
==== Front
iScience
iScience
iScience
2589-0042
Elsevier

S2589-0042(24)01939-4
10.1016/j.isci.2024.110714
110714
Article
Preparation of WSi@SiOx/Ti3C2 from photovoltaic silicon waste as high-performance anode materials for lithium-ion batteries
Niu Yanjie 1
Wei Mengyuan 1
Xi Fengshuo fengshuoxi@126.com
14∗
Li Shaoyuan lsy415808550@163.com
1∗∗
Ma Wenhui mawenhui@ynu.edu.cn
12∗∗∗
Wang Liangtai 1
Li Haoyang 1
Lu Jijun 1
Chen Xiuhua 2
Wei Kuixian 1
Luo Bin 3
1 Faculty of Metallurgical and Energy Engineering/State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China
2 School of Engineering/School of Materials Science and Engineering, Yunnan University, Kunming 650500, China
3 Nanomaterials Centre, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia
∗ Corresponding author fengshuoxi@126.com
∗∗ Corresponding author lsy415808550@163.com
∗∗∗ Corresponding author mawenhui@ynu.edu.cn
4 Lead contact

13 8 2024
20 9 2024
13 8 2024
27 9 11071419 4 2024
7 7 2024
8 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Summary

Silicon anodes hold promise for future lithium-ion batteries (LIBs) due to their high capacity, but they face challenges such as severe volume expansion and low electrical conductivity. In this study, we present a straightforward and scalable electrostatic self-assembly method to fabricate WSi@SiOx/Ti3C2 composites for LIBs. Silicon nanosheets and the ultra-thin oxide layer SiOx serve as sufficient buffers against volume changes, while the layered MXene enhances the electrical conductivity of the composite and promoted Li+/e− transport. Additionally, cationic surfactant-treated Ti3C2 provides more active sites for WSi@SiOx attachment and acts as an intercalating agent, enabling WSi@SiOx to enter the interlayer spaces of Ti3C2. The WSi@SiOx/Ti3C2 electrodes significantly improved electrochemical performance, achieving a capacity of 1,130 mAh g−1 after 800 charge/discharge cycles at 500 mA g−1. This study not only presents a straightforward pathway for high-value utilization of silicon waste but also offers a feasible route for preparing high-performance and cost-effective silicon-based LIBs.

Graphical abstract

Highlights

• A novel and promising method to fabricate WSi@SiOx/Ti3C2 composite was proposed

• SiCWP natural characters of ultrafine particle and sheet-like structure are full used

• The properties of the electrochemical Li+ store/release behavior were clarified

• The WSi@SiOx/Ti3C2 as LIBs anodes exhibit an excellent electrochemical performance

Electrical property; Nanomaterials; Energy materials

Subject areas

Electrical property
Nanomaterials
Energy materials
Published: August 13, 2024
==== Body
pmcIntroduction

With energy shortages and environmental concerns on the rise, the pursuit of sustainable development has gained significant attention. Photovoltaic (PV) power generation, as an exemplary clean energy source, has witnessed a remarkable increase in installed capacity, growing at an annual rate exceeding 35%. Consequently, there has been a corresponding rise in demand for silicon wafers.1 In the PV industry, silicon wafers are primarily produced by the diamond wire slicing of solar-grade silicon (SoG-Si) ingots.2,3 However, approximately one-third of SoG-Si is lost as photovoltaic silicon waste (WSi) during this production process, as shown in Figure 1A.4 In China alone, the annual production of WSi exceeds 200,000 tons.5 In addition, the small flake particle size of WSi leads to significant soil and water pollution.6 While the most common approach for recycling WSi involves its utilization in the preparation of high-purity silicon, its naturally occurring ultrafine particle size-enhanced amorphous silicon oxide layer hinders effective melting and impurity removal.7,8Figure 1 The schematic illustration for the synthetic procedure of the WSi@SiOx/Ti3C2 composite

The demand for high-energy-density energy storage solutions continues to increase. Si, with its high theoretical specific capacity (4,200 mAh g−1), relatively low discharge potential (<0.5 V for Li/Li+), and abundant earth reserves, emerges as a promising anode material for LIBs.9,10,11,12 However, silicon-based anodes experience significant volume expansion (∼300%), which leads to particle fission or pulverization, resulting in the loss of contact between the active material and the conductor during lithiation/de-lithiation.13,14,15 In addition, the recurrent generation and thickening of the solid electrolyte interface (SEI) layer,16 along with the low electrical conductivity of silicon,17 contribute to rapid capacity loss, low initial Coulombic efficiency (ICE), and poor electrochemical performance, posing major challenges for silicon-based anodes.18,19,20 Numerous strategies have been proposed to enhance the electrochemical performance of silicon-based anodes. These include the utilization of nanoscale silicon particles to mitigate substantial expansion and shrinkage phenomena,21,22 as well as the careful design of silicon-based nanostructures, such as one-dimensional silicon nanowires and silicon nanotubes, two-dimensional silicon nanosheets, and three-dimensional silicon nanorods, to improve the structural integrity and electrochemical performance.23,24,25 Moreover, coating the surface of the silicon particles with amorphous silicon oxide has been suggested to suppress volume expansion and enhance electrochemical stability.26,27 However, insufficient control of silicon nanoparticles or nanostructures can lead to severe agglomeration,28 resulting in reduced specific surface area-to-volume ratio of the nanostructures.

