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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 6. Advanced characterisation 6.1. Neutron characterisation of battery materials Emily M Reynolds1,2 and Martin O Jones1,2 1 Science and Technology Facilities Council, North Star Avenue, Swindon SN2 1SZ, United Kingdom 2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom Status Neutrons scatter from elemental nuclei and consequently interact weakly with condensed matter. This property offers considerable advantages in addressing the challenges associated with characterising the complex structure and dynamics of Na battery materials. Neutrons have a highly penetrating nature and can probe magnetic structure, resolve light elements and distinguish elements close to each other in atomic number. As a result, neutron diffraction (ND) can provide an insight into transition-metal (TM) distributions, magnetic structure, and Na diffusion pathways. Total scattering can distinguish between locally ordered TM distributions, observe local structural changes due to oxygen redox, characterise the structure of nanoparticles, and quantify disorder in hard carbon anodes. Inelastic techniques such as inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) can probe dynamics including Na diffusion, and techniques such as reflectometry and small-angle neutron scattering (SANS) allow us to observe the formation of the SEI layer and characterise the change in thickness/porosity of the SEI and electrode materials. Imaging techniques including radiography, tomography, and Bragg edge tomography can reveal the evolution of the components during cycling, and muon spectroscopy can probe the dynamics and energetics of Na diffusion. In addition to these specific techniques, neutrons remain a superior characterisation technique for operando studies of commercial and custom batteries due to their highly penetrating nature. Insight into the structural changes on cycling has revealed the mechanisms responsible for capacity fade [223], cycling-induced cation mixing [224], intermediate phases [224], dependence on cycle rate [225], and has led to the optimisation of commercial materials [226]. While the capabilities of neutron characterisation are vast, there is still scope for improvement through advances in facilities, instruments, and devices. For example, higher flux will increase temporal and spatial resolutions, and permit operando studies at higher cycling rates with an improved signal-to-noise (S/N) ratio. Operando neutron experiments are extremely challenging but are essential for a full understanding of battery operation and failure. Current abilities may be improved by developing a simultaneous characterisation facility that combines neutron scattering with electron paramagnetic resonance (EPR), NMR and X-ray spectroscopy techniques, optimising and improving existing sample environment cells, and developing new cells for scattering techniques currently without operando capabilities. Current and future challenges Disorder is a key feature in the most of the promising electrode materials, whether it be in structure, cation and anion distribution, stacking faults, or defects. Distinguishing between disordered states and correlating these to electrochemical properties is very challenging; however, cation disorder can improve Na transport, prevent unwanted phase changes, and reduce volume changes during cycling [227–229]. Furthermore, the structure of amorphous hard carbon (one of the most promising anode materials), and its Na storage mechanisms are difficult to characterise due to the lack of long-range order. While neutron characterisation, specifically total scattering, is well-equipped to address this, the routine characterisation of disordered materials remains a challenge. The analysis of such data often requires additional techniques to generate disordered models, either driven by physical/thermodynamic constraints, such as the Monte Carlo method, or driven by data, such as the reverse Monte Carlo. In addition, operando total scattering remains challenging, as any additional components from the battery setup interfere severely with data analysis. Neutron characterisation excels in operando studies, despite the low neutron flux, which limits temporal and spatial resolutions. For diffraction, this limits the cycling rate, spatial resolution across an electrode, and the data quality achievable. For other techniques, such as quasi-elastic scattering (QENS), it has inhibited in-situ studies altogether. Cell design is also complex, as the requirements for good quality data and electrochemical cycling comparable to commercial battery operation are different and often incompatible. In general, understanding and characterising the formation of the SEI on electrode materials, its chemical composition, evolution on cycling, and impact on diffusion are huge challenges for the battery community and are still in their infancy. Neutron reflectometry is an excellent technique for studying buried interfacial structures, and the highly penetrating nature of neutrons means that SEI formation can be observed in-situ, which is essential, as the interfacial structures are usually very sensitive to air. A major challenge for this technique is the development of an operando cell that presents a thin electrode with an extremely smooth surface. 56PDF Image | 2021 roadmap for sodium-ion batteries
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