Lithiated Graphdiyne Quantum Dots for Stable Lithium Metal Anodes
2023; Chinese Chemical Society; Volume: 6; Issue: 5 Linguagem: Inglês
10.31635/ccschem.023.202303188
ISSN2096-5745
AutoresHan Shen, Congying Song, Fan Wang, Guo‐Xing Li, Yuliang Li,
Tópico(s)Electrocatalysts for Energy Conversion
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLES25 Sep 2023Lithiated Graphdiyne Quantum Dots for Stable Lithium Metal Anodes Han Shen, Congying Song, Fan Wang, Guoxing Li and Yuliang Li Han Shen Shandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237 , Congying Song Shandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237 , Fan Wang Shandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237 , Guoxing Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Shandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237 and Yuliang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Shandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237 Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.023.202303188 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Here, a facile strategy is proposed for the preparation of lithiated graphdiyne quantum dots (GDY-Li QDs) with conjugated sp- and sp2-hybridized carbons by the self-assembly technique of π–π stacking of lithiated hexaethynylbezene under mild conditions. The as-prepared GDY-Li QDs, containing stacked multialkynyl aromatic backbone and abundant lithium (Li), show an average diameter of about 2.6 nm and good dispersion in the solvents. These distinctive structures endow GDY-Li QDs with superior properties that cannot be matched by traditional QDs, such as strong ion adsorption, Li-ion self-concentration, high Li-ion conductivity, the nanoconfinement effect, and ion solvation regulation. Benefiting from these features, GDY-Li QDs can stabilize Li-metal anodes to effectively suppress Li-dendrite growth and significantly improve its Li plating/stripping coulombic efficiency (99.3% in the carbonate electrolyte). The full cells with GDY-Li QDs protected Li-metal anodes, and LiNi0.8Co0.1Mn0.1O2 cathodes delivered high capacity and excellent cycling stability at high rates, which demonstrates the great potential of GDY-Li QDs for application in fast-charging Li-metal batteries. Download figure Download PowerPoint Introduction Carbon quantum dots (CQDs), a new class of carbon nanomaterials, have attracted extensive attention for their ultrasmall size, large specific surface, abundant functional sites, photoluminescence, and biocompatibility.1–3 Benefiting from these superior characteristics, CQDs have exhibited many applications in the fields of energy storage and conversion, catalysis, chemical sensing, biosensing, and bioimaging.4–6 The general preparation strategies of CQDs include a "top-down" method that stripes carbon sources into CQDs and a "bottom-up" method that synthesizes CQDs from small organic molecules, oligomers, or biomass.7,8 These processes usually involve harsh conditions such as high temperatures or corrosive chemicals, and the prepared CQDs only consist of sp2- or sp3-hybridized carbons in the core.9 Carbon materials containing sp-hybridized carbons have shown numerous unique properties. Graphdiyne (GDY)10,11 is a classic carbon material comprising conjugated sp- and sp2-hybridized carbons.12–14 The presence of the sp-hybridized carbons endows GDY with tunable electronic properties, strong metal-ion adsorption, and uniformly distributed pores. As a result, GDY performs well in the fields of energy storage/conversion, catalysis, electronics, sensing, and separation.15–27 Particularly in the area of rechargeable batteries, the distinctive electronic structure of sp-hybridized carbon in GDY influences the metal-ion storage, transport, and deposition behavior and gives rise to improved battery performance.28–31 Recently, the development of GDY QDs with good dispersion in solvents further promotes the application of GDY in the areas of catalysis, sensors, biology, and rechargeable batteries.32–34 Thus, creating a new class of CQDs composed of sp- and sp2-hybridized carbons with attractive properties can accelerate the development of carbon nanomaterials and broaden the scope of their application. Li-metal is a promising alternative anode for next-generation high-energy-density rechargeable batteries. However, the commercialization of metallic Li anode is severely hindered by dendrite growth and low coulombic efficiency (CE).35,36 Li deposition behavior is highly related to the Li-ion distribution and concentration