What are the requirements for high-quality lithium-ion batteries? Generally speaking, long life, high energy density, and reliable safety performance are the prerequisites for measuring a high-quality lithium-ion battery. Lithium-ion batteries are currently used in all aspects of daily life, but the manufacturer or brand is different. There are some differences in the service life and safety performance of lithium-ion batteries, which are closely related to the production process standards and production materials; the following conditions must be the conditions for high-quality lithium-ion;

1. Long service life

The life of the secondary battery includes two indicators: cycle life and calendar life. Cycle life means that after the battery has experienced the number of cycles promised by the manufacturer, the remaining capacity is still greater than or equal to 80%. The calendar life means that the remaining capacity shall not be less than 80% within the time period promised by the manufacturer, no matter whether it is used or not.

Life is one of the key indicators of power lithium batteries. On the one hand, the big action of replacing the battery is really troublesome and the user experience is not good; on the other hand, fundamentally, life is a cost issue.

The life of a lithium-ion battery means that the capacity of the battery decays to the nominal capacity (at room temperature of 25°C, standard atmospheric pressure, and 70% of the battery capacity discharged at 0.2C) after a period of use, and the life can be considered as the end of life. In the industry, the cycle life is generally calculated by the number of cycles of fully charged and discharged lithium-ion batteries. In the process of use, an irreversible electrochemical reaction occurs inside the lithium-ion battery, which leads to a decrease in capacity, such as the decomposition of the electrolyte, the deactivation of active materials, and the collapse of the positive and negative electrode structures lead to a decrease in the number of lithium ions intercalation and deintercalation. Wait. Experiments show that a higher rate of discharge will lead to a faster attenuation of capacity. If the discharge current is low, the battery voltage will be close to the equilibrium voltage, which can release more energy.

The theoretical life of a ternary lithium-ion battery is about 800 cycles, which is medium among commercial rechargeable lithium-ion batteries. Lithium iron phosphate is about 2,000 cycles, while lithium titanate is said to be able to reach 10,000 cycles. At present, mainstream battery manufacturers promise more than 500 times (charge and discharge under standard conditions) in the specifications of their ternary battery cells. However, after the batteries are assembled into a battery pack, due to consistency issues, the most important factors are voltage and internal The resistance can not be exactly the same, and its cycle life is about 400 times. The recommended SOC usage window is 10%~90%. Deep charging and discharging is not recommended, otherwise it will cause irreversible damage to the positive and negative structure of the battery. If it is calculated by shallow charge and shallow discharge, the cycle life will be at least 1000 times. In addition, if lithium-ion batteries are frequently discharged in high-rate and high-temperature environments, the battery life will be drastically reduced to less than 200 times.

2. Less maintenance, lower use cost

The battery itself has a low price per kilowatt-hour, which is the most intuitive cost. In addition to the aforementioned, for users, whether the cost is really low depends on the “full life cycle cost of electricity.”

“Full life cycle cost of electricity”, the total power of the power lithium battery is multiplied by the number of cycles to get the total amount of power that can be used in the full life cycle of the battery, and the total price of the battery pack is divided by this sum to get the price per kilowatt of electricity in the full life cycle.

The battery price we usually talk about, such as 1,500 yuan/kWh, is only based on the total energy of the new battery cell. In fact, the cost of electricity per unit of life is the direct benefit of the end customer. The most intuitive result is that if you buy two battery packs with the same power at the same price, one will reach the end of life after 50 times of charging and discharging, and the other can be reused after 100 times of charging and discharging. These two battery packs can be seen at a glance which is cheaper.

To put it bluntly, it is long life, durable and reduces costs.

In addition to the above two costs, the maintenance cost of the battery should also be considered. Simply consider the initial cost, select the problem cell, the later maintenance cost and labor cost are too high. Regarding the maintenance of the battery cell itself, it is important to refer to manual balancing. The BMS’s built-in equalization function is limited by the size of its own design equalization current, and may not be able to achieve the ideal balance between the cells. As time accumulates, the problem of excessive pressure difference in the battery pack will occur. In such situations, manual equalization has to be carried out, and the battery cells with too low voltage are charged separately. The lower the frequency of this situation, the lower the maintenance cost.

