US researchers have achieved a 50x speed-up in charge and discharge from Li-ion cells using nano-structured anodes - although scale-up could be tough.
The anodes store ions in silicon, which has a large capacity to absorb lithium, but changes considerably in size during the process.
"The anode structure of a Li-ion battery physically grows and shrinks as the battery charges or discharges," said the Rensselaer Polytechnic Institute. "When charging, the addition of Li ions increases the volume of the anode, while discharging has the opposite effect. These volume changes result in a build-up of stress in the anode."
Too much stress, and the cell fabric deteriorates, permanently cutting charge capacity.
Just before Christmas, Oxfordshire-based Nexeon produced a record-breaking 18650 Li-ion cells with a capacity of 3.2Ah using its silicon anode technology.
The Rensselaer team has developed a nanostructure called a 'nanoscoop', from its resemblance to an ice cream cone (see photo).
Made from an amorphous carbon rod base topped with a thin layer of aluminium and then silicon, the structures are flexible and "able to quickly accept and discharge Li ions at extremely fast rates without sustaining significant damage", claimed the Institute.
The aluminium layer provides a less abrupt transition in stress across the material interfaces, leading to improved structural integrity of the electrode as the silicon expands and contracts.
"Due to their nanoscale size, our scoops can soak and release Li at high rates far more effectively than the macro scale anodes used in today's Li-ion batteries," said Professor Nikhil Koratkar.
The team claims to have demonstrated that its electrode could be charged and discharged at "a rate 40 to 60 times faster than conventional battery anodes, while maintaining a comparable energy density", said the Institute, claiming: "This performance was achieved over 100 continuous charge-discharge cycles."
More specifically, at a 40C charge or discharge rate of 51.2 A/g, the nanoscoop anode delivered 412mAh/g capacity with a power output of 100kW/kg of electrode continuously over 100 charge/discharge cycles.
However, all is not rosy in the nanoscoop garden as the relatively low total mass of the electrode means the technology might not be scalable to real cells.
So the team will attempt to growing longer scoops with greater mass, or develop a method for stacking layers of nanoscoops on top of each other.
Another possibility is scoops on flexible substrates that can be rolled.
Results of the study were detailed in the paper "Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes," published in the journal Nano Letters. 