Dicky Silitonga
email: dsilitonga@gatech.edu

Nico F Declercq

Pascal Pomarède

Fodil Meraghni

Bertrand Boussert

Pooja Dubey

George W Woodruff

Ultrasonic guided waves interaction with cracks in the front glass of thin-film solar photovoltaic module

As the solar photovoltaics power system sees rapid growth in installed capacity and plays a significant role in the future global energy mix, its reliability becomes a crucial factor in maintaining the stability of the electricity supply. Manufacturing imperfections and harsh operating environments may compromise the module's structural integrity, leading to performance deterioration and power loss. Therefore, nondestructive inspection becomes an indispensable part of the quality assurance or maintenance program to detect defects at any stage of the module's lifecycle. Ultrasound is an essential method for material inspection, and ultrasonicguided waves have been long explored as a flaw detection technique on plate-like structures, taking advantage of its long-range detection that yields an efficient inspection process superior to the conventional pulse-echo technique. Inspired by the same idea, this work assesses the prospect of harnessing ultrasonic guided waves, particularly Lamb waves, to detect cracks, as they exist in an actual module. However, unlike the commonly investigated plates, solar photovoltaic modules contain stacks of a-few-microns-thick layers of different materials that add complexities to the structure. The investigated specimen is a thin film photovoltaic module with cracks caused during transportation and handling. It, therefore, represents a real-life research case that may occur in situ. Numerical and experimental methods are performed to reveal various Lamb modes that propagate in the structure, where the results of both methods are mutually confirmed. Unlike other works, this investigation is not confined to the utilization of the non-dispersive mode but attempts to find the defect-sensitive mode that can be used to detect cracks. An analysis of the experimental results reveals the mode most sensitive to cracks, while numerical simulations explain the corresponding phenomena.

Introduction

Solar photovoltaics (PV) has grown exponentially as a clean energy source in recent decades [START_REF]Solar PV -Analysis[END_REF]. It has been proven a practical solution to generate electricity on various scales, from powering a single electronic appliance to being an alternative means of electricity production in remote locations and a source to supply grid systems. It offers flexibility for installation, be it in a dedicated area such as a solar farm, as an additional building feature, or in any other modes of installation, either fixed or equipped with a solar tracker. This versatility leads to an optimistic projection of the future solar photovoltaics module (SPVM) installation. Commercial SPVM generally falls into silicon crystalline and thin film technology, where the latter currently attracts growing attention due to its low production cost potential [START_REF] Thopil | Techno-economic performance comparison of crystalline and thin film PV panels under varying meteorological conditions: a high solar resource southern hemisphere case[END_REF][START_REF] Tsai | Tandem amorphous/microcrystalline silicon thin-film solar modules: developments of novel technologies[END_REF][START_REF] Pouladi | Significant improvement of conversion efficiency by passivation of low-angle grain boundaries in flexible lowcost single-crystal-like GaAs thin-film solar cells directly deposited on metal tape[END_REF]. Furthermore, there still exist many areas open for improvement in the field of thin film solar cells. Besides the standard silicon-based cells, other thin film technologies with improved efficiency are undergoing progressive development, such as gallium nitride (GaN) cells, passivated emitter and rear contact (PERC), and the III-V semiconductors. Although their efficiency is relatively lower compared to the crystalline type modules, the cost-effectiveness of thin film SPVM due to less material usage becomes one of its major selling points in the market. With continuous innovation in thin film technology that tunes up the efficiency of thin film cells, the demand for this type of SPVM is also expected to grow.

As the contribution of PV power generation increases in the future global energy mix [START_REF]IRENA, Future of Solar Photovoltaic: Deployment, Investment, Technology, Grid Integration and Socio-Economic Aspects (A Global Energy Transformation: Paper)[END_REF], the reliability of PV systems becomes a crucial factor in supporting grid stability. Among the efforts to maintain the reliability of PV systems is structural health monitoring (SHM) of individual SPVM. Mechanical defects in the front glass panel may lead to power loss by obstructing sunlight transmission into the cells or damaging the cells and electrical contacts. In addition, the presence of crack may also serve as a precursor to other kinds of defects as it allows moisture to penetrate the solar cell, leading to delamination, corrosion, broken circuit, discoloration, or short circuit [START_REF] Munoz | Early degradation of silicon PV modules and guaranty conditions[END_REF]. Therefore, besides keeping a check on the module's structural integrity, the results of the SHM inspection on SPVM are valuable concerning the module's performance deterioration. Electroluminescence (EL) and infrared (IR) thermography have been the widely used methods for SPVM inspection, mainly for finding hotspot, faulty cells and circuitry [START_REF] Balasubramani | Infrared thermography based defects testing of solar photovoltaic panel with fuzzy rule-based evaluation[END_REF]. Those two methods are effective for cell inspection since their working principles are based on phenomena occurring in the cells, for instance heating (IR) or photons emissions due to carrier recombination (EL). That way, their capability to detect defects on the other module's components outside the electric current path, like the glass, back panel and encapsulant, is limited. Those methods detect cell cracks, yet it may not be able to resolve the overall condition of module's layers. For mechanical defect inspection of the whole structure, ultrasonic technique is more appropriate than the previously mentioned methods because it can detect structural flaws not only in the cells but throughout the module, which includes delamination, air voids and crack [START_REF] Madeti | A comprehensive study on different types of faults and detection techniques for solar photovoltaic system[END_REF]. In addition, ultrasonic technique does not require the module to be energized during inspection.

Conventional ultrasonic scanning technique works by sending ultrasonic signal into the material towards its thickness-wise direction and has two kinds of measurement methods: pulse-echo and transmission. In pulse-echo method, the analyzed signal is the echo coming back to the transducer. On the other hand, transmission method observes the signal received by another transducer at the other side of the structure. Ultrasonic C-scan method collects point-by-point measurements over a given area, producing a map of that enables inspector to locate the exact position and determine the characteristics of the defects. It is a mature and proven method to detect micro-scaled flaws in materials as practiced in various fields. Nevertheless, the inspection process with this conventional ultrasonic bulk wave C-scan method is slow since it needs to examine numerous points to generate a high-resolution image. Hence, the technique is impractical for many SPVM-related use cases, such as final inspection on a fast-moving production line or periodic inspection on a large-scale solar PV power plant.

A promising alternative acoustic method to detect mechanical defects in SPVM is by employing ultrasonic guided waves (UGW). In a plate-like structure, UGW travels in the direction parallel to the plane of principal surfaces, guided by the structure's boundaries, as opposed to the bulk waves that propagate in the thickness-wise direction. Unlike the conventional C-scan method that measures in a point-by-point manner as outlined before, this method benefits from UGW's ability to travel a long distance, interrogating greater area at each measurement which reduces inspection time. Investigators have been using this method to detect discontinuities in plates of various materials, including composites [START_REF] Alleyne | The interaction of Lamb waves with defects[END_REF][START_REF] Hall | Minimum variance guided wave imaging in a quasi-isotropic composite plate[END_REF][START_REF] Castaings | Sizing of impact damages in composite materials using ultrasonic guided waves[END_REF], hence encouraging the authors to adopt the method for inspecting SPVM, which is essentially a multilayered plate that resembles the structure of composites. Nevertheless, an SPVM has a unique structure that differentiates it from other composite materials. In SPVM, multiple thin layers of different materials are sandwiched by substantially thicker plates. Meanwhile, in most laminated plates or composites, the thin layer between two primary plates is a single material, like an adhesive layer. To this end, literature surveys can bring insights since the study of UGW in multilayered plates has been published by many researchers [START_REF] Rose | An approach to guided wave mode selection for inspection of laminated plate[END_REF][START_REF] Ricci | Guided waves for structural health monitoring in composites: a review and implementation strategies[END_REF][START_REF] Cunfu | The propagation of coupled Lamb waves in multilayered arbitrary anisotropic composite laminates[END_REF]. Still, further investigation is necessary to understand the characteristics of UGW in a particular plate constructed with a typical combination of materials and layer thickness, as found in SPVM.

In this paper, we aim to understand the characteristics of UGW in a thin film SPVM with a vision toward its utilization for a non-destructive evaluation technique. Here, our study focuses on how the UGW interacts with cracks as a significant defect that directly affects structural integrity. This investigation combines numerical simulation and experimental methods to describe the propagation characteristics of UGW in the SPVM specimen and the phenomena associated with the interaction of waves with cracks.

Sample description

The SPVM investigated in this work is a thin-film type solar cell manufactured by SHARP (model NA-E135-L5). As stated in the product datasheet, this model features a tandem structure of amorphous and microcrystalline silicon layer. Fig. 1 shows the specimen under a 5-axis scanner arm in our laboratory. The specimen is a full-size module measuring 1401 mm × 1001 mm with an overall thickness of 6.58 mm. A closer look from the front side of the module, as depicted in Fig. 2, shows that solar cells are assembled in a stripes-like pattern that runs in the lengthwise orientation of the module. The bright lines visible from the front side, uniformly spaced at 10.5 mm, are portions of rear metal electrodes that protrude through the thin film layer.

