Synthesis and characterization of thin film polyelectrolytes for solid-state lithium microbatteries Free

The development of energy dense microbatteries has not kept pace with miniaturization of microelectronic devices and microelectromechanical systems (MEMS), which have enumerable technological applications, including distributed sensors, implantable biomedical devices, wearable electronics, and microrobotics.^1–3 For distributed or embedded microelectronic and MEMS devices, energy storage (the battery) ultimately limits miniaturization or practical implementation.^4,5 In their review, Wang et al. provide several apt examples where the size of the battery is several orders of magnitude larger than the active MEMS device.⁴ Lithium ion battery technology is of interest for microbatteries as lithium is the lightest, most electropositive metal—promising very high theoretical energy densities.⁶ However, microbatteries require modifications to the electrochemical cells that comprise conventional lithium ion batteries. Using solid-state materials, microbatteries can eliminate the safety hazards and extensive packaging requirements for conventional cells that employ flammable liquid electrolytes. Planar solid-state thin film batteries show that exceptional performance can be achieved with solid-state materials, but these cells are typically limited by their areal capacity.⁷ Solid-state three-dimensional (3D) microbatteries seek to realize the exceptional performance of solid-state thin film batteries by fabricating conformal, laminated battery components on high surface area supporting the maximize areal capacity while maintaining short diffusional lengths for the mobile Li cation. Many review articles have highlighted the progress that has been made in 3D microbatteries over the past 20 years,^4,8–13 but the synthesis of conformal, solid-state thin film electrolytes remains a significant challenge.¹³ The fabrication of a conformal solid electrolyte is critical as this layer must be uniform and defect-free to prevent short-circuiting and cell failure.⁵ 

Solid-state polymer thin film electrolytes are promising candidate electrolyte materials given their versatile chemical compositions, mechanical flexibility, low temperature synthesis, and good processability. Typical solid polymer electrolytes are unsuitable for conventional, large-format lithium ion batteries due to their low room temperature ionic conductivities—especially relative to liquid electrolytes and other solid-state conductors.^9,14–17 In 3D microbatteries, though, this low conductivity is compensated by the thinness of the electrolyte layer; the key to using these materials is developing a reliable fabrication method for conformal, defect-free thin films over large surface areas. Several approaches have been demonstrated including spray coating, electrostatic layer-by-layer deposition, electropolymerization, and vapor deposition techniques.¹⁸ Both conventional neutral polymers complexed with lithium salt and polyelectrolytes have been prepared.

In this report, we present a novel approach to synthesizing thin film polyelectrolytes using a technique called initiated chemical vapor deposition (iCVD). iCVD synthesizes polymer thin films through heterogeneous polymerization, which is achieved by adsorbing monomer to the substrate surface and generating primary radicals via thermolysis of peroxide initiators at a heated filament array positioned above the substrate; the process is depicted conceptually in Fig. 1.^19,20 The processing science of iCVD is described in detail in several review articles.^21–23 iCVD can deposit conformal polymer thin films onto a variety of substrates with complex nanostructured surfaces, including carbon nanotube forests, nanofibers, micro- and nano-particles, and microfabricated features.^24–29 Many of these geometries have been considered as supports for 3D microbatteries.¹¹ iCVD synthesis of ultrathin polymer electrolytes based on poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) has previously been demonstrated.³⁰ Conductivities of 10⁻¹⁰S cm⁻¹ in 35 nm films were achieved after salt was absorbed into the film. However, in thicker films, less salt could be absorbed compromising the conductivity. In this study, we report an alternative approach where Li cations are introduced as counterions on an anionic polyelectrolyte thin film composed of crosslinked poly(lithium methacrylate). Because monomer salts are involatile and cannot be directly synthesized by iCVD, an approach was developed to first synthesize poly(methacrylic acid) (polyMAA) and then convert it to a polyelectrolyte through ion exchange. The ability of iCVD to controllably incorporate crosslinking into the polymer film is essential to this approach as it makes the polymer films insoluble and physically stable in solution.^31,32 We show the importance of the copolymer compositions (crosslinking density) in terms of physical stability during the ion exchange and lithium ion conductivity. For this initial study, these polyelectrolytes were synthesized on flat planar substrates for efficient chemical and electrochemical characterization; demonstration of these polyelectrolytes in real 3D batteries will be reported in follow-on studies.

