In-situ TEM Investigation of Electron Irradiation Induced Metastable

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In-situ TEM Investigation of Electron Irradiation Induced Metastable States in Lithium-Ion Battery Cathodes: LiFeSiO versus LiFePO 2

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Bin Wei, Xia Lu, Frédéric Voisard, Huijing Wei, Hsien-Chieh Chiu, Yuan Ji, Xiaodong Han, Michel L. Trudeau, Karim Zaghib, George P. Demopoulos, and Raynald Gauvin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00391 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Energy Materials

In-situ TEM Investigation of Electron Irradiation Induced Metastable States in Lithium-Ion Battery Cathodes: Li2FeSiO4 versus LiFePO4 Bin Wei,†,∇ Xia Lu, ‡ Frédéric Voisard,† Huijing Wei,† Hsien-chieh Chiu,† Yuan Ji,§ Xiaodong Han,§ Michel L. Trudeau,

⊥

⊥,

Karim Zaghib,

*

George P. Demopoulos,† and Raynald Gauvin†,*

†

Department of Mining and Materials Engineering, McGill University, Montréal, Québec H3A 0C5, Canada.

‡

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of

Chemical Technology, Beijing 100029, China. §

Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing

100124, China. ⊥

Center of Excellence in Transportation Electrification and Energy Storage (CETEES), Hydro-Québec, 1806,

Lionel-Boulet blvd., Varennes, QC J3X 1S1, Canada. ∇International

Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga, 4715-330 Braga, Portugal

Corresponding authors: *

E-mails: [email protected]; [email protected]

Abstract: Energy storage and conversion in Li-ion batteries involve dynamic Li storage and transport through a series of electrochemical metastable states (or quasi-equilibrium configuration). Therefore, an investigation of these metastable states is helpful to fully understand the lithium storage mechanism. An accurate understanding of the storage mechanism is a key factor in the design and optimization of the next-generation high-performance batteries. Here, we report the results obtained from electron irradiation-induced phase transitions in Li2FeSiO4 and LiFePO4 by electron microscopy. During prolonged irradiation, the crystalline Li2FeSiO4 particles experienced a transition from a monocrystalline structure to an amorphous phase, with a subsequent recrystallization process (monoclinic to orthorhombic phase). The fine structure of the electron energy loss (ELNES) spectra showed the electron beam-sensitive characteristic of Li2FeSiO4 that included the electron beam-induced mass loss (composition changes), formation of intermediate metastable states

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(Li2-xFeSiO4), structural distortion/amorphization and valence state variation, all of which are much less prominent in LiFePO4 under the same flux of electron beam. These findings provide new insights into the structural stability of Li2FeSiO4 and LiFePO4 samples, and is also important guidance in the characterization of electrode materials. Key Words: In-situ TEM, Electron irradiation, Battery cathode, Li2FeSiO4, LiFePO4, EELS.

1. Introduction The performance of lithium-ion batteries (LIBs) is affected by the crystal structure of the electrode materials,1-3 such as the long-order atomic periodicity that is the origin of a plateau-like Li insertion-extraction profiles in LiFePO4 olivine polyanionic cathode due to the stabilized chemical potential (µ).4 On the other hand, the short-order structure (including even amorphous phases) introduces the quasi-continuous changes in the electrode structure, which results in the structural evolution with Li content to produce the sloped electrochemical curves as the ones in Li2FeSiO4 or LiCoO2 cathodes upon electrochemical lithiation/delithiation.5, 6 This structural evolution (instability) introduces a series of meta-stable phases during Li insertion-extraction process. It is of interest to evaluate the effect of these metastable phases on the electrochemical performance of battery materials, such as the two-phase reaction in a LiFePO4 cathode7, 8 and the solid-state reaction in Li2FeSiO4 cathodes.6, 9 However, this conventional perspective on the structural evolution can be obscured,10,

11

if a size effect is considered, because the increasing specific surface area of the

electrode materials enhances the surficial non-faradic Li storage behavior, which always yields a sloped electrochemical curve during electrochemical cycles. With decreasing particle size, the side reactions and surface activity of the electrode materials increase during battery cycles or exposure to


