Development of clay-derived hybrid solidstate electrolyte for potential use in sodium-ion batteries / Nur Sofina Mohamad Johari

Nur Sofina , Mohamad Johari (2022) Development of clay-derived hybrid solidstate electrolyte for potential use in sodium-ion batteries / Nur Sofina Mohamad Johari. PhD thesis, Universiti Malaya.

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Abstract

All-solid-state sodium-ion battery (SIB) is a very attractive technology in achieving a safe and environmentally responsible energy storage device for large scale applications such as grid storage. However, to date, solid-state electrolytes (SSE) have not met the stringent demands that are required to realize the aspiration. Herein, a hybrid SSE (HybSSE) made of halloysite clay-derived Na2ZnSiO4 (Clay-NZS) ceramic and Py14TFSI-NaTFSI ionic liquid (IL) solution has been developed to overcome some of the limitations of pure ceramic electrolytes. Their structural and electrochemical characteristics have been investigated by several testing methods in normal laboratory setup as well as in highthroughput setup. Optimization of sol-gel synthesis allows the Clay-NZS ceramic to become an SSE with ionic conductivity of 5.28 ´ 10-8 S cm-1 at room temperature (RT), where Na2ZnSiO4 synthesised from standard Si source is only able to ionically conduct at extremely high temperature i.e., 9.44 ´ 10-8 S cm-1 at 300°C. The IL solution mixture has been optimized at 10 wt% NaTFSI salt dissolved in Py14TFSI where after infusing the IL solution into the ceramic pores of Clay-NZS through slow soaking, Clay-NZS HybSSE has managed to achieve an ionic conductivity of up to 0.16 ´ 10-3 S cm-1 at RT, which is much higher than the ionic conductivity of the IL solution as a standalone electrolyte itself. Moreover, the optimized IL solution mixture also enabled Clay-NZS HybSSE to achieve an ionic transference number of 0.991 and Na+ ion transference number of 0.506. This shows that the electronic contribution in this material is negligible and Na+ ion is the main ionic species in the conductivity of this HybSSE ion. A total of 22 different dopants (A) were introduced into Clay-NZS at 8 different doping levels (x), using Na2Zn1-xSi1-xA2xO4 stoichiometry. It is found that there is no trend that determines whether an element will improve or deteriorate this HybSSE. The highest ionic conductivity is achieved by Na2Zn0.9Si0.9In0.2O4 HybSSE, with ionic conductivity values of 0.453 ´ 10-3 S cm-1 at RT and 2.27 ´ 10-3 S cm-1 at 48°C. The improved ionic conductivities of the compound now make this HybSSE a viable candidate for quasi-solid SIBs. Moreover, further screening of the electronic conductivity has shown that dendrite growth throughout all tested HybSSEs is not expected to be a concern. On the other hand, the electrochemical stability window (ESW) of undoped Clay-NZS HybSSE is determined to be 1.28 – 3.45 V. Meanwhile, Na2Zn0.9Si0.9In0.2O4 HybSSE showed stability at a bigger window of 4.11 V down to 0.8 V vs Na. Widest ESW is shown by Na2Zn0.9Si0.9Fe0.2O4 HybSSE at a window of 2.14 – 4.79 V. All variations of Clay-NZS HybSSE studied were found to be unstable towards Na metal, due to the ceramic material. Therefore, Na metal is not feasible as an anode for this HybSSE without the use of a suitable buffer. In a nutshell, the results exhibit a significant improvement in this category of electrolyte and should serve as a guide in future research & development required to enable the use of this electrolyte in a full cell of SIB.

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