In Situ SAXS Investigation of Bi-Bi₂O₃/C Anode Materials for Lithium-Ion Batteries Under Operational Conditions

Abstract

Bismuth has become one of the most promising anodes for lithium-ion batteries (LIBs) due to its suitable operating voltage and high volumetric capacity. However, the inevitable volume expansion of bismuth during the charge/discharge cycling process of lithium-ion batteries can lead to severe capacity fading and eventual battery failure. Herein, Bi-Bi₂O₃/C composite materials were prepared using a combination of solvothermal and calcination methods, and a series of characterization tests were conducted to investigate the effects of different secondary calcination times on the microstructure, composition, electrochemical properties, and the internal structural evolution during the initial charge/discharge cycle of the composite materials. In situ synchrotron radiation small angle X-ray scattering (SAXS) was employed to reveal the multi-level nanostructure evolution of Bi-Bi₂O₃/C electrode materials during charge/discharge processes. The present work reveals that these SAXS findings provide some new insights into the theoretical mechanism for the development of LIBs.

Keywords:
Small-angle X-ray scattering; Bi-based anode materials; Nanostructure; Electrochemical performance

1. Introduction

As one of the most important electrochemical energy storage technologies today, lithium-ion batteries (LIBs) play a pivotal role in the clean energy revolution¹-³. With their advantages of high energy density, long cycle life, and low self-discharge rate, LIBs have promoted the popularization of electric vehicles and effectively reduced dependence on fossil fuels and carbon emissions, serving as the core technological foundation for achieving the “dual carbon” goal and energy structure transformation⁴-⁶. Although LIBs are widely used, they still have significant drawbacks and disadvantages⁷-⁹. The current commercial lithium battery energy density is close to the theoretical limit (e.g., graphite anode offer a specific capacity of only 372 mAh g^-1), which is difficult to meet the long range requirements of electric vehicles. The electrode material undergoes volume expansion/contraction during cycling (e.g., silicon electrodes exhibit volume expansion exceeding 300%), resulting in structural pulverization and capacity decay — critical challenges must be addressed¹⁰. For example, relying on excellent electrical conductivity and a stable microscopic skeleton, carbon materials can act as favorable hosts for the reversible insertion and extraction of lithium ions, effectively buffer the volume expansion and reduce electrode polarization¹¹-¹³. Further modification can also greatly improve the rate capability and cycling stability of lithium-ion batteries.

Bismuth (Bi)-based anode materials have become a research hotspot in the field of LIBs due to their unique physicochemical properties and lithium storage mechanism¹⁴-¹⁶. Bi undergoes alloying reaction (Bi + 3Li⁺ + 3e⁻ ↔ Li₃Bi), which enables a theoretical capacity of 386 mAh g^-1, and its operating voltage platform (~0.8 V vs. Li⁺/Li) can suppress lithium dendrite formation while maintaining high energy output efficiency, and many Bi-based materials can be synthesized using MOF-derived strategies¹⁷,¹⁸. At present, research on Bi based anode materials mainly focuses on structural optimization and composite system construction¹⁹. Bi nanoparticles are embedded in sturdy SiOC domains, which alleviates local mechanical strain caused by bismuth alloying during Li⁺ insertion/extraction, and delivers a highly stable capacity of 507 mAh g^-1 after 150 cycles at 50 mA g^-1, and excellent long-term cycling performance of 380 mAh g^-1 at 0.5 A g^-1 after 500 cycles²⁰. Bi₂O₃ anode material attains an impressive initial capacity of approximately 750 mAh g^-1, exhibits excellent rate capability at 1000 mA g^-1 and maintains stable cycling performance over 6000 cycles through a straightforward, commercially viable synthesis route²¹. The core purpose of studying Bi-based anodes is to solve the problem of volume expansion (~300%) of high-capacity materials during cycling, and to design nanostructures such as Bi@C composite materials and optimize interface to enhance cycling stability²². Combining Bi with carbon materials can significantly improve conductivity and buffer volume expansion, resulting in an increase in cycle life²³-²⁶. During the charging and discharging process, Bi nanodots embedded in a porous carbon matrix can effectively adapt to strain changes and shorten the migration distance of Li⁺. The porous carbon forms an effective conductive network. After 100 cycles, the capacity reached 520 mAh g^-1 at 0.2 A g^-1, and the long-term cycling stability reached 380 mAh g^-1 at 0.5 A g^-1 after 500 cycles²⁷. The carbon layer in the dual carbon coating layered structure (Bi/C@C_Ppy) material inhibits the aggregation of Bi particles and reduces the volume change during charge and discharge cycles. After 100 cycles at 0.1 A g^-1, it maintains a specific capacity of 526.4 mAh g^-1. After 900 cycles, it still maintains a specific capacity of 255.6 mAh g^-1 at 0.5 A g-1 ²⁸. However, the structural evolution process of LIBs under working conditions is of great significance for the degradation and improvement of electrode material performance²⁹.

