Lithium Cation Speciation in LiPF6-PC Electrolyte Studied by Two

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C: Energy Conversion and Storage; Energy and Charge Transport

Lithium Cation Speciation in LiPF6-PC Electrolyte Studied by Two Dimensional Heteronuclear Overhauser Enhancement and Pulse Field Gradient Diffusometry NMR Vikas Kumar, R Ravikanth Reddy, Bandaru V. N. Phani Kumar, Chilukuri V. Avadhani, Subramanian Ganapathy, Narayanan Chandrakumar, and Swaminathan Sivaram J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11599 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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The Journal of Physical Chemistry

Lithium Cation Speciation in LiPF6-PC Electrolyte Studied by Two Dimensional Heteronuclear Overhauser Enhancement and Pulse Field Gradient Diffusometry NMR Vikas Kumar,†,  R. Ravikanth Reddy,‡,  B. V. N. Phani Kumar,‡ Chilukuri V. Avadhani,† Subramanian Ganapathy,§ Narayanan Chandrakumarǁ and Swaminathan Sivaram€,* Polymer Science and Engineering Division, CSIR−National Chemical Laboratory, Pune

†

411008, India. ‡NMR,

Inorganic & Physical Chemistry Laboratory, CSIR – Central Leather Research

Institute, Adyar, Chennai-600020, India. §CAS

in Crystallography & Biophysics, University of Madras, Chennai-600025, India.

ǁMRI-MRS

Centre and Department of Chemistry, Indian Institute of Technology Madras

Chennai 600036, Tamil Nadu, India. €Indian

Institute of Science Education and Research, Dr Homi Bhabha Road, Pune-411008,

India. Tel: 020-2590 8434. Both

authors contributed equally to this work.

Corresponding author E-mail address: [email protected]

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ABSTRACT Electrolytic dissociation of lithium hexafluorophosphate (LiPF6) in the non-aqueous cyclic propylene carbonate (PC) has been investigated in the wide range of concentration (0.05 – 3.5 M)

by

7Li

solution

state

Nuclear

Magnetic

Resonance

(NMR)

spectroscopy.

2D-Heteronuclear Overhauser Enhancement Spectroscopy (HOESY) NMR experiments have not only enabled the cation solvation and ion pairing to be directly monitored, but additionally evidence strong anion-solvent interaction at higher concentrations (>1.2 M) of the PC electrolyte. Preliminary analysis of kinetic nOe data has been made to determine crossrelaxation rates for the spatial interaction of the solvent with the Li+ cation and the PF6― anion. The concentration dependence of 7Li NMR self-diffusion coefficient (Dself), determined using very strong pulsed magnetic field gradients (~1700 Gauss/cm), depicts two breaks to mark the solvation and ion pairing events in a distinct manner. This in turn has aided the determination of solvent coordination number and average sizes of solvated and ion-paired clusters. Our results indicate that in the contact ion pair (CIP) dominated electrolyte (> 1.8 M) lithium ion mobility across the solvated and ion-paired environments appears to be inhibited which makes the spectral distinction of solvated and ion-paired environments possible. The concentration dependence of the 7Li NMR spectral and diffusometry data is in striking correspondence with that of bulk conductivity measurements and point to the detrimental effect of CIP aggregates in impeding the ionic conductivity at high salt concentrations. These results have significance in understanding the structure and dynamics of lithium ion solvates that are ubiquitous in the working environment of a lithium ion battery.

2


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INTRODUCTION Lithium-ion batteries (LiBs) are the most commonly used energy storage system for portable electronics and have attracted great interest in recent years, mainly because of their high energy density, high open-circuit voltage, long cycling life, and low self-discharge rate.1,2

A typical

LiB consists of an anode and a cathode sandwiched to a separator membrane and filled with an electrolyte.3,4 The most widely used electrolyte for LIBs is LiPF6 due to its high conductivity. The LiPF6 is dissolved in a non-aqueous carbonate solvent mixture, consisting of a cyclic carbonates, e.g., ethylene carbonate (EC), and one or more linear carbonates, e.g., diethyl carbonate (DEC) or dimethyl carbonate (DMC). The organic carbonate solvents possess high dielectric constant which causes LiPF6 to disassociate, thus enhancing the electrolytic conductivity of the system.

However, the conductivities of this electrolyte solution is

extremely low when compared to their conductivity in aqueous solutions.5 Electrolytes using propylene carbonate (PC, (4-methyl-1,3-dioxolan-2-one)) are also commonly used in LiBs.6,7 PC, besides being liquid at room temperature, has high solubility for LiPF6. PC is especially suited for fundamental experimental studies since it provides access to a much wider concentration of LiPF6 than those afforded by other cyclic carbonate solvents. An understanding of the nature of interaction between the lithium salt and the organic carbonate solvent is of paramount importance. Li-ion complexation in non-aqueous carbonate solvent have been examined by several techniques such as Raman,8,9,10 vibrational,11 infrared9,12 and molecular rotation spectroscopy.13 NMR spectroscopy12,14−18 has also been used to glean valuable atomic level insights from the observed changes in the chemical shifts of

13C

and

17O

of the solvent molecule upon complexation to lithium cation. Molecular

dynamics simulations and density functional theory as well as quantum chemical calculations 3



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have been employed to understand the nature of interaction between the lithium cation and the carbonate solvent. These studies not only complement the experimental findings but have enabled

the

determination

of

thermodynamic

parameters

associated

with

such

interaction.8,12,19−21 By and large, all the studies on neat and mixed electrolytes reported thus far are mostly solvent focused and invariably studied in the solvent rich regime (< 1.4 M). While information on the nature of primary and secondary solvation and the determination of solvent coordination number are provided, information about anion-solvent interaction and ion-pairing are hard to discern if the studies are restricted to concentrations of the electrolyte. It may be noted that for the ion-pairing effects to show up in experimental measurements it is preferable to extend the studies to the salt rich electrolyte regime as the lithium-solvent coordination is by then complete and ion-pairing effects would begin to assume importance. Electrolyte concentrations in the range of 1.4 – 3.0 M are ideally conducive for monitoring the ion-pairing effects.12 In fact, as we show in the present study, cation-solvent, anion-solvent and cation-anion interactions can be individually discerned when the study covers the very low to very high concentration range, namely 0.05 – 3.0 M. It may be noted that super concentrated electrolytes have attracted much attention due to low solvent volatility, enhanced oxidative stability and increased carrier density which are beneficial for advanced lithium ion battery applications.22 Thus, from the viewpoint of their peculiar solution structure, providing an understanding of the physicochemical properties of highly concentrated electrolytes is considered important. This has been the main driving force behind the present study to investigate the PC based LiB electrolyte at much higher concentrations than used before. NMR spectroscopy is an effective molecular characterization tool which aids to probe the solvent, cation and anion environments in the electrolyte at the atomic level. NMR inspection of the solvent molecules is facilitated through the observation of 1H, 13C and 17O nuclei and in the solvent dominated regime the details of cation solvation can be directly inferred from these 4


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studies.14,17,18,21 Similarly, 7Li NMR provides a direct access to probe the lithium cation environments with a very high detection sensitivity due to it high natural abundance (92.6 %). Despite being quadrupolar (I=3/2) a high signal resolution is also guaranteed in the observed NMR spectra because of its small quadrupole moment (Q = -40.1 mbarn)23 and the complete averaging of the electric field gradient tensor in the isotropic solution phase.

