Muon spectroscopy (µSR) is a characterization technique that analyzes the effects of implanted spin-polarized muons on a material. The method detects the influence that the muons have on the atomic, molecular and crystalline matrix of a material. A team of researchers from the UK has now used this technique to depict the diffusion characteristics of lithium ions (Li+) in V- and Nb-doped LiFePO4 materials.
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The use of muons is not a standard characterization method in science. Nevertheless, they did gain popularity recently when muography was used to detect a hidden chamber in the Great Pyramid of Giza. One area of affluent muon research is in their use as a probe to determine the solid-state diffusion behavior of a material.
Diffusion processes are not always well understood in some materials and muons offer a way to effectively measure these processes. Muons are particularly useful for lithium diffusion mechanisms. The diffusion of lithium ions is known to perturb the implanted muons, allowing the diffusion coefficients to be extracted using a series of calculations.
Muon spectroscopy has been used to identify the diffusion characteristics in many lithium-based materials, and the researchers from the UK have now extended this knowledge to vanadium and niobium doped LiFePO4.
The researchers synthesized pure and doped LiFePO4 materials using a pilot-scale continuous hydrothermal flow synthesis (CHFS) reactor and performed the muon spectroscopy experiments on the EMU instrument at the ISIS pulsed muon and neutron source. To analyze the data, the researchers employed a Windows Muon Data Analysis (WiMDA) program and further characterized the materials using X-ray diffraction (XRD).
The spectra produced using this spectroscopy technique was found to arise from the rapid interaction of the paramagnetic iron moments with the lithium and phosphorus nuclear magnetic moments. This interaction allowed the lithium ions to diffuse and be extracted in a similar way to previous studies.
The researchers also tried to account for any iron impurities that could affect the results. They achieved this by employing an exponentially decaying function in their calculations. This, however, was not picked up using XRD and the effect was therefore assumed to be negligible.
The diffusion coefficients obtained for the doped materials by the researchers were found to be close to the values obtained for bulk and nanometric LiFePO4. The exact diffusion coefficient was found to be in the range of 1.8-2.3x10-10cm-2s-1. The fact that the values lie close to undoped LiFePO4 has showcased muon spectroscopy as a versatile technique for measuring families of materials, even when the synthesis of said materials is different.
The doped LiFePO4 materials were also found to possess a higher electrochemical performance than their non-doped counterparts. However, the small difference in diffusion coefficients also told the researchers another point; that the improved performance is not a direct result of increased local lithium ion diffusion. Instead, the researchers believe that other contributing factors, such as a higher electrical conductivity and stabilization arising from the doped structure, are more likely to play a vital role in the enhancement of the doped material.
This observation was only made possible because of muon spectroscopy. Muon spectroscopy can measure self-diffusion with the LiFePO4 material and negated the need for a two-phase delithiation mechanism. This mechanism has previously been used but has prevented an accurate comparison from being made between doped and undoped materials. Therefore, muon spectroscopy can identify the effects of dopants at the local scale.
The results of this research have showcased the versatility that muon spectroscopy can bring in identifying the diffusion behavior of lithium-based materials. Given that lithium-based materials are most prevalent in battery applications, muon spectroscopy could be a useful tool for measuring the diffusion characteristics of many battery materials in the future.
“Mechanistic insights of Li+ diffusion within doped LiFePO4 from Muon Spectroscopy”- Johnson I. D., et al., Scientific Reports, 2018, DOI:10.1038/s41598-018-22435-1