Si-based Composite Anode for Li-ion Batteries with Enhanced Electrochemical Performance via CuCl2•2H2O Assisted Coating of PDA
Silicon (Si) is a promising anode material for Li-ion batteries but its application is limited due to its low electronic conductivity and severe volume change during the lithiation/delithiation process. Recently, coating a carbon layer on the Si surface and doping the alloy phase into the bulk Si have been considered as effective approaches to resolve the aforementioned issues. The present study proposes a facile method via coating the polydopamine (PDA) with the assistance of CuCl2•2H2O followed by a high-temperature annealing process to successfully fabricate the Si-based anode
Introduction<!>Synthesis of Si-Cu3Si@C composite<!>Material Characterizations<!>Electrochemical Measurements<!>1) Microstructure and Morphology<!>2) Electrochemical performance and analysis<!>Conclusions<!>Conflicts of interest
<p>Compared with the traditional anode material, graphite, of lithium-ion batteries (LIBs), silicon (Si) has been becoming an attractive alternative due to its much higher theoretical specific capacity (~3580 mAh/g at room temperature), a low delithiation potential (~0.4 V vs Li + /Li) and abundance in the earth's crust (~23.6%) [1,2].</p><p>Nevertheless, there are two major issues encountered in the commercialization of Si anode [3][4][5]: 1) instability of solid electrolyte interface (SEI) film on the surface of Si and pulverization of materials caused by a huge volume change (~300%) of Si during the lithiation/delithiation process; and 2) a very low electronic conductivity (~10 -5 S cm -1 ) with a non-ideal charge transfer rate due to the nature of a semiconductor. Aiming to alleviate the volume change and enhance the electronic conductivity of Si anode, various methods have been adopted, such as fabricating nanoscale Si-based materials [6,7], developing new binders [8,9], alloying Si with metal elements [10][11][12] and decorating the surface of Si (e.g., coating carbon (C) layer [13][14][15][16]).</p><p>In order to improve the performance of Si anode, one of the promising strategies is to make full use of synergistic effects of different aforementioned methods. Recently, Si-based anode materials with a unique structure of Si-MSi-M@C or Si-MSi@C (M = metal element) have been paid much attention and studied by many groups [17][18][19][20][21][22].</p><p>Specifically, a kind of metal element is doped firstly to the bulk Si, forming an active or inactive phase of metal silicide and resulting in the enhancement of the conductivity of anode materials. Followed by the coating of C layer functioned as a mechanical and conductive buffer matrix, the excellent electrochemical performance of Si anode can be achieved. For example, Lee et al. [19] used a two-step high-energy ball-milling method to fabricate the composite of Si-Cu3Si-CNT/G-C. The final product displayed a high first reversible capacity of 1237 mAh•g -1 and a long cycle durability of 1084 mAh•g -1 over 100 cycles. Pei et al. [21] obtained a core-shell composite of porous Si-Cu3Si-Cu microsphere@C by means of etching/electroless deposition and subsequent C coating.</p><p>The material held a reversible capacity of 627 mAh•g -1 after 200 cycles at 2 A•g -1 .</p><p>However, the reported preparation processes were very complex, in which the hightemperature annealing process is essential for the pyrolysis of C layer from an organic source following the other individual alloying process. To simplify the preparation process and reduce the overall cost for the promotion of large-scale production, a much facile and effective preparation method is highly desired. Polydopamine (PDA), a biopolymer found in mussel adhesive proteins, has attracted much attention since 2007 [23]. PDA is often chosen as the source of C to coat the surface of Si anode [24,25] because (1) PDA can adhere closely and uniformly on the surface of Si and (2) the carbonized PDA thin film containing a high content of nitrogen possesses good conductivity [24][25][26][27]. Interestingly, some recent studies reported that due to the existence of functional groups such as the catechol hydroxyl and amine groups, PDA is also capable of absorbing some metal ions (Cu 2+ [28][29][30],</p><p>Ni 2+ [31], Fe 3+ [32 ] , Pb 2+ [28], Mg 2+ [29]) in the solution that is beneficial to the preparation of a PDA-containing M polymer. This has attracted extensive attention in the fields of environment and energy for removing toxic metal ions in sewage and industrial effluents [28] as well as improving the coating effect of PDA on electrode materials [30,32]. Considering the effect of absorbing metal ions by PDA to diffuse towards into the bulk Si anode and even alloy with it after annealed at high temperature, the Si-MSi-M@C or Si-MSi@C material is possible to synthesize in one step.