Why Is Power Battery Energy Density Important
Why Is Power Battery Energy Density Important
- What is battery energy density
- What is the monomer energy density
- What is the system energy density
- How to increase battery energy density
- System energy density improves the group efficiency of battery packs
Under the condition that the same energy consumption remains constant and the volume and weight of the battery pack are strictly limited, the single maximum mileage of a new energy vehicle mainly depends on the battery energy density.
What is battery energy density
Battery energy density refers to the amount of energy stored in a certain unit of space or mass of matter. The battery energy density is the electrical energy released by the average unit volume or mass of the battery. The battery energy density 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 (Wh/kg)
Battery volumetric energy density = battery capacity × discharge platform/volume, the basic unit is Wh/L (Wh/L)
The greater the battery energy density, the more electricity can be stored per unit volume or weight.
What is the monomer energy density
The battery energy density often refers to two different concepts, one is the battery energy density of a single cell, and the other is the energy density of a battery system.
A cell is the smallest unit of a battery system. M batteries 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 is the energy density at the level of a single cell.
What is the system energy density
The energy density of the system refers to the weight or volume of the entire battery system after the monomer combination is completed. Because the battery system contains 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.
System energy density = battery system power / battery system weight OR battery system volume
The existing lithium-ion battery negative electrode materials are mostly graphite, and the theoretical gram capacity of graphite is 372mAh/g. The theoretical gram capacity of the positive electrode material lithium iron phosphate is only 160mAh/g, while the ternary material nickel-cobalt-manganese (NCM) is about 200mAh/g.
The voltage platform of lithium iron phosphate is 3.2V, and the indicator of ternary is 3.7V. Compared with the two, the battery energy density is higher and lower: a difference of 16%. Of course, in addition to the chemical system, the production process level such as compaction density, foil thickness, etc., will also affect the battery energy density.
Generally speaking, the greater 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 battery energy density.
How to increase battery energy density
- Increase battery size
Battery manufacturers can achieve the effect of power expansion by increasing the size of the original battery. For example, Tesla, a well-known electric car company that took the lead in using Panasonic 18650 batteries, will replace them with new 21700 batteries.
However, changing the size of the battery cell cannot solve the problem. The most important thing is to find the key technology to increase the battery energy density from the positive and negative electrode materials and electrolyte components that make up the battery unit.
- Changes in the chemical system
The battery energy density is limited by the positive and negative electrodes of the battery. Since the energy density of the current negative electrode material is much higher than that of the positive electrode, it is necessary to continuously upgrade the positive electrode material to increase the battery energy density.
- High nickel cathode
Ternary materials generally refer to a large family of nickel-cobalt-manganese oxide lithium oxides, which can change the performance of batteries by changing the ratio of the three elements of nickel, cobalt, and manganese.
In typical ternary materials, 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 battery. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the use of cobalt.
- Silicon carbon negative electrode
The specific capacity of the silicon-based negative electrode material can reach 4200mAh/g, which is much higher than the theoretical specific capacity of the graphite negative electrode of 372mAh/g, so it becomes a powerful substitute for the graphite negative electrode.
At present, the use of silicon-carbon composite materials to increase the battery energy density has become one of the industry-recognized development directions for lithium-ion battery anode materials. The Model 3 released by Tesla uses a silicon carbon anode.
In the future, if you want to break through the 350Wh/kg barrier of a single battery cell, it means that industry peers may need to focus on lithium metal negative electrode battery systems. As for the development of battery cell, here is a latest ranking of top 10 lithium battery cell suppliers in China illustrating the present situation of battery industry.
System energy density improves the group efficiency of battery packs
The battery pack tests the ability of the battery to arrange and combine single cells and modules. It needs to take safety as the premise and maximize the use of every inch of space.
There are mainly the following ways to reduce the size of the battery pack.
- Optimize the layout structure
In terms of external dimensions, the internal layout of the system can be optimized to make the internal components of the battery pack more compact and efficient.
- Topology Optimization
On the premise of ensuring the rigidity and structural reliability through simulation calculation, the weight reduction design is realized. Through this technology, topology optimization and shape optimization can be realized to finally help realize the lightweight of the battery box.
- Material selection
Choosing low-density materials, such as the battery pack cover has gradually changed from the traditional sheet metal cover to the composite material cover, can reduce weight by about 35%.
For 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 weight reduction effect is obvious.