Lithium Batteries,Lithium polymer Batteries,and LISOCL2 batteries manufacture-VAIMA

Paintable Battery

2012-7-2 23:59:43 Category:Professional News

Check for detail

Li-ion batteries power most of our portable electronics by virtue of their high energy and power density. Commercial Li-ion batteries are multilayer devices, fabricated by tightly rolling up sandwiched battery components and packaging them into metal canisters1. Although Li-ion battery packs are compact and volumetrically efficient, the ‘jellyroll’ (Fig. 1a) design strategy limits the batteries to rectangular or cylindrical shapes, which constrains the form factors of devices. Recent efforts on unconventional battery designs have worked towards developing battery technologies that can be inconspicuously accommodated into devices and applications without constraining their form factors2. Some examples are thin and flexible batteries34, stretchable textile energy storage56, paper batteries7, microbatteries8 and transparent batteries9. However, a seamless integration of these energy storage systems into electronic devices and household objects remains a challenge. The advent of smart devices/objects has further generated interest in self-powered electronics1011 with integrated storage. Such energy conversion-storage hybrids will require batteries that can be integrated directly into the object or surface of choice as well as with energy harvesting devices. Printing (or generally, painting) is already considered a viable technique for large-area fabrication of electronic devices (circuits, photovoltaics, displays, etc.) on virtually any type of substrate12. Consequently, there is huge interest in developing a fully paintable energy storage technology. Here we present a paradigm change in battery design and integration. We have developed a fully paintable Li-ion battery that can be simultaneously fabricated and integrated with commonly encountered materials and objects of daily use. Energy harvesting devices, such as solar cells, can then be easily integrated with these batteries to give any surface a standalone energy capture and storage capability. We could easily integrate this technology into standard construction materials (ceramic tiles), common household objects (ceramic mug), stainless steel as well as flexible polymer sheets.

Figure 1: Paintable battery concept.

(a) Simplified view of a conventional Li-ion battery, a multilayer device assembled by tightly wound ‘jellyroll’ sandwich of anode-separator-cathode layers. (b) Direct fabrication of Li-ion battery on the surface of interest by sequentially spraying component paints stencil masks tailored to desired geometry and surface.

We adopted spray-painting technique to assemble batteries (Fig. 1b) due to advantages such as ease of operation and flexibility in formulation from small-scale (aerosol cans) to industrial scale systems (spray guns). Fabrication of batteries by spray painting requires formulation of component materials into liquid dispersions (paints), which can be sequentially coated on substrates to achieve the multilayer battery configuration. Commercial Li-ion batteries have positive and negative electrode materials coated on appropriate current collectors, sandwiching an ion conducting separator (Fig. 1a). Aluminum and copper foils are commonly used current collectors (CC) (positive and negative CC respectively), while electrode materials and separators are chosen based on desired voltage, current capacity, operating temperature and safety considerations131415. We chose Lithium Cobalt Oxide [LiCoO2] (LCO, positive electrode) and Lithium Titanium Oxide [Li4Ti5O12] (LTO, negative electrode), for which the effective cell voltage is∼2.5 V16. Graphite anode or high voltage cathodes could be used to increase the nominal voltage of paintable batteries (∼3.6 V for LCO-Graphite cell). However, graphite based Li-ion batteries have safety concerns1417and LTO was chosen to ensure safer operation due to minimal risk of Li-metal plating if accidentally overcharged. Absence of solid electrolyte interface (SEI) formation, stable and longer cycle life due to low volume change during charging and discharging are other advantages in choosing LTO17.