</p><p>Herein, inspired by the aforementioned studies, we chose PDA and CuCl2•2H2O as reagents and synthesized a PDA-containing Cu X+ coating layer on the surface of pure Si at room temperature. After sintered at 800 ℃, the material with the structure of Si-Cu3Si@C was obtained successfully in a single step. There are three main reasons for choosing Cu as the alloyed metal element: 1) Different from other alloy reactions, no Si-rich silicide phase exists in the Si-Cu system [33] while Si-Cu composites are considered as a high-potential anode candidate for LIBs [34][35][36][37][38][39]; 2) The phase of Cu3Si is the most desired in the bulk anode material because its pseudo capacitance characteristics would provide good structural stability [40], high electronic conductivity (~2×10 4 S cm −1 ) [41] and excellent mechanical ductility [42] during the chemical reactions; 3) Cu3Si is readily obtained via the mutual diffusion property of Si and Cu elements [43] and the mass or molar ratio of Si to Cu in the Cu3Si region of Si-Cu binary phase diagram (shown in Figure S1 [44]) can be adjusted via controlling the amount of CuCl2•2H2O. Consequently, compared with pure Si and the sample coated with PDA only, the modified material with the structure of Si-Cu3Si@C exhibited much better electrochemical properties.</p><!><p>The material preparation process is shown in Figure 1. Firstly, 0.4851 g of Tris base (99.99%, Aladdin) was dissolved in 400 mL of the deionized water and then the hydrochloric acid was added to adjust the pH value to 8.5. Next, 0.2 g of CuCl2•2H2O (99.99%, Aladdin), 0.2 g of Si powder (99.9%, average particle size of 30 -50 nm, Aladdin) and 0.2 g of dopamine hydrochloride (DA, 98%, Aladdin) were added, respectively, into the buffer solvent followed by transferring into a rotary evaporator.</p><p>After stirring for 7.5 h in a room-temperature (28 ℃) water bath in air, the suspension was filtered and washed with deionized water at least five times, and then dried in vacuum overnight to obtain particles of PDA-containing Cu x+ ions (Si@PDA-Cu x+ ).</p><p>Then, with the heating rate of 5 ℃/min, the particles were annealed in a tube furnace at 800 ℃ for 3 h under a high pure argon (Ar) atmosphere (99.99%). The target product of Si-Cu3Si@C was obtained after cooled down to room temperature. For comparison, the Si@PDA and Si@C samples were prepared at similar conditions in the absence of adding CuCl2•2H2O.</p><!><p>Surface morphologies and high-resolution lattices of materials were observed by field emission scanning electron microscopy (FESEM, Tescan MAIA3) and transmission electron microscopy (TEM, Talos F200x G2), respectively. The phase structures were determined by a Rigaku SmartLab 9kW-Advance X-ray diffraction (XRD, Cu Kα) machine from 10° to 80° with a scanning rate of 3°/min. The relative content of phases in the final product was estimated by the Rietveld refinement using a general diffraction/reflectivity analysis program, TOPAS-Academic. X-ray photoelectron spectroscopy (XPS) was conducted to investigate the elements distribution and valence status of samples, which were tested upon the instrument of Thermo ESCALAB 250XI. Thermogravimetric analysis (TGA, TA Q500) was employed to determine the content of coated carbon layer at 10 °C/min from room temperature to 800 ℃ in air.</p><!