- Email: [email protected]
Physicochemical characterization of a new family of small alkyl phosphonium imide ionic liquids
Electrochimica Acta 202 (2016) 100–109
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Physicochemical characterization of a new family of small alkyl phosphonium imide ionic liquids M. Hildera , G.M.A. Girarda , K. Whitbreadb , S. Zavorineb , M. Moserb , D. Nucciaroneb , M. Forsytha , D.R. MacFarlanec , P.C. Howletta,* a b c
Institute for Frontier Materials (IFM), Deakin University, Burwood, Victoria, 3125, Australia Cytec Canada Inc., Niagara Falls, Canada School of Chemistry, Monash University, Victoria, 3800, Australia
A R T I C L E I N F O
Article history: Received 5 January 2016 Received in revised form 10 March 2016 Accepted 22 March 2016 Available online 7 April 2016 Keywords: ionic liquid electrolyte phosphoniumbattery
A B S T R A C T
Despite their promising properties, phosphonium based ionic liquids have attracted little attention as compared to their nitrogen-based cation counterparts. This study focuses on the properties of a family of small phosphonium imide ionic liquids, as well as the effect of lithium salt addition to these. The 6 ionic liquids were either alkyl, cyclic or nitrile functionalised phoshonium cations with bis(trifluoromethanesulfonyl)imide, NTf2, or bis(fluorosulfonyl)imide (FSI) as anion. Amongst the properties investigated were ionic conductivity, viscosity, thermal behaviour, electrochemical stability and the reversibility of electrochemical lithium cycling. All ionic liquids showed very promising properties e.g. having low transition temperatures, high electrochemical stabilities, low viscosities and high conductivities. Particularly the trimethyl phosphonium ionic liquids showed some of the highest conductivities reported amongst phosphonium ionic liquids generally. The combination of electrochemical stability, high conductivity and reversible lithium cycling makes them promising systems for energy storage devices such as lithium batteries. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Generally speaking ionic liquids (ILs) are compounds made up of ions and which have melting points below 100 C, by definition, ideally being liquids at room temperature (room temperature ionic liquids: RTILs) [1,2]. The low melting points of ILs result from weak interactions between the cations and anions [3]. This is usually achieved by combining large asymmetric cations with anions which show some degree of charge delocalisation [4]. Thus ionic liquids are mainly, but not exclusively, organic in nature, the cations in particular. Ionic liquids have many properties which make them particularly attractive for electrochemical applications (e.g. electrolytes for batteries or capacitors) which include [3]: (i) they consist entirely of ions and thus are ion conductive [5]; (ii) in most cases they are electrochemically inert over a wide potential range and thus can support a range of electrochemical processes [3]; (iii) due to their ionic nature they usually have low volatility [6], preventing
* Corresponding author. E-mail address: [email protected] (P.C. Howlett). http://dx.doi.org/10.1016/j.electacta.2016.03.130 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
them from drying out, and lowering their flammability considerably. The electrochemistry of neat ionic liquids has been well studied [7], and metals have been electrodeposited from ionic liquid solutions [8]. This is particularly interesting for reactive electropositive base metals since they are generally not compatible with aqueous electrolytes. Very important here are high voltage applications such as lithium batteries [9]. Since lithium has the highest negative standard reduction potential of all elements (3.05V vs SHE), it is incompatible with aqueous electrolytes and many traditional organic solvents. On the other hand the large electrochemical window of ionic liquids supports the electrochemistry of lithium. While organic solvent electrolytes having these properties are commercially available (solutions of organic carbonates containing lithium hexafluorophosphate), these are flammable, toxic and the solvent’s volatility can lead to evaporation of the electrolyte resulting in device failure [10]. Thus, for lithium energy storage technology ionic liquids are promising candidates to replace solvent-based electrolytes. Of the large variety of ionic liquids, an extensively studied class are those incorporating quaternary ammonium cations, however, less attention has been paid to their analogues, the phosphonium
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
ILs, despite reports that they either match or even outperform the more traditional ammonium derivatives [11–18]. Examples of studied systems include ILs based on tetra-alkyl phosphonium [11,18–31], protic alkyl phosphonium [17,32–34], alkyl-alkoxy phosphonium [20], allyl phosphonium [20,35] and cyclic phosphonium or phosphinanium [36] cations, as well as dicationic alkyl phosphonium [37]. Common anions used include bis(trifluoromethanesulfonyl)imide (NTf2) or bis(fluorosulfonyl)imide (FSI) where, in general, the latter anion shows superior properties compared to the former. Recently, a number of new phosphonium having small alkyl cations and cations with additional functionality such as nitrile groups have become available [38]. The phosphonium ionic liquids investigated in this study were four small alkyl phosphonium ionic liquids, bicyclic phosphonium ionic liquids, as well as two nitrile-functionalised phosphonium ionic liquids. As mentioned above, the small alkyl ionic liquids have promising properties for electrolyte applications (low viscosity, high conductivity). The latter ILs are particularly interesting; while nitrile groups are commonly found in ionic liquids anions (dicyanamide, tetranitrileborate), data on nitrile functionalised cations is more limited. There are a few reports on nitrilefunctionalised pyrrolidinium ionic liquid cations [38]. The nitrile group adds functionality, specific reactivity and also influences other physical properties (e.g. increases hydrophilicity, dipole moment, dielectric constant etc.). Furthermore, the nitrile group adds solvation properties to the positive phosphonium cations which may influence speciation of the Li+[38]. The properties of six such phosphonium ionic liquids are studied in this paper (structures as shown in Fig. 1). A special focus is on evaluating their properties in regards to electrolyte applications in lithium batteries. Thus the effect of the presence of various levels of lithium addition on the properties was also investigated. We also compare it to the properties of P111i4 FSI described recently [39]. 2. Experimental All handling, characterization and preparation of samples and devices was carried out under strictly anhydrous and air-sensitive conditions using glove box and vacuum line techniques.
101
2.1. Samples The ionic liquids triethyl cyanopropyl phosphonium bis (trifluoromethanesulfonyl)imide (e.g. PCNNTf2), triethyl cyanopropyl phosphonium bis(fluorosulfonyl)imide (e.g. PCNFSI), bicyclo[3.1.1], 3,7-dimethyloctane, ethyl, hexyl phosphonium bis (trifluoromethanesulfonyl)imide (e.g. PcycloNTf2), trimethyl, propyl phosphonium bis(trifluoromethanesulfonyl)imide (e.g. P1113NTf2), trimethyl, isobutyl phosphonium bis(trifluoromethanesulfonyl) imide (e.g. P111i4NTf2) and the previously discussed trimethyl isobutyl phosphonium bis(fluorosulfonyl)imide (e.g. P111i4FSI) [39] were received from Cytec (Canada) and their structures are shown in Fig. 1. Matching the anions of the ionic liquids and the lithium salts appropriate amounts of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 = LiN(CF3SO2)2) or lithium bis(fluorosulfonyl) imide (LiFSI = LiN(FSO2)2) were added giving solutions with molalities of 0.5 or 1.0mol Li salt per kg ionic liquid. FTIR are also included in the appendix (Fig. S1). 2.2. Water content The samples were dried using an in-house designed glass vessel consisting of a tube holding the sample (10mL), a bulb connected to the tube via a sintered glass frit, containing sodium hydride as a desiccant (NaH, Sigma Aldrich), and a vacuum connection. After two days of heating the samples under vacuum at 75 C, the water contents of the samples were well below 25ppm as determined by coulometric Karl Fisher titration (Metrohm model 831, Hydranal1 Coulomat AG titrant). 2.3. Differential Scanning Calorimetry (DSC) The phase behaviour (melting, crystallization, and glass transition temperatures, transition enthalpies and transition entropies) were derived from differential scanning calorimetry experiments (Mettler Toledo DSC1) carried out in the range of 95 to 125 C (10 C/min). Temperatures were corrected against a cyclohexane standard (Sigma Aldrich). For thermodynamic transitions (melting, crystallization), the onset temperatures were used, with the enthalpies calculated from the integral of the peak area and the entropy determined by dividing the enthalpy by the
Fig. 1. structures of ionic liquids used in this study.
102
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
onset temperature. The glass transition temperature is reported as the midpoint of the onset and endpoint temperatures as determined by tangent methods. 2.4. Viscosity The viscosities were determined using the rolling ball method from 80 C to 20 C (or above the melting point of 50 C for samples P1113 NTf2 and P111i4 NTf2) using 10 C interval steps, a capillary diameter of 2.5mm and an angle of 60 (Anton Paar Lovis 2000ME). The instrument determined the dynamic viscosity taking the density into account as determined at the above mentioned temperatures (Anton Paar DMA5000).
2.6. Electrochemical Characterisation Cyclic voltammetry (CV) was performed using a SP200 Potentiostat, using a three electrode setup (20mV/s). Platinum wire (APS, 99.95%) was used as a counter electrode and as a quasi-reference electrode (ferrocene was used after each set of experiments to calibrate the potentials). For comparison the potentials were mathematically adjusted to Ag/Ag+ and presented accordingly assuming a standard reduction potential 0.16V vs Ag/ Ag+. The electrochemical window was determined using a glassy carbon working electrode (1.0mm, BAS Japan), while for lithium cycling a nickel working electrode was used (1.5mm, BAS Japan). Measurements were carried out for all liquid samples at 25, 50, 75 and 100 C.
2.5. Ionic Conductivity
2.7. Coin Cell preparation and galvanostatic cycling
The ionic conductivity was determined via impedance methods using an SP200 Impedance/Frequency Response Analyser from Biologic (frequency range: 100mHz–1MHz; potential amplitude: 100mV; temperature range: 20 to 120 C with 10 C intervals; temperature controller: Eurotherm 2240E equipped with a thermo couple and a heating block). The cell constant was obtained using a 0.01 M potassium chloride standard (Sigma Aldrich).
