Interface Instability in LiFePO4âLi3+xP1âxSixO4 All-Solid-State

Subscriber access provided by University of South Dakota

Article 4

3+x

1-x

x

4

Interface Instability in LiFePO—Li P SiO All-Solid-State Batteries Matthias F. Groh, Matthew J. Sullivan, Michael W. Gaultois, Oliver Pecher, Kent J. Griffith, and Clare P. Grey Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01746 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Interface Instability in LiFePO4—Li3+xP1−xSixO4 AllSolid-State Batteries Matthias F. Groh,‡,1 Matthew J. Sullivan,‡,1 Michael W. Gaultois,1 Oliver Pecher,1 Kent J. Griffith,1 and Clare P. Grey*,1 1

University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, UK

Abstract

All-solid-state batteries (ASSBs) based on non-combustible solid electrolytes are promising candidates for safe and high energy storage systems, but it remains a challenge to prepare systems with stable interfaces between the various solid components that survive both the synthesis conditions and electrochemical cycling. We have investigated cathode mixtures based on a carbon-coated LiFePO4 active material and Li3+xP1−xSixO4 solid electrolyte for potential use in all-solid-state batteries. Half-cells were constructed by combining both compounds into pellets by spark plasma sintering (SPS). We report the fast and quantitative formation of two solid solutions (LiFePO4−Fe2SiO4 and Li3PO4−Li2FeSiO4) for different compositions and ratios of the pristine compounds, as tracked by powder X-ray diffraction and solid-state nuclear magnetic resonance; X-ray absorption near edge spectroscopy confirms the formation of iron silicates

ACS Paragon Plus Environment

1

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

similar to Fe2SiO4. Scanning electron microscopy and energy dispersive X-ray spectroscopy reveals diffusion of iron cations up to 40 µm into the solid electrolyte even in the short processing times accessible by SPS. Electrochemical cycling of the SPS treated cathode mixtures demonstrates a substantial decrease in capacity following the formation of the solid solutions during sintering. Consequently, all-solid-state batteries based on LiFePO4 and Li3+xP1−xSixO4 would necessitate iron ion blocking layers. More generally, this study highlights the importance of systematic studies on the fundamental reactions at the active material–solid electrolyte interfaces to enable the introduction of protective layers for commercially successful ASSBs.

1. Introduction Key goals for improved energy storage systems include improving energy density, safety, reliability, and cost. Lithium-based batteries (LIBs) are one of the most promising storage systems due to their high energy densities and voltages.1 However, most commercial LIBs are based on organic (liquid) electrolytes that are flammable and their decomposition (oxidation) products are highly toxic, and a malfunctioning or damaged battery bears the risk of thermal runaway and subsequent explosion. Accordingly, a transition towards safe and reliable solid electrolytes (SEs), based on, for example, oxides, phosphates and sulphides, would be beneficial. In addition, the resulting all-solid-state batteries (ASSBs) offer the possibility of high temperature applications.2,3 Moreover, Li-based solid electrolytes (SEs) rely on lithium cations as the only charge carriers, thus precluding bulk polarization effects originating from concentration gradients of both mobile cations and anions in liquid-electrolyte-based batteries


2

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


during current flow. Therefore, ASSBs could eventually feature higher current densities and faster (dis)charging than traditional LIBs.4 Classic ASSB assembly methods are based on expensive, multistep thin-film deposition techniques, such as chemical vapour deposition, radio frequency sputtering, or pulsed vapour deposition.5 The recent application of spark plasma sintering (SPS) to ASSB assembly has enabled fast, one-step production of high-density materials with robust electrode/electrolyte interfaces and significant increases in electrode thicknesses.6–9 Despite the abundance of well-established active materials and the recent advances in fast lithium ion SEs, suitable combinations of active materials and SEs that can be processed and/or cycled without formation of ion-blocking interfaces or less-active phases are rare, especially via high-temperature processing.10–14 Since these interfaces are buried within the composite electrode, they are non-trivial to investigate and so direct evidence for the local and long range structures that hinder (or indeed improve) ionic transport and electrochemical performance are generally lacking. Given our prior work on the local structures that form in various LIB electrode materials and SEs and the relationship between local structure and ionic-transport15–20 we have explored a series of electrode/SE composites formed with SPS to investigate the nature of the interfaces formed as a function of synthesis condition and compositions and morphologies of the starting powders. In work reported in this paper, we investigated the combination of LiFePO4 (LFP, Figure 1) and the SE Li3+xP1−xSixO4 (LSPO, Figure 2) due to their anticipated chemical compatibility. Indeed, recent computational studies suggested no thermodynamic driving force for a reaction between LFP and Li3PO4, and additionally suggested the formation of a coherent LFP (010)— γ-Li3PO4 (100) interface that is calculated to be stable up to 1500 K.21,22 Thus, in principle, we


