Undoped, Cr-doped, and Nb-doped LiMn1.5Ni0.5O4 (LNMO) is synthesized via a PVP (polyvinylpyrrolidone)-combustion method by calcinating at 1000 °C for 6 h. SEM images show that the morphology of LNMO particles is affected by Cr and Nb doping. Cr doping results in sharper edges and corners and smaller particle size, and Nb doping leads to smoother edges and corners and more rounded and larger particles. The crystal and electron structure is investigated by XRD- and synchrotron-based soft X-ray absorption spectroscopy (sXAS). Cr doping and light Nb doping (LiNb0.02Ni0.49Mn1.49O4) improve the rate performance of LNMO. To explore the reason for rate-performance improvement, we conducted potential intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) tests. The Li+ chemical diffusion coefficient at different state of charge (SOC) is calculated and suggests that both Cr and light Nb doping speeds up Li+ diffusion in LNMO particles. The impedance spectra show that both RSEI and Rct are reduced by Cr and light Nb doping. The cycling performance is improved by Cr or Nb doping, and Cr doping increases both Coulombic efficiency and energy efficiency of LNMO at 1 C cycling. The LiCr0.1Ni0.45Mn1.45O4 remains at 94.1% capacity after 500 cycles at 1 C, and during the cycling, the Coulombic efficiency and energy efficiency remain at over 99.7% and 97.5%, respectively.