1 Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors

3 
voltage, extending this electrochemical window could significantly improve the energy density.
1, 
2, 19
Other organic solvents (adiponitrile,
22, 23
sulfones,
24, 25
and carbonates
25, 26
) have been 
considered for high voltage electrolytes for EDLCs, but typically suffer from higher viscosity 
and lower electrolyte conductivity.
Ionic liquids have generated increased interest due to their 
high stability, but remain limited by high cost, low purity, and low conductivity.
19, 27-30
With prolonged cycling, the capacitance of EDLCs decreases and the resistance increases.
31
The 
performance degrades more rapidly at elevated temperature or higher voltage.
32-34
Degradation is 
typically attributed to decomposition of the electrolyte, and is very sensitive to the electrolyte 
composition, electrode polarity, carbon surface functionality, and trace moisture.
31-35
The long-
term performance of EDLCs can also be limited by the stability of other components in the cell 
including the polymer binders and current collectors (typically aluminum).
32, 34, 35
Commercial 
EDLCs with organic electrolytes operate over a voltage window of ~ 3 V (approximately 1.5 - 
4.5 V vs. Na/Na
+
).
35
Developing higher voltage electrolytes for EDLCs requires careful 
consideration and control of all possible side reactions. For example, extending the positive 
voltage limit beyond 4.5 V vs. Na/Na
+
likely requires strategies to effectively suppress corrosion 
of the aluminum current collector.
36-39
Carbon oxidation may also occur at high voltage.
34
Extending the negative voltage limit below 1.5 V vs. Na/Na
+
also presents certain challenges. 
The solvents most commonly used in lithium-ion batteries and EDLCs (carbonates and ACN) 
passivate electrodes at potentials below about 1.2 V vs. Na/Na
+
.
40-42
Effective passivation of the 
negative electrode is critically important for the operation of lithium-ion batteries, but 
detrimental for double-layer capacitors. Even very thin insulating surface films can reduce the 
double layer capacitance and block small pores.
31-34, 43
Binders based on polytetrafluoroethylene 
(PTFE), which are commonly used in commercial EDLC electrodes, are also reduced below 
about 1.0 V vs. Na/Na
+
.
32
Finally, the stability of the carbon itself with respect to reduction 
and/or intercalation of cations must be considered.
34, 35, 44, 45
In this contribution, the possibility of using ether-based electrolytes to increase the voltage of 
EDLCs is explored. Ethers are highly stable with respect to reduction and, therefore, promising 
solvents for extending the negative voltage limit.
40, 46-49
In particular, this report focuses on an 

4 
electrolyte consisting of NaPF
6
in 1,2-dimethoxyethane (DME or monoglyme). While many salts 
could potentially be used, NaPF
6
salt was chosen because it inhibits corrosion of the aluminum 
current collectors, forms electrolytes with high ionic conductivity, and has a wide 
electrochemical window.
50-54
DME was selected because it has a low viscosity (0.4 cP at 25 
°C),
55
which also promotes high electrolyte conductivity.
56, 57
However, glymes with higher 
molecular weight should all have a similar voltage window. Sodium carboxymethyl cellulose is 
used as an electrode binder primarily for its excellent stability over a wide voltage window.
58-61
This binder also has the advantage of being water-soluble and environmentally benign. The 
DME-based electrolyte shows an electrochemical window up to 3.5 V in full cells with high-
surface-area carbon electrodes. The high voltage performance could significantly increase the 
overall energy density of EDLCs. 

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