3


Page 4 of 33

hypothesized that with optimized sintering of these two materials, composites with minimal grain-boundary resistance might be formed. Moreover, although Li3PO4 displays poor Li ionic conductivity, it can be greatly improved by introducing silicate ions to form LSPO (σ ~ 10– 3

S/cm at 200 ºC, for x = 0.25, 0.5, 0.75).23 In the range 0 ≤ x ≤ 0.55, LSPO adopts the

orthorhombic γ-Li3PO4 structure (Figure 2, left), while it crystallizes in the monoclinic Li4SiO4 structure for 0.65 ≤ x ≤ 1 (Figure 2, right). Thus, coherent interfaces with orthorhombic LFP might be expected in a phosphate-rich composite. In the literature, two solid solutions between LFP and structurally related iron silicates have been reported: LiFePO4−Fe2SiO424 and LiFePO4−Li2FeSiO4.25 Considering the first, an “Fe-rich” complete solid solution (olivine-Li1–yFe1+yP1–ySiyO4) between LFP and Fe2SiO4 was described by Recham et al.,24 which can be rationalized by the homeotypic crystal structures of the compounds (Figure 1). Substitution of all lithium positions in the LFP structure with iron atoms and those of the phosphorus atoms by silicon atoms leads to the Fe2SiO4 (olivine) crystal structure. The second, “Li-rich” solid solution was introduced by Arachi et al., who reported partial substitution of Li2FeSiO4 with phosphorus by direct SPS synthesis to form “monoclinic m-Li2–z’FeSi1–z’Pz’O4” with z’ ≤ 0.2.25 However, they did not discuss the so-formed crystal structure in detail. The parent phase Li2FeSiO4 features several closely related orthorhombic and monoclinic polymorphs, depending on the synthesis conditions.26 More recently, Sirisopanaporn et al. have identified a polymorph of Li2FeSiO4 with a crystal structure homeotypic to (orthorhombic) γ-Li3PO4.26,27 Thus, a solid solution of γ-Li3PO4 and orthorhombic Li2FeSiO4, “o-Li3–zFezP1–zSizO4”, might be anticipated by partial substitution of the 4c Li site in γ-Li3PO4 (Figure 2, left) by iron atoms and simultaneous substitution of the phosphorus atoms by silicon atoms.


4

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1.

Visualisation of the olivine crystal structure of LFP (Pnma); phosphate groups in

grey, octahedral coordination for iron in red. In the homeotypic crystal structure of Fe2SiO4, the lithium (and original Fe) positions are occupied by iron atoms, and the phosphorus position by silicon atoms.

Figure 2.

Visualisation of the crystal structures of γ-Li3PO4 (left, Pmnb) and Li4SiO4 (right,

P21/m), adopted by the Li3+xP1−xSixO4 (LSPO) compositions with x = 0.25 and 0.75, respectively, studied is this work. Phosphate and silicate groups in grey. In the crystal structure of PmnbLi2FeSiO4, which is homeotypic to γ-Li3PO4, the lithium site 4c (light blue) is occupied by iron atoms and the phosphorus position (black) by silicon atoms.


5


Page 6 of 33

Despite the anticipated stability of orthorhombic LFP toward orthorhombic, phosphate-rich LSPO phases, we report herein the rapid formation of the above mentioned solid solutions upon reaction of LFP with LSPO. In order to explore this reactivity, we investigated composites using LSPO in both structure types (orthorhombic γ-Li3PO4 type for Li3.25P0.75Si0.25O4 and monoclinic Li4SiO4 type for Li3.75P0.25Si0.75O4). The nature of the reaction was investigated by powder X-ray diffraction (XRD), Fe K-edge X-ray absorption near edge spectroscopy (XANES), and solidstate magic angle spinning (MAS) 31P nuclear magnetic resonance (NMR) spectroscopy and was found to depend on the composition as well as ratio of the starting materials. Scanning electron microscopy (SEM) measurements were employed to track the migration of the different ions. Finally, SPS-treated cathode mixtures were electrochemically cycled to investigate their performance.

