Electrospinning-technology-application-in-lithium-ion-batteries

Electrospinning technology – application in lithium-ion batteries

Electrospinning technology has the advantages of simple equipment, simple operation and relatively high production efficiency, and is widely used in the preparation of nanofibers. Nanofibers prepared by electrospinning technology, with large specific surface area and being soft, are widely used in catalysts, environmental protection, noise absorption, electronics, medical and lithium-ion batteries.
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Principles of electrospinning technology

As shown in the figure below, the equipment required for electrostatic spinning includes a high-voltage power supply, a collection device, a solution storage device, and a injection device. The principle is to use a high-voltage power supply to form a voltage difference between the solution and the collection device, so that the solution overcomes the surface tension of the liquid and forms a Taylor cone.

When the lithium ion battery voltage exceeds a certain value, liquid is sprayed from the end of the Taylor cone. The sprayed liquid is stretched along the direction of the electric field force, cooled and volatilized by the solvent, and finally forms nanofibers on the collection device. Under normal circumstances, the voltage required for electrostatic spinning is from a few thousand volts to tens of thousands of volts.

principles-of-electrospinning-technology
Principles of electrospinning technology

Factors influencing electrospinning technology

The application of electrospinning technology is affected by a series of process parameters, and the subtle change of process parameters will also have a certain impact on the morphology, structure and properties of the nanofibers. The main impacts fall into four categories:

  1. Solution properties, including viscoelasticity, conductivity, surface tension, etc.
  2. Electrospinning parameters, such as voltage, distance between spinneret needle and collection device and liquid propulsion speed, etc.
  3. Environmental parameters, such as temperature, air humidity, etc.
  4. Collection method.

Factors-influencing-electrospinning-technology

The properties of the solution, electrospinning parameters and environmental parameters will affect the volatilization rate and time of the solvent, the size of the electric field force, the splitting and curing of the fiber, so as to affect the size and uniformity of the fiber diameter. The collection method affects the orientation and shape of the fibers.

In recent years, nanofibers with various unique structures prepared by electrospinning technology have been widely used in the lithium battery industry. Electrospinning technology can be used for the construction of three key materials of lithium battery: cathode materials, anode materials and separators.

Electrospinning technology applications – cathode materials

The cathode is the main lithium ion (Li+) donor in lithium-ion batteries, and it is also a key factor affecting the transmission rate of lithium ions. The development of safe, economical, high-performance and high-capacity cathode materials can effectively promote the application of lithium-ion batteries.

At present, the specific discharge capacity of commercial anode materials (such as LiFePO4) is generally less than 200mAh/g, which is one of the bottlenecks restricting the growing demand for high energy density and low cost of lithium-ion batteries. Among various methods to improve the electrochemical properties of cathode materials, nano-coating and the control of nanostructure morphology by electrospinning technology have been proved to be effective methods.

The researchers successfully synthesized the flower-like Li1.2Ni0.17Co0.17Mn0.5O2 cathode material using high voltage electrospinning technology and heat treatment. This orderly porous flower-like morphology can promote the rapid diffusion of lithium ions, and the assembled battery can have a cyclic discharge capacity of up to 235mAh/g.

Electrospinning, a simple and feasible synthesis method, provides an effective way to design the ideal structure of the cathode of lithium-ion batteries.

Electrospinning-technology-applications-cathode-materials

Some researchers have also synthesized morphology controllable vanadium pentoxide (V2O5) nanostructures (such as porous V2O5 nanotubes, layered V2O5 nanofibers and single crystal V2O5 nanoribons) through the strategy of “electrospinning technology and subsequent annealing treatment”, which are used as high-performance cathode materials for lithium-ion batteries, showing high reversible capacity and excellent cycling performance.

The porous V2O5 nanotubes have a power density of 40.2kW/kg and an energy density of 201Wh/kg. In addition, doping transition metal elements can also improve the performance of the electrode active material, so as to improve the electrochemical performance of lithium-ion batteries.

In addition, the researchers prepared Li2Mn0.8Fe0.2SiO4/ carbon composite nanofibers by combining electrospinning and heat treatment. It was found that iron doping improved the conductivity and purity of the electrode materials, and the carbon nanofiber matrix promoted ion transfer and charge diffusion. The material shows good reversible capacity and excellent cycling performance when used as the cathode of lithium-ion battery.

Electrospinning technology applications – anode materials

In recent years, due to the low energy utilization rate of simple carbon-based anode materials, the lithium ion battery anode structure design has become more complex and fine, with the help of electrostatic spinning/electrostatic spray coating silk film technology can break through the relevant bottleneck.

For example, to address the problems of low capacity utilization and poor cycling performance of titanium dioxide/carbon-based (TiO2/C) anode of lithium ion batteries, researchers prepared dendritic TiO2@mesoporous carbon nanofibers (TiO2@MCNFs) by electrospinning, hydrothermal treatment and carbonization processes (as the figure below).

