P-doped hard carbon microspheres for sodium-ion battery anodes with superior rate and cyclic performance

Abstract

As the first potential anode material used in sodium ion batteries (SIBs), hard carbon has received extensive attention owing to its available resources, inexpensiveness, and high electrochemical properties. The unsatisfactory sodium storage capacity and rate properties constrain its use in real-life applications. Herein, phosphorus (P)-doped hard carbon microspheres (PHCS) with a unique interconnected structure, expanded layer spacing (0.411 nm), and enlarged specific surface area (287.82 m² g⁻¹) are prepared using a facile pyrolysis strategy. They easily achieve a superior sodium storage capacity (293.5 mA h g⁻¹ at 0.1 A g⁻¹), remarkable rate performance (162.5 mA h g⁻¹ at 5 A g⁻¹), and exceptional cyclic stability (more than 2000 cycles at 5 A g⁻¹) when applied as anode materials. In addition, density functional theory (DFT) calculations reveal that P-doping facilitates the adsorption of Na⁺ on the material and lowers its structural resistance, which greatly improves the capacity for sodium storage. This study develops a promising design strategy to prepare P-doped hard carbon for SIB performance-enhanced anodes.

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Introduction

Lithium-ion batteries (LIBs) currently dominate energy storage devices and commercial electric vehicles. The high expense caused by urgent geographical lithium resources seriously impedes the large-scale development of LIBs. This has strongly stimulated the need for alternative batteries that possess both high energy density and earth-abundant elements.^1–4 Notably, sodium-ion batteries (SIBs) are known as the best possible alternative to LIBs because they have the same physical and chemical properties as lithium and rich natural resources.^5–7 As a vital part of SIBs, anode materials play a decisive role in electrochemical performance. Compared to metal compounds,^8–10 alloys,^11,12 and non-metallic materials,¹³ carbon materials are recognized as the best choice based on electrochemical stability, cost-effectiveness, and resource utilization considerations. However, commercial graphite (layer spacing is 0.34 nm) tends to express imperfect rate performance and cycle life owing to the enormous Na⁺ radius (1.02 Å).¹⁴ Interestingly, hard carbon is a disordered structure that combines amorphous/short graphite domains and micropores, which is regarded as the most suitable for storing Na⁺ owing to the rapid Na⁺ transport and volume buffer during the Na⁺ storage process.^15,16 Recently, intensive effects have synthesized various hard carbon with high performance for SIBs, such as hard carbon sheets, hard carbon microspheres, and biomass-derived hard carbon.^17–19 Despite numerous application potentials, how to achieve high sodium storage capacity, excellent rate capability, and superior cyclic stability for hard carbon anode remains a challenge.^20–22

Recent studies of heteroatom (N, O, S, and B)-doped hard carbon have exhibited apparently enhanced electrochemical properties for SIBs.^23–25 It has been proved that heteroatom doping can simply alter the electron donor/acceptor relationship of nearing carbon atoms because it is quite different from the electronegativity of C, benefitting from the increased electron transfer kinetics and rate capability. In particular, P doping enhances the Na⁺ adsorption capability and enlarges the layer spacing for insertion, resulting in improved performance.^26–29 However, the rate capability of carbon materials obtained by single P doping is limited. Alvin et al. reported that P-doped lignin delivers a capacity of only 25 mA h g⁻¹ at 5 A g⁻¹.²⁶ Similarly, the P-doped N-rich nanosheets reported by Zhang et al. show a low rate capacity of 125 mA h g⁻¹ at 3.2 A g⁻¹.²⁷ Therefore, it is still a great challenge to obtain excellent rate performance in single P-doped carbon materials. A unique structure with a large specific surface area can promote the sodium storage rate capacity of hard carbon by accelerating the diffusion and adsorption of Na⁺. In addition, the hard carbon material made from carbonizing phenolic resin has a high yield, can dope heteroatoms into the material in the process of synthesis, and can obtain a large specific surface area through the regulation of the precursor; therefore, it has attracted wide attention.^30–32

In this study, P-doped hard carbon microspheres (PHCS) were synthesized using phenolic resin as a precursor and phytic acid (PA) as a P source. We found that different concentrations of PA could regulate the microstructure of phenolic resin. PHCS2 shows an expanded layer spacing (0.411 nm), an enlarged specific surface area (287.82 m² g⁻¹), and an interconnected carbon sphere structure. When applied as anode material, PHCS2 demonstrates extraordinary rate capability of 293.5 and 162.5 mA h g⁻¹ at 0.1 and 5 A g⁻¹ respectively. Even at 5 A g⁻¹, PHCS2 delivers excellent long-term stability (100.2 mA h g⁻¹ over 2000 cycles). The Na⁺ storage mechanism is investigated via ex situ measurements. In addition, the DFT calculation shows that P-doping could boost the adsorption and diffusion behavior of Na⁺ in hard carbon material. Therefore, our study offers a new path for preparing P-doped hard carbon used in advanced anode electrodes.