The synchrotron radiation-based techniques, including X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and X-ray absorption fine structure (XAFS), etc., have been employed in LIB research to reveal the dynamic structural evolution, chemical valence state changes, and failure mechanisms of electrode materials during charge and discharge processes through high-precision and multi-scale in situ characterization methods, thereby guiding the optimization design of high-performance battery materials³⁰-³³. For example, SAXS is a nondestructive structural analysis technique that obtains information about nanoscale (typically 1-100 nm, extending to several hundred nanometers) structures in a sample by measuring the intensity distribution of the incident X-ray beam scattered by the sample at very small angles (< 5°), based on electron density contrast within the sample. Synchrotron radiation SAXS, with its ultra-high brightness, excellent collimation, and powerful in-situ capability, provides unique structural information for studying the dynamic evolution of nanoscale structures inside lithium-ion batteries (including electrode phase transition, solid electrolyte interface (SEI) growth, lithium deposition, and pore structure changes)³⁴-³⁶. Its value lies in its ability to reveal key structural information in real-time and nondestructive during battery operation, providing direct experimental basis for optimizing and designing electrode materials³⁷-³⁹. The structural changes and subsequent relaxation process of graphite during high-rate discharge were analyzed using in-situ synchrotron radiation XRD. During the high-rate discharge process of 5C, Li deintercalation occurs at different stages, with these stage structures coexisting. Structural relaxation of the staged phases requires extended time periods. This slow change was believed to be the reason for the deterioration of battery performance during high-rate operation⁴⁰. In situ synchrotron SAXS was used to study the structural evolution of a Bi/C anode during its first charge/discharge cycle. The results indicate that during the first lithiation, the Bi/C composite contains mesopores, interspaces, and three different size Bi nanoparticles, each serving as distinct scattering centers. Significantly, the nanostructures of these three distinct Bi nanoparticle types exhibit divergent nanostructural evolution³⁹.

The present study employs Bi-MOF as a precursor to synthesize a Bi-Bi₂O₃/C composite material. The amorphous carbon in the composite effectively suppresses the agglomeration of Bi and Bi₂O₃ particles, significantly enhancing the electrochemical performance of the electrode material. Synchrotron radiation SAXS technology was employed to investigate the evolution of the nanostructure of the Bi-Bi₂O₃/C electrodes during the first charge-discharge cycle. These founding revealed multilevel structural transformations occurring in the electrode material during the electrochemical process, thereby guiding the optimization and design of high-performance battery materials and advancing fundamental research in lithium-ion batteries.

2. Experimental Section

2.1. Preparation of materials

Bismuth-based MOFs were prepared via a solvothermal method. First, 750 mg of benzene-1,3,5-tricarboxylic acid (H₃BTC) was dissolved in 60 mL of anhydrous methanol, and 150 mg of Bi(NO₃)₃ ∙5H₂O was added to the resulting solution. The mixture was stirred at room temperature until clear and transparent (about 30 min), and then transferred to a 100 mL Teflon-lined stainless-steel autoclave for solvothermal reaction at 120 °C for 12 h. The product was obtained as a white precipitate, which was collected by centrifugation at 4000 r/min, washed with anhydrous methanol for three times, and dried under vacuum at 60 °C overnight.

