"Lithium Battery Lecture": A Complete Summary of Key Points on Lithium Battery Self-Discharge

The Effects of Self-Discharge

1. Self-discharge leads to capacity reduction during storage.

Several typical problems caused by excessive self-discharge:

1. Cars fail to start after prolonged parking;

2. Batteries appear normal before storage, but are found to have low or even zero voltage upon shipment;

3. Car GPS devices left in the car in summer show significantly reduced battery power or usage time after a period of use, sometimes accompanied by battery swelling.

2. Metallic impurities in self-discharge can clog separator pores, even puncturing the separator and causing localized short circuits, endangering battery safety.

3. Self-discharge increases the SOC difference between batteries, reducing battery pack capacity.

Due to inconsistent self-discharge, the SOC of batteries within a battery pack varies after storage, leading to decreased battery performance. Customers often find performance degradation after receiving battery packs that have been stored for a period of time. When the SOC difference reaches around 20%, the combined battery capacity is reduced to only 60%~70%.

4. Significant differences in SOC can easily lead to overcharging and over-discharging of the battery.

I. Differentiation between Chemical and Physical Self-Discharge

1. Comparison of High-Temperature Self-Discharge and Room-Temperature Self-Discharge

Physical micro-short circuits are significantly related to time; long-term storage is more effective for selecting batteries with physical self-discharge. Chemical self-discharge, however, is more pronounced at high temperatures, so high-temperature storage should be used for selection.

Storing the battery at 5 days at high temperature and 14 days at room temperature: If the battery's self-discharge is mainly physical, then the ratio of room-temperature self-discharge to high-temperature self-discharge is approximately 2.8; if the battery's self-discharge is mainly chemical, then the ratio of room-temperature self-discharge to high-temperature self-discharge is less than 2.8.

2. Comparison of Self-Discharge Before and After Cycling

Cycling causes the internal micro-short circuits of the battery to melt, thereby reducing physical self-discharge. Therefore: if the battery's self-discharge is mainly physical, the self-discharge after cycling will decrease significantly; if the battery's self-discharge is mainly chemical, there will be no significant change in self-discharge after cycling.

3. Leakage Current Testing under Liquid Nitrogen

Measure the battery leakage current using a high-voltage tester under liquid nitrogen. The following conditions indicate a severe micro-short circuit and high physical self-discharge:

1) The leakage current is unusually high at a certain voltage;

2) The ratio of leakage current to voltage differs significantly at different voltages.

4. Separator Black Spot Analysis

By observing and measuring the number, morphology, size, and elemental composition of black spots on the separator, the magnitude of the battery's physical self-discharge and its possible causes can be determined:
1) Generally, the greater the physical self-discharge, the more numerous and deeper the black spots (especially those penetrating to the other side of the separator);
2) The metallic elemental composition of the black spots indicates possible metallic impurities in the battery.

5. Self-Discharge Comparison at Different SOCs

The contribution of physical self-discharge varies at different SOC states. Experiments have shown that batteries with abnormal physical self-discharge are more easily identified at 100% SOC.

II. Self-Discharge Testing

1. Self-Discharge Detection Methods

1) Voltage Drop Method

This method characterizes the magnitude of self-discharge by the rate of voltage drop during storage. It is simple to operate, but its drawback is that voltage drop does not directly reflect capacity loss. The voltage drop method is the simplest and most practical, and is currently the most widely used method in production.

2) Capacity Decay Method

This method expresses self-discharge as a percentage of capacity reduction per unit time.

3) Self-Discharge Current Method (Isd)

The self-discharge current (Isd) during battery storage is calculated based on the relationship between capacity loss and time.

4) Calculation Method for the Number of Moles of Li+ Consumed by Side Reactions

Based on the influence of the electronic conductivity of the negative electrode SEI film on the rate of Li+ consumption during battery storage, the relationship between Li+ consumption and storage time is derived.

2. Key Points of Self-Discharge Measurement System

1) Selecting an Appropriate SOC

dOCV/dT is affected by SOC. The effect of temperature on OCV is significantly amplified at plateaus, leading to a large error in SOC prediction. For self-discharge testing, a SOC (State of Charge) that is relatively insensitive to temperature changes should be selected, such as: FC1865: self-discharge test at 25% SOC; LC1865: self-discharge test at 50% SOC.

