Zinc chemistry regulated by chitosan-based poly(aprotic/protic ionic liquid)s with multi-anion–cation interactions for highly reversible Zn-ion batteries

Abstract

Aqueous Zn-ion batteries (AZIBs) provide an enticing option for energy storage with cost-effectiveness, integral safety, and environmental benignity. However, the reversibility and lifespan of AZIBs are constrained by the uncontrolled Zn chemistry occurring both within the bulk electrolyte and at the electrode/electrolyte interface. Herein, taking the particular structural feature of chitosan (CS), a series of robust CS-based poly(protic ionic liquid)s (CPPILs) and poly(aprotic/protic ionic liquid)s (CPAPILs) were firstly prepared to regulate Zn chemistry through synergistic anions and cations. The betaine hydrochloride-derived CPAPILs additive (CPAPILs-B) showed a better ability to reorganize the Zn²⁺ solvation structure and enhance ion transport via the carboxylate and chloride anions, while the protonated amine and quaternary ammonium cations in CPAPILs-B are effectively anchored to the Zn anode, providing ample zincophilic sites and a uniform electric field. Consequently, the CPAPILs-B with multi-anion–cation interactions endows Zn//Zn symmetrical cells with long-term cycling durability of 5925 h at 1 mA cm⁻²/0.5 mA h cm⁻² and an ultra-high cumulative plating capacity (CPC) exceeding 7550 mA h cm⁻² at 10 mA cm⁻²/1 mA h cm⁻². This study introduces an eco-friendly, sustainable CPAPILs-B additive and highlights the innovative molecular design and multi-anion–cation synergy as a promising strategy for developing green additives to enhance the reversibility of AZIBs.

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Broader context

Aqueous Zn-ion batteries (AZIBs), as a promising post-lithium battery technology, provide attractive alternatives in new-generation rechargeable batteries due to their easy accessibility, intrinsic safety, and environmental compatibility. However, the reversibility and lifespan of AZIBs are constrained by the uncontrolled Zn chemistry occurring both within the bulk electrolyte and at the electrode/electrolyte interface. Among various strategies, electrolyte additive engineering is the most feasible, economical, and practical method to regulate the electrolyte microenvironment, but most additives are unsuitable for sustainable development and environmental friendliness. Herein, harnessing the simple Brønsted acid–base neutralization reaction, a betaine hydrochloride-derived chitosan-based poly(aprotic/protic ionic liquid)s (CPAPILs-B) multifunctional additive was firstly prepared to regulate Zn chemistry through synergistic multi-anion–cation interactions among the protonated amine, carboxylate groups, quaternary ammonium cations, and chloride ions. As a result, the well-designed CPAPILs-B additive strengthens both hydrogen bonding and interface chemistry, showcasing exceptional performance for AZIBs, including excellent reversibility, superior longevity, and ultra-high cumulative plating capacity. This study not only reveals unique insights into the mechanism of multi-anion–cation interactions but also proposes novel strategies to design efficient bio-based polymer additives, which inject new vitality into the sustainable development of AZIBs.

Introduction

Developing safe, reliable, and cost-effective battery technologies is urgently in demand to efficiently integrate renewable energy, such as solar, wind, and geothermal sources.^1,2 Aqueous Zn-ion batteries (AZIBs), as a promising post-lithium battery technology, are extremely attractive in new-generation rechargeable batteries due to their intrinsic safety, high theoretical capacity (820 mA h g⁻¹ and 5855 mA h cm⁻³), and low redox potential (−0.762 V vs. SHE).^3–5 However, the commercial applications of AZIBs are still hindered by undesirable bottlenecks (excessive dendrite growth, harmful side reactions, etc.), which inevitably facilitate poor reversibility and short circuits of batteries.^6–8 To tackle these pressing issues, numerous intriguing strategies have been conducted, including electrode structural modification,^9–11 separator functional alteration,^12,13 and electrolyte composition design.^14,15 Among these strategies, electrolyte additive engineering stands out as the most viable, economical and practical technique to regulate the electrolyte microenvironment, thereby enhancing battery reversibility and ensuring remarkable electrochemical performance.^16,17

In general, Zn chemistry regulation through additive optimization involves two key aspects: (1) hydrogen bond chemistry within the bulk electrolyte, where transport of H⁺ and OH⁻ is interrupted by substituting highly reducing water in the solvation shell or reorganizing the hydrogen bond network, thereby suppressing side reactions.¹⁸ (2) Interface chemistry at the electrode/electrolyte interface, where additives promote homogeneous Zn deposition and accelerate Zn²⁺ desolvation, forming a protective layer that improves Zn cycling stability.¹⁹ To achieve these effects, ionic liquids (ILs), which can be classified as either aprotic ionic liquids (AILs) or protic ionic liquids (PILs), have proven effective.²⁰ Their structural diversity enables stabilization at the electrode/electrolyte interface, dendrite inhibition, and improved electrochemical performance.^21–23 However, high fluid resistance, pronounced cost implications, and complex synthesis processes of ILs limit their potential for use in AZIBs.

