Lithium-rich cathode materials (positive electrodes) are capable of storing noticeably more charge than those used today in electric vehicles and electronics. This makes them promising for next-generation batteries. However, such cathodes have a long-standing problem: during charge and discharge, transition metal ions, such as nickel and manganese, can shift from their positions in the crystal structure. These ions block the pathways for lithium, slowing its movement and causing irreversible capacity loss after many cycles.
Usually scientists try to preserve strict order in the arrangement of atoms in the cathode, believing that this makes it easier for lithium to move. But what if the opposite approach works?
A group led by Professor Kyojin Ku of Hanbat University (South Korea) tested this by deliberately disrupting the atomic structure of a lithium-rich cathode. The work was published in the journal Energy Storage Materials. The focus was the composition Li1.2Ni0.4Mn0.4O2 - a nickel and manganese compound without expensive cobalt. It usually forms a neat layered structure, where lithium and transition metals occupy separate planes.
To obtain the disordered form, the researchers used a high-energy ball mill - a mechanical grinding method in which a mixture of materials is intensively ground by balls. As a result, lithium and transition metal atoms were distributed more randomly. "To the best of our knowledge, this is the first case in which such a composition has been obtained in a disordered form," Ku notes.
Comparing the disordered material with the ordered one, the scientists found that in the former, charge and discharge proceed more symmetrically, capacity is higher, and it performs better at high currents. In addition, the material showed stability - it retained about 90% of its initial capacity after 50 cycles without a drop in voltage.
Theoretical and computational analysis helped explain the improvements. "By deliberately mixing the atoms into a disordered rock-salt-type structure, we decoupled lithium movement from the harmful movement of transition metals," Ku explains. "Lithium still flows efficiently through a connected network of open pathways, while the transition metals are locked in place and can no longer block these pathways."
The authors emphasize that this is still a laboratory result, but it points to an approach for creating cathodes with higher energy density. "We have shown that cation (positively charged ion) disorder can be used as a tool to solve a problem that has held back an entire family of high-energy battery materials for years," Ku concludes. "If this principle is developed, it will help bring lithium-rich cathodes closer to commercial application and pave the way for more durable batteries for electric vehicles and energy storage."