A Synergistic Effect of Na+ and Al3+ Dual Doping on Electrochemical Performance and Structural Stability of LiNi0.88Co0.08Mn0.04O2 Cathodes for Li-Ion Batteries

The synergistic effect of Na+/Al3+ dual doping is investigated to improve the structural stability and electrochemical performance of LiNi0.88Co0.08Mn0.04O2 cathodes for Li-ion batteries. Rietveld refinement and density functional theory calculations confirm that Na+/Al3+ dual doping changes the lattice parameters of LiNi0.88Co0.08Mn0.04O2. The changes in the lattice parameters and degree of cation mixing can be alleviated by maintaining the thickness of the LiO6 slab because the energy of Al-O bonds is higher than that of transition metal (TM)-O bonds. Moreover, Na is an abundant and inexpensive metal, and unlike Al3+, Na+ can be doped into the Li slab. The ionic radius of Na+ (1.02 Å) is larger than that of Li+ (0.76 Å); therefore, when Na+ is inserted into Li sites, the Li slab expands, indicating that Na+ serves as a pillar ion for the Li diffusion pathway. Upon dual doping of the Li and TM sites of Ni-rich Ni0.88Co0.08Mn0.04O2 (NCM) with Na+ and Al3+, respectively, the lattice structure of the obtained NNCMA is more ideal than those of bare NCM and Li+- and Na+-doped NCM (NNCM and NCMA, respectively). This suggests that NNCMA with an ideal lattice structure presents several advantages, namely, excellent structural stability, a low degree of cation mixing, and favorable Li-ion diffusion. Consequently, the rate capability of NNCMA (83.67%, 3 C/0.2 C), which presents favorable Li-ion diffusion because of the expanded Li sites, is higher than those of bare NCM (78.68%), NNCM (81.15%), and NCMA (83.18%). The Rietveld refinement, differential capacity analysis, and galvanostatic intermittent titration technique results indicate that NNCMA exhibits low polarization, favorable Li-ion diffusion, and a low degree of cation mixing; moreover, its phase transition is hindered. Consequently, NNCMA demonstrates a higher capacity retention (84%) than bare NCM (79%), NNCM (82%), and NCMA (82%) after 50 cycles at 1 C. This study provides insight into the fabrication of Ni-rich NCMs with excellent electrochemical performance.