Hexaazatriphenylene Based Polyimide with Dense Dual Redox Sites as a High‐Performance Organic Cathode for Lithium‐Ion Batteries

Organic polymers are promising candidates as cathode materials for lithium storage, however, suffer from low theoretical capacity due to the presence of multiple inactive components in the polymers. Herein, a novel hexaazatriphenylene‐based polyimide with high theoretical capacity (436 mAh g−1) is developed via the precise design of monomers and controllable synthesis of corresponding polymers. The as‐prepared polymers possess rich edge pyrazine nitrogen (C═N) and carbonyl groups (C═O), well‐defined porosity, and conjugated structure, benefiting for high capacity, rapid ion and charge transport. The resultant polymers electrode achieves a high specific capacity of 303 mAh g−1 at 100 mA g−1, high‐rate capability (171 mAh g−1 even at 8 C, 1 C = 400 mA g−1), and stable cycle performance with a high capacity retention of 93.8% at 500 mA g−1 over 200 cycles. Combined experimental and theoretical calculations reveal that both C═O and C═N sites in the polyimide are served as redox sites for lithium storage, providing high specific capacity. This work offers a novel approach for the development of polymeric cathode materials with dense redox sites for next‐generation energy‐dense batteries.


DOI: 10.1002/admi.202300464
[3][4] However, the further development of energy-dense LIBs with long endurance and quick-charging capability was hindered by limited highcapacity electrode materials, [5,6] especially for cathode materials.Great efforts have been devoted to developing high-performance cathode materials for LIBs, mainly including inorganic cathode materials, and organic cathode materials. [7,8]At present, inorganic materials (e.g., lithium cobaltite, lithium iron phosphate, and ternary materials) dominate the market for commercial LIBs. [9,10]owever, the actual specific capacity of these materials usually hardly exceeds 200 mAh g −1 .As a potential substitute for inorganic materials, organic cathode materials are promising for developing energy-dense batteries due to their high theoretical capacity, diverse structure, reliability, and sustainability. [11,12]enefiting from structural diversity and designability, sustainable nature, and low carbon footprint, organic materials have received increasing attention as promising cathode materials for increasing electrochemical performance and sustainability of energy storage systems. [13]Organic cathode materials are mainly divided into two categories, including organic compound cathode materials, and polymeric cathode materials. [14]Organic compound cathode materials possessed high theoretical capacity (e.g., 418 mAh g −1 for trimeric quinoxaline molecules), [15] however, most of them suffered from dissolution issues, leading to poor cycle stability and low-rate capability. [16]A series of strategies were developed to resolve the dissolution issue, such as the development of insoluble organic compound, [17,18] the complex with carbon nanomaterials, [15,19] and the polymerization of organic compounds. [20,21]Among these strategies, polymerization of organic compound is one of the most efficient approaches to resolve the dissolution issue due to the low solubility of polymers in the electrolyte.Interestingly, pyrazine-based polymers illustrated excellent electrochemical performance for several metal (Li, Na, Mg, Al) ion batteries due to their ordered porosity, conjugated carbon-rich structure, insoluble characters of polymers, and abundant edge pyrazine N. [22,23] The N sites assembled into porous conjugated polymer led to high capacity and rapid kinetics. [19,23]For example, Feng et al. reported a 2D hexaazatrinaphthylene (HATN)-based polymer cathode (2D CCP-HATN) for LIBs, achieving excellent long-term cycle stability and high-rate capability. [19]The construction of polymeric cathode materials using organic compound was able to alleviate its dissolution in the electrolyte, [23] however the addition of non-active groups in the synthetic process of polymers decreased the capacity of polymeric cathode materials.For instance, although the cycle stability and rate-capability performance of 2D CCP-HATN were largely improved, its capacity was obviously reduced to 116 mAh g −1 . [19]Therefore, it is significantly important to develop novel pyrazine-based polymeric cathode materials without sacrificing the high capacity of organic compound, which is promising and desirable for the next-generation density-dense LIBs.
