1 Introduction

Li-ion batteries (LIBs) with merits of high energy density, operating voltage, compact size and environmentally friendliness have emerged as primary energy storage devices in various applications, such as electric vehicles, portable devices, smart grids and emerging electric transportation [1,2,3,4,5,6,7]. However, the current commercially available anode for LIBs remains graphite, which suffers from the low theoretical capacity (372 mAh·g−1 for gravimetric capacity or 719 mAh·cm−3 for volumetric capacity) and inferior rate performance, rendering it insufficient to meet the high energy density requirements of LIBs [8,9,10]. To address the rapid growth in energy demand, it is imperative to design and develop LIBs with larger capacity, higher cycling stability and energy/power densities [11].

Alloy-based anode materials, such as Sb, Sn and Bi, have attracted considerable attention in terms of their ability to deliver high Li-ion-storage capacities [12,13,14]. Bi and its relevant composites, including Bi-based alloys, have been regarded as promising candidates for the LIBs anode due to the high earth abundance, moderate operation voltage and special lamellar structure along c-axis [15,16,17,18]. The (003) interplanar spacing of Bi is 0.395 nm, which is beneficial to the alloying/dealloying reaction of alkaline metal ions during cycling procedure [16, 17, 19,20,21]. Even Bi anode possesses a not very high gravimetric capacity of 385 mAh·g−1, but a high volumetric capacity of ~ 3800 mAh·cm−3 is under consideration due to the two-step intercalation process from Bi to LiBi and further to Li3Bi, which enables the Bi anode to surpass its theoretical capacity [22, 23]. Nevertheless, alloy-semiconductor type anodes reveal the common problem of rapid capacity decay, deriving from the significant volume expansion/shrinkage during the discharge/charge procedure, leading to the pulverization of anode active materials and unstable solid electrolyte interface [24,25,26]. Therefore, strategies were used to mitigate the volume variation problem for alloy-semiconductor type anodes during discharge/charge process, including nanomaterials preparation [27, 28], microstructure design [3, 16, 29] and carbon composites [10, 17, 23, 27]. For instance, Xu et al. [30, 31] developed Bi nanoparticles and carbon framework composites that can efficiently regulate volume exchange and provide superior kinetics during the discharge/charge process. The rational design of structure and meticulous control of morphology in electrode active materials are widely acknowledged as effective strategies for significantly enhancing their electrochemical properties [32,33,34]. Therefore, the investigation of active materials with rational structure and morphology holds great potential for LIBs. Investigations have shown that electrode active materials, characterized by a three-dimensional heterostructure grown directly on conductive substrates, exhibit improved electrochemical performance [35]. This enhancement can be attributed to several factors, such as the increased active surface area, superior electron and charge transfer capabilities, enhanced structural integrity, and the beneficial synergistic effects between the matrix and active materials. Transition metal oxides/hydroxides have garnered considerable attention as potential electrode materials for LIBs [36,37,38,39]. Bi2O2CO3, a Bi-based semiconductor material, has been widely used in the field of healthcare and plays a crucial role in the purification of air and wastewater treatment through photocatalysis [40, 41]. Additionally, nanosheets of Bi2O2CO3 have shown advantageous properties as negative electrode active materials for supercapacitors [42]. However, to the best of our knowledge, there is limited literature available on the in situ growth of flower-like Bi2O2CO3 nanosheets on carbon substrates as negative electrode active materials for Li-ion battery systems.

Beneficial from the aforementioned structural hybrid engineering, in this work, a composite material of flower-like Bi2O2CO3 nanosheets in situ grown on a carbon-based substrate has been synthesized, which significantly improves the storage performance of Li+ by carefully adjusting the structure of the Bi2O2CO3 nanosheets. The presence of a carbon-based substrate and graphene effectively mitigates the stress and strain caused by volume expansion. The Bi2O2CO3 anode, with its distinctive structural advantages, demonstrates exceptional discharge capability of 547 mAh·g−1 after 100 cycles at 0.1 A·g−1. In situ X-ray diffraction (XRD) reveals that the Li storage mechanism of the Bi2O2CO3@C anode undergoes an irreversible conversion reaction, followed by a reversible alloying reaction. To further evaluate the application potential of the Bi2O2CO3@C anode, a full cell structure was established using Li1.20Mn0.54Ni0.13Co0.13O2 (LMLC) as the cathode material. The assembled Bi2O2CO3@C // LMLC full cell exhibits a reversible capacity of 174.1 mAh·g−1 at 0.1 A·g−1.

