In English

Diglyme as an electrolyte solvent for sodium-ion batteries

Kasper Westman
Göteborg : Chalmers tekniska högskola, 2016. 86 s.
[Examensarbete på avancerad nivå]

For storing energy in future sustainable energy systems, sodium-ion batteries (SIB) have emerged as an alternative to the current state-of-the-art lithium-ion batteries (LIBs), since SIBs are potentially cheaper. For LIBs and SIBs alike, the development of stable systems depends on employing stable or metastable electrolytes, forming a SEI. Ether compounds have earlier been investigated for use in electrolytes, due to a presumed high reductive stability. In this project diglyme, an ether solvent, mixed with 1 M NaPF6 has been evaluated for use with Na-metal reference electrodes and a SIB electrode platform comprising Na3V2(PO4)2F3 (NVPF) and hard carbon (HC), and with a model system using Na3V2(PO4)3 (NVP) at both electrodes. Furthermore, this electrolyte has been investigated for electrochemical stability, and for possible degradation mechanisms. Techniques used are galvanostatic cycling (GCPL), cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS), coupling these to ab-initio calculations and physico-chemical characterisation methods. Using these techniques, Na-metal is shown to provide a stable reference electrode. In spite of the electrochemical stability of 1 M NaPF6 in diglyme appearing promising when conducting CV, cells of NVPF|HC are shown to exhibit a c of 99.13 %, a high initial irreversibility (30 %), and an inferior capacity retention, when compared to equivalent cells using a EC50:DMC50-based electrolyte. NVP|NVP cells, on the other hand, show outstanding capacity retention and low initial irreversibility. The problems experienced in NVPF|HC cells are proposed to arise due to a set of different mechanisms: binder degradation, vanadium dissolution-deposition, and possibly reduction of the electrolyte. No coherent indications are given for the formation of an SEI. However, reduction schemes drawn still allow for undetectable reduction products. By independently exploring different voltage regions of the active materials the oxidative stability of 1 M NaPF6 in diglyme is put to question despite earlier indications of stability. Future studies should aim to change the type of binder to avoid losses. NVP should be tested separately with both the active materials NVPF/HC to determine where most of the loss occurs along with further investigations to confirm if there is a reduction of the electrolyte at sodiated HC, since any lack thereof could point towards a future SEI-free system.

Nyckelord: Sodium-ion Batteries, Electrolytes, Diglyme, Electrolyte Degradation, Galvanostatic Cycling, Electrochemistry

Publikationen registrerades 2017-01-19.

CPL ID: 247414

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