Investigation of ion-electrode interactions of linear polyimides and alkali metal ions for next generation alternative-ion batteries

Organic electrode materials offer unique opportunities to utilize ion-electrode interactions to develop diverse, versatile, and high-performing secondary batteries, particularly for applications requiring high power densities. However, a lack of well-defined structure–property relationships for redox-active organic materials restricts the advancement of the field. Herein, we investigate a family of diimide-based polymer materials with several charge-compensating ions (Li+, Na+, K+) in order to systematically probe how redox-active moiety, ion, and polymer flexibility dictate their thermodynamic and kinetic properties. When favorable ion-electrode interactions are employed (e.g., soft K+ anions with soft perylenediimide dianions), the resulting batteries demonstrate increased working potentials and improved cycling stabilities. Further, for all polymers examined herein, we demonstrate that K+ accesses the highest percentage of redox-active groups due to its small solvation shell/energy. Through crown ether experiments, cyclic voltammetry, and activation energy measurements, we provide insights into the charge compensation mechanisms of three different polymer structures and rationalize these findings in terms of the differing degrees of improvements observed when cycling with K+. Critically, we find that the most flexible polymer enables access to the highest fraction of active sites due to the small activation energy barrier during charge/discharge. These results suggest that improved capacities may be accessible by employing more flexible structures. Overall, our in-depth structure–activity investigation demonstrates how variables such as polymer structure and cation can be used to optimize battery performance and enable the realization of novel battery chemistries.