7Li

NMR is

however limiting in that the lithium cations present in different environments cannot be distinguished directly in the NMR spectrum in a single measurement at a given electrolyte concentration. This is due to the extremely rapid lithium exchange mobility which causes a lone chemical shift averaged resonance to be observed at ambient temperature. Nevertheless, this limitation can be overcome by carrying out 7Li NMR experiments as a function of the electrolyte concentration, for which the lithium speciation and changes in their population directly influence the time averaged NMR response. Many of the NMR parameters (chemical shift, spin-relaxation, self-diffusion) are in fact determined as a population weighted average 7Li

of the lithium species present.

NMR experiments designed to determine dynamic

parameters are extremely valuable and, in particular, the determination of 7Li self diffusion coefficients from pulse field gradient NMR experiments has assumed considerable significance24-27 as it provides lithium distinction on a longer time scale (~ 100 ms). Quantitative aspects of lithium solvation and ion pairing are provided when 7Li NMR diffusion experiments are employed to probe the solvent rich and the salt rich regimes more closely as we have done in the present study. In the context of LIB electrolyte characterization in terms of the chemically distinct lithium species present, double resonance experiments involving 7Li and an interacting hetero-spin offer new opportunities. For the lithium cation, both its short range interaction with the solvent and the long range interaction with the anion are spatially dependent and can be fully explored through nuclear Overhauser enhancement (nOe).

Here, the incoherent transfer of

5



magnetization between NMR active spins, which are located in different chemical environments, is mediated through a spatially dependent interaction, which is modulated by the motional dynamics existing in the isotropic solution phase. In general, there is an apprehension that in the case of most quadrupolar nuclei the development of nOe is likely quelled by the extremely rapid quadrupolar driven relaxation. However, in the case of lithium, heteronuclear Overhauser enhancements are favored for both 6Li (I = 1) and 7Li (I = 3/2) due to their much weaker quadrupole interactions and the relatively long relaxation times.28-30 6Li is exceptional due to its very small quadrupole moment (Q = -0.82 mbarn)23 and the increased dipolar character, which conduce to achieving the maximum theoretical enhancement of 339 % upon proton irradiation. In practice an enhancement of about 220 % has been reached and has been reported.30

7Li

is better suited due to the increased sensitivity and the shorter

measurement times offered in the NMR studies. 7Li based HOESY experiments can be readily conducted in a wider multi-nuclear context and further incorporated into the one dimensional difference nOe and the two dimensional HOESY correlation plans. 2D HOESY is elegant as it provides a pictorial view of the interactions being probed through cross-peaks that develop exclusively through nOe and reveled in a single contour plot. 2D HOESY can be conducted in the direct detection mode or the inverse detection mode.31,32 For the LiPF6-PC enormous detection sensitivity is brought into the NMR measurements through the direct detection of 7Li and 19F and for their dipolar correlation with 1H. Furthermore, different HOESY combinations are at once envisaged by exploiting magnetically active nuclei located in the cationic (Li+: 7Li), anionic ( PF6 : 31P, 19F) and the organic solvent (PC: 1H) pools. Thus, HOESY combinations such as (a) 7Li-1H /1H-7Li, (b)

19F-1H/ 31P- 1H

and (c) 7Li-19F are amenable in actual

measurements for studying the cation-solvent (a), anion-solvent (b) and cation-anion (c) interactions at the molecular level in a site resolved manner. Indeed, the application of HOESY

6


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offers new opportunities for incisive electrolyte characterization in LiB research and this is well borne in the study of electrolytes containing ionic liquids.33,34 The present study deals with 7Li NMR studies of an electrolyte solution consisting of LiPF6 in non-aqueous cyclic propylene carbonate (PC) over the salt concentration range of 0.05 – 3.5 M. Although LiPF6-PC electrolyte has been studied using various physiochemical techniques including NMR,8-14 the present work differs from earlier studies in the following perspectives. At the outset, 7Li NMR experiments have been undertaken to monitor the spectral response over a wider concentration range of 0.05 - 3.5 M than done before. This has aided in observing the spectral line shape changes occurring at very high salt concentrations (> 1.8 M) and providing cation speciation by spectral decomposition. Moreover, cutting edge 7Li NMR diffusometry using extremely strong magnetic field gradients (~1700 Gauss/cm) has been employed to observe discontinuities in the concentration dependence of 7Li self diffusion data to portray the solvation and ion-pairing events in a clear manner. Such measurements have also been taken up to aid the quantitative determination of solvent coordination number and the average ion-pair cluster size. Finally, an approach away from conventional NMR has been taken to bring two dimensional heteronuclear Overhasuser enhancement spectroscopy (HOESY) as a powerful LIB electrolyte characterization tool. This is pursued to gather molecular insights and reveal the presence of the cation-solvent, cation-anion and anionsolvent interactions in a site resolved manner than ever done before on a conventional LIB electrolyte. EXPERIMENTAL Materials: Lithium hexafluorophosphate (LiPF6) was purchased from Sigma Aldrich and was used as received. Commercial propylene carbonate (PC) was obtained from Sigma Aldrich and used after distillation. Electrolyte solutions of different concentrations in the range 0.05 3.5 M were prepared by dissolving LiPF6 in propylene carbonate (PC) solvent (0.5 ml) under 7



moisture free inert environment in an argon filled glove box at 25°C at a moisture and oxygen level less than 0.1 ppm.

Homogeneous and clear solutions were carefully transferred

into 5 mm NMR tubes and were immediately used for NMR measurements without any exposure to ambient air by fastening the sample tubes with air tight caps and polyflon sealing. The purity and stability of the electrolyte solution at all the prepared concentrations were ascertained through 31P NMR (Figure S1 in the Supporting Information). A total of 30 samples were prepared for 7Li spectral measurements, while for the self-diffusion studies 13 samples were used. In both cases, the electrolyte adequately covered the very low (0.05 M) to the very high (3.5 M) salt concentration range concentration range. 7Li

NMR and HOESY: 7Li NMR spectra were acquired on a Bruker Avance III HD 400 MHz

NMR spectrometer at the Larmor frequency of 155.506 MHz using a 5 mm multi-nuclear probe head. Typically, 40 free induction decays were accumulated in the single pulse mode with a 30° flip angle pulse (4 μs) and a 1 s relaxation delay and Fourier-transformed with 1 Hz exponential line broadening. Other relevant acquisition parameters were: Time domain data points = 32K; spectral width = 50 ppm. The NMR spectra were acquired on all the samples in quick succession under identical spectrometer conditions with optimally adjusted room temperature shim settings. Two dimensional HOESY experiments were performed interchangeably on the JEOL ECA 500 MHz (7Li-1H, 7Li-19F and 31P-1H) and Bruker AVANCE III HD 400 MHz (7Li-1H, 7Li-19F) spectrometer equipped with a BBFO probe. The respective resonance frequencies were (JEOL ECA 500 MHz) 500.15 (1H), 470.65 (19F), 202.46 (31P) and 194.38 (7Li) MHz and (Bruker 400 MHz) 400.23 (1H) and 376.59 (19F) MHz. The corresponding /2 pulses were typically (JEOL ECA 500 MHz) 13, 13.15, 9.8, and 12 s and (Bruker 400 MHz) 12.8 and 13.2 s. The relevant 2D acquisition and processing parameters are given in the corresponding figure captions. The chemical shifts were referenced externally to TMS (1H), 1M LiCl (7Li), 85% 8