Due to its exceptionally high electron mobility, specific surface area, and tensile strength, as well as remarkable flexibility, graphene has the potential to enhance the electrical conductivity of silicon-based anode materials. Moreover, when combined with silicon-based materials, graphene can effectively mitigate the aggregation and crushing of silicon nanoparticles.29,30 However, the surface of graphene, terminated with sp2-hybridized carbon atoms and lacking of reactive polar groups, results in weak interactions with silicon. Consequently, an unstable interface forms between these two components in the composite material, hindering interfacial electron transfer and cyclic stability. Titanium carbide (Ti3C2Tx) as a typical MXene exhibits a lower Li+ diffusion barrier and a faster Li+ diffusion rate compared to graphene.31,32,33,34,35 Additionally, owing to the abundant end-groups on its surface, Ti3C2Tx demonstrates significant pseudocapacitance beyond its role as a conducting substrate.36,37,38,39 Additionally, it is noteworthy that WSi exhibits diminutive dimensions, characterized by a thickness ranging approximately between 50 and 150 nm, and a planar extent spanning from 0.5 to 1 μm, while manifesting a lamellar morphology.7,40

In this study, we present a facile and scalable electrostatic self-assembly approach based on the mutual attraction between positively charged MXene nanosheets and negatively charged WSi@SiOx for fabricating WSi@SiOx/Ti3C2 composites. These composites, derived from WSi materials sourced from the photovoltaic industry, exhibit excellent performance as anode materials for LIBs. The silicon nanosheets and ultra-thin oxide layer SiOx on the surface of WSi@SiOx/Ti3C2 provided a sufficient buffer against volume changes. The incorporation of layered MXene enhances the electrical conductivity of the composite while facilitating Li+ transport. Moreover, by uniformly adhering to the Ti3C2 surface, WSi@SiOx prevents self-aggregation of layered Ti3C2 and mitigates swelling issues associated with WSi@SiOx expansion/shrinkage during cycling. Furthermore, through rational assembly via electrostatic adsorption between WSi@SiOx and layered Ti3C2 nanosheets, rapid electron/ion transport networks and electrolyte penetration channels are formed within the composite structure, thereby further improving overall performance as LIB anodes. The present study introduces a straightforward and promising approach for the efficient utilization of WSi, offering a viable pathway toward the preparation of high-performance and cost-effective silicon-based LIBs. Moreover, this study proposes an effective method to address the substantial volume of industrial solid waste from silicon waste while also providing prospects for achieving a comprehensive solution for future energy requirements by combining photovoltaic power generation with advanced energy storage technologies.

Results

Synthesis and characterization of the WSi@SiOx/Ti3C2

As shown in Figure 1, WSi is formed during the diamond multi-wire slicing process of SoG-Si ingots for the PV industry due to the shear force between the diamond wires and silicon substrate. Microscopically, the collected WSi exhibits a predominantly lamellar structure with an approximate size of 1 μm, while macroscopically it appears as gray lumpy agglomerates (Figure S1A). These agglomerates are subsequently ball-milled to obtain a black powdery solid (Figure S1B), primarily composed of silicon with a minor oxygen content. The synthesis steps of WSi@SiOx/Ti3C2 composites, as shown in Figure 1, involve preparing positively charged Ti3C2 nanosheets and negatively charged WSi@SiOx followed by interfacial electrostatic self-assembly composite formation between them. Chemical etching of Ti3AlC2 precursor using LiF under acidic conditions results in accordion-like morphology for Ti3C2. Subsequently, etched Ti3C2 is sonicated in anhydrous ethanol to yield few layers of hydroxyl-rich Ti3C2 nanosheets. By employing the esterification reaction between the carboxyl group (-OH) present in the cationic surfactant tetramethylammonium acetate (TMAA) and the hydroxyl group (-COOH) on the surface of Ti3C2, successful functionalization of Ti3C2 with positively charged amino groups from TMAA was achieved, resulting in a positive charge on its surface. Additionally, TMAA acts as an intercalator to expand the interlayer spacing of Ti3C2, which is evident from the leftward shift observed in the X-ray diffraction (XRD) diffraction pattern. In addition, the oxidized silicon waste (WSi@SiOx) (Figure S2) was surface-charged negatively by exploiting the pH-dependent zeta potential of SiOx on its surface.41 Through stirring both components in a buffer solution and leveraging the attractive forces between anions and cations, successful interface electrostatic self-assembly was achieved under relatively mild conditions, which facilitated maintaining the layered structure of MXene.

