Comprehensive Analysis of Lithium Battery Energy Density

What is energy density?

Energy density refers to the amount of energy stored in a certain unit of space or mass of matter.

The lithium battery energy density of a battery is the electrical energy released by the average unit volume or mass of the battery. The energy density of a battery 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

Battery volumetric energy density = battery capacity × discharge platform/volume, the basic unit is Wh/L

The greater the energy density of the battery, the more electricity can be stored per unit volume or weight.

What is monomer energy density?

The lithium battery energy density often refers to two different concepts, one is the 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 vehicle power batteries and energy storage batteries.

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 of power batteries is clarified: in 2020, the lithium 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 at the level of a single cell.

What is 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, the energy density of the battery system is lower than that of the monomer.

System energy density = battery system power/battery system weight OR battery system volume

What exactly limits the lithium battery energy density?

The chemistry behind the battery is largely to blame. Generally speaking, 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 take place, which is equivalent to the two veins of Ren and Du, and their important status can be seen.

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?

The existing lithium ion battery negative electrode materials are mostly graphite, and the theoretical gram capacity of graphite is 372 mAh/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.

According to the barrel theory, the water level is determined by the shortest point of the barrel, and the lower limit of the lithium battery energy density depends on the positive electrode material.

The voltage platform of lithium iron phosphate is 3.2V, and the indicator of ternary is 3.7V. Compared with the two, the 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 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 lithium battery energy density.

How to increase lithium battery 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 be “long-sleeved and good at dancing”. Next, we will explain the two dimensions of monomer and system.

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

Increase battery size

Battery manufacturers can achieve the effect of power expansion by increasing the size of the original battery. The example we are most familiar with is Tesla, a well-known electric car company that took the lead in using Panasonic 18650 batteries and will replace them with new 21700 batteries.

But the “fat” or “longer” lithium battery cells are only a temporary solution, not a permanent cure. The way to draw fire from the bottom of the pot is to find the key lithium technology to increase lithium battery energy density from the positive and negative electrode materials and electrolyte components that make up the battery unit.

Change of chemical system

As mentioned earlier, the energy density of the battery 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 lithium battery energy density.

High Nickel Cathode

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

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 lithium battery energy density of batteries 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 go one step further—break through the 350Wh/kg barrier of a single battery cell, industry peers may need to focus on the lithium metal negative electrode battery system, but this also means that the entire battery manufacturing process is limited. Change and refinement.

System energy density: Improve the group efficiency of battery packs

The grouping of battery packs tests the ability of the battery “siege lions” to deploy single cells and modules. It is necessary to use every inch of space to the greatest extent on the premise of safety.

There are mainly the following ways to “slim down” 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

Through simulation calculations, we realized the weight reduction design under the premise of ensuring rigidity and structural reliability. Through this technology, topology optimization and shape optimization can be realized to finally help realize the light weight of the battery box.

material selection

We can choose low-density materials. For example, the battery pack cover has gradually changed from the traditional sheet metal cover to the composite material cover, which can reduce the 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.

integrated design

Integrated design and structural design are taken into consideration, sharing and sharing structural parts as much as possible, such as anti-collision design, to achieve extremely lightweight

The battery is a very comprehensive product. If you want to improve the performance of one aspect, you may sacrifice the performance of other aspects. This is the basis for understanding the design and development of batteries. Lithium Battery Energy density is not the only measure of battery quality.