Evaluation on electrochemical properties of lithium ions battery-based PMMA-PLA blend incorporation of [EDIMP]TFSI hybrid gel polymer electrolyte

doi:10.21203/rs.3.rs-3089836/v1

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Evaluation on electrochemical properties of lithium ions battery-based PMMA-PLA blend incorporation of [EDIMP]TFSI hybrid gel polymer electrolyte

https://doi.org/10.21203/rs.3.rs-3089836/v1

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This study focuses on developing a novel hybrid gel polymer electrolyte (HGPEs) for lithium-ion batteries. The HGPEs are composed of a hybrid polymer of 80 wt.% PMMA and 20 wt.% PLA, doped with 20 wt.% LiTFSI and incorporated with various contents ionic liquid namely Ethyl-dimethyl-propylammonium bis(trifluoromethylsulfonyl)imide ([EDIMP]TFSI) are successfully prepared and the lithium-ion batteries performance were investigated. This work aims to investigate the influence of the ionic liquid on the electrical properties, cation transference number (t_Li⁺), electrochemical stability window, and charge-discharge performance of the PMMA-PLA-LiTFSI HGPE system. Among the different samples tested, the HGPE containing 20 wt.% [EDIMP]TFSI (E-TFSI 20) exhibited the most promising results. It achieved an optimum ionic conductivity of 3.90 × 10^− 3 S cm^− 1, an increased t_Li⁺ from 0.63 to 0.79, and an extended electrochemical stability window from 4.3V to 5V. Temperature dependence studies revealed that all the HGPE systems followed the Arrhenius characteristic, and their activation energies were calculated. Dielectric studies revealed ionic behavior and suitable capacitance with varying frequencies of the HGPEs system. The most favorable electrolyte was selected based on the highest ionic conductivity observed in each HGPE system. It was utilized in a Li metalǀHGPEsǀGraphite cell configuration. The discharge capacity of the cells using LiTFSI 20 and E-TFSI 20 electrolytes were measured as 152.06 mAh g^− 1 and 71.15 mAh g^− 1, respectively, at a current density of 3.72 A g^− 1.

Ionic liquid

lithium transference number

charge-discharge profile

Hybrid polymer

Lithium-ion batteries (LIBs) based organic liquid electrolyte has been widely employed in consumer and automotive applications. Although the high ionic conductivity of organic liquid electrolyte could potentially create a high energy density and cost-effective battery, it suffers from multiple fatal issues such as thermodynamically unstable, leakage, and flammability which can lead to explosion [1, 2]. These problems have raised concern for the safety usage of LIBs among consumers. The development of polymer electrolytes has been widely investigated as a replacement for liquid electrolytes since their first discovery by Fenton in 1973 [3]. Polymer electrolytes possess properties almost promising as liquid electrolytes, with additional advantages such as easy processability, safety security, good mechanical flexibility, and high ionic conductivity [4–8].

Polymer electrolytes need to meet specific requirements to be suitable for use as electrolytes in LIBs. These requirements include (a) high room temperature ionic conductivity (≤ 10^− 3 S cm^− 1), (b) significant ionic transference number, (c) excellent mechanical strength, (d) broad electrochemical stability, and (e) sufficient dimensional stability to function as separators in batteries[9–11]. The first reported solid-state polymer electrolytes based on polyethylene oxide (PEO) and alkali metal salts have been found to exhibit low ionic conductivity [12]. This limitation has prompted research into gel polymer electrolytes (GPEs) as an intermediate state between liquid and solid electrolytes. GPEs can potentially enhance both safety and ionic conductivity, addressing the limitations of traditional solid-state polymer electrolytes [13]. GPEs are also believed to be the most suitable to act as separators in batteries as they comprise polymer matrix, lithium salt, and plasticizer/ionic liquid. In GPEs, the addition of plasticizer has swollen the polymer matrix in liquid electrolytes, leading them to achieve both characteristics of solid and liquid electrolytes, giving them advantages in device applications [14].