Coin cell prototypes (CR3032) were prepared using Hoisen crimping equipment. NMC (47% Ni; L&F Material Co Ltd) was used as a cathode material. The electrodes were prepared mixing 75% NMC, 10% C65 (TimCal) and 15% Solef PVdF binder (Solvay) with 1Methyl-2-pyrrolidinone (Sigma Aldrich), and applied to Al current collector (battery grade). After evaporation of the solvent, the film was heated in an oven at 100 C overnight (no pressing). The films had a diameter of 1.11cm and a coating weight of 1.5mg (1.13mg of
Table 1 Summary of properties of neat ionic liquids and their binary solutions (0.5mol/kg and 1.0mol/kg). Transition Temperatures in C corrected/heating
Viscosity in mPa s 20 to 80 C 60 /2.5mm
Density in g/cm3 20 to 80 C
Conductivity in mS/cm 20 to 120 C (KCl cor: 0.01M)
PCN NTf2 (neat) PCN NTf2 (0.5mol/kg) PCN NTf2 (1.0mol/kg)
Tg: 71.9 Tg: 61.8 Tg: 48.4
274.6–21.8a 880.5–42.1 613–79.71c
1.392–1.343a 1.444-1.393 1.491–1.437
0.0197–11.76 0.0039-8.90 0.006–5.43
P1113 NTf2 (neat) P1113 NTf2 (0.5mol/kg)
Tm: +39.9os/+42.4p Tm: +35.0os +26.6p1 and +41.6p2 Tm: +23.0os (+37.6p1 and +45.6p2) Tcry: +11.0os
25.66–11.5d 57.7–20.8d
1.395–1.368d 1.445–1.418d
0.00001–28.37 0.00003–19.85
123.9–36.4d
1.486–1.455d
0.000004–13.42
Tm1: +40.8os/+43.4p Tm2: 45.5os/-43.0p Tg1: 60.7 Tcry: -18.2os/-12.1p Tg2: 6.1 Tm: 18.9os +23.5p1 and 33.8p2 Tg:-67.6 Tm: (1.9os/3.9p, 17.6p, 29.7p) Tcry: 23.0os (-16.6p)
21.6–9.4d
1.367–1.341d
0.0004–23.62e
58.0–21.0d
1.418–1.389d
0.005–16.29f
535.3–42.8
1.383–1.338
0.0007–8.84e
Pcyto NTf2 (neat) Pcyto NTf2 (0.5mol/kg)
Tg: 48.7 Tg: 43.4
1042.0–46.1b 875.2–83.3c
1.284–1.243b 1.327–1.293c
0.003–6.35 0.00002–4.99
PCN FSI (neat)
Tg: 88.5 Tcry: 49.7os/39.3p Tm: 8.6os/4.1p Tg: 83.2 Tg: 77.1
230.2–25.58
1.313–1.268
0.1025–23. 50
380.3–35.36 398.5–32.98
1.348–1.304 1.383–1.338
0.0587–17.93 0.0356–16.18
P1113 NTf2 (1.0mol/kg)
P111i4 NTf2 (neat) P111i4 NTf2 (0.5mol/kg)
P111i4 NTf2 (1.0mol/kg)
PCN FSI (0.5mol/kg) PCN FSI (1.0mol/kg) os=onset; p=peak. 25–90 C. 25–80 C. c 40–80 C. d 50–80 C. e +20 to +120 C. f 10 to +120 C. a
b
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
active NMC material). The anode was Li metal (Sigma Aldrich; rolled and brushed in the glovebox using dried hexane, 0.9cm diameter and estimated thickness of 100mm), which was separated from the cathode using a Celgard polyethylene separator provided by MTI Corporation (EQ bsf-0025-60c). Galvanostatic cycling was carried out at room temperature using a Neware battery tester with potential limits of 2.75–4.2V. The charge densities were calculated assuming a capacity of NMC of 150mAh/g for C rates of C/10 and C/5 currents of 0.0169mA (15mA/g) and 0.0338mA (30mA/g) were applied respectively. 3. Results and discussion After water content determination, the ionic liquids were characterized in terms of molecular identification and physicochemical properties. The properties are summarized in Table 1. 3.1. DSC The phase change behaviour for the systems under investigation is summarized in Fig. 2(a)–(f). The neat P1113NTf2 shows simple melting behaviour (melting temperature, Tm: 39.9 C).
103
Upon Li salt addition the DSC traces become more complex showing two distinctive melting transitions. The data suggest the presence of a eutectic melting followed by a broader melting transition (a liquidus) with the final melting temperature of the ionic liquid being lowered by the addition of lithium salts, as is typically observed for mixtures. P111i4NTf2 also shows a single melting transition in its neat form (40.8 C) while lithium salt addition produces a glass transition (Tg) combined with enthalpy relaxation, Fig. 2(c). With increasing salt concentration Tg shifts to higher temperatures. The thermal traces also show that the Li solutions crystallise (a devitrification exotherm) on warming and with increasing salt content the traces gain in complexity, again suggesting the presence of eutectic transition. Neat PCNFSI (Fig. 2b) shows glassy behaviour after heating from a rapid cool in the DSC (Tg = 88.5 C). Upon heating, devitrification and melting transitions are observed (8.6 C). Upon the addition of lithium salt to this IL, the solutions show only glassy behaviour, with increasing Li content leading to higher Tg values. All traces for PCNNTf2 and PcycloNTf2 show solely glass behaviour (2a:-71.9 C; 2f: 48.7 C), with the addition of LiNTf2 continuing to shift Tg to higher temperatures. In general the nitrile functionalised samples show only glass transitions. Neat PCNNTf2 shows the lowest value of any
Fig. 2. DSC traces of neat ILs, and their binary mixtures (0.5 and 1.0 moles/kg): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
104
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
of the NTf2 systems, with a Tg of 71.9 C. While the Tg of PCNFSI is even lower (-88.5 C), it also shows additional phase changes upon heating (devitrification/melting). While the very fluid samples show simple crystallisation/ melting behaviour, in the highly viscous samples this is kinetically hindered. Here the difficulty of molecular motion and energetically driven molecular rearrangements to form crystalline solids is hindered by the high viscosity of the samples, thereby favouring glassy behaviour. This also explains why the addition of lithium salts, which increases the strength of ionic interactions, and viscosity, thus promotes supercooling and glass formation at lower temperatures [40]. Furthermore, the bulky ILs and ILs carrying polar functionality show glass forming behaviour, possibly due to an increase of molecular interactions (PCNNTf2, PCNFSI, PcycloNTf2). It is interesting to note that the relatively symmetric trimethylphosphonium ionic liquids (P111i4NTf2, P1113NTf2) are solids at room
temperature. A direct comparison of the effect of the anion on the phase change properties (FSI vs NTf2 for the P111i4 and PCN cation) can be found in the appendix (Fig. S2). 3.2. Viscosity, ionic conductivity and Walden plots One of the major factors determining the performance of ionic liquids as electrolytes is the mobility of the ions. Fig. 3a–f shows the viscosities and conductivities of the neat ILs as well with 0.5 and 1.0 mol/kg added Li salt of the same anion. Selected conductivity and viscosity data is compiled in tables S1 and S2 (appendix). A few obvious trends can be observed: (i) samples with low viscosity show a high conductivity (ii) with increasing temperature, the viscosity decreases while the conductivity increases; (iii) lithium addition increases the viscosity resulting in lower conductivity.
Fig. 3. Viscosity and conductivity of neat ILs, and their binary mixtures (0.5 and 1.0 mol/kg): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
105
Fig. 4. Walden plot for neat ILs, and their binary mixtures (0.5 and 1.0 moles/kg):): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
The ionic conductivity is typically inversely related to the viscosity via Eq. (1) (a combination of the Nernst-Einstein equation and the Stokes-Einstein equation) with s being the ionic conductivity, e the elementary charge, n the charge carrier density,
r the effective radius and h the viscosity[41]:
s¼
e2 n 6prh
ð1Þ
106
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
viscosity/decrease in conductivity may be a result of a higher degree of intermolecular interactions in the NTf2 cases. Comparing the effect of the cation suggests that the higher molecular weight and/or structural functionalisation results in ILs with higher viscosity and lower conductivity, probably as a result of intermolecular interactions (Fig. 3). As mentioned above, the addition of lithium salts increases the viscosity and reduces the conductivity. With increasing salt addition, this trend continues. Here it is interesting to note that for the two FSI samples the change in viscosity and conductivity with the amount of Li salt addition is identical despite the fact of the different nature of the cation e.g. alkyl vs nitrile. With increasing salt content the increase in viscosity is higher for the NTf2 as compared to the FSI systems: 2 times followed by 4 times for the P111i4NTf2 system. The highest increase in viscosity is observed for PCNNTf2 with 2.4 and 5.6 times the initial value upon addition of 0.5 and 1.0mol/kg. The higher anion mobility of FSI vs NTf2 has been discussed before in terms of the smaller anion size for FSI [42]. Evaluating the findings with respect to previously published data suggests that, particularly the small alkyl phosphonium ionic liquids have very low viscosities and high conductivities, which makes them very attractive for electrolyte applications. Tables S1 and S2 (appendix) provide an extensive tabulation of literature data for comparison. At 50 C (the lowest temperature where all studied systems are liquid), the conductivity ranges from 5–14mScm1 (P111i4FSI: 13.8mScm1; P1113NTf2: 6.7mScm1; P111i4NTf2: 5.0mScm1). With such conductivities they are amongst the highest conductivities reported for phosphonium ionic liquids. The same is observed for the viscosity. At 50 C the most fluid samples are P111i4FSI (18mPas), P111i4NTf2 (26mPas) and P1113NTf2 (30mPas), being some of the lowest viscosities observed amongst phosphonium ionic liquids. It has frequently been shown that ionic conductivity and viscosity often follow either the classic Arrhenius behaviour or Arrhenius behaviour modified by Vogel, Tammann and Fulcher (VTF) behaviour at lower temperatures [3]. lnðs Þ ¼ lnðs 0 Þ
Fig. 5. Electrochemical properties of PCNFSI: (a) electrochemical window and (b) lithium cycling in PCNFSI (0.5mol/kg) showing the 1st 10 cycles. The potentials were measured using a Pt quasi-reference and then mathematically adjusted vs Ag/Ag+ using running ferrocene as a standard.
