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Energy density refers to the amount of energy stored in a unit of space or mass of matter. The energy density of a battery is the average unit volume or mass of electrical energy released by the battery. The energy density of a battery is generally divided into two dimensions: weight energy density and volume energy density.

What is energy density?

Energy density refers to the amount of energy stored in a unit of space or mass of matter. The energy density of a battery is the average unit volume or mass of electrical energy released by 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 x discharge platform/weight, basic unit is Wh/kg (watt hours/kg)

Battery volume energy density=battery capacity x discharge platform/volume, basic unit is Wh/L (watt hours/liter)

The higher the energy density of a battery, the more electricity is stored per 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 individual cells, and the other is the energy density of the battery system.

The battery cell is the smallest unit of a battery system. M battery cells form a module, and N modules form a battery pack, which is the basic structure of automotive power batteries.

The energy density of a single cell, as the name suggests, is the energy density at the level of a single cell.

According to "Made in China 2025", the development plan for power batteries has been clarified: by 2020, the energy density of batteries will reach 300Wh/kg; In 2025, the energy density of batteries will reach 400Wh/kg; By 2030, the energy density of batteries will reach 500Wh/kg. This refers to the energy density at the individual cell level.

What is system energy density?

System energy density refers to the ratio of the total energy of the entire battery system after the completion of individual combinations to the weight or volume of the entire battery system. Because the battery system contains battery management system, thermal management system, high and low voltage circuits, which occupy a portion of the weight and internal space of the battery system, the energy density of the battery system is lower than that of individual cells.

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

What exactly limits the energy density of lithium batteries?

The chemical system behind the battery is the main reason and cannot escape its blame.

Generally speaking, the four components of a lithium battery are crucial: positive electrode, negative electrode, electrolyte, and diaphragm. The positive and negative poles are the places where chemical reactions occur, equivalent to the Ren and Du meridians, and their important position is evident. We all know that the energy density of battery pack systems with ternary lithium as the positive electrode is higher than that of battery pack systems with lithium iron phosphate as the positive electrode. Why is this?

The existing negative electrode materials for lithium-ion batteries are mostly graphite, with a theoretical capacity of 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.

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

The voltage platform of lithium iron phosphate is 3.2V, while the indicator of ternary is 3.7V. Compared to the two, the difference in energy density is 16%.

Of course, in addition to chemical systems, production process levels such as compaction density and foil thickness also affect 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 considered as one of the reference indicators for battery energy density.

In the fourth episode of "Great Power Heavy Industries II", Ningde Times used 6-micron copper foil and advanced technology to improve energy density.

If you can persist in reading each line down until this point. Congratulations, your understanding of batteries has reached a new level.

How to improve energy density?

The adoption of new material systems, precise tuning of lithium battery structures, and improvement of manufacturing capabilities are the three stages for R&D engineers to excel in. Below, we will explain from both individual and system dimensions.

——Individual energy density mainly relies on breakthroughs in chemical systems

1. Increase battery size

Battery manufacturers can achieve battery capacity expansion by increasing the original battery size. Our most familiar example is undoubtedly Tesla, a well-known electric vehicle company that was the first to use Panasonic 18650 batteries, which will be equipped with a new 21700 battery.

However, the phenomenon of battery cells becoming overweight or growing is only a temporary solution, not a root cause. The ultimate solution is to find the key technology to improve energy density from the positive and negative electrode materials and electrolyte components that make up the battery unit.

2. Chemical system transformation

As mentioned earlier, the energy density of a battery is constrained by its positive and negative electrodes. Due to the current energy density of negative electrode materials being much higher than that of positive electrodes, improving energy density requires continuously upgrading positive electrode materials.

High nickel positive electrode

The ternary material generally refers to the large family of nickel cobalt manganese oxide lithium oxides, and we can change the performance of batteries by changing the proportion of nickel, cobalt, and manganese elements.

In the figure, silicon carbon negative electrode

The specific capacity of silicon-based negative electrode materials can reach 4200mAh/g, which is much higher than the theoretical specific capacity of 372mAh/g of graphite negative electrode, making them a powerful alternative to graphite negative electrode.

At present, using silicon carbon composite materials to enhance the energy density of batteries has been recognized as one of the development directions of lithium-ion battery negative electrode materials in the industry. Tesla's Model 3 uses silicon carbon negative electrodes.

In the future, if we want to go further and break through the threshold of 350Wh/kg of individual cells, industry peers may need to focus on lithium metal negative electrode battery systems. However, this also means that the entire battery manufacturing process is undergoing changes and improvements. It can be seen from several typical ternary materials that the proportion of nickel is increasing while the proportion of cobalt is decreasing. 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 usage of cobalt.

3. System energy density: improving the grouping efficiency of battery packs

The grouping test of battery packs is the ability of battery siege lions to deploy individual cells and modules, and it is necessary to prioritize safety and maximize the use of every inch of space.

There are several ways to slim down battery packs.

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.

topological optimization

We achieve weight reduction design through simulation calculations while ensuring rigidity, strength, and structural reliability. Through this technology, topology optimization and morphology optimization can be achieved, ultimately helping to achieve lightweight battery casing.

Material selection

We can choose low-density materials, such as the battery pack cover, which has gradually transformed from a traditional sheet metal cover to a composite material cover, which can reduce weight by about 35%. For the lower box of the battery pack, it has gradually shifted from a traditional sheet metal solution to an aluminum profile solution, reducing weight by about 40% and achieving significant lightweight effects.

Integrated vehicle design

The integrated design of the entire vehicle and the overall structural design of the vehicle should be comprehensively considered, and structural components should be shared as much as possible, such as collision prevention design, to achieve the ultimate lightweighting

Batteries are a comprehensive product, and if you want to improve one aspect of performance, you may sacrifice other aspects of performance. This is the understanding foundation of battery design and development. Power batteries are specialized for vehicle use, so energy density is not the only measure of battery quality.

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