146
6 Nanomaterials for Batteries
6.3.1.2 The Polyanionic Cathode Materials A x M y
(XO m )
n−
z
The polyanionic compound electrode material represented by LiFePO 4 is
widely used in LIBs. In addition, this type of material has drawn much
attention in the research of sodium-ion battery. Due to the good structural
stability and strong anion inducibility of this type of material, their electrochemical platforms usually are higher and their cycle stability is better.
In recent years, the common research systems mainly include phosphate
(NaFePO 4 , Na 3 V 2 (PO 4 ) 3 ), pyrophosphate (Na 2 MP 2 O 7 , Na 4 M 3 (PO 4 ) 2 P 2 O 7 ), fluorophosphate (Na 2 MPO 4 F)(M = Fe, Co, Mn) and sulfate (Na 2 Fe 2 (SO 4 ) 3 ).
The olivine-type NaFePO 4 is concerned because this material has a high theoretical specific capacity of 154 mAh/g and a voltage platform of 2.9 V based on
Fe
2+ /Fe
3+ conversion. However, because of the large gap between the unit cell structures of NaFePO 4 and FePO 4 in charging and discharging process, the material is irreversible when undergoing phase transition. Meanwhile, the one-dimensional sodiumion channel and the lower conductivity affected the application of this structural
material in this structure.
The NASICON type Na 3 V 2 (PO 4 ) 3 is widely studied polyanionic compound material based on the V
3+ /V
4+ variant; this material has a high voltage platform of 3.7 V
and a theoretical specific capacity of 117 mAh/g, in the meantime, this structure
has three-dimensional sodium-ion transmission channels and high conductivity. On
the other hand, a similar Li 3 V 2 (PO 4 ) 3 has a wealth of research experience that can
be directly used for reference, which are conducive to Na 3 V 2 (PO 4 ) 3 as electrode
materials in SIB for research and development. Because of Na 3 V 2 (PO 4 ) 3, material
is prepared in an inert or reducing atmosphere, it can also be modified by means of
various carbon compounds and carbon coatings, in order to exhibit excellent electrochemical performance. At the same time, this material can also be modified by
cation replacement. In addition, there are reports in the literature that Na 3 V 2 (PO 4 ) 3
can also be applied as negative electrodes. Based on the V
2+ /V
3+ valence change, it
has a reversible specific capacity of approximately 100 mAh/g at both 0.3 and 1.6 V
platforms.
Pyrophosphate Na 2 MP 2 O 7 (M = Fe, Mn, Co) is a large material system with
various structural types, high structural stability, and high ionic conductivity. Based
on different transition metal cations and different preparation method, this type of
material has triclinic, orthorhombic, tetragonal, and other different crystal structures. Between Na 2 FeP 2 O 7 and Na 2 MnP 2 O 7 materials, the triclinic is a more stable
structure, but for the Na 2 CoP 2 O 7 , the orthorhombic is the most stable structure. Its
reversible specific capacity is 80 mAh/g at 3.0 V.
Fluorophosphate Na 2 MPO 4 F(M = Fe, Co, Mn) is another kind of polyanionic
compound system, which also has the characteristics of stable structure and carbon
composite modification. In this structural material, sodium ion occupies a pseudooctahedron with 2-dimensional sodium-ion channels. Among them, the carbon
composite Na 2 FePO 4 F material has a reversible specific capacity of 100 mAh/g
at 3.0 V. It has been reported in the literature that the graphene composite NaVPO 4 F
has a high specific capacity of 120.9 mAh/g with retention of 97.7% after 50 cycles.
6 Nanomaterials for Batteries
6.3.1.2 The Polyanionic Cathode Materials A x M y
(XO m )
n−
z
The polyanionic compound electrode material represented by LiFePO 4 is
widely used in LIBs. In addition, this type of material has drawn much
attention in the research of sodium-ion battery. Due to the good structural
stability and strong anion inducibility of this type of material, their electrochemical platforms usually are higher and their cycle stability is better.
In recent years, the common research systems mainly include phosphate
(NaFePO 4 , Na 3 V 2 (PO 4 ) 3 ), pyrophosphate (Na 2 MP 2 O 7 , Na 4 M 3 (PO 4 ) 2 P 2 O 7 ), fluorophosphate (Na 2 MPO 4 F)(M = Fe, Co, Mn) and sulfate (Na 2 Fe 2 (SO 4 ) 3 ).
The olivine-type NaFePO 4 is concerned because this material has a high theoretical specific capacity of 154 mAh/g and a voltage platform of 2.9 V based on
Fe
2+ /Fe
3+ conversion. However, because of the large gap between the unit cell structures of NaFePO 4 and FePO 4 in charging and discharging process, the material is irreversible when undergoing phase transition. Meanwhile, the one-dimensional sodiumion channel and the lower conductivity affected the application of this structural
material in this structure.
The NASICON type Na 3 V 2 (PO 4 ) 3 is widely studied polyanionic compound material based on the V
3+ /V
4+ variant; this material has a high voltage platform of 3.7 V
and a theoretical specific capacity of 117 mAh/g, in the meantime, this structure
has three-dimensional sodium-ion transmission channels and high conductivity. On
the other hand, a similar Li 3 V 2 (PO 4 ) 3 has a wealth of research experience that can
be directly used for reference, which are conducive to Na 3 V 2 (PO 4 ) 3 as electrode
materials in SIB for research and development. Because of Na 3 V 2 (PO 4 ) 3, material
is prepared in an inert or reducing atmosphere, it can also be modified by means of
various carbon compounds and carbon coatings, in order to exhibit excellent electrochemical performance. At the same time, this material can also be modified by
cation replacement. In addition, there are reports in the literature that Na 3 V 2 (PO 4 ) 3
can also be applied as negative electrodes. Based on the V
2+ /V
3+ valence change, it
has a reversible specific capacity of approximately 100 mAh/g at both 0.3 and 1.6 V
platforms.
Pyrophosphate Na 2 MP 2 O 7 (M = Fe, Mn, Co) is a large material system with
various structural types, high structural stability, and high ionic conductivity. Based
on different transition metal cations and different preparation method, this type of
material has triclinic, orthorhombic, tetragonal, and other different crystal structures. Between Na 2 FeP 2 O 7 and Na 2 MnP 2 O 7 materials, the triclinic is a more stable
structure, but for the Na 2 CoP 2 O 7 , the orthorhombic is the most stable structure. Its
reversible specific capacity is 80 mAh/g at 3.0 V.
Fluorophosphate Na 2 MPO 4 F(M = Fe, Co, Mn) is another kind of polyanionic
compound system, which also has the characteristics of stable structure and carbon
composite modification. In this structural material, sodium ion occupies a pseudooctahedron with 2-dimensional sodium-ion channels. Among them, the carbon
composite Na 2 FePO 4 F material has a reversible specific capacity of 100 mAh/g
at 3.0 V. It has been reported in the literature that the graphene composite NaVPO 4 F
has a high specific capacity of 120.9 mAh/g with retention of 97.7% after 50 cycles.