The dew point is the temperature when the moisture condenses. When the water vapor content in the air remains unchanged and the air pressure is constant, the temperature when the air cools to saturation is called dew point temperature (Td), referred to as dew point.
It can also be understood as the temperature when water vapor and water reach equilibrium. The degree of air saturation is indicated by the difference between the actual temperature (t) and the dew point temperature (Td). The air is said to be unsaturated when t>Td, saturated when t=Td, and supersaturated when t<Td. Generally speaking, 0°C and below are referred to as the “frost point” and “dew point,” respectively.
Why is it important to keep an eye on the dew point in the environment used to make lithium batteries?
The production process for lithium-ion batteries has very strict humidity requirements, primarily because excessive water or coarseness control will negatively impact the electrolytes. Lithium salt and organic solvent make up the electrolyte, the battery’s ion transport medium. It ensures that lithium-ion batteries will have high voltage, high specific energy, and other benefits. The electrolyte will be negatively impacted by over-watering:
- Electrolyte metamorphism
The electrolyte, which is made of organic solvent and lithium salt, is the battery’s means of transmitting ions. Lithium-ion batteries are guaranteed to experience the benefits of high voltage and high specific energy because the electrolyte acts as a conductor between the positive and negative electrodes. In order to prevent contact between the battery and air, the battery must be injected with liquid in an environment with less than 1% humidity and sealed right away. The environment of the liquid injection room will be negatively impacted by the electrolyte’s reaction with the water if the moisture content is too high. This will also lower the electrolyte’s quality and result in poor battery performance.
- Battery capacity becomes smaller
As the battery’s moisture content rises, so does its initial discharge capacity. In addition to consuming lithium ions and harming the electrolyte’s functional components, excessive moisture will also cause irreversible chemical reactions in the battery anode. The battery’s energy decreases as lithium ions are used up.
- Increased internal resistance
The internal resistance of batteries tends to increase as their moisture content increases. When a battery is being used, its internal resistance is small, allowing the battery to be discharged and have a very high power; when the internal resistance is large, the battery cannot be discharged and has a relatively low power. Excessive moisture will deteriorate the SEI film in lithium batteries, which will lower the battery’s internal resistance.
- The internal pressure of the battery is too high
Hazardous gases will be produced in the electrolyte when water and LiPF6 react. Excessive moisture causes the battery’s internal pressure to increase, which leads to deformation. If the battery is from a cell phone, it will resemble a drum shell. When the internal pressure is high, the battery can be dangerous because it can burst and cause electrolytes to splash, which could easily injure people.
- Battery leakage
LiPF6 in the electrolyte reacts with water to form gas and hydrofluoric acid, a highly corrosive acid that can eventually erode metal components within the battery and result in a leak. Should the battery leak, the user’s machine will be corroded by the electrolyte and the battery’s performance will quickly deteriorate.
Water greatly affects electrolytes and positive and negative materials. The moisture content of the workshop and glove box must be strictly regulated to guarantee the quality of the battery. This is especially important for certain critical processes, like cell baking, injection, sealing, etc., which must be completed in an environment with less than 1% humidity to prevent moisture from getting into the electrolyte.
At this point, the humidity fluctuation must be reflected using the change in the dew point temperature value. In general, it is best to keep the dew point temperature at or below -45°C.
Since there is less than 1% relative humidity at this level, the dew point is used to represent the water vapor concentration. Even if they can convert display and output values to dew point temperature, the majority of relative humidity measuring instruments lack the resolution and accuracy needed for meaningful measurement at this level.
For example, when the dew point temperature is -50°C, an increase of 5°C to -45°C indicates that the corresponding relative humidity change is only 0.1% – a value that is difficult to distinguish from other interference information.
Dew point measurement principle and method
Polymer moisture sensors, alumina or silicon sensors, and cold mirror hygrometers are common methods for measuring dew points. Every solution has benefits and drawbacks of its own.
The condensation temperature on the reflective surface (mirror) is measured by the cold mirror hygrometer via optical reflection. Under lab conditions, these devices are extremely accurate; however, they are readily impacted by the measurement error known as the Raoult effect when the sampling gas contains solvents that will enter the solution and condense on the mirror. The mirror surface may also sustain damage from strong acids or bases. Although silicon oxide and aluminum oxide sensors are capable of measuring extremely low dew point temperatures, they are also susceptible to environmental pollution and acid-base damage.
Relatively speaking, polymer sensors are resistant to a wide range of chemical contaminants. As a result, the primary methods of dew point monitoring in the environmental monitoring of lithium batteries are currently polymer films and other sensors. The direction of future development will be the laser principle sensor.
In conclusion, ensuring a bone-dry lithium battery workshop, with dew points below -45°C, is crucial. High moisture wreaks havoc, degrading electrolytes, lowering capacity, and inflating pressure, ultimately compromising battery performance and even risking leaks. While cold mirror hygrometers reign supreme in accuracy, they stumble when solvents are present. Polymer sensors, though less precise, stand sturdy against chemical foes, making them the current dew point guardians of battery production. Emerging laser sensors show promise for even sharper monitoring, ensuring the next generation of batteries dances with only the driest air.
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