All polymer precursors—methacrylic acid (MAA, Acros Organics, 99.5% purity), tert-butyl peroxide (TBPO, Acros Organics, 99%), and ethylene glycol diacrylate (EGDA, Monomer-Polymer & Dajac Laboratories, 90% purity)—were used as received. A 1.0M solution of lithium methoxide in methanol was purchased from Sigma-Aldrich and used as received.

iCVD was executed in a previously described vacuum deposition chamber.¹⁹ Vacuum was achieved using a rotary vane vacuum pump (Edwards E2M28). The pressure of the reaction chamber was maintained at 500 mTorr for all depositions using a downstream feedback-controlled throttling valve (MKS Instruments T3bi). Precursors were heated to generate a pressure gradient between the source and the vacuum chamber, resulting in precursor flow. Both MAA and EGDA comonomers were introduced simultaneously to synthesize crosslinked poly(methacrylic acid-co-ethylene glycol diacrylate) (X-PMAA). Flow rates were controlled using mass flow controllers and variable conductance needle valves. MAA flow rates were varied from 6 to 18 sccm, while the flow rates of TBPO (initiator) and EGDA (crosslinker) were fixed at 1 and 0.6 sccm, respectively. The temperature of the substrate was maintained at 20 °C through backside cooling with a recirculating chiller. The filaments were resistively heated to about 250 °C to thermolyze the initiator. The film thickness was monitored in real time using in situ laser interferometry on the silicon wafer, and the deposition was terminated after achieving the desired thickness. Nominal thicknesses were 1 μm.

The as-deposited iCVD X-PMAA polymer films were transformed to lithium-bearing polyelectrolytes (X-PMAA-Li) through the H⁺/Li⁺ ion exchange reaction. A series of X-PMAA films were soaked in 1M LiOCH₃ in methanol for 1 h, removed and rinsed in methanol, and then dried in an oven thoroughly prior to characterization. Figure 1(b) illustrates the change in the polymer's chemical formula during this ion exchange process.

Fourier transform infrared spectroscopy (Bruker Tensor 27) was used to qualitatively determine the chemical structure and composition of the deposited films. Measurements were collected from 4000 to 400 cm⁻¹ with 4 cm⁻¹ resolution; 64 scans were integrated to improve the signal-to-noise ratio. X-ray photoelectron spectroscopy (XPS) was also conducted for quantitative analysis of the film composition. XPS was executed using a monochromatized Al Kα source operating at 15 kV and 10 mA (Kratos Axis Ultra spectrometer). A total of three scans with a pass energy of 160 eV from 1200 to −5 eV with a 1 eV step were integrated for survey spectra. A total of ten scans with a pass energy of 20 eV were integrated for the high resolution spectra of the C 1s, O 1s, and Li 1s regions. Binding energies were calibrated based on aromatic carbon at 284.8 eV. Composition quantification and peak deconvolution were performed using the casaxps software.

For successful electrical testing of thin film materials, custom-fabricated test structures were developed. A schematic of the test structure is provided in Fig. 2. X-PMAA-Li was prepared on a highly doped Si wafer patterned with regions of an insulating oxide layer. An exposed strip of the doped Si surface was one electrode. Following the synthesis of X-PMAA-Li, a gold electrode was deposited atop the polyelectrolyte film perpendicular to and overlapping the exposed Si using RF magnetron sputtering, providing a well-defined electric field of known area for conductivity measurements. Contact was made to the electrically conductive areas of the test structure away from the overlapping area. Inside an Argon-filled glovebox, the sample was loaded into a hermetically sealed box with electrical feedthroughs for impedance measurements, and the box was then removed from the glovebox. Impedance spectroscopy measurements were collected over a frequency range of 3 × 10⁶ to 0.1 Hz (Solarton 1260) and a temperature range of 20–100 °C. Temperature was controlled using an environmental chamber (Tenney Jr.). The samples equilibrated at their test temperatures for 30 min before the impedance measurement. The applied AC potential was 10 mV. The impedance data were analyzed using equivalent circuit modeling, which was executed in zview software (Scribner Associates).

