Two major 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, and everything that is concentrated is the essence. The charge-discharge rate is the speed at which energy is stored and released, preferably in seconds. It is full or released in an instant, and the call comes and goes.

Of course, these are ideals. In fact, they are subject to various realistic factors. We can neither obtain unlimited energy nor achieve instantaneous transfer of energy. How to continuously break through these restrictions and reach a higher level is a problem that needs to be solved by us.

Five, 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 car, people expect the energy density of the battery to reach a whole new order of magnitude, so that the battery life or range of the product will no longer be the main factor that troubles the product.

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

From lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, to lithium-ion batteries, energy density has been continuously increasing. 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 even joked that human progress is stuck in the "battery". Of course, if one day can realize the wireless transmission of global power, and obtain electric energy "wirelessly" everywhere (like a cell phone signal), then humans will no longer need batteries, and social development will naturally not be stuck on batteries.

In view of the current situation 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, Japan and other countries basically point to a value of 300Wh/kg, which is equivalent to an increase of nearly double 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 specific limitations?

We have analyzed earlier that what acts as a carrier of electric energy is actually the lithium element in the battery. Other substances are "waste", but to obtain a stable, continuous and safe electric energy carrier, these "wastes" are indispensable. . For example, in a lithium-ion battery, the mass proportion of lithium is generally a little over 1%, and the remaining 99% are other substances that do not undertake the energy storage function. Edison famously said that success is 99% sweat plus 1% talent. It seems that this principle applies everywhere. 1% is safflower, and the remaining 99% is green leaves. Nothing will work without it.

Then, 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 as possible escape from 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) , Carrying energy again and again.

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 increased (other conditions remain unchanged), and the energy density will also be increased accordingly. Therefore, under a certain volume and weight limit, we hope that more positive electrode active materials, and more.

2. Increase the proportion of negative active materials

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

3. Increase the specific capacity (g capacity) of the cathode material

The proportion of the positive electrode active material has an upper limit and cannot be increased without limit. When the total amount of active material in the positive electrode is constant, only as many lithium ions as possible are extracted from the positive electrode and participate in the chemical reaction, in order to increase the energy density. Therefore, we hope that the mass ratio of the releasable lithium ions to the positive electrode active material will be higher, that is, the specific capacity index will be higher.

This is the reason why we research and choose different cathode materials, from lithium cobalt oxide to lithium iron phosphate, to ternary materials, we are all working towards this goal.

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

4. Increase 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-ion batteries, but if the specific capacity of the negative electrode is further increased, it means that with less mass of the negative electrode material, more lithium ions can be accommodated, thereby Reach the goal of increasing energy density.

Graphite-like carbon materials are used as the negative electrode, and 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. These are the hot topics of current research.

5. reduce weight

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, which account for the proportion of the entire battery weight. Around 40%. If the weight of these materials can be reduced without affecting the performance of the battery, the energy density of lithium-ion batteries can also be improved.It is necessary to conduct detailed research and analysis on electrolytes, isolation membranes, adhesives, substrates and current collectors, shell materials, manufacturing processes, etc., in order to find a reasonable solution. If all aspects are improved, the energy density of the battery can be increased by a certain extent.

From the above analysis, it can be seen that increasing the energy density of lithium-ion batteries is a systematic project. It must start with improving the manufacturing process, improving the performance of existing materials, and developing new materials and new chemical systems, looking for short-term and medium-term And long-term solutions.

6. Charge and discharge rate of lithium-ion battery

The charge-discharge rate of a lithium-ion battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery. Of course, this storage and release process is controllable, safe, and will not significantly affect battery life and other performance indicators.

The magnification index is particularly important when the battery is used as the energy carrier of electric tools, especially electric vehicles. Imagine if you are driving an electric car to work, and you find that the battery is almost out of power halfway, find a charging station and charge it for an hour. If it is not fully charged, it is estimated that everything to be done will be delayed. Or your electric car is climbing a steep slope, no matter how you step on the gas pedal, the car is slow like a tortoise, so you can’t get it up and you can’t wait to get off the cart.

Obviously, the above scenes are what we don't want to see, but it is the current status of lithium-ion batteries. Charging takes a long time and discharging must not be too strong, otherwise the battery will quickly age and even safety problems may occur. However, in many applications, we all need batteries with high-rate charge and discharge performance, so we are stuck here again on the "battery". In order to achieve better development of lithium-ion batteries, it is necessary for us to figure out what factors are limiting the rate performance of the battery.

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

The charge-discharge rate performance of lithium-ion batteries is directly related to the migration ability of lithium ions at the positive and negative electrodes, electrolyte, and the interface between them. All factors that affect the migration speed of lithium ions (these influencing factors can also be equivalent to battery The internal resistance) will affect the charge-discharge rate performance of lithium-ion batteries. In addition, the heat dissipation rate inside the battery is also an important factor affecting the rate performance. If the heat dissipation rate is slow, the heat accumulated during high rate charging and discharging cannot be transferred out, which will seriously affect the safety and life of the lithium ion battery. Therefore, research and improvement of the charge-discharge rate performance of lithium-ion batteries mainly start from two aspects: improving the migration speed of lithium ions and the heat dissipation rate inside the battery.

1. Improve the lithium ion diffusion capacity of the positive and negative electrodes

The deintercalation and intercalation rate of lithium ions in the positive/negative active material, that is, the speed at which lithium ions escape from the positive/negative active material, or enter the active material from the surface of the positive/negative electrode to find a place to "settle home" How fast is the speed? This is an important factor that affects the charge and discharge rate.

