Archives July 2021

What is the capacity degradation of lithium-ion power battery

What is the capacity degradation of lithium-ion power battery

Causes of capacity degradation of lithium-ion power batteries

The internal factors leading to the failure of lithium-ion power batteries mainly include the performance degradation of the positive/negative electrode materials and the aging of the electrolyte decomposition membrane. The external factors include the battery temperature, the intensity of charge and discharge current, and the depth of discharge. Lithium-ion dynamics. When the battery fails, if it can reach the design life specified by the manufacturer, it is called normal failure, otherwise it is called premature failure. The main reasons leading to the premature failure of the hammer ion power battery are excessive use (excessive strength, excessive depth, overload, etc.), external short circuit, internal damage, etc. Excessive use aggravates the irreversible side reactions inside the battery, accelerates the attenuation of battery life, and may even cause fire, explosion, etc.

The ideal working state of the lithium ion power battery is that only Li+ intercalation and deintercalation between the positive and negative electrodes occur, and there are no other side reactions to consume Li+. In the actual use process, as the charge and discharge progress, the lithium-ion power battery will have metal lithium deposition, active material dissolution, electrolyte decomposition and other phenomena, resulting in irreversible loss of the capacity of the lithium-ion power battery. The main mechanisms that cause the capacity degradation of lithium-ion power batteries are:

(1) Cathode material dissolution. The cathode material will dissolve during the use of lithium-ion power batteries. This is mainly caused by factors such as structural defects of the positive electrode material and overcharging during use. With the increase in the number of battery charging and discharging, the dissolution rate of the cathode material is also increasing. The dissolution of the positive electrode material causes the formation of simple metal near the negative electrode, which increases the battery impedance and causes the capacity of the lithium-ion battery to decline.

(2) Phase change of positive/negative electrode materials. There are two types of phase changes in lithium ion battery electrode materials:

①The phase change caused by the deintercalation and intercalation of Li+ during the normal operation of the lithium-ion power battery. This phase change causes physical damage to the positive and negative materials and reduces the electrical contact between the internal materials of the battery.

②The over-use of overcharge and over-discharge during use will cause the phase change of the positive electrode material. This phase change changes the volume structure of the cathode material.

Both of these two phase transitions affect the propagation process of Li+ in the battery, which leads to the degradation of battery capacity.

(3) The electrolyte causes capacity attenuation. The decomposition of the electrolyte causes a series of irreversible reactions in the battery, which produces lithium oxides and LiOH and other deposits, which consumes the electrolyte, which leads to an increase in battery polarization, a decrease in Li+ concentration, and an increase in resistance to expansion.

(4) Overcharge causes capacity loss. When overcharged, Li+ is reduced and deposited on the negative electrode, which thickens the negative SEI film. Inert materials and oxygen are also formed near the positive electrode, which hinders the deintercalation and insertion of lithium ions and causes irreversible loss of battery capacity.

(5) Self-discharge. The self-discharge phenomenon of lithium-ion power batteries is inevitable. Only a small part of the battery capacity loss caused by self-discharge is irreversible loss, and most can be recovered by recharging. The irreversible loss caused by self-discharge is caused by the loss of Li+ and the blockage of the electrode pores by the oxide of the electrolyte.

(6) Formation of SEI interface film. At the beginning of the charge-discharge cycle, an irreversible reaction occurs between the negative electrode material of the lithium-ion power battery and the electrolyte, forming a solid electrolyte membrane (SEI film) on the surface of the negative electrode. Its formation and growth will consume the Li+ and electrolyte inside the battery, leading to a decline in the capacity of the lithium-ion power battery. The growth rate of the SEI film is closely related to the battery life, working temperature, and the specific area of ​​the negative electrode material.

(7) Current collector corrosion. In the process of charging and discharging lithium-ion power batteries, the current collector will corrode and produce a corrosive film. In the case of deep discharge, copper ions will form elemental copper deposits on the surface of the negative electrode during the charging process.These films and deposits hinder the intercalation and deintercalation of lithium ions, resulting in a decline in battery capacity.

