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. Hard carbons are considered the most suitable negative electrode materials for Na-ion batteries. Here, authors use electron paramagnetic resonance spectroscopy to determine the spin nature of mechanochemically-modified hard carbons to predict their charge storage mechanism for sodium-ion batteries.

Hard carbons are the most suitable anode materials for practical sodium-ion batteries (NIBs). Despite various studies, there is still significant scope for improvement in the understanding of the (de)sodiation mechanisms. Here, we study Sn incorporation in waste derived commercial and model sucrose derived hard carbons and its effect on the electrochemical performance. Sn incorporation leads to improved first cycle coulombic efficiency and capacity, specifically increase in the plateau capacity. An improvement from 220 mAh/g to 285 mAh/g and 325 mAh/g is respectively obtained for 7 % and 15 % Sn in hard carbon-Sn composites (HC/Sn). Sn incorporation in both hard carbons has been shown to improve the electrochemical performance, notably achieving a synergy with capacities in excess of that expected from simple addition. For example, 7 % Sn additions tend to increase capacity by 25 %, twice that predicted from simple addition. X-ray diffraction (XRD) studies show that the number of graphene layers in nano-graphitic domains is reduced after Sn incorporation with no change in interlayer spacing. Full cells with commercial benchmark cathodes are also presented along with cost analysis of the Sn doping routes in this study to demonstrate the commercial viability of the strategy. Cost effective anode: Improvements in hard carbon negatives achieved through Sn doping with improvement in electrochemical performance. Specifically, improvement in plateau region of hard carbon is observed with no compromise on initial coulombic efficiency.image

Operando Raman spectroscopy is a well-established technique for monitoring chemical changes in active materials during electrochemical cycling of alkali-ion cells. To date, however, its application to the study of commercial electrodes under realistic operating conditions has been severely limited by cell design constraints. We present here an improved configuration for performing operando Raman spectroscopy on coated metal foil electrodes used in standard laboratory cell testing. Electrochemical modeling predicts much improved lithiation homogeneity compared to a previously used configuration; this observation is validated experimentally for a commercially-sourced graphite electrode. The new configuration delivers improved electrochemical performance at higher specific currents than was previously possible, ensuring that Raman measurements at a single location are representative of the entire electrode. Finally, the broad applicability of the configuration is demonstrated through a study of hard carbon sodium-ion negative electrodes over 50 cycles. These results provide a new configuration for performing reliable, validated operando Raman spectroscopy on commercial battery electrodes, as well as establishing a general methodological framework for the validation of operando spectroscopic techniques to ensure that their performance is relevant to the practical systems to which they are applied.