Polyaniline/Vanadium Pentoxide Layer-by-Layer Electrodes for Electrochemical Energy Storage

The relationship between materials properties, processing methods and performance of layer-by-layer (LbL) films containing polyaniline (PANI) and vanadium pentoxide (V2O5) are investigated. This study offers fundamental principles for designing and manufacturing ultra thin electrodes.;Both PANI and V2O5 are promising electrode materials for electrochemical energy storage. As a composite, the two can have synergistic interactions to form an electrode better than either material alone. Traditional dip LbL assembly is used here to successfully assemble hybrid electrodes containing PANI and V2O5. Assembly conditions are chosen to yield films that grow reliably and have a large cycle thickness. Assembly pH and concentration are critical parameters for this particular LbL system. The influence of molecular weight on the growth of thin films and performance is also investigated. It is found that PANI dominates the electrochromic response. However, the electrochemical response possesses contributions from both PANI and V2O5. Films made from lower molar mass PANI have a charge storage capacity density of 264 mAh/cm3. The films' ability to store charge is also dependent on film thickness, as is the fraction of electrochemically accessible material. These LbL films demonstrate promising performance based on volume. However, the gravimetric capacity, energy and power are not very high due to the mass transport limitations. In addition, cycle life is limited by the volumetric expansion and irreversible formation of non-active side products.;In order to address these limitations, we synthesize PANI nanofibers to replace conventionally synthesized PANI. The resulting electrode is highly porous and significant gains in capacity (3x), specific energy (40x), and specific power (4x) are realized. The thickness of the film increases linearly with respect to the number of layer pairs deposited. The electrochemical performance of this porous LbL system is also dependent on film thickness. Using X-ray photoelectron spectroscopy, it is found that the electrode contains 59 wt% V2O5 and stores 0.8 mol lithium ion per mol of V 2O5. A film thickness of 1.2 mum has optimum results, with a discharge capacity of 320 mAh/g, a specific power of 4000 mW/g, and an specific energy of 886 mWh/g (per mass of active cathode), depending on discharge current. After 100 cycles, the electrode retains 75% of its initial capacity, and volumetric expansion after cycling is successfully suppressed by using this porous structure.;However, conventional dip LbL assembly is not well suited to large-area, large-scale and rapid industrial applications because of its long deposition time and large consumption of solution. A simple and rapid spray-assisted LbL method is applied to produce PANI nanofiber/V2O5 thin film electrodes. This method is based on a four-nozzle system to rapidly spread PANI nanofiber and V2O5 suspensions alternately on to the select surface such as indium tin oxide. After optimization of parameter settings, a water-processable hybrid electrode with high capacity, energy density, power density and good cycle life is fabricated at a speed of 140 sec/layer pair (44 nm). This successful and effective translation from dip- to spray-LbL assembly without loss of thin film performance enables innovative manufacturing processes of thin films.