Poly(ionic liquid)s (PILs) integrate IL moieties within a polymer backbone via covalent bonds, thus offering greater thermal stability, high dielectric constant, and tunable chemical diversity.²⁴ Nevertheless, they often require intricate monomer synthesis and polymerization processes and rely on nonrenewable petroleum-based materials.²⁵ In contrast, chitosan (CS), derived from marine sources, offers sustainability and rich functional groups (–NH₂, –OH, –C–O–C–),²⁶ making it highly suitable for developing bio-based, environmentally friendly electrolyte additives. Despite being water-insoluble, CS can be dissolved through protonation of its amino groups by Brønsted acids, like acetic acid (Ac) and L-lactic acid (LA), yielding CS-based poly(protic ionic liquid)s (CPPILs), which simplifies the design of polymeric additives for AZIBs. To further optimize the molecular design of CPPILs, betaine hydrochloride (BHC), a bio-based carboxylic acid quaternary ammonium salt with distinct ionic properties, is employed to obtain CS-based poly(aprotic/protic ionic liquid)s (CPAPILs).²⁷ The hydrochloride form of betaine not only serves as a carboxylic acid AILs but also acts as a unique solvent for CS, facilitating the direct dissolution of CS in water through a simple Brønsted acid–base reaction and avoiding complex chemical modifications.²⁸

In this study, we developed a series of CPPILs and CPAPILs as ZnSO₄ (ZSO) electrolyte additives, utilizing Ac, LA, and BHC. According to the experimental and simulation results, the BHC-derived CPAPILs (CPAPILS-B) additive with multi-anion–cation interactions has been proven to have more outstanding electrochemical performance. The presence of protonated amine, carboxylate groups, quaternary ammonium cations, and chloride ions is central to the additive's function (Scheme 1). Specifically, the protonated amino groups on CS interact strongly with chloride ions, forming a robust coordination network. Meanwhile, the quaternary ammonium and carboxylate functional groups in BHC enhance Zn²⁺ binding, creating a stabilized microenvironment that mitigates dendrite formation and side reactions. These interactions provide a synergistic effect, strengthening both hydrogen bonds and interface chemistry within the electrolyte. The resulting CPAPILs-B additive displays the best adsorption capability towards the Zn anode due to this multi-ionic network, where protonated amine groups on CS, along with the quaternary ammonium and carboxylate of BHC, coordinate with Zn²⁺ to promote homogeneous Zn deposition. Additionally, the chloride ions in BHC interact with Zn²⁺ to further regulate the solvation sheath and Zn deposition kinetics, thus extending the cell lifespan and improving reversibility. Benefiting from the synergetic effects of multi-anion–cation interactions, the Zn//Zn cell utilizing the CPAPILs-B/ZSO electrolyte achieves an ultralong cycle lifespan of 5925 h at 1 mA cm⁻²/0.5 mA h cm⁻². Moreover, the assembled Zn//Cu cell displays high reversibility with a coulombic efficiency (CE) of 99.8% over 4272 cycles at 1 mA cm⁻²/0.5 mA h cm⁻². Notably, the optimized electrolyte also enabled the Zn//MnO₂ full cell to realize superior capacity retention of 87.3% after 1000 cycles at 1 A g⁻¹. This study not only reveals unique insights to elucidate the mechanism of multi-anion–cation interactions but also proposes novel strategies to design efficient bio-based polymer additives, which inject new vitality into the sustainable development of AZIBs.

Scheme 1 Schematic diagram of the multi-anion–cation interactions mechanism in AZIBs.

Results and discussion

Additives with rich polar functional groups impact the chemical environment and anion–cation interactions in bulk electrolyte and at the electrode/electrolyte interface, thus modulating the electrochemical performance of batteries.²⁹ To better understand the effect of additive structure on hydrogen bond chemistry in dilute electrolytes, three CS-based PILs (CPILs) were facilely prepared by the dissolution of CS in acetic acid (Ac), L-lactic acid (LA) and betaine hydrochloride (BHC) aqueous solution via protonation of its amino groups, which were denoted as CPPILs-A, CPPILs-L, and CPAPILs-B, respectively (Fig. 1a and Fig. S1, ESI†).^30,31 In particular, CPAPILs-B features multi-anion–cation interactions. Fourier transform infrared spectroscopy (FTIR) was first conducted to verify the structure of the three CPILs for structural characterization. The FTIR spectra of CS showcase a broad band spanning from 3600 to 3000 cm⁻¹, which is ascribed to the stretching vibration of –OH and –NH₂, and the peak that corresponds to the deformation mode of the amino is observed at 1598 cm⁻¹ (Fig. S2, ESI†).³² By comparing the band changes between the raw materials and the products, the new bands nearly at 1516 cm⁻¹ in three CPILs correspond to the amide II and the protonated amines, confirming the successful synthesis of the three CPILs.³³

Fig. 1 Physicochemical properties of ZSO and CPIL-optimized electrolytes. (a) Schematic illustration of fabricating the CPILs and the effective mechanism for long-lifespan AZIBs. (b) The cycle lifespan of Zn//Cu cells using different electrolytes at 1 mA cm⁻². (c) Zeta potential, (d) contact angle, and (e) Zn²⁺ transference number of ZSO, CPAPILs-B/ZSO, CPPILs-L/ZSO, and CPPILs-A/ZSO electrolytes. (f) ¹H NMR spectra of different electrolytes. (g) Raman spectrum of the –OH band. (h) Summary of the proportion of hydrogen bonds with CPIL electrolytes. (i) ESP distribution of CPAPILs-B. (j) HOMO and LUMO energy levels of H₂O and different anions. (k) Binding energy of Zn²⁺–H₂O, Zn²⁺–anions, H₂O–H₂O, and CS cation–H₂O.

