In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes

Abstract

•New mechanism of strain accommodation in composite battery electrodes•Very quick screening of optimal binder-electrolyte solution combinations•Stiff binder exhibits small fracture toughness in high-strain composite electrodes•Soft binder with large fracture toughness mitigates high transformation strains Almost all practical composite battery electrodes experience volume changes during cycling, deteriorating their cycling performance. A new key element that the EQCM-D methodology introduces into the energy storage field is a general platform for quantification of high-frequency viscoelastic characteristics of high-strain composite electrodes correlated with their low-frequency resilience and toughness moduli. Fast relaxation ensures effective high-strain accommodation by softened binder in aprotic solution. In contrast, with excessively stiff binder in aqueous solution, the strong electrode/binder interactions tracked dynamically reflect the initial and advanced stages of the mechanical degradation of the polymeric binder up to its complete destruction. The discovered correlation between the fracture toughness of the binder and its high-frequency viscoelastic behavior will serve for a rational design of extremely strained composite alloy-type electrodes such as the Li-Si anode. The high phase-transformation strain developed upon intercalation in the host particles of a composite battery electrode affects the polymeric binder network mechanically, deteriorating the electrode cycling performance. Here, electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to demonstrate a new strain-accommodation mechanism, in high-strain NaFePO4/PVdF electrodes, via relaxation of the binder network surrounding the intercalation particles. Complete mechanical degradation of the polymer network occurs during long-term cycling of NaFePO4 electrodes in aqueous solutions (hard and tough behavior). In contrast, in aprotic solutions, a softened binder easily accommodates the high transformation strain, ensuring excellent electrode cycling performance (soft and tough behavior). Quantification of the high-frequency viscoelastic properties of an operating composite electrode linked to the binder's fracture toughness ensures fast and facile screening of the optimal polymeric binder/electrolyte solution combinations. This methodology should be extremely important for optimization of cycling performance of Li-Si anodes undergoing huge volume changes during cycling. The high phase-transformation strain developed upon intercalation in the host particles of a composite battery electrode affects the polymeric binder network mechanically, deteriorating the electrode cycling performance. Here, electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to demonstrate a new strain-accommodation mechanism, in high-strain NaFePO4/PVdF electrodes, via relaxation of the binder network surrounding the intercalation particles. Complete mechanical degradation of the polymer network occurs during long-term cycling of NaFePO4 electrodes in aqueous solutions (hard and tough behavior). In contrast, in aprotic solutions, a softened binder easily accommodates the high transformation strain, ensuring excellent electrode cycling performance (soft and tough behavior). Quantification of the high-frequency viscoelastic properties of an operating composite electrode linked to the binder's fracture toughness ensures fast and facile screening of the optimal polymeric binder/electrolyte solution combinations. This methodology should be extremely important for optimization of cycling performance of Li-Si anodes undergoing huge volume changes during cycling. LiFePO4 (LFP) is one of the most studied battery electrodes in which Li-ion insertion/extraction occurs as a first-order phase transition (two isostructural phases with different lattice parameters coexist during the intercalation/deintercalation process).1Delmas C. Maccario M. Croguennec L. Le Cras F. Weill F. Lithium deintercalation in LiFePO4 nanoparticles via a domino-cascade model.Nat. Mater. 2008; 7: 665-671Google Scholar, 2Laffont L. Delacourt C. Gibot P. Yue Wu M. Kooyman P. Masquelier C. Tarascon J.M. Study of the LiFePO4/FePO4 two-phase system by high-resolution electron energy loss spectroscopy.Chem. Mater. 2006; 18: 5520-5529Google Scholar, 3Malik R. Zhou F. Ceder G. Kinetics of non-equilibrium lithium incorporation in LiFePO4.Nat. Mater. 2011; 10: 587-590Google Scholar, 4Padhi A.K. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries.J. Electrochem. Soc. 1997; 144: 1188-1194Google Scholar, 5Wang J. Chen-Wiegart Y.C.K. Wang J. 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Res. 2010; 40: 501-529Google Scholar However, when an Li-free host, tailored during the synthesis process to accommodate Li ions, is subjected to electrochemical insertion of foreign Na ions of a significantly larger size than the Li ions (0.102 and 0.076 nm, respectively), the resulting phase-transformation strain in NaFePO4 (NFP) electrode is 3-fold larger than that in the LFP electrode.9Galceran M. Saurel D. Acebedo B. Roddatis V.V. Martin E. Rojo T. Casas-Cabanos M. The mechanism of NaFePO₄ (de)sodiation determined by in situ X-ray diffraction.Phys. Chem. Chem. Phys. 2014; 16: 8837-8842Google Scholar It was recently suggested that the large lattice mismatch between the NFP and FP phases can be mitigated, thereby accommodating the high strain of the lattice, by the formation of an intermediate amorphous phase giving rise to an initial stage of a plastic deformation in NFP electrodes.10Xiang K. Xing W. Ravnsbæk D.B. Hong L. Tang M. Li Z. Wiaderek K.M. Borkiewicz O.J. Chapman K.W. Chupas P.J. et al.Accommodating high transformation strains in battery electrodes via the formation of nanoscale intermediate phases: operando investigation of olivine NaFePO4.Nano Lett. 2017; 17: 1696-1702Google Scholar For more than a 3-fold lower transformation strain in LFP electrodes, the extent of the active mass amorphization is negligibly small. In turn, important and intensively studied Li-Si anodes undergo much more pronounced volume expansion upon lithiation, and the transformation strain thus developed is 20-fold larger in comparison with that for NFP. Crystalline silicon undergoes amorphization during repeated lithiation-delithiation processes.11Limthongkul P. Jang Y.I. Dudney N.J. Chiang Y.M. Electrochemically-driven solid-state amorphization in lithium-silicon alloys and implications for lithium storage.Acta Mater. 2003; 51: 1103-1113Google Scholar These observations indicate that the formation of amorphous phases can serve as a mechanism of strain accommodation on a microstructural level for a variety of intercalation and alloy-type insertion electrodes. However, a variety of structure-sensitive techniques of high spatial resolution (such as those used in the work reported by Xiang et al.10Xiang K. Xing W. Ravnsbæk D.B. Hong L. Tang M. Li Z. Wiaderek K.M. Borkiewicz O.J. Chapman K.W. Chupas P.J. et al.Accommodating high transformation strains in battery electrodes via the formation of nanoscale intermediate phases: operando investigation of olivine NaFePO4.Nano Lett. 2017; 17: 1696-1702Google Scholar) that probe local electrode microstructure are often less informative for tracking structural changes in the composite electrodes on a mesoscopic scale. This inspired us to choose LFP and NFP composite electrodes (with commonly used poly(vinylidene fluoride) [PVdF] binder) cycled in both aqueous and non-aqueous electrolyte solutions for a quantitative study of the mechanical electrode/binder interactions using an electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D).12Arnau A. Antonio A review of interface electronic systems for AT-cut quartz crystal microbalance applications in liquids.Sensors. 2008; 8: 370-411Google Scholar, 13Daikhin L. Sigalov S. Levi M.D. Salitra G. Aurbach D. Quartz crystal impedance response of nonhomogenous composite electrodes in contact with liquids.Anal. Chem. 2011; 83: 9614-9621Google Scholar, 14Höök F.H. Kasemo B. Nylander T. Fant C. Sott K. Elwing H. Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking: a quartz crystal microbalance with dissipation monitoring, ellipsometry, and surface plasmon resonance study.Anal. Chem. 2001; 73: 5796-5804Google Scholar, 15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar, 16Shpigel N. Levi M.D. Sigalov S. Girshevitz O. Aurbach D. Daikhin L. Pikma P. Marandi M. Jänes A. Lust E. et al.In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes.Nat. Mater. 2016; 15: 570-575Google Scholar, 17Levi M.D. Shpigel N. Sigalov S. Dargel V. Daikhin L. Aurbach D. In situ porous structure characterization of electrodes for energy storage and conversion by EQCM-D: a review.Electrochim. Acta. 2017; 232: 271-284Google Scholar, 18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar, 19Shpigel N. Lukatskaya M.R. Sigalov S. Ren C.E. Nayak P. Levi M.D. Daikhin L. Aurbach D. Gogotsi Y. In situ monitoring of gravimetric and viscoelastic changes in 2D intercalation electrodes.ACS Energy Lett. 2017; 2: 1407-1415Google Scholar, 20Eisele N.B. Andersson F.I. Frey S. Richter R.P. Viscoelasticity of thin biomolecular films: a case study on nucleoporin phenylalanine-glycine repeats grafted to a histidine-tag capturing QCM-D sensor.Biomacromolecules. 