2 Experimental

2.1 Materials

Bismuth nitrate pentahydrate, sodium alginate (SA), ethylene glycol, thiourea, and urea are analytical pure grade which purchased from Aladdin online and used without further purification.

2.2 Preparation of Bi@C materials

The aerogel was prepared by cross-linking sodium alginate through Bi ions to form a hydrogel, followed by freeze-drying and subsequent pyrolysis to synthesize Bi@C composites. In a typical synthesis, a proper amount of SA (4 g) and thiourea (8.5 g) were decentralized in 200 mL deionized water and stirring vigorously for 12 h to form a transparent solution. The prepared transparent solution was added dropwise with a medical syringe to a pre-prepared 200 mL solution of 0.1 M bismuth nitrate under continuous stirring. This process facilitated the rapid formation of hydrogel spheres, which were subsequently left undisturbed for 6 h. The hydrogel beads (Bi-alg) were collected by filtration, followed by rinsing with deionized water to remove free impurities. Subsequently, they were placed in a refrigerator set at − 50 °C for 24 h to facilitate the transformation into Bi-alg aerogels through a freeze-drying process. Finally, the dried Bi-alg aerogel was carbonized at 600 °C for 1 h under N2 atmosphere (5 °C·min−1, 80 mL·min−1 N2). After cooling in flowing N2 environment, the obtained samples were recorded as Bi@C.

2.3 Preparation of flowers-like Bi2O2CO3@C

Bi2O2CO3@C composites were synthesized using Bi@C precursor via traditional hydrothermal methods. In subsequently, 0.5 g graphene and 0.4 g urea were dissolved in 70 mL deionized water and ultrasonic dispersion for 10 min. Then, 0.5 g Bi@C composite was added to the mixture and stirred for 60 min to make the mixture dispersion homogeneously. Subsequently, the mixture was transformed to a Teflon-lined autoclave and kept at 180 °C for 10 h. The finally obtained black precipitate was collected by centrifugation and washed thoroughly with deionized water and ethanol more than three times, and dried in a vacuum at 70 °C overnight. The dried black powder obtained was labeled as Bi2O2CO3@C.

2.4 Materials characterization

The phase structure of the as-prepared samples was measured by XRD (Rigaku D/max 2500) operating at 40 kV and 15 mA using Cu Ka1 radiation (λ = 1.541 Å, step size = 0.02°, 5 (°)·min−1 from 10° to 80°). X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) and Raman (Jobin Yvon, T6400) were used to observe the phase composition. The microstructure was investigated by scanning electron microscopy (SEM, Hitachi Su8010) and transmission electron microscopy (TEM, JEOL-2100). Thermogravimetric analysis (TGA, Mettler-Toledo Model TGA/DSC1) and Brunauer–Emmett–Teller (BET, Micrometrics Instrument Corporation) were further conduced.

2.5 Electrochemical measurements

The CR2025-type coin cells were assembled for the electrochemical measurements. The working electrode consists of active materials (80 wt%, Bi2O2CO3@C), Super P (10 wt%) and polyvinylidene difluoride (PVDF) binder (10 wt%). LiPF6 (1.0 M) dissolved in ethylene carbonate/diethyl carbonate/- dimethyl carbonate (EC/DEC/DMC) (1:1:1 in vol%) was used as electrolyte. The mass loading of active material was about 1.2–1.5 mg·cm−1. The metallic Li was employed as the counter electrode for the half cell. For assembled the full cell, the LMLC was used as cathode, which synthesized according to a previous reported, and the mass ratio of the Bi2O2CO3@C anode and LMLC cathode is 0.354 [43]. Furthermore, the pre-Li of Bi2O2CO3@C anode was conducted before assembled full cell. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy test were performed on electrochemical work-station (CHI660E) between 0.01 and 3.0 V (vs. Li/Li+). The Li storage performances were conducted on LAND-BT2013A measurement system.

