Electronically Conductive Polymer Enhanced Solid-State Polymer Electrolytes for All-Solid-State Lithium Batteries

Abstract

All-solid-state lithium batteries (ASSLBs) have gained enormous interest due to their potential high energy density, high performance, and inherent safety characteristics for advanced energy storage systems. Although solid-state ceramic (inorganic) electrolytes (SSCEs) have high ionic conductivity and high electrochemical stability, they experience some significant drawbacks, such as poor electrolyte/electrode interfacial properties and poor mechanical characteristics (brittle, fragile), which can hinder their adoption for commercialization. Typically, SSCE-based ASSLBs require high cell stack pressures exerted by heavy fixtures for regular operation, which can reduce the energy density of the overall battery packages. Polymer–SSCE composite electrolytes can provide inherently good interfacial contacts with the electrodes that do not require high cell stack pressures. In this study, we explore the feasibility of incorporating an electronically and ionically conducting polymer, polypyrrole (PPy), into a polymer backbone, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), to improve the ionic conductivity of the resultant polymer–SSCE composite electrolyte (SSPE). The electronically conductive polymer-incorporated composite electrolyte showed superior room temperature ionic conductivity and electrochemical performance compared to the baseline sample (without PPy). The PPy-incorporated polymer electrolyte demonstrated a high resilience to high temperature operation compared with the liquid-electrolyte counterpart. This performance advantage can potentially be employed in ASSLBs that operate at high temperatures. In our recent development efforts, SSPEs with optimal formulations showed room temperature ionic conductivity of 2.5 × 10⁻⁴ S/cm. The data also showed, consistently, that incorporating PPy into the polymer backbone helped boost the ionic conductivity with various SSPE formulations, consistent with the current study. Electrochemical performance of ASSLBs with the optimized SSPEs will be presented in a separate publication. The current exploratory study has shown the feasibility and benefits of the novel approach as a promising method for the research and development of next-generation solid composite electrolyte-based ASSLBs.

1. Introduction

The increasing need for battery energy storage worldwide has led to a continuing effort to advance battery technologies, particularly lithium-ion battery (LIB) technology [1,2,3]. LIBs have a wide range of applications ranging from large-scale, ground-based energy storage stations to portable electronic devices [4] due to their numerous advantages, such as wide operating voltage windows [5], fast charging capabilities, and long lifespan [6]. However, organic liquid electrolytes used in LIBs are highly flammable, which leads to a significant safety risk for applications such as electric vehicles [7]. A potential solution to mitigate this safety concern is to use solid-state electrolytes in LIBs, since solid-state electrolytes are nonflammable and nonvolatile [8], which can significantly reduce the chance of fire and explosion [9].

Solid-state ceramic (inorganic) electrolytes (SSCEs, e.g., lithium lanthanum zirconium oxide Li₇La₃Zr₂O₁₂ or LLZO [10]) can potentially offer high ionic conductivity, wide electrochemical windows, and excellent mechanical strength [11,12]. However, SSCEs are limited in establishing strong interfacial connections with electrodes, which can lead to poor electrochemical performance when used in LIBs [13]. In addition, SSCEs are highly susceptible to cracking or breaking from mechanical stresses due to their brittle and fragile nature [14,15]. SSCE-based ASSLBs also require a high external pressure exerted by heavy fixtures for regular operation, which can significantly reduce the energy density of the overall battery packages [16,17]. On the other hand, polymer–SSCE composite electrolytes (which we label as solid-state-polymer-electrolytes, or SSPEs, for convenience in this paper) can provide inherently good interfacial contact with the electrodes and, therefore, do not require large external pressures for normal operations of the ASSLBs [18]. Moreover, SSPEs have a large electrochemical stability window, excellent electrode compatibility, and high cyclability [19]. Furthermore, SSPEs can lend flexibility to ASSLBs due to their elastic characteristics [20,21].

