Applications of synchrotron X-rays and neutrons diffraction in energy storage materials research

REN Yang,XIE Yingying,,CHEN Zonghai,MA Zifeng



Applications of synchrotron X-rays and neutrons diffraction in energy storage materials research

REN Yang1,XIE Yingying1,2,CHEN Zonghai1,MA Zifeng2

(1Argonne National Laboratory, Argonne, IL 60439, USA;2Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China)

Synchrotron X-ray and neutron diffraction facilities are very popular and indispensable scientific resources that provide powerful instruments and experimental techniques for both fundamental and applied researches around the world. X-rays and neutrons interact with matter in different and also complementary ways, and recently have been extensively used for studying energy storage materials at the electronic, atomic and molecular levels, and even extended to engineering scale. In this article, we will briefly introduce synchrotron X-ray and neutron scattering techniques and their difference, similarity and complementarity. Advantages of synchrotron high-energy X-rays will also be presented. The unique and powerful capacity of neutron scattering for hydrogen storage material study will be shown. We also present some examples of-/operando study of the atomic structure evolution of Na1–δNi1/3Fe1/3Mn1/3O2and LiNi0.5Mn1.5O4active electrode materials during synthesis and electrochemical intercalation for sodium ion battery and lithium ion battery. Finally, the future perspectives of synchrotron X-ray and neutron diffraction techniques in the field of materials science for energy storage technology will be discussed.

synchrotron; X-ray; neutron diffraction; energy storage technology; electrode material

The energy conservation law tells us that we cannot create or eliminate energy,we just store and convert energy from one type to another for our daily activities. Obviously, high efficient and environmentally benign energy conversion and storage technologies are highly desired and are the focus of growing research and development efforts worldwide. However, physics principles often set up high barriers between performanceand safety, and many competing factors between stability and activity, capacity and cyclability, etc. in active energy materials. Basic knowledge at electronic, atomic and molecular levels is critical not only for better understanding but also for future development of advanced energy materials. Synchrotron X-ray and neutron techniques play a very important role and have broad applications in energy storage materials research.

Since the discovery of X-rays by W.C. ROENTGEN in late 1895, and neutron by J. CHADWICK in 1932, X-rays and neutrons have been widely used in various fields. Especially, the accelerator based synchrotron X-ray and spallation neutron sources have significantly advanced their instrumentational development and applications for research in almost all scientific and engineering disciplines. X-rays are electromagnetic waves and also called photons, while neutrons are subatomic particles. Both X-ray and neutron carry no charge and have spin. Table 1 lists some property and characteristics of X-rays and neutrons for reference.

Generallyspeaking,X-raysinteract with electrons of atoms via the electromagnetic interaction,while neutrons interact with atomic nucleivery short-range strong nuclear forces, and also interact with unpaired electronic spinsthe magnetic dipole interaction [1-5]. Neutrons are important for studying magnetic materials, while X-rays can also probe spinsvery weak relativistic effects. Synchrotron X-ray techniques can be generally categorized into diffraction/scattering, spectroscopy and imaging/ tomography, which can be combined, time-resolved,polarization dependent, coherence related, furtherdetailed. Here we will be mainly focused on the elastic diffraction part for atomic structure characterization, although the particular application of neutron scattering in hydrogen storage materials will be briefly discussed later.

Table 1 Property and characteristics of X-rays and neutrons

1 Å=1´10-10m.

1 Brief introduction to synchrotron X-ray and neutron diffraction

In practice, one of the most important parameters in X-ray and neutron diffraction is the so-called scattering cross section,, whereis the scattering length, which measure the scattering power of X-rays and neutrons by an atom. Fig.1 shows the X-ray and thermal neutron scattering length of some selected elements. One can see that the X-ray scattering length is proportional to the atomic number, because X-rays are interacting with electrons of atoms. X-ray scattering length at photon energy of 10 keV and 100 keV is also plotted for comparison, which indicates that with increasing photon energy the X-ray scattering length is reduced. This factor should be considered when using synchrotron X-rays.In Fig.1, it also can be seen that the neutron scattering length of the sample element with different isotopes can be drastically different, like the hydrogen and deuterium, which even have a sign difference. Such a unique feature makes isotope substitution method very important and useful in neutron scattering experiment. Vanadium has a nearly zero scattering length. This is why people often use vanadium for sample containers in many neutron scattering experiments.