Infrared (IR) spectra were collected on a Bruker Tensor II IR spectrometer with a diamond Attenuated Total Reflectance (ATR) attachment. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku Ultima IV diffractometer equipped with a Cu Kα source (λ = 1.5406 Å) and were baseline-corrected using OriginPro. Solution-state NMR data were collected on a Bruker INOVA 500 MHz spectrometer and are referenced to residual solvent. Magic angle spinning solidstate NMR (MAS SSNMR) 1 H and cross-polarized (CP) 13 C measurements were carried out using a Phoenix NMR HX NB Probe (3.2 mm) located within a Varian INOVA 500 MHz spectrometer. All MAS SSNMR experiments were carried out using samples packed within 35 μL rotors at a spinning speed of 20 kHz. Solid-state UV-Vis spectra were collected using a Shimadzu UV-2600i Spectrophotometer equipped with an ISR-2660 Plus Integration Sphere for solid-state measurements. Surface area data were collected on a Micromeritics 3-flex gas sorption analyzer using ultrapure N2 (99.999%) and a liquid N2 bath. Brunauer-Emmett-Teller (BET) and Langmuir surface areas were determined by linear least squares regression analysis using the linearized forms of the BET and Langmuir equations, respectively. Combustion elemental analysis was performed by Atlantic Microlab Inc. Thermogravimetric decomposition profiles were collected on a Q500 V6.7 thermogravimetric analyzer (TGA) using a temperature ramp of 5.00 °C/min from 40.00 °C to 600.00 °C under an atmosphere of zero grade air (20-22% O2 in N2). Differential scanning calorimetry (DSC) measurements were carried out on a TA instruments DSC Auto 2500 differential scanning calorimeter using a heat-cool-heat procedure, with a heating temperature ramp of 10.00 °C/min from −50.00 °C to 320.00 °C and a cooling ramp of 5.00 °C/min from 320.00 °C to −50.00 °C.
Energy dispersive X-ray scattering (EDS) data were collected at 10.0 kV using a Zeiss Gemini 500 scanning electron microscope equipped with an Oxford Instruments Ultima Max 170 detector (detector type X-max) and processed using the AZtec software. To prepare samples for EDS, the powders were immobilized on carbon tape mounted on an aluminum stub. The samples were blown using compressed air to remove excess material not stuck to the tape and then were coated with a carbon layer. This procedure was adapted from the literature. 1 A 50 mL round-bottom flask equipped with a stir bar was charged with PMDA (1.00 g, 4.58 mmol, 1.00 equiv.), AcOH (20 mL), and di i PrAn (2.59 mL, 13.8 mmol, 3.00 equiv.) in that order. The reaction mixture was allowed to stir for 30 min at room temperature. The reaction mixture was then headed to 105 °C and allowed to stir at 105 °C for 4 h with stirring. Additional AcOH (10 mL) was added when the reaction mixture reached 100 °C to facilitate mixing. A white solid precipitated from the reaction mixture after 90 min. The mixture was allowed to cool to room temperature and filtered. The resulting solid was rinsed with EtOH (50 mL) and deionized (DI) water (50 mL) and dried under vacuum to yield N,N'-bis(2,6-diisopropylphenyl)-pyromellitic diimide (PMDA-di i PrAn) as an off-white solid (2.04 g, 83% yield). 1
This procedure was adapted from the literature. 2 NTCDA (1.34 g, 5.00 mmol, 1.00 equiv.) was suspended in AcOH (70 mL) in a 250 mL roundbottom flask equipped with a stir bar and reflux condenser. Di i PrAn (6.32 mL, 33.5 mmol, 6.70 equiv.) was added to the suspension, and the mixture was heated to reflux (120 °C). The reaction mixture was allowed to stir at reflux for 16 h. The reaction was allowed to cool to room temperature and diluted with DI water (70 mL). The resulting precipitate was collected by filtration and rinsed with DI water (2 × 20 mL) and hexanes (3 × 20 mL). The solid was dried under vacuum at room temperature to yield N,N'-bis(2,6diisopropylphenyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTCDA-di i PrAn) as a pale orange-brown solid (2.05 g, 70% yield). 1    This procedure was adapted from the literature. 4 PTCDA (785 mg, 2.00 mmol, 1.00 equiv.) and imidazole (6.0 g) were added to a 75 mL screw-cap reaction vessel equipped with a stir bar. Di i PrAn (1.51 mL, 8.00 mmol, 4 equiv.) was added, and the flask was sealed. The reaction mixture was heated to 190 °C and allowed to stir at 190 °C for 24 h. At this time, the reaction mixture was allowed to cool to room temperature and then diluted with 95% EtOH (50 mL) and 2 M HCl (60 mL). The resulting mixture was stirred for 3 h and filtered. The resulting solid was rinsed with a 5:6 EtOH/2 M HCl mixture (110 mL), followed by a 1:1 EtOH/DI H2O mixture (100 mL). The solid was extracted with acetone (400 mL). The acetone was collected, dried with anhydrous Na2SO4, and filtered. The solvent was then removed under reduced pressure to yield a purple-red solid, which was further purified by flash column chromatography (SiO2, 50%→100% dichloromethane in hexanes) to yield N,N'-bis(2,6-diisopropylphenyl)-3,4,9,10perylenetetracarboxylic diimide (PTCDA-di i PrAn) as a bright red solid (403 mg, 28%). 1

Polymer synthesis and characterization.
General Procedure A. The following procedure was adapted from literature procedure for synthesizing molecular diimides. 5 A 100 mL round-bottom flask or a 75 mL screw-cap reaction vessel (if the amine boils below 130 °C) was equipped with a stir bar. Imidazole (17.7 g) and the dianhydride were added to the round-bottom flask. If the amine was solid, it was added with the imidazole and the imide to the reaction vessel. If the amine was a liquid, it was added after the solids were added to the reaction vessel. The reaction flask was sealed, and the reaction mixture was heated to 130 °C and allowed to stir at 130 °C for 24 h. The reaction mixture was allowed to cool to room temperature. The resulting solid was then diluted with DI water (300 mL) and centrifuged. The aqueous layer was decanted, and the resulting solid residue was rinsed with acetone (300 mL) and filtered, rinsing with additional acetone. The solid was further dried in air before being Soxhlet washed with chloroform for 48 h at 90 °C. The solid was then transferred to a scintillation vial and dried under vacuum at 120 °C for 16 h prior to characterization. S12 a. Synthesis and characterization of PMDA-pPDA from pyromellitic dianhydride (PMDA) and para-phenylene diamine (pPDA). Figure S10. Scheme for the synthesis of PMDA-pPDA from PMDA and pPDA.
Following General Procedure A, imidazole (17.7 g), PMDA (436 mg, 2.00 mmol, 1.00 equiv.) and pPDA (216 mg, 2.00 mmol, 1.00 equiv.) were added to a 100 mL round-bottom flask and stirred at 130 °C for 24 h. The resulting solid was washed and dried according to the procedure to yield PMDA-pPDA (259 mg, 45% yield) as a light brown solid.        Table S1. Tabulated EDS data for PMDA-pPDA. 1 Excludes hydrogen. Table S2. Tabulated combustion analysis data for PMDA-pPDA. 1 The remaining mass not attributed to C, H, and N was assumed to come from O, as O was not directly analyzed during combustion analysis.