Polymer electrolytes, such as polyethylene oxide (PEO), poly (propylene oxide) (PPO), polyvinylidene fluoride (PVDF), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), and poly-(vinylidene chloride) (PVDC), and copolymers, such as polyvinylidene fluoride hexafluoropropylene copolymer (PVDF-HFP), have been studied [22]. In particular, PVDF-HFP has been studied in ASSLBs [23] since the HFP side chain in PVDF-HFP helps achieve a high amorphous structure and high tolerance to oxidation from fluorine atoms [24]. However, the ionic conductivity of PVDF-HFP-based polymer electrolytes is limited to a low range of 10⁻⁶–10⁻⁸ S/cm at room temperature [25,26,27]. One approach to boost the ionic conductivity is to incorporate a nonconductive cellulose polymer into the PVDF-HFP backbone [28]. Another approach is to incorporate electronically conductive fillers into the polymer backbone [29], where it was theorized that the electron-rich fillers can help increase the dielectric constant of the resultant composite polymer electrolytes, which increases the transport of cations, such as Li ions, in the resultant composite polymer electrolytes. Wen et al. [30] added graphene oxide (GO) in polyethylene oxide (PEO) to increase the ionic conductivity of the composite polymer electrolyte. They found that the functional groups of GO help immobilize the anions, which helps accelerate the rate of Li-ion migration. Similarly, Yuan et al. demonstrated that electronically conductive GO fillers can help increase salt dissociation and ion transport channels, which significantly improves the ionic conductivity of their PEO-based electrolytes [31]. Guo et al. investigated carbon black fillers in the PEO-based polymer electrolyte [32]. They showed that the carbon fillers help increase the bulk dielectric constant of the polymer backbone, leading to enhanced interfacial charge transfer and bulk ionic mobility.

Electronically conductive polymers, such as PPy, have been used to modify Li-ion battery active materials, such as CoP₃ [33], to alleviate large volumetric changes and to promote charge transfer kinetics. To the best of our knowledge, electronically conductive polymers have not been investigated as electronically conductive fillers in polymer electrolytes. In this exploratory research work, a novel process of preparing SSPEs was investigated where, for the first time, a π-conjugated polymer (PPy), both ionically and electronically conductive, is incorporated into an ionically conductive but electronically insulating PVDF-HFP backbone, labelled as PVDF-HFP-PPy (Figure 1), to improve the ionic conductivity of the resultant polymer–SSCE composite electrolyte, which is the main objective of this investigation. In addition to the general benefits of electronically conductive fillers used in SSPEs, the introduction of both electronically and ionically conductive polymers into the polymer backbones can potentially have the following advantages: (i) more uniformly distributed electronically conductive media than the filler particles; (ii) reduced crystallinity or increased amorphousity of the resultant SSPEs, which helps improve polymer segment mobility, leading to increased ionic conductivity [34]; (iii) increased thermal stability of the resultant SSPEs; (iv) increased electrochemical oxidation stability; and (v) an increased dielectric constant, resulting in an increased overall ionic conductivity of the resultant SSPEs. To enhance the SSPE membrane/electrode interfacial contact and to ensure a robust electronic isolation between the anode and the cathode of the ASSLBs, we developed an interfacial coating method where the SSPE membranes were coated with a thin layer of an interfacial property-enhancing polymer (e.g., PEO) that is ionically conducting but electronically insulating. The PPy-incorporated SSPE demonstrated higher ionic conductivity and superior electrochemical performance when used in ASSLBs compared with a baseline SSPE (without PPy), suggesting the electronically conductive polymer enhanced composite polymer electrolytes hold a promising potential for the research and development of next-generation SSPE-based ASSLBs.

