Feb 22, 2019 Pageview:681
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 level, so that the battery life or cruising range is no longer a major factor plaguing 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 ascension is too slow compared to the degree of human demand for energy, relative to the speed of industrial scale. Some people even joked that human progress has been stuck in the "battery". Of course, if one day can achieve global wireless power transmission, wherever you can "wirelessly" get power (like a cell phone signal), then humans no longer need a battery, and social development will naturally not get stuck on the battery.
In view of the current situation that energy density has become a bottleneck. Countries around the world have formulated relevant battery industry policy goals, and expect to lead the battery industry to achieve significant breakthroughs in energy density. The 2020 targets set by the governments or industry organizations of China, the United States, Japan and other countries basically point to the value of 300Wh/kg, which is equivalent to nearly double the current level. The long-term goal of 2030 is to reach 500Wh/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. What are the obvious limitations on the existing chemical systems and structures of lithium-ion batteries?
As we analyzed earlier, the role of the electric energy carrier is actually the lithium element in the battery. Other substances are “waste”, but to obtain a stable, continuous and safe electric energy carrier, these “waste” are indispensable. . For example, in a lithium-ion battery, the mass of lithium is generally more than 1%, and the remaining 99% are other substances that do not bear energy storage. Edison has a famous saying that success is 99% sweat plus 1% talent. It seems that this principle is universal. 1% is safflower, and the remaining 99% is green leaves.
So 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 run out of the positive electrode, move to the negative electrode, and then return to the positive electrode from the original number of the negative electrode (cannot be reduced) the re-transportation of energy.
1. Increase the proportion of positive active materials
Increasing the proportion of positive active materials is mainly to increase the proportion of lithium. In the same battery chemical system, the content of lithium is gone up (other conditions are unchanged), and the energy density will be correspondingly improved. Therefore, under certain volume and weight restrictions, we hope that the positive active material will be more and more.
2. Increase the proportion of negative active materials
This is actually to increase the amount of positive active material, and it needs more negative active material to accommodate the lithium ions that swim and store energy. If the negative active material is insufficient, the extra lithium ions will deposit on the surface of the negative electrode instead of being embedded inside, causing irreversible chemical reactions and battery capacity decay.
3. Improve the specific capacity of the positive electrode material (gram capacity)
The proportion of positive active substances has an upper limit and cannot be increased without limit. In the case of a certain amount of positive active substances, the energy density can be increased only when as many lithium ions as possible are removed from the positive electrode and participate in the chemical reaction. So we want to have a high percentage of removable lithium ions relative to the active anode, which is higher than the capacity index.
This is why we have studied and selected different cathode materials, from lithium cobaltate to lithium iron phosphate to ternary materials.
As previously analyzed, lithium cobaltate can reach 137mAh/g, the actual values of lithium manganate and lithium iron phosphate are all around 120mAh/g, and nickel-cobalt-manganese ternary can reach 180mAh/g. If you want to go up again, you need to research new cathode materials and make progress in industrialization.
4. Improve the specific capacity of the anode material
In contrast, the specific capacity of the anode material is not the main bottleneck of the energy density of the lithium ion battery, but if the specific capacity of the anode is further increased, it means that the cathode material with less mass can accommodate more lithium ions. Achieve the goal of increasing energy density.
The graphite carbon material is used as the negative electrode, and the theoretical specific capacity is 372mAh/g. On the basis of the hard carbon material and the Nano carbon material, the specific capacity can be increased to 600mAh/g or more. Tin-based and silicon-based anode materials can also increase the specific capacity of the negative electrode to a very high level, which is the hot spot 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, accounting for the weight of the entire battery at around 40%. If the weight of these materials can be reduced without affecting the performance of the battery, the energy density of the lithium ion battery can also be increased.
To make a fuss about this, it is necessary to conduct detailed research and analysis on electrolytes, separators, binders, substrates and current collectors, shell materials, manufacturing processes, etc., in order to find a reasonable solution. Improvements in all aspects can increase the energy density of the battery by a whole amount.
From the above analysis, it can be seen that improving the energy density of lithium-ion batteries is a systematic project. Starting from improving manufacturing processes, improving the performance of existing materials, and developing new materials and new chemical systems, look for short-term and medium-term and long-term solutions.
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