It is known that neutrons usually have much higher penetration capacity than X-rays, thus can be used to probe bulky samples, except those containing hydrogen or elements with large neutron absorption. But the availability of high-brilliance synchrotron high energy X-rays with tunable energy generated by high-energy synchrotron radiation sources has significantly advanced the field of materials research, especially for-, operando, studies of function materials, in bulk forms or nanoscale structures, in realistic conditions and in real time. Fig.2(a) shows X-ray scattering cross section of lead as a function of photon energy. Below 1 MeV, the X-ray interactions with matter include the coherent and incoherent scattering and the photoelectron effect, while the nuclear interactions occur at higher energy. Nowadays, X-rays with photon energy of above ~40 keV are considered as high-energy (HE) X-rays. The weak scattering power of high-energy X-rays means a weak absorption, leading to a large penetration power, as shown in Fig.2(b), from which one can see that the lab X-rays with Cu Kradiation can only penetrate a few micron thick ion, while synchrotron HE X-rays with 115 keV can easily penetrate a few millimeter iron, showing bulk sample properties. On the other side, the weak scattering power of high-energy X-rays requires high flux and sometimes bulky samples, in order to perform high quality high-energy X-ray experiments.

High-energy X-rays have many advantages like high penetration, low absorption, small scatteringangle and wide reciprocal space coverage etc. Such advantages make high-energy X-rays particularly suitable for-operando investigation of advanced materials in complex sample environments, e.g. in low and high temperature, under magnetic, electric and stress field, high pressure or with combined external stimuli, or even in chemically hazardous, corrosive and radioactive conditions [6-10].

(a)

(b)

Fig.2 (a) X-ray scattering cross section of lead as a function of photon energy; (b) X-ray transmission as a function of iron thickness for 8 keV and 115 keV photon, respectively

2 Application of neutron scattering for hydrogen containing materials

At first, we want to emphasize a very important and unique feature in neutron scattering that is its ability of studying hydrogen containing materials. Hydrogen is the lightest element, which makes it very difficult for X-rays to probe hydrogen atoms in materials. Unfortunately, it is also very difficult for neutrons to see hydrogen, even it has a reasonable neutron scattering length (Fig.1). We have to point out that Fig.1 plots the coherent scattering length. Hydrogen has a huge incoherent scattering cross section, which gives rise to very high incoherent scattering background in neutron diffraction spectra of hydrogen-containing materials. In order for neutrons to see where hydrogen atoms are located in a compound or system, one has to replace hydrogen (H) by its isotope, deuterium (D).Neutron scattering plays an irreplaceable role and provides fundamental knowledge in this particular field.

Hydrogen molecule is the simplest molecule with quantum states, in which two protons are indistinguishable fermions, thus has an antisymmetric (AS) wave function. If the spins of the two protons are antiparallel, it is called para-hydrogen. If the two spins are parallel, it is called ortho-hydrogen. From Table 1, one can see that the population of para- and othor-hydrogen is temperature dependent. The energy transfer from para-hydrogen to ortho-hydrogen in H2solid is 14.7 meV, which is a fingerprint specifically for molecule hydrogen and its interaction with surroundings.

We have employed neutron scattering techniques to-investigate hydrogen storage capability andmechanism in carbon nanotubes and also dynamic properties of metal hydrides [11-12]. We used a quasielastic neutron scattering (QENS) instrument to monitor hydrogen adsorption in single walled carbon nanotube bundles. The QENS instrument can provide us rich information of hydrogen dynamic behavior and its interaction with absorbents. Molecular hydrogen has quantum rotational energy levels given byE=BJ(1), whereis the rotational quantum number and B the rotational constant (=7.35 meV for solid hydrogen). Fig.5 shows an inelastic neutron spectrum of H­2molecule adsorbed in carbon nanotubes. The shape peak at 14.5 meV corresponds to the quantum rotational transition from=0 (para-) to=1 (ortho-state) of the adsorbed hydrogen molecules, ambiguously indicating that molecule hydrogen is present. This peak becomes weaker and broader with increasing temperature, while the peak position remains almost unchanged. The observed transition energy of 14.5 meV is slightly less than the 14.7 meV found in pure solid hydrogen, implying that adsorbed hydrogen molecules encounter relatively little hindrance for rotation, probably due to the weak Van der Waals interactions between hydrogen and carbon nanotubes. The linear increase of the peak width with temperature increase indicates that the hydrogen molecules become less and less bound to the carbon nanotubes.