3 Results and discussion

Bi2O2CO3@C composites were synthesized via two processes of annealing of Bi-alg in concert with a subsequent hydrothermal process, as depicted in Fig. 1a. The intricate microstructure properties of the Bi2O2CO3@C composites were examined through SEM and TEM observations. Figure 1b–d depicts the flower-like aggregates morphology of Bi2O2CO3 and the composite structure of Bi2O2CO3@C. Obviously, nanosheets self-assembled and anchored on a carbon substrate to form the Bi2O2CO3@C composite structure during the hydrothermal process. Additionally, rough and concave surfaces, as well as porosity, were observed within the carbon matrix due to the release of gases during the pyrolysis process (Fig. 1c). SEM images reveal a relatively homogeneous distribution of flower-like nanosheets on the surface of the carbon matrix, with graphene randomly dispersed among the carbon matrix particles. The size of the flower-like Bi2O2CO3 aggregates is between 0.5–2 μm in diameter. The Bi2O2CO3 nanosheets obtained were further elucidated through TEM analysis, as depicted in Fig. 1e. The size of the nanosheets is distributed in the range of 200–600 nm. The interplanar distance is 0.269 nm, which is corresponding to (022) plane of Bi2O2CO3. Furthermore, the elemental mappings in Fig. 1h, i indicate that Bi2O2CO3 is distributed on the carbon-based substrate. Corresponding energy dispersive spectroscopy (EDS) spectrum (Fig. 1j) validates the composition of Bi2O2CO3@C, primarily consisting of Bi, C and O. The phase structure was further confirmed by XRD results.

Fig. 1
figure 1

a Schematic illustration of fabrication of Bi2O2CO3@C; b–d SEM images with different magnification of Bi2O2CO3@C; e TEM image, f HRTEM image and g local magnified HRTEM image of Bi2O2CO3 nanosheet of Bi2O2CO3@C; h, i elemental mapping of Bi2O2CO3@C; j EDS result of Bi2O2CO3@C

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The phase structural of the Bi2O2CO3@C samples prepared in this work were conducted using powder XRD, followed by a detailed Rietveld refinement analysis (Fig. 2a). The detected diffraction peaks exhibit a remarkable agreement with the expected pattern of the pure orthorhombic structure with a space group of Pn21a (a = 10.94 Å, b = 27.32 Å, c = 10.94 Å). Clearly, the primary diffraction peaks can be attributed to the crystalline structure of Bi2O2CO3 (PDF No. 84–1752). In addition to the mentioned diffraction peaks, no additional diffraction peaks were observed, confirming the high content of Bi2O2CO3 in the samples consisting of a single-phase orthorhombic structure. As illustrated in Fig. 2b, the configuration of the Bi2O2CO3 structure exhibited a stacked arrangement with alternating layer packages oriented perpendicular to the [001] axis. Within each layer, there existed a double sublayer composed of oxygen atoms and carbonate ions, respectively. The metal atoms occupied slightly distorted octahedral sites embedded within the double sublayer, while distinct layer packages were separated by a van der Waals gap. This unique channel structure facilitated the diffusion and storage of ions, while also serving as a buffer for accommodating volumetric changes during the electrochemical reaction.

Fig. 2
figure 2

a XRD pattern of Bi2O2CO3@C; b schematic illustration of crystal structure of Bi2O2CO3; c Raman spectrum; d N2 adsorption–desorption isotherm; e TGA curve; XPS spectra: f survey spectrum, g Bi 4f, h C 1s and i N 1s