2. Methods and Materials

2.1. Sample Preparation

2.1.1. Preparation of SSPE Membranes

The PVDF-HFP copolymer-based membrane (denoted as SSPE-0) and the PVDF-HFP-PPy-based membrane (denoted as SSPE-1) were prepared by the solution casting method, as tabulated in Table S1 (in Supplementary Materials). Briefly, for the enhanced SSPE (SSPE-1), 0.3 g of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Sigma Aldrich, USA), and 0.1 g of polypyrrole (PPy, Sigma Aldrich) in 2 mL anhydrous N,N-dimethylformamide (DMF, Sigma Aldrich) were sonicated for 1 h at 60 °C. After that, 0.6 g of niobium-doped lithium lanthanum zirconate garnet (LLZO, Li_6.5Nb_0.5La₃Zr₂O₁₂, MSE Supplies) and 0.1 g of bis-(trifluoromethane)sulfonimide (LiTFSI) lithium salt (Sigma Aldrich) were added to the above mixture and stirred with a magnetic stirrer for 10 min at 250 rpm at room temperature inside an argon-filled glove box. The obtained slurry was poured onto a releasing paper (4 × 4 cm) and dried at 60 °C for 6 h in a vacuum oven. The baseline membrane (SSPE-0) was prepared similarly without PPy. These SSPE membranes were punched into 16 mm circular disks for analytical and electrochemical evaluations (Figure S1).

2.1.2. Interfacial Property-Enhancing Polymer Coating on SSPE Membranes

A total of 0.6 g of LLZO, 0.3 g of PEO (Sigma Aldrich), and 0.1 g of LiTFSI in 1.0 mL of acetonitrile (Sigma Aldrich) and 1.0 mL of 1,3-dioxolane (DOL, Sigma Aldrich) were added into a ball-mill jar and sealed with electrical tape inside the Ar-filled glovebox. The mixture was ball-milled for 1 h at 400 rpm. After ball-mill mixing, the PEO solution was transferred again into the Ar-filled glovebox, where a thin layer of PEO was coated on both sides of the dried SSPE membranes by simply dipping these membranes into the PEO solution. After being dried at 40 °C under vacuum for 6 h, the PEO-coated SSPE membranes were used for analytical and electrochemical evaluations.

2.1.3. Fabrication of Ni-Rich-Based Cathodes

Ni-rich-based cathodes were prepared by mixing 0.6 g of LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622, MSE Supplies, USA), 0.3 g of LLZO, 0.05 g of conductive carbon black (Super-P C65, MTI, USA), and 0.05 g of poly(vinylidene fluoride) (PVDF, MW 534,000, powder, Sigma Aldrich) in 2 mL of 1-methyl-2-pyrrolidone (NMP, purity ≥ 99%, Sigma Aldrich) with ball-milling for 1 h at 400 rpm. All materials were added to a ball-mill jar in the Ar-filled glove box and sealed with electric tape to avoid air exposure. The mixed slurry was cast onto carbon-coated aluminum foil using a doctor blade and dried at 60 °C for 6 h in a vacuum oven. The dried NMC622 cathode was punched into 12 mm diameter disks with 60% active material content and ~3.0 mg/cm² cathode loading.

2.2. Physical Characterizations

A Thermo Nicolet iS10 (Thermo Fisher Scientific, USA) with a Smart iTX (Thermo Fisher Scientific) connection using a single bounce, 2 mm diameter diamond crystal attenuated total reflectance (ATR) was used to collect Fourier transform infrared (FTIR) spectra. Scanning electron microscopy (SEM, Helios 5 CX, Thermo Fisher Scientific) was used to compare the surface morphologies of the PVDF-HFP baseline electrolyte membrane and the PVDF-HFP-PPy electrolyte membrane. The crystallinity of the samples was analyzed by an X-ray diffractometer (XRD, Empyrean Series 3, Malvern Panalytical, USA) with a Co Kα radiation source (wavelength = 1.78899 Å, 45 kV, 40 mA). To examine the thermal properties of solid-state electrolytes, a TA Q600 instrument (USA) was used to perform thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a heating rate of 10 °C/min under an argon atmosphere.

2.3. Electrochemical Characterizations

ASSLB coin cells were prepared using the PEO-coated SSPE membranes as the solid-state electrolytes, NMC622-LLZO as the cathode, and lithium metal disks (MTIs) as the anodes. It is worth noting that no liquid solvents or electrolytes were added to the ASSLBs. CR2032 coin cell hardware (MTI) was used to prepare the ASSLBs. The coin cells were assembled inside the glove box under an argon atmosphere with H₂O and O₂ concentrations <0.1 ppm. It should be noted that the only pressure applied to the ASSLBs is the spring pressure of the coin cells; 10–100 kPa [35].

