Understanding an organic negative electrode material for sodium-ion batteries

This thesis presents an investigation of sodium naphthalene-2,6-dicarboxylate (Na₂NDC), an organic negative electrode material (ONEM) for sodium-ion batteries (NIBs). The structure of Na₂NDC, solved using 3D electron diffraction (3D ED), featured repeating “organic” naphthalene layers and “inorganic” Na⁺-O layers. The electrochemical redox behaviour suggested a biphasic mechanism. Electrochemical Na⁺ insertion resulted in the loss of long-range order and radical formation, revealed by ex situ powder X-ray diffraction (PXRD) and ex situ electron paramagnetic resonance (EPR) spectroscopy. Operando Raman spectroscopy revealed a well-defined local structure throughout electrochemical Na⁺ insertion and extraction and showed two distinct Raman signatures, confirming a biphasic mechanism. A chemical sodiation procedure was applied to Na₂NDC to enable characterisation without interference from electrode additives and to attempt to improve the initial coulombic efficiency (ICE) in sodium half-cells. The chemical sodiation product was a radical species which lacked long-range order but possessed a well-defined local structure, similar to the electrochemical Na⁺ insertion product. Electrodes fabricated from presodiated Na₂NDC demonstrated an improved ICE, which could be modulated up to 150% by controlling the amount of reducing agent added to Na₂NDC. Ex situ pair distribution function (PDF) analysis confirmed that the chemical and electrochemical Na⁺ insertion products were structurally similar. Na⁺ insertion resulted in the loss of long-range order, accompanied by the intermolecular rearrangement of the local structure, whilst the intramolecular naphthalene framework was retained. A model of the structure of the chemical sodiation product was developed using a multi-technique approach. The model revealed the expansion of the inorganic layer, which featured altered Na⁺ coordination environments and varied Na⁺ occupancies, revealing incomplete Na⁺ insertion. The naphthalene units in the organic layer reorientated to occupy a parallel offset orientation, which resulted in close carbon-carbon distances between neighbouring naphthalene units, this phenomenon was attributed to pancake bonding.