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Raman spectroscopic investigation was performed to investigate the structural features of Bi2O2CO3@C (Fig. 2c). Two peaks were observed in Raman spectrum at 1360 and 1567 cm−1, corresponding to the D-band (disordered graphite) and G-band (crystalline graphite), respectively [44]. The presence of disorder nature in Bi2O2CO3@C, which contributes to enhanced electrochemical performance and safety in LIBs (as lithiation occurs at higher voltages compared to graphitic carbon) [45], is supported by the calculated ID/IG ratio of 1.01 derived from the Raman spectrum. To further investigate the porous structure and the associated pore size distribution of the obtained samples, N2 adsorption–desorption isotherms were employed. As shown in Fig. 2d, it can be observed that Bi2O2CO3@C demonstrate a combination of type-I and type-IV isotherms, indicating the presence of a micro-mesoporous structure [46]. The specific surface area of Bi2O2CO3@C was measured to be 70 m2·g−1, indicating its considerable surface area. The pore size distribution trend of Bi2O2CO3@C was confirmed to be predominantly microporous by the Barrett–Joyner–Halenda (BJH) analysis (Inset of Fig. 2d). The mesopores with sizes ranging from 2 to 5 nm provide significant advantages in achieving superior contact with the electrolyte solution.

TGA of Bi2O2CO3@C was conducted under an O2 atmosphere, as shown in Fig. 2e. The weight loss of Bi2O2CO3@C was observed to occur in three distinct stages. There was a slight mass loss of the sample in the preliminary period of temperature increase (before 200 °C), which can be attributed to the evaporation of water within the composite [47, 48]. In the temperature range of 200–640 °C, a notable reduction in weight was observed, indicating the combustion decomposition of carbon within the composites [49]. Accordingly, the calculated content of carbon and Bi2O2CO3 in the composites is approximately 43% and 48%, respectively. The gravimetric loss in the third stage is a consequence of the thermal decomposition of Bi2O2CO3 into Bi2O3 and CO2.

XPS was employed to investigate the bonding information and chemical constitutions of Bi2O2CO3@C. The survey spectrum (Fig. 2f) results reveal that the elements C, O, N and Bi are present in the Bi2O2CO3@C composites, which is consistent with EDS maps in Fig. 1. High-resolution XPS spectrum of Bi 4f in Fig. 2g exhibits two peaks at 164.80 and 159.48 eV, which are ascribed to Bi 4f5/2 and Bi 4f7/2, respectively [42, 50]. The deconvolution for C 1s spectra (Fig. 2h) of all Bi2O2CO3@C compounds is divided into four peaks derived from C=C, C–OH, C–N and C=O, respectively [51]. High-resolution XPS spectra of N 1s of Bi2O2CO3@C are presented in Fig. 2i, wherein the two peaks centered at 400.55 and 398.67 eV were fitted. These peaks associated with pyrrolic-N and pyridinic-N components, respectively [52, 53]. The introduced N element is conducive to improve the electrical conductivity of the composites and provides active sites for Li storage.

The Li+ storage behavior of Bi2O2CO3@C composites was explored using half coin cells within a potential window of 0.1–3.0 V. The electrochemical reactions were elucidated by the cathodic and anodic peaks observed in the initial five CV curves obtained at a scan rate of 0.1 mV·s−1(Fig. 3a). During the first cathodic scan, a series of reduction peaks ranging from 0.09–0.84 V are observed, which can be attributed to the stepwise lithiation process of Bi. However, these peaks disappear in the subsequent cycles. This process involves the formation of the solid electrolyte interfacial film (SEI layer), followed by the chemical reaction of the formation of Li3Bi. The primary reaction equations corresponding to these processes can be described as Bi + 3Li+  + 3e− → Li3Bi [54]. In the anodic scan, the observed oxidation peak at approximately 0.98 V is primarily attributed to the dealloying process of Bi, which can be represented by the reaction Li3Bi → Bi + 3Li+  + 3e− [55]. The lithiation process of Bi2O2CO3 leads to the degradation of the electrolyte and the generation of the SEI, resulting in an irreversible reduction in the specific capacity during the initial cycle [56]. However, upon the formation of the SEI film, it can be observed from the highly overlapped CV curves in the second to fifth cycles that the Bi2O2CO3@C composite possesses excellent cycling stability. Furthermore, the appearance of broad and weak redox peaks at ~ 0.16 V is associated with the accommodation of Li+ during the interface reaction [57].

