A brief analysis of high performance electrolyte materials for lithium-sulfur batteries has been made

Lithium-sulfur batteries are generally regarded as the future development direction of chemical energy storage in the industry. However, Lithium-sulfur batteries have been facing the key challenges of multi-sulfide lithium shuttling effect and metallic lithium interface instability all the time. Researchers often use lithium nitrate additives to solve the above problems, but the battery system with lithium nitrate, carbon black and elemental sulfur coexistence has safety risks.

But that is now expected to change dramatically. Huamin Zhang, Xianfeng Li and Hongzhang Zhang from the Department of Energy Storage Technology, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, designed and prepared a high-performance electrolyte without lithium nitrate, which realized its application in lithium-sulfur battery devices, which brought a new idea for the design and preparation of electrolyte materials for lithium-sulfur batteries.

So what are the technical breakthroughs of this high-performance electrolyte, the advantages of lithium sulfur batteries and the prospects? Around these issues, the reporter interviewed Zhang Huamin.

First of all, the safety of homemade electrolyte is good. The electrolyte used in previous lithium-sulfur batteries contains a large amount of lithium nitrate, which, when mixed with carbon-sulfur compounds, is similar to that of black powder, so there is a great risk of ignition and explosion. The electrolyte developed by our team does not contain lithium nitrate and has better safety.

Secondly, the stability of self-made electrolyte is good. The electrolyte developed can make the lithium-sulfur battery cycle more than 30 times continuously and steadily. According to the use mode of charging and discharging once a day, the battery can ensure the unmanned aircraft to fly uninterrupted for up to one month. This performance is among the best reported to date.

Again, the capacity of self-made electrolyte plays well. The electrolyte developed adopts dioxy-pentyl ring (DOL) as the solvent to achieve a higher capacity for the first time, so that the energy density of the lithium-sulfur battery can still reach 350Wh/kg during the rapid charge-discharge process of 0.2C.

Electrolyte is one of the key materials to determine the performance of lithium-sulfur batteries and is the bottleneck of the development of lithium-sulfur battery technology. Since October 2013, the team began to take electrolyte as an important breakthrough point, committed to the development of good safety, long life of lithium-sulfur batteries. By August 2016, the team made preliminary progress in the field of electrolyte, developed a class of high performance electrolyte without lithium nitrate, and successfully applied in large capacity (460Ah) lithium-sulfur battery, the research and development lasted 2 years and 10 months.

At present, the Energy Storage Technology Research Department of Dalian Chemical Institute focuses on three important directions in the field of battery research: first, large-scale, fixed battery energy storage technology, including vanadium liquid flow battery, zinc bromide liquid flow battery, zinc nickel liquid flow battery and lead carbon battery; Second, high specific energy, power lithium battery technology, such as lithium sulfur battery. Research is focused on the development of key battery materials such as electrolytes, carbon-sulfur compounds, and membranes. The third is the all-weather, high energy density, high discharge rate battery that can run at the ambient temperature of minus 40℃ above 50℃.

With the goal of industrialization of lithium-sulfur battery devices, the research team has carried out detailed studies on key materials (positive electrode, negative electrode, diaphragm, electrolyte) and key technologies (electrode coating, battery assembly, battery formation, etc.) that limit the development of lithium-sulfur batteries, and made corresponding breakthroughs.

For example, in the aspect of cathode materials, the development of high-performance porous carbon materials with specific surface area of 2300m2/g and pore volume of 8cm3/g, and through the self-created phase conversion molding process, made with a unique three continuous structure of lithium sulfur battery electrode; In the diaphragm material, the ion selection through the membrane with nanoscale ion transport channels has been developed. In terms of battery devices, the 30Ah lithium sulfur primary battery is made, and the energy density reaches more than 900Wh/kg.

There are a number of technical challenges with lithium-sulfur batteries. Our team will continue to work in three areas: continue to improve the cycle life of lithium-sulfur batteries; Continue to improve the safety of lithium-sulfur batteries; Lithium-sulfur primary batteries with an energy density of more than 1200Wh/kg have been developed. Let's start with the energy density of a lithium-sulfur battery. Among the known cathode materials for batteries, lithium metal has the most negative potential and higher specific capacity. In known cathode materials, elemental sulfur has moderate potential and large specific capacity. Lithium-sulfur battery is composed of lithium metal and elemental sulfur, and its theoretical specific energy can reach 2600Wh/kg, which is 35 times of the theoretical specific energy of the current commercial lithium ion battery. The actual energy density of lithium-sulfur batteries can still reach a higher level of 300900Wh/kg after making the battery cell products, while the energy density of commercial lithium-ion power lithium batteries is only 100200Wh/kg. Even if the anode of a lithium-ion battery is replaced with lithium metal, its energy density is still much lower than that of a lithium-sulfur battery. Therefore, lithium-sulfur battery has great technical attraction and is an important research direction in the current energy storage field.

Second, there is the cost of lithium-sulfur batteries. Low material costs are an inherent advantage of lithium-sulfur batteries. According to the existing industrial cell composition and process conditions, the raw material cost of different battery cells can be estimated.

For lithium-sulfur batteries, the cathode material is important carbon sulfur complex, the cost is very low, the cost is important from the metal lithium and electrolyte. Lithium-sulfur battery electrolyte consumption accounts for 30% and 50% of the mass of the cell, 23 times that of lithium-ion battery. With the improvement of the performance of the positive and negative electrodes of the battery, the amount of electrolyte needed for lithium-sulfur batteries will be greatly reduced, and the cost will be further reduced. According to the estimation of 6Ah flexible packaging lithium-sulfur battery in our laboratory, the cost of the cell is about 0.21 yuan /Wh, which is similar to the metal lithium-ion battery with lithium metal as the negative electrode and lithium manganese rich material as the positive electrode, but much lower than the cost of the current commercial lithium ion battery.

However, when choosing a battery, users consider not only its watt-hour cost, but also its cycle life. So far, the 400Wh/kg lithium-ion battery with lithium metal as negative electrode developed by US SIOPOWER (a leading company of lithium-sulfur batteries) has a 50% energy surplus in 160 cycles, which is far from the thousands of cycles of lithium-ion battery with graphite as negative electrode. If you calculate the cost per cycle, the cost advantage of lithium-sulfur batteries is not obvious. Therefore, from the perspective of use cost, the cycle life of lithium-sulfur batteries should be further improved.

Lithium-sulfur batteries involve many technical problems, such as short cycle life, poor stability of lithium metal, flammable electrolyte, etc. As these problems are solved, the applications of lithium-sulfur batteries will be expanded.

At present, the specific energy of lithium-sulfur primary batteries has reached 900Wh/kg, which can be applied in some fields. However, the cycle life of lithium-sulfur secondary batteries is still very short, less than 1/10 of that of lithium-ion batteries, and it still needs to go through a very long development process.

From a practical point of view, lithium-sulfur batteries are likely to be used first in areas such as relatively energy-demanding drones, and gradually to electric vehicles. The development space of the battery is great, but the technical challenges and development opportunities coexist.