Extended Endocyclic Conjugation and N,N‐Bidentate Chelation Alleviate the Capacity–Stability Trade‐Off in Organic Magnesium Battery Cathodes

ABSTRACT

Rechargeable Mg batteries represent an appealing post‐lithium energy‐storage technology, yet their advancement is hampered by the scarcity of cathode materials combining high capacity, rapid kinetics, and long‐term cycling stability. In this study, we propose a molecular design strategy integrating extended endocyclic conjugation with polydentate Mg²⁺ coordination. Using hexaazatriphenylene (HATN), a rigid planar macrocycle featuring extensive π‐conjugation and N,N‐bidentate chelating sites, as the Mg‐storage active center, we constructed polymer cathodes through monothioether and dithioether linkages. Theoretical and experimental analyses reveal that the HATN unit enables high‐capacity, multi‐electron reversible Mg²⁺ storage while maintaining structural stability via efficient charge buffering through strong electron delocalization, offering a notable advantage in a “capacity‒delocalization” evaluation framework. The thioether linkage suppresses dissolution and yields high surface area with hierarchical porosity, boosting interfacial kinetics and Mg²⁺ transport. The resulting polymer cathode delivers a high capacity of 370 mAh g^‒1 at 0.1 A g^‒1, superior rate capability (94 mAh g^‒1 at 5.0 A g^‒1), and exceptional cycling stability (95% capacity retention over 500 cycles at 1.0 A g^‒1). This work presents an innovative molecular‐level design strategy for high‐performance organic Mg‐battery cathodes, advances the mechanistic understanding of multivalent‐ion storage, and provides a new paradigm for rational electrode engineering for multivalent battery systems.

A rational molecular design is established by combining extended endocyclic π‐conjugation and N,N‐bidentate chelation to alleviate the capacity–stability trade‐off for organic magnesium battery cathodes. The rigid hexaazatriphenylene unit enables exceptional electron delocalization and shows clear superiority in the “capacity‒delocalization” evaluation framework. Thioether bridges effectively suppress dissolution and afford hierarchical porous structures boosting Mg²⁺ transport and interfacial kinetics.

1. Introduction

Li‐ion batteries, with their high energy density and extended cycle life, have paved the way for modern energy‐storage systems. Nonetheless, their large‐scale application faces inherent challenges posed by limited lithium reserves and persistent safety concerns. In response, rechargeable Mg batteries (RMBs) are regarded as an attractive candidate, benefiting from the advantages of metallic Mg anode including high crustal abundance (2.1%), high volumetric capacity (3832 mAh cm^‒3), and low‐tendency dendrite‐plating behavior [1, 2, 3]. With recent critical breakthroughs in RMB electrolytes [4, 5], the exploration of high‐performance cathode materials that combine high specific capacity, fast reaction kinetics, and excellent structural stability has become a central task for advancing RMB technology. A major challenge lies in the strong electrostatic interaction between Mg²⁺ cations and the host lattice in conventional inorganic cathodes, which often leads to sluggish Mg²⁺ diffusion and structural degradation, thereby limiting electrochemical performance [6, 7]. Therefore, developing tailored material design strategies that accommodate the unique properties of Mg²⁺ storage is essential for overcoming the current limitations of RMBs.

Unlike conventional inorganic cathode materials that rely on ion intercalation/deintercalation or conversion reactions, organic conjugated polymers offer an advantageous alternative for efficient Mg²⁺ storage owing to their tunable molecular structures and delocalized electron characteristics [8, 9]. Their energy‐storage mechanism typically involves reversible coordination/dissociation of Mg²⁺ at active sites, avoiding the high diffusion barriers of intercalation and structural reorganization inherent in conversion reactions, thereby enabling potentially faster reaction kinetics. Meanwhile, the conjugated structures of these polymers facilitate charge delocalization, which mitigates strong electrostatic interactions with Mg²⁺ and buffers charge density fluctuations during redox processes, thus enhancing structural stability. Besides, the structural versatility of organic materials also provides ample opportunity for precise performance optimization through molecular design. Although research on organic cathodes for RMBs remains in its early stages, recent studies have demonstrated encouraging electrochemical performance and significant potential for further development [9, 10, 11, 12, 13, 14].

