Enhanced Electrochemical Performance of Poly(ethylene oxide) Composite Polymer Electrolyte via Incorporating Lithiated Covalent Organic Framework

The lithiated covalent organic framework (named TpPa-SO3Li), which was prepared by a mild chemical lithiation strategy, was introduced in poly(ethylene oxide) (PEO) to produce the composite polymer electrolytes (CPEs). Li-ion can transfer along the PEO chain or across the layer of TpPa-SO3Li within the nanochannels, resulting in a high Li-ion conductivity of 3.01 × 10−4 S/cm at 60 °C. When the CPE with 0.75 wt.% TpPa-SO3Li was used in the LiFePO4||Li solid-state battery, the cell delivered a stable capacity of 125 mA·h/g after 250 cycles at 0.5 C, 60 °C. In comparison, the cell using the CPE without TpPa-SO3Li exhibited a capacity of only 118 mA·h/g.


Introduction
As the demand for power sources continues to increase, various types of high-performance energy storage devices are developing rapidly [1][2][3]. Solid electrolytes are the key part of solid-state lithium-ion battery (SSLIB), which is believed as one of the next-generation battery systems with high energy density and superior safety [4]. Composite polymer electrolytes (CPEs), such as poly(ethylene oxide) (PEO), are the most promising candidate material for commercial SSLIB [5,6]. However, the low ionic conductivity that resulted from high crystallinity still cannot be satisfied for application. To solve this problem, inorganic particles such as oxides and sulfides were widely used as fillers to decrease the crystallinity of PEO or construct extra Li-ion conductive pathways [7,8]. Traditional efficient fillers have some shortcomings: (1) oxides usually need a high-temperature treatment, such as Li 7 La 3 Zr 2 O 12 (> 900 °C) [9]; (2) most oxides and sulfides are sensitive to water and air [10,11]; (3) the high weight content of fillers hinders the improvement in the overall battery's energy density. Therefore, great efforts in finding better fillers are needed for practical applications.
Covalent organic frameworks (COFs) are a new kind of porous materials with advantages such as highly adjustable structures, light mass, high specific surface area, and unique one-dimensional channel structures [12,13]. Although these features are suitable for use as fillers, COFs are rarely applied in CPEs due to their poor ionic conductivity [14][15][16]. To enhance the intrinsic Li-ion conductivity of COFs, this study prepared a lithiated COF (named TpPa-SO 3 Li) by a mild chemical lithiation strategy. The obtained TpPa-SO 3 Li was selected as the active filler in the CPE for SSLIB. Compared with other fillers without Li-ion conductivity, the TpPa-SO 3 Li provides a rapid Li-ion transfer via the -SO 3 Li groups as Li-ion conducting sites in the nanochannels, which work as the expressway between the PEO chains, leading to enhanced Li-ion conductivity. Moreover, the extra advantages of high chemical stability and low density provide the potential for practical application. Results show that the 1 3 TpPa-SO 3 Li serves as promising filler in CPE and enhances the electrochemical performances of LiFePO 4 ||Li SSLIB.

Experiment Section
TpPa-SO 3 Li was prepared by solvothermal and chemical lithiation methods [13]. TpPa-SO 3 Li, PEO (molecular weight M w ≈ 6 × 10 5 ), and LiClO 4 were dissolved in anhydrous acetonitrile with a certain weight ratio of EO: Li = 18:1. The homogenized suspension was cast into a polytetrafluoroethylene mold and dried at room temperature for 24 h. After further drying at 55 °C for another 12 h under vacuum, the CPE membrane (named (PEO) 18 LiClO 4 / TpPa-SO 3 Li(x), x is the mass fraction of TpPa-SO 3 Li) was obtained. The CPE without TpPa-SO 3 Li was denoted as (PEO) 18 LiClO 4 . Figure 1a shows the chemical structure of TpPa-SO 3 H, which was synthesized by the Schiff base condensation reaction of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Tp) and 2,5-diaminobenzenesulfonic acid (Pa-SO 3 H). The enol-imine form converted to keto-enamine form after condensation. It was lithiated by Lewis acid-base reaction using lithium acetate. As shown in Fig. 1b, the Fourier transform infrared (FT-IR) peaks of TpPa-SO 3 H at 1240, 1575, and 3400 cm −1 represent the C-N, C = C, and enamine N-H bonds, respectively. The peak at 3200 cm −1 , which corresponds to O-H bonds in sulfonates, is found in TpPa-SO 3 H. After being lithiated, this peak significantly weakened, indicating that the H atoms in the -OH groups are replaced by Li atoms. Meanwhile, the O-Li bonds form and show a characteristic peak at 1750 cm −1 , which overlaps with the characteristic peak of C=O bonds. X-ray photoelectron spectroscopy (XPS) spectra ( Fig. 1c and Fig. S1) also confirm the lithium substitution. Due to the splitting of the spin-orbit, double peaks appear in the S 2p spectrum and shift to higher binding energy after being lithiated. It confirms that the environment of the S atom has changed, which is attributed to the substitution of H atoms in the sulfonates by Li atoms [17].