The Bi-Bi₂O₃/C composite was prepared by calcination of the Bi-MOF precursor. As illustrated in Figure 1, the Bi-MOF material was first synthesized and used as the precursor. The Bi-MOF was placed in a quartz boat and carbonized at 700°C for 30 min in a tube furnace under a nitrogen atmosphere, yielding a black powder. Subsequently, the black powder was subjected to secondary calcination in a muffle furnace at 200 °C for various durations before cooling to room temperature. A series of samples were prepared using the same procedure while varying only the secondary calcination time (10, 60, 90, 120, 150, and 300 min): Bi-Bi₂O₃/C-10, Bi-Bi₂O₃/C-60, Bi-Bi₂O₃/C-90, Bi-Bi₂O₃/C-120, Bi-Bi₂O₃/C-150, and Bi-Bi₂O₃/C-300.

Figure 1
Schematic diagram of Bi- Bi₂O₃/C composite synthesis.

2.2. Material characterization

The structural and morphological characteristics of the Bi-Bi₂O₃/C composite were investigated using a combination of advanced characterization techniques. High-resolution transmission electron microscopy (HRTEM, Hitachi H-7650) and field-emission scanning electron microscopy (FESEM, Hitachi S-3400) were employed to examine the material's morphology and microstructure at the nanoscale. Crystallographic analysis was performed using a SmartLab X-ray diffractometer (Rigaku) with Cu Kα radiation (λ = 0.15418 nm) to obtain powder X-ray diffraction (XRD) patterns. The chemical states of constituent elements were determined through X-ray photoelectron spectroscopy (XPS) measurements conducted on an ESCALAB 250Xi spectrometer (Thermo Scientific) with monochromatic Al Kα excitation. Additionally, the textural properties, including specific surface area and pore size distribution, were evaluated by nitrogen physisorption measurements at 77 K using a Micromeritics ASAP-2020 surface area analyzer.

2.3. Electrochemical measurements

The working electrode was fabricated using the following procedure: a homogeneous slurry was prepared by thoroughly mixing the active material (Bi-Bi₂O₃/C), conductive carbon (acetylene black), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 7:2:1, using N-methyl-2-pyrrolidone (NMP) as the solvent to achieve a suitable viscosity. The slurry was then uniformly coated onto a carbon cloth current collector and dried under vacuum at 60 °C for 8 h prior to punching into electrode of defined geometry. The active mass loading of the electrode sheets was controlled at approximately 1.1-1.5 mg cm^-2. A half-cell is assembled as a simplified battery system that uses lithium metal as both the counter electrode and reference electrode, with only one tested electrode (Bi-Bi₂O₃/C anode). The electrolyte consisted of a 1:1:1 volume mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) containing 1 M lithium hexafluorophosphate (LiPF₆) as the conducting salt. A Celgard 2400 polypropylene membrane was used as the separator. Electrochemical characterization was performed using a CHI600C electrochemical workstation for cyclic voltammetry (CV) measurements (0.01~3.0 V vs. Li⁺/Li) and electrochemical impedance spectroscopy (EIS) analysis (frequency range: 10 MHz to 100 kHz). Additionally, the cycling stability and rate performance of the Bi-Bi₂O₃/C electrode were assessed using a LAND battery testing system.

2.4. In Situ SAXS experiment

The in situ SAXS experiments were conducted at the 1W2A beamline of the Beijing Synchrotron Radiation Facility (BSRF). The measurements employed an X-ray beam with a wavelength of 0.154 nm, generated using a storage ring operating at an electron energy of 2.5 GeV with a beam current of approximately 200 mA beam current. A Mar165 CCD detector (2048 × 2048 pixels, 79 µm × 79 µm pixel size) was positioned 1480 mm from the sample for data collection. In situ SAXS characterization was performed concurrently with galvanostatic charge-discharge cycling (voltage range: 0.01~3.0 V). Each SAXS pattern was acquired with an exposure time of 5 s. The two-dimensional scattering images were subsequently processed and converted into one-dimensional scattering profiles using the FIT2D software package for quantitative analysis.

