Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 6072−6076
To Which Extent Is Paramagnetic Solid-State NMR Able To Address Polymorphism in Complex Transition-Metal Oxides? Chiara Ferrara,† Stefania Ferrari,† Marcella Bini,† Doretta Capsoni,† Guido Pintacuda,‡ and Piercarlo Mustarelli*,§ †
Department of Chemistry, Section of Physical Chemistry, University of Pavia, Via Taramelli 16, 271001 Pavia, Italy Centre de RMN à Très Hauts Champs, Institut des Sciences Analytiques, Université de Lyon (ENS-Lyon, UCB Lyon 1, CNRS UMR 5280), 5 rue de la Doua, 69100 Villeurbanne, France § Department of Materials Science, University of Milano - Bicocca, and INSTM, Via Cozzi 55, 20125 Milano, Italy Downloaded via KAOHSIUNG MEDICAL UNIV on October 5, 2018 at 14:25:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: A detailed characterization of the polymorphs constituting cathode materials, both before and after cell cycling, is mandatory to develop more stable and powerful lithium batteries. In many cases, e.g., for transition metal lithium silicates, standard diffraction techniques cannot give a clear-cut response. Here we show that broadband adiabatic fast MAS NMR can give unique information in the case of model Li2(Mn,Fe)SiO4 high-capacity cathode materials. By coupling 7Li and 29Si 1D and 2D spectra, we are able to address polymorphs speciation also in the mixed Mn/Fe compositions, which is a nearly impossible task for X-rays and neutrons diffraction. We finally discuss the conditions under which this approach is useful when applied to rare nuclei such as 29Si.
T
However, these materials suffer of extended polymorphism, due to the small formation energy differences between the different possible crystallographic structures.13−15 Different polymorphs, with potentially different electrochemical behavior, can be prepared depending on synthetic route, composition, sintering temperature, heating/cooling rate,8−10,16 and this issue is exacerbated in M−M′ solid solutions.12 In these polymorphs all the cations occupy distorted tetrahedral positions. The polymorphs can be divided into two main classes, called β and γ, which are stable in different temperature ranges.17 In the β structure (stable at lower temperature) all the tetrahedra point along the same direction, while in the γ phases (stable at higher temperature) the tetrahedra are organized in groups of three units with the central one pointing in the opposite direction with respect to the other two. Where both β and γ forms exist for a given composition, the phase transition from β to γ implies the inversion of half sites. For Mn- and Fe- based materials the most common polymorphs are βII (space group Pmn21), γs (space group P21/n) and γII (space group Pmnb).17 The crystal structures are reported in Figure S1. The main problem limiting complete structural characterization is related to the strong similarities between these crystal
here is increasing need of more performing rechargeable batteries for demanding applications, such as in automotive and smart grids.1 At present, the energy storage market is dominated by Li-ion batteries, which, however, suffer of significant drawbacks, e.g. low volumetric/gravimetric energy density, and reduced cycling capabilities, which are chiefly related to the cathode compartment.2,3 Polyoxyanion systems, such as the olivine LiMPO4 with M transition metal (TM), have been proposed as valid alternative thanks to their structural stability (due to the strong P−O bonds network), the reduced costs and high performances.4−6 In this frame, the family with general formula Li2MSiO4 is particularly attractive for the enhanced stability related to Si− O bonds, the low cost and environmental benignity due to the presence of Si.7 A key feature of these compounds is the possible extraction of two lithium ions per formula unit related to the M2+/M3+ and M3+/M4+ redox processes, which, in principle, should give capacity above 300 mAh g−1. Within this family, Li2FeSiO4 has been early studied thank to its promising electrochemical properties.7 Li2MnSiO4 is even more promising thanks to the high potential of the Mn2+/4+ couple with respect to the Li/Li+.8,9 The major drawbacks of these oxides are low electronic conductivity,10 and structural stability on cycling. To overcome these issues, several strategies were explored, including the synthesis of solid solutions Li2MxM′1−xSiO4 with particular attention for the couple Fe/ Mn.11,12 © XXXX American Chemical Society
Received: August 20, 2018 Accepted: October 2, 2018 Published: October 2, 2018 6072
DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072−6076
Letter
The Journal of Physical Chemistry Letters
Figure 1. Left: 7Li and 29Si 1D fast MAS NMR spectra of sample Mn1 (x = 0). Right: 7Li and 29Si 2D aMAT NMR spectra of the same sample. Symbols mark different NMR lines: central peaks of the silicate polymorphs (∗), secondary impurities (○ and ●). In the 1D spectra, the lines with no marks are spinning sidebands.
