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Lithium-ion batteries have become the standard power source for portable mobile devices, and have developed rapidly in the electric vehicle and energy storage markets in recent years. The requirements for battery safety, energy density, power density, reliability, and cycle life have also been continuously increasing. The performance of lithium-ion batteries is affected by numerous factors, including not only battery design, raw materials, technological level, and equipment precision, but also environmental factors such as temperature, cleanliness, and moisture. Even a small amount of impurities can have an adverse impact on the cycle stability and safety of lithium-ion batteries. Therefore, we must attach great importance to the production process and strictly control the quality. Among them, the control of moisture is very crucial. Commercial lithium-ion batteries are produced in large drying rooms with strict control of environmental moisture, and all components need to be dried before battery assembly. This article organizes and shares the basic knowledge of moisture testing and control of battery electrodes.

1. The Basic Mechanism of How Moisture Harms Battery Performance

If the moisture content in lithium-ion batteries is too high, it will react with the electrolyte. Firstly, the moisture reacts with the lithium salt in the electrolyte to generate HF:

H₂O + LiPF₆ → POF₃ + LiF + 2HF

Hydrofluoric acid is a highly corrosive acid that can corrode the metal parts inside the battery, eventually leading to battery leakage. Moreover, HF destroys the SEI (Solid Electrolyte Interphase) membrane and will continue to react with the main components of the SEI membrane:

ROCO₂Li + HF → ROCO₂H + LiF

Li₂CO₃ + 2HF → H₂CO₃ + 2LiF

Finally, LiF precipitates are produced inside the battery, causing irreversible chemical reactions of lithium ions on the negative electrode of the battery, consuming active lithium ions, and reducing the energy of the battery. When there is enough moisture, more gas is generated, and the internal pressure of the battery will increase, causing the battery to deform under stress, resulting in risks such as battery swelling and leakage.

2. Testing of Residual Moisture in Battery Electrodes

The residual moisture in battery electrodes is generally several hundred ppm, and the moisture content is relatively low and cannot be measured by simple methods. Generally, the Karl Fischer Coulometric method is used to test trace moisture, and its principle is an electrochemical method. When the Karl Fischer reagent in the instrument's electrolytic cell reaches equilibrium, a water-containing sample is injected. Water participates in the redox reaction of iodine and sulfur dioxide. In the presence of pyridine and methanol, pyridinium iodide and pyridinium methyl sulfate are generated. The consumed iodine is electrolytically generated at the anode, so that the redox reaction continues until all the moisture is exhausted. According to Faraday's law of electrolysis, the amount of iodine electrolytically generated is directly proportional to the amount of electricity consumed during electrolysis. The reactions are as follows:

H₂O + I₂ + SO₂ + 3C₅H₅N → 2C₅H₅N·HI + C₅H₅N·SO₃

During the electrolysis process, the electrode reactions are as follows:

Anode: 2I⁻ - 2e⁻ → I₂

Cathode: I₂ + 2e⁻ → 2I⁻

2H⁺ + 2e⁻ → H₂↑

It can be seen from the above reactions that 1 mole of iodine oxidizes 1 mole of sulfur dioxide, requiring 1 mole of water. Therefore, the amount of electricity for electrolyzing iodine is equivalent to that for electrolyzing water. Electrolyzing 1 mole of iodine requires 2 × 96,493 coulombs of electricity, and electrolyzing 1 millimole of water requires 96,493 millicoulombs of electricity. The moisture content in the sample is calculated according to Equation (1):

Where: W --- moisture content in the sample, μg; Q --- electrolytic electricity, mC; 18 --- molecular weight of water.

The general structure of the Karl Fischer Coulometric Trace Moisture Tester is shown in Figure 1, mainly including the Karl Fischer electrolytic cell and the sample heating unit. The electrode sample is placed in a sealed sample bottle, and then the sample bottle is heated at a certain temperature. The moisture in the sample evaporates, and then the water vapor is sent into the electrolytic cell by using dry gas to participate in the reaction. Then, the amount of electricity during the electrolysis process is measured to titrate the moisture content.