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the air. However, it is challenging to characterize these metastable intermediate phases in Li-ion batteries, not only the structural evolution of the electrode materials, but also in respect to the lithium storage and transport mechanism. These characterization studies are important to guide the design and optimization of battery systems, and the exploration of new energy storage device. Polyanionic materials, such as LiFePO4 and Li2FeSiO4 have been widely studied as cathodes for LIBs. The advantage of LiFePO4 is its stable olivine structure, consisting of a vertex/edge-shared FeO6 octahedron with the PO4 tetrahedron in orthorhombic phase. It has a reversible specific capacity of ~ 170 mAh/g with prolonged cycles. Li2FeSiO4 delivers two times higher specific capacity (~ 330 mAh/g), but suffers from poor cyclability, which is probably due to its instable vertex-shared tetrahedral connections in its polymorphs.12 This subtle structural difference affects the electrochemical response to the Fe3+/Fe2+ redox couple plateau around 3.45 V in LiFePO4, while a sloped voltage ranges from 1.5 V to 4.5 V (or higher to 4.8 V) in Li2FeSiO4. These electrochemical results also imply that the structural stability of LiFePO4 is higher than that of Li2FeSiO4 as cathodes in Li-ion batteries, although they are in the same polyanionic family. These differences in structural stability upon delithiation/lithiation can be revealed by electron irradiation, as non-trivial structural damages in the electrode materials are observed during conventional transmission electron microscopy (TEM) studies.13 Indeed, electron irradiation in TEMs can produce defects, phase transitions, material deformation or irreversible damage to the sample.13-16 More specifically, electron irradiation is a useful and versatile procedure to achieve these transformations, such as the superficial relaxation/reconstruction of Li4Ti5O12 (LTO) spinel anode by using a high-energy electron beam (e-beam).17 Therefore, the TEM can provide a method to induce structural changes by electron irradiation, as well as simultaneously allowing spatially imaging at the atomic scale.18 Although the



transformations from a monocrystalline to a polycrystalline phase, even amorphization was reported in ceramic materials,19-21 these phenomena are rarely examined in current battery materials. Moreover, recently, a large number of in-situ TEM and EELS studies of lithium ion batteries were carried out, including the study of crystalline to amorphous transitions of Si-based anodes and phase-conversion reaction of transition metal compounds.22-26 However, the in-situ TEM imaging process always includes a superimposed effect of the electrochemical-triggered morphology and structure evolution, as well as the e-beam irradiation induced contributions to the beam-sensitive battery electrodes. The effect of irradiation to structural change is usually neglected or avoided during TEM characterization. Although these metastable states are not surely to fully appear in the real battery cycles, it is reasonable to assume that the structural changes in the sample caused by the electron beam should be investigated further and should certainly not be neglected.27-30 Determining the metastable states is helpful to understand the intrinsic properties of electrode materials and the electrochemical process. Characterizing these metastable states could ultimately be beneficial to improving the performance of Li-ion batteries.31 In this work, the effect of the electron irradiation on Li2FeSiO4 and LiFePO4 cathode materials are studied. We employed transmission electron microscopy (TEM) in combination with electron energy-loss spectroscopy (EELS) to analyze the e-beam-induced metastable phases in these two cathodes. The results demonstrate that the crystalline Li2FeSiO4 experienced a complex phase transition (flux > 104 e-Å-2s-1), with a transition from a crystalline to an amorphous structure by electron irradiation, and then a recrystallization process that occurs in the amorphous LFS region to form smaller nano-crystals. On the other hand, the amorphization and recrystallization processes for LiFePO4 crystals require higher beam energy (flux > 105 e-Å-2s-1) or longer irradiation time. These


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results can be beneficial to understand some of the differences in the electrochemical performances of these two important battery cathode materials. Results and Discussion The morphology and chemical composition of Li2FeSiO4 (LFS) and LiFePO4 (LFP) nanoparticles are characterized using scanning electron microscope (SEM), transmission electron microscope (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), as shown in Figure 1 and Figure S1. Figure 1(a-c) presents the SEM results of LFS particles, which are synthesized via hydrothermal preparation of an amorphous colloidal precipitate that is subsequently subjected to annealing at 400 °C, 700 °C and 900 °C.5 As shown in Figure1d, the particle size of LFS annealed at 400 °C (denoted as LFS@400) ranges from 10 to 50 nm in the agglomerated secondary particles. Figure 1e shows LFS@700 sample, ranging from 100 ~ 200 nm, which is dominated by the monoclinic phase, with the coexistence of minor orthorhombic phase.5 The crystallinity of LFS@900 nanoparticles (Figure 1f) is characterized using selective area electron diffraction (SAED) pattern, which reveals an orthorhombic (Pmn21) phase as shown in Figure 1g and S2. The EDS mapping indicate the homogeneously distributed silicon, iron and oxygen in the LFS samples as shown in Figure 1h ~ 1k. The EELS analyses, after the background is removed, are shown in Figure S1i and S1j to determine the chemical states of Si and Fe species in the LFS sample.32



Figure 1. SEM and TEM characterizations of Li2FeSiO4 particles. SEM images of LFS sample annealed at (a) 400 °C, (b) 700 °C and (c) 900 °C. Low magnification TEM images of (d) LFS@400, (e) LFS@700and (f) LFS@900. (g) The selective area electron diffraction (SAED) pattern of LFS (LFS@900). (h)The high-angle annular dark field (HAADF) image (LFS@700) and Energy dispersive X-ray spectroscopy (EDS) mapping of (i) O in red, (j) Fe in yellow and (k) Si in cyan.

