Lithium -ion batteries are a type of secondary (rechargeable) batteries commonly used as [630870]

Lithium -ion batteries are a type of secondary (rechargeable) batteries commonly used as
key components in portable electronics, power tools, and hybrid/full electric vehicles [1].
Generally, a lithium -ion battery consist s of four main components: a negative electrode
(an anode ), a positive electrode (a cathode ), an electrolyte and a separator.
In a lithium -ion battery, the anode is the electrode where the oxidation reaction takes place
during the discharge process . The materials that are usually used for manufactu ring the anode are:
metallic lithium, graphitic carbon, hard carbon, synthetic graphite, lithium titanate (Li2TiO 3), tin-
based alloys and silicon -based materials . As expected , the cathode is the electrode where the
reduction reaction takes place during the dis charge cycle of the lithium -ion battery . The materials
which are tipically used for fabricating the lithium -ion battery’s cathode are: an lithium manganese
oxide , an lithium cobalt oxide, iron(II) sulfide (FeS 2), vanadium pentoxide (V2O5), an lithium
nickel cobalt manganese oxide, lithium i ron phosphate (LiFePO 4) or an electronic conducting
polymer [2].
The electrolyte is the medium that enables the back and forth movement of the lithium ions
between the cathode and the anode of the battery during the charge -discharge cycles . Most often,
lithium -ion batteries use liquid electrolytes which consist of a lithium salt, such as lithium
hexafluorophosphate (LiPF 6), lithium perchlorate ( LiClO 4), lithium triflouromethanesulfonate
(LiCF 3SO 3), lithium hexafluoroarsenate ( LiAsF 6) dissolved in a n aprotic organic solvent, such as
mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and
ethylmethyl carbonate (EMC) .
The separator is a thin porous membrane placed between the anode and the cathode which
allows the lithium ions to move freely from one side to the other during the charge -discharge cycles
of the battery , but deny the movement of the electrons through the electrolyte, forcing them to
traverse an external circuit . The separator’s role is to prevent the physical contact of the two
electrodes thereby avoiding any possible electrical short -circuits . Industrially produced separators
can be fabricated from a variety of materials, such as microporous polymer films (e. g.
polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE)), nonwoven fabrics
(commonly made from polymer fi bers), ceramic or naturally abundant materials (e. g. cellulose ,
rubber , wood ) [3].
The working principle of the lithium -ion batter ies is bui ld on a reversible redox -reaction
between the anode material (the negative electrode, the „reductant”) and the cathode material ( the

positive electrode, the „oxidant”) . In a lithium -ion battery, the anode and cathode are physically
separated but connected electrically through an external electrical circuit and ionically through t he
electrolyte. When the battery gets connected to a power source ie during charge, the positive side
of the power source will attract and remove electrons from the cathode material (co mmonly, a
lithium oxide) . As a consequence, t hese el ectrons will flow through the external circuit and reach
the anode material (commonly, a graphite layer) . At the same time, the positively charged lithium
ions formed at the cathode will be attracted by the negative electrode and will flow trough the
electrolyte . Once the lithium ions get to the anode, they will insert between the layers of the anode
material, process known as „intercalation”. When all the positively charged lithium ions get stored
at the anode , the battery is fully charged. As soon as the power source is removed and the battery
gets connected to a load (an electrical power consuming device ) ie during discharge, the lithium
ions will want to go back to the ir stable state of atmos and reintegrate into the cathode material.
Due to this tendency, the lithium ions will migrate from the negative electrode to the positive
electrode through the electrolyte while the electrons acumulated at the anode will flow thorugh the
load back to the cathode, thus producing a n electrical cur rent throughout the load.

Fig.1 Lithium -ions batteries charge and discharge mechanisms

The reaction scheme below shows the redox reactions that occur during the charge and the
discharge of a lithium -ion battery with graphite as the anode material and lithium cobalt oxide
(LiCoO 2) as the cathode material [4]:
LiCoO2Charge
DischargeLi1-xCoO2 + xLi++ xe-Cathode (positive electrode):
Anode (negative electrode): C + xLi++ xe-Charge
DischargeLixC
LiCoO2 + C Li1-xCoO2 + LixC Battery as a whole:Charge
Discharge

Nowadays, most of the lithium -ion batter ies on the market are still using liquid electrolytes
because of their high ionic conductivity. However, s ince lithiu m readly react s with water or other
aqueous electrolytes, in the manufacture of rechargeable lithium -ion batteries only the aprotic
organic electrolytes can be used. Anyway, t his type of electrolytes have a number of disadvantages.
Firstly, the organic aprotic electrolytes have high volatility and flammability and pose important
safety issues as they may leak and cause the battery to explode or catch fire [5]. Secondly , in the
case of lithium -ion batteries that use metallic lithium as an anode it was observed that in a liquid
electrolyte , the lithium ions tend to deposit unevenly on the surface of the lithium metal anode
forming so -called dendrites . Dendrites are tiny, rigid tree -like lithium formations which can grow
on the surface of the lithium anode, expand and pierce the separator material of lithium -ion battery
causing a short circuit in the cell. Also , the liquid electrolytes may corrode or react with the surface
of the electrode materials affecting the efficiency and the lifespan o f cells and if the battery over –
heats due to overvoltage or short circuit conditions there is a risk that the liquid electrolyte will
catch fire or explode [6].
Giving the need to find safer and more reliable electrolytes, researchers have turned their
attention to the development of solid electrolytes. Compared to liquid electrolytes solid electrolytes
have a significant number of advantages:
– Solid electolytes are not flammable, they can not leak accidentally and they suppress
lithium dendrite formation thus being much safer than liquid electrolytes ;