The performance of organic RMB cathodes is fundamentally governed by the design of Mg‐storage‐active units, which typically consists of specific redox‐active groups embedded within a conjugated backbone. During electrochemical reactions, the redox‐active sites reversibly coordinate with Mg²⁺ cations, while the conjugated backbone delocalizes electrons to distribute charge density changes and maintain structural integrity. Consequently, the inherent properties of the redox‐active groups and their compatibility with the conjugated structure critically determine the specific capacity and cycling stability of the cathode material. To date, most studies on organic RMB cathodes have drawn inspiration from monovalent‐ion systems (e.g., Li⁺ and Na⁺), primarily focusing on conjugated carbonyl compounds such as quinones and imides [13, 14, 15, 16]. However, the O atom in carbonyl group (C═O) engages in conjugation through only a single adjacent p orbital, and its high electronegativity limits the extent of charge delocalization. To compensate for the structural instability during Mg²⁺ storage, bulky conjugated structures are often incorporated, which unfortunately dilute the proportion of active sites, leading to an inherent trade‐off between theoretical capacity and structural stability [17, 18, 19].

In this study, to address this challenge, we propose N atoms as more favorable coordination sites for Mg²⁺. Carbonyl O atoms mainly show exocyclic conjugation. By contrast, pyridinic N atoms use their p orbitals for efficient endocyclic conjugation, enabling better orbital overlap. During Mg²⁺ storage, charge is rapidly delocalized over the macrocyclic conjugated system, which greatly improves electron buffering and structural stability while maintaining capacity. The relatively lower electronegativity of nitrogen also facilitates reversible Mg²⁺ dissociation, thereby promoting reaction reversibility. Based on this concept, we selected hexaazatriphenylene (HATN), a rigid macrocycle with an extended π‐conjugated structure, as the Mg‐storage unit (Figure 1a). The HATN skeleton is inherently electron‐deficient, favoring a higher operating voltage, while its three N,N‐bidentate chelating sites perfectly match the coordination preference of Mg²⁺, theoretically enabling a six‐electron transfer per unit for high capacity, stability, and voltage simultaneously. To address the solubility issue of small‐molecule HATN, single‐chain (HATN‐3S) and double‐chain (HATN‐6S) polymers were constructed via a thioether‐bridging strategy. The thioether linkage not only enhances overall stability but also serves as a π‐electron acceptor to modulate the LUMO energy level, optimizing the redox potential. Benefiting from this multi‐level molecular engineering approach, the resulting HATN‐based polymer cathodes exhibit outstanding electrochemical performance, including a high capacity of 370 mAh g^‒1 at 0.1 A g^‒1, excellent rate capability (201 mAh g^‒1 at 1.0 A g^‒1), and remarkable long‐term cycling stability (95% capacity retention after 500 cycles). The present work provides a new design strategy combining extended endocyclic conjugation and N,N‐chelation for high‐performance RMB cathodes from the perspective of electronic structure and coordination chemistry at the molecular level, offering important insights into the storage mechanism of multivalent ions in organic materials.

FIGURE 1.

(a) Proposed six‐electron redox mechanism of HATN in RMBs. (b) Molecular electrostatic potential (MESP) of HATN. (c) Calculated ΔG for the stepwise Mg²⁺ storage of HATN. (d) Comparison of theoretical capacity and charge delocalization of HATN with other Mg‐storage‐active units. (e) Structures and MESP; (f) HOMO/LUMO energy levels and gaps; (g) LUMO orbitals of HATN‐3S and HATN‐6S.