From an electrochemical point of view, there is a complex phase transition process from the initial monoclinic structure to orthorhombic structure in LFS electrodes upon Li insertion and extraction,5 although the different LFS phases have nearly similar total energy of formation.33 As a matter of fact, the structural evolution of LFS is not limited to the electrochemically Li-extraction induced changes. The morphology characterizations shown in Figure 2 reveals structural variations under STEM mode or EELS spectrum acquisition, such as Li+ ion knock-on displacement34 and particle pulverization that is induced by the different irradiation time. These structural changes are


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due to the kinetic energy of the incident 200-300 kV high-energy electrons of the e-beam irradiation, that could cause electronic excitations and displacement of atoms and ions.35 In order to further understand the structural stability of LFS cathode, a nanopore was drilled through the LFS nanoparticle by convergent e-beam due to knock-on collision36 as shown Figure 2 and S3, and we acquire EELS spectra of LFS crystals as a function of irradiation time to check the variations on chemical composition, structural change and the local electronic states in a nano-sized region of LFS particle. Subsequently, the EELS mapping was performed to monitor the changes of the irradiation region. The EELS spectra and mapping of Li L edge, Fe M edge, Si L edge, O K edge and Fe L edge were acquired in both low-loss region and high-loss region as shown in Figure 2, where the fine structure of electron-loss near edge spectrum (ELNES) provides information about the atomic coordination environment, chemical state and chemical composition. After convergent e-beam irradiation (flux:102-104 e-Å-2s-1), a STEM image (Figure 2d) was recorded as marked square in Figure 2c. Figure 2a and 2b show that the intensity of Li K, Fe M, Si L, O K and Fe L edges decreases with the irradiation time, which implies a mass loss phenomena in LFS nanoparticle by the e-beam knock-on damage. The black dot (Figure 2e and 2f) is filled with carbon species (Figure 2h), confirming the elemental loss in LFS nanoparticles induced by the convergent beam. That is, the dark dot represents the Li, Fe, Si and O species that were driven out by the e-beam at different loss rate. The time for disappearance of Li+ ions is shortest, which causes a series of chemical valence variation on the adjacent ions, especially iron.33 In addition, the increased intensity of C K edge is due to the electron beam induced deposition (EBID) of carbon from the hydrocarbons introduced during the LFS synthesis process5 as shown in Figure 2d and 2h.



Figure 2. EELS analysis and EELS mapping on the LFS@700 samples: (a) EELS at low loss range (b) EELS at high loss range. (c) and (d) STEM images of LFS nanoparticles. (e) Li K edge, (f) Fe M edge, (g) Si L edge, (h) C K edge, (i) O K edge and (j) Fe L edge mappings on the e-beam irradiated region.

The Fe M edge threshold for elemental Fe (ca. 54 eV) are situated very close to (overlapping) the Li K edge threshold for elemental Li (ca. 55 eV), it is difficult to clearly separate the Li K edge from the Fe M edge. However, we were able to obtain EELS spectrum of Li K edge and Fe M 2,3 edge within a discernable resolution with LFS nanocrystals in our study. Figure S4 shows the Fe M edges (blue curve) and Li K edge (red curve) after background subtraction.37 There are double peaks in ELNES of Li K edge, where the first one is pre-edge peak located at about 58.7 eV, the other main peak is situated at about 63.4 eV, This is comparable to the case in the Fe M 2,3 ELNES with the pre-edge (around 55 eV) and main peak at about 57 eV. For the Li K edge, the ELNES reflects the different electronic configurations of the central Li ions, the double peaks is the characteristic feature of Li-O bonding.38 The chemical shift of Li K edge for Li+ ion in LFS is about 3.7 eV, with respect to


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elemental Li atom. The pre-edge peak and main peak in the Fe M 2,3 ELNES represent electron excited from the Fe 3p state to unoccupied states within 3d orbitals, which is used to distinguish the valence state of iron.39, 40 The energy split in the M 2,3 ELNES and the intensity of pre-edge structure are related to the ferric and ferrous irons in Li2FeSiO4 sample. In this case, the Fe3+/ƩFe is estimated as ca. 0.093, deriving from two integrated windows of 2.5 eV width at pre-peak and main peak.39 The ratio of ferric iron (Fe3+) in the sample is probably due to the surficial oxidization of ferrous iron (Fe2+) in the LFS nanocrystals as detected by XPS.5 Figure 3a displays the time-resolved ELNES, that show the superposition of Fe M 2,3 edge and Li L edge. The Li L edge is not fully overlapped and located at the tail of Fe M edge, which is readily distinguished. The pre-edge intensity of both edges is different and the peak intensity evolves with irradiation time. During the exposure of LFS nanoparticle to the e-beam, the characteristic features of Fe M

2,3

edge gradually weaken; the Fe M

2,3

edge shows a gradual fading of shoulder peak as

pre-edge, and the energy splitting between the main peak and the pre-peak increases, indicating the changes to the chemical states of iron.41 The results illustrate that ferrous iron (Fe2+) in the pristine sample is oxidized to ferric iron (Fe3+), resulting in the Fe2+/Fe3+ ratio changes within about 2 s and becoming prominent in 5 s, as shown in Figure 3a. Using the method of 2.5 eV window ratio,39 the ratio of Fe2+/Fe3+ in Fe M pre-edge features gradually decreased with irradiation time, which demonstrates an increase of ferric iron (Fe3+) in the LFS matrix probably caused by the loss of Li+ ions. The removal of Li+ ions would simultaneously change the adjacent Fe-O bond lengths, which directly distorts the geometry of FeO4 tetrahedron.42



Figure 3. The irradiation time lapsed EELS of Li2FeSiO4 cathode (a) The electron-loss near edge spectrum (ELNES) of Fe M 2,3 edge and Li K edge with irradiation time (b) The ELNES of (c) Li K and Fe L3-edges. (d) The ELNES of Si L (e), of O K edge and (f) Fe L 2,3 edges from the LFS sample.

