Energy density analysis of lithium batteries, how to improve the energy density of lithium batteries?



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 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: 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 stored in unit volume or weight.

What is monomer energy density?

The energy density of battery often points to two different concepts, one is the energy density of single cell, the other is the energy density of 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 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 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 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 battery management system, thermal management system, high and low voltage circuit, 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 lithium battery are very important: positive electrode, negative electrode, electrolyte and 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 cathode is higher than that of the battery pack system with lithium iron phosphate as 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 energy density of lithium-ion battery 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", 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 battery has reached a new level.

How to improve the energy density?

The adoption of 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.

——The energy density of monomer mainly depends on the breakthrough of chemical system

1. Increase battery size

Battery manufacturers can increase the size of the original battery to achieve the effect of capacity expansion. Our most familiar example is: Tesla, the first famous electric vehicle company to use Panasonic 18650 battery, will replace it with the new 21700 battery.

But cell "getting fat" or "growing up" is only a temporary cure, not a permanent cure. The best way is to find the key technology to improve the energy density from the positive and negative electrode materials and electrolyte composition.

2. Transformation of chemical system

As mentioned earlier, the energy density of a battery is controlled by the positive and negative poles of the battery. At present, the energy density of anode materials is much higher than that of cathode materials, so to improve the energy density of cathode materials, we need to constantly upgrade them.

High nickel cathode

Ternary materials generally refer to the large family of nickel cobalt lithium manganate oxides. We can change the performance of the battery by changing the ratio of nickel, cobalt and manganese.

In Fig

The specific capacity of silicon-based anode material can reach 4200mah / g, which is much higher than the theoretical specific capacity of graphite anode 372mah / g, so it becomes a powerful substitute for graphite anode.

At present, using silicon carbon composite to improve the energy density of lithium-ion battery has been recognized as one of the development directions of lithium-ion battery anode materials. Tesla's model 3 uses silicon carbon anode.

In the future, if you want to make a further progress - to break through the 350wh / kg single cell barrier, peers in the industry may need to focus on the lithium metal negative battery system, but it also means the change and improvement of the whole battery manufacturing process. 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 cell. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the use of cobalt.

3. System energy density: improve battery pack efficiency

The battery pack group test is the battery "siege lions" ability to single cell and module array, need to take safety as the premise, maximize the use of every inch of space.

Battery pack "slimming" mainly in the following ways.

Optimize the layout structure

From the aspect of external dimensions, the internal layout of the system can be optimized to make the internal parts of the battery pack more compact and efficient.

topological optimization

Through simulation calculation, we realize the weight reduction design on the premise of ensuring the stiffness, strength and structural reliability. Through this technology, topology optimization and shape optimization can be realized, and ultimately help to realize the lightweight of the battery box.

Material selection

We can choose low-density materials. For example, the upper cover of battery pack has gradually changed from traditional sheet metal to composite material, which can reduce weight by about 35%. For the lower box of battery pack, the traditional sheet metal scheme has been gradually transformed into the aluminum profile scheme, which reduces 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 should be considered in an all-round way, and the structural parts should be shared as much as possible, such as the anti-collision design, so as to realize the ultimate lightweight

Battery is a comprehensive product. If you want to improve the performance of one aspect, you may sacrifice the performance of other aspects. This is the understanding basis of battery design and development. Power battery belongs to vehicle special purpose, so energy density is not the only measure of battery quality.