Chemistry Reference
In-Depth Information
(a)
(d)
cathode
2-
+e
-
+e
-
(b)
-
O
O
-e
-
O
O
-e
-
O
O
TB6OPO
(c)
0.3 mA
charge
4
charge
discharge
anode
3
discharge
-e
-
-e
-
2Li
Li
+
2Li
+
Li
+
2
+e
-
+e
-
0
40
0
120
160
Capacity/Ah·kg
-1
Figure3.41
(a)Thecoincellofthe''molecularcrystallinesecondarybattery''containing
TB6OPO
asacathode
active material and metal lithium as an anode active material; (b) a propeller working using the force of the
battery; (c) the charge/discharge curves at a current density of 0.27mA/cm
2
in the voltage range of 2.0-4.0V;
and (d) the possible electrochemicalreactions in the cathode and the anode under charge/dischargeprocesses.
Calculatedchargedistributionsoftheanionandtheradicaldianionof
6OPO
areshownbelowthecorresponding
chemicalstructures.
capacity unequivocally demonstrate that
all
of the
TB6OPO
molecules in the cathode participate in the
charge/discharge processes.
Delocalization greatly contributes to the high stability of anion and radical dianion species in the cathode
(Figure 3.41d). The use of organic molecule, unlike polymer and metal complex, as a cathode active mate-
rial has expanded to include closed shell electron donor and acceptor molecules, such as tetrathiafluvalene
(TTF) and TCNQ derivatives, which show discharge capacities of 200
260 Ah/kg.
51
The relationship
between the physical properties of the molecule (i.e. solubility and redox potential and on-site coulombic
repulsion measured by cyclic voltammetry in a solution) and discharge voltage and cyclability have been
carefully investigated.
51
These studies clearly demonstrated a conceptual advance in design criteria of elec-
trode active materials with an increased discharge capacity. These studies also contribute significantly to
practical considerations of avoiding transition metal elements and increasing energy density. A molecular
level understanding of redox processes (charge/discharge processes) occurring inside the secondary battery
in terms of a variety of experimental measurements is the next object for the design of more sophis-
ticated “molecular crystalline secondary battery”. Energy storage devices having multiple functionalities
based on redox-driven changes in physical properties of the molecule are anticipated for next generation
molecular devices.
∼
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