What is the cycle life of lithium ion power battery

What is the cycle life of lithium-ion power battery

Cycle life of lithium-ion power battery

During the use of electric vehicles and other new energy vehicles, the lithium-ion power battery as the power source is in different charging and discharging states, and at the same time is affected by external conditions such as vibration and temperature. As the number of charging and discharging increases, the capacity of lithium-ion power batteries will inevitably have varying degrees of attenuation. The capacity loss of lithium-ion batteries is divided into reversible loss and irreversible loss: reversible loss can be restored by recharging, generally refers to self-discharge; irreversible loss can not be restored by recharging. The cycle life of lithium-ion power batteries is an important parameter for battery production and use, and the attenuation of battery capacity is a long-term and complicated process of change. Accurately detecting or predicting the life state of power batteries is a common concern for automobile manufacturers and users. This section mainly introduces the relevant regulations of lithium-ion power battery life.

The life of a lithium-ion power battery usually includes storage life, service life and cycle life. The storage life refers to the time that the battery has been stored in a static state until it expires. Service life refers to the total discharge time accumulated before the battery fails. Cycle life refers to the total number of charging and discharging of the battery before failure. The cycle life of lithium-ion power batteries is directly related to the cost-effectiveness of the vehicle and the performance of the vehicle. Quick and accurate evaluation of the cycle life of the battery is also one of the key issues that power batteries must solve urgently. At present, most studies often use cycle life to study the life state of lithium-ion power batteries. This article focuses on the cycle life of lithium-ion power batteries.

The cycle life defined in the “USAB Battery Test Manual” is the number of cycles that the battery can perform before it reaches the end-of-life conditions when the battery is subjected to a cyclic charge and discharge test according to the standard charge and discharge system. “GB/Z18333.1–2001 Lithium-ion Battery for Electric Road Vehicles” stipulates that lithium-ion power batteries follow the prescribed charging and discharging system (charged until the voltage reaches 4.2V, and then discharged with 1I3 current to 80% of the rated capacity) Carry out the charge and discharge cycle until the battery capacity decays to 80% of the rated capacity, then the total number of charge and discharge cycles in the whole process is its cycle life. According to IEE1188-1996, when the remaining capacity of the battery is less than 80% of the rated capacity of the new battery, the performance of the battery cannot meet the needs of the vehicle and should be replaced. Therefore, the decay of the battery capacity to 80% of the rated capacity is often used as the end of battery life. SOH is generally used to represent the health of the battery. It is defined as the percentage of the remaining dischargeable capacity of the battery to the rated capacity under standard operating conditions, as shown in the formula.

SOH=Qres/Qrat×100%

In the formula, Qres represents the remaining dischargeable capacity of the battery, and Qrat represents the rated capacity of the battery.

The life of lithium-ion power battery is affected by the design, production, use conditions and other aspects. For lithium-ion power battery packs for vehicles, its life is also affected by the design and consistency of the battery pack. From the perspective of use, vehicle manufacturers and users are most concerned about the life status of the power battery system, not just the life of the battery cells or individual modules. The main factors affecting the life of lithium-ion power batteries include: battery design plan, production level, battery pack structure design, group structure, monomer consistency, vehicle usage conditions, charge and discharge current intensity, depth of discharge, SOC working window, and charging system Wait.

What are the characteristics of lithium ion power battery

What are the characteristics of lithium ion power battery

The main performance of lithium-ion power battery

1.1 Charging and discharging characteristics of lithium-ion power battery

The charging of lithium-ion power batteries needs to take into account safety, reliability and charging efficiency. Usually, the charging method of constant current charging first, and then converted to constant voltage charging to a certain small current is adopted. The main differences between different types of lithium-ion power battery charging methods are:

(1) Different types of lithium-ion power batteries have different currents in the constant current charging stage. According to the different cathode materials and manufacturing processes used in lithium-ion power batteries, there are some differences in the optimal charging current, and the general charging current is between 0.2C and 0.3C.