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H3PO4 (31P) and Trifluro toluene (19F). NMR data were processed using Bruker Topspin 3.5 and JEOL Delta 5.0 softwares. The 7Li NMR spectra at the very high salt concentrations were iteratively fitted to a mixed Gaussian/Lorentzian line shape using the software DMFIT.35 7Li

NMR Diffusometry: 7Li Diffusion experiments were performed at the Larmor frequency

of 194.38 MHz on a Bruker Avance-III NMR spectrometer at the Indian Institute of Science (IISc), Bangalore using specialized pulse field gradient hardware and equipped with a water cooled broadband diffusion probe (Diff-BB) to handle a maximum gradient strength of ~1700 Gauss/cm. The calibration of the PFG strength was carried out using H2O for which the 1H NMR self-diffusion coefficient (at 25°C) is known to be 2.23 x 10-9 m2s-1 reported elsewhere.36 In the present work, the experimentally measured diffusion coefficients span two orders of magnitude (2 x 10-10 to 0.02 x 10-10 m2s-1) over the salt concentration range of 0.1 – 3.0 M (vide infra). Hence the use of a very strong magnetic field gradient as well as the optimization of gradient duration and diffusion period were warranted in order to capture the full spin echo decay. The self-diffusion data were acquired using a stimulated echo ('diffste') sequence instead of the conventional spin echo sequence. This alleviated the issue of signal loss due to T2 relaxation during the diffusion period as the signal evolution is dictated by the longer T1 relaxation associated with the longitudinal magnetization.37 For the diffusion coefficient measurement, 16-40 Gradient strengths (g) ranging from 5% to 95% of maximum gradient strength were used and the gradient duration time () and the diffusion delay () were optimally chosen as 1/5 ms and 20/50 ms, respectively, for the maximum field gradient strengths of ~1700 Gauss/cm. By this way we could achieve a signal attenuation of about 1/20 at the highest value of the field gradient used over that of gradient free signal for a reliable diffusion measurement within 2% error. A relaxation delay in the range 3 – 7.5 s was used depending on the electrolyte concentration. The 7Li spin-echo decay data were analyzed using the Stejskal-Tanner equation.37 9



I  I 0 e  kD Where I 0 is the peak intensity in the absence of gradient pulses, and the parameter

k  (γ  g) 2 (Δ  /3) , where  is the magnetogyric ratio, and D is the translational selfdiffusion coefficient. The diffusion measurements were carried out at 25.5°C with precise temperature control using the Bruker variable-temperature controller. 7Li NMR self-diffusion data were analysed and fitted using Origin 9 graphic and analysis package. The ionic conductivity measurements were carried out on Biologic SP300 electrochemical system at 25°C. RESULTS AND DISCUSSION 7Li

NMR spectra

Figure 1 shows the 7Li-NMR spectra of LiPF6 at a few selected salt concentrations in nonaqueous propylene carbonate solvent. All the 7Li NMR spectra were recorded over the entire concentration range (0.05 – 3.5 M) and these are presented as a stack plot and shown in Figure S2A. A dominant signal with noticeable upfield shift of the resonance with increasing concentration is noticed. As seen, the 7Li NMR spectra are characterized by a single resonance which remains sharp and symmetric down to 1.2 M but becomes broad and slightly asymmetric at higher concentrations. Electrolyte concentrations in the range of 0.1-1.2 M typically correspond to the solvent rich regime in which progressive solvation around the Li+ sites occur to completion with a coordination number of 4 (vide infra). At low salt concentrations (< 1.2 M) the solvent molecules far outweigh the lithium ions present in number and thus all the dissociated lithium is consumed in the solvation shell. Further, the exchange of the propylene carbonate solvent between the Li+ bound and free sites occur on the picosecond timescale at room temperature as determined by coherent two-dimensional infrared spectroscopy.38 For a dynamically averaged cation environment, the resultant 7Li spectrum is a single exchange 10


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narrowed Lorentzian line having a width of 3 Hz and the observed chemical shifts are determined by the population weighted average of the free and solvent bound lithium environments which exist. Solvent domination at 1.2 M and below also implies that the Li+ and PF6 ions are solvent separated and the spatial interaction for Li+ is stronger with the solvent than with the anion. Thus in the solvent rich regime (< 1.2 M) the solvent separated ion pairs (Li+ - (solvent)n – PF6 ) are the dominant species. This is well borne from independent 7Li−1H

and 7Li−19F two dimensional HOESY (Heteronuclear Overhauser Enhancement

Spectroscopy) experiments which are presented and discussed in the next section. Increased line broadening and marked asymmetry of the spectral line shape can be especially noticed at high salt concentrations (>1.8 M) (Figure S2A). Asymmetric line shape characteristics due to quadrupole effects can be ruled out as the 7Li quadrupole moment is rather small and the electric field gradient tensor is completely averaged in the solution state and the line is expected to be narrow and its width determined by the spin-spin relaxation rate. Since optimum shim conditions were maintained throughout the course of data collection on all the samples, the observed line shape changes are not attributable to magnetic field inhomogeneity across the sample volume. These suggest the presence of a second lithium species which are in slow exchange with the lithium species already present. Thus the observed signal line shape is considered to be governed by the two different lithium species present and their relative population in the salt rich electrolyte. A deconvolution of the 7Li spectra in the concentration range 1.8-3.5 M shows that two components best fit the observed line shape with different relative intensities (Figure S2B). Independent 7Li spin-lattice and spin-spin relaxation time (T1 and T2) measurements of a 3 M electrolyte solution (Figure S3) reveal a bi-exponential relaxation behavior for the total integrated signal intensity and additionally support the above interpretation.

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Two dimensional HOESY Cation-solvent interaction through 7Li-1H HOESY The 2D 7Li-1H HOESY spectra of LiPF6-PC electrolyte taken at select concentrations covering the solvent rich regime (0.2 M, 1.0 M and 1.8 M) by using short and long mixing times (m) are shown in Figure 2. The corresponding 1D spectra are also shown alongside the 2D contour plots. The 2D HOESY spectra clearly depict strong cation-solvent interaction at all the above concentrations and additionally show that at shorter mixing times the 1H to 7Li magnetization transfer is selective for the proton resonance at 5.0 ppm whereas at longer mixing times the other protons participate equally in the magnetization transfer process. For the 1.8 M sample all the 7Li-1H dipolar correlations appear with strong cross-peak intensities in the HOESY spectrum at the longer mixing time of 1 s (Figure 2D). Based on the proton signal assignments, which are shown in Figure 2, we note that the early cross-peak development occurs for proton site Hb ( ~ 5 ppm) of the PC molecule. A faster cross-peak development is seen to occur for this proton site whereas for the other protons the 7Li-1H heteronuclear Overhauser enhancement (Figure 3A) occurs at a slower rate. This is well borne from the mixing time dependence of the nOe build-up which has further enabled us to determine the cross relaxation parameters for the site dependent lithium-proton interactions. Anion-solvent interaction through 19F-1H/31P-1H HOESY This anion-solvent interaction can be monitored through

19F-1H

and

31P-1H

2D HOESY

experiments as the nuclei of interest, which provide the spatially dependent dipolar correlation, are located in the PC solvent (1H) and the PF6 anion (19F, 31P).