The distortion of FeO4 tetrahedron is further confirmed by the peak evolution of Li K edge and O K edge with the exposure time. The time lapsed Li K edge are derived by removing the Fe-M edges, as shown in Figure 3b, with the Li K edge intensity employed to monitor the Li content.26, 37 Figure 3b shows that the double peak signals of Li K edge (about 59 eV and 64 eV) decrease with irradiation time, especially at the pre-edge peak region, which implies that the Li+ ions are eliminated from the LFS nanoparticle by e-beam irradiation. It is reasonable to assume from the time-lapsed EELS spectra in Figure 3b that the loss of Li increases with prolonged irradiation time. Figure S5 also shows that the Li pre-edge intensity decreases exponentially with irradiation time. The intensity of Li pre-edge decreases rapidly to about 40 % in the first one second, indicating the fast loss of Li+ ions in the incident region. The loss of Li ions results in the distortions of LiO4 tetrahedra with the nominal structural evolution from the initial Li2FeSiO4 to the e-beam induced delithiated Li2-xFeSiO4.


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The electrochemical delithiation process produces a chemical change of iron that is accompanied by a phase transition in the Li2-xFeSiO4 .5, 32 The time-lapsed ELNES of Fe L2,3 peaks are acquired with a magnified Fe L3 edge spectra in Figure 3c and 3f, where the different shapes of the edge show the distinct changes in the chemical state

of iron in the LFS nanoparticle. The L3 edge centered at 709 eV is used to evaluate the content

of the ferrous state, while the peak intensity centered at 709 eV decreases, a new peak centered at 710.5 eV increases that reveals the content of ferric iron with the irradiation time in LFS samples. Figure 3c shows the change in the L3 edge with e-beam irradiation, which reflects the conversion of valence state form ferrous to ferric irons.43 The ELNES of the Si L2.3 edge and L1 edge (Figure 3d) displays undiscernible change with the irradiation time, and no change in the Si peak is indicative of the structural stability of SiO4 tetrahedron compared to that of FeO4 tetrahedron. The O K edge presented in Figure 3e shows that prominent changes are found at the pre-edge peak centered at about 530 eV, which shrinks with the irradiation time. This pre-edge peak is directly related to the Fe3+-O bond changes in the Li-loss-induced distortion of FeO4 octahedron, which illustrates the participation of lattice oxygen in the electron transfer from the Fe2+ to Fe3+ states, and is similar to the normal electrochemical cycle of LFS electrode.44



Figure 4. TEM images of a representative 900 °C annealed well-crystallized Li2FeSiO4 particle (LFS@900) and LiFePO4 (LFP) nanoparticles under electron beam irradiation. (a) Crystalline phase of LFS before e-beam irradiation. (b ~ d) The HRTEM images show the structural evolution of LFS at incident e-beam position. (e) HRTEM image of LFP without e-beam irradiation. (f) The amorphous appeared with e-beam dose above 106 e/Å2. TEM and STEM images of the LFP nanoparticle before (g,i) and after irradiation (h,j), the e-beam drilled a hole in the nanoparticle.

Electron-beam irradiation seems to rapidly induce the removal of Li+ ions, as well as promote the redox reaction (Fe2+/Fe3+ conversion) in LFS nanoparticles. Prolonged irradiation causes a crystalline-amorphous transition process. In order to investigate these phase transformations, a well-crystallized LFS sample with the dominate orthorhombic phase5 was obtained by annealing at 900 °C (LFS@900). In-situ HRTEM experiments are performed at room temperature with the LFS@900 crystals irradiated by e-beam at different current densities. The flux of the e-beam was typically on the order of 102 ~ 106 electrons/(Å2s). The phase transition and structural change of LFS is clearly captured after several-second irradiation with the 200-kV e-beam. The HRTEM images in Figure 4 highlight the e-beam induced crystalline (Figure 4a) to amorphous transition (Figure 4b) of LFS@900. The time-resolved EELS analysis showed that the Li ions were extracted by the e-beam faster than the other chemical species present in the LFS particles. Moreover, the high energy e-beam leads to the formation of vacancy and interstitial sites in the LFS sample, with the accumulation of