2. Experimental Section 2.1. Solid state synthesis of pristine materials The materials for use in the SPS experiments were prepared as follows with details given in the subsections below: all mixtures of starting materials were ball-milled for 1−6 h in a ZrO2 jar with two grinding balls of the same material (diameter 10 mm) in a SPEX SamplePrep 8000 shaker mill prior to annealing in alumina crucibles under 20–30 mL min–1 flow of Ar gas in a Carbolite GHA Single Zone tube furnace. All subsequent handling was conducted in an argon-filled UNIlab (M. Braun) or VAC glovebox (p(O2)/p0 < 1 ppm, p(H2O)/p0 < 1 ppm). Rietveld-refined powder diffractograms of carbon-coated LiFePO4 and Li3.25Si0.25P0.75O4 can be found in Figures S1−S2, supporting information.


6

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Carbon-coated LiFePO4 (C-LFP): Gram-scale batches (e.g. 1.6 g) of C-LFP were synthesized following a procedure developed by Kobayashi et al.:28 Li2CO3 (99.997% Aldrich), FeC2O4·2H2O (99%, Aldrich), and NH4H2PO4 (99.999%, Aldrich) were mixed according to the stoichiometric ratio of LFP together with 10 wt.-% Ketjenblack carbon (EC-600JD, AkzoNobel) and annealed at 400 °C for 6 h. The duration of ball milling as well as the alternative use of a reducing Ar/H2 atmosphere instead of pure Ar had no significant influence on the particle size of 60−100 nm according to Rietveld refinements. Li3+xP1−xSixO4 (LSPO): LiOH·H2O (99.95%, Aldrich), SiO2 (fumed, dried at 500 °C overnight, Aldrich), and β-Li3PO4 (Aldrich) were mixed according to the desired mass of usually ~2 g and the stoichiometric ratio (x = 0.25 or 0.75), and annealed at 900 ºC for 12 h. 2.2. Spark plasma sintering (SPS) Pristine C-LFP and LSPO powders were ground together in agate mortars and mixed in 1:2 or 2:1 mass ratios. The ground mixtures were loaded into a cylindrical graphite die (10 mm internal diameter) nested in a larger (20 mm internal diameter) die and cold pressed at 38 MPa for 4.5 min in an FCT Spark Plasma Sintering apparatus under vacuum within an argon-filled UNIlab glove box (M. Braun; p(O2)/p0 < 1 ppm, p(H2O)/p0 < 1 ppm). The pressure was increased to 76 MPa whilst heating to 100 °C below Tmax in 2 min and then to Tmax in 2 min for Tmax in the range 400–800 ºC by applying direct current (no pulses). The samples were held at Tmax for a further 3 min before cooling, to yield dense pellets. In contrast, the pellets with a LFP:LSPO mass ratio of 2:1 were heated to Tmax in 2 min and then held for 5 minutes before cooling. For samples examined by electron microscopy, a mixture of LFP/Li3.25P0.75Si0.25O4/C with a mass ratio of 50:35:15 was cold pressed at 62 MPa in a graphite die with 10 mm internal diameter, layered with pure Li3.25P0.75Si0.25O4, and pressed at 62 MPa. The mixture was


7


Page 8 of 33

subsequently heated under vacuum in the FCT SPS apparatus to 700 ºC in 2 min and then to 800 ºC in 3 min. The temperature was maintained at 800 ºC for 3 min before cooling. Phosphorus-substituted Li2FeSiO4 was synthesized directly in the SPS according to Arachi et al. at 900 °C from Li2CO3, FeC2O4·2H2O, SiO2, and NH4H2PO4 with a nominal composition of “Li1.9FeP0.1Si0.9O4”.25 2.3. Control annealing experiments in sealed silica ampoules C-LFP and Li3+xSixP1−xO4 (x = 0.25 or 0.75) were thoroughly mixed in an agate mortar in the mass ratio of 2:1 and sealed in a carbon-coated fused silica ampoule under dynamic vacuum. The ampoules were heated to 800 °C at 5 K·min−1, annealed for seven days, and cooled to room temperature at −5 K·min−1. 2.4. Powder X-ray diffraction (PXRD) In-house data collection was performed at 296(2) K on a Panalytical Empyrean diffractometer equipped with a Ni filter using Cu-Kα radiation (λ = 154.06 pm, 154.43 pm) in Bragg–Brentano setup. Synchrotron PXRD patterns were collected in transmission geometry using 0.35 mm capillaries at beamline I11 at the Diamond Light Source. The X-ray wavelength was determined as 49.5178 pm by refinement of a Si 640c standard pattern, collected separately. Rietveld refinement was performed using the TOPAS Academic software package (version 4.1).29 Due to the complexity of the mixed phases and their similar lattice parameters (and structure types), atomic positions and occupancy factors were usually fixed at their reported values and only the lattice parameters were refined. Thus, compositions based on the PXRD measurements should be taken with care. Phase identification was based on best fit to reference pattern intensities and lattice parameters within the Rietveld refinement as well as the resulting figures of merit.


