Lithium based Battery Chemistry FAQ

Applications: Pacemakers, defibrillators, watches, meters, cameras, calculators, portable, low-power use

Lithium battery chemistry comprise a number of cell designs that use lithium as the anode. Lithium is gaining a lot of popularity as an anode for a number of reasons. In this comparison of anode materials, we can see some reasons why:

Anode	Atomic mass (g)	Standard potential (V)	Density g/cm3	Melting point ºC	Electrochemical Equivalence (Ah/g)
Li	6.94	3.05	0.54	180	3.86
Na	23.0	2.7	0.97	97.8	1.16
Mg	24.3	2.4	1.74	650	2.20
Al	26.9	1.7	2.7	659	2.98
Ca	40.1	2.87	1.54	851	1.34
Fe	55.8	0.44	7.85	1528	0.96
Zn	65.4	0.76	7.1	419	0.82
Cd	112	0.40	8.65	321	0.48
Pb	207	0.13	11.3	327	0.26

Notice that lithium, which is the lightest of the metals, also has the highest electrochemical potential of all the metals, at over 3 V. Some of the lithium cell designs have a voltage of nearly 4 V. This means that lithium has the highest energy density. Many different lithium cells exist because of its stability and low reactivity with a number of cathodes and non-aqueous electrolytes. The most common electrolytes are organic liquids with the notable exceptions of SOCl₂ (thionyl chloride) and SO₂Cl₂ (sulfuryl chloride). Solutes are added to the electrolytes to increase conductivity.

Lithium cells have only recently become commercially viable because lithium reacts violently with water, as well as nitrogen in air. This requires sealed cells. High-rate lithium cells can build up pressure if they short circuit and cause the temperature and pressure to rise. Thus, the cell design needs to include weak points, or safety vents, which rupture at a certain pressure to prevent explosion.

Lithium cells can be grouped into three general categories: liquid cathode, solid cathode, and solid electrolyte. Let's look at some specific lithium cell designs within the context of these three categories.

Liquid cathode lithium cells:

These cells tend to offer higher discharge rates because the reactions occur at the cathode surface. In a solid cathode, the reactions take longer because the lithium ions must enter into the cathode for discharge to occur. The direct contact between the liquid cathode and the lithium forms a film over the lithium, called the solid electrolyte interface (SEI). This prevents further chemical reaction when not in use, thus preserving the cell's shelf life. One drawback, though, is that if the film is too thick, it causes an initial voltage delay. Usually, water contamination is the reason for the thicker film, so quality control is important.

LiSO₂ Lithium–Sulfur Dioxide

This cell performs very well in high current applications as well as in low temperatures. It has an open voltage of almost 3 V and a typical energy density of 240–280 Wh/kg. It uses a cathode of porous carbon with sulfur dioxide taking part in the reaction at the cathode. The electrolyte consists of an acetonitrile solvent and a lithium bromide solute. Polypropylene acts as a separator. Lithium and sulfur dioxide combine to form lithium dithionite:

2Li + 2SO₂ —> Li₂S₂O₄

These cells are mainly used in military applications for communication because of high cost and safety concerns in high-discharge situations, i.e., pressure buildup and overheating.

LiSOCl₂ Lithium Thionyl Chloride

This cell consists of a high-surface area carbon cathode, a non-woven glass separator, and thionyl chloride, which doubles as the electrolyte solvent and the active cathode material. Lithium aluminum chloride (LiAlCl₄) acts as the electrolyte salt.

The materials react as follows:

Location	Reaction
Anode	Li —> Li+ + e-
Cathode	4Li+ + 4e- + 2SOCl2 —> 4LiCl + SO2 + S
Overall	4Li + 2SOCl2 —> 4LiCl + SO2 + S

During discharge the anode gives off lithium ions. On the carbon surface, the thionyl chloride reduces to chloride ions, sulfur dioxide, and sulfur. The lithium and chloride ions then form lithium chloride. Once the lithium chloride has deposited at a site on the carbon surface, that site is rendered inactive. The sulfur and sulfur dioxide dissolve in the electrolyte, but at higher-rate discharges SO₂ will increase the cell pressure.

This system has a very high energy density (about 500 Wh/kg) and an operating voltage of 3.3–3.5 V. The cell is generally a low-pressure system

In high-rate discharge, the voltage delay is more pronounced and the pressure increases as mentioned before. Low-rate cells are used commercially for small electronics and memory backup. High-rate cells are used mainly for military applications.

Solid cathode lithium cells:

These cells cannot be used in high-drain applications and don't perform as well as the liquid cathode cells in low temperatures. However, they don't have the same voltage delay and the cells don't require pressurization. They are used generally for memory backup, watches, portable electronic devices, etc.

LiMnO₂

These account for about 80% of all primary lithium cells, one reason being their low cost. The cathode used is a heat-treated MnO₂ and the electrolyte a mixture of propylene carbonate and 1,2-dimethoyethane. The half reactions are

Anode	Li —> Li+ + e
Cathode	MnIVO2 + Li+ + e —> MnIIIO2(Li+)
Overall	Li + MnIVO2 —> MnIIIO2(Li+)

At lower temperatures and in high-rate discharge, the LiSO₂ cell performs much better than the LiMnO₂ cell. At low-rate discharge and higher temperatures, the two cells perform equally well, but LiMnO₂ cell has the advantage because it doesn't require pressurization.

Li(CF)_n Lithium polycarbon monofluoride

These cells are used in coin cells for watches and memory-back up, nuclear missile batteries, the space shuttle safety system, and other governmental and space applications. The cathode in this cell is carbon monofluoride, a compound formed through high-temperature intercalation. This is the process where foreign atoms (in this case fluorine gas) incorporate themselves into some crystal lattice (graphite powder), with the crystal lattice atoms retaining their positions relative to one another. This is not a stoichiometric reaction, so the proportion of fluorine atoms can vary between 0.8 and 1.2, which is why the half-reactions are also not stoichiometric. This is interesting because it allows a lithium-fluorine reaction, which is probably the most energetic possible by safely storing the fluorine atoms in a graphite matrix. This is similar to how lithium ions are stored in lithium ion batteries. The carbon intercalation makes it safe, but it also reduces the voltage and lowers the electrical current capability.

A typical electrolyte is lithium tetrafluoroborate (LiBF₄) salt in a solution of propylene carbonate (PC) and dimethoxyethane (DME).

Anode	Li —> Li+ + e
Cathode	CFx + xe —> xC + xF
Overall	CFx + xLi —> xLiF+ xC

Note that one of the reaction products is carbon, which lowers the resistance of the cell as the battery is discharged. These cells also have a high voltage (about 3.0 V open voltage) and a high energy density (around 250 Wh/kg). All this and a 7-year shelf life makes them very suitable for low- to moderate-drain use, e.g., watches, calculators, and memory applications.

Solid electrolyte lithium cells:

All commercially manufactured cells that use a solid electrolyte have a lithium anode. They perform best in low-current applications and have a very long service life. For this reason, they are used in pacemakers

LiI₂—Lithium iodine cells use solid LiI as their electrolyte and also produce LiI as the cell discharges. The cathode is poly-2-vinylpyridine (P2VP) with the following reactions:

Anode	2Li —> 2Li+ + 2e
Cathode	2Li+ + 2e + P2VP· nI2 —> P2VP· (n–1)I2 + 2LiI
Overall	2Li + P2VP· nI2 —> P2VP· (n–1)I2 +2LiI

LiI is formed in situ by direct reaction of the electrodes.