Several good supporting polymer matrix systems have been explored to prepare high-performance GPEs [15–19]. Among approaches that have been developed, polymer blending was chosen and prepared through solvent casting. This method is a simple and versatile procedure to prepare various polymer matrices to facilitate ion transport. From our previous work, we found that blending polymethyl methacrylate (PMMA) and polylactic acid (PLA) using a composition weight ratio of 80:20 could contribute more vacancies for ion transportation in a polymer matrix, which results in the enhancement of ionic conductivity from ≤ 10^− 12 S cm^− 1 (pure PMMA) to 10^− 6 S cm^− 1 (PMMA-PLA hybrid polymer) to 10^− 3 S cm^− 1 (PMMA-PLA-20wt.% LiTFSI HGPE) [20]. One of the crucial factors in determining the possible applications of GPEs in LIBs is the Li⁺ cation contribution to the total charge transport in the electrolyte complexes. The low lithium-ion transference number (t_Li+) of LIBs can limit their performance. It can be defined as the proportion of ionic conductivity the lithium-ion provides rather than its counterion [21]. According to Zhao et al. [22], high t_Li+ is better for electrolyte stability to the lithium sheet and battery rate performance. Tsao et al. [23] prepared a polymer membrane without and with the ionic liquid (IL) group-based gel polymer electrolyte using solution casting methods. They reported that adding IL can boost the t_Li+ because it limits the transportation of anions in the electrolyte complexes, thereby providing promising pathways for Li⁺ ions. Thus, it is expected will contribute to excellent battery performance. Furthermore, ILs have gained enormous attention among researchers due to their exciting and potentially practical physicochemical features, including excellent chemical and electrochemical stability, high operating voltage, non-flammability, low vapour pressure, as well as high ionic conductivity [24, 25].

In this study, we prepared homogeneous HGPEs, which compose of PMMA, PLA, lithium bis (trifluoromethylsulfonyl) imide salt (LiTFSI), and Ethyl-dimethyl-propylammonium bis(trifluoromethylsulfonyl)imide ([EDIMP]TFSI) as an ionic liquid. The HGPEs have been prepared by casting method. Electrical impedance spectroscopy (EIS) was employed to measure the electrical properties of the HGPE systems. The lithium-ion (Li⁺) transference number was determined by combining AC impedance measurements with DC polarization techniques. Additionally, linear sweep voltammetry (LSV) was utilized to ascertain the electrochemical stability window of the HGPE systems. It was found that adding IL to the PMMA-PLA-LiTFSI-based HGPE systems increased the t_Li+ and extended the potential window stability of the electrolyte. The cycles performance of the Li|20 wt.% LiTFSI HGPE|graphite and Li|20 wt.% [EDIMP]TFSI|graphite cells have been investigated at different current densities of 3.72 and 11.16 A g^− 1.

Preparation of hybrid gel polymer electrolytes with the incorporation of IL

The components the HGPE are PMMA, PLA, LiTFSI, and [EDIMP]TFSI, which are known as non-flammable and non-volatile; ionic liquids, in particular, have low vapor pressure and high safety performance, as can be seen in Fig. 1. The solution casting method prepared hybrid gel polymer electrolytes containing [EDIMP]TFSI. The PLA was firstly dissolved in the 0.8 ml tetraethylene glycol dimethyl ether (TEGDME) at 120 ℃. After a clear solution was obtained, it was cooled at room temperature before adding it to 20 wt.% LiTFSI salt and various amounts of [EDIMP]TFSI (5 to 35 wt.%). The solution was continued to stir until it completely dissolved. Lastly, PMMA polymer was added to the composite solution and continued stirring at 70 ℃. The polymer solution was again left at room temperature and stirred vigorously until a uniform and viscous solution was obtained. The HGPEs have been stored under an argon atmosphere before future analysis. The sample preparation and designation of HGPEs are demonstrated in Fig. 1.

Figure 1 Schematic diagram of HGPE preparation using solution casting method and its designation.

Preparation of graphite anode

The graphite anode was fabricated by applying a slurry onto a graphite sheet. The slurry was composed of a mixture of graphite, super P, and poly(vinylidene fluoride) (PVDF) binder in N-methyl pyrrolidone (NMP) solvent, with a weight ratio of 80:10:10. The graphite sheet was initially cut to match the size of the coin cell. Then the slurry was applied using a dropping technique. The electrode was subsequently dried in a vacuum oven at 120°C overnight.

Characterization of HGPE-based IL

Ionic conduction measurement

To determine the ionic conductivity values, impedance spectroscopy was employed. The HGPE was placed between two stainless steel (SS) electrodes within a coin cell configuration of SS|HGPEs|SS. Impedance measurements were conducted using a HIOKI 3532-50 LCR Hi-TESTER, applying an AC amplitude of 10 mV. The frequency range for the measurements spanned from 1 MHz to 50 Hz. To explore the influence of temperature on the ionic conductivity of the HGPE, the tests were carried out across a temperature range of 303 K to 393 K for each sample. This allowed for an investigation of the HGPEs ionic conductivity at elevated temperatures.

