N16A-T008 TITLE: Novel Separator Materials for Achieving High Energy/Power Density, Safe, Long-Lasting Lithium-ion Batteries for Navy Aircraft Applications.

TECHNOLOGY AREA(S): Electronics, Materials/Processes, Sensors

ACQUISITION PROGRAM: Navy and Marine Corps Small Tactical Unmanned Air Systems, PMA 263

OBJECTIVE: Develop and demonstrate novel, tailored, designer separator materials with optimized properties to maximize lithium-ion cell/battery performance, life, safety and reliability.

DESCRIPTION: A typical lithium-ion cell consists of a positive electrode, such as LiFePO4 coated on an aluminum current collector, and a negative electrode, such as carbon coated on a copper foil current collector. The electrodes are separated by a porous plastic film (separator) soaked by an electrolyte liquid. The key function of the separator is to prevent electrical contact between the positive and negative electrode, thereby preventing electrical shorting. The separator has an additional role as a charge-carrier facilitator.

During discharge, for example, the anode supplies Li+ ions to the separator and electrons to the external circuit. The positive Li-ions are inserted into the cathode electrode and are charge compensated by negatively charged electrons in the external circuit, resulting in usable electrical power of the cell/battery. Thus, the separator has to allow ions to transport, but block the flow of electrons. In other words, functionally the separator must be a good ionic conductor, but be a poor electronic conductor.

Separator materials play an important role in achieving high energy and power density and ensure the safety of the battery [1-2]. Cells with high resistance separators perform poorly during high rate discharge and contribute to an increase in charge time. Larger pore sizes of the separator will allow more shorts during high-temperature storage; smaller pore sizes impact cycle life at low temperature. Thinner separators contribute to increasing the capacity by virtue of lower resistance. However, if they are too thin, there may not be enough required mechanical strength. The separator is exposed to volatile, flammable, organic, corrosive electrolyte liquid and operate in a reducing and oxidizing environment. Thus, the designer separator materials should have low resistance, uniform pore structure, and superior oxidation-resistance properties.

In case of rapid internal increase due to electrical (overcharge, short circuit) or mechanical (nail penetration, crush) abuse, the separator has to be shut down, which requires the process to be irreversible to ensure safety [3]. The shutdown prevents thermal runaway events such as those that has contributed to failure of the commercial airliner batteries. The high-temperature melt integrity feature will preserve the safety of the cell during extended overcharge or exposure to higher temperature. The separator must block the lithium-metal dendrite from penetrating through and causing internal shorts. It is to be noted that dendrite growth leading to puncturing the separator and creating internal shorts is one of the major root cause failures of fielded Li-ion batteries. Thus, separators with excellent shut-down features combined with structural integrity are vital to achieving thermal stability and ensuring safe operation of the cell/battery. Overall, the separator and its material properties have a significant impact on the aspects of reliability, safety, high-performance, and longevity of the Li-ion battery.

The majority of the separators used in Li-ion batteries, however, are derived from spin-off technologies and are not specifically developed or optimized for Li-ion batteries. The only advantage is that they are produced in large volume at a relatively low cost. The need is the development of tailor-made novel separator materials with the required chemical, mechanical, and electrochemical properties that will improve the performance, longevity, and most importantly, improve safety without adversely affecting cost.

The goal of the effort is to develop novel separator materials tailored for Li-ion batteries with the following features influencing the design considerations: electronic insulator/high ionic conductivity, physical strength, chemical resistance, mechanical stability, wettability, pore size optimization, dendrite migration prevention, impurity particulate reduction, rapid shut-down, and thermal stability [4-7].

PHASE I: Develop and demonstrate novel separator materials with optimum properties tailored for Li-ion battery applications as proof of concept. Demonstrate feasibility through analytical methods and construct a Li-ion cell for comparison with a baseline control.

PHASE II: Fully develop the concept into a safe, high-performance Li-ion battery prototype by integrating the innovative separator materials in cells/modules/pack, to demonstrate the gain and response to failure modes in a lab environment.

PHASE III DUAL USE APPLICATIONS: Demonstrate the functionality of the Li-ion battery product that meets the electrical needs of aircraft in a safe and effective manner in an operational environment. Obtain flight certification and transition the representative technology to appropriate Navy platforms (Ex. UAS, F/A-18E/F, F-35) and commercialize. Due to ~ 1/3 weight and ~ 3X energy in comparison to current lead-acid batteries, Li-ion batteries have become the energy storage system of choice. The performance improvement combined with safety is very attractive for Navy aircraft applications. Improvements made under this topic will tremendously benefit commercial aviation, consumer, and automobile markets.

REFERENCES:

1. Arora, P., and Zhang, Z., Battery separators, Chem. Rev. 2004, 104, 4419-4462 and references therein.

2. Spotnitz, R., Handbook of battery materials, J.O. Besenhard, Editor, Wiley: Amsterdam and New York, 1999.

3. Laman, F.F., Gee, M.A., Denovan, J. J. Electrochem. Soc, 140 (1993) L 51.

4. NAVSEA S9310-AQ-SAF-010, Navy lithium battery safety program responsibilities and procedures, (15 July 2010), Retrieved from http://everyspec.com/USN/nAVSEA/NAVSEA S9310-AQ-SAF-010 4137/.

5. MIL-PRF-29595A- Performance Specification: Batteries and Cells, Lithium, Aircraft, General specification For (21 Apr 2011) [Superseding MIL-B-29595]. Retrieved from http://www.eveyspec.com/MIL-PRF/MIL-PRF-010000-29999/MIL-PRF-29595A 32803/.

6. MIL-STD-810G � Department of Defense Test Method Standard: Environmental Engineering Considerations Laboratory Tests (31 Oct 2008). Retrieved from http://everyspec.com/MIL-STD/MIl-STD-0800-0899/MIl-STD-810G_12306/.

7. MIL-PRF-461F � Department of Defense Interface Standard: Requirements For the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment (10 Dec 2007). Retrieved from http:// everyspec.com/MIL-STD/Mil-STD-0300-0499/MIl-STx-461F_19035/.

KEYWORDS: Li-ion battery; Safety System; High energy density; Dendrites; Separator; High Power Density

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TPOC-2: 812-854-4082

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