179
Components and Materials for Electrochemical Supercapacitors
ESR, 120 kW.kg –1 ) came from a combination of an amorphous MnO 2 cathode
and PEDOT anode that contained 15 to 20 wt% CNT to maximize conductivity. The combination of high pseudocapacitance and high power carbon
offers intriguing design possibilities. The trade-off is more complex systems
and design criteria that can prevent the long cycle lives expected of ECs if not
properly managed [28].
To increase energy density, another group of hybrid devices utilizing lithium storage with EDLC storage mechanisms [17] was explored. The major
challenge in this approach is how to overcome the poor diffusion coefficient
and poor electronic conductivity of lithium compared to EDLC and other
fast redox materials. A lithium salt (Li + BF 4 or Li + PF 6 ) was used along with a
pre-doped lithium (carbon electrode) anode and an AC cathode [106]. The
pre-doped lithium source acts as the anode on discharge. The chemisorbed
lithium charges oxidize and are released into the electrolyte. To compensate,
Li + will temporarily adsorb onto the carbon cathode surface. A commercialized version of this design is made by Fuji Heavy Industries and its performance characteristics can be seen in Table 5.7.
Wang et al. [17] investigated an alternative design in which a composite
titanium dioxide on CNT cathode was used to adsorb Li + ions released from
a lithium-doped mesoporous templated carbon anode during discharge.
Titania (TiO 2 ) was treated onto the porous CNT substrate in a thin layer and
exhibited a high chemical activity toward lithium. The use of CNTs and mesoporous carbon could improve conduction, surface area, and ion conduction
via ordered spacing. The resulting ES had an energy density of 25 Wh.kg –1
and a power density of 3 kW.kg –1 power and was stable for over 1000 cycles
[17].
Naoi et al. [106] used a composite anode made of nanocrystalline lithium titanate (Li 4 TiO 14 , 5 to 20 nm) chemisorbed to carbon fibers (200 m 2 .g –1 )
and an AC cathode to achieve 55 Wh.kg –1 and 10.3 kW.kg –1 , respectively.
Lithium titanate was chosen because it exhibits high Coulombic efficiency
(95%) of its theoretical (175 mAh.g –1 ) storage capacity even at higher power.
In this case, a storage capacity of 158 mAh.g –1 was achieved. The composite
also showed a low strain during charge discharge cycles that enhanced the
cycle life.
Using composite electrodes, the challenges such as the power limitations
created by the low Li + diffusion coefficient (<10 − 6 cm 2 .s –1 ) and poor electronic conductivity (<10 −10 mS.cm –1 ) of lithium titanate in bulk form could
be addressed [106]. In a composite electrode, the nanocrystalline particles
reduce diffusion problems, while the highly conductive nanofibers promote
conductivity [106]. Along with higher energy density compared to other lithium doping techniques, the lithium titanate device maintained 90% energy
density stability after 9000 cycles [106].
Components and Materials for Electrochemical Supercapacitors
ESR, 120 kW.kg –1 ) came from a combination of an amorphous MnO 2 cathode
and PEDOT anode that contained 15 to 20 wt% CNT to maximize conductivity. The combination of high pseudocapacitance and high power carbon
offers intriguing design possibilities. The trade-off is more complex systems
and design criteria that can prevent the long cycle lives expected of ECs if not
properly managed [28].
To increase energy density, another group of hybrid devices utilizing lithium storage with EDLC storage mechanisms [17] was explored. The major
challenge in this approach is how to overcome the poor diffusion coefficient
and poor electronic conductivity of lithium compared to EDLC and other
fast redox materials. A lithium salt (Li + BF 4 or Li + PF 6 ) was used along with a
pre-doped lithium (carbon electrode) anode and an AC cathode [106]. The
pre-doped lithium source acts as the anode on discharge. The chemisorbed
lithium charges oxidize and are released into the electrolyte. To compensate,
Li + will temporarily adsorb onto the carbon cathode surface. A commercialized version of this design is made by Fuji Heavy Industries and its performance characteristics can be seen in Table 5.7.
Wang et al. [17] investigated an alternative design in which a composite
titanium dioxide on CNT cathode was used to adsorb Li + ions released from
a lithium-doped mesoporous templated carbon anode during discharge.
Titania (TiO 2 ) was treated onto the porous CNT substrate in a thin layer and
exhibited a high chemical activity toward lithium. The use of CNTs and mesoporous carbon could improve conduction, surface area, and ion conduction
via ordered spacing. The resulting ES had an energy density of 25 Wh.kg –1
and a power density of 3 kW.kg –1 power and was stable for over 1000 cycles
[17].
Naoi et al. [106] used a composite anode made of nanocrystalline lithium titanate (Li 4 TiO 14 , 5 to 20 nm) chemisorbed to carbon fibers (200 m 2 .g –1 )
and an AC cathode to achieve 55 Wh.kg –1 and 10.3 kW.kg –1 , respectively.
Lithium titanate was chosen because it exhibits high Coulombic efficiency
(95%) of its theoretical (175 mAh.g –1 ) storage capacity even at higher power.
In this case, a storage capacity of 158 mAh.g –1 was achieved. The composite
also showed a low strain during charge discharge cycles that enhanced the
cycle life.
Using composite electrodes, the challenges such as the power limitations
created by the low Li + diffusion coefficient (<10 − 6 cm 2 .s –1 ) and poor electronic conductivity (<10 −10 mS.cm –1 ) of lithium titanate in bulk form could
be addressed [106]. In a composite electrode, the nanocrystalline particles
reduce diffusion problems, while the highly conductive nanofibers promote
conductivity [106]. Along with higher energy density compared to other lithium doping techniques, the lithium titanate device maintained 90% energy
density stability after 9000 cycles [106].