Cation Transference Number (t_Li+) measurement

The lithium-ion transport number of hybrid gel polymer electrolytes was measured using a Li|HGPE|Li cell configuration, where lithium metal acted as non-blocking electrodes. The methodology described was determined according to the method described by Evans [26] to calculate t_Li+, which involved a combination of DC potentiostatic measurements and AC impedance spectroscopy. To begin, the HGPE was polarized at 100 mV using DC polarization. The polarization process could be characterized by measuring the initial current (Io) and steady-state current (Iss). Subsequently, impedance spectroscopy was performed over a frequency range of 100 kHz to 10 mHz to analyze the film resistance of the HGPE before (R_o) and after (R_ss) the polarization tests. By integrating the data obtained from both measurements, the t_Li⁺ was calculated using a specific relation:

$${t}_{{Li}^{+}}=\frac{{I}_{s}(\varDelta V-{I}_{o}{R}_{o})}{{I}_{0}(\varDelta V-{I}_{ss}{R}_{ss})}$$

where ΔV is the applied voltage across the HGPE during DC polarization. This approach allowed for the determination of t_Li⁺ and its contribution to the overall behavior of the HGPE.

Linear Sweep Voltammetry (LSV)

The electrochemical stability of HGPEs was investigated using linear sweep voltammetry. For this purpose, coin cells were assembled with HGPE samples sandwiched between SS electrodes as the working electrodes, while lithium metal served as the counter and reference electrode. The assembly process was conducted within an inert dry box. During the experiments, the scan rate was set to 10 mV s^− 1, and the electrochemical behavior of the HGPE was analyzed through the applied voltage sweep. This allowed for assessing the HGPE's electrochemical stability and provided insights into its performance under varying voltage conditions from 2.8 to 6.5 V.

Cell fabrication and Battery testing

For the assembly of the half-cell lithium-ion battery, the highest conducting HGPE samples were placed between the graphite anode and the lithium metal cathode, as illustrated in Fig. 2. All the cell assembly procedures were conducted within a dry box filled with argon gas to ensure a controlled and moisture-free environment.

Figure 2 Arrangement of half-cell graphite electrode

The coin cell configuration used in this study was Li metal|HGPEs|graphite, where the lithium metal acted as the anode, the HGPE served as the electrolyte, and graphite was used as the cathode material. To house the cells, CR 2032 coin cell holders were employed. The charge-discharge performance of the cells was evaluated under two different current densities: 3.72 A g^− 1 and 11.16 A g^− 1. The tests involved cycling the cells between 0.01 V and 2 V for 10 cycles. These experiments allowed for examining the cell's performance, capacity retention, and cycling stability under different operating conditions.

Ionic conduction properties studies

Figure 3 presents the frequency dependence of the AC conductivity for the HGPEs with and without ionic liquid (IL) at room temperature. The plot reveals two distinct regions characterized by different conductivity behaviors. The low-frequency dispersive region is attributed to electrode polarization effects at the blocking electrodes. At lower frequencies, a significant amount of charge accumulates at the interface between the electrode and the electrolyte. This accumulation reduces the mobility of ions, consequently leading to a decrease in conductivity.

On the other hand, the frequency-independent plateau region represents the DC conductivity. At higher frequencies, the period of the applied field is too short for significant charging to occur at the electrode-electrolyte interface. As a result, the AC conductivity tends to stabilize and assume a frequency-independent value equivalent to the DC conduction. By extrapolating the DC plateau on the conductivity axis, the DC conductivity of the system can be determined. This extrapolation allows for estimating the conductivity under steady-state conditions, providing valuable insights into the material's overall conductive properties [27].

It also can be seen from Fig. 3(a) that the dc conductivity increases from 10^− 4 S cm^− 1 optimum value of 1.02 × 10^− 3 S cm^− 1, corresponding to a sample containing 20 wt.% of LiTFSI. This phenomenon might be due to the number of charge carriers increased by dissociating ion pairs and ion aggregates [Li⁺---TFSI⁻] that improve ionic conductivity. The highest conducting sample from PMMA-PLA-LiTFSI (LiTFSI 20) HGPEs were chosen for further improvement by adding various content of IL, namely [EDIMP]TFSI, as illustrated in Fig. 3(b). It was shown from the figure that the ionic conductivity raised up to 3.90 × 10^− 3 S cm^− 1 with an increase of IL content up to 20 wt.% [EDIMP]TFSI (E-TFSI 20). The possible reason for such behavior can be discussed due to the increment in mobile charge species generation by the IL. The IL contains mobile EDIMP⁺ and TFSI⁻ ions, thereby augmenting the pool of charge carriers responsible for conduction. The increased ionic conductivity observed in the HGPEs can be attributed to the exceptional self-dissociation and ion transport properties demonstrated by [EDIMP]TFSI. This indicates that [EDIMP]TFSI can dissociate and transport ions effectively, thereby enhancing ionic conductivity in the HGPEs system [28].