The cross-sectional anatomy of the SPVM specimen received from the manufacturer is illustrated in Fig. 2. It is a multilayered structure with glass plates sandwiching the solar cell assembly. Note that the illustration is not to scale since the physical dimension of the solar cell assembly is much thinner than the glasses. Underneath the upper glass is a layer of TCO (transparent conductive oxide) coating, followed by silicon layers and rear metal electrodes. EVA (ethylene vinyl acetate) is an encapsulant to seal the cells, prevent moisture ingress, and bond the back glass to complete the module assembly. The solar cell itself is of a tandem type; thus, the Si part is a micromorph that consists of microcrystalline Silicon (μc-Si) bottom cell and amorphous Silicon (a-Si) top cell.

Our specimen contains defects that occurred due to transportation and handling activities. There are cracks of varying severity and density throughout the front glass plate. In some areas, there is also delamination visible from the front side. Since the defects are caused by realworld activities instead of artificially induced damages, this investigation is expected to deliver results that are more pertinent to the actual operations of the SPVM. However, there exist consequences of these actual defects, i.e., their randomness in shape and size, since they occurred in an uncontrolled manner. Therefore, it is essential to obtain the dimensional detail of defects, in this case, the cracks under scrutiny. This task, presented in the result section of this paper, was accomplished by employing Scanning Acoustic Microscopy (SAM). Cracks were observed through SAM imaging with a 400 MHz transducer that scanned an area where a crack exists at a resolution of 2.5 μm.

Numerical simulations of the UGW dispersion in the structure

A model of a multilayered plate was numerically simulated to provide preliminary information on the behavior of UGW prior to experiments and to verify the results of experimental data processing. DISPERSE, a dispersion curves calculator software was employed for this task. The simulated model was a multilayered plate mimicking the structure of the investigated SPVM specimen in terms of materials and thickness of the layup, except that the model is a stack of infinite plates. The infinite character of the model was approached in the experiments by measuring in areas away from edges to alleviate the reception of edge reflection. In addition, all measurements were made in the direction parallel to the visible lines, representing cell assembly patterns, since the received wave was intended to have traveled along a uniformly flat structure. Otherwise, the waves would have encountered a periodic structure of solar cell patterns along their propagation path in a line between transducers.

Although the types of glass, TCO, and rear electrodes are undisclosed by the manufacturer, shortlisted materials commonly used in commercial SPVM can be selected to build the model. The typical solar panel front cover material is soda lime float glass [START_REF] Allsopp | Towards improved cover glasses for photovoltaic devices[END_REF]. The actual thickness measurement of our specimen's glass panels is 3.2 mm, consistent with the standard thickness figure of industrial glasses available in the market. As the transparent conductive oxide, FTO (fluorine-doped tin oxide) is the widely chosen material in the market owing to its low-cost [START_REF] Way | Fluorine doped tin oxide as an alternative of indium tin oxide for bottom electrode of semi-transparent organic photovoltaic devices[END_REF][START_REF] Anderson | Thin films for advanced glazing applications[END_REF][START_REF] Klochko | Development of semi-transparent Fig. 10. Power flow distribution and particle displacement normal to surface plane, at wavenumber k = 0.04 mm -1 . The vertical axis defines the thickness-wise position in the structure (midplane = 0.0, top glass surface = 3.2, bottom glass surface = -3.2), horizontal axis defines the normalized magnitude of each parameter. ZnO/FTO solar thermoelectric nanogenerator for energy efficient glazing[END_REF]. For the rear electrode, Cu, Ag, Al, and Mo are some of the suitable metals. When simulating the model with those metals as the rear contact layer, there was no noticeable difference in the Cu, Ag, and Mo dispersion curves. The Al showed different curves where one higher frequency mode deviated off the position of the same mode in other metals' dispersion curves. Of those three possible metal choices, Mo is more favorable in CIGS or perovskite solar cells [START_REF] Li | Efficiency improvement of CIGS solar cells by a modified rear contact[END_REF][START_REF] Abdy | Investigation of metal-nickel oxide contacts used for perovskite solar cell[END_REF][START_REF] Behrouznejad | A study on utilizing different metals as the back contact of CH3NH3PbI3 perovskite solar cells[END_REF][START_REF] Salhi | The photovoltaic cell based on CIGS: principles and technologies[END_REF]. Meanwhile, our module is based on Si solar cells. Thus, it can be assumed that the metal in our specimen is either Cu or Ag, both being standard materials for Si-based solar cell rear electrodes [START_REF] Tous | Large area copper plated silicon solar cell exceeding 19.5% efficiency[END_REF][START_REF] Ur Rehman | Crystalline silicon solar cells with nickel/copper contacts[END_REF]. The simulations showed that selecting any of those two metals as the element of the simulated model does not make any difference in the resulting dispersion curves. Table 1 describes the constituents of the specimen with their corresponding mechanical properties as input values for calculating dispersion curves in DISPERSE.

Experiment

We studied the characteristics of UGW in SPVM by observing dispersion curves of the modes propagating in the structure. For this purpose, a setup as schematized in Fig. 3 was designed to experiment as Alleyne and Cawley [START_REF] Alleyne | A two-dimensional Fourier transform method for the measurement of propagating multimode signals[END_REF]. It allowed the acquisition of signals from a series of equally spaced points which were then processed with a 2D Fourier transform procedure in MATLAB to plot dispersion curves in a frequency-wavenumber space. This setup used two contact transducers: one as a stationary emitter and another as a receiver mounted on a moving scanner arm. The arm of a 5-axis ultrasonic scanner (POLAR C-Scan) carried the receiver along a 100 mm line path while the system recorded a time-domain waveform every 0.25 mm. Thus, every run of the experiment registered waveforms from 400 points. The receiver started at 30 mm from the emitter, and the final distance between transducers at the end of the scanning stroke length was 130 mm.

The emission signal was a 5-burst sinusoidal pulse of 500 kHz frequency, generated by a waveform generator (Stanford Research Systems DS345), which then underwent a 10x amplification before reaching the emitting transducer. Both transducers (Panametrics V103) were of longitudinal wave type, positioned at normal incidence concerning the plane of the plate by placing the transducers' face in contact with the front glass surface. Water was introduced as a couplant on both transducers, considering the practical reason that it is a standard substance for cleaning SPVM, thus ensuring the safety and possibility for future adaptation of the proposed technique to the regular module cleaning activities.

During the experiment, the scanner was set to line scanning mode in the direction parallel to the solar cell strips. This direction was chosen to receive the waves that traveled in a uniformly flat structure between the transducers, undistracted by the solar cell's strip patterns that the waves would bump into should they propagate towards other directions. Therefore, the condition of the received waves in the experiment could approximate that of the infinite flat plate geometry model on the numerical simulation. Measurements were conducted on pristine and cracked areas to study the influence of cracks on the propagation of UGW in SPVM by comparing and analyzing the resulting dispersion curves. Areas with cracks of different severity levels were investigated in this work, as shown in Fig. 4: (b) one crack, (c) two cracks, and (c) a shattered area. In the case of investigating one and two cracks, transducers were placed such that the defect was located in the 30 mm gap between the transducers at the receiver's initial position.

Results

The crack image obtained from SAM presented in Fig. 5 shows a line of one-pixel width. Since the resolution of the SAM is limited to 2.5 μm/ pixel, the crack can be of any width up to that size and is classified as a microcrack. In the SPVM defect terminology, any cracks less than 30 μm in width are microcracks [START_REF] Da Silva | Review on the sources of power loss in monofacial and bifacial photovoltaic technologies[END_REF][START_REF] Chiou | Micro crack detection of multi-crystalline silicon solar wafer using machine vision techniques[END_REF]. From visual observation of the specimen, the crack in the investigated areas penetrates through the entire front glass thickness. In contrast, the back panel glass is perfectly intact. Recalling that our experimental setup involved contact transducers placed on the top glass surface and that the transducers were optimized for longitudinal waves, i.e., perturbation out of the element's plane, the receiver would predominantly pick up vertical displacement of the surface. Therefore, in the experiment, we observed mainly Lamb waves by picking up out-of-plane motion. Accordingly, horizontal shear modes were either weakly probed or undetected in the received signals.

DISPERSE calculation yielded 13 Lamb modes within the range up to 1.5 MHz, plotted in frequency-wavenumber (f-k) and frequency-phase velocity (f-v ph ) diagrams in Fig. 6(a) and (b), respectively. Whatever mode is considered, it is not necessarily symmetric or anti-symmetric throughout its spectrum since the plate structure does not form any symmetry [START_REF] Zhang | Piezoacoustic wave spectra using improved surface impedance matrix: application to high impedance-contrast layered plates[END_REF] in either geometry or material properties. Evidence of this is shown in Fig. 6(c), where mode 1 is symmetric about the horizontal midplane at the low wavenumber region but anti-symmetric at a higher part of the curve, as demonstrated by the numerically simulated shape of plate displacement. This behavior also occurs in other modes, although some retain the shape throughout the curve. Therefore, to designate the modes in Fig. 6, plain numbers from 1 to 13 are assigned without mentioning any symmetric or anti-symmetric characteristic.