Due to differences in battery capacity, the actual SOC of the battery fluctuates, with a tolerance of approximately 4%. Therefore, the change in the slope of the OCV curve within a 5% tolerance range is examined. The slopes of LC1865 at 53% and 99.9% SOC are very stable, at 3.8mV/%SOC and 10mV/%SOC, respectively. The slope of FC1865 is relatively stable between 25% SOC; of course, a fully charged state is also a simple and practical point for measuring self-discharge.

2) Selection of Starting Time

For FC1865 at 25% SOC (or other SOC values), the voltage change was observed hourly after charging. After 20 hours, the voltage drop rate was basically consistent, indicating that polarization had largely recovered. Therefore, 24 hours was selected as the starting time for the self-discharge test.

For LC1865 at 50% SOC, the voltage change rate fluctuated slightly within a range of 0.01 mV/h after 14 hours, indicating that polarization had largely recovered. Selecting 24 hours as the starting point for self-discharge is feasible.

3) Storage Temperature and Time

The Influence of Storage Temperature and Time on Self-Discharge (LC1865H)

Within the study period, self-discharge showed a significant linear relationship with both time and temperature. The self-discharge model can be fitted as: Self-discharge = 0.23*t + 0.39*(T-25). (The above values ​​and relationships are related to the battery system, and the constants will change accordingly. The same applies to other relationships below.)

At room temperature, due to the reduced chemical reaction rate, the abnormal points of physical self-discharge are more pronounced. 14D storage can predict 28D results very well.

3. Improvements to the Self-Discharge Measurement System

1) Voltage Measurement Temperature

The effect of ambient temperature on self-discharge: FC1865: For every 1°C increase, the voltage decreases by 0.05mV; LC1865: For every 1°C increase, the voltage decreases by 0.17mV.

2) Voltmeter Selection

For voltmeter selection, since self-discharge studies focus on changes at the 0.1mV level, the traditional 4.5-digit voltmeter (accurate to 1mV, resolution to 0.1mV) is no longer suitable. Therefore, the 6.5-digit Agilent 34401A voltmeter (accurate to 0.1mV, resolution to 0.01mV or even higher) is selected. Furthermore, the repeatability of this instrument is quite good.

4. Determination of Self-Discharge Standards

1) Theoretical Calculation

2) Simulation of 1mV Difference

The balance results of a 1mV self-discharge difference (1mV over 28 days, 0.5mV over 14 days) after 3 years are simulated by artificially adjusting a 10% SOC difference. None of the three battery packs experienced overcharge safety issues, but the voltage difference during discharge was already very large (1200mV). The battery with the highest self-discharge was over-discharged to 2.5V, resulting in a 10% capacity loss in the battery pack.

Factors Affecting Self-Discharge and Key Control Points

I. Metal Impurities in Raw Materials

1. Mechanism of Influence of Metal Impurities

In the battery: Metal impurities undergo chemical and electrochemical corrosion reactions, dissolving into the electrolyte: M → Mn+ + ne-; Subsequently, Mn+ migrates to the negative electrode and undergoes metal deposition: Mn+ + ne- → M; As time increases, metal dendrites continuously grow, eventually penetrating the separator, leading to a micro-short circuit between the positive and negative electrodes, continuously consuming electricity, resulting in a voltage drop.

Note: The above is only the most common form; there may be many other influencing mechanisms.

2. Degree of Influence of Different Types of Metal Shavings

(1) Addition of different types of metal shavings to the positive electrode slurry

The degree of influence can be qualitatively ranked as follows: Cu > Zn > Fe > Fe2O3

Note: In principle, any metal impurity (such as FeS, FeP2O7, etc., not listed above) will have a significant impact on self-discharge, with the influence generally being strongest in elemental metals.

