Rechargeable batteries are an essential part of our green energy future, from electric vehicles to large-scale energy storage for the grid. Creating bigger, better, faster-charging, more efficient batteries is key, but with our growing dependency on batteries, so is making them more environmentally-friendly.
Currently, the most popular form of batteries, lithium-ion batteries, use lithium cobalt oxide as a cathode. The cobalt must be mined and the cathodes are made in a high-temperature and energy-intensive process. Both the making and recycling of the materials is expensive.
Researchers at Rice University and City College of New York think they've found the perfect solution: a cathode that is made from an extract from the madder plant. Using plant material instead of metals makes for a battery that is cheaper to produce and less harmful to the environment.
“Green batteries are the need of the hour, yet this topic hasn’t really been addressed properly,” lead researcher Arava Leela Mohana Reddy said. “This is an area that needs immediate attention and sustained thrust, but you cannot discover sustainable technology overnight. The current focus of the research community is still on conventional batteries, meeting challenges like improving capacity. While those issues are important, so are issues like sustainability and recyclability.”
The madder plant is a climbing vine that is a good source of purpurin, an organic dye that has been used since ancient times in fabrics. Now it is getting a new use as a green cathode.
Rice University explains, "Reddy and his colleagues came across purpurin while testing a number of organic molecules for their ability to electrochemically interact with lithium and found purpurin most amenable to binding lithium ions. With the addition of 20 percent carbon to add conductivity, the team built a half-battery cell with a capacity of 90 milliamp hours per gram after 50 charge/discharge cycles. The cathodes can be made at room temperature, he said."
Agricultural waste could be a good source of purpurin as well as other suitable molecules, which could drive costs down even further and make good use of a waste stream.
“We’re interested in developing value-added chemicals, products and materials from renewable feedstocks as a sustainable technology platform,” said George John, one of the researchers and a professor of chemistry at the City College of New York. “The point has been to understand the chemistry between lithium ions and the organic molecules. Now that we have that proper understanding, we can tap other molecules and improve capacity.”
The researchers hope to eventually create a totally green battery that uses not just the plant dye cathode, but also organic molecules for the anode and an electrolyte that doesn't break those molecules down. Reddy says that a prototype of a completely organic battery could be ready in just a few years.