The morphology and microstructure of the synthesized WSi@SiOx/Ti3C2 composites were characterized using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Figures 2B and 2E show the SEM images of multilayer Ti3C2 and monolayer Ti3C2 TEM images after etching, respectively. The exfoliation of multilayer Ti3C2 resulted in the formation of a disordered monolayer Ti3C2 structure with a size range from nanometer to micrometer, featuring a surface rich in pores that facilitate ion exchange. However, this monolayer Ti3C2 is susceptible to folding, leading to poor cycling stability. To address this issue, ultrasonic treatment and cationic surfactant modification were employed on the multilayer Ti3C2 to obtain positively charged Ti3C2. TMAA was adsorbed on the surface of Ti3C2 as a molecular template for controllable adsorption of WSi@SiOx nanosheets (Figure 2C). Further characterization by low and high magnification TEM images revealed that WSi@SiOx with an average diameter of 1∼3 μm was attached to the upper surface of the Ti3C2 nanosheets (Figures 2F and 2G), while high-resolution TEM (HRTEM) lattice images showed a lattice spacing of 0.31 nm corresponding to the (1 1 1) plane of Si (Figures 2G and S3). Elemental analysis confirmed successful coupling between Si and Ti3C2 via features corresponding to Si, Ti, and oxygen elements in Figure S4 (energy spectral analysis image corresponding to Figure 2F). Additionally, TEM imaging demonstrated uniform formation of an ultrathin oxide layer on silicon waste surfaces (∼5 nm thickness) after oxidation modulation which effectively alleviates chalking issues caused by silicon volume expansion while reducing side reactions with electrolytes. Furthermore, employing interfacial electrostatic assembly, WSi@SiOx was securely immobilized onto Ti3C2 nanosheets to establish a stable sandwich structure.Figure 2 The morphology and microstructure of WSi, Ti3C2, and WSi@SiOx/Ti3C2

(A) SEM image of silicon cutting waste.

(B) SEM image of etched Ti3C2.

(C) SEM image of WSi@SiOx/Ti3C2.

(D) TEM image of oxidized silicon waste.

(E) TEM image of Ti3C2 nanosheets.

(F) Low-magnification TEM image of WSi@SiOx/Ti3C2.

(G) High-magnification localized TEM image of WSi@SiOx/Ti3C2.

(H) TEM image and elemental mapping analysis for WSi@SiOx/Ti3C2.

The XRD plot in Figure 3A reveals the crystallinity and phase composition of WSi@SiOx/Ti3C2, exhibiting a prominent peak at 6.0° corresponding to the (0 0 2) facet of Ti3C2. Additionally, the XRD diffraction pattern (Figure S5) demonstrates that, following chemical etching, the primary peak of Ti3AlC2 located at 39° (JCPDS52-0875) disappears while the (0 0 2) peak broadens and shifts to the left at around 9.3°, indicating successful etching of Ti3C2.42 In order to elucidate the chemical bonding state between WSi@SiOx and Ti3C2, the surface group evolution was investigated using FTIR spectroscopy (Figure 3B). Characteristic peaks at approximately 3,448 cm−1, 1,630 cm−1, and 1,410 cm−1 were observed in both WSi@SiOx and WSi@SiOx/Ti3C2 samples, corresponding to the stretching vibrations of O-H, C=O, and C=C bonds respectively. The presence of asymmetric stretching and bending vibrations at 1,110 cm−1 can be attributed to Si-O-Si bonds. Additionally, a newly formed peak at 949 cm−1 was detected in the WSi@SiOx/Ti3C2 sample indicating the formation of a Si-O-Ti bond.43 This observation suggests that a chemical bond has been established between WSi@SiOx and Ti3C2 nanosheets which is stronger than a physical bond; thus emphasizing the significance of covalent bonding for enhancing structural stability in composites. Furthermore, a slight broad peak is observed at approximately 21° in WSi@SiOx/Ti3C2 due to amorphous SiO2 on its surface. Moreover, Raman spectroscopy analysis (Figure 3C) exhibits a strong Stokes shift at 510 cm−1, which corresponds to typical Raman characteristics of crystalline silicon/Si material. Moreover, Raman spectral analysis (Figure 3C) reveals a prominent Stokes shift at 510 cm−1, indicating Raman signatures consistent with crystalline Si/Si materials. Furthermore, distinct defect-induced (D) peaks are observed in the Raman spectra, characterized by elevated intensity ratios (ID/IG) reaching up to 1.05 within the D and G bands, suggesting a significant degree of C graphitization within the Ti3C2 composites.44,45 Simultaneously, the enhancement of the graphite crystal structure correlates with increased electrical and thermal conductivity, thereby meeting the necessary criteria for anode applications in LIBs. This improvement facilitates the formation of high-quality SEI films and provides ample lithium storage capacity, ultimately enhancing the cycling stability of these composites.Figure 3 Composition and valence analysis of WSi@SiOx/Ti3C2 composites, WSi@SiOx, and Ti3C2

(A) XRD patterns.

