Elsevier

Nano Energy

Volume 22, April 2016, Pages 406-413
Nano Energy

Communication
In operando observation of temperature-dependent phase evolution in lithium-incorporation olivine cathode

https://doi.org/10.1016/j.nanoen.2016.01.031Get rights and content

Highlights

  • Laboratory X-ray source is employed to probe the electrochemical reaction with high time resolution.

  • For the first time, we demonstrate the existence of intermediate phases during the lithiation/delithiation processes at low temperature.

  • Such intermediate phases between LiFePO4/FePO4 can efficiently inhibit the degradation of ion diffusion coefficient with decreasing of reaction temperature.

Abstract

LiFePO4 is one of the most outstanding cathodes for the high performance lithium-ion battery, while it is restricted by its unsatisfactory low temperature performance. Here we detect the structural dynamics and reaction routes of LiFePO4 via operando condition with high rates, well reproducibility over cycles and low temperature in common laboratory X-ray without the synchrotron light source. The intermediate phases between LiFePO4 and FePO4, driven by the overpotential and limited ion transfer rate along the b direction at low temperature, are captured. Our results demonstrate that the existence of intermediate can greatly improve the diffusion kinetics of LiFePO4. The deep understanding of reaction routes of LiFePO4 at low temperature will guide the further material optimization design. Besides LiFePO4, such high time resolution in-situ X-ray diffraction testing method with laboratory source is available to understand the reaction mechanisms of other electrochemical reaction system.

Introduction

Rechargeable lithium-ion batteries (LIBs) show great potential in electrical vehicles, portable electronics, and back-up for wind energy [1], [2], [3], [4], [5]. Olivine LiFePO4 is one of the most promising and safest cathode materials for LIBs [6]. Since the seminal work by Goodenough et al. [7], over 4000 papers have been reported to enhance the performance and understand the lithiation/delithiation mechanisms of LiFePO4 [8], [9], [10], [11], [12]. At room temperature, the phase transition between LiFePO4 and FePO4 is considered as a two-phase reaction with a theoretic volume change of 6.8%, due to the limited Li+ solubility and miscibility gap in LiFePO4 and FePO4 [13], [14], [15]. Such a large volume change during the phase transition processes traditionally leads to a poor cycling performance in LIBs. However, LiFePO4 displays excellent high-rate performance after nanosized [10], [16], [17], [18], [19], [20], [21]. In numerous studies, the theoretical calculation was used to understand the correlation between the structure and the ionic/electronic transportation properties of LiFePO4, which were further employed to explain the nanosized effect in LiFePO4 [22], [23], [24]. It is found that the Li+ can move quickly in the tunnels along the b direction. During the cycling, the phase growth is much faster than its nucleation process [25]. Thus, only few particles react, while the majority of particles keep in the stable LiFePO4 or FePO4 state. This is known as the “Domino-cascade mechanism” [9].

Ceder׳s group proposed a nonequilibrium solid solution path to understand the high-rate performance of nanosized LiFePO4. During cycling, the high C-rate arouse high overpotential, which changes the Li+ reaction route from the thermodynamic control to kinetics control. Instead of undergoing nucleation and two-phase growth processes, the overpotential induces a nonequilibrium LixFePO4 (0<x<1) solid solution path [26]. Recently, Grey׳s group demonstrated the nonequilibrium facile phase transformation route by high temporal resolution in-situ synchrotron X-ray diffraction (XRD) [27]. It provides a new understanding on the high rate capability of electrode materials undergone two phase reactions, during which the intermediate phase forms with a really short life time. The statement above is further confirmed by X-ray absorption near edge structure investigation [28]. As mentioned above, the research on LiFePO4 has achieved great progresses on both the electrochemical performance and reaction mechanisms at room temperature. However, further application of LiFePO4 is still limited by the unsatisfactory low temperature performance. Therefore, detecting the structural dynamics and reaction routes of LiFePO4 at low temperature is meaningful to understand and further optimize the electrochemical performance of LiFePO4.

Herein, we develop an approach to in situ probe the reaction in a customized electrochemical cell at high rates and adjustable temperatures for multiple cycles. A two-dimensional XRD (XRD2) [29], which is available in common laboratory without the synchrotron light source, is employed to probe the electrochemical reaction with high time resolution. In this work, the phase transformation routes of LiFePO4/FePO4 at different temperatures (253, 273, 293, and 313 K) with various cyclic voltammetry (CV) scan rates (1.4, 2.8, and 4.2 mV s−1) and galvanostatic charge/discharge rates (1, 2, and 5 C) are investigated by in-situ XRD2 (Co Kα radiation, λ=1.7902 Å, Bruker D8 DISCOVER). We demonstrate the existence of intermediate phases during the lithiation/delithiation processes at low temperature. Moreover, the dynamics determined phase transformation between LiFePO4/FePO4 at low temperature with/without overpotential is further investigated. At a temperature of 273 K, the ion diffusion rate along the b direction is greatly limited, which leads to the accumulation of potential, another channel is forced to open, which results in the stronger polarization and the formation of intermediate phases at lower temperature. It is found that, such intermediate phases between LiFePO4/FePO4 can efficiently inhibit the degradation of ion diffusion coefficient with the decreasing of reaction temperature. Meanwhile, more intermediate phases form during the discharge process than the charge process, due to the high activation energy for the phase transition from FePO4 to LiFePO4 [30].

Section snippets

Material synthesis

The LiFePO4/C composites were obtained through a sol–gel process followed by sintering. In the preparation, stoichiometric amount of LiNO3, FeCl2·4H2O, and citric acid were dissolved in deionized H2O to form a homogeneous solution. Ethylene glycol and NH4H2PO4 solution were then added into the above solution. After drying at 60 °C for 24 h, the precursor was heated at 600 °C for 1 h in N2 atmosphere to obtain the LiFePO4/C composites.

