Study Of Lithium Ion Batteries In Nanomechanics Field Biology Essay

The characteristics and compositions of nanomaterials are comparatively different from that of other regular engineering materials, such as concrete, soils, steel, and asphalt. There is a number of evidence that indicates that nanomaterials are widely used for many engineering fields because it can be physically and chemically manipulated for specific engineering applications. There are many nanotechnologies associated with microelectronics and computer technology, medical and health, aerospace and aviation, etc that have been practically and successfully applied for the improving the development of society. However, the progress of nanotechnology has never been an isolated topic, it generally supported by the principle of nanomechanics, which is exceptionally complicated subject. The definition of nanomechanics is defined as "the branch of mechanics that studies the fundamentals of materials (elastic and inelastic behavior in continuum and atomistic/molecular approaches) at the nanometer scale" (Voyiadjis). This review paper will introduce the Lithium-ion batteries, and the ultimate purpose of this paper is concentrated in the analysis of the Li insertion/extraction process in LiFePO4/FePO4. Additionally, the synthesis of LiFePO4 by using hydrothermal method with different carbon resources also will be presented.

Key words: LiFePO4, Lithium-ion battery, hydrothermal method, SEM, TEM.

Introduction

With the rapid development of technology, the traditional batteries such as lead-acid battery, nickel-hydrogen battery, etc are no longer satisfied with the environmental issue and market demand. Instead, Lithium-ion battery has been considered as a green and high performance battery in the contemporary society. Currently, the cathode materials such as LiCoO2, LiMnO4, and LiNiO2 are commercially used in the Lithium-ion batteries. However, the considerable volume of literatures found that using LiFePO4 as a cathode material in Lithium-ion batteries were significantly strengthen than other cathode materials in terms of low price, environmental advantages, non-toxicity, and good thermal stability (Doyle, Takeuchi, Abraham). However, the low electronic conductivity and diffusion rate of Li+ are both inherent disadvantages of LiFePO4, and those weaknesses probably become large obstacles to restrains its development. Therefore, in order to get a better understanding of the characteristics of LiFePO4, it is important to investigate Li insertion/extraction in charge/discharge process of LiFePO4/FePO4.

Crystal structures of LiFePO4

Theoretically, the properties and functions of nanomaterials can be determined by its crystal structures in some extent. The LiFePO4 now is recognized as a cathode material in Lithium-ion batteries is also benefits from its stable crystal structures. The nanostructures of LiFePO4 can be seen in the Figure 1 shown below. The Figure 1 clearly demonstrates that LiFePO4 crystal was formed by the olivine-type structure, and the space group is called Pnma. More specifically, the oxygen atoms (O) were slightly distorted and arranged by the hexagonal packed. Fe and Li were located in the central of O plane so that formed FeO6 octahedra and LiO6 octahedra, respectively. Phosphate atoms (P) were located in the central of tetrahedral oxygen plane to formed PO4 tetrahedron. One thing need to be emphasized that is each PO4 tetrahedron sharing two oxygen atoms with FeO6 octahedra and LiO6 octahedra in the bc plane at same time (Ramana, Mauger, Gendron, Julien, and Zaghib, 2008). The one of the most notable structural attributes of LiFePO4 that is shared both edges and oxygen atoms, which is quite complicated. Fortunately, this special structure is provided space for the possibility of Li insertion/extraction in LiFePO4.

Figure 1: Crystal structures of LiFePO4

Theory of LiFePO4 charge/discharge process

LiFePO4 has been under intense study as an efficient cathode in Lithium-ion batteries. Many researches illustrates that the process of charge and discharge in the LiFePO4 system are great different from other traditional lithium cathode materials. More specifically, the charge and discharge process of LiFePO4 system generally includes two phases: LiFePO4 and FePO4, respectively. The charge and discharge process can be concluded by the following chemical equations:

LiFePO4 – xLi+ –xe- → xFePO4 + (1-x) LiFePO4 (Charge)

FePO4 + xLi+ + xe- → xLiFePO4 + (1-x) FePO4 (Discharge)