><p>The electrochemical performance was evaluated with CR2025-type coin cells. The working electrodes were prepared by coating uniform slurries on the Cu foil as the current collector. The slurries were prepared by mixing the active material, Super P and CMC-Na (sodium carboxymethyl cellulose, viscosity: 1000-1400 mPas, Aladdin)/PAA (polyacrylic acid, Mv ~ 3,000,000, Aladdin) in the mass ratio of 6:2:1:1 in deionized water. The electrodes were then dried in vacuum at 80 °C for 12 h followed with 150 °C for 1 h. Then, the foils were cut into small wafer electrodes with the diameter of 12 mm. With the Li foil as a counter electrode, the coin cells were assembled in an argon-filled glovebox (H2O ≤ 0.5 ppm, O2 ≤ 0.5 ppm) (MBRAUN, Germany). The electrode separator was a Cellgard 2400 membrane and the electrolyte was a 1 M LiPF6 solution in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1 with 10% (volume) FEC (fluoroethylene carbonate) as an addictive. Lastly, the cells were charged and discharged with the voltage range from 0.01 V to 1 V at room temperature using a battery testing system (Land CT-2001A, China). Cyclic voltammetry (CV) was conducted with a scanning rate of 0.1 mV•s -1 (Biologic, VMP300, France). The SEM pictures of as-prepared samples are shown in Figure 2 (a-c). Compared with the Si@C and Si-Cu3Si@C samples (Figures 2(b) and (c)), the pure Si particles (Figure 2(a)) aggregated severely that is a typical feature of nano materials. It can be seen that the particles of Si@C and Si-Cu3Si@C were with a quasi-spherical shape, showing no obvious difference in morphology. Figure 2(d) shows the XRD patterns of pure Si, Si@C and Si-Cu3Si@C samples, in which the standard XRD patterns of Si (PDF#04-006-4528) and Cu3Si (PDF#00-059-0263) were used as reference. Clearly, the crystalline phase in Si@C was only Si (PDF#04-006-4528), indicating that the coated PDA layer after calcination did not introduce any new phase. Different from Si@C, in addition to the typical diffraction peaks of Si, two new peaks appeared at 44.6° and 45.1° in the XRD pattern of Si-Cu3Si@C sample, corresponding to the phase of Cu3Si. This confirms our speculation that the Cu 2+ ions adhered on the coated PDA layer may behave differently from other metal ions reported in literature [32,48], which moved into the bulk Si and alloyed with it during the high-temperature pyrolysis process of organic carbon layer. In order to estimate the amount of Cu3Si in Si-Cu3Si@C, the refinement results of XRD patterns are shown in Figure S2. The relative content was 5%.</p><!><p>To investigate the feature of coated C layer, Raman test was conducted and the corresponding spectra of Si, Si@C and Si-Cu3Si@C are shown in Figure 2(e). Two peaks at 511.4 and 949.5 cm -1 are ascribed to the vibration bands of crystalline Si [49,50] in all three samples. Different from the spectrum of pure Si, there are two additional peaks at 1359 and 1586 cm -1 , respectively, representing D band and G band of carbonaceous materials [50] in Si@C and Si-Cu3Si@C samples. The D band arises from the disordered structure of C, related to the breathing motion of sp2-rings, while the G band results from the graphitic C, representing an in-plane bond stretching of sp2hybridized C atoms [51][52][53]. Considering the presence of D and G bands, there should be a significant fraction of C with a ring-like structure in the pyrolytic PDA layer [51].</p><p>Generally, the ratio of ID/IG is used to express the graphitic degree of C. The higher intensity of ID refers to the lower graphitic degree of C with more disordered structure [54]. In Figure 2(e), the values of ID/IG of Si@C and Si-Cu3Si@C are very close and relatively high, revealing the existence of some pinholes and defects in the C layer [55].</p><p>Figure 2(f) provides the information of the TGA curves of Si, Si@C and Si-Cu3Si@C to determine the content of coated carbonized PDA layer (c-PDA). Obviously, the tendency of mass change of pure Si is not similar to that of the coated samples, where its mass gained a constant increment before 600 ℃ that is attributed to the oxidation of SiOx layer on the surface. In contrast, both Si@C and Si-Cu3Si@C have a three-stage changing process in their TGA curves. To be specific, there was a slight drop in mass before 400 ℃ that may be due to the absorption of water [21]. Then, a sharp decrease can be seen for both materials, indicating the initial oxidation of coated C layer. Finally, the mass increased at further higher temperatures when the c-PDA layer was thoroughly consumed and the pure Si was exposed to the air again. Note that the c-PDA content in Si-Cu3Si@C (~8.8 %) was