A scientist at the Savannah River National Laboratory has developed a new kind of anode for lithium-ion batteries that is expected to increase the energy density four-fold—€”enough to enable the battery to power an electric car for 300 miles on a single charge.
It has long been known that the energy storage capacity of lithium-ion batteries is limited by the widely used graphite anode. For a plug-in hybrid passenger car with a 40-mile pure electric driving range, 12 kWh electricity must be stored in the Li-ion battery pack. With the typical graphite anode, the battery pack of that vehicle will weigh 150 kilos and will cost $12,000. "To build electrical vehicles with a 300-mile driving range per single charge, we must develop batteries with three to four times higher energy density and ten times lower cost," says SRNL's Ming Au. "This scale of increase calls for transformative, rather than incremental approaches to battery development."
In a typical graphite anode, lithium-ions are sandwiched into the carbon layer structure—€”a structure known as lithium intercalation—€”with every six carbon atoms accommodating one Li atom. That structure gives the graphite anode a maximum capacity of 372 Ah/kg, which converts to a theoretical energy density of 0.38 kWh/kg. In practice, that means that today's Li-ion batteries provide 0.08 kWh/kg of energy.
Dr. Au developed several low cost nanostructured anodes that increase energy density to 1.3 to 4.3 kWh/kg, a three-to five-fold increase over graphite anodes. His solution uses nanorods—€”structures less than 100 nanometers in diameter—€”of various metals and metal oxides. These nanorods have the advantage of large surface areas for lithium ions to access, which means they can bind a higher number of lithium cations than the conventional graphite design. They also have the flexibility to withstand the expansion and contraction of multiple charge/discharge cycles, which contribute high energy and power densities with expected longer cyclic life.
In addition to the improvement in energy density, Dr. Au's method of producing this new type of anode simplifies fabrication and eliminates safety and environmental concerns. Most carbon-based anodes are fabricated through a series of processes of mixing carbon, binder and conductive additives in organic solution, pasting the slurry onto the current collector and baking to remove the solvent. This method involves intensive labor, along with fire safety measures and environmental emission controls, resulting in high cost. Au's direct depositing of nanorods eliminates these processes.
In one study, aluminum nanorods were directly formed on a titanium substrate. When the aluminum nanorod anode was tested with a lithium cathode in a LiPF6 propylene- and dimethyl-carbonate electrolyte, it showed initial discharging capacity that is four times higher than graphite anodes. Cobalt oxide nanorods were also formed on a titanium substrate and tested with even greater results.
Studies of these formations have shown that numerous inexpensive metals and metal oxides can be considered for nanorod formations to elevate charge capacities of the anodes in lithium-ion batteries. Further studies are ongoing to explore other nanostructure such as nanoporous hollow spheres of metals and metal oxides, understand the mechanism of lithium reaction in anode and sustain the high-energy density through multiple charge-discharge cycles.
The replacement of graphite anodes is inevitable as the demand for battery power increases for transportation markets. Lithium-ion battery development will continue using additional metals and metal oxides with various nanostructures.