Page 556 - Handbook of Battery Materials

530 17 Liquid Nonaqueous Electrolytes

simultaneously serving as solvents (SO 2 Cl 2 ,SOCl 2 ) or cosolvents (SO 2 ) in primary
or secondary lithium batteries. Recent developments in solvents include methane
sulfonylchloride (MSC) [53], boric acid esters of glycol such as 1,3-propylene glycol
boric ester (BEG-1) [57], ethylene sulfite (ES) [58], and ethyl methyl carbonate
(EMC), and MPC [55].

17.2.2
The Salts

The most commonly used salt in secondary lithium-ion batteries is lithium
hexafluorophosphate (LiPF 6 ). This salt is not the best at all, but it combines most
of the essential and often mutually exclusive properties that are required from a
good electrolyte-building salt. The list below shortly summarizes the properties
and demands that have to be fulfilled most preferably:
• intrinsic thermal stability
• electrochemical stability (high oxidation limit and low reduction limit of the
anion)
• good solubility of the salt in appropriate solvents
• chemical stability with the solvent
• high conductivity of electrolyte solutions
• inertness of the anion toward all cell components (separator, electrodes, casing)
• low molecular weight
• low cost of the salt
• nontoxicity.
As an example of contradictory requirements, LiCl may be considered. The last
three requirements are perfectly fulfilled by LiCl, but its solubility in suitable solvent
blends is low because of its high lattice energy and strong ion–ion interaction
forces, even in solvents of high permittivity. Therefore the first electrolytes used
for primary lithium cells [59, 60] were based on lithium salts with coordinatively
−
saturated molecular anions, such as ClO 4 , or anions based on Lewis acids XF n as
−
−
−
−
well as the corresponding Lewis bases XF n , for example, BF 4 ,AsF 6 , and PF 6 .
An extensive survey of salts used in lithium-ion batteries can be found in Ref.
[24]. A brief summary of salt properties with additional remarks on the state of the
art is given in the following subsections.
17.2.2.1 Lithium Perchlorate
Lithium perchlorate (LiClO 4 ) is sufficiently soluble (beyond 1 M in organic solvents,
e.g., EC/DMC) and forms electrolyte solutions with good conductivity (about
9mS·cm −1 in EC/DMC at ambient temperature) [61]. In organic solvents LiClO 4
forms thicker solid electrolyte interface (SEI) layers than LiPF 6 or LiBF 4 ,but they are
less resistive. This fact is attributed to the highly resistive LiF on the surface which
is formed by hydrogen fluoride (HF) generated by hydrolysis of fluorine-containing
anions, for example, LiBF 4 and LiPF 6 , with traces of moisture and the existing SEI