The diffusion and movement of lithium ions in the positive/negative poles is basically similar to that of a marathon. There are both slow and fast runners. In addition, the length of the road chosen by each is different, which severely restricts the time for the end of the race (everyone After running). So, we don’t want to run a marathon. It’s best for everyone to run 100 meters. The distance is short enough so that everyone can reach the finish line quickly. In addition, the runway should be wide enough, not crowded with each other, and the road should not be winding and winding. The best is to reduce the difficulty of the game. As a result, the referee made a beep, and thousands of troops rushed to the finish line. The game ended quickly and the magnification performance was excellent.

At the cathode material, we hope that the pole piece should be thin enough, that is, the thickness of the active material should be small, which is equivalent to shortening the distance of the race, so we hope to increase the compaction density of the cathode material as much as possible. Inside the active material, there must be enough pore gaps to allow lithium ions to be used for the competition. At the same time, these "tracks" must be evenly distributed, not where there are some places, and some places are not. This requires optimizing the structure of the positive electrode material. Change the distance and structure between particles to achieve uniform distribution. The above two points are actually contradictory. Increasing the compaction density will reduce the thickness, but the particle gap will become smaller, and the runway will appear crowded. On the contrary, maintaining a certain particle gap is not conducive to making the material thinner. Therefore, it is necessary to find a balance point to achieve the best lithium ion migration rate
The positive electrode materials of different materials have a significant effect on the diffusion coefficient of lithium ions. Therefore, choosing a cathode material with a relatively high lithium ion diffusion coefficient is also an important direction for improving rate performance.

The processing idea of ​​the negative electrode material is similar to that of the positive electrode material. It mainly starts from the structure, size, thickness and other aspects of the material to reduce the concentration difference of lithium ions in the negative electrode material and improve the diffusion capacity of lithium ions in the negative electrode material. Take carbon-based anode materials as an example. In recent years, research on nano-carbon materials (nanotubes, nanowires, nanospheres, etc.), replacing the traditional anode layered structure, can significantly improve the specific surface area, internal structure and Diffusion channel, thereby greatly improving the rate performance of the negative electrode material.

2. Improve the ionic conductivity of the electrolyte

Lithium ions play a race in the positive/negative material, but swim in the electrolyte.

In swimming competitions, how to reduce the resistance of water (electrolyte) has become the key to speed improvement. In recent years, swimmers generally wear shark suits. This type of swimsuit can greatly reduce the resistance formed by water on the surface of the human body, thereby improving the athlete's competition performance, and has become a very controversial topic.

Lithium ions need to shuttle back and forth between the positive and negative electrodes, just like swimming in a "swimming pool" formed by the electrolyte and the battery case. The ionic conductivity of the electrolyte is like the resistance of water, and the swimming speed of lithium ions is very large. Influence. At present, the organic electrolytes used in lithium-ion batteries, no matter whether it is a liquid electrolyte or a solid electrolyte, the ionic conductivity is not very high. The resistance of the electrolyte has become an important part of the overall battery resistance, and its impact on the high-rate performance of lithium-ion batteries cannot be ignored.

In addition to improving the ionic conductivity of the electrolyte, it is also necessary to focus on the chemical stability and thermal stability of the electrolyte. When charging and discharging at a high rate, the electrochemical window of the battery has a very wide range. If the chemical stability of the electrolyte is not good, it is easy to oxidize and decompose on the surface of the positive electrode material, which affects the ionic conductivity of the electrolyte. The thermal stability of the electrolyte has a great impact on the safety and cycle life of lithium-ion batteries, because a lot of gas is generated when the electrolyte is thermally decomposed. On the one hand, it poses a hidden danger to battery safety. The SEI film has a destructive effect and affects its cycle performance.

Therefore, choosing an electrolyte with high lithium ion conductivity, good chemical and thermal stability, and matching the electrode material is an important direction to improve the rate performance of lithium ion batteries.

3. Reduce the internal resistance of the battery

This involves several different substances and the interface between substances, and the resistance value they form, but all have an impact on the conduction of ions/electrons.

Generally, a conductive agent is added to the positive electrode active material to reduce the contact resistance between the active materials, the active material and the positive electrode matrix/current collector, improve the electrical conductivity (ionic and electronic conductivity) of the positive electrode material, and enhance the rate performance. Conductive agents of different materials and shapes will all affect the internal resistance of the battery, thereby affecting its rate performance.

The current collectors (tabs) of the positive and negative electrodes are the carrier for the lithium-ion battery to transfer electric energy to the outside world, and the resistance value of the current collector also has a great influence on the rate performance of the battery. Therefore, by changing the material, size, extraction method, connection process, etc. of the current collector, the rate performance and cycle life of the lithium-ion battery can be improved.

The degree of infiltration between the electrolyte and the positive and negative materials will affect the contact resistance at the interface between the electrolyte and the electrode, thereby affecting the rate performance of the battery. The total amount of electrolyte, viscosity, impurity content, and the pores of the positive and negative materials will change the contact impedance between the electrolyte and the electrode, which is an important research direction for improving the rate performance.

During the first cycle of lithium ion batteries, as lithium ions are inserted into the negative electrode, a solid electrolyte (SEI) film will be formed on the negative electrode. Although the SEI film has good ionic conductivity, it will still diffuse lithium ions. There is a certain hindrance, especially when charging and discharging at a high rate. As the number of cycles increases, the SEI film will continue to fall off, peel off, and deposit on the surface of the negative electrode, causing the internal resistance of the negative electrode to gradually increase, which becomes a factor affecting the cycle rate performance. Therefore, controlling the change of the SEI film can also improve the rate performance of the lithium-ion battery during long-term cycling.

In addition, the liquid absorption and porosity of the separator also have a greater impact on the passability of lithium ions, and will also affect the rate performance (relatively small) of the lithium ion battery to a certain extent.

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