Tailoring a multifunctional polyglutamic acid–tragacanth gum binder for enhancing the lithium storage performance of red phosphorus anodes

Abstract

Red phosphorus, with its high theoretical specific capacity and safe lithiation potential, is a promising anode for lithium-ion batteries. However, challenges such as significant volume expansion, dissolution of lithium polyphosphides (Li_xpPs), and low electronic conductivity hinder its practical application. In this study, we propose a multifunctional hydrogen-bond enhanced cross-linked binder, polyglutamic acid–tragacanth gum (PGA–TG). The PGA–TG binder not only exhibits strong mechanical properties to inhibit the volume expansion of phosphorus particles but also demonstrates a high affinity for phosphorus, thereby facilitating the capture of soluble Li_xpPs and enhancing the long-cycle performance. Therefore, the PGA–TG-based electrode achieves a lower volume expansion of 19.4% compared with the PVDF-based electrode (233%). Additionally, the PGA–TG-based electrode delivers high reversible capacity of 1575.91 mA h g⁻¹ after 150 cycles at 260 mA g⁻¹ and 1442 mA h g⁻¹ after 280 cycles at 1 A g⁻¹. This work presents a facile and effective binder design strategy to address the multiple challenges associated with phosphorus anodes in lithium-ion batteries.

---

New concepts

The practical implementation of red phosphorus (RP) anodes in lithium-ion batteries is hindered by challenges such as significant volume expansion, dissolution of lithium polyphosphides (Li_xpPs), and low electronic conductivity. To address these issues, we developed a multifunctional binder system, polyglutamic acid–tragacanth gum (PGA–TG). This cross-linked network, formed through intermolecular hydrogen bond, provides a robust mechanical framework that mitigates the volume expansion of phosphorus particles. The PGA–TG binder also features an abundance of oxygen-containing groups, enhancing its affinity with phosphorus and improving phosphorus utilization. Furthermore, the adsorption properties of PGA–TG effectively suppress the dissolution of Li_xpPs into the electrolyte. The binder's conducive network and improved electrolyte wettability accelerate electron and ionic transport within the electrode. Consequently, the PGA–TG-based electrode demonstrates excellent long-cycle reversible capacity and rate performance. In this work, we explore the potential of binder engineering as a pragmatic solution to enhance the electrochemical performance of RP anodes, providing an example for enhancing the performance of RP anodes in lithium-ion batteries.

Introduction

The burgeoning electric vehicle market necessitates the development of next-generation batteries with long mileage, enhanced safety, economic viability, and ultra-rapid charging capabilities.^1–4 One of the key aspects for this endeavor is the evolvement of anode materials, such as red phosphorus (RP) emerging as one of the promising candidates, due to its high theoretical specific capacity (2596 mA h g⁻¹), safe lithiation potential (0.7 V vs. Li⁺/Li) and low cost.^5–8 However, RP still faces intrinsic challenges like other alloy electrode materials (Si anodes and S cathodes). Phosphorus undergoes reversible lithiation and delithiation processes via an alloying reaction (P + 3Li ↔ Li₃P), which results in a volume expansion as high as 300%.^9,10 These processes involves multiphase and multi-step reactions, accompanied by the issues of slow kinetics and dissolution of intermediate products (lithium polyphosphides, abbreviated as Li_xpPs) into the electrolyte,^11,12 thereby diminishing the active phosphorus content. It is even worse that the low electronic conductivity of RP combined with its slow kinetics hinders fast charging capability and output power.¹³

To mitigate these issues, researchers have proposed various strategies, size regulation, nanoscale confinement, surface coatings, and elemental doping.^14–20 While these approaches hold promise, they often entail complex fabrication processes and are not cost-effective. A strategy of binder design would be a facile and economical path for improving the electrochemical behavior of RP anodes, as it influences the features of the P particle surface, the P/electrolyte interface and the transport of Li ions and electrons. Binders basically serve as adhesive agents that integrate active materials and conductive additives to the current collector, ensuring the integrity of electrodes. As battery technologies evolve, the expectations from binders extend to their role in curbing volume expansion, facilitating ionic and electronic transport, and enabling self-healing properties.^21,22 Polyvinylidene fluoride (PVDF), the most widely used binder, falls short in its adhesive interaction with active phosphorus and its mechanical properties, failing to effectively inhibit the volume expansion of RP anodes.²³ Moreover, the solvent N-methyl-2-pyrrolidone (NMP), utilized in the PVDF binder, is toxic and volatile,²⁴ posing significant health and environmental risks. The inherent limitations in the electronic and ionic conductivities of PVDF further impede the fast charging performance of the phosphorus anode.²⁵ Worse still, recent studies have also highlighted a critical side reaction between PVDF and Li₃P, which not only degrades the PVDF structure but also leads to the loss of active RP.²⁶ Therefore, it is imperative to tailor a binder for P-based anodes that ensures cost-effectiveness and safety. This binder must address the shortcomings of PVDF and enhance the electrochemical performance of P-based anodes, making it a critical component in the advancement of next-generation lithium-ion batteries for electric vehicles.

