Lithium sulfur battery
Encyclopedia
The lithium–sulfur battery (Li–S battery) is a rechargeable
galvanic cell
with a very high energy density
. By virtue of the low atomic weight of lithium
and moderate weight of sulfur
, Li–S batteries are relatively light; about the density of water. They were demonstrated on the longest and highest-altitude solar-powered airplane flight in August, 2008. Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and the low cost of sulfur
. There is much interest in using them for electric vehicles.
surface (and incorporation into polysulfide
s) during discharge, and reverse lithium plating
to the nominal anode while charging. This contrasts with conventional lithium-ion cells, where the lithium ions are intercalated
in the anode and cathodes, and consequently Li-S allows for a much higher lithium storage density. Polysulfides are reduced
on the anode surface in sequence while the cell is discharging:
Across a porous diffusion separator, sulfur polymers form at the nominal cathode
as the cell charges:
These reactions are analogous to those in the sodium–sulfur battery.
For experimental purposes most batteries are constructed with a carbon and sulfur cathode and a lithium anode. Sulfur as a raw material has the advantage for mass production that it is very cheap, but it lacks electroconductivity. Sulfur alone being at 5*10−30 S cm−1 at 25°C. The carbon coating on the sulfur then provides the electroconductivity missing from pure sulfur. The solution to this problem is carbon nanofibers. The carbon materials provide an effective electron conduction path and structural integrity. The downside of carbon nanofibres is the high cost.
Each sulfur atom can host two lithium ions. Typically, in lithium-ion batteries, for every host atom, only 0.5–0.7 lithium ions can be accommodated.
and other safety circuitry is sometimes used along with voltage regulator
s to control cell operation and prevent rapid discharge
.
has produced Li–S cells with 84% of the theoretical maximum energy density for Li–S that suffer minimal degradation during charge cycling, and thus potentially offering four times the gravimetric energy density of lithium-ion. The team accomplished this through use of a mesoporous carbon cathode, full of deep pits. Sulfur and carbon were milled and heated together, causing the low surface tension
sulfur to seep into the pits, with just enough room to expand. The composite was then heated to bake off residual sulfur from the surface. To further trap the polysulfides in the cathode, the surface was functionalized and coated with polyethylene glycol
to repel the hydrophobic polysulfides and keep them trapped in the pits. In a "worst case scenario" test using a glyme solvent known for its affinity for dissolving polysulfides, a traditional LiS cathode lost 96% of its sulfur over 30 cycles, while the new cathode lost only 25%.
Rechargeable battery
A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...
galvanic cell
Galvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...
with a very high energy density
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
. By virtue of the low atomic weight of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
and moderate weight of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, Li–S batteries are relatively light; about the density of water. They were demonstrated on the longest and highest-altitude solar-powered airplane flight in August, 2008. Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and the low cost of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. There is much interest in using them for electric vehicles.
Chemistry
The chemical processes in the Li–S cell include lithium dissolution from the anodeAnode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
surface (and incorporation into polysulfide
Polysulfide
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2nSn...
s) during discharge, and reverse lithium plating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
to the nominal anode while charging. This contrasts with conventional lithium-ion cells, where the lithium ions are intercalated
Intercalation (chemistry)
In chemistry, intercalation is the reversible inclusion of a molecule between two other molecules . Examples include DNA intercalation and graphite intercalation compounds.- DNA intercalation :...
in the anode and cathodes, and consequently Li-S allows for a much higher lithium storage density. Polysulfides are reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
on the anode surface in sequence while the cell is discharging:
- S8 → Li2S8 → Li2S6 → Li2S4 → Li2S3
Across a porous diffusion separator, sulfur polymers form at the nominal cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
as the cell charges:
- Li2S → Li2S2 → Li2S3 → Li2S4 → Li2S6 → Li2S8 → S8
These reactions are analogous to those in the sodium–sulfur battery.
For experimental purposes most batteries are constructed with a carbon and sulfur cathode and a lithium anode. Sulfur as a raw material has the advantage for mass production that it is very cheap, but it lacks electroconductivity. Sulfur alone being at 5*10−30 S cm−1 at 25°C. The carbon coating on the sulfur then provides the electroconductivity missing from pure sulfur. The solution to this problem is carbon nanofibers. The carbon materials provide an effective electron conduction path and structural integrity. The downside of carbon nanofibres is the high cost.
Each sulfur atom can host two lithium ions. Typically, in lithium-ion batteries, for every host atom, only 0.5–0.7 lithium ions can be accommodated.
Degradation
One of the primary shortfalls of most Li–S cells is intermediary reactions with the electrolytes. While S and are relatively insoluble in most electrolytes, many of the intermediary polysulfides are not. The dissolving of into electrolytes causes irreversible loss of active sulfur material. The majority of research on Lithium-sulfur batteries in 2010 is to improve the choice of electrolytes to minimize this side reaction.Safety
Because of the high potential energy density and the nonlinear discharge and charging response of the cell, a microcontrollerMicrocontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
and other safety circuitry is sometimes used along with voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...
s to control cell operation and prevent rapid discharge
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
.
Recent advances
Research conducted at the University of WaterlooUniversity of Waterloo
The University of Waterloo is a comprehensive public university in the city of Waterloo, Ontario, Canada. The school was founded in 1957 by Drs. Gerry Hagey and Ira G. Needles, and has since grown to an institution of more than 30,000 students, faculty, and staff...
has produced Li–S cells with 84% of the theoretical maximum energy density for Li–S that suffer minimal degradation during charge cycling, and thus potentially offering four times the gravimetric energy density of lithium-ion. The team accomplished this through use of a mesoporous carbon cathode, full of deep pits. Sulfur and carbon were milled and heated together, causing the low surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
sulfur to seep into the pits, with just enough room to expand. The composite was then heated to bake off residual sulfur from the surface. To further trap the polysulfides in the cathode, the surface was functionalized and coated with polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
to repel the hydrophobic polysulfides and keep them trapped in the pits. In a "worst case scenario" test using a glyme solvent known for its affinity for dissolving polysulfides, a traditional LiS cathode lost 96% of its sulfur over 30 cycles, while the new cathode lost only 25%.
External links
|