Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 400

Fig. 14 Crystal structures of some typical polyanion compounds: a NASICON-type
Na 3 V 2 (PO 4 ) 3 (R-3c), [ 27 ] b Na 3 V 2 (PO 4 ) 2 F 3 (P42/mnm), [ 28 ] c Na 2 FeP 2 O 7 projected along
the [011] direction (P1); [ 29 ], and d Na 2 FePO 4 F; [ 30 ] e cubic NaMnFe(CN) 6 [ 31 ]

Fluorophosphate materials commonly possess higher operating voltages than
the corresponding phosphate counterparts [ 28 , 37 , 38 ] due to the strong inductive
effect of the fluorine element. Na 3 V 2 (PO 4 ) 2 F 3 (space group of P42/mnm) was first
reported by Meins et al. [ 39 ]. In the structure, [V 2 O 8 F 3 ] bi-octahedral units are
linked by the fluorine atoms while the oxygen atoms are all interconnected through
the [PO 4 ] units, leading to the formation of the 3D formwork (Fig. 14 b). Na ions
have two types of sites in the tunnel: Na1 sites are fully occupied and Na2 sites are
half occupied [ 28 ]. The charge/discharge profiles of Na 3 V 2 (PO 4 ) 2 F 3 showed two
voltage plateaus at about 3.7 and 4.2 V (Fig. 17 a), with an average voltage of
~3.95 V [ 28 ]. The reversible capacity was ~110 mAh g -1 and showed almost no
decline after 30 cycles (Fig. 17 b). Thus, the energy density of Na 3 V 2 (PO 4 ) 2 F 3 is
comparable to that of LiFePO 4 and LiMn 2 O 4 used in Li-ion batteries. Another
high-voltage fluorophosphate is NaVPO 4 F[ 37 ]. Its structure is similar to that of
Na 3 Al 2 (PO 4 ) 3 F 2 (space group I4/mmm) [ 39 ]. NaVPO 4 F was found to have two
voltage plateaus at 3.0 and 4.0 V, respectively, and a reversible capacity of
~80 mAh g -1 during charge/discharge [ 28 ]. This value is much lower than the
theoretical capacity (142.5 mAh g -1 ). Hence, there should still be a very large
improvement margin by adopting different synthesis methods or structural

tailoring.
A new family with the general formula Na 3 V 2 O 2x (PO 4 ) 2 F 3-2x was also studied
for high voltage Na intercalation/deintercalation [ 38 ]. This type of compounds
have similar crystal structure with Na 3 V 2 (PO 4 ) 2 F 3 (tetragonal symmetry, P42/
mnm space group) (Fig. 14 b). Na 3 V 2 O 2x (PO 4 ) 2 F 3-2x showed two voltage plateaus