2. Results and Discussion

The active unit plays a decisive role in determining the electrochemical properties of organic electrodes. To meet the specific requirements of divalent Mg²⁺ storage, HATN was selected as the active unit, which features a rigid planar π‐conjugated framework, electron‐deficient characteristics, and N,N‐bidentate chelation sites [20]. We first systematically evaluated its potential as a cathode material for RMBs with density functional theory (DFT) calculations. As illustrated in Figure 1a, each HATN unit can accommodate three Mg²⁺ cations, corresponding to a six‐electron redox reaction, yielding a high theoretical specific capacity of 418 mAh g^‒1. Molecular electrostatic potential (MESP) analysis (Figure 1b) revealed pronounced negative potential regions around the N atoms, with adjacent N atoms forming ideal N,N‐bidentate chelation sites that provide a favorable chemical environment for reversible Mg²⁺ coordination/dissociation. Meanwhile, the extended endocyclic π‐conjugation of HATN facilitates effective charge delocalization, which helps to buffer electron density fluctuations during Mg²⁺ storage/release, thereby enhancing structural stability. Based on the most stable coordination conformation, the Gibbs free energy change (ΔG) for the stepwise Mg‐storage reactions was further calculated. The results (Figure 1c) showed that the ΔG values for the sequential incorporation of one, two, and three Mg²⁺ cations are ‒1.79, ‒3.03, and ‒4.10 eV, respectively, confirming that the multi‐electron process is thermodynamically spontaneous and proceeds smoothly. These findings theoretically support the feasibility of HATN as a high‐capacity Mg‐storage active unit.

The storage of high‐charge‐density Mg²⁺ cations often induces significant local charge fluctuations within the active units, making enhanced charge delocalization crucial for maintaining structural stability. Compared to reported conjugated carbonyl‐based active units (e.g., quinones and imides), HATN utilizes N atoms as coordination centers, which enables more efficient orbital overlap via two adjacent p orbitals and extended endocyclic conjugation. To quantitatively evaluate this advantage, we introduced normalized multicenter bond order (nMCBO) as a key metric to describe the extent of electron delocalization. Figure 1d compares the performance of typical Mg‐storage‐active units in a two‐dimensional (2D) plot with theoretical specific capacity versus nMCBO (detailed data in Tables S1 and S2). The results revealed that conventional carbonyl‐based units are predominantly located in either the “high‐capacity‒low‐delocalization” or “low‐capacity‒high‐delocalization” regions, highlighting their inherent performance trade‐off. This trade‐off is difficult to alleviate through simple configurational adjustments (e.g., PQ vs. AQ) [21]. In sharp contrast, HATN exhibits both a high nMCBO value (0.56) and a high theoretical capacity (418 mAh g^‒1), positioning it in the upper‐right “high‐capacity‒high‐delocalization” region of the plot. This clearly demonstrated the superiority of the molecular design strategy integrating extended endocyclic conjugation and N,N‐chelation.

To address the solubility issue of small‐molecule HATN in electrolytes, we constructed polymers using monothioether (HATN‐3S) and dithioether (HATN‐6S) bridging approaches and evaluated their properties through DFT calculations. As illustrated in Figure 1e, the MESP distribution of the HATN unit remain consistent with that of the monomer after polymerization, preserving the key Mg‐storage characteristics, while the thioether bridging groups exhibit weakly positive electrostatic potentials. Although the incorporation of thioether chains results in a decrease in theoretical capacity (HATN‐3S: 375 mAh g^‒1; HATN‐6S: 339 mAh g^‒1), polymerization effectively increases molecular weight, mitigating dissolution and thereby enhancing cycling stability. HOMO/LUMO energy level analysis (Figure 1f) indicated that the thioether groups function as π‐electron acceptors, lowering the LUMO energy level of the polymers and contributing to an elevated discharge voltage. Concurrently, the band gaps of the polymers are significantly reduced (from 3.60 eV for the HATN monomer to 2.98 eV for HATN‐3S and 3.08 eV for HATN‐6S), indicating that thioether bridging enhances the overall electron delocalization to some extent. Figure 1g further demonstrated that the LUMO orbital is predominantly localized on the HATN unit, confirming the electron‐accepting and delocalization features governed by the active centers. In summary, the designed HATN‐based polymers not only retain excellent Mg‐storage sites but also possess enhanced electron delocalization and optimized energy level properties, establishing a theoretical basis for achieving high‐performance Mg²⁺ storage.