There are multiple approaches to selecting the most appropriate mode in Lamb waves inspection techniques. A standard method is by exciting a mode in its non-dispersive frequency range, and another is by selecting a specific mode sensitive to particular defects [START_REF] Alleyne | Optimization of lamb wave inspection techniques[END_REF]. By examining the f-v ph curves in Fig. 6(b), it is evident that mode 2 is an excellent choice for the non-dispersive mode excitation method owing to its flat velocity profile over a broad frequency regime. It exhibits an insignificant velocity change (5523 m/s ± 0.6%) over a broad frequency band between 100 and 400 kHz. Being a lower frequency mode is also an advantage of mode 2, allowing it to travel greater distances. Moreover, by exciting mode 2 in the lower part of its non-dispersive band (100 kHz-200 kHz), the occurrence of mode conversion, if any, will be evident since the only possible converted Lamb mode is unmistakably mode 1. Indeed, selecting and analyzing the first appearing modes at low frequency, or the fundamental modes, are the standard practice in Lamb waves-based inspection technique [START_REF] Lowe | Low-frequency reflection characteristics of the s0 Lamb wave from a rectangular notch in a plate[END_REF][START_REF] Lowe | The low frequency reflection characteristics of the fundamental antisymmetric Lamb wave a0 from a rectangular notch in a plate[END_REF][START_REF] Wandowski | Circular sensing networks for guided waves based structural health monitoring[END_REF].

Although the technique of exciting pure non-dispersive modes is simple and mature, it is, for our purposes, insufficiently sensitive to minor defects due to the relatively long wavelength of the low frequency these fundamental modes have. This shortcoming is known and becomes the reason why researchers have explored techniques that exploit other, more sensitive modes [START_REF] Cirtautas | Selection of Higher Order Lamb Wave Mode for Assessment of Pipeline Corrosion[END_REF][START_REF] Satyarnarayan | Circumferential higher order guided wave modes for the detection and sizing of cracks and pinholes in pipe support regions[END_REF], to detect microcracks in materials like metals and composites, although this far has not been explored in SPVM research.

When such modes are used, a non-dispersive nature is no longer the primordial focus but rather the selection of modes that appear to undergo substantial changes upon interaction with certain defects. Thus, any mode may be suitable, offering a wide range of frequency and wavelength choices. On the other hand, while selecting the nondispersive modes requires a relatively straightforward analysis of the curves, discovering the damage-sensitive modes necessitates an extended effort. In this work, the approach to finding the sensitive modes was carried out experimentally. The modes of interest are those with sensitivity to cracks, under the focus of this paper, as stated in the introductory section.

Dispersive nature of the propagating UGW in the SPVM specimen can be discerned from the change in time signal waveform shape at different distances, as depicted in Fig. 7. It shows the waveforms of UGW signals obtained experimentally from three distances between the emitter and receiver in the undamaged area. The amplitude is normalized to emphasize the evolution of the waveform shape over the propagation distance due to the dispersive nature of UGW. At the emitter-receiver distance of 130 mm, the first arrival of the signal appears at 14 μs, which yields 9290 m/s of wave velocity. As the discussion proceeds, it will be shown that this velocity is within the range of the velocity of the fastest propagating mode in the specimen during the experiments. That mode lies in a higher frequency band beyond the fundamental modes.

The experimental dispersion curves were plotted as a color-scaled diagram in MATLAB by performing a 2-D Fourier transform to the time-space matrix of waveforms acquired from a series of positions. Four diagrams of dispersion curves from the measurements on areas with different conditions are presented in Fig. 8. To enable comparison with simulation results, the curves generated by DISPERSE software are overlaid on the diagrams. Those diagrams demonstrate that there are 4 Lamb wave modes excited in the experiments, as indicated by the highintensity color traces, which coincide with 4 of the curves in Fig. 6(a): mode 1, mode 2, mode 5, and mode 11. The non-visible modes are either non-existent or considerably less dominant relative to the appearing modes, which can be related to the perpendicular angle of incidence of the emission signal. Changing the angle of incidence by placing the transducers in an oblique orientation will shift the dominance of and generate the invisible modes in the present experiments [START_REF] Cirtautas | Selection of Higher Order Lamb Wave Mode for Assessment of Pipeline Corrosion[END_REF][START_REF] Park | Selective generation of lamb wave modes in a finite-width plate by angle-beam excitation method[END_REF][START_REF] Xue | Impact of Wedge Parameters on Ultrasonic Lamb Wave Liquid-Level Sensor[END_REF]. However, our work stayed as close as possible to real applications in which normal incidence is expected. Therefore, despite the absence of some theoretical modes, the plots in Fig. 8 still indicate that numerical results agree with the experimental counterparts, hence validating the methods employed in this work.

The color plots in Fig. 8 show modes 1 and 2 consistently appearing in all cases, regardless of damage severity, inferring their insensitivity to crack types assessed in this investigation. The plots also demonstrate that the defects do not induce any mode conversion in all modes throughout the spectrum. In contrast to the first two modes, modes 5 and 11 exhibits characteristic behavior as their intensity diminishes with increasing damage severity, with mode 11 being the most impacted. To quantitatively present this observation, the mean of pixel values in the high-intensity areas in Fig. 8 are calculated and plotted as bar charts in Fig. 9. The mean value is taken to represent the level of intensity strength around a select area. One can compare mode intensities at different cracking conditions by inspecting the charts. The areas in Fig. 8 considered for the calculations are rectangles bounded vertically from wavenumber (k) 0.03/mm to 0.05/mm for all modes; and horizontally within the frequency (f) range of 30 kHz-80 kHz for mode 1, 150 kHz-200 kHz for mode 2, 580 kHz-630 kHz for mode 5, and 1100 kHz-1160 kHz for mode 11. Among those modes, mode 11 shows a consistent decline in intensity as the crack severity escalates.

Common sense may lead to an argument that a higher frequency wave has a shorter wavelength which is indeed more sensitive to discontinuity. Nevertheless, due to the dispersive nature of Lamb waves, the wavelengths of higher-order modes are not necessarily shorter than that of the lower-order modes because the velocities vary throughout their frequency spectra. To prove that argument, a physical example can be taken from the frequency and velocity relation of the first arrived signal, as discussed earlier in Fig. 7. Among the four propagating modes, the relatively high velocity of 9290 m/s found in Fig. 7 at 130 mm emitter-receiver distance is only possible in mode 11 at 1.2 MHz, see Fig. 6(b). With that velocity and frequency pair, the calculated wavelength is 7.7 mm, which is within the wavelength range of mode 5, a mode which lies in the frequency regime far below mode 11, see Table 2. The wavelength of each propagating mode in Table 2 is calculated by selecting the frequency range with a high-intensity color trace in Fig. 8. Then, the value of phase velocity at those selected frequency ranges is extracted from Fig. 6(b). The wavelength is calculated through the relation λ = v ph /f. Despite being within a similar wavelength range, modes 5 and 11 have distinct characteristics regarding their propagation mechanism. Those modes have a different profile of power flow distribution that affects their sensitivity to defects on the plate layers. DISPERSE simulation results, exhibited in Fig. 10, reveal that most waves' energy is located at the lower layer in mode 5. Meanwhile, the opposite profile prevails in mode 11. Likewise, in terms of particle displacement in the direction normal to the surface, i.e., the motion to be probed by the contact transducer, mode 5 creates more dominant displacement in the lower portion of the structure. At the same time, the reverse applies to mode 11.

All the cracks in our specimen exist in the upper glass, or the module's front panel, with the rear glass perfectly intact. Thus, in the case of mode 11, a more significant portion of the wave energy passes through the defective upper glass, scattered and attenuated, diminishing the transmitted energy at the other side of the defect. The portion of the wave energy that propagates in the lower glass, although being undisturbed by defects, only carries a low intensity. Therefore, even if the mode wave can reconstruct downstream of the crack, the wave's overall energy intensity picked up by the receiver is substantially reduced. In addition, the middle layers contain soft EVA polymer with high acoustic absorption, suppressing energy exchange between the lower and upper parts of the structure. Eventually, discontinuities in the upper part of the structure, or the front glass panel of our investigated specimen, are more impactful to mode 11. Therefore, analyzing the behavior of this mode is appropriate for detecting front panel cracks, which is one of the critical failure modes in SPVM operations. From the practical point of view, utilizing this mode is also advantageous because the measurement is

Fig. 1 .

 1 Fig. 1. The solar module specimen under the 5-axis ultrasonic scanner.

Fig. 2 .

 2 Fig.2. Anatomy of the module with details of solar cell structure, materials, and thickness. Notice that the metal portion that protrudes into the Si layer are the lines visible from the front side through the TCO (the schematic is drawn according to the information from SHARP Energy Solutions Europe).

Fig. 3 .

 3 Fig. 3. Schematic of the experimental setup to obtain dispersion curves.

Fig. 4 .

 4 Fig. 4. Conditions investigated in the experiments.

Fig. 5 .

 5 Fig. 5. SAM observation: (a) The section containing a typical crack investigated in this work; (b) SAM image of the crack; (c) Further magnification of the SAM image showing the crack as a line of one-pixel width.

Fig. 6 .

 6 Fig. 6. Plots of dispersion curves generated from DISPERSE simulation: diagrams of frequency versus (a) waveform and (b) phase velocity. The number corresponds to the nearby mode curve, and the red curves are the propagating modes appearing in the experiments. (c) Shapes of mode 1 at 100 kHz and 285 kHz demonstrate different symmetry behaviors within the same mode. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7 .

 7 Fig. 7. Waveforms of signals on the undamaged area at three different emitterreceiver distances.

Fig. 8 .