Table 1. Iso = Isotropic Chemical Shift (ppm) and Aniso = Anisotropy Parameter (ppm)b Pmn21
P21/n
7
29
Li
sample
Pmnb
7
Si
29
Li
7
Si
29
Li
Si
iso
aniso
iso
aniso
iso
aniso
iso
aniso
iso
aniso
iso
aniso
Mn1
−122
−820
7453
880
700
−101
−800
7617
900
−115
700
a
a
a
a
−
−
−
−
Mn05
−105
800
a
a
a
a
−
−
−
−
Mn025
−106
900
a
a
a
a
−
−
−
−
−
−
−
−
−1250 1000 −1250 1000 −800 850 −700 1100 820 −1100
7747
Mn075
−93 −47 −95 −58 −55 1 −52 2 −57 −6
3386
1000
−
−
−
−
Fe1
a
a
Complex manifolds due to Fe−Mn substitutions are observed. See Figures 3, S5, and S6 and related discussion. bIn the P21/n polymorph there are two non-equivalent sites for lithium and only one for silicon.
were possible for mixed samples,12 as well as for the sister compound Li2CoSiO4.21 Our aim, here, is to explore how far we can push the analytic capabilities of solid-state NMR to investigate extended polymorphism in TM-based oxides. The samples of the family Li2Mn1−xFexSiO4 were prepared following our previous experience,22 in order to obtain (when possible) the coexistence of more than one polymorph for each composition. The NMR study was performed both on 7Li and 29Si nuclei, to get an insight on both the ions involved in the electrochemical process, and the species involved in the structure skeleton. The use of abundant 7Li as the NMR probe allowed us to strongly reduce the experimental acquisition time. The use of 29Si NMR (natural abundance 4.7%) is proposed here on such system for the very first time. 29Si measurements were performed on isotopically enriched samples (see Supporting Information). Advanced solid-state NMR techniques were needed to manage paramagnetic interaction: the combined use of ultrafast MAS, and a new 2-D sequence (aMAT),23 based on adiabatic pulses (SHAPs)24 and able to achieve 100% spin inversion on ∼1 MHz chemical
structures that imply very similar diffraction patterns, leading to ambiguous and nonunique indexing of the patterns and thus to many controversial determinations, chiefly in case of mixed M/M′ compounds.12 Figure S2a,b show the simulated XRD patterns for all the above polymorphs, and the experimental laboratory XRD results obtained for the Li2Mn1−xFexSiO4 (x = 0, 0.25, 0.50, 0.75, 1) samples, which in the following we will name Mn1 (x = 0), Mn075 (x = 0.25), Mn05 (x = 0.5), Mn075 (x = 0.25) and Fe1 (x = 1). As even Rietveld refinement is not able to fully discriminate among all the possible polymorphs, a local probe such as solid-state NMR is mandatory to highlight local distortions and changes in the relative orientations of the tetrahedra. 6Li fast MAS NMR has been applied to the study of Li2FeSiO4,18,19 Li2MnSiO4,19,20 and of some samples of the series Li2Mn1−xFexSiO4.12 Despite of its low natural abundance (7.59%) and the strong hyperfine interaction with the TM unpaired electrons (see Supporting Information), 6Li led to well-resolved spectra for the Fe and Mn end-members of the series, where the authors fully addressed the chemical shifts and the anisotropy patterns of all the involved polymorphs. However, no clear-cut conclusions 6073
DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072−6076
Letter
The Journal of Physical Chemistry Letters
Figure 2. Left: 7Li and 29Si 1D fast MAS NMR spectra of sample Fe1 (x = 1). Right: 7Li and 29Si 2D aMAT NMR spectra of the same sample. An asterisk marks the central peaks of the silicate polymorph, and no secondary impurities are detected. In the 1D spectra, the lines with no marks are spinning sidebands.
