Energy density refers to the amount of energy stored in a certain unit of space or mass of matter. The energy density of a battery is the electrical energy released by the average unit volume or mass of the battery. The energy density of a battery is generally divided into two dimensions: gravimetric energy density and volumetric energy density.
Battery weight energy density = battery capacity × discharge platform/weight, the basic unit is Wh/kg (watt-hour/kg)
Battery volumetric energy density = battery capacity × discharge platform/volume, the basic unit is Wh/L (watt-hour/liter)
The greater the energy density of a battery, the more electricity can be stored per unit volume, or weight.
What is monomer energy density?
The energy density of batteries often points to two different concepts, one is the energy density of a single cell, and the other is the energy density of the battery system.
A cell is the smallest unit of a battery system. M cells form a module, and N modules form a battery pack, which is the basic structure of a vehicle power battery.
The energy density of a single cell, as the name suggests, is the energy density at the level of a single cell.
What is system energy density?
System energy density refers to the weight or volume of the entire battery system after the monomer combination is completed. Because the battery system includes the battery management system, thermal management system, high and low voltage circuits, etc., which occupy part of the weight and internal space of the battery system, the energy density of the battery system is lower than the energy density of the monomer.
System energy density = battery system charge/battery system weight OR battery system volume
What exactly limits the energy density of lithium batteries?
The chemistry behind the battery is the main reason.
Generally speaking, the four parts of a lithium battery are very critical: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. The positive and negative electrodes are where chemical reactions take place, which are equivalent to the two veins of Ren and Du, and their importance is evident. We all know that the energy density of the battery pack system with ternary lithium as the positive electrode is higher than that of the battery pack system with lithium iron phosphate as the positive electrode. Why is this?
Existing lithium-ion battery anode materials are mostly graphite, and the theoretical gram capacity of graphite is 372mAh/g. The theoretical gram capacity of the cathode material lithium iron phosphate is 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, and this indicator of ternary is 3.7V. Compared with the two, the energy density is high and the difference is 16%.
Of course, in addition to the chemical system, the production process level, such as compaction density, foil thickness, etc., also affects 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 regarded as one of the reference indicators of the battery energy density.
CATL adopted 6-micron copper foil and improved the energy density by using advanced technology.
If you can stick to each line, read all the way to this point. Congratulations, your understanding of batteries has reached a new level.
How to improve energy density?
The adoption of new material systems, the fine-tuning of the lithium battery structure, and the improvement of manufacturing capabilities are the three stages for R&D engineers to "gracefully dance". Below, we will explain from the two dimensions of the single and the system.
——Individual energy density, mainly relying on breakthroughs in chemical systems
1. Increase the size of the battery
Battery manufacturers can achieve the effect of capacity expansion by increasing the size of the original battery. We are most familiar with the example: Tesla, the well-known electric car company that took the lead in using Panasonic's 18650 battery, will replace it with a new 21700 battery.
However, the "fat" or "long" cells are only a temporary solution, not a root cause. The method of drawing wages from the bottom of the kettle is to find the key technology to improve the energy density from the positive and negative electrode materials and electrolyte components that constitute the battery unit.
2. Changes in the chemical system
As mentioned earlier, the energy density of the battery is controlled by the positive and negative electrodes of the battery. Since the energy density of the negative electrode material is much higher than that of the positive electrode, it is necessary to continuously upgrade the positive electrode material to improve the energy density.
High nickel cathode
Ternary materials generally refer to the big family of nickel cobalt lithium manganate oxides. We can change the performance of the battery by changing the ratio of nickel, cobalt, and manganese.
Silicon carbon anode in figure
The specific capacity of silicon-based anode materials can reach 4200mAh/g, which is much higher than the theoretical specific capacity of graphite anodes of 372mAh/g, so it has become a powerful substitute for graphite anodes.
At present, the use of silicon-carbon composite materials to improve the energy density of batteries has become one of the development directions of lithium-ion battery anode materials recognized by the industry. The Model 3 released by Tesla uses a silicon carbon anode.
In the future, if you want to go a step further and break through the 350Wh/kg barrier for single cells, industry peers may need to focus on lithium metal negative-type battery systems, but this also means that the entire battery manufacturing process Change and refinement. It can be seen from several typical ternary materials in China that the proportion of nickel is getting higher and higher, and the proportion of cobalt is getting lower and lower. The higher the nickel content, 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.
3. System energy density: improve the group efficiency of battery packs
The grouping of battery packs tests the ability of battery "siege lions" to arrange single cells and modules. It is necessary to take safety as the premise and maximize the use of every inch of space.
There are mainly the following ways to "slim down" the battery pack.
Optimize the layout
In terms of external dimensions, the internal arrangement of the system can be optimized to make the arrangement of components inside the battery pack more compact and efficient.
We realize weight reduction design through simulation calculation on the premise of ensuring rigidity and structural reliability. Through this technology, topology optimization and topography optimization can be achieved and ultimately help achieve lightweight battery boxes.
We can choose low-density materials. For example, the top cover of the battery pack has been gradually transformed from a traditional sheet metal top cover to a composite material top cover, which can reduce the weight by about 35%. For the lower box of the battery pack, the traditional sheet metal solution has been gradually transformed into an aluminum profile solution, reducing the weight by about 40%, and the lightweight effect is obvious.
Vehicle integrated design
The integrated design of the whole vehicle and the design of the whole vehicle structure are taken into consideration, and the structural parts are shared as much as possible, such as the anti-collision design, so as to achieve the ultimate lightweight
The battery is a very comprehensive product. If you want to improve the performance of one aspect, you may sacrifice the performance of other aspects. This is the basis for understanding battery design and development. Power batteries are dedicated to vehicles, so energy density is not the only measure of battery quality.