3. Results and Discussion

The Bi-Bi₂O₃/C composite was first obtained by calcining the Bi-MOF precursor at 700 °C for 30 minutes under a nitrogen atmosphere, followed by secondary calcination for different durations. The resulting samples—Bi-Bi₂O₃/C-10, Bi-Bi₂O₃/C-30, Bi-Bi₂O₃/C-60, Bi-Bi₂O₃/C-90, Bi-Bi₂O₃/C-120, and Bi-Bi₂O₃/C-300—are shown in Figure 2a-f (scale bar 5µm). As shown in the SEM images, the Bi-Bi₂O₃/C composites retained the layered structure of the original Bi-MOF, with nanoparticles decorating the surface. The particle size of the Bi-Bi₂O₃/C composites remained approximately 3-4 µm, and showed no significant variation with increasing calcination time. EDS analysis was conducted on the Bi-Bi₂O₃/C-10 composite, and the results were presented in Figure 2g-j. EDS confirmed the presence of three elements: carbon (C), oxygen (O), and bismuth (Bi). In addition, during the process of preparing Bi-MOF, a 500 mL Teflon-lined stainless-steel autoclave was used to expand the experiment by 5 times, and the obtained samples were the same. The main factor that may limit amplification in the current process is the high energy consumption in high-temperature carbonization.

Figure 2
SEM image of different Bi-Bi₂O₃/C sample: (a) Bi-Bi₂O₃/C-10 (b) Bi-Bi₂O₃/C-30 (c) Bi-Bi₂O₃/C-60 (d) Bi-Bi₂O₃/C-90 (e) Bi-Bi₂O₃/C-120 (f) Bi-Bi₂O₃/C-300, (g-j) EDS mapping images of Bi-Bi₂O₃/C-10 composite.

To further investigate the microstructure of the Bi-Bi₂O₃/C composite, the Bi-Bi₂O₃/C sample was characterized by TEM. Figure 3a-d presents TEM images at increasing magnifications (scale bars: 2 μm, 1 μm, 200 nm, and 100 nm, respectively). Carbonization and secondary calcination yielded a composite comprising a carbon matrix with uniformly embedded granular particles, which were identified as Bi and Bi₂O₃ nanoparticles. The carbon framework acted as a physical support structure, effectively preventing nanoparticle aggregation.

Figure 3
TEM images of Bi-Bi₂O₃/C-30 composites at different magnifications: (a) 2 µm (b) 1 µm (c) 200 nm (d) 100 nm.

To investigate the coexistence of Bi and Bi₂O₃ phases in the Bi-Bi₂O₃/C composites, XRD characterization was performed. Figure 4 shows the XRD patterns of Bi-MOF and the as-prepared Bi-Bi₂O₃/C composites. The Bi-MOF precursor exhibits distinct diffraction peaks in the low-angle region (10-30°), characteristic of its crystalline framework. After carbonization, these peaks completely disappear, indicating the decomposition of the MOF structure. The results demonstrate that, compared with the Bi-MOF precursor, the secondary-calcined Bi-Bi₂O₃/C composites exhibit distinct crystalline peaks. The characteristic diffraction peaks observed at 27.2°, 37.9°, 39.6°, 48.7°, 62.2°, and 64.5° correspond to the (012), (104), (110), (202), (116), and (122) reflections of metallic Bi phase (PDF#85-1329), respectively. A broad peak in the 2θ range of 20~40° was attributed to amorphous carbon. Additional characteristic peaks at 27.9°, 31.7°, and 32.7° were assigned to the tetragonal β-Bi₂O₃ phase (PDF#78-1793). With increasing calcination time, the diffraction peaks became progressively sharper, indicating enhanced crystallinity. However, when the secondary calcination time was extended to 300 min, the crystallinity of the Bi phase decreased, likely due to partial oxidation of Bi into Bi₂O₃ nanoparticles and reduced particle size.

Figure 4
XRD plots of Bi-MOF and different Bi-Bi₂O₃/C composites.