 8 Fig. 8. Dispersion curves of Lamb waves obtained from experiments on 4 areas with different conditions (refer to Fig. 4). Out of 13 theoretical modes according to DISPERSE simulation within 0-1.5 MHz range, four modes appear: modes 1, 2, 5, and 11. Although the dispersion curves obtained from the undamaged area (top left) and the area with one crack (top right) look similar, the color trace along Mode 11 is slightly weaker in the area with crack, indicating the decrease in the mode's intensity. When the guided waves travel through two cracks, color trace along Mode 11 noticeably fades out, implying further suppression in its intensity (bottom left). In the shattered area (bottom right), Mode 11 is no longer visible in the diagram and even the intensity of Mode 5 is significantly reduced. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9 .

 9 Fig. 9. The relative intensity of the bright areas in Fig. 8, in the range of k = 0.03 mm -1 to k = 0.05 mm -1 , for mode 1 (30 kHz-80 kHz), mode 2 (150 kHz-200 kHz), mode 5 (580 kHz-630 kHz), and mode 11 (1100 kHz-1160 kHz). Error bars indicate standard errors from 3 experiments. Mode 11 demonstrates consistent intensity reduction with increased crack severity in the solar module's front panel.

Table 1

 1 Layers specifications of the specimen model for numerical simulations.

	Layer	Material	Thickness (mm)	Density (g/cm 3 )	Young's modulus	Poisson's ratio
					(GPa)
	1	Glass (Soda	3.2	2.5	73	0.24
		lime)
	2	TCO (FTO)	0.0015	6.8	150	0.33
	3	a-Si + μcSi	0.0020	2.3	181	0.22
	4	Rear
		electrode:
		Cu	0.0015	7.8	110	0.34
		-0r -
		Ag	0.0015	10	76	0.37
	5	EVA	0.175	0.94	0.02	0.45
	6	Glass (Soda	3.2	2.5	73	0.24
		lime)

Table 2

 2 Acoustic parameters of the propagating Lamb modes.

	Mode	f (kHz)	v p (m/s)	λ (mm)
	1	20-200	2147-5538	10.7-277
	2	125-250	5545-5597	22.2-44.8
	5	530-700	5032-5443	7.2-10.3
	11	1100-1200	6300-29000	5.7-24.2

Acknowledgement

The authors appreciate the contribution of SHARP Energy Solutions Europe for providing valuable information regarding product details.

Data availability

Data will be made available on request.

made on the front side, where accessibility will not become an issue.

Conclusion

This work demonstrates the selection of damage-sensitive Lamb mode to detect cracks in the front glass of SPVM. Experiments were carried out to extract dispersion curves, and the results agree with numerical simulations. This has validated the capability of the experimental technique based on the 2D Fourier transform to capture the propagating modes in a structure with multilayers of micron-thickness films as in SPVM. It is beneficial from the perspective of nondestructive evaluation since the technique is based on an experimental approach familiar with NDT of plate structures in general and applied to SPVM. This method can be embedded in automated mechanisms or module-cleaning devices in prospective practical implementations.

The identification of damage-sensitive modes involved analysis of the intensity of modes as they interact with defects of different severity levels. Results show that a mode undergoes remarkable intensity decay when the waves propagate through areas with increasing cracks. Therefore, that mode can be exploited as a damage indicator to detect cracks and gain information on their severity. Unlike the standard approach of Lamb waves-based inspection that utilizes the lowfrequency fundamental modes, the selected damage-sensitive mode is located at the higher frequency band at 1.1 MHz-1.2 MHz.

CRediT authorship contribution statement

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Aza-Michael Addition

Jirong Wang

Chi Zhang

Yong Zhang

Gong Chen

Rinaldo Poli

Xiaolin Xie

Zhigang Xue

Facile Assembly of C-N Bond-Containing Polymer Electrolytes Enabled by Lithium Salt-Catalyzed

Keywords: polymer electrolyte, aza-Michael addition, lithium metal batteries, lithium salt

Assembling polymer electrolytes (PEs) with lithium metal anodes is a promising strategy to address safety and specific capacity concerns. Molecular design on conductive polymer matrices avoids inherent high crystallinity, poor ion transport and combustion drawbacks by adjusting the chain structure and functional groups. However, conventional structural modulation methods are difficult to be conducted via in-situ polymerization and cell assembly, thus hampering the further performance-enhancement of PEs in lithium batteries.

Herein, we report a simple aza-Michael addition method induced by lithium salts for designing and fabricating high-performance PEs. This method fulfils the in-situ structure modulation of polyether-based PEs without any introduction of non-electrolytic components. The obtained polyether electrolytes exhibit a more amorphous crosslinked structure, leading to a favorable medium for ion migration and excellent flexibility. In addition, the introduction of C-N bonds imparts excellent non-flammability to the material, as well as the ability to interact with sulfolane. Hence, the in-situ constructed gel polymer electrolyte (GPE) is characterized by high ionic conductivity (1.76 × 10 -4 S cm -1 at 30 o C) and excellent interface stability with the electrodes. The assembled Li/LiFePO4 (LFP) battery based on this newly designed GPE exhibits a high initial specific discharge capacity and a good rate performance (112.7 mAh g -1 at 5 C).

Long-term stability was also demonstrated, with a capacity retention of 86.8% after 500 cycles at 1C.

INTRODUCTION

Lithium metal batteries (LMBs) have great potential for providing high energy density and long lifespan to numerous energy-storage devices. [START_REF] Chen | Opportunities and Challenges of High-Energy Lithium Metal Batteries for Electric Vehicle Applications[END_REF][START_REF] Li | Functional Polymers for Lithium Metal Batteries[END_REF][START_REF] Wu | Guidelines and Trends for Next-generation Rechargeable Lithium and Lithium-ion Batteries[END_REF][START_REF] Yoon | Challenges and Strategies towards Practically Feasible Solid-State Lithium Metal Batteries[END_REF] Solid electrolytes, especially polymer electrolytes (PEs), have been identified as promising candidates to escape from the safety hazards and highly active interface reaction caused by conventional liquid electrolytes. [START_REF] Liu | Upgrading Traditional Liquid Electrolyte via In Situ Gelation for Future Lithium Metal Batteries[END_REF][START_REF] Wang | Advances in Host Selection and Interface Regulation of Polymer Electrolytes[END_REF][START_REF] Wu | Polymer Electrolytes and Interfaces toward Solid-State Batteries: Recent Advances and Prospects[END_REF][START_REF] Wang | Reviewing the Current Status and Development of Polymer Electrolytes for Solid-State Lithium Batteries[END_REF] However, the most used polyether-based electrolyte system is limited by the upper cut-off voltage and slack Li + migration due to its high crystallinity, [START_REF] Chen | Inhibition of Crystallization of Poly(ethylene oxide) by Ionic Liquid: Insight into Plasticizing Mechanism and Application for Solid-State Sodium Ion Batteries[END_REF][START_REF] Zhang | Nanoconfinement and Salt Synergistically Suppress Crystallization in Polyethylene Oxide[END_REF] tight lithium ion (Li + ) affinity [START_REF] An | Progress in Solid Polymer Electrolytes for Lithium-Ion Batteries and Beyond[END_REF][START_REF] Mindemark | Beyond PEO-Alternative Host Materials For Li + -Conducting Solid Polymer Electrolytes[END_REF][START_REF] Tang | Advances in Composite Polymer Electrolytes for Lithium Batteries and Beyond[END_REF] and poor interface characteristics, [START_REF] Wu | Polymer Electrolytes and Interfaces toward Solid-State Batteries: Recent Advances and Prospects[END_REF][START_REF] Verma | A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries[END_REF] thus hampering its practical application in powering consumer electronics. In addition to the segment mobility and stability, bulk polyether is easy ignitable, and the conventional flame-retardant strategy through addition of small-molecule flame retardants yields poor compatibility with the electrodes, deteriorating the battery performance. [START_REF] Zhu | Constructing Flame-Retardant Gel Polymer Electrolytes via Multiscale Free Radical Annihilating Agents for Ni-Rich Lithium Batteries[END_REF][START_REF] Long | Thermotolerant and Fire-proof Gel Polymer Electrolyte toward High-Performance and Safe Lithium-Ion Battery[END_REF][START_REF] Mi | Topology Crafting of Polyvinylidene Difluoride Electrolyte Creates Ultra-Long Cycling High-Voltage Lithium Metal Solid-State Batteries[END_REF] Inserting multiple functional groups in, or modulating the structure of the polymer matrix is an effective strategy to attenuate some inherent limitations of the polyether chains. [START_REF] Meng | Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries[END_REF][START_REF] Wang | Structure Code for Advanced Polymer Electrolyte in Lithium-Ion Batteries[END_REF] However, the poor ion-conduction ability of the solid polymer electrolytes and interfacial issues with the electrodes still require the help of liquid components and in-situ polymerization methods. [START_REF] Li | Functional Polymers for Lithium Metal Batteries[END_REF][START_REF] Wu | Polymer Electrolytes and Interfaces toward Solid-State Batteries: Recent Advances and Prospects[END_REF][START_REF] Ding | Polymer Electrolytes and Interfaces in Solid-State Lithium Metal Batteries[END_REF] Gel polymer electrolytes (GPEs) prepared through in-situ polymerization methods can circumvent the issues of low ionic conductivity, poor electrolyte-electrode infiltration and contact, and high interfacial impedance. [START_REF] Liu | Upgrading Traditional Liquid Electrolyte via In Situ Gelation for Future Lithium Metal Batteries[END_REF][START_REF] Ma | In Situ Preparation of Gel Polymer Electrolyte for Lithium Batteries: Progress And Perspectives[END_REF][START_REF] Wang | Molecular Engineering of A Gel Polymer Electrolyte via In-Situ Polymerization for High Performance Lithium Metal Batteries[END_REF][START_REF] Chen | Self-Enhancing Gel Polymer Electrolyte by In Situ Construction for Enabling Safe Lithium Metal Battery[END_REF] More importantly, the liquid phase can be carefully chosen and designed according to the structure and components of the polymer skeleton. For instance, the use of deep eutectic solvent (DES) as the plasticizer can simultaneously improve the ionic conductivity and nonflammability of the polymer electrolyte, [START_REF] Pei | Li-N Interaction Induced Deep Eutectic Gel Polymer Electrolyte for High Performance Lithium-Metal Batteries[END_REF][START_REF] Wang | In Situ Cross-Linked Plastic Crystal Electrolytes for Wide-Temperature and High-Energy-Density Lithium Metal Batteries[END_REF][START_REF] Navarro-Suárez | Perspective-Semi-Solid Electrolytes Based on Deep Eutectic Solvents: Opportunities and Future Directions[END_REF][START_REF] Wu | Deep Eutectic Solvents for Boosting Electrochemical Energy Storage and Conversion: A Review and Perspective[END_REF] and its unique advantages of low-cost, simple preparation and flexible designability provide great possibility for enhancing the performance of GPEs. [START_REF] Wu | Lidfob Initiated In Situ Polymerization of Novel Eutectic Solution Enables Room-Temperature Solid Lithium Metal Batteries[END_REF][START_REF] Fu | A Dual-Salt Coupled Fluoroethylene Carbonate Succinonitrile-Based Electrolyte Enables Li-Metal Batteries[END_REF][START_REF] Ogawa | Lithium Salt/Amide-Based Deep Eutectic Electrolytes for Lithium-Ion Batteries: Electrochemical, Thermal And Computational Study[END_REF] Moreover, unwanted parasitic electrochemical reactions between the liquid plasticizer and the electrode are eliminated via the deliberate installation of intermolecular interactions between the polymer segments and polar liquid molecules. However, the precise GPE structural design relying on in-situ polymerization is unrealistic because of the difficult-to-modulate polymerization process in an enclosed battery. In addition, the conventional initiations used in in-situ LMBs polymerizations are often stimulus-responsive (heating, irradiation, etc.), and the introduction of side reactions and non-electrolytic components may exert an adverse influence on the battery performance. [START_REF] Hwang | Room Temperature Cross-Linkable Gel Polymer Electrolytes for Lithium Ion Batteries by In Situ Cationic Polymerization of Divinyl Ether[END_REF][START_REF] Lu | Dendrite-Free, High-Rate, Long-Life Lithium Metal Batteries with a 3D Cross-Linked Network Polymer Electrolyte[END_REF][START_REF] Wei | In Situ Gel Polymer Electrolyte with Inhibited Lithium Dendrite Growth and Enhanced Interfacial Stability for Lithium-Metal Batteries[END_REF] Hence, designing an efficient structure-modulation strategy for in-situ fabrication of gel polymer electrolytes is significantly necessary for high-performance LMBs.

