Page 182 - Lindens Handbook of Batteries

BATTERY ELECTROLYTES 7.7

were found to depend on both the salt and solvent, and most interpretations of oxidation reaction
mechanisms are still unresolved. 10
Chapter 14 of this volume sets out some rules for electrolytes that are to be used in lithium anode
batteries. The rules are:
1. The electrolyte must be aprotic, that is, have no reactive protons or hydrogen atoms, although
hydrogen atoms may be in the molecule.
2. It must have low reactivity with lithium (or form a protective coating on the lithium surface to
prevent further reaction) and the cathode.
3. It must have good ionic conductivity.
4. It should be liquid over a broad temperature range.
5. It should have favorable physical characteristics, such as low vapor pressure, stability, nontoxic-
ity, and nonflammability.
To expand on these rules, the first is a criterion that has already been discussed in terms of water and
other contaminants in the electrolyte, however, it is broader because any Brønsted acid can contribute
protonic species which can penetrate passive films and corrode the lithium (or lithiated graphite or other
alloys), thus making the system unstable. The formation of gaseous hydrogen at the anode surface
further disrupts the passive film during bubble formation. The second rule is critical to any active metal
anode, since the polar solvents useful for electrolytes are at best only metastable to the anode metal.
The author has shown by thermodynamic calculations that even propylene and ethylene carbonates are
capable of highly exothermic reactions with lithium metal to produce lithium carbonate and the cor-
11
responding alkene. A recognition of the importance of the passivating film (or the Solid Electrolyte
12
Interphase [SEI] layer) is demonstrated in the book Lithium Batteries: Solid Electrolyte Interphase
and many other writings referred to therein. Additives to the electrolyte have frequently been employed
13
to improve the SEI. Reactions with the cathode are also of great concern to the electrochemist study-
ing lithium and lithium-ion batteries. Again, the solvents are only metastable as demonstrated by cal-
11
culations of simple reactions and the mechanisms are poorly understood. However, the exothermicity
of solvent reaction with strong oxidants can be very great as can be seen by DSC measurements of
14
electrolyte-charged positive electrode samples. The third rule relates to the practicality of making
cells with usable current output in high energy density (and power density) configurations for ambient
operation. The conductivity of the electrolyte should be at least 3 mS/cm to make practical electrolytes,
although higher values are very desirable and allow thicker, shorter electrodes to be used. Many liquid
electrolytes exceed this value at room temperature and will be discussed later. However, the attempts to
produce solid polymer electrolytes with adequate conductivity at room temperature have yet to succeed.
The best of these types have achieved conductivities of the order of 0.1 mS/cm, but adequate designs
have not yet been fully developed to utilize these interesting materials. Rule 4 is also important and
has made necessary the use of solvent mixtures to satisfy both high and low temperature requirements.
Even so, extreme high or low temperature operation has necessitated special purpose electrolytes,
which do not perform well at the opposite extremes. Finally, rule 5 has many ramifications. The lack of
toxicity has been a relative goal as some electrolytes evince moderate toxicity and may even have reac-
tion products of high toxicity on exposure to ambient environments. An example is the frequently used
LiAsF salt, which can be converted to toxic arsenic oxide on exposure to room atmosphere. Low vapor
6
pressure is moderated through the use of solvent mixtures. If a component has a high vapor pressure,
it is usually found in modest concentration in the solvent mixture to suppress the vapor pressure, but
may contribute to the level of conductivity of the electrolyte or improve low-temperature performance.
Stability is really a reference to stability within the cell environment, as one of the most common salts
used in lithium-ion cells, LiPF , is thermally, as well as photochemically, unstable in many solutions,
6
but in the cell environment, it can maintain stability over many years of operation. Nonflammability or
flame retardancy is generally not observed with organic solvents. However, phosphorus and halogen
substituents (such as fluorine) can confer these properties, and a number of workers are active in this
field at present. No agreed-upon materials have yet been found. As in other property studies, an admix-
ture of a flame-retardant chemical is the most likely to be successful. 15