Enhanced electrochemical performance of LiNiO₂ by B doping

The application markets for portable electronics, battery-operated electric vehicles, and large-scale energy-storage grids have been expanding rapidly for the past ten years, which has attracted massive attention to the investigation and development of batteries with high energy density, long cycle life, high safety, and low cost. A commonly used lithium-ion battery consists of intercalation-type materials, such as LiCoO₂ as cathode and graphite as an anode. Owing to technical difficulties, including high cost, low stability, and the poor safety of Li, the large-scale application of the high-energy Li anode is still premature. A more common strategy than the one mentioned above for improving the energy density of Li-ion batteries is to develop a cathode material with high specific capacity and low cost, such as LiNi_1–x–yCo_xMn_yO₂ (NCM) and LiNi_1–x–yCo_xAl_yO₂ (NCA). Among the NCMs and NCAs, Co is more expensive and less abundant than Ni, Mn, and Al. Presently, high-nickel, low-cobalt NCMs, and NCAs have attracted huge attention as suitable cathodes for both academic and industrial purposes. LNiO₂ can be regarded as the Ni content increasing to 100% for NMCs and NCAs, which stood as the “holy grail” of layered cathodes. This study aims to investigate the structural and electrochemical stability of LiNiO₂ and B-doped LiNiO₂. In this study, Ni(OH)₂ was synthesized by a coprecipitation method using a continuous stirred tank reactor (CSTR). LiNiO₂ and B-doped LiNiO₂ were synthesized by high-temperature solid-state sintering. The crystal structure, surface morphology, and electrochemical performance were investigated by X-ray diffraction (XRD), Rietveld refinement, scanning electron microscopy (SEM), constant current charge–discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). XRD and Rietveld refinement results indicate that B-doping could slightly increase the lattice parameters and unit cell volume due to the occupancy of B in the tetrahedral site. Meanwhile, the LiO₆ slab distance increases, consequently favoring the transportation of Li⁺ during (de)-intercalation. SEM images suggest that LiNiO₂ and B-doped LiNiO₂ consist of primary grains with a similar size, and the secondary particle in both samples has an average size of 10 µm. Long-term cycling data show that B-doping could improve capacity retention. The capacity retention at 40 mA·g⁻¹ is 77.5% for the B-doped sample, whereas a value of 66.6% is obtained for LiNiO₂. The dQ/dV vs V curves and EIS results suggest the suppression of impedance growth by B-doping.