The HATN‐3S and HATN‐6S polymers were successfully synthesized following the route shown in Figure S1 and systematically characterized. Infrared (IR) spectra (Figure 2a) showed the emergence of an Ar─S characteristic peak at 1050 cm^‒1 upon thioether bridging [13], while the disappearance of the o‐phenylene absorption peak of HATN at 756 cm⁻¹ confirmed the successful formation of the bridged structure. The intensified C═C/C═N peaks within 1540–1720 cm⁻¹ [22, 23] indicated an increased dipole moment of the C═N─C structure induced by thioether linkage. The C─N vibrational peaks at 1180‒1365 cm^‒1 [24, 25] and the strong peak at 1084 cm^‒1 corresponding to the N═Q═N quinoid imine structure (where Q denotes the fused conjugated structure) [26] remained intact, suggesting the preservation of the HATN unit after polymerization. Raman spectroscopy (Figure 2b) further corroborated the structural features. The peaks at 1515 and 1390 cm^‒1 are assigned to C═C/C═N and C─C/C─N vibrations, respectively [24, 27]. The Ar─S─Ar peaks for HATN‐3S appeared at 624 and 1074 cm^‒1, while those for HATN‐6S were blueshifted to 647 and 1110 cm^‒1, reflecting stronger p‐π conjugation and a greater C─S bond force constant resulting from the dual thioether bridges. X‐ray photoelectron spectroscopy (XPS) results showed that the C 1s spectra (Figure 2c) exhibited characteristic peaks at 284.8 (C─C/C═C), 285.3 (C─S), and 286.0 eV (C─N/C═N) [24], consistent with the chemical structures of HATN‐3S and HATN‐6S. The N 1s spectra (Figure 2d) displayed peaks at 399.3 (C═N─C) and 400.6 eV (C─N─C) [28, 29, 30], with the latter ascribed to slight surface reduction of C═N─C during synthesis. The characteristic doublet in the S 2p spectra (Figure 2e) also confirmed the successful incorporation of S atoms into the polymer chains [13, 31]. Morphological characterization revealed features beneficial for RMB cathode applications. TEM images (Figure 2f) indicated that both HATN‐3S and HATN‐6S consisted of particles approximately 200 nm in size. Surface area and pore structure analysis demonstrated that both polymers possessed high specific surface areas (474.4 and 795.5 m² g^‒1 for HATN‐3S and HATN‐6S, respectively) and abundant hierarchical porosities, with average pore sizes of 2.41 and 1.94 nm. Pore size distribution (Figure S4) further confirmed the presence of micropores and mesopores. This unique morphology and hierarchical pore structure provide ideal pathways for electrolyte wetting and Mg²⁺ transport, enhancing interfacial charge transfer capability and Mg‐storage reaction kinetics when used as RMB cathodes.

FIGURE 2.

(a) IR, (b) Raman, (c) C 1s, (d) N 1s, and (e) S 2p XPS spectra of HATN, HATN‐3S, and HATN‐6S. (f) TEM images and (g) N₂ adsorption–desorption isotherms of HATN‐3S and HATN‐6S.

