Part of the reason that lithium-ion batteries are so popular in everything from cellular phones to automobiles is their relatively quick charge times, reasonable capacity, and resistance to fatigue. Unfortunately, Li-ion batteries are still somewhat expensive as their manufacturing process requires a good amount of energy and some of their usual metal companions, such as cobalt and nickel, are not entirely inexpensive.
Researchers at the Pacific Northwest National Laboratory (PNNL) with help from the U.S. Department of Energy are working on developing Li-ion batteries which can perform at similar levels, but cost much less to produce. The cost reduction will come from a change in both production methods and materials used.
Rather than the typical lithium metal oxide construction, the PNNL team looked to materials to replace the oxide and expensive cobalt or nickel with a phosphate and manganese or iron. Lithium metal phosphate batteries are not unheard of, but the PNNL team wanted something without such complicated production methods and high costs.
Materials scientist at PNNL, Daiwon Choi speaks of the team's technique in a PNNL news release, "This method is a lot simpler than other ways of making lithium manganese phosphate cathodes. Other groups have a complicated, multi-step process. We mix all the components and heat it up."
The simpler process he spoke of involves nothing much more than paraffin wax and oleic acid (soap), and heat. They began by mixing the lithium, phosphate, and manganese together with melted wax and oleic acid. Paraffin wax is made of long and mostly inert molecule chains which helped direct the crystal growth, while the oleic acid, as a surfactant, helped spread the important constituents evenly.
Next they raised the temperature of the mixture. By 400 degrees Celsius, the wax and soap had evaporated away from the mixture, leaving tiny lithium manganese phosphate (LMP) crystals of approximately 50 by 2000 by 2000 nanometers. For comparison, a human hair is about 50,000 to 60,000 nanometers in diameter. They further raised the temperature to bond the crystals together and form a plate of cathode material.
In theory, LMP should be rather competitive with typical Li-ion metal oxide batteries, with a capacitance of about 170 milliAmp hours in one gram of material. In past tests, researchers had been able to get up to 120 milliAmp hours with lithium metal phosphate-based batteries. Choi and colleagues where able to get 168 milliAmp hours per gram of material in their best case charge/discharge tests. But the number dipped as low as 54 milliAmp hours during a fast/fast "real world" test cycle.
These numbers are far from disappointing for PNNL. The team was able to engineer a way to create LMP crystals with simplicity and without high cost. The work may pave the way for other alternative lithium-based composite batteries. They also plan further work in refinement of the carbon backing used as the positive electrode for the LMP crystal plates.