Fig. 3
figure 3

a CV curves of Bi2O2CO3@C at a scan rate of 0.1 mV·s−1; b GCD curves tested at 0.1 A·g−1; c cycling properties at 0.1 A·g−1; d c-rate performance at various current densities; e long-term cycling properties at 0.5 A·g−1

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The electrochemical characteristics of Bi2O2CO3@C composites can be evaluated by performing galvanostatic charge–discharge experiments within the voltage window of 0.01–3.00 V. Figure 3b illustrates the galvanostatic charge–discharge profiles of the Bi2O2CO3@C cathode during the 1st, 2nd, 4th, 6th, 8th and 10th cycles, at a current density of 0.1 A·g−1. As depicted in Fig. 3b, the discharge profiles display two distinct plateaus at 0.69 and 0.08 V, which can be attributed to the processes of Bi → Li3Bi and the interface reaction during the electrochemical reaction at a current density of 0.1 A·g−1. Similarly, the charge profiles reveal voltage plateaus at 0.89 and 0.12 V, which are associated with the dealloying process and interface reaction. These observations are in strong agreement with the CV measurements. In the initial cycle, the discharge and charge capacities are recorded as 1183 and 781 mAh·g−1, respectively. Actually, the discharge capacity of Bi-based anode materials all shows rapid decline due to the formation of the SEI layer during the alloying or conversion reaction procedure, which is consistent with the reported results [54, 55]. With increasing cycling, the specific capacity during discharge reaches a stable state, while the Coulombic efficiency stays at around 98% from the second cycle. The cycling behavior of the Bi2O2CO3@C was investigated at a current density of 0.1 A·g−1, as depicted in Fig. 3c. At a current density 0.1 A·g−1, the discharge specific capacity of Bi2O2CO3@C stabilized at 547 mAh·g−1 after 100 cycles, while the Coulombic efficiency remained above 99% throughout the duration of the experiment, suggesting the excellent reversibility of the anode materials. Additionally, the energy density of the Bi2O2CO3@C cathode is 396 mWh·g−1 during the first cycles, decreasing to 343 mWh·g−1 after 100 cycles at 0.1 A·g−1. It has been reported in the literature that bare Bi2O2CO3 anode was tested in cycling at 0.5 A·g−1 and the discharge specific capacity of the battery decayed from 156 mAh·g−1 in the first cycle to 34.8 mAh·g−1 after 300 cycles, as shown in Fig. 3e [58]. During the extended cycle test at an elevated current density to 0.5 A·g−1, a remarkable reversible capacity of 394 mAh·g−1 was maintained experience 550 galvanostatic charge–discharge (GCD) cycles (Fig. 3e). The rate performance of Bi2O2CO3@C was further investigated under various current densities, such as 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 A·g−1, as shown in Fig. 3d. When operating at a current density of 0.1 A·g−1, the capacity of Bi2O2CO3@C experienced a decline in the initial ten cycles, which can be attributed to an activation (de)lithiation process. However, despite this initial decrease, the average Coulombic efficiency consistently maintained a stable value of approximately 98%. Remarkably, the Bi2O2CO3@C composite exhibited impressive reversible capacities of 466.1, 404.3, 343.3, 276.9, 209.8 and 498.0 mAh·g−1 at 0.5, 1.0, 2.0, 5.0 and 0.1 A·g−1, respectively. These results collectively highlight the exceptional stability, reversibility, and high Coulombic efficiency of the Bi2O2CO3@C composite, signifying its promising potential as a prospective anode material for LIBs.

To further elucidate the Li storage mechanism of the Bi2O2CO3@C anode, it was incorporated as an anode material (loaded onto a circular pure beryllium disk) and assembled it with a Li foil cathode in a specially designed setup. This method was conducted to recording the phase variation of the Bi2O2CO3@C composite material in real-time during the battery charge–discharge cycling tests, as shown in Fig. 4. As the discharge process proceeds, the diffraction peaks corresponding to Bi2O2CO3 (24.4°, 30.7° and 33.2°) progressively decrease and eventually disappeared completely. The decreased peak intensity corresponds to the conversion of Bi2O2CO3 into discharge products, namely Bi (27.7° and 40.2°), and Li3Bi (23.5°, 38.5° and 45.3°). The characteristic diffraction peaks associated with Bi during the charging and discharging process (corresponding to the position of the voltage around 0.7 V of the charging/discharging curve) can be clearly observed in the figure. This observation confirmed the conversion of Bi2O2CO3 into Bi during the discharge phase. Conversely, during the charging phase, the noticeable peaks at 23.5°, 38.5° and 45.3° gradually diminish, confirming the phase transition of Li3Bi. The diffraction peaks of Bi2O2CO3 disappeared when the charge reached below 0.7 V during the initial discharge and did not reappear during the subsequent charging and discharging process, which indicates that the transformation reaction of the Bi2O2CO3@C composite material is irreversible within the cycling process. The underlying reaction mechanism of the Bi2O2CO3@C composites could be described as follows Eqs. (1-3)

