By Battery Power Online Staff
October 5, 2020 | Researchers at Argonne National Laboratory presented a model, lithium-rich cathode system having no oxygen redox this summer in a June issue of the Journal of Power Sources (DOI: 10.1016/j.jpowsour.2020.228335).
Their first principles calculations and experimental data on a bespoke Li-rich system showed immunity to oxygen activity and significant energy inefficiency. “The results directly implicate transition metal migration as the fundamental, sole process responsible for hysteresis in this material,” they wrote.
“Lithium-rich oxides offer the possibility of more sustainable and cost-effective options over many current technologies as they can be made predominantly of Earth-abundant elements, with manganese being the most promising candidate,” said Jason R. Croy, a physicist in Argonne’s Chemical Sciences and Engineering (CSE) division and first author on the paper.
Lithium-rich oxides are a promising class of cathode materials that contain substantially more lithium than currently used oxides and are composed of high amounts of Earth-abundant elements, such as manganese. The complex way that elements arrange themselves at the atomic level in these oxides, including lithium, manganese, titanium, oxygen and fluorine, leads to unique processes when a battery operates.
Most notable is the participation of oxygen in helping to generate current, a process that does not occur in traditional lithium-ion batteries, but is one of the main reasons why lithium-rich oxides can utilize larger amounts of lithium. One drawback of these cathodes is the appearance of an energy inefficiency, or “hysteresis,” between charge and discharge: the amount of energy needed to charge the battery is more than the energy obtained on discharge.
Oxygen has generally been blamed for this inefficiency, but to explore that assumption the Argonne team designed an experiment based on a model, lithium-rich oxide: Li1.2Ti0.4Cr0.4O2. This material is a model because it contains substantial amounts of lithium and displays a hysteresis between charge and discharge; however, oxygen does not participate in the charge/discharge processes of the cell. Through the design of this system, along with theoretical calculations and measurements of working cells, the team showed that the movement of elements, in this case titanium and chromium, between certain sites within the oxide was the sole cause of hysteresis in this material—not the processes directly involving oxygen.
“The results presented in this work clearly demonstrate that anion redox in Li-rich oxides is not a prerequisite for hysteresis and energy inefficiency,” the authors write in their conclusions. “Chemical inhomogeneities (e.g., local ordering and short-range order) as well as electronic structure can have a significant impact on defect formation and the subsequent electrochemical properties, in the absence of anion redox or loss. Such defects, being local phenomena, are expected to play an important role in all classes of Li-rich oxides, including disordered rock salts.”
There is an “essential need,” the authors say, to develop materials devoid of transition metal migration to interstitial sites in both layered and disordered oxides alike.