Then Mg cells were assembled using Mg metal anode and Mg(TFSI)₂/MgCl₂‐DME electrolyte to evaluate the electrochemical performance of the HATN‐based polymer cathodes. Open‐circuit voltage (OCV) tests (Figure S6) revealed that the small‐molecular HATN electrode exhibited a high voltage decay rate of 21.2 mV h^‒1 over 72 h, indicating significant active material dissolution in the electrolyte. In contrast, the OCV decay for HATN‐3S and HATN‐6S was almost negligible, confirming that the polymerization strategy effectively suppressed the dissolution of active material. The cyclic voltammetry (CV) curves of HATN, HATN‐3S, and HATN‐6S (Figure 3a; additional cycles in Figures S7 and S8) all exhibited multiple pairs of redox peaks, consistent with the DFT‐predicted stepwise Mg‐storage mechanism of the HATN unit (Figure 1c). Due to the π‐electron‐accepting nature of the S atoms, the redox peak potentials (O1/R1, O2/R2, and O3/R3) of HATN‐3S and HATN‐6S were slightly higher than those of HATN, a trend that aligns with the LUMO energy level calculations (Figure 1f) and contributes to enhanced operating voltage and energy density of the batteries. Kinetic analysis (Figures S8‒S10) indicated that the charge‐storage process in the HATN polymers is governed by a combination of capacitive behavior and diffusion, with pseudocapacitive contributions being dominant. Quantitative analysis further suggested (Figures 3a and S10) that the capacitive contribution ratios at 1.0 mV s⁻¹ reached 94% for HATN‐3S and 87% for HATN‐6S. The CV results not only validated the reversible Mg‐storage capability of the HATN unit but also indicated that the high specific surface area and porous structure of HATN‐3S and HATN‐6S effectively facilitate interfacial charge transfer. Galvanostatic intermittent titration technique (GITT) measurements (Figure S11) further showed that both HATN‐3S and HATN‐6S exhibited relatively high Mg²⁺ diffusion coefficients (10^‒8 to 10^‒13 cm² s^‒1). Collectively, these results indicated that the outstanding rate performance of the HATN polymers in subsequent tests arises from the synergistic effects of surface‐dominated pseudocapacitive behavior (ensuring rapid interfacial reactions) and the abundant porous structure (promoting Mg²⁺ transport).

FIGURE 3.

Electrochemical performance of HATN, HATN‐3S, and HATN‐6S cathodes in RMBs. (a) CV curves at 1.0 mV s^‒1, capacitive and diffusion‐controlled contributions. Cycling performance at (b) 0.1 and (c) 0.2 A g^‒1 of HATN‐3S and HATN‐6S (25°C). (d) Cycling performance at different temperatures and (e) charge/discharge curves at different current densities of HATN‐3S. (f) Rate performance and (g) long‐term cycling at 1.0 A g^‒1 of HATN‐3S and HATN‐6S. (h) 3D Ragone plots and (i) comparison of long‐term cycling stability with reported high‐performance RMB cathode materials.

The theoretical predictions were convincingly validated by electrochemical performance. As given in Figure 3b (corresponding charge/discharge profiles in Figure S12), HATN‐3S and HATN‐6S delivered high capacities of 370 and 309 mAh g^‒1 (0.1 A g^‒1), respectively, approaching their theoretical values (375 and 339 mAh g^‒1). At 0.2 A g^‒1 (Figure 3c), they retained capacities of 307 and 268 mAh g^‒1, stabilizing at 297 and 262 mAh g^‒1 after 100 cycles, corresponding to outstanding capacity retention rates of 97% and 98%, respectively. Moreover, HATN‐3S demonstrates exceptional wide‐temperature adaptability with robust electrochemical performance across both elevated and low‐temperature regimes. At 45°C (0.2 A·g^‒1), it retains a stable capacity of 306 mAh·g^‒1 over 50 consecutive cycles; even at −25°C (0.1 A·g^‒1), the cathode still delivers a reliable capacity of 207 mAh·g^‒1 after 50 cycles, highlighting its outstanding thermal tolerance for practical RMB applications. Benefiting from enhanced charge delocalization and high specific surface area, both materials exhibited outstanding rate capability. A progressive increase in the current density from 0.1 to 5.0 A g^‒1 (Figure 3f) revealed that HATN‐3S and HATN‐6S maintained reversible capacities of 94 and 93 mAh g^‒1. Upon returning to 0.1 A g^‒1, the capacities rapidly recovered to their initial levels (charge/discharge profiles in Figures 3e and S13), demonstrating exceptional reversibility and reaction kinetics for Mg²⁺ storage. Long‐term cycling stability was evaluated at 1.0 A g⁻¹ (Figure 3g). After 500 cycles, HATN‐3S and HATN‐6S retained reversible capacities of 186 and 160 mAh g^‒1, respectively, with remarkable capacity retention rates of 95% and 96%. This confirmed that the polymer backbone and extended conjugation significantly enhanced structural and cycling stability through effective charge delocalization. Compared with recently reported high‐performance RMB cathodes (Figure 3h,i and Tables S3‒S6), the HATN‐based polymers in this work demonstrated superior overall performance in key metrics including specific capacity, energy output, and cycle life. Notably, HATN‐3S achieved significantly higher active site utilization than most reported systems, yielding a high energy density of 325 Wh kg^‒1 (at a power density of 105 W kg^‒1). Furthermore, it exhibited an exceptionally low average capacity fade rate of merely 0.01% per cycle over 500 long‐term cycles. These outstanding electrochemical results strongly affirmed the effectiveness of the design strategy combining extended endocyclic conjugation and multidentate Mg²⁺ coordination.

