Two performance indicators of lithium-ion batteries: energy density and charge-discharge rate

Briefly analyze the two performance indicators of lithium-ion batteries: energy density and charge-discharge rate

Energy density refers to the amount of energy that can be stored per unit volume or weight. Of course, the higher the index, the better. Everything that is concentrated is the essence. The charge and discharge rate is the speed of energy storage and release, preferably in seconds. It is filled or released in an instant, and it can come and go as soon as it is called.

Of course, these are all ideals, and in fact, they are subject to various practical factors. It is impossible for us to obtain infinite energy, nor to realize the instantaneous transfer of energy. How to continuously break through these limitations and reach a higher level is a difficult problem that we need to solve.

(A) The energy density of lithium-ion batteries

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 vehicle, people expect the energy density of the battery to reach a whole new level, so that the battery life or mileage of the product will no longer be the main factor that plagues the product.

From lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, to lithium-ion batteries, the energy density has been continuously improved. However, the speed of improvement is too slow compared to the speed of industrial-scale development and the degree of human demand for energy. Some people even joke that human progress is stuck in the "battery". Of course, if one day global power transmission can be achieved wirelessly, and power can be obtained "wirelessly" anywhere (like a mobile phone signal), then human beings will no longer need batteries, and social development will naturally not be stuck on batteries.

In response to the status quo 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, and Japan basically point to a value of 300Wh/kg, which is equivalent to nearly doubling the current basis. The long-term goal in 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. As far as the existing chemical system and structure of lithium-ion batteries are concerned, what are the obvious limitations?

We have analyzed earlier that what acts as an electrical energy carrier is actually the lithium element in the battery, and other substances are "waste", but to obtain a stable, sustainable and safe electrical energy carrier, these "wastes" are indispensable. . For example, in a lithium-ion battery, the mass proportion of lithium is generally a little more than 1%, and the remaining 99% of the components are other substances that do not undertake energy storage functions. Edison famously said that success is 99% perspiration plus 1% talent. It seems that this principle applies everywhere. 1% is safflower, and the remaining 99% is green leaves.

So to improve the energy density, the first thing we think about is to increase the proportion of lithium elements, and at the same time let as many lithium ions run out from the positive electrode, move to the negative electrode, and then return from the negative electrode to the positive electrode (it can't be less) , the cycle of transporting energy.

1. Increase the proportion of positive active material

Increasing the proportion of positive active materials is mainly to increase the proportion of lithium elements. In the same battery chemical system, the content of lithium elements increases (other conditions remain unchanged), and the energy density will also increase accordingly. So under certain volume and weight constraints, we hope that there are more positive active materials, and more.

2. Increase the proportion of negative active material

This is actually to match the increase of positive active materials, and more negative active materials are needed to accommodate the lithium ions that swim over and store energy. If the active material of the negative electrode is not enough, the extra lithium ions will be deposited on the surface of the negative electrode instead of being embedded inside, resulting in an irreversible chemical reaction and battery capacity decay.

3. Improve the specific capacity (gram capacity) of the cathode material

The proportion of positive active materials has an upper limit and cannot be increased indefinitely. When the total amount of positive active materials is constant, only as many lithium ions as possible can be deintercalated from the positive electrode to participate in chemical reactions, in order to improve the energy density. Therefore, we hope that the mass ratio of the lithium ions that can be deintercalated relative to the positive active material is higher, that is, the specific capacity index is higher.

This is why we research and select different cathode materials, from lithium cobalt oxide to lithium iron phosphate, to ternary materials, all of which are rushing towards this goal.

As previously analyzed, lithium cobalt oxide can reach 137mAh/g, the actual values of lithium manganate and lithium iron phosphate are all around 120mAh/g, and the ternary nickel cobalt manganese can reach 180mAh/g. If we want to improve further, we need to study new cathode materials and make progress in industrialization.

4. Improve the specific capacity of anode materials

Relatively speaking, the specific capacity of the negative electrode material is not the main bottleneck of the energy density of the lithium-ion battery, but if the specific capacity of the negative electrode is further improved, it means that the negative electrode material with less mass can accommodate more lithium ions, thereby achieve the goal of increasing energy density.

Using graphite-like carbon materials as the negative electrode, 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, which are hot directions of current research.

5. Lose weight

In addition to the active materials of 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 proportion of the entire battery weight around 40%. If the weight of these materials can be reduced without compromising the performance of the battery, it could also improve the energy density of lithium-ion batteries.

To make a fuss in this regard, it is necessary to conduct detailed research and analysis on electrolytes, separators, binders, substrates and current collectors, shell materials, manufacturing processes, etc., so as to find a reasonable solution. If all aspects are improved, the overall energy density of the battery can be increased by a certain extent.

From the above analysis, it can be seen that improving the energy density of lithium-ion batteries is a systematic project. We should start from improving the manufacturing process, improving the performance of existing materials, and developing new materials and new chemical systems. and long-term solutions.