To further investigate the chemical composition and surface electronic structure of the Bi-Bi₂O₃/C composite, XPS characterization was performed. Figure 5a displays the full XPS spectrum of Bi-Bi₂O₃/C-30, revealing characteristic peaks corresponding to Bi 5d, Bi 4f, C 1s, Bi 4d, and O 1s. This confirms the coexistence of Bi, O, and C elements in the composite, consistent with the EDS results. Notably, the carbon component exhibits a highly graphitic. The high-resolution Bi 4f spectrum (Figure 5b) displays two characteristic peaks at binding energies of 159.0 eV and 164.3 eV, attributed to Bi³⁺ 4f_7/2 and Bi³⁺ 4f_5/2, respectively. No metallic Bi⁰ signal was detected, likely due to surface oxidation under ambient conditions. The O 1s spectrum (Figure 5c) can be deconvolved into three components: the peak at 529.6 eV is assigned to Bi-O bonds; the peak at 533 eV to C=O bonds, and the broad peak between 533 and 534 eV to chemisorbed oxygen species. The high-resolution C 1s spectrum (Figure 5d) shows three distinct peaks at 284.5 eV, 286.0 eV, and 289.0 eV, which are assigned to C-C bonds, C-O/C=O bonds, and O-C=O bonds, respectively.

Figure 5
(a) XPS survey (b) high-resolution Bi 4f spectra (c) high-resolution O 1s spectra and (d) high-resolution C 1s spectra of the Bi-Bi₂O₃/C-30 composites.

Figure 6 shows the pore structure and pore size distribution of the samples characterized by nitrogen adsorption/desorption measurements. As shown in Figure 6a, all samples exhibit typical Type IV isotherms with distinct hysteresis loops in the medium-to-high relative pressure (P/P₀) range of 0.4-1.0, indicating well-developed mesoporosity. The specific surface areas of Bi-Bi₂O₃/C-30, Bi-Bi₂O₃/C-60, Bi-Bi₂O₃/C-90, and Bi-Bi₂O₃/C-300 are 104.03, 112.64, 148.25, and 144.11 m²/g, respectively. The specific surface area initially increases with calcination time, which can be attributed to the gradual evolution of porosity during secondary calcination. A large specific surface area facilitates lithium-ion transport and electrolyte infiltration, thereby improving the electrochemical performance of the composites. The pore size distribution curves are shown in Figure 6b. The pore sizes are predominantly distributed in the 2-10 nm range (the sharp single peak is at ~ 3-4 nm), confirming the typical mesoporous structures of the composites. The similarity in pore size distributions across all samples indicates that secondary calcination primarily affects the surface area rather than the pore architecture. The Bi-Bi₂O₃/C-90 exhibits the most favorable pore architecture for ion transport, as evidenced by its highest specific surface area and optimal pore size distribution. This mesoporous architecture facilitates Li⁺ transport and electrolyte infiltration, thereby improving the ion transport capability of composites. The average pore diameters of Bi-Bi₂O₃/C-30, Bi-Bi₂O₃/C-60, Bi-Bi₂O₃/C-90, and Bi-Bi₂O₃/C-300 calculated using the BJH method are 7.62, 5.76, 5.44, and 5.40 nm, respectively.

Figure 6
(a) BET curves of Bi-Bi₂O₃/C with different secondary calcination temperatures, (b) Pore size distribution curves of Bi-Bi₂O₃/C with different secondary calcination temperatures.

