LiFePO4-Carbon Core-shell Structured Nanowires

Rui Xu

University of Rochester
Dept. of Mechanical Engineering, Material Science Program,
OPT 307: Electron Microscopy
Spring 2010
Final Project

 

Introduction
Material Preparation
Analysis Methodology
Results and Discussion
Conclusion
References
Acknowledgements

 

Introduction

LiFePO4-C core-shell structured nanowires are promising electrode materials for lithium ion batteries. Their core-shell structure, as characterized in this project, is the key factor that enabled them to overcome the drawbacks of conventional LiFePO4 particles. Their special structure equipped the material with improved electronic conductivity and lithium ion mobility. As a result, LiFePO4-C core-shell structured nanowires can provide high energy-density, high power-density and long life which are needed for a very wide array of applications including mobile computers and phones, electric vehicles, military weapons and spaceships. 

The core-shell structured nanowires I prepared are about 100 nm in diameter. They were prepared using an electrospinning method and a successive heat treatment. This material has a LiFePO4 core with a shell of carbon. I used LiAc, Fe(NO3)3, H3PO4 and PAN (Peroxyacetyl Nitrate) polymers as the reagents for preparing the LiFePO4-C nanowires. The nanowires obtained by using electrospinning method were then heated in the air first (for stabilization) and thereafter in N2 atmosphere for carbonization and reaction. 

This study aims to examine the size and shape of the as-prepared LiFePO4-C core-shell structured nanowires. The composition of the material was also analyzed in an attempt to identify preliminarily the content of each element.  

Since Padhi et al. demonstrated the reversible electrochemical lithium insertion–extraction for LiFePO4 in 1997 (More...)

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Material Preparation

The setup for electrospinning is shown in Figure 1 [1]. It consisted of a high voltage supplier (Dongwen, Tienjing, China), a syringe pump (Harvard Pump11, U.S.A.), and a plastic syringe equipped with a 22 gauge stainless steel needle. Carbon paper was used to collect the composite fiber. The polymer used was PAN (Polyacrylonitrile) obtained from Aldrich. The salts used that contain Li, Fe and P elements were LiC2H3O2•2H2O, Fe(NO3)3•9H2O and H3PO4 from VWR. Solvents used ware deionized water, methanol or DMF (Dimethylformamide). The distance between the tip of the syringe and the carbon paper collector was 7 cm (for methanol and DMF solution) and 13cm (for water solution) and the applied field was 0.8-1 kV/cm (for methanol and DMF solution) or around 1.5 kV/cm (for water solution). Solution feeding speed was 0.1-0.3 mL/h. Temperature was around 25 ℃ in roomI prepared LiFePO4–carbon core-shell structured nanowires for lithium ion battery cathodes using electrospinning method.environment. To obtain the final product of LiFePO4 nanowires, the precursor fibers underwent a heat treatment for the salts to react in N2 mixed atmosphere for 3h at 600℃. 

                                                                                      
                                                                                      
Figure 1: The setup of electrospinning used for preparing the core-shell structured nanowires.

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Analysis Methodology

The equipment used for the scanning electron microscope analysis and tranmission electron microscope analysis in this project include a Zeiss SUPRA40-VP model SEM and a FEI Tecnai F20 TEM at the Institute of Optics at the University of Rochester. Prior to SEM analysis, the sample needs to be sputter coated with gold because LiFePO4 nanowires are insulators. An Everhart-Thornley secondary electron detector (SE2) was used for the collection of micrographs. Using an Electron Flight Simulation software, the interation zone of the electron beam with the nanowires can be seen. X-ray microanalysis was conducted using energy-dispersive x-ray spectroscopy (EDS detector) capable of producing a spectrum characteristic to the analyzed sample with associated quantitative results. Prior to TEM analysis, the sample holder needs to be plasma cleaned in order to reduce hydrocarbon contamination on the sample during observation in TEM. In addition, the LiFePO4-C nanowires underwent an x-ray mapping in STEM mode for examining the core-shell distribution of elements within a single nanowire. The SEM and TEM micrographs can be colorized using Photoshop to emphasize the observed nanowires. 

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Results and Discussion

The as-prepared LiFePO4-C samples were not all fine core-shell structured nanowires. Figure 2 shows the micrographs of the LiFePO4-C samples taken at various magnifications, acquired using the InLens detector. From those micrographs, we can see that the diameter of those nanowires is about 100 nm, and the length of them is more than 10 micrometers. It can be seen that at a magnification as high as more than 50k, we can still obtain very nice micrographs. Figure 3 shows the micrograph and colorized micrograph of the LiFePO4-C sample obtained at 30 kV. In the colorized image, the nanowires are emphasized by green color.

                       
Figure 2: The SEM images of LiFePO4-C nanowires taken at various magnifications

Figure 3:  The colorized micrograph of the LiFePO4-C sample

In Figure 4, the interaction zone of the electron beam (20 kV, 5.3 mm) with the sample surface is simulated by the Electron Flight Simulation software. From Brian's tutorial, we know that ultimately image formation in an SEM is dependent on the acquisition of signals produced from the interaction of the specimen and the electron beam. These interactions can be broken down into two major categories: the elastic collisions and inelastic collisions of the electron beam on the sample. The electron beam is not immediately reflected off in the way that light photons might be in a light microscope. Rather the energized electrons penetrate into the sample for some distance before they encounter an atomic particle with which they collide. In doing so the primary electron beam produces what is known as a region of primary excitation. Various signals are produced from this zone, and it is the size and shape of this zone that ultimately determines the maximum resolution of a given SEM working with a particular specimen. In this simulation image, we can see that the electron beam penetrate into 2.3 micrometers of the surface of the LiFePO4-C core-shell structured nanowires, which can give information of several layers of the nanowires. 

