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How does graphite play a role in lithium-ion batteries?

by:dcfpower     2021-03-17

Graphite, the key raw material for lithium-ion batteries, was predicted by the media in 2015 to have a large demand for graphite, so it is necessary to meet the growing demand for lithium-ion batteries for electric vehicles. Since large electric vehicle batteries require approximately 25 kilograms (55 pounds) of graphite for lithium-ion anodes, it is speculated that graphite may be in short supply. Although prices and consumption have been sluggish, there are signs that demand is shrinking.

Graphite comes from the Greek 'graphein'. It has heat resistance, electrical and thermal conductivity, chemical passivation (corrosion resistance), and is lighter than aluminum. In addition to lithium ion anodes, advanced graphite is also used in fuel cells, solar cells, semiconductors, LEDs, and nuclear reactors. It is expensive to produce anode grade graphite with a purity of 99.99%, and the process generates waste. The final cost is not the material, but the purification process. Recycling old lithium ions to recover graphite will not solve this problem because it is a cumbersome purification process.

Carbon and graphite are related substances. Graphite is an allotrope of carbon, a structural modification that occurs by combining elements in different ways. Graphite is the most stable form of carbon. Diamond is a metastable carbon allotrope. It is known for its excellent physical properties. It is not as stable as graphite, which is soft and plastic. The diameter of carbon fiber is about 5-10 microns, which is one-tenth the thickness of a human hair. The carbon atoms are held together in microscopic crystals and are very strong. They are woven in a textile fashion and mixed with a polymer matrix, which is a hardened form of carbon fiber, which is as strong as steel but lighter. These materials are used in golf clubs and bicycle frames, as well as car and airplane body parts to replace aluminum. Carbon fiber is widely used in Boeing 787 and Airbus 350X. Currently, graphite for batteries only accounts for 5% of global demand.

There are two forms of graphite: natural graphite from mines and synthetic graphite from petroleum coke. Both types are used for lithium ion anode materials, and 55% tend to balance synthetic and natural graphite. Manufacturers prefer synthetic graphite because of its consistency and purity better than natural graphite. This situation is changing. Through modern chemical purification technology and heat treatment, the purity of natural graphite reaches 99.9%, while the purity of synthetic equivalent reaches 99.0%. Purified natural flake graphite has a higher crystal structure, and has better electrical and thermal conductivity than synthetic materials. Switching to natural graphite can reduce production costs while having the same or better lithium ion performance. Lithium-ion synthetic graphite sells for about US$10,000 per ton, while spherical graphite made from natural flakes sells for US$7,000 (2015 price). Unprocessed natural graphite is much cheaper. In addition to cost, natural graphite is more environmentally friendly than synthetic graphite; it is also the basis of graphene and is a scientist's dream. As of the end of 2016, natural graphite accounted for 60-65% of the market share; synthetic ingredients were about 30%, while alternatives such as lithium titanate, silicon and tin were about 5%.

Graphene

Graphene is an allotrope of carbon in the form of a two-dimensional hexagonal lattice. Graphene is present in a sheet of pure carbon, only one atom thick. It is flexible, transparent, impervious to moisture, stronger than diamond, and more conductive than gold. Experts suggest that graphene is a miracle material that can improve many products, including batteries. It is said that graphene anodes retain energy better than graphite anodes and guarantee charging time ten times faster than current lithium-ion batteries.

For traditional graphite anodes, lithium ions accumulate around the outer surface of the anode. Graphene has a more elegant solution by passing lithium ions through the micropores of graphene flakes with a size of 10-20 nm. This guarantees the best storage area and easy extraction. Once available, it is estimated that this battery can store ten times more electrical energy than lithium-ion batteries with conventional graphite anodes. The graphene is further improved by adding vanadium oxide to the cathode. The experimental battery with this enhanced function is said to be recharged within 20 seconds and maintain 90% capacity after 1000 cycles. Graphene is also tested in supercapacitors to improve specific energy density, as well as solar cells. The image below shows the unique lattice of graphene visible using scanning probe microscopy (SPM).

Scanning probe microscopy (SPM) shows graphene images

For decades, scientists have conducted theoretical research on the miracle of graphene, but No commercial product exclusively uses this obvious miracle material. The miracle of graphene has probably been used unknowingly in pencils and other products for centuries, so a better understanding of its mechanism will ultimately lead to product improvements.

Related information: Graphene battery price analysis The impact of graphene on the development of drone battery technology The emergence of graphene batteries has caused doubts in the industry. What do you think?

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