Poly( β -amino ester) based solid polymer electrolytes for lithium-ion batteries

In this paper, we present the synthesis of poly( β -amino ester)-based solid polymer electrolytes (SPE) from off-stoichiometric acrylate-amine formulations using one-step, catalyst and solvent free aza-Michael addition. By varying the monomers, the pendant functionality of the polymer chain structure could be altered. All synthesized polymers yield freestanding and easy to handle electrolyte films and hence are evaluated as a new class of SPEs. The SPE with 1,4-butanediol diacrylate and propylamine showed the highest conductivity of 1.15 (cid:1) 10 (cid:3) 7 S cm (cid:3) 1 at 30 (cid:4) C with 10 wt% lithium bis(trifluoromethanesulfonyl) imide. Because of the presence of the various functional groups in the structure, the polymer chain aids in the movement of both the anion and the cation.


| INTRODUCTION
Finding a suitable electrolyte is a major challenge in lithium-ion battery development.Solid polymer electrolytes (SPEs) have received a lot of attention as safer alternatives to liquid electrolytes off late and the field has grown rapidly.SPEs provide the adaptability and flexibility of polymer chemistry, enabling simple synthesis of customized materials with specific functionalities for specific applications.From the economic and commercial perspective, a low-cost electrolyte with good ionic conductivity and improved dimensional and mechanical stabilities, are the problems to be overcome.Materials based on poly(ethylene oxide) (PEO) have dominated the SPEs for long.2][3][4][5] In this study, poly (β-amino ester)s (PBAEs) via aza-Michael addition reaction are being considered as potential host material for SPEs.
The aza-Michael addition reaction is one of the most fundamental addition reactions in organic chemistry that can be executed in mild reaction conditions, in the absence of toxic metal catalysts and by-products.This 1,4-addition reaction comprises of a nucleophile, commonly an amine (Michael donor), and an electron deficient alkene molecule (Michael acceptor) such as acrylates, acrylamides, vinyl ketones, vinyl sulfones, and so on (Figure 1).A range of mechanical and thermal properties for the final materials can be realized by choosing the structure of the monomers and the stoichiometry of the formulations.Various polymer topologies can be easily generated through these reactions.][8][9] Hence, we present an evaluation of a new class of SPEs based on PBAEs using aza-Michael addition in the presence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).We use off-stoichiometric multifunctional short chain acrylates and amines as monomers and synthesize crosslinked polymer electrolyte films through a one-step, dual-curing, catalyst, and solvent free approach.UV-irradiated crosslinking is a quick and simple way to mechanically stabilize a material, which results in easy-to-handle polymer films.Ethanolamine and propylamine were the amines used in this study.The choice to use ethanolamine was motivated by polymer systems that contain hydroxyl groups (like polyvinyl alcohol, poly(hydroxyethylacrylate) and poly(hydroxyethylmethacrylate)) and are capable of participating in hydrogen bonding.These systems have the ability to dissolve huge amounts of salt and exhibit intriguing ion transport behavior around room temperature, despite having glass transition temperatures above room temperature, due to the presence of hydroxyl groups.1][12] Propylamine, on the other hand, has the same chain length as ethanolamine but does not have the hydroxyl group.The modularity of our polymer system provides us with the liberty to compare between homologue structures-with and without hydrogen bonding.It has also been previously proposed that a polymer backbone containing nitrogen atoms can lower the binding energies of cations/anions, which aids in the dissociation of lithium salts.4][15] The nitrogen atoms, we believe, can accelerate the dissociation of lithium salts, whereas the polyester segments can help to effectively transport ions.
Hot pressing or solvent casting is the usual methods for producing SPE films.Hot pressing is a dry procedure to prepare solvent-free polymer electrolyte films although there is always a possibility of inhomogenous mixing in these films.In contrast, solvent casting imparts homogenous mixing but residual solvents can affect the ionic conductivity at times. 10,16Since our film preparations rely on solvent free and homogenous monomer/salt mixtures, the measured conductivities are only influenced by the polymer structure.The synthesized films are analyzed via differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and infrared spectroscopy (IR).Additionally, we also examine the modularity of these PBAEs.

