Role of structural defects in olivine cathodes

The high rate capability and structural stability of the olivine phosphates attracted a lot of interest as promising cathode materials for high energy density batteries. Alteration on these cathode materials, for instance, reducing particle size, conductive coating and metal ion doping were performed in order to improve the conductivity and to obtain high specific capacity. Wide range of transition metal ions with range of valences (M1+ to M5+) was successfully doped both in M1 site (Li) and M2 site (M) of olivine LiMPO4 (M = Fe, Mn, Co and Ni) cathode. The large charge difference between the doped supervalent ions and M cations at the M2 site, limited the use of supervalent cations in the olivines. However, the supervalent dopants (namely; Cr3+, Ti4+, Nb5+) are reported to be successfully substituted in the olivine with an improvement in electrical and ionic conductivity. The amount of defect can be reduced by low concentration of dopants, choosing suitable synthesis method and optimized reaction conditions. Charge compensation vacancies accomplished through aliovalent doping reduces the grain size and widens the Li+ migration path resulting in faster Li+ diffusion. However, the drastic improvement in electrical conductivity for the aliovalent doping is still unclear. Rather writing a lengthy standard review, this manuscript intends to describe briefly the lattice defects owing to metal ion doping and its influence in improving the cathode performance of the olivine phosphates. This gives a new approach in this field.