The cathodes of these batteries consist of various lithium
transition metal oxides such as Lithium Nickel Manganese
Cobalt Oxide (NMC), while the anodes are most often made from
graphite, a form of carbon.
When fi rst produced, these batteries are in a state of discharge,
all the lithium ions are located at the metal oxide cathode, and
the battery has no ability to produce current and therefore needs
to be charged through the application of an external power
source. During this process lithium at the cathode undergoes a
reaction called oxidation, where the lithium loses some negatively
charged electrons. An equivalent quantity of Li2+ will move into
the electrolyte and migrates towards to anode, were they become
embedded in the porous structure of the anode in a process called
intercalation along with electrons in a process called reduction.
During discharge/powering of a device, oxidation occurs at the
anode, where lithium ions become detached and travel back
towards the cathode through the electrolyte. During this process,
electrons are released from the anode and, in turn, flow through
the wires of a connected device and provide an electrical current.
BATTERY
DISCHARGING
Li +
Anode (-)
Li +
e -
Electrolyte
Cathode (+)
e -
Li +
CHARGING
Anode (-)
e -
Electrolyte
Li +
When the cathode becomes saturated with lithium ions the
reaction then stops, and must therefore be recharged once more
through the application of an external current moving the Lithium
ions back to the anode.
Between the cathode and anode is the electrolyte which usually
in the case of a Li-Ion battery consists of a solution comprising
of organic solvents and lithium salts. As the electrolyte solution
already contains lithium ions, they do not have to make a complete
journey from cathode to anode and vice versa, speeding up the
process.
e -
BATTERY
e -
+
Li
Cathode (+)
Li +
e -
Why does the battery life decrease over time?
Over time, physical and mechanical changes occur to the
electrodes, diminishing their capacity. The anodes can
chemically react with electrolyte solvents such as dimethyl
carbonate. When a battery is new, reactions occur between
the graphite and organic solvents of the electrolyte forming a
layer called the solid electrolyte interface (SEI) which consists
of lithium oxide and lithium carbonates. This prevents further
side reactions from occurring, while remaining permeable to
lithium ions. However, during charge/discharge cycles, new
sites become available for reaction at the anode, resulting in a
process which continuously reduces the capacity of the battery
through the increasing thickness of the SEI layer. The cathode
can also undergo similar degradation through the formation of
a restrictive layer which is enhanced under high temperatures
called electrolyte oxidation. Another process that can diminish
the capacity of a battery is through thermal decomposition of
the electrolyte.
As such, rechargeable Li-ion batteries have a limited life and
manufactures often state an estimated life expectancy on
batteries after which they should be replaced.
About the scientist:
Dr Richard Cunningham has a PhD in medicinal chemistry from Queen’s University Belfast where he also worked as post-doctoral researcher for a number of years before working
in the US at the Mitchell Cancer Institute in Alabama. Among his areas of expertise are organic synthesis, nucleosides, nucleotides, cellular biology, vitamins, aminoglycosides, drug
delivery, phosphorous chemistry and chemical analysis. He is now the director of quality at Liquid Sciences LLC in the UK.
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