192
Electrochemical Supercapacitors for Energy Storage and Delivery
mate contact generated through drying or annealing is not possible or is
insufficient to prevent power limitation.
Even with efficient current collection through the electrode–collector
interface, an active material can exhibit large electronic resistances. Internal
resistance within an electrode paste is caused by low conductivity and insufficient percolation to provide enough complete conduction pathways across
the electrode layer thickness. These internal resistances are characteristic to
a particular active material and can vary based on the quality of binding by
the electrode production technique. Poor binding can also result in increased
resistance and device failure during operation.
To reduce internal resistance, electrode pastes contain polymeric binding agents (PTFE, Nafion, PVdF, or PVB) and conductive carbon additives
(acetylene black, Super P) [127–129]. Binders increase durability and prevent
degradation of the collector–electrode interface over time. However, one
downside of using these agents is an increase in internal electrode resistance
that reduces device power [31]. Optimal balance of internal resistance and
sufficient paste stability leads to loading of about 3 to 5% by weight.
Internal resistance within the electrode paste matrix is reduced by the
enhanced percolation provided by 10 to 20% of conductive carbon filler.
Collector metals and conductive pastes contribute to a non-negligible weight
within a device. Dead weight is an important factor that cannot be overlooked
when designing electrode materials. Any non-capacitive weight in a cell reduces
device performance. As a result, optimization of cell design for minimal paste
additives should be used and collector metals should be thin and lightweight.
With this in mind, a hybrid collector electrode material with CNTs was
demonstrated. The highly conductive nanotubes provided sufficient current
collection for pseudocapacitive [50] and EDLC devices [47]. In both cases, electrodes made of CNTs deposited on lightweight paper substrates replaced the
heavy metal collectors and carbon paste additives and enhanced the specific
capacitance of the overall system, compared with results from CNTs deposited on metal substrates. A CNT paper electrode of 30 Wh.kg –1 and 200 kW.kg –1
was demonstrated [47]. The downside is that CNTs offer low energy density
compared to AC materials and cost more than metal collector substrates.
4.6 Sealants
Proper sealing is a key component of cell assembly. Gas evolution from electrolyte degradation, corrosion, and surface oxidation of electrode and packaging can be a problem over time [1]. Depending on the ion type and solvent,
issues can vary. A proper seal will prevent water and gas from entering a
cell. Another key reason for proper sealing is to prevent shunt resistances
between neighboring electrodes and cells. A shunt current significantly
Electrochemical Supercapacitors for Energy Storage and Delivery
mate contact generated through drying or annealing is not possible or is
insufficient to prevent power limitation.
Even with efficient current collection through the electrode–collector
interface, an active material can exhibit large electronic resistances. Internal
resistance within an electrode paste is caused by low conductivity and insufficient percolation to provide enough complete conduction pathways across
the electrode layer thickness. These internal resistances are characteristic to
a particular active material and can vary based on the quality of binding by
the electrode production technique. Poor binding can also result in increased
resistance and device failure during operation.
To reduce internal resistance, electrode pastes contain polymeric binding agents (PTFE, Nafion, PVdF, or PVB) and conductive carbon additives
(acetylene black, Super P) [127–129]. Binders increase durability and prevent
degradation of the collector–electrode interface over time. However, one
downside of using these agents is an increase in internal electrode resistance
that reduces device power [31]. Optimal balance of internal resistance and
sufficient paste stability leads to loading of about 3 to 5% by weight.
Internal resistance within the electrode paste matrix is reduced by the
enhanced percolation provided by 10 to 20% of conductive carbon filler.
Collector metals and conductive pastes contribute to a non-negligible weight
within a device. Dead weight is an important factor that cannot be overlooked
when designing electrode materials. Any non-capacitive weight in a cell reduces
device performance. As a result, optimization of cell design for minimal paste
additives should be used and collector metals should be thin and lightweight.
With this in mind, a hybrid collector electrode material with CNTs was
demonstrated. The highly conductive nanotubes provided sufficient current
collection for pseudocapacitive [50] and EDLC devices [47]. In both cases, electrodes made of CNTs deposited on lightweight paper substrates replaced the
heavy metal collectors and carbon paste additives and enhanced the specific
capacitance of the overall system, compared with results from CNTs deposited on metal substrates. A CNT paper electrode of 30 Wh.kg –1 and 200 kW.kg –1
was demonstrated [47]. The downside is that CNTs offer low energy density
compared to AC materials and cost more than metal collector substrates.
4.6 Sealants
Proper sealing is a key component of cell assembly. Gas evolution from electrolyte degradation, corrosion, and surface oxidation of electrode and packaging can be a problem over time [1]. Depending on the ion type and solvent,
issues can vary. A proper seal will prevent water and gas from entering a
cell. Another key reason for proper sealing is to prevent shunt resistances
between neighboring electrodes and cells. A shunt current significantly