In this work, we designed a hydrogen-bond enhanced cross-linked binder, poly glutamic acid–tragacanth gum (PGA–TG). The PGA–TG binder system capitalizes on the water solubility of both PGA and TG, eliminating the need for toxic solvents of NMP. As shown in Fig. 1a, the rich presence of carboxyl, hydroxyl, and amino groups in PGA and TG facilitates the formation of intermolecular hydrogen bonds, endowing the binder with a robust 3D cross-linked structure and exceptional mechanical properties. These properties are pivotal in mitigating the volume expansion of RP anodes. Furthermore, the PGA–TG binder demonstrates the enhanced affinity for RP, facilitated by P–O–C bond. On the one hand, it will drive the binder to coat on the phosphorus surface uniformly, reducing side reactions. On the other hand, the greater affinity leads to stronger adhesion avoiding the detachment of active materials during the cycling process. The ability of the PGA–TG binder to adsorb soluble Li_xpPs and its high lithium transference number (t_Li⁺) further promote the rapid conversion of Li_xpPs and enhance the electrochemical performance of P-based anodes. Therefore, the RP electrode using the PGA–TG binder delivers excellent reversible capacity of 1575.91 mA h g⁻¹ after 150 cycles at 260 mA g⁻¹ and 1442 mA h g⁻¹ after 280 cycles at 1 A g⁻¹.

Fig. 1 (a) The formation of the PGA–TG 3D network and its role in enhancing the performance of phosphorus anodes. (b) Hydrogen bond of N⋯HO and its bond energy calculated by DFT simulations. (c) FTIR spectra of PGA, TG and PGA–TG. (d) FTIR spectra of RP-PGATG, RP and PGA–TG. (e) The 180° peeling tests of PGA–TG, PGA, TG, and PVDF-based electrodes. (f) Load–displacement curves and the (g) reduced modulus and hardness of the electrodes. (h) Adsorption effect of different binders on LiP₅, LiP₇.

Results and discussion

Compatibility between PGA–TG and RP

The molecular structures and electrostatic potential distributions of PGA and TG, rich in oxygen-containing groups, are depicted in Fig. S1a and b (ESI†). It reveals that the nitrogen atom in PGA is electron deficient (shown in blue color), while the negative charge concentrates on the O atom in TG (shown in red color), suggesting the possibility of the hydrogen bond between N–H and O atoms. Similarly, the red O atom region in PGA and the blue O–H region in TG can form hydrogen bonds as well. Therefore, hydrogen bonds can be easily and widely formed between PGA and TG. Fig. 1b and Fig. S1c (ESI†) illustrate different types of intermolecular hydrogen bonds and their bond energies simulated by density functional theory (DFT) calculations, which corroborates our preceding analysis. Subsequently, we further confirmed the presence of hydrogen bonds through Fourier transform infrared (FTIR) spectra (Fig. 1c). The vibration peaks at 1230 and 1030 cm⁻¹ can be assigned to C–O and C–O–C, respectively.²⁷ The vibration peaks at 1590 cm⁻¹ and 1390 cm⁻¹ correspond to symmetric and antisymmetric stretching vibrations of the carboxylate group,²⁸ while the peaks at 1738 cm⁻¹ and 2930 cm⁻¹ are attributed to the asymmetric stretching of C=O and stretching vibrations of the –CH₂ group.^29,30 The vibration peak at 3346 cm⁻¹ can be assigned to the stretching vibration of O–H in TG, and the vibration peak at 3272.9 cm⁻¹ can be assigned to the stretching vibration of N–H in PGA.^27,28 Different from it, the vibration peak assigned to O–H and N–H shifts and becomes broader, proving the existence of hydrogen bonds in PGA–TG.^29,31 Therefore, PGA and TG can obtain a robust cross-linked network by generating intermolecular hydrogen bonds.

The PGA–TG binder exhibits an excellent affinity for phosphorus, as evidenced by the P–O–C vibration peak at 1002 cm⁻¹ in the Fourier transform infrared (FTIR) spectra (Fig. 1d).³² It indicates that the presence of P–O–C bonds enhances the interaction between RP and the PGA–TG binder. To quantify the adhesion strength, 180° peeling tests were conducted, revealing that the PGA–TG-based electrode demonstrates the highest maximum peeling force of 2.9 N, compared with TG (2.5 N), PGA (2.1 N) and PVDF (1.8 N)-based electrodes, respectively (Fig. 1e). The average peeling force (Fig. S2, ESI†) also shows that the PGA–TG-based electrode has the highest average value of 2.9 N, significantly outperforming other binders. The enhanced adhesion strength is attributed to the 3D cross-linked network and the presence of P–O–C bonds.

To accommodate the substantial volume expansion of phosphorus during cycling, binders with excellent mechanical properties are needed. Nano-indentation tests (Fig. 1f) were performed to measure these properties, and the reduced modulus and hardness were calculated using the Oliver–Pharr method.³³ The PGA–TG-based electrode shows the highest value for both the reduced modulus and the hardness (Fig. 1g). Specifically, the PGA–TG-based electrode exhibits the highest hardness of 0.098 GPa among the TG-based electrode (0.055 GPa), PGA-based electrode (0.081 GPa) and PVDF-based electrode (0.057 GPa). The reduced modulus of the PGA–TG-based electrode still shows a maximum value of 6.043 GPa among those of TG (3.066 GPa), PGA (5.401 GPa), and PVDF (4.687 GPa)-based electrodes. These outstanding mechanical properties suggest that PGA–TG has sufficient strength to mitigate the volume expansion, maintaining the electrode's integrity and making it more suitable for red phosphorus anodes.