As the backbone support, the dendritic TiO2@MCNFs composite material has a large number of exposed nano-tio2 lattices, which can provide intrinsic crystal channels for lithium ion transport. Its interwoven carbon nanofiber skeleton has high structural integrity and mechanical flexibility. As a anode material, the dendritic TiO2@MCNFs has excellent initial discharge capacity (1932mAh/g) and excellent cycling performance (617mAh/g reversible capacity after 100 cycles).

The unique structure and excellent electrochemical properties of dendritic carbon matrix composites provide a new idea for the development of practical electrospinning carbon nanofiber anode materials doped with heteratom nitrogen, sulfur, phosphorus and boron. For example, nitrogen-doped carbon nanofibers modified with silicon nanoparticles (W-Si@N-CNFs) and nitrogen-doped carbon nanofibers with open channels (N-CNFO) were prepared by electrospinning.

preparation-of-dendritic-TiO2@MCNFs
Preparation of dendritic TiO2@MCNFs

Metal oxide materials with high theoretical specific capacity are also considered to be promising anode materials. The use of transition metal nanoparticles to improve surface electrochemical reactivity is conducive to further improving battery performance. Such as iron oxide (Fe2O3) -carbon fiber composite, manganese oxide (MnO) -carbon fiber composite, Li4Ti5O12 nanofibers coated with highly conductive titanium nitride (TiN) /TiOxNy layer.

In addition, special fiber structures such as hollow nickel oxide (NiO) nanofibers prepared by electrospinning and core-shell silicon/carbon base (Si/C) @CNF non-woven fabrics prepared by coaxial electrospinning can also significantly improve the electrochemical properties of anode materials.

Electrospinning technology applications – separators

Electrospun nanofiber membranes (single-layer, multilayer, composite and modified) have the characteristics of porous structure, high porosity and large specific surface area, and are ideal candidate materials for cell membranes to improve ion transport efficiency. As a special functional polymer with excellent comprehensive properties, polyimide (PI) has been developed as electrospun nanofiber membrane.

The researchers prepared a robust fluorinated polyimide (FPI) nanofiber film through the electrospinning/thermal crosslinking process, with a diaphragm with high mechanical strength (31.7MPa), small average pore size and narrow pore size distribution, showing good performance in preventing the growth and penetration of lithium dendrites, and can be assembled into a safe and reliable lithium-ion battery.

Combining the advantages of different fiber layers, nanofibrous membranes with a multilayer structure can be prepared by adjusting the spinning sequence, and as a multilayer separator, more excellent performance can be obtained in terms of mechanical strength, thermal stability and electrochemical performance.

Some researchers have fabricated a new sandwich structure PVDF/polym-phenylene isophthalamide (PMIA)/PVDF nanofiber battery separator with strong mechanical strength (tensile strength up to 13.96MPa) and thermal stability through sequential electrospinning technology.

Adding two or more organic polymers or inorganic fillers to the electrospinning solution to prepare composite nanofibrous membranes is another effective way to improve the performance of separators.

Since different polymers or inorganic fillers have different physical and chemical properties and electrochemical properties, compared with a single polymer precursor, the comprehensive performance of composite membranes containing multiple polymer materials is improved.

For example, researchers prepared lignin/polyacrylonitrile composite fiber membranes (L-PANs) by electrospinning. Thanks to the high porosity (74%) and good electrolyte wettability of L-PANs, the assembled batteries showed good rate performance and cycle performance. Due to the low preparation cost and simple process of L-PANs, they can be used as ideal candidate materials for lithium-ion battery separators.

Preparation-of-PVDF-HFP-PDA-composite-membrane
Preparation of PVDF-HFP-PDA composite membrane

In order to further improve the mechanical and electrochemical properties of the electrospun membrane, another effective method is to post-treat the electrospun membrane (including modifying its chemical structure or surface morphology) in order to obtain a modified separator with excellent comprehensive properties.
The researchers modified and grown a thin layer of polydopamine (PDA) functional layer on the surface of PVDF-HFP nanofibers by electrospinning and dip coating methods, forming a unique core-shell structure (as shown in the figure above), which serves as a high-safety modified separator.

Cycle stability and rate performance, and the entire reaction process is carried out in an environmentally friendly aqueous solution, which can meet the safe use requirements of large lithium-ion batteries.

Summary

As a new technology that was gradually researched and applied around the world at the end of the 20th century, the application of electrospinning technology in the field of lithium-ion batteries has gradually begun.

Compared with several technologies such as high-energy ball milling and vapor deposition, electrospinning technology has the advantages of simple principle, convenient operation and low preparation cost, and has gradually become one of the commonly used methods for battery materials construction.

However, in commercial applications, this technology still has many challenges, such as the problem of mass production, how to achieve precise control of nanostructures, etc., all of which need further optimization and improvement.

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