Leading the charge to better batteries

To prevent the planet from catastrophic warming, climate experts agree that we need to achieve carbon-neutrality by mid-century. A key step will be switching from fossil fuels to renewable energy sources such as wind, solar, and hydropower.

“There is an ever-increasing demand for sustainable renewable energy as well as emerging applications in electrical vehicles (EVs), future electronic systems, and grid power storage,” says Ho Seok Park, director of the Center for 2D Redox Energy Storage at Sungkyunkwan University (SKKU) in Suwon, South Korea.

Today’s batteries lack the capacity to increase the driving range of EVs and to reliably store large amounts of renewable energy for long periods, so Park and his colleagues at SKKU are striving to build better batteries that employ new materials and comprise novel charge storage mechanisms. “This work is key to overcoming the performance and safety limitations of existing energy storage materials,” Park says.

Oxygen boost

One key aspect of building a better battery is to boost its energy density. The more energy a battery can hold, the further an electric vehicle can run between charges, or the longer it can power household appliances between sunny or windy spells.

With that goal in mind, Park and his team at the Center for 2D Redox Energy Storage (2DRES) are exploring new materials with which to make electrodes — the ‘ends’ of the battery between which a charge travels to generate electricity. In recent years, he has focused on two-dimensional elementary nanomaterials. These make use of a totally different mechanism compared with existing materials, Park says. “For instance, their surface charge storage kinetics is much faster.”

In 2020, Park set up the 2DRES centre dedicated to the study and development of these nanomaterials. “Our centre is aimed at achieving unprecedented properties and new mechanisms of 2D elementary materials through the manipulation of their surface chemistry and multi-scale structure,” he says.

Park is particularly interested in a powder known as 2D black phosphorus (BP), an elemental two-dimensional semiconductor.

2D BP is considered a promising battery material because theoretically it can achieve a high capacity of nearly 2,600 milliampere hour per gram (mAh/g). Batteries comprising the material, however, present a number of challenges: they take a long time to charge, release their energy slowly, and only last a few charge cycles before going flat.

But in 2019, Park’s lab found a way to overcome these obstacles by controllably oxidizing surface atoms of 2D BP.

“When we incorporate oxygen atoms into the surface of 2D BPs, they achieve unprecedented performance and new surface redox mechanisms that existing batteries cannot demonstrate,” he explains. For instance, oxidized black phosphorus can hold four times the amount of charge and can be charged and discharged at a rate that is nearly 3.5 times quicker than non-oxidized black phosphorous.¹

“It’s like adding toppings to a pizza to give it a new flavour,” says Park.

Testing transition metals

Won-Sub Yoon, head of the Department of Energy Science and the director of one of the BK21FOUR programmes at SKKU, is also exploring alternative materials for electrodes.

“The energy density of current lithium-ion batteries is approaching the technological limit,” says Yoon. “There is not much room for improvement.”

Yoon, a battery engineer, is particularly interested in replacing graphite commonly found in existing electrodes with transition metals such as nickel and molybdenum. These materials show “abnormally high charge capacities,” he says — up to four times higher than electrodes in today’s lithium-ion batteries, especially when they have been engineered on the nanoscale.

For instance, when Yoon and his colleagues built an anode made of molybdenum dioxide and introduced nanopores into the compound, its capacity to store lithium ions more than doubled to 1,814 milliampere hour per gram (mAh/g).²

Similarly, when his team came up with a new method to make a nanosructured nickel hydroxide anode, the charge capacity increased nearly three-fold to 1,422 mAh/g from its theoretical capacity of 511 mAh/g.³

“Most future applications like electric vehicles demand much higher energy densities than current ones,” says Yoon. “Nano-engineered transition metal oxides give us entirely new opportunities to make breakthroughs on battery performances, allowing us to develop the next generation of lithium-ion batteries.”

Coating cathodes

Another SKKU professor, Young-Jun Kim, is adopting a slightly different approach to building better batteries, by considering the overall electrode design. This involves enhancing both the energy density of an electrode’s active component, as well as minimizing the volume of inactive parts such as binders, conducting agents, and separators.

“The most significant issue in increasing battery performance is to find an optimal balance between the active and passive components,” explains Kim, who is the director of SKKU’s Advanced Center for Convergent Energy Storage System.

“When there are a lot of inactive components, it reduces the space for the battery’s active materials, which reduces its energy density,” he says.

Take carbon black, for example. The fine powder is added to help enhance the electronic conductivity of cathodes in the majority of rechargeable batteries today. But its addition must also be accompanied by an increase in binder material, all of which reduces the amount of active material in an electrode. It also greatly complicates the slurry mixing process.

In 2021, Kim and his team developed a novel way to coat graphene on to cathodes without the substance disintegrating into the liquid electrolyte or solvent. The result? Only a quarter of the usual amount of conducting agent was required, which in turn led to a 30% increase in electrode volume.⁴

“The cathode’s volumetric capacity hasn’t changed much in the last 20 years,” says Kim. “Although 30% may not seem like much, it represents a significant technical leap when compared to the current rate of technological advancement.”

The SKKU researchers now want to push new battery technology even further and improve other performance aspects. “These include finding a way to charge batteries quickly without sacrificing energy density,” while also perfecting safety, and improving materials to reduce costs, says Kim.

They also hope that their research can be applied to more than just lithium-ion batteries, and used to improve metal and solid-state batteries too, as well as hybrid capacitors.

“SKKU’s work will definitely contribute to the fundamental battery science, as well as practical applications,” says Park.