Effect of lithium extraction on the stabilities, electrochemical properties, and bonding characteristics of LiFePO4 cathode materials: A first-principles investigation

The impact of lithium extraction on the structural stabilities, electronic structures, bonding characteristics, and electrochemical performances of LiFePO4 compound was investigated by first-principles technique. The results demonstrated that the partition scheme of electrons not only affects the calculated atomic charges but also the magnetic properties. In FePO4 and LiFePO4 compounds, all Fe ions take high spin arrangements and have large magnetic moments (MMs), while the MMs of other ions are very small. The magnetisms of LixFePO4 compounds are mainly originated form Fe ions. It was found that the changes in d band electrons of the transition metals do play an important role in determining the voltage of a battery (versus Li/Li+). Furthermore, the variations in d band electrons also provide us a method to control the density of states (DOS) and carrier concentration at the Fermi energy. Our calculations confirmed that the substitution of Fe by Co and Ni ions leads to a voltage increase by about 0.70 V and 1.23 V respectively. According to the bond populations, it can be identified that strong covalent bonds are formed between O and P ions. The P–O bonds are much stronger than Fe–O ones. The partial DOSs further revealed that the covalent bonds in LixFePO4 are derived from the orbital overlaps between O2s,2p and P3s,3p states, and the overlap between Fe3d and O2p states. Such covalent bonds are of particularly importance for the excellent thermodynamic stabilities of the two-ends structures of LixFePO4.