High voltage cathodes promise significantly higher energy yield for lithium-ion batteries
Among battery researchers, lithium cobalt phosphate is regarded as the material of the future. The reason for this is that it operates at a higher voltage than the lithium-iron phosphate used to date and therefore achieves a higher energy density – 800 watt-hours per kilogram instead of the 600 watt-hours previously used.
Until now, however, the production of this promising high-voltage cathode material has been laborious, energy-intensive and not very efficient: drastic conditions with temperatures of 900 degrees Celsius are required. In addition, the crystals formed under these conditions vary in size and must first be ground into nanocrystalline powder in a second, energy-intensive step. These are not all disadvantages: the resulting grains have sufficient ionic conductivity only in one direction. The chemical reaction between electrode material and electrolyte in the battery is slow on most of the surface.
The microwave synthesis developed by the TUM’s junior researcher Jennifer Ludwig solves all these problems in one fell swoop: it only takes a small microwave oven and half an hour to produce high-purity lithium cobalt phosphate. The reagents are heated together with a solvent in a Teflon container. Just 600 watts of power are enough to generate the necessary temperature of 250°C and stimulate crystallization.
The resulting flat platelets have a diameter of less than one micrometer, a thickness of a few hundred nanometers, and the axis of maximum conductivity is oriented towards the surface. “This shape ensures better electrochemical performance because the lithium ions only have to travel short distances in the crystal,” explains Ludwig.
The chemist was also able to solve another problem in her experiments: At temperatures of more than 200°C and under high pressure, the desired lithium cobalt phosphate is not always produced, but rather a hitherto unknown complex cobalt hydroxide hydrogen phosphate. Jennifer Ludwig succeeded in elucidating the reaction pathway, isolating the chemical compound and determining its structure and properties. Since this compound is unsuitable as a battery material, Ludwig modified the reaction conditions in such a way that only the desired lithium-cobalt phosphate is produced.
“With the new manufacturing process, we can now produce the high-performance lithium-cobalt phosphate crystals in a single process step, tailor-made and of high quality,” comments Professor Tom Nilges, holder of the chair for synthesis and characterization of innovative materials at TUM. “This means that a further hurdle on the way to new high-voltage materials has been overcome.”
Jennifer Ludwig’s work was supported by car manufacturer BMW, among others. Research work was conducted in cooperation with the Lawrence Berkeley National Laboratory (LBNL), Stanford Synchrotron Radiation Lightsource (SSRL) and the Walther-Meißner-Institut (WMI). Jennifer Ludwig received the Evonik Research Prize for the development of her new synthesis process, which is awarded annually by the chemical group to outstanding young scientists.
More information:
https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.7b01152
https://www.sciencedirect.com/science/article/pii/S0378775316317554
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