Most high-energy and high-rate reserve batteries for strategic and tactical military applications are oxyhaline-based. The current reserve battery anode consists of metallic lithium (Li), and the cathode depolarizer consists of the oxychlorides, thionyl chloride (SOCl2) or sulfuryl chloride (SO2Cl2). A common feature of these technologies has been the environmental and safety issues associated with metallic lithium. Even a slight presence of moisture could cause corrosion on the surrounding facility and the generation of toxic hydrochloric acid gas.
The objective of this SERDP Exploratory Development (SEED) project was to develop a low-cost, safe, and environmentally benign high-energy and high-rate reserve battery with equivalent energy density to that of the metallic lithium-oxychloride reserve battery. The intent was to replace the metallic lithium anode with the high-energy, high-density material, magnesium (Mg), and replace the oxychloride depolarizer with the high-voltage, high-capacity cathode material lambda-manganese dioxide (λ-MnO2) or molybdenum oxide (MoO3).
Four approaches were employed to develop a new Mg/metal oxide reserve battery. The first approach was to develop new electrolyte solutions to eliminate dangerous hydrogen evolution and excessive passivation of the Mg anode, which is a common problem with aqueous Mg-based batteries. This was accomplished by using environmentally benign, non-aqueous electrolyte solutions. The use of non-aqueous electrolyte solutions has the advantage of being stable at high voltages, thereby resulting in a higher voltage battery. The second approach was to create a new technology for the synthesis of high energy density λ-MnO2 cathode material to optimize rate capability and deliverable capacity. Again, the non-aqueous electrolyte solutions are a key factor in developing a stable, high-voltage cathode. The third approach focused on cathode processing technology with emphasis on thin film technology. The fourth approach optimized electrolyte solutions and anode materials that will be used in the fuel cells, ensuring that the technology is highly conductive over the entire military range of -40°C to 65°C, highly stable upon storage at temperatures up to 70°C, and successful in eliminating hazardous volatile organic compound (VOC) components and electrolyte degradation products.
With the proper processing techniques, MaxPower, Inc. was able to produce a cell with a reasonable capacity that could be used to determine the energy of a bipolar battery. The results indicate that multiple types of metal oxide cathodes can be produced using the film deposition method, which produces highly porous thin films. The anode, on the other hand, presented the greatest challenge. The Mg oxide layer forms quickly in air, and many of the electrolyte compositions were not able to break down the oxide layer sufficiently to produce the optimal system results. The oxide layer could be removed under an inert atmosphere. However, having to complete the assembly of a battery under an inert gas in large volumes will increase the cost of producing such a battery.
The highest energy results were produced by the MoO3 cathode, which ideally in a bipolar stack with 0.05 mm thick Mg plates could produce 9.8 watt-hours in a D-cell arrangement. This number is lower than a D-cell lithium oxychloride cell, but high enough that it can be used for certain military applications as an adequate replacement for the lithium oxychloride cell. Even though the Mg/metal oxide battery could not replace the entire fleet of lithium-oxychloride batteries and cells, a partial replacement would still lead to an overall decrease in the environmental burden.
An Mg/metal oxide electrochemical power source with an organic electrolyte solution can provide the Department of Defense with a next generation reserve battery that is safer and more environmentally friendly than current reserve batteries based on metallic lithium and oxychloride technologies. In addition, the costs for the new reserve battery could be significantly less and the associated environmental risks will be reduced or eliminated.