Page 176 - Lindens Handbook of Batteries

CHAPTER 7

BATTERY ELECTROLYTES

George E. Blomgren

7.1 INTRODUCTION

Previous editions of this Handbook did not include a chapter on electrolytes. The individual chapters
on each battery type included information needed to understand the electrochemical operation of
the systems under discussion. This edition also includes some information about the electrolytes for
each battery type within the chapter describing the system, but this chapter attempts to broaden the
horizon on the electrolyte component because of the key role of electrolytes as the bridging phase
between the electrodes. Each electrode has a contribution to the battery impedance that is due to
the electrode-electrolyte interface, as well as a separate contribution due to the impedance of the
electrolyte itself beyond the region of the electrical double layer. This impedance term becomes
especially important under high-current conditions when mass transport of ions through the electro-
lyte frequently becomes the limiting process through the Nernst equation relating the concentration
of ions at the electrode interface to the electrode polarization.
Since most batteries of the 19th and much of the 20th century utilized aqueous electrolytes, the
discussion begins with the various aqueous electrolytes of importance in batteries. Some specialized
electrolytes are omitted in the discussion in order to focus on those of major importance.
Beginning in the late 1950s, electrolytes were developed that had good stability with lithium
metal. This opened the door to lithium primary batteries, which became available in the 1970s. After
major safety difficulties with rechargeable lithium metal batteries became evident, the lithium-ion
battery, using lithiated carbon as the negative electrode, made its appearance in the early 1990s, and
the rechargeable lithium-ion battery industry was born. An overview of the electrolytes used in both
battery types is given. In addition, newly developed ionic liquid electrolytes, which offer the pos-
sibility of low flammability to enhance safety, are also discussed.
While lithium metal batteries did not achieve success as rechargeable wet cells, the use of
ceramic or glassy solids as lithium-ion conductive electrolytes has permitted the use of lithium metal
to produce high cycle-life rechargeable batteries, mostly as thin film cells (see Chap. 27). New work
on these solid inorganic electrolytes offers the hope of making high-capacity lithium metal cells and
will also be discussed. Similarly, new approaches to dry polymer electrolytes have renewed interest
in this type of cell. Therefore, an introduction to these types of electrolytes is also presented.

7.2 AqUeOUs eleCTROlyTes

Aqueous electrolytes can be broken up on the pH scale into alkaline, neutral (or mildly acidic), and
strong-acid electrolytes. The alkaline electrolytes are usually very strong with pH values close to 13.
Neutral electrolytes are generally composed of salts of strong acids and bases. Additives of weaker

7.1