The electrochemical performance of the ASSLB coin cells was galvanostatically charged and discharged using a battery testing system (Neware, China, Model BTS4000) at 80 °C (maintained by a Tenney Environmental Chamber, USA, TUJR-A-F4T). The testing temperature (80 °C) was chosen because it is known that increased temperatures lead to accelerated segmental dynamics and improved ion transport of the SSPEs [36]. At this elevated temperature (80 °C), we evaluate (i) the feasibility and (ii) high temperature tolerance of our SSPE-based ASSLBs. The ASSLB cells were cycled between 2.5 and 4.3 V at 0.05 C-rate for the first two cycles and 0.1 C-rate for further cycles.

Cyclic voltammetry (CV) profiles of the ASSLBs were carried out using a potentiostat/galvanostat electrochemical workstation (Corrtest Instruments Corp., China, Model CS310) with a scan rate of 0.05 mVs⁻¹ between a voltage window of 2.5 and 4.3 V (vs. Li/Li⁺) at 80 °C.

Ionic conductivities of the SSPE/PEO membranes were measured using the electrochemical impedance spectroscopy (EIS) method [37]. The SSPE/PEO membranes were sandwiched between stainless steel (SS) plates. Silver paste was used to bond the SSPE membranes to the SS plates to minimize the contact resistance. It should be noted that the only pressure applied to the membranes was the spring pressure of the coin cells. A potentiostat/galvanostat electrochemical workstation (Corrtest Instruments Corp., Model CS310) was used for EIS at a frequency range of 10⁵ to 10⁻² Hz and an AC voltage of 10 mV at room temperature. The electrical resistivities of the SSPE/PEO membranes were determined by DC resistance measurement.

Dielectric characteristics of the SSPEs were assessed to gain some insights into the electronically conductive polymer on the properties of the bulk electrolyte. The dielectric constant, ${\epsilon }_r^{\prime }$, and the dielectric loss, ${\epsilon }_r^{″}$, were calculated using the following equations [38]:

$${\epsilon }_r^{\prime }=-{\displaystyle \frac{Z^{″}d}{2\pi f{\left|Z\right|}^2{\epsilon }_{0}A}}$$

(1)

$${\epsilon }_r^{″}={\displaystyle \frac{{\epsilon }_r^{\prime }}{tan\theta }}$$

(2)

Here, ε₀ is vacuum permittivity, f is frequency, θ is phase angle, and d and A are thickness and area of the SSPE membranes, respectively. The real (Z′) and the imaginary (Z″) components of the impedance were measured using EIS at a frequency range of 10⁵ to 10⁻² Hz and an AC voltage of 10 mV at room temperature.

Coin cells for linear sweep voltammetry (LSV) of the SSPE membranes were prepared using a 15.8 mm diameter SS disk as the working electrode, the PEO-coated SSPE membranes as the solid-state electrolytes, and a 0.6 mm-thick lithium metal disk as the counter and reference electrode. The LSV of the SSPE membranes was measured using a PalmSens4 workstation (Basi, USA) at a voltage window of 3.0–5.0 V (vs. a Li reference electrode) with a scan rate of 5 mVs⁻¹ at room temperature.

The leakage currents of the SSPE-based ASSLB coin cells were measured by first charging the cells to 4.3 V at a constant current (CC) of 0.05 C-rate, followed by a constant voltage (CV) charge for 2 h. Since the cells were fully charged at the sufficiently low current of 0.005 C-rate, the tapering current during the CV charge was used to indicate the leakage currents of the SSPE-based ASSLBs.

3. Results and Discussion

The electrical resistivity (which is reversely proportional to electrical conductivity) and ionic conductivity of SSPEs with different PVDF-HFP to PPy ratios are summarized in Table S2 (in Supplementary Materials). The SSPE with PVDF-HFP:PPy = 3:1 demonstrated the highest room temperature ionic conductivity among the SSPE samples evaluated. The data suggest that there is an optimal PPy concentration when introduced into the polymer backbone to yield an optimal ionic conductivity. This trend is also observed in polymer electrolytes with formulations described in Table S3 (in Supplementary Materials). We ascribe this optimal PPy concentration to the tendency of PPy to agglomerate at high concentrations. This composition, PVDF-HFP:PPy = 3:1, was used to prepare the SSPE membranes with extensive physical, chemical, and electrochemical characterizations.