Figure 1 Schematic Diagram of the Structure of the Karl Fischer Moisture Tester

Michael Stich and others used the device shown in Figure 1 to conduct a more detailed study on the moisture drying behavior of battery electrodes. During the experimental process, they dried the electrodes in two steps: First, the sample was rinsed with dry argon at room temperature, and the amount of water released from the sample was monitored. Subsequently, the sample was placed in the heating unit and heated to 120 °C for 12 minutes to evaporate the water in the sample and measure the moisture content. At the same time, to verify the accuracy of the experimental data, the moisture content of the empty bottle without the sample (Blank value) was first measured, and the drift value of the moisture (Drift value) was considered. Figure 2 shows the time evolution of the released moisture content, moisture release rate, blank value, and drift value during the moisture determination process of the LiFePO₄ electrode sample. The blank value mainly appears in the first few seconds of the measurement, mainly due to the water adsorbed on the walls of the empty glass bottle and the moisture in the gas phase. During the heating process of the test, it is impossible for all the moisture in the sample to evaporate into the electrolytic cell to participate in the reaction in a short time. Michael fitted the moisture release rate from 6 to 15 minutes when testing the sample according to the empirical formula (2) characterizing the film drying process:

After curve fitting, the parameters a, b, k₁, k₂ in the formula are obtained, and then the remaining moisture in the sample after the experimental test is extrapolated and calculated as shown in Figure 3.

In this way, the moisture measured by the Karl Fischer method can be divided into three parts:

1. The moisture detected within the first 3 minutes of rinsing with dry argon at room temperature.

2. The moisture detected by rinsing with dry argon during the 13-minute heating process at 120 °C.

3. The remaining water after the experiment, which is calculated by fitting the moisture determination rate curve and extrapolating.

3. Control of Residual Moisture in Battery Electrodes

In the production process of lithium-ion batteries, the negative electrode is generally an aqueous slurry, and the positive electrode is generally an oil-based slurry. After the slurry is coated, the battery electrodes are dried for the first time. The main purpose of this step is to remove the solvent in the slurry to form battery electrodes with a microscopic porous structure. After this drying step, relatively high moisture still remains in the electrodes. Moreover, during the subsequent electrode processing, due to the characteristics of porous and high specific surface area, the electrodes are prone to absorb moisture from the ambient air. Therefore, the control of residual moisture in battery electrodes is a very crucial step. Currently, there are mainly two drying processes to remove the residual moisture:

1. Before the battery is wound or laminated, the battery electrodes are vacuum dried. Generally, the drying temperature is 80 - 150 °C. The battery electrodes are usually dried in rolls or stacks, and gas replacement is carried out multiple times during the process. Besides the drying procedures such as heating, vacuum degree, and gas replacement, the size of the electrode roll or the number of stacked sheets also has a significant impact on the drying effect and needs to be carefully considered.

2. Before the battery is filled with electrolyte, the assembled battery is vacuum dried. Since the battery contains components such as diaphragms at this time, the drying temperature is generally 60 - 80 °C, and gas replacement is carried out multiple times. At this time, the drying temperature is relatively low, the various components of the battery are assembled together, and the reserved injection port is small, all of which are not conducive to moisture removal.

Michael Stich and others used the method described in Part 2 to study the drying process of various battery electrodes. The moisture content of the three parts included in the drying experiment is shown in Figure 4. Among them, the graphite negative electrode is an aqueous slurry, and the residual moisture content in the electrode is relatively high, while the moisture content in the positive electrode varies greatly. The main factors affecting the drying behavior of electrode electrodes include the specific surface area of the electrode, the hydrophilicity of the material, and the binding strength with water. For example, nanomaterials have a large specific surface area and are more likely to absorb water. Therefore, the electrode drying procedure needs to be set according to the characteristics of the electrode material to achieve a better drying effect.