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these defects resulting in the loss of the long-range periodicity of orthorhombic LFS. These defects initially aggregate locally33 and then lead to the amorphization of crystalline LFS at a flux of 5×104 e/Å2 s for 10 ~ 20 seconds, as shown by the white dash circle in Figure 4b. If the incident e-beam spot increases to a larger size (flux: 3×104 ~ 4×104 e Å-2s-1), the area of amorphous region also increases (see Figure 4c). The observed nanocrystals (possibly LFS or delithiated ones) are observed by the lattice fringes (black dash circle) in the magnified HRTEM image in Figure 4d. These nanocrystals were possibly present in the pristine LFS before irradiation or newly formed after irradiation, and may play an important role to promote a subsequent phase transition process, as discussed later. The electron irradiation effect induced amorphization of LiFePO4 (LFP) nanocrystals were also investigated with e-beam flux of 2.5×105 e/(Å2s) for several seconds. Figure 4e and 4f show the TEM images of a LFP sample before and after e-beam irradiation, but with a flux of e-beam that is one order magnitude larger than that for amorphization of LFS. In addition, the parallel e-beam could also drill a nanopore in nanoparticles (Figure 4g and 4h), and with a convergent e-beam irradiation (Figure 4i and 4j), the crystalline structure could also change to amorphous state. Figure 4j shows that a nanopore is produced at the centered position of the convergent e-beam and the crystal lattice gradually disappears. It should be noted that the temperature increases with a 200-kV TEM has been estimated to be about 10 ~ 45 K.45-47 This temperature rise is clearly not high enough for an amorphous transition, so irradiation-enhanced diffusion and displacement defects as a result of ionization effect (radiolysis) has been previously proposed as a mechanism to account for e-beam irradiation induced amorphization.46 In addition to knock-on or radiolysis effects, a phase transformation mechanism of the induced electric-field-driven cations diffusion have been proposed



recently.47 The e-beam induced the electric field reuslted in massive atomic displacements, which is the primary mechanism for amorphization and recrystallization process. This phase transition phenomenon from a crystalline to an amorphous structure was also demonstrated in electrochemical delithiated Li2-xFeSiO4 during extraction of more than one Li+ ion.48, 49 The removal of lithium ions from their originally occupied sites induces disorder in the crystal structure. When all the lithium ions are nearly fully extracted, the crystalline Li2-xFeSiO4 becomes completely amorphous FeSiO4, which easily forms a glassy state.31,33 This amorphous transition was experimentally characterized as having a negative influence on the electrochemical performance of a Li2FeSiO4 cathode.48

Figure 5. Structural transition of 400 °C (a, b and c) and 700 °C (d, e and f) annealed well-crystallized Li2FeSiO4 nanoparticles under electron beam irradiation. (a) and (d) The single nanoparticle before irradiation. (b) The amorphous structure after irradiation for 10 s. (c) and (e) The aforementioned amorphous phase recrystallizes during further irradiation (60 s). (f) HRTEM image shows the delithiated LFS nanocrystals formed again inside the amorphous structure. The insets are Fast Fourier Transform (FFT) pattern.

When the e-beam was focused on the low-temperature LFS samples (< 900 °C), a crystal to


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amorphous phase transition also occurred, and recrystallization of the metastable LFS@400 and LFS@700 samples was observed by TEM, which is likely correlated with the smaller particle size (less than 200 nm vs. 500 ~ 1000 nm for LFS@900 sample) 5 and the lower energetics of monoclinic phase with respect to the orthorhombic phase.33 A dose rate and prolonged irradiation time dependences on the phase transition is complicated. The incident electrons could cause the localized electronic excitations, which contribute to the induced electric field, especially for light elements. The density of the excitations increases with the increase of the dose rate, resulting in the electric field dependence of massive atomic displacements.46,47 We summarize the dose and particle size dependent phase transformation on LFS and LFP as shown in Table S2. Upon continuous irradiation dose, recrystallization process in the interior of the amorphous LFS matrix is observed (see Figure 5 and S6), which indicates that the LFS nanoparticles experience a gradual delithiated and atomic displacement process with a crystalline-amorphous-polycrystalline transition. In addition, the nanocrystal (20-100 nm) changes from the initial monoclinic structure (LFS) to the orthorhombic structure (Li2-xFeSiO4) after irradiation is completed (see Figure 5a, 5b and 5c), and illustrates the significant morphological and crystallographic differences compared to the LFS@900 sample, as discussed previously. The lattice spacing in the nanoparticle before irradiation (Figure 5a) is 0.53 nm, corresponding to the (101) planes of monoclinic phase with the space group of P21. Note that (1) the XRD patterns of the LFS@400 and LFS@700 samples have a monoclinic-dominated phase compositions5 and (2) the LFS with P21 space group has a much lower total energies than that of LFS with a P21/n space group, although the P21/n demonstrates high symmetry.33 Under prolonged e-beam irradiation, the crystal structure of LFS@400 nanoparticle (20-30 nm) transformed to an amorphous phase, as shown in