The black spots on the separator of the metal scrap battery are deep in morphology (penetrating to the other side) and numerous:

The metal element composition of the black spots on the separator matches the types of metals added, indicating that the metal elements on the black spots on the separator do indeed originate from metal impurities:

(2) Adding different types of metal scraps to the negative electrode slurry

The influence of metal impurities in the negative electrode slurry is less than that in the positive electrode slurry; among them, Cu and Zn have a significant effect on self-discharge; Fe and iron oxide did not show a significant effect.

3. Key control of metal impurities

(1) Establishing a test method for magnetic metal impurities

① Weigh the powder with an electronic scale and put it into a polytetrafluoroethylene ball mill jar

② Put the prepared magnet into the powder and add ultrapure water

③ Stir the ball mill at a speed of 200±5 rpm for 30±10 minutes

④ After stirring, remove the magnet (avoid direct contact with hands or other tools)

⑤ The positive electrode active material adsorbed on the surface of the magnet is washed with ultrapure water and then cleaned with ultrasound. 15±3 seconds.

⑥ Repeat steps ⑤ multiple times – Lithium iron phosphate: 20 times; other materials: 5-8 times.

⑦ Transfer the cleaned magnet to a 100ml beaker. (To prevent contamination by foreign matter)

⑧ In the beaker, pour 6ml of dilute aqua regia (hydrochloric acid:nitric acid = 3:1), then add ultrapure water to the same concentration as the magnet. Heat for approximately 20 minutes.

⑨ Transfer the heated solution to a 100ml volumetric flask, rinse at least 3 times, and transfer the rinsing solution to the volumetric flask as well. Finally, dilute to volume with ultrapure water.

⑩ Send the prepared solution to AAS for quantitative analysis of the contents of iron, chromium, copper, zinc, nickel, and cobalt (for lithium iron phosphate, also measure lithium).

Measure the magnetic metal impurity content of the raw materials:

Lithium iron phosphate:

Impurities include Fe, Cr, Ni, Al, P, etc., and the impurity metal should be stainless steel.

KS6:

The main magnetic metallic impurity is Al, with a small amount of Mg.

(2) Iron removal from raw materials with excessively high metal impurity content

(3) Improvement of self-discharge due to iron removal from raw materials

II. Process Dust and Metal Scrap

1. Potential sources of dust and metal scrap in the process

2. Measures to reduce and eliminate dust and metal scrap

3. Examples

Electrode sheet shedding was significantly reduced after using an automatic winding machine:

Electrode core short-circuit rate was significantly reduced after using an automatic winding machine:

Improvement of self-discharge due to automatic winding machine:

Non-metallization and 5S initiatives throughout the workshop and production line:

III. Battery Moisture

1. Mechanism of moisture's influence on self-discharge

As shown in the figure above, when H2O is present in the battery, it first reacts with LiPF6 to produce corrosive gases such as HF; simultaneously, it reacts with solvents to produce gases such as CO2, causing battery expansion; HF reacts with many substances in the battery, such as the main components of SEI, damaging the SEI film; generating CO2 and H2O; CO2 causes battery expansion, and the newly generated H2O participates in the reaction with LiPF6 and solvents, forming a vicious chain reaction! Consequences of SEI film damage:

1) Solvent enters the graphite layer and reacts with LixC6, causing irreversible capacity loss;

2) Repairing the damaged SEI consumes Li+ and solvent, further causing irreversible capacity loss.

2. Moisture Measurement

Improvement of solid moisture measurement methods:

The original methanol immersion method has poor repeatability and reproducibility; moreover, the testing cycle is long (immersion 24 hours), making it unsuitable for online control.

The Karl Fischer furnace + moisture analyzer improves accuracy and precision, passes MSA; the testing time is approximately 5 minutes, suitable for online monitoring.

3. Moisture Control

(1) Optimize the core baking process to improve water removal efficiency

(2) Develop a small-roll baking process to enhance water removal efficiency

(3) Construct an automated assembly line to reduce core water absorption

(4) Control water absorption during battery electrolyte injection

(5) Optimize the manufacturing process to reduce work-in-process inventory

IV. Improvement Effects

1. Voltage tends to stabilize

2. Self-discharge failure rate decreases

3. Self-discharge trend gradually stabilizes

4. Mean and median self-discharge values ​​decrease