1. Long lifespan
The lifespan of secondary batteries includes two indicators: cycle lifespan and calendar lifespan. Cycle life refers to the remaining capacity of a battery that is greater than or equal to 80% after experiencing the manufacturer's promised number of cycles. The calendar lifespan refers to the remaining capacity, whether used or not, within the time period promised by the manufacturer, which shall not be less than 80%.
The lifespan is one of the key indicators of power lithium-ion batteries. On the one hand, replacing the battery is indeed a cumbersome task and the user experience is not good; On the other hand, fundamentally, lifespan is a cost issue.
The lifespan of lithium-ion batteries refers to the capacity decay of the battery after a period of use to 70% of its nominal capacity (room temperature 25 ℃, standard atmospheric pressure, and discharge at 0.2C), which can be considered as the end of its lifespan. In the industry, the cycle life of lithium-ion batteries is generally calculated based on the number of cycles at full charge and discharge. During use, irreversible electrochemical reactions occur inside lithium-ion batteries, leading to a decrease in capacity, such as electrolyte decomposition, deactivation of active materials, collapse of positive and negative electrode structures, and a decrease in the number of lithium ion insertion and extraction. Experiments have shown that higher rate discharge leads to faster capacity decay. If the discharge current is lower, the battery voltage will approach the equilibrium voltage, which can release more energy.
The theoretical lifespan of ternary lithium-ion batteries is about 800 cycles, which is considered moderate in commercial rechargeable lithium-ion batteries. Lithium iron phosphate has about 2000 cycles, while lithium titanate is said to have up to 10000 cycles. At present, mainstream battery manufacturers promise to produce ternary battery cells with a specification of more than 500 times (charging and discharging under standard conditions). However, after the battery cells are assembled into battery packs, due to consistency issues, the important thing is that the voltage and internal resistance cannot be exactly the same, and their cycle life is about 400 times. The recommended SOC usage window is 10%~90%. It is not recommended to perform deep charging and discharging, as it may cause irreversible damage to the positive and negative electrode structures of the battery. If calculated based on shallow charging and discharging, the cycle life should be at least 1000 times. In addition, if lithium-ion batteries are frequently discharged at high rates and high temperatures, their lifespan will significantly decrease to less than 200 times.
2. Low cost
The battery itself has a low price per kilowatt hour, which is the most intuitive cost. As mentioned earlier, whether the cost is really low for users depends on the "full lifecycle electricity cost".
The cost of electricity per kilowatt hour throughout the entire life cycle of a power lithium battery is obtained by multiplying the total amount of electricity that can be utilized throughout the battery's life cycle by the number of cycles. The total price of the battery pack is divided by this sum to obtain the price per kilowatt hour of electricity throughout the entire life cycle.
The battery prices we usually refer to, such as 1500 yuan/kWh, are only priced based on the total energy of the new battery cells. In fact, the cost of electricity throughout the entire lifecycle is the direct benefit for end customers. The most intuitive result is that buying two battery packs with the same amount of electricity at the same price, one will reach the end of its lifespan after being charged and discharged 50 times, and the other will be reusable after being charged and discharged 100 times. It's clear at a glance which one is cheaper and which one is more expensive for these two battery packs.
Simply put, it means long lifespan, durability, and reduced costs.
In addition to the above two costs, the maintenance cost of the battery also needs to be considered. Simply considering the initial cost and selecting problematic battery cells, the maintenance and labor costs in the later stage are too high. For the maintenance of the battery cell itself, it is important to refer to manual balancing. The balancing function of BMS is limited by the size of its designed balancing current, which may not achieve ideal balance between battery cells. Over time, there may be a problem of excessive voltage difference in the battery pack. In such situations, manual balancing is necessary to charge the battery cells with low voltage 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 amount of energy contained per unit weight or volume; The average unit volume or mass of electricity released by a battery. Generally, under 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 have a smaller volume and lighter weight than nickel cadmium and nickel hydrogen batteries.
Battery energy density=Battery capacity× Discharge platform/battery thickness/battery width/battery length.
Power density refers to the numerical value of the maximum discharge power corresponding to a unit weight or volume. In the limited space of road vehicles, only by increasing density can the overall energy and power be effectively improved. In addition, the current national subsidies, which use energy density and power density as thresholds to measure subsidy levels, further strengthen the importance of density.
However, there is a certain contradictory relationship between energy density and safety. As energy density increases, safety will always face more challenging updates.
4. High voltage
Due to the fact that graphite electrodes are mostly used as negative electrode materials, the voltage of lithium-ion batteries is mainly determined by the material characteristics of the positive electrode materials. The upper voltage limit of lithium iron phosphate is 3.6V, and the highest voltage of ternary lithium and manganese lithium ion batteries is about 4.2V (the next article will explain why the highest voltage of lithium-ion batteries cannot exceed 4.2V). Developing high-voltage battery cells is a technological route for improving the energy density of lithium-ion batteries. To increase the output voltage of the battery cell, a positive electrode material with high potential, a negative electrode material with low potential, and a stable electrolyte with high voltage are required.
5. High energy efficiency
Coulombic efficiency, also known as charging efficiency, refers to the ratio of battery discharge capacity to charging capacity during the same cycle. The percentage of discharge specific capacity to charging specific capacity.
As for the positive electrode material, it is the lithium insertion capacity/lithium removal capacity, that is, the discharge capacity/charging capacity; Regarding the negative electrode material, it is the delithiation capacity/lithium insertion capacity, that is, the discharge capacity/charging capacity.
During the charging process, electrical energy is converted into chemical energy, while during the discharge 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, which directly reflects the performance of the battery.
From a professional physics perspective, Coulombic efficiency and energy efficiency are different, one is the ratio of electricity to work done.
