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Material strategies for high energy layered cathodes with improved stability for Li-ion batteries: investigation of doping strategies and solid-state concepts using a combined in- situ/operando approach

Subject Area Synthesis and Properties of Functional Materials
Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 416542991
 
Final Report Year 2024

Final Report Abstract

The main objective of this cooperative project on high energy cathodes is understanding the systematic correlation of redox processes induced by Li ion and electron removal and induced degradation (involving the electrolyte), occurring in the bulk and at surfaces, and to develop strategies to avoid/reduce these unwanted side reactions (in the second phase). As cathodes Ni-rich NCMs (lithium nickel cobalt manganese oxides) are compared to Li/Mn-rich HE-NCM layered oxides to elucidate the central relevance of oxide to transition metal redox reactions. The complimentary expertise of the project consortium in synthesis and structural studies (BIU), analysis of chemical and electronic structure (OF TUDa), and of chemical variation during polarisation (PC TUDa) allows a rather full characterisation of differently prepared electrodes also in their electrochemical behaviour. As materials we investigated commercial NMC powders and thin films as well as BIU prepared and SO2/NH3 gas treated and modified HE- NCMs with improved stability. Using our analytical approach we developed and established essential and reliable methods to discriminate bulk from surface effects. For XPS a mechanical surface scratching procedure allows a so far impossible analysis of bulk properties. XPS and XANES measurements at synchrotrons allow to use changes of excitation energy to vary surface sensitivity. For Raman spectroscopy we evaluated the potential of resonance effects and developed an in-situ/ operando cell with optionally increased sensitivity exploiting SERS effects. One central insight comparing different measurements is the participation of O 2- in the involved redox reactions of all high-energy cathodes. XPS and XANES indicate that the formal oxidations states of 4+ of the metals are not reached and that the oxide ions are oxidized. The relative changes in the oxidation states and their origin (formation of different phases, changes of hybridisation, non-rigid bond effects) cannot be assigned yet (work in progress). A further central insight is the degradation involving surface reactions induced by oxidized oxide ions and high valent transition metal ions. Surprisingly, the surface vs. bulk composition after electrochemical oxidation indicates surface oxide phases with reduced metal ions. Reduced surfaces are also formed on HE-NCMs after SO2/NH3 gas phase treatment. Also, first oxide ALD layers show improved stability. These results are supported by CV and impedance measurements, indicating a degradation by reaction of the cathodes with electrolyte components at high potentials; preferentially are then formed Li-ion permeable layers with reduced reactivity. Summarizing, based on the so far gained insights from this project it is evident that the properties of the CEI phase boundary are critical for the properties of Li-ion batteries and that they must be specifically adjusted. The presented approach promises interesting insights and perspectives but involves a high level of effort because of the inherent complexity in synthesis and characterisation.

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