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11
Separators

Werner B¨ ohnstedt

11.1
General Principles

The separator – the distance-keeping component between the positive and the
negative electrode of a galvanic cell – does not directly participate in the electro-
chemical processes of electricity storage. As a ‘passive’ element it has naturally
attracted little scientific interest; its significance lies in the technical challenge to
build batteries ever more compact and long-lasting. A decisive breakthrough was
achieved only in the second half of the twentieth century by the development of
sufficiently stable synthetic materials. The know-how of the chemical industry in
selecting suitable plastics and their processing was combined with the experience
of the battery industry regarding the unique conditions of use; an independent
separator industry developed which, since the late 1960s, from the combina-
tion of these two aspects, has given essential impulses to the advancement of
batteries.
A comprehensive modern survey of separators for electrochemical power sources
exists only in incomplete parts [1–3], and textbooks on batteries treat this important
element only as a side aspect [4–11]. This section is an attempt to describe, besides
some fundamental aspects, the development history of the battery separator,
competing systems of the present day with their advantages and weaknesses, and
also future development trends.

11.1.1
Basic Functions of the Separators

Separators serve two primary functions: while having to keep the positive electrode
physically apart from the negative in order to prevent any electronic current passing
between them, they also have to permit an ionic current with the least possible
hindrance. These two opposing requirements are best met by a compromise: a
porous nonconductor.
The electronic insulation – the origin of the term ‘separator’) – has to be durable,
that is, it must be effective over many years over a wide range of temperatures and

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.