19F-1H and 31P-1H 2D HOESY

spectra, acquired for the 1.0 and 2.0 M electrolyte samples with a mixing time of 1 s are shown in Figure 4 and Figure S4, respectively. The corresponding 1D spectra are shown along with 12


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the

2D

contour

plots.

concentrations (< 1 M) no

It

is

19F-1H

noticed

that

for

electrolyte

samples

at

lower

HOESY cross-peak development occurs as judged by

2D spectra taken with 1 sec mixing time. On the other hand, cross-peaks develop in both the 19F-1H

and 31P-1H 2D HOESY spectra at 1 M and above and signals the emergence of anion-

solvent interactions which manifest in the observed dipolar correlations between the anion (19F, 31P)

and the PC solvent (1H). Anion-solvent interactions are thus revealed from our HOESY

data and these begin to show up at 1 M and become prominent at higher concentration. As seen in Figure 4B, cross-peak intensities are seen to be more strongly emphasized when the electrolyte concentration is increased to 2 M (Figure 4B). The 19F-1H HOESY observations are also duplicated in the complementary 31P-1H HOESY spectra to lend further support. It may be noted that anion-solvent interactions have been shown to exist in the LIB electrolytes by Raman spectroscopy8-10 and theoretical calculations.

8-10,19-21

However, direct NMR

evidence has so far been lacking. In this context the results presented in Figure 3 assume considerable significance since our data clearly establishes that strong anion-solvent interactions do exist in the case of cyclic PC. Further insights are provided from the mixing time dependence of the nOe build up which aid the determination of the cross relaxation parameters for the site dependent anion-proton interactions. These are discussed below. The build of the 19F-1H HOESY cross peaks as a function of the mixing time is shown in Figure 3B. The mixing time dependence of the cross-peak intensities show that anion-solvent interactions are felt on all the protons of the propylene carbonate with slightly different nOe buildup rates. Our finding that anion-solvent interactions prevail in LiPF6/PC is well supported by recent DFT study on the LiPF6/EC electrolyte.39 Cation-anion interaction through 7Li-19F HOESY 2D HOESY results pertaining to 7Li-19F, which aid in probing the cation-anion interaction are presented in Figures 5A and 5B for low (0.2 M) and high concentrations (2 M), respectively, 13



at a mixing time of 1 s. The HOESY results show that while cation-solvent interactions dominate at early concentrations (Figure 2A) and proceed further to higher concentrations (Figures 2B- 2D), the cation-anion interaction, which is implicated in the formation of contact ion pairs (CIP), begin to assume importance only at concentrations of 2 M (Figure 5B). This is evident from the absence of any 7Li-19F cross-peaks at 0.2 M (Figure 5A) even at the long mixing time of 1 s and the detection of strong cross peaks at 2 M (Figure 5B) for the same mixing time. Thus it is observed that for the LiPF6-PC electrolyte CIP becomes the dominant species only at a high electrolyte concentration of 2 M and above whereas at 1 M, which is typically used in LIBs, CIP accumulation is way far too less to outweigh the solvated cations. Our observations are consistent with the earlier findings6,12,16 that for the cyclic carbonate CIP are favored only at high salt concentrations. From 7Li NMR results of LiPF6-PC electrolyte we discern that the lithium cation speciation into the solvent separated ion pair (SSIP) and CIP types occurs in such a way that their dominance prevails in well demarcated regions of electrolyte concentrations, namely, (0.1 – 1 M) and (2.0 – 3.0 M), respectively. We thus reckon from out HOESY studies that CIP formation is quite significant at higher salt concentrations of 1.8 M and above which is further supported by our 7Li NMR diffusion studies (vide infra). From the point of view of the conventionally used electrolyte concentration of 1 M in LIBs, the CIP accumulation at lower concentrations is far too less to inhibit the lithium ion mobility in the electrolyte solvent in any regressive manner. It is clear from the aforementioned discussion that the dissociation of LiPF6 in PC initially favors Li+ ions which are solvated and are separated from the PF6 counter-ions. It is now known that in cyclic carbonates solvation is readily favored due to the Li ion binding to the carbonyl sites of the solvent molecules and, thus, represents the first event in lithium speciation.17 To what limits of salt concentration the solvation would proceed to completion depends on the nature and geometrical details of the ion-solvent interaction. However, due to 14


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the similarity of molecular structure and geometry in most of the cyclic carbonates the choice of around 1 M seems sufficient to ensure that the competing effects of contact ion formation are minimized and the electrolyte performance under ideal battery conditions is not unduly compromised. In the present study of LiPF6-PC electrolyte, the 7Li NMR spectroscopic distinction of the SSIP and CIP environments directly from the observed spectra at very electrolyte concentrations (2.5 – 3.5 M) is somewhat surprising because these two lithium environments in a homogeneous electrolyte solution is expected to exist in a chemically exchanging dynamic equilibrium. In such a situation the individual distinction of the lithium cation in SSIP and CIP environments would be precluded in the NMR experiment due to rapid chemical exchange occurring in the fast exchange limit, namely, exch 1.8 M) and a distinct break in the 7Li NMR diffusion data (vide infra) are in favor of discounting CIP species which are entirely monomers in the solution. The NMR observations suggest that a significant population CIP aggregates exist in the electrolyte solution in the salt rich regime of 2.0 – 3.0 M. The 2D HOESY connectivities established in LiPF6- PC electrolyte are presented in Table 1. Transient nOe and kinetic analysis

15



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We have monitored and analyzed the Overhauser enhancement build up in the 7Li-1H

and 19F-1H HOESY experiments to further glean insights on the specifics of the

interaction that the propylene carbonate solvent has with Li+ cations and PF6― anions. This was achieved by observing 7Li or 19F directly and repeating the 2D experiment at different mixing times. In each HOESY experiment the use of nonselective pulses to obtain selective nOes entails the resolution of dipolar correlations at the intersection of the chemical shifts of the two interacting nuclei, namely 7Li or

19F,

and 1H. This

allows us to measure the complete set of nOe build up curves40,41 for the four protons of the PC molecule from the observed cross-peak intensities. A stack plot of the signal build up in the 7Li-1H HOESY experiment as a function of the mixing time is shown in Figure S5. Figure 3 shows the complete transient Overhauser response for the 7Li-1H and 19F-1H interactions, which were determined at the four different proton sites from the two independent 2D experiments. These transient nOe data lend for a kinetic analysis as described below. Heteronuclear Overhauser enhancement occurs due to the spatially dependent dipolar interaction between the observed (7Li,

19F)

and the perturbed (1H) spins, and

their time dependence imparted by the motional dynamics in solution phase. For the 16


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transient nOe experiment involving of 1H and

19F

spins, the build up curves can be

analyzed using Solomon30,40 rate equations, originally derived for a coupled spin ½ pair and solved for given initial conditions. For the transient nOe experiment involving 1H

and 7Li (I = 3/2) spins, however, it may be anticipated that further theoretical

considerations would apply and these will be the subject of a future study. Additional interplay from exchange effects, such as that due to solvation dynamics, and 1H-1H cross-relaxation effects complicate the kinetic analysis of the transient nOe data and these must be considered in the analysis of the complete build up curves. Because of the above complications, we have refrained from carrying out an analysis of our build up curves using the two-spin Solomon model. Instead, we have analyzed the kinetic data in the initial nOe build up region for which a linear rate approximation (ISm < 1) holds.30 This is written as: S ( m )  I eq  I NI    IS S eq  S eq  S N S (n.a.) 