The two alkylphosphonium NTf2 samples (P111i4NTf2 and P1113NTf2) show very low viscosities and high conductivities in their liquid state. The conductivities also show dramatic changes when the samples change from solid to partial melt to liquid (Fig. 3). Comparing the effect of the anion suggests that the viscosities are lower for the FSI ILs, as compared to the NTf2 analogues (a direct comparison is presented in the appendix in Fig. S3). This effect however is much more dominant for the P111i4 cation as compared to the PCN cation. At 50 C the viscosity is not a strong function of the anion (62mPas for PCNNTf2 as compared to 59mPas for PCNFSI) while the viscosity is nearly halved when going from NTf2 to FSI for the P111i4 cation (30mPas vs 18mPas). At the same time, the conductivity is increased by 2.5 times when changing from NTf2 to the FSI anion (at 50 C: 13.8mScm1 for P111i4FSI as compared to 5.0mScm1 for P111i4NTf2; 5.0mScm1 for PCNFSI as compared to 1.9mScm1 for PCNNTf2). The increase of
B T T0
ð2Þ
where s0 is the pre-exponential factor, B a pseudo activation energy (related to the free volume available to support mobility), and T0 the ideal glass transition temperature [43] T0 is lower than Tg (T0 Tg 20–50K) [3]. It has also been suggested for polymers p that s0 is temperature dependent (e.g. s0 = A/T or s0 = A/ T) [44]. A more correct approach would be considering ion dissociation energies including diffusion of the dissociated species by introducing a T-T0 exponent. However over the short temperature range measured there would be too many parameters to be taken into account. Good fits of the liquid state experimental data to the VTF equation could be obtained in most cases. The parameters are presented in the appendix (Table S3 and S4). However, the parameters derived do not follow a clear trend in all cases, principally because different temperature ranges are involved in the fits, because of the different liquid ranges. The interrelation of viscosity and conductivity can be further examined via the Walden plot [45]. From Eq. (3), the molar conductivity (L) and viscosity (h) should be inversely proportional
s c
¼ L ¼ constant=h
ð3Þ
A more detailed description recognises a fractional exponent a in the viscosity dependence such that L ¼ constant=ha where a is typically in the region of 0.8. Consequently, for a system dissociated 100% into mobile ions, a plot of the logarithm of the molar conductivity vs the logarithm of the fluidity w should be a straight line passing through the origin (fluidity w = 1/h).
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
107
Fig. 6. Galvanostatic cycling data of a battery device at different C rates (NMC75/C6510/PVdF15 | PCNFSI-LiFSI(1 M) | Li).
Deviations towards lower conductivity from this ideal line suggest that the IL may be less than 100% dissociated into independently mobile ions. Typically it is thought that this deviation is caused by strong short range interactions resulting in formation of transient ion pairs or aggregates. When neutral ion aggregates are formed, the conductivity is lowered, even in low viscosity samples. This has been studied in detail by a number of authors [46]. The Walden plots of the samples are shown in Fig. 4. Three obvious observations can made: (i) all samples lie quite close to the ideal line; (ii) the addition of lithium salts do not seem to significantly affect the ion association in these samples and (iii) while the FSI samples have superior properties regarding viscosity and ion conductivity, the NTf2 samples show a lower degree of ion association (Fig. S4). Overall, despite having significantly different cation structures, both PCNNTf2 and P111i4NTf2 show high conductivities. The reason for the high conductivity for the PCNNTf2 may be the lack of significant ionic interactions. The data pretty much lie on the line of ideally dissociated KCl solution suggesting weak interactions, not affecting the conductivity adversely. Similarly, the interactions in P111i4FSI may be lower than in the NTf2 analogue, given the closer proximity to the ideal Walden line, making this the sample with the highest ion conductivity. 3.3. Electrochemical properties In general the potential limits of an IL are determined by cation reduction and anion oxidation. The electrochemical windows were determined for neat ILs in the liquid state at 25, 50, 75 and 100 C using a glassy carbon working electrode as shown in Fig. 5 (detailed information in Table S5). All ILs presented here have an electrochemical window of at least 5.7V. It also seems that the reduction potential limits are not exclusively determined by the cation while the limits on the oxidation side are not solely governed by the anion. For example, the oxidation limits at 50 C range from +2.4 to +2.9V vs Ag/Ag+ for the range of NTf2 ionic liquids studied here. Thus it appears that the decomposition reactions are determined by the complex ionic interactions that occur for each cation-anion couple, rather than simply by the electrochemistry of the isolated ions. Also, it is evident that, with increasing temperature, the cathodic and anodic potential limits and hence the electrochemical window, decrease for most samples. Fig. 5a shows the example of PCNFSI.
This trend is expected since an increasing temperature contributes to the activation energy for the decomposition. With increasing temperature, not only do the currents increase, but also the slope of the voltammetry peak increases reflecting the increase in conductivity. The cationic decomposition region is important since this is where lithium electrochemistry takes place. In order to investigate this, the 0.5mol/kg Li+ samples were cycled at 25, 50, 75 and 100 C using a nickel working electrode. Fig. 5b shows an example of lithium cycling in PCNFSI. All systems show similarly excellent lithium cycling, Fig. 6, suggesting that these ionic liquids all support reversible lithium charging and discharging and are thus promising candidates as lithium battery electrolytes. In particular, as can be noted from Fig. 5b, the Li redox behaviour is highly stable with minimal current decay evident over 10 successive cycles. This suggests the formation of a stable solid electrolyte interface layer (SEI), with some evidence of its formation between 2.5 and 3.0 V vs. Ag/Ag+, as well the successive cycling of an active and uniform Li deposit (i.e. avoiding the formation of so-called ‘dead lithium’ morphologies) [47]. The apparent coulombic efficiency decreases at higher temperatures which may result from the decrease in the electrochemical window as previously discussed. It should be pointed out that the fact the nitrile functionalised phosphonium ionics liquids show high electrochemical stability is rather surprising. Most nitrile groups in ionic liquids are present in the anion (e.g. dicyanamide) and their low oxidative stability is limited by the oxidation of the anion to a neutral dimer. However, data regarding nitrile functionalised cations is much more limited. It has been previously observed that introducing nitrile groups into the IL cations changes the physiochemical properties immensely (maybe due to formation of hydrogen bonds) [48]. It was also shown that the nitrile functionalised ILs have a large electrochemical window [48] thus suggesting that reduction of the nitrile group does not limit the reductive stability of IL cations containing this moiety. The positioning of the nitrile group at the end of the long alkyl chain could result in an orientation of the phosphonium cation at the electrode surface that somehow limits the reduction step of these cations; presumably this could be related to a well defined ionic structuring at the electrode interface, as suggested by many authors [49]. This may also be correlated with the partial negative charge on the nitrogen atom of the nitrile group increasing with increasing number of CH2 spacers [50].
108
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
Thus the PCNFSI was thought to be a promising candidate to prepare battery prototypes based on NMC as a cathode material and lithium metal as the anode as the anode (see experimental section for details). PCNFSI-LiFSI (1M) has been selected as an electrolyte. Fig. 6 shows the performance matches that of commercial standard electrolytes, producing high capacities and efficiencies of >98%.
at Monash University. MF and DRM also acknowledge funding through the Australian Laureate Fellowships scheme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.03.130.
4. Conclusions References A variety of phosphonium ionic liquids were characterized in terms of their physiochemical behaviour. The effect of lithium salt addition (matching the IL anion: LiNTf2, LiFSI), the cation (small alkyl vs bulky/functionalised) and the anion (NTf2, FSI) were studied. While samples followed the expected trends (Tg increases by addition of Li salt, Tm decreases with the addition of lithium salt, viscosity increases with Li salt addition and decreasing temperature, conductivity increases with decreasing Li salt content and increasing temperature) some other interesting observations were made. The properties depend on both the anion and the cation. In general the FSI ILs have lower phase change transitions, higher conductivities, lower viscosities and highly reversible Li cyclability as compared to the NTf2 samples. Tg for PCNFSI is 89 C while for the NTf2 analogue it is 73 C. Similarly Tm is 12 C for P111i4FSI while it is 41 C for P111i4NTf2. Comparing the viscosities and conductivities at 50 C (the highest temperatures for which we have both viscosity and conductivity data across all samples), it can be seen that the FSI samples have superior properties to the NTf2 samples. While when changing from NTf2 to FSI, the drop in viscosity is more dominant for the P111i4 cation (the viscosity of the PCN samples seems more or less unaffected by the anion), the conductivity increases 2.5 times for both cations. When comparing the effects of the cation structure, with increasing molecular weight the interactions increase, resulting in higher viscosities and lower conductivities. In general the most promising systems (e.g. low viscosities and high conductivities) are shown by small alkyl phosphonium ionic liquids (at 50 C: P111i4FSI: 18.4mPas, 13.8mScm1; P111i4NTf2: 30mPas, 5mScm1; P1113NTf2: 25.5mPas, 6.7mScm1). As such they belong to the most fluid and conductive phosphonium ionic liquids. Adding lithium salts to the ionic liquids increases the viscosity and lowers the conductivity. This is roughly linearly proportional to the amount of salt added. Furthermore, the ILs investigated here are electrochemically stable and have large electrochemical windows ranging from 5.6V up to 7.0V, while all samples support the electrochemistry of lithium which makes them potential candidates for lithium battery electrolytes; the onset for lithium reduction is at potentials more negative than 3.2V vs Ag/Ag+. The electrochemical properties of the more fluid samples (P1113NTf2, P111i4NTf2, PCNFSI) were much more promising in terms of reversible cyclability. This is possibly a result of the higher diffusivity, since determining the electrochemical performance for the more viscous samples was more challenging (PCNNTf2, PcycloNTf2). Acknowledgements This work was carried out as part of an Australian Research Council (ARC) funded Linkage Project with Cytec Canada Inc. The authors would like to thank Cytec for funding and supplying samples of phosphonium ionic liquids also including input in discussion regarding the progress of the project. Also special thanks goes to Dr. Peter Newman (Deakin University) and John Taylor (Vacuum Technology) for technical assistance. Also thanks to Dr. Aminah Noor for assistance with viscometer measurements
