As new energy vehicles, energy storage, communications, and data centers have developed rapidly in recent years, the development of large-capacity lithium-ion batteries has greatly increased. Lithium-ion batteries have faced higher energy density requirements in various fields.
The active energy storage material of lithium-ion batteries is a positive electrode material and a negative electrode material. A higher discharge voltage and discharge capacity increase the energy density of the positive electrode. A high capacity and low lithium removal voltage are the characteristics of negative electrode materials.
Positive and negative electrode materials are currently being upgraded in the third-generation lithium-ion batteries with the main development goal of improving energy density. Further increasing energy density will lead to the development of batteries with lithium metal negative electrodes in the future.
What is Battery Energy Density?
As a proportion to a battery’s weight, energy density is a measure of its energy density. A watt-hour is a unit of electrical energy equivalent to the consumption of one watt for an hour. A watt-hour is typically expressed as Watt hours per kilogram (Wh/kg).
As opposed to energy density, power density is a measure of how quickly energy can be delivered, not how much energy is stored.
A battery’s energy density is the electrical energy released by the average unit volume or mass of the battery in relation to the amount of energy stored in it.
Batteries have two dimensions of energy density: weight and volume.
How to calculate the energy density of lithium batteries?
Energy density (Wh/L) = battery capacity ×discharge platform voltage/ volume the basic unit is Wh/L
Battery weight Energy density = battery capacity × discharge platform/ weight the basic unit is Wh/kg
The platform voltage of iron batteries: 3.2V; the platform voltage of ternary lithium batteries is generally 3.7V.
Prismatic or other volume = length×width ×height
The greater the energy density of the battery, the more power is stored per unit volume or weight.
Do you know energy density of these rechargeable batteries?
We can see from the chart above that lithium cells have the highest energy density, which is why they are widely used all over the world and can be used in a variety of applications.
What exactly limits the energy density of lithium batteries?
The main reason is the battery’s chemical system.
There are four major components of a lithium battery: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. As the second vein of the governor, the positive and negative poles are the places where chemical reactions occur, and their importance can be seen.
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 so?
The most commonly used material for lithium ion battery anodes is graphite, which has a theoretical gram capacity of 372mAh/g. The cathode material’s theoretical gram capacity – lithium iron phosphate – is significantly lower at 160mAh/g, with a voltage platform of 3.2V, while the ternary material nickel-cobalt-manganese (NCM) contains approximately 200mAh/G and a voltage platform of 3.7V. Applying the barrel theory, which implies that the water level is determined by the shortest part of the barrel, it is evident that the lower limit of energy density in lithium-ion batteries depends on the cathode material; this can specifically be seen when comparing the two mentioned phases—thereby producing a difference of 16% in energy density.
Energy density will also be affected by the level of production technology, including compaction density and foil thickness, in addition to the chemical system. Generally, the greater the density of the main material, the greater the capacity of the battery in a limited space, so the density of the main material is also regarded as one of the reference indicators of battery energy density.
In the fourth episode of “Great Heavy Weapon II”, the Ningde Era used 6 micron copper foil to increase energy density.
How to raise Energy Density of Lithium Battery?
Introducing the new material system, fine-tuning the lithium battery structure, and improving manufacturing capabilities are three stages for R&D engineers to “dance with long sleeves”.
Chemical breakthroughs mainly responsible for increasing monomer energy density
01 Increase Battery size
It is possible for battery manufacturers to achieve the effect of power expansion by increasing the size of their original batteries. Tesla, the well-known electric vehicle company that pioneered the use of Panasonic 18650 batteries, will be replacing its batteries with 21700 batteries.
A battery’s fatness or lengthening, however, is a symptom, not a cure. In order to figure out how to draw the bottom of the kettle, you need to find the key technology to improve the energy density of the positive and negative electrodes that make up the battery cell, as well as the electrolyte composition.
02 Change Chemical system
It has been mentioned earlier that the battery’s energy density depends on its positive and negative electrodes. In order to increase the energy density of the cathode, it is necessary to continuously upgrade the cathode material since the current anode material has a much higher energy density than the cathode material.
High nickel positive electrode
Generally, ternary materials are lithium nickel-cobalt-manganese oxide oxides. The ratio of nickel, cobalt, and manganese can affect the battery’s performance.
As can be seen from several typical ternary materials in Figure 5, the proportion of nickel is getting higher and higher, while the proportion of cobalt is getting lower and lower. Increasing the proportion of nickel will increase the specific capacity of the cell, as well as reducing the use of cobalt due to a scarcity of cobalt resources.
Silicon carbon negative electrode
It has become a powerful substitute for graphite anodes because it has a specific capacity of 4200mAh/g, which is much higher than the theoretical specific capacity of 372mAh/g.
Anodes for lithium-ion batteries are increasingly being made from silicon-carbon composite materials to increase their energy density. Tesla’s Model 3 uses silicon-carbon negative electrodes.
It is likely that peers in the industry will have to focus on lithium metal negative electrode battery systems in the future if they want to break through the 350Wh/kg threshold for single cells, but this also means changing and refining the entire production process of batteries.
03 System energy density: improve the grouping efficiency of battery packs
Batteries are grouped to test the ability of the battery “siege lions” to line up single batteries and modules while taking safety into account.
To “slim down” the battery pack, there are mainly two ways.
Optimize the arrangement structure
A more compact and efficient battery pack can be achieved by optimizing the internal layout of the system.
As a result of simulations and calculations, we achieve a weight reduction design that ensures structural stability and rigidity. With this technology, topology optimization and morphology optimization can be achieved, resulting in a lighter battery box.
We can choose low-density materials, such as the upper cover of the battery pack, which gradually changed from the traditional sheet metal upper cover to the composite upper cover, which can reduce weight by about 35%. It has gradually changed the upper box from a sheet metal to an aluminum profile scheme, reducing weight by about 40%, making the battery pack appear lighter.