Essential for powering the technology we use every day, batteries are in almost everything – from electric cars to the device you’re using to read this article. The most common type of battery is the lithium-ion battery (LIB), but the world’s supply of lithium is finite, and drastically depleting. Some believe there could be a global shortage within just 2-3 years.
It’s clear that we need additional types of batteries if increased electrification is going to be used to tackle our reliance on fossil fuels – batteries whose performance can compete with LIBs, without using any lithium. Luckily, that solution may come in the form of the sodium-ion battery (SIB).
First developed three decades ago, SIBs are an improved alternative to their more widespread lithium-ion counterparts. In fact, their working principle is the same as LIBs. However, the cation that enables the battery to be charged and discharged multiple times is sodium instead of lithium.
One key focus of SIB research is to develop high-performing anode materials that can store sodium ions. Currently, research in the area is focusing on investigating the use of hard carbon for this purpose.
Hard carbon can be produced cheaply and sustainably by heating certain widely-available organic materials such as lignin and sucrose to very high temperatures (in the region of 800–2000 °C). Charge can then be stored, and these storage mechanisms have already been extensively studied.
The challenge arises when attempting to characterise the different storage mechanisms, due in part to the variety and complexity of hard carbon structures.
In this study, published in the Journal of Materials Chemistry, a team from the Faraday Institution, led by ISIS-based researchers Gabriel Perez and Emily Reynolds, used small angle neutron scattering (SANS) to observe the insertion of sodium in a commercially available type of hard carbon throughout its discharge cycle. To do this, they used a bespoke electrochemical cell developed by Innes McClelland with his co-supervisor Peter Baker and supervisor Professor Serena Cussen at the University of Sheffield during his ISIS facility development studentship. Although designed for in operando muon spectroscopy studies, they successfully used it for SANS experiments with a few minor modifications. Due to the ISIS long shutdown, their experiments were carried out at SINQ, the Swiss Spallation Neutron Source.
“SANS can provide information about the nanoscale morphology and composition of porous materials, such as hard carbon, as well as the content in the pores,” says Gabriel Perez. “Therefore, this technique is well-suited for this investigation. In particular, neutrons offer the possibility of distinguishing carbon from sodium, and are highly penetrating allowing them to probe the bulk of the material even if other materials, such as the cell components that aren’t of interest, are on the way of the beam.”
They found that there are three distinct regions in which the hard carbon stores sodium during the discharge of a sodium-ion battery. This evidence directly supports a currently debated model for the sodium-ion insertion processes, building up better understanding of the way that SIBs store charge.
A visual representation of the 3-phase discharge mechanism, taken from the paper.
This research will help improve sodium-ion batteries, making them more efficient by designing hard carbon with structures that can maximise electrochemical performance. This is crucial to developing a commercially available alternative to LIBs, to support an increased need for batteries despite the world’s lithium shortage.
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