Results and discussion

Material synthesis

The preparation for P-doped hard carbon microspheres (PHCS) is illustrated in Scheme 1. First, phenolic resin is prepared using resorcinol and formaldehyde as raw material, and phytic acid (PA) as a catalyst and dopant. Then, phenolic resin formed a continuous polymer chain in the presence of PA at 95 °C. Over time, the polymer chains intertwine to form a cross-linked network, and after the reaction lasts for 6 h, the cross-linked network self-assembles to form microspheres. At this point, the reaction is completed, and P-doped phenolic resin-based hard carbon microsphere precursors are prepared (Fig. S1†). PHCS are obtained by drying and carbonization. More detailed information is provided in the experimental section (ESI†).

Scheme 1 Preparation process for PHCS.

Material characterization

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are shown in Fig. 1a–d and Fig. S2.† The produced hard carbon materials are microspheres with particle sizes ranging from 500 nm to 4 μm. The obtained HCS agglomeration is serious, and microspheres adhered to each other without adding PA (Fig. 1a). As the PA content increased, agglomeration decreased significantly, and pores gradually appeared between the microspheres. When the amount of PA was 2 mL, the dispersion of microspheres (PHCS2) is relatively good, and the surface is smooth (Fig. 1b). Notably, microspheres are not loosely adjacent to each other, and a small part of the area between microspheres and microspheres closely adheres to each other (Fig. 1c). This means that it is not only beneficial to accommodate more electrolytes but also to promote structural reversibility and the rapid flow of electrons. However, excessive PA (PHCS3) leads to the embedding of carbon microspheres in the carbon layer derived from the cross-linking reaction (Fig. S2b†). Moreover, large numbers of hard carbon microspheres clump together to form larger bulk structures, which are disadvantageous to Na⁺ transport and storage.³³ The properties of sodium storage can be improved by adjusting the PA content to control the microscopic morphology and pore structure. From the high-resolution TEM (HRTEM) image (Fig. 1e), PHCS2 presents disordered amorphous in a long range, and a part of lattice fringes can be seen at the edges, indicating the existence of local order in the disordered structure of PHCS2. Compared to graphite (0.34 nm), the layer spacing for PHCS2 is about 0.411 nm, implying that a larger layer spacing encourages Na⁺ transport and storage, which enhances stability. Evidently, all elements are evenly distributed on PHCS2, as shown in the energy dispersing spectra (EDS) mappings (Fig. 1f–j).

Fig. 1 The microstructure for HCS and PHCS2. (a) SEM image for HCS. (b and c) SEM and (d) TEM images for PHCS2. (e) HRTEM image for PHCS2. (f) HAADF image and (g–j) corresponding elemental mappings for PHCS2.

The effects of PA addition on crystal characteristics and defects for hard carbon materials were determined using X-ray diffraction (XRD) and Raman spectrum. XRD patterns show the characteristic peak (002) of amorphous carbon material near 22°.³⁴ It is noteworthy that the (002) peak gradually moves from 22.7° to 21.3° (Fig. 2a). The corresponding layer spacing can be obtained using the Bragg equation.^35,36 When PA was added, the layer spacing enlarged in the range of 0.392–0.416 nm (Table S1†); thus, PA was successfully doped into the materials. P-doping results in a larger space, which facilitates the storage of sodium.³⁷ The internal defects of the materials were detected by the Raman spectrum. Fig. 2b shows that the D-peak and G-peak appear at 1348 cm⁻¹ and 1590 cm⁻¹, which are correlated with the degree of disorder and order, respectively.³⁸ In general, I_D/I_G can estimate the level of defects in carbon materials. It can be counted that after adding PA, the I_D/I_G is 0.951, 0.957, 0.971, and 0.976, respectively, corresponding to the increase in defects. In addition, the electron paramagnetic resonance (EPR) test further shows that the introduction of PA led to the creation of rich edge defects (Fig. S3†). Abundant edge defects can greatly promote the adsorption of Na⁺ on the surface of the material, thus significantly improving sodium storage capacity.²⁴

Fig. 2 Structure characterization for PHCS and HCS. (a) XRD patterns. (b) Raman spectra. (c) XPS spectra. (d and e) High-resolution spectra of C 1s and P 2p for PHCS2. (f) N₂ adsorption–desorption isotherms.