Moreover, the incorporation of the IL into the system introduced supplementary pathways that facilitated the movement of ions along the surface of the polymer matrix. This additional network of pathways contributed to the enhanced ion mobility within the hybrid polymer matrix [29]. This was made possible by the proximity of the ions to each other. As a result, the mobile charge carriers could migrate within the polymer matrix. Moreover, the Li + and TFSI- ions could also migrate, leading to increased ionic conductivity due to their enhanced mobility [30]. In addition, the ionic liquid has a plasticization effect on the system, resulting in increased flexibility of the polymer chains. This enhanced flexibility promotes the movement of ions within the polymer backbone, thereby contributing to improved ion conductivity [31]. However, it should be noted that beyond a 20 wt.% concentration of the ionic liquid, the ionic conductivity starts to decrease. This decrease can be attributed to the restricted movement of ions within the rigid polymer matrix, limiting their ability to contribute to ion conductivity [15, 32].

Figure 3 Variation of AC conductivity with frequency of (a) PMMA-PLA-LiTFSI and (b) PMMA-PLA-LiTFSI-IL based HGPEs at room temperature.

To understand the ionic conduction mechanism, the temperature dependence of electrical conductivity was investigated across a range from ambient temperature to 393K. Figure 4(a) depicts the temperature-dependent conductivity for the PMMA-PLA-based hybrid polymer electrolytes with varying LiTFSI contents, while Fig. 4(b) illustrates the conductivity for different [EDIMP]TFSI contents. In both figures, no significant or abrupt changes in ionic conductivity were observed as the temperature increased. This absence of sudden changes suggests the absence of phase transitions within the investigated temperature range for all the polymer electrolyte complexes. The smooth and gradual changes in conductivity with temperature indicate a continuous and stable ionic conduction mechanism throughout the temperature range studied [33, 34].

Figure 4 Temperature dependence studies of (a) LiTFSI various content and (b) [EDIMP]TFSI various content at 303K to 393K.

In both cases, an increase in ionic conductivity with temperature was observed, which can be attributed to the expansion of the polymer matrix. This expansion creates additional free volume and empty spaces, providing pathways for ion migration. It is important to note that this phenomenon indicates the decoupling of charge carriers in the HGPEs from the segmental motion of the polymer chains. As a result, ion transport primarily occurs through the gel electrolyte rather than along the polymeric chains [35]. This behavior is commonly observed in liquid electrolytes and gel-type polymer electrolytes. Furthermore, as the temperature rises, the vibrational energy of polymer segments becomes sufficient to overcome the hydrostatic pressure imposed by neighboring atoms. This allows the segments to create small regions of increased space, enabling enhanced vibrational motion.

Consequently, the ionic conductivity increases with temperature. The difference in ionic conductivity values between Fig. 4(a) and Fig. 4(b) can be attributed to the appearance of new ionic carrier mobilities, such as [Li⁺---TFSI⁻] and [Li⁺---TFSI⁻, EDIMP⁺---TFSI⁻], respectively. The regression values being close to unity suggest that the temperature-dependent ionic conductivity of both HGPEs systems, with and without IL, follows the Arrhenius characteristic. This empirical relationship has been reported in previous studies and provides a suitable description of the conductivity behavior with changing temperatures [36–38].

Table 1 presents the Arrhenius parameters for the HGPEs system based on PMMA-PLA-LiTFSI and PMMA-PLA-LiTFSI-IL. In the HGPEs, the Li⁺ ions tend to dissociate from the polymer chain, allowing other Li⁺ ions from the surrounding environment to occupy the vacant sites with polymer chain segmental motion. The movement and coordination of Li⁺ ions within the polymer complex require energy, referred to as activation energy (E_a). The table indicates that as the content of both LiTFSI and [EDIMP]TFSI increases, the E_a values decrease. For PMMA-PLA-LiTFSI, E_a decreases from 7.44 × 10^− 2 to 5.99 × 10^− 2 eV, while for PMMA-PLA-LiTFSI-IL, E_a decreases from 3.18 × 10^− 2 to 2.79 × 10^− 2 eV. This decrement pattern can be explained by introducing lithium salt and IL, which leads to an increased amorphous region within the polymer matrix. The amorphous phase's presence implies that the particles' arrangement in the polymer matrix is less organized. This arrangement allows the attractive bonds between molecules to break more quickly, requiring less energy for ion hopping and coordination with the ester group of the hybrid polymer [39]. It can be concluded that the hopping process is more favorable with a lower E_a value, indicating that a lower E_avalue corresponds to higher ionic conductivity in the PMMA-PLA hybrid polymer when LiTFSI is used as a dopant salt and [EDIMP]TFSI is employed as the IL.

