What is energy density?
Energy density refers to the amount of energy stored in a certain unit of space or mass. The energy density of a battery is the energy released by the average unit volume or mass of the battery. The energy density of a battery is generally divided into two dimensions: the weight energy density and the volume energy density.
Battery weight energy density = battery capacity × Discharge platform / weight, basic unit is wh / kg (watt hour / kg)
Battery volume energy density = battery capacity × Discharge platform / volume, basic unit: WH / L (watt hour / L)
The higher the energy density of the battery, the more power is stored in unit volume or weight.
What is monomer energy density?
The energy density of a battery often points to two different concepts, one is the energy density of a single cell, the other is the energy density of the battery system.
The cell is the smallest unit of a battery system. M cells form a module, N modules form a battery pack, which is the basic structure of a vehicle power battery.
Single-cell energy density, as the name suggests, is the energy density of a single cell level.
According to "made in China 2025", the development plan of power battery is made clear: in 2020, the energy density of battery will reach 300wh / kg; In 2025, the battery energy density will reach 400wh / kg; By 2030, the battery energy density will reach 500wh / kg. This is the energy density at the level of a single cell.
What is system energy density?
System energy density refers to the ratio of the power of the whole battery system to the weight or volume of the whole battery system. Because the battery system contains a battery management system, thermal management system, high and low voltage circuits, which occupy part of the weight and internal space of the battery system, the energy density of the battery system is lower than that of the monomer.
System energy density = battery system capacity/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 important: a positive electrode, a negative electrode, an electrolyte, and a diaphragm. The positive and negative poles are the places where chemical reactions take place, which is equivalent to the two vessels of Ren and Du. We all know that the energy density of the battery pack system with lithium ternary as the cathode is higher than that of the battery pack system with lithium iron phosphate as a cathode. Why?
The current anode materials for Li-ion batteries are mostly graphite, and the theoretical capacity of graphite is 372mah / g. The theoretical capacity of LiFePO4 is only 160 MAH / g, while that of NCM is about 200 MAH / g.
According to the barrel theory, the water level depends on 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 LiFePO4 is 3.2V, and the index of ternary is 3.7V. Compared with the two phases, the energy density is high, and the difference is 16%.
Of course, in addition to the chemical system, the level of production technology, such as compaction density, foil thickness, will also affect the energy density. Generally speaking, the higher the compaction density, the higher the capacity of the battery in the limited space, so the compaction density of the main material is also regarded as one of the reference indexes of the battery energy density.
In the fourth episode of "great power heavy equipment II", the Ningde era adopted 6-micron copper foil, which made use of advanced technology to improve energy density.
If you can stick to reading each line all the way to here. Congratulations, your understanding of the battery has reached a new level.
How to improve the energy density?
The adoption of a new material system, the fine-tuning of lithium battery structure, and the improvement of manufacturing capacity are the three stages for R & D engineers to be good at "Long Sleeve Dance". Next, we will explain from monomer and system dimensions.
- Single energy density, mainly relying on the breakthrough of the chemical system
1, increase the battery size
Battery manufacturers can achieve the effect of power expansion by increasing the original battery size. Our most familiar example is that it is the first to use the well-known electric car of Panasonic 18650 battery to exchange new 21700 batteries.
But the battery "gain fat" or "long" is just the standard, not to cure this. The bottom of the clavicle is a key technique that improves energy density from the positive and negative material and the electrolyte component constituting the battery unit.
2, chemical system change
As mentioned earlier, the energy density of the battery is subject to the positive and negative electrodes of the battery. Since the energy density of the current negative electrode material is much larger than the positive electrode, the positive electrode material is continuously upgraded by increasing the energy density.
High nickel positive
The ternary material refers to the lithium oxide of nickel-manganese acid, which can change the performance of the battery by changing the ratio of the three elements of nickel, cobalt, manganese.
On the silicon-carbon negative electrode
The specific capacity of silicon-based negative electrode material can reach 4200 mAh / g, which is much higher than 372 mAh / g of the graphite negative poor than the capacity, thus becoming a strong substitute for graphite negative electrodes.
At present, the way the battery energy density is improved with silicon-carbon composite materials is one of the leading directions of the industry's recognized lithium-ion battery negative electrode material. The Model3 released by Tesla is a silicone carbon negative electrode.
In the future, if you want a hundred feet to further - break through the gate of the monomer cell 350WH / kg, the industry's peers may need to focus on the lithium metal negative pole-type battery system, but this also means the entire battery production process. Rapture and refinement. It can be seen in several typical three-membered materials, and nickel proportion is increasing, and the proportion of cobalt is getting lower and lower. The higher the content of nickel, it means that the higher the specific capacity of the battery. In addition, due to the scarce of cobalt resources, the ratio of nickel is increased, and the amount of use of cobalt will be reduced.
3, system energy density: improve the assembly efficiency of battery packs
The group test of the battery pack is the ability of the battery to "siege" to the monomer battery and the module and needs to maximize every inch of space with safety.
The "slimming" of the battery pack has the following ways.
Optimized arrangement
From the dimensions of the shape, the arrangement of the internal parts can be optimized, allowing the internal parts of the battery pack to be more compact and efficient.
Topology Optimization
We have achieved weight loss design by simulation calculation in ensuring rigidity and structural reliability. Through this technology, topology optimization and morphological optimization can ultimately help to achieve a lightweight battery case.
Selection of materials
We can choose a low-density material, such as the battery pack, which has gradually transformed from a conventional sheet metal upper cover to a composite cover, which can weigh about 35%. For the battery pack, the case has gradually transformed from a conventional sheet metal solution to an aluminum profile, and the weight is reduced by about 40%, and the lightweight effect is obvious.
Whole-integrated design
The whole vehicle integrated design is considering the design of the vehicle structure, like sharing, shared structural parts, such as anti-collision design, achieving the ultimate lightweight
The battery is a very comprehensive product, you have to improve the performance of a certain aspect, may sacrifice other performance, which is the basis for battery design and development. Power batteries belong to car dedicated, so the energy density is not a unique scale that measures the quality of the battery.
