A novel polymer electrolyte with in situ polymerization and a high concentration of lithium salts for lithium metal batteries

Abstract

A novel polymer electrolyte is prepared by in situ polymerization for lithium metal batteries. The polymer electrolyte with in situ polymerization can maintain good inter-facial contact property between the electrodes and electrolyte. It is proved that the interface contact between the Li metal and the polymer electrolyte is very good. And the electrolyte is joined to the cathode electrode by in situ polymerization. As a result, the inter-facial resistance of the electrodes and electrolyte is small. Herein, the polymer electrolyte is amorphous. Thus, the novel polymer electrolyte has a high ionic conductivity (3.4 × 10⁻³ S cm⁻¹) at room temperature and an electrochemical stability window (5 V, vs. Li/Li⁺). Furthermore, the polymer electrolyte is stable on the lithium metal anode. The polymer electrolyte also has good mechanical properties and can adapt to the changes of material volume and stress during charge and discharge. Thus, it can maintain the interface stability. As a result, the LiFePO₄/polymer electrolyte/Li battery has good cycling stability and rate performance.

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Introduction

With the rapid development of electric vehicles, unmanned aircraft and smart grids in the past several years,^1–3 portable power sources with a high energy density have become a neccessity.^4,5 Lithium metal batteries, which consist of a lithium metal anode, cathode and electrolyte, are considered to be the most promising mobile power source.^6–9 However, the development of lithium metal batteries with liquid electrolytes has been limited by the problems of lithium dendrites, leakage, and flammability.^10–12 The emergence of polymer electrolytes has brought good news to the development of lithium metal batteries. Polymer electrolytes can alleviate or even solve the above problems.^13–16 In addition, polymer electrolytes also have the advantages of excellent mechanical strength, easy processing, and low cost.^17,18

Polymer electrolytes usually consist of a polymer as the matrix and lithium salts.^14,19–28 Therefore, polymer electrolytes have good mechanical properties and film formation. In addition, polymer electrolytes with a high Young's modulus can effectively prevent the formation of lithium dendrites, thus improving the cycling stability of lithium metal. Polymer matrix materials mainly include polyvinylidene fluoride (PVDF),^20,29–31 polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP),^32–35 polymethyl methacrylate (PMMA),^35–38 polyethylene oxide (PEO),^23,27,39,40 aliphatic polycarbonate (APC),²¹etc. However, the ionic conductivity of the above polymer electrolytes at room temperature is low (about 10⁻⁷ S cm⁻¹). They can undergo normal operation at high temperatures (60–80 °C), and they cannot satisfy the actual needs. Although the ionic conductivity of polymer electrolytes at room temperature can be improved by the modification of the polymer matrix or the addition of inorganic nanoparticles, it is still difficult to meet the practical requirements.^29,40,41

To improve the ionic conductivity of polymer electrolytes at room temperature, in situ polymer electrolytes are introduced. Polymer electrolytes can significantly improve the interface properties and electrochemical performances of lithium secondary batteries.^42–45 In addition, the preparation process of in situ polymer electrolytes eliminates the complex sequence of membrane drying and solvent evaporation. As a result, the preparation of electrolytes and the assembly of batteries can be carried out at the same time. Thus, this method can effectively reduce the cost and is compatible with the existing industrial system of lithium secondary batteries. As a result, it has a good application prospect. Through simple acid treatment, Zhang et al.⁴⁶ introduced –COOH and –OH groups on the surface of carbon nanotubes, and then the 1,3-dioxy amyl ring (DOL) was induced to realise ring-opening. Thus, flexible electrolyte films on the surface of acidic carbon nanotubes were obtained. The electrolyte film has excellent ion selectivity, which can not only seal polysulfide in the cathode electrode, but also allow lithium ions to pass through. As a result, the cycle performance of lithium–sulfur batteries was improved. Although many studies have shown that in situ polymerization can provide good interface compatibility for lithium secondary batteries, now most of the in situ polymerization technologies require an additional non-electrolyte monomer, initiator or high temperature environment, etc. In order to upgrade the liquid electrolyte of traditional lithium–sulfur batteries in a simple way, a new strategy was developed. Lithium hexafluorophosphate (LiPF₆) was used to catalyze the ring-opening reaction of DOL, and a new type of quasi solid polymer electrolyte was generated.⁴⁷ Compared with other common methods, it uses commercial electrolyte materials, and the polymerization conditions are mild. Additional catalysts are not required. The quasi solid polymer electrolyte shows good adaptability in secondary batteries (such as sulfur, LiFePO₄ and LiNi_0.6Co_0.2Mn_0.2O₂). However, the quasi solid polymer electrolyte not only has poor mechanical properties, but also is decomposed at high temperatures (∼60 °C). Therefore, it is urgent to improve the mechanical properties and high temperature properties of the quasi solid polymer electrolyte.

