Cu Doping Increases Capacity Retention in LiNi<sub>0.6</sub>Mn<sub>0.2</sub>Co<sub>0.2</sub>O<sub>2</sub> (NMC622) by\nAltering the Potential of the Ni-Based Redox Couple and Inhibiting\nParticle Pulver

To\ndiscern the influence of Cu<sup>2+</sup> as a dopant on both\nthe structural and electrochemical characteristics of LiNi<sub>0.6</sub>Mn<sub>0.2</sub>­Co<sub>0.2</sub>O<sub>2</sub> (NMC622), Cu<sup>2+</sup>(aq) was added to the coprecipitation synthesis from the\nconstituent ions. At 5 mol % Cu<sup>2+</sup>, a single-phase Cu-NMC\nproduct results, evidenced by an increase in <i>d</i>-spacing\nalong the [003] and [104] directions and a slight increase in the\ncrystal volume of the <i>R</i>–3<i>m</i> hexagonal (rock-salt superstructure) lattice. XRD data and high-resolution\nTEM imaging support Cu<sup>2+</sup> doping primarily on the transition\nmetal 3b Wyckoff sites. Galvanostatic cycling of Cu-NMC shows a reversible\ngravimetric capacity of 102 mAh/g compared to 136 mAh/g for undoped\nNMC. Despite the lower capacity, the discharge capacity retention\nof Cu-NMC is 89% after 100 cycles compared to only 70% for NMC. XPS\nanalysis reveals that this lower capacity is due to an increase in\nthe concentration of Ni<sup>3+</sup> ions at the surface, while XRD\ndata collected at the top and bottom of charge show a smaller decrease\nin crystalline domain size for Cu-NMC (40.5% decrease) compared to\nNMC (74.7% decrease), translating to pulverization of the secondary\nparticles.