Figure 5b. The initial Fast Fourier Transform (FFT) pattern is replaced by an amorphous diffraction ring, where the diffused halo ring is characteristic of an amorphous phase (insets in Figure 5a and 5b). Further irradiation results in recrystallization of the amorphous LFS matrix, in which nano-sized crystallites (Li2-xFeSiO4 or silicate) are formed. Randomly oriented nano-crystallites are nucleated in the amorphous LFS nanoparticle (Figure 5c). The FFT patterns (insets in Figure 5e) displayed some diffraction pattern among the amorphous rings. Various polymorphs of LFS with different annealing temperatures have been reported, mainly including monoclinic and orthorhombic space groups.5,50 Interestingly, the e-beam irradiation induces structural transformation from monoclinic to orthorhombic phase in the above LFS@400 sample and also in LFS@700 nanoparticle (Figure 5d ~5f). The initial nanoparticle exhibits monoclinic structure, as confirmed by spacing distance and FFT pattern (Figure 5d), and e-beam irradiation leads to the complete disappearance of crystal lattices, i.e., the monoclinic phase is completely transformed to an amorphous structure. Then, upon further irradiation the recrystallization process occurred in the newly formed amorphous phase region. Electron irradiation provided the continuous driving force to form the nucleated nanoparticle inside the LFS amorphous matrix. As a result, the initial nanoparticle is composed of many new nanocrystals (3 ~ 5 nm), as shown in Figure 5c and 5e. The HRTEM image of a newly formed nano-crystallite is shown in Figure 5f, with inter-planar d-spacing distances of 0.24 nm (2d = 0.48 nm), and the corresponding crystallographic structure consists of (102) planes and orthorhombic (Pmn21) [4ത12] zone axis. The initial monoclinic structure (P21) of LFS thus transforms to a stable orthorhombic structure. As matter of fact, this metastable phase transition has also been demonstrated during charging of LFS electrodes at low temperature.5,51, 52


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In addition, because knock-on damage requires a threshold beam energy, the nanopores are formed by e-beam with flux above 106 e/(Å2s), as shown in Figure 5e and Figure S7. The metastable states (amorphous phase and nanocrystal) of LFS are clearly seen around the edge of nanopore, the focused e-beam induced metastability results in surface reconstruction,53 relaxation,17 amorphization and recrystallization.

It is interesting to note that the e-beam induced amorphization and recrystallization of LFS and LFP samples is dependent on the particle size, beam energy and accumulated dose, which is summarized in Table S2. A flux of 102~104 e/(Å2s) is typical for conventional HRTEM, and the LFS and LFP nanoparticles maintains the single-crystal structure without obvious amorphous transition at this dose rate. With increasing flux to 2×104 e/(Å2s), the amorphization of LFS occurs within several seconds. On the other hand, the LFP gradually becomes amorphous at flux of 105 e/(Å2s). The recrystallization process of LFS and LFP sample (10-300 nm) occurs under a total dose of 106 e/Å2, but the recrystallization was not observed in LFS@900 sample (Size > 500 nm). With one order of magnitude larger flux of 106 e/(Å2s), a nanopore in both LFS and LFP is produced by e-beam. The e-beam triggered phase transition involves a series of Li+/electron hopping and structural changes that is an intricate process. Figure S8 shows the details of nucleation process, morphology change and structural evolution of orthorhombic LFS samples (LFS@700) under flux of 104 ~ 105 e/(Å2s). The amorphous structure is formed in localized areas of the orthorhombic LFS particle first, as shown in Figure S8a and S8b. Then, upon further e-beam irradiation, the nucleation of orthosilicate nanocrystals (circled) takes place in the amorphous matrix of the LFS sample (see Figure S8c), with nucleation occurring near the surface region. The crystallized nuclei gradually become a larger crystal cluster and more nanocrystals nucleate, with the nanocrystals visible in lattice structure



(circles in Figure S8c). Finally, the delitiated LFS or silicate crystallites grow bigger in the amorphous matrix (Figure S8d). The lattice strips of crystallized crystallite become distinguishable in the amorphous region, as shown in Figure S8e. The d-spacing in the newly-formed nano-crystallites is ca. 0.24 nm, which is assigned to the (102) planes of orthorhombic phase. In fact, during the synthesizing process, the amorphous LFS colloidal precipitate following the same growth mechanism as crystallization process.54 Moreover, the temperature sensitive LFS polymorph transforms to an orthorhombic structure by ball-milling due to the local stress and frictional heating.55 During the electrochemical cycling, the temperature sensitive Li2-xFeSiO4 also experiences a series of metastable states, transitioning form the monoclinic to orthorhombic structures.56 All these phase transitions, which occur during synthesis and carbon coating, influence the Li storage and transport behavior in LFS cathodes.

Figure 6. LiFePO4 nanoparticle under electron beam irradiation. (a) TEM image of a LFP particle with a nanopore. The inset is SAED pattern. (b) LFP nanoparticle after amorphous transition. The amorphous LFP appeared with e-beam dose


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above 105 e/Å2.The insets are FFT patterns.(c-f) with prolonged e-beam irradiation (20-200 s), the amorphization and recrystallization processes in LFP particle. The inset in f is SAED pattern, showing polycrystalline rings.

