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Improving electrochemical energy storage is one of the major issues of our time. The search for new battery materials together with the drive to improve performance and lower cost of existing and new batteries is not without its challenges. Success in these matters is undoubtedly based on first understanding the underlying chemistries of the materials and the relations between the components involved. A combined application of experimental and theoretical techniques has proven to be a powerful strategy to gain insights into many of the questions that arise from the "how do batteries work and why do they fail" challenge. In this Review, we highlight the application of solid-state nuclear magnetic resonance (NMR) spectroscopy in battery research: a technique that can be extremely powerful in characterizing local structures in battery materials, even in highly disordered systems. An introduction on electrochemical energy storage illustrates the research aims and prospective approaches to reach these. We particularly address "NMR in battery research" by giving a brief introduction to electrochemical techniques and applications as well as background information on both in and ex situ solid-state NMR spectroscopy. We will try to answer the question "Is NMR suitable and how can it help me to solve my problem?" by shortly reviewing some of our recent research on electrodes, microstructure formation, electrolytes and interfaces, in which the application of NMR was helpful. Finally, we share hands-on experience directly from the lab bench to answer the fundamental question "Where and how should I start?" to help guide a researcher's way through the manifold possible approaches. ; This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 655444 (O.P.). K.J.G. thanks the Winston Churchill Foundation of the United States and the Herchel Smith Scholarship for financial support.

Operando $^{23}$Na solid-state NMR and pair distribution function analysis experiments provide insights into the structure of hard carbon anodes in sodium-ion batteries. Capacity results from "diamagnetic" sodium ions first adsorbing onto pore surfaces, defects and between expanded layers, before pooling into larger quasi-metallic clusters/expanded carbon sheets at lower voltages. ; J. M. S. acknowledges funding from EPSRC 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.).

Alluaudite sodium iron sulfate Na$_{2+2x}$Fe$_{2−x}$(SO$_4$)$_3$ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe$^{3+}$/Fe$^{2+}$ redox reactions (3.8 V vs Na/Na$^+$ ), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), $^{23}$Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe$^{3+}$/Fe$^{2+}$ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions. ; The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the "Element Strategy Initiative for Catalysts & Batteries" (ESICB) project. The synchrotron XRD experiments were performed under KEK-PF User Program (No. 2013G670). Crystal structures and the Fourier difference maps were drawn by VESTA.65 G.O. acknowledges financial support from JSPS Research Fellowships under "Materials Education Program for the Future Leaders in Research, Industry, and Technology" (MERIT) project. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 655444 (O.P.). R.P. gratefully acknowledges financial support through the Marie Curie Actions People Program of the EU's Seventh Frame work Program (FP7/2007-2013), under the grant agreement n.317127, the 'pNMR project'. K.J.G. gratefully acknowledges funding from The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. This work made use of the facilities of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. ; This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.6b01091

We have developed and explored an external automatic tuning/matching (eATM) robot that can be attached to commercial and/or home-built magic angle spinning (MAS) or static nuclear magnetic resonance (NMR) probeheads. Complete synchronization and automation with Bruker and Tecmag spectrometers is ensured via transistor-transistor-logic (TTL) signals. The eATM robot enables an automated "on-the-fly" re-calibration of the radio frequency (rf) carrier frequency, which is beneficial whenever tuning/matching of the resonance circuit is required, e.g. variable temperature (VT) NMR, spin-echo mapping (variable offset cumulative spectroscopy, VOCS) and/or in situ NMR experiments of batteries. This allows a significant increase in efficiency for NMR experiments outside regular working hours (e.g. overnight) and, furthermore, enables measurements of quadrupolar nuclei which would not be possible in reasonable timeframes due to excessively large spectral widths. Additionally, different tuning/matching capacitor (and/or coil) settings for desired frequencies (e.g. $^{7}$Li and $^{31}$P at 117 and 122MHz, respectively, at 7.05 T) can be saved and made directly accessible before automatic tuning/matching, thus enabling automated measurements of multiple nuclei for one sample with no manual adjustment required by the user. We have applied this new eATM approach in static and MAS spin-echo mapping NMR experiments in different magnetic fields on four energy storage materials, namely: (1) paramagnetic $^{7}$Li and $^{31}$P MAS NMR (without manual recalibration) of the Li-ion battery cathode material LiFePO$_{4}$; (2) paramagnetic $^{17}$O VT-NMR of the solid oxide fuel cell cathode material La$_{2}$NiO$_{4+δ}$; (3) broadband $^{93}$Nb static NMR of the Li-ion battery material BNb$_{2}$O$_{5}$; and (4) broadband static $^{127}$I NMR of a potential Li-air battery product LiIO$_{3}$. In each case, insight into local atomic structure and dynamics arises primarily from the highly broadened (1-25MHz) NMR lineshapes that the eATM robot is uniquely suited to collect. These new developments in automation of NMR experiments are likely to advance the application of in and ex situ NMR investigations to an ever-increasing range of energy storage materials and systems. ; 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.). D.M.H. acknowledges funding from the Cambridge Commonwealth Trusts. J.L. gratefully acknowledges Trinity College, Cambridge (UK) for funding. K.J.G. gratefully acknowledges funding from the Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. M.B. is the CEO of NMR Service GmbH (Erfurt, Germany), which manufactures the eATM device; M.B. acknowledges funding of the Central Innovation Programme for small and medium-sized enterprises (SMEs; Zentrales Innovationsprogramm Mittelstand, ZIM) of the German Federal Ministry of Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie, BMWi) under the Grant No. KF 2845501UWF. DFT calculations were performed on (1) the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council and (2) the Center for Functional Nanomaterials cluster, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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. ; This work was supported by STFCBatteries.org 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.