Anodes for Lithium-Ion Batteries

Graphite

Abbreviation: C (derived from the abbreviation for the chemical element carbon).

Currently, anodes based on carbon (graphite) are mostly used in Lithium-Ion Battery. They have a low electrode potential and hardly expand during charging.

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Lithium titanium spinel

Synonyms: lithium titanate, lithium titanium oxide. Abbreviations: LTO or as chemical formula: Li4Ti5O12

LTO anodes offer a higher discharge rate and performance at different operating temperatures than graphite. Compared to graphite anodes, they are considered even safer because of their high potential and built-in overcharge protection, and have a long service life. However, LTO anodes tend to have a slightly lower energy density and are more expensive than graphite anodes.

LTO anodes are Battery Cathodes. They are often used in conjunction with high voltage manganese-based materials because of the high potential compared to lithium. LTO has a nominal voltage of 2.4 V, is fast-chargeable and can be discharged with high electrical currents of 10C. LTO is a very safe material, has excellent discharge characteristics at low temperatures and even at temperatures of -30 °C, 80 percent of the capacity is still usable with LTO electrodes - significantly more than with other materials.

The specific energy density is low at around 60 Wh/kg and competes less with other lithium-ion systems than with nickel-cadmium batteries. LTO reaches a voltage of 2.8 V when charged, but only 1.8 V when discharged.

Silicon and silicon-carbon composites

The chemical element silicon is considered to hold out hope for significantly higher energy densities when used as a material for the anode of lithium-ion battery cells. Its volumetric energy density exceeds that of graphite - the currently used industrial standard - by almost three times. However, despite intensive university and industrial research, the introduction of silicon or silicon-containing compounds (SiOx) in commercial battery systems has so far only succeeded in small proportions of a few percent of the anode's active material.1

Silicon has the property of expanding strongly when the battery is charged. When it is discharged, it shrinks again. After a few charging and discharging cycles, the thin silicon layers or silicon particles are pulverised and the storage of lithium ions no longer works. Other components of a cell are also stressed by the volume changes. These effects lead to a much too short life span of the cells.

Various approaches are currently being tried to get a grip on these problems. In order to be able to control the expansion of the silicon, which occurs when lithium ions are stored in the material, the silicon is formed into tiny wires in the PorSSi joint project of the BMBF's Battery 2020 funding initiative. The scientists are deliberately giving these free space into which they can grow.2 The joint project ProSiSt is also trying to give the silicon more room to "breathe". However, the scientists are proceeding somewhat differently than in the PorSSi project. They want to develop a coating process to apply the silicon to the arrester foils. In a further step, they then want to create space to "breathe" by introducing void structures such as trenches into the silicon layer using lasers or etching processes.3

Silicon-carbon composites

Silicon-carbon composites (Si-C composites): Another approach to compensate for the volume change of silicon already exists on a laboratory scale. One does not use pure silicon as anode material, but composites of silicon (chemical symbol Si) and carbon (chemical symbol C). This composite material has properties that differ significantly from those of the basic components. Carbon serves as a spacer and electrical conductor between the silicon particles. In this way, the high storage capacity of silicon can be used and at the same time a considerably improved service life can be achieved.

Lithium metal

In primary cells, pure lithium metal is used in the anode, which has a very high energy density. In secondary cells, metallic lithium has hardly been used so far, because when charging the battery there is a risk that the lithium does not deposit evenly on the anode, but grows in the form of tree-like structures (dendrites). In unfavourable cases, these can bore through the separator and trigger a short circuit. The dendrites can also detach from the electrode, causing the battery to lose capacity.

A company has already brought a secondary battery with a lithium metal anode and a Polymers Electrolyte onto the market.1 Scientists worldwide are working on making lithium metal usable as an anode material for secondary LIB - especially with ceramic Solid-sate-electrolyte.

If an anode made of lithium metal is used in the battery types described here, strictly speaking, it is no longer a lithium-ion battery, but a lithium battery, since lithium ions are not intercalated in this anode (intercalation), but lithium ions are reduced to metallic lithium.

Other research anode materials

Lithium alloys with aluminium with magnesium with silicon with tin

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