Electron paramagnetic resonance as a tool to determine the sodium charge storage mechanism of hard carbon.

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.

Enabling scale-up of mesoporous silicon for lithium-ion batteries: a systematic study of a thermal moderator.

The volume expansion of silicon during cycling of a lithium-ion battery (LIB) leads to degradation and capacity loss. Porous silicon can address many of the issues faced by silicon active materials and has previously been shown to have excellent cyclability. Recently we have uncovered the mechanisms underpinning the pore evolution in magnesiothermic reduction (MgTR) of silica and further demonstrated that it has the potential to produce porous silicon in a scalable and economic manner [J. Mater. Chem. A, 2020, 8, 4938]. However, the scalability of MgTR is affected by the large excess heat produced during reaction. Although previous studies have shown that NaCl can be used as a thermal moderator to mitigate this issue, this has not been systematically investigated, leading to a lack of knowledge on scalability of MgTR. Here, by carefully investigating the roles of NaCl, we show that the NaCl ratio and reduction temperature are the critical factors for controlling scale-up and the product properties. We identified the upper temperature limit of NaCl as a thermal moderator. Further, we systematically showed how the amount of NaCl and the reduction temperature affect the porous properties of the product silicon. Our results have established pathways for scaling-up this method such that it can now be taken forward to target specific porous silicon properties.