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 Li_1-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., Li_1.211Mo_0.467Cr_0.3O₂with 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.

 

References

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