N-methylpyrrolidone (NMP) is a highly polar non-polar substanceProton transferSolvent, appearing as a colorless transparent or slightly yellow oily liquid with a slight amine odor, has a mild irritation to the skin.

NMP has the characteristics of high boiling point, strong polarity, low viscosity, strong solubility, no corrosion, low toxicity, low volatility, non flammability, good chemical and thermal stability, degradability, and recyclability. It is easily soluble in water, ethanol, acetone, etcethyl acetateChloroform and benzene can dissolve most organic and inorganic compounds, polar gases, natural substances, and合成高分子化合物.

At present, it is mainly used in coatings, lithium batteries, plastics, chemical production agents, agricultural chemicals, pigments, cleaning agentsinsulating materialWaiting for the industry. Here we mainly discuss the application of NMP in the lithium battery industry.

Application of NMP in the field of lithium batteries

Lithium ion batteries are currently widely used in many fields. In addition to laptops, mobile phones, electric bicycles, and electric vehicles in daily life, lithium-ion batteries also have important applications in energy storage.

According to industry research data, overall, NMP accounts for about 3% -6% of the manufacturing cost of lithium-ion batteries. Compared with positive and negative electrodes, electrolytes, and separators, NMP has a relatively small cost share, but its role cannot be ignored. The market size and demand for NMP have also grown with the growth of lithium-ion batteries, especially energy storage batteries and power batteries.

The important role of NMP in lithium battery production

The use of NMP in lithium battery production is to dissolve/swell PVDF and dilute the slurry, with a concentration requirement generally above 99.9%. NMP, as one of the main auxiliary materials in the manufacturing process of lithium-ion batteries, directly affects the quality of slurry coating and environmental protection requirements of lithium-ion batteries.

NMP is the positive electrode binder PVDF (polyvinylidene fluoride) for lithium batteries）The supporting solvent is also a necessary supporting solvent for PVDF (polyvinylidene fluoride). NMP can interact with water PVDF、 Positive and negative electrode materialsDue to their mutual solubility, NMP is widely used in the preparation of lithium batteries. When making electrode sheets, NMP serves as a solvent to fuse various electrode materials such as binders, positive electrode active materials, and conductive agents together, allowing the binders to fully contact and evenly distribute with other substances.

Analysis of Economic and Environmental Benefits of NMP

Benefiting from the rapid development of the new energy vehicle industry in 2021, NMP prices have continued to rise, from 15000 yuan/ton at the beginning of 2021 to over 40000 yuan/ton currently.

In the production process of lithium batteries, NMP, as an expensive solvent, generates high-temperature NMP waste gas during the coating process. If effective recycling is not carried out, it will not only result in waste of raw materials, but also cause serious pollution to the environment. China also has relevant control requirements for NMP emissions, as outlined in the 'Clean Production Evaluation Index System for the Battery Industry'The benchmark value of NMP for exhaust gas pollutant index in the (trial) is 1.5mg/m3. Emission Standards for Pollutants in the Battery Industry》The limit requirement for the organized emission concentration of NMP waste gas is set at 50mg/m3. Therefore, strengthening NMP emission control and recycling in the production process of lithium batteries has significant economic and environmental benefits.

Production and Recovery Control of NMP in Lithium Battery Production Process

Analysis of the production process of lithium batteries：

From the above figure, it can be seen that NMP is mainly used and discharged in the positive electrode preparation process. The main processes using NMP are as follows:

coatingUsing a coating machine to apply the positive electrode slurry (lithium nickel cobalt manganese oxide)A mixture of conductive carbon black, solvent NMP, and binder PVDF is uniformly coated on both sides of the aluminum foil;

The dryer adopts a closed negative pressure exhaust dryer (drying channel). During the coating process, only a small amount of NMP evaporates at the inlet and outlet of the coating machine, and the exhaust gas in the dryer is led to the exhaust gas treatment through the induced draft fanHigh altitude emissions after facility treatment; NMP waste gas has a very small amount of unorganized volatilization in the workshop due to insufficient drying and other reasons.

bakeAfter coating is completed, it is dried in a vacuum dryerBake and dry the positive electrode sheet internally at a temperature of 120 ± 5 ℃; Remove the NMP component from the electrode. The dried gas enters the NMP recovery device for recovery.

In summary, NMP is mainly used in the coating process of lithium battery production, followed by NMP recovery control.

