Anion-redox chemistry has unlocked unprecedented energy densities in cathode materials by enabling additional redox reactions beyond traditional cation-based mechanisms. For example, Li-rich layered oxides like Li1.2Ni0.13Co0.13Mn0.54O2 exhibit capacities exceeding 300 mAh/g due to reversible oxygen redox activity at high voltages (>4.5 V). This represents a ~50% increase compared to conventional NMC cathodes. However, challenges such as voltage hysteresis and oxygen release must be addressed to harness this potential fully.
Recent advancements in doping strategies have improved the reversibility of anion-redox reactions. For instance, doping with elements like Ru or Sb has been shown to reduce voltage hysteresis from ~0.5 V to <0.2 V while maintaining capacities above 250 mAh/g after 100 cycles. These dopants stabilize the oxygen lattice by forming strong covalent bonds with oxygen anions, preventing irreversible oxygen loss during cycling.
In-situ spectroscopic techniques such as X-ray absorption spectroscopy (XAS) and Raman spectroscopy have provided critical insights into anion-redox mechanisms. XAS studies reveal that reversible oxygen redox occurs via the formation of peroxo-like (O2)n- species rather than irreversible O2 gas evolution. Raman spectroscopy further confirms the stability of these species during cycling, even at high charge states (>4.6 V).
Practical implementation of anion-redox cathodes requires addressing safety concerns related to oxygen release at high voltages. Recent work on surface coatings like Al2O3 and Li3PO4 has demonstrated significant improvements in thermal stability, reducing exothermic heat release by ~40% during thermal runaway tests while maintaining high energy densities.
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