The Michael addition is one of the most attractive click reactions due to its atom economy, mild reaction conditions and available raw materials, all of which conform to the principles of green chemistry. [START_REF] Mather | Addition Reactions in Macromolecular Design for Emerging Technologies[END_REF][START_REF] Rulev | Reaction: Achievements and Prospects[END_REF] The main participants include a proton-bearing nucleophilic groups such as a thiol, an alcohol, or an amine, and an electrophilic alkene. Thereinto, the inherent basicity of amines triggers the addition process without the need of a catalyst, [START_REF] Rulev | Reaction: Achievements and Prospects[END_REF][START_REF] Akinc | Parallel Synthesis and Biophysical Characterization of A Degradable Polymer Library For Gene Delivery[END_REF][START_REF] Peyrton | Reaction as A Greener, Safer, and More Sustainable Approach to Biobased Polyurethane Thermosets[END_REF] which makes this reaction possible for the in-situ construction of crosslinked polymer electrolytes via utilizing bis(primary amine) and crosslinking monomer. Moreover, the reaction rate can be easily regulated by lithium salt, [START_REF] Zhou | Lithium Chloride Catalyzed Aza-Michael Addition of Pyrazoles to α,β-Unsaturated Imides[END_REF][39][START_REF] Azizi | Liclo4 Accelerated Michael addition of Amines to Α,Β-Unsaturated Olefins Under Solvent-Free Conditions[END_REF] which is the inherent component of electrolyte. Meanwhile, the properties of the crosslinked matrix can be modulated by the variety of diamine and the diamine/diacrylate molar ratio, [START_REF] Naga | Synthesis of Polymer Networks by Means of Addition Reactions of Tri-Amine and Poly(Ethylene Glycol) Diacrylate or Diglycidyl Ether Compounds[END_REF][START_REF] Gao | Hyperbranched Poly(Beta-Amino Ester) Based Polyplex Nanopaticles for Delivery of CRISPR/Cas9 System and Treatment of HPV Infection Associated Cervical Cancer[END_REF][START_REF] Liu | Poly(beta-Amino Esters): Synthesis, Formulations, and Their Biomedical Applications[END_REF] forming in-situ a favorable electrolyte network for ion-conduction. In addition, this reaction introduces a large fraction of C-N bonds into the polymer chains, which endows the corresponding electrolyte with unique physical and flame-retardant properties and provides abundant coordination points to interact with the plasticizer. [START_REF] Pei | Li-N Interaction Induced Deep Eutectic Gel Polymer Electrolyte for High Performance Lithium-Metal Batteries[END_REF][START_REF] Feng | One-Pot, Solventand Catalyst-Free Synthesis of Polyphosphoramide as An Eco-Benign and Effective Flame Retardant for Poly(lactic acid)[END_REF] Here, we introduce a structural modulation of in-situ polyether-based electrolytes via a simple and mild double aza-Michael addition reaction (Figure 1). In comparison with crosslinked polymer (PPEGDA) formed via the conventional radical-initiated polymerization of a PEO-based diacrylate monomer (PEGDA), the aza-Michael process is more favorable without any introduction of non-electrolytic components and side reactions, and the obtained polymer network with the introduction of abundant C-N bonds is more amorphous and nonflammable. In addition, the polar groups (C-N and C=O) in this polymer skeleton can interact with DES to fabricate an advanced GPE with good ionic conduction ability, a wide electrochemical window (5.5 V vs Li/Li + , 60 o C) and stable interfacial characteristics. As a result, the assembled Li/LFP batteries exhibit obviously improved rate performance and cycling stability with respect to the pure polyether and DES systems, and the optimized lithium symmetric cell based on this GPE can steadily work at higher current density (0.5 mA cm -2 ) without obvious dendrite-growth. The present contribution provides a simple and novel avenue for the in-situ regulation of the electrolyte structure and electrochemical performance of advanced and safe LMBs.

EXPERIMENTAL SECTION

Materials. Poly(ethylene glycol) diacrylate (PEGDA, Mn = 600, Aladdin), 1,4butanedioldiacrylate (BDA, 90%, Innochem) and methyl acrylate (MA, 90% , Innochem) were purified by the columns of neutral aluminium oxide (Al2O3) to remove residual polymer inhibitors and then stored in a refrigerator. Propylamine (1-AP, 99%, Aladdin), 1,3diaminopropane (DAP, 98%, Innochem), diethylene glycol bis(3-aminopropyl) ether (DGBE, 98%, Aladdin), poly(propylene glycol) bis(2-aminopropyl ether) (PPGBAE, Mn = 400, Aladdin), and sulfolane (SL, 98%, Innochem) were used directly without any purification. Before in-situ polymerization, above corresponding reagents required three freeze-pump-thaw cycles under argon flow. Bis(trifluoromethane)sulfonimide lithium (LiTFSI, 99%, Aladdin), lithium difluoro(oxalato)borate (LiDFOB, 99.9%, Innochem), lithium perchlorate (LiClO4, 99%, Innochem), lithium hexafluorophosphate (LiPF6, 99%, Innochem) and lithium tetrafluoroborate (LiBF4, 99%, Innochem) were dried in a vacuum oven and then transferred to glovebox for storage. The used cellulose separator (TF4840) has a thickness of 40 μm, a diameter of 16 mm and an areal density of 48 g m -2 . The thickness of the in situ obtained electrolyte film is around 80 μm to 100 μm. Lithium iron phosphate (LiFePO4), lithium polyacrylate (PAALi, Mn = 450000, Aladdin) and acetylene black (AB) were used for the preparation of anode material.

Simulation Experiment of Lithium Salt-Catalyzed Aza-Michael Addition Reaction.