at the electrode surface and the Li-ion transport pathway in the batteries.37,38 Particularly on the nanoscale, the realization of homogenous distribution and concentration of Li ions can certainly enable the uniform and dendrite-free deposition of Li. Additionally, the fabrication of an interphase composed of nanosized and Li-concentrated materials on the Li-metal, especially the nanosized Li-rich materials, can significantly enhance the local Li-ion concentration and transport kinetics, optimizing the Li-ion distribution on the nanoscale,39–41 which also benefit the uniform Li deposition and fast Li-ion transport. Owing to the quantum-sized advantages of QDs, traditional CQDs with sp2-hybridized carbons have been used to modify the Li-metal anode to achieve Li-ion adsorbing at the nanoscale level and homogenize Li-ion concentration.42,43 Nevertheless, the relatively weak interactions between sp2-hybridized carbons and Li ions attenuate the ability of concentration and homogenization of Li ions near the anodes, especially under high-rate conditions. Moreover, these CQDs cannot provide an additional Li source and fail to significantly self-enhance the local Li-ion concentration. GDY-based QDs containing numerous sp-hybridized carbons provide much stronger adsorption with Li ions than traditional CQDs and more effectively regulate Li-ion distribution and transport behavior under harsh conditions. Therefore, developing Li-rich GDY QDs with abundant sp-hybridized carbons can provide enough Li source and strong interactions with Li ions to realize stable Li-metal anodes. Multialkynyl aromatic compounds with sp- and sp2-hybridized carbons are a class of monomers for preparing GDY derivatives.44,45 The high reactivity of terminal alkynes not only triggers the coupling reactions but also provides a possibility to metallize the terminal alkynes by metals46,47 (Li, sodium, potassium, etc., referred to as metal-acetylides) and change their intrinsic properties. Owing to the high self-assembly tendency of the highly π-conjugated backbone, the metal-acetylides with aromatic structure can be stacked with each other via π–π interactions to generate the metallized GDY QDs with sp-hybridized carbons (Scheme 1a). As a proof of concept, we demonstrate for the first time a strategy to design and synthesize self-assembled lithiated GDY QDs (denoted as GDY-Li QDs) through π–π stacking of lithiated hexaethynylbezene (HEB-Li) under mild conditions. GDY-Li QDs contain stacked multialkynyl aromatic backbone and abundant Li. Their unique structures allow the features of homogenizing Li-ion distribution and self-concentration of Li ions in the nanoscale region, making them compatible with Li-metal anodes. Owing to the good dispersion in the solvents, an ultrathin GDY-Li QDs layer can be fabricated on the Li-metal surface to tackle the issues of Li-metal anodes. The intrinsic Li-rich feature of GDY-Li QDs provides a Li reservoir to promote local Li-ion concentration and diffusion, and the strong Li-ion adsorption on alkynes of GDY-Li QDs further dynamically enhances the Li-ion concentration and homogenizes Li-ion distribution, thus overcoming the diffusion-limited current for Li deposition. Quantum-sized Li-acetylide-containing dots render a nanoconfinement of Li ions that enables uniform Li-ion transport and deposition; the interactions between electrolyte components and GDY-Li QDs change Li-ion solvation/desolvation behavior and optimize the solid electrolyte interphase (SEI) compositions. Benefiting from these features, the GDY-Li QDs-protected Li metal (denoted as Li@GDY-Li) shows uniform and dendrite-free Li deposition and a high Li plating/stripping CE of 99.3% in the carbonate electrolyte. The Li@GDY-Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) full cell exhibits greatly enhanced cycling stability and capacity retention at the high rate of 3 C. Scheme 1 | Synthesis and structure of self-assembled GDY-Li QDs. (a) General preparation strategy of self-assembled metallized GDY QDs. (b) Synthesis route of HEB-Li. (c) The charge density distribution (left: isosurface, 0.03 e Å−3; right: slice) of HEB and HEB-Li molecules. (d) Schematic illustration of the formation of self-assembled GDY-Li QDs. Download figure Download PowerPoint Experimental Methods Synthesis of GDY-Li QDs Hexakis[(trimethylsilyl)ethynyl]benzene (HEB-TMS) was synthesized as previously reported.10 After removing the protecting groups by tetrabutylammonium fluoride (TBAF), 60 mg hexaethynylbenzene (HEB) was transferred into the glove box, then dissolved in dried and degassed tetrahydrofuran (THF). 