Figure 4:  The interaction zone of the electron beam (20 kV, 5.3 mm) with the sample surface

Figure 5 shows the EDS result of the sample, from which it can be seen that the sample contains a large amount of carbon and gold and Fe, P and O elements. Gold is from sample coating, and most carbon is from carbon tape. The atomic percent of Fe and P are approximately 1:1, which is well in accordance with the composition of LiFePO4 nanowires. The content of oxygen is not precisely the amount of O in the LiFePO4 nanowires, but also contains that result from contamination, environment and so on. As a result, the content of oxygen is very high in the sample. For EDS, Li has too samll an atomic number and cannot be analyzed with the EDS detector.

Figure 5:  EDS result of the LiFePO4-C sample 



TEM micrographs can demonstrate clearly the core-shell structure of the as-prepared LiFePO4-C samples (as shown in Figure 6). It can be seen from Figure 7 that the nanowires in the samples are different in detail. In the first micrograph of the figure, the shell of the nanowire is about 15 nm in thickness, and the core is about 80 nm in diameter. We can also observe that there are more than one layer in the shell on the right. In the second micrograph, the nanowire has a more thicker shell than that of the previous one. It can also be seen that the shell on one side has more than one layers. The third micrograph shows a nanowire with shells of moderate thickness. A detailed observation shows that the core and shell of this nanowire both have more than one layers. 

Figure 6:  TEM micrographs of the as-prepared core-shell structured LiFePO4-C nanowires

Figure 7:  TEM micrographs of the as-prepared core-shell structured LiFePO4-C nanowires

Figure 8 shows the dark field STEM micrograph of the same two nanowires shown in figure 7. In the contrast of figure 7, this micrograph shows the core in a lighter color than the shell. Both of figure 7 and 8 are in well accordance with the fact the sample has a core consisting of LiFePO4 and a shell consisting of carbon. Figure 9 is the mapping result of the core-shell structured sample obtained in the STEM mode. The mapping region is shown in figure 9a. Figure 9b shows the mapping result for carbon, while figure 9c, 9d and 9e are for Fe, P and O. It is reasonable that there are more Fe, P and O elements in the core of the nanowires, but it seems quite puzzling that there are also more carbon in the core than in the shell, as indicated by figure 9b, because the sample has a carbon shell and it appears that there should be more carbon in the two sides of the nanowire than in the center of it. This phenomenon can be explained when taking into consideration that this micrograph is a flat image taken from outside the nanowire, while the shell of the nanowire exists all around the core. The contents of carbon shown in the center of the nanowire in the micrograph also contain that exists in the shell, and as a certain amount of polymers which contains carbon may remain in the LiFePO4 core, the carbon amount is higher in the center of the nanowire in the micrograph.

Figure 8:  Dark field STEM micrograph of the as-prepared core-shell structured LiFePO4-C nanowires

Figure 9:  Mapping

Figure 10 shows the EDS result taken in the TEM. As the analyzing region is within one nanowire, the signal is much weaker than the EDS result taken in the SEM, but it can also be seen the existence of Fe, P and O in the sample. 

Figure 10:  EDS results of the LiFePO4-C core-shell structured nanowires

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Conclusion

The LiFePO4-C core-shell structured nanowires were prepared and characterized by various SEM and TEM techneques. Prior to SEM analysis, the sample needs to be sputter coated with gold because LiFePO4 nanowires are insulators. An Everhart-Thornley secondary electron detector (SE2) was used for the collection of SEM micrographs. The diameter of the as-prepared nanowires is about 100 nm, and the length of them is more than 10 micrometers. Using an Electron Flight Simulation software, the interation zone of the electron beam with the nanowires can be seen. X-ray microanalysis was conducted using energy-dispersive x-ray spectroscopy (EDS detector) and shows that the as-prepared nanowires contains Fe, P and O elements and the quantitative results shows that the atomic percent of Fe and P are approximately 1:1, which is well in accordance with the composition of LiFePO4 nanowires. TEM micrographs demonstrate clearly the core-shell structure of the as-prepared LiFePO4-C samples and it can be seen that the as-prepared nanowires in the samples are different in detail. Prior to TEM analysis, the sample holder needs to be plasma cleaned in order to reduce hydrocarbon contamination on the sample during observation in TEM. From the mapping result of the core-shell structured sample obtained in the STEM mode, it can be seen that there are more Fe, P and O elements in the core of the nanowires, which is reasonable because the core of the nanowires consists of LiFePO4. There are also more carbon in the core than in the shell. This phenomenon can be explained when taking into consideration that this micrograph is a flat image taken from outside the nanowire, while the shell of the nanowire exists all around the core. The contents of carbon shown in the center of the nanowire in the micrograph also contain that exists in the shell, and as a certain amount of polymers which contains carbon may remain in the LiFePO4 core, the carbon amount is higher in the center of the nanowire in the micrograph. 

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References

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Acknowledgements

I would like to thank Brian McIntyre for his expert guidance, patience and for use of the TEM. I would also like to thank Andreas Liapis for his assistance in using the SEM. Special thanks to my advisor, James Li, as well as Jianglan Shui and my friends for their support in this project.

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