IEEE Power Electronics Magazine - September 2023 - 53

E
nergy storage devices (ESDs) have become an
essential component in renewable energy systems
for higher reliability, given the fluctuating nature
of renewable energy sources such as solar, wind,
and biomass. Thus, various types of ESDs have
been invented over the past years, and rechargeable batteries
play an increasingly vital role in storing energy in
response to the fluctuating nature of the renewable sources
and increasing demand for reliable renewable energy
sources. ESD options for longer-term and infrequent utilization
can be listed primarily as below [1]:
■ Rechargeable batteries
■ SC banks
■ Flywheels
■ Thermal energy storage system
■ Fuel cells
■ Compressed air energy storage system
■ Superconductive Magnetic Energy Storage (SMES)
■ Pumped hydro-storage
In this review article, mainly the lithium based rechargeable
battery technologies and supercapacitors have been investigated
and compared.
I. Li-ion Battery and Supercapacitor:
Structural Comparison
Lithium-ion (Li-ion) batteries totally rely on chemical
reactions where submerging the positive side and the
negative side in a liquid electrolyte solution and separating
them using a micro-perforated separator, allowing
only ions to pass through [3]. Since, lithium is
extremely reactive in its elemental form, generally in a
lithium-ion battery, lithium metal oxide such as lithiumcobalt
oxide (LiCoO2) is used in the cathode and lithium-carbon
compounds are used in the anode as those
materials allow intercalation letting the electrodes to
have easy lithium-ions movement in and out of their
structures [2]. Figure 1 depicts the structure of a
Li-ion cell.
At the cathode, from the half-reaction of reduction,
LiCoO2 is formed and at the anode, and from the half-reaction
of oxidation, graphite (C6) and lithium ions are formed
as depicted below.
Reduction(cathode) CoO2 + Li+ + e− → LiCoO2
Oxidation (anode) LiC6 → C6 + Li+ + e−
(1)
(2)
Then, the full reaction (left to right = discharging, right to
left = charging) is given by:
LiC6 + CoO2 →← C6 + LiCoO2
(3)
Reduction always takes place at the cathode and oxidation
always occur at the anode. But the anode and cathode
change during the charging and discharging process
of the battery and can be illustrated as depicted in
Figure 2 [27], [28].
FIG 1 Structure of a Li-ion battery [2].
II. Supercapacitor Structure
Supercapacitors do not rely on chemical reactions like batteries
and instead, they store potential energy electrostatically.
They have bigger electrode plates with less distance
between them and those plates are coated with a porous
substance such as powdery activated charcoal and are
soaked in an electrolyte separated by a very thin insulating
material to separate the collection of positive and negative
charges forming on each side's plates. Figure 3 shows the
practical formation of a supercapacitor and the two separate
series capacitors formed inside due to the electrical double
layer. This allows the device to store and release energy efficiently
and store the static electricity for later use [2], [5].
FIG 2 Li-ion battery charging and discharging processes [27],
[28].
FIG 3 Structure of a supercapacitor and the formation of two
series capacitors inside [3].
September 2023 z IEEE POWER ELECTRONICS MAGAZINE 53
IEEE Power Electronics Magazine - September 2023

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