1.0 mL (1.6 M, 6.0 equiv) n-butyllithium (dissolved in hexane) was added dropwise into the solution and vigorously stirred for 2 h. The mixture was left to stand for another 2 h until it became cloudy. Then it was separated by ultrahigh-speed centrifugation. The dark-brown solid part was cleaned by oxygen- and moisture-free THF three times. Cells assembling and electrochemical tests All the cells in this work were assembled in CR2032-type coin cells with Celgard2325 separator, and the electrolyte used here was 1 M LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC) (v/v = 1:1) with 15% fluoroethylene carbonate, because carbonate electrolytes are more compatible with high-voltage cathode materials. The cells were tested using a LAND and a Chenhua CHI660E test system at room temperature. Electrochemical impedance spectroscopy (EIS) tests were performed at the open-circuit potential over a frequency range of 10−2–105 Hz and an amplitude at 5 mV. The Li or Li@GDY-Li foils for Li||Li and Li||Cu cells were punched into disks with a diameter of 10 mm, and those for full cells were 16 mm. To fabricate cathodes, LiNi0.8Co0.1Mn0.1O2 (NCM811) powder, super P, and PVDF were fully mixed at a mass ratio of 7:2:1 in 1-methyl-2-pyrrolidinone into a uniform slurry. The slurry was cast onto carbon-coated Al foil, then dried under vacuum. The dried foil was punched into disks with diameter of 12 mm as the cathode. The area loading of the NCM811 cathodes was ∼1.5 mg cm−2. The full cells were run with galvanostatic charge and discharge within a voltage of 3.0–4.3 V. Results and Discussion HEB, the precursor of GDY prepared from the desiliconization of HEB-TMS, contains conjugated terminal alkynes (sp-hybridized carbons) and benzene ring (sp2 hybridized carbons) (Scheme 1b). The highly conjugated structure leads to an improved acidity of terminal alkynyl H atoms, and these terminal alkynyl H atoms can be easily substituted by Li via lithiation agents under mild reaction conditions. Thus, n-butyllithium was used as the lithiation agent here to react with HEB to afford HEB-Li. HEB-Li shows distinct properties from HEB at molecular level. Theoretical calculation results show that the minimum energy of the HEB-Li is 126.9 kJ mol−1, much lower than that of the unlithiated HEB (169.2 kJ mol−1), suggesting that Li substitution of terminal alkynyl H atoms greatly enhances the molecular stability. The charge density distributions of HEB and HEB-Li molecules show that electron clouds on alkynes decrease after lithiation (Scheme 1c), due to the stronger electron-withdrawing property of substituted Li. Owing to the highly π-conjugated structure and good molecular stability, HEB-Li will subsequently self-assemble into GDY-Li QDs through π–π stacking and gradually be precipitated from the solution (Scheme 1d), which is comprised of abundant conjugated benzene rings, alkynyl groups, and terminal Li in both bulk and surface of the material. To make the morphology and structure of GDY-Li QDs clear, a series of microscopic and spectroscopic measurements were carried out. Transmission electron microscopy (TEM) image of the dispersed GDY-Li QDs in Figure 1a describes them as well-distributed ultrafine particles, with an average diameter of 2.6 nm in size distribution statistic (Figure 1b). The high-resolution TEM (HR-TEM) image of GDY-Li QDs exhibits parallel fringes with an average distance of 0.21 nm (Figure 1c), which is assigned to the d-spacing of (100) in the rhomboid unit of periodically assembled GDY-Li QDs.9 The lattice fringes with an approximate distance of 0.37 nm are occasionally observed ( Supporting Information Figure S1), corresponding to the interlayer space of π–π stacking.45 The X-ray diffraction (XRD) pattern of GDY-Li QDs displays two peaks at 20.6° (002) and 32.8° (100) (Figure 1d), which are in agreement with the HR-TEM results. The three-dimensional (3D) image of atomic force microscope (AFM) shows that the height of the GDY-Li QDs is around 3 nm (Figure 1e). All of these results reveal the quantum-size nature of GDY-Li QDs with high crystallization. Figure 1 | Microscopic and spectroscopic characterizations of GDY-Li QDs. (a) TEM image of dispersed GDY-Li QDs. (b) Size distribution of GDY-Li QDs. (c) HR-TEM image of the lattice fringes of GDY-Li QDs (inset: top view of the rhomboid unit). (d) XRD patterns of HEB-TMS and GDY-Li QDs. (e) 3D AFM image of GDY-Li QDs (inset: height profile along the arrow). (f) UV–vis (green) and fluorescence (orange) spectra of GDY-Li QDs/THF mixture (inset: optical photos of GDY-Li QDs/THF under sunlight, 638 nm laser, and 365 nm radiation). (g) 13C NMR and (h) 7Li MAS solid NMR spectra of GDY-Li QDs. (i) C 1s and (j) Li 1s XPS spectra of GDY-Li QDs. (k) FT-IR spectra of HEB-TMS, HEB and GDY-Li QDs. (l) Raman spectra of HEB-TMS, HEB, and GDY-Li QDs. Download figure Download PowerPoint The quantum-sized property was also verified by UV–vis and fluorescence spectra (Figure 1f). The absorption and emission peaks located at 340 and 533 nm are attributed to the highly π-conjugated structure of GDY-Li QDs.48 GDY-Li QDs solution (dispersed in THF) presents a clear Tyndall phenomenon (inset of Figure 1f), indicating the uniform colloidal dispersion of GDY-Li QDs in the solvent. The dispersion of GDY-Li QDs in the solvent also shows strong fluorescence. Solid-state nuclear magnetic resonance (NMR) spectroscopy was performed to investigate the composition of GDY-Li QDs. Figure 1g shows two dominant carbon peaks at 137 and 56 ppm in the 13C NMR spectrum, which are assigned to aryl and alkynyl carbon, respectively. 7Li magic-angle-spinning (MAS) NMR shows symmetric peaks of alkynyl-Li in Figure 1h, suggesting the successful lithiation of HEB. The X-ray photoelectron spectra (XPS) give detailed information about the elemental composition and electrochemical states of GDY-Li QDs. The survey spectrum affirms the existence of C and Li in GDY-Li QDs ( Supporting Information Figure S2). The mainly deconvoluted peaks at 284.4, 284.9, and 285.5 eV in the high-resolution C 1s XPS spectrum correspond to C=C (sp2-hybridized carbon), C≡C (sp-hybridized carbon), and C-Li species, respectively (Figure 1i).10 It is worth noting that the peak of sp-hybridized carbon in C≡C-Li with decreased electron clouds shifts to higher binding energy compared with that of sp-hybridized carbon in C≡C-H,49 which is consistent with the charge density distribution results in Scheme 1c. The peak at 55.2 eV in the Li 1s XPS spectrum is attributed to alkynyl Li (Figure 1j).50 Fourier transform infrared (FT-IR) spectroscopy was employed to track the transformation from HEB-TMS to GDY-Li QDs. After removing the TMS groups of HEB-TMS, the new peaks at 3276 and 2106 cm−1 appear, which correspond to the C-H and C≡CH stretching vibrations of the terminal alkyne of HEB, respectively (Figure 1k). When the terminal alkynyl H atoms of HEB are substituted by Li, the peak of C-Li located at 3278 cm−1 appears, and the peak of C≡C stretching vibration shifts to a lower wavenumber (2104 cm−1), owing to the stronger electron-withdrawing effect of substituted Li. The successful preparation of GDY-Li QDs was also verified by Raman spectra. As shown in Figure 1l, compared to HEB-TMS which presents sharp and intense Raman peaks at 1309.5, 1510.6, and 2146.3 cm−1, HEB shows broader ones at 1342.8, 1582.2, and 1934.6 cm−1. After the lithiation of HEB and the self-assembly of HEB-Li, GDY-Li QDs display peaks around 1335.7, 1575.9, and 1934.4 cm−1, corresponding to D band, G band, and terminal C≡C stretching vibrations, respectively.10 We note that the peak of dialkynyl stretching vibration around 2200 cm−1 is not observed, demonstrating that the coupling reaction of HEB is hindered because the Li substitution blocks the terminal alkynes and reduces their reaction activities. The quantum-sized feature, intrinsic abundant Li, and the unique characteristics of conjugated sp-hybridized carbons enable GDY-Li QDs with great potential to stabilize Li-metal anodes. Benefiting from its good dispersion in solvents, the GDY-Li QDs layer can be simply fabricated on the Li-metal surface through the drop-casting approach (Figure 2a). The AFM image shows that the thickness of the as-prepared GDY-Li QDs layer is around 10 nm (Figure 2b). The thickness can be easily tuned by changing the dosage of the GDY-Li QDs solution ( Supporting Information Figure S3). GDY-Li QDs contain lots of terminal alkynyl Li, which not only provide a Li reservoir to promote local Li-ion concentration but also boost the Li-ion diffusion in GDY-Li QDs protective layer. The Arrhenius plots of Li-ion conductivity show that the freestanding GDY-Li QDs film is ion-conductive, reflecting the enhanced Li-ion transport kinetics in GDY-Li QDs ( Supporting Information Figure S4). The hybridization ways of carbon atoms determine the distribution of electron clouds and in turn influence their interaction with metal ions. The presence of sp-hybridized carbons in GDY-Li QDs greatly