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

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%.

How to increase energy density?

Increase battery size

Battery manufacturers can achieve the effect of capacity expansion by increasing the original battery size. The most familiar example is that Tesla, a well-known electric car company that was the first to use Panasonic's 18650 battery, will replace it with a new 21700 battery.

Chemical system reform

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 greater than that of the positive electrode, increasing the energy density requires continuous upgrading of the positive electrode material.

High nickel cathode

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

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 battery cell. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the amount of cobalt used.

Silicon carbon anode

The specific capacity of silicon-based anode materials can reach 4200mAh/g, which is much higher than the theoretical specific capacity of graphite anodes of 372mAh/g, so it has become a powerful substitute for graphite anodes.

System energy density: improve the efficiency of battery packs

Optimized layout structure: From the aspect of dimensions, the internal layout of the system can be optimized to make the internal components of the battery pack more compact and efficient.

We realize weight reduction design under the premise of ensuring rigidity and structural reliability through simulation calculation. Through this technology, topology optimization and topography optimization can be achieved and ultimately help to achieve lightweight battery cabinets.

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 lightweight effect is obvious.

Integrated vehicle design: The integrated design of the vehicle and the structure design of the vehicle are considered in a comprehensive manner, sharing and sharing structural parts as much as possible, such as anti-collision design, to achieve the ultimate lightweight

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