8

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In the case of the Rietveld refinements involving o-Li3–zFezP1–zSizO4, the Fe and Li atoms share position 4c and their site occupancy factors were refined while coupled in addition to Si/P to achieve charge neutrality. The atomic parameters of all atoms (except for the above mentioned split positions) in this phase have been refined individually for mixture A (see below, m(CLFP):m’(Li3.25P0.75Si0.25O4) = 1:2), while in the direct synthesis case, only the atomic parameters of the split position of Li/Fe were refined. A synopsis of the lattice parameters of all compounds in each refinement can be found in Table S1, supporting information. Crystal structures were visualized using the Diamond software package.30 2.5. Electron microscopy and energy dispersive X-ray spectroscopy (EDX) Secondary and backscattered electron micrographs were collected using a field emission gun scanning electron microscope (Camscan MX2600) operating at an accelerating voltage of 20 kV. 20 nm of Pd was sputter-deposited on specimens to minimize charging. Energy dispersive X-ray (EDX) spectra were collected using an Oxford Instruments Inca x-act detector. The spatial resolution of each spectrum collected at 20 kV in the materials studied here is estimated to be 2– 4 µm, which is smaller than the distance between points examined by EDX.31 2.6. Galvanostatic cycling experiments Composite cathode powders (m(C-LFP):m(Li3.25P0.75Si0.25O4) = 1:2) prepared by SPS as described above and a pristine C-LFP reference were ground with Ketjenblack carbon (EC600JD AkzoNobel), SuperP carbon (Timcal), and PTFE spheres (Aldrich) in a 3:3:3:1 mass ratio. Thin pellets (20 mg, average thickness 225 µm) were subsequently prepared in a 13 mm die by cold pressing at 45 MPa for 1 min. Coin cells (2032-type) were assembled in an argonfilled UNIlab glove box (M. Braun; p(O2)/p0 < 1 ppm, p(H2O)/p0 < 1 ppm). Metallic lithium foil (Aldrich) was used as the anode and a solution of 1 mol·L–1 LiPF6 in ethylene


9


Page 10 of 33

carbonate/dimethyl carbonate (EC/DMC in a 1:1 volume ratio, Aldrich) as the liquid electrolyte. Room temperature galvanostatic cycling measurements at a nominal rate of C/10 (17 mA·g–1) were performed using a Bio-Logic potentiostat/galvanostat instrument running EC-Lab® software (version 10.40). The lower and upper potential limitations were 2.5 V and 4.2 V in all cases. 2.7. Solid-state NMR spectroscopy SPS-treated composite pellets as well as according mixtures of starting materials were ground in agate mortars and packed into 1.3 mm ZrO2 rotors under argon atmosphere inside a glove box (M. Braun; p(O2)/p0 < 1 ppm, p(H2O)/p0 < 1 ppm).

31

P magic angle spinning (MAS) NMR

experiments were performed at sample spinning speeds of 50 and 60 kHz (to distinguish overlapping signals) on a 200 MHz (4.7 T) Bruker Avance I spectrometer using a Bruker 1.3 mm double resonance MAS probe at ambient temperature. 31P shifts were referenced to 85% H3PO4 using solid NH4H2PO4 (δ = 0.8 ppm;32 99.999%, Aldrich) as a secondary chemical shift reference.33 The latter compound was used for pulse optimization as well. 31P NMR signal line shapes were measured with rotor-synchronized Hahn echo pulse sequences. LFP resonates at a significantly greater

31

P frequency with an associated much shorter spin–lattice relaxation time

(T1) constant than LSPO. Accordingly, 31P spectra were measured with a recycle delay of 15 ms and offset frequency of ~300 kHz (≈ 3700 ppm) to measure the LFP component34 as well as with a recycle delay of 3.2 s and offset frequency of −2 kHz (≈ −25 ppm) for the LSPO component, relative to the Larmor frequency of the primary reference.23 Bruker Topspin was used for raw data handling and processing.35


