Why lithium hexafluorophosphate occupies a position in lithium battery electrolyte

The four key materials of lithium-ion batteries include positive electrode, negative electrode, battery separator, and electrolyte. The electrolyte transports ions and ionic compounds between the positive and negative electrodes of the battery. Its performance directly determines the conductivity, capacity and output of lithium-ion batteries. Voltage. The electrolyte is generally prepared in a certain proportion from high-purity organic solvents, solutes and a small amount of additives. Among them, lithium hexafluorophosphate is one of the electrolyte solute materials.

What is lithium hexafluorophosphate

Lithium hexafluorophosphate is the most important part of the cost of the electrolyte, accounting for about 43% of the total cost of the electrolyte. The production technology threshold of lithium hexafluorophosphate is relatively high, especially the production of high-purity crystalline lithium hexafluorophosphate. It can be said that lithium hexafluorophosphate, as a cutting-edge material in the lithium battery industry, is an important part of the electrolyte. As an electrolyte material, the overall performance is good, but the disadvantage is that the thermal stability is not good, and it is easy to deliquescence, so it needs to be stored at low temperature and isolated from the air.

Irreplaceability of lithium hexafluorophosphate

Lithium hexafluorophosphate has moderate ion transfer number, moderate dissociation constant, good oxidation resistance and good aluminum foil passivation ability in commonly used organic solvents, and can be matched with various positive and negative electrode materials. Therefore, it has become the most important electrolyte lithium salt used in commercial energy storage.

Researchers are constantly trying to develop new lithium salts in order to replace lithium hexafluorophosphate, but so far they have not been successful. Therefore, it is expected that for a long period of time in the future, lithium hexafluorophosphate will still be the only electrolyte salt used on a large scale, and its uniqueness mainly depends on the three elements of lithium, phosphorus and fluorine.

Lithium is the lightest alkali metal element and the metal element with the smallest molar mass. It is also the metal element with the lowest redox potential, the largest mass energy density, and the highest electrochemical equivalent. These characteristics determine that lithium is a high specific energy electrode material.

Fluorine is the most electronegative element in nature and the most active element among non-metallic elements, and it is also the element with the highest standard electrode potential. The combination of fluorine and lithium constitutes an electrochemical reversible battery, with a maximum potential of 5.93V and the highest specific energy of the battery. At the same time, the radii of lithium and fluorine are extremely small, making them suitable as electrode materials for lithium batteries.

In addition, the association ability of hexafluorophosphate is poor, so the conductivity of its electrolyte is relatively high, which is higher than that of general inorganic lithium salts. Lithium hexafluorophosphate has strong electrochemical stability, and the stable voltage of the cathode reaches 5.1V, which is much higher than the 4.2V required by lithium-ion batteries. It does not corrode the current collector, and its comprehensive performance is stronger than other lithium salts.

How to make lithium hexafluorophosphate

Lithium hexafluorophosphate is very unstable, decomposes at around 60°C, and is also prone to deliquescence. Generally, potassium hexafluorophosphate products should be prepared in non-aqueous solvents such as anhydrous hydrogen fluoride and low alkyl ethers. Moreover, if lithium hexafluorophosphate develops in the direction of lithium-ion batteries and power batteries, the requirements for its purity, stability, and consistency are very high. At the same time, the production process of lithium hexafluorophosphate involves harsh working conditions such as low temperature, strong corrosion, no water and no dust, and the process is extremely difficult.

The preparation methods of lithium hexafluorophosphate mainly include gas-solid reaction method, organic solvent method and hydrogen fluoride solvent method. At present, the mainstream lithium hexafluorophosphate preparation method is the hydrogen fluoride solvent method, which accounts for more than 80% of all industrial production methods, and many large enterprises adopt this method to realize industrial production. Therefore, we mainly introduce the hydrogen fluoride solvent method that realizes continuous and automated production.