Due to the low cost, environmental friendliness, good elasticity, excellent flexibility, and profound thermal resistance, thermoplastic polyurethane (TPU) has been widely applied to industrial and civil applications. In recent years, TPU has also been applied in polymer electrolytes.^48–52 Due to the very low ionic conductivity of the TPU polymer electrolyte (∼10⁻⁵ S cm⁻¹ at 60 °C), Tao et al.⁵⁰ prepared a polymer electrolyte by mixing TPU and PEO, and the ionic conductivity of the TPU/PEO polymer electrolyte was improved (5.3 × 10⁻⁴ S cm⁻¹ at 60 °C). The TPU/PEO polymer electrolyte has high temperature stability and good mechanical properties. However, the TPU/PEO polymer electrolyte still does not work at room temperature. Therefore, Chen et al.^51,52 prepared TPU-based gel polymer electrolytes. The ionic conductivity of TPU-based gel polymer electrolytes is approximately 10⁻⁴ S cm⁻¹ at room temperature. TPU-based gel polymer electrolytes have a wide electrochemical stability window (about 5 V) and high temperature adaptability (2 °C–85 °C).

In this paper, a novel polymer electrolyte with in situ polymerization was prepared. The soft chain of TPU, the unit formed by DOL ring-opening polymerization and LiFSI, provides an effective channel for ion transfer. The lithium-ion transference number of LEUI3 is better than that of the reported pure PEO-based solid polymer electrolytes and conventional carbonate electrolyte (1.0 M LiPF₆-EC/DEC, v/v = 1 : 1).^53–56 To improve the mechanical properties of the DOL-based quasi solid polymer electrolyte, TPU is introduced into the quasi solid polymer electrolyte. It is demonstrated that the interaction between DOL, DME, TPU and LiFSI makes the polymer electrolyte transform from a quasi-solid to a solid. At the same time, the thermal stability of the polymer electrolyte is improved and the polymer electrolyte with in situ polymerization could maintain good interface contact between the electrodes and electrolyte. It is proved that the interface contact between the Li metal and the polymer electrolyte is very good. And the electrolyte is joined to the cathode electrode by in situ polymerization. As a result, the interface resistance of the electrodes and electrolyte is small. Furthermore, the polymer electrolyte has high ionic conductivity at room temperature and an electrochemical stability window. In addition, the polymer electrolyte is stable to the lithium metal anode. The LiFePO₄/the polymer electrolyte/Li battery also has good cycling stability and rate performance.

Experimental

A certain amount of TPU was added into a mixed organic solution of DOL and DME (1 : 1, v/v), and the solution was stirred at 50 °C for 4 h to ensure that the TPU was completely dissolved. Different masses of lithium bis(fluorosulfonyl)imide (LiFSI) (1, 2, 3, and 4 mol L⁻¹) were added to the above solutions. The preparation process is shown in Fig. 1. According to the content of LiFSI, the electrolyte films were labeled as LEUI1, LEUI2, LEUI3, and LEUI4, respectively. LiFSI can not only provide ion transport for the polymer electrolyte but also trigger DOL to undergo ring-opening polymerization.

Fig. 1 Diagram of the preparation process of the polymer electrolytes.

An X-ray diffractometer (XRD, Ultima, IV) with Cu Kα radiation was used to analyze patterns of the polymer electrolytes. The scanning range was 2θ = 10°–60°, and the scanning speed was 4° min⁻¹. The morphologies of the polymer electrolytes and the metal Li after cycling were analyzed using a scanning electron microscope (SEM, Bahens, S-3400N). The disassembled battery was placed in N-methylpyrrolidone (NMP). It can easily remove the cathode electrode and separate the electrolyte from the metal Li.

The chemical structures of the polymer electrolytes and the bond exchange reactions between DOL, DME, TPU and LiFSI were analyzed by Fourier-transform infrared spectroscopy (FTIR, Nicolet iS 50 ART) with an average of 24 scans and a scanning resolution of 4 cm⁻¹. Nuclear magnetic resonance (NMR, Ascend 400 MHz with DMSO) spectroscopy was performed to analyze the chemical structures of the polymer electrolytes.

