Introduction: The consistency of self discharge is an important part of the influencing factors. After a period of storage, batteries with inconsistent self discharge will experience significant differences in SOC, which will greatly affect their capacity and safety. Studying it can help improve the overall level of our battery pack, achieve longer lifespan, and reduce product defect rates.


A battery with a certain amount of charge will lose some capacity after being stored for a period of time at a certain temperature, which is called self discharge. Simply put, self discharge refers to the loss of capacity of a battery when it is not in use, such as the negative electrode's charge returning to the positive electrode on its own or the battery's charge being lost through side reactions.

The importance of self discharge

At present, lithium batteries are becoming increasingly widely used in various digital devices such as laptops, digital cameras, and digital camcorders. In addition, they also have broad prospects in automobiles, mobile base stations, energy storage stations, and other fields. In this case, the use of batteries no longer appears separately like in mobile phones, but more in the form of series or parallel battery packs.

The capacity and lifespan of a battery pack are not only related to each individual battery, but also to the consistency between each battery. Poor consistency will greatly affect the performance of the battery pack.

The consistency of self discharge is an important part of the influencing factors. After a period of storage, batteries with inconsistent self discharge will experience significant differences in SOC, which will greatly affect their capacity and safety. Studying it can help improve the overall level of our battery pack, achieve longer lifespan, and reduce product defect rates.

Self discharge motor mechanism

锂钴石墨电池电极反应如下：

When the battery is open circuited, the above reaction does not occur, but the battery level will still decrease, mainly due to self discharge of the battery. The main reasons for self discharge are:

a. Internal electron leakage caused by local electron conduction or other internal short circuits in the electrolyte.

b. External electronic leakage caused by poor insulation of battery sealing rings or gaskets, or insufficient resistance between external lead shells (external conductors, humidity).

c. The reaction between electrodes/electrolytes, such as corrosion of the anode or reduction of the cathode due to electrolyte or impurities.

d. Localized decomposition of electrode active materials.

e. The electrode is passivated due to the decomposition products (insoluble substances and adsorbed gases).

f. Mechanical wear of electrodes or increased resistance between electrodes and current collectors.


The impact of self discharge

1. Self discharge leads to a decrease in storage capacity during the storage process

Several typical problems caused by excessive self discharge:

1. The car has been parked for too long and cannot be started;

2. The voltage and everything else were normal before the battery was stored, but low or even zero voltage was found during shipment;

3. In summer, when the car GPS is placed in the car, after a period of use, the battery level or usage time may feel significantly insufficient, and even accompanied by battery swelling.

2. The self discharge of metal impurities can cause blockage of the diaphragm aperture, and even puncture the diaphragm, resulting in local short circuits and endangering battery safety

3. Self discharge leads to an increase in SOC difference between batteries and a decrease in battery capacity

Due to inconsistent self discharge of batteries, there are differences in SOC among the batteries in the battery pack after storage, resulting in a decrease in battery performance. Customers often notice performance degradation after receiving a battery pack that has been stored for a period of time. When the SOC difference reaches around 20%, the capacity of the combined battery is only 60% to 70%.

4. Significant differences in SOC can easily lead to overcharging and overdischarging of the battery

1、 The distinction between chemical and physical self discharge

1. Comparison between High Temperature Self Discharge and Room Temperature Self Discharge

The relationship between physical micro short circuit and time is obvious, and long-term storage is more effective for selecting physical self discharge; Chemical self discharge is more significant at high temperatures, and high-temperature storage should be used for selection.

According to the storage method of 5D at high temperature and 14d at room temperature: if the battery self discharge is mainly physical self discharge, then the room temperature self discharge/high temperature self discharge ≈ 2.8; If the self discharge of the battery is mainly chemical self discharge, then the normal temperature self discharge/high temperature self discharge is less than 2.8.


2. Comparison of self discharge before and after cycling

Cycling can cause micro short circuits and melting inside the battery, thereby reducing physical self discharge. Therefore, if the battery's self discharge is mainly physical self discharge, the self discharge after cycling will be significantly reduced; If the self discharge of the battery is mainly chemical self discharge, there is no significant change in the self discharge after cycling.

