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High performance lithium (Li) ion batteries are rechargeable batteries that are widely used in a variety of consumer electronics such as laptops and smartphones. They are also used in electric vehicles and grid storage as a result of their advantageous properties as compared to other types of batteries1.
Li-ion batteries are composed of electrodes that are made up of light weight lithium and carbon (C) or carbon-based material2. Due to the high reactivity of Li, large amounts of energy can be stored in its atomic bonds, allowing these batteries to have a high energy density (ED), which refers to the amount of energy that can be stored in a given space2.
While typical nickel-metal hydride (NiMH) batteries and lead acetate batteries can only store 100 watt-hours per kilogram and 25 watt-hours per kilogram, respectively, Li-ion batteries can store 150 watt-hours of electricity per kilogram2. Li-ion batteries sustain for hundreds of charge/discharge cycles, and only lose 5% of the charge per month, whereas the NiMH batteries will lose 20% of their charge during the same time period2.
Unlike some other batteries that have a memory effect, which is known as the need for the battery to be completely discharged before charging, Li-ion batteries do not succumb to this property2. As impressive as the Li-ion batteries may sound, some of their disadvantages include high flammability, relatively quick degradation and extreme sensitivity to high temperatures2.
In the Li-ion batteries, the negatively charged electrode, or anode, is made up of graphite or other carbon based materials that have a low ED3. To create even more efficient Li-ion batteries, current research is being focused on Li metal anode-based battery technologies in an effort to increase the capacity of Li-ion batteries up to five to ten times its normal capacity3.
Despite its advantages, such as low redox potential, high specific capacity, low density and low cost, the Li metal anode-based batteries are not yet used for practical applications due to the uncontrollable dendrite growth and low coulombic efficiency (CE) 1.
Dendrites are uncontrollable microscopic fibers that resemble tree sprouts, which are developed during the charge cycles of Li metal anode-batteries3. These dendrites not only reduce the performance of the batteries, but also present a safety threat due to their ability to catch fire when short-circuited3.
To solve this 40 year old dendrite problem, Researchers from the University of California’s Department of Chemistry and Department of Chemical Engineering, have developed an interfacial coating that can suppress the growth of Li dendrites. Chao Wang’s team used a new strategy of in situ formation of interfacial coating on the Li metal anode by incorporating a chemically reducible material, methyl viologen into the electrolyte allowing for stable cycling of Li metal anode. Methyl viologen can be dissolved into the electrolytes when they are in their charged states1,3.
A highly uniform, stable and ionically conductive interfacial coating was able to be formed onto the surface as a result of the electrochemical reduction that occurs following the treatment of the lithium metal layer with 0.5 wt % methyl viologen in the ether electrolyte1.
The viologen coating allowed for better control of the flow of Li ions, while also suppressing the growth of Li dendrites that resulted in the formation of a stable solid electrolyte interface (SEI)1. The Li-metal anion batteries with the viologen-coated ether-based electrolyte showed a lifetime of 300 cycles with a CE of 99.1%, and 400 cycles with a CE of 98.2 %, at a current density of 1 mA per cm2. This data was particularly impressive, as the recorded lifetimes were shown to be more than three times the lifetime of the control ether electrolyte-based batteries that were not coated with viologen1.
The Researchers believe that their new approach will greatly improve the lifetime and CE of Li metal anodes, both in ether-based and carbonate electrolytes1,3. The low cost, easy manipulation and compatibility with current Li ion batteries allow this new approach to have a promising potential to change the future of the battery industry3.
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References:
- Haiping Wu, Yue Cao, Linxiao Geng, Chao Wang. In Situ Formation of Stable Interfacial Coating for High Performance Lithium Metal Anodes. Chemistry of Materials, 2017.
- "How Lithium-ion Batteries Work." HowStuffWorks. 14 Nov. 2006. Web. http://electronics.howstuffworks.com/everyday-tech/lithium-ion-battery.htm.
- "New Battery Coating Could Improve Smart Phones and Electric Vehicles1." ScienceDaily. ScienceDaily, 17 Apr. 2017. Web. https://www.sciencedaily.com/releases/2017/04/170417144938.htm.
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