Atomic force microscopy was used to characterize the morphology of iCVD polymer films (NTMDT AFM Microscope). Data were acquired in an intermittent-contact tapping mode and further analyzed in gwyddion software. Wettability of the polymer film was conducted via the sessile drop video contact angle system (VCA Optima Surface Analysis system). Two microliters of de-ionized water was used to wet the surface of the polymer films to perform the water contact angle measurements.

A series of crosslinked X-PMAA films were synthesized to demonstrate the ability to control copolymer composition. The films were crosslinked with EGDA to impart insolubility to the films, which is needed to prevent the film from dissolving into solution during the subsequent ion exchange process. Crosslinking is also expected to be an important consideration when these films are integrated into thin film or 3D microbatteries as the crosslinking leads to greater stiffness, and the film will be able to better support residual film stresses of subsequent electrode and current collector films that are deposited. However, the incorporation of EGDA reduces the concentration of mobile charge carriers. The flow rate of MAA was used to modulate crosslinking density, while all other flow rates and deposition control parameters remained constant. The deposition conditions are summarized in Table I.

The flow rate of MAA is directly correlated to its reduced partial pressure, which defines the surface concentration of adsorbed species.³³ However, surface concentrations are difficult to quantify as the depositions are operated in a pressure regime where multilayer, multicomponent adsorption occurs.¹⁹ In Fig. 3, the thin film deposition rate is plotted against the reduced partial pressure, showing a positive correlation between deposition rate and partial pressure. This trend is expected since the rate of polymerization is proportional to the total monomer surface concentration, which increases with P_MAA/P_MAA,sat.²⁰ 

The compositions of the four crosslinking density X-PMAA films (1 μm thick) were assessed using FTIR. Figure 4(a) provides the FTIR spectra in the fingerprint regions for all four iCVD polymer films. A comparison of spectra over the entire mid-infrared region (4000–650 cm⁻¹) is provided in Fig. S1,⁵⁸ including polyMAA and poly(ethylene glycol diacrylate) (polyEGDA) homopolymers. The principle difference among these spectra occurs within the 1700–1800 cm⁻¹ window. The absorption peaks centered at 1738 and 1700 cm⁻¹ are attributed to the stretching vibration of C˭O in ester moieties of EGDA and carboxylic acid moieties of MAA, respectively.³⁴ PolyMAA and polyEGDA both show their respective single C˭O stretch mode, whereas both carbonyl stretch modes are present in the crosslinked films, indicating that EGDA and MAA were successfully copolymerized. These spectra confirm the expected qualitative trend in the film composition through the relative integrated areas of these two absorption modes. Reliable quantification was not possible given practical difficulties in establishing reproducible baselines for integration and the different absorption cross sections, but the qualitative trends are still revealing. X1-PMAA, which was synthesized with the highest partial pressure of MAA, should have the highest MAA content relative to EGDA. Among the four crosslinked copolymer films, X1-PMAA has the highest A_{C˭O, acid}/A_{C˭O, ester}, where A is the integrated absorbance associated with a particular vibrational mode. X4-PMAA, synthesized with the smallest P_MAA/P_MAA,sat, has the lowest ratio. There is a direct qualitative trend between this ratio and the partial pressure of MAA in the feed, which shows that controlling the relative flow rates into the deposition chamber is an effective means to tune copolymer composition.

The ability to deprotonate the X-PMAA and exchange H⁺ with Li⁺ was also confirmed using FTIR. Figure 4(b) shows FTIR spectra for the same set of films after the ion exchange processes (labeled as X#-PMAA-Li). The peak associated with the stretching vibration of C˭O from EGDA is present, but the carbonyl stretch associated with MAA has disappeared. A new prominent absorption at 1563 cm⁻¹ is present in all of the exchanged spectra. It is assigned to the asymmetric stretching vibration of the C˭O bond in the carboxylate anion, confirming that the ion exchange was indeed successful.^35–37 The relative ratio of this absorbance to the absorbance at 1738 cm⁻¹ associated with the EGDA crosslinker is the largest for X1-PMMA-Li, which was synthesized with the largest P_MAA/P_MAA,sat. The trend in this relative ratio qualitatively matches the trend in the relative ratios of the carbonyl absorbance in the two comonomers.³⁸ 

