ORNL develops new hydrothermal synthesis method

Researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) have developed a new hydrothermal synthesis method that makes a cobalt-free cathode material (LiNi0.9Mn0.05Al0.05O2, NMA) with more uniform, round, tightly packed particles than is common in today’s cathodes, maintaining more stability throughout a charging cycle. The result is a more affordable battery from a faster, less wasteful process that uses less toxic material.

NMA half cells show 200 mAh g−1 capacity with 96% capacity over 100 cycles. NMA-LTO full cells shows capacity of 186 mAh g−1 with 81% retention over 200 cycles. A paper on the new method is published in the Journal of Power Sources.

The drive toward decarbonization and the demand for electric cars has increased the focus on sustainably producing energy-dense cathodes. However, traditional processing presents challenges.

The first obstacle is a reliance on cobalt, a rare metal mined and refined abroad. This dependence on foreign sources poses risks to American manufacturing supply chains and transportation infrastructure.

The availability of cobalt is not the only complication. The balance of other metals common in cathodes can also make the manufacturing process longer and more hazardous.

For example, high nickel concentration has led to the widespread use of a chemical mixing method for cathode production that requires large quantities of ammonia for corrosive reactions. Using the toxic chemical increases costs, heightens health and environmental concerns, and wastes large amounts of water to reduce acidity.

Instead of continuously stirring cathode materials with chemicals in a reactor, the ORNL hydrothermal synthesis approach crystalizes the cathode using metals dissolved in ethanol. The ethanol is safer to store and handle than ammonia, and afterward it can be distilled and reused.

The hydrothermal synthesis method is also much faster, said ORNL lead researcher Rachid Essehli. The time required to make particles and prepare for the next cathode batch drops from as many as a few days to 12 hours.

In addition, the material produced has more uniform, round, tightly packed particles that are ideal for a cathode, Essehli said. Although the ORNL team has previously identified other cobalt-free combinations that work, the material developed through this study was better at maintaining stability throughout the battery charge cycle.

Because its properties are similar to those of today’s cobalt-based cathodes, the new material can be seamlessly integrated into existing battery manufacturing processes. A patent is pending on the technology, which is ready to be scaled up for commercial production by industry, Essehli said.

The research was funded by the DOE Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office. It used resources of ORNL’s Center for Nanophase Materials Sciences and the Advanced Photon Source at Argonne National Laboratory. Both are DOE Office of Science user facilities.