New composite materials, developed by researchers at Stanford University, could be the key to ultra-light weight and fire resistant lithium-ion batteries, unlocking the potential for safer and higher capacity batteries for electric vehicles and a 20 per cent boost in efficiency.
The researcher team based at the Stanford University’s Department of Materials Science and Engineering have used innovative composite materials in lithium-ion batteries, that have allowed for the same amount of energy storage while significantly reducing the weight of the batteries, while also delivering improvements in fire safety.
In a paper published in the journal Nature Energy, the research team detailed how they targeted ‘current collector’ components within lithium-ion batteries, which assist in the process of charging and discharging the batteries, but do not contribute to the amount of energy that can be stored.
The researchers said that current collectors, which are generally made from metals like copper or aluminium, contribute as much as 15 to 50 per cent of the weight of a lithium-ion battery, and that by achieving reductions in this weight it would be possible to make substantial improvements to the energy density of battery devices.
“Our strategy for the design of next-generation [current collectors] holds enormous promise for practical battery applications by providing safer lithium batteries with even higher specific energies,” the paper says.
The team, led by Stanford’s professor Yi Cui, used innovative composite materials using polyimide to achieve the same level of performance, while dramatically reducing the weight. The researchers estimated that the new current collectors would provide a boost of between 16 to 26 per cent in the energy density of lithium-ion batteries.
“The current collector has always been considered dead weight, and until now it hasn’t been successfully exploited to increase battery performance,” professor Yi Cui said.
“But in our study, making the collector 80% lighter increased the energy density of lithium-ion batteries – how much energy they can store in a given weight – by 16-26%. That’s a big jump compared to the average 3% increase achieved in recent years.”
In their paper, the researchers say that the current collectors have an important role in battery performance, but have a disproportionate contribution to battery weight, particularly without adding to battery storage capacity.
“Although [current collectors] in batteries play a critical role in efficient electron transfer and mechanical support for electrode materials, they are generally inactive components that cause considerable dead weight yet do not contribute to cell capacity. Thus, the battery industry is constantly trying to decrease the thickness of these metal foils to achieve a lightweight [current collectors],” the paper says.
The researchers were also able to incorporate a layer of triphenyl phosphate based flame retardant, which is just 9 micrometres thick and sandwiched between super thin layers of copper, allowing the materials to self-extinguish any fires that might be triggered within the batteries, such as when a short-circuit is caused, providing a significant improvement in the safety of the batteries.
“People have also tried adding fire retardant to the battery electrolyte, which is the flammable part, but you can only add so much before it becomes viscous and no longer conducts ions well,” professor Cui added.
By using a composite materials approach, where the copper current collectors were layered on the fire retardant material, the researchers were able to use layers of copper just 500 nanometres thick, allowing for a seven-fold reduction in the amount of copper used in the battery current collectors.
The research is likely to contribute to the development of lighter-weight lithium-ion batteries, which has been a critical requirement for electric vehicles, where battery storage represents a significant proportion of the weight of the vehicle.
At its recent ‘battery day’ event, Tesla announced that the company is aiming to make substantial improvements in battery designs, that work to boost the capacity of lithium-ion battery packs, while reducing the weight and cost.
Tesla unveiled its own innovation to battery technology, with CEO Elon Musk announcing that the company had developed a ‘tab-less’ battery design, that allowed for more energy to be drawn from the battery, as well as facilitating faster battery charging, by eliminating electrical ‘choke points’ within a battery cell.