| Polymer synthesis and film preparation
The polymer films were prepared with a one-step, dualcuring process in the absence of any metal catalyst or solvent.All the films were made from off-stoichiometric (1.3:1) acrylate-amine formulations, 2 wt% 2,2-dimeth oxy-2-phenylacetophenone (DMPA), required amount of 1,1,1-trimethylol propane triacrylate (TMPTA) and the required amount of LiTFSI.The reaction is an aza-Michael addition of multifunctional acrylates and amine monomers with an excess of acrylate groups followed by photopolymerization of the unreacted acrylate groups and crosslinking (Figure 2).According to the Carothers equation, the expected average chain length at full conversion (before crosslinking) would be four with acrylate end groups.
The mold used for these films are made of easily available plastic sheets and double-sided tape (Figure S1).This process of film preparation can easily be duplicated to make films of various thickness, in our project between 93 and 288 μm.All the films produced by this method are freestanding, amorphous, transparent, and easy to manipulate (Figure 3).No laborious hot pressing or evaporation of solvents is employed in the making of these films.
In order to establish that the salt addition did not change the reactivity and hence the extent of the reaction, the aza-Michael addition of 1,4-butanediol diacrylate (1,4-BDA) and ethanolamine (EA), without subsequent photopolymerization, was also performed both with and without LiTFSI.The comparative nuclear magnetic resonance (NMR) spectra of 1,4-BDA + EA + LiTFSI after 15 min and of 1,4-BDA + EA after 15 min show that the reactions proceed in a similar manner irrespective of the salt.It is also evident from the spectra that there are acrylate end groups in the system (as intended), peaks between 5.7-6.5 ppm, which will be utilized later during photopolymerization.(Figures S2 and S3).

| Optimization of salt content
To determine the ideal salt concentration for these systems, 1,4-BDA + EA films with LiTFSI concentrations between 5 and 20 wt% were prepared and their conductivities were studied via impedance spectroscopy.The molar ratio of [carbonyl oxygen]:[Li] for the different salt concentrations were 11:1 (5 wt%), 5:1 (10 wt%), 3:1 (15 wt %), and 2:1 (20 wt%).The molar ratio of [N/OH]: [Li] were 16:1 (5 wt%), 8:1 (10 wt%), 5:1 (15 wt%), and 3:1 (20 wt%).These ratios were defined by the number of moles of 1,4-BDA/2 or number of moles of EA divided by the number of moles of LiTFSI.The temperature dependence of the conductivity shows up as curved lines in the Arrhenius plot and follows the Vogel-Tamann-Fulcher (VFT) relationship.Both the cation and the anion can potentially be coordinated to the different functional groups present in the polymer chain, namely the carbonyl group, the nitrogen atom, and the hydroxyl group.
From the EIS data it can be seen that there is not a linear trend linking the room temperature conductivity to the salt concentration (Figure 4).Nevertheless, it is observed that the higher the salt concentration, higher is the T g (Figure 5).Since films with 10 wt% LiTFSI show the highest room temperature conductivity this composition was selected for all further formulations (Figure 6).It is interesting to note here that hydrogen bonding systems like polyvinyl alcohol which have pendant hydroxyl groups show high conductivity values at very high salt concentrations corresponding to [OH]:[Li] of 4:1. 10,12imilarly, polyester and polycarbonate systems based on ε-caprolactone and trimethylene carbonate which have

| Optimization of crosslinker concentration
To determine the optimal concentration of the crosslinker, films with TMPTA concentrations between 1 and 3 and 10 wt% LiTFSI were prepared and their conductivities were studied via impedance spectroscopy (Figure 7).All the films exhibited similar T g s and there were minute differences in the conductivities but since the film with 2 wt% TPMTA had the best conductivity at room temperature, it is the concentration that was used in all future formulations.