Due to the multi-step redox process, some of the producing Li_xpPs intermediates will dissolve in the electrolyte,^10,13 causing the loss of active phosphorus and thus detrimental to long cycle stability. Based on DFT calculations, Fig. 1h illustrates the binding energy between different binders and LiP₅/LiP₇. PVDF shows relatively low binding energy, while PGA and TG exhibit stronger adsorption of Li_xpPs through electrostatic interactions. It indicates that the PGA–TG binder can effectively immobilize Li_xpPs within the electrode, facilitating long-cycle capacity retention.

Electrochemical performance

Binders play a pivotal role in the structural integrity and electrochemical performance of lithium-ion batteries. They must be both structurally and electrochemically stable to ensure the life span and cycling performance of batteries. To verify the stability and electrochemical inertness of our PGA–TG binder, we prepared electrodes comprising a 50 wt% binder and 50 wt% Super P and subjected them to cyclic voltammetry (CV) analysis. The results, as shown in Fig. S4a and b (ESI†), reveal that both PGA–TG and PVDF binders exhibit excellent cycling stability, with highly overlapped curves in subsequent cycles, indicating their potential for use in lithium-ion batteries. PVDF is renowned for its exceptional electrochemical stability,³⁴ a key factor in its widespread use as a binder in lithium-ion batteries. That is, no redox reaction occurs when PVDF is used as a binder, and the peaks in CV curves are all attributed to active materials (conductive carbon and red phosphorus). In our study, the first cycle of the PGA/TG-Super P was compared with that of PVDF-Super P, as depicted in Fig. S5 (ESI†).³⁵ The CV curves of PGA/TG-Super P closely resemble those of the PVDF-Super P, with the peak locations and curve shapes essentially overlapping. Notably, no new peaks are observed, indicating that PGA–TG does not participate in the lithiation or delithiation reactions, making it a viable candidate as a binder in phosphorus anodes for lithium-ion batteries.

The electrochemical performances were further tested through galvanostatic charge–discharge cycling in the voltage range of 0.01–3 V vs. Li⁺/Li. The specific capacity was calculated based on the total weight of the P/G composite electrode. Prior to rate and cycling tests at a current of 260 mA g⁻¹, the batteries were activated at 50 mA g⁻¹ for 1 cycle. A stepwise activation involving 50 mA g⁻¹ for 1 cycle, 100 mA g⁻¹ for 3 cycles, and 400 mA g⁻¹ for 3 cycles was conducted before 1 A g⁻¹ long cycle tests.

Fig. 2a illustrates the cycling performance at 260 mA g⁻¹, where the PGA–TG-based electrode outperforms all other electrodes, maintaining a high specific capacity of 1575.91 mA h g⁻¹ after 150 cycles. This capacity significantly exceeds that of the PGA-based electrode (1180.7 mA h g⁻¹), PVDF-based electrode (1143.7 mA h g⁻¹), and TG-based electrode (793.3 mA h g⁻¹). Notably, the PGA–TG, PGA, and TG-based electrodes exhibit higher initial reversible specific capacities than the PVDF-based electrode, attributed to the enhanced contact between the binder and phosphorus due to the presence of oxygen-containing groups. The TG-based electrode suffers from severe capacity degradation due to its poor mechanical properties. In contrast, PGA–TG is able to maintain a more impressive capacity retention at high rates. Fig. 2b and c compare the charge–discharge curves of the PGA–TG and PVDF-based electrodes at different cycles. The PGA–TG-based electrode displays a lower overpotential of 0.47 V in the first cycle compared with the PVDF-based electrode (0.58 V). Fig. 2d and e present the CV curves at a scan rate of 0.1 mV s⁻¹ between 0.01 V and 3 V vs. Li⁺/Li. The cathodic peaks at around 0.33 V and 0.68 V correspond to the multi-step lithiation process of phosphorus (xLi + p → Li_xP, x = 1–3), while the anodic peaks at around 1.00 V and 1.25 V are associated with the multi-step delithiation of Li_xP (x = 1–3).³⁶ The polarization between the two major peaks of the PGA–TG-based electrode is 0.57 V, significantly smaller than that observed in the PVDF-based electrode (0.66 V). The lower overpotential indicates faster lithium ion and electron transfer kinetics in the PGA–TG-based electrode.

Fig. 2 (a) Long cycle performance of PGA–TG, PGA, TG, PVDF-based electrodes at 260 mA g⁻¹. Galvanostatic charge and discharge curves of the (b) PGA–TG-based electrode and (c) PVDF-based electrode at 260 mA g⁻¹. CV curves of the (d) PGA–TG-based electrode and (e) PVDF-based electrode at a scan rate of 0.1 mV s⁻¹ between 0.01 and 3.0 V vs. Li⁺/Li. (f) Contact angle between PGA–TG, PGA, TG, PVDF and RP. (g) Comparison of the initial coulombic efficiency of the PGA–TG-based electrode and the PVDF-based electrode. (h) Top view and (i) cross section morphologies of the PGA–TG-based electrode and PVDF-based electrode.

