Page 183 - Lindens Handbook of Batteries

7.8 PRINCIPLES OF OPERATION

Solvent properties of the most common solvents for lithium primary batteries are given in
Chap. 14 of this volume, and the most common solvents for lithium-ion rechargeable batteries are
given in Chap. 26. Additional information is also found in Chap. 27. A more complete listing of
solvents can be found in Ref. 16. As alluded to earlier, mixtures of two or more of these solvents
are usually employed to obtain the best combination of properties for the intended application. A
high dielectric constant, high-viscosity solvent such as propylene carbonate (permittivity = 64.4,
viscosity = 2.5 cP) can be mixed with a low dielectric constant, low-viscosity solvent such as
dimethoxyethane (permittivity = 7.2, viscosity = 0.455 cP) to give a mixed solvent of intermediate
dielectric constant and intermediate viscosity with very good solvation properties toward lithium
salts. Ideal mixing rules adapted from traditional physical chemistry generally predict the mixed
17
solvent properties to within a few percent for aprotic solvents. Certain empirical parameters,
such as donor and acceptor numbers, have been usefully employed to help choose cosolvents to
improve properties, while concepts such as ion association from physical chemical studies may
also be useful to understand the conductivity and viscosity of electrolyte solutions. 16
The choice of electrolyte salt is also very important for thermal and electrochemical stability of
the electrolyte solution as well as affecting the conductivity of the electrolyte solution. It is not widely
recognized, but many times the anodic window of an electrolyte is determined by the reactivity of
the anion, which can play a catalytic role in the oxidation of the solution. Thus, a one-electron oxi-
dation of the anion at the positive electrode to form a neutral free radical leads to attack of a solvent
molecule in a chain reaction. Chain termination usually results from radical combination, but often
great damage is done to the electrolyte, which becomes evident on further cycling. These reactions
10
are especially important for rechargeable systems. The salt may also be unstable at the negative
electrode. For example, the tetrachloroaluminate anion may deposit aluminum in an exchange reac-
tion with lithium. Furthermore, the salt may effect the SEI of the graphite anode in a beneficial way
as exemplified by the LiBOB (lithium bis-oxalatoborate) salt in some lithium-ion battery electrolytes
(see Chap. 26). One of the most important effects of salts in lithium-ion batteries is in the effect on
the collector substrate for the positive electrode. Aluminum is the most commonly used collector in
lithium-ion batteries and the choice of salt is very important, particularly at longer times. The pit-
ting corrosion potential is similar for a group of salts at short times, but the effect at longer times
18
shows that only a few salts such as LiPF and LiBF are stable with aluminum. Thermal stability
4
6
is affected by the salt as well as the solvent choice. One of the critical thermal aspects is related to
the onset temperature of the dissolution of the SEI layer, which is very sensitive to the choice of
electrolyte salt. These matters are discussed in Ref. 15. In summary, rechargeable cells are the most
sensitive to the choice of salt, with LiPF the preferred salt in most cases. A wider choice is avail-
6
able for primary batteries, in part because the cathode is never charged and thus the collector is not
exposed to high overpotentials. LiCF SO , LiPF , LiBF , LiBr, LiI, LiN(CF SO ) , and LiClO
3
4
6
3
3
4
2 2
have all been used in primary lithium batteries. Dahn and Ehrlich discuss the conductivity of vari-
ous electrolyte salts in different solvents in Chap. 26. It should be emphasized, however, that in all
cases, the conductivity of electrolytes with organic solvents (as well as the inorganic solvent sys-
tems discussed below) are at least an order of magnitude poorer than aqueous electrolyte solutions,
especially those with enhanced proton conductance such as acids and bases. This has a profound
effect on cell design for the nonaqueous systems. For high-rate cells, electrodes are much thinner,
the corresponding electrode areas are much greater, and separators are much thinner. This makes the
cells more expensive to manufacture than aqueous cells of similar rate capability. However, because
of the high voltage or high capacity of the chosen nonaqueous systems, the cost of a given cell size
per watt hour of energy is mitigated.
Many additives have been developed for lithium-ion batteries for the purpose of improving safety,
extending the calendar life, and extending cycle life of cells. The subject is complicated because
the level of additive is generally small (1% or less of the electrolyte) unless the additive is for the
purpose of adding flame retardancy to the electrolyte, when the level is generally 5% or higher.
Furthermore, extensive chemical analysis before and after cycling has not often been performed,
so the effects are generally left as empirical findings. Also, the long-term effects of the additive are
generally not described. Their use in batteries is discussed in Chap. 26 of this volume as well as
in Ref. 15.