[1] S.A. Forsyth, J.M. Pringle, D.R. MacFarlane, Ionic liquids—An overview, Australian Journal of Chemistry 57 (2004) 113–119. [2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nature Materials 8 (2009) 621–629. ski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, [3] M. Galin Electrochimica Acta 51 (2006) 5567–5580. [4] S. Tang, G.A. Baker, H. Zhao, Ether- and alcohol-functionalized task-specific ionic liquids: Attractive properties and applications, Chemical Society Reviews 41 (2012) 4030–4066. [5] A. Hofmann, M. Schulz, T. Hanemann, Effect of conducting salts in ionic liquid based electrolytes: Viscosity, conductivity, and li-ion cell studies, International Journal of Electrochemical Science 8 (2013) 10170–10189. [6] F. Endres, S. Zein El Abedin, Air and water stable ionic liquids in physical chemistry, Physical Chemistry Chemical Physics 8 (2006) 2101–2116. [7] P. Hapiot, C. Lagrost, Electrochemical reactivity in room-temperature ionic liquids, Chemical Reviews 108 (2008) 2238–2264. [8] F. Endres, Ionic liquids: Solvents for the electrodeposition of metals and semiconductors, ChemPhysChem 3 (2002) 144–154. [9] A. Lewandowski, A. Swiderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies, Journal of Power Sources 194 (2009) 601–609. [10] R. Chen, H. Zhang, F. Wu, Applications of ionic liquids in batteries, Progress in Chemistry 23 (2011) 366–373. [11] Y. Li, X. Wu, Q. Wu, H. Ding, W. Yan, Ammonium- and phosphonium-based temperature control-type polyoxometalate ionic liquids, Dalton Transactions 43 (2014) 13591–13595. [12] D.J. Abdallah, R.G. Weiss, The influence of the cationic center, anion, and chain length of tetra-n-alkylammonium and -phosphonium salt gelators on the properties of their thermally reversible organogels, Chemistry of Materials 12 (2000) 406–413. [13] C.V. Manohar, D. Rabari, A.A.P. Kumar, T. Banerjee, K. Mohanty, Liquid-liquid equilibria studies on ammonium and phosphonium based ionic liquidaromatic-aliphatic component at T=298.15 K and p=1 bar: Correlations and apriori predictions, Fluid Phase Equilibr 360 (2013) 392–400. [14] A. Stojanovic, D. Kogelnig, L. Fischer, S. Hann, M. Galanski, M. Groessl, R. Krachler, B.K. Keppler, Phosphonium and Ammonium Ionic Liquids with Aromatic Anions: Synthesis, Properties, and Platinum Extraction, Australian Journal of Chemistry 63 (2010) 511–524. [15] R.K. Blundell, P. Licence, Quaternary ammonium and phosphonium based ionic liquids: a comparison of common anions, Physical Chemistry Chemical Physics 16 (2014) 15278–15288. [16] P.J. Carvalho, S.P.M. Ventura, M.L.S. Batista, B. Schroder, F. Goncalves, J. Esperanca, F. Mutelet, J.A.P. Coutinho, Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations, J. Chem. Phys. 140 (2014). [17] U.A. Rana, R. Vijayaraghavan, M. Walther, J.Z. Sun, A.A.J. Torriero, M. Forsyth, D. R. MacFarlane, Protic ionic liquids based on phosphonium cations: comparison with ammonium analogues, Chemical Communications 47 (2011) 11612– 11614. [18] J.S. Luo, O. Conrad, I.F.J. Vankelecom, Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids, Journal of Materials Chemistry 22 (2012) 20574–20579. [19] V. Armel, D. Velayutham, J.Z. Sun, P.C. Howlett, M. Forsyth, D.R. MacFarlane, J. M. Pringle, Ionic liquids and organic ionic plastic crystals utilizing small phosphonium cations, Journal of Materials Chemistry 21 (2011) 7640–7650. [20] K. Tsunashima, Y. Sakai, M. Matsumiya, Physical and electrochemical properties of phosphonium ionic liquids derived from trimethylphosphine, Electrochemistry Communications 39 (2014) 30–33. [21] C.M.S.S. Neves, P.J. Carvalho, M.G. Freire, J.A.P. Coutinho, Thermophysical properties of pure and water-saturated tetradecyltrihexylphosphonium-based ionic liquids, Journal of Chemical Thermodynamics 43 (2011) 948–957. [22] K. Yoshii, K. Yamaji, T. Tsuda, K. Tsunashima, H. Yoshida, M. Ozaki, S. Kuwabata, Physicochemical properties of tri-n-butylalkylphosphonium cation-based room-temperature ionic liquids, Journal of Physical Chemistry B 117 (2013) 15051–15059. [23] M.D. Green, C. Schreiner, T.E. Long, Thermal, Rheological, and Ion-Transport Properties of Phosphonium-Based Ionic Liquids, J. Phys. Chem. A 115 (2011) 13829–13835. [24] F.J. Deive, M.A. Rivas, A. Rodriguez, Study of thermodynamic and transport properties of phosphonium-based ionic liquids, Journal of Chemical Thermodynamics 62 (2013) 98–103.
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109 [25] T.J. Stockmann, Z.F. Ding, Tetraoctylphosphonium Tetrakis(pentafluorophenyl) borate Room Temperature Ionic Liquid toward Enhanced Physicochemical Properties for Electrochemistry, Journal of Physical Chemistry B 116 (2012) 12826–12834. [26] A. Garcia, L.C. Torres-Gonzalez, K.P. Padmasree, M.G. Benavides-Garcia, E.M. Sanchez, Conductivity and viscosity properties of associated ionic liquids phosphonium orthoborates, J. Mol. Liq. 178 (2013) 57–62. [27] M. Quiroz-Guzman, D.P. Fagnant, X.Y. Chen, C.J. Shi, J.F. Brennecke, G.S. Goff, W. Runde, Synthesis and characterization of the thermodynamic and electrochemical properties of tetra-alkyl phosphonium oxalate ionic liquids, Rsc Advances 4 (2014) 14840–14846. [28] X.Y. Liu, W. Afzal, J.M. Prausnitz, Unusual trend of viscosities and densities for four ionic liquids containing a tetraalkyl phosphonium cation and the anion bis(2,4,4-trimethylpentyl) phosphinate, Journal of Chemical Thermodynamics 70 (2014) 122–126. [29] M. Blahusiak, S. Schlosser, Physical properties of phosphonium ionic liquid and its mixtures with dodecane and water, Journal of Chemical Thermodynamics 72 (2014) 54–64. [30] M.S. AlTuwaim, K.H.A.E. Alkhaldi, A.S. Al-Jimaz, A.A. Mohammad, Temperature Dependence of Physicochemical Properties of Imidazolium-, Pyroldinium-, and Phosphonium-Based Ionic Liquids, J. Chem. Eng. Data 59 (2014) 1955– 1963. [31] G. Adamova, R.L. Gardas, M. Nieuwenhuyzen, A.V. Puga, L.P.N. Rebelo, A.J. Robertson, K.R. Seddon, Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure, Dalton Transactions 41 (2012) 8316–8332. [32] K. Tsunashima, M. Fukushima, M. Matsumiya, Physicochemical Properties of Trialkylphosphonium-Based Protic Ionic Liquids, Electrochemistry 80 (2012) 904–906. [33] L. Timperman, H. Galiano, D. Lemordant, M. Anouti, Phosphonium-based protic ionic liquid as electrolyte for carbon-based supercapacitors, Electrochemistry Communications 13 (2011) 1112–1115. [34] L. Timperman, M. Anouti, Transport Properties of Tributylphosphonium Tetrafluoroborate Protic Ionic Liquid, Ind. Eng. Chem. Res. 51 (2012) 3170– 3178. [35] K. Tsunashima, Y. Ono, M. Sugiya, Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond, Electrochimica Acta 56 (2011) 4351– 4355. [36] S.I. Lall-Ramnarine, J.A. Mukhlall, J.F. Wishart, R.R. Engel, A.R. Romeo, M. Gohdo, S. Ramati, M. Berman, S.N. Suarez, Cyclic phosphonium ionic liquids, Beilstein Journal of Organic Chemistry 10 (2014) 271–275. [37] A.K. Ziyada, C.D. Wilfred, M.A. Bustam, Synthesis and physiochemical properties of supramolecular phosphonium-based symmetrical dicationic ionic liquids, Journal of Applied Sciences 11 (2011) 1356–1360.
109
[38] D.B. Zhao, Z.F. Fei, R. Scopelliti, P.J. Dyson, Synthesis and characterization of ionic liquids incorporating the nitrile functionality, Inorganic Chemistry 43 (2004) 2197–2205. [39] G. Girard, M. Hilder, H. Zhu, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D. MacFarlane, P.C. Howlett, Electrochemical and physicochemical properties of small phosphonium cation ionic liquid electrolytes with high lithium salt content, Physical Chemistry Chemical Physics 17 (2015) 8706– 8713. [40] P.D. Gujrati, M. Goldstein, Viscous-Liquids and the Glass-Transition .9. NonConfigurational Contributions to the Excess Entropy of Disordered Phases, Journal of Physical Chemistry 84 (1980) 859–863. [41] J.P. Southall, H.V.S.A. Hubbard, S.F. Johnston, V. Rogers, G.R. Davies, J.E. McIntyre, I.M. Ward, Ionic conductivity and viscosity correlations in liquid electrolytes for incorporation into PVDF gel electrolytes, Solid State Ionics 85 (1996) 51–60. [42] O. Borodin, W. Gorecki, G.D. Smith, M. Armand, Molecular Dynamics Simulation and Pulsed-Field Gradient NMR Studies of Bis(fluorosulfonyl)imide (FSI) and Bis[(trifluoromethyl)sulfonyl]imide (TFSI)-Based Ionic Liquids, Journal of Physical Chemistry B 114 (2010) 6786–6798. [43] M. Dahbi, F. Ghamouss, F. Tran-Van, D. Lemordant, M. Anouti, Comparative study of EC/DMC LiTFSI and LiPF6 electrolytes for electrochemical storage, Journal of Power Sources 196 (2011) 9743–9750. [44] T.Y. Wu, L. Hao, C.W. Kuo, Y.C. Lin, S.G. Su, P.L. Kuo, I.W. Sun, Ionic conductivity and diffusion in lithium tetrafluoroborate-doped 1-methyl-3pentylimidazolium tetrafluoroborate ionic liquid, International Journal of Electrochemical Science 7 (2012) 2047–2064. [45] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, How ionic are room-temperature ionic liquids? An indicator of the physicochemical properties, Journal of Physical Chemistry B 110 (2006) 19593–19600. [46] W. Xu, E.I. Cooper, C.A. Angell, Ionic liquids: Ion mobilities, glass temperatures, and fragilities, Journal of Physical Chemistry B 107 (2003) 6170–6178. [47] J.-I. Yamaki, S.-I. Tobishima, Handbook of Battery Materials, in: J.O. Besenhard (Ed.), Wiley-VCH, Weinheim, 1999, pp. 339. [48] Q.H. Zhang, Z.P. Li, J. Zhang, S.G. Zhang, L.Y. Zhu, J. Yang, X.P. Zhang, Y.Q. Deng, Physicochemical properties of nitrile-functionalized ionic liquids, Journal of Physical Chemistry B 111 (2007) 2864–2872. [49] A. Elbourne, K. Voitchovsky, G.G. Warr, R. Atkin, Ion structure controls ionic liquid near-surface and interfacial nanostructure, Chemical Science 6 (2015) 527–536. [50] K.C. Lethesh, K. Van Hecke, L. Van Meervelt, P. Nockemann, B. Kirchner, S. Zahn, T.N. Parac-Vogt, W. Dehaen, K. Binnemans, Nitrile-Functionalized Pyridinium, Pyrrolidinium, and Piperidinium Ionic Liquids, Journal of Physical Chemistry B 115 (2011) 8424–8438.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Physicochemical characterization of a new family of small alkyl phosphonium imide ionic liquids M. Hildera , G.M.A. Girarda , K. Whitbreadb , S. Zavorineb , M. Moserb , D. Nucciaroneb , M. Forsytha , D.R. MacFarlanec , P.C. Howletta,* a b c