(1)

where S is the observed magnetization with its equilibrium value being Seq, Ieq is the equilibrium I spin magnetization, I and S denote their magnetogyric ratios, NI and NS denote the number of I and S spins which are involved in the interaction and (n.a.) 17



is the natural abundance of the S spin (7Li = 0.9241).42 Equation (1) shows that in the initial rate approximation the rate of change of magnetization is proportional to the cross-relaxation rate and therefore IS can be determined if the other parameters in equation 1 are known. The equilibrium magnetization ratio is (Ieq/ Seq ) can be estimated from the composition of the solvent and the ionic species (Li+,PF6― ) present at the known electrolyte concentration, which aids in estimating the cross-relaxation rate IS from the linear analysis using equation 1. We wish to remark that the absolute values of IS are not determined in our analysis for the 7Li-1H interaction but their relative magnitudes are undoubtedly recovered from the linear analysis, thus providing a bench mark for a meaningful comparison. In the initial rate approximation, the rate of change of magnetization is essentially governed by the cross-relaxation rate and hence can be determined directly from the slope of the straight line graph. Similarly, for the cation-solvent interaction from 7Li-1H HOESY measurements, a linear analysis of the kinetic nOe data seems valid since the auto relaxation rate RH1 tends to RLi 1 and the slope determined by linear regression may be considered to be proportional to the cross-relaxation rate HLi. We have accordingly carried out a linear regression analysis of the 7Li-1H

and 19F-1H transient nOe data using equation 1 and these are shown in Figure 3 as inserts

above the mixing time build up curves. The cross relaxation parameters extracted from such an analysis are summarized in Table 2.

18


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The results in Table 2 show that cation-solvent and anion-solvent interactions are distinctly demarcated from the values for the slope which show nearly an order of magnitude difference between the two data sets. The changes in the slope can be readily noticed in the linear plots of Figure 3 as well. In the case of 2 M LiPF6-PC electrolyte we have already inferred from 2D HOESY spectra that cation-solvent and anion-solvent interactions coexist. However, a comparative measure of the relative strength of these interactions could not be assessed. The cross-relaxation parameters determined from the kinetic analysis clearly show that the cation-solvent interaction occurs on a stronger footing and, by comparison, the anionsolvent interaction is weaker. The final results further show that the cross-relaxation rates determined for the different protons are markedly different, especially noticed in the 7Li-1H kinetic nOe analysis, and evidences site dependent spatial interaction which varies across the four proton sites. This is revealed from the value for Is which has been estimated using  = [S2 –(RI1-RS1)2]1/2 with due inclusion of the I (1H) and S (7Li,

19F)

spin longitudinal

relaxation rates which were determined from independent inversion recovery T1 measurements (Table 2, Fig. S8). From the cross-relaxation rates determined for the 7Li-1H interaction we ascertain that the interaction of the solvent with lithium cation is strongest at Hb site and is full affirmity with

17O

NMR studies which have shown the solvent binding occurs at ethereal

oxygen site.17 It may be remarked that to a first degree the cross relaxation rates determined from the kinetic HOESY analyses are qualitatively interpretable as a reflection of the average heteronuclear proximity, thus serving to assess the cation-solvent and the anion-solvent interactions in a comparative manner and to know if solvation occurs preferentially at certain atomic sites. Further interpretation of the cross-relaxation rate in terms of inter-nuclear distances is however formidable since the dipolar interaction is intermolecular in nature and the combined effects of rotational and translational motions impart complex time dependence. Theoretical descriptions43-45 of intermolecular nOe have shown that the spectral density 19



Page 20 of 47

function is no more Lorentzian and the deduction of intermolecular distance information from the observed cross relaxation rate is rendered difficult owing to the r-n dependence, n varying between 1 and 6 depending on the frequency of the nuclei and their dynamics. Further interpretation of the accurately determined cross relaxation rates in terms of internuclear distances would require molecular dynamics simulations and adaptation of motional models which provide a comprehensive description of the complex rotational and translational dynamics in the LIB electrolyte. 42,46,47 7Li

NMR Diffusometry

We additionally provide lithium cation speciation in LiPF6-PC electrolyte through independent dynamic 7Li NMR measurements based on 7Li self-diffusion coefficient (Dself). Variableconcentration Dself data depict distinct profiles for the spin-echo decay due to their respective translational cation mobilities. It is noted that all the observed spin-echo decays in the investigated Li concentration range (0.1 – 3 M) exhibited single exponential behaviors indicating fast exchange of Li between SSIP and CIP pool environments (Figure S6). However, 7Li-1H 2D HOESY data for 3 M at mixing times 0.1 and 1 s reveals the presence of two Li species which are better resolved at higher magnetic field (Figures S7). This clearly shows the presence of two distinct cation species which are in slow exchange and is further evidenced by 7Li T2 decay exhibiting bi-exponential characteristic (Figure S3B).

The

distinction is very clear in T2 decay plot than in T1 plot and this can be ascribed to the fact that while the former additionally involves spectral density at zero frequency when compared to latter, causing slow motions to bear the observation. We notice that the spin-echo decay in diffusion data clearly exhibit a single exponential behavior at all concentrations upto 3 M as shown in Figure S6. Considering that we have used diffusion times in the range of 20-50 ms 20


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in the present studies any exchange occurring on a time scale which is shorter than this period would not reveal any bi-exponential decay attributable to the translation mobilities of two distinct cation species. On the other hand, although the cation exchange occurs on the shorter time scale of the diffusion experiment, the two species may be considered to be long lived in the relaxation window which characterizes their rotational mobility decoupled in the relaxation experiment and hence distinguished. The 7Li NMR determined self-diffusion (Dself) as a function of variable-concentration for the binary mixture LiPF6/PC as function of [LiPF6] depicted in Figure 6A. The numerical values of the diffusion coefficients determined from the analysis of spin echo decays are presented in Table 3. The plot shows a variation which can be distinctly characterized by Li ion in two distinct environments with different translational degree of freedom. From the plot of Dself versus concentration (Figure 6A), two distinct breaks are apparent at 0.37 M and 1.20 M and are attributable to onset of SSIP and CIP species, respectively. A plateau in Dself is noted below a critical concentration of 0.37 M and is attributable to the translational mobility of both free Li as well as partially solvated Li species as we may assume that these species share a common diffusivity. However, changes in Dself above 0.37 M are prominent and these signal the onset of solvated Li species and the observed variation of Dself in the 0.37 - 1.2 M region is fully governed by the lithium-solvent coordination in a dynamically exchanging environment. A much greater retardation to lithium translational mobility is observed beyond 1.2 M and can be ascribed to the formation of contact-ion pair species. Overall, the distinctly different slopes that we have observed in the 7Li self-diffusion plot over the entire concentration range of 0.1 – 3 M are in excellent accord with our 7Li spectral line shape observations and the two dimensional heteronuclear 7Li-1H and 7Li-19F 2D HOESY results (Figures 2 and 5). It may be noted that in the solvent rich regime (0.1 – 1.2 M) the Li ion is in fast exchange between its free/PC-solvated and the smaller population of CIP environments so that the 21



Page 22 of 47

observed self-diffusion is considered as a weighted average on the experimental NMR time scales (diffusion time ~ 20 ms). Such an adaptation for the NMR determined self-diffusion is reasonable in the framework of two-site exchange model as reported elsewhere.48,49 As we have noted earlier, this is also reflected in the 7Li spectra which are exchange narrowed to a single Li resonance in the concentration range 0.1 M – 3.0 M due to dynamic exchange of Li ion between free/solvated and CIP environments. The observed diffusion data can therefore be analyzed using the equation Dobs = Df pf + Db pb

(2)

Where Df and Db represent diffusion coefficients corresponding to Li in SSIP and CIP environments, respectively.