Fig. 2f shows the contact angle measurements between the binders and RP. The PGA–TG binder exhibits the smallest contact angle of 36.79°, indicating the best affinity for RP. This enhanced affinity allows the PGA–TG binder to spread more evenly on the surface of phosphorus particles, protecting the electrode from side reactions with the electrolyte. Fig. 2g demonstrates that the initial coulombic efficiency (ICE) of the PGA–TG-based electrode is 86.84%, higher than that of the PVDF-based electrode (67.79%), indicating the less side reactions of the PGA–TG-based electrode. Additionally, greater affinity helps to build a faster Li⁺ transfer network, which is beneficial to capacity performance. To verify the effectiveness of binders on restraining volume expansion, the morphology of the electrodes before and after cycling was analyzed (Fig. 2h and i and Fig. S6, ESI†). The PGA–TG-based electrode maintains a uniform particle distribution and no cracks are observed even after 50 cycles, in contrast to the visible cracking and particle aggregation observed in the PVDF-based electrode. The PGA-based electrode and the TG-based electrode also show a certain extent of particle aggregation and tiny cracks. The PGA–TG-based electrode also exhibits the smallest volume expansion ratio of 19.4% among the PVDF-based electrode (233%), TG-based electrode (75%), and PGA-based electrode (61%).

Fig. 3a shows that even at a high current density of 1 A g⁻¹, the PGA–TG-based electrode maintains an outstanding reversible specific capacity of 1442 mA h g⁻¹ after 280 cycles. This performance is superior to that of the PGA-based electrode (1149.3 mA h g⁻¹), TG-based electrode (381.8 mA h g⁻¹), and PVDF-based electrode (685.6 mA h g⁻¹), respectively. Fig. 3b and Fig. S7 (ESI†) compare the overpotential after cycling. In the 8th cycle, the overpotential of all the electrodes shows tiny differences of 0.48 V (PGA–TG-based electrode), 0.52 V (PGA-based electrode), 0.56 V (TG-based electrode), and 0.81 V (PVDF-based electrode), respectively. Nevertheless, in the 100th cycle, the distinctions are quite significant. The PGA–TG-based electrode and the PGA-based electrode exhibit the smallest polarization of 0.47 V, while the PVDF-based electrode exhibits the biggest polarization of 1.22 V. The TG-based electrode provides an intermediate value of 0.88 V. The lower overpotential demonstrates the enhanced electrochemical kinetics of the PGA–TG-based electrode. The improved kinetics also leads to the prominent high-rate performance of the PGA–TG-based electrode (Fig. 3c), where it delivers specific capacities of 2000.9, 1899.6, 1801.3, 1718.7, 1636.7, and 1468.5 mA h g⁻¹ at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g⁻¹, respectively. This performance is significantly better than that of the other electrodes, with a high capacity retention of 73.4% as the current density increases from 0.1 A g⁻¹ to 5 A g⁻¹, while those are only 43.9%, 65.8%, and 48.2% for PGA, TG and PVDF-based electrodes.

Fig. 3 (a) Long cycle performance of PGA–TG, PGA, TG, PVDF-based electrodes at 1 A g⁻¹. (b) Galvanostatic charge and discharge curves of the (b) PGA–TG-based electrode and (c) PVDF-based electrode at 1 A g⁻¹. (c) Rate performance of the PGA–TG, PGA, TG, and PVDF-based electrodes. (d) The hysteresis (ΔE_p) of the PGA–TG-based electrode and PVDF-based electrode. (e) Conductivity of the PGA–TG-based electrode and PVDF-based electrode. (f) The wettability of the electrolyte on different electrodes. (g) Comparison of electrochemical performance between this work and other pervious works about P-based anodes for LIBs.

The remarkable high-rate performance of the PGA–TG-based electrode can be attributed to its lower hysteresis, as shown in Fig. 3d. The hysteresis (ΔE_p) of the PGA–TG-based electrode is significantly lower than that of the PVDF-based electrode, indicating faster kinetics. This improved kinetics is further supported by the enhanced electronic and lithium ion conductivity, with the PGA–TG-based electrode achieving a higher electrical conductivity of 8.38 kS mm⁻¹, compared to that of the PVDF-based electrode (1.85 kS mm⁻¹), as shown in Fig. 3e. Fig. 3f demonstrates that the PGA–TG-based electrode exhibits the smallest contact angle with the electrolyte, indicating the best wettability, which is conducive to fast lithium-ion conduction within the electrode. Compared with other reported work about P-based anodes in lithium-ion batteries, the PGA–TG-based electrode still exhibits outstanding performance (Fig. 3g). It indicates that PGA–TG enhances the Li⁺ storage properties, highlighting the importance of the binder.^26,36–43

