Energy density analysis of lithium-ion batteries

It can be said that energy density is the biggest bottleneck that restricts the development of lithium ion batteries. Whether it's a mobile phone or an electric car, people expect the battery's energy density to reach a completely new level, so that product life time or mileage is no longer the main factor that troubles the product.

From lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, to lithium-ion batteries, energy density has been increasing. However, the speed of increase relative to the speed of development of the industrial scale, relative to the degree of human demand for energy, is too slow. Some people even argue that human progress is stuck in the "battery" here. Of course, if the world were to be able to transmit electricity wirelessly and wirelessly anywhere(like cell phone signals), then humans would no longer need batteries, and social development would naturally not be stuck on them.

In view of the fact that energy density has become a bottleneck, all countries in the world have formulated relevant battery industry policy goals and expect to lead the battery industry to make significant breakthroughs in energy density. The 2020 targets set by governments or industry organizations such as China, the United States, and Japan basically point to the value of 300 Wh/kg, which is equivalent to nearly double the current level. The long-term goal for 2030 is to reach 500 Wh / kg, or even 700 Wh / kg. The battery industry must have a major breakthrough in the chemical system before it is possible to achieve this goal.

There are many factors that affect the energy density of lithium-ion batteries. What are the obvious limitations on the existing chemical systems and structures of lithium-ion batteries?

What we have analyzed before is that the lithium element in the battery is actually used as an electric energy carrier. Other substances are "waste", but they are to obtain stable, continuous, and safe electric energy carriers. These "wastes" are also indispensable. of. For example, in a lithium ion battery, the quality of lithium is generally a little more than 1 %, and the remaining 99 % of the components are other substances that do not perform energy storage functions. Edison had a famous saying that success is 99 % sweat plus 1 % talent. It seems that this truth is universally correct. 1 % is red flower, and the remaining 99 % of green leaves are no less.

So to increase the energy density, the first thing we want to do is to increase the proportion of lithium, while allowing as many lithium ions as possible to run from the positive pole, move to the negative pole, and then return from the negative pole number to the positive pole(can't be reduced).), Constantly moving energy.

1. Increase the proportion of positive polar active substances

The increase in the proportion of positively active substances is mainly to increase the proportion of lithium. In the same battery chemical system, the content of lithium is increased(other conditions are unchanged), and the energy density will also be correspondingly increased. Therefore, under certain volume and weight restrictions, we hope that there are more positive polar active substances and more.

2. Increase the proportion of negatively active substances

This is actually to match the increase in positive polar active substances, and more negative polar active substances are needed to accommodate the lithium ions that swim over and store energy. If the negative electrode is not active enough, the additional lithium ions will be deposited on the negative electrode surface rather than embedded inside, with irreversible chemical reactions and battery capacity attenuation.

3. Increase the specific capacity(in grams) of positive material

The proportion of positive polar active substances is limited and can not be increased indefinitely. Under the condition that the total amount of positive polar active substances is certain, only as many lithium ions as possible can be deem bedded from the positive pole and participate in chemical reactions to increase the energy density. Therefore, we hope that the quality of deem bedded lithium ions relative to positive polar active substances is higher, that is, higher than the capacity index.

This is why we study and choose different positive materials, from lithium cobalt acid to lithium iron phosphate to ternary materials, all geared towards this goal.

As previously analyzed, lithium cobalt acid can reach 137mAh/g, lithium manganese acid and lithium iron phosphate have actual values of about 120mAh/g, and nickel cobalt manganese can reach 180mAh/g. If we want to upgrade, we need to study new positive materials and make progress in industrialization.

4. Increase 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 the lithium ion battery. However, if the specific capacity of the negative electrode is further increased, it means that more lithium ions can be accommodated with less mass of the negative electrode material. To achieve the goal of increasing energy density.

Using graphite carbon materials as negative poles, the theoretical specific capacity is 372 mAh/g. The hard carbon materials and nano carbon materials studied on this basis can increase the specific capacity to 600 mAh/g or more. Tin-and silicon-based negative electrode materials can also increase the specific capacity of the negative pole to a very high level, which is a hot research direction.

5. Lose weight

In addition to the positive and negative active substances, electrolytes, isolation membranes, binders, conductive agents, collecting fluids, matrices, shell materials, etc., are all "dead weights" of lithium ion batteries, accounting for the entire battery weight. About 40 %. If you can reduce the weight of these materials without affecting the performance of the battery, then you can also increase the energy density of lithium ion batteries.

In this regard, we need to study and analyze in detail the electrolyte, isolation film, binder, Matrix and collection fluid, shell material, manufacturing process, etc., in order to find a reasonable solution. By improving all aspects, the overall energy density of the battery can be increased by one amplitude.

From the above analysis, it can be seen that increasing the energy density of lithium-ion batteries is a systematic project. It is necessary to find short-, medium-and long-term solutions from the aspects of improving the manufacturing process, improving the performance of existing materials, and developing new materials and new chemical systems..

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