Figure 3. Left: 7Li 1D fast MAS. Right: 29Si 2D aMAT spectrum of the Mn025 (x = 0.75) sample. Green and blue sideband patterns refer to P21/n and Pmn21 polymorphs, respectively. Symbols mark different NMR lines: central peak of the silicate polymorphs (∗), secondary impurity (○). In the 1D spectra, the lines with no marks are spinning sidebands.
terms. The 7Li line-widths of both isotropic and sidebands peaks increase by roughly one order-of-magnitude from Mn1 (∼10 ppm) to Fe1 (∼100 ppm). This is due to anisotropic bulk magnetic susceptivity (ABMS) related to the g-tensor anisotropy of d6 configuration, rather than to relaxation effects.18,19 Such a big line-width difference is not observed in 29Si spectra because of concurrent contributions, such as isotropic chemical shift due to the bigger Si electronic cloud. Figure 1 shows the 1D and 2D 7Li and 29Si NMR spectra of sample Mn1. We clearly evidenced the presence of the three polymorphs Pmn21, P21/n, and Pmnb. In particular, the P21/n space group has two nonequivalent crystallographic positions for lithium (named 1-P21/n and 2-P21/n) with multiplicity 1:1, and only one for silicon. The best-fits of the 1D MAS NMR spectra (left) are well supported by the isotropic slices of the
shift range with anisotropies in excess of 100 kHz, made it possible to obtain unsurpassed information on this class of compounds. Figure S3 shows the 1D 7Li and 29Si fast MAS NMR spectra of all the samples under study. The 7Li spectra show a spinning sideband manifold extended on about 4000 ppm, which is due to through-space electron−nucleus magnetic dipolar interaction.18,20,25 The region of isotropic peaks is marked with stars. The isotropic chemical shifts are given by Fermi-contact (hyperfine interaction transferred to chemical bonds) for Mn1 (Mn2+, d5 configuration),20 and by a combination of Fermicontact and pseudocontact (due to anisotropy of the g-tensor) terms with roughly the same intensity for Fe1 (Fe2+, d6 configuration).18 The chemical shifts of the mixed samples are due to differently weighed combinations of these two 6074
DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072−6076
Letter
The Journal of Physical Chemistry Letters 2D indirect dimension (right). Table 1 reports the 7Li chemical shifts and the anisotropy values, in excellent agreement with the 6Li results of the group of Dominko.20 We report here, for the first time in the literature, the corresponding tensor elements of 29Si. The relative amounts of the polymorphs, obtained by fitting the entire 7Li and 29Si sidebands manifolds, are reported below in Figure 4 together with the results of XRD Rietveld analysis. Figure 2 reports the same spectra of Figure 1 for the sample Fe1. Here, the spectra are simpler, since only the P21/n polymorph is observed. The apparent intensity difference of sites 1 and 2 is due to their different anisotropy (see Table 1). Again, the parameters of 7Li chemical shift tensor are in excellent agreement with previous literature.18 Noteworthy, fast MAS (60 kHz) applied to 7Li gave us about the same resolution of previous 6Li experiments with a sensitivity increase of about one order-of-magnitude due to the different isotopic abundances. Figure 3 shows the 7Li 1D fast MAS spectrum and the 29Si 2D aMAT spectrum of the Mn025 (x = 0.75) sample. Besides a diamagnetic impurity (3% Li2SiO3, see Rietveld refinements in the Supporting Information), the 7Li spectrum clearly shows the presence of two polymorphs, Pmn21 and P21/n, whose chemical shifts are very similar to those reported by Sirisopanaporn et al. for the sample xFe = 0.8.20 The 29Si spectrum is characterized by a complex manifold which is only partially resolved on the isotropic axis. As a result of the aMAT sequence, the one-dimensional spectrum is decomposed into a series of isotropic peaks, each of which correlates with a resolved sideband pattern.23 The number of isotropic peaks is due to all the possible (Mn, Fe) configurations around the Si sites. As in both the polymorphs Si tetrahedra are surrounded by four TM distorted tetrahedra, a maximum of 16 different configurations per polymorph are possible (see Figure S4), which can be grouped in terms of the