3 In operando synchrotron X-ray study of sodium-ion battery materials

synchrotron X-ray diffraction (XRD) has proven to be a powerful technique to probe the sodium insertion mechanism in sodium ion batteries. We have used in situ/operando high-energy X-ray diffraction (HE-XRD) to understand the phase transformation of NaNi1/3Fe1/3Mn1/3O2materials during Na ion intercalation. During the operando experiments, high-energy X-rays are directed to transmit through a perforated 2032-type coin-cell containing our newly synthesized NaNi1/3Fe1/3Mn1/3O2materials. TheHE-XRD patterns of the NaNi1/3Fe1/3Mn1/3O2during charge and discharge are illustrated in Fig.4.

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(b)

Fig.4HE-XRD patterns collected during cycling of Na1–δNi1/3Fe1/3Mn1/3O2electrode (a) between 2.0 V and 4.0 V and (b) between 2.0 V and 4.3 V [13]

TheHE-XRD data clearly indicate the existence of a nonequilibrium solid-solution reaction during charging when the cut-off voltage of charge was set to 4.0 V as evident in Fig.4(a). During the discharge, the XRD pattern indicates an exact opposite electrode evolution compared with charge, suggesting that the phase transformation of Na1–δNi1/3Fe1/3Mn1/3O2is reversible through an O3–P3–P3–O3 sequence during cycling. We also can get the change in lattice parameters of Na1–δNi1/3Fe1/3Mn1/3O2during cycling. Clearly, the structure of Na1–δNi1/3Fe1/3Mn1/3O2is very stable when cycled in the voltage range of 2—4.0 V.

By contrast, as evident in Fig.4(b), the phase transformation of Na1–δNi1/3Fe1/3Mn1/3O2is completely different when the cutoff voltage is set to 4.3 V. The phase change follows the sequence of O3 to P3 when charged from 3.2 V to 4.0 V, but in the charge voltage range of 4.0—4.3 V, the (003) P3 shifts to higher angle and splits into two again. These two peaks eventually merge to a new (003) peak, indicating a new nonequilibrium phase, which is assigned to a monoclinic O3′ phase consisting of a distorted lattice in comparison to an ideal hexagonal cell.Note that a two-phase reaction with the coexistence of P3 hexagonal and O3′ monoclinic phases occurs in the charge voltage range of 4.0—4.1 V.XRD shows a reversible evolution of O3–P3–O3′ during charge. The lattice parameters in critical phase change were calculated and are summarized in Table S2 of the Supporting Information. The lattice parameters for such O3′ monoclinic phases are=8.5394 Å,=5.3697 Å, and=2.4631 Å. Interestingly, a single O3′ monoclinic phase is maintained to cutoff voltage of 4.3 V. During discharge from 3.5 to 3.0 V, the (003) peak shifts to a lower angle, and the O3′ monoclinic phase transforms to P3′ monoclinic phase. The O3 phase is recovered via O3′–P3′–O3 during discharge.

The nature of the structural evolution of P2-Na0.67[Mn0.5Fe0.5]O2and P2-Na0.67[Mn0.65Ni0.15Fe0.2]O2upon cycling were studied by using combined X-ray and neutron powder diffraction (XRPD and NPD) [14]. The phase diagram of the two compositions as a function of sodium content is determined using operando diffraction experiments, and the structure of the unknown high voltage phase is solved by X-ray pair distribution function (PDF) analysis. The good fit between the average structure, determined by diffraction, and the measured PDF is a consequence of the low occupancies and weak scattering coefficient of sodium atoms. The structure of the high voltage phase of the nickel substituted oxide was similarly determined, using X-ray PDF analysis on a chemically oxidized “Z”-Na0.1Fe0.2Mn0.65Ni0.15O2sample (Fig.5). The sodium content reached by chemical oxidation of P2-Na0.67Fe0.2Mn0.65Ni0.15O2is slightly lower than what is achieved electrochemically.

