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Investigating Sodium Storage Mechanisms in Tin Anodes: A Combined Pair Distribution Function Analysis, Density Functional Theory and Solid-State NMR Approach

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Stratford, JM 
Allan, PK 
Pecher, O 
Borkiewicz, OJ 


The alloying mechanism of high-capacity tin anodes for sodium-ion batteries is investigated using a combined theoretical and experimental approach. Ab initio random structure searching (AIRSS) and high-throughput screening using a species-swap method provide insights into a range of possible sodium-tin structures. These structures are linked to experiments using both average and local structure probes in the form of operando pair distribution function analysis, X-ray diffraction, and 23Na solid-state nuclear magnetic resonance (ssNMR), and ex situ 119Sn ssNMR. Through this approach, we propose structures for the previously unidentified crystalline and amorphous intermediates. The first electrochemical process of sodium insertion into tin results in the conversion of crystalline tin into a layered structure consisting of mixed Na/Sn occupancy sites intercalated between planar hexagonal layers of Sn atoms (approximate stoichiometry NaSn3). Following this, NaSn2, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn3, but has no tin atoms between the layers. NaSn2 is broken down into an amorphous phase of approximate composition Na1.2Sn. Reverse Monte Carlo refinements of an ab initio molecular dynamics model of this phase show that the predominant tin connectivity is chains. Further reaction with sodium results in the formation of structures containing Sn-Sn dumbbells, which interconvert through a solid-solution mechanism. These structures are based upon Na5-xSn2, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, Na15Sn4, can store additional sodium atoms as an off-stoichiometry compound (Na15+xSn4) in a manner similar to Li15Si4.



0306 Physical Chemistry (incl. Structural)

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Journal of the American Chemical Society

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American Chemical Society
United States Department of Energy (DOE) (via University of California) (7057154)
European Commission (655444)
Engineering and Physical Sciences Research Council (EP/K014560/1)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (696656)
Engineering and Physical Sciences Research Council (EP/P022596/1)
This work was supported by through the STFC Futures Early Career Award (J.M.S.). J.M.S. acknowledges funding from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. DOE under Contract no. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program subcontract no. 7057154, and the European Commission under grant agreement no. 696656 (Graphene Flagship). P.K.A. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship and a Junior Research Fellowship from Gonville and Caius College, Cambridge. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 655444 (O.P.). M.M. and A.J.M. acknowledge the support from the Winton Programme for the Physics of Sustainability. A.J.M. and C.J.P. were supported by Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (Grant no. EP/G007489/2). C.J.P. is also supported by the Royal Society through a Royal Society Wolfson Research Merit award. Calculations were performed using the Archer facility of the UK national high performance computing service, for which access was obtained via the UKCP consortium and funded by EPSRC grant no. EP/K014560/1.
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