(The proportion of the two-stage charging capacity to the total capacity is different. The extension of the constant current charging time helps to shorten the total charging time and facilitate the practical application of electric vehicles.

The voltage of lithium-ion power battery is stable in the middle of the discharge, and the voltage drops rapidly in the latter part of the discharge. Effective control should be carried out at the later stage of discharge to prevent over-discharge. If overdischarge occurs, not only the lattice structure of the positive electrode material will change, but the negative copper current collector will also be oxidized, which will cause irreversible damage to the battery. Related standards stipulate that when a series of power battery packs are discharged, in order to prevent overdischarge of a certain cell, when the voltage of a certain cell reaches the discharge cut-off voltage, a discharge protection circuit should be used to terminate the discharge of the battery pack.

1.2 Discharge capacity of lithium-ion power battery

“GB/Z183331-2001 Lithium-ion battery for electric road vehicles” stipulates that after the lithium-ion battery is fully charged according to the charging method given by the manufacturer, it is allowed to stand for 1~5h at a temperature of (20±5)℃, and discharge at a current of 113A. The capacity is the rated capacity of the battery. The electrode material, charging voltage, and working temperature of lithium-ion power batteries have important effects on the discharge capacity.

1.2.1 The influence of electrode material on discharge capacity

It can be seen from Figure 1-1 that at different discharge temperatures, the discharge capacity of C/LiCoO2 batteries is greater than that of C/LiMn2O4 batteries. Under the condition of 21℃, the discharge capacity of C/LiCoO2 battery is 18.6% higher than that of C/LiMn2O4 battery, and the average discharge voltage of the latter reaches 3.9V. Therefore, when discussing the energy density of lithium-ion power batteries, batteries with different electrode materials should be treated differently.

Discharge capacity of lithium-ion power battery under different conditions

Figure 1-1 Discharge capacity of lithium-ion power battery under different conditions

1.2.2 The influence of operating temperature on discharge capacity

The 18650-type C/LiMn2O4 battery and C/LiCoO2 battery were subjected to the discharge test under the condition of -20℃~60℃, and the results obtained are shown in Figure 1-1. It can be seen from Figure 1-1 that the operation of lithium-ion batteries under low temperature conditions is significantly affected, and the average discharge voltage and discharge capacity are significantly smaller; when the operating temperature is higher than 20°C, as the temperature increases, the two types of lithium-ion batteries The average working voltage and discharge capacity of the battery no longer change significantly.

1.2.3 The influence of charging voltage on discharge capacity

By increasing the end-of-charge voltage, the discharge capacity and specific energy of the lithium-ion power battery can be increased. Take a certain type of 7Ah square lithium-ion battery as an example. Charge the battery to 4.1V and 4.2V with 1A current at a temperature of 25°C, and then use different powers for constant power discharge. The discharge capacity is shown in Figure 1-2 and the figure. Shown in Figure 1-3. You can see from it

The relationship between the end-of-discharge voltage of a certain lithium-ion power battery and the energy released

Figure 1-2 The relationship between the end-of-discharge voltage of a certain lithium-ion power battery and the energy released

After increasing the end-of-charge voltage, the battery discharge energy increases, and the battery discharge energy increases by 25% when the charging voltage is 4.2V at 45W discharge power. At the same time, it can be seen that as the discharge power increases, the energy released by the battery decreases.

When the charge termination voltage of lithium-ion power battery is high, it will cause partial decomposition of the positive electrode material, deterioration of electrolyte performance, and oxidation of the diaphragm, which accelerates the aging process of the battery and shortens the service life of the battery. Therefore, the charge termination voltage must be strictly controlled.

