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SCIENTISTS in the US have found a way to use a simple manufacturing technique to build a high-capacity battery material that protects itself from damage.
The scientists from Brookhaven National Laboratory, Berkeley Lab and SLAC National Accelerator Laboratory say test batteries incorporating this cathode material would be capable of recharging quickly, and hold enough capacity for long-distance automotive applications.
One of the issues with current batteries is that they break down under the constant wear and tear of the chemical reactions that provide the power. Lithium-ion rechargeable batteries work by shuttling lithium ions between positive and negative electrodes bathed in an electrolyte solution.
As lithium ions move into the cathode, chemical reactions generate electrons that can be routed to an external circuit for use. Recharging pulls lithium ions back out of the cathode and sends them to the anode.
A good potential cathode material is nickel, which is a reactive metal. However, reactive metal also tend to be unstable, resulting in destructive side reactions with the electrolyte.
Scientists from Berkeley Lab explored using spray pyrolysis, a standard manufacturing technique for making powders, to make their cathode material, and ended up with a nickel-rich material that performed much better than expected without breaking down.
The scientists were unable to explain how the material worked, and so went to SLAC and Brookhaven to use more advanced imaging techniques to study the micro- and nanoscale details of the material.
X-ray imaging at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC and electron microscopy at Brookhaven’s Center for Functional Nanomaterials (CFN) revealed that the cathode material was made of spherical particles.
These micron-diameter spheres were in turn made up of stacks of smaller, faceted nanoscale particles.
A key characteristic is that the spherical particles have more nickel on the inside, allowing them to store more energy, but less nickel on the surface, which can cause issues with the electrolyte. In fact, the surface of the particles was enriched with manganese, which formed an effective barrier to protect the inner, nickel-rich structure.
Despite this barrier, lithium ions were able to enter the material to react with the nickel. Further examinations with an aberration-corrected scanning transmission electron microscope found the particles had flat faces which allowed them to pack tightly together. However, slight misfits between the surfaces meant the packing of atoms at the interfaces between the tiny particles is slightly less dense than the perfect lattice within each individual particle. These interfaces became the pathway for lithium ions to go in and out. Meanwhile, the larger electrolyte molecules are unable to infiltrate the material through those gaps.
This breakthrough means it is potentially possible to make lithium-ion batteries that are cheaper and have higher energy density.