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Using biomass to improve lithium batteries, green life will depend on it in the future.

- Nov 09, 2018 -

At present, many processes for obtaining fuel or chemical raw materials through biomass refining are quite mature. For example, ethanol obtained by microbial fermentation of sugar can be used not only as a car fuel in a certain proportion, but also in reaction with animal and vegetable oils to obtain biodiesel which can replace diesel. Ethanol can also be used as a raw material for many chemical products. For example, it can be dehydrated to obtain ethylene, and an important plastic polyethylene can be synthesized.

However, although these methods are effective, they are mainly used for biomass derived from agricultural products such as starch, sugar, and oil, so it is inevitable to compete for land for food production. With one-ninth of the world’s population still failing to escape hunger, these means of using biomass energy are inevitably criticized. Therefore, researchers in many fields believe that in order to truly develop biomass energy, it is necessary to transfer the target to lignocellulosic biomass.

At present, many processes for obtaining fuel or chemical raw materials through biomass refining are quite mature.However, although these methods are effective, the biomass derived from agricultural products such as starch, sugar and oil is the main material they used, so it is inevitable to compete with the food production for land. With one-ninth of the world’s population still failing hunger, these means of using biomass energy are inevitably criticized. Therefore, researchers in many fields believe that in order to develop biomass energy, it is necessary to transfer the target to lignocellulosic biomass.

The so-called lignocellulosic biomass refers to the biomass composed of three natural polymer compounds of cellulose, hemicellulose and lignin, which constitute the backbone of green plants and can be a synonym for the plant biomass. Lignocellulosic biomass is not only rich in reserves, but they are not digested by the body. Therefore, if the biomass energy raw materials are converted from crops to lignocellulose, not only can the dependence on agricultural production be greatly reduced, but also the waste processed by agricultural products, such as straw and bagasse, can be helped. It is theoretically not difficult to take this step: cellulose can be hydrolyzed to glucose by chemical or biological means. As long as this step is completed, we can use inexhaustible lignocellulose resources. Perfectly interface with mature biomass energy technologies.

However, in practice, it is the crucial first step in the hydrolysis of cellulose, which is a headache for researchers. Due to the strong hydrogen bonds between the molecules, cellulose forms dense crystals, which makes them insoluble in most solvents. If a solution cannot be formed, hydrolysis of cellulose is difficult to carry out efficiently. Fortunately, in recent years, researchers have found that many ionic liquids can dissolve a certain proportion of cellulose by destroying hydrogen bonds between cellulose molecules, and some ionic liquids can even completely swallow whole lignocellulosic biomass. Once the solution is formed, it is possible to further process the cellulose to better utilize the treasures that nature has given us.

The Ionic liquids not only help us make better use of biomass, but we can also play a role in another energy-related application, a lithium-ion battery that almost everyone can't live without.

In 2016, the famous mobile phone manufacturer South Korea's Samsung's Note7 mobile phone crashed several times in the near future after charging, which not only caused property damage to users, but also caused the airlines of various countries to promulgate the Samsung Note 7 mobile phone for security reasons. The ban" became a sensational news. The culprit of this series of accidents is the lithium-ion battery used in mobile phones.

The safety accidents of lithium-ion batteries are frequent, due to its "soft ribs" - electrolytes. When a lithium ion battery is discharged, lithium ions originally embedded in the negative electrode of the battery (usually graphite) are deintercalated and moved toward the positive electrode (usually a lithium-containing compound such as cobalt lithium oxide or lithium iron phosphate); and when the battery is charged, The above process reverses, and lithium ions move from the positive electrode to the negative electrode and re-embed in the graphite. In order to ensure that the lithium-ion battery can work properly, we need to provide a liquid medium that allows lithium ions to move freely between the two electrodes. This medium is the electrolyte. Not only lithium-ion batteries, but other types of batteries are also inseparable from the key component of electrolytes.

The most commonly used as a battery electrolyte is an aqueous solution of an ionic compound. For example, the electrolyte used in the most common disposable carbon-zinc dry battery is a paste formed by dissolving ammonium chloride or zinc chloride in water; and the electrolyte of a disposable alkaline battery is an aqueous solution of strong alkali such as potassium hydroxide. This is why the battery is named alkaline battery. Then, as long as the lithium salt is dissolved in water, can we not get the electrolyte of the lithium ion battery? Unfortunately, this method of trial and error is not feasible here, because the working voltage of lithium-ion batteries is too high to electrolyze water into hydrogen and oxygen. Therefore, in lithium-ion batteries, we can only use organic solvents instead of water to dissolve lithium salts. However, these organic solvents, although not electrolyzed, have the disadvantage of being flammable. Once the lithium-ion battery fails, the organic solvent is ignited, and the consequences are naturally unimaginable.

Despite the repeated safety accidents of lithium-ion batteries, we still have to rely on it. This is because lithium-ion batteries have the advantages of recharging and high energy density. In portable electronic devices such as mobile phones and notebook computers, other types are available. The battery is really not competent. Especially with the use of lithium-ion batteries for new vehicles such as electric vehicles and energy storage equipment for solar energy and wind energy, the application range will be further expanded. Therefore, it is imperative to improve the safety of lithium ion batteries.

Since the root cause of the safety hazard of lithium-ion batteries is the flammable organic solvent in the electrolyte, can we replace it with a solvent that does not burn? As mentioned earlier, it is not feasible to use an aqueous solution as an electrolyte, so the remaining choice is naturally an ionic liquid. In fact, ionic liquids did not live up to the expectations of scientists. A 2010 study from Canada showed that if a ionic liquid with a mass fraction of 40% was added to a conventional lithium-ion battery electrolyte, the performance of the battery was not significantly affected, but the flammability of the electrolyte was significantly reduced. Even if you face a bright flame, it will not burn. With such electrolytes, the safety hazard of lithium-ion batteries may be completely history. When 40% of the ionic liquid is added to the organic solvent commonly used in the electrolyte of the lithium ion battery, the solvent becomes no longer flammable, thereby greatly improving the safety of the lithium ion battery. However, although ionic liquids have shown unique advantages in many fields, a considerable number of related applications remain in the laboratory research stage. Although the reasons behind this are various, there are several common "blockers" worth noting.

First, ionic liquids are a little more expensive than traditional solvents. It is estimated that the current price of ionic liquids is about 20 US dollars per kilogram. If you want to replace traditional organic solvents with ionic liquids, the price should be reduced to at least 2.5 US dollars per kilogram, which will make the producers who are accustomed to budget calculations feel beneficial. Mapable.

Secondly, although many ionic liquids are liquid at room temperature, their viscosity is high and their fluidity is much worse than that of traditional organic solvents, which may cause inconvenience to the production and use process.

Third, most of the ionic liquids developed in the early days are easily hygroscopic, which is also a trouble in many occasions. For example, many ionic liquids are good solvents for cellulose, but once a small amount of water is mixed, the solubility of cellulose decreases linearly. In addition, some ionic liquids, such as the aforementioned hexafluorophosphate, meet once. The water will decompose and release the highly toxic hydrofluoric acid, which is a serious threat to the user's personal safety. If the use of ionic liquids must be carried out under extremely dry conditions, they will also greatly reduce their own advantages. In addition, how to recycle and reuse ionic liquids and how to reduce the toxicity of certain ionic liquids are all worthy of attention. Undoubtedly, there are still many difficulties to overcome in the future development of ionic liquids. However, decades of practice have taught us that these flowing salts are indeed promising. The continuous development of advanced ionic liquids will definitely bring us a greener life.