Modification technology for synthesis process of lithium manganese iron phosphate

1、Synthesis process

At present, there are many methods to prepare lithium manganese iron phosphate, 

such as solid phase synthesis method, liquid phase method (sol gel method, coprecipitation method, hydrothermal method/solvothermal method and spray drying method) and other methods. 

The following summarizes the research progress on the preparation methods of lithium manganese iron phosphate cathode materials.

①Solid-phase synthesis method

Solid phase synthesis method generally uses lithium compounds, iron manganese sources, and carbon sources as the main raw materials.

 The raw materials are mixed uniformly according to stoichiometry,

 and then protected gas (such as nitrogen, argon, etc.) is introduced for pretreatment at a lower temperature to pre decompose the raw materials. 

The finished product is then baked at high temperature.

The solid-phase synthesis method for preparing lithium manganese iron phosphate cathode materials is relatively easy to industrialize,

 but the final particle size is large and unevenly distributed, and the reaction process time is long and energy consumption is high.

②Sol gel method

The sol gel method is generally to dissolve all kinds of raw materials together in water or organic solvent, stir for a long time to make each component form a uniform sol,

 then raise the temperature to make the system gel, and then calcine at high temperature to get the finished product.

The sol gel method can obtain particles of uniform size, but the drying process is more complex, and the roasting also takes a long time, and the process conditions are difficult to control.

③Co precipitation method

The co precipitation method is commonly used to first synthesize manganese iron precursor,

 and then add lithium and phosphorus sources for ball milling and calcination to obtain the finished product. 

Compared with traditional solid-phase methods, co precipitation method is beneficial for the uniform distribution of Mn and Fe in particles,

 which is very important for improving energy density.

This method makes product quality easier to control, simple, and easy to achieve mass production.

④Hydrothermal method/solvothermal method

Hydrothermal/solvothermal methods are widely used to prepare nanoscale cathode materials. Common solvents include water, ethylene glycol, and water/ethanol.

However, there are many influencing factors of hydrothermal/solvothermal methods, 

such as solvent selection, pH, and even the order of reactant addition, which have a significant impact on the morphology of the products. 

Compared to solid-phase synthesis, hydrothermal method has the advantages of low cost and low energy consumption, 

while solvothermal method can control the particle size, but the reaction needs to be carried out under high temperature and high pressure, 

and the use of a large amount of organic solvents can cause serious environmental pollution, making it unsuitable for mass production.

⑤Spray drying

The spray drying method is to disperse the materials to be dried into mist like tiny droplets through the atomizer, 

conduct heat exchange with the hot air flow, evaporate a lot of water, and then obtain powder or fine granular finished products or semi-finished products.

The spray drying method has the advantages of short residence time of materials in the drying chamber, simple production process and convenient operation control. 

But it is difficult to control the particle size of the product, and it is difficult to obtain a uniform coating layer.

2、Modification technology

In response to the low conductivity of lithium manganese iron phosphate, the current main method of improving its conductivity is through modification.

 The mainstream modification methods include surface coating, ion doping, and microstructure control.

① Surface coating modification

Epi coating includes carbon coating, graphene coating, and metal oxide coating. The carbon coating methods include in-situ carbon coating and non in-situ carbon coating.

In situ carbon coating is a precursor formed by mixing a carbon source with other raw materials, drying, and other methods.

 Finally, high-temperature sintering is performed to decompose and dehydrogenate the carbon source, uniformly coating the surface of lithium manganese iron phosphate cathode material.

Non in situ carbon coating is achieved by mixing precursors obtained by drying other raw materials with carbon sources, 

and then sintering them at high temperatures to obtain LiMnxFe1-xPO 4/C composite materials. 

The materials obtained by this method are generally unevenly coated and have poor electrochemical performance. 

The morphology of the coated carbon generated by high-temperature sintering of carbon sources with different molecular weights and types varies, resulting in different coating effects.

Graphene coating is a process of in-situ growth on graphene during the preparation of lithium manganese iron phosphate nanocrystals. 

Graphene is further crosslinked through oxygen-containing groups of oxidized graphite (GO) as crosslinking sites, 

constructing an interconnected conductive network and greatly reducing the internal resistance of the entire electrode.

Metal oxide coating is a modification of LiMnxFe1-xPO4/C powder prepared by carbon coating using vanadium trioxide (V2O3). 

The addition of V2O3 can increase the carbonization degree of the carbon coating, thereby improving the conductivity of the material.

②Ion doping modification

Ion doping can be modified by doping with carbon, nickel ions, or trivalent vanadium ions.

 The use of different ions as dopants can cause Li or M defects in the lattice of lithium manganese iron phosphate, 

creating vacancies or changing the interatomic bond length in the material lattice,

 facilitating the movement of Li+in the lattice and improving the electrochemical performance of the material. 

Among samples doped with different elements, Ni2+is the most promising positive electrode material, providing excellent electrochemical performance in various aspects.

This result is attributed to several factors:

(1) appropriate morphology of the sample, 

(2) good connectivity provided by in-situ generated carbon, and

 (3) the presence of Ni2+and Fe2+in the olivine structure improves structural stress.

③ Microscopic morphology control modification

Microscopic morphology control can also be referred to as nanoscale modification.

 Phosphate positive electrode materials with different morphologies and particle sizes have varying degrees of impact on the electrochemical and physical properties of batteries.

The excellent electrochemical performance of nano materials is mainly achieved by controlling the microstructure of lithium iron phosphate particles. 

The smaller particles allow the electrolyte to be fully wetted, and the Li+transfer path in the material is shorter, 

thereby accelerating the Li+migration rate and improving the material's charge and discharge capacity and rate performance.