Anode

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What is a anode?
Anode is said to be negative electrode in the battery. It has lower voltage wrt to Lithium metal when compared with cathode counterpart. 

   Graphite is used as anode and it is minimum potential intercalation electrode. It generally comprises carbon, silicon blended carbon or lithium titanate (LTO). Previously Lithium metal was used as a anode but problems like lithium plating and dendrite formation were common with this anode.

What are advanatages and disadvantages of Lithium metal as anode?
It can give the best voltage difference with a cathode for lithium ions since the potential for Li ions for lithium metal is practically zero (lowest electrochemical potential). It has highest theoretical capacity of 3860 mAh g−1.

 Lithium plating and dendrite formation leading to internal short-circuit of cells, which is one of the reason for thermal runaway and explosion hazards.

What are requirements of an anode?
Excellent porosity and conductivity, good durability and light weight, low voltage with respect to lithium metal.

 

Typical Voltage of anode?

The voltage of anode is always less than the cathode and it is measured wrt Lithium metal, therefore Li metal has 0V. Materials have more than one oxidation and reduction potential, generally the lowest potential at which we can utilise completely is considered the voltage. For a graphite it is 0.05V and silicon it is 0.2V 

 

What are active anode materials?

Graphite is one of the mostly used as anode material in the industry. Acetylene black is another carbon form, carbon blended with silicon is the next generation anode because of high capacity over carbon. Silicon itself has superior capacity but the volume expansion while charging discharging makes it unsuitable for applications. Lithium titanate is also used as anode with a very stable capacity for thousands of cycles but the voltage w.r.t. to lithium is more than carbon and even costlier. Therefore its a costlier alternative with less power to carbon.

 

How is the electrode made for anode?

Graphite is mixed with conductive carbon, CMC (dispersant) and SBR (binder) in the ratio of 80-96:0-10:2-5:2-5 with water as solvent. The slurry is then coated on copper foil to make the anode electrode.

What are the binders used with anode?

Like cathode the binder is PVDF when solvent is NMP but there is an alternative which is water soluble binder, Styrene Butadiene Copolymer (SBR) and it is used along with Carboxy Methyl Cellulose (CMC).

What are the foils for anode coating?

Copper foils are generally used for coating anode with thickness as less as 6-25 microns. For avoiding high anti-corrosion when different electrolyte are used we can also use a nickel foil but it is costly.

What are the advanṭages of Lithium titanate?

SEI film and lithium plating formation is very low at low temperature. LTO has a spinel structure which gives it the stability. Thermal stability at high temperature.

Silicon anode?

Crystalline silicon  has a theoretical specific capacity of 4200 mAh/g, more than ten times that of anodes such as graphite (372 mAh/g). Each atom of the silicon 4 element can bind up to 4.4 atoms as compared to 1 atom per 6 carbon atoms for the fully lithiated state of graphite LiC6. But lithiation increase volume to 320% which makes them unsuitable for application.

 

Anode classification?

They are classified depending on the nature of reaction which can be intercalation, alloying or conversion. Ex Graphite (Intercalation), Si (Conversion) and Sn (alloying)