less than that in Si@C (~11.3 %). This may explain that the hydrolysis of Cu 2+ ions caused the pH value of buffer solution to deviate from the optimal value (pH = 8.5 [23]) for self-polymerization of PDA. The pH value of the buffer solution with 0.2 g of CuCl2•2H2O was ~6.6 (shown in Figure S3). The detailed microstructure of Si, Si@C and Si-Cu3Si@C materials can be elucidated by the TEM images and the corresponding HRTEM images in Figure S4 and Figure 3(a-c), respectively. It can be seen that there was an amorphous SiO2 layer with the thickness of 1 -2 nm on the surface of pure Si (in Figure S4(c)). According to the HRTEM image, a pyrolytic amorphous carbon layer was coated uniformly on the surface of the Si-Cu3Si material with the thickness of 8 -10 nm while the Si@C sample had a carbon layer with the thickness of 16 -18 nm (seen in Figure S4(   The surface chemical compositions of Si@PDA-Cu x+ and Si-Cu3Si@C samples are consistent to the previous study [45]. Among them, the C element was the majority in the C-C bond, i.e., 55.62 % in Si@PDA-Cu x+ and 71.17 % in Si-Cu3Si@C (seen in Table 1), respectively. The C-N and C=N bonds further confirmed the expected Ndoped C layer. There were still some C-O and C=O bonds in Si-Cu3Si@C, but the amount was relatively low (seen in Table 1), indicating that oxygen still remained in the c-PDA layer even after sintered at 800 ℃ for 3 hours. The formation of O-C=O bonds may be attributed to the presence of oxygen-containing contamination species [21] while the π→π* satellite is just a typical symbol of materials with a structure similar to benzene ring [56]. The high-resolution spectrum of N 1s in Si@PDA-Cu x+ has three distinct peaks assigned to pyridinic-N (at 398.57 eV), pyrrolic-N (at 399.92 eV) and graphitic-N (at 401.4 eV) in which the relative content of pyrrolic-N is the most prominent (~ 65.92 %, seen in Table 1). However, after the annealing process, the peak of pyrrolic-N disappeared while the peaks of pyridinic-N (at 398.42 eV) and graphitic-N (at 400.97 eV) remained in Si-Cu3Si@C. The obvious increment in the relative content of pyridinic-N from 29.04 % to 40.27 % revealed that the appearance of N at the edge or defect of c-PDA (the proposed structure can be seen in Figure 1) can be beneficial to form a disordered structure of C layer with the enhanced conductivity [45]. The Cu 2p spectra in Figure 4(e) show that the Cu element in Si@PDA-Cu x+ and Si-Cu3Si@C coexisted with the chemical valences of +1 and +2, where the satellites at 943.62 eV determined the existence of Cu 2+ (2p3 at 934.47 eV and 2p1 at 954.27 eV) in both materials. Generally, it is not easy to directly distinguish the peaks of Cu + and Cu 0 only by XPS due to their very close bind energy and here we preferred to assign the peaks at 932.52 and 952.22 eV to Cu 2p3 (Ⅰ) and Cu 2p1 (Ⅰ) respectively based on the peak of Cu LMM and the corresponding Auger parameters (seen in Figure S5)</p><p>and the existence of a weak satellite at 948.32 eV in Si-Cu3Si@C. This result, where the chemical valence of absorbed Cu ions by PDA was not all with the divalent oxidation state but partially +1 valence, is quite different from the reported work [29].</p><p>It may be due to a certain degree of reducibility of PDA such that some Cu 2+ ions were reduced to Cu + , which shares similar properties of some noble metal ions [57]. Thus, we named the material of Si@PDA absorbing Cu ions as Si@PDA-Cu x+ in which Cu x+ represents Cu + /Cu 2+ ions. Figure 4(f) shows the spectra of O 1s and the results mutually confirmed with the analyses of Si 2p and C 1s spectra. Combined with the previous microstructural characterizations, we can draw a conclusion that the Cu ions from CuCl2•2H2O were absorbed successfully by the PDA layer. After sintered at 800 ℃ for 3 h in the Ar atmosphere, the Cu ions alloyed with Si to produce the Cu3Si crystallites while the amorphous N-doped C layer was formed on the surface. The CV curves of Si, Si@C and Si-Cu3Si@C samples in the initial two cycles are presented in Figures 5(a-c), showing similar lithiation/delithiation processes.</p><!