binomial C(n,r) coefficients (n = 4, 0 ≤ r ≤ 4) [1, 4, 6, 4, 1]. Clement et al. discussed the system LiFexMn1−xPO4 (32 configurations per polymorph) and showed by means of DFT calculations that most of the possible configurations are equivalent in terms of chemical shifts. However, as pointed out by the same Authors, DFT calculations on the TM−O−P pathways contributions were affected by large uncertainties.23 The same must be expected for TM−O−Si pathways, as 29Si is very similar to 31P from the NMR point of view. By considering the symmetry of the 29Si spectra reported in Figure S3, we can infer that Fe for Mn substitution takes place in a nearly random fashion. Moreover, the comparison of the 29 Si chemical shifts of samples Mn1 (Figure 1) and Fe1 (Figure 2) indicates that each substitution accounts for 800−900 ppm shift downfield. By considering the relative amounts of P21/n (78%) and Pmn21 (22%) polymorphs determined by 7Li spectra (see Figure 3, left, and Figure 4), we were able assigning the sideband patterns of the 29Si aMAT spectrum to the different SiO4(MnrFe4‑rO4) (r = 0−4) configurations. In some case two spectral manifolds correspond to the same configuration, as expected also from the results of Clement et al.23 Full details are given in Table S1. Within this assignment, we obtained that P21/n and Pmn21 account for 67% and 33%, respectively, in excellent agreement with 7Li NMR results. The best-fit of the entire sidebands patterns of Figure 3-right shows that the relative intensities of the r = 0−4 configurations match well with the binomial distribution
B (r ) =
n! pn q n − r r! (n − r )! Fe Mn
where pFe is the probability that a FeO4 group, rather than a MnO4 one, is connected to a SiO4 tetrahedron. We obtained pFe ≅ 0.85 for both the polymorphs, in acceptable agreement with the nominal composition of the sample (xFe = 0.75). The same sidebands analysis could not be performed on the 7Li aMAT spectrum (Figure S5) as, in this case, the Mn for Fe substitution accounts for chemical shifts less than 10 ppm, due to the small electronic cloud of lithium,19 which are smaller than the inhomogeneous broadening of the isotropic spectral line.19,23 Figures S6 and S7 show the 7Li 1D fast MAS, and 7Li and 29 Si 2D aMAT spectra for the samples Mn050 and Mn075, respectively. Here, the worst signal-to-noise ratio does not allow a clear-cut analysis as for sample Mn025. However, by indexing the chemical shifts of the sidebands patterns according to those of Mn025, we obtained again polymorphs ratios in good agreement with the 7Li data (see Figure 4). In all
Figure 4. Polymorphs amounts for the various samples, as determined by 7Li, 29Si NMR, and XRD Rietveld profile analysis. x = 0 corresponds to sample Mn1.
cases, the sidebands manifolds follow the binomial distribution (Mn050: pFe (P21/n) ≅ 0.65; pFe (Pmn21) ≅ 0.55. Mn075: pFe (P21/n) ≅ 0.15; pFe (Pmn21) ≅ 0.45). In case of Mn050 and Mn075 we estimated uncertainties >10% and >15%, respectively. Figure 4 shows the polymorphs speciation for all the examined samples obtained by 7Li, 29Si NMR and by the XRD Rietveld analysis (Figure S8). The two NMR nuclear probes are in very good internal agreement, whereas the XRD analysis tends to overestimate the presence of Pmnb for low Fe contents, and of P21/n in the case of samples Mn05 and Mn025. Noteworthy, under our preparation conditions, the formation of P21/n is favored for increasing Fe content, in agreement with the findings of Sirisopanaporn et al.12 In case of the sample Fe1, both NMR (7Li and 29Si) and XRD do reveal 100% P21/n. In conclusion, we demonstrated that fast MAS 2D aMAT NMR is a powerful tool to address polymorphs speciation in complex transition-metal oxides, where standard diffraction techniques can fail due to strong phase similarities. Fast MAS rotation allows using 7Li instead the relatively rare 6Li, with substantial improvements in the signal-to-noise ratio. This NMR approach can be applied also to rare nuclei such as 29Si, 6075
DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072−6076
