New materials for the energy transition

As we move towards net-zero emissions we need to reconsider how we generate, store, transport and use energy. At the heart of all these applications are the materials and devices which enable us to move towards greener technologies. At the Cavendish, together with the wider Cambridge community, we are developing materials and devices for the energy transition, including energy efficient computing, energy generation, transmission and storage. This not only includes research within our Energy Materials research theme but also two new graduate courses focused on this highly interdisciplinary area.

I lead the Functional Energy Materials (FEM) group, an interdisciplinary team working at the interface between physics, chemistry and materials. We focus on developing and understanding materials for next generation energy devices including rechargeable batteries, low temperature cooling, and optoelectronic materials. The group uses a wide range of measurement techniques to explore the interplay between structure and a material’s properties in complex oxide systems. We are interested in developing materials for next-generation, beyond-lithium-ion batteries.

Recently, we have explored the role of the Jahn-Teller electronic transition in nickel (Ni³⁺) oxides in layered sodium-transition metal oxides, which are of interest as next-generation battery materials. The energy lowering Jahn-Teller transition lifts the degeneracy of partially occupied bands in a material and reduces the symmetry of the transition metal site. In NaNiO₂, the local Jahn-Teller distortion is ordered throughout the structure resulting in symmetry lowering, which is suppressed on heating above 500 Kelvin. Jahn-Teller transitions are also observed in other technologically relevant materials including undoped high temperature cuprate superconductors and manganese oxides with ‘colossal magnetoresistance’, which is when the electrical conductivity of a material changes dramatically when placed in a magnetic field. Such materials could be used in developing magnetic memory devices. 

Our initial focus was on understanding the nature of the high temperature transition in NaNiO₂. Through experiments at several international facilities – the Spallation Neutron Source in the USA, the ISIS Pulsed Neutron and Muon Source in the UK, the European Synchrotron Radiation Facility in France and the Diamond Lightsource in the UK – we studied this as a function of both temperature and pressure. Our work resulted in the first observation of a displacive rather than an order-disorder Jahn-Teller transition and we have subsequently demonstrated similar behaviour in the lithium-containing analogue, LiNiO₂, a parent system for many next-generation lithium-ion batteries. 

Using our knowledge of NaNiO₂ we explored the structures formed when NaNiO₂ is cycled in a battery. On removing sodium ions (i.e. charging in a battery), the presence of the Jahn-Teller distortion gives rise to complex structures in the Na_xNiO₂ phases which limit reversibility and enhance degradation. The Na_xNiO₂ structures were first reported in the 1980s, however their exact structures had not been fully determined due to their complex nature caused by the combination of Na vacancy, Ni charge and Jahn-Teller ordering. Using high-intensity, high-resolution and low noise measurements we have successfully determined the structure of the de-sodiated phases. In all phases we find complex combinations of Ni charge ordering, Jahn-Teller distortions and Na vacancy ordering. 

Using tools developed in our work on NaNiO₂, we can show that whilst the Jahn-Teller distortion is suppressed in Na_xNiO₂ the Ni sites still retain some Jahn-Teller characteristics which give rise to the complex phase diagram not observed in other layered transition metal oxides. In ongoing work, we are exploring how we can use our understanding of Na_xNiO₂ to minimise degradation and improve battery performance. 

A long-standing project in the Functional Energy Materials group is exploiting divalent batteries, which have the advantage of driving two electrons through the external circuit for every ion transported across the battery. At the turn of the century, divalent magnesium-ion batteries were demonstrated to reversibly cycle multiple times, albeit at much lower operating voltages and capacities than commercial lithium-ion batteries. Since then, there have been extensive reports of both higher voltage and higher capacities in magnesium-ion systems. Our work in this area has shown that the promising electrochemical performance often arises from degradation, rather than magnesium-ion transport, highlighting the need for detailed post-cycling studies in magnesium batteries.

In addition to batteries, we are also interested in developing alternatives to cryogenic gases. Helium gas is typically used for cooling to very low temperatures. However, helium is a biproduct of natural gas extraction and in recent years there have been shortages in supply. In addition to allowing for low temperature physics experiments, helium is used to cool the superconducting magnets in magnetic resonance imaging (MRI) scanners and high energy particle accelerators, as well as in many quantum computing applications that require cooling to low temperatures. In this area we focus on magnetic cooling using changes in the ‘magnetocaloric effect’, changes in temperature driven by changing spin configurations on application of a magnetic field. To optimise performance, we use geometrically frustrated magnetic systems with lanthanide ions to maximise the entropy available and the accessible temperatures for cooling. 

Collaborating with Mike Zhitomirsky (CEA, Grenoble) and Claudio Castelnovo (TCM, Cavendish), we have experimentally verified the theoretically predicted enhancement of the magnetic cooling rate for some geometries of magnetic lattices, whilst also showing that the best materials for low temperature cooling will be those with the weakest magnetic interactions. In complementary experimental work we have been able to demonstrate free spin (weakly interacting) dominated magnetocaloric effect in gadolinium (Gd³⁺) containing dense double perovskites. Following on from our initial reports, we are now exploiting the chemical diversity of double perovskites to tune the magnetic and magnetocaloric properties. 

Our work combining physics, chemistry, and materials science allows us to understand the processes occurring in energy materials. Using facilities both in Cambridge and elsewhere we can build a detailed understanding of the relationship between structure and properties in materials relevant for the energy transition.