The LiFePO4 particles with size of 100-200 nm (Figure 6 and Figure S9) demonstrate significant stronger resistance to the e-beam irradiation compared to that for Li2FeSiO4. The electrochemical cycling studies of LFP resulted in a conventional two-phase separation reaction with the extraction of Li+ ions. However, a metastable phase of LixFePO4 is present under non-equilibrium conditions.57 The HRTEM and in situ TEM observation also showed the presence of this metastable phase (ordered staging structure and disordered solid solution zone) at the LiFePO4-FePO4 interface. This metastable phases is characterized by an ordered staging structure and disordered solid solution zone.58, 59 High power laser is also capable of destroying the delithiated LiFePO4 crystal structure to generate the disordered amorphous phase, which recrystallizes into the metastable FePO4 structure.60 Figure 6 and S10 shows the electron irradiation effect to the LFP nanoparticle. HRTEM observation (Figure 6a) shows the lattice fringes of LiFePO4, corresponding to the crystal faces of (2ത10) and (010) with inter-planar d-spacing of 0.405 nm and 0.61 nm (S.G.: Pmnb), respectively. The LFP loses Li+-ions due to e-beam irradiation, as well as experiencing irradiation damage and metastable transformation.61 The hole drilling phenomena in LFP samples were observed with an e-beam flux of 106 ~ 108 e/(Å2s). A nanopore is produced due to the displacement of ions by high-energy e-beam bombardment inducing ionization plus sputtering, as shown in Figure 6a. The size and shape of the nanopore depends on beam energy and irradiation time.62 There are no obvious crystalline structure changes on the edge of the nanopore, which is different from what is observed in LFS orthosilicate. But the LFP shows a similar e-beam induced amorphization and recrystallization process. We directly visualized an amorphous transition in LFP samples (see Figure6c and 6d), the single-crystal structure completely become amorphous upon a total dose of 106 e/Å2. With further



e-beam irradiation, recrystallization in the amorphous matrix occurred at a dose of 107 e/Å2 (see Figure 6e and 6f). These transformations happen with a flux closed to 105 e/(Å2s). These findings show that LFP has a much higher dose threshold for transformation than that of LFS. In other words, LFP is more resistant to amporhization and recrystallization process under e-beam irradiation, which suggests that it has probably a greater structural stability and excellent electrochemical performance.

Conclusion In summary, through comparing the irradiation-induced metastable states in Li2FeSiO4 and LiFePO4, a dose dependent transformation process of amorphization and recrystallization is demonstrated in LFS and LFP with different particle size by varied e-beam flux and dose. An amorphous transition in LFS and LFS is observed, but the dose (2×104 e/Å2) for the amorphrization of Li2FeSiO4 is lower than that (6×105 e/Å2) of LiFePO4. It was also found that the e-beam can induce a recrystallization of the amorphous LFS and LFP particles with size ranging from 30-200 nm (LFS@400 and LFS@700 samples) to produce nano-crystallites. These results are helpful to understand the phase evolution and the role that metastable phases can play in the structural changes in LFS and LFP. The EELS results demonstrated that mass losses and valence-state changes in LFS are clearly related to the e-beam energy. The e-beam extracts Li+ ions from the LFS nanoparticles, while the valence state of iron changes from ferrous to ferric with this loss of Li. Both LFS and LFP show knock-on damage under higher energy electron irradiation. From an application point of view, our results demonstrate that the structural stability of LFP is better than that of LFS to Li extraction-induced structural changes, thus supporting the choice of LFP as cathode for Li-ion batteries. The investigation of the metastable states using e-beam irradiation can be helpful for


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understanding the intermediate process in a battery material and the electrode reaction kinetic.

Methods Synthesizing LFS nanoparticles. The pristine Li2FeSiO4 (LFS) crystalline nanoparticles are synthesized via preparation of an amorphous colloidal precipitate. In a typical procedure, stoichiometric amounts (0.015 mol) of lithium acetate hydrate (CH3COOLi.2H2O) powder, iron (III) nitrate hydrate (Fe (NO3)39H2O), and tetraethyl orthosilicate (TEOS) were dissolved in distilled water (70 ml) with continuously stirring. After 10 min stirring, 3.75 ml of ethylene glycol (EG) was added to the solution. After stirring for 15 min, 3.75 ml of anhydrous ethylenediamine (EN) was added to the solution. The obtained dark red solution mixture was stirred for 30 min, and then the dark red suspension was transferred into an autoclave and heated at 180 for 3 hours. The different LFS polymorphs were prepared by annealing hydrothermally prepared pristine LFS samples at different temperatures (400 °C, 700 °C and 900 °C). For example, with annealing temperature of 900 °C, the size of pristine LFS particles (ranging from5-20 nm) became to 500 ~ 1000 nm. Commercial LFP nanoparticles (99.0%, Sigma-Aldrich, USA) have been used for the SEM and TEM characterizations

SEM characterization and EDS mapping.