10

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.8. Fe K-edge XANES Fe K-edge spectra were collected in transmission geometry at the Diamond Light Source using beamline B18. Samples were handled in a UNIlab glove box (M. Braun; p(O2)/p0 < 1 ppm, p(H2O)/p0 < 1 ppm); powders were finely ground, intimately mixed with dried cellulose, and pressed to form pellets of appropriate optical density for transmission experiments. Transmission spectra were collected at room temperature (296(2) K) with an energy step size of 0.3 eV through the absorption edge. The incident photon energy was calibrated using Fe metal foil (E0 = 7112 eV) positioned behind the sample; no drift in energy was seen during the experiment. 3. Results and discussion Carbon-coated LiFePO4 (C-LFP) and Li3+xP1−xSixO4 (x = 0.25; 0.75) were combined via SPS into cathode composites for Li-ion ASSBs at temperatures ranging from 400 °C to 800 °C. Depending on electrolyte composition and mass ratio m(C-LFP):m’(LSPO) of the starting materials, we observed the formation of different solid solutions upon thermal treatment. This work examines three different mixtures: A: m(C-LFP):m’(Li3.25P0.75Si0.25O4) = 1:2 B: m(C-LFP):m’(Li3.25P0.75Si0.25O4) = 2:1 C: m(C-LFP):m’(Li3.75P0.25Si0.75O4) = 2:1 Although a maximised ratio of AM to SE would be the ideal case for an ASSB, mixture A represents a typical ratio that had proven to be successful in literature,6 while B and C were chosen to investigate the influence of the mass ratio of the active material as well as of the silicate content on the reactivity between the two components.


11


Page 12 of 33

3.1. Powder X-Ray Diffraction (PXRD) Despite the anticipated possible formation of coherent LFP (010)—γ-Li3PO4 (100) interfaces,21,22 powder X-ray diffraction (PXRD) following SPS processing revealed that C-LFP rapidly reacted upon sintering of cathode mixtures A, and the effect becomes more pronounced with increasing SPS dwell temperature (Figure 3, Figure S3, supporting information). Further, the splitting of the LSPO PXRD reflections confirms a chemical reaction is occurring between C-LFP and LSPO to form, on the basis of the Rietveld refinement of the sample prepared at 800 o

C (Figure 4), a silicate-depleted LSPO phase with a composition close to γ-Li3PO4 and thus with

smaller lattice parameters (accounting for a phase fraction of 33% for the 800 oC sample) and a second phase with a structure closely related to γ-Li3PO4 but with significantly increased lattice parameters leading to a shift of the LSPO reflections to lower diffraction angles. Rietveld refinement indicates an iron substitution of approximately 8% on the Li 4c site of the latter phase (for the 800 oC sample), which agrees with the occupation of the respective sites in the crystal structure of Pmnb-Li2FeSiO4,27 suggesting that this phase is part of a solid solution between γLi3PO4 and the homeotypic orthorhombic Li2FeSiO4 phase. This solid solution o-Li3–zFezP1– zSizO4

(z ≈ 0.1 from Rietveld refinement) is proposed to be an alternative to the reported solid

solution “m-Li2–z’FeSi1–z’Pz’O4” (z’ ≤ 0.2) described by Arachi et al,25 a suggestion that will be explored by solid state NMR spectroscopy.


12

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3.

PXRD patterns of C-LFP/Li3.25P0.75Si0.25O4 composites mixed in a 1:2 mass ratio

(A) before and after sintering at a range of SPS dwell temperatures. Full patterns in Figure S3, supporting information. Note the decrease of the LFP reflections and the splitting of the LSPO reflections upon increased sintering temperatures.

In addition, weak reflections of the “Fe-rich” complete solid solution of LFP and Fe2SiO4, olivine-Li1–yFe1+yP1–ySiyO424 are visible. By comparing the distinct differences in the intensity distribution of the Bragg reflections of known members of the olivine Fe-rich solid solution Li1– yFe1+yP1–ySiyO4,

we propose that the formed member phase has a silicate content of

approximately y = 0.4.


13


Figure 4.

Page 14 of 33

Rietveld refinement of a powder X-ray diffractogram (Cu-Kα radiation; λ =

154.06 pm, 154.43 pm) of a C-LFP/Li3.25P0.75Si0.25O4 composite mixed in a 1:2 mass ratio (sample A) after SPS-treatment at 800 °C. The ratio of refined phases is indicated by their weight percentages. The very low intensities (and thus small phase fractions – 5% following treatment at 800 °C) of the olivine-Li1–yFe1+yP1–ySiyO4 reflections at this composition, as well as due to the overlapping reflections with the other phases, meant that the PXRD results were not conclusive. In addition, poorly crystalline phases not observable by XRD or different compounds with a structure similar to γ-Li3PO4 may be present, further motivating the use of

31

P MAS NMR to

confirm the presence of the various solid solutions (see section 3.2). We next investigated Fe-richer compositions by increasing the mass ratio of C-LFP in the cathode mixture to m(C-LFP):m’(Li3.25P0.75Si0.25O4) = 2:1 (B). Again, we found increased consumption of both constituents of the composite upon sintering with increasing SPS dwell temperature (Figure S4, supporting information). Due to the increased amount of C-LFP, we could clearly attribute the newly formed compounds to a mixture of silicate-depleted LSPO with lattice parameters close to pure γ-Li3PO4 and a member of the Fe-rich olivine solid solution Li1– yFe1+yP1–ySiyO4