The concentration of the additive is strictly controlled to be below 3.2 g L⁻¹, as exceeding this threshold can alter the dispersion state of the electrolyte, leading to an unstable micro-environment (Fig. S3, ESI†). The Zn//Cu half cells using ZSO electrolyte showed a poor cycle lifespan (154 h) while using CPIL electrolytes with a concentration of 1.6 g L⁻¹ manifested excellent cycle lifespans at 1 mA cm⁻², which were 4272 (CPAPILs-B/ZSO), 3900 (CPPILs-L/ZSO) and 2746 h (CPPILs-A/ZSO), respectively (Fig. 1b). These optimal CPIL electrolytes are much better than those assembled with ZSO and other concentrations of CPIL electrolytes and present a much higher initial CE (over 85%) than ZSO electrolytes (63.2%) (Fig. S4–S6, ESI†). The remarkable and highly reversible Zn//Cu cells (even over 4000 cycles) indicate that the CPAPILs-B with multi-anion–cation interactions can enhance the electrochemical performance. Compared with different molecular designs, CPPILs-L containing hydroxyl groups presents a stronger hydrogen bond reconstruction ability than CPPILs-A, which gives the battery better cycle durability. The CPAPILs-B integrating a quaternary ammonium salt structure not only has hydrogen bond donors and acceptors but also has a strong adsorption capacity with Zn metal anodes, presenting the best cyclic reversibility in the Zn²⁺ plating/stripping process.

The zeta potential of electrolytes increases from −13.2 (ZSO) to 4.8 (CPAPILs-B/ZSO), −0.9 (CPPILs-L/ZSO), and −7.8 mV (CPPILs-A/ZSO), indicating that the disturbance of charge-rich CPILs to ZSO electrolyte will change the electric field and affect the Zn²⁺ ion transport process (Fig. 1c and Table S2, ESI†). The addition of CPAPILs-B, CPPILs-L, and CPPILs-A causes only a minor reduction in the pH of the ZSO electrolyte from 4.3 to 3.8, 3.8, and 4.1, respectively, which can be attributed to the carboxyl group (Fig. S7, ESI†). The ionic conductivity of the electrolyte exhibited a slight decrease but remained within the same order of magnitude, indicating that the impact of additives on the conductivity is negligible (Fig. S8 and Table S3, ESI†). Furthermore, the decrease in contact angle indicates enhanced interfacial compatibility at the electrode/electrolyte interface, which leads to a diminished energy barrier for Zn²⁺ ion interfacial transport kinetics (Fig. 1d).³⁴ Meanwhile, the Zn²⁺ transference number (t_Zn²⁺) in the CPAPILs-B/ZSO electrolyte system is 0.45, which is markedly higher compared to ZSO (0.26), CPPILs-L/ZSO (0.42), and CPPILs-A/ZSO (0.36) electrolytes, respectively (Fig. 1e and Fig. S9–S12, ESI†). The steady increase in t_Zn²⁺ confirms the positive effect of not only the addition of CPILs but also the designed multi-anion–cation interactions in accelerating Zn²⁺ ion transport and promoting uniform Zn deposition. Nuclear magnetic resonance (NMR) results demonstrate that when CPILs are incorporated into the ZSO electrolyte, the ¹H peaks shift from 4.76 to 4.61 ppm, reflecting that CPILs can break the interaction of Zn²⁺ and H₂O to reduce solvated water content (Fig. 1f).³⁵ Within the Raman spectrum, the range of 3000 to 3800 cm⁻¹ often displays three distinct peaks related to the O–H stretching vibration, representing strong, medium, and weak hydrogen bonds, respectively (Fig. 1g, h and Fig. S13, ESI†).³⁶ Compared to the ZSO electrolyte, the addition of the three CPILs increase the proportion of strong hydrogen bonds while decreasing the proportion of medium and weak hydrogen bonds, with the extent of influence following the order: CPAPILs-B > CPPILs-L > CPPILs-A. This phenomenon arises because the hydrophilic groups (such as the quaternary ammonium salt structure in CPAPILs-B and the hydroxyl groups in CPPILs-L) engage in stronger hydrogen bonding interactions with H₂O to stabilize water clusters in bulk electrolytes.³⁷

Density functional theory (DFT) was employed to analyze the interactions between CPILs, Zn²⁺ ions and H₂O molecules.^38,39 The minimum electrostatic potentials with CPAPILs-B, CPPILs-L, and CPPILs-A are −3.39, −2.07, and −1.99 eV, respectively, which exceed H₂O (−1.61 eV), indicating the vital influence of CPILs on the reorganization of the Zn²⁺ solvation shell (Fig. 1i and Fig. S14, ESI†). As per molecular orbital theory, the slight discrepancies between the unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels indicate the electron transfer capability.⁴⁰ The H₂O molecule has a larger band gap (12.8 eV) compared to the three CPILs: 5.2 eV for CPAPILs-B, 8.4 eV for CPPILs-L, and 8.9 eV for CPPILs-A, indicating strong adsorption of these CPIL molecules on the Zn anode surface, thereby facilitating the establishment of a water-deficient inner Helmholtz plane (IHP) (Fig. 1j). Besides, the binding energy of various components is calculated to be −4.4 eV (Zn²⁺–H₂O), −16.5 eV (Zn²⁺–SO₄²⁻), −16.9 eV (Zn²⁺–Cl⁻), −23.2 eV (Zn²⁺–BHC⁻), −17.2 eV (Zn²⁺–LA⁻), and −16.2 eV (Zn²⁺–Ac⁻), respectively (Fig. 1k). It can be deduced that with the enhancement of the interaction between Zn²⁺ ions and the electronegative groups in three CPIL additives, the participation of anions in the Zn²⁺ solvated structure is dominant. Furthermore, the binding energy of CS⁺–H₂O (−0.7 eV) is lower than that of H₂O–H₂O (−0.2 eV), indicating a reconstruction of the internal H₂O–H₂O hydrogen bond network and an increase in the amount of free H₂O bonded to the protonated amine groups in CS.