3. High energy density/high power density

Energy density refers to the energy contained in a unit weight or unit volume; the electric energy released by the average unit volume or mass of a battery. Generally, in the same volume, the energy density of lithium-ion batteries is 2.5 times that of nickel-cadmium batteries and 1.8 times that of nickel-hydrogen batteries. Therefore, when the battery capacity is equal, lithium-ion batteries will be better than nickel-cadmium and nickel-hydrogen batteries. Smaller size and lighter weight.

Battery energy density=battery capacity× discharge platform/battery thickness/battery width/battery length.

Power density refers to the value of the maximum discharge power per unit weight or volume. In the limited space of road vehicles, only by increasing the density can the overall energy and overall power be effectively improved. In addition, the current state subsidies use energy density and power density as the threshold to measure the level of subsidies, which further strengthens the importance of density.

However, there is a certain contradiction between energy density and safety. As energy density increases, safety will always face newer and more difficult challenges.

4. High voltage

Since graphite electrodes are basically used as anode materials, the voltage of lithium-ion batteries is mainly determined by the material characteristics of the cathode materials. The upper limit of the voltage of lithium iron phosphate is 3.6V, and the maximum voltage of ternary lithium and lithium manganate batteries is about 4.2V (the next part will explain Why can’t the maximum voltage of Li-ion battery exceed 4.2V). The development of high-voltage batteries is a technical route for lithium-ion batteries to increase the energy density. To increase the output voltage of the cell, a positive electrode material with a high potential, a negative electrode material with a low potential and an electrolyte with a stable high voltage are required.

5. High energy efficiency

Coulomb efficiency, also called charging efficiency, refers to the ratio of battery discharge capacity to charging capacity during the same cycle. That is, the percentage of discharge specific capacity to charge specific capacity.

For the positive electrode material, it is the lithium insertion capacity/delithium capacity, that is, the discharge capacity/charge capacity; for the negative electrode material, it is the lithium removal capacity/lithium insertion capacity, that is, the discharge capacity/charge capacity.

During the charging process, electrical energy is converted into chemical energy, and during the discharging process, chemical energy is converted into electrical energy. There is a certain efficiency in the input and output of electrical energy during the two conversion processes, and this efficiency directly reflects the performance of the battery.

From the perspective of professional physics, Coulomb efficiency and energy efficiency are different. One is the ratio of electricity and the other is the ratio of work.

The energy efficiency of the storage battery and the Coulomb efficiency, but from the mathematical expression, there is a voltage relationship between the two. The average voltage of charge and discharge is not equal, the average voltage of discharge is generally less than the average voltage of charge

The performance of the battery can be judged by the energy efficiency of the battery. From the conservation of energy, the lost electrical energy is mainly converted into heat energy. Therefore, the energy efficiency can analyze the heat generated by the battery during the working process, and then the relationship between internal resistance and heat can be analyzed. And it is known that energy efficiency can predict the remaining energy of the battery and manage the rational use of the battery.

Because the input power is often not used to convert the active material into a charged state, but part of it is consumed (for example, irreversible side reactions occur), so the Coulomb efficiency is often less than 100%. But as far as current lithium-ion batteries are concerned, the Coulomb efficiency can basically reach 99.9% and above.

Influencing factors: electrolyte decomposition, interface passivation, changes in the structure, morphology, and conductivity of electrode active materials will reduce the Coulomb efficiency.

In addition, it is worth mentioning that battery decay has little effect on Coulomb efficiency and has little to do with temperature.

The current density reflects the size of the current passing per unit area. As the current density increases, the current passed by the stack increases, the voltage efficiency decreases due to internal resistance, and the Coulomb efficiency decreases due to concentration polarization and other reasons. Eventually lead to a reduction in energy efficiency.

6. Good high temperature performance

Lithium-ion batteries have good high-temperature performance, which means that the battery core is in a higher temperature environment, and the battery’s positive and negative materials, separators and electrolyte can also maintain good stability, can work normally at high temperatures, and the life will not be accelerated. High temperature is not easy to cause thermal runaway accidents.