The final conclusion from the FTIR data is that ion exchange of the X-PMAA is complete as the statistically significant absorbance from the carbonyl stretch of carboxylic acid at 1700 cm⁻¹ could not be clearly distinguished in any of the spectra, even for the X1-PMAA-Li film with the lowest crosslinking density. The 1 μm-thick films are stable in the methanolic solution and undergo deprotonation while remaining tethered to the substrate; there does not appear to be a limit on the penetration depth of the Li⁺ cations as is observed in other crosslinked neutral polymer thin films that rely on physical entrapment of lithium salts within the polymer matrix.³⁹ 

The compositions of the polymer films were analyzed quantitatively using x-ray photoelectron spectroscopy (XPS). The XPS survey spectrum of X1-PMAA with the lowest crosslinking density is presented in Fig. 5(a) as a representative example. The survey spectrum for X1-PMAA consists solely of carbon and oxygen peaks. The composition is 76 at. % carbon and 24 at. % oxygen (C:O ratio is 3.1). This is higher than the theoretical C:O ratio of 2.0 based on the repeat units’ chemical structure, which may be due to impurities in the monomer precursors (they were not purified prior to iCVD synthesis) and/or due to adsorption of adventitious carbon from the atmosphere.^40,41 Moreover, the survey spectrum suggests that films are dense and free of macroscopic defects since Si peaks associated with the underlying substrates are not observed.

High resolution spectra of the same film are provided in Figs. 5(b)–5(d). These spectra are deconvoluted into the component peaks based on their respective bonding environments. In Table II, their positions and integrated areas are compared to poly(methacrylic acid) and poly(methyl acrylate) homopolymers, which have a similar chemical structure.⁴² Notably, the methyl acrylate repeat unit is compositionally equivalent to one half of the EGDA crosslinker as hydrogen atoms are not detected. The chemical structures of the two monomers and their distinct carbon and oxygen bonding environments are provided in Fig. S2.⁵⁸ The two component peaks observed in the O 1s high resolution spectra of X-PMAA are consistent with the expectations from the literature,⁴² as O 1s high resolution spectra of both poly(methacrylic acid) and poly(methyl acrylate) consist of two component oxygen peaks at 532.3 and 533.8 eV of equal integrated areas. The C 1s high resolution spectra for X-PMAA consist of four components. Three of these components (C1–C3) are common to both poly(methacrylic acid) and poly(methyl acrylate). The C4 component is associated with alkoxy carbons in the EGDA comonomer. In X1-PMAA, it comprises 4.7% of the carbon high resolution spectrum, whereas it would be 25% in a homopolymer of polyEGDA. This is a further indication that the EGDA and MAA have been successfully copolymerized. The consistency between the component peaks in X1-PMMA and representative homopolymer also supports our claim that iCVD is an effective synthetic tool to deposit copolymer thin films with compositional control, while retaining critical pendent organic functionalities.

Due to the elemental compositions of methacrylic acid and ethylene glycol diacrylate being identical (excluding H), quantification of copolymer compositions using the XPS survey spectra was impossible. Instead, X-PMAA compositions were quantified using the area of the C4 carbon peak at 287 eV that is unique to the EGDA component. The comparison of the high resolution spectra of C 1s among the four different X-PMAA-Li films is shown in Fig. S3 in the supplementary material.⁵⁸ The quantification results are included in Table I, showing that EGDA content in the films ranges from 9% to 27%. The quantification methodology for these films is provided in the supplementary material.⁵⁸ 