Adverse side reactions with thermodynamic advantages can significantly affect the lifetime of AZIBs, resulting in the generation of insulating by-products (4Zn²⁺ + SO₄²⁻ + 6OH⁻ + xH₂O → Zn₄SO₄(OH)₆·xH₂O) and exacerbating the hydrogen evolution reaction (HER) (2H⁺ + 2e⁻ → H₂).^41,42 Thus, the role of additives in inhibiting the HER, preventing interface corrosion, and reducing side reactions is an important indicator that determines the lifespan of the battery. Linear sweep voltammetry (LSV) shows that the potential observed in the CPAPILs-B/ZSO electrolyte is significantly lower than those in other electrolytes due to the optimization of the Zn²⁺ solvated structure, thus demonstrating the efficient inhibition of hydrogen evolution (Fig. 2a).⁴³ Tafel plots demonstrate that the corrosion currents (j₀) vary greatly in different electrolytes, ranging from 1.68 (pure ZSO electrolyte) to 0.60 (CPPILs-A/ZSO), 0.55 (CPPILs-L/ZSO) and 0.53 mA cm⁻² (CPAPILs-B/ZSO), certifying that CPILs endow the electrolyte with the capability to restrain the corrosion (Fig. 2b).⁴⁴ Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) unveil surface roughness values of 10.9 (ZSO), 0.5 (CPAPILs-B/ZSO), 0.9 (CPPILs-L/ZSO), and 3.3 μm (CPPILs-A/ZSO) following the immersion of Zn foil in different electrolytes at room temperature for a duration of 15 days (Fig. S15–S19, ESI†). The Zn foil surface in three CPIL-containing electrolytes displays a smooth and compact morphology but appears rough with prominent Zn dendrites and numerous by-products in ZSO electrolyte, which further confirms the distinguished performance of CPIL additives in reducing adverse concomitant reactions.

Fig. 2 Interface characterization of Zn anodes. (a) LSV curves, (b) Tafel plots, and (c) chronoamperometric curves of Zn//Zn cells in ZSO, CPAPILs-B/ZSO, CPPILs-L/ZSO, and CPPILs-A/ZSO electrolytes. (d) Initial Zn²⁺ nucleation overpotential and (e) EDL capacity at the Zn anode surface in different electrolytes. (f) Arrhenius behavior of temperature-dependent resistances of Zn metal anodes in different electrolytes. (g) In situ optical microscopy, (h) SEM and (i) AFM images of the Zn deposition process under 10 mA cm⁻².

Chronoamperometry (CA) results display a distinctive 2D diffusion pattern of Zn²⁺ in the ZSO electrolyte, accompanied by an increased current density, which corresponds to the arbitrary nucleation of Zn²⁺ and the formation of sharply growing dendrites (Fig. 2c). Conversely, in CPIL electrolytes (especially CPAPILs-B/ZSO), the current density manifests a consistent 3D diffusion pattern after a short nucleation stage, implying that H₂O-poor IHP can notably modify the diffusion mode of Zn²⁺ ions.⁴⁵ The nucleation overpotential (NOP) in the ZSO electrolyte (112.80 mV) is notably larger than those in the CPAPILs-B/ZSO (85.42 mV), CPPILs-L/ZSO (90.25 mV), and CPPILs-A/ZSO (96.15 mV) electrolytes, respectively (Fig. 2d). The decrease in nucleation energy barriers results from the adsorption of CPILs at the interface, which enlarges the size of the crystal nucleus and facilitates the uniform Zn deposition.⁴⁶ By analyzing the cyclic voltammetry (CV) curves of Zn//Zn symmetrical cells at various scan rates, the electric double layer (EDL) capacitances can be obtained, which are 304 (ZSO), 189 (CPAPILs-B/ZSO), 203 (CPPILs-L/ZSO), and 212 μF cm⁻² (CPPILs-A/ZSO), respectively (Fig. 2e and Fig. S20–S23, ESI†). The reduction in EDL capacitance can be attributed to the electrostatic repulsion of CPILs, which diminishes Zn²⁺ adsorption and, consequently, reduces the charge storage capacity.⁴⁷ In addition, the change in interface impedance over time serves as a metric for assessing the chemical stability of the electrode/electrolyte interface in evaluating the sustained performance of the battery.⁴⁸ In light of the changes in the electrochemical impedance spectroscopy (EIS) curve, the activation energy (E_a) was determined using the Arrhenius equation: 1/R_ct = A exp(−E_a/RT), which serves as an indicator for evaluating the difficulty of Zn²⁺ desolvation (Fig. 2f and Fig. S24, ESI†).⁴⁹ The E_a value of the CPAPILs-B/ZSO electrolyte (28.87 kJ mol⁻¹) is lower than those of ZSO (35.21 kJ mol⁻¹), CPPILs-L/ZSO (29.16 kJ mol⁻¹) and CPPILs-A/ZSO electrolytes (30.64 kJ mol⁻¹), indicating that the addition of CPAPILs-B makes the desolvation process of Zn²⁺ ions easier and enhances the Zn²⁺ ion transfer kinetics. Given the preceding analysis, the battery using the CPAPILs-B/ZSO electrolyte has the best basic electrochemical performance, which is credited to the synergistic regulation of hydrogen bond chemistry and interface chemistry by the multi-anion–cation interactions of protonated CS molecules and the quaternary ammonium salt structure. To further monitor the evolving morphologies of the Zn anode surface throughout the electrochemical process, an in situ optical microscope was employed to conduct a 30-minute plating test at 10 mA cm⁻² (Fig. 2g).⁵⁰ In the ZSO electrolyte, bubbles are emitted immediately, and dendrite clusters continue to grow disorderly over time, while in the CPAPILs-B/ZSO electrolyte, the deposition process is uniform and stable. Then, SEM images exhibit the randomly distributed protrusions caused by Zn²⁺ aggregation in the ZSO electrolyte, whereas the Zn anode surface maintains a smooth and dense texture in the CPAPILs-B/ZSO electrolyte (Fig. 2h). Moreover, the energy dispersive spectroscopy (EDS) spectra of the Zn anode surface in the case of the CPAPILs-B/ZSO electrolyte displays a uniform distribution of Zn, N, C, S, and O elements, which is due to the N-rich CPAPILs-B being anchored on the Zn anode, making the electric field uniform and forming a stable and dense interface (Fig. S25 and S26, ESI†). The 3D and planar AFM images offer further confirmation that optimizing the electrolyte with CPAPILs-B effectively mitigates dendrite growth and interface corrosion, while also reducing the generation of by-products (Fig. 2i and Fig. S27, ESI†). The in situ EIS curves of Zn//Zn symmetrical cells in ZSO and CPAPILs-B/ZSO electrolytes were obtained by a continuous charge–discharge test model to evaluate the interface chemistry at the electrode/electrolyte interface during prolonged Zn deposition (Fig. S28, ESI†).⁵¹ It was observed that in the ZSO electrolyte, the transfer impedance of Zn//Zn cells increases with successive plating/stripping cycles, while in the CPAPILs-B/ZSO electrolyte, the impedance change gradually tends to be constant. The X-ray photoelectron spectroscope (XPS) coupled with Ar⁺ sputtering was utilized to analyze the surface and in-depth composition of the Zn anode after 50 cycles at 2 mA cm⁻²/1 mA h cm⁻² (Fig. S29, ESI†). Before Ar⁺ sputtering, the N 1s spectrum of the Zn anode surface in the CPAPILs-B/ZSO electrolyte displays signals corresponding to C–N/NH₃⁺ (400.5 eV) and Zn–N (399.5 eV), while the C 1s spectrum exhibits peaks for C=O (289.3 eV) and C–O/C–N (286.9 eV).⁵² As the Ar⁺ sputtering process continues, the signals of these elements gradually diminish or disappear from the spectrum. The results further indicate that CPAPILs-B is only adsorbed onto the Zn anode surface and forms a nitrogen-rich interface protective layer, thereby mitigating side reactions and reducing the risk of cell short-circuiting due to Zn dendrite growth.

