Every year, Apple unveils a new iPhone complete with a longer-lasting battery.

In the newest model, Apple touts the battery in the iPhone XS Max lasts up to 1.5 hours longer than the iPhone X, which debuted less than a year ago. It’s easy to take for granted how quickly battery technology improves.

At night, people across the world plug in their iPhones and other smartphones before nodding off to sleep. Most assume the battery will be at 100 percent in the morning, ready for another day of texting and streaming. People who have electric vehicles may charge their batteries in the same manner at home or at work.

Implicit in these routines are the assumption that these sleeker, better-performing batteries will operate safely and without overheating. That myth was busted in 2016 when reports surfaced of the Samsung Galaxy Note 7 catching fire. There were also cases where electric vehicles caught fire or even exploded due to battery failures.

The question for battery researchers is: How can they develop higher performing and smaller batteries while maintaining high safety standards? In order to generate more power, batteries generally have to heat up more, which potentially leads to those battery failures. Batteries play a key role in our gadget-filled society, and our reliance on them will only increase as society shifts to more renewable energy sources.

One Virginia Tech chemistry professor and his lab are tackling these challenges. Feng Lin, an assistant professor in the College of Science, has had a productive year with over 10 published papers on battery research in 2018. Lin’s work covers the gamut of chemistry and materials science battery research, looking at micro-scale and basic science aspects, as well as macro-scale and applied science.

Two of Lin's graduate students, Xiaona Pan (front) and David Kautz, work in the glovebox in the lab.

Graduates students in the Feng Lin Lab
Two of Lin's graduate students, Xiaona Pan (front) and David Kautz, work in the glovebox in the lab.

Why batteries fail

The battery community often prioritizes advances in energy density, but Lin focuses on safety first. In papers published by Nano Letters and Nature Communications this year, the Lin Lab looked at what happens at the atomic level when a battery combusts.

“We are taking chemical and physical approaches that were rarely implemented in the battery research previously,” Lin said. “This was a very specialized study about battery safety in trying to identify the chemical origin of safety problems.”

Lin’s postdoc, Linqin Mu, is the first author for both studies.

"We explained the global behaviors of batteries from the perspective of chemical bonding," Mu said. "Now, we can really use the insight to develop advanced battery materials with superior safety and lifetime." 

In their research, Mu and Lin created conditions to mimic a real battery, charged the battery particles, and then observed what happens during overheating. Oxygen gas is released from overheated particles in the cathode, and that gas can induce cell overheating and electrolyte combustion and lead to a complete failure of batteries.

In order to observe these phenomena, Lin teamed up with Yijin Liu and other researchers at SLAC National Accelerator Laboratory in California. This large facility, which Lin estimated to be around the size of the Drillfield, accelerates electrons close of the speed of light and generates light “synchrotron X-rays.” Using the synchrotron X-ray method, Lin’s group and their SLAC collaborators can visualize the changes of the particles in three dimensions at a microscopic resolution.

The research design not only allowed the researchers to fundamentally understand the chemical failures in batteries, but it also allowed them to study the effects of phase change on battery materials. Phase change refers to the solid, liquid, and gas phases and the transitions from one phase to another. Lin said phase change can be detrimental to batteries, and the release of oxygen gas can trigger phase changes, which worsen properties such as energy density and cycle life.

“This study is not about designing a new material,” Lin said. “It’s a study to understand what happens at the chemical and atomic level when overheating in a battery happens. And then in the future, can we design a material better to inhibit this phenomenon?”

The Lin Lab has applied the knowledge learned from this study and developed a range of lithium-ion and sodium-ion battery cathode materials, and the studies have been published in Energy & Environmental Science, Advanced Energy Materials, Journal of Materials Chemistry A, ACS Applied Materials & Interfaces, etc. These studies are driven forward by Lin’s postdoc and graduate students.

“We are excited about our results,” Lin said. “We are still building our foundation, and I am really glad to see our team continue to grow at Virginia Tech.”

Creating a new electrolyte

Building off the research in battery safety, Lin and his lab are developing new, better-performing batteries.

A battery consists of three basic components: anode, cathode, and electrolyte. The electrolyte serves as the medium that facilitates ion movement in between the anode and cathode. Rechargeable lithium-ion batteries are very common in household and smartphone use, and those batteries see lithium ions flow between the anode and cathode.

When an appliance needs power, electrons flow from the anode into the device. These negatively charged electrons continue flowing into the cathode, and positively charged lithium ions move from the anode to the cathode via the electrolyte in order to maintain a neutral charge overall.

“Lithium cannot move in the air, so you need an electrolyte to be the medium,” Lin said. “Like a fish, it can’t swim through the air. At the least, it can’t move for very long.”

The electrolytes of today are mostly liquid and flammable. To address this potential safety hazard, Lin, who is a faculty member with the Macromolecules Innovation Institute (MII), has teamed up with fellow polymer experts in MII to design polymer-based, nonflammable electrolytes that are neither flammable liquids nor impermeable solids.

Lin’s collaboration with MII director and chemistry professor Timothy Long was recently featured in the Journal of Power Sources. Lin’s graduate student Xiaona Pan was the first author on the study.

“Fish don’t swim in air or concrete, but the question is can we design a gel for the fish to go through?” Lin said. “That’s what we’re trying to do, either with a ceramic material or polymer-based material, so the lithium ion can still conduct.

“The cathode work we’ve been doing shows that flammable liquid plus oxygen gas plus local overheating can lead to an explosion. The gel could get rid of the liquid.”

Between smartphones, tablets, and laptops, not to mention cars and other larger applications, batteries are everywhere. You’re probably reading this story using battery power. 

Lin sees the tremendous research opportunities that abound in the battery space. From understanding why our batteries fail to applying that knowledge and developing better batteries, Lin and his group have started to shine in this emerging field.

“I am excited to work with a group of talented postdocs, graduate students, and undergraduate students,” Lin said. “We are excited to be surrounded by so many polymer experts on campus, and with the collaborations with professors Timothy Long, Robert Moore, Louis Madsen, and Michael Schulz, we hope to see more collaborative studies surface in the future.”