Fig. 4
figure 4

a The 1st charge/discharge cycle curves and corresponding in situ 2D maps of XRD; b 3D map with full contour maps; c individual contour maps of Bi2O2CO3@C anode in the 1st discharge/charge cycle

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In the discharge process:

$$ {\text{Bi}}_{{2}} {\text{O}}_{{2}} {\text{CO}}_{{3}} + {\text{ 6Li }} = {\text{ 2Bi }} + {\text{ Li}}_{{2}} {\text{CO}}_{{3}} + {\text{ 2Li}}_{{2}} {\text{O}} $$
(1)
$$ {\text{Bi }} + {\text{ 3Li}} ^+ \, + {\text{ 3e}}^{ - } = {\text{ Li}}_{{3}} {\text{Bi}} $$
(2)

In the charge process:

$$ {\text{Li}}_{{3}} {\text{Bi }} = {\text{ Bi }} + {\text{ 3Li}}^{ + } + {\text{ 3e}}^{ - } $$
(3)

The phase transition process was further illustrated by the 3D contour plot of in situ XRD pattern, as depicted in Fig. 4b. The significant changes in peak intensity (indicated by the arrows) within the plot confirmed the generation of Bi and Li3Bi as the final discharge products. Enlarged XRD in Fig. 4c presents detailed variations in peak intensity within different ranges. In summary, the charge storage mechanism of Bi2O2CO3@C composites as an anode for LIBs was determined using in situ XRD characterization techniques. This mechanism involves the initial irreversible conversion of Bi2O2CO3 to Bi, followed by an alloying reaction between Bi and Li+, resulting in the formation of Li3Bi, which is subsequently dealloying to regenerate Bi.

CV at various scan rates (0.2–1.0 mV·s−1) were performed for the purpose of further investigating the Li storage mechanism of Bi2O2CO3@C composites. CV profiles of the Bi2O2CO3@C anode exhibited similar shapes at different scanning rates, indicating rapid kinetic behavior [59], as illustrated in Fig. 5a. The equation governing the relationship between the peak current (i, mA) and scan rate (v, mV−1) is given by Eq. (4), with a and b being adjustable parameters that can be determined by fitting a lgv-lgi curve. The peak currents of the reduction peak (Peak I) and oxidation peak (Peak II) were utilized in Eq. (5) to determine the reaction kinetic parameter b value for Bi2O2CO3@C composites (Fig. 5a). A reaction process that is primarily governed by diffusion is linked to the b value of ~ 0.5, while a reaction process primarily controlled by capacitance is typically associated with the b value close to 1 [60]. By performing a linear regression analysis, Fig. 5b was generated, and the obtained b values for Peak I and Peak II are 0.69 and 0.60, respectively, implying the prevalence of a capacitive dominant process in the Bi2O2CO3@C electrode with exceptional kinetic properties. It is widely acknowledged that Eqs. (4, 5) can effectively differentiate between the contributions of pseudo-capacitive control behavior and diffusion control behavior. Figure 5c presents the contribution of the pseudo-capacitor control process at a scan rate of 0.8 mV·s−1, which account for 56.27%. Notably, the pseudo-capacitive contributions exhibit a significant increase with the escalation of scan rates (Fig. 5d), thereby providing a possible explanation for the remarkable rate performance observed. Consequently, the involvement of capacitance contribution plays a significant role in the (de)lithiation process of the Bi2O2CO3@C electrode, resulting in enhanced rate and capacity performance of the Bi2O2CO3@C composites.

$$ i = av^{b} $$
(4)
$$ i\left( V \right) \, = k_{{1}} v + k_{{2}} v^{{0.{5}}} $$
(5)

where i, v, i(V), k1v and k2v0.5 represent the peak current, scan rate, total current, surface induced capacitive-controlled current and diffusion-controlled current, respectively.

