Emma Kendrick

Na2CoSiO4 as a cathode material for sodium-ion batteries: structure, electrochemistry and diffusion pathways

The importance of developing new low-cost and safe cathodes for large-scale sodium batteries has led to recent interest in silicate compounds. A novel cobalt orthosilicate, Na2CoSiO4, shows promise as a high voltage (3.3 V vs. Na/Na+) cathode material for sodium-ion batteries. Here, the synthesis and room temperature electrochemical performance of Na2CoSiO4 have been investigated with the compound found to yield a reversible capacity greater than 100 mA h g-1 at a rate of 5 mA g-1. Insights into the crystal structures of Na2CoSiO4 were obtained through refinement of structural models for its two polymorphs, Pn and Pbca. Atomistic modelling results indicate that intrinsic defect levels are not significant and that Na+ diffusion follows 3D pathways with low activation barriers, which suggest favourable electrode kinetics. The new findings presented here provide a platform on which future optimisation of Na2CoSiO4 as a cathode for Na-ion batteries can be based.

The re-emergence of sodium ion batteries: testing, processing, and manufacturability

With the re-emergence of sodium ion batteries (NIBs), we discuss the reasons for the recent interests in this technology and discuss the synergies between lithium ion battery (LIB) and NIB technologies and the potential for NIB as a “drop-in” technology for LIB manufacturing. The electrochemical testing of sodium materials in sodium metal anode arrangements is reviewed. The performance, stability, and polarization of the sodium in these test cells lead to alternative testing in three-electrode and alternative anode cell configurations. NIB manufacturability is also discussed, together with the impact that the material stability has upon the electrodes and coating. Finally, full-cell NIB technologies are reviewed, and literature proof-of-concept cells give an idea of some of the key differences in the testing protocols of these batteries. For more commercially relevant formats, safety, passive voltage control through cell balancing and cell formation aspects are discussed.

Investigation of the Sodiation and Desodiation of Hard Carbon by Electrochemical Testing and X-Ray Computed Tomography

Sodium ion diffusion plays a key role within the charging and discharging mechanisms in a sodium ion battery. The mobility of sodium ions is important to allow full sodiation of hard carbon materials and enables faster charge and discharge rates for batteries. This investigation studies the diffusion of sodium ions by combining a physical characterisation technique and an electrochemical testing method for a hard carbon composite electrode. Untreated and sodiated hard carbon composite electrodes have been imaged on microscale using x-ray computed tomography to visualise volume changes, and simulate tortuosity and sodium ion concentration distribution by using the MATLAB plug-in TauFactor. Furthermore a comparison to calculated apparent diffusion coefficients determinded by galvanostatic intermittent titration technique was done. The results indicate a correlation of an increasing tortuosity due to the sodiation of hard carbon and the decrease in sodium ion mobility.

Microstructural Analysis of the Effects of Thermal Runaway on Li-Ion and Na-Ion Battery Electrodes

Thermal runaway is a phenomenon that occurs due to self-sustaining reactions within batteries at elevated temperatures resulting in catastrophic failure. Here, the thermal runaway process is studied for a Li-ion and Na-ion pouch cells of similar energy density (10.5u2009Wh, 12u2009Wh, respectively) using accelerating rate calorimetry (ARC). Both cells were constructed with a z-fold configuration, with a standard shutdown separator in the Li-ion and a low-cost polypropylene (PP) separator in the Na-ion. Even with the shutdown separator, it is shown that the self-heating rate and rate of thermal runaway in Na-ion cells is significantly slower than that observed in Li-ion systems. The thermal runaway event initiates at a higher temperature in Na-ion cells. The effect of thermal runaway on the architecture of the cells is examined using X-ray microcomputed tomography, and scanning electron microscopy (SEM) is used to examine the failed electrodes of both cells. Finally, from examination of the respective electrodes, likely due to the carbonate solvent containing electrolyte, it is suggested that thermal runaway in Na-ion batteries (NIBs) occurs via a similar mechanism to that reported for Li-ion cells.