【Technical Field】
The present application claims the benefit of Korean
Patent Application No. 10-2021-0090243 on July 9, 2021 and
Korean Patent Application No. 10-2022-0083460 on July 7, 2022
with the Korean Intellectual Property Office, the disclosure
of which are herein incorporated by reference in their
entirety.
The present disclosure relates to a positive electrode
for a lithium-sulfur battery and a lithium-sulfur battery
comprising the same.
【Background Art】
As the scope of application of lithium secondary
batteries expands not only to portable electronic devices and
communication devices, but also to electric vehicles (EV) and
electric storage systems (ESS), demands for high capacity of
lithium secondary batteries used as a power source thereof
have increased.
Among various lithium secondary batteries, a lithiumsulfur battery is a battery system using a sulfur series
material including a sulfur-sulfur bond as a positive
electrode active material, and using lithium metal, a carbonbased material having lithium ion
intercalation/deintercalation, or silicon, tin or the like
forming an alloy with lithium as a negative electrode active
material.
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Sulfur, which is a main material of a positive electrode
active material in a lithium-sulfur battery, has advantages
that it has a low atomic weight, is readily supplied due to
abundant resources, is low in price, is not toxic, and is
environmental-friendly.
In addition, a lithium-sulfur battery has theoretical
discharging capacity of up to 1,675 mAh/g, which is obtained
from a conversion reaction of lithium ions and sulfur
(S8+16Li++16e-→8Li2S) in a positive electrode, and when using
lithium metal (theoretical capacity: 3,860 mAh/g) as a
negative electrode, has theoretical energy density of 2,600
Wh/kg. This is a very high numerical value compared to
theoretical energy density of other battery systems currently
under study (Ni-MH battery: 450 Wh/kg, Li-FeS battery: 480
Wh/kg, Li-MnO2 battery: 1,000 Wh/kg, Na-S battery: 800 Wh/kg)
and a lithium ion battery (250 Wh/kg), and therefore, the
lithium-sulfur battery, among secondary batteries that have
been developed so far, is attracting attention as a high
capacity, environmental-friendly, and low-cost lithium
secondary battery, and various studies thereon are being made
as a next-generation battery system.
In a lithium-sulfur battery, since sulfur used as a
positive electrode active material is a nonconductor having
no electrical conductivity with electrical conductivity of
5×10-30 S/cm, there is a problem in that electrons generated
by an electrochemical reaction are difficult to migrate.
Accordingly, sulfur is composited with a carbon material
capable of providing an electrochemical reaction site, and
used as a sulfur-carbon composite.
However, in addition to an elution problem of lithium
polysulfide (Li2Sx, x=8, 6, 4, 2) produced during a charge and
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discharge process of a lithium-sulfur battery, there is a
problem in that electrochemical reactivity of a sulfur-carbon
composite, which is a positive electrode active material,
decreases due to low electrical conductivity of sulfur that
is a positive electrode active material and lithium sulfide
(Li2S) that is a discharged product thereof.
Accordingly, although having high initial discharge
capacity during the actual operation, since a lithium-sulfur
battery has capacity and charge and discharge efficiency
properties rapidly declining as a cycle progresses, and energy
density and cycle life characteristics are reduced as well,
it is difficult to secure sufficient performance and driving
stability, and thus commercialization has not been successful.
In order to implement a lithium-sulfur battery having
energy density and cycle life characteristics at a
commercially feasible level, various techniques to improve
electrochemical reactivity of a sulfur-carbon composite, which
is a positive electrode, specifically a positive electrode
active material, have been proposed.

【Claim 1】
A positive electrode for a lithium-sulfur battery,
comprising:
a positive electrode current collector; and
a positive electrode active material layer positioned on
at least one surface of the positive electrode current
collector,
wherein the positive electrode active material layer
comprises a sulfur-carbon composite having a plurality of
island-shaped carbon coating layers on its surface as a
positive electrode active material.
【Claim 2】
The positive electrode for a lithium-sulfur battery
according to claim 1, wherein each of the plurality of islandshaped carbon coating layers comprises reduced graphene oxide.
【Claim 3】
The positive electrode for a lithium-sulfur battery
according to claim 2, wherein the reduced graphene oxide has
a specific surface area of 700 m2/g or more.
【Claim 4】
The positive electrode for a lithium-sulfur battery
according to claim 3, wherein the reduced graphene oxide has
a specific surface area of 700 to 1500 m2/g.
【Claim 5】
The positive electrode for a lithium-sulfur battery
according to claim 2, wherein the reduced graphene oxide is
included in an amount of 0.5 to 3% by weight, based on the
total weight of the positive electrode active material.
【Claim 6】
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The positive electrode for a lithium-sulfur battery
according to claim 1, wherein the positive electrode active
material has a specific surface area of 20 to 50 m2/g.
【Claim 7】
The positive electrode for a lithium-sulfur battery
according to claim 1, wherein the positive electrode active
material has a pore volume of 0.3 to 0.5 cm3/g.
【Claim 8】
The positive electrode for a lithium-sulfur battery
according to claim 1, wherein a pore size in the positive
electrode active material is 30 to 70 nm.
【Claim 9】
The positive electrode for a lithium-sulfur battery
according to claim 1, wherein a ratio of a specific surface
area of the positive electrode active material relative to a
specific surface area of a sulfur-carbon composite that does
not include a carbon coating layer is 1 to 3.
【Claim 10】
The positive electrode for a lithium-sulfur battery
according to claim 1, wherein the positive electrode active
material layer further comprises a carbon bridge connecting
the positive electrode active materials.
【Claim 11】
The positive electrode for a lithium-sulfur battery
according to claim 10, wherein the carbon bridge is a crumpled
structure or has a fibrous shape.
【Claim 12】
A method for preparing a positive electrode for a
lithium-sulfur battery, comprising steps of:
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preparing a reduced graphene oxide dispersion by
dispersing reduced graphene oxide into a dispersion medium;
preparing a sulfur-carbon composite having a plurality
of island-shaped carbon coating layers on its surface by
mixing the reduced graphene oxide dispersion and a sulfurcarbon composite and drying a mixture;
preparing a positive electrode slurry composition
including the sulfur-carbon composite having the plurality of
island-shaped carbon coating layers; and
applying the positive electrode slurry composition on at
least one surface of a positive electrode current collector
to obtain the positive electrode of claim 1.
【Claim 13】
The method for preparing a positive electrode for a
lithium-sulfur battery according to claim 12, wherein the
dispersion medium comprises a lower alcohol.
【Claim 14】
A method for preparing a positive electrode for a
lithium-sulfur battery, comprising steps of:
preparing a pre-dispersion including reduced graphene
oxide;
preparing a positive electrode slurry composition by
adding the pre-dispersion to a mixture including a sulfurcarbon composite and a binder; and
applying the positive electrode slurry composition on at
least one surface of a positive electrode current collector
to obtain the positive electrode of claim 1.
【Claim 15】
A lithium-sulfur battery comprising:
the positive electrode for a lithium-sulfur battery
according to claim 1;
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a negative electrode; and
an electrolyte.