Nanorod Gradient Cathode: Preventing Electrolyte Penetration into Cathode Particles

Layered nickel-rich cathode particles for lithium-ion batteries can fail and severely limit the cycling performance via cracking from anisotropic strain which allows electrolyte penetration and the formation of electrically insulating material and a decreased capacity. Self-assembled layered nanorod gradient (NRG) Li[Ni0.81Co0.06Mn0.13]O2 cathode particles cycle more stably with improved performance compared to its constant concentration counterpart. NRG cathode material was synthesized with a Ni-rich bulk (for higher lithium storage) and a radially columnar nanorod comprised surface and benchmarked against the widely used constant concentration (CC) LI[Ni0.82Co0.14Al0.04]O2 cathode and in both half- and full-cells. Through a combination of in situ and time-resolved X-ray diffraction (XRD), cross-section scanning electron microscopy imaging (SEM), and high-resolution transmission electron microscopy (HR-TEM), we confirm that the enhanced durability of the NRG material is attributed to its radially columnar concentration graded nanorods at the surface. These nanorods function as a buffer to diminish abrupt stress from the high Ni-content bulk during the H2 → H3 phase transition by suppressing crack propagation to preserve particle coherency, enabling reversibility of the cathode particle. Notably, we show that electrolyte infiltration into the reactive Ni-rich bulk and subsequent formation of the electrically insulating rock-salt nanostructure (NiO) along the cracks are prevented, thereby minimizing impedance increase during long-term cycling. Furthermore, the increased Mn concentration at the outer surface of the nanorods also enhances the thermal stability by delaying the layered to rock-salt phase transition on the surface.