(B) FTIR spectra.

(C) Raman spectra.

(D–F) High-resolution XPS spectra of O 1s, Ti 2p, and Si 2p for the WSi@SiOx/Ti3C2 composites.

The chemical bonding state of the WSi@SiOx/Ti3C2 composite was further investigated using X-ray photoelectron spectroscopy (XPS). The typical XPS survey spectra (Figure S6A) confirmed the presence of C, Ti, F, and Si. F is derived from LiF used for Ti3C2 etching and plays a crucial role in controlling the homogeneous transport and deposition of Li+,46 leading to more uniform Li-metal deposition and dissolution, thereby improving the cell’s long-cycle stability at room temperature. High-resolution O 1s XPS spectra (Figure 3D) showed peaks at 531.8, 531.0, and 529.5 eV corresponding to different O states Si-O-Si, Si-O-Ti, and Ti-O respectively. The particular O valence state (Si-O-Ti; 531.0 eV) suggests that there exists a chemical bond between WSi@SiOx and Ti3C2 which is consistent with FTIR spectroscopy results. Furthermore, XPS survey spectra of Ti 2p (Figure 3E) revealed no evidence of Ti-OH indicating that hydroxyl groups on the surface of Ti3C2 reacted with carboxyl groups on cationic surfactant TMAA by esterification successfully accessing positively charged amino groups onto the surface of Ti3C2. The WSi@SiOx and Ti3C2 materials exhibit specific surface areas of 40.05 and 32.37 m2 g−1, respectively (Table S1). By incorporating layered Ti3C2, the specific surface area of the WSi@SiOx/Ti3C2 composites significantly increased to 67.24 m2 g−1 (Table S1), facilitating rapid diffusion of Li+ within the electrodes and thereby enhancing Li+ storage performance.

Electrochemical performance of the WSi@SiOx/Ti3C2 and comparative material

To evaluate the electrochemical performance of the WSi@SiOx/Ti3C2 anode for LIB, Figure 4A presents the first three CV curves obtained at a scan rate of 0.1 mV s−1. In the inaugural cycle, a prominent reduction peak at 1.21 V was observed, which gradually diminished in subsequent cycles.47,48 This phenomenon can be ascribed to the irreversible electrolyte interaction between electrolyte and functional groups on the surface of MXene, leading to the formation of a robust SEI film. Furthermore, successive cycles revealed the emergence of a cathodic peak at 0.2 V, indicating of the lithiation process involving silicon and resulting in LixSi (0 ≤ x ≤ 4.4).49,50 On the anodic trajectory, two distinct oxidation peaks were observed around 0.34 and 0.82 V respectively, resembling the cyclic voltammetry (CV) profile exhibited by silicon porous nanosheets (Si p-NSs),43 as illustrated in Figure S7B. Figure 4B reveals the charge-discharge voltage profiles of the WSi@SiOx/Ti3C2 electrode over different cycles at a current density of 200 mA g−1 between 0.01 and 2 V (vs. Li/Li+). The initial discharge and charge capacities were measured as 2,085 mAh g−1 and 1,618 mAh g−1, respectively, resulting in a Coulombic efficiency of 77.6% at a current density of 200 mA g−1. This irreversible capacity loss can be primarily attributed to the electrochemical activation decomposition of the electrolyte, leading to the formation of a SEI layer. However, during the second cycle, an impressive increase in Coulombic efficiency was observed, reaching 89.52%. Furthermore, from the twelfth cycle onwards, no significant changes in Coulombic efficiency were observed, indicating that the WSi@SiOx/Ti3C2 nanosheets exhibit highly reversible Li+ storage behavior.Figure 4 Electrochemical performance of the WSi@SiOx/Ti3C2 and comparative material

(A) Initial CV curve of WSi@SiOx/Ti3C2 electrode.

(B) Charge-discharge voltage curves of WSi@SiOx/Ti3C2 electrodes.

(C and D) Specific capacity and rate performance of Ti3C2, WSi@SiOx and WSi@SiOx/Ti3C2 electrodes at 200 mA g−1.

(E) Long cycle performance of WSi@SiOx/Ti3C2 electrodes at 500 mA g−1.

