As Christmas is fast approaching one item that won't feature highly on anyone's wish list is a Li-ion battery. But Li-ion batteries are more precious to us than we might realise – with mobile phones, tablets, and many more items coming with a built-in Li-ion battery pack that can be recharged at will, often wirelessly.

Improvements in Li-ion battery technology mean that they are also starting to be used in electric and hybrid vehicles. By 2018 the global demand for Li-ion battery power (for consumer and electric vehicle use) is expected to reach 100 GW hours.[1]  They may well also have an important role to play as storage devices for renewable energy. As our Christmas lights shine in the dwindling daylight it begs the question “what happens to our renewable energy supply when the wind doesn't blow or the Sun doesn't shine?"

“One of the most important scientific problems to solve in our society is how to convert and store clean energy. In order to accomplish a paradigm shift in this field, we need to understand the fundamental dynamical processes that govern the transfer of energy on an atomic scale. Although Li-ion batteries are one of the great achievements of modern materials electrochemistry, we are currently reaching the limits in performance using the available electrode and electrolyte materials. In order to build better batteries, a breakthrough in science and technology is needed. Only recently, technical developments at especially large-scale experimental facilities such as ISIS, have opened new possibilities for studying such material's properties in a straightforward manner." 

 Jun Sugiyama, Toyota Central Research & Development Laboratories, Japan and Dr. Martin Månsson, KTH Stockholm.​

Batteries are made up of one of more cells, each of which contains a positive electrode (anode), a negative electrode (cathode) and an electrolyte between them. When a device is turned on, a stream of positive ions flows across the cell, with a flow of negative electrons flowing though the electric circuits. With a non-rechargeable battery, the chemical reactions involved only work in one direction; when the battery is flat, it's dead. Rechargeable batteries rely on different chemical reactions that can be reversed.

A lot of painstaking research has taken place all over the world investigating which reversible reactions make the best batteries, and how to improve performance. We're constantly demanding more from our electronics - higher power ratings, longer battery lives, quicker charging times. We want our batteries to be smaller, cheaper, and safer. In 2013 the new Boeing 787 Dreamliner aeroplanes were grounded just weeks after delivery, due to an issue with their Li-ion battery packs catching fire. 2016 saw the recall of Samsung's Galaxy Note 7 for the same reason. Lithium is a highly reactive and flammable metal; Li-ion batteries use protective circuits to guard against problems such as over-charging and thermal runaway, which could quickly lead to fire.

We make a lot of demands on battery technology: smaller, safer, cheaper, longer lasting, faster charging. Key to achieving these is a better understanding of how the materials we use in batteries work, and how they can be improved.  Neutrons are a crucial tool in this work, in combination with other imaging techniques, as they give us a better view of how light elements such as lithium are moving around, and better resolution between some of the other elements that are commonly found in Li-ion batteries (such as iron, nickel, cobalt and magnesium). And neutrons allow us to observe reactions on a real-life scale - it's now possible to put a full-sized battery into a neutron beam to see what's going on inside as it charges and discharges over several hours. Muons are useful in battery research, too, probing how fast ions move . Scientists have been using ISIS neutron and muon source to conduct research into Li-ion batteries since the 1980s; the examples below include some of the more recent investigations to hit the scientific headlines.

The structure of lithium electrodes

In 1990, Rossouw et al published (in Materials Research Bulletin[2]) their determination of the structure of Li_xMnO_y electrodes, which used powder X-ray and neutron diffraction studies. They noted at the time that “Recent announcements by the Sony Corporation and Moli Energy and patent applications by leading battery manufacturers have raised expectations for the commercial development of a Li/“Li_xMnO_y" battery as a competitor to the well-established nickel-cadmium system".

Their experiments collected time-of-flight neutron data on ISIS Neutron and Muon Sources' High Resolution Powder Diffractometer (HRPD), which obns a diffraction pattern as a function of the time-of-flight of the neutrons. The results showed, unequivocally, that the “Li_xMnO_y"electrode samples were characterized by a spinel phase and Li₂MnO₃. This led to the important conclusion that the good cycling (charge/discharge) characteristics of the cells could be attributed to the spinel component. Since then, the spinel LiMn₂O₄ has been widely used as a cathode material in Li-ion batteries, including in vehicles such as the all-electric Nissan Leaf and the Chevy Volt hybrid[3].

The structure of lithium copper nitride battery materials

In 2011, Powell et al published the results of their experiments combining Nuclear Magnetic Resonance (NMR) and powder neutron diffraction studies of the structure of lithium copper nitride battery materials, in Physical Chemistry Chemical Physics[4]. Their results shed light on the atomic structure of three lithium copper nitride materials, of interest because lithium nitride has the highest reported lithium conductivity for a crystalline material.

