Oxygen Stability of Disordered, Multi-Lithium Oxide Cathodes for Li-Ion Batteries
Authors: Self Ethan Craig , Ruther Rose Emily , Veith Gabriel M , Nanda Jagjit 
Published: February 2020

Development of high energy density, low-cost, and safe electrode materials is one of the key enablers for advanced batteries for transportation. Currently, one major technical barrier towards development of high energy density lithium-ion batteries is the lack of robust, high-capacity cathodes. Traditional layered LiMO2 (M = Mn, Co, Ni, etc.) cathodes cannot reversibly cycle their entire Li supply (e.g., x ≤ 0.5 in Li1-xCoO2). When charged beyond ~4.3 V vs. Li/Li+, many lithium-transition metal oxide cathodes undergo irreversible structural changes with concomitant oxygen gas evolution, resulting in irreversible capacity loss and voltage fade during cycling.[1-3]Understanding and addressing these structural instabilities is of vital importance to design next-generation cathode materials which can better utilize their Li supply without sacrificing cycle life.
The scientific community has largely focused on developing well-ordered cathodes due to the notion that cation mixing reduces the electrode’s reversible capacity. Challenging this widely-held belief, recent work[2, 4-6] has shown that disordered Li-excess cathodes exhibit high capacities and good cycling stability (e.g., Li1.211Mo0.467Cr0.3O2with 266 mAh/g over 10 cycles). The excellent performance of these cathodes has been attributed to anion redox activity and formation of a percolating Li diffusion channel which is not energetically accessible in traditional materials.
Motivated by these recent developments, the focus of the present work is to investigate the structural evolution of disordered cathodes during charge/discharge cycling. Multi-lithium cathodes containing transition metals which can access multiple oxidation states during lithium extraction are prepared and characterized. For example, Li2MoO3 (theoretical capacity of 340 mAh/g) can be charged to MoO3 (corresponding to oxidation of Mo4+ to Mo6+) at modest potentials (~2.5 V vs. Li/Li+) that are compatible with standard electrolytes. Furthermore, Li2MoO3 has been shown to have superior oxygen stability compared to Li2MnO3, offering opportunities to prepare composite layered-layered cathodes with the general formula xLi2MoO3•(1-x)LiMO2.[7]

In this presentation, results will be presented on disordered, multi-lithium cathodes including Li2MoO3 and Li2MoO3•(1-x)LiMO2. Various synthesis procedures to produce these materials will be discussed, and standard electrochemical half-cell experiments will be coupled with advanced in-situ characterization methods including Raman spectroscopy, mass spectrometry, X-ray absorption spectroscopy (XAS), and X-ray diffraction (XRD) to evaluate the cathodes' oxidative stability. Furthermore, the effects of fluorine doping on cycling stability and oxygen lattice stability will be discussed.

Acknowledgements

This research has been supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program.

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