Table 1 List of ionic conductivity, σ, regression value, R², and activation energy, E_a for HGPEs system with and without IL.

The dielectric permittivity of the HGPEs was investigated to evaluate their capacity for storing electrical energy in an electric field. Fig. 5 presents the dielectric constant (ε_r) and dielectric loss (ε_i) as a function of different LiTFSI and [EDIMP]TFSI contents in the HGPEs at selected frequencies. The figures indicate that both dielectric permittivity parameters increase with the addition of lithium salt and IL to the hybrid polymer systems, enhancing the HGPE's ability to store electrical energy. This increment in dielectric permittivity can be attributed to the increased density of charge carriers provided by LiTFSI and [EDIMP]TFSI, which enables the charges to move within the polymer chain and contribute to its electrical conductivity [28,40]. Furthermore, the plots closely follow the trends observed in ionic conductivity, as discussed earlier. However, beyond 20 wt.% of LiTFSI and [EDIMP]TFSI, a decline in both ε_r and ε_i is observed. This can be explained by the reassociation of ions and the formation of neutral ion pairs, which reduces the number of charge carriers and consequently diminishes the HGPE's ability to store electrical energy [41,15]. Another factor contributing to the decrease in ε_r and ε_i is the tendency of dipoles in the macromolecules to align themselves in the direction of the applied electric field, especially within the low-frequency range. This alignment of dipoles can limit the material's ability to store electrical energy [42].

Figure 5 Dielectric properties studies of HGPEs system for without and with IL (a) and (b) dielectric constant, and (c) and (d) dielectric loss, respectively.

Table 1

List of ionic conductivity, σ, regression value, R², and activation energy, E_a for HGPEs system with and without IL.
wt.%	LiTFSI			[EDIMP]TFSI
wt.%	σ (S cm^− 1)	R²	E_a (× 10^− 2 eV)	σ (S cm^− 1)	R²	E_a (× 10^− 2 eV)
5	2.03 × 10^− 4	0.99	7.44	1.56 × 10^− 3	0.96	3.18
10	5.63 × 10^− 4	0.99	7.34	1.86 × 10^− 3	0.98	3.09
15	6.45 × 10^− 4	0.99	6.85	2.78 × 10^− 3	0.96	2.95
20	1.02 × 10^− 3	0.97	5.99	3.90 × 10^− 3	0.97	2.79
25	7.90 × 10^− 4	0.99	6.68	3.77 × 10^− 3	0.98	2.81
30	5.75 × 10^− 4	0.99	7.22	3.56 × 10^− 3	0.96	2.89
35	5.39 × 10^− 4	0.97	7.28	3.15 × 10^− 3	0.96	3.40

Lithium-ion transport number studies

The transference number indicates the correlation between the surface polarity of materials and their ion transport property [43]. In addition, a higher Li⁺ transport number can reduce concentration polarization during charge-discharge cycles and sustain the power density of the electrolyte [44, 45]. Therefore, the t_Li⁺ of the highest conducting HGPEs sample, both with and without IL, was measured at room temperature. Incorporating [EDIMP]TFSI into the PMMA-PLA-LiTFSI complex significantly impacts the system's ionic conductivity, resulting in an increase in the total current involved and generated. This enhancement in conductivity can be attributed to the contribution of Li⁺ ions in the polymer complexes. To measure the t_Li⁺, the LiTFSI 20 and E-TFSI 20 samples were placed between lithium metal electrodes, as illustrated in Fig. 6. Lithium metal electrodes were chosen as non-blocking electrodes since the charge carriers in this HGPEs system are lithium ions (Li⁺). This electrode configuration allows for the passage of Li⁺ through the electrode, enabling the measurement of the number of Li⁺ transports.

Figure 6. Schematic diagram of Li⁺ transport number measurements of HGPEs using lithium metal as blocking electrodes.