Herein, we developed a novel hexaazatriphenylene (HAT)based polyimide with high-density pyrazine nitrogen and carboxyl groups, which was prepared by the solvothermal reaction between hexaazatriphenylene hexacarboxylic acid (HCB-HAT) and p-phenylenediamine.After polymerization, the HAT units were linked by amide bonds, producing HAT-based polyimide with abundant pyrazine N and C═O groups as redox sites for lithium storage.The maximum theoretical capacity of the prepared polymer reached 436 mAh g −1 , addressing the solubility issues of organic compound cathode via polymerization without sacrificing its high capacity.Benefiting from abundant redox site, porous conjugated and stable structure, the resultant HAT-based polyimide (HCBHAT-PH@CNT) electrode achieved a high capacity of 303 mAh g −1 at 100 mA g −1 , high-rate capability (171 mAh g −1 even at 8 C, 1 C = 400 mA g −1 ), and long-term stability over 1600 cycles at 2.5 C. Ex situ Fourier transform infrared spectrometer (FT-IR) and X-ray photoelectron spectroscopy (XPS) spectra combined with density functional theory (DFT) calculations revealed that both C═O and pyrazine N in the polyimide served as redox sites for lithium storage, achieving high specific capacity.Compared with previouslyreported organic polymer cathodes, HCBHAT-PH@CNT cathode possessed abundant dual redox sites, which achieved longterm stability combined with high actual capacity.The capacity of HCBHAT-PH@CNT cathode was also higher than that of inorganic cathode materials. [9,10]This work gives an insight of lithium storage in polyimide cathode with dense redox sites, which will open up new chances to develop high-capacity cathode materials with high-rate capability and robust stability for nextgeneration density-dense LIBs and beyond.

Results and Discussion
Figure 1 schematically illustrates the synthesis route of HATbased polyimide coated carbon nanotube (denoted as: HCBHAT-PH@CNT), which was prepared by the hydrothermal reaction of HCBHAT and p-phenylenediamine (PH) monomers in the presence of carboxyl functionalized carbon nanotube (CNT) dispersion at 200 °C for 48 h.The carboxyl functionalized CNT was used to enhance the electronic conductivity of HAT-based polyimide.Besides, the carboxyl functionalization of CNT is beneficial to increasing its dispersion for in situ growth of polyimide coating on the surface of CNT.As reported, in situ polymerization of monomer containing redox groups in the presence of carbon nanomaterials (e.g., SWCNT, Ketjenblack) is a promising strategy to increase the utilization of redox sites and electronic conductivity, resulting in improved cycling stability and high rate capability. [24,25]To clearly illustrate the successful preparation of HCBHAT-PH and highlight the key role of CNT in increasing electrochemical performance of HCBHAT-PH@CNT, the pure HCBHAT-PH was also prepared as the control following the same protocol without adding CNT.For preparing HCBHAT-PH, the key HCBHAT monomer was synthesized according to earlier report (Scheme S1, Supporting Information). [26]FT-IR spectrum of HCBHAT in Figure S1 (Supporting Information) shows the characteristic peak at 1741 cm −1 stem from C═O stretching vibration of carboxyl groups, and peaks at 3300-2500 cm −1 ascribed to stretching vibration of O−H, [26] proving the existence of carboxyl groups in the HCBHAT.The precise structure of HCB-HAT was further confirmed by 13 C nuclear magnetic resonance (NMR) (Figure S2, Supporting Information), in line with the reported data. [27]he formation of HCBHAT-PH was characterized by FT-IR and solid 13 C NMR. Compared with HCBHAT and PH monomers, FT-IR spectrum of HCBHAT-PH (Figure 2a) shows new peaks at 1727 and 1670 cm −1 , corresponding to symmetry and asymmetry characteristics peaks of C═O in imide rings. [28,29]dditionally, stretching vibrations of C═N (1540 cm −1 ) and C─N─C (1381 cm −1 ) for amide bonds were also observed. [28,29]eanwhile, the peak at 1741 cm −1 associated with C═O from ─COOH of HCBHAT, and the peak at 3373 cm −1 ascribed to NH 2 groups of PH disappeared.Above results indicate the successful formation of HCBHAT-PH via the imide reaction between ─COOH groups from HCBHAT and ─NH 2 groups from PH. Solid 13 C NMR spectrum of HCBHAT-PH (Figure 2b) exhibits the signals at 173.2 and 149.4 ppm, corresponding to the C of C═O and C─N in the imide moiety, respectively.The peak at 120.4 ppm corresponded to the phenyl C in the linked units (labeled as 1, 2, and 5 sites in Figure 2b). [30]The NMR results further confirmed the successful preparation of HCBHAT-PH.