><p>Specifically, due to the formation of SEI film [58,59], there was only a small and broad cathodic peak at ~0.75 V in the 1 st cycle, which is much obvious for Si@C and Si-Cu3Si@C. A sharp cathodic peak at ~0.05 V in the 1 st cycle is due to the generation of crystalline Li3.75Si (c-Li3.75Si) [60,61]. Besides, there are two anodic peaks located at approximately 0.36 and 0.52 V, respectively, indicating a two-stage transition from the rich-Li LixSi (0 ≤ x ≤ 3.75) alloy to the final status of amorphous Si (a-Si) [62]. In the 2 nd cycle, the peak positions showed no apparent change but the current associated with the two anodic peaks increased, proving that the electrode materials were undergoing an activation process [63]. Different from the 1 st cycle, a broad and distinguishable cathodic peak appeared at ~0.16 V in the 2 nd cycle, which is related to the reaction producing the amorphous Li-Si alloy [62]. Overall, the processes of coating the c-PDA and doping the Cu3Si phase do not change the reaction mechanism between Si anode and Li + ions. in the voltage window from 0.01 to 1.00 V (vs Li/Li + ). It is shown that the profile was with a lithiation plateau at ~0.08 V and a delithiation plateau at ~0.40 V, respectively.</p><p>The pure Si presented the highest initial reversible capacity (IRC) and initial coulombic efficiency (ICE) of 1.83 mAh•cm -2 (3155 mAh•g -1 ) and 90.1%, respectively. In contrast, Si@C and Si-Cu3Si@C performed worse in terms of IRC and ICE. The IRC and ICE of Si@C were 1.48 mAh•cm -2 (2552 mAh•g -1 ) and 85.1%, while those of Si-Cu3Si@C were 1.44 mAh•cm -2 (2483 mAh•g -1 ) and 83.2%, respectively. The reason of codegradation of IRC and ICE is mainly attributed to the existence of c-PDA that is much inactive to Li + ions than Si and more likely to form SEI films because of aforementioned defects.  a-b) show the cycle performance of Si, Si@C and Si-Cu3Si@C samples with the same mass loading of 0.58 mg•cm -2 . The first 2 cycles were under the current density of 0.14 mA•cm -2 (240 mA•g -1 ) for the activation process of the materials while the following cycles were under the current density of 0.28 mA•cm -2 (480 mA•g -1 ) in the voltage range from 0.01 to 1 V. It can be seen clearly that the three samples presented different degradation scenarios during the 100 cycles. To be specific, the capacity of Si declined from 1.80 mAh•cm -2 (3103 mAh•g -1 ) to 1.41 mAh•cm -2 (2431 mAh•g -1 ) with the capacity retention of 78.3%, while the capacity of Si@C declined from 1.50 mAh•cm -2 (2586 mAh•g -1 ) to 1.25 mAh•cm -2 (2155 mAh•g -1 ) with the capacity retention of 83.3%, and the capacity of Si-Cu3Si@C declined from 1.38 mAh•cm -2</p><p>(2379 mAh•g -1 ) to 1.26 mAh•cm -2 (2172 mAh•g -1 ) with the capacity retention of 91.3%.</p><p>Though the pure Si always possessed the highest capacity, the huge volume change of Si during the reaction was not well-controlled, resulting in the worst structural stability.</p><p>In contrast, though with a certain degree of reduction in capacity, the improved capacity retention of Si@C proves that the coating of the PDA layer has taken positive effect on alleviating the volume change of Si. The best cycle stability, which is exactly what an idea cell needs when the capacity can perform a certain value [19], was achieved in Si-Cu3Si@C when doping the phase of Cu3Si into the bulk Si due to the synergistic effect of c-PDA and Cu3Si phase. Moreover, Figures 6(c) shows the rate performance of Si, Si@C and Si-Cu3Si@C samples with close initial area capacity adjusted by the mass loading. The samples were tested at different current densities of 0.14, 0.35, 0.70, 1.05, and 1.40 mA•cm -2 in turn and back from 0.35 mA•cm -2 for every 10 cycles. It can be seen that, with increasing the current density, the attenuation degree of Si was the highest and that of Si-Cu3Si@C was the lowest. Especially, when the current density was increased to 1.40 mA•cm -2 , Si-Cu3Si@C maintained the capacity of ~0.98 mAh•cm -2 (~1508 mAh•g -1 ), which was higher than the ones of Si@C with ~0.90 mAh•cm -2 (~1475 mAh•g -1 ) and Si with ~0.78 mAh•cm -2 (~1345 mAh•g -1 ), respectively. Overall, the Si-Cu3Si@C sample exhibited the best electrochemical performance, which was not only attributed to the coating of c-PDA layer but also the doping of Cu3Si. The involvement of Cu3Si could further improve the electronic conductivity of the material, reduce the polarization of the reaction, and promote the