Here pf and pb denote the free and bound fraction of Li,

respectively, so that pf + pb = 1. By inserting the values of pf = cf/ct and pb = cb/ct, the equation 2 can be reframed as Dobs = Df (cf/ct) + Db(cb/ct) = Db + (Df – Db )cf/ct

(3)

Here, cf and cb are the free and bound concentration of Li+ species, while ct (= cf + cb) is the total Li ion concentration. Alternatively, cf denotes the critical onset concentration for SSIP formation. A linear plot of Dobs (above the inflection point or critical solvation concentration, where both SSIP and CIP species are present) versus 1/ct yields the intercept Db, while the slope yields the product of cf and (Df – Db). The hydrodynamic radius (RH) can be obtained from the knowledge of Db, given by Stokes-Einstein relation, RH = kBT/6D

(4)

where all the other symbols have their usual meaning. A quantitative description of variable-concentration 7Li NMR diffusion data in the PC solvent dominated concentration range (0.1 M – 1.2 M) can be made with the aid of two-site model as follows: The observed Dself data plotted against 1/ct fitted to the linear equation (3) 22


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(see inset of Figure 6B) yielded Df = 2.02 x 10-10 m2s-1 and Db = 0.94 x 10-10 m2s-1 (by considering the break as cf = 0.37 M). A viscosity of PC at 25°C~ 2.55 mPas was considered.6 The hydrodynamic radius calculated from the values of Df (using equation 4) is 4.24 Å and is indicative of the size of solvated lithium ion and in good agreement with the Stokes radius of 4.4 Å for the Li+ ion in PC reported earlier.6 The consequent effective volume of lithium ions Veff (Li+) volume 319 Å3 calculated from the hydrodynamic dimension (4.24 Å), which is reasonable agreement with its reported value of 357 Å3 found elsewhere.6 In the present study, the coordination number of Li+ for the PC solvent has been quantitatively determined to be about 4 from the ratio of the calculated Veff (Li+) (= 319 Å3) and the volume of a PC molecule (= 82.8 Å3).6 The same value of coordination number has been reported for LiPF6-PC system from ab initio molecular dynamics (AIMD) simulation.50,51 17O

NMR studies of LiPF6 in mixed solvents of ethylene carbonate and dimethyl carbonate

also show that in the fully ethylene carbonate bound limit four EC molecules bind to the Li+ cation in the solvation sheath.17 We wish to remark that NMR studies by us and others39,51 show that in dilute cyclic carbonate solvents (PC, EC) there is a four-fold coordination of the solvent molecules around Li+. Previous studies39 on non-dilute LiPF6-EC electrolyte solution by infrared femtosecond spectroscopy show that the solvent coordination number is reduced to 2 due to the presence of anions in the first coordination sphere which tend to modify the tetrahedral structure around Li+. This variance is indeed striking but it must be borne in mind that while NMR determined coordination number is derived from a dynamically averaged response, the free and solvated PC are individually distinguished and analysed by femtosecond spectroscopy for which the resolving power is very high. Furthermore, our calculations show that the average dimension of the CIP (composed of Li+ and PF6-) deduced from Db is 9.2 Å which is definitely larger than the size of a single ion-pair and thus shows evidence for cluster formation. 23



A distinct demarcation in 7Li self-diffusion in the salt rich regime (1.2 – 3 M) with a distinct break beyond complete solvation, at around 1.2 M, characterizes the CIP formation and its increasing dominance to impede the translational mobility of the lithium ion. In this regard, the observed Dself (> 1.2 M) is mostly contributed by the slowly moving CIP species which determine the observed 7Li spin-echo decay data and the data fit we have obtained based on a single component fit. (Figure S6). From our observation of two distinct regions for 7Li selfdiffusion, namely 0.1 – 1.2 M and 1.2 – 3 M corresponding to the solvation and CIP dominating regions, respectively, and the distinct break in Dself observed at 1.2 M, we can recognize that two distinct lithium species co-exist in the electrolyte solution. The lithium speciation provided by 7Li NMR finds a striking comparison with the conductivity data determined over the same electrolyte concentration range. The results are shown on the same plot of Figure 6A. The enhanced conductivity in the solvation regime (0.2-1.2 M) during which SSIPs are the dominant species is at once evident (Figure 6A and Table 3). Conductivity maximization at 1.4 M for which complete solvation has been achieved is revealed from the data and, more interestingly, the sharp decrease in conductance is fully reflected in the increased population of CIPs at high salt concentrations (Figure 6A). As inferred from our 7Li NMR and HOESY results, the detrimental effects of CIPs to impede lithium mobility in the electrolyte solution occur beyond 1.4 M and the actual conductivity at any given concentration in the salt rich regime is likely determined by the relative composition of the SSIPs and CIPs in the electrolyte solution. We additionally remark that while CIP dominance at high salt concentrations (> 2 M) is largely responsible for the rapid drop in ionic conductivity, the coexistence of anion-solvent interactions, which we have inferred from our 19F-1H

HOESY experiments, would additionally cause a dent in the lithium ion conductivity.

A previous study39 based on femtosecond infrared spectroscopy and DFT calculations has shown that in non-dilute LiPF6 ethylene carbonate solutions the PF6― anions influence the local 24


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structure of lithium cation and lead to the formation of cation/anion solvated weak complexes. The ionic conductivity drop of lithium/carbonate electrolyte solutions at high concentrations, which was not well correlated with change in solution viscosity, was explained based on the formation of such complexes.39 We have additionally measured solvent (1H) and anion (19F) NMR diffusion self-diffusion coefficients 0.1 – 3 M for the sake of completeness (Figure S9). It may be noted that while 7Li NMR diffusion data show two distinct breaks attributable to the onset of SSIP and CIP, no such breaks are could be detected in the anion and solvent diffusion data. Our anion and solvent diffusion data matches well with those reported earlier on various lithium salts including LiPF6.24,26,27 The striking similarity between 7Li self-diffusion and bulk conductivity further bears on how the local translational diffusion affects the bulk conductivity in the actual battery cell. Studies on assessing SSIP and CIP and their roles in influencing the overall conductivity (or mobility) of Li species in the battery cell, where the latter has an adverse effect on mobility are extremely important. Minimizing detrimental effects CIPs by a judicious choice of additives i. e., co-solvents, ionic liquids, hydrophilic copolymers etc are synthetically attractive and a full level systematic diffusion approach (with the aid of strong field gradients) will be quite rewarding to provide more accurate lithium ion speciation, the local dynamics and their link to the bulk electrolyte conductivity. The scheme-1 portrays the proposed microstructures of Li+ in LiPF6/PC binary mixture. CONCLUSIONS 7Li-NMR spectroscopy has been utilized to study the lithium cation speciation in the electrolyte