Institute for Frontier Materials (IFM), Deakin University, Burwood, Victoria, 3125, Australia Cytec Canada Inc., Niagara Falls, Canada School of Chemistry, Monash University, Victoria, 3800, Australia
A R T I C L E I N F O
Article history: Received 5 January 2016 Received in revised form 10 March 2016 Accepted 22 March 2016 Available online 7 April 2016 Keywords: ionic liquid electrolyte phosphoniumbattery
A B S T R A C T
Despite their promising properties, phosphonium based ionic liquids have attracted little attention as compared to their nitrogen-based cation counterparts. This study focuses on the properties of a family of small phosphonium imide ionic liquids, as well as the effect of lithium salt addition to these. The 6 ionic liquids were either alkyl, cyclic or nitrile functionalised phoshonium cations with bis(trifluoromethanesulfonyl)imide, NTf2, or bis(fluorosulfonyl)imide (FSI) as anion. Amongst the properties investigated were ionic conductivity, viscosity, thermal behaviour, electrochemical stability and the reversibility of electrochemical lithium cycling. All ionic liquids showed very promising properties e.g. having low transition temperatures, high electrochemical stabilities, low viscosities and high conductivities. Particularly the trimethyl phosphonium ionic liquids showed some of the highest conductivities reported amongst phosphonium ionic liquids generally. The combination of electrochemical stability, high conductivity and reversible lithium cycling makes them promising systems for energy storage devices such as lithium batteries. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Generally speaking ionic liquids (ILs) are compounds made up of ions and which have melting points below 100 C, by definition, ideally being liquids at room temperature (room temperature ionic liquids: RTILs) [1,2]. The low melting points of ILs result from weak interactions between the cations and anions [3]. This is usually achieved by combining large asymmetric cations with anions which show some degree of charge delocalisation [4]. Thus ionic liquids are mainly, but not exclusively, organic in nature, the cations in particular. Ionic liquids have many properties which make them particularly attractive for electrochemical applications (e.g. electrolytes for batteries or capacitors) which include [3]: (i) they consist entirely of ions and thus are ion conductive [5]; (ii) in most cases they are electrochemically inert over a wide potential range and thus can support a range of electrochemical processes [3]; (iii) due to their ionic nature they usually have low volatility [6], preventing
* Corresponding author. E-mail address: [email protected] (P.C. Howlett). http://dx.doi.org/10.1016/j.electacta.2016.03.130 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
them from drying out, and lowering their flammability considerably. The electrochemistry of neat ionic liquids has been well studied [7], and metals have been electrodeposited from ionic liquid solutions [8]. This is particularly interesting for reactive electropositive base metals since they are generally not compatible with aqueous electrolytes. Very important here are high voltage applications such as lithium batteries [9]. Since lithium has the highest negative standard reduction potential of all elements (3.05V vs SHE), it is incompatible with aqueous electrolytes and many traditional organic solvents. On the other hand the large electrochemical window of ionic liquids supports the electrochemistry of lithium. While organic solvent electrolytes having these properties are commercially available (solutions of organic carbonates containing lithium hexafluorophosphate), these are flammable, toxic and the solvent’s volatility can lead to evaporation of the electrolyte resulting in device failure [10]. Thus, for lithium energy storage technology ionic liquids are promising candidates to replace solvent-based electrolytes. Of the large variety of ionic liquids, an extensively studied class are those incorporating quaternary ammonium cations, however, less attention has been paid to their analogues, the phosphonium
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
ILs, despite reports that they either match or even outperform the more traditional ammonium derivatives [11–18]. Examples of studied systems include ILs based on tetra-alkyl phosphonium [11,18–31], protic alkyl phosphonium [17,32–34], alkyl-alkoxy phosphonium [20], allyl phosphonium [20,35] and cyclic phosphonium or phosphinanium [36] cations, as well as dicationic alkyl phosphonium [37]. Common anions used include bis(trifluoromethanesulfonyl)imide (NTf2) or bis(fluorosulfonyl)imide (FSI) where, in general, the latter anion shows superior properties compared to the former. Recently, a number of new phosphonium having small alkyl cations and cations with additional functionality such as nitrile groups have become available [38]. The phosphonium ionic liquids investigated in this study were four small alkyl phosphonium ionic liquids, bicyclic phosphonium ionic liquids, as well as two nitrile-functionalised phosphonium ionic liquids. As mentioned above, the small alkyl ionic liquids have promising properties for electrolyte applications (low viscosity, high conductivity). The latter ILs are particularly interesting; while nitrile groups are commonly found in ionic liquids anions (dicyanamide, tetranitrileborate), data on nitrile functionalised cations is more limited. There are a few reports on nitrilefunctionalised pyrrolidinium ionic liquid cations [38]. The nitrile group adds functionality, specific reactivity and also influences other physical properties (e.g. increases hydrophilicity, dipole moment, dielectric constant etc.). Furthermore, the nitrile group adds solvation properties to the positive phosphonium cations which may influence speciation of the Li+[38]. The properties of six such phosphonium ionic liquids are studied in this paper (structures as shown in Fig. 1). A special focus is on evaluating their properties in regards to electrolyte applications in lithium batteries. Thus the effect of the presence of various levels of lithium addition on the properties was also investigated. We also compare it to the properties of P111i4 FSI described recently [39]. 2. Experimental All handling, characterization and preparation of samples and devices was carried out under strictly anhydrous and air-sensitive conditions using glove box and vacuum line techniques.
101
2.1. Samples The ionic liquids triethyl cyanopropyl phosphonium bis (trifluoromethanesulfonyl)imide (e.g. PCNNTf2), triethyl cyanopropyl phosphonium bis(fluorosulfonyl)imide (e.g. PCNFSI), bicyclo[3.1.1], 3,7-dimethyloctane, ethyl, hexyl phosphonium bis (trifluoromethanesulfonyl)imide (e.g. PcycloNTf2), trimethyl, propyl phosphonium bis(trifluoromethanesulfonyl)imide (e.g. P1113NTf2), trimethyl, isobutyl phosphonium bis(trifluoromethanesulfonyl) imide (e.g. P111i4NTf2) and the previously discussed trimethyl isobutyl phosphonium bis(fluorosulfonyl)imide (e.g. P111i4FSI) [39] were received from Cytec (Canada) and their structures are shown in Fig. 1. Matching the anions of the ionic liquids and the lithium salts appropriate amounts of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 = LiN(CF3SO2)2) or lithium bis(fluorosulfonyl) imide (LiFSI = LiN(FSO2)2) were added giving solutions with molalities of 0.5 or 1.0mol Li salt per kg ionic liquid. FTIR are also included in the appendix (Fig. S1). 2.2. Water content The samples were dried using an in-house designed glass vessel consisting of a tube holding the sample (10mL), a bulb connected to the tube via a sintered glass frit, containing sodium hydride as a desiccant (NaH, Sigma Aldrich), and a vacuum connection. After two days of heating the samples under vacuum at 75 C, the water contents of the samples were well below 25ppm as determined by coulometric Karl Fisher titration (Metrohm model 831, Hydranal1 Coulomat AG titrant). 2.3. Differential Scanning Calorimetry (DSC) The phase behaviour (melting, crystallization, and glass transition temperatures, transition enthalpies and transition entropies) were derived from differential scanning calorimetry experiments (Mettler Toledo DSC1) carried out in the range of 95 to 125 C (10 C/min). Temperatures were corrected against a cyclohexane standard (Sigma Aldrich). For thermodynamic transitions (melting, crystallization), the onset temperatures were used, with the enthalpies calculated from the integral of the peak area and the entropy determined by dividing the enthalpy by the
Fig. 1. structures of ionic liquids used in this study.
102
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
onset temperature. The glass transition temperature is reported as the midpoint of the onset and endpoint temperatures as determined by tangent methods. 2.4. Viscosity The viscosities were determined using the rolling ball method from 80 C to 20 C (or above the melting point of 50 C for samples P1113 NTf2 and P111i4 NTf2) using 10 C interval steps, a capillary diameter of 2.5mm and an angle of 60 (Anton Paar Lovis 2000ME). The instrument determined the dynamic viscosity taking the density into account as determined at the above mentioned temperatures (Anton Paar DMA5000).
2.6. Electrochemical Characterisation Cyclic voltammetry (CV) was performed using a SP200 Potentiostat, using a three electrode setup (20mV/s). Platinum wire (APS, 99.95%) was used as a counter electrode and as a quasi-reference electrode (ferrocene was used after each set of experiments to calibrate the potentials). For comparison the potentials were mathematically adjusted to Ag/Ag+ and presented accordingly assuming a standard reduction potential 0.16V vs Ag/ Ag+. The electrochemical window was determined using a glassy carbon working electrode (1.0mm, BAS Japan), while for lithium cycling a nickel working electrode was used (1.5mm, BAS Japan). Measurements were carried out for all liquid samples at 25, 50, 75 and 100 C.
2.5. Ionic Conductivity
2.7. Coin Cell preparation and galvanostatic cycling
The ionic conductivity was determined via impedance methods using an SP200 Impedance/Frequency Response Analyser from Biologic (frequency range: 100mHz–1MHz; potential amplitude: 100mV; temperature range: 20 to 120 C with 10 C intervals; temperature controller: Eurotherm 2240E equipped with a thermo couple and a heating block). The cell constant was obtained using a 0.01 M potassium chloride standard (Sigma Aldrich).
