Spray-on lithium-ion batteries

Researchers at Rice University in the US have developed a lithium-ion battery that can be spray painted onto virtually any surface.

The rechargeable device consists of  painted-on layers that represent the components in a traditional battery. To prove that any surface can be used, the materials were airbrushed onto ceramic bathroom tiles, flexible polymers, glass, stainless steel and a beer stein just to see how well they bonded to the substrate.

The complete battery consists of five layers - two current collectors, a cathode, an anode and a polymer separator. The first layer is a positive current collector comprising high-purity, single-wall carbon nanotubes with carbon black particles dispersed in N-methylpyrr-olidone.

The second layer is the cathode of lithium cobalt oxide, carbon and ultrafine graphite powder mixed with a binder. The third layer is a polymer separator paint containing Kynar Flex resin, poly methyl methacrylate (PMMA) and silicon dioxide dispersed in a solvent.

Layer four acts as an anode consisting of lithium titanium oxide and ultrafine graphite, again in a binder. The final layer is the negative current collector of commercially available conductive copper painted diluted with ethanol.

During the experiment at Rice, one battery was connected to a solar cell and, using a laboratory light as an energy source, a fully charged battery bank powered 40 LEDs at 2.4 V for six hours. After 60 charge and discharge cycles, the batteries showed only a small storage capacity loss.

Giving some background on the battery’s development, Rice graduate student Neelam Singh said one of the hardest parts of the research was the time-consuming  formulation and mixing and testing paints for the five layers.

The researchers said the batteries were very consistent in their capacities - within ±10% of the target.

“The most difficult challenge,” said Singh, “was achieving mechanical stability and the separator played a critical role. We found that the nanotube and the cathode layers were sticking very well but if the separator was not mechanically stable, it would peel off the substrate.  PMMA gave the right adhesion to the separator.”

In the future she sees the possibility of integrating painted batteries with recently developed printed solar cells to create an energy-harvesting combination. They also envisage their batteries as snap-together tiles that can be configured in any number of ways.

She said they are looking for electrolytes that would make it easier to create painted batteries in the open air. With present technology, lithium-ion looks like the battery of today and the immediate future.  Although its history goes back to 1912, it had to wait until the 1970s for a practical rechargeable device to become available.

The energy density of lithium-ion is about twice that of nickel-cadmium and there is potential for higher energy densities, according to Siomar Batteries. The high cell voltage of 3.6 allows battery pack designs with only one cell, whereas a nickel-based pack would need three 1.2 V cells in series.

There is no memory effect and no need for planned cycling. The self-discharge rate is less than half that of nickel-cadmium and lithium is more environmentally friendly when it comes to disposal.

However, despite the positives, lithium-ion does have some disadvantages. A protection circuit to limit the peak voltage of each cell during charging and to prevent the cell voltage from dropping too low on discharge is a vital component. The cell temperature also has to be monitored so that the difference between charge and discharge is limited to between one and two degrees.

Capacity deterioration can be seen after a year even if the battery is not used and the device sometime fails completely after two or three years. However, this phenomenon is not just peculiar to lithium-ion. Nickel-metal-hydride devices, for example, are very sensitive to ageing, especially if they are exposed to high temperatures.

Storing in a temperature of around 15°C with the battery on a 40% charge is the manufacturer’s recommendation for reducing the effects of ageing.

Siomar summarises lithium-ion as:

Advantages:

• High energy density with potential for higher capacities
• No need for priming when new
• Low self-discharge - less than half that of nickel-based batteries
• Low maintenance and no memory
• Special cells can provide very high current, eg, power tools

Limitations:

• Requires a protection circuit
• Subject to ageing
• Transport restrictions but not to personal batteries
• About 40% more expensive to make than nickel-cadmium
• Not a fully mature technology with metals and chemicals changing

A variation on lithium-ion is lithium polymer, a technology going back to the 1970s when a dry solid polymer electrolyte was first used. The plastic-like film electrolyte is non-conductive but allows ion exchange. It replaces the traditional porous separator which is soaked with electrolyte.

Again, according to Siomar,  the design is simpler with a cell of just one millimetre. However, the battery is a poor conductor with the internal resistance too high, making it unable to deliver the current bursts needed for many modern devices and provide the power to spin up the hard drives of mobile computing equipment, for instance. Heating the cell to 60°C and higher improves the conductivity but this is unworkable for portable applications.

As a compromise, gelled electrolyte has been added with commercial cells using a separator/electrolyte membrane made of a porous polyethylene or polypropylene separator filled with a polymer that gels on filling with the liquid electrolyte.

Lithium-ion polymer has failed to be the godsend that was expected of it. Its superiority and low manufacturing costs have not been achieved, neither have there been improvements in capacity. In fact, the capacity is slightly less than that of the standard lithium-ion battery.  Where lithium-ion-polymer finds its market niche is in wafer-thin batteries for applications including credit cards.

Lithium in some form or another and in some combination appears to be the metal of the future for high output batteries. They already power mobile phones, tablets and portable computers and banks of them are providing the electrical energy source for electric and hybrid cars.

Lithium is a metal that is widely available and relatively safe. While lead acid is going to be hard to shift as the choice for conventional motor vehicles and many stationary power applications, lithium-ion seems certain to go on developing into wider and more exciting uses as the technology matures.