Different lithium salts (0.23 mmol, including LiClO4, LiTFSI, LiDFOB, LiBF4 and LiPF6) were dissolved separately in MA (4.65 mmol), and then mesitylene (1 wt%) was dripped into the solution as a reference reagent. 1-AP (2.32 mmol) was added quickly to the bottle, and the mixed solution was stirred at room temperature. The aliquots were taken from the reaction mixture at different time and then dissolved in CDCl3 for NMR test. The control group is without lithium salt, and other operation are same to the above process. (MA, δA = 6.41, 6.12, 5.84 ppm; Momo-Adduct, δB1 = 2.88 ppm, δC1 = 2.58 ppm, δD1 = 2.54 ppm, δE1 = 3.76 ppm, δF1 = 1.52 ppm, δG1 = 0.91 ppm; Bi-Adduct, δB2 = 2.76 ppm, δC2 = 2.45 ppm, δD1 = 2.37 ppm, δE2 = 3.68 ppm, δF2 = 1.43 ppm, δG2 = 0.85 ppm).

LiClO4 was be selected for the qualitative research of linear polymerization by aza-Michael addition. Firstly, LiClO4 (0.05 mmol) and BDA (1 mmol) were uniform mixing, then 1-AP (1 mmol) was dripped into the bottle to proceed the aza-Michael addition. The solution was monitored by 1 H NMR spectrum after 0.6 h, 6 h and 18 h of reaction, respectively. The only difference of control group is that no lithium salt was added, and other operations are same to the above process. The control group did not add LiClO4, but other process was the same. (BDA, δa = 6.41, 6.12, 5.84 ppm; Momo-Adduct, δb1 = 1.52 ppm, δc1 = 2.59 ppm, δd1 = 2.53 ppm, δe1 = Then, a certain mass of DES was mixed with PEGDA600 (1 mmol) and amine monomers (1mmol, including DAP and DGBE) to prepare the reaction precursor. During the battery assembly process, a drop of the precursor liquid was added to the cellulose membrane of the battery. Finally, the battery was assembled in-situ, reserved in a vacuum oven at 60 °C for 12 hours to make the reaction complete. Finally, the in-situ GPE membrane is named as X-PEGDA600-DES Y (X, the abbreviation of amine monomer; Y, the mass-percentage value of DES relative to overall mass).

Calculation of Crosslinking Density. Dynamic mechanical properties were characterized by the Diamond DMA. Standard samples with diameter of 1cm and thickness of 1 mm were tested in compression mode, while the tested temperature ranges from -80 to 80 °C and the frequency is 1

Hz. The temperature corresponding to the maximum loss modulus was recorded as Tg. The formula for calculating the crosslinking density (ρ) is as follow: ρ = E'/ϕRT. Thereinto, E′ is the storage modulus at Tg + 30 °C, ϕ is the front factor which can be approximated to 1 in the Flory theory, R is the gas constant at Tg + 30 °C, and T is the absolute temperature at Tg + 30 °C.

Fabrication of Li/LFP Cells. The LiFePO4 cathode was prepared by casting a slurry of LFP, PAALi, and AB in deionized water on Al foil, and the weight ratio of LFP, PAALi, and AB is 80:10:10. The casted cathode film was dried at 80 °C for 12 h and punched into small plates with a diameter of 16 mm. The mass loading of the active material was ~1.5 mg cm -2 . CR2032 coin cells were assembled in the glove box (O2 < 0.1 ppm; H2O < 0.1 ppm) under an Ar atmosphere with TF080 as the separator and lithium plates as the anode. The corresponding precursor was injected into the cell, and the cell was then assembled and heated for testing. The cycling performance of the cell was tested at the voltage range from 2.5 V to 4.0 V vs. Li/Li + on the LAND CT2001A test system (Wuhan Land Electronics Co. Ltd., China) at 60 o C.

Characterization. The thermostability of the solid and gel polymer electrolytes were tested from 30 °C to 800 °C with a heating rate of 10 °C/min through a thermogravimetric analyzer (TGA, PerkinElmer 4000), and the whole process is under a nitrogen (N2) atmosphere. Their glass transition temperature (Tg) was measured by a differential scanning calorimetry (DSC, Q2000) with a heating rate of 10 °C/min from -90 °C to 100 °C under a N2 atmosphere.

Electrochemical Performance.

The ionic conductivities (σ) of the polymer electrolytes were evaluated using an electrochemical workstation (Autolab PGSTAT302N, Netherlands), and the tested temperature ranges from 30 to 80 °C. σ can be calculated from the following equation:

σ 𝑖 = 𝐿 𝑅 𝑖 ×𝑆
. Thereinto, the film thickness (D) and area (S) were measured after the end of the EIS measurement, and Ri represents the impedance value at different temperature.

The electrolyte membranes were sandwiched between the lithium and steel sheets to in-situ form the batteries, and their electrochemical stability windows were conducted on a test (60°C) of linear sweep voltammetry (LSV) under a potential from 0 to 7 V. The Li|SPE or GPE|Li cell was assembled to reflect the condition of lithium-ion migration under a polarization voltage (ΔV) of 10 mV. The current (I0), interfacial resistance (R0) at the outset, and current (Is), interfacial resistance (Rs) under the stable condition were all recorded to measure the lithium-ion transference number (tLi + ), which can be calculated by equation:

𝑡 𝐿𝑖 + = 𝐼 𝑠 (𝛥𝑉-𝐼 0 𝑅 0 ) 𝐼 0 (𝛥𝑉-𝐼 𝑠 𝑅 𝑠 )
.

Surface Properties of Lithium Anode.

The lithium symmetric cells assembled with corresponding electrolytes were used for the analysis of interfacial performance after the lithium-deposition cycling (60°C). The surface morphology of the lithium anode was detected by a scanning electron microscopy (SEM, Nova NanoSEM 450). X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W) was applied to analyze the element composition of lithium anodes after the longtime chargedischarge cycling.

RESULTS AND DISCUSSION

Effect of Lithium Salts on the Aza-Michael Addition. In order to ensure a satisfying crosslinked rate for the subsequent construction of polymer electrolytes, the effect of various lithium salts (LiTFSI, LiClO4, LiDFOB, LiBF4 and LiPF6) on the reaction rate was first studied based on monofunctional propylamine (1-AP) and methyl acrylate (MA), in a 1:2 molar ratio, and the detailed calculation process (equation S1 in Supporting Information) and resulting conversion values by [START_REF] Chen | Opportunities and Challenges of High-Energy Lithium Metal Batteries for Electric Vehicle Applications[END_REF] H NMR spectra are listed in Tables S1-S6 (Supporting Information), Figures S1-S5 (Supporting Information) and Figure 2a. In the early stages (< 1 h), the reaction was fast no matter whether salt was present or not, which is consistent with a fast addition of the primary amine to consume one MA equivalent and generate the intermediate secondary amine.

The consumption of the second MA equivalent to yield the final tertiary amine product, on the other hand, was much slower, proceeding to less than 10% after 5 h in the absence of lithium salt. A dramatic drop of reactivity on going from a primary to a secondary amine in aza-Michael additions is well established, [START_REF] Wu | Effects of Chemistries Of Trifunctional Amines On Mechanisms Of Michael Addition Polymerizations With Diacrylates[END_REF][START_REF] Cheng | Michael Addition Polymerization of Trifunctional Amine and Acrylic Monomer: A Versatile Platform for Development of Biomaterials[END_REF] hence the lithium salts mainly play an accelerated role in the second addition step (Figure S6), which can be also confirmed by the quantification of the secondary amine and tertiary amine according to equations S2 and S3. This proposition of lithium salt-catalyzed aza-Michael bis-addition was further clarified by density functional theory (DFT), and the calculation results indicate that the acceleration effect of lithium salts on amine addition not only results from the decrease in activation barrier via the coordination between Li + and C=O group, but also contributes to the stabilization of transition state and intermediate. The computational method and results are detailed in the Supporting Information (Figures S7-S10). According to above results, LiTFSI was chosen as the salt, and three different diamines, including 1,3-diaminopropane (DAP), diethylene glycol bis(3-aminopropyl ether) (DGBE) and poly(propylene glycol) bis(2-aminopropyl ether) (PPGBAE) were used to react with PEGDA for preparing crosslinking polymer electrolyte (Figure 1). As shown in Figure 2b and Figure S11, the addition of LiTFSI significantly shortens the solidification time, while the crosslinked rate was also found to depend on the length of the diamine ether due to the decreased concentration of reactive groups in the reaction mixture. For the shortest-chain DAP, the solidification was efficient after only 5 min at room temperature, while the longer the amino chain (DGBE and PPGBAE), the slower the reaction, thus a heating stimulus was needed for the curing process.

