Getting the Skinny on Batteries

Blemishes are a thing of beauty in graphene paper. Graphene is the world's thinnest material. Researchers at Rensselaer Polytechnic Institute (RPI), have made the grapheme into paper and then zapped it with a laser or camera flash to blemish it with countless cracks, pores, and other imperfections. This turns the graphene into an anode material that can be charged or discharged 10 times faster than conventional graphite anodes used in today's lithium (Li)-ion batteries.

^{Li-ion battery. Image Credit: HowStuffWorks}

We all know rechargeable Li-ion batteries. They are the industry standard for mobile phones, laptop and tablet computers, electric cars, and a range of other devices. The batteries have a high energy density and are able to store large amounts of energy. But due to their low power density, they take about an hour to charge and are unable to quickly accept or discharge energy.

The Need

Electric cars run on Li-ion batteries, but they cannot be totally dependent on them yet for high-power functions such as accelerations and braking. The RPI team sought to solve this problem by creating a new battery that can hold large amounts of energy, but also quickly accept and release this energy. The over-arching goal is to create a battery that will alleviate the need for the complex pairing of Li-ion batteries and super-capacitors in electric cars. This means a simpler, better-performing automotive engine based solely on high energy, high-powered Li-ion batteries. The new batteries could shorten the time it takes to charge portable electronic devices.

^{Difference between electric and gas powered cars. Image Credit: hybridcars.com/electric-car}

"Li-ion battery technology is magnificent, but truly hampered by its limited power density and its inability to quickly accept or discharge large amounts of energy. By using our defect-engineered graphene paper in the battery architecture, I think we can help overcome this limitation," said Koratkar, the John A. Clark and Edward T. Crossan Professor of Engineering at Rensselaer. "We believe this discovery is ripe for commercialization, and can make a significant impact on the development of new batteries and electrical systems for electric automobiles and portable electronics applications."

^{Graphene. Image Credit: Wikipedia}

The Process

The study, published in ACS Nano, discusses the use of graphite in today's Li-ion batteries. Graphene is a derivative from graphite, made of an atom-thick sheet of carbon atoms arranged like a Nanoscale chicken-wire fence. The graphite was slow to charge because lithium ions could only physically enter or exit the battery's graphite anode from the edges, and slowly work their way across the length of the individual layers of graphene. The solution the RPI team developed was to create a sheet of graphene oxide papers, about the thickness of printer paper. The sheet was exposed to either a laser or a flash from a digital camera, which gave off enough heat to cause "mini-explosions" within the paper. The explosions happen as the oxygen is expelled from the paper, leaving the graphene covered with blemishes. The process also causes the paper to expand five-fold in thickness, creating large voids between the individual graphene sheets.

^{Graphene paper blemishes. Image Credit: Rensselaer Polytechnic Institute}

The lithium ions can now use the pores and cracks as shortcuts to move quickly into or out of the graphene. This greatly increased the battery's overall power density. The experimental anode material could charge 10 times faster than conventional anodes in Li-ion batteries, without incurring a loss in energy density. The robust graphene paper was able to perform successfully after more than 1,000 charges/discharge cycles. The process of making the graphene paper anodes for Li-ion batteries can easily be scaled up and the paper can be made in many shapes and sizes. The photo-thermal exposure by laser or camera flashes is an easy and inexpensive process to replicate.

The next step for the research team is to pair the graphene anode material with a high-power cathode material to construct a full battery that can be used in applications such as communication devices, electronics, and electric cars.

Resources

Batteries Made From World's Thinnest Material Could Power Tomorrow's Electric Cars