THESIS
2021
1 online resource (xv, 58 pages) : illustrations (chiefly color)
Abstract
Sodium metal batteries (NMBs) are promising alternatives to lithium batteries as intermittent
energy storages systems for large-scale grid powers due to their low-cost and high natural
abundance of Na. However, Na dendrite growth in the organic liquid electrolytes (OLEs) and
leakage of liquid induces safety risks. One of the effective strategies to improve the safety of NMBs
is to replace OLEs with quasi-solid electrolytes (QSEs). QSE contains a unique hybrid structure
with a polymer host with one or more liquid plasticizers and Na salt. Although QSE can eliminate
electrolyte leakage and reduce dendritic growth, the typical Na salts used in QSE still encounter
problems such as risk of explosion (sodium perchlorate, NaClO
4), decomposition when exposed to
moisture (sodium hexafluorophosph...[
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Sodium metal batteries (NMBs) are promising alternatives to lithium batteries as intermittent
energy storages systems for large-scale grid powers due to their low-cost and high natural
abundance of Na. However, Na dendrite growth in the organic liquid electrolytes (OLEs) and
leakage of liquid induces safety risks. One of the effective strategies to improve the safety of NMBs
is to replace OLEs with quasi-solid electrolytes (QSEs). QSE contains a unique hybrid structure
with a polymer host with one or more liquid plasticizers and Na salt. Although QSE can eliminate
electrolyte leakage and reduce dendritic growth, the typical Na salts used in QSE still encounter
problems such as risk of explosion (sodium perchlorate, NaClO
4), decomposition when exposed to
moisture (sodium hexafluorophosphate, NaPF
6), and corrosion to the aluminum current collector
(sodium bis(trifluoromethanesulfonyl)imide, NaTFSI). New strategies, such as developing new
salts, or using mixed salts are needed to solve these problems.
In this thesis, the effect of mixed salts NaTFSI and sodium-difluoro(oxalate)borate (NaDFOB) were
studied in polymer-based QSE, and a free-standing dual-salt polymer electrolyte (DSPE) was
obtained via a simple solution casting method. Several aspects related to DSPE were investigated:
materials characterization of DSPE; the electrochemical performances of DSPE based on the molar
ratio of NaDFOB to NaTFSI from 0, 5, 10, 20, 40, and 100 mol.%; and the interfacial analysis of
electrode cycled with DSPE. It was found that DSPE with 20 mol.% of NaDFOB (DSPE-20)
delivered a high ionic conductivity (0.3 mS cm
-1), an extended electrochemical window (4.5 V),
and improved capacity retention in the battery of 98.3% in 100 cycles compared with 54.7% of
using DSPE with 0 mol.% of NaDFOB (DSPE-0) and 95.6% of using DSPE with 100 mol.% of
NaDFOB (DSPE-100). The improved performance of DSPE-20 was due to the high ionic
conductivity given by NaTFSI, and a stable cathode electrolyte interphase (CEI) with Na
xBO
yF
z
that decomposed by NaDFOB preferentially among the components in DSPE as confirmed by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS),
and density functional theory (DFT) calculation.
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