The gelation time was quantified by the crossover between the storage and loss modulus, as shown in Figure 2c-2e, the gelation times of DAP/PEGDA600 systems with and without LiTFSI addition are 83.7 min and 9.5 min at 30 o C, respectively, while longer time about 40 min was observed in DGBE/PEGDA600 system under 60 o C heating, which is consistent with above vial experiments. The FT-IR spectra shown in Figure S12 confirms the complete crosslinking in DGBE/PEGDA600 system after 1.5 h heating, and there is almost no residual C=C stretching vibration in the LiTFSI-added system, while a peak is clearly observed for the non-catalyzed one. Most noteworthy is that the obtained crosslinked products are directly usable as lithiumdoped solid polymer electrolytes. Therefore, this facile method paves the way to directly obtain polymer electrolytes for LMBs without the need to introduce any non-electrolytic additives. Performance Evaluation of Solid PEs Obtained from Aza-Michael Reaction. The physicochemical properties and conducting power of the obtained polymer electrolytes were evaluated to reveal the superiority of the aza-Michael reaction in the structural design and construction of polyether-based LMBs. In comparation, the PPEGDA600 was also prepared via conventional AIBN-initiated radical polymerization. As shown in Figure 3a, the flexibility of the obtained membranes was examined by a folding test. The PPEGDA600 film fractures upon bending, while the crosslinked membranes obtained via the aza-Michael reaction is softer, being freely bent without damage. This feature is due to the lower crosslinking density (ρ) in DAP-PEGDA600 and DGBE-PEGDA600 membranes relative to PPEGDA600 (Figure S13), which can be calculated according to the rubber elasticity theory (Table S8). [START_REF] Zhao | Renewable Epoxy Networks Derived from Lignin-Based Monomers: Effect of Cross-Linking Density[END_REF][START_REF] Nouailhas | Synthesis and Properties of Biobased Epoxy Resins. Part 1. Glycidylation of Flavonoids by Epichlorohydrin[END_REF] In view of the gelation rate for in-situ battery preparation and the content of ethyl oxide (EO) group, DGBE-PPEGDA600 system was chosen for further study. The inherent crystallinity of the polyether electrolyte is interrupted by the crosslinking structure and the introduction of C-N bonds, thus the adduct of aza-Michael reaction exhibits a lower glass transition temperature (Tg) relative to the C-C crosslinking in PPEGDA600, lowering the Tg value to -45.3 o C (Figure 3b). The increase of the segment mobility is also reflected in the ion conduction (Figure 3c) and mechanical properties (Figure S14). The ionic conductivity of DGBE-PPEGDA600 is higher than that of PPEGDA600, and the elongation of DGBE-PEGDA600 (79.9%) is higher than that of PPEGDA600 (33.1%), which is consistent with the above crosslinking density and folding results. In addition, the insertion of C-N bonds leads to improved redox stability, with a high oxidation voltage of 4.9 V and tLi+ (Figures S15 andS16).

The advantages of the DGBE-PPEGDA600 electrolyte also reflect in the in-situ prepared lithium metal batteries. As shown in Figure 3d, the as-assembled Li/LFP cell can steadily work at 0.1 C (60 o C) for 100 cycles with a high-capacity retention of 95.1%, while PPEGDA600 retains only an 82.1% capacity after 75 cycles under the same operating conditions, indicating that the formed interface between the DGBE-PPEGDA600 electrolyte and the electrode is stable.

The relatively higher voltage of PPEGDA600 in lithium plating-stripping tests carried out for 600 h with a current density of 0.05 mA cm -2 also confirms that the C-N inserted polyether is a better Li + carrier than the conventional one (Figure 3e). More interestingly, the introduction of N atom endows the polyether-based electrolyte with an excellent fire-retardant ability. As shown in Figure 3f, the decomposition temperature of DGBE-PPEGDA600 with 50% remaining (Td, 50%) is higher than PPEGDA600, and the PPEGDA600 film immediately catches fire, whereas the DGBE-PPEGDA600 material cannot be ignited after several seconds (Figure 3g and Movie S1 in Supporting Information). This is because the inserted N atoms generate inert gases (e.g. NH3), [START_REF] Wang | Reviewing the Current Status and Development of Polymer Electrolytes for Solid-State Lithium Batteries[END_REF] which cover the polymer surface, thus restraining the propagation of oxygen radicals. This nonflammability property paves a simple way for the fabrication of high safety polymer electrolytes via structural design.

Performance Optimization via C-N Bonding with Deep Eutectic Solvent. In addition to the flexibility and fire retardancy properties, the structural design with the insertion of C-N bonds also endows the material with the ability to interact with a liquid plasticizer, thus constructing a gel polymer electrolyte for better ion migration. As a common additive in liquid electrolyte, sulfonyl-containing compounds can improve the rate performance of corresponding batteries, and the interface stability between the electrode and the electrolyte is significantly enhanced via their reductive decomposition. Moreover, the interaction between this compound and lithium salt forms deep eutectic solvent (DES), which can decrease the total viscosity of the precursor and improve the electrode infiltration, which are both beneficial for in-situ batteries construction.

Hence, sulfolane was chosen to premix with LiTFSI and LiDFOB (SL:LiTFSI:LiDFOB = 5:1:0.5), and the formed DES was used in aza-Michael system for preparing gel polymer electrolyte. LiDFOB is added for further interface-reinforcement due to its easy generation of Fand B-containing solid electrolyte interface. [START_REF] Huang | Modulation of Solid Electrolyte Interphase of Lithium-Ion Batteries by lidfob and libob Electrolyte Additives[END_REF] Indeed, gels with improved electrochemical performance could be obtained by adding different content of DES, the obtained gel electrolytes are named PPEGDA-DESx and DGBE-PEGDA600-DESx, respectively, where x represents the weight fraction of DES. As shown in Figure S17, the introduction of the DES yields a marked increase of the ionic conductivity and of the electrochemical stability window (around 5.5 V), and the ion-migration ability increases with the DES content. Although the observed decomposition temperature of the gel electrolytes is lower than the solid ones, it is sufficiently high to allow battery processing. Relative to PPEGDA-DES, the gel electrolyte obtained by the combination of DES and DGBE-PEGDA600 matrix exhibits greater advantage. For example, the critical current density (CCD) of the lithium symmetric batteries based on DGBE-PEGDA600-DES60 (1.08 mA cm -2 ) is higher than that of PPEGDA-DES60 (0.75 mA cm -2 ) (Figure S18), and the high crosslinking density Li|PPEGDA-DES60|Li and Li|DES|Li cells short-circuit at the current density of 0.5 and 0.3 mA cm -2 (Figure S18), respectively, whereas the lithium symmetric cells based on DGBE-PEGDA600-DES60 can steadily work at different current densities, and the more conductive DGBE-PEGDA600-DES60 exhibits a lower overpotential (Figure 4a). Similar tendency was also observed in the rate performance of Li/LFP batteries, as shown in Figure 4b, the Li|DGBE-PEGDA-DES60|LFP cell delivers better cycling stability and higher specific capacities than that in Li|PPEGDA-DES60|LFP cell. Above results are ascribed to the C-N bonds and polar carbonyl groups in DGBE-PEGDA600 matrix, which can interact with the SL molecules and with the lithium salts, thus decreasing the drastic reactions and stabilizing the electrode-electrolyte interface.

When DES weight fraction reaches 80 wt%, a high ionic conductivity value of 1.76 × 10 -4 S cm -1 was obtained at 30 o C. The charge-discharge process of the resulting Li|DGBE-PEGDA-DES80|LFP cell was significantly enhanced with a slight capacity decay as the current increases.

High specific discharge capacities of 112 mAh g -1 and 79 mAh g -1 were obtained at 5 C and 10 C, respectively (Figure 4c). The narrow voltage platform at different rates also reflects the excellent charge-discharge reversibility of the Li|DGBE-PEGDA600-DES80|LFP battery (Figure 4d). Further increased current-tolerance is obtained in the Li|DGBE-PEGDA600-DES80|Li cell with the critical current density of 1.08 mA cm -2 (Figure 4e), and long-term cycling stability is shown under the relatively high current density of 0.5 mA cm -2 for 300 h (Figure 4f). The Li|DGBE-PEGDA600-DES80|Li battery can also steadily cycle for 600 h (0.1 mA cm -2 ) without any short-circuit (Figure S19). In addition, the full battery assembled with insitu formed DGBE-PEGDA600-DES80 has a high initial discharge capacity of 145.2 mAh g -1 at 0.5 C and can cycle for 500 cycles with the capacity retention rate of 86.8% (Figure 4g), without showing any obvious deteriorating of the voltage platform even after 500 cycles (Figure S20). This indicates a good compatibility of this electrolyte with the lithium anode and LFP cathode. Interfacial Characterization. To gather additional information on the electrolyte-electrode interface of the Li|DGBE-PEGDA600-DES80|Li cell, the lithium anode was analyzed by X-ray photoelectron spectroscopy (XPS). For comparison, the lithium symmetric battery assembled with pure DES was also studied. Both cells were cycled for 100 hours at a current density of 0.1 mA cm -2 . As shown in Figures 5a and5d, signals attributable to C-F bonds (C 1s, 293.8 eV; F 1s, 689.0 eV) were clearly observed on the lithium surface of the GPE system, whereas these signals were barely visible for the DES system. [START_REF] Fu | Lithium Nitrate Regulated Sulfone Electrolytes for Lithium Metal Batteries[END_REF] Instead, a strong LiF peak (F 1s) appears at 685. 0 eV. This phenomenon indicates that there is extensive reduction of the TFSI -and DFOB - anions in the DES system to generate LiF. The effect of the DGBE-PEGDA600 matrix to fix the anions through interacting with the C-N bonds and the polar carbonyl groups inhibits this reduction reaction. Moreover, the introduction of polymer skeleton leads to the formation of ROCO2Li in the solid electrolyte interface (SEI) layer by reacting with the lithium anode, which is suggested by the observation of a peak (O 1s spectrum) (Figure 5b). The presence of this organic component in the SEI layer was further supported by the N 1s spectrum. As shown in Figure 5c, the N-containing components in the DES system are mainly derived from the decomposition of TFSI -and an extra signal attributable to LixNOy (403.12 eV, N 1s) was detected for the GPE system. In addition, the higher content of Li2SO3, LiS and S-S derivatives in the GPE system than that in DES is attributed to the decomposition of the dissociated SL molecules (Figure 5e), [START_REF] Yao | Vinyl Ethylene Sulfite as A New Additive in Propylene Carbonate-Based Electrolyte for Lithium Ion Batteries[END_REF][START_REF] Parimalam | Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: lipf6, libf4, lidfob, libob, and litfsi[END_REF] which are beneficial for stabilizing the electrolyte-electrode interface. [START_REF] Xu | Interface Layer Formation in Solid Polymer Electrolyte Lithium Batteries: An XPS Study[END_REF][START_REF] Li | Composition Analysis of the Solid Electrolyte Interphase Film on Carbon Electrode of Lithium-Ion Battery Based on Lithium Difluoro(Oxalate)Borate and Sulfolane[END_REF] In addition, the use of LiDFOB provides a robust B-containing SEI composition (Figure 5f). [START_REF] Xu | Investigation and Application of Lithium Difluoro(oxalate)borate (LiDFOB) as Additive to Improve the Thermal Stability of Electrolyte for Lithium-Ion Batteries[END_REF] According to the above results, we can clarify that the in-situ fabrication of the Ncontaining crosslinked polymer electrolyte not only decreases the drastic reaction between the active lithium metal and anions, but also provides helpful organic SEI components. The organicinorganic composite SEI layer of GPE exhibits higher stability than the cell interface assembled with DES, thus providing excellent long-cycle stability and good interfacial compatibility for LMBs. Scanning electron microscopy (SEM) was then used to explore the anode surface morphology. As shown in Figures 5g-5i, after the same time of charging and discharging, the lithium assembled with GPE shows a smooth and flat surface without any evident dendrite growth (Figure 5g), while the lithium anode in liquid DES system exhibits tiny cracks and irregularities (Figure 5h). These results further confirm the polymer matrix effect on dendrite suppression. When using a DES electrolyte devoid of LiDFOB, evident and uneven protrusions appear on the lithium metal surface (Figure 5i), indicating the need of a small amount of LiDFOB in the design of the DES component. The results of both XPS and SEM confirm the positive effect of the C-N bond-containing polymer on the dendrite suppression and cell stability.

