What is energy density?

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

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

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

What is the energy density of the monomer?

The energy density of a battery often points to two different concepts, one is the energy density of a single cell, and the other is the energy density of the battery system.

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

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 for power batteries is defined: 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 refers to the energy density of a single cell level.

What is the system energy density?

The system energy density refers to the weight or volume of the entire battery system compared to the weight or volume of the entire battery system after the monomer combination is completed. Because the battery system contains battery management system, thermal management system, high and low voltage circuits, etc. occupy part of the weight and internal space of the battery system, the energy density of the battery system is lower than the energy density of the monomer.

System energy density = battery system power / 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 critical: positive electrode, negative electrode, electrolyte, and diaphragm. The positive and negative poles are the places where chemical reactions take place, which are equivalent to the two veins of Ren and Du, and their important status is evident. 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?

Most of the current anode materials for lithium-ion batteries are graphite, and the theoretical gram capacity of graphite is 372 mAh/g. 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 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. Comparing the two phases, the energy density is high and the difference is 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 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 regarded as one of the reference indicators for the energy density of the battery.

In the fourth episode of "Great Powers II", CATL adopted 6-micron copper foil and used advanced technology to increase the energy density.

If you can stick to each line, read it all the way to here. Congratulations, your understanding of batteries has reached a level.