Assemble lithium-ion half cells using Bi-Bi₂O₃/C composite material as the anode material and conduct electrochemical performance testing. As shown in Figure 7a, the CV curves of the first to third cycles of Bi-Bi₂O₃/C-90 composite material were measured at a scan rate of 0.1 mV s^-1 in the range of 0.01~3.0 V (vs. Li⁺/Li). There are three redox peaks near 0.54, 0.70, and 1.30 V in the figure. The two reduction peaks near 0.54 and 0.70 V indicate the lithiation reaction of the electrode, during which the bimetallic alloy is formed to produce LiBi and Li₃Bi, while the peak near 1.30 V corresponds to the oxidation reaction to produce Li₂O. During the reduction process, there are two distinct peaks around 0.90 V, attributed to the reduction reaction that converts LiBi, Li₃Bi, and Li₂O to Bi. The second and third cycle CV curves show excellent overlap in both shape and intensity, indicating high reversibility. During the first cycle of charge/discharge, the SEI film is formed, resulting in a different CV curve compared to the subsequent cycles. Figure 7b presents the initial galvanostatic charge/discharge curves of the Bi-Bi₂O₃/C anode materials at a current density of 100 mA g^-1. As shown in the figure, the Bi-Bi₂O₃/C-90 sample delivers the highest initial discharge specific capacity of 1743 mAh g^-1among all samples. In comparison, the initial discharge capacities of Bi-Bi₂O₃/C-10, Bi-Bi₂O₃/C-30, Bi-Bi₂O₃/C-60, Bi-Bi₂O₃/C-120, and Bi-Bi₂O₃/C-300 are 1372, 1525, 1598, 1655, and 1655 mAh g^-1, respectively. Their corresponding initial charge capacities are 975, 1400, 1238, 1230, and 1280 mAh g^-1, with Coulombic efficiencies of 71.1%, 91.8%, 77.5%, 73.9%, and 77.3%, respectively. However, Bi-Bi₂O₃/C-90 exhibits a relatively low initial Coulombic efficiency of only 70.3%, whereas Bi-Bi₂O₃/C-30 demonstrates superior performance in this regard. Additionally, all Bi-Bi₂O₃/C anode materials exhibit a charge plateau near 0.9 V, while two discharge plateaus appear around 0.70 V and 1.3 V. These results are consistent with the conclusions drawn from the CV curves.

Figure 7
(a) CV curves of the Bi-Bi₂O₃/C-90, (b) Initial galvanostatic charge/discharge curves, (c) cycle performance at 100 mA g^-1, (d) rate capability, (e) Galvanostatic charge/discharge curves of the Bi-Bi₂O₃-90 at various current densities, (f) Nyquist plots of the different Bi-Bi₂O₃/C.

Figure 7c shows the cycling performance of Bi-Bi₂O₃/C anode materials at a current density of 100 mA g^-1. Bi-Bi₂O₃/C-30 maintained the highest reversible capacity during the first 80 cycles. However, Bi-Bi₂O₃/C-90 exhibited good cycling stability, with a reversible capacity of 1019 mAh g^-1 after 100 cycles, corresponding to 58.5% capacity retention to its initial discharge capacity. The capacity is better compared to the performance of Bi-based lithium-ion batteries in recent years⁴²,⁴³, which may be due to the assembly of lithium-ion half cells and the use of carbon cloth as the substrate. Figure 7d shows the rate performance of Bi-Bi₂O₃/C anode materials. The Bi-Bi₂O₃/C-90 electrode material delivers reversible capacities of 1356, 1207, 1068, and 811 mAh g^-1 at current densities of 50, 100, 200, and 500 mA g^-1, respectively, and recovers to 1300 mAh g⁻¹ when the current density returns to 50 mA g⁻¹. Figure 7e shows the galvanostatic charge/discharge curves of Bi-Bi₂O₃/C-90 at various current densities. These results indicate that Bi-Bi₂O₃/C-90 electrode material has good rate performance and high reversibility. Figure 7f shows the EIS curves of different Bi-Bi₂O₃/C electrode materials. In comparison, Bi-Bi₂O₃/C-90 exhibits lower charge transfer resistance, faster electrochemical reaction rate, and better diffusion ability.

The Bi-Bi₂O₃/C-30 composite material, which delivered the best initial cycling performance, was chosen as the anode material for LIB and assembled into a lithium-ion half-cell for in situ synchrotron radiation SAXS experiment during the initial charge/discharge cycle. Figure 8a displays the SAXS curves of the Bi-Bi₂O₃/C-30 anode recorded during the first discharge process at a current density of 100 mA g^-1. As shown in the figure, the scattering intensity undergoes pronounced variation with decreasing voltage. Figure 8b shows the experimental SAXS curves together with the fitted curves (solid lines) derived from the tangent-by-tangent (TBT) method³⁸,³⁹,⁴¹. It is clear that the fitted curves match the raw experimental data excellently, verifying the reliability of the fitting approach. Figure 8c shows the evolution of the average radius of gyration (Rg) for the Bi-Bi₂O₃/C-30 anode as a function of voltage during the first discharge, obtained from the fitted results. The average Rg reaches its maximum at 2.3 V and then decreases gradually, which reflects the size evolution of the average scattering nanoparticles in the electrode. Figure 8d displays the variation of normalized volume fraction (NVF) obtained from the fitted data. NVFs exhibit a distinct bimodal distribution, and nanoparticles with Rg values of approximately 0.7 nm and 7 nm possess higher volume fraction.