Reaction kinetics

Li–Li and steel–steel symmetric cells were assembled to study the intrinsic ion conductivity of PGA–TG and PVDF. Solutions of PGA–TG and PVDF were evenly cast on polypropylene (PP) separators, with detailed procedures provided in the Experimental section. Fig. S8 (ESI†) illustrates the average thickness of the PVDF@PP and PGA–TG@PP composite separators. The ionic conductivity (σ) and the lithium-ion transference number (t_Li⁺) were calculated using eqn (1) and (2):⁴⁴where L is the thickness of the separator, S is the effective area of contact between the separator and the electrode, R_b is the bulk resistance, and σ is the ionic conductivity.where I₀ and I_s are the initial and steady-state currents, R₀ and R_s are the initial and steady-state resistances, ΔV is the applied voltage (here, it is 10 mV). PGA–TG demonstrated a higher t_Li⁺ of 0.53 and a lithium-ion conductivity of 0.65 × 10⁻³ S cm⁻¹, indicating more effective lithium-ion conduction (Fig. 4a–c and Table S1, ESI†). The enhanced t_Li⁺ is attributed to the strong adsorption between PGA–TG and PF₆⁻ due to the presence of the N atom.⁴⁵ Fig. S9 (ESI†) presents the simulation results of the binding energy between the binder and PF₆⁻, showing that PGA–TG has a higher binding energy of 1.53 eV compared with PVDF (1.17 eV). Consequently, the PGA–TG-based electrode facilitates free, fast, and efficient movement of Li⁺. The slightly lower ion conductivity of PGA–TG is attributed to the thicker separator and the immobilization of PF₆⁻.

Fig. 4 EIS spectrums before and after polarization, and the corresponding I–t curves of (a) PGA–TG and (b) PVDF using Li–Li symmetric cells. (c) Impendence plots of PGA–TG and PVDF using steel–steel symmetric cells. (d) CV curves of the PGA–TG-based electrode at various scan rates from 0.1 to 1 mV s⁻¹ between 0.01 and 3.0 V vs. Li⁺/Li. (e) CV curves of the PVDF-based electrode at various scan rates from 0.1 to 1 mV s⁻¹ between 0.01 and 3.0 V vs. Li⁺/Li. (f) The corresponding plots (I_pvs. v^1/2) at each redox peak. (g) The potential response curves of PGA–TG, PGA, TG, and PVDF electrodes during GITT tests. (h) Diffusion coefficients of Li⁺ calculated from the GITT pattern at the discharge process. (i) EIS spectra of the PGA–TG, PGA, TG, PVDF-based electrodes after cycling at current densities of 50 mA g⁻¹ for 1 cycle and 260 mA g⁻¹ for 1 cycle.

A series of cyclic voltammograms (CV) at different scan rates were measured to evaluate the lithium-ion diffusion coefficient for PGA–TG and PVDF-based electrodes (Fig. 4d–e). The Randles–Sevcik equation was used to compare the Li⁺ diffusion coefficient:⁴⁶

Ip = 2.69 × 105n3/2ADLi+1/2v1/2CLi (3)

where I_P is the peak current (mA), n is the number of charge-transfer, A is the surface area of the electrode, D_Li⁺ is the Li⁺ diffusion coefficient, v is the scan rate (mV s⁻¹), and C_Li is the concentration of Li⁺. By fitting I_p and v^1/2, we obtained a linear relationship whose slope reflects the relative magnitude of D_Li⁺. As shown in Fig. 4f, the cathodic and anodic slopes of the PGA–TG-based electrode are 3.74 and 5.15, respectively, larger than those of the PVDF-based electrode (2.14 and 2.70). This indicates that the PGA–TG-based electrode exhibits faster Li⁺ diffusion with a larger D_Li⁺. Additionally, the galvanostatic intermittent titration technique (GITT) was employed to compare D_Li⁺ using eqn (4):⁴⁷where τ is the titration time, m_B is the mass of the active material, V_M and M_B are the molar volume and molar mass, A is the electrode surface area, E_S is the quasi-thermodynamic equilibrium potential difference before and after the current pulse, and E_τ is the potential difference during the current pulse. Fig. 4g shows the potential response curves of the PGA–TG, PGA, TG, PVDF based-electrodes during GITT tests. The calculated results, presented in Fig. 4h, show that the PGA–TG-based electrode has the largest D_Li⁺ in the range of 10⁻¹³–10⁻¹⁰ cm² s⁻¹, confirming that the cross-linked PGA–TG promotes the transport of Li⁺ ions within the electrode. The decreased charge transfer resistance of the PGA–TG-based electrode observed after two cycles further supports this finding (Fig. 4i).

Conclusions

This study presents the hydrogen bond enhanced cross-linked binder, PGA–TG, which significantly improves the electrochemical performance of red phosphorus anodes in lithium-ion batteries. The enhanced mechanical properties of PGA–TG and the presence of the P–O–C bond significantly suppress the volume expansion of phosphorus. The strong adsorption of soluble Li_xpPs by PGA–TG also increases the utilization rate of active phosphorus, promoting the improvement of reversible specific capacity. Meanwhile, the GITT and variable scan rate CV also demonstrate that PGA–TG effectively promotes electron and Li-ion conduction within the electrode, enhancing the reaction kinetics. As a result, PGA–TG not only boosts the initial coulombic efficiency but also significantly improves the lithium-ion conductivity, achieving a small overpotential and high reversible capacities of 1575.91 mA h g⁻¹ after 150 cycles at 260 mA g⁻¹ and 1442 mA h g⁻¹ after 280 cycles at 1 A g⁻¹. This work provides an insight for enhancing the performance of P anodes by simple and effective design of the binder in lithium-ion batteries.

