A New Tool for NMR Crystallography: Complete C-13/N-15 Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR

International audienceNMR crystallography of organic molecules at natural isotopic abundance (NA) strongly relies on the comparison of assigned experimental and computed NMR chemical shifts. However, a broad applicability of this approach is often hampered by the still limited H-1 resolution and/or difficulties in assigning C-13 and N-15 resonances without the use of structure-based chemical shift calculations. As shown here, such difficulties can be overcome by C-13-C-13 and for the first time N-15-C-13 correlation experiments, recorded with the help of dynamic nuclear polarization. We present the complete de novo C-13 and N-15 resonance assignment at NA of a self-assembled 2'-deoxyguanosine derivative presenting two different molecules in the asymmetric crystallographic unit cell. This de novo assignment method is exclusively based on aforementioned correlation spectra and is an important addition to the NMR crystallography approach, rendering firstly H-1 assignment straightforward, and being secondly a prerequisite for distance measurements with solid-state NMR

Graphite anodes for Li-ion batteries – an electron paramagnetic resonance investigation

Graphite is the most commercially successful anode material for lithium (Li) ion batteries: its low cost, low toxicity and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterised. However, questions remain regarding its electronic structure and whether the electrons occupy localised states on Li or delocalised states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterising the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphite. In this work, ex situ variable-temperature (10-300 K), variable frequency (9-441 GHz) EPR was carried out to extract the g-tensors and linewidths, and understand the effect of metallicity on the observed EPR spectra of charged graphite at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to 7Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.T.I. and C.P.G. were supported by an ERC Advanced Investigator Grant for C.P.G. (EC H2020 835073). E.N.B. was supported by the Engineering Physical Sciences Research Council (EPSRC) via the National Productivity Interest Fund (NPIF) 2018. K.M. was supported by the Faraday Institution Degradation Project (FIRG001 and FIRG024). The Pulsed EPR measurements were performed at the Centre for Pulse EPR at Imperial College London (PEPR), supported by the EPSRC grant EP/T031425/1

DNP-enhanced NMR of Lithium Dendrites: Selective Observation of the Solid–Electrolyte Interphase

Li metal anodes represent the ultimate energy density, but to address safety issues caused by dendrite formation, it is critical to understand the solid–electrolyte interphase (SEI) layer which forms on the metal surface. Dynamic nuclear polarisation (DNP) boosts sensitivity in NMR by harnessing the greater polarisation of unpaired electrons, however typical exogenous organic radicals are non-selective, could react with the SEI, and require cooling the sample to cryogenic temperatures. We instead exploit the inherent conduction electrons to hyperpolarise lithium metal at room temperature, utilising the Overhauser mechanism by which DNP was first discovered. This permits selective enhancement of the organic and inorganic SEI components, revealing their chemical nature and spatial distribution, via the 7Li, 1H and 19F NMR spectra.<br /

Bulk Fatigue Induced by Surface Reconstruction in Layered Ni-Rich Oxide Cathodes for Liion Batteries

Ni-rich layered cathode materials are among the most promising candidates for high energy density Li-ion batteries. However, the low cobalt containing materials suffer from rapid degradation, the underlying mechanism of which is still poorly understood. We herein report a novel structure-drive degradation mechanism for the NMC811(LiNi0.8Mn0.1Co0.1O2) cathode, in which a proportion of the material exhibits a lowered accessible state-of-charge (SoC) at the end of charge after repetitive cycling, i.e. becomes fatigued. Ex-situ and operando long- duration high-resolution X-ray diffraction enabled by a laser-thinned coin cell design clearly shows the emergence of the fatigued phase and the increase in its population as the cycling progresses. We show that the fatigue degradation is a structure-driven process rather than originating solely due to kinetic limitations or inter-granular cracking. No bulk phase transformations or increase in Li/Ni antisite mixing were observed by diffraction; no significant change in the local structure or Li-ion mobility of the bulk were observed by 7Li solid-state NMR spectroscopy. Instead, we propose that the fatigue process is a result of the high interfacial lattice strain between the reconstructed surface and the bulk layered structure when the latter is at SoCs above a distinct threshold of ~75 %. This mechanism is expected to be universal to Ni-rich layer cathodes, and our findings provide a fundamental guide for designing effective approaches to mitigate such deleterious processes.</div