2012; 13: 2322-2332Google Scholar This acoustic technique12Arnau A. Antonio A review of interface electronic systems for AT-cut quartz crystal microbalance applications in liquids.Sensors. 2008; 8: 370-411Google Scholar, 15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar, 20Eisele N.B. Andersson F.I. Frey S. Richter R.P. Viscoelasticity of thin biomolecular films: a case study on nucleoporin phenylalanine-glycine repeats grafted to a histidine-tag capturing QCM-D sensor.Biomacromolecules. 2012; 13: 2322-2332Google Scholar is ideally suited for a real-time mesoscopic mechanical characterization of composite batteries electrodes comprising expanding/contracting intercalation particles embedded into a continuous network of a polymeric binder. As illustrated in Figure S1, the polymeric binder in a composite electrode binds together neighboring intercalation particles that are arranged naturally in layers. From electrochemical and mechanical points of view, this porous multilayer active mass can be regarded as an interphase between the quartz crystal surface and the contacting electrolyte solution. The bottom layer of the electrode is rigidly attached to the Au-coated quartz crystal surface serving as a current collector (“no-slip” condition at the crystal/bottom layer interface).12Arnau A. Antonio A review of interface electronic systems for AT-cut quartz crystal microbalance applications in liquids.Sensors. 2008; 8: 370-411Google Scholar, 13Daikhin L. Sigalov S. Levi M.D. Salitra G. Aurbach D. Quartz crystal impedance response of nonhomogenous composite electrodes in contact with liquids.Anal. Chem. 2011; 83: 9614-9621Google Scholar, 14Höök F.H. Kasemo B. Nylander T. Fant C. Sott K. Elwing H. Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking: a quartz crystal microbalance with dissipation monitoring, ellipsometry, and surface plasmon resonance study.Anal. Chem. 2001; 73: 5796-5804Google Scholar, 15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar, 16Shpigel N. Levi M.D. Sigalov S. Girshevitz O. Aurbach D. Daikhin L. Pikma P. Marandi M. Jänes A. Lust E. et al.In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes.Nat. Mater. 2016; 15: 570-575Google Scholar, 17Levi M.D. Shpigel N. Sigalov S. Dargel V. Daikhin L. Aurbach D. In situ porous structure characterization of electrodes for energy storage and conversion by EQCM-D: a review.Electrochim. Acta. 2017; 232: 271-284Google Scholar, 18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar, 19Shpigel N. Lukatskaya M.R. Sigalov S. Ren C.E. Nayak P. Levi M.D. Daikhin L. Aurbach D. Gogotsi Y. In situ monitoring of gravimetric and viscoelastic changes in 2D intercalation electrodes.ACS Energy Lett. 2017; 2: 1407-1415Google Scholar, 20Eisele N.B. Andersson F.I. Frey S. Richter R.P. Viscoelasticity of thin biomolecular films: a case study on nucleoporin phenylalanine-glycine repeats grafted to a histidine-tag capturing QCM-D sensor.Biomacromolecules. 2012; 13: 2322-2332Google Scholar The top layer of the multilayered assembly is bound to the rest of the active mass by the same binder, which forms a continuous network throughout the entire electrode depth. The multilayered assembly on the quartz crystal surface is probed by the transverse acoustic waves generated by the crystal oscillating in MHz-range frequency on multiple odd harmonics (overtone orders, n, from 3 to 13); both intercalation-induced gravimetric and viscoelastic changes in the battery electrodes can be sensitively tracked.16Shpigel N. Levi M.D. Sigalov S. Girshevitz O. Aurbach D. Daikhin L. Pikma P. Marandi M. Jänes A. Lust E. et al.In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes.Nat. Mater. 