Results and Discussions
Scanning electron microscopy (SEM) images show that the TpPa-SO 3 H and TpPa-SO 3 Li have similar stacked particle structures (Fig. 1d, e). These particles have an average diameter of less than 100 nm and are formed by stacking porous nanosheets. The X-ray diffraction (XRD) pattern of TpPa-SO 3 H (Fig. 1f) shows the characteristic peaks at 2θ = 4.6° and 26.2° that correspond to (100) and (001) planes, respectively. This confirms the ordered pores with a pore size of 1.88 nm and an interlayer distance of 3.4 Å. This indicates that the lithiation process has basically no effect on the layered structure and morphology. The crystalline structure of TpPa-SO 3 Li can be verified by the transmission Fig. 1 a Chemical structure of TpPa-SO 3 X (X = H, Li). b FT-IR spectra of TpPa-SO 3 H and TpPa-SO 3 Li. c XPS spectra of S 2p in TpPa-SO 3 H and TpPa-SO 3 Li. SEM images of (d) TpPa-SO 3 H and (e) TpPa-SO 3 Li. f XRD patterns of TpPa-SO 3 H and TpPa-SO 3 Li electron microscopy (TEM) image (Fig. S2) as well. The Li content obtained by the inductively coupled plasma optical emission spectrometer is about 1.53 wt.%, which is a little lower than the theoretical value of 2.33 wt.%.
TpPa-SO 3 Li can be uniformly dispersed in the casting solution without sedimentation (Fig. S3). As shown in Fig. 2a and Fig. S4, the CPEs are flexible with a thickness of ~ 50 μm and have a uniform surface without visible defects or pellets (Fig. 2b, c). In addition, there is no obvious difference in the surface morphology of CPEs with or without TpPa-SO 3 Li. The uniform dispersion of TpPa-SO 3 Li in PEO can be demonstrated by the cross-sectional energydispersive spectrometer (EDS) mapping of S (Fig. 2d). The XRD characterization was performed to study the crystallinity of CPEs in Fig. 2e, indicating that the introduction of TpPa-SO 3 Li with various ratios has no effect on the crystallinity of CPEs either [5]. After adding 0.75 wt.% TpPa-SO 3 Li, the melting enthalpy of CPEs changes from 88.03 J/g to 91.88 J/g. (Fig. 2f). Correspondingly, the crystallinity of CPEs increased from 41.19 to 42.99%, which suggests that the crystallinity of CPEs is hardly affected.
To study the influence of TpPa-SO 3 Li on the ionic conductivity, this work assembled SS|CPE|SS cells (SS represents stainless steel) and performed electrochemical impedance spectroscopy (EIS). Figure 3a shows the typical spectra of (PEO) 18 LiClO 4 /TpPa-SO 3 Li(0.75) at different temperatures. The resistance decreases rapidly with the increase in temperature. At 50 °C and above, the semicircle disappears, indicating a decreased interface resistance because of the improved interface interaction. As shown in Fig. 3b, with the increase in the TpPa-SO 3 Li content, the ionic conductivity shows a trend of initially rising and then falling with inflection points at 0.75 wt.% TpPa-SO 3 Li content. Taking the curve at 60 °C as an example, the ionic conductivity ranges from 1.62 × 10 −4 to 3.01 × 10 −4 S/cm as the filling amount increases from 0 to 0.75 wt.%. When the content is further increased and exceeds the threshold, the conductivity drops rapidly to even lower than that of (PEO) 18 LiClO 4 . This phenomenon can be explained by the percolation behavior. It is attributed to aggregation, phase separation, and bubbles caused by excess TpPa-SO 3 Li, which goes against ion conduction. Moreover, due to the low density and high specific surface area of TpPa-SO 3 Li, the ionic conductivity can be improved at a very low content, and the threshold of the percolation behavior is only about 0.75 wt.%.