Materials characterization

Materials characterization was conducted by field emission

Results and discussion

The LiFePO4/C nanoparticles show an average particle size of ~43 nm, which is consistent with the XRD Rietveld refinement data (Figure S1). The LiFePO4/C exhibits a BET surface area of 23 m2 g−1 and a carbon content of 8.9 wt% (Figure S2). To explore the electrochemical performance and structural dynamics of LiFePO4, the CV curves were measured at different scan rates and temperatures under operando conditions (Figure 1a–c). It is observed that, the lower the temperature is, the flatter the

Conclusion

We firstly in-situ investigate the structural dynamic and observe the formation of intermediate phases of LiFePO4 with high time resolution with laboratory X-ray source. Our experiments allow the electrochemical process to be probed in a wide temperature ranging from 253 to 313 K and the intermediate phases of LiFePO4 are captured under the relatively low temperature. The formation mechanism of intermediate phases is further proposed, which is due to the limited ion transfer rate and the

Acknowledgments

M. Yan and G. Zhang contributed equally to this work. This work was supported by the National Basic Research Program of China (2013CB934103, 2012CB933003), the National Natural Science Foundation of China (51521001, 51272197), the International Science & Technology Cooperation Program of China (2013DFA50840), The Hubei Science Fund for Distinguished Young Scholars (2014CFA035), The National Science Fund for Distinguished Young Scholars (51425204) and the Fundamental Research Funds for the

Mengyu Yan received his B.S. degree in Material Chemistry from China University of Geosciences in 2012 and he is currently working toward the Ph.D degree in Material Science at Wuhan University of Technology. His current research interests include nanoenergy materials and devices.

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    Mengyu Yan received his B.S. degree in Material Chemistry from China University of Geosciences in 2012 and he is currently working toward the Ph.D degree in Material Science at Wuhan University of Technology. His current research interests include nanoenergy materials and devices.

    Guobin Zhang received his B.S. degree in Inorganic Non-metallic Materials Engineering from Inner Mongolia University of Science & Technology in 2014 and he is currently working toward the Master. degree. His current research interests include nanomaterials for Li-ions batteries, supercapacitors, sodium batteries and in-situ characterization.

    Qiulong Wei Wei received his B.S. degree in Department of Materials Science and Engineering from Wuhan University of Technology in 2011. He has joined State Key Laboratory of Advanced Technology for Materials Synthesis and Processing for four years. He is currently working toward the Ph.D. degree. His current research involves the design and synthesis of nanomaterials for achieving both high energy density and power density electrochemical energy storage device, including the lithium-ion battery, sodium ion battery and the hybrid capacitor.

    Xiaocong Tian received his first B.S. degree in Materials Physics from Wuhan University of Technology in 2011 and the second B.S. degree in English from Huazhong University of Science & Technology in the same year. He is currently working toward the Ph.D. degree and his current research focuses on the energy storage materials and devices.

    Kangning Zhao received his B.S. degree in Department of Materials Science of Engineering from Wuhan University of Technology in 2012. He has joined WUT-Harvard Joint Nano Key Laboratory for two years. He is currently working toward the Ph.D. degree. His current research involves the nanomaterials achieving high energy density and power density for lithium ion battery and sodium ion battery.

    Qinyou An received his Ph.D degree in Material Science at Wuhan University of Technology. His current research interests include nanoenergy materials and devices. He is now a full associate professor at Wuhan University of Technology.

    Dr. Liang Zhou received his Ph.D (2011) degrees from Department of Chemistry, Fudan University. After graduation, he worked as a postdoctoral research fellow at Nanyang Technological University for 1 year and The University of Queensland for 3 years. He is now a full professor at Wuhan University of Technology and an honorary associate professor at the Australian Institute for Bioengineering and Nanotechnology, The University of Queensland. His research interests include functional nanomaterials for electrochemical energy storage. Up to now, he has published over 50 papers with a total ISI citation of > 2000 and an h-index of 22.

    Yunlong Zhao received his B.S. degree in Material Science from the Wuhan University of Technology in 2012 and he is currently working toward the Ph.D degree. His current research interests include nanomaterials for Li-ions batteries, supercapacitors, Li-sulfur batteries and Li-air batteries.

    Chaojiang Niu received his M.S. degree in Material Chemistry from Wuhan University of Technology in 2009.He is currently working toward the Ph.D. degree and his current research focuses on the energy storage materials and devices.

    Wenhao Ren received his B.S. degree in material science from Wuhan University of Sciecne and Technology in China in 2012, and He is currently working toward the Ph.D. degree. His current research involves nanoenergy materials and devices.

    Liang He is anassistant professor of the State Key Laboratory of Advanced Technology for Materials Synthesisand Processing at Wuhan University of Technology. He received his Ph.D. from Tohoku University (Japan) in2013.His current research interests include the microfabrication and characterization of micro/nano structures and devices for use in MEMS(Micro Electro Mechanical Systems).

    Liqiang Mai is Chair Professor of Materials Science and Engineering at Wuhan University of Technology (WUT) and Executive Dean of International School of Materials Science and Engineering at WUT. He received his Ph.D. from WUT in 2004. He carried out his postdoctoral research in the laboratory of Prof. Zhonglin Wang at Georgia Institute of Technology in 2006-2007 and worked as advanced research scholar in the laboratory of Prof. Charles M. Lieber at Harvard University in 2008-2011. His current research interests focus on nanowire materials and devices for energy storage. He received the National Natural Science Fund for Distinguished Young Scholars, the First Prize for Hubei Natural Science Award and so forth.

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    These authors contributed equally to this work.

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