Figure 5 also presents XPS spectra for the post-ion exchange X1-PMAA-Li film, clearly showing that Li has been incorporated into the films. In both the survey and high resolution Li 1s spectra for X1-PMAA-Li, Li peaks are clearly resolvable. Quantification of the survey spectra revealed that this film was 65 at. % C, 22 at. % O, and 13 at. % Li. XPS spectra for the complete set of films are provided in Fig. S4 in the supplementary material.⁵⁸ In the high resolution Li 1s spectra [Fig. 5(d)], the peak position of 55 eV is consistent with literature assignments of Li⁺ in poly(acrylic acid) binders used in lithium ion batteries.⁴³ Given the copolymer composition of X1-PMAA (Table I), the atomic percentage of Li closely matches the theoretical Li composition expected for fully ion exchange of the MAA (12.2%), which indicates that the exchange is complete (at least to the penetration depth of the XPS tool) within the 1 h soak duration used for ion exchange. This has also been confirmed for the other film compositions. The survey spectrum also reveals that Na was introduced into the film, likely as impurities in the methanolic LiOCH₃ solution or as contamination from the sodium borosilicate glassware. The small peak at 1070 eV is associated with Na 1s, but quantification showed that Na is <0.4 at. % of film composition. An additional peak at 507 eV is attributed to Na KLL. The Na content was not included in the compositional analysis above.

Importantly, the C:O ratio in X-PMAA (3.1) and X-PMAA-Li (3.2) is very close, which suggests that the ion exchange process only exchanges H⁺ for Li⁺ in the carboxylic acid functionality and does not result in other undesirable chemical reactions within the film. The component peaks in the C 1s spectra for X-PMAA and X-PMAA-Li are also the same [Fig. 5(b)], reinforcing the conclusion that undesirable modifications of the film chemistry do not occur. However, the O 1s spectrum of the post-ion exchange X1-PMAA-Li film is significantly different from that of the X1-PMAA. In the O 1s spectrum of X1-PMAA, the binding energies of the two component peaks are 532.0 and 533.3 eV. In the O 1s spectrum of X1-PMAA-Li, three component peaks (531.5, 532.5, and 533.4 eV) are present but with distinct integrated areas. The larger O 1s peak at 531.5 eV is assigned to the equivalent bonding environments of the oxygen atoms in lithium carboxylate resulting from the ion exchange process.⁴⁴ The peaks at 532.5 and 533.4 eV are consistent with the binding energies prior to ion exchange and are associated with the EGDA residues that are unaffected by ion exchange.

The surface morphology of iCVD polymer films was characterized via atomic force microscopy (AFM), presented in Figs. 6(a) and 6(b). In both chemical states, the films are continuous, smooth, and pinhole-free over the analyzed area (10 × 10 μm²). The as-deposited X2-PMAA film has a root-mean-square roughness of 0.4 nm, whereas roughness increases to 4.3 nm for the X2-PMAA-Li. An elevated surface roughness after the ion exchange process is consistently observed for all film compositions. The roughness may develop as the films swell in LiOCH₃ solution for ion exchange and then collapse as the solvent is eliminated during the drying step. However, compared to previously published studies of thin film polymer electrolytes,^30,45 our films show comparable roughness. This nanometer-level roughness is acceptable for integration with real electrodes and for further electrochemical characterization. The critical feature is the absence of pinholes that would lead to electrical shorting, precluding electrochemical characterization.

The effect of ion exchange on the surface energies of X2-PMAA films was assessed using water contact angle measurements, which are provided in the inset of Figs. 6(a) and 6(b). Both film surfaces are hydrophilic due to the carboxylic acid moieties of methacrylic acid. Prior to ion exchange, the water contact angle was 51°, which decreased to 24° after ion exchange. Converting the methacrylic acid to lithium methacrylate salts makes the film more hydrophilic, resulting in higher surface energy relative to the neutral X-PMAA.⁴⁶ 

Another important question is the effect of ion exchange on the underlying electrode. Fused carbon fiber mats, commonly utilized in fuel cell electrodes, are often proposed as multifunctional scaffolds for microbatteries, providing a high surface area support while also able to intercalate Li⁺ and function as the anode and current collector.⁴⁷ To address the effect of the lithium methoxide ion exchange solution on carbonaceous materials, conventional graphite electrodes were exposed to the lithium ion exchange solution and processing conditions. The electrochemical performance of the initial lithiation step and subsequent cycle was compared for the exposed and nonexposed graphite. The voltage profiles are provided in Fig. S5,⁵⁸ showing that the ion exchange process does not have an obvious chemical or physical effect on graphite that alters the energetics and kinetics of lithiation. The voltage curves are essentially identical, and irreversible capacities are similar, as well, suggesting that this ion exchange process will be compatible with graphitic electrodes.

