Olivine-Based Cathode Materials
Résumé
The lithium insertion compounds built with polyanionic groups such as (SO4)2−,
(PO4)3−, (P2O7)4−, (MoO4)2− or (WO4)2− are considered as potential positive
electrode materials for use in lithium rechargeable batteries [1, 2]. Yet in this
family, olivine phosphate and Nasicon-like frameworks are currently the subject of
many investigations. In particular, LiFePO4 (LFP) has received a great deal of
interest because this cathode material realizes the highest capacity (≈170 mAh g−1)
at moderate current densities [3]. In addition, it presents several advantages with
regard to low cost, non-toxicity, tolerance on abuse, and high safety, which are
determinant with respect to cobalt-oxide-based materials for large-scaled applications
such as hybrid electric vehicles (HEV). Nevertheless, the bulk electronic
conductivity of olivine is quite low, which may result in losses in the specific
capacity during high-rate discharge. To increase the electrochemical performance, it
is a common practice in the production of Li-ion battery cathodes to manipulate the
active material by (i) adding carbon additives to a olivine matrix [1, 4], (ii) surface
coating of particles with thin layers of carbon [5–7] or reducing the particle size [8].
Still, there have been numerous efforts through the years to decrease the size of
the particles from a few microns to this “nano” range, for several reasons. One is the
increase of the effective contact area of the powder with the electrolyte. A larger
effective contact surface with the electrolyte means a greater probability to drain Li+