| Modularity of the system
To support our claim that these systems can be easily modified by changing the monomers and their functionalities, we attempted to prepare additional films, by replacing 1,4-BDA with 1,3-butanediol diacrylate and ethanolamine with propylamine, in the following combinations (Figure 8).
Propylamine was selected as it is almost the same chain length as ethanolamine without the hydroxyl group.1,3-Butanediol diacrylate was chosen as it is a structural isomer of 1,4-BDA which would help in introducing some irregularities in the polymer structure because of the pendant methyl group with atactic stereochemistry.All the films were prepared using 10 wt% of LiTFSI, 2 wt% of TMPTA, and 2 wt% of DMPA.These films were then analyzed further with EIS, DSC, and IR.
When we replaced the ethanolamine with propylamine, we noticed a decrease in T g and increase in conductivity which is to be expected since the primary conduction mechanism is related to segmental motion (Figures 9 and 10).One explanation for the reduced conductivity in the case of EA polymer could be the strong interaction of the ions with the hydroxyl group.It has previously been shown that the presence of hydroxyl end groups in poly (propylene oxide) systems influences the anion and cation solvation significantly, and that these groups can serve as preferred coordination sites for both the anion and the cation. 20The same trend was also observed for 1,3-BDA system where propylamine shows higher conductivity than ethanolamine (Figures 11  and 12).
When the polymer chain has a pendant methyl group (1,3-BDA, films C & D), conductivity drops by nearly one order of magnitude.One plausible explanation is the steric hindrance imposed by the pendant group, restricting the mobility of ions, which in turn results in poor conductivity as compared with the structure lacking pendant methyl groups (1,4-BDA, films A & B).
When comparing the conductivities of all the films, it was observed that 1,4-BDA + PA had the best conductivity at room temperature amongst all the four polymer electrolytes, which correlates with the fact that it also had the lowest T g .Similarly, 1,3-BDA + EA exhibited the least conductivity at room temperature and it also had the highest T g of all the four films (Table 1 and Figure 13).If the conductivity is plotted versus a temperature corrected for T g (shifted temperature scale of T-T g . 21) there is a possibility to compare ionic mobility related to functionality rather than segmental motion, Figure 14.There are only minor differences between the different structures but with a slight tendency for the hydroxyl containing polymers to positively contribute to the ionic mobility.This tendency is strongest in the sterically hindered system of 1,3-BDA (comparing black and red lines) and less so in the 1,4-BDA (blue and green lines).When we look at the IR spectra of these polymer electrolyte films, one uncommon aspect that is witnessed is that the C O stretching at 1724 cm À1 does not show the same shift associated with carbonyl coordination of LiTFSI as seen in polyester-based electrolyte systems, that is, a shoulder at higher frequency. 22Instead, the coordination seems noticeable in the C O stretching peak between the frequency range 1100-1250 cm À1 .From the IR spectra of the films with EA with and without salt, we can clearly observe the intermolecular hydrogen bonded OH absorption band, which is not prominent in the films with PA (Figures 15 and 16).Also, by scrutinizing the frequency range between 3000 and 4000 cm À1 , we can state that we have tertiary amines in our polymer structure as the N H stretching peaks are absent.This also correlates with the aspect that we had the acrylates in excess.In the spectrum of the EA film with salt, we notice an increase in intensity for a slightly broader and misshapen peak in the frequency range 1510-1680 cm À1 .This can be the carbonyl peak at 1724 cm À1 , which shifts to a lower frequency when coordinated, as we can distinctly notice a reduction in intensity for the peak at 1724 cm À1 .The increased intensity broad peak is also visible in the PA film with salt but is quite small as compared with the EA film, which can be an indication to the fact that LiTFSI coordinates more strongly in the EA system due to the presence of the hydroxyl group.Furthermore, we also spot a rise in intensity and additional shoulders for the C N stretching peak in the frequency range 960-1100 cm À1 (Figures 17 and 18).This can be credited to the interaction of LiTFSI with the nitrogen atom.These observations suggest that both the carbonyl oxygens and the nitrogen atoms interact strongly with the lithium salt, which is beneficial for LiTFSI dissociation.Similar behavior can also be seen in the IR spectra of 1,3-BDA + EA and 1,3-BDA + PA films (Figures S4  and S5).

| CONCLUSION
In summary, we present a SPE host system based on the aza-Michael addition reaction where structural variations can be easily introduced.This methodology generates zero side products, is highly atom efficient and produces freestanding films in a single step.By merely changing the monomers, we also demonstrated our assertion that this new class of SPEs can be especially modular.Moreover, we saw the diverse interactions of LiTFSI with these polymers though IR spectroscopy indicating the influence of hydrogen bonding and ionic interactions.The best ionic conductivity achieved in the systems investigated by us was 1.15 Â 10 À7 S cm À1 at 30 C which is significantly lower than the required conductivity for application in a battery (1 Â 10 À3 S cm À1 at ambient temperature).However, the modularity of the presented system offers us the opportunity to explore other polymer systems based on PBAEs wherein we can achieve better conductivities.

| Polymerization and electrolyte film preparation
In an argon-filled glovebox, DMPA (27 mg) was dissolved in 1,4-BDA (1.05 g).To this mixture TMPTA (27 mg) was added followed by the desired amount of LiTFSI.After the dissolution of LiTFSI, EA (0.25 g) was added to get a yellowish mix.An immediate exotherm is noticed.After 15 min, this mix was poured into the plastic mold made of plastic sheets and double-sided tape (7-8 cm Â 7-8 cm dimension) and irradiated in a UV chamber with a UV lamp of 365 nm wavelength for 1 h to obtain the polymer electrolyte films. of 10 C min À1 for all the steps under the flow of nitrogen gas.The second heating ramp was used for the glass transition temperature (T g ) measurements.