The t_Li⁺ can be evaluated by measuring the final steady-state current (I_ss) and initial current (I_o) under a constant potential bias. For ideal electrolytes under nominal polarization voltage, the cations transport can be calculated as t_Li⁺ = I_ss/I_o [46]. However, there is a change in interfacial resistance has occurred during impedance studies. To ensure the accuracy of the measurements, certain modifications to the method were implemented to account for the interface between the electrolyte and the lithium metal. Bruce and Vincent have introduced new techniques to determine the number of lithium-ion transports in polymer complex-based HGPEs by combining AC impedance and DC polarization measurements [26, 47–49]. Before the DC polarization test, the AC impedance response of the Li|HGPE|Li cell was analyzed. A constant voltage pulse (ΔV) was applied to the cells until the polarization current reached a steady state, and subsequently, the impedance response of the cells was re-examined. A double AC measurement was employed to determine the resistances, measuring the resistance before (R_o) and after (R_ss) the DC polarization.

Figure 7 depicts the impedance plot of HGPEs system without and with the presence of IL, respectively, whereas inset the figures show the current polarization results. The t_Li⁺ of this study was calculated using Eq. (1), and it was found that the addition of IL into the HGPEs system has improved the t_Li⁺ from 0.62 to 0.79. The enhancement of t_Li⁺ to a higher number could be attributed to the strong electrolyte absorption (refers to the ability of the electrolyte to absorb and retain ions) through the cell and molecular interactions between the hybrid polymer and the charged ions (Li⁺---TFSI⁻ and EDIMP⁺---TFSI⁻) in the polymer complex [50]. Moreover, adding [EDIMP]TFSI may favor the re-dissociation of ion pairs and weaken the interactions involving Li⁺ and the functional group of hybrid polymer, thus boosting the number of free Li⁺ ions [51]. This flexible and delocalized nature also facilitates the transport of Li⁺ ions in the polymer backbone. However, according to Ghosh et al. [52], achieving a high transference number of lithium ions without any anion trapper additives is unexpected since they believed a large anion molecular size could influence the lithium ion transport number.

Meanwhile, Appetecchi et al. [53] claimed that the t_Li⁺ can be achieved as high as 0.8 by studying the unique interaction between host polymer (PMMA or PAN), solvent (EC and PC), and lithium salt (LiTFSI). Furthermore, Shah et al. investigated the effect of anion size on transference number based three different lithium salt (LiFSI, LiTFSI, and LiBETI). They found that the t_Li⁺ was above 0.6 for all the electrolytes and was increasing function of anion size [54]. Moreover, Libo et al. reported t_Li⁺ of LiTFSI-P(VdF-HFP) based HGPE without and with IL revealed 0.8 for both systems [55]. Another finding was found with higher t_Li⁺ of 0.76 and 0.62 based hybrid gel polymer electrolyte system reported by Li et al. [56] and Isa et al. [57], respectively. Hence, it can be concluded that the present results are compatible with previous studies.

Figure 7 Lithium transference number of Li/HGPE/Li for (a) LiTFSI 20 and (b) E-TFSI 20.

Electrochemical stability window studies

To ensure the suitability of an electrolyte material for practical battery devices, it is crucial to investigate its electrochemical stability and its interactions with the electrodes in lithium-ion batteries. As many lithium-based electrode chemistries operate at high voltages, the electrolyte material needs to possess a wide electrochemical stability window that extends beyond the operating potential of the electrodes. This is essential for the battery to function effectively within the typical temperature range of approximately 40 to 60 ℃ [58]. A linear sweep voltammetry test was performed to assess the potential window stability of the HGPEs. Figure 8 illustrates the linear sweep voltammograms of the HGPEs sample LiTFSI 20 and E-TFSI 20 at a scan rate of 10 mV s^− 1. The plots reveal that the sudden increase in current during the potential sweeping towards anodic values signifies the anodic decomposition voltage of the HGPEs. It also can be seen that the HGPE with the presence of IL exhibited a potential stability of 5 V, which is greater than HGPE without IL with 4.3 V at room temperature. The enhancement of the decomposition voltage of the HGPE with the addition of IL into the system can be elucidated due to the formation of a stable passive layer and electrolyte decomposition at the inert electrode interface [59–61]. It is worth noting that a stable passive layer enhances the potential window by providing a protective barrier that prevents unwanted reactions and ensures the stability of the electrochemical system, allowing for a broader range of operating voltages and improving overall performance and reliability [62]. In addition, forming an anodic stability window correlates with irreversible oxidation of the salt anion [63]. This analysis provides insights into the ability of the electrolyte to withstand higher voltages, which is essential for the stable operation of lithium-ion batteries.

Figure 8 Electrochemical stability curve of HGPEs.