Figure 8
(a) The SAXS curves of Bi-Bi₂O₃/C-30 anode material during the first discharge process. (b) SAXS experimental intensities (symbols) versus calculated values (solid lines). (c) Average Rg variation of scattered nanoparticles. (d) Normalized volume fraction of scatterers during the initial discharge obtained by the TBT method.

Using the TBT method, we obtained the size distribution of scattering objects at different hierarchical levels in the Bi-Bi₂O₃/C-30 anode, as shown inFigure 9a. The scattering nanoparticles can be categorized into five levels. The variations in the radii of Rg for these five levels of scatterers with decreasing voltage are displayed inFigures 9b-f, respectively. By combining the Rg evolution of scatterers inFigures 9 and 9d with the bimodal distribution observed from the NVF results, it is clear that three types of nanostructures play critical roles during the charge/discharge process. As shown in Figure 9b, the scattering nanoparticle (~1 nm) exhibits a continuous increase in size from 2.7 V to 1.5 V, which can be attributed to Li⁺ insertion and SEI formation. The SEI layer thickens with increasing lithium uptake. Subsequently, the size begins to decrease at 1.5 V, which is likely caused by the tensile stress induced by lithium intercalation, leading to crack formation on the SEI surface and a corresponding reduction in size. This phenomenon contributes to the irreversible high capacity observed during the first discharge.

Figure 9
(a) The discrete distribution, and (b-f) the physical Rgs of the polydisperse scatterers and their evolutions with the decreasing voltage.

In Figure 9c, the scattering nanoparticle corresponds to the mesopores in the Bi-Bi₂O₃/C-30 composite, as confirmed by the pore size distribution curves in Figure 6b. During discharge, the mesopore size gradually decreases, which is likely caused by irreversible Li⁺ intercalation. The scattering objects in Figure 9d are attributed to the gaps between Bi and Bi₂O₃ nanoparticles dispersed in the carbon matrix. As shown in Figures 9 and 9f, these two levels of scatterers represent Bi or Bi₂O₃ nanoparticles with different sizes. The nanoparticle size in Figure 9e remains relatively stable, indicating that smaller nanoparticles possess superior electrochemical stability. The nanoparticle size in Figure 9f increases continuously owing to Li⁺ insertion. Assuming the largest nanoparticles in this level are spherical, their diameter (~80 nm) agrees well with the larger nanoparticles observed inFigure 3d. This comprehensive analysis, coupled with the nitrogen adsorption-desorption results, provides deeper insights into the structural evolution and electrochemical behavior of the Bi-Bi₂O₃/C-30 anode during cycling.

Figure 10a presents the SAXS curves of Bi-Bi₂O₃/C-30 anode during the first charge, collected at a current density of 100 mA g^-1. A universal trend can be observed: the scattering intensity decreases with the voltage rises during charging (inset in Figure 10a). Figure 10b demonstrates that the original SAXS data are in good agreement with the fitting curves obtained by the TBT method in. Data fitting enables the analysis of the average Rg evolution of the Bi-Bi₂O₃/C anode throughout the first charging process (Figure 10c). This variation trend differs distinctly from that during discharge, further demonstrating the irreversible nature of the first discharge reaction. As illustrated in Figure 10d, the NVFs changes during the initial charge are completely opposite to those during the discharge. Specifically, the peak at ~0.7 nm weakens with increasing voltage, whereas the peak around 7 nm gradually intensifies as the voltage rises.

Figure 10
(a) The SAXS curves of Bi-Bi₂O₃/C-30 anode material during the first charge process. (b) SAXS experimental intensities (symbols) versus calculated values (solid lines). (c) Average Rg variation of scattered nanoparticles. (d) Normalized volume fraction of scatterers during the initial discharge obtained.