Fig. 5
figure 5

a CV profiles of Bi2O2CO3@C at different scan rates; b lgi-lgν curve; c capacitive separation at 0.8 mV·s−1 of Bi2O2CO3@C electrode; d contribution ratios of capacitive; e Nyquist plot and f corresponding bode plot of Bi2O2CO3@C after different cycles; g GITT curves; h, i variation of \({D}_{{{\text{Li}}}^{+}}\) values during charge and discharge

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The Nyquist plots of the Bi2O2CO3@C electrode at the initial and 100th cycled states are presented in Fig. 5e, wherein the semicircle observed at medium and high-frequency ranges signifies the charge transfer resistance (Rct) and surface resistance (Rs), while the oblique line corresponds to the Warburg resistance (Rw). After 100 cycles, the impedances of the Bi2O2CO3@C electrode were observed to decrease in comparison with the fresh state, which is attributed to the formation of the SEI layer. The SEI is facilitated by the presence of flexible carbon multi-chambers and interconnected architectures [61]. Additionally, the reduced particle size resulting from repeated interaction with Li+ also contributes to the improved impedance behavior. According to the concept that ideal pseudo-capacitive materials exhibit alternating current (AC) impedance characteristics with a phase angle of 90° or less in the Nyquist plot [62], the Bi2O2CO3@C electrode demonstrates a steeper slope in the low-frequency region, as depicted in Fig. 5e. In the corresponding Bode plots (Fig. 5f), the phase angle of Bi2O2CO3@C approaches nearly 90° at a frequency near 0.01 Hz. By employing the commonly used equivalent circuit for analysis, it becomes evident that the influence of Warburg impedance accounts for the slope observed in the high-frequency range of Bi2O2CO3@C, approaching 45°–60°. The Warburg impedance is associated with the diffusion process, explaining the role of the electrochemical diffusion process in the behavior of Bi2O2CO3@C.

To better understand the characterization of Li+ diffusion behavior in Bi2O2CO3@C electrode, the galvanostatic intermittent titration technique (GITT) was performed to evaluate the (de)lithiation process within the cell. The GITT analysis was conducted to determine accurate Li+ diffusion rates in Bi2O2CO3@C under a current density of 0.1 A·g−1, the intermittent discharge/charge procedure lasting 10 min, and subsequent resting intervals of 60 min (as illustrated in Fig. 5g), the Li+ diffusion coefficient (\({D}_{{{\text{Li}}}^{+}}\)) was determined using Eq. (6) [32, 34]:

$$ D_{{\text {Li}^{ + }}} = \frac{4}{\pi \tau }\left( {\frac{{m_{\text B} V_{\text m} }}{{M_{\text B} S}}} \right)^{2} \left( {\frac{{\Delta E_{\text S} }}{{\Delta E_{\uptau } }}} \right)^{2} $$
(6)

where \(\tau \), \({m}_{{\text{B}}}\), \({M}_{{\text{B}}}\), \({V}_{{\text{m}}}\) and S represent relaxation time, weight of active material, molar mass, molar volume and active surface area, respectively. \(\Delta {E}_{{\text{S}}}\) is the voltage change in response to a pulse, and \(\Delta {E}_{\tau }\) is the voltage change associated with constant current charging (discharging). And the values of \(\Delta {E}_{{\text{S}}}\) and \(\Delta {E}_{\tau }\) can be directly obtained from the GITT curves. As shown in Fig. 5h, i, the measured diffusion coefficient (\({D}_{{{\text{Li}}}^{+}}\)) for the charging process of Bi2O2CO3@C, which ranges from 10–10 to 10–8 cm2·s−1. Additionally, the \({D}_{{{\text{Li}}}^{+}}\) value for the discharging process is slightly lower than the charging process. For visual comparison, graphene was subjected to GITT tests under identical conditions. The \({D}_{{{\text{Li}}}^{+}}\) range for graphene during the charging process is from 10–13 to 10–10 cm2·s−1, while the \({D}_{{{\text{Li}}}^{+}}\) range for the discharging process is from 10–12 to 10–10 cm2·s−1. The higher \({D}_{{{\text{Li}}}^{+}}\) values obtained can be attributed to the enhanced reactivity of the flower-like Bi2O2CO3 nanosheet and the short Li+ diffusion distance within Bi2O2CO3@C. In summary, the incorporation of the in situ grown flower-like Bi2O2CO3 nanosheet within the carbon matrix facilitates remarkable Li+ storage kinetics.

