It can be said that energy density is the biggest bottleneck restricting the development of current lithium-ion batteries. Whether it is a mobile phone or an electric car, people expect the energy density of the battery to reach a new order of magnitude, making the product’s battery life and mileage no longer a problem. The main factor of the product.
From lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, to lithium-ion batteries, energy density has been continuously increasing. However, the speed of improvement is too slow compared to the speed of industrial-scale development and the degree of human demand for energy. Even if some people fine salt, the progress of human beings is stuck on the “battery” to obtain electricity (like a cell phone signal), then humans no longer need batteries, and social development will naturally not be stuck on batteries.
In view of the current situation that energy density has become a bottleneck, countries around the world have formulated relevant battery industry policy goals, hoping to lead the battery industry to achieve significant breakthroughs in energy density. The 2020 goals set by governments or industry organizations in China, the United States, Japan and other countries basically point to 300Wh/kg, or even 700Wh/kg. The battery industry must have a major breakthrough in the chemical system to achieve this goal.
There are many factors that affect the energy density of lithium-ion batteries. As far as the existing chemical system and structure of lithium-ion batteries are concerned, what are the specific limitations?
We have analyzed earlier that what acts as a carrier of electric energy is actually the lithium element in the battery. Other substances are “waste”, but to obtain a stable, continuous and safe electric energy carrier, these “wastes” are indispensable. . For example, in a lithium-ion battery, the mass proportion of lithium is generally a little over 1%, and the remaining 99% are other substances that do not undertake the energy storage function. Edison famously said that success is 99% sweat plus 1% talent. It seems that this principle applies everywhere. 1% is safflower, and the remaining 99% is green leaves. Nothing will work without it.
Then to increase the energy density, the first thing we think of is to increase the proportion of lithium, and at the same time let as many lithium ions as possible escape from the positive electrode, move to the negative electrode, and then return the original number from the negative electrode to the positive electrode (it cannot be reduced) , Carrying energy again and again.
1. Increase the proportion of positive active materials
Increasing the proportion of active materials in the positive electrode is mainly to increase the proportion of lithium. In the same battery chemical system, the content of lithium is increased (other conditions remain unchanged), and the energy density will also be increased accordingly. Therefore, under a certain volume and weight limit, we hope that more positive electrode active materials, and more.
2. Increase the proportion of negative active materials
This is actually in order to cope with the increase in positive active materials, and more negative active materials are needed to accommodate the swimming lithium ions and store energy. If the negative electrode active material is not enough, the extra lithium ions will be deposited on the negative electrode surface instead of being embedded inside, causing irreversible chemical reactions and battery capacity degradation.
3. Increase the specific capacity of the cathode material (g capacity)
The proportion of positive electrode active material has an upper limit and cannot be increased without limit. With a certain total amount of positive electrode active material, only as many lithium ions as possible are extracted from the positive electrode and participate in chemical reactions in order to increase the energy density. Therefore, we hope that the mass ratio of the releasable lithium ions relative to the positive electrode active material will be higher, that is, the specific capacity index will be higher.
This is so fast that we are researching and choosing the reasons for different cathode materials, from lithium cobalt oxide to lithium iron phosphate, to ternary materials, all of which are rushing toward this goal.
As previously analyzed, lithium cobalt oxide can reach 137mAh/g, the actual values of lithium manganate and lithium iron phosphate are both around 120 mAh/g, and nickel cobalt manganese ternary can reach 180mAh/g. If you want to improve further, you need to research new cathode materials and make progress in industrialization.
4. Improve the specific capacity of the negative electrode material
Relatively speaking, the specific capacity of the negative electrode material is not the main bottleneck of the energy density of lithium-ion batteries, but if the specific capacity of the negative electrode is further increased, it means that with less mass of the negative electrode material, more lithium ions can be accommodated, thereby Reach the goal of increasing energy density.
Graphite-like carbon materials are used as the negative electrode, and the theoretical specific capacity is 372mAh/g. The hard carbon materials and nano-carbon materials studied on this basis can increase the specific capacity to more than 600mAh/g. Tin-based and silicon-based anode materials can also increase the specific capacity of the anode to a very high level. These are the hot topics of current research.
5. Weight loss
In addition to the active materials of the positive and negative electrodes, electrolytes, separators, binders, conductive agents, current collectors, substrates, shell materials, etc., are the “dead weight” of lithium-ion batteries, which account for the proportion of the entire battery weight. Around 40%. If the weight of these materials can be reduced without affecting the performance of the battery, the energy density of lithium-ion batteries can also be improved.