"Popcorn" particle paths promise better lithium-ion batteries

Researchers at Sandia National Laboratories confirmed the particle by particle mechanism by which lithium ions moving in and out of the electrodes of lithium-iron-phosphate (LiFePO4, or LFP), findings, the better performance in lithium-ion batteries in electric vehicles, medical equipment and machines could do.

The research is reported in an article titled "Intercalation pathway LiFePO4Electrode revealed in n particles of nanoscale charge status mapping" in the journal of nano letters, 2013, 13 (3), pp. 866-872. Authors are Sandia physicist Farid El Gabaly and William Chueh from Stanford University.

LFP, a natural mineral olivine family is one of the newer materials in lithium-ion batteries and is known as safer and longer-lasting than the lithium cobalt oxide (LiCoO2) used composite in smartphones, laptops and other consumer electronics.

While LFP material for these reasons is of interest to researchers and battery maker, is the process by which ions and the LFP move lithium, battery saves and lifts his energy is not well understood. This proves to be an obstacle for the material wide-spread adoption.

Cathode materials such as LFP are critical in the search for higher capacity, long life, lithium-ion batteries for applications where no batteries to replace, in consumer electronics are as easy or as often as they. Larger applications where lithium cobalt oxide could be replaced by LFP battery cells are electric vehicles and aircraft.

Popcorn-like particle movements seen on microscopy technique

Complete observation battery cross sections, researchers have created important insights on a controversy over the process that limited unloading rates and charge the battery.

Previous attempts to speed optimizing charge/discharge included coating the particles to increase their electrical conductivity and to accelerate reduction of grain size to their transformation, but the initiation process that well may have overlooked the critical speed determining step in such a way that lithium without moving a particle to its interior.

Use X-ray microscopy, ultra-thin slices of a commercial battery review Sandia of researchers evidence found that recharge and discharge in LFP is limited by the introduction of PHASE transformation or nucleation and is independent of the size of the particles.

The LFP electrode forms a mosaic of homogeneous particles, the lithium-rich or lithium poor condition are. The Sandia research confirmed the particle-particle, or mosaic, way of the phase formation by adding lithium ions at the cathode. The results contradict previous assumptions.

"A theory of the spread said that if all particles were exposed to lithium, they would discharge all start slowly along in a concurrent PHASE transformation," said El-Gabaly. "We have now seen that the process rather like popcorn." A particle is completely discharged then the next, and they go one like popcorn, absorb the lithium."

Understanding lithium ion charge helps cut and dice

Lithium ions move and electrode materials battery are charged and discharged. A lithium-ion battery is charged, extracted an external voltage source lithium-ion from the cathode (positive electrode) material in a process known as "Delithiation." The lithium ions move through the electrolyte and inserted (unofficial) in the material (negative electrode) anode in a process known as "Lithiation." The same process in reverse order happens when energy from the battery discharged.

"We observed that there were only two phases, where the particles had either lithium or not," El Gabaly said. "Many previous studies researchers have focused a particle on the understanding of the charging within."

El-Gabaly and his colleagues Sandia took a piece just a bit thicker mapped as a human hair from a commercial battery, only a layer of LFP particles, and the locations of lithium in about 450 particles, when the battery to different States was free of charge.

He said "Our discovery was possible thanks to the mapping of the lithium in a relatively large ensemble of particles".

Many tools, facilities contribute to the research

The researchers could build a standard button cell battery from raw materials, the use of Sandias cell battery prototyping facility in New Mexico, equipped with the largest Department of energy plant for the production of small lots of lithium-ion cells. The battery was divided into Sandias Livermore, California, plant by a new method of slices layers, then charged and tested for normal behavior to get the spatial arrangement of the cathode to the anode.

The Sandia Researchers went to Lawrence Berkeley Laboratory, the materials with State-of-art Scan national to characterize transmission X-ray microscopy (STXM) at advanced light source (ALS), and returned to Sandias California Web site for study by transmission electron microscopy (TEM).

"The x-ray spectroscopy from the ALS tells you what is inside a single particle or where the lithium, but it has low spatial resolution. We want to tell the electron microscopy of the same segment to us where the particles over the entire level of the battery, were distributed", said Cameron, former Sandia Truman fellow, that of energy is lead author of the journal article and assistant professors and Center fellow Precourt Institute at Stanford University.

Sandia research team and others presented their technical findings on the most recent materials research society spring meeting in San Francisco. As a result of this presentation El Gabaly, said other researchers are using the results to validate theoretical models. The team can also with industry partners, how a company has already indicated a strong interest in the implementation of similar studies on other, more complex battery materials Sandia.

The research team at Sandia was internally funded with support from the Sandia Truman Fellowship in national security science and technology, and by the Department of energy Office of science, which also supports ALS.

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