A continuing program of research and development is devoted to the design, fabrication, and testing of polymeric composite- material devices that are multifunctional in the sense that they both store electrochemical energy and bear mechanical loads. It is anticipated that if designed to exhibit sufficient structural and energy efficiencies, these devices could enable significant reductions in the weights of systems in which they could be used, by supplanting electrically inert structural components and conventional batteries while providing supplementary power for lightload applications.
Not only can a device of this type be regarded in its entirety as both a structural component and an electrochemical cell or battery, in addition, each component (the electrodes and electrolyte) of the device is designed to have a desired combination of electrochemical properties and mechanical strength. In the fabrication of the device, these components are integrated by use of costeffective molding processes and other processes that are commonly used in the manufacture of composite-material objects and that can be scaled up to mass production.
The figure presents a partly schematic cross-section of a representative basic structural polymeric composite battery comprising multiple cell layers. Each cell layer consists of an anode sublayer made of a carbon-fiber fabric, a separator sublayer made of a glass fabric, a cathode sublayer in the form of a metal mesh coated with a cathode material, and a structural solid polymer electrolyte that fills the spaces between the aforementioned sublayers, binding all sublayers and layers together. The cathode and anode sublayers both bear mechanical loads and act as electriccurrent collectors. The separator sublayer provides additional structural support while ensuring electronic isolation of the electrode sublayers. The polymer electrolyte transfers mechanical loads to and from the other components and conducts ions between electrodes.
The designs of these structural polymeric composite batteries incorporate lithiumbased electrochemistry, which is chosen because it offers high energy density and compatibility with polymer-based electrolytes. The carbon-fiber fabric anode material (typically, a carbon paper, a mat of nonwoven carbon fibers, or a bidirectional woven carbon fabric) is chosen partly because carbon fibers exhibit sufficient stiffness and strength to afford the desired mechanical reinforcement and sufficient electrical conductivity for transport of electrons into and out of each cell. In addition, for lithium ion battery chemistry, carbon fibers can serve as media for intercalation of lithium ions.
In a cathode, the metal mesh serves as a current collector. The mesh is coated with a thin film comprising a mixture of an active cathode material and a carbon powder. The composition of the mixture and the processing thereof are optimized to obtain high electrochemical capacity, electrical conductivity, rechargeability, and mechanical integrity. LiCoO2 and LiFePO4 are active cathode materials that have been evaluated thus far. The chosen carbon powder is acetylene black, which is highly electrically conductive and is used to optimize the electronic conductivity of the cathode coating film.
For a given battery, electric power and mechanical strength can be increased by using a more processable electrolyte resin that performs well as a thin film. Reducing the thickness of the electrolyte increases the current by increasing the rate of conduction of ions between electrodes. In addition, the ability to fabricate a structural polymeric composite battery using only a small quantity of polymer electrolyte as a binder would enable the incorporation of a relatively large volume fraction of structural electrode materials; as a result, such a battery could have both greater strength and higher charge/discharge capacity than would otherwise be achievable.
The polymer electrolytes being developed for use in structural polymeric composite batteries can be characterized as load-bearing ion-conductive resins and nanocomposites of those resins. The specific resins receiving the most attention have been polymerized vinyl ester derivatives of poly(ethylene glycol) (PEG). A broad selection of monomers has been complexed with lithium triflate and thermally cured as solvent-free polymers. The etheric oxygen groups of PEG are capable of dissociating and transporting donor salt ions in the absence of solvents, while mechanical strength is provided by crosslinked vinyl ester networks. By varying the proportions, molecular structures, and functionalities of the vinyl ester and PEG constituents, it has been possible to tailor structural and electrolytic properties over wide ranges.
An additional benefit of the specific choice of resins is that their viscosities are low enough that vacuum-assisted resintransfer molding (VARTM) can be used as a processing technique for distribution of monomers through stacked layers and sublayers. In comparison with manuallayup processing techniques, VARTM results in less void content, makes it possible to use higher volume fractions of fibers, and is more readily scalable to mass production.
At the beginning of a typical fabrication process, the various layers and sublayers are stacked and enclosed in a vacuum bag equipped with ports for inflow and outflow of resin. During VARTM, the suction applied to the outflow port causes the resin to be pulled from a reservoir, through the inflow port, through the layers and sublayers, and then through the outflow port. The anode, cathode, and separator sublayers become wetted by the resin. The vacuum bagging causes compaction of the layers, while the glass fabric separators maintain electrical isolation between anodes and cathodes in the face of the compaction. After VARTM, the composite is cured at a temperature of 80 °C in an oven overnight.
This work was done by J. F. Snyder, R. H. Carter, K. Xu, E. I. Wong, P. A. Nguyen, E. H. Hgo, and E. D. Wetzel of the Army Research Laboratory.
This Brief includes a Technical Support Package (TSP).
Structural Composite Batteries
(reference ARL-0022) is currently available for download from the TSP library.
Don't have an account? Sign up here.