2016; 15: 570-575Google Scholar, 17Levi M.D. Shpigel N. Sigalov S. Dargel V. Daikhin L. Aurbach D. In situ porous structure characterization of electrodes for energy storage and conversion by EQCM-D: a review.Electrochim. Acta. 2017; 232: 271-284Google Scholar, 18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar, 19Shpigel N. Lukatskaya M.R. Sigalov S. Ren C.E. Nayak P. Levi M.D. Daikhin L. Aurbach D. Gogotsi Y. In situ monitoring of gravimetric and viscoelastic changes in 2D intercalation electrodes.ACS Energy Lett. 2017; 2: 1407-1415Google Scholar EQCM-D produces two coupled output responses, the resonance frequency f/n and the resonance bandwidth W/n or, equivalently, the dissipation factor D = W/fo; all the output responses are functions of n; fo stands for fundamental frequency. Quantification of the viscoelastic changes in the electrodes is reached via fitting a suitable acoustic load impedance model to the experimental frequency and dissipation factor changes (Δf/n and ΔD, respectively).15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar, 18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar, 19Shpigel N. Lukatskaya M.R. Sigalov S. Ren C.E. Nayak P. Levi M.D. Daikhin L. Aurbach D. Gogotsi Y. In situ monitoring of gravimetric and viscoelastic changes in 2D intercalation electrodes.ACS Energy Lett. 2017; 2: 1407-1415Google Scholar, 20Eisele N.B. Andersson F.I. Frey S. Richter R.P. Viscoelasticity of thin biomolecular films: a case study on nucleoporin phenylalanine-glycine repeats grafted to a histidine-tag capturing QCM-D sensor.Biomacromolecules. 2012; 13: 2322-2332Google Scholar, 21Voinova M.V. Rodahl M. Jonson M. Kasemo B. Viscoelastic acoustic response of layered polymer films at fluid-solid interfaces: continuum mechanics approach.Phys. Scripta. 1999; 59: 391-396Google Scholar All the applications of the EQCM-D method in the energy storage and conversion field reported so far (including theoretical and experimental backgrounds of the method) have been recently reviewed by us.18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar The challenging issue of the electrochemical/mechanical coupling in high-strain intercalation hosts has never been discussed in either the available literature or our previous EQCM-D studies. This work is focused on recording and quantification of the effect of moderate and high transformation strains on the viscoelastic properties of composite LFP/PVdF and NFP/PVdF electrodes, respectively, cycled in both aqueous and non-aqueous electrolyte solutions. A mesoscopic view of the different transformation strains in LFP/PVdF and NFP/PVdF electrodes caused by the related microstructural changes in the lattice parameters (atomic-scale view) is shown schematically in Figures 1B and 1A , respectively. We believe that successful quantification of the viscoelastic properties of the high-strain NFP/PVdF electrode (the highest transformation strain among the topotactic intercalation hosts) can promote this methodology for speedy and facile optimization of the binder/electrolyte solution combinations in order to significantly improve the cycling performance of extraordinary high-strain anodes. Figure 2 presents the morphology and structure of the LFP/PVdF electrodes used for this study, obtained by scanning electron microscopy (EM), atomic force microscopy (AFM), and scanning EM-focused ion beam (SEM-FIB) measurements. Tilted images of the electrodes (Figures 2A and 2E) and especially the cross-section of the FIB-cut electrodes (Figures 2B and 2D) clearly show the individual layers of intercalations particles bound together and to the quartz crystal surface (Figure 2B) by PVdF binder. Figure 2F gives an impression of flexibility of the multilayered electrode assembly. The various images of the composite electrode shown in this figure were used for a schematic presentation of the multilayered electrode structure shown in Figure S1. Comparing the images in Figures 2B and 2E, we conclude that the top layer (facing electrolyte solution) may contain less strongly bound intercalation particles compared with the bottom layer (facing quartz crystal surface). The implication is that the top layer is more compliant than the bottom layer (see the sloping versus vertical displacement profiles schematically shown in Figure S1). This is further proved by direct measurements of Δf/n and ΔD changes for the coated versus uncoated (neat) quartz crystal using the acoustic multilayer formalism15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar (a special rule for the combination of acoustic load impedances of the individual layers, formally resembling in-series and in-parallel combinations of the elements of equivalent electrical circuits analogs of the electrochemical impedance of composite electrodes). As expected, composite electrodes with a multilayer structure have a larger dissipation factor D than the uncovered crystal (see Figure S2), and the specific overtone order dependence of D reflects the particular mechanism of the oscillation-energy dissipation, which is linked to the multilayer electrode structure. Previously, we described in detail how this analysis is performed.22Dargel V. Jäckel N. Shpigel N. Sigalov S. Levi M.D. Daikhin L. Presser V. Aurbach D. In situ multilength-scale tracking of dimensional and viscoelastic changes in composite battery electrodes.ACS Appl. Mater. Interfaces. 