The effect of PA content on elemental composition was researched via X-ray photoelectron spectroscopy (XPS). Fig. 2c and Table S2† demonstrate the element categories and levels. PHCS samples contain C, O, and P, among which C content is in the range of 81.96%–86.37%, while HCS contains only C (84.50%) and O (15.50%). In addition, the levels of P increase from 1.83% to 4.41% and O increases from 11.82% to 13.93% with the addition of PA. Fig. 2d depicts the high-resolution spectrum for C 1s, which comprised C–C/C=C (284.9 eV), C–P (286.1 eV), and C=O (287.9 eV).³⁹ The high-resolution spectrum for P 2p is made up of P–C and P–O bonds (Fig. 2e), which are placed in 132.9 eV and 133.8 eV, respectively.⁴⁰ Furthermore, the implication of PA amount on the specific surface area and pore structure was investigated by performing N₂ adsorption and desorption experiments. The isothermal chart is listed in Fig. 2f. From the experimental data, it can be observed that the presence of PA can greatly change the specific surface area of the sample (Table S1†), but the excessive amount generates a less specific surface area (from 287.820 for PHCS2 to 46.269 m² g⁻¹ for PHCS3). Moreover, PHCS2 and PHCS1 have abundant micropore structure, and their pore diameter is mainly distributed in the range of 0.4–0.6 nm (Fig. S4†), indicating that rich micropores promote sodium storage performance. In contrast, neither HCS nor PHCS3 has a microporous structure. This demonstrates that controlling the amount of PA significantly affects the specific surface area and pore structures of PHCS.

Electrochemical performance

To investigate the Na⁺ storage characteristics of the samples, electrochemical tests were performed on assembled CR2032-typical batteries. The cyclic voltammetry (CV) graphs for HCS, PHCS1, PHCS2, and PHCS3 are illustrated in Fig. 3a and S5.† During the first negative sweep, an irreversible peak occurs near 0.7 V. This corresponds to the formation of a solid electrolyte interface (SEI) film. Moreover, we can clearly observe that the irreversible peak intensity at 0.7 V increases significantly with the increase in P doping, indicating that the formation of SEI is mainly owing to the irreversible reaction between P and Na.^4,41 Moreover, a couple of visible redox peaks appears in the CV graph classified as the insertion/desertion behavior for Na⁺ below 0.1 V.^42,43Fig. 3b demonstrates the galvanostatic charge/discharge graph. It can be observed that the total capacity comprises two parts: the plateau region (below 0.1 V, insertion for Na⁺) and the slope region (above 0.1 V, adsorption for Na⁺). As the PA amount increases, both the plateau and slope regions increase. Among them, PHCS2 has the largest plateau and slope region. The discharge and charge capacities for HCS, PHCS1, PHCS2, and PHCS3 were 199.0/131.8, 221.1/115.2, 413.9/293.5, and 355.0/206.3 mA h g⁻¹, leaving an initial coulombic efficiency (ICE) of 66.22%, 52.10%, 67.95%, and 58.10%, respectively (Table S3†). In general, the ICE depends mainly on the decomposition of the electrolyte on the hard carbon surface and the generation of SEI. Obviously, the appropriate concentration of P doping increases the conductivity and ion diffusion kinetics, thus greatly reducing the formation of SEI, while too high P doping leads to more irreversible Na⁺ consumption, and too low leads to slow electron and ion diffusion.⁴⁰ This means that the amount of PA can affect the storage performance of sodium, and a certain amount of PA can improve the ability of Na⁺ diffusion and adsorption by expanding the layer spacing and increasing the edge defect level. However, excessive PA causes a lot of stacking of the cross-linking network, leading to a reduction in the internal pore structure. This closed structure is not favorable for Na⁺ storage and reduces only the sodium storage capacity.³ Therefore, the sample derived from 2 mL PA (PHCS2) has the best sodium storage performance. The cyclic capabilities of all samples at 0.2 A g⁻¹ are shown in Fig. 3c. Notably, the sodium storage capacity of PHCS2 is 245.3 mA h g⁻¹ over 60 cycles, while HCS, PHCS1, and PHCS3 are only 124.3, 106.1, and 166.7 mA h g⁻¹, respectively. The rate capabilities are further investigated, as illustrated in Fig. 3d. It is found that PHCS2 displays optimal rate capability, providing reversible capacities of 293.5, 254.8, 234.8, 217.4, 199.7, and 162.5 mA h g⁻¹ at the range from 0.1 to 5 A g⁻¹. PHCS2 exhibits higher capacities under different current densities (Table S3†). This prominent rate capability is preferable for many published studies on hard carbon (Fig. 3e and Table S4†).^{17,19,31,32,44–49} Simultaneously, by investigating the sodium storage characteristics for the samples at various pyrolysis temperatures, 800 °C was confirmed as the best (Fig. S6†). In addition, the cycling durability at 1 A g⁻¹ was investigated to assess its value in use. As depicted in Fig. S7,† the reversible capacity for PHCS2 still holds 150 mA h g⁻¹ over 1000 cycles (retains 76.48% before the cycle). In comparison, the reversible capacities for HCS, PHCS1, and PHCS3 are 83.7, 64.8, and 130.2 mA h g⁻¹ (PHCS3 only 800 cycles because of battery failure), respectively. Additionally, PHCS2 could remain 100.2 mA h g⁻¹ at 5 A g⁻¹ over 2000 cycles (Fig. 3f), with only 0.05% degradation for each cycle and a 71.4% total capacity maintained.