solution of LiPF6 in non-aqueous organic propylene carbonate. The dominance of the solvated and solvent separated ion pairs (SSIPs) in the solvent rich regime (< 1.2 M) and that of the contact ion pairs (CIPs) in the high salt rich regime (> 1.2 M) of electrolyte concentration have been established from two-dimensional hetronuclear Overhauser enhancement spectroscopy 25



and through 7Li NMR self-diffusion measurements. Various combinations of 2D HOESY, by exploiting NMR active nuclei located on the solvent (PC), cation (Li+) and anion (PF6), have enabled to probe the spatial proximities between them. Specifically, HOESY results show that the cation-solvent interaction occurs in the vicinity of the ethereal oxygen. Furthermore, definite anion-solvent interactions are also found to exist. The observed self-diffusion data over the full concentration range reflects two species with a clear demarcation of the translational mobilities. The diffusion data in the solvent rich region (0.1 – 1.2 M) based on the two phase model has enabled the determination of a coordination number of four for the Li+-PC solvent interaction. We further infer that the CIP species are not entirely monomeric in nature but rather occur as molecular aggregates. Our spectral and dynamic 7Li NMR diffusion measurements and their comparison of with conductivity data serve to establish the molecular basis for understanding bulk conductivity behavior of the LiPF6-PC electrolyte and additionally validate the presently used 1 M electrolyte concentration as Li-ion battery industrial standard for optimal battery performance.

ASSOCIATED CONTENT Supporting Information 1) 31P (161.976 MHz) NMR spectra of 0.35 M (A), 2.5M (B) and 3 M (C) LiPF6 in propylene carbonate. All these spectra depict a septet J-splitting pattern centered at -144.33 ppm due to 1J (31P-19F)

(706 Hz) for the PF6 ion. The absence of signals with associated multiplicities in

the -7.99 to –20.34 ppm PO3 F2 and PO2 F2 indicates that there is no decomposition of PF6 , showing the exceptional purity and stability of the electrolyte solutions. 2) (A) Stack plot of 7Li

NMR spectra at all the concentrations of LiPF6/PC in the entire range 0.2 - 3.5 M. (B).

Deconvolution of the spectra for 0.4, 1.2, 2.0, 2.8, 3.0 and 3.2 M are shown. 3) 7Li signal recovery and decay as a function of the relaxation delay time for 3 M electrolyte solution of 26


Page 26 of 47

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LiPF6 and propylene carbonate in the inversion-recovery T1 (A) and Carr-Purcell-MeiboomGill T2 (B) experiments. Data points correspond to the total integrated intensity of the signal at each  delay. The dash lines through data points denote the theoretical fit to single component relaxation behaviour in each case. The solid blue line denotes the theoretical fit of the experimental data to relaxation involving two components. 4)

31P-1H

HOESY contour plot

(25°C) of 2 M LiPF6-PC electrolyte at m = 1 s. 16 transients with a recycle delay of

1.5 s

were coadded in each of the 128 experiments in the t1 domain with increments of 330 s using a 400/15 ppm acquisition spectral width. The 8192 x 4096 data matrix was processed in the magnitude mode using exponential window function in both F2 and F1 dimensions. The 31P (fluorine coupled, septet with 1J: 31P-19F = 706 Hz) and 1H NMR spectra along with the proton signal assignments are shown along the orthogonal axes. 5) Evolution of 1H magnetization as a function of mixing time m used in the 7Li-1H HOESY experiments (2 M LiPF6 in PC) depicting the Overhauser enhancement build-up. These stack spectra were extracted along the F1 dimension from the observed cross-peaks. 6) 7Li spin echo decay as a function of the applied magnetic field gradient strength. The symbols denote the experimental data and the continuous lines represent the data fitting to Stejskal-Tanner equation, I  I 0 e  kD , where I 0 is the peak intensity in the absence of gradient pulses, D is the translational self-diffusion coefficient and

k  (γ  g) 2 (Δ  /3) where  is the magnetogyric ratio and  and  are gradient duration and diffusion period, respectively. D was determined at each electrolyte concentration in the range 0.1 - 3 M. 7) 7Li-1H HOESY contour plot of 3 M LiPF6-PC electrolyte at 25° C (a) m= 0.1 s; (b) m=1 s. 16 transients with a recycle delay of 1.5 s were coadded in each of the 128 experiments in the t1 domain with increments of 330 s using a 25/10 ppm acquisition spectral width. The 8192 x 4096 data matrix was processed in the magnitude mode using exponential window function in both F2 and F1 dimensions. The 7Li and 1H NMR spectra along with the 27



proton signal assignments are shown along the orthogonal axes. 8) Magnetization recovery profiles for the solvent (1H: PC), anion (19F: PF6― ) and cation (7Li: Li+) recorded for 2M LiPF6/PC electrolyte. The symbols denote the experimentally determined values, while the solid represents the best of the experimental data to the equation M(t) = Mo[1 – 2exp(-/T1)]. 9) Self-diffusion coefficient determined for the cation (7Li: Li+), anion (19F: PF6― ) and solvent (1H: PC) LiPF6-PC electrolyte over the concentration range 0.1 - 3.0 M. The symbols denote the experimentally determined values. Only the 7Li self-diffusion data has been subjected to detailed analysis (Figure. 1 (A), (B), vide text).

AUTHOR INFORMATION Corresponding Author E-mail [email protected] ORCID Vikas Kumar: 0000-0002-3824-4321 R. Ravikanth Reddy: 0000-0002-1259-0082 B. V. N Phani Kumar: 0000-0001-5490-4345 Chilukuri V. Avadhani: 0000-0003-2787-1829 Subramanian Ganapathy: 0000-0003-3625-8719 Narayanan Chandrakumar: 0000-0002-7089-3823 Swaminathan Sivaram: 0000-0002-1059-6122 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

28


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This work was supported by CSIR under TAPSUN Grant NWP0056-D. RRKR is thanks to CSIR, New Delhi for the grant of a Senior Research Fellowship. V. K thanks UGC, India for the grant of a research fellowship. RRKR and BVNPK are grateful to Dr. B. Chandrasekaran, Director, CSIR-CLRI for the extended support. S.G. thanks the Council of Scientific and Industrial

Research,

New

Delhi

for

support

under

Emeritus

Scientist

Scheme

(HRDG:21(0701)/07/EMR-II). Dr. S. Sivaram acknowledges financial support from CSIR, New Delhi (Bhatnagar Fellowship) and Indian National Science Academy, New Delhi. Authors are indebted to Prof. S. Ramakrishnan, IPC, IISc-Bangalore for providing access to specialized NMR facilities to carry out diffusion measurements.

