Lithium iron phosphate modification technology

1、Synthesis process

The performance of LiFePO4 cathode material depends to some extent on the morphology, particle size, 

and atomic arrangement of the material, so the preparation method is particularly important. 

At present, there are mainly solid-phase and liquid-phase methods, 

among which solid-phase methods include high-temperature solid-phase reaction method,

 carbon thermal reduction method, microwave synthesis method, and pulsed laser deposition method; 

Liquid phase method includes sol gel method, hydrothermal method, precipitation method and solvothermal method.

1） High-temperature solid-phase method

① Lithium carbonate and lithium hydroxide are used as lithium sources, while ferrous oxalate, ferrous oxalate, iron oxide, and iron phosphate are used as iron sources. 

The phosphate ions mainly come from ammonium dihydrogen phosphate.

②The typical process flow is: after ball milling and drying the raw materials,

 they are heated to a certain temperature in an inert or reducing atmosphere in a muffle furnace or tube furnace at a certain heating rate, reacted for a period of time, and then cooled.

③ The advantages of high-temperature solid-phase method are simple process and easy industrialization,

 but the product particle size is difficult to control, uneven distribution, irregular morphology, and requires the use of inert gas protection during the synthesis process.

2） Carbon thermal reduction method

1) This method is an improvement of the high-temperature solid-phase method, 

which directly uses high valent iron oxides such as Fe2O3, LiH2PO4, and carbon powder as raw materials, mixes them in a stoichiometric ratio, 

and sinter them at 700 ℃ for a period of time in an argon atmosphere in a box type sintering furnace, and then naturally cools them to room temperature.

2) The initial charge and discharge capacity of the experimental battery made using this method is 151mAh/g. 

This method is currently being applied by a few companies. 

Due to its simple and controllable production process and the use of one-time sintering, it provides another way for LiFePO4 to move towards industrialization.

3) The material prepared by this method has lower capacity and rate performance compared to traditional high-temperature solid-phase methods.

3）Hydrothermal synthesis method

1) Synthesis of LiFePO4 using Na2HPO4 and FeCl3, followed by hydrothermal synthesis of LiFePO4 with CH3COOLi.

2) Compared with the high-temperature solid-phase method,

 the hydrothermal synthesis method has a lower temperature of about 150-200 degrees, 

a reaction time of only about 1/5 of the solid-phase reaction, and can directly obtain lithium iron phosphate without the need for inert gas. 

The product has the advantages of small grain size and uniform phase, making it particularly suitable for the field of high power discharge. 

However, this synthesis method is prone to Fe dislocation in the formation of olivine structure, which affects the electrochemical performance. 

Additionally, the hydrothermal method requires high-temperature and high-pressure equipment, making industrial production more difficult.

4）Liquid-phase co precipitation method

The raw materials of this method are evenly dispersed, and the precursor can be synthesized under low temperature conditions;

 Add LiOH to a mixed solution of (NH4) 2Fe (SO4) and H3PO4 to obtain a co precipitate.

 After filtration and washing, perform heat treatment under an inert atmosphere to obtain LiFePO4; 

The product exhibits good cycling stability, and Japanese companies have adopted this technology route, but due to patent issues, it has not yet been widely applied.

5） Atomization pyrolysis method

The atomization pyrolysis method is mainly used for synthesizing precursors.

 The raw materials and dispersants are stirred at high speed to form a slurry, 

and then subjected to pyrolysis reaction in an atomizing drying device to obtain the precursor, which is then burned to obtain the product.

6) Oxidation-reduction method

This method can obtain electrochemically excellent nanoscale lithium iron phosphate powder, 

but its process is complex and cannot be mass-produced, only suitable for laboratory research.

In addition, there are emulsion drying method, microwave sintering method and sol gel method.

2、Modification technology

LiFePO4 has the problems of low ion diffusion rate and poor conductivity, which greatly affect its rate performance and low-temperature performance.

Since Good enough conducted research on LiFePO4 cathode materials in 1997, 

numerous researchers have conducted in-depth and extensive exploration of modification strategies for LiFePO4 materials. 

So far, the modification methods of LiFePO4 mainly include ion doping, surface coating, morphology control, and adding lithium supplementation materials.

1）Ion doping

Ion doping mainly refers to the doping of certain highly conductive metal ions into the LiFePO4 lattice coated with carbon layers,

 in order to reduce the resistance of Li+diffusion along one-dimensional paths and improve the cycling and rate performance of LiFePO4 materials. 

According to the position occupied by doping ions, LiFePO4 doping modification can be divided into Li site doping,

 Fe site doping, O site doping, and Li and Fe site co doping. 