Charge-discharge profile studies

The PMMA-PLA-20 wt.% LiTFSI and PMMA-PLA-20 wt.% LiTFSI-20 wt.% IL hybrid gel polymer electrolytes are tested in Li/graphite half-cell lithium-ion batteries at room temperatures. Figure 9 shows the charge-discharge curves of Li|HGPE|graphite cells at the first cycle with different current densities of 3.72 A g^− 1 and 11.16 A g^− 1. The cut-off potential for charging and discharging are 2.0 and 0.01 V, respectively. It can be noticed from the Fig. 9 that the current work does not show profiles as typical graphite anode profiles. According to the literature, this issue could have several possible reasons. They include (a) surface contamination that could affect the intercalation and deintercalation processes of lithium ions in the graphite structure, (b) structural defects which affect lithium ion diffusion and intercalation, (c) electrolyte materials effects on the interfacial processes, lithium-ion solvation, or solid-electrolyte interface (SEI) formation, and (d) cycling conditions, including the voltage range, current density, and temperature [64–66]. However, for this work, we assumed chemical reactions during the charge-discharge process cause the present electrochemical profiles.

It can be observed that the cycling profiles at 3.72 A g^− 1 and 11.16 A g^− 1 for Li|LiTFSI 20|graphite cell show small changes compared to Li|E-TFSI 20|graphite cell. The phenomenon of electrode activation polarization, which refers to changes in charge-discharge profiles, occurs due to the slower lithiation kinetics of the electrode at higher current densities [67]. In simpler terms, when the current density increases, the electrode's capacity to absorb lithium ions becomes limited, leading to decreased capacity and efficiency. This is a well-known challenge in lithium-ion batteries and is primarily caused by the aging process of the anode electrode and the formation of the solid electrolyte interface (SEI) layer. These factors contribute to the loss of active lithium within the battery system [68].

Figure 9 Galvanostatic charge-discharge curves of Li/graphite half cells with (a) LiTFSI 20 and (b) E-TFSI 20 HGPE at different current density.

Figure 10 illustrates the discharge performance curves of the Li|HGPE|graphite cell at the current density of 3.72 A g^− 1. It can be seen from the figure the first discharge capacity for Li|LiTFSI 20|graphite and Li|E-TFSI 20|graphite cells are 152.06 mAh g^− 1 and 71.15 mAh g^− 1, respectively. It is worth noting that the performance of the graphite anode with the addition of IL was initially lower than without IL, which is not as expected since IL shows the best performance as an electrolyte. Chatterjee et al. [67] investigated the cyclability of lithium-ion battery cells with and without the inclusion of IL additives in half-cell graphite configuration. Interestingly, it was observed that the performance of cells without the IL additive was superior during the initial cycles. The researchers proposed that this difference in performance could be attributed to the deterioration of the electrolyte on the graphite electrode caused by the presence of IL, which is associated with a lower energy level of the lowest unoccupied molecular orbital (LUMO).

Moreover, it is widely recognized that the electrolyte plays a crucial role in facilitating the movement of lithium ions between the electrodes in lithium batteries. The addition of IL in the HGPE systems may have interacted with the electrolyte and affected its stability or reactivity. It is suggested that the addition of IL caused the degradation of the electrolyte on the graphite electrode. In another study by Chengyong et al. [69], it was highlighted that ionic liquid electrolytes based on bis(trifluoromethanesulfonyl)imide (TFSI) and a lithium salt, although demonstrating excellent cycling performance for lithium batteries, do not perform effectively in Li-ion batteries. This limitation is primarily attributed to the behavior of the cations present in these ionic liquids, which tend to undergo severe intercalation and/or reductive decomposition in graphite before the Li⁺ ions during the initial cycle. In other words, the cations in TFSI-based ionic liquids react with graphite before the lithium ions can, limiting their performance in Li-ion batteries. Wang et al. [61] also agreed that ionic liquid-based electrolytes cannot form a stable SEI layer between the electrode and electrolyte, which is crucial for battery performance, especially in cycle performance. Moreover, Table 2 presents the performance of lithium-ion batteries using a gel polymer electrolyte system comprising lithium salt and ionic liquid, as reported in the literature. Based on the information provided in the table, it can be inferred that the current findings for both the LiTFSI 20 and E-TFSI 20 systems exhibit discharge capacities that are comparable to those reported in previous studies.

