6.3 The Theory and Research Progress of Sodium-Ion Batteries
151
Therefore, the structure of Na 2 Ti 3 O 7 is very stable. As another oxide of titanium,
the large-diameter amorphous TiO 2 nanowire can also releasably resorb sodium and
reversibly deintercalate 0.4Na at a voltage platform of 1.5 V (vs. Na
+ /Na), in other
words, it has a reversible specific capacity of 150 mAh/g.
Li 4 Ti 5 O 12 in LIBs is superior to its “zero strain” characteristics, and it also has
a higher voltage, this can avoid the production of SEI film, so it has the best cycle
stability, which is most widely used as anode material. Although it is a spinel type
material, its lithium insertion mechanism is that lithium ions intercalate into excess
oxygen octahedral voids, and thus can also be applied in SIBs.
P2 materials have received extensive concern as cathode materials in SIBs, but
due to the lack of sodium in the initial composition of the material, the first Coulomb
efficiency is affected (above 100%), which limits the application of the material
in full cells. On the other hand, this initial sodium vacancy can also be used as a
place for the sodium storage of the negative electrode. Based on the low voltage
potential of Ti
3+ /Ti
4+ , the P2 type titanium-based material can also be used as anode
electrodes in SIBs. For example, P2 − Na 0.66 [Li 0.22 Ti 0.78 ] has a specific capacity
of 116 mAh/g with a 75% capacity retention rate after 1200 cycles. The Coulomb
efficiency is nearly 100% during the entire charge–discharge cycle. Further in situ
XRD testing shows that the material is removed. Embedded sodium is only 0.77%
of the volume change, and it can also be a “zero strain” anode material, which has a
certain application prospect.
NaTi 2 (PO 4 ) 3 is a titanium-based NASICON compound with a theoretical capacity
of 133 mAh/g, which has attracted attention because of its low cost and environmental
friendliness. At the same time, due to its high sodium-insertion potential (~2.1 V)
and the stability of the water environment, this material is also used in the negative
electrode materials of new generation aqueous sodium-ion batteries.
6.3.2.3 Metal and Alloy Anode Materials
Metals can store sodium ions by forming an alloy with sodium ions. The alloy as
anode materials in SIBs can provide a very high specific capacity, but it can also bring
about a corresponding problem of large volume expansion. This is in contrast to the
anodes in LIBs. In fact, the effect of volume change is even more pronounced than
that of alloy anode materials in LIBs because of the larger radius of sodium ions.
Cheerier et al. found that the same volume expansion is found by calculating the
relationship between volume energy density and volumetric expansion in the alloy.
Under the conditions, the volumetric energy density of the sodium alloy compound
is half that of the lithium alloy compound, which means that if it is desired to reach
the specific capacity (~360 mAh/g) of the graphite anodes in LIBs, the sodium alloy
compound needs to have 150% volume expansion rate.
In addition to traditional metallic materials, black phosphorus also has the alloying
reaction characteristic of sodium inlay. NaP 7 , NaP, Na 2 P, and Na 3 P are formed in
sequence during charge and discharge. The insertion of up to three sodium ions can
151
Therefore, the structure of Na 2 Ti 3 O 7 is very stable. As another oxide of titanium,
the large-diameter amorphous TiO 2 nanowire can also releasably resorb sodium and
reversibly deintercalate 0.4Na at a voltage platform of 1.5 V (vs. Na
+ /Na), in other
words, it has a reversible specific capacity of 150 mAh/g.
Li 4 Ti 5 O 12 in LIBs is superior to its “zero strain” characteristics, and it also has
a higher voltage, this can avoid the production of SEI film, so it has the best cycle
stability, which is most widely used as anode material. Although it is a spinel type
material, its lithium insertion mechanism is that lithium ions intercalate into excess
oxygen octahedral voids, and thus can also be applied in SIBs.
P2 materials have received extensive concern as cathode materials in SIBs, but
due to the lack of sodium in the initial composition of the material, the first Coulomb
efficiency is affected (above 100%), which limits the application of the material
in full cells. On the other hand, this initial sodium vacancy can also be used as a
place for the sodium storage of the negative electrode. Based on the low voltage
potential of Ti
3+ /Ti
4+ , the P2 type titanium-based material can also be used as anode
electrodes in SIBs. For example, P2 − Na 0.66 [Li 0.22 Ti 0.78 ] has a specific capacity
of 116 mAh/g with a 75% capacity retention rate after 1200 cycles. The Coulomb
efficiency is nearly 100% during the entire charge–discharge cycle. Further in situ
XRD testing shows that the material is removed. Embedded sodium is only 0.77%
of the volume change, and it can also be a “zero strain” anode material, which has a
certain application prospect.
NaTi 2 (PO 4 ) 3 is a titanium-based NASICON compound with a theoretical capacity
of 133 mAh/g, which has attracted attention because of its low cost and environmental
friendliness. At the same time, due to its high sodium-insertion potential (~2.1 V)
and the stability of the water environment, this material is also used in the negative
electrode materials of new generation aqueous sodium-ion batteries.
6.3.2.3 Metal and Alloy Anode Materials
Metals can store sodium ions by forming an alloy with sodium ions. The alloy as
anode materials in SIBs can provide a very high specific capacity, but it can also bring
about a corresponding problem of large volume expansion. This is in contrast to the
anodes in LIBs. In fact, the effect of volume change is even more pronounced than
that of alloy anode materials in LIBs because of the larger radius of sodium ions.
Cheerier et al. found that the same volume expansion is found by calculating the
relationship between volume energy density and volumetric expansion in the alloy.
Under the conditions, the volumetric energy density of the sodium alloy compound
is half that of the lithium alloy compound, which means that if it is desired to reach
the specific capacity (~360 mAh/g) of the graphite anodes in LIBs, the sodium alloy
compound needs to have 150% volume expansion rate.
In addition to traditional metallic materials, black phosphorus also has the alloying
reaction characteristic of sodium inlay. NaP 7 , NaP, Na 2 P, and Na 3 P are formed in
sequence during charge and discharge. The insertion of up to three sodium ions can