In Situ Solidified Gel Polymer Electrolytes for Stable Solid−State Lithium Batteries at High Temperatures

Abstract

Lithium metal batteries have attracted much attention due to their high energy density. However, the critical safety issues and chemical instability of conventional liquid electrolytes in lithium metal batteries significantly limit their practical application. Herein, we propose polyethylene (PE)−based gel polymer electrolytes by in situ polymerization, which comprise a PE skeleton, polyethylene glycol and lithium bis(trifluoromethylsulfonyl)imide as well as liquid carbonate electrolytes. The obtained PE−based gel polymer electrolyte exhibits good interfacial compatibility with electrodes, high ion conductivity, and wide electrochemical window at high temperatures. Moreover, the assembled LiFePO₄//Li solid−state batteries employing PE−based gel polymer electrolyte with 50% liquid carbonate electrolytes deliver good rate performance and excellent cyclic life at both 60 °C and 80 °C. In particular, they achieve high specific capacities of 158.5 mA h g⁻¹ with a retention of 98.87% after 100 cycles under 80 °C at 0.5 C. The in situ solidified method for preparing PE−based gel polymer electrolytes proposes a feasible approach for the practical application of lithium metal batteries.

1. Introduction

Over the past 30 years, rechargeable lithium−ion batteries have had tremendous success. With the demand for high energy density, lithium metal batteries have attracted widespread attention due to the high theoretical specific capacity of lithium as well as its lowest electrochemical potential [1,2,3,4,5]. However, organic liquid electrolyte−based lithium metal batteries usually exhibit dendritic structural evolution and continuous parasitic reactions at the electrode–electrolyte interface, leading to safety issues and low coulombic efficiency [6,7,8,9,10]. Significant efforts have been made to overcome these challenges, of which the exploration of safe and stable electrolytes compatible with lithium metal is particularly critical and indispensable [11,12,13,14,15,16,17]. Solid−state electrolytes with high stability have been considered as optional alternatives to conventional liquid electrolytes for achieving safer and more stable energy storage systems [18,19,20].

Among various solid electrolytes, gel polymer electrolytes with liquid organic electrolytes possess high ionic conductivity under room temperature and can form a flexible interface with the electrode [21,22,23,24]. Generally, the polymer films of gel polymer electrolytes are prepared by an ex situ method and then immersed in a liquid electrolyte for gelation. However, the gel polymer electrolyte only contacts with the top of the electrode and therefore the entire mass of active material cannot be directly utilized. Thus, additional liquid electrolyte is still required to ensure interfacial wettability when utilizing ex situ prepared gel polymer electrolytes [25,26].

In situ preparation of gel polymer electrolytes is considered a feasible method to address the aforementioned interfacial problems and active material utilization issues [27,28,29,30,31,32]. Gelation could be achieved in the presence of liquid electrolyte−containing monomers injected into the battery. This method takes advantage of low viscosity, ease of handling, and good wettability, which enable gel polymer electrolytes with enhanced interfacial contact with electrodes, creating well−connected pathways for ionic transport [27,29,31].

Herein, we present polyethylene (PE)−based gel polymer electrolytes composed of PE skeleton, polyethylene glycol (PEG), lithium bis(trifluoromethylsulfonyl)imide and liquid carbonate electrolyte (LCE) for high temperature solid−state lithium metal batteries. Poly(ethylene glycol) methyl ether acrylate (PEGMEA) is used as the monomers for in situ polymerization due to the formed PEG possessing good compatibility with lithium metal as well as high ionic conductivity [19], which enables the obtained gel polymer electrolyte to possess both good chemical stability and high ion conductivity at high temperatures. The PE−based gel polymer electrolyte with 50% liquid carbonate electrolytes (PE−50%LCE@PEG) shows a high ionic conductivity of 1.73 × 10⁻⁴ S cm⁻¹ under 60 °C and good electrochemical stability with the lithium metal anode. The PE−50%LCE@PEG−based Li–Li symmetrical battery exhibits cycling stability for 1200 h at 0.1 mA cm⁻² and 0.1 mAh cm⁻² at 60 °C. Furthermore, the LiFePO₄/PE−50%LCE@PEG/Li cell exhibits excellent cyclic performance with no capacity decay over 150 cycles at 0.5 C under 60 °C. In addition, the cell shows a high initial reversible capacity of 160.3 mA h g⁻¹ under a higher temperature of 80 °C at 0.5 C and delivers excellent cycling stability for 100 cycles. This in situ solidified PE−based gel polymer electrolyte exhibits promising potential application in lithium metal batteries.