The temperature of the lithium-ion battery shows the thermal state of the battery, and the essence of it is the result of the heat generation and heat transfer of the lithium-ion battery. Studying the thermal characteristics of lithium-ion batteries, and their heat generation and heat transfer characteristics under different conditions, can make us realize the important way of exothermic chemical reactions inside lithium-ion batteries.

Unsafe behaviors of lithium-ion batteries, including battery overcharge and overdischarge, rapid charge and discharge, short circuit, mechanical abuse conditions, and high temperature thermal shock, can easily trigger dangerous side reactions inside the battery and generate heat, directly destroying the negative and positive electrodes Passivation film on the surface.

When the cell temperature rises to 130°C, the SEI film on the surface of the negative electrode decomposes, causing the high-activity lithium carbon negative electrode to be exposed to the electrolyte to undergo a violent oxidation-reduction reaction, and the heat that occurs makes the battery enter a high-risk state.

When the internal temperature of the battery rises above 200°C, the passivation film on the positive electrode surface decomposes the positive electrode to generate oxygen, and continues to react violently with the electrolyte to generate a large amount of heat and form a high internal pressure. When the battery temperature reaches above 240°C, it is accompanied by a violent exothermic reaction between the lithium carbon negative electrode and the binder.

The temperature problem of lithium-ion batteries has a great impact on the safety of lithium-ion batteries. The environment of use itself has a certain temperature, and the temperature of the lithium ion battery will also appear when it is used. The important thing is that temperature will have a greater impact on the chemical reaction inside the lithium-ion battery. Too high temperature can even damage the service life of the lithium-ion battery, and in severe cases, it will cause safety problems for the lithium-ion battery.

7. Good low temperature performance

Lithium-ion batteries have good low-temperature performance, which means that at low temperatures, the lithium ions and electrode materials inside the battery still maintain high activity, high residual capacity, reduced discharge capacity degradation, and large allowable charging rate.

As the temperature drops, the remaining capacity of the lithium-ion battery decays into an accelerated situation. The lower the temperature, the faster the capacity decay. Forcible charging at low temperatures is extremely harmful, and it is very easy to cause thermal runaway accidents. At low temperatures, the activity of lithium ions and electrode active materials decreases, and the rate at which lithium ions are inserted into the negative electrode material is severely reduced. When the external power supply is charged at a power exceeding the allowable power of the battery, a large amount of lithium ions accumulate around the negative electrode, and the lithium ions embedded in the electrode are too late to get electrons and then directly deposit on the surface of the electrode to form lithium elemental crystals. The dendrite grows, penetrates the diaphragm directly, and pierces the positive electrode. Causes a short circuit between the positive and negative electrodes, which in turn leads to thermal runaway.

In addition to the severe deterioration of the discharge capacity, lithium-ion batteries cannot be charged at low temperatures. During low-temperature charging, the intercalation of lithium ions on the graphite electrode of the battery and the lithium plating reaction coexist and compete with each other. Under low temperature conditions, the diffusion of lithium ions in graphite is inhibited, and the conductivity of the electrolyte decreases, which leads to a decrease in the intercalation rate and makes the lithium plating reaction more likely to occur on the graphite surface. The main reasons for the decrease in the life of lithium-ion batteries when used at low temperatures are the increase in internal impedance and the degradation of the capacity due to the precipitation of lithium ions.

8. Good security

The safety of lithium-ion batteries includes not only the stability of internal materials, but also the effectiveness of battery safety auxiliary measures. The safety of internal materials refers to the positive and negative materials, diaphragm and electrolyte, which have good thermal stability, good compatibility between the electrolyte and the electrode material, and good flame retardancy of the electrolyte itself. Safety auxiliary measures refer to the safety valve design of the cell, the fuse design, the temperature-sensitive resistance design, and the sensitivity is appropriate. After a single cell fails, it can prevent the fault from spreading and serve the purpose of isolation.

9. Good consistency

Through the “barrel effect” we understand the importance of battery consistency. Consistency refers to the battery cells used in the same battery pack, the capacity, open circuit voltage, internal resistance, self-discharge and other parameters are extremely small, and the performance is similar. If the consistency of the battery cell with its own excellent performance is not good, its superiority is often smoothed out after the group is formed. Studies have shown that the capacity of the battery pack after grouping is determined by the smallest capacity cell, and the battery pack life is less than the life of the shortest cell.