After establishing the deposition parameters to control the copolymer composition of X-PMAA and demonstrating successful incorporation of Li⁺ into the films, the lithium ion conductivity of the film was characterized by impedance spectroscopy. To determine the potential contribution of H⁺ transport, impedance spectroscopy was performed for X1-PMAA prior to ion exchange [Fig. S6 (Ref. 58)]. This film is purely capacitive in nature, indicating that negligible proton conduction occurs. This is expected given the high energy barrier for dissociation of the acid in a completely dry state, and any residual unexchanged protons are expected to contribute negligibly to the conductivity measurements. Impedance spectra of the X1-PMAA-Li exchanged film with the lowest crosslinking density are presented as Nyquist plots in Fig. 7. The Nyquist plots for X-PMAA-Li consist of two key features: a partial semicircle at relatively high frequencies and a near vertical tail at low frequencies. The semicircle is a result of parallel processes of ionic transport and dielectric polarization in the bulk polymer film, whereas the vertical line at low frequencies is due to the double layer capacitance at the blocking electrode interfaces. The Si electrode is theoretically a nonblocking electrode as Li can alloy with Si, but the reaction kinetics are sufficiently slow that it can be satisfactorily modeled as a blocking electrode.

As seen in Fig. 7, the general features of the spectra remain unchanged over the entire tested temperature range, but the diameter of the semicircle decreases with increasing temperature indicating that the film resistance decreases (conductivity increases). All spectra were modeled by the equivalent circuit shown in the inset of Fig. 7, where the parallel RQ_bulk component describes the resistance of the bulk film in parallel with its dielectric capacitance, whereas Q_dl is the double layer capacitance of blocking electrodes. Q designates that constant phase elements were used to model capacitance rather than ideal capacitors due to nonuniformities and other nonidealities in the material and test structure. Values for the equivalent circuit elements were obtained by fitting the impedance spectra to the equivalent circuit, and lithium ion conductivity was calculated by σ = h/RA, where h is the film thickness, R is the measured resistance, and A is the overlapping area of the blocking electrodes.

Ionic conductivities for all X-PMAA-Li compositions as a function of temperature are provided in Fig. 8. The films were all nominally 1 μm in thickness, and the lowest resistance that could be measured with our impedance spectrometer was on the order of $10−9−10−10S/cm$⁠, thus limiting the lowest temperature measurement of the samples with the highest crosslinking densities. For all samples, the temperature dependence of conductivity follows Arrhenius behavior, which is generally associated with an ion-hopping mechanism.^48,49 This is typical of ion transport in solid polyelectrolytes, where ions coordinate with immobile anionic species on the segments of a polymeric chain. It is also the dominant mode of transport in polymer glasses and crystalline polymers, where there is limited segmental mobility.^9,50 The characterization temperatures in this study were well below the glass transition temperature (T_g) of X-PMAA. The T_g of these thin crosslinked films could not be experimentally characterized, but they are expected to be higher than the T_g of linear poly(methacrylic acid) homopolymer, which is 228 °C.⁵¹ 

The activation energy for each copolymer composition was determined through linear regression of the conductivity data; they are tabulated in Table III along with the conductivities at 70 °C. Examining data in both Fig. 8 and Table III, it can be concluded that ionic conductivity is a strong function of polymer composition.^52,53 In glasses, it is quite common for very small changes in the concentration of charge carriers to alter ionic conductivities by orders of magnitude.⁵⁴ Incorporation of EGDA crosslinker in these films influences both the pre-exponential factor and activation energy in the Arrhenius equation. The cation concentration and average length of separation between the MAA groups are key parameters in the pre-exponential factor.⁵⁴ Lithium cations are hosted at the carboxylate moieties in MAA. As crosslinking density increases, the EGDA composition increases at the expense of MAA, and the concentration of Li cations in the exchanged film decreases (Table III). The distance between these sites also increases as MAA is diluted, increasing the jump distance for the diffusing Li cations. Both of these effects are responsible for the pre-exponential factor decreasing with increasing crosslinking density. The activation energy represents the energy required for the cation to jump between sites. As the MAA composition is reduced, the permittivity of the film will decrease, resulting in greater coulombic potential energy between the fixed carboxylate anion and lithium cation that must be overcome to free Li⁺ from the site so that it can diffuse and contribute to conductivity.³⁶ 

