Voltage windows, liquid electrolytes

In comparison with aqueous electrolytes, liquid nonaqueous electrolytes offer larger liquid ranges, down to below -150 °C [23] and up to above 300 °C [24], voltage windows up to more than 5 V, (see... 

For in situ x-ray diffraction measurements, the basic construction of an electrochemical cell is a cell-type enclosure of an airtight stainless steel body. A beryllium window, which has a good x-ray transmission profile, is fixed on an opening in the cell. The cathode material can be deposited directly on the beryllium window, itself acting as a positive-electrode contact. A glass fiber separator soaked in liquid electrolyte is then positioned in contact with the cathode followed by a metal anode (3). A number of variations and improvements have been introduced to protect the beryllium window, which is subject to corrosion when the high-voltage cathode is in direct contact with it. 

Carbonate-containing liquid electrolytes are primarily chosen for their ability to dissolve lithium salts and their relatively low viscosity (which facilitates Li-ion diffusion between electrodes). Their flammability has in part led to interest in the use of room-temperature ionic liquids (ILs) as replacements. ILs can potentially operate in a higher voltage window relative to carbonates and also have the added benefit of being more thermally stable and having low vapor pressure. The main drawback of this class of compounds is a high viscosity. Additionally, carbonates may have to be introduced at certain voltages to form a suitable SEI for operation. 

Advances in the understanding of ion desolvation and transport mechanisms have furthered the utility of these materials. However, environmental toxicity and safety issues associated with organic electrolytes coupled with their still limited operational potential windows shifted the focus to the development of ionic liquids as electrolytes for new ESs. With ionic liquid electrolytes, operating voltages can be increased to 3.5 V or more without instability issues arising. Moreover, ionic liquids have well defined ion sizes and do not have ion salvation and desolvation mechanisms that plague aqueous and organic electrolyte systems. 

In comparison with aqueous electrolytes, liquid nonaqueous electrolytes offer larger liquid ranges, from below —150 °C [33] to above 300°C [34], voltage windows up to more than 5 V (see Section 17.4.1), a large range of acid-base properties, and often better solubility for many materials (electrolytes and nonelectrolytes), better compatibility with electrode materials, and increased chemical stability of the solution. Their drawbacks are lower conductivity, higher cost, flammability, and environmental problems. 

No Electrode materials Electrolyte (solid/liquid) Specific capacitance Voltage window Cycles Ref. 

Alternatively, the salt LiAlCLi dissolved in the inorganic liquid SO2 has been proposed [7] this Li -ion electrolyte has a good room temperature ffu = 7 x 10 S/cm and is nonflammable its electrolyte window may be too small to be competitive with the carbonates, but it deserves to be explored for Li batteries of lower voltage. 

It is important to choose an electrolyte with a wide electrochemically stable range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical disassociation window of water is around 1.23 V at room temperature. If water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V if acetonitrile is the solvent, the electrode potential window is around 2.0 V with an ion liquid, the electrode potential window can be as high as 4.0 V. Therefore, different solvents have different potential windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors. 

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