Analysis of the current development of lithium-ion batteries

1, what is the current industry's more recognized line of development of lithium electricity?

Through the unremitting efforts of research and development personnel and engineers, from lead-acid batteries, nickel-metal hydride batteries, nickel-cadmium batteries, to lithium iron phosphate batteries, and now the mainstream three-yuan batteries, every time the promotion is a generation of efforts. Based on improving the safety, energy density and ratio performance of lithium-ion batteries, and combining with the current status of battery development, a development route of lithium-ion batteries in the future is summarized.

In 2020, it is a multi-cation electrode, mainly NCM and NCA composite cathode materials, and the negative electrode is mainly C and some Silicon carbon complexes. The energy density is about 300-350 WH / kg.

From 2020 to 2025, it is dominated by all-solid lithium-ion batteries, and lithium metal negative poles or Silicon carbon negative poles. The energy density is 400wh / kg, while sodium-ion batteries are developed. Sodium is cheaper than lithium, but it is larger than lithium ions and has liquid memory.

After 2025, lithium-sulfur batteries are mainly used-& GT; Lithium metal batteries-& GT; & GT; Lithium-air batteries are mainly developed. Such batteries have a higher energy density and the desirability of materials is becoming more and more convenient. However, there are currently more difficulties and it is necessary to continue to overcome. Lithium-sulfur batteries use sulfur as the positive electrode of the battery. Lithium as a negative lithium battery. The elemental sulfur is rich in reserves in the earth and has the characteristics of low price and friendly environment. Lithium-sulfur batteries using sulfur as a positive material have higher material theory than capacity and battery theory that energy, reaching 1675 mAh/g and 2600 Wh / kg, respectively, which is much higher than commercially widely used ternary batteries.

And sulfur is an environmentally friendly element, basically no pollution to the environment is a very promising lithium battery; Lithium metal batteries, which replace graphite with lithium metal foil, can contain more ions, but usually, lithium metal foil reacts negatively with electrolytes, causing electrolytes to overheat and even cause combustion. This technology can reduce the current lithium batteries. The size is reduced by half. In theory, if the battery volume is unchanged, the range of electric vehicles with lithium metal batteries will double; Lithium-air battery is a battery that uses lithium as an anode and uses oxygen in the air as a cathode reactant. Lithium-air batteries have a higher energy density than lithium-ion batteries because their cathode(mainly porous carbon) is very light, and oxygen is obtained from the environment without being stored in the battery. Theoretically, oxygen is not limited as a cathode reactant., The battery's capacity depends only on the lithium electrode, and its specific energy is 5.21 kWh / kg (including oxygen quality), or 11.4 kWh / kg(excluding oxygen).

2, what are the basic requirements of energy carriers?

(1) The relative mass of atoms is small;

(2) The ability to gain and lose electrons is strong;

(3) The proportion of electronic transfer should be high.

3, what are the main indicators of batteries?

(1) Capacity;

(2) Energy density;

(3) charge and discharge ratio;

(4) Voltage;

(5) Life expectancy;

(6) Internal resistance;

(7) Self-discharge;

(8) Operating temperature range.

4, What are the properties of positive materials(LFP, NCM, LiCo, etc.)?

(1) Higher Redox reaction potential, high output voltage;

(2) High lithium content and high energy density;

(3) Structural stability in chemical reactions;

(4) High conductivity;

(5) Good chemical stability and thermal stability, not easy to decompose and react;

(6) Cheap prices;

(7) The production process is relatively simple and suitable for large-scale production;

(8) Environmentally friendly and low pollution.

5, what are the characteristics of negative materials(Li, C, AL, lithium titanate, etc.)?

(1) Layered structures or tunnel structures that facilitate deembedding;

(2) Stable structure, good reversibility of charge and discharge and cyclic performance;

(3) as many lithium ions as possible are inserted and deembedded;

(4) Low Redox potential;

(5) The first irreversible discharge capacity is low;

(6) Good compatibility with electrolyte solvents;

(7) Low price and easy access to materials;

(8) Good safety;

(9) The environment is friendly.

6. What are the ways to increase battery energy density?

(1) Increase the proportion of positive and negative active substances;

(2) Increase the specific capacity(in grams) of positive and negative polar materials;

(3) Lose weight and slim down.

7, how to improve the charge and discharge ratio of lithium-ion batteries?

(1) Improve the lithium ion diffusion capability of the positive and negative poles;

(2) Improving the Ionic conductivity of electrolytes;

(3) The reduced internal resistance of the battery(ohmic internal resistance and polarization internal resistance).

8. What factors influence the cycle life of lithium-ion batteries?

(1) Negative metal lithium deposition;

(2) Decomposition of positive polar materials;

(3) The formation and re-consumption of SEI;

(4) The influence of electrolytes is mainly manifested in: the total amount is reduced, impurities are present, and water infiltrates;

(5) diaphragm obstruction or destruction;

(6) The positive and negative materials fall off;

(7) External use factors.