To elucidate the Mg‐storage mechanism of HATN‐3S and HATN‐6S, we first employed Raman spectroscopy to monitor their structural evolution during charge/discharge processes. Ex situ Raman spectra (Figure 4a) revealed a noticeable intensity decrease of the C═C/C═N vibration band (∼1515 cm⁻¹) upon discharge, along with a significant enhancement of the C─C/C─N band (∼1390 cm^‒1). These spectral features recovered to their original intensities after charging. Further in situ Raman measurements (Figure 4b) demonstrated the continuous reversibility of this process. As discharge progressed, the C═C/C═N peak gradually weakened while the C─C/C─N peak strengthened accordingly. These changes were fully reversed during the subsequent charging process. These Raman results indicated that the C═N groups function as active centers participating in reversible redox reactions (C═N ↔ C─N), with the conjugated backbone effectively delocalizing charge (C═C ↔ C─C) to maintain structural stability. N 1s XPS spectra (Figure 4c) provided direct evidence of valence changes in N atoms. The intensities of the C═N─C and C─N─C peaks exhibited alternating trends during cycling. Upon discharge, the C═N─C peak decreased while the C─N─C peak intensified, indicating the reduction of C═N─C to C─N─C upon electron uptake and subsequent coordination with Mg²⁺. This process was fully reversible upon charging. The pronounced intensity variation of the Mg signal (Figure 4d) further corroborated the high Mg‐storage capacity of the HATN unit. Additionally, other structural features in the Raman spectra, such as the Ar─S─Ar peaks at 624 and 1074 cm⁻¹, remained largely unchanged. Meanwhile, the S 2p XPS spectra (Figure S14) showed no significant shift or alteration in peak shape over repeated cycles, confirming the high stability of the polymer backbone and the thioether structure during Mg²⁺ storage.

FIGURE 4.

(a) Ex situ Raman spectra of HATN‐3S and HATN‐6S. (b) In situ Raman spectra of HATN‐3S. Ex situ (c) N 1s and (d) Mg 2p XPS spectra of HATN‐3S and HATN‐6S. (e) EDS spectra, (f) elemental composition, and (g) elemental mapping images of HATN‐3S in charge/discharge states. (h) Calculated structures and ΔG of MgCl⁺ storage and Mg²⁺ storage mechanism for HATN unit.