NMP recycling control measures

The principle of NMP recovery process: The NMP gas emitted from the coating machine in the coating workshop is extracted by a suction fan and enters the NMP recovery equipment through a pipeline. Cooling water and chilled water coils are used to condense NMP from the air (cooling temperature of about 5 ℃), and then collected and purified to achieve the purpose of recovery.

The NMP recycling process flow is shown in the following figure:

During the production process of lithium-ion batteries, the NMP in the exhaust gas discharged by the coating machine has a high temperature. In the early stage, water and gas are used to cool and condense the exhaust gas, and most of the NMP is recovered through gas gas heat exchange. The residual exhaust gas enters the later stage treatment system and is discharged in compliance with standards after treatment.

Process flow: After cooling the exhaust gas, process water is used to wet the packing inside the tower. The 304 stainless steel packing has a large specific surface areaThe exhaust gas and process water are fully contacted and fused on the packing to the bottom for a single recovery. The exhaust gas is continuously sent into the storage tank as the number of fusion cycles increases, and the concentration of NMP waste liquid continues to rise until it reaches the set concentration (≥ 80%) and is discharged into the storage tank; Part of the tail gas is sent to the secondary absorption tower and washed with low concentration process water again to meet the final emission standards.

In the commonly used NMP recovery process, the NMP liquid concentration sensor product is installed on the circulating pipeline in the spray tower for online real-time monitoring of the concentration of NMP solution.

The important role of NMP solution concentration detection in lithium battery coating machine process

In the manufacturing process of lithium battery electrodes, the solvent NMP in the coating slurry will form a gaseous state after heating, and NMP gas is toxic. Moreover, NMP is a flammable and explosive liquid, and there is a possibility of explosion when mixed with air. Therefore, a device is particularly needed in this process to achieve automatic detection of NMP concentration, in order to ensure the personal safety of workers.

The NMP solution online concentration meter uses the principle of refraction to measure the refractive index of NMP solution. The refractive index corresponds to the concentration of NMP solution, and accurate solution concentration values can be obtained through conversion.

The current process recovery principle is to use cooling water to cool the high-temperature air containing NMP through a water air heat exchanger, so that NMP in the exhaust gas is separated from the air in liquid form and recovered. The real-time concentration value of NMP is controlled by an online concentration sensor, which greatly improves the recovery efficiency and automation technology productivity. The air separated from NMP is subjected to heat exchange and recycling through a gas air heat exchanger, achieving energy-saving and environmental standards.

The waste gas from the production equipment is adsorbed by the absorption unit, and the NMP in the waste gas is basically dissolved in water or water vapor. After gas-liquid separation and rinsing, the NMP in the waste gas is adsorbed.

Based on the recycling device, there are assembled circulation pumps， The NMP online concentration sensor is installed in the pipeline of the circulating pump to detect the concentration of NMP liquid in real time, which can effectively reduce the waste of water resources and monitor the discharge of finished NMP recovery liquid.

NMP solution online concentration sensors are usually installed on the stage circulation (internal circulation) loop, where the absorbed solvent (such as water) comes from the second stage loop. The solvent has already absorbed the target component in the exhaust gas in the second stage circulation loop. After further absorption in the stage circulation loop, the absorption capacity of the solvent will decrease. The concentration meter monitors the concentration of the target component in the solvent in real time, determines the absorption capacity of the solvent for the target component, decides whether to discharge the internal circulation solution, and whether to add fresh absorption solvent to improve recovery efficiency.

NMP solution online concentration sensor Application advantages:

1. Improve measurement accuracyReduce measurement error (± 0.5%): NMP solution online concentration meter with precise automatic temperature compensationFunction ensures accurate measurement of NMP solution concentration in different measurement environments.

2. Improve control accuracyBy outputting standard control signals to control the opening and closing of the discharge valve, continuous closed-loop detection and control are carried out to ensure the stability and accuracy of the NMP solution concentration value.

3. Improve recycling efficiency: To avoid a decrease in the ability of high concentration internal circulation wastewater to exchange and dilute with exhaust gas during the recycling process, resulting in insufficient recovery of NMP in the exhaust gas and discharge outside the tower, reducing recycling efficiency and causing NMP waste.

4. Can achieve 24-hour continuous monitoringImprove automation level, reduce labor intensity, and lower labor costs.