Lithium-ion batteries work by shuttling lithium ions between a graphite-based negative electrode and a layered transition metal oxide positive electrode. During charging, lithium ions are extracted from the positive electrode and inserted into the negative electrode. This process is reversed during discharging to produce electricity. Key to this process is the electrolyte, which contains lithium ions to enable rapid ion transport within the cell and forms protective interfaces on the electrodes. Research focuses on developing new electrode materials and electrolytes to improve batteries' energy density, lifetime, and safety.

1. Theworkingprincipleofthelithium-ion battery!
Lithium-ion batteries belong to the group of batteries that generate electrical energy by
converting chemical energy via redox reactions on the active materials, i.e. the negative (anode)
and a positive electrode (cathode), in one or more electrically connected electrochemical cells.
Lithium-ion batteries can be further divided into primary (non-rechargeable) and secondary
(rechargeable) batteries, depending on whether or not they are rechargeable by applying an
electric current.
In conventional lithium-ion batteries, Li -ions are shuttled between the positive electrode (usually a
layered transition metal oxide material) and a graphite-based negative electrode according to the
“rocking chair” principle (cf. video).
The term discharge is used for the process in which the battery supplies electrical energy to an
external load. The electrolyte in this system contains additional Li -ions to ensure rapid transport
of the ionic charge within the cell.
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2.
Besidesionconduction, theelectrolyte fulfillsother importantpurposes:
Support of the formation of effective interphases (e.g., solid electrolyte interphase, SEI or cathode
electrolyte interphase, CEI) which:
enable the battery to function
are well Li -ion conducting (rate!)
are protective against further electrolyte decomposition
Contribute to cell safety – being inert to other materials, such as:
Separator
Current collectors
Conductive additives, Binders
Cell casing
When discharged, the Li -ions
are in the positive electrode
material. Thus, the positive
electrode is the source of the
Li -ions necessary for the
conversion of electrical energy
into chemical energy. To allow
Li -ions to migrate from the
positive electrode to the
negative electrode, the
electrolyte is also enriched with
Li -ions.
In the very beginning of the first
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Step1-Initialstate(state
ofcharge(SOC)0%)
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Step2-FormationofSEIandCEI
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3. charging process, electrons
migrate from the positive
electrode⊕ material (oxidation)
via an external conductor into
the negative electrode⊕ material
(reduction). To ensure charge
neutrality, Li -ions de-intercalate
from the positive electrode
material into the electrolyte and
migrate through the electrolyte
to the negative electrode
material for subsequent storage.
As a result of these reactions,
boundary phases, the so-called
SEI and CEI⊕, are formed at the
interfaces between electrolyte /
negative electrode surface and
electrolyte / positive electrode
surface, respectively. These
interphases are built up from
insoluble electrochemically
induced decomposition products
of electrolyte components and
Li -ions originating from the
positive electrode and enable a
reversible cycling of the battery.
After the formation of the SEI
and CEI, further Li -ions de-
intercalate from the positive
electrode material into the
electrolyte and migrate through
it to the negative electrode
material to be subsequently
incorporated into the latter.
After the formation of the SEI
and CEI⊕, further Li ions de-
intercalate from the positive
electrode material into the
electrolyte and migrate through
it to the negative electrode
material to be subsequently
incorporated into the latter.
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Step3-Electrode
reactions
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4. Positive electrode:
LiMO → Li MO + x·e +
x·Li
Negative electrode:
C + x·e + x·Li → Li C
Overall cell reaction:
C + LiMO → Li C + Li
MO
Depending on the number of Li -
ions embedded in the negative
electrode (depending on the
state of charge, SOC), it
changes colour from black over
red (early SOC) to gold (100%
SOC).
During discharge, the reverse
reactions are taking place. The
electrode⊕ reactions are:
Positive electrode = “cathode”
(reduction)
Li MO + x·e + x·Li → LiMO
Negative electrode = “anode”
(oxidation)
2 (1-x) 2
–
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6
– +
x 6
6 2 x 6 (1-x)
2
Step4-Colourchangeduring intercalation/de-
intercalationintographite
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Step5-Discharge
(1-x) 2
– +
2
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5. Li C → C + x·e + x·Li
After discharge (SOC 0%), the
Li -ions are re-stored in the
positive electrode material from
which they originally came. The
back and forth movement of Li -
ions reminds of the movement of
a rocking chair, which is why this
principle was called the “rocking
chair principle”.
Especially the first cycle (charge
and discharge) is associated
with an irreversible loss of Li -
ions in the SEI and CEI but also
in the negative electrode
material. As a result, fewer Li -
ions are now able to be stored in
the negative electrode in the
following charge cycle, which
leads to a reduced capacity of
the battery.
Different aging processes take
place in a lithium-ion battery,
which reduces the performance
of the battery over the period of
use and depends strongly on the
cell chemistry and the intended
use of the battery. Especially the
right choice of the electrolyte
has an enormous influence on
these aging mechanisms and
underlines once more the
importance of tailor-made
electrolytes.
In order to optimize lithium-ion batteries with respect to the specific energy and energy density,
lifetime and safety, many efforts have been made to further expand the application possibilities of
LIBs. Especially the increasing demands for both high specific energy and energy density lithium-
ion batteries particularly for automotive applications, raise the research efforts all over the world.
The energy density and specific energy of batteries by definition is the amount of energy stored in
x 6 6
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Step6-Rockingchair principle
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6. a given system per unit volume and per unit mass, respectively. The product of the specific
capacity and the mean discharge voltage gives the specific energy and this relation finds
expression in equation 1:
E = C · U (1)
According to equation 1, it appears reasonable that most of the current research focuses on new
positive electrode materials with higher operation voltages (high-voltage approach) and/or
increased specific capacity (high-capacity approach). The high-voltage cathode materials are
highly restricted by the narrow electrochemical stability window state-of-the-art carbonate-based
electrolytes (≈1.0 – 4.4 V vs. Li/Li ) and reinforce the design of intrinsically stable electrolytes or
suitable electrolyte additives to enable high-voltage lithium-ion batteries.
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