Journal of Power Sources 164 (2007) 397–414 Review Thermal activated (thermal) battery technology Part II. Molten salt electrolytes Patrick Masset a,∗ , Ronald A. Guidotti b a Karl Winnacker Institut der Dechema e.V., Theodor-Heuss Allee 25, 60486 Frankfurt am Main, Germany b Sierra Nevada Consulting, 1536 W. High Pointe Ct. Minden, NV 89423, USA Received 31 March 2006; received in revised form 24 September 2006; accepted 24 October 2006 Available online 1 December 2006 Abstract This article gives an overview of the important properties and design characteristics of electrolyte used in thermally activated (thermal) batteries. The basic physical properties of the main compositions are reviewed. The properties of electrolytes such as melting point, ionic conductivity, surface tension, density, thermal characteristics, and moisture sensitivity were analyzed in relation with the functioning of the batteries. Solubility data of alkali metals, sulphides, and oxides were compiled and analyzed. The important parameters of separator pellets are discussed in terms of both electrical and mechanical properties as they pertain to thermal-battery design and functioning. A number of lower-melting electrolytes are presented along with key physical properties for possible use in applications requiring lower operating temperatures such as borehole power supplies. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermal batteries; Molten salts; Electrolyte Contents 1. Introduction ............................................................................................................ 398 2. Lithium-halide electrolytes ............................................................................................... 398 2.1. Electrochemical window ........................................................................................... 398 2.2. Ionic conductivity ................................................................................................. 400 2.3. Density .......................................................................................................... 402 2.4. Surface tension ................................................................................................... 402 2.5. Thermal properties ................................................................................................ 403 3. Hygroscopicity of salts .................................................................................................. 404 3.1. MX·nH 2 O hydrates (M = Li, Na, K and X = F, Cl, Br, I) ............................................................... 404 3.2. Drying procedures ................................................................................................ 405 4. Retention of the electrolyte ............................................................................................... 405 4.1. Binder ........................................................................................................... 405 4.2. Mechanical properties of the separator .............................................................................. 406 5. Solubility phenomenon in molten salts .................................................................................... 406 5.1. Solubility of lithium and electronic conductivity ..................................................................... 406 5.2. Solubility of sulfur-based species ................................................................................... 407 6. Lower-melting electrolytes ............................................................................................... 408 6.1. Alkali halide systems .............................................................................................. 408 ∗ Corresponding author. Tel.: +49 697 564 362; fax: +49 697 564 388. E-mail address: masset@dechema.de (P. Masset). 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.10.080