(a)

(b) (c)

Fig.5 Fit of the PDF curves of chemically oxidized Z-Na0.1Fe0.2Mn0.65Ni0.15O2using a 4´4´10 supercell; (b) comparison of the short range experimental PDF of Z-Na0.1Fe0.2Mn0.65Ni0.15O2and Z-Na0.14Fe0.5Mn0.5O2, showing the evolution in the MO6/MO4 ratio (yellow shading) and the contraction of the bilayer thickness (lavender shading); (c) representation of the two structures (reprint from [14], Published by the Royal Society of Chemistry)

4 Complementary neutron and synchrotron study of high-voltage spinel cathode for lithium-ion batteries

Research efforts are in progress world-wide to develop reliable, high-performance cathode materials for advanced lithium-ion batteries, paving the way to a secure and sustainable energy future. Among the cathode materials, LiCoO2and LiTMO2(TM is a mixture of transition metal elements and/or Al), LiMn2O4and LiFePO4are the most widely used materials and stands for three different types of structures, such as layered-, spinel- and olivine-structure, respectively. To improve the energy and power density, one wants to increase the capacity and/or the operating voltage. LiNi0.5Mn1.5O4is a type of spinel material with discharge voltage plateau around 4.7 V, which is among the highest ones. Among the claimed high voltage materials, LiNi0.5Mn1.5O4is the few demonstrating good cycle and calendar life, being a good candidate for 5 V lithium ion batteries. However, the material produces by different synthesis methods demonstrate different electrochemical properties. It is known that, depending on the synthetic routes, LiNi0.5Mn1.5O4has a structure of face-centered spinel (Fd3m) or primitive cubic crystal (P4332), where face-centered spinel (Fd3m) performance better than primitive (P4332) [15]. Even the material produced from the same synthetic route, the variation on the calcination process (temperature and heat or cooling rate) will also result in differences in performance. As we know that X-ray can hardly distinguish Ni and Mn, thus will give almost identical diffraction patterns for the two structures. To understand the differences in property, we need to have sufficient information on the material structure change at different calcination condition or during the calcination temperature change by employing both neutron and high-energy X-ray diffraction techniques.

As we mentioned previously, synchrotron high-energy X-rays are very suited for in-situ probing materials formation and transformation of bulky samples. It is a straightforward experiment to monitormaterial synthesis process during high temperature calcination. Fig.6 is a photo of the experimental setup for-high-energy X-ray diffraction during solid state material synthesis at high temperature at Beamline 11-ID-C, Advanced Photon Source (APS) at Argonne National Laboratory. A commercial furnace was used and HE X-rays penetrate through a 2 mm thick pellet of a mixture of Ni0.25Mn0.75CO3precursor and Li2CO3in a molar ratio of 4∶1. The sample was heated up to 800 ℃ at a heating rate of 1 ℃·min-1, the wavelength of X-ray used was 0.1078 Å. A 2D X-ray detector was used to collect the X-ray diffraction (XRD) patterns with a speed of one spectrum per minute.

Fig.7(a) shows a contour plot of in situ HEXRD patterns illustrating the structural evolution of the material during the heating process from the room temperature to 800 ℃, which clearly shows that a major reaction occurred at about 500 ℃. The diffraction peaks from the starting material completely disappeared and a set of new peaks of the final product emerged. Fig.7(b) depicts the 1D covariance analysis of adjunct HEXRD patterns in real time space, Three downward peaks at about 290 ℃, 390 ℃, and 540 ℃ suggest that three different reactions occur at the specific temperatures. From the HE-XRD data, there seems to be no structural change above 540 ℃. However, it is know that the formation of the ordered and disordered spinel structures is strongly dependent on the high temperature synthesis procedure.

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(b)

Fig.7 (a) A contour plot of the HE-XRD patterns collected during the solid state sysnthesis; (b) 1D covariance analysis of the HE-XRD patterns

In order to investigate high temperature structure changes and possible migration mechanism of transition metal ions, we have carried outneutron diffraction measurements of the same mixture for LiNi0.5Mn1.5O4at various high temperatures at the neutron instrument VULCAN at the SNS of ORNL. Because neutron diffraction data collection time is much longer than HE-XRD measurements, the furnace had to be held at specific temperatures for 3 h to collect a complete neutron diffraction pattern. Fig.8 shows the in-situ neutron diffraction patterns at 600, 700, 800 and 900 ℃.