The symmetrical batteries (stainless-steel/polymer electrolyte/stainless-steel) were assembled to test the ionic conductivity of the polymer electrolytes at different temperatures by the AC impedance method (CHI-760E). Linear sweep voltammetry (LSV) experiments were used to investigate the electrochemical stability of the polymer electrolytes with lithium metal as the counter and reference electrodes and stainless-steel serving as the working electrode. AC impedance was used to measure the interface resistance between the electrolyte and the metal Li in the symmetrical batteries (Li/electrolyte/Li). Then, the Li/polymer electrolyte/Li batteries were used to test the stability of the polymer electrolyte against lithium metal. The electrochemical performances of the Li/polymer electrolyte/LiFePO₄ batteries were tested using a Xinwei battery-testing equipment (Shenzhen, China) within a voltage range of 3 to 4 V. The battery was tested with LiFePO₄/LEUI3/Li using 2032 type coin cells. The active material content of cathode was about 5 mg cm⁻². The thickness of LEUI3 and Li is about 40 μm and 0.6 mm respectively.

Results and discussion

The XRD images of the polymer electrolytes with the different concentrations of LiFSI were tested as presented in Fig. 2. It is very obvious that only one broad peak can be observed in the 2θ range (as shown in Fig. 2). In other words, all four polymer electrolytes are amorphous. In addition, it also shows that LiFSI is not precipitated, even when the concentration of LiFSI is as high as 4 mol L⁻¹. The polymer electrolyte is amorphous, which can greatly improve the ionic conductivity of the polymer electrolyte at room temperature.

Fig. 2 XRD images of the polymer electrolytes.

Furthermore, the morphologies of the polymer electrolytes were tested as displayed in Fig. 3. As shown in Fig. 3, the surfaces of the four polymer electrolytes are uniform and dense without grains. The result is consistent with the conclusion of the XRD test. The surface of the polymer electrolyte membrane is uneven with obvious ups and downs, and the uneven surface is aggravated with the increase of LiFSI, which is caused by the high content of lithium salt. The reasons are as follows: on the one hand, the higher the concentration of LiFSI is, the faster the polymerization rate; on the other hand, it is caused by the cross-linking between the polymer chains. As a result, the surface of the film is not smooth. In addition, the polymer electrolyte film also has high flexibility and good mechanical properties (Fig. S1†), which is beneficial for inhibiting the growth of lithium dendrites.⁵⁷

Fig. 3 SEM images of the polymer electrolytes: (a) LEUI1, (b) LEUI2, (c) LEUI3, and (d) LEUI4.

The FTIR curves of the pristine polymers and the polymer electrolytes were measured as displayed in Fig. 4. As shown in Fig. 4a, the peaks in the range of 1000–1300 cm⁻¹ are the characteristic absorption peaks of ether groups, which appear in DOL, DME and TPU, and are attributed to the stretching vibration of C–O. The peaks at 3325 cm⁻¹ and 1701 cm⁻¹ respectively correspond to the stretching vibration of N–H and the stretching vibration of C=O of TPU. And the peaks at 1597 cm⁻¹ and 1529 cm⁻¹ correspond to the absorption peak of the benzene ring of TPU, as shown in Fig. 4a. The absorption peaks corresponding to the bending vibration of C–H at 665 cm⁻¹ and 915 cm⁻¹ of DOL disappear in LEUI1, as displayed in Fig. 4a. However, a new peak appears in the range of 900–970 cm⁻¹, indicating the formation of poly-DOL.⁵⁸ The results of NMR also confirm the formation of poly-DOL,⁴⁷ as shown in Fig. S2.† The absorption peak at 1372 cm⁻¹ corresponding to the C–H bending vibration of DME and the absorption peaks corresponding to the benzene ring and carbonyl group of TPU disappear after polymer formation, indicating that DOL and DME have been polymerized with TPU, forming the electrolyte with a cross-linked network structure. In addition, it can be seen from Fig. 4b that the peak strength of benzene ring and carbonyl group gradually decreases with the increase of lithium concentration. When the lithium concentration reaches 4 M, the peak disappears. It indicates that the degree of the cross-linking of the polymer electrolyte is related to the concentration of lithium salt.

Fig. 4 FTIR curves of the pristine polymers and the polymer electrolytes.

