How to increase lithium batteries’ energy density?

lithium batteries’ energy density

Table of Contents

What is the battery energy density?

The energy density of a battery is the electrical energy released per unit volume or mass of the battery. The greater the energy density of the battery, the more power can be stored per unit volume. It is an important indicator of evaluating battery performance and is crucial for the development of electric vehicles, mobile devices, and renewable energy.

What factors determine battery energy density?

The battery’s energy density is determined by the positive and negative electrodes of the battery, but only the positive and negative active materials cannot guarantee that the battery can generate electricity. There must be many inactive substances, such as conductive auxiliary agents, binders between active powders, and isolation. Membrane, cathode and anode foil, insulating and fixing tape, aluminum-plastic film shell or steel aluminum shell, etc.

What limits lithium batteries' energy density?

The battery chemistry is the main reason. Generally speaking, the four parts of a lithium battery are very critical: positive electrode, negative electrode, electrolyte, and diaphragm. The positive and negative electrodes are where chemical reactions occur, equivalent to the Ren and Du meridians, and their importance is evident. For example, the energy density of a battery pack system using ternary lithium as the positive electrode is higher than that of a battery pack system using lithium iron phosphate as the positive electrode. Why is this? Existing lithium-ion battery anode materials are mostly graphite, and the theoretical gram capacity of graphite is 370mAh/g. The theoretical gram capacity of the cathode material lithium iron phosphate is only 159mAh/g, while the ternary material nickel cobalt manganese (NCM) is about 201mAh/g. According to barrel theory, the water level is determined by the shortest part of the barrel, and the lower limit of lithium-ion batteries’ energy density depends on the cathode material. The voltage platform of lithium iron phosphate is 3.2V, while the voltage platform of ternary is 3.7V. Compared with the two, the energy density differs by about 16%. 

Performances-of-different-Li-ion-battery-technologies

In addition to the chemical system, the level of production technology such as compaction density, foil thickness, etc. will also affect the energy density. Generally, the greater the compaction density, the higher the battery capacity in a limited space, so the compaction density of the main material is also regarded as one of the reference indicators of battery energy density.

For lithium battery manufacturers, how can they improve energy density?

The adoption of novel material systems, fine-tuning of lithium battery structures, and improvement of manufacturing capabilities are three key directions that R&D engineers focus on.

Putting them into practice includes the following aspects:

Increase the battery size

Battery manufacturers can expand power by increasing the original battery size. The example we are most familiar with is: Tesla, the well-known electric car company that was the first to use Panasonic’s 18650 batteries, will replace it with the new 21700 batteries. However, “getting fatter” or “growing” in battery cells only treats the symptoms, not the root cause. The way to get rid of the problem is to find the key technology to improve energy density from the positive and negative electrode materials and electrolyte components that make up the battery unit.

Improve the battery chemical system

As mentioned earlier, the energy density of the battery is limited by the positive and negative electrodes of the battery. Since the current energy density of negative electrode materials is much greater than that of positive electrodes, increasing energy density requires continuous upgrading of positive electrode materials.

Increase the proportion of positive active material

Increasing the proportion of positive active material is mainly to increase the proportion of lithium elements. In the same battery chemical system, as the content of lithium elements increases (other conditions remain unchanged), the energy density will also increase accordingly. Therefore, within certain volume and weight constraints, we hope to have more positive active materials, and then some.

Increase the proportion of negative active material

This is actually to cope with the increase in positive active materials, which requires more negative active materials to accommodate swimming lithium ions and store energy. If the negative active material is not enough, the excess lithium ions will be deposited on the surface of the negative electrode instead of being embedded inside, causing irreversible chemical reactions and battery capacity fading.

Increase the specific capacity (gram capacity) of cathode materials

The proportion of positive active material has an upper limit and cannot be increased without a limit. When the total amount of positive active material is constant, only as many lithium ions as possible can be deintercalated from the positive electrode and participate in chemical reactions can the energy density be increased. Therefore, we hope that the mass proportion of deintercalable lithium ions relative to the cathode active material should be high, that is, the specific capacity index should be high. This is why we study and select different cathode materials, from lithium cobalt oxide to lithium iron phosphate to ternary materials, all with this goal in mind. As previously analyzed, lithium cobalt oxide can reach 137mAh/g, the actual values of lithium manganate and lithium iron phosphate are around 120mAh/g, and nickel cobalt and manganese ternary can reach 180mAh/g. If we want to improve further, we need to research new cathode materials and make progress in industrialization.

Improve the specific capacity of negative electrode materials

Relatively speaking, the specific capacity of the negative electrode material is not the main bottleneck in lithium-ion batteries’ energy density. However, if the specific capacity of the negative electrode is further increased, it means that the negative electrode material with less mass can accommodate more lithium ions, thus achieving the goal of increasing energy density. Using graphite-like carbon materials as negative electrodes, the theoretical specific capacity is 372mAh/g. On this basis, the hard carbon materials and nanocarbon materials studied can increase the specific capacity to more than 600mAh/g. Tin-based and silicon-based negative electrode materials can also increase the specific capacity of the negative electrode to a very high level. These are current hot research directions.

Lose weight and lose weight

In addition to the active materials of the positive and negative electrodes, the electrolyte, isolation film, binder, conductive agent, current collector, matrix, case material, etc. are all the “dead weight” of the lithium-ion battery, accounting for the proportion of the entire battery weight. Around 40%. If the weight of these materials can be reduced without affecting battery performance, the energy density of lithium-ion batteries can also be increased.

From the above analysis, it can be seen that improving lithium-ion batteries’ energy density is a systematic project. If lithium battery manufacturers can improve in each of the above aspects, the overall energy density of the battery can be increased to a certain extent, thereby finding short-term solutions. , mid-term, and long-term solutions.

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