CONCLUSIONS

In summary, we have developed a simple and efficient aza-Michael double addition reaction catalyzed by lithium salts. The obtained network structure exhibits relatively low crosslinking density and Tg. Moreover, the insertion of C-N bonds endows the polyether-based polymer electrolyte with additional favorable properties relative to the conventional electrolyte, including excellent flame-retardancy, facile ion transport and better compatibility with a polar SLcontaining DES. Hence, a novel and advanced gel polymer electrolyte was prepared by in-situ incorporation of the DES into the reaction system. The liquid DES further improves the Li + mobility, and its strong interaction with the nitrogen-containing polymer network enhances the interfacial stability. The assembled lithium symmetric battery can withstand a high current density (critical current density of 1.08 mA cm -2 ) and steadily works for 300 h at the current density of 0.5 mA cm -2 . XPS and SEM were used to determine the constitution and morphology of the lithium anode after cycling. In addition, the as-assembled Li/LFP cell could be cycled for

Figure 1 .

 1 Figure 1. C-N bond-containing non-flammable polymer electrolytes via lithium salt-catalyzed aza-Michael addition.

2 .

 2 89 ppm, δf1 = 1.75 ppm, δg1 = 4.19 ppm, δh1 = 0.91 ppm; Linear Polymer, δb2 = 1.42 ppm, δc2 = 2.44 ppm, δd1 = 2.37 ppm, δe2 = 2.77 ppm, δf2 = 1.71 ppm, δg2 = 4.09 ppm, δh1 = 0.85 ppm). Preparation of Solid Polymer Electrolytes. Preparation of DGBE-PEGDA600 and DAP-PEGDA600: Equimolar DAP or DGBE and PEGDA600 were mixed with LiTFSI:LiDFOB (2:1), and the EO/Li = 20:1, to form a precursor solution. The precursor was then reserved in a vacuum oven at 60 °C for 12 h. Preparation of PPEGDA600: PEGDA600 was mixed with LiTFSI:LiDFOB (2:1), and the EO/Li = 20:1, to form a precursor solution. 0.5 wt% AIBN was then added and heated in a vacuum oven at 60 °C for 12 h. Fabrication of Gel Polymer Electrolytes. LiTFSI (2 mmol), LiDFOB (1 mmol), and SL (15 mmol) were mixed and stirred vigorously at room temperature to form the liquid-state DES.

Figure 2 .

 2 Figure 2. (a) Time evolution of the vinyl conversion for the aza-Michael addition of 1-AP to MA catalyzed by different lithium salts (10%, room temperature). (b) Digital images of the reaction between different diamines and PEGDA accelerated by LiTFSI. (c) Storage and loss modulus of DAP-PEGDA600 system without addition of LiTFSI at different time. Storage and loss modulus of DAP-PEGDA600 and DGBE-PEGDA600 system with addition of LiTFSI at different time.

Figure 3 .

 3 Figure 3. (a) Photographs of PPEGDA600, DAP-PEGDA600 and DGBE-PEGDA600 before and after folding. (b) DSC curves of PPEGDA600 and DGBE-PEGDA600. (c) Temperature-dependent ionic conductivities of PPEGDA and DGBE-PEGDA600. (d) Cycling performance of DGBE-PEGDA600 with a rate of 0.1 C at 60 o C. (e) Voltage profile for plating/stripping experiments in Li symmetric cell cycled at the current density of 0.05 mA cm -2 at 60 o C. (f) TGA curves of PPEGDA and DGBE-PEGDA600. (g) Combustion experiments of PPEGDA and DGBE-PEGDA600.

Figure 4 .

 4 Figure 4. (a) Galvanostatic cycling curves of lithium symmetric cells based on PPEGDA600-DES60 and DGBE-PEGDA600-DES60 at different current densities. (b) Rate performance of Li|DGBE-PEGDA600-DES60|LFP and Li|PPEGDA600-DES60|LFP cells. (c) Rate performance of Li|DGBE-PEGDA600-DES80|LFP. (d) Charge-discharge curves of Li|DGBE-PEGDA600-DES80|LFP cell at different rates. (e) CCD of Li|DGBE-PEGDA600-DES80|Li cell. (f) Galvanostatic cycling profiles of the Li|DGBE-PEGDA600-DES80|Li symmetrical cell at the current density of 0.5 mA cm -2 . (g) Long cycling profiles of the Li|DGBE-PEGDA600-DES80|LFP cell at 0.5 C and corresponding charge-discharge curves. All tests were carried out at 60 °C. (h) Photographs of the lamps lightened by the flat and folded Li|DGBE-PEGDA600-DES80|LFP pouch cell.

  Therefore, the flexibility of the branched DGBE-PEGDA600 skeleton permits the assembly of pouch cells for practical application. As shown in Figure4h, the Li|DGBE-PEGDA600-DES80|LFP pouch cell was assembled and tested. The initial open-circuit voltage delivered by the cell, after being charged to the upper cutoff voltage of 4.2 V, was 3.24 V. Abuse experiments were then conducted. There was no internal short circuit or failing after a 90 o bend, demonstrating a satisfactory flexibility and safety of the DGBE-PEGDA600-DES80 electrolyte for advanced lithium metal batteries.

Figure 5 .

 5 Figure 5. (a) Substance analysis of the SEI layer of the lithium metal anode in the Li|DGBE-PEGDA600-DES80|Li cell by (a) C 1s, (b) O 1s, (c) N 1s, (d) F 2p, (e) S 2p and (f) B 1s XPS. SEM images of the micromorphology of lithium metal anodes based on (g) DGBE-PEGDA600-DES80, (h) pure DES and (i) DES without LiDFOB under 100 and 20 μm scales. All the lithium anodes were tested after 100 h cycling at the current density of 0.1 mA cm -2 .

ACKNOWLEDGMENT J.W and C.Z contribute equally. Z.X and J.W are grateful to the National Natural Science Foundation of China (Grant No. 51973073), the Fellowship of China Postdoctoral Science Foundation (2021M701303), and the analytical and testing assistance from the Analysis and Testing Center of HUST for support of this work. R.P is grateful to the CALMIP mesocenter of the University of Toulouse for the allocation of computational resources.

of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Chi Zhang -Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Yong Zhang -Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Gong Chen -Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Rinaldo Poli -CNRS, LCC (Laboratoire de Chimie de Coordination), UPS, INPT, Université de Toulouse, 205 route de Narbonne, F-31077 Toulouse, Cedex 4, France Xiaolin Xie -Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ORCID Rinaldo Poli: 0000-0002-5220-2515 Xiaolin Xie: 0000-0001-5097-7416

was able to lighten up the bulb even after bending. This study provides an extremely promising guide for the structural design of in-situ polymer electrolytes and for the preparation of highperformance and safe gel PE systems for LMBs.

ASSOCIATED CONTENT

Supporting Information

Notes

The authors declare no competing financial interest.