Element
Line  Figure S18. Scheme for the synthesis of NTCDA-pPDA from NTCDA and pPDA.
Following General Procedure A, imidazole (17.7 g), NTCDA (268 mg, 1.00 mmol, 1.00 equiv.) and pPDA (108 mg, 1.00 mmol, 1.00 equiv.) were added to a 100 mL round-bottom flask stirred at 130 °C for 24 h. The resulting solid was washed and dried according to the procedure to yield NTCDA-pPDA (235 mg, 69% yield) as a dark brown solid.        Table S3. Tabulated EDS data for NTCDA-pPDA. 1 Excludes hydrogen. Table S4. Tabulated combustion analysis data for NTCDA-pPDA. 1 The remaining mass not attributed to C, H, and N was assumed to come from O, as O was not directly analyzed during combustion analysis.
Following General Procedure A, imidazole (17.7 g), PTCDA (392 mg, 1.00 mmol, 1.00 equiv) and pPDA (108 mg, 1.00 mmol, 1.00 equiv.) were added to a 100 mL round-bottom flask and stirred at 130 °C for 24 h. The resulting solid was washed and dried according to the procedure to yield PTCDA-pPDA (321 mg, 69% yield) a deep burgundy solid.      Figure S32. Thermogravimetric decomposition profile in air for PTCDA-pPDA, indicating that it is stable below 450 °C. The gradual weight loss from this material upon heating is likely due to the loss of solvent/water, given its microporosity ( Figure S78 and Figure S79).     Figure S35. Scheme for the synthesis of PTCDA-chex from PTCDA and chex.
Following General Procedure A, imidazole (17.7 g), PTCDA (392 mg, 1.00 mmol, 1.00 equiv.) and en (100 μl, 1.00 mmol, 1.00 equiv.) were added to a 75 mL screw-cap reaction vessel and stirred at 130 °C for 24 h. The resulting solid was washed and dried according to the procedure to yield PTCDA-en (418 mg, quantitative yield) as a purple-black solid.

a. Solution CV/RDE experiments.
For the electrochemical evaluation of the molecular analogues (PMDA-di i PrAn, NTCDAdi i PrAn, and PTCDA-di i PrAn), each of the small molecules was prepared as a 1 mM stock solution in anhydrous DMF. The solution was then filtered to remove any undissolved material and divided into four separate vials. To each vial, a different perchlorate salt was added (TBAP, KClO4, NaClO4, or LiClO4) at 0.1 M concentration. The solutions were then added into a threecompartment cell, where the outer compartments contained blank electrolyte solution (no analyte species). The solution was thoroughly degassed with Ar to remove any dissolved oxygen. For the electrochemical measurements, a 5 mm glassy carbon electrode was used as the working electrode, a coiled Pt wire was used as the counter electrode, and a house-made Ag/AgCl (3 M KCl solution) electrode was used as the reference. Cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments were carried out to characterize each solution. Following the measurements, sublimed ferrocene was added to the solution to serve as an internal reference and to correct for solution resistance. The potential scales for the figures presented in the manuscript were then converted to estimated potentials vs. SHE (Fc/Fc + = 0.400 V vs. SHE).