The 2 orders of magnitude difference in conductivity within this set of film compositions highlights the importance of controlling the crosslinking density of these deposited films. Films with even lower crosslinking densities will have superior conductivity, but such films are generally not physically stable during the ion exchange process, delaminating from the substrate. The film compositions reported in this study could all be reliably ion exchanged without film delamination. The maximum conductivity was observed in X1-PMAA-Li, consisting of 91 mol. % lithium methacrylate and 9% EGDA. Its lithium conductivity is 6 × 10⁻⁹S cm⁻¹ at 20 °C and 1 × 10⁻⁵S cm⁻¹ at 100 °C. While this conductivity is significantly lower than what can be achieved in state-of-the-art polyelectrolytes,¹⁷ it is important to compare these materials to other thin polyelectrolytes developed for 3D microbatteries. The architecture of 3D microbatteries and requirement for conformal electrolyte layers put significant constraints on the synthesis and integration of thin polymer electrolytes. Several fabrication methods have been developed for thin film polymer electrolytes, with solution casting, electropolymerization, and chemical vapor deposition (both iCVD and plasma polymerization methods) being the most common.^{30,45,55–57} Conductivities of similar orders of magnitude have been observed for both polyelectrolytes and conventional polymer electrolytes, where neutral polymers complex lithium salts. Future development will emphasize materials with higher conductivities, which can be achieved by synthesizing films with softer fixed anions to reduce the binding energies to the Li⁺ ions, while simultaneously trying to maximize the concentrations of mobile lithium cations.

One important benefit in our approach using ion exchanged polyelectrolytes is that the films are single ion conductors with near unity transference number since the polyelectrolyte backbone is immobile. This eliminates transport limitations due to salt concentration polarization of conventional salt-loaded polymer electrolytes, where the lithium salt anion also has high mobility, often with higher diffusivities than Li⁺. Moreover, films with a wide range of thicknesses can be prepared and are expected to have identical lithium ion conductivities as the mobile charge carrier is hosted by the MAA units that are found throughout the polymer bulk. As long as ion exchange solution can infiltrate the entire film, the mobile charge carriers (Li⁺) can be introduced to impart ionic conductivity. Ultrathin (<100 nm) films can be prepared for nanoscopic microbattery designs, or thicker films can be prepared for rougher substrates with large features requiring passivation.

The integration of this polyelectrolyte thin film into full electrochemical cells in microbattery formats will be demonstrated in future publications. The physical properties of these polymer thin films require reoptimizing the fabrication processes for electrode depositions. However, to illustrate the capability of iCVD to prepare conformal electrolytes on relevant microbattery electrode architectures, X-PMAA was deposited on a mat of carbon fibers. Figure S7 (Ref. 58) provides SEM micrographs of the bare and coated carbon fibers, showing that thin film electrolyte conformally coats the carbon fibers with a coaxial morphology. The coating thickness is tunable through deposition duration. For microbattery fabrication, the next step would be deposition of the cathode and current collector.

In this work, crosslinked poly(methacrylic acid-co-ethylene glycol diacrylate) films were synthesized by iCVD, which can be transformed into single Li⁺ ion conducting polyelectrolytes via an ion exchange process. Prior to incorporation of lithium counterions, these films are insulators. After ion exchange, films display room temperature ionic conductivities as high as 6 × 10⁻⁹S cm⁻¹. The polymer chemistry was comprehensively characterized using FTIR and XPS, and film composition was related to the trends observed in lithium ion conductivities. It was shown that crosslinking density strongly influenced lithium ion conductivity—in terms of both the pre-exponential factor and activation energy in the Arrhenius relationship. While the film conductivities should be further enhanced to achieve greater 3D microbattery performance, this study has provided a basic understanding of requirements for polyelectrolyte thin films prepared by iCVD. The smooth surface morphology (nanometer-level surface roughness) and absence of pinhole defects is an important result. Future studies will emphasize increasing the lithium cation concentration and altering the polymer chemistry to promote dissociation of the counterion, resulting in higher lithium ion conductivities.

This material is based upon work supported by the National Science Foundation (NSF) under Grant No. CBET-1604471.