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6 Nanomaterials for Batteries
6.3 The Theory and Research Progress of Sodium-Ion
Batteries
6.3.1 Cathode Material
Sodium-ion batteries (SIBs), superior to rich reserve and low cost of sodium, have
been of great concern in energy storage systems. However, the primary hindrance—
large ionic radius of sodium and its slow electrochemical kinetics—make it difficult to
develop suitable and high-performance electrodes for SIBs, which is key to improving
the energy density of SIBs and facilitating their commercialization. Recently, a lot of
research into cathode materials for SIBs have been made, including transition metal
oxides, polyanions, organic materials and polymers, and amorphous materials.
6.3.1.1 The Transition Metal Oxides
Compared with other electrode materials, the transition metal oxide is attractive
because of their species materials, diversity, structure stability, and good electrochemical performances. In LIBs, layered oxide LiMO 2 (M is transition metal Co, Mn,
Fe, Ni, etc.) has received widespread attention and application due to the excellent
electrochemical properties. Similarly, in the sodium-ion batteries, a similar transition
metal oxide Na x MO 2 is also a research hotspot (Hamani et al. 2011; Pan et al. 2013;
Ma et al. 2011). The radius of sodium ion (1.02 Å) is larger than that of lithium ion
(0.76 Å), so the difference between radius enables them to occupy oxygen ions in the
crystal structure of the oxide electrode. In the transition metal oxide structure, lithium
ions occupy tetrahedral oxygen vacancies and octahedral oxygen vacancies, whereas
sodium ions are unable to occupy tetrahedral oxygen vacancies due to their large ionic
radius. Sodium transition metal mainly contains three different oxide structures: P2
type, P3 type, O3 type. The crystal structure of three types of materials is shown in
Fig. 6.6.
The type O 3 material is a typical a-NaFeO 2 structure. This structure of material is
widely used as positive electrodes in LIBs such as LiCoO 2 , Li-rich, ternary, and so on.
Similarly, type O3 material also received widespread attention in SBIs. For example,
LiCrO 2 cathode material in LIBs shows a very poor electrochemical performance,
but NaCrO 2 in SIBs has a reversible specific capacity of 120 mAh/g (0.5 Na). For
LiCrO 2 , Cr
4+ O 4
2− tetrahedrons are easily formed in the LiCrO 2 crystal lattice, thus
destroying the original layered crystal structure. For NaCrO 2 , the larger interlayer
distances will prevent the movement of Cr
4+ , thereby avoiding the formation of
tetrahedral coordination. The inert gas is used in the NaCrO 2 preparation process,
and Cr(III) is reduced hard. The material can be further modified by in situ carbon
coating, leaving more space for improvement in electrochemical performance (Kim
et al. 2012).
P2 and O3 are layered materials, while P2-type layered oxides tend to have higher
specific capacity and better cycling properties. For example, Kamaba’s research
6 Nanomaterials for Batteries
6.3 The Theory and Research Progress of Sodium-Ion
Batteries
6.3.1 Cathode Material
Sodium-ion batteries (SIBs), superior to rich reserve and low cost of sodium, have
been of great concern in energy storage systems. However, the primary hindrance—
large ionic radius of sodium and its slow electrochemical kinetics—make it difficult to
develop suitable and high-performance electrodes for SIBs, which is key to improving
the energy density of SIBs and facilitating their commercialization. Recently, a lot of
research into cathode materials for SIBs have been made, including transition metal
oxides, polyanions, organic materials and polymers, and amorphous materials.
6.3.1.1 The Transition Metal Oxides
Compared with other electrode materials, the transition metal oxide is attractive
because of their species materials, diversity, structure stability, and good electrochemical performances. In LIBs, layered oxide LiMO 2 (M is transition metal Co, Mn,
Fe, Ni, etc.) has received widespread attention and application due to the excellent
electrochemical properties. Similarly, in the sodium-ion batteries, a similar transition
metal oxide Na x MO 2 is also a research hotspot (Hamani et al. 2011; Pan et al. 2013;
Ma et al. 2011). The radius of sodium ion (1.02 Å) is larger than that of lithium ion
(0.76 Å), so the difference between radius enables them to occupy oxygen ions in the
crystal structure of the oxide electrode. In the transition metal oxide structure, lithium
ions occupy tetrahedral oxygen vacancies and octahedral oxygen vacancies, whereas
sodium ions are unable to occupy tetrahedral oxygen vacancies due to their large ionic
radius. Sodium transition metal mainly contains three different oxide structures: P2
type, P3 type, O3 type. The crystal structure of three types of materials is shown in
Fig. 6.6.
The type O 3 material is a typical a-NaFeO 2 structure. This structure of material is
widely used as positive electrodes in LIBs such as LiCoO 2 , Li-rich, ternary, and so on.
Similarly, type O3 material also received widespread attention in SBIs. For example,
LiCrO 2 cathode material in LIBs shows a very poor electrochemical performance,
but NaCrO 2 in SIBs has a reversible specific capacity of 120 mAh/g (0.5 Na). For
LiCrO 2 , Cr
4+ O 4
2− tetrahedrons are easily formed in the LiCrO 2 crystal lattice, thus
destroying the original layered crystal structure. For NaCrO 2 , the larger interlayer
distances will prevent the movement of Cr
4+ , thereby avoiding the formation of
tetrahedral coordination. The inert gas is used in the NaCrO 2 preparation process,
and Cr(III) is reduced hard. The material can be further modified by in situ carbon
coating, leaving more space for improvement in electrochemical performance (Kim
et al. 2012).
P2 and O3 are layered materials, while P2-type layered oxides tend to have higher
specific capacity and better cycling properties. For example, Kamaba’s research