The chemical equations shown above demonstrate that the charge and discharge process of LiFePO4 is mainly based on the reactions between the LiFePO4 and FePO4. However, the most important observation in this case is that some portion of LiFePO4 and FePO4 does not participated in the reactions, and so that lithiaiton mechanism in LiFePO4 is essentially included as two-phase growth process involving the coexistence of both phases (LiFePO4/ FePO4). Although the performance of LiFePO4 has been considerable improved for the past decades, while the lithiaiton/delithiation mechanisms still causes many controversial arguments. The main debate of this material focuses on the mechanisms of Li insertion/extraction in LiFePO4, which is critical to battery performance. On the other hand, some models that have been investigated try to fully explain the mechanism of this process, which include mosaic model, shell-core model, and domino-cascade model, etc. However, the mechanisms of Li insertion/extraction still under argument because of lack of direct experiment evidences (Sun, Qiao)

Experimental Program

The experimental program is considered as one of the most important factors that significantly affect the results and conclusions of experiment. Thus following the principle and regulation of this experiment ensures that the studies are correctly planned and can be adequately carried out, and are fully and precisely reported. Basically, the process of this experiment can be divided into the following sections: objective, synthesis of LiFePO4 and LiFePO4/C, XRD analysis of LiFePO4 and LiFePO4/C, SEM analysis of LiFePO4 and LiFePO4/C, Electrochemical Performances of LiFePO4 and LiFePO4/C, and conclusions.

1.1 Objective

As we mentioned in the previous section, the low electronic conductivity and diffusion rate of Li+ are both inherent disadvantages of LiFePO4. In order to make an obvious improvement of the electrochemical performance of LiFePO4, three different carbon sources (critic acid, PVA, and glucose) doping of LiFePO4 by using hydrothermal method were investigated.

1.2 Synthesis

The performance of electrochemical and the percentages of LiFePO4 content were relatively low in the Iron-Phosphate minerals. Therefore, the almost LiFePO4 in batteries were generally produced by artificial synthesis, which mainly include hydrothermal method, co-precipitation method, and solid-state method (Sun, Qiao). Each synthesis approach has its own advantages and disadvantages in terms of different materials. In this experiment, LiFePO4 and LiFePO4/C composites were synthesized by using hydrothermal method. In order to investigate the relationships and differences between LiFePO4 and LiFePO4/C, each sample was being labeled. "The LiFePO4 without being roasted was marked as sample a. The unroasted sample synthesized with 5% citric acid, PVA or glucose was marked as b’, c’, or d’. The corresponding roasted samples were marked as b, c, and d, respectively" (Liu, Zhang, Jiang, Ma, and Wang, 2012).

1.3 XRD analysis of LiFePO4 and LiFePO4/C

It is well known that X-ray diffraction (XRD) techniques and approaches have been popularly used for identification of the crystalline phase of the products in the nanomechanics field. The XRD patterns of LiFePO4 and LiFePO4/C can be seen in Figure 1 shown below. More specifically, Figure 2 demonstrates that all samples have very similar XRD patterns, expect for the sample a, which was the only one contains imparities Li3PO4 and Fe2 (PO4)OH. Besides that, the peak intensity of sample a was the weakest one among the other investigated samples. The differences between doped samples and undoped sample can be concluded as the following reasons. Carbon sources added in LiFePO4 will provide an optimistic dispersion and reducibility environment for the synthesis of LiFePO4. With that in mind, the carbon restricts oxidation of Fe2+ to Fe3+ during the hydrothermal synthesis process. Based on this planned function of carbon sources, sample a LiFePO4 without added carbon sources that will causes some of Fe2+ oxidized to Fe3+ during the hydrothermal synthesis process. These non-conductive imparities of Li3PO4 and Fe2 (PO4)OH will be attached on the surface of LiFePO4 particles, which have negative impacts for electrochemical performance of LiFePO4 in the batteries eventually (Liu, Zhang, Jiang, Ma, and Wang, 2012).