2. Experimental Section

2.1. Synthesis of PE−Based Gel Polymer Electrolytes

Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI, Sigma) and polyethylene glycol methyl ether acrylate (Sigma, 480 g mol⁻¹) monomer with dibenzoyl peroxide (Aladdin) initiator were mixed to form a homogeneous solution. The EO:Li ratio was controlled at 18:1 and the mass of initiator was 0.2% of PEGMEA. At the same time, a LCE solution of 1 M LiPF₆ in EC/DMC/EMC (1/1/1, v/v/v) was also prepared. The gel polymer electrolyte liquid precursor solutions were prepared by mixing different amounts of LCE (x = 40%, 50%, 60%) with the PEGMEA/LiTFSI solution, where x is the mass percent of PEGMEA. Then, the above liquid precursor solutions were incorporated into PE separator and then heating cured at 60 °C for 12 h, resulting in PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes.

2.2. Physical Characterization

Scanning electron microscopy (SEM, Hitachi S−4800) was used to observe the microstructure of the samples. A Nicolet 6700 spectrometer was used to collect Fourier transform infrared spectroscopy (FT−IR) spectra of the PEGMEA precursors and different electrolytes. Thermogravimetric analysis (TG209F1) was carried out under N₂ atmosphere with a heating rate of 10 °C min⁻¹.

2.3. Electrochemical Measurements

A Solartron 1470E electrochemical workstation was used to test electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and direct current polarization. EIS experiments were used to determine the ionic conductivity of the electrolytes using stainless steel (SS)/gel polymer electrolytes/SS symmetric cells and calculated based on Equation (1):

$$\sigma =L/(S\cdot R)$$

(1)

where L is the thickness, R represents the bulk resistance, and S represents the area.

The activation energy (E_a) of the electrolytes was obtained by the Arrhenius Equation (2):

$$\sigma \left(T\right)=A\exp \left(\frac{-E_a}{RT}\right)$$

(2)

where σ is the ionic conductivity, A is the frequency factor, R is the molar gas constant, and T is the absolute temperature. The electrochemical stability window of SS//Li cells was ascertained by linear scanning voltammetry (LSV) with a scan rate of 1 mV s⁻¹ within 3~6 V. The Li⁺ transference number ($t_{L{i}^{+}}$) of the polymeric solid electrolyte was determined by direct−current polarization on a Li//Li symmetric cell and calculated from the Bruce–Vincent–Evans Equation (3):

$$t_{L{i}^{+}}=\frac{I_{ss}\left(\Delta V-I_0{R}_0\right)}{I_0\left\{\Delta V-I_{ss}{R}_{ss}\right\}}$$

(3)

where ΔV is the polarization voltage of 10 mV, I₀ is the initial current, I_ss is the steady−state current. R₀ and R_ss are the initial and steady−state interfacial resistance after the polarization process. Additionally, Li//Li symmetric batteries with various electrolytes were assembled to test the stability with lithium anode on commercial battery testing equipment (LAND Wuhan Electronics Co., Ltd., Wuhan, China).

2.4. Battery Fabrication and Evaluation

LiFePO₄, PVDF/LiClO₄ and Super p were mixed together with a mass ratio of 8:1:1 in N−methyl−2−pyrrolidone to prepare cathode slurry, which was coated on aluminum foil and vacuum dried at 60 °C for 24 h to obtain the cathode. The specific area capacity of the cathode was about 0.3 mAh cm⁻². The PE separator was sandwiched between the cathode and lithium metal. The gel polymer electrolyte liquid precursor solutions were injected into the cell and direct in situ solidified at 60 °C for 12 h to fully form PE−based gel polymer electrolytes in the battery. The charging–discharging of assembled LiFePO₄//Li cells were tested by the commercial battery testing system (Wuhan LAND Electronics Co., Ltd.) from 2.8–3.8 V at 30 °C, 60 °C and 80 °C, respectively.