To confirm the feasibility of integrating Bi2O2CO3@C into complete LIBs, LMLC was utilized as the cathode to assemble a full cell. The schematic diagram of the full cell arrangement is depicted in Fig. 6a. The full cell was initially charged to 4.8 V for activation and subsequently exhibited a rechargeable capacity of 174.1 mAh·g−1 at a rate of 0.1 A·g−1. The initial Coulombic efficiency was recorded to be 91.4% (Fig. 6b). The irreversible capacity can be attributed to the formation of the cathode-electrolyte interface (CEI) on the cathode side, while the capacity degradation on the anode side follows a typical pattern. The capacity was consistently maintained at around 100 mAh·g−1 after 200 cycles at a current density of 0.5 A·g−1, as depicted in Fig. 6b. Constant-current discharging at various current densities (Fig. 6d) demonstrated substantial capacity delivery for the full cell across different current densities, with values of 174.1, 148.1, 107.2, 73.3 and 39.2 mAh·g−1 at 0.1, 0.2, 0.5, 1 and 2 A·g−1, respectively. Upon returning to a current density of 0.1 A·g−1, the reversible capacity remarkably recovered to 146.3 mAh·g−1, highlighting the impressive ability of the Bi2O2CO3@C electrode to withstand varying rates in the full cell. Moreover, the Coulombic efficiency reaches more than 98%, indicating that the full cells composed of Bi2O2CO3@C//LMLC possess promising prospects in low current density application domains. Additionally, compared to previously reported anodes, Bi2O2CO3@C exhibits excellent performance in full cell tests (Table S1). According to Fig. 6b, it is evident that a full Li-ion battery can effortlessly power a red-light emitting diode (LED). The remarkable electrochemical performance observed in the full Li-ion battery suggests the possibility of a novel anode for LIBs, indicating its potential feasibility.

Fig. 6
figure 6

a Schematic illustration of Bi2O2CO3@C//LMLC full cell; b cycle performance at 0.5 A·g−1 and (inset) an optical image showcasing a full LIB energizing a red-LED; c discharge performance under different current densities; d GCD profiles with varied current densities

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4 Conclusion

Flower-like Bi2O2CO3 composites anchored on a carbon substrate were successfully prepared using a simple method and employed as a highly promising anode material for LIBs. Anchoring flower-like Bi2O2CO3 nanosheets onto a carbon substrate is beneficial for enhancing the electrical conductivity of Bi2O2CO3 and mitigate both volume expansion and agglomeration of the nanosheets. The unique and novel flower-like structure of the Bi2O2CO3@C anode enables it to provide a stable reversible capacity of 547 mAh·g−1 at 0.1 A·g−1 after 100 cycles. Additionally, the composites exhibit excellent rate performance, with specific capacities of 548.1, 466.1, 404.3, 343.3, 276.9, 209.8 and 498.0 mAh·g−1 at current densities of 0.1, 0.5, 1.0, 2.0, 5.0 and 0.1 A·g−1, respectively. Even after 550 cycles at a current density of 0.5 A·g−1, it maintains a reversible specific capacity of 394 mAh·g−1. The Li-ion storage mechanism and kinetic reaction of the Bi2O2CO3@C composites were explored using in situ XRD, scanning method and GITT. Additionally, the Bi2O2CO3@C//LMLC full cell achieved an exceptional discharge specific capacity of nearly 100 mAh·g−1 at 0.5 A·g−1, demonstrating its practical application potential. It also maintained a high Coulomb efficiency of 99% over 200 cycles at 0.5 A·g−1. The exciting findings of this study have the potential to significantly advance the development and construction of high-performance negative electrode materials for advanced LIBs and other rechargeable batteries.