24

with approximately y = 0.4, on the basis of the cell parameters as well as

intensity patterns extracted from the Rietveld refinement of the sample following SPS-treatment at 800 °C (Figure 5; see also the comparisons in Figures S4 and S5, supporting information). This reaction is complete (i.e. full reaction of Li3.25P0.75Si0.25O4 to form γ-Li3PO4 and a Sisubstituted olivine) if the sample is SPS-treated at 800 °C or annealed at the same temperature in an evacuated fused silica ampoule for several days (Figures S4–S6, supporting information),


14

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


confirming both the thermodynamic stability of the observed phases, and that in this system, fast SPS processing achieves the thermodynamic products at 800 °C within minutes.

Figure 5.


154.06 pm, 154.43 pm) of a C-LFP/Li3.25P0.75Si0.25O4 composite mixed in a 2:1 mass ratio (B) after SPS-treatment at 800 °C. Note the residual C-LFP and graphite from the die set. The ratio of refined phases is indicated by weight percentage.

The

final

system

investigated

was

a

more

Si-rich

composite

with

m(C-

LFP):m(Li3.75P0.25Si0.75O4) = 2:1 (C). Upon sintering of this composite, C-LFP and Li3.75P0.25Si0.75O4 react and, as before, the extent of reaction increases with SPS dwell temperature (Figure S7, supporting information) resulting in a large number of different phases (Figure 6). As expected for the higher silicon content, a silicate-rich member of the olivine-Li1– yFe1+yP1–ySiyO4

24

solid solution is formed with a composition close to its endmember Fe2SiO4,

based on extracted lattice parameters and the best fit of the intensity pattern. Subsequently, LSPO is depleted of silicon and a phosphorus-rich member of Li3+xP1−xSixO4 can be detected with a composition again close to pure Li3PO4 (Figure 6, see additional Rietveld refinement and


15


Page 16 of 33

comparison in Figures S8 and S9, supporting information). In addition, we identify FeO (Wuestite)36 and lithium metasilicate Li2SiO3 as minor by-phases.37 Finally, we detected a small fraction of the Pmnb modification of Li2FeSiO4.27 Thus, both cases A and C lead to the additional formation of o-Li3–zFezP1–zSizO4/Li2FeSiO4 along with olivine-Li1–yFe1+yP1–ySiyO4. The o-Li3–zFezP1–zSizO4/Li2FeSiO4 formation is suppressed in case B, presumably due to the much lower Si content. In contrast to A and B, we found residual LSPO starting material, Li3.75P0.25Si0.75O4. Thus, SPS processing of a sample of m(C-LFP):m(Li3.75P0.25Si0.75O4) = 2:1 (C) at 800 °C for five minutes does not result in a quantitative reaction of the electrolyte. However, a sample prepared by annealing an analogous mixture in an evacuated fused silica ampoule for several days led to complete consumption of both starting materials to form silicate-rich olivine-Li1–yFe1+yP1–ySiyO4 (Fe2SiO4), phosphorus-rich Li3+xP1−xSixO4 (Li3PO4), Li2FeSiO4, and a minor by-phase with distinct Bragg peaks that most likely corresponds to yet another (unknown) solid solution (Figures S7–S9, supporting information). This suggests that these phases are the thermodynamically favoured products, while FeO and lithium metasilicate can be viewed as (metastable) decomposition products of Li2FeSiO4, which only occur during SPS processing, possibly due to local temperature gradients and hotspots.


16

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6.


154.06 pm, 154.43 pm) of a C-LFP/Li3.75P0.25Si0.75O4 composite mixed in a 2:1 mass ratio (C) after SPS-treatment at 800 °C. The ratio of refined phases is indicated by weight percentage.

3.2. NMR Studies The formation of poorly-ordered phases is rather common at lower temperatures and short processing times, particularly for silicates, and they may not necessarily be readily detected using XRD. Accordingly,

31

P MAS solid state NMR measurements were conducted to support

the conclusions of the PXRD results concerning the formation of solid solutions by the reaction of LFP and LSPO. The latter two materials are easily distinguished by their paramagnetic LFP shows a larger

31

31

P NMR signals;

P NMR shift of ~3500 ppm compared to LSPO, which

features a sharp signal at 0 ppm consistent with the paramagnetic and diamagnetic local environments surrounding the phosphorus nuclei in the two phases, respectively.23,34 The major source of the large shifts is the hyperfine interaction, which is a measure of the spin density transferred from the Fe2+ ions to the P 2s orbitals, via the intervening oxygen ions, the total shift depending on the number of paramagnetic ions in the local coordination shell and factors such as the Fe-O-P bond angles and the degree of orbital overlap.38–40 The proposed members of the olivine solid solution Li1–yFe1+yP1–ySiyO4 similarly exhibit