The relationship between the end-of-charge voltage of lithium-ion power batteries and the energy released

Figure 1-3 The relationship between the end-of-charge voltage of lithium-ion power batteries and the energy released

1.2.4 The influence of discharge current on discharge capacity

Take a 35Ah square lithium-ion power battery as a sample, and use 5 different discharge currents to conduct a discharge test at an ambient temperature of 25°C. The results are shown in Figure 1-4. It can be seen from the curve in Figure 2-5 that as the discharge current increases, the discharge capacity of the square lithium-ion power battery decreases. This is because lithium-ion power batteries use organic electrolytes, and the internal resistance of the battery is greater than that of other types of batteries, so the discharge performance of lithium-ion power batteries under high current conditions is poor.

The relationship between discharge current and discharge capacity of a certain type of lithium-ion power battery

  Figure 1-4 The relationship between discharge current and discharge capacity of a certain type of lithium-ion power battery

1.3 Internal resistance of lithium-ion power battery

Internal resistance is an important parameter of lithium-ion power batteries, an important indicator of battery health, and one of the key data in the research of lithium-ion power batteries. Its value has an important influence on the charging and discharging efficiency of the lithium-ion power battery and the thermal characteristics of the battery.

The internal resistance of lithium-ion power batteries is greatly affected by factors such as state of charge (SOC) and temperature. Under the same conditions, the internal resistance of lithium-ion power batteries is larger than that of power batteries with other structures. For example, the single cell internal resistance of a 10Ah valve-regulated lead-acid battery is 2~3mΩ, while the internal resistance of a lithium-ion power battery cell of 8~10Ah is 10mΩ. The large internal resistance causes the specific energy of the battery to drop rapidly at high power output. There are two main reasons for this:

(1) The cathode materials of lithium-ion power batteries mostly use oxides or salts, and their electronic conductivity is worse than that of metals.

(2) Lithium-ion power batteries use organic materials as electrolyte solvents, and the diffusion rate of lithium ions is also affected by the material lattice.

1.3.1 The influence of temperature on internal resistance

Studies have shown that the internal resistance of lithium-ion power batteries remains basically unchanged in the temperature range of 20°C to 50°C, but the internal resistance increases rapidly in a low temperature environment. At 0°C, the internal resistance at room temperature doubles to -10°C. When the internal resistance increases by more than 2 times. Therefore, the heating and heat preservation of the lithium-ion power battery pack should be strengthened when used in a low-temperature environment.

1.3.2 The influence of SOC on internal resistance

Figure 1-5 and Figure 1-6 show the charge and discharge internal resistance of a certain type of lithium manganese battery and lithium iron phosphate battery in different SOC states measured at a room temperature of 25°C. Through analysis, it can be seen that the internal resistance of the battery increases significantly when the SOC is low, and the internal resistance increases rapidly as the SOC decreases when the SOC is less than 40%. The internal resistance of the battery is the smallest and relatively stable when the SOC is less than 40%, and it has a certain degree of stability. The characteristics of the platform are conducive to working as a vehicle power battery. At the same time, it can be seen that the internal resistance of lithium-ion power battery charging and discharging is not much different. The maximum difference between charge and discharge internal resistance of lithium manganate battery is 4.5%, and the maximum difference between charge and discharge internal resistance of lithium iron phosphate battery is about 5%.

 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium manganate battery

Figure 1-5 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium manganate battery

 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium iron phosphate battery

Figure 1-6 The relationship between charge and discharge internal resistance and SOC of a certain type of lithium iron phosphate battery

What is a lithium ion power battery pack

What is a lithium ion power battery pack

1.1 Lithium-ion power battery pack

1.1.1 Lithium-ion power battery assembly Lithium-ion power battery assembly (PACK) is to connect single cells in series and parallel, and assemble them with the protection control board, charge and discharge ports, and housing. Some products also assemble the charger together. When designing the battery pack structure, various factors must be considered comprehensively, while taking into account the main performance. The following aspects should be considered in battery assembly:

(1) Security. Flame-retardant materials must be used when assembling.