Figure 2: XRD patterns of LiFePO4 and LiFePO4/C samples.(a) LiFePO4; (b) LiFePO4/5% critic acid; (c) LiFePO4/5% PVA; (d) LiFePO4/5% glucose; (d’)LiFePO4 precursor /5% glucose.

1.4 SEM analysis of LiFePO4 and LiFePO4/C

In order to obtain a better understanding of LiFePO4 and LiFePO4/C, the crystal structures and particle size of those two composites were observed by using Scanning Electron Microscopy (SEM). Figure 3 shown below clearly indicate the crystal structures of unroasted LiFePO4/C with adding carbon sources of citric acid, PVA, and glucose, respectively. As we can seen in Figure 3, it is found that the LiFePO4/C with added glucose has the most homogenous distribution with particle size about 100 nm. The crystal structures of b’ and c’ were relatively irregular compared to the d’. Figure 4 shows the crystal structures of roasted LiFePO4/C with adding carbon sources of citric acid, PVA, and glucose, respectively. Similar to the results of Figure 3, the LiFePO4/C with added glucose has the most homogenous distribution compare to other carbon sources. However, the most notable differences between Figure 3 and Figure 4 that is the roasting process causes crystal structures of LiFePO4/C from rectangular body to spherical particles, which was a good phenomenon.

From the analysis of LiFePO4 and LiFePO4/C by the procedures of XRD and SEM, the results indicate that roasting process can significantly influence on the crystal structures of samples. In addition, LiFePO4/C cathode material synthesized with glucose was considered as the most reasonable alternative so far. The electrochemical performance of the LiFePO4/C will be detailed introduced in the next section.

Figure 3: SEM patterns of LiFePO4 and LiFePO4 /C samples. (b’) LiFePO4 precursor /5% critic acids; (c’) LiFePO4 precursor /5% PVA; (d’) LiFePO4 precursor /5% glucose.

Figure 4: SEM patterns of LiFePO4 and LiFePO4 /C samples. (b) LiFePO4/5% critic acid; (c) LiFePO4/5% PVA; (d) LiFePO4/5% glucose.

1.5 Electrochemical Performances of LiFePO4 and LiFePO4/C

In order to successfully implement the long-term strategy and application of LiFePO4/C cathode material synthesized with glucose, the electrochemical performances of this material also need to be identified in this experiment. Figure 5 and Figure 6 are the summary of initial charge-discharge curves of specimens at 0.1C and cycle performance of specimens at 0.2C, respectively. The combination of Figure 5 and Figure 6 indicate that LiFePO4 with added glucose have the most stable charge and discharge platform at 3.5V. With that in mind, LiFePO4 with added glucose also shows the best cycle performance among the all other specimens. More specifically, as we can seen in Figure 4, indicating the observable difference between curves corresponding to the glucose doped in LiFePO4 and those corresponding to the other carbon sources. As expected, the results illustrate that the discharge capacity of LiFePO4 with added glucose has the maximum values, which is 115.5 mAh·g-1. In addition, Figure 5 shows that with the increase in cycle number, the specific capacity of the specimen LiFePO4 (a) and LiFePO4 with added critic acids (b) and PVA(c) are decreased. On the contrary, the specific capacity of LiFePO4 with added glucose is keeping at the same level after 20 cycles. This phenomenon demonstrate that glucose doped in LiFePO4 have the best cycle performance compare to other alternatives.

Figure 5: Initial charge-discharge curves of samples at 0.1C. (a) LiFePO4; (b) LiFePO4/5% critic acids; (c) LiFePO4/5% PVA; (d) LiFePO4/5% glucose.

Figure 6: Cycle performance of samples at 0.2C. (a) LiFePO4; (b) LiFePO4/5% critic acids; (c) LiFePO4/5% PVA; (d) LiFePO4/5% glucose.