3.1. Physical Characterizations

3.1.1. FTIR Analysis of Polymers and SSPEs

FTIR of SSPEs and their polymer components was conducted to identify functional groups of the polymers. Figure 2a shows FTIR spectra of PEO, PPy, PVDF-HFP, PEO-coated PVDF-HFP (SSPE-0/PEO), and PEO-coated PVDF-HFP (SSPE-1/PEO) samples. The FTIR peaks from 871 to 650 cm⁻¹ are attributed to the presence of α-, β-, and γ- phases of the PVDF-HFP copolymer in both SSPE-0 and SSPE-1 membranes. Other peak characteristics of the PVDF-HFP copolymer are assigned to C-F₃ symmetric stretching (1050 cm⁻¹), C-F₂ symmetric stretching (1130 cm⁻¹), C-F asymmetric stretching (1180 cm⁻¹), C-H₂ wagging vibration (1390 cm⁻¹), and C-H stretching (3020–2800 cm⁻¹) [39,40]. The characteristic peak of PEO at 2870 cm⁻¹ is present in the SSPE-0/PEO and SSPE-1/PEO membranes, suggesting the uniform coating of PEO on the polymer electrolyte membranes. The intensity scale of the PPy FTIR spectrum is enlarged by six times. The presence of PPy in the SSPE membranes is not visible in the FTIR spectra with the real intensity scale. In the SSPE-0 and SSPE-1 spectra, the peak at 1660 cm⁻¹ is ascribed to the stretching of the double-bonded sulfur with oxygen (S=O) in the LiTFSI salt [41]. The FTIR data show that there are no new chemical bonds in SSPE-1, suggesting that PPy was physically mixed with the PVDF-HFP backbone.

3.1.2. XRD Analysis of Polymer Membranes

The X-ray diffraction (XRD) patterns on polymer membranes in Figure 2b reveal the crystalline phases for PPy, PVDF-HFP, and PVDF-HFP-PPy membranes where no Li salt and LLZO were added. The α- PVDF-HFP copolymer exhibits distinctive peaks at 21.39° and 23.42°, corresponding to α (1 0 0) and (α + β) (0 2 0) crystal planes, respectively [42]. The XRD pattern of PPy shows broad peaks at 12.93° and 30.06°, corresponding to (0 0 2) and (1 0 2) crystal planes [3], respectively, indicating the amorphous nature of PPy. The XRD pattern of PVDF-HFP-PPy shows characteristic peaks from both PVDF-HFP and PPy, as expected. However, the intensities of these peaks are noticeably reduced. Since it is known that XRD peak intensity is associated with the degree of crystallinity of the material [43], this observation suggests that the introduction of PPy into the PVDF-HFP backbone resulted in a reduced crystallinity or increased amorphousity of the resultant PVDF-HFP-PPy polymer composite. This is consistent with the reported studies where nanowire fillers (analogous to PPy) not only favor the generation of amorphous regions but also offer continuous active pathways along the interfaces for fast Li⁺ transportation and superior ionic conductivity compared to nanoparticles [44,45]. It is widely recognized that the increased amorphous phase helps improve polymer segment mobility [46], leading to increased ionic conductivity. We believe the net effect of increased amorphousity in the final polymer composite (SSPE-1) contributed to the enhanced Li⁺ conduction and the improved performance of SSPE-1-based ASSLBs, as discussed later.

3.1.3. SEM Analysis of SSPEs

Figure 2c,d compare the SEM surface morphologies of the SSPE-0 and SSPE-1 membranes. It can be seen that the introduction of PPy into the PVDF-HFP backbone rendered a much smoother and more uniform surface morphology in SSPE-1 compared with the baseline SSPE-0. We attribute the observed surface morphology of SSPE-1 to the homogeneously mixed PPy to fill in some of the voids in the backbone polymer. We believe the improved SSPE surface morphology contributed to better interfacial contacts and more uniform current distribution, which helps achieve a superior electrochemical performance in the SSPE-1-based ASSLBs, as discussed below.

