Structural and Electronic Properties of Cu2+ Doped LiMn2O4 Cathode Material for Rechargeable Lithium Ion Battery: A DFT Study

Spinel LiMn2O4 offers a potentially attractive alternative to the commercialized LiCoO2 because of its low cost, non-toxicity, good rate performance and superior safety, but experiences severe capacity fading as a result of Jahn-Teller cubic phase transition to unstable tetragonal phase especially when operated at elevated temperature (>55oC). Among the various techniques carried out to overcome these setbacks is doping with cations to raise the oxidation state of Mn thereby decreasing the amount of unstable Mn3+. This study focuses on the investigation of the structural and electronic properties of Cu2+ doped LiMn2O4 using ab-initio computational method. The structural properties of both un-doped LiMn2O4 and Cu2+ doped LiMn2O4 were collectively obtained using both generalized gradient approximation (GGA) employing pseudo-potentials, plane wave basis sets and XcryDen. From the electronic properties of undoped LiMn2O4, it was revealed that in LiMn2O4, only Mn3+ ions contribute to conduction, even though Mn3+ ions and Mn4+ ions exist together. Increasing the number of Mn3+ brings it a unique challenge of instability. Due to this instability, avoiding the Mn3+ state of manganese prevents Jahn-teller distortion, resulting in a reduced phase transition. Based on the results from the first-principles calculation of Cu2+ doped LiMn2O4, the Cu valence in doped spinel LiCu0.25Mn0.75O4 is Cu2+ at a fully lithiated state. When half of the Li ions are extracted, the Cu2+ ions oxidized to Cu3+ ions. Since Cu2+ is a di-valent element, the dopant act as electrochemically active centres for valence change, leaving two trivalent ions to only take on +4 oxidation states stably, thus increasing Mn-O bonding strength which ultimately suppressed Jahn-Teller distortions. However, previous studies show that Cu2+ substitution shrinks the lattice because the ionic radius of Cu2+ is smaller than that of the host Mn3+ ions, thereby the reversible capacity of Cu-doped spinel may be reduced as Cu ions can only provide one electron per ion.