Lithium iron sulfide battery

Lithium iron sulfide battery of type IEC FR6, size AA

The lithium iron sulfide battery or lithium iron sulfide cell is a type of lithium battery and, as a galvanic cell, is one of the non-rechargeable batteries ( primary cells ). The anode ( negative pole ) consists of metallic lithium and the cathode (positive pole) consists of iron (II) disulfide . The electrolyte is a solution of lithium iodide (LiI) in a mixture of organic solvents . The nominal voltage of a cell is 1.5 V, so it can be used as a replacement for alkaline manganese batteries and zinc-carbon batteries . Advantages over alkaline-manganese batteries are higher capacity , especially when discharging with high currents and at low temperatures, and longer shelf life. The disadvantage is the significantly higher price, which is due to the more complex construction.

Round cells in sizes Mignon (AA, IEC FR6) and Micro (AAA, IEC FR03) have been commercially available since 1992 and 2004, respectively. In 1979 lithium iron sulfide batteries were also introduced as button cells and as an inexpensive alternative to silver oxide zinc batteries for watches. Due to the falling silver price , button cells made of lithium iron sulfide were soon withdrawn from the market because they were not competitive due to the higher price.

Electrochemistry

In the lithium iron sulfide cell, the oxidation of lithium and the reduction of iron sulfide provide the electrical energy. The theoretical capacity of lithium is 3.86  Ah / g, of iron sulfide (FeS) 0.61 Ah / g and of iron disulfide (FeS ₂ ) 0.89 Ah / g. The reactions taking place in the cell are complex and are shown schematically below.

When discharging, metallic lithium (Li) is oxidized to lithium ions in the anode. The oxidation number of lithium increases from ± 0 to + I.

Anode reaction: ${\displaystyle \mathrm {4\,Li\rightarrow 4\,Li^{+}+4\,e^{-}} }$

When discharging, iron (II) disulfide (FeS ₂ ) is reduced to iron (Fe) and sulfide ions (S ²⁻ ) in the cathode . The oxidation number of iron decreases from + II to ± 0, the (formal) oxidation number of sulfur decreases from −I in the disulfide ion (S ₂ ²⁻ ) to −II in the sulfide ion (S ²⁻ ). Alternatively, iron (II) sulfide (FeS) can also be used as the cathode material. In this case, only iron sulfide is reduced to iron during the discharge. The oxidation number of iron decreases from + II to ± 0. In practice, only FeS _{2 is used} as the cathode material, as the cell voltage and capacity are higher in this case.

Cathode reaction (FeS ₂ ): ${\displaystyle \mathrm {FeS_{2}+4\,e^{-}\rightarrow Fe+2\,S^{2-}} }$

Cathode reaction (FeS): ${\displaystyle \mathrm {FeS+2\,e^{-}\rightarrow Fe+S^{2-}} }$

The overall reaction can be represented as follows:

Overall reaction (Li-FeS ₂ ): ${\displaystyle \mathrm {4\,Li+FeS_{2}\rightarrow Fe+2\,Li_{2}S} }$

Overall reaction (Li-FeS): ${\displaystyle \mathrm {2\,Li+FeS\rightarrow Fe+Li_{2}S} }$

A mixture of different organic solvents ( ethylene carbonate , ethylene glycol dimethyl ether ) with lithium iodide (LiI) is used as the electrolyte .

construction

The commercially available cylindrical Li-FeS ₂ round cells in the sizes Micro (AAA) with the IEC designation FR03 and Mignon (AA) with the IEC designation FR6 are so-called wound cells. This means that the electrodes are foils and are wound into a roll with the microporous separator made of polyethylene or polypropylene in between . The anode foil consists of lithium and the cathode foil consists of a metallic carrier coated with iron disulfide. The cell is equipped with a thermal switch ( PTC resistor ) to protect against an external short circuit . Due to the high current flow during a short circuit, the cell heats up considerably. When the activation temperature of the thermal switch is reached, its resistance increases and the flow of current and thus the generation of heat is limited.

properties

The nominal voltage of the lithium iron sulfide cell is 1.5 V. The open-circuit voltage is around 1.8 V. The load voltage is higher than that of alkali-manganese cells with the same load, and the discharge characteristic is flatter. This explains the higher load capacity of lithium iron sulfide cells compared to alkali manganese cells when used in applications with high power or power requirements (e.g. digital cameras and flash units for cameras). The lower the power or power requirement of the application, the lower the advantage of the lithium iron sulfide cell. Lithium iron disulfide cells can be operated in the temperature range from approximately -40 ° C to 60 ° C. As with all battery systems, the capacity that can actually be drawn increases with increasing temperature. The lithium iron sulfide battery is characterized by its very good shelf life (according to the manufacturer, 10–15 years).

literature

• Lucen F. Trueb, Paul Rüetschi: Batteries and accumulators - Mobile energy sources for today and . Springer, Berlin 1998 ISBN 3-540-62997-1 .
• David Linden, Thomas B. Reddy (Eds.): Handbook of Batteries . 3. Edition. McGraw-Hill, New York 2002 ISBN 0-07-135978-8 .

Web links

• Datasheet lithium iron disulfide cell FR6 (English; PDF file; 92 kB)
• Datasheet lithium iron disulfide cell FR03 (English; PDF file; 77 kB)