The cycling performance and stability of Ti3C2, WSi@SiOx, and WSi@SiOx/Ti3C2 electrodes were further investigated at a current density of 200 mA g−1 (Figure 4C). It is evident that the WSi electrode exhibits rapid capacity decay, which can be attributed to its significant volume expansion. In contrast, the cycling stability of the WSi@SiOx electrode improves due to the presence of an oxide layer on its surface mitigating volume expansion issues. Notably, the WSi@SiOx/Ti3C2 electrode demonstrates significantly enhanced cycling stability with a capacity retention of 1,205 mAh g−1 after 200 cycles at 200 mA g−1 without notable degradation from cycle 12 onwards, surpassing that of commercial graphite negative electrodes (372 mAh g−1). This phenomenon can be attributed to the establishment of covalent chemical bonds, particularly Si-O-Ti, between the nanostructured silicon sheets and MXene, which significantly bolster the cycling durability of the composite. Furthermore, the arrangement of silicon nanosheets within MXene multilayers acts to mitigate volume expansion-induced pulverization and material shedding, owing to the restraining effect exerted by MXene’s multilayer elasticity on silicon’s volume changes. Moreover, this capacity outperforms both WSi@SiOx and Ti3C2 electrodes individually. The initial rapid capacity decay within the first 12 cycles can be primarily attributed to Li+ depletion caused by SEI layer formation and electrolyte decomposition during early cycles. The exceptional specific capacity observed in the WSi@SiOx/Ti3C2 anode can be ascribed to both excellent electrochemical activity exhibited by WSi@SiOx and essential synergistic effects provided by Ti3C2. The rate performance of WSi@SiOx/Ti3C2, WSi@SiOx, and Ti3C2 from 100 mA g−1 to 2 A g−1 is illustrated in Figure 4D. With an increase in current density from 100 mA g−1 to 2 A g−1, the capacity gradually decreases from 1,722 mAh g−1 to 566 mAh g−1. Notably, a remarkable recovery of high capacity (1,009 mAh g−1) is observed upon sudden adjustment back to the initial current density of 100 mA g−1, highlighting the exceptional rate performance exhibited by the WSi@SiOx/Ti3C2 anode. In contrast, at every current density tested, the capacity of WSi@SiOx/Ti3C2 surpasses that of both Ti3C2 and WSi@SiOx electrodes significantly, indicating that coupling Ti3C2 MXene with silicon nanoparticles enhances overall performance. Furthermore, the long-term cycling stability of WSi@SiOx/Ti3C2 was evaluated at a current density of 500 mA g−1 (Figure 4E). Remarkably, the WSi@SiOx/Ti3C2 electrode exhibited an impressive reversible capacity of 1,130 mAh g−1 after undergoing 800 cycles, surpassing the performance of previously reported Ti3C2-based and Si-based anodes (Table S2; Figure S8). These exceptional results can be attributed to the distinctive layered mesoporous nanostructure of the WSi@SiOx/Ti3C2 electrodes, which facilitates efficient transport of Li+ ions and electrons while effectively mitigating volume expansion associated with nanoscale silicon.

The reaction kinetics can be investigated using EIS. In the high-frequency region, the semicircles correspond to contact resistance and charge transfer resistance (Rct), while the diagonal lines in the low-frequency region are associated with ion diffusion capacity within the electrode, forming a Nyquist diagram as depicted in Figure 5A. The observed Rct values for WSi@SiOx/Ti3C2 (Rct = 167.4 Ω) in Figure 5A surpass those of Ti3C2 electrodes (Rct = 101.6 Ω) but fall short of the Rct values exhibited by WSi@SiOx electrodes (Rct = 887.2 Ω) in the high-frequency domain. Additionally, the DLi+ of the WSi@SiOx, Ti3C2, and WSi@SiOx/Ti3C2 electrodes were further calculated using the EIS method, as shown in Figure S9, revealing that impedance slopes are ranked in descending order as Ti3C2, WSi@SiOx/Ti3C2, and WSi@SiOx in the low-frequency spectrum. These results confirmed that after incorporating layered Ti3C2 as a conductive substrate, the WSi@SiOx/Ti3C2 electrode exhibits reduced charge transfer impedance and enhanced electron transfer rate. Additionally, EIS plots of WSi@SiOx/Ti3C2 before and after cycling for 800 cycles at a current density of 500 mA g−1 were compared in Figure 5B, revealing a slight increase in semicircle radius in the high-frequency region post-cycling, corresponding to slightly increased charge transfer resistance (Rct = 40.13 Ω). Moreover, an increased slope of slash line was observed in the low-frequency region indicating improved ion diffusion ability within the electrodes, thus demonstrating excellent cycling stability for WSi@SiOx/Ti3C2 composites.Figure 5 The reaction kinetics performance of the WSi@SiOx/Ti3C2 and comparative material

(A) EIS spectra of WSi@SiOx/Ti3C2, WSi@SiOx, and Ti3C2.

(B) EIS spectra of WSi@SiOx/Ti3C2 before and after cycling.

(C) CV curves of WSi@SiOx/Ti3C2 electrodes at different scan rates.

(D) CV curves with corresponding capacitance contributions at 0.8 mV s−1.

(E) Relationship between peak current and scan rate.

(F) Capacitance contribution at different scan rates.