To evaluate the effect of synergistic regulation of multi-anion–cation interactions on enhancing the reversibility of battery plating/stripping processes, Zn//Cu asymmetric cells were constructed. The failure occurred in the Zn//Cu cell with ZSO electrolyte after 158 cycles due to rapid voltage fluctuations, whereas incorporating CPAPILs-B markedly improved stability and reversibility, extending the cell's lifespan to 2100 cycles (Fig. 3a). Moreover, the CV curve of the Zn//Cu cell with CPAPILs-B/ZSO electrolyte exhibits lower Zn nucleation overpotential and larger integrated peak areas, which can be attributed to the fact that CPAPILs-B can regulate the Zn²⁺ solvation structure and adsorb onto the Zn anode surface, resulting in enhanced Zn deposition kinetics and improved interfacial activity (Fig. S30, ESI†).⁵³ Additionally, compared to the ZSO electrolyte, the rate performance shows that the Zn//Zn cell using CPAPILs-B/ZSO electrolyte exhibits lower polarization and higher stability after 3 cycles (Fig. 3b). Depth of discharge (DOD) was determined under conditions of 1.7% DOD_Zn, 3.4% DOD_Zn, 8.6% DOD_Zn, and 17.1% DOD_Zn to evaluate Zn anode utilization.⁴⁷ The Zn//Zn cell with ZSO electrolyte short-circuits at a lower current density, whereas the addition of CPAPILs-B to the ZSO electrolyte greatly improves Zn anode utilization, leading to stable cycling performance (Fig. S31, ESI†). The exceptionally long cycle life surpassing 5925 h (246 days) is evidenced by the Zn//Zn cell using the CPAPILs-B/ZSO electrolyte at 1 mA cm⁻²/0.5 mA h cm⁻², markedly outperforming the ZSO electrolyte (Fig. 3c). Furthermore, the Zn//Zn cell also exhibits stable cycle performance in CPPILs-L/ZSO (3840 h) and CPPILs-A/ZSO (2366 h) electrolytes, aligning with the assessment of physicochemical and electrochemical properties discussed above (Fig. S32, ESI†). Besides, the Zn//Zn cell with ZSO electrolyte fails within 180 h, exhibiting a high polarization of 400 mV under 10 mA cm⁻²/1 mA h cm⁻². In contrast, the battery using CPAPILs-B/ZSO electrolyte demonstrates an extended cycle life of 1510 h and an unprecedented cumulative plating capacity (CPC) of up to 7550 mA h cm⁻² (Fig. 3d). Even under the severe condition (17.1% DOD_Zn), the Zn//Zn cell using the CPAPILs-B electrolyte can still function reliably for 578 h at 10 mA cm⁻² (Fig. S33, ESI†). Moreover, a shelving-recovery test (stand 8 h after every 10 cycles) was performed to simulate real conditions, and the cell with CPAPILs-B/ZSO electrolyte shows outstanding stability, lasting over 1570 h at 5 mA cm⁻²/1 mA h cm⁻², far exceeding the 150 h of the ZSO electrolyte (Fig. 3e).

