What is energy density?
Energy density (Energydensity) is the amount of energy stored in a unit of a certain amount of space or mass of material. The energy density of a battery is also 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 / liter)
The higher the energy density of a battery, the more power is stored in a 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 modules 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 at the individual 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 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 exactly limits the energy density of lithium 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 very critical: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. The positive and negative electrodes are where the chemical reaction takes place, which is equivalent to the Rendu, and its importance is evident. We all know that the ternary lithium as the cathode of the battery pack system energy density is higher than the lithium iron phosphate as the cathode of the battery pack system. Why is this?
Existing lithium-ion battery cathode materials are mostly graphite, graphite's theoretical gram capacity of 372mAh/g. The cathode material lithium iron phosphate 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 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.
In the fourth episode of "The Greatest Hits II", Ningde Times used 6 micron copper foil to improve the energy density by using advanced process level.
If you can stick to each line and read down all the way to here. Congratulations, you have taken your understanding of batteries to the next level.
How can energy density be improved?
The adoption of new material systems, the fine-tuning of lithium battery structures, and the improvement of manufacturing capabilities are the three arenas where R&D engineers "dance long and hard". In the following, we will explain from two dimensions: monomer and system.
--Monomer energy density, mainly relying on the breakthrough of chemical system
1、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.
2、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 the figure 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. Several typical ternary materials can be seen in the middle, the proportion of nickel is getting higher and higher, and the proportion of cobalt is getting lower and lower. The higher the 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.
3、System energy density: improve the efficiency of battery pack formation
Battery pack formation is a test of the battery "siege lion" on the ability of the single cell and module array, the need to take safety as the premise, to maximize the 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 rigid strength and structural reliability to achieve the weight reduction design. 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 obvious effect of lightweighting.
Integrated design of the whole vehicle
The integrated design of the whole vehicle and the structural design of the whole vehicle are considered as a whole, and the structural parts are shared and common as far as possible, such as anti-collision design, to achieve the ultimate lightweighting.
Battery is a very all-round product, you have to improve the performance of one aspect, may sacrifice the performance of other aspects, which is the basis of understanding of battery design and development. The power battery belongs to the vehicle-specific, so the energy density is not the only measure of battery quality.
Translated with www.DeepL.com/Translator (free version)