| Electrochemical impedance spectroscopy
Ionic conductivity was measured using impedance spectroscopy with a Schlumberger SI 1260 impedance gain-phase analyzer at a frequency range of 1-10 MHz with the amplitude set to 10 mV.Electrolyte films with 12 mm diameter were punched out, sandwiched between stainless steel electrodes, and sealed in CR2032 coin cells.The thickness of the SPEs was the average of five different measurements using a Mitutoyo digital indicator.The cells were heated to 100 C for 1 h and cooled overnight to anneal the electrolyte with the electrode surfaces before the measurements were carried out from room temperature to 100 C in the intervals of 10 C. The samples were equilibrated at every temperature for ca.Twenty minutes before a new recording was performed.The resistance was evaluated with ZView (Scribner Associates) using a modified Debye equivalent circuit.

| Infrared spectroscopy
Infrared spectroscopy (FT-IR) measurements were performed on the polymers (with and without LiTFSI) using a Shimadzu IRTracer-100 FT-IR spectrometer with an attenuated total reflectance (ATR) accessory and LabSolutions IR software.The absorption spectra were recorded in the frequency range 400-4000 cm À1 at 4 cm À1 resolution.The number of scans for each spectrum was 45.

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I G U R E 2 Example reaction scheme displaying the sequential stages involved in the aza-Michael addition.Highlighted in green is the potential ion coordinating sites.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 3 Homogenous and transparent polymer electrolyte film from mold prior punching into a 12 mm diameter film.[Color figure can be viewed at wileyonlinelibrary.com] carbonyl oxygens exhibit high conductivity at salt concentrations which correspond to [carbonyl oxygen]: [Li] of 5-6:1. 17,18Additionally, PEO-based systems, the most extensively studied SPEs, exhibit high conductivity at [ether oxygen]:[Li] of 12-16:1. 19These previous results are comparable to our best composition (10 wt% salt) with a molar ratio of [carbonyl oxygen]:[Li] 5:1 or [N/ OH]:[Li] 8:1.

F I G U R E 5
Differential scanning calorimetry data (2nd heating) for different salt concentrations in 1,4-butanediol diacrylate and ethanolamine electrolyte films.LiTFSI, lithium bis(trifluoromethanesulfonyl)imide.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 4 Electrochemical impedance spectroscopy data for different salt concentrations in 1,4-butanediol diacrylate and ethanolamine electrolyte films.LiTFSI, lithium bis(trifluoromethanesulfonyl)imide.[Color figure can be viewed at wileyonlinelibrary.com]

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I G U R E 6 T g and room temperature conductivity for different salt concentrations in 1,4-butanediol diacrylate and ethanolamine electrolyte films.F I G U R E 7 Electrochemical impedance spectroscopy data for different crosslinker concentrations in 1,4-butanediol diacrylate and ethanolamine electrolyte films with 10 wt% lithium bis(trifluoromethanesulfonyl)imide.TMPTA, 1,1,1-trimethylol propane triacrylate.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 8 Polymer structures of the different combinations.

T A B L E 1 9 F
Glass transition temperatures and room temperature conductivities of the different polymer electrolyte films in the order of decreasing conductivity.I G U R E 1 3 Electrochemical impedance spectroscopy data for different polymer electrolyte films.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 1 4 Electrochemical impedance spectroscopy data for different polymer electrolyte films plotted versus (T-T g ).[Color figure can be viewed at wileyonlinelibrary.com]

1 H 4 . 4 |
NMR spectra were recorded using a JEOL eclipse +400 MHz spectrometer with delta 5.3.1 software and CDCl 3 as the solvent.Differential scanning calorimetry DSC measurements were done using Mettler Toledo DSC 3+ with STARe software.Samples weighing between 5 and 10 mg were hermetically sealed in aluminium pans in an argon-filled glovebox.AÀ100-80 C cooling/heating/cooling/heating cycle was used with a ramping speed F I G U R E 1 5 Infrared spectroscopy spectra of 1,4-BDA + EA films with and without lithium bis(trifluoromethanesulfonyl)imide.F I G U R E 1 6 Infrared spectroscopy spectra of 1,4-BDA + PA films with and without lithium bis(trifluoromethanesulfonyl)imide.