A new in situ battery test cells

As mentioned above, one of the big advantages of using neutrons to investigate battery materials is that they can work on larger samples, and in 2013 Roberts et al[5] published their design for a new Li-ion battery test cell that could be used for in situ experiments in the neutron beamline. The new design allows for the construction of test cells with conventional electrochemical performance - in other words, experiments on them will be as close to real-world conditions as possible. When Li-ion batteries are in use, significant structural changes occur in the electrode materials, which are extremely important in understanding the electrochemistry of the cell and the performance of the battery.

This new test battery is an improvement on the one used in the first in situ neutron diffraction measurement of a Li-ion battery, which was carried out by the same team in 1998. It was designed for use on ISIS Neutron and Muon Sources' POLARIS instrument, where the intense neutron flux and a large detector solid angle mean experiments can take place at high rates of charging and discharging.

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The search for alternatives to lithium graphite anodes

2014[6] saw the publication of results from studies of FeNb₁₁O₂₉ as a possible new material for anodes in Li-ion batteries. Pinus et al conducted neutron diffraction experiments that developed our understanding of the resulting lithium atom structure in the compound. They found that it accepted lithium well both chemically and electrochemically, with very good reaction reversibility, making it an interesting candidate for an anode material.

A wound cell for in situ material analysis​

​In 201​6 B​rant  et al developed a new, large format cell for in situ neutron diffraction experiments, allowing high quality patterns to be collected every 15 minutes, whilst maintaining conventional electrochemical performance. It will enable experiments investigating (for example) perovskite-type oxides, which a re interesting due to their compositional and structural flexibility, which means they can be fine-tuned for particular applications. 

Improving battery material synthesis

The energy storage capacity of Li-ion batteries is primarily limited by the cathode, so this is where research is focusing to bring the next generation of Li-ion batteries that will be needed to power electric cars. One of the most important factors in how well a material performs as an electrode is how it is made, and so Luo et al[7] have developed a simple, 'one pot' method for producing one particular lithium-rich transition metal oxide. Their experiments involved collecting neutron powder diffraction data on POLARIS.

Theirs is not the first experiment at ISIS Neutron and Muon Source to look into innovative ways of producing battery materials. In 2013 a team of researchers led by Vilas Pol[8] used X-ray diffraction on NIMROD to investigate the structure of carbon spheres produced when polyethylene (which could come from carrier bag waste) is heated above 700°C. The properties of carbon materials depend on their structure on the nm scale, which is dependent on the method of manufacture. The carbon spheres investigated in these experiments had a hardness approaching that of diamond, and could be used to extend the life of Li-ion batteries. In research carried out at the Argonne National Laboratory, the team found that heating the carbon spheres to an extremely high temperature, e.g. 2800°C, increased the carbon (graphitic) layering and improved their electrochemical properties still further.

In 2014, Ashton et al[9] used a microwave to synthesise LiFePO₄ for use as a cathode material. Their motivation was two-fold. In part they were looking for more energy-efficient ways to synthesis materials. Rather than heating a furnace for several days, they've got the reaction time down to 15 minutes in the microwave. They were also interested in creating a nanostructure, where the lithium diffusion distance is much shorter and the large surface area offers more opportunities to insert/remove lithium from the cathode, leading to improved electrochemical performance. Using muon spectroscopy on Emu to investigate the diffusion of lithium through the nanostructure, they were able to confirm that the lithium still diffused effectively inside the nanoparticles.

Scientists whose research at ISIS Neutron and Muon Souce looked at battery materials won awards in 2016 for their work and we're looking forward to more exciting science in future. Now researchers are using ISIS to investigate materials that could well make up the batteries hiding under your tree in a few Christmases' time.  

References​

[1] Yoshino, Akira. "The Birth of the Lithium‐Ion Battery." Angewandte Chemie International Edition 51.24 (2012): 5798-5800. http://onlinelibrary.wiley.com/doi/10.1002/anie.201105006/full   

[2] Rossouw, M. H., et al. "Structural aspects of lithium-manganese-oxide electrodes for rechargeable lithium batteries." Materials research bulletin 25.2 (1990): 173-182.

[3] http://www.batteriesinternational.com/michael-thackeray/ 
[4] Powell, Andrew S., et al. "Structure, stoichiometry and transport properties of lithium copper nitride battery materials: combined NMR and powder neutron diffraction studies." Physical Chemistry Chemical Physics 13.22 (2011): 10641-10647.

5] Roberts, Matthew, et al. "Design of a new lithium ion battery test cell for in-situ neutron diffraction measurements." Journal of Power Sources 226 (2013): 249-255. 

[6] Pinus, Ilya, et al. "Neutron Diffraction and Electrochemical Study of FeNb11O29/Li11FeNb11O29 for Lithium Battery Anode Applications." Chemistry of Materials 26.6 (2014): 2203-2209.

[7] Luo, Kun, et al. "One-pot Synthesis of Lithium-rich Cathode Material with Hierarchical Morphology." Nano Letters (2016).

[8] http://www.sciencedirect.com/science/article/pii/S000862231301021X

[9] http://pubs.rsc.org/en/content/articlepdf/2014/ta/c4ta00543k​

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