Likewise, five levels of scatterers are still observed, as shown in Figure 11a. The smaller Bi nanoparticles in Figure 11b show no obvious variation and exhibit excellent stability. During charging, Li⁺ is extracted, and the mesopores in the Bi-Bi₂O₃/C-30 composite in Figure 11c gradually increase with rising voltage. As shown in Figure 11d, the interspaces between nanoparticles also slightly increase with increasing voltage. The sizes of the scatterers (Bi and Bi₂O₃ nanoparticles) in Figures 11 and 11f decrease with increasing voltage, and the trend is more pronounced for larger nanoparticles. it is thus clear that reducing the nanoparticle size of electrode materials is beneficial for alleviating volume changes during cycling.

Figure 11
(a) Discrete distributions. (b-f) Rgs of these polydisperse scatterers during the first charging process and their variation with the increasing voltage.

4. Conclusion

In this work, Bi-Bi₂O₃/C composites were successfully derived from Bi-based MOF precursor via carbonization and secondary calcination, and were further employed as the electrode materials for lithium-ion batteries (LIBs). Systematic characterization and electrochemical measurements reveal that the as-prepared composites possess a well-defined microstructure, abundant mesopores, and excellent electrochemical performance. Notably, the Bi-Bi₂O₃/C-90 sample exhibits the optimal electrochemical performance, delivering initial discharge and reversible capacities of 1743 and 1207 mAh g^-1 at a current density of 100 mA g^-1, respectively. In situ synchrotron radiation SAXS was further applied to investigate the nanostructural evolution of Bi-Bi₂O₃/C-30 sample during the initial charge/discharge processes. The results demonstrate that the Bi-MOF precursor provides a favorable structure template for the derived electrode material, while the in situ formed carbon skeleton effectively alleviates the aggregation and stacking of nanoparticles, thereby significantly enhancing the electrochemical performance. The abundant mesopores and enlarged internal voids in Bi-Bi₂O₃/C material not only effectively accelerate lithium ions transport but also effectively alleviate the volume expansion during cycling, synergistically leading to improve electrochemical performance. Moreover, the synergistic effect between Bi and Bi₂O₃ effectively boosts the discharge capacity. The smaller nanoparticles exhibit superior structural stability, providing solid evidence that nanostructure engineering is beneficial to improving the cycling stability of LIBs. The electrode materials developed in this study effectively improved the electrochemical performance of Bi-based anodes, which is of great significance for understanding the complex structural evolution occurring in Bi-based electrode for LIBs.

For future research, based on the in-situ structural mechanism revealed in this work, dedicated research on long-term cycling stability and structural degradation mechanisms will be carried out in the follow-up study, to further improve and supplement the practical performance evaluation of this Bi-based material. In addition, more advanced in situ characterization techniques should be combined to gain in-depth insights into the lithium storage mechanism of Bi-based materials and further optimize their electrochemical performance. With the progress in material design and processing technology, Bi-based electrodes are expected to become promising candidates for next generation LIBs with high energy density and high safety. Furthermore, with the continuous improvement of synchrotron radiation source performance (higher brightness, higher coherence), the advancement of detector technology (faster response and higher sensitive), the optimization of in situ electrochemical cell design, and the development of advanced data analysis methods, synchrotron radiation technologies will continue to play an irreplaceable role in the fundamental research of lithium-ion batteries.

5. Acknowledgment

The authors are grateful to Beijing Synchrotron Radiation Facility (BSRF) for offering beam time at the beamlines 1W2A. This research was funded by the Project of Education Department of Heilongjiang Province (No. 145409329), and the College Students Innovation and Entrepreneurship Training Program of Heilongjiang Province (No. X202410232005).

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• Data Availability
  The dataset supporting the results of this study is not publicly available.

Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The dataset supporting the results of this study is not publicly available.

Publication Dates

• Publication in this collection
  05 June 2026
• Date of issue
  2026

History

• Received
  19 Dec 2025
• Reviewed
  27 Apr 2026
• Accepted
  04 May 2026