Coin cell prototypes (CR3032) were prepared using Hoisen crimping equipment. NMC (47% Ni; L&F Material Co Ltd) was used as a cathode material. The electrodes were prepared mixing 75% NMC, 10% C65 (TimCal) and 15% Solef PVdF binder (Solvay) with 1Methyl-2-pyrrolidinone (Sigma Aldrich), and applied to Al current collector (battery grade). After evaporation of the solvent, the film was heated in an oven at 100 C overnight (no pressing). The films had a diameter of 1.11cm and a coating weight of 1.5mg (1.13mg of
Table 1 Summary of properties of neat ionic liquids and their binary solutions (0.5mol/kg and 1.0mol/kg). Transition Temperatures in C corrected/heating
Viscosity in mPa s 20 to 80 C 60 /2.5mm
Density in g/cm3 20 to 80 C
Conductivity in mS/cm 20 to 120 C (KCl cor: 0.01M)
PCN NTf2 (neat) PCN NTf2 (0.5mol/kg) PCN NTf2 (1.0mol/kg)
Tg: 71.9 Tg: 61.8 Tg: 48.4
274.6–21.8a 880.5–42.1 613–79.71c
1.392–1.343a 1.444-1.393 1.491–1.437
0.0197–11.76 0.0039-8.90 0.006–5.43
P1113 NTf2 (neat) P1113 NTf2 (0.5mol/kg)
Tm: +39.9os/+42.4p Tm: +35.0os +26.6p1 and +41.6p2 Tm: +23.0os (+37.6p1 and +45.6p2) Tcry: +11.0os
25.66–11.5d 57.7–20.8d
1.395–1.368d 1.445–1.418d
0.00001–28.37 0.00003–19.85
123.9–36.4d
1.486–1.455d
0.000004–13.42
Tm1: +40.8os/+43.4p Tm2: 45.5os/-43.0p Tg1: 60.7 Tcry: -18.2os/-12.1p Tg2: 6.1 Tm: 18.9os +23.5p1 and 33.8p2 Tg:-67.6 Tm: (1.9os/3.9p, 17.6p, 29.7p) Tcry: 23.0os (-16.6p)
21.6–9.4d
1.367–1.341d
0.0004–23.62e
58.0–21.0d
1.418–1.389d
0.005–16.29f
535.3–42.8
1.383–1.338
0.0007–8.84e
Pcyto NTf2 (neat) Pcyto NTf2 (0.5mol/kg)
Tg: 48.7 Tg: 43.4
1042.0–46.1b 875.2–83.3c
1.284–1.243b 1.327–1.293c
0.003–6.35 0.00002–4.99
PCN FSI (neat)
Tg: 88.5 Tcry: 49.7os/39.3p Tm: 8.6os/4.1p Tg: 83.2 Tg: 77.1
230.2–25.58
1.313–1.268
0.1025–23. 50
380.3–35.36 398.5–32.98
1.348–1.304 1.383–1.338
0.0587–17.93 0.0356–16.18
P1113 NTf2 (1.0mol/kg)
P111i4 NTf2 (neat) P111i4 NTf2 (0.5mol/kg)
P111i4 NTf2 (1.0mol/kg)
PCN FSI (0.5mol/kg) PCN FSI (1.0mol/kg) os=onset; p=peak. 25–90 C. 25–80 C. c 40–80 C. d 50–80 C. e +20 to +120 C. f 10 to +120 C. a
b
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
active NMC material). The anode was Li metal (Sigma Aldrich; rolled and brushed in the glovebox using dried hexane, 0.9cm diameter and estimated thickness of 100mm), which was separated from the cathode using a Celgard polyethylene separator provided by MTI Corporation (EQ bsf-0025-60c). Galvanostatic cycling was carried out at room temperature using a Neware battery tester with potential limits of 2.75–4.2V. The charge densities were calculated assuming a capacity of NMC of 150mAh/g for C rates of C/10 and C/5 currents of 0.0169mA (15mA/g) and 0.0338mA (30mA/g) were applied respectively. 3. Results and discussion After water content determination, the ionic liquids were characterized in terms of molecular identification and physicochemical properties. The properties are summarized in Table 1. 3.1. DSC The phase change behaviour for the systems under investigation is summarized in Fig. 2(a)–(f). The neat P1113NTf2 shows simple melting behaviour (melting temperature, Tm: 39.9 C).
103
Upon Li salt addition the DSC traces become more complex showing two distinctive melting transitions. The data suggest the presence of a eutectic melting followed by a broader melting transition (a liquidus) with the final melting temperature of the ionic liquid being lowered by the addition of lithium salts, as is typically observed for mixtures. P111i4NTf2 also shows a single melting transition in its neat form (40.8 C) while lithium salt addition produces a glass transition (Tg) combined with enthalpy relaxation, Fig. 2(c). With increasing salt concentration Tg shifts to higher temperatures. The thermal traces also show that the Li solutions crystallise (a devitrification exotherm) on warming and with increasing salt content the traces gain in complexity, again suggesting the presence of eutectic transition. Neat PCNFSI (Fig. 2b) shows glassy behaviour after heating from a rapid cool in the DSC (Tg = 88.5 C). Upon heating, devitrification and melting transitions are observed (8.6 C). Upon the addition of lithium salt to this IL, the solutions show only glassy behaviour, with increasing Li content leading to higher Tg values. All traces for PCNNTf2 and PcycloNTf2 show solely glass behaviour (2a:-71.9 C; 2f: 48.7 C), with the addition of LiNTf2 continuing to shift Tg to higher temperatures. In general the nitrile functionalised samples show only glass transitions. Neat PCNNTf2 shows the lowest value of any
Fig. 2. DSC traces of neat ILs, and their binary mixtures (0.5 and 1.0 moles/kg): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
104
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
of the NTf2 systems, with a Tg of 71.9 C. While the Tg of PCNFSI is even lower (-88.5 C), it also shows additional phase changes upon heating (devitrification/melting). While the very fluid samples show simple crystallisation/ melting behaviour, in the highly viscous samples this is kinetically hindered. Here the difficulty of molecular motion and energetically driven molecular rearrangements to form crystalline solids is hindered by the high viscosity of the samples, thereby favouring glassy behaviour. This also explains why the addition of lithium salts, which increases the strength of ionic interactions, and viscosity, thus promotes supercooling and glass formation at lower temperatures [40]. Furthermore, the bulky ILs and ILs carrying polar functionality show glass forming behaviour, possibly due to an increase of molecular interactions (PCNNTf2, PCNFSI, PcycloNTf2). It is interesting to note that the relatively symmetric trimethylphosphonium ionic liquids (P111i4NTf2, P1113NTf2) are solids at room
temperature. A direct comparison of the effect of the anion on the phase change properties (FSI vs NTf2 for the P111i4 and PCN cation) can be found in the appendix (Fig. S2). 3.2. Viscosity, ionic conductivity and Walden plots One of the major factors determining the performance of ionic liquids as electrolytes is the mobility of the ions. Fig. 3a–f shows the viscosities and conductivities of the neat ILs as well with 0.5 and 1.0 mol/kg added Li salt of the same anion. Selected conductivity and viscosity data is compiled in tables S1 and S2 (appendix). A few obvious trends can be observed: (i) samples with low viscosity show a high conductivity (ii) with increasing temperature, the viscosity decreases while the conductivity increases; (iii) lithium addition increases the viscosity resulting in lower conductivity.
Fig. 3. Viscosity and conductivity of neat ILs, and their binary mixtures (0.5 and 1.0 mol/kg): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
105
Fig. 4. Walden plot for neat ILs, and their binary mixtures (0.5 and 1.0 moles/kg):): (a) PCNNTf2; (b) PCNFSI; (c) P111i4NTf2; (d) P111i4FSI; (e) P1113NTf2; (f) PcycloNTf2.
The ionic conductivity is typically inversely related to the viscosity via Eq. (1) (a combination of the Nernst-Einstein equation and the Stokes-Einstein equation) with s being the ionic conductivity, e the elementary charge, n the charge carrier density,
r the effective radius and h the viscosity[41]:
s¼
e2 n 6prh
ð1Þ
106
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
viscosity/decrease in conductivity may be a result of a higher degree of intermolecular interactions in the NTf2 cases. Comparing the effect of the cation suggests that the higher molecular weight and/or structural functionalisation results in ILs with higher viscosity and lower conductivity, probably as a result of intermolecular interactions (Fig. 3). As mentioned above, the addition of lithium salts increases the viscosity and reduces the conductivity. With increasing salt addition, this trend continues. Here it is interesting to note that for the two FSI samples the change in viscosity and conductivity with the amount of Li salt addition is identical despite the fact of the different nature of the cation e.g. alkyl vs nitrile. With increasing salt content the increase in viscosity is higher for the NTf2 as compared to the FSI systems: 2 times followed by 4 times for the P111i4NTf2 system. The highest increase in viscosity is observed for PCNNTf2 with 2.4 and 5.6 times the initial value upon addition of 0.5 and 1.0mol/kg. The higher anion mobility of FSI vs NTf2 has been discussed before in terms of the smaller anion size for FSI [42]. Evaluating the findings with respect to previously published data suggests that, particularly the small alkyl phosphonium ionic liquids have very low viscosities and high conductivities, which makes them very attractive for electrolyte applications. Tables S1 and S2 (appendix) provide an extensive tabulation of literature data for comparison. At 50 C (the lowest temperature where all studied systems are liquid), the conductivity ranges from 5–14mScm1 (P111i4FSI: 13.8mScm1; P1113NTf2: 6.7mScm1; P111i4NTf2: 5.0mScm1). With such conductivities they are amongst the highest conductivities reported for phosphonium ionic liquids. The same is observed for the viscosity. At 50 C the most fluid samples are P111i4FSI (18mPas), P111i4NTf2 (26mPas) and P1113NTf2 (30mPas), being some of the lowest viscosities observed amongst phosphonium ionic liquids. It has frequently been shown that ionic conductivity and viscosity often follow either the classic Arrhenius behaviour or Arrhenius behaviour modified by Vogel, Tammann and Fulcher (VTF) behaviour at lower temperatures [3]. lnðs Þ ¼ lnðs 0 Þ
Fig. 5. Electrochemical properties of PCNFSI: (a) electrochemical window and (b) lithium cycling in PCNFSI (0.5mol/kg) showing the 1st 10 cycles. The potentials were measured using a Pt quasi-reference and then mathematically adjusted vs Ag/Ag+ using running ferrocene as a standard.
