Hard carbon is a promising negative electrode material for rechargeable sodium-ion batteries due to the ready availability of their precursors and high reversible charge storage. The reaction mechanisms that drive the sodiation properties in hard carbons and subsequent electrochemical performance are strictly linked to the characteristic slope and plateau regions observed in the voltage profile of these materials. This work shows that electron paramagnetic resonance (EPR) spectroscopy is a powerful and fast diagnostic tool to predict the extent of the charge stored in the slope and plateau regions during galvanostatic tests in hard carbon materials. EPR lineshape simulation and temperature-dependent measurements help to separate the nature of the spins in mechanochemically modified hard carbon materials synthesised at different temperatures. This proves relationships between structure modification and electrochemical signatures in the galvanostatic curves to obtain information on their sodium storage mechanism. Furthermore, through ex situ EPR studies we study the evolution of these EPR signals at different states of charge to further elucidate the storage mechanisms in these carbons. Finally, we discuss the interrelationship between EPR spectroscopy data of the hard carbon samples studied and their corresponding charging storage mechanism.

The direct observation of real time electrochemical processes is of great importance for fundamental research on battery materials. Here, we use electron paramagnetic resonance (EPR) spectroscopy to monitor the electrochemical reaction of sodium ions with few-layer MoS₂ and its composite with carbon nanotubes (CNTs), thereby uncovering new details of the reaction mechanism. We propose that the sodiation reaction takes place initially in structural defects at the MoS₂ surface that have been created during the synthetic process (ultrasonic exfoliation), leading to a decrease in the density of Mo⁵⁺ at low symmetry sites that can be related to the electrochemical irreversibility of the process. In the case of the few-layer MoS₂/CNTs composite, we found metallic-type conduction behavior for the electrons associated with the Mo paramagnetic centers and improved electrochemical reversibility. The reversible nature of the EPR spectra implies that adsorption/desorption of Na⁺ ions occurs on the Mo⁵⁺ defects, or that they are neutralized during sodiation and subsequently created upon Na⁺ extraction. These effects help us to understand the higher capacities obtained in the exfoliated samples, as the sum of electrosorption of ions and faradaic effects, and support the suggestion of a different reaction mechanism in the few-layer chalcogenide, which is not exclusively an insertion process.

The structure-directing role of the inorganic secondary building unit (SBU) is key for determining the topology of metal–organic frameworks (MOFs). Here we show that organic building units relying on strong π interactions that are energetically competitive with the formation of common inorganic SBUs can also play a role in defining the topology. We demonstrate the importance of the organic SBU in the formation of Mg₂H₆(H₃O)(TTFTB)₃ (MIT-25), a mesoporous MOF with the new ssp topology. A delocalized electronic hole is critical in the stabilization of the TTF triad organic SBUs and exemplifies a design principle for future MOF synthesis.

Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and has certain advantages over X-ray crystallography; which requires high quality crystals. The technique relies on the design of binding probes that target a functional group, for example, the thiol group of a cysteine residue within a protein. The unpaired electron is typically supplied through a nitroxide radical and sterically shielded to preserve stability. Pulsed electron paramagnetic resonance (EPR) techniques allow small magnetic couplings to be measured (e.g., <50 MHz) providing information on single label probes or the dipolar coupling between multiple labels. In particular, distances between spin labels pairs can be derived which has led to many protein/enzymes and nucleotides being studied. Here, we summarise recent examples of spin labels used for pulse EPR that serve to illustrate the contribution of chemistry to advancing discoveries in this field.