Moreover, Ti3C2 exhibits significant pseudocapacitive properties attributed to the surface redox reaction, which significantly impacts its capacity contribution. To further investigate the Li+ storage mechanism of the WSi@SiOx/Ti3C2 electrode, CV measurements were conducted at various scan rates ranging from 0.2 to 1 mV s−1, as illustrated in Figure 5C. The Li+ storage mechanism in this case encompasses three primary types: diffusive contribution arising from alloying reactions, capacitive contribution based on surface redox reactions, and capacitive contribution originating from double-layer charging. The determination of capacitive contribution involves measuring the relationship between current (i) and scan rate (v).51(Equation 1) i=avb

Both factors a and b are adjustable parameters. Generally, when b = 0.5, it indicates a diffusion-controlled process (battery-mode energy storage mechanism), while b = 1 signifies a fully capacitively controlled process (capacitive-mode energy storage mechanism). Based on the CV measurements presented in Figure 5C, the anode peaks (peak A) exhibit b values of 0.69 each (Figure 5E), indicating that the contribution of capacitance-based energy storage mechanism cannot be disregarded for the WSi@SiOx/Ti3C2 electrode. Compared to WSi@SiOx (Figure S7), layered Ti3C2 (Figure S10) significantly enhances the pseudocapacitive performance of WSi@SiOx/Ti3C2 composite anode. The capacitance contribution as a proportion of total Li+ storage can be quantitatively confirmed based on the relationship between current value i(v), capacity contribution (k1v), and diffusion contribution (k2v1/2) at a fixed voltage: i(v)=k1v+k2v1/2, as shown in Figure 5D where the capacitance contribution to total Li+ storage at a scan rate of 0.8 mV s−1 is 60.6%.52 Furthermore, the proportion of capacitor-controlled capacity increases with scan rate (Figure 5F), indicating that capacitive behavior can store/release Li+ more efficiently at higher scan rates due to fast charging/discharging characteristics of capacitive storage mechanism. These results demonstrate that energy storage process of WSi@SiOx/Ti3C2 composite anode conforms to battery capacitive two-mode energy storage mechanism.(Equation 2) Si+xLi+↔LixSi(batteryreaction)

(Equation 3) Ti3C2+xLi++e−↔Ti3C2Lix(capacitivereaction)

The WSi@SiOx component primarily contributes to the high-capacity energy storage battery section through the Li-Si alloy reaction (reaction 2). On the other hand, Ti3C2 contributes to the capacitor section through surface redox reaction (Reaction 3). Additionally, the Si-O-Ti bond between Ti3C2 and WSi@SiOx plays a crucial role as a dual auxiliary. In terms of cell mode storage, it facilitates efficient interfacial charge transfer and ensures high interfacial mechanical stability. For pseudo-capacitive storage, the Si-O-Ti bond further polarizes atomic charges within the composite, thereby enhancing Li+ ion adsorption. The combination of these factors significantly contributes to the exceptional electrochemical performance of this cell. The galvanostatic intermittent titration technique (GITT) was employed to further investigate the lithium ion diffusion coefficient (DLi+) of WSi@SiOx/Ti3C2 composites, using a current density of 200 mA/g, a pulse duration of 5 min, and an interval of 1 h. As shown in the Figure S11, the GITT results revealed DLi+ values ranging from 10−10 to 10−12 cm2 s−1, which are consistent with the EIS results and provide additional evidence for the enhanced lithium diffusion kinetics observed in the WSi@SiOx/Ti3C2 composite electrode.

In order to investigate the cycling stability of both WSi@SiOx/Ti3C2 and WSi materials, the cycling cells were disassembled simultaneously after 200 cycles at a current of 200 mA g−1 to reveal the morphological changes of the WSi@SiOx/Ti3C2 and WSi electrode. SEM analysis revealed that even after 200 cycles, no substantial damage occurred on the electrode surface of WSi@SiOx/Ti3C2 composite (Figure 6B).53 Cross-sectional SEM image displayed that compared with initial thickness of 15.4 μm, only a modest increase in thickness by merely 11.1% was observed for WSi@SiOx/Ti3C2 electrode which is significantly lower than that observed for WSi electrode. For the cycled WSi anode, cracks emerged on the previously smooth and compact surface of the WSi electrode, resulting in a remarkable increase in thickness by 109.5% from 15.3 μm to 32.1 μm after 200 cycles (Figure 6D). The contrasting results indicate that ultra-thin oxide layer present on WSi@SiOx/Ti3C2 composite’s surface along with its stable sandwich structure play a crucial role in mitigating volume expansion as well as maintaining structural integrity.Figure 6 Electrode thickness of electrode plane and cross-section before and after cycling 200 cycles at a current of 200 mA g−1

(A and B) SEM images of WSi@SiOx/Ti3C2 electrode.

(C and D) SEM images of WSi electrode.

