A different route is proposed to reduce the Mn³⁺ content in LiMn₂O₄ spinel. Manganese ions are responsible for both the Jahn–Teller distortion and the dissolution of Mn in the electrolyte, the main causes of low cycling stability and limited rate capability. For this, Cl₂ was used as an oxidising agent to promote partial oxidation of Mn³⁺ without destroying the spinel structure. The X-ray diffraction (XRD) pattern recorded with Mo K_α1 confirmed the formation of a spinel defective in Mn, with Li_1.06Mn_1.94O₄ stoichiometry. The Mn vacancies were occupied by excess Li together with the presence of a layered Li₂MnO₃ phase as an impurity. Combining SEM images and EDX spectra, it was possible to differentiate both components as independent phases with different morphology. The spinel has a remarkable rate capability in the half-cell configuration from 1C to 50C, delivering an average capacity of 130 to 96 mA h g⁻¹, respectively. Also, it has excellent cycling stability, as revealed by the capacity retention values, between 98–96% from 0.5 (500 cycles) to 10C (1000 cycles), maintaining high capacity values, from 138 to 127 mA h g⁻¹, respectively. Furthermore, it can deliver high power, 26 196 W kg⁻¹, with a high energy of 322 W h kg⁻¹. To our knowledge, our spinel delivers the highest specific energy for lower power values reported so far. These excellent properties can be attributed to both its Mn deficiency, which decreases Mn³⁺, and the good transport properties obtained from CV and EIS techniques. The first, together with the high crystallinity and micrometric particle size, would mitigate the dissolution of Mn and enhance the structure stability. On the other hand, the high rates that the electrode supports can be ascribed to the high values of Li⁺ diffusion coefficient averaged to the different stages of the process, around 4 × 10⁻¹⁰ cm² s⁻¹. Spinel also provides satisfactory performance in full cells using MCMB as an anode and a positive/negative ratio close to 1 without the need for a prelithiation process. Thus, for power values of 10 567 W kg⁻¹, it supplied an energy of 196 W h kg⁻¹.