Experimental section

Preparation of phosphorus/graphite (P/G) composites

Commercial red phosphorus (98.5%, Energy Chemical) was washed with 10 wt% NaOH solution to remove surface oxides. Specifically, 10 g of commercial red phosphorus was added to 150 mL of cooled 10 wt% NaOH solution and stirred at 55 °C for 6 hours to ensure complete reaction. The mixture was then washed with deionized water until the liquid reached neutrality, followed by vacuum drying in an oven at 60 °C overnight. The product was subsequently stored in a glove box for further use. Subsequently, the alkali-washed red phosphorus and graphite (99.999%, Aladdin) were mixed at a mass ratio of 7 : 3 and then underwent a high-speed ball-milling process under an argon atmosphere at 400 rpm for 12 hours.

Preparation of the binder

50 wt% PGA (90.0%, Haosogbio) and 50 wt% TG (USP, Aladdin) were added in deionized water under magnetic stirring at room temperature for 8 h to obtain a homogenous 1.5% PGA–TG solution. Other binders were prepared by dissolving different powder into the corresponding solvent (1.5 wt% TG, 2% PGA in deionized water and 2% PVDF in NMP).

Preparation of electrodes

To prepare the P/G electrode, the P/G composite, super P and binder were mixed with a mass ratio of 8 : 1 : 1 in the solvent. The slurry was then coated evenly on the surface of the Cu collector and dried at 60 °C under vacuum overnight. The dried electrode was punched into circular discs with a diameter of 12 cm. The mass loading of the active material (P/G) was 0.4–0.8 mg cm⁻², and the weight of Cu discs was 8.34 mg for each electrode sheet. The PGA/TG-super P and PVDF-super P electrodes were prepared by mixing PGA–TG (and PVDF) and super P with a mass ratio of 1 : 1, following the same conditions as mentioned above.

Electrochemical measurement

The electrochemical performance was examined by assembling coin-type (CR 2032) cells, using lithium foil as the anode and the PP separator. The half-cells were assembled in an Ar-filled glove box with a water/oxygen (H₂O/O₂) content lower than 1 ppm. To assemble the Li–Li and steel–steel symmetric cells, PGA–TG@PP and PVDF@PP were prepared. 2% PGA–TG and 2% PVDF were blade-coated onto the PP separator and then dried at 70 °C overnight, and the dried separator was punched into circular discs with a diameter of 16 cm. The average thickness of both separators was measured using a micrometer screw gauge. For each type of separator, three individual samples were selected, and each sample was measured 3 times to ensure data reliability and reproducibility. The used electrolyte is 1 M LiPF₆ in EC/DEC (1 : 1, V%) with 5% wt% FEC (purchased from Canrd Technology Co., Ltd). The cycling performance, rate capability, and GITT were tested using a Neware battery testing system (CT-4000) between 0.01 and 3.0 V. GITT was conducted at a constant current pulse for 15 min at 100 mA g⁻¹, followed by a 2 h relaxation rest after each discharge step. For the values used in eqn (4), τ is the titration time (900 s), m_B is the mass of the active material, V_M is the molar volume (11.52 cm³ mol⁻¹), M_B is the molar mass (31 g mol⁻¹), and A is the electrode surface area (1.13 cm²). E_S is the quasi-thermodynamic equilibrium potential difference before and after the current pulse, and E_τ is the potential difference during the current pulse, and these two parameters were derived from the GITT test curve information. The CV measurements were carried out on Ivium electrochemical workstation, EIS and I–t measurements were conducted using an electrochemical workstation (CHI660E). The electrochemical testing temperature was 30 °C.

Materials characterization

The FTIR spectra were obtained using a Bruker TENSOR II FTIR spectrometer. The nanoindentation test was performed using an American-Agilent-Nano indenter G200. The 180° peeling test was performed using a microcomputer control electron universal testing machine. The contact angle was measured using a static contact angle goniometer (JC2000D2M). The contact angles between red phosphorus and the binder were measured using dripping binder solution onto a phosphorus sheet. The liquid drop in Fig. 2f is the binder which was used to prepare the P/G electrode (1.5 wt% PGA–TG, 1.5 wt% TG, and 2% PGA in deionized water and 2% PVDF in NMP). The thin slice was fabricated through the cold pressing of nano-scaled red phosphorus after the ball milling at 400 rpm for 12 hours. Fig. 3f shows the dripping of the electrolyte onto the P/G electrode. Scanning electron microscopy (SEM) images were obtained using an FESEM, Hitachi-S4800. The electrical conductivity of the electrode was tested using a four-point probe (Rooko).