Discussion

In conclusion, the present study reports a simple and scalable electrostatic assembly method based on the mutual attraction between positively charged Ti3C2 nanosheets and negatively charged WSi@SiOx, aiming to obtain WSi@SiOx/Ti3C2 composites as anode materials for LIBs. By uniformly adhering WSi@SiOx onto the surface of Ti3C2, self-agglomeration of Ti3C2 nanosheets is effectively prevented. Moreover, TMAA-treated Ti3C2 not only increases the number of active sites for WSi@SiOx attachment but also acts as an intercalating agent that expands and maintains the interlayer spacing of Ti3C2. Consequently, this allows for the entry of WSi@SiOx into the interlayer spaces of Ti3C2, minimizing re-stacking of MXene nanosheets and aggregation issues associated with lithiation/delithiation processes in silicon-based materials. Additionally, sheet-like silicon and ultra-thin SiOx provide sufficient cushioning against volume changes while incorporating Ti3C2 enhances conductivity within the composite and facilitates Li+ transport. As a result, overall performance improvements are achieved in terms of cycling stability and multiplicity when using WSi@SiOx/Ti3C2 composites as LIBs anode materials. The composite showed a capacity of 1,130 mAh g−1 after 800 charge/discharge cycles at 500 mAg-1. This study presents a straightforward yet promising approach toward high-value utilization of WSi while offering a viable route for preparing cost-effective silicon-based LIBs.

Limitations of the study

Typically, the esterification reaction demonstrates optimal conditions within the temperature range of 50°C–100°C. Deviations from this range, whether excessively high or low, can have a detrimental impact on both reaction efficiency and product quality. Furthermore, this study has only investigated half-cells, while future research will focus on conducting comprehensive studies to evaluate the performance of lithium-ion full batteries.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER	
Chemicals, peptides, and recombinant proteins	
	
WSi	LONGi Silicon Materials Ltd (Chuxiong, China)	N/A	
Ti3AlC2 (200 mesh)	Jilin Eleven Technology Co	CAS196506-01-1	
sodium hydroxide (NaOH)	aladdin	CAS1310-73-2	
lithium fluoride (LiF)	aladdin	CAS7789-24-4	
hydrochloric acid (HCl)	aladdin	CAS7647-01-0	
sodium alginate	aladdin	CAS9005-38-3	
conductive carbon black (Super P)	aladdin	CAS308068-56-6	
anhydrous ethanol	aladdin	CAS64-17-5	
Tetramethylammonium Acetate (TMAA) powder with a purity of 98%	aladdin	CAS10581-12-1	
Phosphate Buffer Solution (PBS) at pH 6.6 with a 50% aqueous solution	aladdin	P684573	
	
Software and algorithms	
	
OriginLab	Analyze and graph	https://www.originlab.com	
	
Other	
	
NEWARE CT-4008-5V-12A	Charging and discharging	https://www.neware-technology.com	

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Fengshuo Xi (fengshuoxi@126.com).

Materials availability

The study did not generate new unique materials. The readers can buy the chemicals to remake the materials as mentioned in the text.

Data and code availability

• Data: All data reported in this paper will be shared by the lead contact upon request.

• Code: This paper does not report the original code.

• Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Experimental model and subject details

There are no experimental models (animals, human subjects, plants, microbe strains, cell lines, primary cell cultures) used in the study.

Method details

Materials and reagents

The WSi utilized in this study was provided by LONGi Silicon Materials Ltd (Chuxiong, China). Ti3AlC2 (200 mesh) was procured from Jilin Eleven Technology Co. Analytical grade reagents including sodium hydroxide (NaOH), lithium fluoride (LiF), hydrochloric acid (HCl), sodium alginate, conductive carbon black (Super P), and anhydrous ethanol were employed. Tetramethylammonium Acetate (TMAA) powder with a purity of 98% and Phosphate Buffer Solution (PBS) at pH 6.6 with a 50% aqueous solution were obtained from Aladdin.

WSi@SiOx/Ti3C2 electrostatic self-assembly

The initial step involved the comminution of pristine WSi agglomerates into WSi powders, which underwent a subsequent cleansing procedure using a 0.1 mol/L solution of sodium hydroxide in ethanol for 2 h at ambient conditions to remove any residual organic constituents. Subsequently, the desiccated WSi powders were oxidized in a muffle furnace at a sustained temperature of 300°C for 1 h. Notably, the resulting oxidized WSi@SiOx exhibited a weight gain of 18%, indicating an oxidation rate of approximately 15.8%. This precisely orchestrated process aimed to finely modulate the thickness of the oxide layer surrounding the laminar WSi flakes, thus yielding the desired partially oxidized WSi@SiOx material. To obtain negatively charged WSi@SiOx, 3 g of WSi@SiOx was dispersed in pH = 6.6 PBS (200 mL) and stirred for 2 h before filtration.54,55 For Ti3C2 modification, 2 g LiF was stirred with 40 mL of 9 M hydrochloric acid at a speed of 400 rpm for half an hour firstly; then, another batch consisting of equal amounts was prepared by adding and stirring continuously for a duration of 24 h after introducing Ti3AlC2 into the mixture.56 The resulting mixture underwent washing with anhydrous ethanol before being filtered and sonicated in another batch containing 200 mL anhydrous ethanol at a power level set to reach approximately four hundred watts for 15 min. This process led to abundant presence of hydroxyl groups (-OH) on the surface area of Ti3C2. Subsequently, esterification occurred between carboxyl group (-COOH) present in TMAA and hydroxyl group (-OH) on Ti3C2 surface when stirring together with fifteen weight percent aqueous solution containing one point five grams Ti3C2 rich in hydroxyl groups along with TMAA. As a result, positively charged amino ions were introduced onto the surface area making it positively charged. The MXene surfaces exhibit abundant positively charged amino groups under acidic or neutral conditions, while the zeta potential of silicon varies with pH due to its silicon oxide layer. To enhance the bonding affinity between these materials, increasing the negative charge density on the silicon surface is crucial. Analysis based on the silica Zeta-pH diagram revealed maximal attraction between these materials at pH 6.6 under acidic conditions.55,56,57,58 Consequently, 1 g of positively charged Ti3C2Tx and 3 g of negatively charged WSi@SiOx were stirred in 300 mL of PBS at pH 6.6 for 12 h. The interaction between anions and cations facilitated interfacial electrostatic self-assembly, resulting in the composite formation. The WSi@SiOx/Ti3C2 composites were subsequently obtained via vacuum filtration.