Fig. 3 Sodium storage properties for PHCS and HCS. (a) CV graph for the PHCS2 electrode at 0.1 mV s⁻¹. (b) Galvanostatic charge–discharge graph for the first cycle at 0.1 A g⁻¹. (c) Cyclic capability at 0.2 A g⁻¹. (d) Rate capability at range of 0.1–5 A g⁻¹. (e) Comparisons for rate capability with other hard carbon materials. (f) Cyclic performance for PHCS2 at 5 A g⁻¹.

Kinetic sodium storage process

CVs at various scan rates were employed to study the kinetics and mechanism for Na⁺ storage in PHCS2. Fig. 4a demonstrates the CV graph for PHCS2 at 0.1–5 mV s⁻¹. The locations of peaks 1 and 2 do not change evidently, even as the scan rate is enhanced, meaning that the PHCS2 electrode produces very small polarization and has very fast Na⁺ storage kinetics. The connection between peak current (i) and scan rate (v) is explained based on the following equation: i(v) = av^b, where a is a constant term, v is the scan rate, and i(v) is the peak current. By calculating the value of b, the control process of Na⁺ storage can be further determined. If b-values are 0.5 and 1, they represent diffusion control and capacitive behavior, respectively.⁵⁰ The computed b-values are 0.81 and 0.85 for peaks 1 and 2, respectively (Fig. S8†), suggesting a capacitive behavior in PHCS2. Therefore, according to equation i(v) = k₁v + k₂v^1/2,^51,52 the contribution of the capacitive behavior in the PHCS2 electrode was further calculated, where k₁v represents capacitive and k₂v^1/2 represents diffusion-controlled. Fig. 4b demonstrates the capacitive contribution of 72.5% for PHCS2 at 1 mV s⁻¹. Similarly, Fig. 4c demonstrates that the capacitive contributions are 63.3%, 66.2%, 68.3%, 72.5%, 79.7%, and 92.2% at 0.1–5 mV s⁻¹. In comparison, we further investigated the sodium storage kinetics of HCS (Fig. S9†). The CV curves of HCS at different scan rates are similar to those of PHCS. The calculated b-values are 0.80 and 0.84, respectively, indicating the same capacitive behavior control. However, its capacitive contributions are lower than those of PHCS2, which are 62.6%, 65.9%, 68.0%, 71.7%, 77.5%, and 88.9%, respectively. The high capacitive contribution of PHCS2 is due to its unique interconnected structure, increased defect levels, and expanded layer spacing, facilitating rapid electron and ion transport. Hence, a fast capacitive behavior is regarded as the main consideration for the excellent rate capability of PHCS2, in particular at a high rate.

