Page 254 - Handbook of Battery Materials

224 8 Metallic Negatives

8.3.5
Lithium (Li)
Most battery systems in which lithium is applied as anode material belong to the
group using nonaqueous electrolytes, but there is one system that works with water
serving as solvent and reactant as well. This is only possible because lithium forms
a passive layer in solutions containing higher amounts of caustic [104–107].
Nevertheless, the water is decomposed with evolution of hydrogen but un-
der controlled conditions and with a reasonable reaction rate. With water as
the active cathode material, the battery system – used in military underwater
applications – can be designed as: (−) Li/KOH/H 2 O (+) [108].
The regular discharge reactions are:

+
Anode: Li → Li + e − (8.11)
− − 1
Cathode: H 2 O + e → OH + / 2 H ↑ (8.12)
2
1
Overall reaction: Li + H 2 O → LiOH + / 2H 2 ↑ (8.13)
The electrochemical equivalent of 3860 Ah kg −1 is the highest of all metal anodes
and the OCV of 2.7–2.8 V (depending on electrolyte concentration) is also rather
high.

8.3.6
Magnesium (Mg)
Magnesium alloys as anode materials are found in (at least) four types of primary
cells: the ‘dry cell’ type, (−) Mg/KOH/MnO 2 (+) [109], magnesium/air batteries
(−) Mg/KOH/O 2 (+) including a particular version operating on oxygen dissolved
in sea water [110–112], water-activated reserve batteries, (−) Mg/KOH/K(+), where
K = AgCl [113], CuCl, PbCl 2 ,Cu 2 I 2 [114], CuSCN, CuO [115], and again MnO 2 [116],
and finally cells equipped with organic cathodes on copper current collectors (−)
Mg(Zn)/chlorides, perchlorates/organic active material (e.g., 2-nitrophenylpyruvic
acid) (+) [117]. All of these cells are primaries (AB1) using the advantage of a high
−1
electrochemical equivalent (2200 Ah kg ) and relatively high OCVs (e.g., 2.8 V for
the Mg/MnO 2 -system).
The dominating discharge reactions (for the Mg/air system as an example) are:
Anode: Mg → Mg 2+ + 2e − (8.14)
−
Cathode: 1/2O 2 + H 2 O + 2e → 2OH − (8.15)
Overall reaction: Mg + 1/2O 2 + H 2 O → Mg(OH) (8.16)
2
Side reaction: Mg + 2H 2 O → Mg(OH) + H 2 ↑ (8.17)
2
The most important anodes for battery use are ternary Mg/Al/Zn-alloys (AZ 61,
AZ 63 – Norsk Hydro), an alloy containing 1% of lithium (AZ 21), and the rather
complex alloy AZ 31 (Mg-3 Al-1 Zn-0.2 Mn-0.15 Ca) [118].