“Rivet” cations enhance mechanical stability of Ni-rich layered cathode materials for Li-ion batteries

The Mg2+ and Ni2+ ions at the Li site (A-site) in Ni-rich NMC layered cathode materials improve their cycling stability by reducing intragranular cracking. These cations act as “rivets” due to their ability to suppress gliding of the close-packed layers and associated mechanically-destructive phase transitions. In the present study the impact of the Mg2+ and Ni2+ ions on the mechanical properties of the single-crystal Mg-doped NMC811 materials has been investigated quantitatively with in situ micropillar compression, nanoindentation and pillar splitting in a scanning electron microscope. Here we show that the increase in cumulative occupancy of the “rivet” cations NiA and MgA at the A-site in the layered R3̄m structure results in an increase of the critical resolved shear stress of the basal {0001} < 112̄0> slip system activation, hardness, fracture toughness K1c and Young’s modulus. Density functional theory calculations demonstrate that the Ni/Mg for Li substitution contributes synergistically to the enhancement of mechanical properties in NMC811. Mg²⁺ and Ni²⁺ ions at Li sites enhance Ni-rich NMC stability by acting as structural rivets that suppress layer gliding and cracking. Here, the authors show that Mg²⁺ and Ni²⁺ at Li sites synergistically enhance the mechanical strength and toughness of single-crystal NMC811.