Fig. 3 Cyclic stability of Zn plating/stripping in ZSO and CPAPILs-B/ZSO electrolytes. (a) CE of the Zn//Cu cells at 5 mA cm⁻²/1 mA h cm⁻². (b) The rate capability of Zn//Zn cells in ZSO and CPAPILs-B/ZSO electrolytes. Long-term galvanostatic charging/discharging at (c) 1 mA cm⁻²/0.5 mA h cm⁻² and (d) 10 mA cm⁻²/1 mA h cm⁻², respectively. (e) Shelving-recovery performance of Zn//Zn cells in ZSO and CPAPILs-B/ZSO electrolytes under 5 mA cm⁻²/1 mA h cm⁻². (f) and (g) XRD patterns of Zn foil after 50 cycles at 2 mA cm⁻²/1 mA h cm⁻² with ZSO and CPAPILs-B/ZSO electrolytes. (h) Comparison of cyclic reversibility and CPC with state-of-the-art electrolyte additives reported recently.

Typically, the deterioration of battery efficiency is mainly ascribed to adverse HER and the accumulation of irreversible residual products (3Zn + ZnSO₄ + 11H₂O → Zn₄SO₄(OH)₆·5H₂O + 3H₂) driven by the Domino effect.⁵⁴ The product of the Zn anode after cycling was analyzed through X-ray diffraction (XRD), providing an in-depth understanding of the effect of the addition of CPAPILs-B on the formation of the Zn deposition crystal plane and insulation by-products (Fig. 3f and g). After 50 cycles at 2 mA cm⁻²/1 mA h cm⁻², the XRD pattern of the Zn anode shows a peak corresponding to Zn₄SO₄(OH)₆·5H₂O, indicating that the Zn anode is severely corroded in the ZSO electrolyte, which would significantly reduce the CE of the cell. Conversely, in the CPAPILs-B/ZSO electrolyte, the XRD pattern exclusively exhibits the characteristic peaks of metallic Zn without any additional peaks. Besides, the diffraction signals at 36.2°, 38.9°, and 43.1° are attributed to the Zn (002), (100) and (101) crystal planes, respectively. The I₍₀₀₂₎/I₍₁₀₀₎ ratios of Zn foil cycled in the ZSO and CPAPILs-B/ZSO electrolytes are 2.6 and 4.1, demonstrating a preference for the exposure of the (002) basal planes in the CPAPILs-B/ZSO electrolyte. This enhancement is attributed to protonated amine and quaternary ammonium cations in CPAPILs-B, which create an electrostatic shielding layer that evens out the Zn²⁺ concentration field and electric field, resulting in Zn (002)-preferential orientation and effectively inhibiting the formation of side reactions.^55,56 In summary, compared with different types of additives, the Zn//Zn cell with CPAPILs-B shows excellent long-term stability, exceeding most of the reported electrolyte additives, achieving unparalleled CPC performance (Fig. 3h and Tables S4–S7, ESI†). These conclusions underscore the critical importance of employing CPAPILs-B to ensure stable and controlled Zn chemistry, highlighting the positive influence of coordinated multi-anion–cation regulation on electrochemical energy storage devices.

Computational simulations were subsequently performed to capture the molecular motion and Zn²⁺ solvated structure in the bulk phase and to examine the variations in the electric field and Zn²⁺ ion concentration field during the dynamic growth of Zn dendrites at the electrode/electrolyte interface. Based on ab initio molecular dynamics (MD) simulations, the composition and distribution of the primary solvation sheath (PSS), as well as the coordination number (CN) and radial distribution functions (RDFs) were obtained from the calculation results (Fig. 4a and b). In the ZSO electrolyte, the Zn–O distance peak at approximately 1.94 Å away from Zn²⁺ is ascribed to the presence of H₂O molecules within the PSS. Additionally, a sharp peak around 1.84 Å in the Zn–O distance corresponds to the SO₄²⁻ ions present in the Zn²⁺ solvated structure. Similarly, in the CPAPILs-B/ZSO electrolyte, peaks associated with Zn–O originating from SO₄²⁻ and H₂O still manifest at the same distance. The Zn–O peak derived from BHC⁻ emerges at 1.82 Å from Zn²⁺, indicating the inclusion of BHC⁻ molecules into the PSS. Additionally, the CN of Zn–O (H₂O) in the PSS decreases from 5.6 in pure ZSO electrolyte to 5.2 in CPAPILs-B/ZSO electrolyte, implying that the addition of CPAPILs-B can regulate the CN of water molecules. In the RDFs of Zn²⁺–Cl and Zn²⁺–O for BHC⁻ identified peaks at 1.82 and 2.30 Å, respectively, with corresponding CN of 0.18 and 0.13. The total CN of the CPIL electrolytes is quite low, with CPAPILs-B/ZSO at 0.31, CPPILs-L/ZSO at 0.34, and CPPILs-A at 0.44. This indicates that only a minimal amount of CPIL molecules can engage in the Zn²⁺ solvated structure, thereby enhancing the desolvation effect without significantly impacting Zn²⁺ ion transport kinetics (Fig. S34a and S35a, ESI†). Among them, CPAPILs-B/ZSO has the lowest total CN value due to the strong electrostatic repulsion and space steric effect of quaternary ammonium cations in CPAPILs-B, and the steric hindrance effect of methyl in CPPILs-A is inferior to that of hydroxyl in CPPILs-L. Subsequently, the analysis of the RDFs and CN of Zn–O (SO₄²⁻) confirmed that the CPIL molecules have less influence on the Zn²⁺ solvated structure, but minimize the generation of alkaline sulfate by-products related to SO₄²⁻. Simulated structures of the ZSO and CPAPILs-B/ZSO electrolytes reveal three typical configurations of ion clusters containing varying quantities of Zn²⁺ (n = 1, 2, 3) (Fig. 4c and d). It was discovered that BHC⁻ is less common within these clusters and only replaces a portion of the solvated water molecules, which is consistent with prior conclusions. These unpaired anions and cations are believed to effectively interrupt the hydrogen bond network, ultimately improving electrolyte stability. Similar phenomena were also observed in two other CPIL-optimized systems, which were explained by the different steric hindrance effects of these anions (Fig. S34b and S35b, ESI†).⁵⁷ Furthermore, the substitution of a single BHC⁻ anion for an H₂O molecule in the solvation sheath results in a significant decrease in the electrostatic potential value, signaling a reduction in the repulsive forces within the newly formed solvation structure, which facilitates enhanced Zn²⁺ transport (Fig. 4e). Finally, the simulation results of different electrolyte systems quantify and analyze the evolution of the solvation structure, and clarify the influence of the multi-anion–cation interactions mechanism in the CPAPILs-B/ZSO electrolyte on the Zn deposition kinetics and interface behavior.