2017; 9: 27664-27675Google Scholar, 23Dargel V. Shpigel N. Sigalov S. Nayak P. Levi M.D. Daikhin L. Aurbach D. In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes.Nat. Commun. 2017; 8: 1389Google Scholar For the electrodes used in this work, the experimental frequency and dissipation changes of the electrode measured in air are presented in Figure S2, whereas the extracted model viscoelastic parameters are indicated in Table S1 (see also comment following Figure S2). Viscoelastic diagnostics of neat PVdF films in aqueous and non-aqueous solutions were performed to characterize the intrinsic viscoelastic properties of the binder, which are affected neither by the multilayered structure of the pristine electrode nor by the strongly expanded/contracted intercalated/deintercalated electrode particles. The details of this simple test have been reported previously. 22Dargel V. Jäckel N. Shpigel N. Sigalov S. Levi M.D. Daikhin L. Presser V. Aurbach D. In situ multilength-scale tracking of dimensional and viscoelastic changes in composite battery electrodes.ACS Appl. Mater. Interfaces. 2017; 9: 27664-27675Google Scholar, 23Dargel V. Shpigel N. Sigalov S. Nayak P. Levi M.D. Daikhin L. Aurbach D. In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes.Nat. Commun. 2017; 8: 1389Google Scholar The data for neat PVdF films in contact with 2 M aqueous solutions of Li2SO4 and 1 M solutions of LiPF6 and NaPF6/EC + DMC (with related comments) are presented in Figure S3. The major conclusion is that PVdF binder is stiff in both aqueous solutions, but swells and softens in the aprotic solutions we used (swelling and softening of the binder is easily recognized by this EQCM-D test).18Shpigel N. Levi M.D. Sigalov S. Daikhin L. Aurbach D. In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage and conversion by electrochemical quartz crystal microbalance with dissipation monitoring.Acc. Chem. Res. 2018; 51: 69-79Google Scholar, 22Dargel V. Jäckel N. Shpigel N. Sigalov S. Levi M.D. Daikhin L. Presser V. Aurbach D. In situ multilength-scale tracking of dimensional and viscoelastic changes in composite battery electrodes.ACS Appl. Mater. Interfaces. 2017; 9: 27664-27675Google Scholar The behavior of single-layer (further termed thin) electrodes with a loading mass ∼35 μg/cm2 is remarkably simple in all the four relevant electrolyte solutions used. Cyclic voltammetry (CV) curves for both LFP and NFP electrodes in aqueous and aprotic solutions (Figures 3A and 3D , respectively) are strictly quasi-reversible. Due to the quasi-Nernstian behavior of LFP with respect to Li ions in the solution,24Levi M.D. Sigalov S. Salitra G. Elazari R. Aurbach D. Daikhin L. Presser V. In situ tracking of ion insertion in iron phosphate olivine electrodes via electrochemical quartz crystal admittance.J. Phys. Chem. C. 2013; 117: 1247-1256Google Scholar, 25Sauvage F. Baudrin E. Morcrette M. Tarascon J.-M. Pulsed laser deposition and electrochemical properties of LiFePO4 thin films.Electrochem. SolidState Lett. 2004; 7: A15-A18Google Scholar the anodic peak potential during Li-ion extraction in pure Na2SO4 solution is shifted toward a more negative potential with respect to the extraction peak potential in the Li2SO4 solution. In contrast, the potential of Na-ion insertion is shifted toward much more negative potentials compared with that of Li-ion insertion, and the extraction of Na ions occurs in two well-separated steps. The shape of the CV curves characterizing Na-ion insertion/extraction into/from the FePO4 host is in excellent agreement with previous reports.24Levi M.D. Sigalov S. Salitra G. Elazari R. Aurbach D. Daikhin L. Presser V. In situ tracking of ion insertion in iron phosphate olivine electrodes via electrochemical quartz crystal admittance.J. Phys. Chem. C. 2013; 117: 1247-1256Google Scholar, 25Sauvage F. Baudrin E. Morcrette M. Tarascon J.