3.1.4. Thermal Properties of Polymer Membranes

Figure 2e shows thermogravimetric analysis (TGA) of PVDF-HFP and PVDF-HFP/PPy membranes where no Li salt and LLZO were added. It can be seen that onset temperature of the steep-weight-loss (T_onset = 467 °C) of PVDF-HFP/PPy was increased by 25 °C from that of the baseline PVDF-HFP (T_onset = 446 °C). It is interesting to observe that the residual weight percent at the final temperature for PVDF-HFP/PPy is higher than for PVDF-HFP (35% vs. 20%). These observations are consistent with reported work where epoxy resin-based ceramic fillers significantly slowed down the decomposition rate and increased the residual weight of the composite polymer [47]. It is known that temperature changes can have a significant impact on the mechanical properties of polymers [48]. As the temperature increases, the polymer’s Young’s modulus typically decreases [49], making the material more flexible and less stiff (more prone to cracking). The increased thermal stability with the incorporation of PPy into the polymer backbone offers an additional advantage for our approach, since SSPEs play a critical role in the thermal stability of SSPE-based ASSLBs [50]. Figure 2f shows differential scanning calorimetry (DSC) curves of SSPE-0 and SSPE-1. Both membranes exhibited an endothermic peak at 146 °C, corresponding to their melting point. Interestingly, the endothermic peak of SSPE-1 is substantially smaller than SSPE-0, which is ascribed to the increased amorphous nature of SSPE-1 compared to the baseline SSPE-0. There are no significant exothermic reactions (due to polymer decompositions) of the two types of SSPE samples until 460 °C, indicating the high thermal stability of these SSPEs.

Figure 2. (a) FTIR of SSPEs, (b) XRD of SSPEs, (c,d) SEM images of SSPE−0 and SSPE−1, respectively, (e) TGA, and (f) DSC of SSPEs, polymer membranes, and polymer components, as indicated.

Figure 2. (a) FTIR of SSPEs, (b) XRD of SSPEs, (c,d) SEM images of SSPE−0 and SSPE−1, respectively, (e) TGA, and (f) DSC of SSPEs, polymer membranes, and polymer components, as indicated.

3.2. Electrochemical Characterizations

3.2.1. Ionic Conductivity of SSPEs

Table 1. Room temperature ionic conductivities and electrical resistivities of PEO-coated SSPEs.

Sample ID	PEO-Coated SSPE	Electrical Resistivity r (Ω × cm)	Ionic Conductivity s (S/cm)
SSPE-0/PEO	PVDF-HFP/PEO	4.48 × 107	1.8 × 10−6
SSPE-1/PEO	PVDF-HFP-PPy/PEO	4.30 × 107	4.5 × 10−6

summarizes ionic conductivities and electrical resistivities (which are reversely proportional to electrical conductivities) of PEO-coated SSPE membranes (SSPE/PEO) measured at room temperature (RT), where LLZO was used as the ceramic solid electrolyte (CSSE) in the SSPEs. The ionic conductivity of SSPE-0/PEO was 1.8 × 10⁻⁶ S/cm. After incorporating the functional PPy into the polymer backbone, the ionic conductivity of SSPE-1/PEO increased by 2.5 times (4.5 × 10⁻⁶ S/cm). It is worth noting the only pressure applied to the membranes is the spring pressure of the coin cells. The increased ionic conductivity of SSPE-1/PEO is ascribed to the reduction of crystallinity (increase of amorphousity) of the backbone copolymer with the addition of PPy (Figure 2b), because higher ionic conductivity is generally associated with higher segment mobility of the amorphous phase in the polymer electrolytes [51,52]. As can be seen from Table 1, the electric resistivities of SSPE-0/PEO and SSPE-1/PEO were comparable, indicating the effectiveness of the thin PEO layer coating to ensure robust electronic separation between the anode and the cathode in ASSLBs. In our recent development efforts, SSPEs with optimal formulations showed a room temperature (23 °C) ionic conductivity of 2.5 × 10⁻⁴ S/cm (see Table S3 in Supplementary Materials). This conductivity value is comparable to those in the literature [32,53]. The data also showed, consistently, that incorporating PPy into the polymer backbone helped boost the ionic conductivity with various SSPE formulations, consistent with the current work. Electrochemical performance of the optimized SSPE-based ASSLBs will be presented in a separate publication.