migration rate of Li + ions. Furthermore, as shown in Figure 6(d), the Si-Cu3Si@C sample was tested for long cycle at the current density of 1.4 mA•cm -2 (2500 mA•g -1 ) in which the initial 5 cycles</p><p>were for activation at the current density of 0.14 mA•cm -2 (250 mA•g -1 ). The capacity degraded rapidly at the subsequent 15 cycles after the activation process, and then became steady hereafter. After 400 cycles, Si-Cu3Si@C still exhibited the capacity of 0.75 mAh•cm -2 (1339 mAh•g -1 ) with the capacity retention of 65.8% corresponding to the 6 th cycle. Compared with the similar work reported in the recent 5 years as listed in Table 2, our proposed Si-Cu3Si@C material shows advantages especially in terms of capacity and cycle stability. Besides, according to the literature [19], the general requirements (i.e. from Samsung SDI and LG Chem.) for a commercial Si-based anode material are: IRC > 1000 mAh•g −1 ; ICE > 80%; and capacity retention after 100 cycles > 85%. Given these criteria, the electrochemical performance of Si-Cu3Si@C is promising. Si-Cu3Si@C samples at the current density of 0.28 mA•cm -2 (480 mA•g −1 ). SEM images of electrodes after 100 cycles at 0.28 mA•cm -2 (480 mA•g −1 ) of (d) Si, (e) Si@C and (f) Si-Cu3Si@C samples.</p><p>The cause of the improved cycle stability of Si-Cu3Si@C sample was revealed by examining its dQ/dV curves at the 3 rd , 20 th , 50 th and 100 th cycles. Figures 7(a-c) show the typical dQ/dV plots of Si-based anode materials during the delithiation process, in which three distinct anodic peaks (denoted as A1 at ~0.28 V, A2 at ~0.45 V, and A3 at ~0.48 V) originate from the transformation of a-Li3.5Si to a-Li2Si, c-Li3.75Si to a-Li2Si, and a-Li2Si to a-Si [66,67], respectively. The appearance of the A2 peak proves the formation of crystalline Li3.75Si phase when the materials were in the state of full lithiation. However, this is not desirable because the formation of c-Li3.75Si would adversely affect the cycle performance of Si-based anode [68,69], causing much severe volume change, more cracks, and faster capacity degradation. Among three electrode samples, the A2 peak of Si-Cu3Si@C sample exhibited the lowest relative intensity at the 3 rd cycle (Figure 7(c)) and then disappeared in the subsequent cycles. Based on the observation, the presence of Cu3Si phase in the bulk Si is inferred to be capable of reducing the amount of Li3.75Si phase during the reaction, which is beneficial to the structural stability of electrode. Conversely, the A2 peaks in Si and Si@C samples (shown in Figures 7(a, b)) exhibited much higher relative intensity and then declined in the subsequent cycles. It should be noted that the A2 peak of pure Si was still very strong even after 20 cycles. This may explain why the capacities degraded rapidly in the early stage but gradually tended to steady in the later stage for Si and Si@C (seen in Figures 6(a-c)). As there is a close relationship between the cycle stability and structural integrity of electrode, the morphology evolution of electrodes after 100 cycles at the current density of 0.28 mA•cm -2 (480 mA•g −1 ) was further studied by SEM as shown in Figures 7(d-f). A homogenous and complete surface of electrodes is highly desired because the formation of fragmentation and cracks on the surface would cause the disconnection between active materials or active materials and conductive agent to increase the contact resistance and polarization, leading to the fast degradation of the capacity. When compared with Si and Si@C, the structural integrity of Si-Cu3Si@C electrode surface after 100 cycles was much better as shown in Figures 7(d-f). The SEM images could serve as evidence that the issue of huge volume fluctuation of Si anode could be effectively suppressed by the synergistic effect of doped Cu3Si phase and coated c-PDA layer, resulting in the most stable cycle performance of Si-Cu3Si@C.</p><!><p>Si-based anode materials of Si-MSi-M@C or Si-MSi@C for LIBs have been intensively studied in recent years due to their unique structure and outstanding electrochemical performance, but the corresponding synthesis processes are complex.</p><p>Inspired by the characteristics of the reaction between PDA and some specific metal</p><!><p>There are no conflicts to declare.</p>