Electrolyte, which plays the crucial role of transporting ions between the cathode and the anode of a battery, has been both the strength and the weakness of lithium-ion (Li-ion) batteries. Currently, the carbonate electrolyte that is inside conventional Li-ion batteries has a decent level of conductivity and is low in cost. However, its high flammability poses a serious safety risk.
Recently, a team of researchers at Stanford University has developed a new type of electrolyte in the process of boosting the performance of lithium-metal batteries. Their study shows that this electrolyte is safer and more efficient than the mainstream electrolytes for Li-ion and other kinds of lithium batteries. Lithium-metal batteries, which are different from Li-ion batteries, have the potential to achieve a very high level of energy density. Despite this advantage, they are still a niche concept in the battery market due to their incompatibility with most available electrolytes. This breakthrough by the Stanford team could thus lead to a renewed interest in the development of lithium-metal batteries for various applications.
Changing the material composition of the anode is one way to increase the energy density of lithium batteries. The anode of Li-ion batteries is made of graphite and copper, whereas the anode of lithium-metal batteries contains metallic lithium. The replacement of graphite and copper with metallic lithium can significantly raise the energy density and the storage capacity of a battery while not increasing the weight. However, metallic lithium is a highly reactive element (i.e., more so than sodium and magnesium). Uneven clumps of lithium ions tend to build up on the surface of a lithium-based anode due to unwanted reactions with the electrolyte. These clumps will then become dendrites, or crystalline protrusions. Eventually, dendrites can grow to the point of piercing the separator of the battery. When this happens, the whole device fails and may even catch fire.
Many battery manufacturers and scientists at research institutions around the world are attracted by the potential of lithium-metal batteries. However, commercialization of this battery technology involves the laborious task of discovering material combinations and structural designs that not only enhance the performance of the battery but also minimizes the fire hazard. Zhiao Yu, a member of the Stanford team and a co-lead author of the study on the new electrolyte, said that he and his colleagues have adopted the approach of using additives to improve the existing electrolytes instead of designing a new formula from scratch.
The Stanford team tested the hypothesis that the inclusion of fluorine atoms can improve the stability of commercially available liquid electrolytes, especially for their use in lithium-metal batteries. This idea seems counterintuitive on the surface because fluorine is extremely reactive, toxic, and flammable. However, fluorinated compounds are remarkably stable, and a plethora of them have been synthesized for a wide range of applications. In the medical domain, fluorinated compounds are used in dental care, drug manufacturing, and CT scanning.
Yu noted that fluorine is already a common additive in the electrolytes for lithium batteries. What his team has done is to take advantage of the ability of this element to attract electrons and form new molecular structure. The compound that his team has created is called “fluorinated 1, 4-dimethoxylbutane” or FDMB. It is designed to be an electrolyte solvent that is easy and cheap to mass produce. Yu pointed out that other new electrolytes with similar potentials are more costly in terms of raw materials or manufacturing processes.
Regarding safety, the Stanford team has found that the FDMB electrolyte surpasses conventional carbonate electrolyte in this area. As the above video shows, the carbonated electrolyte on the left instantly caught fire when came in contact with the flame of the lighter. In contrast, the FDMB electrolyte on the right was able to remain in its original state for at least three seconds during the combustibility or flammability test.
Moreover, the FDMB electrolyte achieved impressive results in performance tests. The experimental batteries containing this electrolyte kept 90% of their initial charge following 420 cycles of charging and discharging. Previously, lithium-metal batteries would fail after around 30 cycles.
Yi Cui, another member of the Stanford team, said that the FDMB electrolyte could be the key to the market entry of lithium-metal batteries. According to Cui, the coulombic efficiency of his team’s experimental batteries are near the commercially viable level of 99.9% (i.e., 99.52% for the half cell configuration and 99.98% for the full cell configuration).
The Stanford team also tested the FDMB electrolyte in anode-free pouch cells. Compared with other battery cell packages, pouch cells is lighter in weight and could later be adopted for batteries used in consumer electronics and drones. Currently, anode-free lithium batteries still lag behind conventional Li-ion batteries in terms of performance. However, the experimental anode-free pouch cells built by the Stanford team retained 80% of their initial capacity after 100 cycles. This result is the best so far for this kind of battery.
(Source: TechNews. Photo credit: Stanford University.)