TEM study of fracturing in spherical and plate-like LiFePO4 Particles

As I mentioned in the previous section, tremendous efforts have been accomplished in order to overcome the low electronic conductivity and diffusion rate of Li+ limitations of LiFePO4. At present, LiFePO4 with added glucose has been considered as a successful approach, which widely used for improving the performance of lithium ion batteries. However, for further higher energy density applications such as plug-in hybrid electric vehicles (PHEVs), the doping on the LiFePO4 particles "must remain reasonably intact over many moderately deep charge-discharge cycles. The possibility of particle fracture due to the stresses of repeated delithiation and relithiation also bears relevance to cycle life". In addition, many TEM studies on these materials indicate that the thickness of a carbon layer about 10nm is sufficient to improve the electrochemical performance of LiFePO4. However, the micro cracks occur both include in the two phases: LiFePO4 and FePO4, while differing only in the orientation of fracture surfaces (Gabrischa, J. Wilcoxb, and M. Doeff). The following TEM studies focused on the fracture mechanism can provide useful information for the further leaning of the performance of lithium-ion batteries.

Figure 7. a) Image of a particle retrieved from cell 1, showing (100) lattice planes and an intact carbon surface layer. b) Corresponding diffraction pattern in the [011] zone axis orientation showing spot splitting at some outer reflections (see arrow).

Figure 8. Image and diffraction patterns taken from LiFePO4 particles retrieved from cell 1 (Fig. 8a, b) and cell 2 (Fig. 8c, d). The fracture surfaces are oriented parallel to (100) and (010) planes.

Figure 9. Image and diffraction pattern taken from Li0.5FePO4 synthesized from hydrothermally produced LiFePO4. The fracture surface is oriented parallel to (100) planes. In the diffraction pattern, spot splitting typical for the two-phase microstructure is observed.

The Figures 7, 8, 9 shown above were observed by TEM by using "a JEOL 3010 operated at 200kV at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory and a JEOL 2010 at the University of New Orleans operated at 200kV. Tilting experiments for imaging in dark-field conditions were performed at the Technical University of Berlin/Germany using a Philips CM300 operated at 200kV". Again, the fractures observation in LiFePO4 subjected to electrochemical cycling and to chemical delithiation is the purpose of this experiment test.

More specifically, Pay attention in Figure 7, which shows a particle from cell 1 in [011] orientation. "In the diffraction patterns of the cycled particles, spot splitting at reflections further away from the incident beam was occasionally observed". From this observation, it demonstrating that microstructure has been formed during the both two phases. In addition, some FePO4 does not fully discharged because coulombic inefficiencies or cycling losses. Similar to the Figure 7, specimens of particles with micro fractures taken from cell 1 and cell 2 can be seen in Figures 8 a and c, combine with the corresponding diffraction patterns in Figures 8 b and d, respectively. The fracture surfaces are oriented parallel to (010) and (100) planes in the electrochemically cycled LiFePO4. Figure 9 shows an example of a fracture observed in the chemically delithiated specimen. As we can seen in the figure, the fracture surfaces was observed "the fracture observed in other particles of this sample are parallel to (100) planes in agreement with the original published data". Because extensive fractures observed in the "chemically delithiated material is considerably higher than in the electrochemically cycled powders, underlining the correlation between internal stresses and particle fracturing" (Gabrischa, J. Wilcoxb, and M. Doeff).

The short conclusion in this experiment can be concluded as: extensive fractures and the dislocation density were significantly higher after chemical delithiation than after electrochemical cycling. This phenomenon can influence on the effect of lithium extraction rate on the magnitude of internal stresses.

Conclusions

In conclusion, the analysis and discussions above indicate that all carbon sources have significant influence on the charge-discharge performance of LiFePO4, and glucose doped in LiFePO4 is considered as the one of the best practical and reliable approaches to enhance the rate capacity of LiFePO4. As mentioned in the previous section, although LiFePO4 has its own advantages as a cathode material in lithium ion batteries while the low electronic conductivity and slow diffusion rate of Li+ are both inherent disadvantages of LiFePO4, and the electrochemical performance of LiFePO4 in the batteries can be improved by using LiFePO4 with added glucose. In addition, microstructure fractures during the two phase process need to be overcome for the furthering learning of lithium ion batteries.