3. Testing leakage current under liquid nitrogen

Using a high-voltage tester under liquid nitrogen to measure battery leakage current, if the following conditions occur, it indicates severe micro short circuit and large physical self discharge:

1) At a certain voltage, the leakage current is too high;

2) The ratio of leakage current to voltage varies greatly under different voltages.

4. Black spot analysis of diaphragm

By observing and measuring the number, morphology, size, and elemental composition of the black spots on the separator, the size of the physical self discharge of the battery and its possible reasons can be determined: 1) In general, the larger the physical self discharge, the more black spots there are, and the deeper the morphology (especially when it penetrates to the other side of the separator); 2) Determine the possible metal impurities in the battery based on the metal element composition of the black dots.

5. Comparison of self discharge of different SOC

The contribution of physical self discharge will vary under different SOC states. Through experimental verification, it is easier to distinguish batteries with physical self discharge anomalies at 100% SOC.

2、 Self discharge test

1. Self discharge detection method

1) Voltage drop method

Use the rate of voltage drop during storage to characterize the magnitude of self discharge. This method is simple to operate, but the disadvantage is that the voltage drop cannot intuitively reflect the loss of capacity. The voltage drop method is the simplest and most practical method currently widely used in production.

2) Capacity attenuation method

Expressed as the percentage decrease in capacity per unit time.

3) Self discharge current method Isd

Calculate the self discharge current Isd during battery storage based on the relationship between capacity loss and time.

4) Calculation method for Li+moles consumed by side reactions

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


2. Key points of self discharge measurement system

1) Select the appropriate SOC

DOCV/dT is affected by SOC, and the effect of temperature on OCV is significantly amplified at the platform, resulting in significant SOC prediction errors. Choose SOC testing self discharge that is relatively insensitive to temperature changes, such as FC1865: 25% SOC testing self discharge; LC1865:50% SOC self discharge measurement.

Due to differences in battery capacity, there are fluctuations in the actual SOC of the battery, with a tolerance of about 4%. Therefore, we will investigate the changes in the slope of the OCV curve within a tolerance range of 5%. The slopes at 53% and 99.9% SOC of LC1865 are very stable, with values of 3.8mV/% SOC and 10mV/% SOC, respectively. The slope is relatively stable at FC1865~25% SOC; Of course, a fully charged state is also a simple and practical self discharge measurement point.

2) Selection of starting time

Under FC1865 25% SOC (or other SOC values), the voltage changes every hour after charging. After 20 hours, the rate of voltage drop is basically the same, indicating that polarization has been basically restored. Therefore, 24 hours was selected as the starting time for the self discharge test.

After 14 hours at 50% SOC in LC1865, the voltage change rate fluctuates within a small range of 0.01mV/h, indicating that polarization has been basically restored. Therefore, 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 research interval, there is a significant linear relationship between self discharge and 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 following other relationships are also related.)

At room temperature, due to the decrease in chemical reaction rate, the anomalous points of physical self discharge are more pronounced. The storage of 14 bits can predict the results of 28 bits very well.

3. Improvement of self discharge measurement system

1) Measure voltage and temperature

The influence of measuring voltage and environmental temperature on self discharge: FC1865: For every 1 ℃ increase, the voltage drops by 0.05mV; LC1865: For every 1 ℃ increase, the voltage drops by 0.17mV.

2) Selection of voltmeter

In terms of selecting a voltmeter, due to the fact that self discharge studies changes at the 0.1mV level, traditional 4-bit and a half voltmeter (accurate to 1mV, resolution to 0.1mV) is no longer suitable. Therefore, a six and a half bit Agilent 34401A voltmeter (accurate to 0.1mV, resolution to 0.01mV or even higher) is chosen. In addition, the repeatability of the measuring instrument is also quite good.

4. Determination of self discharge standards

1) Theoretical calculation

2) 1mV differential simulation

Balance results were obtained by artificially adjusting the 10% SOC difference to simulate a 1mV (28 day 1mV, 14 day 0.5mV difference) self discharge difference after 3 years of use. The safety issue of overcharging did not occur in any of the three battery groups, but the voltage difference during discharge was already very large (1200mV). The self discharging large battery was overdischarged to 2.5V, resulting in a 10% loss of PACK capacity.