While conductive Cu paints are commercially available, a conductive Al paint would require the use of Al micro-powders, which are dangerous in use (form explosive aerosols) and have a high degree of surface oxidation. So, Al paints were not considered viable. Single-walled nanotube (SWNT) current collectors have been used in batteries418 due to their high electrical conductivity and electrochemical stability at potentials above 1 V vs. Li/Li+. We found that high concentrations (∼0.5–1% w/v) of SWNTs can be readily dispersed without the use of surfactants1920 or polymeric binders2122 by bath ultrasonication in 1-methyl-2-pyrrolidone (NMP) to form viscous, highly consistent inks suitable for spray painting. We chose NMP due to its ability to solvate pristine SWNTs without altering its electronic properties or requiring any post-treatments such as surfactant removal23. A 20% w/w of Super PTM (SP) conductive carbon additive lowers the sheet resistance of the spray-painted SWNT films (∼2 mg/cm2) up to 10Ω/□, sufficient for use as current collectors.

LCO paint was made by adding a mixture of LCO, SP carbon and ultrafine graphite (UFG) into Polyvinylidine fluoride (PVDF) binder solution in NMP. Spray painted electrodes with only SP carbon as conductive additive had poor capacity retention, possibly due to inhomogeneous distribution of the small SP carbon particles (∼50 nm) in films composed of far larger LCO particles (7–10 µm). Addition of UFG (particle size ≤5 µm, comparable to LCO) gave more homogeneous distribution of conducting pathways, improving capacity retention24 (details in supplementary information, section SI-1 and Fig. S1).

In Li-ion polymer batteries, well-controlled microporosity of polymer separators is crucial for optimal electrolyte uptake and formation of a microporous gel electrolyte (MGE) with high ionic conductivity, which is necessary for complete capacity utilization and its retention upon cycling2526. Thus, obtaining the right morphology in a spray-painted separator was considered the most crucial step for realization of a paintable Li-ion battery. Besides, adhesion of the separator to various substrates is key to making the paintable battery mechanically robust. We could obtain microporous separators with good adhesion characteristics from a paint prepared by blending Kynarflex®-2801 (Kynarflex) polymer with poly(methyl methacrylate) (PMMA) and fumed SiO2 (27:9:4 ratio by wt.) in a 8:1 (by vol.) mixture of acetone and N,N-Dimethylformamide (DMF). Kynarflex was used due to its good solubility in low boiling solvents and electrochemical stability in a wide voltage window27, while PMMA was used to promote adhesion to a variety of substrates. Kynarflex-PMMA separators fabricated from paints in acetone had good adhesion, but had a fibrous morphology with very high porosity and excessive electrolyte uptake which made them mechanically unstable. We found that by adding DMF to the paint, the microporosity and electrolyte uptake could be tailored to make the separators mechanically robust upon electrolyte addition. This, however, also reduced the ionic conductivity of MGE by a factor of∼4 at 11% DMF content (Fig. 2e). A further addition of 10% w/w fumed SiO2 to the separator helped offset this loss in conductivity and gave the best compromise between mechanical stability, porosity and ionic conductivity (Fig. 2f–g) (details on optimization of polymer separator in supplementary information, section SI-2 and Figs. S2S4).

Figure 2: Electrochemical characterization of individual components of paintable Li-ion battery.

Composite electrodes: Charge-discharge curves and specific capacity vs. cycle numbers for spray painted (a, b), LCO/MGE/Li half-cell cycled between 4.2−3 V vs. Li/Li+ at C/8 and (c, d), LTO/MGE/Li half-cell cycled between 2-1 V vs. Li/Li+ at C/5. Both half cells show desired plateau potentials and good capacity retention.Polymer separator optimization: (e) Addition of DMF to polymer paint gave a mechanically robust separator but drastically reduced ionic conductivity. (f) Addition of SiO2 (at∼11% DMF) helped recover the ionic conductivity while maintaining mechanical robustness. Ionic conductivities were calculated from impedance spectra in supplementary Fig. S4. (g) High frequency region of electrochemical impedance spectrum of a typical optimized polymer measured at 23°C. The separator shows low ionic resistance, with ionic conductivity∼1.24×10−3S/cm.

Figure 3: Characterization of spray painted Li-ion cells.