enhances their capability for adsorbing Li ions. The sp-hybridized carbon with stronger electronegativity prefers to adsorbing metal ions compared with sp2-hybridized carbon, evidenced by the GDY-based materials.31,51 GDY-Li QDs include stacked HEB-Li segments with conjugated sp- and sp2-hybridized carbons (Figure 2c) show more efficient ionic adsorption than traditional CQDs only composed of sp2- or sp3-hybridized carbons. Density functional theory (DFT) calculations demonstrate two adsorption geometries52,53 (corner of the angular diyne with sp-hybridized carbons, top of the benzene ring center with sp2-hybridized carbons) of Li ions with the HEB and HEB-Li molecules (Figure 2d), and the adsorption at corner sites exhibits larger binding energy for both molecules, indicating that sp-hybridized carbons have stronger interaction with Li ions than sp2-hybridized carbons. More importantly, the lithiation of HEB significantly increases the binding energy with Li ions (−5.46 eV for HEB-Li, −2.05 eV for HEB), indicating that HEB-Li possesses much stronger Li-ion affinity at the molecular level compared with HEB. The strong Li-ion adsorption of GDY-Li QDs can dynamically enhance the local ion concentration. Together with the intrinsic Li-rich feature and fast Li-ion diffusion in GDY-Li QDs, the GDY-Li QDs layer will overcome the diffusion-limited current for Li deposition and effectively suppress the growth of Li dendrites. Linear sweep voltammetry (LSV) was used to experimentally demonstrate the increased local Li-ion concentration and enhanced Li-ion transport enabled by the GDY-Li QDs layer so as to overcome the diffusion-limited current compared to that of bare Li at a voltage range between −0.1 and −0.3 V (Figure 2e), owing to a higher local Li-ion concentration improved by GDY-Li QDs layer. Above a voltage of −0.3 V in the LSV, the significantly increased current density of Li@GDY-Li over bare Li electrodes can be attributed to the enhanced Li-ion transport in the GDY-Li QDs layer. Figure 2 | Li deposition behavior on Cu and Cu@GDY-Li electrodes. (a) Illustration of the GDY-Li QDs layer comprising self-assembled GDY-Li QDs. (b) AFM image of GDY-Li QDs layer (inset: height profile along the arrow). (c) Chemical structure of self-assembled GDY-Li QDs. (d) Charge density difference and binding energy calculation of Li ions with HEB-Li and HEB in corner and top adsorption geometries, respectively. (e) LSV curves of Li@GDY-Li and Li electrodes. (f) X and (g) Y axes of Li-ion density distribution on Li@GDY-Li and bare Li surfaces. (h) Surface mesh simulation of Li-ion distribution on bare Li and Li@GDY-Li surfaces. (i-l) SEM images of deposited Li on (i, k) bare Cu and (j, l) Cu@GDY-Li at 0.5 mA cm−2 and 2 mA h cm−2 from (i, j) top view and (k, l) cross-sectional view. Download figure Download PowerPoint Abundant lithiophilic sites, including heteroatoms, metals, and so on, are crucial for uniform Li nucleation and deposion.54–57 Alkynes and terminal-Li provide GDY-Li QDs with abundant sites for Li adsorption and nucleation. In addition, the GDY-Li QDs render a nanoconfinement effect for Li-ion concentration and transport. Driven by the Li-ion affinity of GDY-Li QDs, Li ions prefer to diffuse towards the QD region, leading to a redistribution of Li ions at quantum-sized level. Together with the enriched Li in QDs, uniform Li-ion distribution and diffusion at the nanoscale are achieved. Molecular dynamics (MD) simulation was carried out to investigate the Li-ion motion near the electrode surface. As shown in Figure 2f,g, well-distributed Li ions are observed near the GDY-Li QDs layer along the X and Y axes, while more rags and fluctuations of the density profile are presented on the bare Li surface. Surface mesh results further confirm homogenized Li ions near the GDY-Li QDs layer. The tiny spheres in Figure 2h represent Li ions, and the colorful faces represent the surface of cavities between Li ions. Therefore, fewer faces near the GDY-Li QDs layer reflect greater homogeneous Li-ion distribution. The uniform Li-ion distribution and transport on the nanoscale enabled by the GDY-Li QDs layer effectively suppress the Li-dendrite growth. Figure 2i–l demonstrates the scanning electron microscopy (SEM) morphologies of deposited Li on bare Cu and Cu@GDY-Li. The uniform, compact, and dendrite-free Li deposition is clearly observed on the Cu@GDY-Li electrode from both top and cross-sectional views (Figure 2j,l). In contrast, the deposited Li on the bare Cu electrode shows a mossy surface and