(2) Tightness. The battery pack may cause malfunctions in humid or rainy environments, and even cause safety hazards

(3) Heat dissipation. The battery pack will generate heat during the charging and discharging process, the electronic components in the protection circuit will also generate heat, and the battery pack will also generate heat due to factors such as ambient temperature. The main measures to improve the heat dissipation of the battery pack include: minimizing the heat generation of the heat source and optimizing the structure of the heat dissipation system.

(4) Shock absorption. CR foam can be used, and flame-retardant materials with good mechanical strength can be used, so that safety and shock absorption can be taken into account.

(5) The layout of high current lines should be as short as possible.

1.2 Parameters and characteristics of lithium-ion power battery packs

The lithium-ion power battery pack is composed of multiple lithium-ion battery cells combined in series and in parallel. The group voltage, group power and group watt-hour capacity are the fundamental reasons for the grouping of batteries. Due to the impact of group use, battery packs have additional characteristics compared to single cells. The relationship between the parameters of the lithium-ion power battery pack and the cell parameters includes: the group voltage is equal to the sum of all cell voltages plus the voltage drop generated on the connecting conductor, the latter is often ignored in the analysis; the group internal resistance is equal to all the cells in series The sum of the internal resistance plus the resistance of the connecting conductor; the group watt hour capacity is approximately equal to the product of the minimum unit capacity and the total number of units; the group ampere-hour capacity and group life are the same as the minimum unit; the group power is equal to The sum of the power of all the monomers; other performance indicators of the battery pack also often depend on the poorer relevant indicators in the monomer.

As an energy supply device for new energy vehicles such as pure electric vehicles, hybrid electric vehicles, and extended-range electric vehicles, the most important characteristics of lithium-ion power batteries are high power and high energy. These two performance indicators are actually contradictory. In order to increase the power, it is necessary to increase the charging and discharging current. When designing the battery structure, it is necessary to consider increasing the equivalent reaction area and reducing the contact impedance, which requires an increase in the volume and weight of the battery. Lithium-ion power battery needs to be designed according to the optimization index to make the two main indexes meet the actual demand. From the point of view of use, lithium-ion power battery applications have the following characteristics:

(1) Tandem group application. The voltage of lithium-ion power battery cells is limited. When applied to new energy vehicles, several battery cells need to be connected in series to meet the requirements of load voltage supply, so the external characteristics of the battery pack are also restricted by the characteristics of the battery cells. .

(2) Large power capacity. The working current of the power battery often reaches tens to hundreds of amperes, and special current detection and protection devices should be designed for it.

(3) High charging and discharging intensity. The current in the charging process is controlled by the charging strategy. The state of the power battery is relatively stable and the current is determined by the characteristics of the load when it is used. The more violent the electrochemical power behavior of the battery fluctuates, the more difficult it is to effectively implement the hardware protection method based on voltage changes. When the lithium-ion power battery is applied to the vehicle, the discharge current of the power battery during most of the driving process is 0.3C~1.5C, and occasionally there is inverter charging during deceleration such as braking. The voltage fluctuation range and frequency of the battery pack are very disordered, and the hardware Management protection is easy to be disturbed and make wrong judgments.

(4) Large energy capacity. The capacity of traditional low-power batteries is often limited to less than a few ampere hours, the discharge process is stable, the working time is long, and the capacity (unit: Ah) is relatively easy to measure. Lithium-ion power battery has a large energy capacity, and the calculation of power is more complicated. When the vehicle is fully loaded, the power battery discharges quickly and the current changes drastically. The capacity measurement needs to be more accurate and timely to calculate the remaining power of the battery pack.

(5)The gradient voltage is high. Lithium-ion power battery packs are composed of multiple single cells connected in series to achieve a higher stack voltage, and the voltage value of each connection terminal increases step by step like a staircase, which is called a gradient voltage. Therefore, it is necessary to design an appropriate solution to integrate a single protection circuit.

How does the lithium ion power battery work

How does the lithium ion power battery work

  1. The working principle of lithium ion power battery

At present, lithium-ion power batteries have been widely used in electric vehicles, and the research on their performance has become a hot spot in the industry.

