Structural batteries: “massless” energy storage

In recent years, people have sought to harness the world’s natural sustainable resources for our energy needs, such as sunlight, wind and tides. But what if we could also make use of other things that are all around us? What if the buildings that we live and work in could be turned into batteries?

This might sound fantastical, but it could soon be a reality. Recent research has shown that concrete, the ubiquitous material made of cement, gravel and sand, can be functionalised for mass energy storage. Scientists from the Chalmers University of Technology in Gothenburg, Sweden have developed a lab-scale prototype of a rechargeable cement-based battery that contains embedded electrodes.

The prototype was made by adding small amounts of short carbon fibres to the cement mixture, and then implanting a metal-coated carbon fibre mesh, which contains an iron anode and a nickel cathode. Significantly, it is estimated to perform ten times better than previous attempts at concrete batteries.

Power in the walls

This research is part of the wider field of “structural batteries”, in which the battery becomes part of a load-bearing structure, meaning that the mass of the battery is essentially negligible. In other words, the material of a structure is itself the energy storage device, and there is no need for a separate battery. For this reason, structural batteries are said to provide “massless” energy storage.

The challenge is to provide batteries with both good electrical and mechanical properties. Early endeavours struggled to find the right balance, due to poor electrical insulation or low tensile stiffness. However, researchers are now starting to make major strides and envisage that structural batteries could also find use in electrical vehicles. Considering that the battery of the Tesla model S contributes to approximately one-quarter of the total car mass, it would be highly beneficial to the efficiency of future cars if this weight could be eliminated (see our blog Tesla: driving battery development differently).

With an eye to the electric vehicle field, Chalmers have published another paper in collaboration with the KTH Royal Institute of Technology in Stockholm. In this work, a cell has been developed that comprises a carbon fibre electrode and a lithium iron phosphate electrode, separated by a glass fibre fabric and impregnated with a structural battery electrolyte (which includes a porous polymer and liquid electrolyte mixture).

The battery has an energy density of 24 Wh kg−1, which is around 20% of common Li-ion batteries, though it is hoped that in practice the weight-loss opportunities could provide further advantages. It also has an elastic modulus of 25 GPa and tensile strength exceeding 300 MPa, which are comparable with many commonly used materials. And there is optimism that the elastic modulus and energy density might each be tripled in the next wave of research.

The future is light

As well as cars, structural batteries could be applied to produce lighter mobile phones and laptops. Meanwhile, Airbus have recently applied for patents in technology that could eventually allow the wings and fuselage of aircraft to store energy, allowing for lighter and more efficient planes.

Today, rechargeable batteries are a major part of our everyday lives and developments in this field have been impressive, both in terms of the science and the speed of progress. Structural batteries could represent the next major step in this fast-moving area – leading to a world where everyday structures are suitable for energy storage, and where the need for heavy separate battery packs is a thing of the past. Batteries won’t disappear any time soon, but they might become invisible.