What is energy density?
Energy density (Energydensity) refers to the amount of energy stored in a unit of a certain space or mass matter. The energy density of a battery is the electric energy released by the average 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 greater the energy density of the battery, the more electricity stored per unit volume or weight.
What is the energy density of the monomer?
The energy density of a battery 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.
The battery cell is the smallest unit of a battery system. M batteries form a module, and N modules form a battery pack. This is the basic structure of a vehicle power battery.
The energy density of a single cell, as the name implies, is the energy density of a single cell level.
According to "Made in China 2025", the development plan for power batteries is defined: 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. This refers to the energy density of a single cell level.
What is the system energy density?
The system energy density refers to the weight or volume of the entire battery system compared to the weight or volume of the entire battery system after the monomer combination is completed. Because the battery system contains battery management system, thermal management system, high and low voltage circuits, etc. 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 power / battery system weight OR battery system volume
What limits the energy density of lithium batteries?
The chemical system behind the battery is the main reason.
Generally speaking, the four parts of a lithium battery are very critical: positive electrode, negative electrode, electrolyte, and diaphragm. The positive and negative poles are the places where chemical reactions occur, which are equivalent to the two veins of Ren and Du, and their important status 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?
Most of the current anode materials for lithium-ion batteries are graphite, and the theoretical gram capacity of graphite is 372 mAh/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 the ternary index is 3.7V. Comparing the two phases, 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., 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 regarded as one of the reference indicators for the energy density of the battery.
In the fourth episode of "Great Power Heavy Equipment II", CATL adopted 6-micron copper foil and used advanced technology to increase the energy density.
If you can stick to each line, read it down and read it till here. Congratulations, your understanding of batteries has reached a level.
How to increase 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 "long-sleeve good dance". Below, we will explain from the two dimensions of monomer and system.
——Monomer energy density, mainly depends on the breakthrough of the chemical system
1. Increase the battery size
Battery manufacturers can achieve the effect of capacity expansion by increasing the original battery size. The most familiar example is that Tesla, a well-known electric car company that was the first to use Panasonic's 18650 battery, will replace it with a new 21700 battery.
However, the “fatness” or “growth” of batteries is only a temporary cure, not a permanent cure. The method of drawing salaries from the bottom of the kettle is to find the key technology to increase the energy density from the positive and negative materials that constitute the battery cell and the electrolyte composition.
2. Changes in the chemical system
As mentioned earlier, the energy density of the battery is restricted by the positive and negative electrodes of the battery. Since the energy density of the current negative electrode material is much greater than that of the positive electrode, increasing the energy density requires continuous upgrading of the positive electrode material.
High nickel cathode
Ternary materials generally refer to the large family of nickel cobalt manganese oxide lithium oxides. We can change the performance of the battery by changing the ratio of the three elements of nickel, cobalt, and manganese.
Silicon carbon anode in the picture
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 increase the energy density of batteries has become one of the development directions of lithium-ion battery anode materials recognized in the industry. The Model 3 released by Tesla uses a silicon carbon anode.
In the future, if you want to go a step further-breaking the 350Wh/kg threshold for single cells, industry counterparts may need to focus on lithium metal negative battery systems, but this also means that the entire battery manufacturing process Change and diligence. It can be seen from several typical ternary materials that the proportion of nickel is higher and higher, and the proportion of cobalt is lower and lower. The higher the nickel content, the higher the specific capacity of the battery 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 packs
The group test of battery packs is the ability of the battery "siege lions" to lay out single cells and modules. It is necessary to take safety as the premise and make the most of every inch of space.
There are mainly the following ways to "slim down" the battery pack.
Optimize the layout structure
From the aspect of dimensions, the internal layout of the system can be optimized to make the internal components of the battery pack more compact and efficient.
We realize weight reduction design under the premise of ensuring rigidity and structural reliability through simulation calculation. Through this technology, topology optimization and topography optimization can be achieved and ultimately help to achieve lightweight battery cabinets.
We can choose low-density materials. For example, the battery pack cover has gradually changed from a traditional sheet metal cover to a composite material cover, which can reduce weight by about 35%. Regarding the lower box of the battery pack, it has gradually changed from the traditional sheet metal solution to the aluminum profile solution, reducing the weight by about 40%, and the lightweight effect is obvious.
Integrated vehicle design
The integrated design of the whole vehicle and the structural design of the whole vehicle are considered comprehensively, and the structural parts are shared and shared as much as possible, such as anti-collision design, to achieve the ultimate lightweight
The battery is a very comprehensive product. If you want to improve one aspect of performance, you may sacrifice other aspects of performance. 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.