The spinel of nominal composition Li4Mn5O12 has been prepared by the wet chemistry technique and characterized by structural analysis (X-ray diffractometry, Raman spectroscopy), magnetic properties including electron spin resonance (ESR), and thermal properties (specific heat). Both structural and magnetic analyses reveal that the physical and electrochemical properties are importantly influenced by the presence of Li2MnO3 impurity phase. The presence of this additional phase reduces the average oxidation state of manganese according to a disproportionation reaction so that the final composition of the spinel phase is Li3.8Mn5.2O12 (or Li1.27Mn1.73O4). Because of the important geometric frustration of the magnetic interactions and dilution of the antiferromagnetic interactions, no magnetic ordering is observed in the temperature range investigated. The anomalous magnetic properties, including the Dysonian profile of the ESR line, show that the material is metallic. The Sommerfeld constant is 308 mJ/(K2) per mole of Li1.27Mn1.73O4, which shows that this material belongs to the class of heavy-fermion systems like LiV2O4 or LiTi2O4. These heavy fermions are the minority-spin tgV electrons of Mn3+ ions that have an reduced effective masse m/m0 ) 467. The electrochemical properties show that the specific capacity of Li3.8Mn5.2O12 at charge rate 1C is 163 mA h/g, a large value that is possibility due to the to insert Li up to the composition Li6.8Mn5.2O12. The origin of the disproportionation, and the flat voltage in the lithiation process, are discussed in the framework of the stability of the Mott insulator phase with respect to the metallic phase.