dendritic morphology. Even at a high deposition capacity of 5 mA h cm−2, the GDY QDs layer still enables a uniform and smooth Li deposition without any Li-dendrite growth ( Supporting Information Figure S5). The interactions between electrolyte components and the GDY-Li QDs layer change the Li-ion solvation structure and desolvation behavior. In turn, they influence the SEI compositions and structure. DFT calculations of HEB-Li adsorbed with EC, diethyl carbonate (DEC), and PF6− are depicted in Figure 3a. All the components of electrolyte can be adsorbed at the corner of an angular diyne triangle and undergo the charge transfer with HEB-Li, which weakens the Li-ion solvation structure at the Li@GDY-Li surface and accelerates the desolvation process. Especially for PF6−, the high binding energy with HEB-Li (−3.58 eV) will aggregate the anion on the surface of the GDY-Li QD layer and drive the formation of an anion-derived SEI layer. The changes in the Li-ion solvation structure can be verified by MD simulations (Figure 3b,c). At the surface of bare Li, the radii for Li ions with oxygen on EC and DEC are 2.125 and 2.025 Å, calculated by the radial distribution function (RDF) (Figure 3b), which increase to 2.375 and 2.175 Å near the GDY-Li QDs layer on Li@GDY-Li, respectively (Figure 3c). The increased radii indicate the weakened Li-ion solvation structure enabled by GDY-Li QDs. Raman spectra further experimentally confirm the efficient desolvation process (Figure 3d,e). When mixing GDY-Li QDs with the electrolyte, the peaks of EC and DEC shift to lower wavenumbers (Figure 3d), which suggests that more "free" solvents are generated, indicating the desolvation enforcement of GDY-Li QDs. The peak of PF6− also undergoes a redshift due to the interaction with GDY-Li QDs (Figure 3e). Thus, both the simulation and experimental results demonstrate accelerated Li-ion desolvation, and more "free" Li ions are released through the competitive adsorption of electrolyte components on the GDY-Li QDs layer, which is beneficial for improving the fast-charging performance of Li-metal batteries. The regulated Li-ion solvation structure and aggregation of anions on the anode surface alter the formation of SEI layers. The chemical compositions of SEI were detected by XPS. The F 1s XPS spectra (Figure 3f) can be deconvoluted into two peaks at 684.8 eV (LiF) and 688.1 eV (C-F).58 The relative intensity of LiF in the SEI layer on Li@GDY-Li is higher than that of bare Li anode, which is because more PF6− participates in the early formation of the SEI layer on Li@GDY-Li, owing to the strong interaction with the GDY-Li QDs layer. The LiF-rich components merged with the GDY-Li QDs layer further strengthen the toughness of the protective layer and block the further decomposition of the electrolyte, which is evidenced by the P 2p XPS spectra that shows much lower intensity of P peaks in the SEI of Li@GDY-Li (Figure 3g and Supporting Information Figure S6). The C 1s XPS spectrum demonstrates that the composition of the protective layer remains similar to the GDY-Li QDs layer on the Li@GDY-Li after cycling ( Supporting Information Figure S7), further confirming the mitigated decomposition of the electrolyte. The morphology of SEI formed on bare Li and Li@GDY-Li after cycling was investigated by SEM. The surface of the bare Li-metal shows a number of spherical particles that keep growing to form a porous film after extended cycles (Figure 3h,j and Supporting Information Figure S8), indicating the formation of a loose and vulnerable SEI layer. While the surface of Li@GDY-Li remains smooth and flat, the formed SEI is tightly incorporated into the GDY-Li QDs layer to further improve the mechanical property of the protective layer (Figure 3i,k and Supporting Information Figure S8). Figure 3 | Characterization of the SEI layers and Li-ion solvation structure. (a) Charge density difference and binding energy calculation of HEB-Li with the electrolyte. (b, c) RDF of the oxygen on EC, DEC, and fluorine on PF6- around Li ions near (b) Li and (c) Li@GDY-Li surfaces. (d, e) Raman spectra of the electrolyte with/without GDY-Li QDs. (f) F 1s and (g) P 2p XPS spectra of the SEI layers. (h, i) SEM images of SEI formed on (h) Li and (i) Li@GDY-Li. (j, k) Schematic illustrations of SEI formation on (j) Li and (k) Li@GDY-Li. Download figure Download PowerPoint The dendrite-free Li deposition and strengthened SEI enabled by the GDY-Li QDs layer significantly improve the electroche
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