Micro-Textured Deposition in Sodium Metal Electrodes

With the imperishable betterment of renewable energy technologies, the constraint of raw material components, a viable candidate for a large-grid storage system is Sodium (Na)-ion battery with an abundance of Na raw materials. Among the many anode candidate materials, sodium metal with the earth-abundancy, lowest redox potential (−2.714 V vs standard hydrogen electrode) and highest theoretical capacity (1165 mAh/g), possesses the most future potential. However, they suffer from a change in mesoscale morphology and chemical composition triggered by sodiation in electrodes, which in turn contributes substantially to the irreversible capacity and poor coulombic efficiency of the battery. Similar to lithium, Na metal also suffers from an intrinsic large dimensional expansion, high electrolyte consumptions, and mechanical instability. Another major problem is the Na plating during the repeated electroplating/stripping process, which causes unavoidable dead Na and leads to abnormal dendritic growth with the possibility of short circuits, thermal runaway, and potential safety hazards. Before considering ways to manipulate the sodium deposition for safe battery operation, the plating and stripping mechanism of Na metal should be well explored. In this study, we have focused on understanding the texture and morphology of deposited Na in a variety of atypical shapes and sizes and their dependency on applied current density. By means of Copper (Cu)|Na cells, we have also studied electrochemical voltage profile and linked the deposition overpotential to the non-uniform sodium nucleation sites which later leads to dendritic growth during cycling. The uneven current density and Na⁺ concentration gradient in the inhomogeneous small nucleus kind of deposition lead to further heterogeneity with a long cycling process. Incorporation of different Na salts and electrolyte additives with a different functional group shows the evolution of different types of structural growth indicating the significant role of anionic and functional groups in the morphology evolution. With long term galvanostatic cycling experiments, micro-textured structures of plated/stripped Na were visualized using Scanning Electron Microscopy (SEM) and linked the morphological changes with the electrochemical responses and voltage fluctuation in the plating/stripping process. The elemental composition of the deposited structures was confirmed with Energy-dispersive X-ray spectroscopy (EDS) analysis. In-situ electroanalytical approaches such as Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were used to understand the transfer kinetics, the charge transfer resistance, and interfacial resistance evolution during recurring Na insertion/de-insertion process. Detailed investigation of the deposited sodium morphology in Cu or Na metal along with the non-destructive in-situ analysis of the dynamic processes occurring inside a battery during the charge-discharge process can open new opportunities for improving sodium-metal battery response for practical utilization.