Page 393 - Handbook of Battery Materials

364 12 Lithium Intercalation Cathode Materials for Lithium-Ion Batteries

In the initial work, fewer than 0.7 lithium ions were extracted per formula unit
of LiFePO 4 even at very low current densities, which corresponds to a reversible
capacity of <120 mAh g −1 [94]. The lithium extraction/insertion occurred via a
two-phase mechanism with LiFePO 4 and FePO 4 as end members without much
solid solubility. The limitation in capacity was attributed to the diffusion-limited
transfer of lithium across the two-phase interface and poor electronic conductivity
due to the corner-shared FeO 6 octahedra. Nevertheless, because Fe is abundant,
inexpensive, and environmentally benign, olivine LiFePO 4 attracted immense
interest as a potential cathode. Recognizing that the limited reversible capacity
and low rate capability may be linked to the poor electronic conductivity, re-
searchers investigated the possibility of improving the electronic conductivity by
coating the LiFePO 4 powder with conductive carbon [97]. However, LiFePO 4 is a
one-dimensional lithium-ion conductor with the lithium-ion diffusion occurring
along edge-shared LiO 6 chains (b axis). It was therefore suggested that both intimate
contact with conductive carbon and particle size minimization are necessary to
optimize the electrochemical performance [98, 99]. Consequently, with a reduction
in particle size and coating with conductive carbon, reversible capacity values of
∼160 mAh g −1 were realized. Also, doping of LiFePO 4 with supervalent cations
5+
4+
4+
like Ti ,Zr ,Nb , and organometallic precursors of the dopants was reported
8
to increase the electronic conductivity by a factor of 10 [100]. Although this report
attracted significant interest, subsequent investigations have suggested that the
formation of a percolating nano-network of metallic iron phosphides may play a
role in enhancing electronic conductivity [101].
Recognition of the importance of both the decrease in particle size and improve-
ment in electronic conductivity has generated a flurry of activity in investigations
into the solution-based synthesis of LiFePO 4 to minimize the particle size and coat-
ing the LiFePO 4 particles with conductive species such as carbon and conducting
polymers [102–111]. Of these investigations, microwave-assisted hydrothermal and
solvothermal approaches are appealing as they offer single-crystal LiFePO 4 with
◦
high crystallinity at significantly low temperatures of 230–300 C within a relatively
short reaction time of 5–15 min [108–111]. The products obtained by such ap-
proaches exhibit unique nanorod-like morphologies with excellent crystallinity (see
the TEM fringe pattern), as seen in Figure 12.19, with the easy lithium diffusion
direction (b axis) perpendicular to the long axis, which is beneficial for achieving
high rate capability. In addition, the width and length of the nanorods depend on
the synthesis conditions (e.g., reactant concentration), which could help to tune the
rate capability and volumetric energy density.
While decreasing the particle size to the nanometer level has been suc-
cessful in reducing the diffusion length of lithium-ions and overcoming the
lithium-ion transport limitations in LiFePO 4 , the pristine LiFePO 4 nanorods still
suffer from poor electronic conductivity. In this context, addition of conducting
polymers and nano-networking with multi-walled carbon nanotubes (MWCNTs)
(Figures 12.20–12.22) have been found to offer significantly improved electro-
chemical performances [108, 110]. Figure 12.22 compares the discharge capacities