Skip to main content

Anionic redox processes: A transformational approach for advanced energy materials

Periodic Reporting for period 3 - ARPEMA (Anionic redox processes: A transformational approach for advanced energy materials)

Reporting period: 2018-10-01 to 2020-03-31

Throughout its history, the Li-ion battery technology has relied on cationic redox reactions as the sole source of energy storage capacity. This is no longer true. Through a totally new chemical approach, he demonstrated that the extraordinary increase in energy storage capacity offered by the Li-rich layered oxides was due to the cumulative contribution of both cationic and anionic reversible redox processes. However, as is often the case with new discoveries, the fundamental science at work needs to be rationalized and understood. Specifically, what are the mechanisms for ion and electron transport in these Li-driven anionic redox reactions? Or how to push the anionic redox activity to its limit, while preventing the material stability against release of O2 at high potential? . This can be viewed both as a blessing and a challenge.

In short this new way of extracting electrons from framework structures has the potential to trigger transformational changes in the way we store and convert energy. It opens wide new research horizons for advanced electrode materials that will enable to design a new generation of Li-ion batteries with substantial increases in energy storage capacity. They should enable the rapid development of electric transportation and facilitating the use of renewable energy resources. Both of these aspects will heavily contribute to lower CO2 emissions and make our lives and our environment more pleasant with a viable legacy to our successors.

So the overall objective of the project is centered towards the development of a new generation of Li-ion batteries based on sustainable electrode materials enlisting both cationic and anionic redox activities, and exhibiting substantial increases (20 - 30%) in energy storage capacity. Additional it is aimed to the implementation of the anionic redox concept to Na-ion battery electrodes, a technology with presently only a few well-performing materials offering modest capacities (140 mAh/g), together with the design of water splitting catalysts having the feasibility to surpass current water splitting efficiencies .
Through this first period key achievements were realized and they are listed below.

+ Provide, as published in pour 2016’s science paper, the first direct visualization of the (O-O) peroxo-like dimers in high capacity layered Li-rich oxides by High Resolution Electron Microscopy, hence ending the long remaining controversies about the role of the anionic network.

+ Demonstrate (Nature Materials 2017) the feasibility to trigger this novel anionic redox process in oxides having three dimensional (3D) rather than two dimensional (2D) crystal structures. Thus, by freeing the structural dimensionality constraint, he opens wide the rich crystal oxide chemistry for designing high energy density electrodes for the next generation of Li-ion batteries.

+ Design a novel Li3IrO4 phase that push the limit of the anionic redox activity to 3.7 e- per nd metals – a record among all the cathodes so far investigated. This work submitted to Nature Energy, show that the O/M parameter delineates the boundary between the material’s maximum capacity and its stability, hence providing valuable insights for further developing high capacity materials.
+ Isolate a Na-rich phase Na2IrO3 phase which can reversibly cycle 1.5 Na+ per formula unit while not suffering from oxygen release nor cationic migrations. This work published in Chemistry of Materials turns out to be an impetus for the design of high energy Na-rich materials based on more sustainable elements than Ir as we are presently investigating.
+ Provide a rationalization of the anionic redox process by showing that a strong M–(O2) covalence is an absolute condition to ensure high electrochemical reversion reversible and to prevent O2 gas release from the structure at high states of charge, which is crucial application-wise. Such finding was discussed in our 2016 Nature Material paper.

+ Demonstrate enhanced OER activity in La2LiIrO6 when compared to the state-of-the-art IrO2 catalysts in acidic environment. This large OER activity is triggered by an activation step in acidic media corresponding to the delithiation/oxidation of the surface. The mechanistic of the process was deduced by combined DFT calculations and HRTEM measurements. This work, published in Nature Energy (2016), established for the first time the correlation existing between the OER activity stability for perovskites when triggering the surface oxygen redox

+ We unravel the poor kinetics of anionic-driven poor kinetics which keeps deteriorating further with cycling and we also find that voltage fades faster if oxygen is kept oxidized for longer Such findings, which are in fact harsher for LR-NMC, were published as editor’s choice in the Journal of the Electrochemical Society(2016). The paper conveys caution that anionic redox risks practical problems; hence, when chasing larger capacities with this class of materials, we encourage considering real-world applications.
Altogether, these advances published in hig impact journals (Nature materials, Science) journals have been widely cited and received unsolicited coverage in press worldwide.
Through this first 18 months, exciting results were obtained some of which be beyond our initial expectations. Among them are 1) the finding that this novel anionic redox process, first described in layered compounds,, can be generalized to three dimensional oxides, hence, widely expanding the crystal chemical landscape for designing high energy density electrodes an 2) the discovery of a new phase Li3IrO4 showing the highest capacity (3.6 e- per Ir metal) ever reported for any positive electrode materials till now. Application-wise, such new class of materials having exacerbated capacity enables the next generation of Li-ion batteries with substantial increases (20-30%) in energy storage capacity provide that we could replace Ir by low cost and abundant 3d chemical elements.

Such a work should enable to meet the challenge of producing better batteries for boosting the use of renewable energy and the proliferation of electric vehicles. At the end it will help in improving our life quality and, simultaneously, protect our planet.
Strategy to activate anionic redox
Voltage profiles for the first three cycles of Beta-Li2IrO3
Structural evolution of Li3IrO4
Effect of the rate capability of Li2Ru0.75Sn0.25O3
Structural changes in the oxygen sublattice