Lithium-ion batteries are increasingly used in mobile phones, notebook
computers, digital cameras, electric vehicles, etc. due to their advantages such
as high energy density, low self-discharge current, high safety, large current
charge and discharge, high number of cycles, and long life. Automobiles, special
aerospace, special equipment and other fields.
LiFeP04 basic performance
LiFeP04 basic structure
LiFeP04 charging and discharging principle
The charging and discharging process of lithium iron phosphate battery is
carried out between the two phases of LiFePO4 and FePO4, as shown in Figure 2.
Its specific mechanism: During the charging and discharging process, Li+ is
inserted and detached back and forth between the two electrodes. During
charging, Li+ comes out of the positive electrode and migrates to the crystal
surface. Under the action of the electric field force, it passes through the
electrolyte and then through the separator. It migrates through the electrolyte
to the surface of the negative electrode crystal and is embedded in the negative
electrode lattice. The negative electrode is in a lithium-rich state. At the
same time, electrons flow to the positive electrode through the positive
conductor, flow to the current collector of the negative electrode through the
external circuit, and then flow to the negative electrode through the negative
conductor, so that the charge of the negative electrode reaches balance. After
the lithium ions are released from the positive electrode, lithium iron
phosphate is converted into iron phosphate; while the discharge process is
reversed.
Regarding the electrochemical reaction during the charge and discharge
process of lithium iron phosphate, there are three main classic models: the
interface migration model proposed by Padhi, the radial model and the Mosaic
model proposed by Andersson. Among them, the interface migration model is shown
in Figure 3.
When Anderson et al. used neutron powder diffraction to study the first
capacity loss of lithium iron phosphate materials, they believed that the
remaining unreacted LiFePO4 and FePO4 were the cause of the capacity loss. From
this, he proposed a radial model, as shown in Figure 4(a). However, since few
reactions occur uniformly, a mosaic model is proposed based on this, as shown in
Figure 4(b).
LiFeP04 modification
Due to the poor conductivity and low lithium ion diffusion coefficient of
the lithium iron phosphate cathode material itself, domestic and foreign
researchers have conducted a lot of research in these aspects and achieved some
good results. Its modification research is mainly in three aspects: doping
method, coating method and material nanotechnology.
Doping method
The doping method mainly refers to doping some metal ions with good
conductivity into the cation positions in the lithium iron phosphate crystal
lattice, changing the size of the crystal grains, causing lattice defects of the
material, thereby improving the conductivity of electrons in the crystal grains
and the lithium The diffusion rate of ions is thereby improved to improve the
performance of LiFePO4 materials. At present, the doped metal ions mainly
include Ti4+, CO2+, Zn2+, Mn2+, La2+, V3+, and Mg2+.
coating method
Coating carbon on the surface of LiFePO4 materials is an effective method
to improve electronic conductivity. Carbon can play the following roles: ①
inhibit the growth of LiFePO4 grains and increase the specific surface area; ②
enhance inter-particle and surface electrons The conductivity reduces the
occurrence of battery polarization; ③ acts as a reducing agent to avoid the
generation of Fe and improves product purity; ④ acts as a nucleating agent to
reduce the particle size of the product; ⑤ adsorbs and maintains the stability
of the electrolyte.
Nanomaterials
Compared with limitations in conductivity, the diffusion of lithium ions in
lithium iron phosphate materials is the most important and decisive control step
for battery discharge. Due to the olivine structure of LiFePO4, the diffusion
channel of lithium ions is one-dimensional, so the particle size can be reduced
to shorten the lithium ion diffusion path, thereby improving the lithium ion
diffusion rate.
The main advantages of nanomaterials are: ① Nanomaterials have a high
specific surface area, which increases the reaction interface and can provide
more diffusion channels; ② The material has many defects and micropores, and the
theoretical lithium storage capacity is high; ③ Due to the small size of
nano-ions The size effect reduces the insertion and extraction depth and stroke
of lithium ions; ④ The gaps between the aggregated nanoparticles relieve the
stress of lithium ions during deintercalation and improves cycle life; ⑤ The
superplasticity and creep properties of nanomaterials make them It has strong
volume change tolerance and can lower the glass transition temperature of the
polymer electrolyte.
Conclusion
The three modification methods of ion doping, coating, and material
nanonization have greatly improved the shortcomings of lithium iron phosphate
cathode materials in terms of low conductivity, slow lithium ion diffusion rate,
and poor low-temperature discharge performance. Among them, ion doping changes
the particle size by doping ions with good conductivity, causing lattice defects
of the material, thereby improving the electronic conductivity of the material
and the diffusion rate of lithium ions; the coating is mainly carbon coating.
Suppressing the growth of LiFeP04 grains increases the specific surface area,
thereby enhancing the conductivity between particles and surface electrons; on
the one hand, the nanonization of the material increases the specific surface
area of the material, providing more diffusion channels for interface reactions,
and on the other hand On the one hand, it shortens the ion diffusion distance,
reduces the stress on lithium ions when they are deintercalated, and improves
the cycle life.
In addition, there are still some shortcomings in the modification of
lithium iron phosphate cathode materials. For example, there are still
differences in the conductivity and lithium ion diffusion rate of ion doping to
improve the material; the preparation process and production cost of
nanomaterials are relatively high; in addition, in addition to considering In
addition to feasibility studies under laboratory conditions, large-scale
industrial production requirements must also be considered, all of which require
further research. Therefore, comprehensively improving the comprehensive
performance of lithium iron phosphate through the above methods is still one of
the main development directions of current and future research and applications
in this field.
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