X-ray diffraction (XRD) pattern of HCBHAT-PH (Figure 2c) displays three peaks at 9.9°, 27.2°, and 45.2°, corresponding to d-plane spacings of 8.23, 3.27, and 2.0 Å, respectively.The interfacial spacing of 3.27 Å associated with the strongest diffraction peak is smaller than that of - stacking (3.3-3.4Å), suggesting the reduced interfacial spacing and the enhanced intermolecular interactions of HCBHAT-PH, [31] in line with previously-reported typical -conjugated systems. [32,33]The calculated Brunauer-Emmett-Teller (BET) specific surface area (SSA) of HCBHAT-PH is 49.2 m 2 g −1 (Figure S3a, Supporting Information).Besides, its pore size distribution curves in Figure S3b,c (Supporting Information) show the presence of both micropores (7.8 Å) and mesopores (15.3 nm).The mesopores in HCBHAT-PH are beneficial for fast ion transport.Above results indicated that the asprepared HCBHAT-PH was porous conjugated polymer.Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure 2d and Figure S4 (Supporting Information) illustrate the stacked bulk structure for HCBHAT-PH due to the strong - interlayer interactions.The clearly even distribution of C, N, and O elements over the HCBHAT-PH in energy-dispersive X-ray spectroscopy (EDX) elemental mapping images in Figure S4g-i (Supporting Information) suggest the presence of uniform and abundant redox sites (C═N and C═O), which is able to enlarge the capacity for lithium storage significantly.Thermogravimetry (TGA) curve of HCBHAT-PH hardly (Figure S5, Supporting Information) shows mass loss below 440 °C, suggesting its high thermal stability.The mass loss of 3.4% below 110 °C can be attributed to the evaporation of adsorbed H 2 O from the porous polymers, which was further proved by elemental analysis.The measured element contents of C, N, O, and H for HCBHAT-PH are close to their theoretical values after excluding the effect of adsorbed water (Table S1, Supporting Information).
To increase the electronic conductivity of HCBHAT-PH for rapid-charging, the HCBHAT-PH was tethered on the surface of carboxyl functionalized CNTs, obtaining HCBHAT-PH@CNT.The as-prepared HCBHAT-PH@CNT exhibited a tubular structure after HCBHAT-PH coating on CNTs (Figure 2e,f).Compared with pure HCBHAT-PH with - interaction stacking, HCBHAT-PH@CNT possessed a looser structure, which was conducive to the exposure of redox sites, benefiting for increasing the utilization rate of redox sites.The TEM image of HCB-HTA@CNT (Figure 2f) shows the presence of a low contrast polymer coating on the CNT surface compared with pure CNT (Figure S6, Supporting Information). [30]And a layer of 5-7 nm HCBHAT-PH was coated on the surface of CNT.The complex structure was formed via strong - interactions between CNT and HCBHAT-PH due to its abundant aromatic HAT structures.Moreover, scanning transmission electron microscopy (STEM) and corresponding EDX mapping images show even distribution of carbon, nitrogen, and oxygen on the surface of CNTs (Figure S7, Supporting Information), suggesting uniform coating of HCBHTA-PH on the surface of CNTs due to absence of nitrogen in the pure CNTs.Owing to the uniform HCBHAT-PH coating on the CNT surface, the low charge transfer ability of HCBHAT-PH could be largely improved (vide infra).The CNT not only endowed the HCBHAT-PH with high charge transfer ability, but also improved the utilization of redox sites via exposing uniform and thin HCBHAT-PH coating on the CNT surface with high SSA, as verified by higher SSA of HCBHAT-PH@CNT (119.3 m 2 g −1 ) than that of HCBHAT-PH (Figure S3, Supporting Information).The high electrical conductivity and fast ion diffusion behavior enable HCBHAT-PH@CNT as a promising cathode material for lithium storage.
