Researchers from Rice University have used boron nitride to engineer lithium-ion batteries which work at far greater temperatures – and oil and gas technology could benefit
Researchers at Texas’ Rice University have engineered a new electrolyte and separator for rechargeable lithium-ion batteries, allowing the cells to work across a far greater temperature range.
Central to their innovation is the use of hexagonal boron nitride (h-BN), an atom-thin compound often referred to as “white graphene.” The resulting composite – which apparently resembles toothpaste – is used alongside conventional electrodes to improve the stability of the battery at high temperatures and voltages.
The team’s paper, published in the Advanced Energy Materials journal in April, reports that the test battery remained stable for over 600 cycles at a temperature of 120°C, experiencing a capacity loss of 3%.
Rice materials scientist Pulickel Ajayan stated in a press release that the battery continued to work from room temperature up to 150°C – one of the widest recorded envelopes for li-ion batteries. That is more remarkable when one considers that traditionally, storing a battery anywhere above 30°C is considered an “elevated” temperature.
Most importantly, as the study’s lead author Rice graduate student Marco-Túlio Rodrigues told InnovOil, there are no commercial li-ion batteries on the market that can sustain temperatures above 80°C. “Even if they manage to avoid a complete failure above this temperature,” he said, “They will not be able to perform nearly as well as they did under milder conditions.”
Given that h-BN is chemically and mechanically resistant, even at very high temperatures, Rodrigues and the team believe that it could help offer increased stability to the battery’s electrolyte layer, helping it to function at higher temperatures.
It possesses a number of desirable qualities for battery applications: it is not a conductor, nor known to be an ionic conductor, and is already used as a component in ceramics for high-temperature applications. Rodrigues explained: “It’s fairly inert, so it shouldn’t react with any chemicals, it won’t expand or contract a lot and the temperature isn’t a problem. That made it perfect.”
Even more promisingly, tests showed that the mix of h-BN, a piperidinium-based ionic liquid and a lithium salt actually appeared to help catalyse a better reaction from the chemicals around them. Rice postdoctoral researcher and co-author Hemtej Gullapalli added: “Even though the boron nitride, which is a very simple formulation, is not expected to have any chemical reaction, it gives a positive contribution to the way the battery works. It actually makes the electrolyte more stable in situations when you have high temperature and high voltages combined.”
What makes the team’s battery interesting is the fact that it not only survives at 150°C, but it works better under these conditions. In an email to InnovOil, Marco Rodrigues said: “Basically, once you can overcome the safety problems and the long-term stability of every single component of the battery, high temperatures actually help, accelerating the chemical reactions associated to energy storage in the cell, increasing the power output. Since our devices were optimised to withstand these harsh conditions, the performance will not degrade.”
Yet that performance boost is a double-edged sword. Perhaps the greatest issue standing in the way of wider commercialisation of these cells is that an average room temperature is probably too low for the battery to operate in. Rodrigues conceded that “right now, due to intrinsic limitations on some of our components, our batteries do not work at their full capacity at room temperature and lower, reaching their peak beyond 50°C.”
For now, that is a pretty major hurdle to overcome – and especially problematic when looking at deployment in EVs – but it does leave the technology very well suited to other applications.
In conventional batteries, separators are made of plastics or polymers, which can melt or break down at higher temperatures. If that happens, or the separator is punctured, the battery will heat up and potentially ignite. Yet the construction of the Rice team’s battery prevents this from happening.
That stability makes batteries inherently safer – a concern which has kept li-ion technology away from applications where they would otherwise be useful. Because the electrolyte’s components are non-flammable, “if there’s a failure, it’s not going to catch fire,” explained Gullapalli.
Greater safety opens new doors for li-ion cells, as the paper’s abstract hints: “The ease of [the Rice team] formulation along with superior thermal and electrochemical stability of this system extends the use of Li-ion chemistries to applications beyond consumer electronics and electric vehicles.”
Power for producers
Industrial equipment and aerospace applications appear to be at the forefront of the team’s thinking. Rodrigues specifically noted the requirements of oil and gas drillers looking for robust batteries to power sensors on wellheads. “They put a lot of sensors around drill bits, which experience extreme temperatures,” he said. “It’s a real challenge to power these devices when they are thousands of feet downhole.”
Currently, many industrial devices which do use batteries use non-rechargeable cells, posing issues with disposal and the fact that they require a complete pack change between discharges.
In his email, Rodrigues explained that while some might be used for oil and gas applications, they were still hindered by reach and safety. “MWD can be done nowadays by powering sensors with wirelines and, if the temperature is not too high, some specific types of non-rechargeable batteries,” he said. “The problems here are that wirelines have limited reach downhole and that both options can intrinsically provide only so much power. Having a remote power source that can operate at high power and even be recharged by harvesting units could allow information to be gathered at much longer timescales.”
This would be possible by linking rechargeable batteries to conventional sources – e.g. mud pulse turbines – allowing them to use and store energy would otherwise be wasted. As Rodrigues alludes to, this opens up the possibility of much longer operating periods for some equipment, and slashes the need to retrieve, recharge and re-deploy.
While the Rice team’s battery is unlikely to be suitable for applications at the very extremes of production, a 150°C operating threshold does carry it into high temperature (HT) territory – an area which is crying out for greater innovation around powering equipment.
For ever more cost-conscious operators, Rodrigues was confident that while these batteries would be more expensive than conventional li-ions, they would not be prohibitive. He added: “Our concept was developed towards specialty batteries, able to power under conditions so far impossible to be tackled by current technologies.” So, although the final product may potentially be more expensive, “the benefits generated by operations lacking remote powering capability would overcome the additional costs.”
However the technology is commercialised, it looks promising enough to take li-ion technology into realms it has never been before. With the 80°C goal already surpassed, Ajayan said that the team’s next step would be “to break this barrier and create stable batteries at twice this temperature limit or more.”