3. Results and Discussion

The flowable precursor solutions containing PEGMEA, LiTFSI, different amounts of LCE (x = 40%, 50%, 60%) and initiator can be directly polymerized and transformed into solid−state LCE@PEG after thermally cured (Figure S1). Figure 1a displays the preparation schematic diagram of the PE−x%LCE@PEG (x = 40%, 50%, 60%) gel polymer electrolytes. A 16 μm−thick PE separator is selected as skeleton, which can be completely infiltrated by the transparent and flowable precursor solution. After in situ thermal curing, PE−x%LCE@PEG solid electrolytes formed. Figure 1b exhibits the FT−IR absorption spectra of PEGMEA, PEGMEA@LCE, PEG, PE−x%LCE@PEG (x = 40, 50, and 60) electrolytes. Clearly, the C=C stretching vibration of acrylate group in PEGMEA at 1600~1690 cm⁻¹ disappears after in situ polymerization. Moreover, the C=O in the gel membrane guarantees a strong interaction with the liquid electrolyte. Figure 1c shows the TG curves of PEG, PE, PE@PEG, PE−50%LCE@PEG and LCE. The LCE evaporates rapidly at relatively low temperature and the weight loss reaches up to 41.88% when the temperature increases to 100 °C. In contrast, the weight loss of PE−50%LCE@PEG is nearly zero at 100 °C, indicating that the liquid electrolytes have been effectively immobilized in the polymerized PEG matrix and become thermally stable. Furthermore, PEG is thermally stable and its decomposition temperature can reach about 300 °C. By further incorporating with the PE separator, the PE−50%LCE@PEG gel polymer electrolyte exhibits good thermal stability at high temperature.

The morphology of PE and PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes were observed by SEM. As shown in Figure 2a, the surface of the pristine PE separator shows a porous structure, which can well accommodate the precursor solution before heating curing. After in situ polymerization, the pores in PE separator store LCE@PEG well to form a gel polymer electrolyte, and the surface of the electrolyte is uniform and flat (Figure 2b and Figure S2). The thickness of the PE−50% LCE@PEG electrolyte is about 18 μm with a dense structure (Figure 2c).

The ionic conductivity of gel polymer electrolytes is measured by EIS tests (Figure S3), showing 1.21 × 10⁻⁴, 1.73 × 10⁻⁴, and 2.11 × 10⁻⁴ S cm⁻¹ for PE−x%LCE@PEG (x = 40, 50, and 60) at 60 °C, respectively. The activation energies of PE−x%LCE@PEG (x = 40, 50, and 60) are calculated to be 0.286, 0.281, and 0.22 eV, respectively (Figure 3a). Based on LSV testing (Figure 3b), no obvious oxidation peak for the PE−x%LCE@PEG was observed until 4.3 V vs. Li/Li⁺, which is slightly higher than that of the liquid carbonate electrolyte at 4.1 V (Figure S4). The higher electrochemical window can be ascribed to the strong interaction between the C=O groups of PEGMEA and the anions in the electrolytic salt [19,33]. Moreover, the $t_{L{i}^{+}}$ values for PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes are 0.415, 0.425, and 0.5, respectively (Figure 3c and Figure S5), exceeding those of commercial liquid electrolytes (0.2–0.4) [34]. The high $t_{L{i}^{+}}$ could be due to the C=O groups in the PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes limiting the movement of anions, which could further reduce the concentration polarization and improve the performance of the cells [35,36,37]. The electrochemical stability of gel polymer electrolytes with lithium metal anode is further investigated in Li/Li symmetric cells. The critical current densities of PE−x%LCE@PEG (x = 40, 50, and 60) are 1.1, 1.1, and 0.9 mA cm⁻², respectively (Figure 3d), indicating the PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes possess good ability to inhibit lithium dendrites [38]. In addition, the long−term cyclic performances of Li//Li symmetric cells employing PE−x%LCE@PEG (x = 40, 50, and 60) and LCE are shown Figure 3e and Figure S6. Clearly, PE−50%LCE@PEG gel polymer electrolyte displays the best cycling stability up to 1200 h at 0.1 mA cm⁻² and 0.1 mAh cm⁻². The symmetrical cell with LCE suffers a short circuit after 875 h. The excellent interfacial stability is due to the confinement of polymer to the liquid phase, which reduces the interfacial reactions between lithium metal and reactive electrolyte components [39].