To identify the dominant charge carrier (Mg²⁺ or MgCl⁺), TEM and EDS analyses were performed on HATN‐3S electrodes in charged and discharged states (Figure 4e–g). Quantitative EDS showed a marked difference in Mg content (8.2% charged vs. 22.4% discharged, variation of 14.2%), while Cl varied only slightly (2.2% vs. 4.0%, variation of 1.80%). This indicated Mg²⁺ is the primary charge carrier rather than MgCl⁺, and residual Mg and Cl in the charged state likely came from adsorbed electrolyte. Electrochemical tests in Cl‐free Mg[B(hfip)₄]₂/G2‐DME electrolyte further supported this mechanism (Figures S16 and S17). At 0.1 A g^‒1, HATN‐3S and HATN‐6S delivered 314 and 253 mAh g^‒1, respectively, corresponding to 85% and 82% of their capacities in Cl‐containing electrolyte, confirming that storage is mainly governed by Mg²⁺. DFT calculations provided thermodynamic validation (Figure 4h and details in Figure S18). Coordination with MgCl⁺ gave a positive ΔG value (+5.38 eV), indicating thermodynamic non‐spontaneity. This is mainly due to steric hindrance and increased interionic repulsion resulting from the bulky MgCl⁺ when attempting to coordinate within the confined adjacent N sites of HATN. In contrast, coordination with Mg²⁺ yielded a favorable ΔG of ‒1.79 eV, demonstrating a clear thermodynamic preference of the bidentate chelating sites in HATN for divalent Mg²⁺ and confirming the spontaneous Mg²⁺ storage. In summary, EDS, Cl‐free electrolyte performance, and DFT calculations consistently established Mg²⁺ as the predominant charge carrier in the HATN‐based polymers.

Combined spectroscopic and theoretical analyses were employed to systematically elucidate the robust electron‐delocalization capability and reversible reaction mechanism of the HATN polymers from an electronic structure perspective. Ex situ (Figure 5a) and in situ IR spectra (Figure 5b) revealed that during discharge, the absorption peak corresponding to the quinoid imine structure (N═Q═N, 1084 cm^‒1) gradually decreased in intensity, whereas the C─N vibrational peaks (1240 and 1365 cm⁻¹) strengthened significantly. Concurrently, the C─N band at 1180 cm⁻¹ underwent a blue shift to 1190 cm⁻¹ due to the electron‐withdrawing effect of Mg²⁺ coordination. All spectral features recovered to their initial states upon charging, confirming the high reversibility. Unlike the Raman spectra, the C═C/C═N skeletal vibrations intensified during discharge, suggesting polarization and rearrangement of the electronic structure, which will be further elucidated by subsequent DFT calculations. Photoluminescence spectra (Figure 5c) showed that both pristine and cycled HATN‐3S/HATN‐6S exhibited a characteristic π–π* transition peak at 486 nm, indicating that the conjugated backbone remained intact throughout the electrochemical process. A slight decrease in fluorescence intensity in the discharged state suggested that Mg²⁺ coordination promotes electron transfer from HATN to Mg²⁺, leading to fluorescence quenching. The IR, Raman, and photoluminescence spectra collectively demonstrated that the outstanding Mg‐storage performance of HATN polymers arises from their stable conjugated framework, reversible redox activity, and efficient electron delocalization.

FIGURE 5.

(a) Ex situ IR spectra of HATN‐3S and HATN‐6S. (b) In situ IR spectra of HATN‐3S. (c) Ex situ fluorescence spectra of HATN‐3S and HATN‐6S. (d) Electron density difference of HATN after discharge (top: contour plot; bottom: isosurface plot). (e) Total atomic dipole moment corrected Hirshfeld population (ADCH) atomic charges of C and N in charge/discharge states. (f) LOL‐π images of HATN and HATN‐3Mg (isovalue = 0.3).

DFT calculations delivered electronic‐level insights into the coordination mechanism and charge distribution in HATN upon Mg²⁺ storage. Electron density difference contour plot (Figure 5d, top; Figures S19 and S20) revealed increased electron density around N atoms and decreased density in bonding regions such as C─N and C─C, rationalizing the enhanced IR absorption (increased bond vibrational dipole moment) and weakened Raman signals (decreased bond polarizability) observed experimentally. Significant electron accumulation between Mg and N confirmed the N,N‐chelation ability. Isosurface plot (Figure 5d, bottom) further illustrated effective electron delocalization into the π‐system. ADCH charge analysis (Figure 5e; Tables S7 and S8) reveals a distinctive “coordination at N, buffering at C” cooperative mechanism within the HATN polymers. Although N atoms serve as the primary electron‐injection sites, the cumulative charge variation on C atoms (HATN‐3S: 2.76 e; HATN‐6S: 1.88 e) exceeds that on N atoms (1.21 and 1.56 e, respectively). This pronounced charge buffering effect originates from the fused‐ring scaffold of HATN, namely three pyrazine rings are annulated to form a rigid, planar macrocycle, with all constituent atoms participating in the extended π‐conjugated system. LOL‐π profiles (Figure 5f) further demonstrated uniformly distributed π‐electron clouds in both pristine and Mg‐coordinated states, confirming maintained delocalization throughout cycling. These results collectively highlighted that the high capacity, rate capability, and cycling stability of HATN polymers arise from the synergy between efficient electron delocalization across the conjugated framework and stable bidentate Mg²⁺ coordination.

