Structural Transformations in High-Capacity Li₂Cu_0.5Ni_0.5O₂ Cathodes

Cathode materials that can cycle >1 Li⁺ per transition metal are of substantial interest for increasing the overall energy density of lithium-ion batteries. Li₂Cu_0.5Ni_0.5O₂ has a very high theoretical capacity of ∼500 mAh/g assuming both Li⁺ ions are cycled reversibly. The Cu^2+/3+ and Ni^2+/3+/4+ redox couples are also at high voltage, which could further boost the energy density of this system. Despite such promise, Li₂Cu_0.5Ni_0.5O₂ undergoes irreversible phase changes during charge (delithiation) that result in large first-cycle irreversible loss and poor long-term cycling stability. Oxygen evolves before the Cu^2+/3+ or Ni^3+/4+ transitions are accessed. In this contribution, X-ray diffraction, transmission electron microscopy (TEM), and transmission X-ray microscopy combined with X-ray absorption near edge structure (TXM–XANES) are used to follow the chemical and structural changes that occur in Li₂Cu_0.5Ni_0.5O₂ during electrochemical cycling. Li₂Cu_0.5Ni_0.5O₂ is a solid solution of orthorhombic Li₂CuO₂ and Li₂NiO₂, but the structural changes more closely mimic the changes that the Li₂NiO₂ endmember undergoes. Li₂Cu_0.5Ni_0.5O₂ loses long-range order during charge, but TEM analysis provides clear evidence of particle exfoliation and the transformation from orthorhombic to a partially layered structure. Linear combination fitting and principal component analysis of TXM–XANES are used to map the different phases that emerge during cycling <i>ex situ</i> and <i>in situ</i>. Significant changes in the XANES at the Cu and Ni K-edges correlate with the onset of oxygen evolution