Lithium-ion power battery is a high-performance secondary battery composed of four parts: positive electrode, negative electrode, separator and electrolyte: the positive electrode undergoes a reduction reaction when the battery is discharged, and most transition metal oxides such as LCoO2、Lix2NiO2、LixMn2O4 are used; there are many negative electrodes. Using carbon materials, oxidation reaction occurs during discharge; the diaphragm provides electronic isolation for the positive and negative electrodes; the electrolyte is generally an organic solution such as LiAsF6, which is a transport medium for ion movement. When charging, Li+ is deintercalated from the positive electrode through the electrolyte and inserted into the negative electrode. At the same time, the electronic compensation charge is supplied from the external circuit to the carbon negative electrode to maintain the electric balance of the negative electrode. On the contrary, Li+ is deintercalated from the negative electrode and inserted into the positive electrode through the electrolyte. It can be seen that the lithium-ion power battery uses Li+ to reciprocally intercalate and de-intercalate between the positive and negative electrodes for charging and discharging. It is a lithium ion concentration difference battery. Its general working principle is shown in Figure 2-1. The electrode reaction expression is shown in the formula diagram (1-1), formula (1-2), and formula (1-3).

The working principle of lithium-ion power battery

Figure 2-1 The working principle of lithium-ion power battery

Electrode reaction expression

Formula diagram

According to the different cathode materials, lithium-ion power batteries mainly include lithium manganese oxide batteries, lithium cobalt oxide batteries, lithium iron phosphate batteries, and ternary material lithium-ion batteries; according to different electrolyte materials, lithium-ion power batteries are divided into liquid lithium-ion There are two major categories of batteries and polymer lithium-ion batteries. Compared with power batteries of other structures, the main advantages of lithium-ion power batteries are:

(1) The working voltage is high. According to the different cathode materials, the operating voltage range of lithium-ion power batteries is 3.2~3.7V, which is about three times the operating voltage of other types of batteries such as nickel-cadmium batteries.

(2) Higher than energy. The theoretical specific energy of lithium-ion power batteries is as high as 200Wh/kg, and the actual specific energy is higher than 140Wh/kg, which is about twice that of nickel-hydrogen batteries.

(3) High capacity and energy conversion efficiency

(4) Long storage and cycle life. In a suitable environment, the lithium-ion power battery can be stored for more than 5 years, the number of deep-cycle charge and discharge can reach more than 1,000 times, and the cycle life is up to 10,000 times at low depth of discharge. The life characteristics are much better than other types of batteries.

(5) The self-discharge is small. When the ambient temperature is (20±5)℃, the monthly self-discharge rate of lithium-ion batteries is only 5%-9%, which greatly alleviates the problem of power loss caused by self-discharge when traditional secondary batteries are placed.

(6) No memory effect

(7) Wide operating temperature range. Lithium-ion power battery can work in the temperature range of -20~60℃. However, we should try our best to provide a suitable working temperature for the lithium-ion power battery, because the working environment of high temperature (≥40℃) and low temperature (≤0℃) will damage the electrical performance of the lithium-ion power battery and accelerate the lithium-ion power battery. The life of the decay.

(8) High environmental protection, lithium ion power battery does not contain cadmium, lead, mercury and other harmful substances, has low environmental pollution, and is a true green battery

The excellent electrical performance of lithium-ion power batteries lays the foundation for their application in electric vehicles and accelerates the research and development of new energy vehicles. However, it is not a perfect car power battery. Its main disadvantages are: large internal resistance, and large internal resistance causes the lithium-ion power battery to rapidly decrease its energy at high power output; due to the charging and discharging of the lithium-ion power battery It has a wide range and requires special protection circuits to prevent overcharge and overdischarge of the battery; poor compatibility with ordinary batteries, which is mainly due to the large difference in voltage between batteries; poor overcharge and overdischarge resistance.