Page 374 - Handbook of Battery Materials

12.3 Lithium-Ion Battery Electrodes 345

12.3
Lithium-Ion Battery Electrodes
Rechargeable lithium batteries involve a reversible insertion/extraction of lithium
ions into/from a host electrode material during the charge–discharge process. The
lithium insertion/extraction process, which occurs with a flow of ions through the
electrolyte, is accompanied by an oxidation/reduction (redox) reaction of the host
matrix assisted by a flow of electrons through the external circuit (Figure 12.2).
The open-circuit voltage V oc of such a lithium cell is given by the difference in
the lithium chemical potential between the cathode (µ C ) and the anode (µ A )as
V oc = (µ A − µ C )/F
−1
where F is the Faraday constant (F = 96485 C mol ). Figure 12.3 gives a schematic
energy diagram of a lithium-ion cell at open circuit. The cell voltage is determined
+
by the energies involved in both the electron transfer and the Li -ion transfer.
While the energy involved in electron transfer is related to the redox potential of
+
the ion involved in the cathode and anode, the energy involved in Li -ion transfer
is determined by the crystal structure and the coordination geometry of the site
+
into/from which the Li ions are inserted/extracted [9]. The energy separation E g
between the lowest unoccupied molecular orbital (LUMO) and the highest occupied
molecular orbital (HOMO) of the electrolyte defines the stability window of the
electrolyte. Therefore, thermodynamic stability considerations require the redox
energies of the cathode (µ C )and anode(µ A ) to lie within the band gap E g of the
electrolyte, as shown in Figure 12.3. An anode with a µ A above the LUMO will
reduce the electrolyte, and a cathode with a µ C below the HOMO will oxidize the
electrolyte unless an appropriate solid electrolyte interfacial (SEI) layer is formed
to prevent such reactions. Thus, the electrochemical stability requirement imposes
a limitation on the cell voltage as given by
V oc = (µ A − µ C )/F ≤ E g

− Load −
e e

Anode Cathode

Li +

+
Li
C
Li x 6 Electrolyte Li 1-x CoO 2
Figure 12.2 Illustration of the charge–discharge process
involved in a lithium-ion cell consisting of graphite as the
anode and layered LiCoO 2 as the cathode.