343
Perspectives and Challenges
9.4 Electrolyte Innovations
Increasing the operating voltages of ES devices is an extremely practical way
to improve energy storage density. Although aqueous electrolytes are preferred from a material handling and processing perspective, they are stable
only up to a potential of ~1.0 V, above which decomposition in the form of
gas evolution will occur. Organic and nonaqueous electrolytes can be used
and will increase the stable potential window up to ~2.5 V. The most common organic electrolytes are acetonitrile and propylene carbonate, with the
former more commonly used due to its higher ionic conductivity.
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.
The primary technical challenge facing their application is the limited
ionic conductivity arising from the large sizes and structures of the constituent ion species. The initial belief was that ionic liquids were not effective for operations at low temperature because of these factors. Recent
developments, however, demonstrated that with appropriate design and
material selection, ionic liquid-based ESs can perform reasonably over a
broad range of operating temperatures. Using carefully selected mixtures
of anionic and cationic species, for example, the temperature-dependent
behaviors and physical properties of ionic liquids can be modified significantly [57].
The development of ionic liquid electrolyte-based ESs is a promising
approach but it is still in an infancy stage. With further research on carefully
selected ionic liquid combinations and compatibility investigations with various electrode active materials, marked progress is expected for the ES industry.
9.5 Development of Computational Tools
On a fundamental level, we still lack a clear understanding of the specific
mechanisms of energy storage in ES devices. Detailed computational tools
including molecular scale modeling of the physical and chemical processes of
charging, storage, and discharge are important, especially for new component
materials such as composite active electrodes (carbons and psuedocapacitive
Perspectives and Challenges
9.4 Electrolyte Innovations
Increasing the operating voltages of ES devices is an extremely practical way
to improve energy storage density. Although aqueous electrolytes are preferred from a material handling and processing perspective, they are stable
only up to a potential of ~1.0 V, above which decomposition in the form of
gas evolution will occur. Organic and nonaqueous electrolytes can be used
and will increase the stable potential window up to ~2.5 V. The most common organic electrolytes are acetonitrile and propylene carbonate, with the
former more commonly used due to its higher ionic conductivity.
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.
The primary technical challenge facing their application is the limited
ionic conductivity arising from the large sizes and structures of the constituent ion species. The initial belief was that ionic liquids were not effective for operations at low temperature because of these factors. Recent
developments, however, demonstrated that with appropriate design and
material selection, ionic liquid-based ESs can perform reasonably over a
broad range of operating temperatures. Using carefully selected mixtures
of anionic and cationic species, for example, the temperature-dependent
behaviors and physical properties of ionic liquids can be modified significantly [57].
The development of ionic liquid electrolyte-based ESs is a promising
approach but it is still in an infancy stage. With further research on carefully
selected ionic liquid combinations and compatibility investigations with various electrode active materials, marked progress is expected for the ES industry.
9.5 Development of Computational Tools
On a fundamental level, we still lack a clear understanding of the specific
mechanisms of energy storage in ES devices. Detailed computational tools
including molecular scale modeling of the physical and chemical processes of
charging, storage, and discharge are important, especially for new component
materials such as composite active electrodes (carbons and psuedocapacitive