Theoretical calculations

Density functional theory (DFT) calculations were conducted using the Dmol3 module in Materials Studio. The generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) was used for exchange–correlation processing. The Grimme method was used for correction. The calculation accuracy was 10⁻⁶. The electrostatic potential distribution of PGA and TG was obtained by calculating and analyzing the electron density. For the calculation of binding energy, the species used are exhibited in Fig. S3 (ESI†). The binding energy was calculated in accordance with the following equation:

E_b = E_complex − E_polymer − E_adsorbate

where E_b represents the binding energy, E_complex is the energy of polymer–adsorbate composites (such as PVDF–PF₆⁻ and PVDF–LiP₅), E_polymer is the energy of PGA, TG or PVDF, and E_adsorbate is the energy of adsorbates (LiP₅, LiP₇, and PF₆⁻).

Hydrogen bond energy was calculated in accordance with the following equation:

E_H = E_sys − E_PGA − E_TG

where E_H represents the hydrogen bond energy, E_sys is the energy of the PGA–TG system with a kind of hydrogen bond, E_PGA is the energy of PGA, and E_TG is the energy of TG.

Author contributions

Yanting Li: conceptualization, methodology, investigation, data curation, formal analysis, visualization, and writing – original draft. Xu Liang, Shaojie Zhang, Yidian Dong, Yujie Wang, Yiming Zhang, Haochen Gong, Hui Rong, and Xinpeng Han: investigation. Moyuan Cao, Kar Ban Tan, Anjie Dong, and Fengmin Jin: resources. Bin Zhang and Jie Sun: resources, validation, funding acquisition, and writing – review and editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22279089 and 22409146), the Major Science and Technology Projects of Yunnan Province (No. 202402AF080004), the Natural Science Foundation of Distinguished Young Scholar in Tianjin (23JCJQJC00300), and the Municipal Key R&D Program of Ningbo (2023Z109).