29



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Organometallics, 2000, 19, 2868–2873. [30] Neuhaus, D.; Williamson, N. P. The Nuclear Overhauser Effect in Structural and Conformational Analysis; Verlag Chemie: New York, 1989. [31] Walker, O.; Mutzenhardt, P.; Canet, D. Heteronuclear Overhauser Experiments for Symmetric Molecules Magn. Reson. Chem. 2003, 41, 776–781. [32] Bauer,

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[49] Pettersson, E.; Topgaard, D.; Stilbs, P.; Söderman, O. Surfactant/Nonionic Polymer Interaction. A NMR Diffusometry and NMR Electrophoretic Investigation. Langmuir 2004, 20, 1138–1143. [50] Ganesh, P.; Jiang, D.-E.; Kent, P. R. C. Accurate Static and Dynamic Properties of Liquid Electrolytes for Li-Ion Batteries from Ab Initio Molecular Dynamics. J. Phys. Chem. B 2011, 115, 3085–3090. [51] Xing, L.; Zheng, X.; Schroeder, M.; Alvarado, J.; Cresce, A. V. D.; Xu, K.; Li, Q.; Li, W. Deciphering the Ethylene Carbonate–Propylene Carbonate Mystery in Li-ion Batteries. Acc. Chem. Res., 2018, 51, 282–289.

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Figure 1. 7Li-NMR spectra of LiPF6 in propylene carbonate at select salt concentration in the range 0.2-3.5 M.

37



Figure 2.

7Li-1H

HOESY contour plot of LiPF6-PC electrolyte at 25 °C. (A) 0.2 M, m= 1 s;

(B) 1 M, m= 1 s, (C) 1.8 M, m= 0.25 s and (D) 1.8 M, m= 1s. 16 transients with a recycle delay of 1.5 s were coadded in each of the 128 experiments in the t1 domain with increments of 345 s using a 25/15 ppm acquisition spectral width. The 8192 x 4096 data matrix was processed in the magnitude mode using exponential window function in both F2 and F1 dimensions. The 7Li and 1H NMR spectra along with the proton signal assignments are shown along the orthogonal axes.

38


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Figure 3. Variation of the cross-peak intensities for the different protons of propylene carbonate determined in the (A) 7Li-1H and (B) 19F-1H HOESY experiments against the mixing time for the different protons of the propylene carbonate solvent at 2.0 M concentration. Insets depict the linear fits of HOE intensities for different protons of PC.

39



Figure 4. 19F-1H HOESY contour plot of LiPF6-PC electrolyte at 25 °C. (A) 1 M, m= 1 s; (B) 2 M, m= 1 s. 16 transients with a recycle delay of 5 s were coadded in each of the 128 experiments in the t1 domain with increments of 100 s using a 50/25 ppm acquisition spectral width. The 8192 x 4096 data matrix was processed in the magnitude mode using exponential window function in both F2 and F1 dimensions. The 19F and 1H NMR spectra along with the proton signal assignments are shown along the orthogonal axes. The fluorine appears as a doublet due to scalar coupling to 31P (JF-P = 710 Hz).

40


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Figure 5. 7Li-19F HOESY contour plot of LiPF6-PC electrolyte at 25 °C. (A) 0.2, 1 M, m= 1 s; (B) 2 M, m= 1 s. 16 transients with a recycle delay of 1.5 s were coadded in each of the 128 experiments in the t1 domain with increments of 15 s using a 25/128 ppm acquisition spectral width. The 8192 x 4096 data matrix was processed in the magnitude mode using exponential window function in both F2 and F1 dimensions. The fluorine appears as a doublet along the indirect dimension due to scalar coupling to 31P (JF-P = 710 Hz).

41



Figure 6. (A) 7Li self-diffusion coefficient determined for the LiPF6-PC electrolyte over the concentration range 0.1 - 3 M. The symbols denote the experimentally determined values and the straight lines are drawn to show the two well demarcated regions of 7Li self-diffusion behavior with distinct breaks at 0.37 and 1.2 M. 7Li self-diffusion data is also compared with conductivity data in the similar Li concentration range. (B) 7Li self-diffusion data plotted against 1/[LiPF6] in the range 0.1 - 1.2 M showing two distinct regions indicated by the straight lines and the break observed at 0.37 M. Linear-fit of the diffusion data in the concentration range 0.1 - 1.2 M using equation (3) is shown in the insert graph. 42


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Table 1. 2D HOESY connectivity established in LiPF6- PC electrolyte Concentration (M) 0.2

7Li-1H

7Li-19F

19F-1H

31P-1H

xx

-

-

-

1.0

xx

-

xx

-

1.8

xx

xx

xx

xx

2.0

xx

xx

xx

xx

3

xx

xx

xx

xx

The HOESY connectivity established is indicated against the different experiments (xx).

43



Page 44 of 47

Table 2. Relative cross-relaxation parameters for cation-solvent and anion-solvent interactions in LiPF6-PC electrolyte (2 M, 25°C) from 2D HOESY data analysis.a Proton

Slope from linear fit (S) /10-2 s-1

Site

7Li-1H

 = [S2 –(RI1-RS1)2]1/2 TH1 (s)b

/10-2 s-1 19F-1H

7Li-1H

IHeq/ILi eq

IHeq/IFeq

19F-1H

Ha

16.78 (1.10)

1.32 (0.05)

16.780

0.946

0.691 (0.008)

49.211

3.1337

Hb

5.87 (0.14)

0.78 (0.05)

5.838

0.701

1.270 (0.007)

16.404

1.0446

Hd

7.07 (0.25)

1.07 (0.14)

7.070

0.649

0.762 (0.007)

16.404

1.0446

Hc

7.29 (0.20)

1.03 (0.11)

7.289

0.567

0.752 (0.001)

16.404

1.0446

TLi 1 (s)

0.661 (0.002)

TF1 (s)

1.612 (0.002)

aDetermined

from linear regression analysis of normalized cross-peak intensities [S(m)/Seq] of 2D HOESY spectra recorded at different mixing times and analyzed using equation 1. bSpin-lattice relaxation times determined from inversion recovery experiments (Figure S8). H F Values in parentheses denote standard error estimates. IHeq/ILi eq and Ieq/Ieq correspond to the values determined for 2 M LiPF6-PC electrolyte.

44


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Table 3. 7Li NMR self-diffusion coefficients (Dself) and conductivity data of LiPF6-PC at different concentrationsa Concentration (M) 0.1 0.15 0.35 0.5 0.6 0.7 0.8 0.9 1 1.2 1.5 2 2.5 3 -

Dself -10 /10 m2s-1 2.0825 (0.0046) 2.05 (0.0025 2.0860 (0.0040) 1.9070 (0.0032) 1.6135 (0.0015) 1.4874 (0.0022) 1.5302 (0.0045) 0.8235 (0.0014) 1.2832 (0.0057) 1.2945 (0.0039) 0.9784 (0.0022) 0.6904 (0.0019) 0.0201 (0.0001) 0.0205 (0.00008) -

Concentration (M) 0.2

Conductivity S/m 0.00241

0.4

0.004

0.6

0.00521

0.8

0.00588

1

0.00667

1.2

0.0069

1.4

0.00699

1.6

0.00633

1.8

0.00562

2

0.00526

2.2

0.00392

2.4

0.00299

2.6

0.0014

2.8

0.0007

3

0.0006

aValues

in parentheses denote the corresponding error in the measured self-diffusion coefficients

45



Scheme 1. 7Li NMR depicting the two proposed microstructures of Li+ in a solution of LiPF6 in PC.

46


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TOC Graphic


Lithium Cation Speciation in LiPF6-PC Electrolyte Studied by Two

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