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It is that part of the battery wherein the electrons move towards the cathode during discharge and move from the cathode upon charging. The anode on conjunction with the cathode produce the necessary output required from the battery.
Initially, metallic Lithium was hypothesized as the material of choice for anodes in lithium batteries due to its high specific energy per weight. Later, this hypothesis was refuted due to the high instability of Lithium metal. Growth of lithium dendrites led to catastrophic results. This shifted the focus to safer anodes, like coke and graphite, with lesser specific energy capacity. Graphite usage has superseded that of any other material because of its ability to produce a flatter discharge curve. Research and Development in the field are ongoing to produce optimized anodic materials.
Graphite remains one of the most commercially attractive anode materials for Li-ion batteries. Electrochemically active graphite is popularly known as meso-carbon micro bead (MCMB). As there is little scope to further increase the capacity of graphite anode, research has turned to other new materials including metal oxides such as cobalt oxide, copper oxide and Li metal alloys.
The anode material is a blend of graphite and silicon anode that maximizes the amount of lithium that can be reversibly at the anode upon discharge. Blending allows for increased capacity when compared to the industry standard of graphite alone whilst still maintaining good electronic conductivity and low electrochemical potential (with respect to Li metal). The blending of silicon with the silicon increases the maximum amount of lithium that can be intercalated within the blended anode structure. The availability, performance and potential (vs. Li metal) of each of the blended materials are all preferable when compared to current graphite based anodes. The geometry of the blended material has been engineered to result in no significant swelling of, or stack pressure generation by the anode on prolonged cycling and therefore Li-ion cells can be constructed as cylindrical, flat or prismatic cells with thin-walled cases or in any other cell configurations.
The total market for anode materials for all applications of Li-ion batteries exceeded 76,000 tons in 2015 and the compound annual growth rate (CAGR) amounted to 14% in the period from 2005 to 2015. m 2005 to 2015. Approximately 40% of the total global demand of the anode active materials (ca. 30,400 tons) was used in Li-ion batteries for HEVs, PHEVs and BEVs. The largest share of the market is taken by graphite and is almost equally divided between natural graphite and artificial graphite.
The market for anode active materials is expected to grow from 76,000 to more than 250,000 tons in 2025. The expected distribution of the market among various anode active materials in 2025 is estimated to be headed by artificial graphite which may likely increase to 52%, that of natural graphite likely to decrease to 24% and the share of other anode active materials is projected to grow to 24%.
Silicon and Tin Anodes:
Crystalline silicon or tin anode 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 group 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.
The lattice distance between the atoms of silicon and tin multiplies as it accommodates lithium ions (lithiation), reaching 320% of the original volume. The expansion causes large anisotropic stresses to occur within the electrode material, leading to fractures and crumbling of the material and ill-fated detachment from the current collector. Prototypical lithium-silicon batteries lose most of their capacity in as little as 10 charge-discharge cycles. A solution to the capacity and stability issues posed by the significant volume expansion upon lithiation is critical to the success of silicon and tin anodes. Nanowire morphology can overcome the problems associated with the stability of these anodes by providing conducting pathways for the ions and decreasing the disruption in volume change. Nanoparticles (with polymeric binders) are being used to achieve similar effects to offset the cost incurred upon use of nanowires.
