Energy density of lithium-ion batteries

It can be said that energy density is the biggest bottleneck limiting the development of lithium-ion batteries. Whether it is a mobile phone or an electric vehicle, people expect the energy density of the battery to reach a new order of magnitude so that the battery life or mileage of the product is no longer the main factor that affects the product.

From lead-acid batteries, to nickel-cadmium batteries, to nickel-metal hydride batteries, to lithium-ion batteries, energy density has been improving all the time. But the rate of improvement is too slow compared to the pace of development on an industrial scale, and compared to the scale of human energy needs. Some people even joke that human progress is stuck in the "battery". Of course, if one day the global wireless transmission of energy can be achieved, with "wireless" access to electricity everywhere (like mobile phone signals), then human beings will no longer need batteries, and social development will of course not be stuck in the battery.

In response to the situation where energy density has become a bottleneck, countries around the world have formulated relevant policy goals for the battery industry, hoping to lead the battery industry to achieve significant breakthroughs in energy density. The 2020 targets set by governments or industry organisations in China, the United States, Japan and other countries basically point to a value of 300Wh/kg, which is almost a doubling on the current basis. The long-term target for 2030 is 500Wh/kg or even 700Wh/kg, and the battery industry will need a major breakthrough in chemistry to achieve this.

There are many factors that affect the energy density of Lithium Batteries, in terms of the existing chemical system and structure of lithium-ion batteries, what are the obvious limitations?

We have analysed before, as a carrier of electric energy, in fact, is the lithium element in the battery, other substances are "waste", but to obtain a stable, sustainable, safe carrier of electric energy, this "waste" is indispensable. For example, in a lithium-ion battery, the mass of lithium is generally a little more than 1%, and the remaining 99% of the components are other substances that do not have the function of storing energy. Edison has a famous saying, success is 99% sweat plus 1% talent, it seems that this truth is universal ah, 1% is red flowers, the remaining 99% is green leaves, which is not.

So to increase the energy density, our first thought is to increase the proportion of lithium elements, while allowing as many lithium ions as possible to run out of the positive electrode, move to the negative electrode, and then have to return from the negative electrode (can not be reduced), the cycle of handling energy.

1. Increase the amount of positive active substances

Increasing the proportion of positive active materials is mainly to increase the proportion of lithium, in the same battery chemistry system, the content of lithium goes up (other conditions remain unchanged), and the energy density will also be correspondingly improved. So, within certain volume and weight limits, we want more positive active material and more active material.

2. Increase the amount of negative active material

This is because, in order to keep up with the increase in positive active material, more negative active material is needed to accommodate the lithium ions that swim over and store energy. If there is not enough negative active material, the extra lithium ions will be deposited on the negative surface instead of being embedded inside, causing irreversible chemical reactions and loss of battery capacity.

3. Improving the specific capacity of the positive electrode material (gram capacity)

The amount of positive active material is limited and cannot be increased indefinitely. For a given amount of positive active material, only as many lithium ions as possible are removed from the positive electrode to participate in chemical reactions to increase the energy density. Therefore, we hope that the mass fraction of removable lithium ions relative to the positive active material is high, i.e. the specific capacity index is high.

This is the reason why we are studying and selecting different cathode materials, from lithium cobaltate to lithium iron phosphate and then to ternary materials, towards this goal.

Previously analysed, lithium cobaltate can reach 137mAh/g, lithium manganate and lithium iron phosphate actual values are about 120mAh/g, nickel cobalt manganese ternary can reach 180mAh/g. If you want to go up, you need to study new cathode materials and make progress in industrialisation.

4. Improving the specific capacity of the anode material

Relatively speaking, the specific capacity of the negative electrode material is not the main bottleneck of the energy density of Lithium Iron Phosphate Battery, but 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, thus achieving the goal of increasing the energy density.

With graphite carbon materials as the negative electrode, the theoretical specific capacity is 372 mAh/g, and the hard carbon materials and nanocarbon materials studied on this basis can increase the specific capacity to more than 600 mAh/g. Tin- and silicon-based anode materials can also increase the specific capacity of the anode to a very high order, which is the current research focus.

5. Weight reduction

In addition to the active materials of the positive and negative electrodes, the electrolyte, insulating film, binder, conductive agent, fluid collector, matrix, casing material, etc. are the "dead weight" of lithium-ion batteries, accounting for about 40% of the weight of the entire battery. If the weight of these materials can be reduced without affecting the performance of the battery, the energy density of Li-ion batteries can be improved.

In this regard, it is necessary to carry out detailed research and analysis on electrolyte, insulating film, binder, matrix and fluid collector, casing material, manufacturing process, etc., in order to find a reasonable programme. If all aspects are improved, the energy density of the battery as a whole can be improved by one degree.

From the above analysis, it can be seen that improving the energy density of lithium-ion batteries is a systematic engineering to improve the manufacturing process, improve the performance of existing materials, and develop new materials and new chemical systems from these aspects, searching for short, medium and long term solutions.