Low-cost electrodes that store more lithium than the ones used in today’s lithium-ion batteries could enable electric car drivers to go farther between charging stops. For that reason, researchers have examined many Li-based electrode materials, searching for better performers.
A family of Li-rich layered transition metal oxides looks especially promising. Compared with common commercial electrode materials, these oxides could, in principle, store 30% or more charge on a volume basis. But the voltage of batteries made from these materials drops substantially with repeated charging. Understanding the electrochemical processes that cause these batteries to fail is key to overcoming this problem.
By combining synchrotron-based X-ray microscopy, spectroscopy, and scattering techniques, researchers have now pinned down the complex interplay between changes in the crystal structure and redox potentials of Li1.17Ni0.21Co0.08Mn0.54O2, a member of the family of Li-rich layered oxides (Nat. Commun. 2017, DOI: 10.1038/s41467-017-02041-x).
The material was prepared by chemists at Samsung, incorporated into battery cathodes, and analyzed via the X-ray methods by a large team that includes Michael F. Toney of SLAC National Accelerator Laboratory and Wanli Yang of Lawrence Berkeley National Laboratory.
The analyses show that as lithium ions migrate from the cathode to the anode upon charging, transition metal ions move to fill the lithium vacancies, but not all of the metal ions move back during discharge. The incomplete ion shuttling leads to microscopic structural changes that alter oxygen’s bonding geometry, lowering oxygen’s redox potential and causing the drop in voltage.