-M. Pulsed laser deposition and electrochemical properties of LiFePO4 thin films.Electrochem. SolidState Lett. 2004; 7: A15-A18Google Scholar, 26Casas-Cabanas M. Roddatis V.V. Saurel D. Kubiak P. Carretero-González J. Palomares V. Serras P. Rojo T. Crystal chemistry of Na insertion/deinsertion in FePO4-NaFePO4.J. Mater. Chem. 2012; 22: 17421Google Scholar The existence of two anodic peaks for Na-ion extraction and only one cathodic peak for Na-ion insertion is explained by formation of the intermediate phase Na0.7FePO4, with related volume changes,26Casas-Cabanas M. Roddatis V.V. Saurel D. Kubiak P. Carretero-González J. Palomares V. Serras P. Rojo T. Crystal chemistry of Na insertion/deinsertion in FePO4-NaFePO4.J. Mater. Chem. 2012; 22: 17421Google Scholar in contrast to the direct transformation between the LiFePO4 and FePO4 phases.27Maxisch T. Ceder G. Elastic properties of olivine LixFePO4 from first principles.Phys. Rev. B. 2006; 73: 1-4Google Scholar EQCM-D signatures of the thin LFP and NFP composite electrodes in both aqueous and aprotic solutions (Figures 3B, 3C, 3E, and 3F, respectively) are amazingly simple. During both Li- and Na-ion insertion/extraction the dissipation factor remains constant. Thus, Δf/n does not depend on n and practically coincides with the frequency change (designated FFaraday); Δf/n is then transformed into the related mass-density change due to the inserted (intercalated) Li ions (Δm) using the Sauerbrey equation15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google ScholarΔm = −C∙Δf/n,(Equation 1) where Δf/n is the change in the frequency normalized by the overtone order and C = Zq/(2fo2) is the mass sensitivity constant where fo is the fundamental frequency and Zq is the acoustic wave impedance of AT-cut quartz equal to 8.8 × 105 g/(cm2s). For a 5-MHz crystal, C = 17.7 ng/(cm2Hz). FFaraday = Δf/n is calculated using the same Equation 1 relating Δm to the intercalation charge obtained by integration of the CV curves (using the Faraday law). The coincidence of the experimental values of Δf/n with FFaraday (Figures 3B and 3C) implies that the mass-per-charge ratios of the intercalated Li and Na ions are 7 and 23 μg/cm2 per μmol/cm2 of inserted Li and Na ions, respectively, indicating a simple intercalation mechanism. A similar gravimetric limit was also reached with LFP/LiPF6 in EC + DMC and NFP/NaPF6 in EC + DMC solutions (the CV curves are shown in Figure 3D whereas the related changes of Δf/n and ΔD are presented in Figures 3E and 3F). The same mass-per-charge ratios were found for the LFP and NFP electrodes in aprotic solutions. The conclusion is that once a sufficient amount of the polymer binder reliably attaches a single layer of intercalation particles to the quartz crystal surface, the only effect of Li-ion intercalation on the EQCM-D response is the change in the electrode mass due to Li-ion insertion/extraction. This implies that both LFP and NFP electrodes operate in the thin-layer mode. Operation in this mode reduces the viscoelastic contribution to both the frequency and dissipation changes with respect to the contribution to the measured frequency change caused by the mass of the inserted ions.15Johannsmann D. The Quartz Crystal Microbalance in Soft Matter. Springer, 2015: 1-387Google Scholar Once the wavelength of sound (depending on the shear modulus and overtone order, the estimated range implies a length ≥1 μm) is larger than the electrode thickness (200 nm in this case), the EQCM-D response is reduced to only a frequency change due to the mass of the intercalated ions. When a gravimetric characterization of the intercalation process is the desired goal, the electrodes should be fabricated to work in a thin-layer mode. The fact that thin LFP and NFP electrodes in both aqueous and non-aqueous solutions behave gravimetrically during ion intercalation, and the experimental frequency change is equal to that calculated for neat (unsolvated) ions, two important conclusions imply that upon intercalation of Li and Na ions into composite LFP and NFP electrodes the only source of gravimetric change is the mass of the inserted unsolvated cations. No additional accumulation/loss of mass occurs during cycling these electrodes in the potential window used herein, implying that there are no side reactions that form surface species and there is no dissolution of active mass during the short experiments thus performed. These important conclusions, derived from EQCM-D experiments with very thin electrodes, should be true for any type of composite LFP and NFP electrodes, because they reflect the intrinsic properties of these systems (the active masses in the rel