Improving Lithium-ion Batteries

Improving Lithium-ion Batteries

Dr. Nikhil Koratkar

Battery Research in the Koratkar Lab at RPI

Materials Science and Nanotech

November 20, 2015

Battery technology is becoming increasingly important to our society. Instead of using batteries to only power simple electronics, we are attempting to engineer better batteries that can power cars, houses, and advanced technology such as cell phones and computers. Since 2012, research in the Koratkar Lab has led to discoveries that improve lithium-ion batteries.

While trying to improve batteries one must consider the charge rate, power/energy density, and weight of the battery. By improving the charge rate, the battery can reach full charge faster. By increasing the energy density. the battery can last longer by storing more energy. In order to be used in portable technology such as phones, the battery must be light weight. By improving these areas a better battery can be made. Although, making improvements means finding a balance between all of the different factors.

At RPI, Dr. Koratkar focused on improving the electrode material within lithium-ion batteries. Since the cathode is made of lithium cobalt oxide and it works very well, he focused mainly on improving the anode which consists of sheets of graphene (layers of graphite that are just one atom thick). When the battery is discharged, lithium ions from the cathode migrate to the graphene. When the batter is recharged, the lithium ions return to the cathode.   Inherently, the graphene sheets can only accept a limited amount of lithium ions. So to improve the battery, one can add more graphene, or find a way to get the graphene to accept more Lithium ions. Dr. Kortakardeveloped a process that creates an open pore structure within the sheets of graphene. This creates more space to store the lithium ions and more surface area for the ions to diffuse into the graphene.

Process

Step 1: allow the graphite to undergo oxidation for 96 hours

Step 2: Ultrasonicate the graphene oxide in water to get graphene oxide paper

Step 3: Remove the oxygen and some carbon (CO₂) by using a thermal shock (quickly heat to 700 degrees Celsius for 45 seconds); this thermal shock can also be delivered by photons (similar to a camera flash)

As the thermal shock removes CO₂ it creates vacancies or defects in the carbon structure of the graphene (the C in the CO₂ comes from the graphene).  The thermal shock also disrupts the graphene structure making it porous. After testing this new material in the lab it was discovered that the capacity greatly increased. The original capacity of the graphite sheets was 370 mAh/g while the capacity of the new porous graphene sheets was 900 mAh/g. This means the new material can hold many more lithium ions in the anode. The reason for this is in the traditional material (graphite sheets) when the positive lithium ions combined with electrons in the anode they did not want to stick to the graphite sheets so they just hovered between them. Since the new material has vacancies, when the lithium ions diffuse into the new material they stick in the spots where the vacancies are. In comparison, the traditional material was LiC₆ while the new material is Li₃C₈. This research in the lab showed that the more defective the anode material (up to a certain point) is the higher the capacity/ energy density.

In addition to improving the energy density, Dr. Koratkar  improved the charge rate. Charging a battery is limited by how fast the lithium ions diffuse into the anode. In the porous material the ions can diffuse everywhere instead of just on the edge like the original graphite sheets. This means the battery can be charged for less time because the ions diffuse faster. This was proven when the charge time was decreased to 24 seconds and the power density increased. Ideally, thisporous anode could eventually be optimized to the point that it could be  used to start a car. Since a lot of power is needed to start a car, in order to use a battery it must have a very high power density in a short amount of time. However, in order to apply this to a realistic situation the volume of the battery had to be normalized. So, in the lab the defected graphene was compressed to a realistic size for an anode. The compression closed some cracks and pores so when a charge was applied the power density was not as great but the energy density was still high. This shows that while designing a new idea some desired results may be compromised. In this case, normalizing volume is not good for graphene when a high power density is desired.

Concepts/ Vocabulary

Energy density- The amount of energy stored in a given system or region of space per unit volume or mass

Power density- the amount of power per unit volume

Power=energy/time

Economies of scale- a proportionate savings in cost gained by an increased level of production

Example: The cost of implementing the process to make graphene porous is justified by the increase in production of these new lithium-ion batteries.  (In this case we would want to produce morebecause they have more advantages then traditional lithium-ion batteries)

Oxidation- combining with oxygen; electrons are lost

Ultrasonicate in water- breaks open the graphene oxide into layers with an ultrasound and water

-Phones need a battery to last 1,000 charge cycles while cars need a battery that can last 10,000 charge cycles

-Patent ideas first then publish

-Ideas are useful but in order to be used in production for manufacturing they must be able to be scaled up

-for the production of drones the battery that powers them must be light weight

Government:

Funding for researching lithium-ion technology comes from grants (National Science Foundation, and NY State energy research development authority)

Connections

SME project/Mr. Chiappone: Scaling up; high rate manufacturing at low costs

Materials Science and Engineering/ Professors Ed Palermo and Chaitanya Ullal: The concept of vacancies in materials; Structure defines function

Dr. Linhardt: scaling up ideas that work on the small scale; using grants to fund research; patenting ideas in order to publish them

Mr. Silva: Improving a traditional version of an idea to make it work better; scaling up ideas for large scale production

Ms. Moldoff: utilizing grants/funding from the government