Fig. 4 Theoretical calculations of the synergistic effect of multi-anion–cation interactions. MD simulation snapshot, RDFs of Zn²⁺–O, and radius-dependent CN of Zn²⁺–H₂O in (a) ZSO and (b) CPAPILs-B/ZSO electrolytes. Atomic configuration of Zn²⁺ solvated structures in (c) ZSO and (d) CPAPILs-B/ZSO electrolytes. (e) Electrostatic potential mapping of the Zn²⁺–6H₂O and Zn²⁺–5H₂O–BHC⁻ solvation structures. (f) Phase-field simulation of dendrite morphology grown from 0 to 400 s on the Zn anode. 1D evolutions of the Zn²⁺ concentration profile along the x-axis across the tip of the dendrite in (g) ZSO and (h) CPAPILs-B/ZSO electrolytes. The images in the inset show the 2D map of the Zn²⁺ concentration at t = 400 s. 1D evolutions of the electric field profile along the x-axis across the tip of the dendrite in (i) ZSO and (j) CPAPILs-B/ZSO electrolytes. The images in the inset show the 2D map of the local electric field at t = 400 s.

The changes in the electric field and concentration gradient during Zn deposition were analyzed using the finite element method (FEM). Snapshots of Zn dendrite growth simulation from 0 to 400 s show that Zn dendrites grow rapidly at the tips in the ZSO electrolyte (Fig. 4f, top). In contrast, the uniform electric and Zn²⁺ concentration fields in the CPAPILs-B/ZSO electrolyte result in slower Zn dendrite growth, attributed to the advantageous effect of the adsorbed nitrogen-rich interface protective layer (Fig. 4f, bottom). The simulation of the electric field and Zn²⁺ ion concentration during Zn dendrite growth shows a uniform concentration gradient and electric field distribution, ensuring consistent Zn²⁺ nucleation and deposition (Fig. S36 and S37, ESI†). In order to elucidate these findings, one-dimensional (1D) profiles of Zn²⁺ concentration along the x-axis of the dendritic tip were constructed, as illustrated in the 2D inset diagrams (Fig. 4g and h). In the ZSO electrolyte, the Zn²⁺ ion concentration at the dendrite tip of the Zn anode surface rises rapidly, whereas with CPAPILs-B added, this concentration is effectively suppressed. After similar data processing, the electric field change (E_x) along the dendrite tip at various time points shows that the maximum E_x in the ZSO electrolyte (0.03 V μm⁻¹) is 3 times that in the CPAPILs-B/ZSO electrolyte (0.01 V μm⁻¹) (Fig. 4i and j). This disparity arises from the sharper tip morphology and larger curvature in the ZSO electrolyte, resulting in a higher local electric field near the tip, which further promotes the self-accelerating growth of dendrites. Hence, it can be inferred that the interface protective layer with a faster Zn²⁺ diffusion rate and higher Zn²⁺ transfer efficiency can notably inhibit dendrite expansion by modulating the concentration gradient and electric field.

Ultimately, to assess the superior electrochemical performance and practicability of incorporating the CPAPILs-B additive in Zn//MnO₂ cells, δ-MnO₂ nanosheets were synthesized through electrodeposition and confirmed through examination of SEM images, XRD patterns, and EDS images (Fig. S38 and S39, ESI†).⁵⁸ The CV profiles of the Zn//MnO₂ full cells reveal a reduced potential gap between the oxidation and reduction peaks in the CPAPILs-B/ZSO electrolyte compared to the ZSO electrolyte, indicating that the CPAPILs-B can participate in the redox reactions, leading to decreased electrochemical polarization and enhanced reaction kinetics (Fig. 5a).⁵⁹ Moreover, the peak area of the CPAPILs-B/ZSO electrolyte exceeds that of the ZSO electrolyte at various scan rates, indicating that CPAPILs-B both maintains redox reactions effectively and enhances ion migration and charge transfer, thereby improving redox kinetics (Fig. S40, ESI†). As expected, the Zn//MnO₂ full cell with CPAPILs-B/ZSO electrolyte shows outstanding rate performance and superior reversibility with an initial capacity of 194.5 mA h g⁻¹ from 1 to 5 A g⁻¹, which is ascribed to the synergistic effects of multi-anion–cation interactions providing continuous protection to the Zn anode (Fig. 5b). Besides, the discharge/charge behavior of the Zn//MnO₂ cells using both ZSO and CPAPILs-B/ZSO electrolytes have similar curves (Fig. 5c and Fig. S41, ESI†).