To demonstrate the feasibility of PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes in rechargeable lithium metal batteries, LiFePO₄//Li cells were assembled employing PE−x%LCE@PEG (x = 40, 50, and 60) and LCE, as shown in Figure 4a and Figure S7. The PE−50%LCE@PEG cell shows excellent cycling stability over 150 cycles with no capacity decay at 0.5 C under 60 °C. However, the LCE−based cell shows dramatic decrease in capacity after 30 cycles under the same conditions. Furthermore, the rate capabilities of LiFePO₄/PE−50%LCE@PEG cells were assessed at various current densities from 2.8–3.8 V (Figure 4b). The reversible capacities for PE−50%LCE@PEG cells are 164.8, 165, 160, 157, 152, 142, 63.6 mA h g⁻¹ at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 3 C, and 5 C, respectively. The corresponding charge–discharge curves are presented in Figure 4c. With the increase of current density, the discharge voltage plateau decreases slowly without serious polarization until 3 C.

In addition, the cyclic performances of LiFePO₄/PE−x%LCE@PEG/Li cells and LiFePO₄/LCE/Li cells at 0.5 C under 80 °C are also evaluated, as shown in Figure 4d and Figure S8. For the commercial liquid electrolyte−based cell, it is difficult to operate at a high temperatures of 80 °C for a long cycle life [40]. Nevertheless, the PE−50%LCE@PEG based cell can stably cycle for 100 cycles at 0.5 C under 80 °C, exhibiting a reversible discharging specific capacity of 158.5 mA h g⁻¹ at the 100th cycle with a capacity retention of 98.87%. Moreover, the LiFePO₄//Li battery with the PE−50%LCE@PEG electrolyte also possesses outstanding electrochemical performances under 0.5 C at 30 °C, showing a comparable performance with LCE−based cell (Figure S9). Furthermore, the lithium anode of the LiFePO₄/PE−50%LCE@PEG/Li cell after cycling shows a smooth and compact surface without cracks, and few lithium dendrites are observed (Figure S10). However, after cycling, the lithium anode of LiFePO₄/LCE/Li cell becomes rough and full of dendrites.

Furthermore, a series of flexibility and safety tests were performed on the LiFePO₄/PE−50%LCE@PEG/Li pouch cells. As shown in Figure 5, the pouch batteries with PE−50%LCE@PEG can continuously light up the yellow LED in the flat state, folded state, several fold and even cut, which displays the good reliability and safety of PE−50%LCE@PEG for flexible solid−state lithium batteries.

4. Conclusions

A series of PE−x%LCE@PEG (x = 40, 50, and 60) gel polymer electrolytes were successfully synthesized to realize high−temperature solid−state lithium batteries. The PE−50%LCE@PEG gel polymer electrolyte shows a high ionic conductivity of 1.73 × 10⁻⁴ S cm⁻¹ at 60 °C with a high Li⁺ transference number of 0.425. The critical current density of PE−50%LCE@PEG gel polymer electrolyte−based Li/Li symmetric cell is up to 1.1 mA cm⁻², and the cell exhibits cycling stability for 1200 h at 0.1 mA cm⁻² and 0.1 mAh cm⁻². The assembled LiFePO₄/PE−50%LCE@PEG/Li solid−state batteries show outstanding cycling stability with no capacity decay over 150 cycles at 0.5 C under 60 °C. In addition, the cell exhibits a high initial reversible capacity of 160.3 mA h g⁻¹ under a higher temperature of 80 °C at 0.5 C and exhibits excellent cycling stability for 100 cycles, demonstrating promising potential application for high−temperature solid−state lithium metal batteries.