Mechanical Aspects of Structural Composite Batteries

February 1, 2008

Army Research Laboratory, Adelphi, Maryland

In furtherance of the development described in the immediately preceding article, an experimental study focusing on mechanical aspects of structural composite batteries was performed. In this study, battery component materials, battery components, and fully assembled structural composite batteries were subjected to tensile tests. Limited electrochemical tests were also performed.

The components and materials selected for testing were the following:

• Polymer Electrolyte

  The polymer electrolyte was a vinyl ester random copolymer comprising 80 weight percent of methoxy poly(ethylene glycol) 550 monoacrylate and 20 weight percent ethoxylated pentaerythritol tetraacrylate.

• Anodes

  Two anode materials were selected: a plain woven and a nonwoven carbon fabric. The relevant electrochemical properties of these materials were known from prior tests. The thicknesses and areal mass densities of these materials were known from manufacturers' specifications.

• Cathodes

  Each cathode consisted of a film of a composite cathode material on a metal substrate. The composite film consisted of 72 weight percent of LiFePO4 (the active cathode material), 8 weight percent of acetylene black (to ensure electrical conductivity throughout the film, and 20 weight percent of poly(ethylene oxide) 200k (serving as a binder). The film was cast on each metal substrate using acetonitrile as a solvent.

  Three metal substrate materials were chosen: a woven mesh made of 304 stainless steel, a perforated foil made of 304 stainless steel, and an expanded foil made of 316L stainless steel. The proportions of open surface area of these materials are all ~30 percent — a value chosen because it affords a good compromise between retaining in-plane stiffness and providing open surface area for deposition of the composite cathode material.

• Separators

  The separators were made of a glass fiber cloth having a thickness of 0.11 mm, an areal mass density of 99 g/m², a weave lineal density of about 64 yarns per inch (~24 yarns per centimeter), and an effective fiber volume fraction of about 35 percent.

• Structural Composite Batteries

  Four structural composite batteries having the configuration shown in the figure were fabricated according to the procedure described in the last paragraph of immediately preceding article. In two of the batteries, designated for mechanical testing, the anodes were made of the woven carbon fabric. In all of the batteries, the perforated stainless steel was used as the cathode substrate material (the other cathode substrate materials were subjected to tensile tests but not incorporated into batteries).

  Two of the batteries were designated for electrochemical tests. In one of these batteries, the anodes were made of the woven carbon fabric; in the other battery, the anodes were made of the non-woven carbon fabric.

One of the conclusions drawn from the results of the tensile tests was that the specific stiffness of a structural composite battery containing a perforated-foil cathode substrate can be expected to exceed that of an otherwise equivalent battery containing a woven mesh or expanded-foil substrate. However, the tensile tests also drew attention to issues of design and fabrication that must be addressed in order to exploit synergies among the component materials to realize the full potential for strengthening and stiffening.

The electrochemical tests were limited to measurements of electrical resistances; further electrochemical tests could not be performed because these measurements revealed anode-to-cathode short-circuiting. An investigation to determine the cause of the short-circuiting was in progress at the time of reporting the information for this article.

This work was done by E. L. Wong, D. M. Baechle, K. Xu, R. H. Carter, J. F. Snyder, and E. D. Wetzel of the Army Research Laboratory.

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