(670g) Molten Salt Batteries: Mechanics and Electrolyte Transport

Molten salt batteries are unique power sources with large energy and power densities per unit mass. The electrolyte used to transport charged ions between electrodes is a solid at room temperature which eliminates self-discharge over long storage times. To activate a molten salt battery, the electrolyte is melted by heat generated from an exothermic, pyrotechnic reaction. Because the battery will only remain active while the electrolyte is liquid, insulation is used on all sides of the cell stack.

To model the performance of molten salt batteries, mechanical behavior of the cell (cathode, separator, and anode) and the insulation must be understood. For example, the insulation acts like a spring to maintain a compressive force on the cell stack during activation, yet this material is subject to stress relaxation during battery aging at room temperature. At the electrolyte melting transition, the initially porous separator compresses significantly, redistributing the electrolyte and also determining the ionic resistance across the cell. The initial separator composition affects this behavior greatly, including electrolyte concentration and separator porosity. The insulation at this transition expands in thickness nonlinearly, determining the stress state of the activated battery, which is also investigated. The effects of these mechanical changes on electrolyte transport and battery performance will be discussed. To understand implications of design variables such as materials choice or initial stress state, these mechanical behaviors are included in a comprehensive thermal, electrochemical, and mechanical model for molten salt battery activation.

Sandia is a multi-mission laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.