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(11) Kokalj, A.; Dominko, R.; Mali, G.; Meden, A.; Gaberscek, M.; Jamnik, J. Beyond One-Electron Reaction in Li Cathode Materials: Designing Li2MnxFe1‑xSiO4. Chem. Mater. 2007, 19, 3633−3640. (12) Sirisopanaporn, C.; Dominko, R.; Masquelier, C.; Armstrong, A. R.; Mali, G.; Bruce, P. G. Polymorphism in Li2(Fe,Mn)SiO4: A Combined Diffraction and NMR Sudy. J. Mater. Chem. 2011, 21, 17823−17831. (13) Arroyo-deDompablo, M. E.; Dominko, R.; Gallardo-Amores, J. M.; Dupont, L.; Mali, G.; Ehrenberg, H.; Jamnik, J.; Moran, E. On the Energetic Stability and Electrochemistry of Li2MnSiO4 Polymorphs. Chem. Mater. 2008, 20, 5574−5584. (14) Kalantarian, M. M.; Asgari, S.; Mustarelli, P. Theoretical Investigation of Li2MnSiO4 As a Cathode Material for Li-Ion Batteries: a DFT Study. J. Mater. Chem. A 2013, 1, 2847−2855. (15) Kalantarian, M. M.; Asgari, S.; Capsoni, D.; Mustarelli, P. An ab initio Investigation of Li2M0.5N0.5SiO4 (M, N = Mn, Fe, Co, Ni) As LiIon Battery Cathode Materials. Phys. Chem. Chem. Phys. 2013, 15, 8035−8041. (16) Dominko, R.; Arcon, I.; Kodre, A.; Hanzel, D.; Gaberscek, M. In-situ XAS Study on Li2MnSiO4 and Li2FeSiO4 Cathode Materials. J. Power Sources 2009, 189, 51−58. (17) Islam, M. S.; Dominko, R.; Masquelier, C.; Sirisopanaporn, C.; Armstrong, A. R.; Bruce, P. G. Silicate Cathodes for Lithium Batteries: Alternatives to Phosphates? J. Mater. Chem. 2011, 21, 9811−9818. (18) Mali, G.; Sirisopanaporn, C.; Masquelier, C.; Hanzel, D.; Dominko, R. Li2FeSiO4 Polymorphs Probed by 6Li MAS NMR and 57 Fe Mössbauer Spectroscopy. Chem. Mater. 2011, 23, 2735−2744. (19) Mali, G.; Rangus, M.; Sirisopanaporn, C.; Dominko, R. Understanding 6Li MAS NMR Spectra of Li2MSiO4 Materials (M = Mn, Fe, Zn). Solid State Nucl. Magn. Reson. 2012, 42, 33−41. (20) Mali, G.; Meden, A.; Dominko, R. 6Li MAS NMR Spectroscopy and First-Principles Calculations as a Combined Tool for the Investigation of Li2MnSiO4 Polymorphs. Chem. Commun. 2010, 46, 3306−3308. (21) Armstrong, A. R.; Lyness, C.; Menetrier, M.; Bruce, P. G. Structural Polymorphism in Li2CoSiO4 Intercalation Electrodes: A Combined Diffraction and NMR Study. Chem. Mater. 2010, 22, 1892−1900. (22) Bini, M.; Ferrari, S.; Ferrara, C.; Mozzati, M. C.; Capsoni, D.; Pell, A. J.; Pintacuda, G.; Canton, P.; Mustarelli, P. Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials. Sci. Rep. 2013, 3, 3452. (23) Clement, R. J.; Pell, A. J.; Middlemiss, D. S.; Strobridge, F. C.; Miller, J. K.; Whittingham, M. S.; Emsley, L.; Grey, C. P.; Pintacuda, G. Spin-Transfer Pathways in Paramagnetic Lithium Transition-Metal Phosphates From Combined Broadband Isotropic Solid-State MAS NMR Spectroscopy and DFT Calculations. J. Am. Chem. Soc. 2012, 134, 17178−17185. (24) Kervern, G.; Pintacuda, G.; Emsley, L. Fast Adiabatic Pulses for Solid-State NMR of Paramagnetic Systems. Chem. Phys. Lett. 2007, 435, 157−162. (25) Mustarelli, P.; Massarotti, V.; Bini, M.; Capsoni, D. Transferred Hyperfine Interaction and Structure in LiMn2O4 and Li2MnO3 Coexisting Phases: an XRD and NMR-MAS Study. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 12018−12024.
if significant isotopic sample enrichment is provided. Of course, optimal results can be reached by 100% enrichment, which however calls for very expensive reactants. If proper reference spectra were provided, it could be possible to fully address chemical shift tensors even in case of complex polymorphism. Further improvements can be obtained by combining NMR results with DFT calculations, provided that optimized potentials for paramagnetic oxides (to date not yet available) are produced.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02569.
■
Full experimental details, paramagnetic NMR theory, crystal structure of the involved polymorphs, 1D NMR spectra of all the investigated samples, analysis of the possible configurations for Mn for Fe substitutions, 2D aMAT NMR spectra of all the investigated samples, a table with NMR chemical shifts, and X-ray Rietveld refinements of all the investigated samples (PDF)
AUTHOR INFORMATION
Corresponding Author
*(P.M.) E-mail:
[email protected]. ORCID
Marcella Bini: 0000-0001-7099-0650 Guido Pintacuda: 0000-0001-7757-2144 Piercarlo Mustarelli: 0000-0001-9954-5200 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072−6076