Table 2

Previous study on Lithium-ion batteries based GPEs.
GPE	σ (s cm^− 1)	LSV (V)	C_sp (mAh g^− 1)	References
SiO₂/P(MMA-AN-VAc) + LiTFSI + PYR₁₄TFSI/VC	1.20 × 10^− 3	5.3	142.2	[70]
PVdF-HFP/EMIMFSI/LiTFSI	~ 3.8 × 10^− 4	~ 4.7	~ 141.2	[71]
B4MePyTFSI/LiTFSI/P(VdF-HFP)	2.01 × 10^− 4	5.5	160	[72]
P(VDF-co-HFP)/TAIC/[EMIm][TFSI]/LiTFSI	1.40 × 10^− 3	-	45.1	[73]
EC + PC/PVdF/EMI-Tf/LiPF₆	6.40 × 10^− 3	-	~ 90	[74]
Bis(AEA4)/LiTFSI/MPPipTFSI	0.64 × 10^− 3	4.8	124	[75]
PMMA-PLA-LiTFSI	1.02 × 10^− 3	4.3	152.06	Current work
PMMA-PLA-LiTFSI-[EDIMP]TFSI	3.90 × 10^− 3	5.0	71.15	Current work

Figure 10. Discharge capacity versus number of cycles for HGPE systems at 3.72 A g^− 1.

Table 2 Previous study on Lithium-ion batteries based GPEs.

A new HGPEs system based PMMA-PLA-LiTFSI-[EDIMP]TFSI was successfully developed for lithium-ion batteries. Introducing an ionic liquid into the systems has improved the electrolytes’ performance in ionic conduction and electrical properties. Sample E-TFSI 20 shows the optimum ionic conductivity of 3.90 × 10^− 3 S cm^− 1, corresponding to the lowest E_a value. The temperature-dependent study on all HGPE systems demonstrated adherence to Arrhenius behavior, where the ionic conductivity increased with temperature without sudden decline. Incorporating the IL ([EDIMP]TFSI) into the HGPE resulted in an increase in both the lithium-ion transference numbers (from 0.62 to 0.79) and the electrochemical stability (from 4.3 V to 5 V) for LiTFSI 20 and E-TFSI 20 samples. Regarding lithium-ion battery performance, it can be inferred that using the ionic liquid-based [EDIMP]TFSI is incompatible with the graphite anode. This incompatibility led to potential chemical reactions occurring at the interface between the electrode and electrolyte, thereby impeding the movement of Li⁺ ions during the charge-discharge process.

Acknowledgement

The authors would like to thank University Malaysia Pahang (UMP) for providing financial support under UMP Internal Grant (RDU223304) and Postgraduate Research Scheme (PGRS) No. UMP.05/26.10/03/PGRS210355 (PGRS210355), Faculty of Industrial Sciences and Technology, UMP, Center for Advanced Intelligent Materials, UMP for the electrode materials, and Centre for the Ionics University of Malaya for the laboratory facilities for the completion of this work.

Ethical Approval

This work is not about both human and/or animal studies.

Competing interests

The authors declare no competing interests.

Authors' contributions

N.F.Mazuki: methodology, writing—original, draft preparation, investigation, writing—reviewing and editing. M.Z. Kufian: Writing—reviewing and editing, data curation. A.S. Samsudin: project administration, supervision, funding acquisition, conceptualization, investigation, writing—reviewing, and editing.

Funding

UMP Internal Grant (RDU223304) and Postgraduate Research Scheme (PGRS) No. UMP.05/26.10/03/PGRS210355 (PGRS210355)

Availability of data and materials

The data supporting the findings of this study areavailable within the article.

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You are reading this latest preprint version

Evaluation on electrochemical properties of lithium ions battery-based PMMA-PLA blend incorporation of [EDIMP]TFSI hybrid gel polymer electrolyte

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Preparation of hybrid gel polymer electrolytes with the incorporation of IL

Preparation of graphite anode

Characterization of HGPE-based IL

Ionic conduction measurement

Cation Transference Number (t_Li+) measurement

Linear Sweep Voltammetry (LSV)

Cell fabrication and Battery testing

Result and discussion

Ionic conduction properties studies

Lithium-ion transport number studies

Electrochemical stability window studies

Charge-discharge profile studies

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Evaluation on electrochemical properties of lithium ions battery-based PMMA-PLA blend incorporation of [EDIMP]TFSI hybrid gel polymer electrolyte

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Preparation of hybrid gel polymer electrolytes with the incorporation of IL

Preparation of graphite anode

Characterization of HGPE-based IL

Ionic conduction measurement

Cation Transference Number (tLi+) measurement

Linear Sweep Voltammetry (LSV)

Cell fabrication and Battery testing

Result and discussion

Ionic conduction properties studies

Lithium-ion transport number studies

Electrochemical stability window studies

Charge-discharge profile studies

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Cation Transference Number (t_Li+) measurement