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What is Energy Density of Li-Po Battery?

Intro

Battery life=available power↑÷energy consumption↓

Under the condition that the same energy consumption remains the same, and the volume and weight of the battery pack are strictly limited, the maximum mileage of a single new energy vehicle mainly depends on the energy density of the battery.

What is Energy Density?

The energy density of a power battery can be divided into weight energy density and volume energy density from different dimensions, that is, the total electrical energy released per unit mass or volume. It is an important indicator to measure the capacity of the battery, that is, the electrical energy that the battery can store per unit weight and volume.

Weight energy density = battery capacity × discharge weight, the basic unit is Wh/kg (watt-hour/kg)

Battery volume energy density = battery capacity × discharge volume, the basic unit is Wh/L (watt-hour/liter)

The energy density of the battery is proportional to the unit weight and volume. The industry points to two concepts of the energy density of the power battery, namely the energy density of the monomer and the energy density of the system.

What is the Monomer Energy Density?

The smallest unit of a battery system is a battery cell. The basic structure of an automobile power battery is: the battery module is composed of multiple batteries, and the battery pack is composed of multiple modules.The energy density of a monomer refers to the energy density of a single cell.

“In the industry information of “Made in China 2025”: The development plan of power batteries will achieve a battery energy density of 300Wh/kg by 2020; in 2025, the battery energy density will reach 400Wh/kg; in 2030, the battery energy density will reach 500Wh/kg.”The energy density of the battery mentioned in this paragraph refers to the energy density of a single cell.

What is the System Energy Density?

The power ratio of the entire battery system composed of a single unit to the weight or volume of the entire battery system is the energy density of the system.Since the high and low voltage circuits, battery management systems, thermal management systems, etc. inside the battery system will also occupy the volume and weight of the battery system, the monomer energy density of the power battery is higher than the energy density of the system.

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

What Exactly Limits the Energy Density of Li-Po batteries?

The chemical system behind the battery is the main reason. Generally speaking, the four parts of a lithium battery are very critical: the positive electrode, the negative electrode, the electrolyte, and the diaphragm.The positive and negative poles are the places where chemical reactions occur, which are equivalent to the second veins of the governor, and their important status can be seen.

We all know that the energy density of a battery pack system with ternary lithium as the positive electrode is higher than that of a battery pack system with lithium iron phosphate as the positive electrode.Why is this?

Most of the existing lithium-ion battery anode materials are mainly graphite, with a theoretical gram capacity of 372mAh/g of graphite.The theoretical gram capacity of the cathode 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 level of the water level is determined by 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 lithium iron phosphate is 3.2V, and the ternary index 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 and foil thickness 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 of the battery energy density.

Factors Affecting the System Energy Density

Number of cells

The current number of mainstream batteries has reached about 100, or even more. Increasing the number of batteries can effectively increase the energy density of the system contained in each kilogram of battery.

Using Voltage

The use of higher-voltage batteries and the use of higher-voltage cathode materials can improve the energy density of the system: the current mainstream high-voltage cathode materials are mainly lithium iron phosphate, ternary lithium, etc.; At the current technical level, ternary lithium has a higher energy density than lithium iron phosphate and lithium iron phosphate.

How to Increase the Energy Density?

The adoption of the new material system, the fine-tuning of the lithium battery structure, and the improvement of manufacturing capabilities are the three stages for R&D engineers to “dance with long sleeves”.Below, we will explain from the two dimensions of monomer and system—Monomer energy density, mainly relying on breakthroughs in 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 vehicle company that took the lead in using Panasonic 18650 batteries, will be replaced with a new 21700 battery. However, the “fatness” or “lengthening” of the battery is only a symptom, not a cure.The way to draw the bottom of the kettle is to find the key technology to improve the energy density from the positive and negative electrode materials that make up the battery cell and the composition of the electrolyte.

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 current energy density of the anode material is much larger than that of the cathode, it is necessary to continuously upgrade the cathode material to increase the energy density.

High Nickel Positive Electrode

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

Silicon Carbon Negative Electrode

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 has become a powerful substitute for graphite anode.

At present, the use of silicon-carbon composite materials to increase the energy density of batteries is one of the development directions of lithium-ion battery anode materials recognized by the industry.The Model3 released by Tesla uses a silicon-carbon negative electrode. In the future, if you want to go one step further-breaking through the threshold of 350Wh/kg for single cells, peers in the industry may need to focus on lithium metal negative electrode battery systems, but this also means the entire battery production process changes and refinement.

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