3. Conclusion

This study demonstrates a molecular design strategy integrating extended endocyclic conjugation and multidentate Mg²⁺ coordination to develop HATN‐based thioether‐bridged polymer cathode materials (HATN‐3S and HATN‐6S) for high‐performance RMBs. Theoretical calculations reveal that the rigid planar π‐conjugated structure of HATN units, coupled with their bidentate N,N‐chelation sites, enables high‐capacity and reversible six‐electron Mg²⁺ storage. The highly delocalized electron system ensures structural stability, outperforming conventional carbonyl‐based counterparts in the 2D evaluation framework of capacity versus delocalization capability. The thioether‐bridging strategy effectively suppresses material dissolution, while the high specific surface areas (HATN‐3S: 474.4 m² g⁻¹; HATN‐6S: 795.5 m² g^‒1) and hierarchical porous structures facilitate interfacial reactions and ion transport. Electrochemically, HATN‐3S provides a high reversible capacity of 370 mAh g^‒1 at 0.1 A g^‒1 and exhibits excellent cyclability with 95% capacity retention after 500 cycles at 1.0 A g^‒1, as well as remarkable rate performance (94 mAh g^‒1 at 5 A g^‒1). Mechanistic studies confirm that reversible transformation between C═N and C─N and efficient π‐electron delocalization across the conjugated skeleton are key to high reversibility and structural integrity. Electronic structure analysis further reveals that charge is primarily delocalized over the entire π‐system rather than localized at N sites, forming an effective charge buffering effect. Moreover, EDS, Cl‐free electrolyte tests, and thermodynamic calculations consistently identify Mg²⁺ as the dominant charge carrier. By delving into the electronic structure and coordination chemistry of Mg‐storage‐active units, this work presents a promising class of organic cathodes for RMBs and deepens the mechanistic understanding of multivalent‐ion storage in conjugated polymers, providing a rational design strategy and a material paradigm for future development.

Author Contributions

Hongda Gui: investigation, writing – original draft, conceptualization. Ze Wang: investigation. Jiaming Jiang: investigation. Ting Li: writing – review and editing and funding acquisition. Yuliang Cao: writing – review and editing. Fei Xu: writing – review and editing, funding acquisition, conceptualization, supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: Detailed experimental procedures and synthetic schemes. nMBCO and theoretical capacity calculations of active unit. Magnesium storage performance of polymers with various active units FTIR spectra of raw materials, intermediates and final products. Material characterizations (XRD, pore size analysis, SEM, and EDS) of HATN‐3S and HATN‐6S. OCV measurements of HATN, HATN‐3S, and HATN‐6S. Electrochemical kinetics (CV at varied scan rates, relationship between peak currents and scan rates, capacitive and diffusion‐controlled contributions, GITT, and charge–discharge curves) of HATN‐3S and HATN‐6S. Performance comparison with recently reported rechargeable magnesium battery electrode materials. Energy density values at different power densities. Ex situ XPS S 2p spectra reveal the redox mechanism. Symmetric Mg plating/stripping behavior of the Mg[B(hfip)₄]₂/G2‐DME electrolyte. Cycling performance and charge–discharge curves of HATN‐3S and HATN‐6S. Theoretical calculations comprising Gibbs free energies along the reaction pathway, electron density difference maps, ADCH charges, and HOMO–LUMO energy diagrams for HATN, HATN‐3S, HATN‐6S, and their Mg²⁺‐associated states.

Data Availability Statement

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