. The acid and water are required to be accurately measured, and the amount of water is fine-tuned according to the moisture content, oxidation degree and air humidity of the lead powder, so that high-quality lead paste can be produced. Using new lignin produced in Nanjing, its low-temperature performance is equivalent to that of imported lignin, and the lead paste does not become thinner. The appropriate dosage is 0.3%.

1.5.1 Green Plate Process

The green plates do not need to be dried on the surface after coating. If the surface drying is not well controlled, it will inevitably affect the bonding strength between the grid and the active material. In factories that mainly use manual coating, the plates should be pressed and acid-soaked immediately after coating, and then enter the curing room. Especially in summer, water is lost quickly. The water content of the plates entering the curing room is less than 8%, and the curing is often poor. There is a large time difference between the plates entering the curing room, and the curing room is generally closed after 8 hours. If the curing room does not maintain high humidity before closing, the water content of the plates entering the curing room will be too different, and the curing effect will be inconsistent.

In the first stage of curing, the green plate is an exothermic reaction, and the temperature should be low and the humidity should be high (35~40℃, humidity 100%), so that the plate temperature is not too high and the active material is well combined with the grid. The second stage is medium temperature and high humidity (60±2℃, humidity 95%~100%). In the third stage of drying, the temperature should rise slowly and the humidity should decrease slowly. Drying can be completed when the moisture content is 1%, and it is not necessary to dry to less than 0.5%. Because, if it is not used for tank formation immediately, the humidity of the air will also make the moisture content of the green plate higher than 1%, and a small amount of moisture content will begin to present a weak alkaline environment when the plate is formed, which is conducive to the formation of a-PbO2. In the curing process, the supply of environmental oxygen is very important, and insufficient oxygen will affect the curing effect.

1.5.3 Cell Formation and Battery Activation Process

It is generally believed that the plates should be thoroughly treated during the tank formation stage, which can shorten the time for re-discharging and activating the battery. In fact, it is not necessary to thoroughly treat the plates during the tank formation stage. As long as there are no white spots on the surface and the positive and negative plates are of normal color, they can be removed from the tank. In this way, the strength of the positive plate is good, and after being installed in the tank, it is thoroughly treated during the battery charge and discharge activation stage, which is beneficial to prolonging the life. Since the spongy lead of the negative plate reacts strongly with the air in a humid environment, it should be impregnated with an antioxidant after the tank formation water washing. The battery is activated by using a constant current and constant voltage current limiting method, which is controlled by a computer. In this way, the loss of the cooked plate during the washing, drying, assembly process and reaction with the electrolyte can be compensated during the activation charge. The charge and discharge system is adopted to make the active substances of the plate uniformly treated.

In winter, in workshops without temperature control, the formation current should be appropriately increased. It should be noted that during the activation process, the electrolyte of the plates and separators should be saturated, and the battery should not lack acid, otherwise the battery cannot be activated. The on-duty personnel should check and replenish acid frequently, and the electrolyte height in each plug should be consistent.

Now we use high-frequency, high-power positive and negative pulse automatic chargers produced by Zhangjiagang Zhongheng Power Supply Co., Ltd. to activate batteries, which speeds up the process and improves the quality. Taking 16 6-DZM-17 batteries as an example, the procedure is as follows:

(1) Charging with an average current of 5.4A, positive pulse of 9A, negative pulse of 0, time of 3h, and a charge of 16.2Ah

(2) Charging with an average current of 4.1A, positive pulse of 8.5A, negative pulse of 5A, time of 4h, and a charge of 16.4Ah

(3) Charging with an average current of 4.2A, positive pulse 7A, negative pulse 0, conversion voltage 256V, and charging capacity of 2Ah

(4) Charge at 4A and 240V for 1h, charging 1.5Ah

The above 4 steps take a total of 9 hours to charge, with a total charge of 34.1Ah

(5) Discharge at 8.5A until the final voltage reaches 10.5V. The discharge time is greater than 125min. Check the capacity.

(6) Charging at 6A constant current, conversion voltage 235V, charging time 3h, charging capacity 18Ah

(7) Charging with an average current of 4.1A, positive pulse 8.5A, negative pulse 5A, charging time 3h, and a charged capacity of 16.4Ah

(8) Charging at a constant voltage of 240V and a current of 4A for 1h, with a charge of 1.5Ah

The total charging time for steps (5) to (8) above is 8 hours, and the amount of electricity charged is 35.9Ah.

(9) End of battery activation 1.5.4 Assembly pressure

Generally, the assembly pressure is required to be 35~45Kpa, and the AGM compression is about 15%. As long as the battery slot does not deform, it should be assembled as tightly as possible, and the assembly pressure should be 50~55Kpa. Because the AGM shrinks after adding the electrolyte, the AGM compression is preferably 20%.

2 Effects and Discussion

2.1 Effect
6-DZM-10 battery has been fully inspected by the National Battery Quality Supervision and Inspection Center (inspection report number W2004-023), and all performances meet the technical requirements of JB/T10262-2001 standard. Among them, several indicators are as follows:

(1) 2h rate capacity (25℃): 11.00Ah

(2) -10℃ low temperature capacity: According to standard test, the actual capacity is not less than 0.9C2, the actual capacity is 1.050C2

(3) Overcharge characteristics: According to the standard test, the actual capacity is not less than 0.95C2, and the actual capacity is 1.125C2

(4) Over-discharge characteristics: According to the standard test, the actual capacity is not less than 0.75C2, and the actual capacity is 1.040C2

(5) High current discharge characteristics: According to the standard test, the battery terminal voltage is greater than 8.40V, and the actual voltage is 11.76V

(6) Cycle life: According to the standard test, the number of cycles is not less than 350 times. After 386 times, the capacity is still 11Ah.

Recently, 6-DZM-10 batteries were randomly sampled on the production line and tested using a PC-controlled comprehensive battery tester purchased from Shenyang Yaohua Automation Research Institute. The results are as follows:

(1) 15A high current discharge, discharge for 10 minutes (5 minutes as required by the standard), battery terminal voltage 12.14V (greater than or equal to 8.40V according to the standard)

(2) 22.2A high current discharge, final voltage 10.50V, discharge time 25min26s

2.2 Discussion

Improving and enhancing the overcharge, over-discharge and high-current discharge resistance of DZM batteries is a comprehensive system technical issue. The key is to select and optimize active material additives and electrolyte additives that match them, correctly design the ratio of positive and negative active materials, grid thickness ratio, grid geometry, use thin plates, tight assembly and other process measures, strictly control the production process, achieve chemical consistency and physical consistency, so that the quality of mass-produced products can be guaranteed.

Improving and enhancing the overcharge, over-discharge and high-current discharge resistance of DZM batteries is not only technically significant, but also economically significant. Improving battery performance, extending battery life, and minimizing quality problems during the warranty period are the greatest economic benefits. Today, various disciplines in the field of technical sciences are mutually infiltrating. Drawing on theories and technologies from related disciplines is inspiring and helpful for improving the performance of lead-acid batteries.