Material characterization

Morphology, internal structure, and elemental analysis were conducted using a transmission electron microscope (TEM, Tecnai G2 TF30 S-Twin, FEI) equipped with an energy dispersive X-ray spectrometer (EDS, Bruker Quantax), as well as a scanning transmission electron microscope (STEM). Surface morphology and microstructure observations were performed using a field emission scanning electron microscope (FE-SEM, QUANTA 200, FEI). X-ray diffraction measurements (XRD, X′pert 3, PANalytical) were utilized to characterize the material’s structure. The surface state information of the WSi@SiOx/Ti3C2 composites was determined via X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo Fisher Scientific). Specific surface area analysis was carried out using the Brunauer-Emmett-Teller method (BET, Nova2200e, Quantachrome). Fourier transform infrared spectroscopy (FTIR, TENSOR27, Bruker) spectrometer was used for FTIR characterization over a range of 500–4000 cm−1. Raman spectra were analyzed on a JY-HR800micro Raman spectrometer using a 532 nm laser as excitation source.

Electrochemical measurements

The active material (Ti3C2, WSi@SiOx, WSi@SiOx/Ti3C2 and WSi), conductive carbon black, and sodium alginate binder were mixed in deionized water to form a homogeneous slurry with a composition of 70 wt %, 15 wt %, and 15 wt % respectively. This slurry was then uniformly coated onto a copper foil as the working electrode. Subsequently, the entire electrode was dried at 80°C for 12 h under vacuum conditions. Afterward, the electrode was cut into discs and further dried at 80°C for an additional 2–4 h in a vacuum oven. CR2032 button half-cells were prepared inside an argon-filled glove box using lithium metal foil as the counter electrode and polyethylene (PE) film as the diaphragm. The electrolyte consisted of vinyl carbonate and diethyl carbonate (EC:DEC = 1:1 v/v) with a concentration of 1 M LiPF6. All cells underwent testing at various current densities (Li/Li+) within a voltage range of 2.0–0.01 V using an electrochemical test system (XWJ Neware Tech., BTS3000, China). The Galvanostatic Intermittent Titration Technique (GITT) involves a measurement current of 200 mA/g accompanied by a pulse duration lasting 30 min, interspersed with a 2-h resting interval. Cyclic voltametric (CV) curves and electrochemical impedance spectra (EIS) were measured using an electrochemical workstation (Ametek, PMC-1000dc, USA).

Quantification and statistical analysis

No methods were used to determine whether the data met the assumptions of the statistical approach.

Additional resources

Our study has not generated or contributed to a new website/forum or has not been part of a clinical trial.

Supplemental information

Document S1. Figures S1–S11 and Tables S1–S3

Acknowledgments

Financial support for this work from the Yunnan Province Basic Research General Program (202201AT070442 ); 10.13039/501100001809 National Natural Science Foundation of China (no. 52204314 , 52274408 ); Yunnan Xingdian Talents Support Plan (XDYC-QNRC-2022-0596 ); Young Elite Scientists Sponsorship Program by CAST (2022QNRC001 ); Yunnan Fundamental Research Projects (no. 202201BE070001-002 , 202201AW070014 ); and the Program for Innovative Research Team in University of Ministry of Education of China (no. IRT_17R48 ).

Author contributions

Conceptualization: F.X.; methodology: Y.N.; formal analysis and investigation: W.M., L.W., H.L., J.L., X.C., and K.W.; writing—original draft preparation: Y.N. and M.W.; writing—review and editing: F.X. and B.L.; funding acquisition: F.X. and S.L.; supervision: J.Y., F.X., and W.M.; All authors have read and agreed to the published version of the manuscript.

Declaration of interests

The authors declare no competing interests.

Supplemental information can be found online at https://doi.org/10.1016/j.isci.2024.110714.
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