To study the effect of the lithiation sites, (PEO) 18 LiClO 4 /TpPa-SO 3 H(0.75) was prepared with the same method, and its ionic conductivity was tested at different temperatures. As shown in Fig. 3c, the ionic conductivity of (PEO) 18 LiClO 4 /TpPa-SO 3 H(0.75) is significantly lower than that of (PEO) 18 LiClO 4 . Since TpPa-SO 3 H does not have Li-ion conductivity, its uniform dispersion in CPEs may form micro-domains that are not conducive to the Li-ion. In addition, the H-ions that dissociated from the sulfonates hinder the Li-ions from entering the nanochannels of TpPa-SO 3 H by steric hindrance and Coulomb In addition to the high ionic conductivity, sulfonates in TpPa-SO 3 Li serve as single Li-ion conduction sites, which is beneficial to increase the Li-ion transference number (t Li + ) [13]. Chronoamperometry and EIS were carried out at 60 °C to measure the t Li + using Li|CPE|Li cells. As shown in Fig. 3d  The improvements in the ionic conductivity and t Li + benefit from the structure and composition of TpPa-SO 3 Li. As shown in Fig. 3e, the unique one-dimensional channel structure provides rapid Li-ion transfer and shortens the transfer distance. Meanwhile, lithiation sites in the channels significantly reduce the Li-ion diffusion barrier.
To explore the practical application of the CPEs, this work assembled LiFePO 4 |CPE|Li cells and measured their electrochemical performance at 60 °C and 0.5 C with activation at 0.1 C for five cycles. As shown in Fig. 4b, the polarization voltage of LiFePO 4 |(PEO) 18 LiClO 4 |Li significantly increases with the cycle process, while LiFePO 4 |(PEO) 18 LiClO 4 / TpPa-SO 3 Li(0.75)|Li exhibits a stable polarization voltage of 0.11 V (Fig. 4c). This proves that it has the potential to be applied in SSLIB.
EIS spectra at the reduction state were tested to study the interfacial stability. As shown in Fig. 4e, LiFePO 4 |(PEO) 18 LiClO 4 |Li shows two overlapped semicircles in the middle-and high-frequency range. Moreover, the impedance increases rapidly during cycling, which means a serious interface reaction between (PEO) 18 LiClO 4 and electrodes. After three cycles, LiFePO 4 |(PEO) 18 LiClO 4 /TpPa-SO 3 Li(0.75)|Li shows only one semicircle (Fig. 4f), and the corresponding impedance is smaller than that of the initial state. Therefore, the addition of TpPa-SO 3 Li effectively improved the interface stability between the CPE and electrodes.

Conclusions
The TpPa-SO 3 Li was prepared via a mild chemical lithiation strategy and applied as active filler in the PEO-based CPE. The -SO 3 Li groups in TpPa-SO 3 Li work as Li-ion conducting sites in the nanochannels, providing an expressway between the PEO chains. Therefore, high ionic conductivity of 3.01 × 10 −4 S/cm was obtained at 60 °C. When the CPE with only 0.75 wt.% TpPa-SO 3 Li was assembled in the SSLIB, it delivered a reversible capacity of 125 mA·h/g after 250 cycles at 0.5 C and 60 °C. This strategy provides a convenient method to prepare Li-ion conducting COFs and provides a new path for lightweight CPEs.