Figure 4 Drying Moisture Content of Various Electrode Materials

Subsequently, Michael Stch also studied the moisture absorption process of the electrodes. After drying various electrode materials at 80 °C for 12 hours, a part of the moisture was removed. The moisture content of the dried electrodes was measured by the Karl Fischer moisture tester, and the content was all below 700 μg/g, and that of the glass fiber diaphragm was 1040 μg/g. Then they were placed in an atmospheric environment with a relative humidity of 40%, and their moisture absorption behavior was examined. The results are shown in Figure 5. It can be seen from the figure that most of the moisture absorption occurs within the first hour. The graphite negative electrode absorbs more than 80% of the moisture within the first hour, and even the moisture absorption percentage of the glass fiber diaphragm and the LiFePO₄ positive electrode is higher. The coating thickness of the LiMn₂O₄ and Li(NiCoMn)O₂ positive electrodes is relatively thin, and the water absorption rate is relatively low, and the total water content of the LiCoO₂ positive electrode is low. It is difficult to dry the battery electrodes, but it is easy to absorb moisture. Therefore, the control of the humidity of the production environment of the battery is very important, especially after the electrodes are dried, the assembly and processing of the battery need to strictly control the environmental moisture.

4. The Impact of Residual Moisture in Battery Electrodes on Battery Performance

Michael Stich and others took graphite/LiFePO₄ button batteries as an example to study the impact of moisture on battery performance. Batteries with high moisture content had severe cycle capacity attenuation and increased internal resistance, as shown in Figure 6. The main reason for the capacity attenuation may be related to the continuous accumulation of LiF with poor conductivity in the SEI, the dissolution of Fe ions caused by the acidity formed by the hydrolysis of LiPF₆, and the continuous reduction of the concentration of LiPF₆ in the electrolyte. Figure 6b is the AC impedance spectrum of the graphite/LiFePO₄ button battery in the discharged state. The button battery with high moisture content has an additional semicircle at a higher frequency, and the second semicircle frequency is about 100 Hz to 1 Hz. These semicircles are attributed to the thickening of the SEI and the charge transfer resistance, indicating the formation of an interface film with high resistance. The ohmic resistance of the battery did not change significantly, indicating that the conductivity of the electrolyte was not affected by the hydrolysis of LiPF₆.

Niu Junting and others conducted a more systematic study on the relationship between the residual moisture in battery electrodes and battery performance. The cycle performance curves of batteries assembled with positive electrodes with different moisture contents are shown in Figure 7. In the first 50 cycles, the capacity attenuation rates of batteries with different electrode moisture contents were close, and the cycle was stable. The batteries with the moisture content of the positive electrode between 0.4‰ and 0.5‰ had good cycle performance. After 200 cycles of charging and discharging at 1C current, the discharge capacity of the battery still remained 92.9% of the initial capacity. As the cycle progressed, the capacity of the batteries with the moisture content of the positive electrode exceeding 0.6‰ decayed rapidly, and the performance deteriorated. This may be due to the fact that the amount of moisture precipitated from each battery electrode at the initial stage of charging and discharging was not much different. As the cycle progressed, more moisture in the battery electrodes with higher moisture content (exceeding 0.6‰) diffused into the electrolyte and reacted with the lithium salt in the electrolyte to produce highly corrosive HF, which destroyed the structure of the lithium battery, resulting in capacity attenuation. Especially as the charging and discharging process progressed, the batteries with higher HF content decayed faster.

It can be seen from the comparison of the rate performance from 1C to 5C in Figure 8 that the batteries with the moisture content of the battery electrodes in the range of 0.3‰ to 0.6‰ had a relatively high and close discharge specific capacity. As the discharge rate increased (2C to 5C), the moisture content of the battery electrodes exceeded 0.6‰, and the capacity attenuation rate increased.

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