b. Coin cell fabrication.
For electrochemical measurements performed in the solid state, the polymeric materials were incorporated into 2032 coin cells. First, the active material, Super P carbon (Imerys Graphite & Carbon, dried in vacuum oven at 60 °C overnight), and PVDF (Kynar Flex, dried in vacuum oven at 60 °C overnight), were mixed at a ratio of 6:3:1 (by wt) in NMP. The resulting slurry was then spread on a carbon paper current collector using the doctor blade method and dried in a vacuum oven at 60 °C for 2 h and then at 110 °C overnight. The electrode was then punched into disks (3/16'' diameter for CV tests, 1/4'' for activation energy measurements, and 3/8'' for all other electrochemical tests) with an average active mass loading of ~1 mg/cm 2 . The disks were used as the cathode within the assembled coin cells. The cells were assembled with a fiber glass separator, a 1/2'' disk metal anode, and 80 μL of an electrolyte solution consisting of the corresponding metal ion PF6 − salt (1 M and 0.8 M LiPF6 for Li batteries, 1 M NaPF6 for Na batteries, and 0.8 M KPF6 for K batteries) in EC:DEC (1:1 v:v). Cell assembly was conducted in an Ar-filled glovebox with O2 levels below 0.5 ppm.
For Li batteries, lithium metal foil was used as received. For Na batteries, Na metal cubes were made into a foil by first removing the oxide layer with a ceramic knife. The Na metal was then placed in a plastic bag and pressed flat using a pasta maker. The resulting foil was then punched into disks. For K batteries, K metal chunks were made into a foil by first removing the oxide layer with a ceramic knife. The K metal was then placed in a plastic bag which had been washed with acetone and then rolled flat with a pasta maker. The resulting foil was then punched into disks.
Solid polymer electrolyte cells were made by dissolving LiTFSI and polyethylene oxide (PEO, average MV 600,000) in MeCN and stirring the resulting solution for 24 h. The ratio of Li + to ethylene oxide units was 0.01. Following mixing, the MeCN was allowed to evaporate at room temperature until a thick, gel-like solution was obtained. The solution was then dropped onto Celgard separators. The separators were then dried in a vacuum oven at 60 °C overnight, and the resulting solid polymer electrolyte was stored in the glovebox until use.
The resulting batteries were examined using galvanostatic charge/discharge, CV, and potentiostatic electrochemical impedance spectroscopy (PEIS). Galvanostatic charge/discharge measurements were performed using a Neware battery test station. CV and PEIS measurements were performed with a Biologic SP150 potentiostat.

c. Cyclic voltammetry studies to determine diffusion-limitations.
CV experiments were performed at slow scan rates (1.00, 0.75, 0.50, 0.25, and 0.10 mV/s) to determine the kinetic processes limiting the current response throughout the charge/discharge process. Because surface-controlled kinetics exhibit a current i that scales with the scan rate ν (i.e., i ∝ ν), while diffusion-limited kinetics have a current that scales with ν 1/2 , the following relationship (eq. S1) can be used over a range of potentials employed in CV experiments to determine the contribution of each to the total current: eq. S1 where k1 is an experimentally determined constant related to the surface-limited current contribution, and k2 is an experimentally determined constant relating to the diffusion-limited current. Integration of the diffusion-limited current term relative to the total integrated current provides the percentage of the charge arising from diffusion-limited processes.

d. Activation energy measurements.
For activation energy measurements, three-electrode coin cells were employed and assembled following a procedure described in the literature. 6 Briefly, a third electrode was incorporated into the cell to serve as a reference electrode. The reference electrode consisted of a stainless-steel strip with one end embedded in a small piece of Li foil. The reference electrode was placed between the working electrode (polymer slurry) and the counter electrode (Li metal disk). To prevent contact between the reference, counter, and working electrode, separators were placed above and beneath the reference electrode. Additionally, Kapton tape was place on the cell wall and base, under the stainless-steel strip, to prevent contact between the working electrode and reference electrode at the cell edge.
For activation energy measurements, the cells were first pre-cycled 5 times at 1 mV/s by CV. PEIS measurements were then performed at the E1/2 of the first redox couple (between the neutral and −1 state) of each polymer. Before PEIS measurements were conducted, the cell was allowed to equilibrate for 1 h at a given temperature while being held at the potential used in the PEIS measurements. A potential amplitude of 5 mV was employed and a frequency range of 5 MHz to 50 mHz was used. The high frequency semi-circle portion of the Nyquist plot was fit with S42 a modified Randles-circuit (Rs + (Q1/Rct) where Rs is solution resistance, Q1 is a constant phase element, and Rct is the charge transfer resistance) using ZFit. From the fits, the Rct was extracted for the calculation of the activation energy.
The PEIS measurements were conducted for each cell at four different temperatures to generate an Arrhenius plot from the measured charge transfer resistance (ln(1/Rct) vs. 1/T, where T is temperature in Kelvin. From the slope of the obtained line, the activation energy, Ea, was calculated using eq. S2: where R is the universal gas constant.

