Process synthesises high-voltage cathodes for Li-ion batteries

Hope is pink: The powder that Jennifer Ludwig carefully pours into a glass bowl and which glows pink in the light of the laboratory lamp has the potential to significantly improve the performance of future batteries. “Lithium cobalt phosphate can store substantially more energy than traditional cathode materials,” explains the chemist.

Working in the group of Tom Nilges, head of the Professorship of Synthesis and Characterisation of Innovative Materials, the chemist has developed a process to produce the pink powder quickly, with minimal amounts of energy and in the highest quality.

Battery researchers have considered lithium cobalt phosphate a material of the future for some time. It operates at higher voltages than the traditionally employed lithium iron phosphate and thus attains a higher energy density – 800 watt hours per kilogram instead of just under 600 watt hours.

Previously, however, producing the promising high-voltage cathode material necessitated a very complex, energy-intensive and inefficient process under drastic conditions with temperatures of 800°C. “And the crystals that form under these conditions vary in size and must be ground to nanocrystalline powder in a second, energy-intensive step,” reports Ludwig.

Furthermore, the resulting grains possess sufficient ionic conduction in only one direction. Over most of the surface the chemical reaction between the electrode material and the electrolyte in the batteries progresses very slowly.

The microwave synthesis developed by Jennifer Ludwig solves all of these problems at once: Obtaining the high-grade lithium cobalt phosphate requires merely a microwave oven and 30 minutes of time.

The reactants are placed in a Teflon receptacle together with a solvent and are then heated. A mere 600W are sufficient to achieve the 250°C required to stimulate the formation of crystals.

The flat platelets created in the process measure less than one micrometer across and are only a few hundred nanometers thick, with the axis of maximum conductivity oriented towards the surface. “This shape ensures better electrochemical performance because the lithium ions need to move only short distances within the crystals,” explains Ludwig.

The chemist was also able to solve another problem during her experiments: At temperatures over 200°C and under great pressure, instead of the desired lithium cobalt phosphate a hitherto unknown cobalt hydroxide hydrogen phosphate complex occasionally formed.

Jennifer Ludwig succeeded in elucidating the reaction process, isolating the compound and determining its structure and properties. Since the new compound is unsuitable as a battery material, she modified the reaction so that only the desired lithium cobalt phosphate is produced.

“With this new production process, we can now create bespoke, high-performance, platelet-shaped lithium cobalt phosphate crystals in a high quality,” says Professor Nilges. “Thus, a further hurdle on the path to new high-voltage materials has been taken.”