Fig. 4 Sodium ion kinetic study for PHCS2 electrode. (a) CV graph at 0.1–5 mV s⁻¹. (b) Capacitance ratio diagram at 1 mV s⁻¹. (c) Diffusion and capacitive contributions. (d) EIS plots. (e) Discharge and charge graph for the GITT test. (f) Calculated diffusion coefficients of Na⁺.

Electrochemical impedance spectroscopy (EIS) was applied to study the kinetic processes of PHCS2. The equivalent circuit is fitted with a semicircle and a diagonal, as illustrated in Fig. 4d. Moreover, the semicircle diameter is the charge transfer resistance (R_ct) and the diagonal diameter is the diffusion impedance (W₁). As depicted in Fig. S10,† the R_ct gradually decreases with the addition of PA. PHCS1 and PHCS2 have similar and smallest R_ct (5.5 Ω), while the W₁ of PHCS2 is lower than that of PHCS1, implying that the Na⁺ diffusion barrier is the smallest in PHCS2. This is because the introduction of the P element could strengthen rapid electron transport, and the abundant microporous structure also facilitates the conduction of electrolytes.

To further evaluate the diffusion rate for Na⁺ in different samples, the diffusion coefficients for Na⁺ during charging and discharging were measured and calculated using the galvanostatic intermittent titration technique (GITT) with a pulse current of 0.1 A g⁻¹ for 10 min and rest 1 h (Fig. 4e and S11†). Furthermore, the diffusion coefficients for all samples were acquired based on Fick's second principle: ,⁵³ where m_B represents the mass of active material, τ represents the pulse duration, S represents the active surface area, and V_M and M_B represent the molar volume and molecular mass of carbon, respectively. ΔE_s and ΔE_τ values were obtained from the GITT curve. As shown in Fig. 4f, PHCS2 illustrates an extremely fast Na⁺ diffusion coefficient (10⁻¹¹–10⁻⁶ cm² s⁻¹) during charging and discharging, which is faster than that of all other samples (10⁻¹¹–10⁻⁷ cm² s⁻¹ for HCS, 10⁻¹¹–10^−6.8 cm² s⁻¹ for HCS1, and 10⁻¹²–10⁻⁶ cm² s⁻¹ for HCS3) (Fig. S12†). This implies that the appropriate amount of PA is conducive to obtaining the fastest Na⁺ diffusion kinetics and that the rapid diffusion coefficient facilitates the superior rate capability for PHCS2. Moreover, it can be found that the Na⁺ diffusion coefficients in the plateau region are lower than those in the slope region, which has the same trend as most reported results,^54,55 reflecting that the adsorption diffusion rate for Na⁺ is faster than that of intercalation.

Sodium storage mechanism

To explain the Na⁺ storage mechanism, PHCS2 states at different potentials were analyzed using ex situ XRD and ex situ Raman (Fig. 5a). Fig. 5b demonstrates the ex situ XRD for PHCS2, where the position of the (002) peak (21.5°) at the open-circuit voltage is consistent with the original powder position and there is no significant shift from 2.5 V discharge to 0.1 V, which is attributed to the adsorption behavior for Na⁺ at 2.5 V–0.1 V (slope region). As the discharge proceeds, the (002) peak gradually moves to a low location (20°), starting from 0.1 V and reaching a minimum when discharging to 0.01 V, leading to an increasing layer spacing of the PHCS2 electrode (from 4.13 to 4.42 nm), which illustrates that the intercalation behavior of Na⁺ below 0.1 V (plateau region) causes an enlargement of the carbon layer spacing.^56,57 When charging to the maximum voltage, the (002) peak gradually restores to its pre-cycle situation. Simultaneously, the level of defects in the PHCS2 electrode was measured via ex situ Raman spectroscopy. Fig. 5c demonstrates that the value of I_D/I_G decreases from 0.975 to 0.947 during discharging to 0.1 V, that is, the defect level decreases significantly for a large number of defects occupied by Na⁺. Moreover, the value of I_D/I_G is basically unchanged below 0.1 V, indicating that the storage mechanism based on intercalation behavior does not cause defect changes. When recharging to the maximum voltage, the defect level gradually returns to the previous value before the cycle, indicating an exceptionally stable PHCS2 electrode throughout the cycle. As per the results of ex situ XRD and ex situ Raman, an adsorption-intercalation mechanism of Na⁺ can be concluded (Fig. 5d). During high-potential discharge, Na⁺ ions are mainly stored in defective parts of the surface. The intercalation behavior is obvious at low potentials (below 0.1 V), and a large number of Na⁺ ions are inserted into the carbon layer or nanovoids. The Na⁺ ions are continuously dislodged from the carbon layer and surface upon charging to 3.0 V.