1. Gas-solid reaction method

The gas-solid reaction method is the earliest preparation method of lithium hexafluorophosphate, which was proposed by American scientists in 1950. The gas-solid reaction method preparation process mainly includes two steps:

LiF (solid) + HF (gas) → LiHF2 (solid) → LiF (porous) + HF (gas)
LiF (porous) + PF5 (gas) → LiPF6

The synthesis method is simple to operate and carried out at high temperature and high temperature, but the generated lithium hexafluorophosphate will cover the surface of the corroded lithium to form a dense protective film, which prevents the further progress of the reaction, resulting in a large amount of unreacted lithium fluoride in the final product , the product purity is relatively low. If it is further purified, the process and cost will be increased, and the purity is not easy to guarantee. If porous LiF is used to react with high-purity PF5 gas, lithium hexafluorophosphate with a purity of 99.9% can be prepared, but the preparation cost is relatively high.

2. Hydrogen fluoride solvent method

The hydrogen fluoride solvent method is currently the most widely used preparation method for lithium hexafluorophosphate. The hydrogen fluoride solvent method is to dissolve lithium halide in anhydrous hydrogen fluoride, and then introduce high-purity PF5 gas to react to generate potassium hexafluorophosphate crystals, and then obtain lithium hexafluorophosphate products through separation and drying.

3. Organic solvent method

The preparation process of the organic solvent method is similar to the hydrogen fluoride solvent method. The difference is that the purity of the product prepared by the organic solvent is only 90% to 95%. The product is easy to absorb the organic solvent, and it is difficult to further remove it and it is not easy to produce solid lithium hexafluorophosphate.

Analysis and forecast of lithium hexafluorophosphate demand

The data shows that the demand for lithium hexafluorophosphate in 2020 is about 35,000 tons, the predicted demand is about 55,000 tons by 2022, and the predicted demand for lithium hexafluorophosphate is about 100,000 tons by 2025.

Production capacity analysis of lithium hexafluorophosphate

In terms of supply, the global lithium hexafluorophosphate production capacity is mainly concentrated in China, Japan and South Korea, with China accounting for the highest proportion. In 2020, the total production capacity of China’s lithium hexafluorophosphate industry will be about 56,500 tons.

China is the second country to industrialize lithium hexafluorophosphate after Japan. With the rapid expansion of China’s lithium hexafluorophosphate production capacity, China’s import dependence on lithium hexafluorophosphate has dropped significantly, and China has become the largest producer of lithium hexafluorophosphate. According to the data, China’s lithium hexafluorophosphate shipments account for about 70% of the world’s total.

Price analysis of lithium hexafluorophosphate

In 1998, the price of lithium hexafluorophosphate in China was 5 million to 7 million yuan/ton, and in 2004 it was 600,000 yuan/ton. Due to the production of lithium hexafluorophosphate by Jinniu in 2003, and the production of lithium hexafluorophosphate by Japan’s Sentian in China, the price of lithium hexafluorophosphate dropped to about 250,000 yuan/ton in 2013.

After 2013, due to the successive production of lithium hexafluorophosphate manufacturers, lithium hexafluorophosphate has overcapacity, and the price has gradually dropped. By the second quarter of 2015, the price reached the lowest point of 80,000 yuan/ton. In the second half of 2015, due to the rapid increase in production capacity of lithium-ion batteries, the prices of materials related to power batteries rose sharply. The price of lithium hexafluorophosphate as a raw material for lithium battery electrolytes began to soar, and rose to 400,000 yuan in just a few months in 2016 / tons or more. In the later period, the production capacity of lithium hexafluorophosphate increased, and the price began to drop again. By 2020, the price of lithium hexafluorophosphate will return to 80,000 yuan/ton. the

After 2020, the capacity utilization rate of lithium hexafluorophosphate continues to increase. With the unexpected growth of downstream demand, lithium hexafluorophosphate has changed from the past state of overcapacity, showing the coexistence of high start-up and low inventory. On July 12, 2021, the market price of lithium hexafluorophosphate in China has risen to 385,000 yuan/ton, a month-on-month increase of 22.22%, an increase of 259.81% from the beginning of the year, and a year-on-year increase of 431.03%.

Summary

The new energy industry is a hot area for the future development of the world. Lithium-ion batteries are an important part of the new energy industry such as Powerwall, Portable power station, home power storage, etc. The demand will also face high growth in the next few years. As the main electrolyte of the electrolyte, lithium hexafluorophosphate still has a large gap between demand and production capacity.