The two alkylphosphonium NTf2 samples (P111i4NTf2 and P1113NTf2) show very low viscosities and high conductivities in their liquid state. The conductivities also show dramatic changes when the samples change from solid to partial melt to liquid (Fig. 3). Comparing the effect of the anion suggests that the viscosities are lower for the FSI ILs, as compared to the NTf2 analogues (a direct comparison is presented in the appendix in Fig. S3). This effect however is much more dominant for the P111i4 cation as compared to the PCN cation. At 50 C the viscosity is not a strong function of the anion (62mPas for PCNNTf2 as compared to 59mPas for PCNFSI) while the viscosity is nearly halved when going from NTf2 to FSI for the P111i4 cation (30mPas vs 18mPas). At the same time, the conductivity is increased by 2.5 times when changing from NTf2 to the FSI anion (at 50 C: 13.8mScm1 for P111i4FSI as compared to 5.0mScm1 for P111i4NTf2; 5.0mScm1 for PCNFSI as compared to 1.9mScm1 for PCNNTf2). The increase of
B T T0
ð2Þ
where s0 is the pre-exponential factor, B a pseudo activation energy (related to the free volume available to support mobility), and T0 the ideal glass transition temperature [43] T0 is lower than Tg (T0 Tg 20–50K) [3]. It has also been suggested for polymers p that s0 is temperature dependent (e.g. s0 = A/T or s0 = A/ T) [44]. A more correct approach would be considering ion dissociation energies including diffusion of the dissociated species by introducing a T-T0 exponent. However over the short temperature range measured there would be too many parameters to be taken into account. Good fits of the liquid state experimental data to the VTF equation could be obtained in most cases. The parameters are presented in the appendix (Table S3 and S4). However, the parameters derived do not follow a clear trend in all cases, principally because different temperature ranges are involved in the fits, because of the different liquid ranges. The interrelation of viscosity and conductivity can be further examined via the Walden plot [45]. From Eq. (3), the molar conductivity (L) and viscosity (h) should be inversely proportional
s c
¼ L ¼ constant=h
ð3Þ
A more detailed description recognises a fractional exponent a in the viscosity dependence such that L ¼ constant=ha where a is typically in the region of 0.8. Consequently, for a system dissociated 100% into mobile ions, a plot of the logarithm of the molar conductivity vs the logarithm of the fluidity w should be a straight line passing through the origin (fluidity w = 1/h).
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
107
Fig. 6. Galvanostatic cycling data of a battery device at different C rates (NMC75/C6510/PVdF15 | PCNFSI-LiFSI(1 M) | Li).
Deviations towards lower conductivity from this ideal line suggest that the IL may be less than 100% dissociated into independently mobile ions. Typically it is thought that this deviation is caused by strong short range interactions resulting in formation of transient ion pairs or aggregates. When neutral ion aggregates are formed, the conductivity is lowered, even in low viscosity samples. This has been studied in detail by a number of authors [46]. The Walden plots of the samples are shown in Fig. 4. Three obvious observations can made: (i) all samples lie quite close to the ideal line; (ii) the addition of lithium salts do not seem to significantly affect the ion association in these samples and (iii) while the FSI samples have superior properties regarding viscosity and ion conductivity, the NTf2 samples show a lower degree of ion association (Fig. S4). Overall, despite having significantly different cation structures, both PCNNTf2 and P111i4NTf2 show high conductivities. The reason for the high conductivity for the PCNNTf2 may be the lack of significant ionic interactions. The data pretty much lie on the line of ideally dissociated KCl solution suggesting weak interactions, not affecting the conductivity adversely. Similarly, the interactions in P111i4FSI may be lower than in the NTf2 analogue, given the closer proximity to the ideal Walden line, making this the sample with the highest ion conductivity. 3.3. Electrochemical properties In general the potential limits of an IL are determined by cation reduction and anion oxidation. The electrochemical windows were determined for neat ILs in the liquid state at 25, 50, 75 and 100 C using a glassy carbon working electrode as shown in Fig. 5 (detailed information in Table S5). All ILs presented here have an electrochemical window of at least 5.7V. It also seems that the reduction potential limits are not exclusively determined by the cation while the limits on the oxidation side are not solely governed by the anion. For example, the oxidation limits at 50 C range from +2.4 to +2.9V vs Ag/Ag+ for the range of NTf2 ionic liquids studied here. Thus it appears that the decomposition reactions are determined by the complex ionic interactions that occur for each cation-anion couple, rather than simply by the electrochemistry of the isolated ions. Also, it is evident that, with increasing temperature, the cathodic and anodic potential limits and hence the electrochemical window, decrease for most samples. Fig. 5a shows the example of PCNFSI.
This trend is expected since an increasing temperature contributes to the activation energy for the decomposition. With increasing temperature, not only do the currents increase, but also the slope of the voltammetry peak increases reflecting the increase in conductivity. The cationic decomposition region is important since this is where lithium electrochemistry takes place. In order to investigate this, the 0.5mol/kg Li+ samples were cycled at 25, 50, 75 and 100 C using a nickel working electrode. Fig. 5b shows an example of lithium cycling in PCNFSI. All systems show similarly excellent lithium cycling, Fig. 6, suggesting that these ionic liquids all support reversible lithium charging and discharging and are thus promising candidates as lithium battery electrolytes. In particular, as can be noted from Fig. 5b, the Li redox behaviour is highly stable with minimal current decay evident over 10 successive cycles. This suggests the formation of a stable solid electrolyte interface layer (SEI), with some evidence of its formation between 2.5 and 3.0 V vs. Ag/Ag+, as well the successive cycling of an active and uniform Li deposit (i.e. avoiding the formation of so-called ‘dead lithium’ morphologies) [47]. The apparent coulombic efficiency decreases at higher temperatures which may result from the decrease in the electrochemical window as previously discussed. It should be pointed out that the fact the nitrile functionalised phosphonium ionics liquids show high electrochemical stability is rather surprising. Most nitrile groups in ionic liquids are present in the anion (e.g. dicyanamide) and their low oxidative stability is limited by the oxidation of the anion to a neutral dimer. However, data regarding nitrile functionalised cations is much more limited. It has been previously observed that introducing nitrile groups into the IL cations changes the physiochemical properties immensely (maybe due to formation of hydrogen bonds) [48]. It was also shown that the nitrile functionalised ILs have a large electrochemical window [48] thus suggesting that reduction of the nitrile group does not limit the reductive stability of IL cations containing this moiety. The positioning of the nitrile group at the end of the long alkyl chain could result in an orientation of the phosphonium cation at the electrode surface that somehow limits the reduction step of these cations; presumably this could be related to a well defined ionic structuring at the electrode interface, as suggested by many authors [49]. This may also be correlated with the partial negative charge on the nitrogen atom of the nitrile group increasing with increasing number of CH2 spacers [50].
108
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109
Thus the PCNFSI was thought to be a promising candidate to prepare battery prototypes based on NMC as a cathode material and lithium metal as the anode as the anode (see experimental section for details). PCNFSI-LiFSI (1M) has been selected as an electrolyte. Fig. 6 shows the performance matches that of commercial standard electrolytes, producing high capacities and efficiencies of >98%.
at Monash University. MF and DRM also acknowledge funding through the Australian Laureate Fellowships scheme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.03.130.
4. Conclusions References A variety of phosphonium ionic liquids were characterized in terms of their physiochemical behaviour. The effect of lithium salt addition (matching the IL anion: LiNTf2, LiFSI), the cation (small alkyl vs bulky/functionalised) and the anion (NTf2, FSI) were studied. While samples followed the expected trends (Tg increases by addition of Li salt, Tm decreases with the addition of lithium salt, viscosity increases with Li salt addition and decreasing temperature, conductivity increases with decreasing Li salt content and increasing temperature) some other interesting observations were made. The properties depend on both the anion and the cation. In general the FSI ILs have lower phase change transitions, higher conductivities, lower viscosities and highly reversible Li cyclability as compared to the NTf2 samples. Tg for PCNFSI is 89 C while for the NTf2 analogue it is 73 C. Similarly Tm is 12 C for P111i4FSI while it is 41 C for P111i4NTf2. Comparing the viscosities and conductivities at 50 C (the highest temperatures for which we have both viscosity and conductivity data across all samples), it can be seen that the FSI samples have superior properties to the NTf2 samples. While when changing from NTf2 to FSI, the drop in viscosity is more dominant for the P111i4 cation (the viscosity of the PCN samples seems more or less unaffected by the anion), the conductivity increases 2.5 times for both cations. When comparing the effects of the cation structure, with increasing molecular weight the interactions increase, resulting in higher viscosities and lower conductivities. In general the most promising systems (e.g. low viscosities and high conductivities) are shown by small alkyl phosphonium ionic liquids (at 50 C: P111i4FSI: 18.4mPas, 13.8mScm1; P111i4NTf2: 30mPas, 5mScm1; P1113NTf2: 25.5mPas, 6.7mScm1). As such they belong to the most fluid and conductive phosphonium ionic liquids. Adding lithium salts to the ionic liquids increases the viscosity and lowers the conductivity. This is roughly linearly proportional to the amount of salt added. Furthermore, the ILs investigated here are electrochemically stable and have large electrochemical windows ranging from 5.6V up to 7.0V, while all samples support the electrochemistry of lithium which makes them potential candidates for lithium battery electrolytes; the onset for lithium reduction is at potentials more negative than 3.2V vs Ag/Ag+. The electrochemical properties of the more fluid samples (P1113NTf2, P111i4NTf2, PCNFSI) were much more promising in terms of reversible cyclability. This is possibly a result of the higher diffusivity, since determining the electrochemical performance for the more viscous samples was more challenging (PCNNTf2, PcycloNTf2). Acknowledgements This work was carried out as part of an Australian Research Council (ARC) funded Linkage Project with Cytec Canada Inc. The authors would like to thank Cytec for funding and supplying samples of phosphonium ionic liquids also including input in discussion regarding the progress of the project. Also special thanks goes to Dr. Peter Newman (Deakin University) and John Taylor (Vacuum Technology) for technical assistance. Also thanks to Dr. Aminah Noor for assistance with viscometer measurements