The electrochemical stabilization windows of the polymer electrolytes with different concentrations of LiFSI are exhibited in Fig. 5a. As can be seen from Fig. 5a, the decomposition potential of the polymer electrolytes increased with the increase of the salt concentration at the beginning. When the lithium salt concentration was increased to 3 M, the decomposition potential of the polymer electrolytes did not change significantly. The decomposition potential of LEUI3 is about 5 V (vs. Li/Li⁺), which can meet the requirements of most cathode materials.

Fig. 5 (a) Electrochemical stability window, and (b) ionic conductivities of the polymer electrolytes.

In addition, the ionic conductivities of the polymer electrolytes at different temperatures were tested, as shown in Fig. 5b. It is obvious that the ionic conductivity of the polymer electrolytes increased first and then decreased with the increase of LiFSI, as shown in Fig. 5b. The reason is that too much lithium salt could hinder the movement of the chain segment of the polymer electrolyte, and then affect the migration of lithium ions. Polymer electrolytes were prepared by cross-linking polymerization and DOL ring-opening polymerization, as shown in Fig. 4 and Fig. S2.† The interactions of the cross-linking polymerization are strong. Furthermore, polymer electrolytes have many ether groups, which is beneficial for the conduction of lithium ions. Thus, polymer electrolytes all have high ionic conductivity at room temperature.⁴⁷ It can be seen from Fig. 5b that the ionic conductivities of the polymer electrolytes increased with the increase of temperature. The reason is that high temperature accelerates the movement of the chain segment, thus improving the migration rate of lithium ions. There is a linear relationship between the ionic conductivity and temperature, and there is no transition between the crystalline and amorphous phases. It indicated that the relationship between the ionic conductivity of the polymer electrolyte and temperature follows the Arrhenius formula. Therefore, the Arrhenius formula can be used to calculate the activation energy of the polymer electrolyte. LEUI3 had the highest ionic conductivity of the four polymer electrolytes in the test temperature range, as displayed in Fig. 5b. Its ionic conductivity was 3.4 × 10⁻³ S cm⁻¹ at room temperature.

The polymer electrolyte should not only have high ionic conductivity at room temperature and electrochemical stability window, but also be able to form a stable interface with the lithium metal anode. Polymer electrolytes can be used when these conditions are met. Thus, the Li/the polymer electrolyte/Li symmetrical cells were assembled to test the stability between the polymer electrolyte and the lithium metal anode, as displayed in Fig. 6a. As shown in Fig. 6a, the Li/the polymer electrolyte/Li cells are charged/discharged for 0.5 h during each process at the current density of 0.5 mA cm⁻². The potential of the LEUI1/Li symmetric cell showed asymmetric fluctuation at about 600 h, which was caused by the generation of lithium dendrites. Later, the polarization potential of the LEUI1/Li symmetric cell becomes stable again. It is because LEUI1 has good mechanical properties.^51,59 In addition, in situ polymerization can also form a good interface between the electrode and the electrolyte. However, the other three polymer electrolytes still maintain a stable polarization potential without the asymmetric fluctuations. In particular, the LEUI3/Li symmetric cell not only maintains a stable polarization potential, but also has a low polarization potential, even after cycles of 700 h. It indicates that lithium plating/stripping is reversible and Li dendrite growth has been significantly restrained in LEUI3. The reason is that LEUI3 has high ionic conductivity and can form a good interface contact with the lithium metal anode.

Fig. 6 (a) The potential profile for the Li/LEUI/Li symmetric cell at current density 0.5 mA cm⁻² and (b) the impedance spectra for the Li/the electrolyte/Li symmetric cells.

To prove the above conclusion, the impedance spectra for the Li/the electrolyte/Li symmetric cells were obtained at room temperature, as shown in Fig. 6b. The equivalent circuit diagram is also shown in Fig. 6b. The diameter of the semicircle corresponds to the interface resistance of the electrolyte/Li. The interface resistance is made up of two parts: (i) the interface contact resistance (R_f) between the metal Li and the electrolyte, and (ii) the charge-transfer resistance (R_c).^60–62 The analysis of the electrochemical impedance spectroscopy of Li/LEUI3/Li yielded R_f = 13 Ω and R_c = 38 Ω, which are compared with R_f = 16 Ω and R_c = 34 Ω for Li/liquid electrolyte/Li. Thus, the charge-transfer resistance for the LEUI3 polymer electrolyte was higher than that of the liquid electrolyte, as shown in Table 1. However, the interface contact resistance (R_f) for the LEUI3 polymer electrolyte was lower than that of the liquid electrolyte. That is to say, the interface contact property between the Li metal and the LEUI3 polymer electrolyte was very good.

