As one of the remarkable cathode materials for lithium-ion batteries, the disordered rocksalt Li-excess Li_1.3Nb_0.3Mn_0.4O₂ (LNMO) revealed attractive behaviors. Herein, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) for the as-prepared LNMO were reported. The symmetrical bending and stretching vibrations of Li-O and Mn-O appeared in the range 480-700 cm^-1. The reaction enthalpy ( ) is found to be 306.78 J/g. The dielectric properties were also studied at various frequencies and temperatures. At low frequencies, the LNMO has high permittivity as well as losses. The AC conductivity and the dielectric modulus at various frequencies and temperatures were also discussed. Different dielectric parameters were given for the LNMO sample. All general outcome results of LNFO emphasize the importance of such lithium-ion battery cathode material in the applications.