– Solid electrolytes have higher thermal stability and wider operating temperatures
ranging from -50°C to 200°C where organic liquid electrolytes may fail due to freezing,
boiling, or decomposition [7].
– solid electrolytes have higher energy density and power density because they allow the
use of lithium ion methal as anode [8] which is preferred due to its high specific
capacity and low electrochemical potential ( -3.04 V vs standard hydrogen electrode)
[9].
– if a solid electrolyte is used, the existence of a separator in the lithium ion battery is no
longer necessary ;
– Batteries which use solid state electrolytes have longer cycle lives and require less
packaging, enabling the operation of higher series voltages compared with those of
liquid electrolytes [10].
– Stabilitatea electrochimica a electrolitilor solizi?
Two types of materials are mainly used for the manufacture of solid electrolytes: inorganic
ceramics and organic polymers. The most evident difference between these two types of materials
is related to the mechanical properties. Cera mics have higher modulus of elasticity , are hard and
brittle, which makes them more suitable for rigid battery design. In contrast, the polymeric
electrolytes are soft and have a lower modulus of elasticity, which makes them more adequate for
flexible battery designs. Also, polymers are easier to process than ceramics, which can lead to a
decrease in the manufacturing price of the battery , while ceramic have the advantage that they
have greater stability at high temperatures and other aggressiv e environments [11]. Another aspect
worth mentioning is the interfacial resistance that appears at the electrolyte / cathode interface
which can lead to a large internal resistance of the battery and result in a loss of energy and power
density . It was proved that using a flexible polymer electrolyte will ensure a lower interface
resistance while the rigid inorganic ceramic electrolyte s have a large r interfacial resistance due to
insufficient contact with the electrodes [12]. Another difference between the two types of solid
electrolytes arises from the way in which ionic conduction is achieved .
Ionic conduction in ceramics consists in the movement of ions from one site to another via
point defects called vacancies in the crystal lattice. At room temperature very little ion movement
takes place, since the atoms are at relatively low energy states , so a high temperature is needed for
ion conduction to take place. As a consequence, ceram ic solid electrolytes are more adequate for

high-temperature applications although there are some ceramics which exhibit l ithium ionic
conduction even at relatively low temperatures , such as: Li 2S–P2S5 sulfide glass , Li 2S–SiS 2 sulfide
glass, Li 2S–GeS 2–P2S5 sulfide glass , (Li, La) TiO 3 perovskite. On the other hand, polymer
electrolytes are obtained by dissolving Li salt in a high molecular weight polymer host. In some
polymer electrolytes, lithium salts are directly solvated by the polymer chains, while in others
besides thepolymer and the lithium salt, a solvent is added to form a polymer gel . it is believed
that in polymers, ionic conduction occurs only in the amorphous phase m above the glass transition
temperature, Tg, where polymer chain motion creates a dynamic, disordered environment that
plays an essential role in facilitating ion transport [13].
Intensively studied as promising candidates for the manufacturing of lithium polymer solid
electrolytes have been the materials based on polyethylene oxide (PEO). However the PEO based
solid electrolyte s usually show low ionic conductivit ies of only 10-8-10-4 S∙cm-2 at room
temperature, which is not sufficie nt for practical application. This is due to the high crystallinity
of the polyethylene oxide which can restrain the movement of the lithium ions due to its stiff
structure and low free volume especially at low temperature [14].

Solid polymer electrolytes have been known to suppress lithium dendrite formation; however,
the low lithium ion conductivity and high interface resistance between lithium and the polymer electrolyte
at room temperature limits their use for conventional batteries.

When the battery is fully charged, all the lithium ions are stored between
layers of graphene (sheets of carbon one atom thick) in the graphite electrode
ut forces the electrons to traverse an external circuit where they can move freely

During discharge, an oxidation half-reaction at the anode produces positively charged lithium
ions and negatively charged electrons

Bibliography

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A separator is a permeable membrane placed between a battery's anode and cathode

The materials used as electrolytes include LiPF6, 25,26 LiClO4, 27,28 LiAsF6 29 and LiCF3SO3. 3

Liquid electrolytes in lithium -ion batteries consist of lithium salts, such as LiPF
6, LiBF
4 or LiClO
4 in an organic solvent , such as ethylene carbonate , dimethyl carbonate , and diethyl carbonate .