31

P

signals with large positive (hyperfine) shifts. The reported shifts for both the y = 0.35 and 0.75 composition were similar to that of LFP (3400 ppm), the dominant contribution to the shifts in both sets of compounds arising from the hyperfine shift from the Fe2+ ions.24 Unlike LFP, however, Li1–yFe1+yP1–ySiyO4 exhibits a broad, featureless

31

P NMR signal.24 Surprisingly, the

31

P signal of Li1–yFe1+yP1–ySiyO4 was not affected by a change of the composition from y = 0.35


17


Page 18 of 33

to 0.75, which was attributed to the formation of local clusters of LFP-like composition within the structure that would maintain a similar environment for the phosphorus atoms even close to compositions of the Fe2SiO4 endmember.24 For the second proposed solid solution, o-Li3–zFezP1–zSizO4, between the orthorhombic compounds γ-Li3PO4 and Li2FeSiO4, no prior NMR experiments have been reported in the literature to the best of our knowledge. Thus, we directly synthesised a member of this solid solution (nominal “Li1.9FeP0.1Si0.9O4”; Figure S10, supporting information) via SPS according to the methods of Arachi et al.25 The associated X-ray diffractogram was found to consist predominantly of o-Li3–zFezP1–zSizO4 (refined with z = 0.564(4)) among the minor by-phases γLi3PO4, FeO, Li2SiO3, and graphite from the die set. The measured 31P NMR spectrum contains two signals (Figure S11, supporting information), a sharp one at 0 ppm that we attribute to diamagnetic γ-Li3PO4 (or a similar member of the LSPO solid solution), and one at ~2200 ppm that we attribute to o-Li3–zFezP1–zSizO4. The smaller zSizO4

31

P NMR hyperfine shift of o-Li3–zFezP1–

compared to LFP can be rationalised with the differences in the local environment

surrounding the phosphate anions (Figure S12, supporting information). The phosphate ions in LFP are connected to an additional iron site (named “Fe1” in Figure S12, supporting information) compared to Li2FeSiO4 and thus five instead of four iron ions are bonded to the anion. On varying the compositions of the two solid solutions, these structural differences become even larger: in the olivine solid solution, all adjacent lithium sites will be gradually substituted by additional iron atoms. In Li3–xFexP1–xSiyO4 however, the iron atoms are substituted by lithium ions leading to fewer iron atoms around the phosphate ions. If the substitution proceeds via the formation of local clusters as described above for the olivine structures, the shift of ~2200 ppm could be expected for a wide compositional range of o-Li3–zFezP1–zSizO4.24


18

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We have recorded 31P NMR spectra for all three investigated cases A–C in two steps, the first with a short recycle delay and a carrier (= excitation/offset) frequency similar to the Larmor frequency of 31P in LFP (left and middle in Figure 7 (sample A), Figure 8 (sample B), Figure 9 (sample C)) and the second with a longer recycle delay and a carrier frequency close to the signal of LSPO (right hand side in respective figures). For all investigated combinations of starting materials, a decrease of the sharp LFP signal at ~3500 ppm with increasing SPS dwell temperature is detected, consistent with the consumption of C-LFP as discussed earlier (section 3.1). The decrease of the LFP signal correlates with the silicate content of the electrolyte (Figure S13, supporting information) supporting the formation of iron silicate species through the consumption of Fe from LFP and Si from LSPO. The signal of the olivine solid solution Li1–yFe1+yP1–ySiyO4, expected based on the XRD results, seems to be broadened to the extent that no clear signal can be detected above the (sharper) line shape from the remaining LFP. However, in the case of A, the remaining signal around 3500 ppm is unlikely to be due to LFP since this phase is no longer detected in the XRD pattern following treatment at 800 °C (cf. section 3.1) and thus it probably stems from Li1– yFe1+yP1–ySiyO4

(y = 0.4), its weak intensity being consistent with its very small phase fraction

(5%). Additionally, the signal is slightly shifted to higher ppm-values. We suggest that the above described local LFP-like clusters24 in olivine-Li1–yFe1+yP1–ySiyO4 are at least partially suppressed under SPS conditions, leading to

31

P local environments with more Fe ions in their local

coordination shells and a larger 31P hyperfine shift. Only in the case of A was a clear additional

31

P signal at ~2200 ppm (Figure 7) detected,

which we attribute to the second solid solution o-Li3–zFezP1–zSizO4 (z ≈ 0.1). Although the solid solution o-Li3–zFezP1–zSizO4 occurs also in case C with its silicate-rich endmember Li2FeSiO4,


19


Page 20 of 33

(cf. section 3.1), only the higher P:Si ratio of case A leads to the partial substitution of silicon by phosphorus and thus the additional signal at ~2200 ppm. In addition, the phase fraction of Li2FeSiO4 in case C is low (~8 wt.-%), thus a slight phosphorus substitution to form o-Li3– zFezP1–zSizO4

would only result in a negligible 31P signal.