From Fig.8, one can see that the diffraction patterns at 600 ℃ can be described by the disordered spinel structure with space group of Fd-3m, in agreement with the HE-XRD data. However, at 700 ℃, a set of new peaks, labeled by arrows, were observed, and these peaks disappeared again when the temperature was increased to 800 ℃ and above. Rietveld refinement analysis indicated that the extra peak at 700 ℃ belonged to the ordered LiNi0.5Mn1.5O4spinel (with spacegroup P4332). In the disordered spinel, Ni ions and Mn ions randomly occupied 50% of octahedral sites in the FCC oxygen framework without forming a long term ordering between Ni and Mn. On the other hand, in the ordered spinel, one Ni ion pairs with three Mn ions to form a stable repeating unit in the FCC oxygen framework, resulting in extra peaks in the neutron diffraction pattern. This implies that the ordered spinel is energetically more stable than the disordered spinel. The phase transition from the ordered spinel to the disordered spinel at high temperatures above 700 ℃ was primarily driven by the entropy, which tends to maximize the randomness of the system. The disordered spinel at 600 ℃ and below might be due to the random distribution of Ni and Mn in the Ni0.25Mn0.75CO3precursor. Right after the formation of the spinel, the mobility of transition metal ions is very low. The migration of transition metal ions was only enabled at a temperature around 700 ℃ to allow for the transition of the metastable disordered spinel into a stable ordered spinel at about 700 ℃.

5 Summary

Advanced functional materials are the pivots for the development and commercialization of energy storage technologies for modern society, which become increasing sophisticated in order to fine tune many competing interactions and also to optimize multiple controlling parameters. Fundamental knowledge at various levels is always the key driver for rational material design and discovery. Synchrotron X-ray and neutron scientific user facilities have been extensively employed to tackle material issues in almost every scientific and engineering disciplines, and have greatly advanced our knowledge of energy storage materials at electronic, atomic and molecule levels. Particularly, recent and future improvements in synchrotron and neutron sources and instrumentation have provided us with a large set of state of the art techniques for-, operando, studies of energy storage materials in realistic conditions and in real time, with increasing temporal and spatial resolution. With the help of synchrotron and neutron facilities, we have gained much deeper insights in the energy storage materials and have witnessed flourishing activities with innumerable breakthroughs and discoveries for future energy storage technology developments.

Acknowledgment

Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under Contract No. DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.We would like thank all our collaborators and users for their excellent work using high-energy x-rays at the Beamline 11-ID-C, Advanced Photon Source. We are very grateful to Charles Kurtz, Guy Jennings and Richard Spence for their technical supports.

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同步辐射X-射线和中子衍射在储能材料研究中应用

任 洋1，颉莹莹1, 2，陈宗海1，马紫峰2

（1Argonne National Laboratory, Argonne, IL 60439, USA；2上海交通大学化学工程系，上海200240）

同步辐射X-射线和中子散射设施是国际上非常流行和不可或缺的科学资源，可为基础研究和应用研究提供强大的工具和实验技术。X-射线和中子以不同又互补的方式与物质相互作用，近年来已经广泛应用于电子、原子和分子水平，乃至工程尺度上对储能材料的研究。本文简要介绍了同步辐射X-射线和中子衍射技术及其差异性、相似性和互补性，对同步辐射高能X-射线的优点也进行了阐述。我们展示了中子散射独特和强大的能力，及其在储氢材料研究中的应用。分别介绍了利用同步辐射X-射线和中子衍射技术原位研究钠离子电池和锂离子电池中Na1–δNi1/3Fe1/3Mn1/3O2和LiNi0.5Mn1.5O4等电极活性物质在合成和电化学脱嵌过程中的结构演变规律的若干案例。最后，展望了同步辐射X-射线和中子衍射技术在储能科学研究中的前景。

同步辐射；X-射线；中子衍射；储能技术；电极材料

10.12028/j.issn.2095-4239.2017.0111

TM 911

A

2095-4239（2017）05-855-09

2017-06-21; Revised date: 2017-06-26.

Fundation: Natural Science Foundation of China (21676165), and the U.S. Department of Energy, under Contract (No. DE-AC02-06CH11357973).

The first author and corresponding author: REN Yang (1964—)，male，researcher, research interests: synchrotron radiation and neutron scattering techniques and applications, condensed matter physics, phase change and energy storage, E-mail: ren@aps.anl.gov.