According to the type and quantity of doping ions, it can be divided into single ion doping, double ion doping, and multi ion doping.

2） Surface coating

LiFePO4 has extremely poor conductivity. By coating the material surface with structurally stable and high-performance conductive/ion conductive materials,

 the electronic and ion conductivity between LiFePO4 material particles can be improved.

 LiFePO4 coating modification can control particle size, reduce resistance during Li+migration, improve the overall electronic conductivity and ion transport rate of the material, 

and further enhance the rate and low-temperature performance of the material.

The main types of coating agents include carbon materials, metal or metal oxide materials, and ion conductive materials.

① Carbon material coating: Coating LiFePO4 material with conductive substances is an important measure to improve its rate and low-temperature performance, 

among which carbon material is the simplest and cheapest excellent material. Carbon coating is generally divided into two types. 

One is in-situ coating, which involves adding a carbon source during the preparation of LiFePO4 to fully mix the material at the molecular angle, and then calcining it.

 Another type of non in situ coating is to prepare the precursor first, then mix it with carbon and lithium sources, and finally calcine it.

Carbon coating mainly has the following functions: carbon can act as a reducing agent to prevent the oxidation of Fe2+in LiFePO4 materials; 

Carbon coating can improve the electrical contact between LiFePO4 materials and enhance their conductivity; 

Carbon coating to some extent hinders direct contact between particles and can effectively inhibit particle growth and enlargement.

② Metals and metal oxides: Many researchers have also studied and prepared LiFePO4 cathode materials modified with metal or metal oxides and carbon composites. 

Metal material coatings have the following advantages: 

introducing metal coatings into LiFePO4 can fix the three-dimensional electronic transition network and maintain the integrity of the material structure;

 Metal coating materials can prevent the continuous growth of LiFePO4, control particle size, and shorten the transmission distance of Li+; 

The conductivity of metal coated materials is significantly improved, and the tap density is also increased.

But the metal coating has oxidation problems, and the introduced metals are generally precious metals, which are not suitable for large-scale production.

 The metal oxide coating strategy can also improve the electrochemical performance of LiFePO4 cathode materials.

 Its cost is relatively low, but the conductivity of the metal oxide coating layer is not as high as that of the metal coating layer,

 so further improvements are needed in these strategies and the coating agents used in the future.

③ Ionic conductive materials: Ionic conductive materials have high ionic conductivity and Li+storage capacity,

 which can provide additional Li+for the battery during charging and discharging. 

In addition, the introduction of ion conductive materials can effectively suppress Li/Fe anti site defects and improve the electrochemical performance of LiFePO4 cathode materials.

 Ionic conductor material coating modification of LiFePO4 can effectively improve the electrochemical performance of positive electrode materials due to its excellent ion conductivity, 

while also improving the defects of low tap density and lack of ion conductivity caused by carbon coating.

 It is a very promising method for coating modification of lithium iron phosphate materials.

3） Shape control

The preparation of nanoscale lithium iron phosphate helps to shorten the diffusion path of lithium ions and improve the discharge performance of the material at high currents. 

By preparing nanoscale particles, the contact area with the electrolyte is increased, the active sites for reaction are increased, and the specific surface area of the material is increased. 

As the particle size decreases, the potential value of lithium iron phosphate increases, the electrode polarization decreases, and the cyclic reversibility of the material will be improved.

4） Add lithium supplement material

During the first charging process of LiFePO4 batteries, 

approximately 5% to 20% of lithium in the positive electrode material is consumed due to the formation of a solid electrolyte interface (SEI) on the negative electrode surface, 

resulting in low first cycle Coulombic efficiency and significant irreversible capacity loss. 

To solve the above problems, lithium supplement materials can be added to the positive electrode material of lithium iron phosphate. 

During the charging process of the battery, the lithium supplement materials decompose and release excess lithium, 

compensating for the irreversible lithium loss caused by the formation of SEI film on the negative electrode.

 Lithium replenishment materials usually have the characteristics of strong lithium replenishment ability, easy synthesis, strong stability, and low cost. 

Common lithium iron phosphate positive electrode replenishment materials include Li2O, LiF, Li3N, and Li2S.

At present, the ways to supplement active lithium in electrode materials are mainly divided into two categories: 

positive electrode lithium supplementation and negative electrode lithium supplementation.

 In commercial production, positive electrode lithium replenishment technology is a very promising development direction for LiFePO4 modification.

 In the future, more suitable positive electrode lithium replenishment additives can be developed to solve the problem of low initial coulombic efficiency 

and discharge specific capacity of positive electrode materials in practical applications.