Table 1 Simulated impedance parameters for electrochemical impedance spectroscopy of Li|Li cells in the LEUI3 polymer electrolyte and liquid electrolyte

	R b/Ω	R f/Ω	R c/Ω
Liquid electrolyte	3	16	34
LEUI3 polymer electrolyte	2	13	38

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Through the above research, it is found that the LEUI3 has excellent electrochemical performance. In addition, the lithium-ion transference number of LEUI3 is 0.35 (Fig. S3†). Therefore, the battery was assembled with LEUI3 as the electrolyte, LiFePO₄ and lithium metal as the cathode and anode electrodes, respectively. And the charge and discharge test of the battery was carried out at 0.2 C and 25 °C. The results are shown in Fig. 7. The initial discharge capacity of the battery was 140.3 mA h g⁻¹, as displayed in Fig. 7a. LEUI3 and the electrodes were activated during the first 10 cycles. Thus, the discharge capacity of the battery was increased in the first 10 cycles. After 10 cycles, the specific discharge capacity of the battery was maintained at about 153 mA h g⁻¹. In addition, the cycling performances of the LiFePO₄/LEUI3/Li battery were also measured, as shown in Fig. 7b. It is very obvious that the discharge capacity of the battery was increased in the initial cycles, and the discharge capacity stabilized after a few cycles. The reasons are as follows: on the one hand, it is due to the good interface performance; on the other hand, it is due to the activation of the electrolyte and electrodes.⁶³ The coulombic efficiency of the battery was 88.3% in the initial cycle, and the coulombic efficiency was maintained at above 99.5% for the rest of the cycles, as shown in Fig. 7b. The formation of the stable interface layer on the lithium metal surface results in the low coulombic efficiency of the battery in the initial cycle. The stable interface layer can improve the stability of the lithium metal anode and avoid the formation of lithium dendrites and dead lithium, as shown in Fig. S4.†

Fig. 7 (a) The charging–discharging curves and (b) the cycling performances of the LiFePO₄/LEUI3/Li battery (0.2 C, 25 °C).

Moreover, the rate performance of the LiFePO₄/LEUI3/Li battery was tested, as depicted in Fig. 8. As can be seen from Fig. 8, the discharge specific capacity of the battery gradually decreased with the increase of the C rate. The reason is that the polarization gradually increases with the increase of the C rate, resulting in the attenuation of the discharge specific capacity of the battery. The discharge specific capacity of the battery was maintained above 138 mA h g⁻¹ at 0.5 C. The reasons are as the follows: on the one hand, LEUI3 has high ionic conductivity at room temperature; on the other hand, it is due to the excellent interface properties between the polymer electrolyte and electrodes. When the current returned to 0.1 C, the specific discharge capacity of the battery quickly recovered, indicating that the battery has a good rate performance.

Fig. 8 Rate performance of the LiFePO₄/LEUI3/Li battery (25 °C).

Conclusions

In summary, a new polymer electrolyte with a high concentration of lithium salts was prepared by in situ polymerization for lithium metal batteries. The effect of the concentration of LiFSI on the properties of the electrolyte was investigated. It was found that the polymer electrolyte had an excellent performance when the concentration of LiFSI was 3 M. The results show that in situ polymerization is beneficial for the formation of a stable interface between the electrodes and electrolyte. Herein, the polymer electrolyte film had a cross-linked network structure. And it had high flexibility and good mechanical properties, which is beneficial for inhibiting the growth of lithium dendrites. Moreover, the polymer electrolyte was amorphous. Thus, the novel polymer electrolyte had a high ionic conductivity (3.4 × 10⁻³ S cm⁻¹) at room temperature and electrochemical stability windows (5 V, vs. Li/Li⁺). It could meet the requirements of most cathode materials. In addition, the specific discharge capacity of the LiFePO₄/LEUI3/Li battery was maintained at about 153 mA h g⁻¹ at 0.2 C and 25 °C, and the coulombic efficiency was maintained at above 99.5%. Furthermore, the rate performance of the LiFePO₄/LEUI3/Li battery was also very good. And the new polymer electrolyte also has the potential to be commercialized.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the foundation of the State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2022-K65), the Open Fund of Anhui Province Key Laboratory of Optoelectronic and Magnetism Functional Materials (Grant No. ZD2021005 and ZD2022010), the Anhui Provincial excellent scientific research and innovation team (Grant No. 2022AH010062) and the Natural Science Foundation of Anhui Education Department (Grant No. KJ2021A0645 and 2022AH051045).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2py01208a

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