Flexible Hybrid Battery Pseudocapacitor, Year 1

Much progress has been made in the past few years in energy storage systems including high-energy batteries and ultra capacitors. Both have their own strengths. Lithium batteries have been developed with high energy densities and ultra capacitors have been produced with high power densities. The problem is that the lithium batteries have low power density and ultra capacitors have low energy densities. So applications are specific to each type of device. It is proposed to produce a "first of its kind" hybrid battery/pseudocapacitor device combining both high energy density and high power density. Specifically, this research will utilize aligned sheets of multi-walled carbon nanotubes as the device electrodes. A unique aspect of this device will be a liquid ion electrolyte common to both electrodes. This electrolyte will be produced here at MSFC. The electrodes will be manufactured at North Carolina State University (NCSU), with whom the P.I. has worked for the past five years. The packaging and testing of the device will be performed in the Microfabrication Laboratory in the Aviation and Missile Research, Development and Engineering Center (AMRDEC), with whom the P.I. is presently involved in another project. The common electrolyte will be selected and/or synthesized at MSFC. This device has many possible applications for unmanned and manned spaceflight, as well as DOD and commercial applications. Carbon nanotubes are being utilized in a wide variety of applications due to their superior mechanical, thermal and electrical properties. Previous work by the P.I. with North Carolina State resulted in highly aligned carbon nanotube tape (CNT tape) with superior mechanical and thermal properties compared to traditional carbon fiber composites and carbon nanotube nonwoven composites. They have extended this research into the area of piezoelectricity producing CNT tape using P(VDF-TrFE) as the matrix material. It is now proposed to use the aligned carbon nanotubes without a binding matrix for the electrodes in a hybrid battery/pseudocapacitor. NCSU also added a coating of pyrolitic carbon to some Si-coated CNT sheets. Both types were assembled in coin cells using lithium metal foil as the counter electrode. Galvanostatic charge-discharge profiles of the anodes were produced for both types of sheets using a current density of 50 mA/g. The CNT-Si sheet reached 3322 and 2487 mA/g for its discharge and charge capacities, respectively and had an initial coulombic efficiency (CE) of 75%. By comparison, the first discharge and charge capacities were 2270 and 1801 mA/g and had and initial CE of 79% for the CNT-Si-C sheet. This improvement of CE was attributed to the outer layer of carbon. Silicon fragmentation can cause a decrease in lithium capacity. It is posited that the carbon layer prevents this fragmentation. It is proposed to use vanadium oxide coated CNT sheet as the capacitor electrode. Vanadium oxide has a high capacitance and is cheap compared to some other high capacitance oxides such as ruthenium oxide. Atomic layer deposition (ALD) will be used to coat the CNT sheets with thin layers of vanadium oxide. Previous studies have shown that a 10 nm layer is most effective, since it retains a high capacitance, while still allowing electron transfer from the carbon nanotubes to the electrolyte. NCSU has an ALD instrument, which will be used to coat the CNT sheets. We propose to use a common electrolyte for both the battery and capacitor electrodes. This will be either a single ionic liquid containing a lithium salt or a combination of ionic liquids containing lithium salt. For applications such as supercapacitors and batteries the appeal of ILs is self-evident because their inherent ionic conductivity makes them candidates as electrolytes. However, the nature and properties of a particular IL can have significant effects on its performance for a specific device, and these must be taken into consideration. The structure and composition of the electrical double layer is highly dependent on the size and charge density of the cations and anions, as well as any specific interactions these ions may have with a given surface. The electrical conductivity is determined by the mobility of the ions, which in turn depends on the viscosity of the IL. The electrical window of the IL; i.e., the difference between the oxidation and reduction potentials, is also an important factor; generally the wider the window the greater the energy storage capacity. In addition, the compatibility of the ions of the IL with the electrode materials (e.g., Faradaic interactions) must be taken into account. Lastly, factors such as hydrophilicity vs. hydrophobicity and thermal stability are also important. Advantages of ILs over traditional electrolytes include; low volatility, low melting point and low viscosity. Ionic liquid chemistry is considered "Green Chemistry" due mainly to the low vapor pressure and thus low volatility of these liquids. If need be, an ionic liquid will be synthesized to serve as the common electrolyte. Initially, we will use LiFePO4 (Lithium Iron Phosphate) along with ionic liquids such as 1-butyl-3-methylimidazolium tetrafluoroborate (BMI-BF4) and 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMI-TFSI) as starting common electrolytes. These are on hand. If need be, others will be tried and if necessary, a tailored ionic liquid will be produced. Coin cells will be produced and tested at the AMRDEC laboratory. These are easy to manufacture and AMRDEC lab has a precision disc cutter and hydraulic crimping machine for making these cells. Cyclic voltammetry, galvanostatic cycling, potentio-electrochemical impedance spectroscopy and battery testing will be performed. Flexible pouch cells will then be produced utilizing the same testing methodology as for the coin cells. For the optional second year we will concentrate on developing a solid state electrolyte. More »