Environmental Engineering Reference
In-Depth Information
Chemical energy storage devices (batteries) and electrochemical capacitors
(ECs) are among the leading EES technologies today.
1
Both are based on elec-
trochemistry, and the fundamental difference between them is that batter-
ies store energy in chemical reactants capable of generating charges, whereas
electrochemical capacitors store energy directly as charges. Although the elec-
trochemical capacitor is a promising technology for electrical energy storage—
especially considering its high power capability—its energy density is too low
to be considered for large scale energy storage. For this reason, electrochemical
capacitors will not be covered in this chapter. The discussion in this chapter
focuses on rechargeable battery technologies for large-scale energy storage.
2
Among various rechargeable battery technologies, four specific chemistries
(lead-acid, sodium-sulfur (NaS), vanadium redox, and lithium ion (Li-ion)
are reviewed in detail.
3
One case study based on the NaS system is discussed
to show practical applications of batteries for large-scale energy storage appli-
cations.
4
Vanadium redox batteries (VRBs) are also reviewed because they
can scale up to much larger storage capacities and show great potential for
longer lifetimes and lower per-cycle costs than conventional batteries requir-
ing refurbishment of electrodes. Finally lithium ion batteries are reviewed
because they display very high potential for large-scale energy storage.
A battery contains one or more electrochemical cells; these may be con-
nected in series or arranged in parallel to provide the desired voltage and
power. The anode is the electronegative electrode from which electrons are
generated to do external work. The cathode is the electropositive electrode to
which positive ions migrate inside the cell and electrons migrate through the
external electrical circuit. The electrolyte allows the flow of ions, for example,
lithium ions in Li-ion batteries allow flow from one electrode to another. The
flow is restricted to electrons and not ions. The electrolyte is commonly a
liquid solution containing a salt dissolved in a solvent. The electrolyte must
be stable in the presence of both electrodes.
The current collectors allow the transport of electrons to and from the
electrodes. They are typically metals and must not react with the electrode
or electrolyte materials. The cell voltage is determined by the energy of the
chemical reaction occurring in a cell. The anode and cathode are, in practice,
complex composites. They contain, besides the active material, polymeric
binders to hold together the powder structure and conductive diluents such
as carbon black to give the whole structure electronic conductivity so that
electrons can be transported to the active material. In addition, these com-
ponents are combined to ensure sufficient porosity to allow the liquid elec-
trolyte to penetrate the powder structure and permit the ions to reach the
reacting sites.
Secondary or Rechargeable Batteries
Secondary or rechargeable batteries are widely used in many applica-
tions such as starting, lighting, and ignition (SLI) automotive applications;