The LSPO signal at ~0 ppm is affected by the ratio of starting materials, the SPS dwell temperature, and the used recycle delay. For both silicate-poor mixtures (A and B), a decrease of the LSPO signal (measured with the long recycle delay of 3.2 s) occurs with higher SPS dwell temperatures (Figure 7, right and Figure 8, right) due to the formation of the paramagnetic olivine-Li1–yFe1+yP1–ySiyO4 solid solution. However, the LSPO signal intensity is strongly influenced by the recycle delay, especially in the case of m(C-LFP):m(Li3.25P0.75Si0.25O4) = 1:2 (A, Figure 7). After SPS treatment, the NMR signal at ~0 ppm shows significantly increased intensity when recorded with the shorter 15 ms recycle delay, even though the overall amount of LSPO has decreased. This counterintuitive behaviour can be rationalized with a faster magnetization relaxation for a fraction of LSPO after processing by SPS. Indeed, measurements of the spin–lattice relaxation time (T1) for these signals indicate a second component with a short T1 in the order of milliseconds that was absent in the pristine electrolyte. We attribute this to the paramagnetic relaxation induced by the migration of paramagnetic Fe2+ ions from LFP into the LSPO phase, and as well from residual Fe2+ ions in the new-formed Li3PO4 phase that result from the original LFP particles, after iron ions have moved into LSPO. This migration of Fe2+ ions is supported by SEM and EDX measurements, described below (section 3.4). The above-mentioned formation of Li3PO4 at the position of the former LFP particles leads to an increase of the diamagnetic

31

P signal at ~0 ppm (at both recycle delays) in the case of the


20

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


silicate-rich mixture C (Figure 9), because the amount of formed Li3PO4 exceeds the parallel consumption of LSPO for olivine-Li1–yFe1+yP1–ySiyO4.

Figure 7.

31

P MAS NMR spectra recorded with a recycle delay of 15 ms and offset

frequency of ~300 kHz (≈ 3700 ppm) for m(C-LFP):m(Li3.25P0.75Si0.25O4) = 1:2 (A) before and after sintering at 600 °C and 800 °C (left and middle), and with a recycle delay of 3.2 s and offset frequency of −2 kHz (≈ −25 ppm; right). The red arrows indicate the offset frequencies. Spectra were recorded with otherwise identical measurement conditions. Spinning sidebands are denoted with asterisks. Please note the enhanced LSPO intensities for the shorter recycle delay time for samples sintered at higher temperatures.


21


Figure 8.

Page 22 of 33

31


frequency of ~300 kHz for m(LFP):m(Li3.25P0.75Si0.25O4) = 2:1 (B) before and after sintering at 600 °C and 800 °C (left and middle), and with a recycle delay of 3.2 s and offset frequency of −2 kHz (right). The red arrows indicate the carrier frequencies. Spinning sidebands are denoted with asterisks. Please note the enhanced LSPO intensities for the shorter recycle delay time after sintering at higher temperatures.


22

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9.

31


frequency of ~300 kHz (≈ 3700 ppm) for m(C-LFP):m(Li3.75P0.25Si0.75O4) = 2:1 (C) before and after sintering at 600 °C and 800 °C (left and middle), and with a recycle delay of 3.2 s and offset frequency of −2 kHz (≈ −25 ppm; right). The red arrows indicate the carrier frequencies. Spectra were recorded with otherwise identical measurement conditions. Spinning sidebands are denoted with asterisks.

3.3. XANES The Fe K-edge XANES is characteristic of the local geometry and electronic structure, and has been extensively used to characterize the Fe chemical environment in many materials, including phosphates and silicates.41–43 Accordingly, Fe K-edge X-ray absorption measurements were performed to investigate the nature of change for Fe in the LFP-LSPO composites studied here (Figure 10). No significant change in the absorption energy was observed (

Interface Instability in LiFePO4âLi3+xP1âxSixO4 All-Solid-State

Recommend Documents

Interface Instability in LiFePO4âLi3+xP1âxSixO4 All-Solid-State