Abstract: According to the barrel theory, the height of the water level is determined by the shortest part of the barrel, and the lower limit of the energy density of lithium-ion batteries depends on the cathode material.
What determines the range of a new energy vehicle? The range of a new energy vehicle mainly depends on the available power and the energy consumption of the whole vehicle.
Range ↑= available power ↑÷ energy consumption ↓
Under the same energy consumption, the volume and weight of the battery pack are strictly limited, the single maximum driving range of new energy vehicles mainly depends on the energy density of the battery.
What are the little secrets of energy density?
What is energy density?
Energy density refers to the amount of energy stored in a certain amount of space or mass per unit. The energy density of a battery is the average amount of electrical energy released per unit volume or mass of the battery. The energy density of a battery is generally divided into two dimensions: weight energy density and volume energy density.
Battery weight energy density = battery capacity × discharge platform / weight, the basic unit is Wh/kg (watt-hour/kg).
Battery volume energy density = battery capacity × discharge platform/volume, the basic unit is Wh/L (Watt-hour/L).
The higher the energy density of a battery, the more power is stored per unit volume or weight.
What is single-cell energy density?
The energy density of a battery often points to two different concepts, the energy density of a single cell and the energy density of a battery system.
A cell is the smallest unit of a battery system; M cells make up a module and N cells make up a pack, which is the basic structure of an automotive power battery.
Single-cell energy density, as the name implies, is the energy density of a single cell level.
According to "Made in China 2025", the development plan of power battery is clear: in 2020, the battery energy density will reach 300Wh/kg; in 2025, the battery energy density will reach 400Wh/kg; in 2030, the battery energy density will reach 500Wh/kg. here refers to the energy density of the single-cell level.
What is system energy density?
System energy density refers to the power of the entire battery system after the completion of the single-cell combination than the weight or volume of the entire battery system. Because the internal battery system contains a battery management system, thermal management system, high and low voltage circuit, etc. occupies part of the weight and internal space of the battery system, so the energy density of the battery system is lower than the energy density of the single cell.
System energy density = battery system power/battery system weight OR battery system volume
What limits the energy density of the battery?
What exactly limits the energy density of Li-ion batteries? The chemical system behind the battery is the main reason is hard to escape blame.
Generally speaking, four parts of a lithium battery are critical: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. The positive and negative electrodes are where the chemical reactions occur, equivalent to the Ren-Du, and their importance is evident.
Figure 1: Structure of square shell battery cell
We all know that the energy density of the battery pack system with ternary lithium as the cathode is higher than that of the battery pack system with lithium iron phosphate as the cathode. Why is this?
The existing lithium-ion battery cathode material is mostly graphite, which has a theoretical gram capacity of 372mAh/g. The cathode material lithium iron phosphate has a theoretical gram capacity of only 160mAh/g, while the ternary material nickel cobalt manganese (NCM) is about 200mAh/g.
According to the barrel theory, the water level is determined by the shortest part of the barrel, and the lower limit of the energy density of lithium-ion batteries depends on the cathode material.
The voltage platform of lithium iron phosphate is 3.2V, while this indicator of ternary is 3.7V, and the energy density of the two comparisons is high and low immediately: 16% difference.
Of course, in addition to the chemical system, the level of production processes such as compaction density, foil thickness, etc., will also affect the energy density. Generally speaking, the higher the compaction density, the higher the capacity of the battery in a limited space, so the compaction density of the main material is also seen as one of the reference indicators of battery energy density.
If you can insist on reading down each line all the way to here. Congratulations, your understanding of batteries has been taken to the next level.
How to improve the energy density?
The adoption of new material systems, the fine-tuning of lithium battery structure, and the improvement of manufacturing capability are the three stages where R&D engineers can "dance long and hard". In the following, we will explain from two dimensions: monomer and system. --Single-cell energy density, mainly relying on the breakthrough of a chemical system
01、Increase the size of the battery
Battery manufacturers can increase the size of the original battery to achieve the effect of power expansion. We are most familiar with the example than the first to use Panasonic 18650 battery well-known electric car company Tesla will change the new 21700 battery.
But the battery cell "fat" or "long" is only a cure, not the root cause. The bottom solution is to find the key technology to improve energy density from the positive and negative electrode materials and electrolyte composition of the battery cell.
02、Change of chemical system
As mentioned earlier, the energy density of the battery is limited by the positive and negative electrodes of the battery. As the current energy density of the negative electrode material is much greater than the positive electrode, so to improve the energy density we must constantly upgrade the positive electrode material.
High nickel cathode
Ternary materials generally refer to the large family of lithium nickel cobalt manganese oxide, we can change the performance of the battery by changing the ratio of the three elements of nickel, cobalt, and manganese.
In several typical ternary materials in Figure 2, we can see that the proportion of nickel is getting higher and the proportion of cobalt is getting lower. The higher content of nickel means the higher the specific capacity of the cell. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the amount of cobalt used.
Figure 2. Comparison of gram capacity of different cathode materials
Silicon carbon cathode
The specific capacity of silicon-based cathode material can reach 4200mAh/g, which is much higher than the theoretical specific capacity of graphite cathode of 372mAh/g, thus becoming a strong alternative to graphite cathode.
At present, the use of silicon-carbon composites to enhance the energy density of batteries is one of the industry's recognized directions for the development of anode materials for lithium-ion batteries. The Model 3 released by Tesla uses silicon-carbon anode.
In the future, if you want to go a step further - break through the single cell 350Wh/kg barrier, industry peers may need to look at the lithium metal cathode type battery system, but this also means that the entire battery production process iteration and refinement.
03, system energy density: improve the efficiency of the battery pack into a group
Battery pack formation is a test of the battery "siege lion" of the single-cell and the ability to array the module, the need for safety as the premise, the maximum use of every inch of space.
Battery pack "slimming" mainly has the following ways.
Optimize the layout structure
In terms of external dimensions, the internal arrangement of the system can be optimized to make the internal parts of the battery pack more compact and efficient
Topology optimization
We use simulation to ensure the rigidity and reliability of the structure while achieving weight reduction. Through this technology, topology optimization and shape optimization can be achieved to help achieve the final lightweight of the battery box.
Material selection
We can choose low-density materials, such as the upper cover of the battery pack has been gradually changed from the traditional sheet metal upper cover to composite upper cover, which can reduce the weight by about 35%. For the lower box of the battery pack, it has been gradually changed from the traditional sheet-metal solution to the aluminum profile solution, which can reduce the weight by about 40%, with the obvious effect of lightweight.
Integrated design of the whole vehicle
The integrated design of the whole vehicle and the structural design of the whole vehicle is considered as a whole, and the structural parts are shared and common as much as possible, such as the anti-collision design, to achieve the ultimate lightweight.
Is the higher the energy density the better?
The battery is a very comprehensive product, you want to improve the performance of one aspect, may sacrifice the performance of other aspects, this is the basis of understanding battery design and development. The power battery belongs to the vehicle-specific, so energy density is not the only measure of battery quality.
MANLY Battery, a joint-stock high-tech enterprise focusing on R&D, production, and sales of lithium-ion batteries, the application fields of lithium batteries we produce cover 5G Internet of Things, consumer/medical electronics, smart cards/RFID tags, solar energy storage systems, oil drilling exploration, AGV automatic guidance If you have any lithium battery needs, please contact us at sales@manlybatteries.com.