Fig. 5 Performance and practical application of Zn//MnO₂ full cells. (a) The CV curves at 0.1 mV s⁻¹ in ZSO and CPAPILs-B/ZSO electrolytes. (b) Rate capability and (c) charge–discharge curves of the cells in ZSO and CPAPILs-B/ZSO electrolytes. (d) EIS spectra of Zn//MnO₂ cells using ZSO and CPAPILs-B/ZSO electrolytes before and after 1 cycle. Self-discharge curves of Zn//MnO₂ cells with (e) ZSO and (f) CPAPILs-B/ZSO electrolytes. (g) Long-term cycling performance of the Zn//MnO₂ cells at 1 A g⁻¹. (h) Timer lighted by the Zn//MnO₂ pouch cell with CPAPILs-B/ZSO electrolytes.

In comparison to the ZSO electrolyte, the Zn//MnO₂ cell demonstrates a marginally higher charge transfer resistance (R_ct) in the CPAPILs-B/ZSO electrolyte before cycling, a phenomenon that may be ascribed to the adsorption of CPAPILs-B (Fig. 5d). After one cycle, the R_ct in the high-frequency region decreases and the gradient in the low-frequency region increases, reflecting the positive impact of the nitrogen-rich protective layer formed in the CPAPILs-B/ZSO electrolyte. During the in situ impedance test, the full cell with CPAPILs-B exhibits a stable impedance, highlighting its excellent ability to regulate the electrochemical environment (Fig. S42, ESI†). The resistance of ZSO electrolyte gradually increases, indicating that the internal electrochemical environment fluctuates violently. The self-discharge characteristics of full cells were assessed by measuring the voltage decline after 24 h of standing in a fully charged state (Fig. 5e and f). The full cell with CPAPILs-B/ZSO electrolyte retains 87.6% CE, outperforming the ZSO electrolyte (83.7%), indicating a reduction in side reactions. Additionally, the CPAPILs-B/ZSO electrolyte demonstrates improved cycling performance, with the Zn//MnO₂ full cells maintaining 87.3% capacity retention after 1000 cycles at 1 A g⁻¹, compared to just 62.2% with ZSO electrolyte before cell failure (Fig. 5g). Furthermore, the Zn//MnO₂ cell with CPAPILs-B exhibited long-term stability over 2500 cycles with a capacity retention of 94.4%, while a rapid capacity loss appeared in the pure electrolyte (Fig. S43, ESI†). To further simulate real operating conditions, the full cell was assembled with a high mass loading (4.8 mg cm⁻²) of MnO₂ as the cathode, and the corresponding N/P ratio was 10.1 (Fig. S44, ESI†).⁶⁰ Notably, the Zn//MsnO₂ full cell with CPAPILs-B/ZSO electrolyte delivers a capacity retention of 71.5% after 200 cycles at 1 A g⁻¹, significantly outperforming the ZSO electrolyte (41.8%). Moreover, the Zn//MnO₂ pouch cell with CPAPILs-B/ZSO electrolyte lights up a timer, showcasing its commercial potential for AZIBs (Fig. 5h).

Conclusions

In summary, taking a particular structural feature of chitosan (CS), a series of robust CS-based poly(ionic liquid)s (CPILs) were prepared in water via mild protonation reaction using organic carboxylic acids, which were first used to regulate Zn chemistry. Through simulation calculation and experimental verification, the fully bio-based betaine hydrochloride-derived poly(aprotic/protic ionic liquid)s (CPAPILs-B) has the best performance. It is believed that the carboxylate and chloride anions in CPAPILs-B are capable of substituting one solvated H₂O molecule, which advances the desolvation process and reconstructs the hydrogen bond network, thus improving the Zn²⁺ ion transport kinetics. Additionally, the coexisting protonated amine and quaternary ammonium cations are preferentially anchored onto the surface of the Zn anode to form a nitrogen-rich interface protective layer, which can homogenize the Zn²⁺ ion flux and electric field. As a result, the Zn//Zn symmetrical cell with CPAPILs-B/ZSO sustains superior longevity of 5925 h at 1 mA cm⁻²/0.5 mA h cm⁻² and 1510 h at 10 mA cm⁻²/1 mA h cm⁻² with lower voltage hysteresis. The Zn//Cu asymmetrical cell displays excellent plating/stripping reversibility with a high average CE of 99.8% over 2100 cycles at 5 mA cm⁻²/1 mA h cm⁻². Notably, the Zn//MnO₂ full cell using the CPAPILs-B/ZSO electrolyte demonstrates excellent cycling stability, retaining 87.3% capacity after 1000 cycles at 1 A g⁻¹. This work delivers a convenient method of a simple process to prepare bio-based multi-functional additives, provides in-depth insight into the molecular design concept, and elucidates the synergistic regulation mechanism of multi-anion–cation interactions in AZIBs.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52203083, 22275041, 21574030), the Youth Science and Technology Top notch Talents Project of Guizhou Provincial Department of Education ([2024]314), Guizhou Province Basic Research Program (Natural Science ZK [2024]078, Z2024021), Guizhou Province Science and Technology Achievement Application and Industrialization Plan (Major Project) (2023-010), the Open Project of Guizhou Research Institute for Dual Carbon and New Energy Technology Innovation and Development (DCRE-2023-15), and the Specific Natural Science Foundation of Guizhou University (X2022009). The authors extend their gratitude to the Theoretical and Computational Chemistry Team from Shiyanjia Lab (https://www.shiyanjia.com) for providing invaluable assistance. The authors also thank SCI-GO (https://www.sci-go.com) for the XRD analysis and Big Data Center for materials at Guizhou University for computational support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee05442c

‡ These authors contributed equally to this work.

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