9, the internal material reaction decomposition temperature of lithium-ion batteries?

(1) SEI membrane decomposition, electrolyte exothermic reaction, 130 °C;

(2) Electrolyte decomposition, heat production, 130 °C -250 °C;

(3) Positive material decomposition produces a large amount of gas and oxygen, 180 °C -500 °C;

(4) The reaction of binders and negative polar active substances, 240 °C -290 °C.

Generally due to overcharging, high-power discharge, internal short circuit, external short circuit, vibration, collision, fall, impact, etc., causing a short circuit, a process that produces a large amount of heat and gas.

Some of the most promising lithium battery materials of the future

(1) Silicon-carbon composite negative electrode material, high energy density, industrialization 400 WH / kg or more, but serious volume expansion, poor circulation;

(2) Lithium titanate, more than 10,000 cycles, the volume change of 1 %, no dendrite formation, excellent stability, rapid charging, but high price, low energy density, about 170 WH / kg;

(3) Graphene, which can be used in cathode materials and positive additives, has excellent conductivity, fast ion transfer, poor first effect, about 65 %, poor circulation, and high price;

(4) Lithium-rich manganese batteries, with an energy density of about 900wh/kg, are rich in raw materials but have low primary effects, low safety, and poor circulation, and low magnification performance;

(5) NCM ternary materials, generally at 250 Wh/kg, with a silica negative electrode, about 350 Wh/kg;

(6) CNTs, carbon nanotubes, superior electrical conductivity, excellent thermal conductivity;

(7) Coating the diaphragm, basement + PVDF + Bomushi, improving membrane resistance to contractility, low heat conduction, prevent all heat out of control;

(8) High voltage electrolyte, this goes without saying, with the energy density of the energy material, the voltage also increases accordingly;

(9) Waterborne binders, for environmental protection and health.

Prelithiation. Before we talk about this, let's talk about the first effects of semi-batteries (positive polar materials, negative metallic lithium tablets) and full-battery.

This is the first effect of lithium cobalt acid half battery. If you do not understand that the whole battery and half battery are not the same, you understand that this is the first effect of positive material.

The first charge capacity of the semi-battery is slightly higher than the first discharge capacity, that is, lithium ions that are deembedded from the positive pole when charging, and not 100 % return to the positive pole when discharging. The first discharge volume/first charge capacity is the first efficiency of this semi-battery.

The first efficiency of the ternary is the lowest, generally 85 to 88 %; Lithium cobalt acid is second, generally, 94 ~ 96 %; lithium iron phosphate is slightly higher than lithium cobalt phosphate, which is 95 % to 97 %. The first effect of the cathode material is mainly due to the change in the structure of the cathode material after de-embedding. There is not enough lithium placement and lithium ions can not return at the first discharge.

The difference between a graphite battery half-battery and a positive pole is that graphite is a positive electrode and a metallic lithium sheet is a negative electrode. Therefore, the first effect of graphite is significantly lower than that of positive material. The main reason is that lithium ions pass through the electrolyte. The SEI film will be formed on the surface of graphite. It consumes a lot of lithium ions. The lithium ions dedicated to the SEI film cannot return to the negative pole.

The first efficiency of the whole battery, after the battery is injected with liquid, it needs to go through the process of converting(charging only) and dividing capacity(with charging and discharging). In general, the first step of converting and dividing capacity is the charging process. The sum of the two capacities is the first time the whole battery is filled with capacity; The second step of the capacity separation step is generally to discharge from the full state to the air, so this step capacity is the discharge capacity of the full battery. Combining the two, an algorithm for the first efficiency of the whole battery is obtained:

Full battery first efficiency = capacity second stage discharge capacity /(converted into filling capacity + capacity first stage filling capacity)

In order to reduce deviations in daily life, the second complete discharge capacity is taken as the battery capacity.

To sum up, we can draw a conclusion. If the battery positive uses a ternary material with an initial efficiency of 88 %, the negative electrode uses a graphite material with an initial efficiency of 92 %. For this full battery, the first efficiency is 88 %, that is, when the positive pole effect is 88 % and the negative pole effect is 92 %, the first effect of the full battery is 88 %, which is equal to the lower positive pole.

Of course, in addition to the effect of battery materials on the first effect, the specific surface area of electrode materials is also an important factor. The larger the specific surface area of graphite, the larger the SEI membrane formed, the more lithium ions need to be consumed, and the lower the first effect. In addition, it is also related to the battery's conversion into a charging system, and filling in the appropriate SOC will also affect the battery's first effect to some extent.

For a full battery, the SEI film formed at the interface of the negative electrode during the formation of consumes lithium ions deintercalated from the positive electrode and reduces the capacity of the battery. If we can find a lithium source from the outside of the positive electrode material, the formation of the SEI film consumes the lithium ion of the external lithium source, so that the lithium ion of the positive electrode deintercalation is not wasted in the formation process, and finally, the full battery can be improved. capacity. This process of providing an external lithium source is pre-lithiation.