e. Conductivity measurements.
Pressed-pellet conductivities were measured using a two-point probe. The sample powder was placed between two stainless-steel screws and two copper contacts encased in a Teflon tube, such that the entire contact area (A, 0.0919 cm 2 ) was covered by the powder. A normal pulse voltammogram was carried out with a Gamry Instruments Interface 1010e potentiostat, where pulses of 100-500 mV were applied across the device, and the current responses were used to generate I-V plots. A Fluke 75III multimeter was used in cases where a reliable current response could not be obtained due to the high resistance of the material (PTCDA-en) even with thinner pressed pellets. The slope of the plot corresponded to the resistance, R, of the sample, and the following equation was used to calculate the conductivity (σ): where t is the thickness of the layer of material between the two metal contacts.    Figure S54. The solid lines/filled circles correspond to the first reduction peak potential (0 → −1) and the dotted lines/open circles correspond to the second reduction peak potential (−1 → −2). Figure S56. Difference in the reduction peak potential values between the first reduction (0 → −1) and second reduction (−1 → −2) from the cyclic voltammograms in Figure S54 as a function of polymer and charge compensating ion of the pPDA-based polymers. The x-axis is labelled by the tetracarboxylic acid dianhydride used in the synthesis of the polymer.                 Figure S71. The slopes of the plots were used to determine the activation energy of charge transfer in the PTCDA-based polymers. These measurements were repeated at least three times for each polymer to determine the error associated with the activation energies calculated from this measurement.    . When cycled in the electrolyte containing DOL:DME, PTCDA-en delivers a higher capacity upon initial cycling as would be expected in a lower dielectric solvent where the Li + would interact less strongly with the solvent molecules. However, a lower cycling stability is also observed, likely due to the reduced stabilizing effects in a lower dielectric solvent.       Figure S82 for the three polymers is relatively shortranged. showing PTCDA-en has a sharp aliphatic peak while PTCDA-chex has a broad aliphatic peak and PTCDA-pPDA has no aliphatic peak. This suggests that the aliphatic segments in PTCDAen are flexible and able to dynamically adopt more conformers than those of PTCDA-chex.

Computational structural models.
Density functional theory (DFT) calculations were carried out at the WB97XD level of theory using Gaussian16. 7,8 The standard Def2-SVP basis set was used for geometry optimizations and the expanded Def2-TZVP basis set was used for single point and population density with natural bonding orbital (NBO) calculations. 9 The calculated structures were then depicted using CYLview20 10 .
a. PTCDA-based polymers. Figure S85. DFT-calculated structure of a portion of PTCDA-pPDA, suggesting that the polymer is not fully conjugated.

Tabulated calculated data for polymers and molecular analogues.
Table S12. Theoretical capacities, initial capacities, and capacity retention after 100 cycles for all studied polymers in this work.
The binding energies of the charge compensating ions to the small molecule analogues were estimated based on the observed reduction potentials relative to the TBA + binding energy. The potential shift relative to the reduction potential in the presence of TBA + (ΔE) was used to calculate the binding energy (ΔGbinding) by the following equation: ∆ = ∆ Eq. S4 Table S13. Changes in the second binding energies (ΔGbinding) of each charge balancing ion in comparison to the binding energy of TBA + calculated from Eq. S4.