[1] S.A. Forsyth, J.M. Pringle, D.R. MacFarlane, Ionic liquids—An overview, Australian Journal of Chemistry 57 (2004) 113–119. [2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nature Materials 8 (2009) 621–629. ski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, [3] M. Galin Electrochimica Acta 51 (2006) 5567–5580. [4] S. Tang, G.A. Baker, H. Zhao, Ether- and alcohol-functionalized task-specific ionic liquids: Attractive properties and applications, Chemical Society Reviews 41 (2012) 4030–4066. [5] A. Hofmann, M. Schulz, T. Hanemann, Effect of conducting salts in ionic liquid based electrolytes: Viscosity, conductivity, and li-ion cell studies, International Journal of Electrochemical Science 8 (2013) 10170–10189. [6] F. Endres, S. Zein El Abedin, Air and water stable ionic liquids in physical chemistry, Physical Chemistry Chemical Physics 8 (2006) 2101–2116. [7] P. Hapiot, C. Lagrost, Electrochemical reactivity in room-temperature ionic liquids, Chemical Reviews 108 (2008) 2238–2264. [8] F. Endres, Ionic liquids: Solvents for the electrodeposition of metals and semiconductors, ChemPhysChem 3 (2002) 144–154. [9] A. Lewandowski, A. Swiderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies, Journal of Power Sources 194 (2009) 601–609. [10] R. Chen, H. Zhang, F. Wu, Applications of ionic liquids in batteries, Progress in Chemistry 23 (2011) 366–373. [11] Y. Li, X. Wu, Q. Wu, H. Ding, W. Yan, Ammonium- and phosphonium-based temperature control-type polyoxometalate ionic liquids, Dalton Transactions 43 (2014) 13591–13595. [12] D.J. Abdallah, R.G. Weiss, The influence of the cationic center, anion, and chain length of tetra-n-alkylammonium and -phosphonium salt gelators on the properties of their thermally reversible organogels, Chemistry of Materials 12 (2000) 406–413. [13] C.V. Manohar, D. Rabari, A.A.P. Kumar, T. Banerjee, K. Mohanty, Liquid-liquid equilibria studies on ammonium and phosphonium based ionic liquidaromatic-aliphatic component at T=298.15 K and p=1 bar: Correlations and apriori predictions, Fluid Phase Equilibr 360 (2013) 392–400. [14] A. Stojanovic, D. Kogelnig, L. Fischer, S. Hann, M. Galanski, M. Groessl, R. Krachler, B.K. Keppler, Phosphonium and Ammonium Ionic Liquids with Aromatic Anions: Synthesis, Properties, and Platinum Extraction, Australian Journal of Chemistry 63 (2010) 511–524. [15] R.K. Blundell, P. Licence, Quaternary ammonium and phosphonium based ionic liquids: a comparison of common anions, Physical Chemistry Chemical Physics 16 (2014) 15278–15288. [16] P.J. Carvalho, S.P.M. Ventura, M.L.S. Batista, B. Schroder, F. Goncalves, J. Esperanca, F. Mutelet, J.A.P. Coutinho, Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations, J. Chem. Phys. 140 (2014). [17] U.A. Rana, R. Vijayaraghavan, M. Walther, J.Z. Sun, A.A.J. Torriero, M. Forsyth, D. R. MacFarlane, Protic ionic liquids based on phosphonium cations: comparison with ammonium analogues, Chemical Communications 47 (2011) 11612– 11614. [18] J.S. Luo, O. Conrad, I.F.J. Vankelecom, Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids, Journal of Materials Chemistry 22 (2012) 20574–20579. [19] V. Armel, D. Velayutham, J.Z. Sun, P.C. Howlett, M. Forsyth, D.R. MacFarlane, J. M. Pringle, Ionic liquids and organic ionic plastic crystals utilizing small phosphonium cations, Journal of Materials Chemistry 21 (2011) 7640–7650. [20] K. Tsunashima, Y. Sakai, M. Matsumiya, Physical and electrochemical properties of phosphonium ionic liquids derived from trimethylphosphine, Electrochemistry Communications 39 (2014) 30–33. [21] C.M.S.S. Neves, P.J. Carvalho, M.G. Freire, J.A.P. Coutinho, Thermophysical properties of pure and water-saturated tetradecyltrihexylphosphonium-based ionic liquids, Journal of Chemical Thermodynamics 43 (2011) 948–957. [22] K. Yoshii, K. Yamaji, T. Tsuda, K. Tsunashima, H. Yoshida, M. Ozaki, S. Kuwabata, Physicochemical properties of tri-n-butylalkylphosphonium cation-based room-temperature ionic liquids, Journal of Physical Chemistry B 117 (2013) 15051–15059. [23] M.D. Green, C. Schreiner, T.E. Long, Thermal, Rheological, and Ion-Transport Properties of Phosphonium-Based Ionic Liquids, J. Phys. Chem. A 115 (2011) 13829–13835. [24] F.J. Deive, M.A. Rivas, A. Rodriguez, Study of thermodynamic and transport properties of phosphonium-based ionic liquids, Journal of Chemical Thermodynamics 62 (2013) 98–103.
M. Hilder et al. / Electrochimica Acta 202 (2016) 100–109 [25] T.J. Stockmann, Z.F. Ding, Tetraoctylphosphonium Tetrakis(pentafluorophenyl) borate Room Temperature Ionic Liquid toward Enhanced Physicochemical Properties for Electrochemistry, Journal of Physical Chemistry B 116 (2012) 12826–12834. [26] A. Garcia, L.C. Torres-Gonzalez, K.P. Padmasree, M.G. Benavides-Garcia, E.M. Sanchez, Conductivity and viscosity properties of associated ionic liquids phosphonium orthoborates, J. Mol. Liq. 178 (2013) 57–62. [27] M. Quiroz-Guzman, D.P. Fagnant, X.Y. Chen, C.J. Shi, J.F. Brennecke, G.S. Goff, W. Runde, Synthesis and characterization of the thermodynamic and electrochemical properties of tetra-alkyl phosphonium oxalate ionic liquids, Rsc Advances 4 (2014) 14840–14846. [28] X.Y. Liu, W. Afzal, J.M. Prausnitz, Unusual trend of viscosities and densities for four ionic liquids containing a tetraalkyl phosphonium cation and the anion bis(2,4,4-trimethylpentyl) phosphinate, Journal of Chemical Thermodynamics 70 (2014) 122–126. [29] M. Blahusiak, S. Schlosser, Physical properties of phosphonium ionic liquid and its mixtures with dodecane and water, Journal of Chemical Thermodynamics 72 (2014) 54–64. [30] M.S. AlTuwaim, K.H.A.E. Alkhaldi, A.S. Al-Jimaz, A.A. Mohammad, Temperature Dependence of Physicochemical Properties of Imidazolium-, Pyroldinium-, and Phosphonium-Based Ionic Liquids, J. Chem. Eng. Data 59 (2014) 1955– 1963. [31] G. Adamova, R.L. Gardas, M. Nieuwenhuyzen, A.V. Puga, L.P.N. Rebelo, A.J. Robertson, K.R. Seddon, Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure, Dalton Transactions 41 (2012) 8316–8332. [32] K. Tsunashima, M. Fukushima, M. Matsumiya, Physicochemical Properties of Trialkylphosphonium-Based Protic Ionic Liquids, Electrochemistry 80 (2012) 904–906. [33] L. Timperman, H. Galiano, D. Lemordant, M. Anouti, Phosphonium-based protic ionic liquid as electrolyte for carbon-based supercapacitors, Electrochemistry Communications 13 (2011) 1112–1115. [34] L. Timperman, M. Anouti, Transport Properties of Tributylphosphonium Tetrafluoroborate Protic Ionic Liquid, Ind. Eng. Chem. Res. 51 (2012) 3170– 3178. [35] K. Tsunashima, Y. Ono, M. Sugiya, Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond, Electrochimica Acta 56 (2011) 4351– 4355. [36] S.I. Lall-Ramnarine, J.A. Mukhlall, J.F. Wishart, R.R. Engel, A.R. Romeo, M. Gohdo, S. Ramati, M. Berman, S.N. Suarez, Cyclic phosphonium ionic liquids, Beilstein Journal of Organic Chemistry 10 (2014) 271–275. [37] A.K. Ziyada, C.D. Wilfred, M.A. Bustam, Synthesis and physiochemical properties of supramolecular phosphonium-based symmetrical dicationic ionic liquids, Journal of Applied Sciences 11 (2011) 1356–1360.
109
[38] D.B. Zhao, Z.F. Fei, R. Scopelliti, P.J. Dyson, Synthesis and characterization of ionic liquids incorporating the nitrile functionality, Inorganic Chemistry 43 (2004) 2197–2205. [39] G. Girard, M. Hilder, H. Zhu, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D. MacFarlane, P.C. Howlett, Electrochemical and physicochemical properties of small phosphonium cation ionic liquid electrolytes with high lithium salt content, Physical Chemistry Chemical Physics 17 (2015) 8706– 8713. [40] P.D. Gujrati, M. Goldstein, Viscous-Liquids and the Glass-Transition .9. NonConfigurational Contributions to the Excess Entropy of Disordered Phases, Journal of Physical Chemistry 84 (1980) 859–863. [41] J.P. Southall, H.V.S.A. Hubbard, S.F. Johnston, V. Rogers, G.R. Davies, J.E. McIntyre, I.M. Ward, Ionic conductivity and viscosity correlations in liquid electrolytes for incorporation into PVDF gel electrolytes, Solid State Ionics 85 (1996) 51–60. [42] O. Borodin, W. Gorecki, G.D. Smith, M. Armand, Molecular Dynamics Simulation and Pulsed-Field Gradient NMR Studies of Bis(fluorosulfonyl)imide (FSI) and Bis[(trifluoromethyl)sulfonyl]imide (TFSI)-Based Ionic Liquids, Journal of Physical Chemistry B 114 (2010) 6786–6798. [43] M. Dahbi, F. Ghamouss, F. Tran-Van, D. Lemordant, M. Anouti, Comparative study of EC/DMC LiTFSI and LiPF6 electrolytes for electrochemical storage, Journal of Power Sources 196 (2011) 9743–9750. [44] T.Y. Wu, L. Hao, C.W. Kuo, Y.C. Lin, S.G. Su, P.L. Kuo, I.W. Sun, Ionic conductivity and diffusion in lithium tetrafluoroborate-doped 1-methyl-3pentylimidazolium tetrafluoroborate ionic liquid, International Journal of Electrochemical Science 7 (2012) 2047–2064. [45] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, How ionic are room-temperature ionic liquids? An indicator of the physicochemical properties, Journal of Physical Chemistry B 110 (2006) 19593–19600. [46] W. Xu, E.I. Cooper, C.A. Angell, Ionic liquids: Ion mobilities, glass temperatures, and fragilities, Journal of Physical Chemistry B 107 (2003) 6170–6178. [47] J.-I. Yamaki, S.-I. Tobishima, Handbook of Battery Materials, in: J.O. Besenhard (Ed.), Wiley-VCH, Weinheim, 1999, pp. 339. [48] Q.H. Zhang, Z.P. Li, J. Zhang, S.G. Zhang, L.Y. Zhu, J. Yang, X.P. Zhang, Y.Q. Deng, Physicochemical properties of nitrile-functionalized ionic liquids, Journal of Physical Chemistry B 111 (2007) 2864–2872. [49] A. Elbourne, K. Voitchovsky, G.G. Warr, R. Atkin, Ion structure controls ionic liquid near-surface and interfacial nanostructure, Chemical Science 6 (2015) 527–536. [50] K.C. Lethesh, K. Van Hecke, L. Van Meervelt, P. Nockemann, B. Kirchner, S. Zahn, T.N. Parac-Vogt, W. Dehaen, K. Binnemans, Nitrile-Functionalized Pyridinium, Pyrrolidinium, and Piperidinium Ionic Liquids, Journal of Physical Chemistry B 115 (2011) 8424–8438.