Composite electrolytes based on electrospun PVDF and ionic plastic crystal matrices for Na-metal battery applications

Tiago Correia Mendes,Faezeh Makhlooghiazad,Frederick Nti,Jennifer M Pringle,Ju Sun,Maria Forsyth,Sneha Subhas Malunavar

doi:10.1088/2515-7639/abeed2

Abstract

Sodium ion batteries are widely considered to be a feasible, cost-effective, and sustainable energy storage alternative to Lithium, especially for large-scale energy storage applications. Next generation, safer electrolytes based on ionic liquid (IL) and organic ionic plastic crystals (OIPCs) have been demonstrated as electrochemically stable systems which show superior performance in both Li and Na applications. In particular, phosphonium‐based systems outperform most studied nitrogen‐based ILs and OIPCs. In this study triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P1i444][FSI]) OIPC mixed with 20 mol% of NaFSI or NaTFSI were combined with an electrospun polyvinylidene fluoride (PVDF) support to create self-standing electrolyte membranes, and their thermal phase behaviour and ionic conductivity were investigated and compared with the bulk electrolytes. The ability of the solid-state composite electrolytes to support the cycling of sodium metal with good efficiency and without breakdown were examined in sodium metal symmetrical coin cells. The sodium transference number was determined to be 0.21. The electrochemical performance of Na/Na3V2(PO4)3 cells incorporating the composite electrolytes, including good cycling stability and rate capability, is also reported. Interestingly, the mixed anion systems appear to outperform the composite electrolyte containing only FSI anions, which may relate to electrolyte interactions with the PVDF fibres.

Highlights

Society is fast approaching a transition towards a renewable energy economy supported by a robust energy storage network
In the Organic ionic plastic crystals (OIPCs)/polyvinylidene fluoride (PVDF) composite electrolyte, the cold crystallisation and subsequent melting peak are significantly larger, indicating that the PVDF fibre surfaces act as a nucleation site to encourage the crystallisation on heating, whereas in the absence of the fibres this process is more sluggish and the melt occurs before much of the material can crystallise
Solid-state composite electrolyte membranes based on the OIPC [P1i444][FSI] with 20 mol% NaFSI or NaTFSI and PVDF nanofiber mats have been prepared and characterised

Summary

Introduction

Society is fast approaching a transition towards a renewable energy economy supported by a robust energy storage network. Despite the larger ion size and sometimes slower ion dynamics in the electrolyte, Na ion batteries (NIBs) can surprisingly outperform the high rate capability of the best Li ion chemistries [5] These properties of NIBs make them an excellent candidate to supersede Li ion chemistries for many applications where the higher energy density provided by Lithium is not necessary [6, 7]. This includes large-scale grid applications, mobility applications (lower driving range affordable EVs, electric bikes, buses etc.), telecommunication backup and many others; wherever raw materials cost and sustainability of enormous production volumes is critical, and high power applications where high rate performance is needed

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