Shedding Light on New Battery Materials: the vision of combining experiments with computation
“Seeing” individual atoms is a tricky business. At such tiny length scales illumination by individual packets of light, called photons, will not work. Their wavelength, around 500 nanometres, (about 150th of a human hair) is simply too large to resolve atomic scale features in materials. To see how nature works at the atomic scale, transmission electron microscopy (TEM) uses a shorter wavelength particle, the electron. However, in the life-sciences this technique has proved unsuitable. Many biological molecules are too delicate: imaging them in an electron beam is like imaging a Ming vase with an artillery barrage.
This year’s (2017) Nobel Prize in Chemistry was awarded to Jacques Dubochet, Joachim Frank and Richard Henderson for the development of cryo-electron microscopy. To avoid damage to the bio-molecule from the electron beam “barrage” they deep freeze biological molecules to -200oC using liquid nitrogen. This not only protects the molecule from the beam, but quite literally freezes it allowing them to make a 3-dimensional image of its behaviour exactly at the moment it was frozen.
Cryo-electron microscopy has recently spread from biology to materials science for seeing the atomic scale problems within developmental lithium-ion batteries. Yi Cui’s research group in Stanford have employed cryo-electron microscopy to visualise individual lithium atoms in a dendrite. Lithium dendrites form as lithium repeatedly moves between the electrodes as a battery is charged and discharged. Dendrites cause large volume changes in electrodes, cracking materials and leading to reduced battery lifetimes. Moreover, they are also capable of catastrophically short circuiting the battery. Seeing them in such unprecedented detail is a first step to mitigating their formation. It is a promising technique for visualising many other technological battery problems and timely, since in October of this year the UK Government announced £246 million investment in battery research and development through its Faraday Research Challenge, which forms part of the government’s Industrial Strategy Challenge Fund (ISCF).
Seeing material degradation and failure is a necessary step in understanding energy materials, but, to fully exploit this, real understanding requires a conceptual model. Hence experiment is made all the more powerful when combined with simulation. Moreover, “Trial and error” still plays a large part in the discovery of new materials. From the initial idea, the material must be synthesised before its suitability as a battery can be measured. Synthesis is slow, difficult and expensive. To cover the vast design space and accelerate these new battery materials from concept to market, the only possible recourse is rational design using computers. High-throughput computation accelerates the design process by suggesting then screening new materials from the atom up, allowing us to ask "what if?" without the time and expense of manufacturing and categorizing samples. In recognition of the need for computational simulation the UK has invested in 6 new Tier-2 High-performance Computing Centres which came online in April 2017, encompassing HPC Midlands+ operated by 7 universities including the University of Birmingham.
At Birmingham we have expertise in using high-throughput computation to discover and categorise new battery materials at the atomic scale. We collaborate with world-leading battery experimentalists predicting the structure and properties of novel electrode materials for both lithium-, and the in-development but cheaper, sodium-ion batteries. In 2014 our predictions of high-capacity germanium electrodes went on to be experimentally verified . This year, with experimental colleagues we used high-throughput computation to show how an electrode made from tin for a sodium-ion battery behaves.
An ability to computationally predict new energy materials, and suggest the most promising candidates for synthesis, followed by their examination with this unprecedented resolution, is a vision more insightful than just “seeing” atoms alone.