References

1. J. B. Goodenough and Y. Kim, Chem. Mater., 2009, 22, 587–603.
2. M. Peng, K. Shin, L. Jiang, Y. Jin, K. Zeng, X. Zhou and Y. Tang, Angew. Chem., Int. Ed., 2022, 61, e202206770.
3. B. Scrosati, J. Hassoun and Y.-K. Sun, Energy Environ. Sci., 2011, 4, 3287.
4. M. Ue, K. Sakaushi and K. Uosaki, Mater. Horiz., 2020, 7, 1937–1954.
5. R. Amine, A. Daali, X. Zhou, X. Liu, Y. Liu, Y. Ren, X. Zhang, L. Zhu, S. Al-Hallaj, Z. Chen, G.-L. Xu and K. Amine, Nano Energy, 2020, 74, 104849.
6. X. Han, H. Gong, H. Li and J. Sun, Chem. Rev., 2024, 124, 6903–6951.
7. H. Jin, Y. Huang, C. Wang and H. Ji, Small Sci., 2022, 2, 2200015.
8. Y. Sun, L. Wang, Y. Li, Y. Li, H. R. Lee, A. Pei, X. He and Y. Cui, Joule, 2019, 3, 1080–1093.
9. C. Marino, A. Debenedetti, B. Fraisse, F. Favier and L. Monconduit, Electrochem. Commun., 2011, 13, 346–349.
10. C. Peng, H. Chen, G. Zhong, W. Tang, Y. Xiang, X. Liu, J. Yang, C. Lu and Y. Yang, Nano Energy, 2019, 58, 560–567.
11. A. Allagui, A. R. Baboukani, A. S. Elwakil and C. Wang, Electrochim. Acta, 2021, 395, 139149.
12. B. Liu, Q. Zhang, L. Li, Z. Jin, C. Wang, L. Zhang and Z.-M. Su, ACS Nano, 2019, 13, 13513–13523.
13. C. Liu, M. Han, Y. Cao, L. Chen, W. Ren, G. Zhou, A. Chen and J. Sun, Energy Storage Mater., 2021, 37, 417–423.
14. A. Rabiei Baboukani, I. Khakpour, E. Adelowo, V. Drozd, W. Shang and C. Wang, Electrochim. Acta, 2020, 345, 136227.
15. X. Liu, B. Xiao, A. Daali, X. Zhou, Z. Yu, X. Li, Y. Liu, L. Yin, Z. Yang, C. Zhao, L. Zhu, Y. Ren, L. Cheng, S. Ahmed, Z. Chen, X. Li, G.-L. Xu and K. Amine, ACS Energy Lett., 2021, 6, 547–556.
16. Y. Cao, S. Zhang, B. Zhang, C. Han, Y. Zhang, X. Wang, S. Liu, H. Gong, X. Liu, S. Fang, F. Pan and J. Sun, Adv. Mater., 2022, 35, 2208514.
17. S. A. He, Q. Liu, Z. Cui, K. Xu, R. Zou, W. Luo and M. Zhu, Small, 2021, 18, 2105866.
18. S. Zhang, C. Liu, H. Wang, H. Wang, J. Sun, Y. Zhang, X. Han, Y. Cao, S. Liu and J. Sun, ACS Nano, 2021, 15, 3365–3375.
19. S. Zhang, Y. Zhang, Z. Zhang, H. Wang, Y. Cao, B. Zhang, X. Liu, C. Mao, X. Han, H. Gong, Z. Yang and J. Sun, Adv. Energy Mater., 2022, 12, 2103888.
20. M. Han, S. Zhang, Y. Cao, C. Han, X. Li, Y. Zhang, Z. Yang and J. Sun, J. Energy Chem., 2022, 70, 276–282.
21. Q. He, J. Ning, H. Chen, Z. Jiang, J. Wang, D. Chen, C. Zhao, Z. Liu, I. F. Perepichka, H. Meng and W. Huang, Chem. Soc. Rev., 2024, 53, 7091–7157.
22. G. T. Pace, M. L. Le, R. J. Clément and R. A. Segalman, ACS Energy Lett., 2023, 8, 2781–2788.
23. W. Xiao, Q. Sun, M. N. Banis, B. Wang, W. Li, M. Li, A. Lushington, R. Li, X. Li, T. K. Sham and X. Sun, Adv. Funct. Mater., 2020, 30, 2000060.
24. J. S. Kim, W. Choi, K. Y. Cho, D. Byun, J. Lim and J. K. Lee, J. Power Sources, 2013, 244, 521–526.
25. J. T. Li, Z. Y. Wu, Y. Q. Lu, Y. Zhou, Q. S. Huang, L. Huang and S. G. Sun, Adv. Energy Mater., 2017, 7, 1701185.
26. X. Liang, X. Wang, G. Li, Q. Xiang, C. Zhou, A. Chen, X. Li, S. Zhang, Y. Cao, M. Han, C. Han, H. Gong, H. Wang, Y. Zhang and J. Sun, J. Power Sources, 2022, 541, 231686.
27. D. Nugroho, W.-C. Oh, S. Chanthai and R. Benchawattananon, Nanomaterials, 2022, 12, 3277.
28. Z. Li, X. Li, Y. Yang, Q. Li, J. Gong, X. Liu, B. Liu, G. Zheng and S. Zhang, Int. J. Biol. Macromol., 2024, 261, 129929.
29. J. Liu, D. G. D. Galpaya, L. Yan, M. Sun, Z. Lin, C. Yan, C. Liang and S. Zhang, Energy Environ. Sci., 2017, 10, 750–755.
30. Y. Yang, R. Wang, X. Ai, D. Liu, C. Niu and T. Li, Int. J. Biol. Macromol., 2024, 257, 128343.
31. L. Hu, X. Zhang, P. Zhao, H. Fan, Z. Zhang, J. Deng, G. Ungar and J. Song, Adv. Mater., 2021, 33, 2104416.
32. Y. Zhang, H. Tao, J. Li and X. Yang, Ionics, 2020, 26, 3377–3385.
33. W. C. Oliver and G. M. Pharr, J. Mater. Res., 2011, 7, 1564–1583.
34. P. Saxena and P. Shukla, Adv. Compos. Hybrid Mater., 2021, 4, 8–26.
35. R. M. Gnanamuthu and C. W. Lee, Mater. Chem. Phys., 2011, 130, 831–834.
36. X. Li, G. Chen, Z. Le, X. Li, P. Nie, X. Liu, P. Xu, H. B. Wu, Z. Liu and Y. Lu, Nano Energy, 2019, 59, 464–471.
37. Z. Yu, J. Song, M. L. Gordin, R. Yi, D. Tang and D. Wang, Adv. Sci., 2015, 2, 1400020.
38. M. Han, X. Liang, C. Han, C. Zhou, Q. Xiang, Y. Cao and J. Sun, Adv. Funct. Mater., 2023, 34, 2308325.
39. H. Liu, Y. Zou, L. Tao, Z. Ma, D. Liu, P. Zhou, H. Liu and S. Wang, Small, 2017, 13, 1700758.
40. J. Zhou, Z. Jiang, S. Niu, S. Zhu, J. Zhou, Y. Zhu, J. Liang, D. Han, K. Xu, L. Zhu, X. Liu, G. Wang and Y. Qian, Chem, 2018, 4, 372–385.
41. L. Sun, Y. Zhang, D. Zhang, J. Liu and Y. Zhang, Nano Res., 2018, 11, 2733–2745.
42. H. Liu, S. Zhang, Q. Zhu, B. Cao, P. Zhang, N. Sun, B. Xu, F. Wu and R. Chen, J. Mater. Chem. A, 2019, 7, 11205–11213.
43. T. Wang, F. Cheng, N. Zhang, W. Tian, J. Zhou, R. Zhang, J. Cao, M. Luo, N. Li, L. Jiang, D. Li, Y. Li, K. Liang, H. Liu, P. Chen and B. Kong, Adv. Eng. Mater., 2021, 23, 2001507.
44. Z.-l Wang, J.-h Jiang, J.-h Lu, A.-b Wang, Z.-q Jin and W.-k Wang, J. Cent. South Univ., 2022, 28, 3681–3693.
45. J. Luo, Q. Huang, D. Shi, Y. Qiu, X. Zheng, S. Yang, B. Li, J. Weng, M. Wu, Z. Liu, Y. Yu and C. Yang, Adv. Funct. Mater., 2024, 2403021.
46. Y.-H. Huang, F.-M. Wang, T.-T. Huang, J.-M. Chen, B.-J. Hwang and J. Rick, Int. J. Electrochem. Sci., 2012, 7, 1205–1213.
47. E. Deiss, Electrochim. Acta, 2005, 50, 2927–2932.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh01615g

---