3.2.2. Linear Sweep Voltammetry (LSV) and Leakage Current Measurement of SSPEs

The linear sweep voltammetry (LSV) was conducted to assess the oxidative stability of the PEO-coated SSPE membranes. Li/SSPE/SS cells were used for LSV at room temperature. Figure 3a shows both SSPEs exhibited an oxidation onset voltage V_onset ~4.5 V (vs. Li). However, the SSPE-0 baseline showed a much sharper rise in current beyond V_onset compared with SSPE-1, suggesting an improved oxidation stability of the PPy-enhanced SSPE-1. It is noted that the V_onset value can depend on the scan rate. However, since different SSPE samples were under the same scan rate, the difference in V_onset can serve as an indicator of relative oxidation stability between the two different SSPE samples. Shin et al. reported that their polymer electrolytes with oxide fillers exhibited a higher electrochemical oxidation potential at 90 °C [54]. The data suggest PPy may serve equivalently as a filler in the composite polymer electrolyte, rendering a similar benefit of electrochemical stability enhancement in SSPE-1 as the oxide fillers.

To determine the electronic isolation property of the SSPEs coated with PEO, the Li/NMC622 cells were further investigated with leakage current measurement, where the leakage current (tapering current) was measured as a function of time at a constant charge voltage of 4.3 V. Figure 3b shows the leakage current for both SSPE-0- and SSPE-1-based cells decayed vs. time at a similar rate and magnitude. This observation suggests the PPy-enhanced SSPE-1/PEO has a similar electronic isolation property to the baseline SSPE-0 when used in ASSLBs. This result shows the effectiveness of the PEO interfacial coating on the electronically conductive PPy-enhanced SSPE-1 to afford a robust electronic isolation between the anode and the cathode in the SSPE-1-based ASSLB cells.

3.2.3. Dielectric Constant and Dielectric Loss of SSPEs

Figure 3c,d show SSPE-1/PEO exhibited a ~10 times higher dielectric constant and ~2 times higher dielectric loss at low frequencies than the SSPE-0/PEO baseline, respectively. The higher dielectric constant is attributed to the increased polarization of the polymer matrix by the free mobile charges provided by the conductive media (PPy) [38,55], which can help larger salt charge dissociation, resulting in enhanced interfacial and charge transfer properties, whereas the higher dielectric loss of heat dissipation can help enhance the amorphous phase of the polymer matrix, which also helps ionic mobility [28].

Figure 3. (a) Linear sweep voltammograms (LSV), (b) leakage current, (c) dielectric constant, and (d) dielectric loss of the SSPEs.3.2.4. Electrochemical Performance of SSPE-Based ASSLB Coin Cells.

Figure 3. (a) Linear sweep voltammograms (LSV), (b) leakage current, (c) dielectric constant, and (d) dielectric loss of the SSPEs.3.2.4. Electrochemical Performance of SSPE-Based ASSLB Coin Cells.

Figure 4a shows the initial charge/discharge voltage profiles of Li/NMC622 coin cells using the baseline SSPE-0 (PVDF-HFP) and the PPy-incorporated SSPE-1 (PVDF-HFP-PPy) membranes. The SSPE membranes were coated with PEO. No liquid solvents or electrolytes were added to these ASSLB coin cells. No extra pressure, other than the coin cell spring pressure, was applied to the ASSLBs. The ASSLB coin cells were initially cycled between 2.5 V and 4.3 V at a 0.05 C-rate at 80 °C for the first two cycles. The SSPE-1-based ASSLB coin cells showed a consistently higher discharge specific capacity than the SSPE-0 counterpart, exemplified by their representative performance displayed in Figure 4a, where the SSPE-1-based cell delivered a specific capacity of >121 mAh/g, which is nearly 20% higher than the SSPE-0-based cell (104 mAh/g). The superior ability of the SSPE-1-based ASSLBs to deliver a higher specific capacity is attributed to increased ionic conductivity (Table 1) due to an increased amorphous phase (Figure 2b), resulting in an increased polymer segmental mobility and increased dielectric constant (Figure 3c).

