The role of portable electronics in the modern age continues to grow and as a result the demand for energy dense batteries continues to increase. The military is not immune to this trend and EMPF continues its role in the identification and solution of manufacturing technology issues affecting batteries and power electronics systems used in DoD applications. Recent battery and power system projects the EMPF has conducted include development of the SDV battery, the PRC-112 survival radio battery pack, the Integrated Power System (IPS), the MOFA reserve battery, and the DSU-33 thermal battery

Unique to the military is the need for munitions batteries that may be stored for up to twenty years without maintenance. Reserve batteries are batteries designed to be stored for years, even decades, without performance degradation. Reserve batteries are stored in an inert state and can be activated within a fraction of a second with no degradation of battery capacity or power. The demand for reserve batteries by the consumer market place is minimal. Typical Reserve batteries are thermal batteries and liquid reserve batteries. More information about thermal batteries can be found in the October 2006 issue of the EMPFasis Newsletter.

The typical liquid reserve battery is kept inert during storage by keeping the electrolyte separate from the electrodes. The electrolyte is kept in a glass or metal ampoule inside the battery case. Prior to use, the battery is activated by breaking the ampoule and allowing the electrolyte to flood the electrodes. The ampoule is broken either mechanically or by the high g shock experienced from being shot from a cannon.

Over the past few years there have been a number of advancements in reserve battery technologies. Among these advances are superhydrophobic nanostructured materials, bimodal lithium reserve battery, and ceramic fiber separator for thermal batteries. Nano powders are a hot field of material science research and a number of battery advancements are a direct result. In batteries, nano manufactured electrode powders have greater surface area than traditionally processed electrode powders. Increased surface area, allows higher currents to be discharged from the battery. The use of nano- power manufacturing techniques for the powders used in electrodes has increased the current rate for which batteries can be designed and can be used to decrease the recharge time for secondary (rechargeable) batteries.

Of particular interest to reserve battery designers is the emerging “superhydrophobic nanostructured material” technology for use in reserve batteries. The battery electrodes are formed on nanostructured silicon surfaces that are treated with an appropriate fluorocarbon polymer honeycomb structure. The resulting honey comb surface has a high contact angle (>120º) that does not allow the electrolyte to penetrate beyond the surface. “Electrowetting” changes the surface contact angle by application of a trigger voltage pulse. The electrolyte can now penetrate the honey comb structure and come into contact with the electrodes. Thus, the cell becomes electrochemically active. The predicted activation time is approximately 1 millisecond.

The advantage of “superhydrophobic nanostructured material” is there is no need for separate storage space in the battery case for storing the an electrolyte ampoule. Removal of the ampoule storage space will increase the overall energy density of the battery design. The self life for batteries using this technology is predicted to be 15 to 20 years.

A second solution to reducing the space used to separate electrolyte from the electrodes is the bimodal lithium reserve battery. The electrodes in this oxyhalide battery are stored in a low molar (neutral) concentration of thionyl chloride electrolyte (1.0M LiAlCl4/SOCl2). A small ampoule of high salt concentration (acid) electrolyte (5M LiCl + LiAlCl4/ SOCl2) is kept separated. The cell is “activated” when the acid electrolyte is injected into the neutral electrolyte. The acid electrolyte removes the passivation layers that built up on the electrodes and creates a more conductive electrolyte than the original neutral electrolyte. The activation time for the battery to operate in high current mode is less than 4 milliseconds.

The advantage of the Bimodal lithium reserve battery is reduced storage space for the electrolyte ampoule used to activate the battery. This reduces the battery size and thus increases its energy density. The second advantage is that the cell is “active” in a “low current mode” during storage and can be used in the low power mode for periodic checks of the electronics system.

Another advancement is the development of a new manufacturing processes for thermal batteries. This new development uses a modified paper- making process to make ceramic fiber separators (CFS) instead of pressed powder separator pellets. Production of pressed MgO based powder separators less than 10 mils thick and greater than a few inches in diameter is extremely difficult. Thinner separators can be manufactured using the paper – making method than pressed powder method. Thin CFS separators reduce the cell stack height which increases the overall energy density of the thermal battery. In addition, thin separators are desirable because of lower IR drops between electrodes. Low IR cell drops result in better overall battery conductivity. It is predicted that this CFS continuous production method will produce separators 40 times faster than the current pellet manufacturing method and will reduce thermal battery production costs by 25%.