Fig. 5 Studies of sodium storage mechanism in PHCS2 electrode. (a) The different test voltages at 0.1 A g⁻¹. (b) Corresponding ex situ XRD and (c) Ex situ Raman spectra. (d) Schematic diagram for Na⁺ storage.

DFT calculations

To study deeply the effect of P doping on Na⁺ storage properties, density functional theory (DFT) was employed to study different models (pure carbon, defect carbon, and P-doped defect-carbon). First, the adsorption energy (E_a) of Na⁺ in different models was calculated (Fig. 6a–c). In pure carbon, the E_a is only 0.47 eV, indicating that pure carbon is unfavorable for Na⁺ adsorption. A higher negative E_aEa (−1.24 eV) is obtained owing to the rearrangement of hybrid orbitals caused by the defects, and this facilitates the adsorption of Na⁺. In the P-doped defect-carbon, rehybridization causes larger structural changes owing to the larger electronegativity of P,²⁹ forming more Na⁺ adsorption sites (E_a = −1.27 eV). Finally, the diffusion energy barriers (E_bs) for Na⁺ across defect-carbon and P-doped defect carbon were further calculated. The trajectory of Na⁺ in the carbon layer is S1 to S2† (Fig. 6d and e). Furthermore, the actual layer spacing of defect-carbon and P-doped defect-carbon are simulated separately according to the layer spacing calculated by XRD (Fig. 6f). By DFT calculation, the corresponding E_bs are shown in Fig. 6g. It is found that the E_b in the P-doped defect carbon decreases from 0.55 eV to 0.46 eV as the layer spacing increases (0.39–0.41 nm), highlighting that P doping can significantly modify the diffusion kinetics of Na⁺.⁵⁸ Thus, DFT calculation shows that P doping could boost Na⁺ adsorption and improve the diffusion kinetics for Na⁺ by expanding the layer spacing.

Fig. 6 DFT calculations. (a–c) Top and side views of Na-ion adsorbed in pure carbon, defect-carbon, and P-doped defect-carbon, respectively. (d and e) Top view of Na-ion diffusion path (from S1 to S2†) in defect-carbon sheet and P-doped defect-carbon sheet, respectively. (f) Side view of two models with different layer spacings. (g) Diffusion energy barriers. The gray, violet, and orange balls are C, P, and Na, respectively.

Conclusions

In conclusion, we illustrate a simplified carbonization route to produce P-doped hard carbon microspheres (PHCS) using the precursors of phytic acid/phenolic resin. Benefiting from its interconnected carbon spherical structure, high specific surface area (287.82 m² g⁻¹), and expanded layer spacing (0.411 nm), the synthesized PHCS2 exhibits remarkable rate performance of 293.5 and 162.5 mA h g⁻¹ at 0.1 and 5 A g⁻¹, respectively. Furthermore, PHCS2 demonstrates an outstanding cyclic performance (100.2 mA h g⁻¹ over 2000 cycles) at 5 A g⁻¹. Its exceptional electrochemical behaviors are mostly based on its high Na⁺ adsorption capability and fast Na⁺ diffusion according to DFT calculation, EIS, and GITT. Moreover, the Na⁺ storage mechanism (adsorption-intercalation) in hard carbon materials was further corroborated via ex situ XRD and ex situ Raman analysis. Therefore, this study could offer a facile approach for constructing and synthesizing P-doped carbon materials with advanced electrochemical performance.

Author contributions

Sheng Wu: methodology, investigation, data curation, software, writing-original draft. Handong Peng: investigation, data curation, software. Le Huang: Investigation, software. Yongsi Liu: investigation, data curation, software. Yanxue Wu: investigation, data curation. Lei Liu: data curation, investigation, software. Wei Ai: data curation, investigation, software, supervision. Zhipeng Sun: funding acquisition, project administration, writing-review & editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Guangdong University of Technology Hundred Talents Program (No. 263118136).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01296d

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