Figure 4b shows cyclic voltammetry (CV) of the Li/NMC622 cells using SSPE-0 and SSPE-1 membranes. The cells were scanned at 0.05 mVs⁻¹ between 2.5 and 4.3 V (vs. Li/Li⁺) at 80 °C. The SSPE-0-based cell showed oxidation and reduction peaks at 4.1 and 3.4 V, respectively. In comparison, the SSPE-1-based cell showed oxidation and reduction peaks at 3.9 and 3.5 V, respectively. Thus, the SSPE-1-based cell showed a smaller potential difference (ΔE = 0.4 V) between oxidation and reduction peaks than the SSPE-0 (ΔE = 0.7 V), indicating improved reaction kinetics in the SSPE-1-based ASSLB cells.

After the two formation cycles at a 0.05 C-rate, the ASSLB coin cells were cycled at a higher current of 0.1 C-rate for extended cycles to expedite the cycle data acquisition. Figure 4c shows the SSPE-1-based ASSLB cells exhibited consistently higher discharge specific capacity vs. cycling than the SSPE-0 baseline, maintaining 86% capacity retention after 30 cycles. Furthermore, the SSPE-1-based ASSLB cells also exhibited higher Coulombic efficiency than the SSPE-0 baseline during cycling. The superior electrochemical performance of SSPE-1-based ASSLB cells is attributed to an increased dielectric constant, ionic conductivity, and oxidation stability of SSPE-1 as a result of incorporating the electronically conductive polymer (PPy) into the polymer backbone.

Figure S2 (in Supplementary Materials) shows cycle performance of a representative Li/NMC622 coin cell with conventional Li-ion battery liquid electrolyte, 1M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 vol%, cycled at 2.5–4.3V at 80 °C. The liquid electrolyte-based cell experienced a rapid capacity fade vs. cycling when cycled at 80 °C. In contrast, the SSPE-1-based ASSLB coin cells demonstrated the capability of cycling stably at as high as 80 °C, highlighting an important advantage of our approach that might potentially be applied to applications (automotive, medical, industrial, military, etc.) that require a resilience of battery operation at high temperatures [56].

Figure 4. (a) Voltage profiles, (b) CV, and (c) cycle performance of SSPE−based ASSLB coin cells.

Figure 4. (a) Voltage profiles, (b) CV, and (c) cycle performance of SSPE−based ASSLB coin cells.

4. Conclusions

A novel process of preparing SSPEs was investigated in this exploratory study where, to the best of our knowledge, for the first time, an ionically and electronically conductive polymer (PPy) was incorporated into an ionically conductive polymer backbone (PVDF-HFP). The novel SSPE demonstrated improved material and electrochemical properties: (i) improved surface morphology, (ii) increased thermal stability, (iii) increased electrochemical oxidation stability, and (iv) an increased dielectric constant, leading to an increased overall ionic conductivity of the resultant membranes. Thin film coatings of a nonelectronic conducting polymer (PEO) on the SSPEs provided good interfacial contacts between the SSPE and the battery electrodes. The novel SSPE-based Li/NMC622 ASSLB cells demonstrated superior electrochemical reaction kinetics, discharge specific capacity, and cycle performance than the baseline. The SSPE-based ASSLB coin cells demonstrated a high resilience to high temperature operation compared with the cells with a liquid electrolyte counterpart, highlighting an important advantage of our approach to be potentially applied to applications that require battery operation at high temperatures. In our recent development efforts, SSPEs with optimal formulations showed a room temperature ionic conductivity of 2.5 × 10⁻⁴ S/cm. The data also showed, consistently, that incorporating PPy into the polymer backbone helped boost the ionic conductivity with various SSPE formulations, consistent with the current study. This is very encouraging, since the drastically increased ionic conductivity of the optimized SSPE, by two orders of magnitude, is expected to significantly improve the room-temperature or low-temperature performance of ASSLBs using the optimized, high ionic conductive SSPEs. The current exploratory study has shown the feasibility and benefits of the novel approach as a promising method for the research and development of next-generation SSPE-based ASSLBs.