Influencing factors and control points of self-propelled movies


1、 Raw material metal impurities

1. The influence mechanism of metal impurities

In the battery, metal impurities undergo chemical and electrochemical corrosion reactions and dissolve into the electrolyte: M → Mn++ne -; Afterwards, Mn+migrates to the negative electrode and undergoes metal deposition: Mn++ne - → M; As time goes by, metal dendrites continue to grow and eventually penetrate the membrane, causing micro short circuits between the positive and negative electrodes, continuously consuming electricity and leading to a decrease in voltage.

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


2. The degree of influence of different types of metal shavings

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

Qualitatively rank the degree of impact: Cu>Zn>Fe>Fe2O3

Note: In principle, any metal impurities (such as FeS, FeP2O7, etc. not listed above) will have a significant impact on self discharge, and the degree of impact is generally the strongest for metallic elements.


The black spots on the separator of metal chip batteries have a deep morphology (penetrating to the other side) and a large quantity:

The metal element composition of the diaphragm black spot matches the type of metal added, indicating that the metal elements on the diaphragm black spot do indeed come from metal impurities:

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

The influence of metal impurities in the negative electrode slurry is not as significant as that in the positive electrode slurry; Among them, Cu and Zn have a significant impact on self discharge; Fe、 No significant effect was observed on iron oxide.

3. Key control of metal impurities

(1) Establish a testing method for magnetic metal impurities

① After weighing the powder with an electronic scale, put it into a PTFE ball mill jar

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

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

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

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

⑥ The technique of item ⑤ is repeated multiple times - lithium iron phosphate: 20 times; Other materials: 5-8 times

⑦ Transfer the cleaned magnet to a 100ml beaker. (Prevent the entry of foreign objects)

⑧ In a beaker, pour 6ml of diluted aqua regia (hydrochloric acid: nitric acid=3:1), and then add ultrapure water with a magnetic immersion degree. Then heat it up for about 20 minutes

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

⑩ The prepared solution is sent to AAS for quantitative analysis of the content of iron, chromium, copper, zinc, nickel, and cobalt (with lithium iron phosphate and an additional lithium element measured).

Measure the magnetic metal impurity content of raw materials:

Lithium iron phosphate:

The impurity components include Fe, Cr, Ni, Al, P, etc. The impurity metal should be stainless steel.

KS6：

The main component of magnetic metal impurities is Al, with a small amount of Mg.


(2) Removing iron from raw materials with high levels of metal impurities

(3) Improvement of self discharge by removing iron from raw materials

2、 Process dust and metal shavings

1. Potential sources of dust and metal shavings in the manufacturing process

2. Take measures to reduce and eliminate dust and metal shavings

3. Example

After using the automatic winding machine, there was a significant improvement in the material dropping of the polarizer:

After using the automatic winding machine, the short-circuit rate of the core is significantly reduced:

Improvement of self discharge by automatic winding machine:

Non metalization and 5S action of the entire workshop and production line:

3、 Battery moisture

1. The influence mechanism of moisture on self discharge

As shown in the above figure, when H2O is present in the battery, it first reacts with LiPF6 to produce corrosive gases such as HF; Simultaneously reacting with solvents to produce gases such as CO2, causing battery expansion; HF will react with numerous substances in the battery, such as the main component of SEI, and damage the SEI film; Generate CO2 and H2O, etc; CO2 causes battery expansion, and the regenerated H2O participates in reactions such as LiPF6 and solvents; Forming a malignant chain reaction!

The consequences of SEI film damage:

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

2) The repair of damaged SEI requires the consumption of Li+and solvents, further causing irreversible capacity loss.


2. Moisture measurement


Improvement of Solid Moisture Measurement Method:

The repeatability and reproducibility of the original methanol soaking measurement method are poor; And the testing cycle is long (soaking for 24 hours), which cannot be used for online control.

Switching to a Karl Fischer heating furnace and moisture analyzer has improved accuracy and precision, and MSA has passed; The testing time is about 5 minutes, suitable for online monitoring.


3. Moisture control

(1) Optimize the baking process of the core and improve the water removal effect

(2) Develop small roll baking process to improve water removal efficiency

(3) Build an automatic assembly line to reduce the absorption of water by the core

(4) Control water absorption during battery injection process

(5) Optimize the production process and reduce the backlog of work in progress

4、 Improvement effect

1. Voltage tends to stabilize

2. Decrease in self discharge failure rate

3. The self discharge trend gradually stabilizes

4. The mean and median of self discharge decrease