I will borrow a piece of article to tell you about the main pre-lithification method, and I have only seen one, is a negative spray lithium powder method.

1, negative pole into the method in advance

We can separate the negative electrode into a negative electrode and then assemble it with the positive electrode after the negative electrode forms the SEI membrane. This can avoid the loss of the pair of polar lithium ions and greatly increase the first efficiency and capacity of the whole battery.

Negative plates and lithium plates are soaked in electrolytes and charged with external electrical connections. In this way, it can be ensured that the lithium ions consumed at the time of conversion are derived from metallic lithium plates rather than positive poles. After the negative electrode is finished, it is assembled with the positive electrode. The core does not need to be converted again so that the lithium ion of the positive pole will not be lost due to the formation of the negative electrode into the SEI membrane, and the capacity will be significantly increased.

The advantage of this pre-lithification method is that it can be normalized into a process with maximum simulation while ensuring that the formation effect of the SEI membrane is similar to that of the whole battery. However, the two processes of the early conversion of negative polar films and the assembly of positive and negative polar films are too difficult to operate.

2, negative electrode spraying lithium powder method

Since it is difficult to operate lithium by using negative electrode tablets alone, people think of lithium supplementation methods that spray lithium powder directly on negative electrode tablets. First, a stable metallic lithium powder particle is produced. The inner layer of the particle is metallic lithium, and the outer layer is a protective layer with good lithium ion conductivity and electron conductivity. During the prelithification process, the lithium powder is first dispersed in an organic solvent, then the dispersed body is sprayed on a negative electrode, and then the residual organic solvent on the negative electrode is dried so that a prelithified negative electrode is obtained. Subsequent assembly work is consistent with normal processes.

When it is formed, lithium powder sprayed on the negative electrode will be consumed in the formation of the SEI membrane, so as to maximize the retention of lithium ions removed from the positive electrode and increase the capacity of the whole battery.

The disadvantage of adopting this pre-lithification method is that safety is difficult to guarantee and the material and equipment are expensive to reform.

3, negative three-layer electrode method

Due to the limitations of equipment and processes, high-cost transformation for pre-lithification is not a priority for battery plants. If pre-lithification can be completed in a familiar manner in battery plants, then the generalization will be greatly enhanced. The three-layer electrode method described below makes it easier to operate a battery factory. The core of the three-layer electrode method is the processing of copper foil,

Compared with normal copper foil, the copper foil of the three-layer electrode method is coated with the metal lithium powder required for late transformation. In order to protect the lithium powder from reacting with air, a layer of the protective layer is applied; The negative pole is directly painted on the protective layer.

When the core completes the injection, the protective layer will dissolve in the electrolyte, allowing the metal lithium to come into contact with the negative electrode, and the lithium-ion consumed to form the SEI membrane will be supplemented by the metallic lithium powder. This method has no harsh requirements for the processing conditions of the battery factory, but the stability of the protective layer in the electrode receiving and discharging rolls, roller pressure, cutting and other positions is a great challenge to the development of electrode materials, and the metal lithium powder becomes a negative material after it disappears. The guarantee of adhesion is also quite difficult.

4, very lithium material method

The small partners who work in the company must have learned that even what works in laboratory conditions can be difficult to move to the company's large-scale production. The cost of reforming the equipment, the cost of mass input of materials, and the cost of control of the processing environment may become fatal injuries that cannot be promoted by new technologies. For lithium-electric technology, the equipment has been a basically mature industry, the enterprise's preferred pre-lithification plan will certainly be a way to directly promote without many on-site changes, or even take it. The lithium material method is very rich and meets the needs of the battery factory.

The so-called positive lithium method can be simply understood as a material. When it is formed, the number of lithium ions released by her positive pole is several times that of the number of lithium ions that can be released by the currently used material. When the negative pole effect is lower than the positive pole, there will be too many lithium ions lost to the negative pole when it is formed, resulting in the failure of the positive pole effective space to be filled by lithium ions after discharge, resulting in the waste of the positive polar lithium space. If a small amount of high-gram lithium-rich material is added to the positive electrode, this can provide more lithium ions for the formation of the SEI membrane. There is no need to worry that lithium-rich materials cannot be re-embedded during discharge(because lithium ions provided by lithium-rich materials have been completely consumed when they are converted).

The various pre-lithification methods described above are all aimed at full-cell batteries with a negative first effect lower than a positive one. After the full-battery pre-lithification, the first time the maximum efficiency can only reach the level of a semi-battery of positive material. For batteries with lower positive first effects, the above method is basically powerless, because, at this time, the first effect of the whole battery is limited by the fact that there is no longer enough room for lithium after positive charging. Even if lithium is filled by the outside world, it can not be embedded in the positive pole., so there is no effect.

The page contains the contents of the machine translation.