The energy efficiency of a battery is related to its Coulombic efficiency, but from a mathematical expression, there is a voltage relationship between the two. The average voltage of charging and discharging is not equal, and the average voltage of discharging is generally lower than the average voltage of charging
The performance of a battery can be judged by its energy efficiency. According to the conservation of energy, the lost electrical energy is significantly converted into thermal energy. Therefore, energy efficiency can analyze the heat generated during the battery's operation, and thus analyze the relationship between internal resistance and heat. And the known energy efficiency can predict the amount of remaining energy in the battery, and manage the reasonable use of the battery.
Because the input electricity cannot be used entirely to convert the active substance into a charged state, but is partially consumed (such as through irreversible side reactions), the Coulombic efficiency is often less than 100%. But for current lithium-ion batteries, the Coulombic efficiency can generally reach 99.9% or above.
Influencing factors: electrolyte decomposition, interface passivation, and changes in the structure, morphology, and conductivity of electrode active materials can all reduce Coulombic efficiency.
Furthermore, it is worth mentioning that battery attenuation has little effect on Coulombic efficiency and is not closely related to temperature.
The current density reflects the amount of current passing through a unit area. As the current density increases, the current passing through the stack increases. The voltage efficiency decreases due to internal resistance, while the Coulomb efficiency decreases due to concentration polarization and other reasons, ultimately leading to a decrease in energy efficiency.
6. Good high-temperature performance
Lithium ion batteries have good high-temperature performance, which means that the battery cells are in a high temperature environment, and the positive and negative electrode materials, separators, and electrolyte can maintain good stability. They can work normally at high temperatures, and their lifespan will not accelerate. High temperatures are less likely to cause thermal runaway accidents.
The temperature of lithium-ion batteries displays the thermal state of the battery, which is essentially the result of heat generation and transfer in lithium-ion batteries. Studying the thermal characteristics of lithium-ion batteries, as well as their heat generation and transfer characteristics in different states, can help us understand the important pathways through which exothermic chemical reactions occur within lithium-ion batteries.
The unsafe behavior of lithium-ion batteries, including overcharging and discharging, rapid charging and discharging, short circuits, mechanical abuse conditions, and high-temperature thermal shock, can easily trigger dangerous side reactions inside the battery and generate heat, directly damaging the passivation film on the negative and positive electrode surfaces.
When the temperature of the battery cell rises to 130 ℃, the SEI film on the surface of the negative electrode decomposes, causing a severe oxidation-reduction reaction of the highly active lithium carbon negative electrode when exposed to the electrolyte. The resulting heat puts the battery in a high-risk state.
When the local temperature inside the battery rises above 200 ℃, the passivation film on the surface of the positive electrode decomposes and the positive electrode undergoes oxygen evolution, which continues to react violently with the electrolyte, generating a large amount of heat and forming high internal pressure. When the battery temperature reaches above 240 ℃, there is also a violent exothermic reaction between the lithium carbon negative electrode and the binder.
The temperature issue of lithium-ion batteries has a significant impact on the safety of lithium-ion batteries. The environment in which it is used also has a certain temperature, and lithium-ion batteries may also experience temperature during use. Importantly, temperature can have a significant impact on the chemical reactions inside lithium-ion batteries. Excessive temperature can even damage the lifespan of lithium-ion batteries, and in severe cases, it can lead to safety issues.
7. Good low-temperature performance
Lithium ion batteries have good low-temperature performance, which refers to the high activity of the lithium ions and electrode materials inside the battery at low temperatures, high residual capacity, reduced discharge capacity, and allowed for high charging rates.
As the temperature decreases, the remaining capacity of lithium-ion batteries decays rapidly. The lower the temperature, the faster the capacity decay. Forced charging at low temperatures poses great risks and can easily lead to thermal runaway accidents. At low temperatures, the activity of lithium ions and electrode active substances decreases, and the rate of lithium ion insertion into the negative electrode material severely decreases. When charging with an external power source exceeding the allowable power of the battery, a large amount of lithium ions accumulate around the negative electrode, and the lithium ions that are not embedded in the electrode can directly deposit on the surface of the electrode after receiving electrons, forming lithium elemental crystals. Dendritic growth, directly penetrating the diaphragm and piercing towards the positive electrode. Causing a short circuit between the positive and negative poles, leading to thermal runaway.
Besides severe degradation of discharge capacity, lithium-ion batteries cannot be charged at low temperatures. During low-temperature charging, the insertion 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, leading to a decrease in insertion rate. On the surface of graphite, the lithium plating reaction is more likely to occur. The main reasons for the decrease in lifespan of lithium-ion batteries when used at low temperatures are the addition of internal impedance and the capacity degradation caused by lithium ion precipitation.
8. Good security
The safety of lithium-ion batteries includes both the stability of internal materials and the effectiveness of auxiliary measures for cell safety. The safety of internal materials refers to the positive and negative electrode materials, separators, and electrolytes, which have good thermal stability, good compatibility between electrolyte and electrode materials, and good flame retardancy of the electrolyte itself. Safety auxiliary measures refer to the design of safety valves, fuses, temperature sensitive resistors for battery cells, with appropriate sensitivity. After a single battery cell failure, they can prevent the spread of faults and serve as isolation measures.
9. Good consistency
We understand the importance of battery consistency through the "barrel effect". Consistency refers to the application of cells in the same battery pack, with small differences in parameters such as capacity, open circuit voltage, internal resistance, and self discharge, and similar performance. If the consistency of battery cell monomers with excellent performance is poor, their superiority is often smoothed out after grouping. Research has shown that the capacity of a battery pack after grouping is determined by the minimum capacity of the battery cells, and the lifespan of the battery pack is smaller than the lifespan of the shortest battery cells.