Batteries that charge quickly and last a long time would be a priceless invention for engineers in this world. But despite decades of research and innovation, a fundamental understanding of how exactly batteries work at the smallest scale has remained a question that bothers scientists even today.
In a paper published in 2016 in the journal Science, a team led by William Chueh, an assistant professor of materials science and engineering at Stanford and a faculty scientist at the Department of Energy’s SLAC National Accelerator Laboratory, has devised a way to look closely as never before into the electrochemical reaction that fuels the most common rechargeable cell in use today: the lithium-ion battery.
A Li-ion battery is a rechargeable battery commonly used for portable electronics and electric vehicles and has a wide scope for military and aerospace applications. During discharge in the batteries, lithium ions travel from the negative electrode through an electrolyte to the positive electrode and back when charging. They typically use graphite at the negative electrode and an intercalated lithium compound at the positive electrode.
Visualizing the foundations of batteries – tiny particles typically measuring less than 1/100th of a human hair in size – the team members have highlighted a process that is far more convoluted than once thought. Both the method they developed to observe the battery in real-time and their refined understanding of the electrochemistry could have widespread applications for battery design and management for years to come.
MOTIVATION BEHIND THE RESEARCH
At the heart of every lithium-ion battery is a simple chemical reaction. The positively charged lithium ions reside on the lattice-like structure of a crystal electrode. As the battery is discharging, it receives negatively charged electrons in the process. So discharging transfers energy from the cell to wherever the electric current dissipates its energy, usually in the external circuit. This process is called lithiation. In the reverse process of charging, also called delithiation, electrons are removed and the ions are freed. The external circuit has to provide electric energy for charging to occur, and this energy is then (with some loss) stored as chemical energy in the cell.
These two basic processes are hampered by an electrochemical Achilles heel. The ions do not insert uniformly across the surface of the particles. Instead, certain areas take on more ions, and others fewer. As a part of the area of the crystal lattice become overburdened with ions, these inconsistencies eventually lead to mechanical stress on the particles. Tiny fractures are thus developed on the surface, leading to deteriorating battery performance and shortening its life.
This research work thus lays out a path toward suppressing the phenomenon and coming up with batteries that are much long-lasting and charge faster than today’s conventional models.
The study visualizes the charge/discharge reaction in real-time, something scientists refer to as operando, at fine detail and scale. The team utilized brilliant X-rays and cutting-edge microscopes at Lawrence Berkeley National Laboratory’s Advanced Light Source.
Chueh and his team fashioned a transparent battery using the same active materials as ones found in smartphones and electric vehicles. This was a very, very small battery, holding ten billion times less charge than a smartphone battery, but it allows a clear view of what’s happening at the nanoscale. It was designed and fabricated in collaboration with Hummingbird Scientific. It consists of two very thin, transparent silicon nitride “windows.” The battery electrode, made of a single layer of lithium iron phosphate nanoparticles, sits on the membrane inside the gap between the two windows. A salty fluid, known as an electrolyte, flows in the gap to deliver the lithium ions to the nanoparticles.
It was discovered that lithiation is significantly more uniform than delithilation. X-ray microscopy technique was also one of the advancements which was developed in collaboration with Berkeley Lab Advanced Light Source scientists Young-sang Yu, Tolek Tyliszczak and David Shapiro.
“The improved uniformity lowers the damaging mechanical stress on the electrodes and improves battery cyclability,” Chueh said. Referring to the catalysts, memory devices, and so-called smart glass, which transitions from translucent to transparent when electrically charged Chueh said ” Beyond batteries, this work could have a far-reaching impact on many other electrochemical materials.”
The microscope can reveal the never seen before dynamics at the nanoscale which affect the energy research across the board. Ford-Stanford Alliance, U.S. Department of Energy and Office of Basic Energy Sciences provided funding for this work. Bazant was supported by the Global Climate and Energy Project and he was also the visiting professor at Stanford.
Bazant said “What we’ve learned here is not just how to make a better battery, but offers us a profound new window on the science of electrochemical reactions at the nanoscale.”