Structure, Electrochemistry, and Phase Evolution of Al-Substituted Na2/3[Ni1/3-yMn2/3-zAl y + z ]O2 as a Sodium-Ion Battery Cathode Material

(NaNM) composition are desirable for their rate capability and energy density. Unfortunately, these materials are hindered by their rapid capacity loss during charge/discharge cycling. While prior work has shown the benefits of Al doping in improving its capacity retention, it remains unclear whether and where Al is substituted in the bulk crystalline phase, especially given the honeycomb ordering of Ni and Mn in NaNM, and how Al-substitution affects the structural evolution during electrochemical cycling. In this work we synthesize Al-substituted NaNM via two substitution strategies-aliovalent and "isovalent" substitution. Complementary X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) are used to determine the effects of Al-substitution strategies on the crystal structure and show a preference of Al to substitute Ni, not Mn. Operando XRD is used to track phase evolution of the Al-substituted phases and reveals a reduced lattice mismatch during the high voltage phase transition as Al content increases. This work provides insights into the complex elemental substitution in NaNM and serves as the basis for developing future substitution strategies.