The EDS results were carried out in a FEI Titan

TEM with Super-X detectors and a Hitachi SU8230 equipped with Bruker flatQuad silicon drift detector. SEM characterizations were performed in a Hitachi SU8230 and a Hitachi SU8000 SEM with cold field emission gun.

In situ TEM and EELS characterization. The LFS nanoparticles were dispersed in ethanol by ultrasonic method. Then a drop of the ethanol was deposited on a PELCO TEM grid (copper grid



with lacey carbon film). High energy electron irradiation and in-situ HRTEM observation were performed in a JEOL 2010F TEM, a FEI F20 TEM and a FEI Titan G2 TEM. The e-beam with flux of 102 ~ 106 e/(Å2s) was used for the different irradiation doses and HRTEM characterization. The electron dose rate measured on FEI TEM with TEM or STEM mode (parallel or convergent beam) can be directly read out. For JEOL TEM, the current density of e-beam is measured as 10-500 pA/cm2, and then the unit is converted from pA/cm2 to e/(Å2s). The EELS spectra and EELS mapping were performed in a Hitachi HF 3300 equipped with a cold field emission gun and a GIF Quantum ER with dual-EELS. The spectra were gathered using time-resolved EELS, in dual EELS. The total exposure time was 20 s for low loss and high loss spectra, and 5 s for Li and about 10 s for Fe and Si speciation.

Associated content Supporting Information: TEM image and EELS analysis of LiFePO4 and Li2FeSiO4 samples. The selected area electron diffraction patterns of LFS sample. High resolution TEM image of amophizaiton and recrystalizaiton process on Li2FeSiO4 samples. EDS mapping of LiFePO4 samples

Acknowledgements This work is supported through a Hydro-Québec/Natural Sciences & Engineering Research Council of Canada (NSERC) Collaborative R&D research grant (463484-2014). X. Lu thanks the funding supports from the National Natural Science Foundation of China (11704019).

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Table of Contents (TOC)



Figure 1. SEM and TEM characterizations of Li2FeSiO4 particles. SEM images of LFS sample annealed at (a) 400 °C, (b) 700 °C and (c) 900 °C. Low magnification TEM images of (d) LFS@400, (e) LFS@700and (f) LFS@900. (g) The selective area electron diffraction (SAED) pattern of LFS (LFS@900). (h)The high-angle annular dark field (HAADF) image (LFS@700) and Energy dispersive X-ray spectroscopy (EDS) mapping of (i) O in red, (j) Fe in yellow and (k) Si in cyan. 207x164mm (120 x 120 DPI)


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Figure 2. EELS analysis and EELS mapping on the LFS@700 samples: (a) EELS at low loss range (b) EELS at high loss range. (c) and (d) STEM images of LFS nanoparticles. (e) Li K edge, (f) Fe M edge, (g) Si L edge, (h) C K edge, (i) O K edge and (j) Fe L edge mappings on the e-beam irradiated region. 533x470mm (96 x 96 DPI)



Figure 3. The irradiation time lapsed EELS of LFS cathode (a) The electron-loss near edge spectrum (ELNES) of Fe M 2,3 edge and Li K edge with irradiation time (b) The ELNES of (c) Li K and Fe L3-edges. (d) The ELNES of Si L (e), of O K edge and (f) Fe L 2,3 edges from the LFS sample. 550x379mm (96 x 96 DPI)


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Figure 4. TEM images of a representative 900 °C annealed well-crystallized LFS@900 particle and LFP particles under electron beam irradiation. (a) Crystalline phase of LFS before e-beam irradiation. (b ~ d) The HRTEM images show the structural evolution of LFS at incident e-beam position. (e) HRTEM image of LFP without e-beam irradiation. (f) The amorphous appeared with e-beam dose above 106 e/Å2. TEM and STEM images of the LFP nanoparticle before (g,i) and after irradiation (h,j), the e-beam drilled a hole in the nanoparticle. 332x150mm (96 x 96 DPI)



Figure 5. Structural transition of 400 °C (a, b and c) and 700 °C (d, e and f) annealed well-crystallized LFS nanoparticles under electron beam irradiation. (a) and (d) The single nanoparticle before irradiation. (b) The amorphous structure after irradiation for 10 s. (c) and (e) The aforementioned amorphous phase recrystallizes during further irradiation (60 s). (f) HRTEM image shows the delithiated LFS nanocrystals formed again inside the amorphous structure. The insets are Fast Fourier Transform (FFT) pattern. 228x152mm (120 x 120 DPI)


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Figure 6. LiFePO4 nanoparticle under electron beam irradiation. (a) TEM image of a LFP particle with a nanopore. The inset is SAED pattern. (b) LFP nanoparticle after amorphous transition. The amorphous LFP appeared with e-beam dose above 105 e/Å2.The insets are FFT patterns.(c-f) with prolonged e-beam irradiation (20-200 s), the amorphization and recrystallization processes in LFP particle. The inset in f is SAED pattern, showing polycrystalline rings. 304x202mm (96 x 96 DPI)


In-situ TEM Investigation of Electron Irradiation Induced Metastable

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