The aim of this master thesis was to establish if sodium is trapped in aluminium current collectors, which in turn could affect the capacity fade in sodium-ion battery systems. In the case of lithium-ion batteries, previous studies have shown that a trapping mechanism, where lithium diffuses through the active material and current collectors, can explain the capacity fade observed for several systems. However, no such reports have been published in the sodium case, motivating this pioneering investigation. Contact samples of sodium and aluminium current collector material confirmed the uptake of sodium as shown by ICP-AES analyses. The uptake of sodium in the aluminium was equivalent to a charge of 0.4 µAh after 70 days of contact at 55°C. The main characterisation method was galvanostatic plating and stripping of sodium on aluminium in a pouch-cell configuration. When using a bare aluminium working electrode with a metallic sodium counter electrode in a 1 M NaPF6/diglyme electrolyte, the galvanostatic cycling showed coulombic efficiency instabilities. It was concluded that a more stable, high efficient plating-stripping would be needed to quantify the effects of sodium trapping with the employed electrochemical methods. Coulombic efficiency values that exceeded 100 % were attributed to the oxidation of disconnected (detached) sodium from previous plating cycles. On consecutive cycles some of the disconnected sodium got reconnected, resulting in coulombic efficiency values well over 100 %.

<p>Sodium (Na)-ion battery displays many properties similar to Lithium (Li)-ion battery, such as operating principles and capacity, which noticeably compressed the Na-ion battery cathode exploration period. Having said that, anode materials of Na-ion battery is still underperforming as commercial graphite is inadequate in storing bulky Na ions. In the search for anode materials, both alloy-type and Na metal anode materials have gained popularity as these materials can absorb more charges and have higher storage capacity. It is essential to remember that such materials exhibit massive volume expansion upon sodiation and hence experience considerable mechanical stress upon cycling, leading to fractures and pulverization of the electrodes. In addition to electrode stability, ionic motions between the electrode and electrolyte are pivotal in determining the battery response. The decomposition of the electrolyte cocktails forms a passivation layer on the electrode surface, known as solid electrolyte interphase (SEI), which can rupture and regenerate in unstable cycles. Rickety SEI can cause the consumption of active Na and the formation of local hotspots for notorious dendrite growth, leading to short battery durability.</p> <p><br></p> <p>In the first part of the thesis, Tin (Sn) has been selected as an exemplar system to study the dynamic changes in a Na-ion battery. Higher ion-uptake capabilities of Sn electrode come with a price of large structural and morphological changes and can be controlled by careful charting of non-active phases such as binder and suitable electrolyte solution. This work comprehensively studies the technical challenges associated with Sn with different binder domains and in different liquid electrolyte environments. Parallelly, the sensitivity of the Na-Sn system towards the operating potential window and the crosstalk between the working electrode (alloying and de-alloying) and the counter electrode (plating and stripping) has been untied. Also, a fundamental understanding of the materials-transport-interface interactions during thermal abuse tests and their implication on the safety aspects of Na-ion batteries has been addressed. </p> <p><br></p> <p>Following that, the morphological stability of the Na metal anode is investigated based on the distinct electrochemical reactions arising from the composition of different liquid electrolytes. The role heterogeneity in the SEI layer of Na metal for the growth of dendritic patterns has been discussed. A unified framework incorporating a detailed electrochemical study of various electrolyte formulations, cognizant of the reactions and kinetics at the electrode-electrolyte interface, has been developed. To mechanistically counter the heterogeneity implications and synergistically leverage the electrolyte-additive-driven improvement in ionic transport, a flux-homogenizing separator has been introduced to extend the battery cycling. Based on this synergistic approach, the complex interplay between the homogeneity in SEI composition, electrodeposition/dissolution morphology, and cell performance in Na-metal-based batteries has been identified.</p> <p><br></p> <p>This work tried to offer fresh insights on fundamental mechanisms governing the evolution of the electrode-electrolyte interphases and their role in determining electro-chemo-mechano-thermal stability for future research endeavors in the Na-ion battery field. </p>