To test its electrochemical performance, the half-cell with HCBHAT-PH@CNT cathode was constructed using Li foil as the counter electrode.CV curve of HCBHAT-PH@CNT electrode (Figure 3a) displays two pairs of different redox peaks at 1.99/1.81and 3.05/2.61V (vs Li/Li + ), suggesting the presence of multi-step redox reactions, which could be caused by different redox sites in the HCBHAT-PH@CNT.It should be noted that the HCBHAT-PH@CNT electrode does not exhibit a clear charge-discharge voltage plateau, which may be caused by the multi-step lithium storage process.Moreover, the first five CV curves almost overlap (Figure 3a), indicating that HCBHAT-PH@CNT electrode has highly reversible and stable electrochemical behavior, as further proved by stable and reversible charge-discharge curve in Figure S8 (Supporting Information).In contrast, although the pure HCBHAT electrode showed two similar redox peaks in the first 5 cycles (Figure S9a, Supporting Information), its charge-discharge curves show poor stability (Figure S9b, Supporting Information), which may be caused by the unstable structure of organic HCBHAT electrode in the electrolyte.In view of the presence of high-density redox sites (pyrazine N and carbonyl groups) in the HCBHAT-PH and high conductivity provided by CNT, the HCBHAT-PH@CNT electrode achieved a high capacity of over 303 mAh g −1 after the first cycle activation at 100 mA g −1 (Figure S10, Supporting Information), which reached to about 70% of the theoretical capacity of the HCBHAT-PH polymers (436 mAh g −1 ), illustrating high utilization of redox sites.Compared with HCBHAT electrode, the cycle stability of HCBHAT-PH based electrodes was largely improved (Figure 3d).The stable and high performance of HCBHAT-PH based electrodes may be ascribed to enhanced intrinsic conductivity by the extended -conjugated system of HCBHAT-PH, and the facilitated ion transport ability benefited from its layer-by-layer - stacking and porous structure. [34]Additionally, HCBHAT-PH@CNT electrode exhibited higher capacity than that of HCBHAT-PH electrode (CNT also served as the conductive additive) (Figure 3d), suggesting the key role of in situ coating polymer structure for increasing the utilization rate of redox sites in HCBHAT-PH, which can be further verified by continuously enhanced capacity with increasing addition of CNT (Figure S11, Supporting Information).And the optimized content of CNT in HCBHAT-PH@CNT electrode (40 wt.%) is obtained (Figure S11, Supporting Information).Figure 3b,c displays the discharge/charge capacities of HCBHAT-PH@CNT electrode at various current densities.The HCBHAT-PH@CNT electrode achieved higher specific capacity than HCBHAT-PH electrode at various current densities.Besides, the capacity retention of HCBHAT-PH@CNT electrode maintained at a high level (>65%) from 0.75 to 8 C, which is higher than that of HCBHAT-PH electrode (51%), exhibiting a higher rate performance.The capacities of the HCBHAT-PH@CNT electrode are 262, 234, 206, 182, and 171 mAh g −1 at 0.75, 1.25, 2.5, 4, and 8 C, respectively.When the current density recovered to 0.75 C, the discharge capacity of HCBHAT-PH@CNT returned to its initial level, indicating that the HCBHAT-PH@CNT electrode had excellent rate capability, which was superior to most previously reported or-ganic electrodes for lithium storage (Table S2, Supporting Information).
As shown in Figure 3d, HCBHAT-PH@CNT electrode exhibited a high-capacity retention of 93.8% after 200 cycles at 1.25 C, indicating excellent cycle stability of HCBHAT-PH@CNT electrode.Besides, HCBHAT-PH@CNT electrode also showed high stability with a high capacity of 246 mAh g −1 at 1 C over 200 cycles (Figure S12, Supporting Information).The excellent cycle stability was achieved at a mass loading of 0.6-0.8mg cm −2 for electrode active material (HCBHAT-PH).To highlight its superiority, the cycle performance was further measured at high mass loading.As shown in Figure S13 (Supporting Information), the HCBHAT-PH@CNT electrode exhibited superior cycle stability even at high mass loading (1.2 and 1.5 mg cm −2 ).The outstanding cycle stability combined with high capacity of HCBHAT-PH@CNT electrode is superior to most previously-reported organic polyimide cathode (Table S3, Supporting Information).More importantly, HCBHAT-PH@CNT exhibited large energy density and power density at various current densities (Table S4, Supporting Information).For example, HCBHAT-PH@CNT achieved a high energy density of 607.0 Wh kg −1 , which was comparable to those of most previously-reported HAT-based cathode materials for LIBs (Table S5, Supporting Information).Additionally, the HCBHAT-PH@CNT and HCBHAT-PH electrodes exhibited better cycle stability than organic HCBHAT electrodes by using CNTs as the conductive additive, highlighting the superiority of HCBHAT-PH polymers for high cycle stability.The disassembled cycling cells with different electrodes were further characterized.As given in Figure S14 (Supporting Information), the separator of HCBHAT-PH based cell after cycling retains its original color, indicating a stable structure of HCBHAT-PH in the electrolyte.However, the separator of the HCBHAT based battery after cycling has a distinct tan stain, suggesting the dissolution of HCBHAT molecules from the electrode surface in the electrolyte.The insolubility of HCBHAT-PH in the electrolyte contributed to the excellent cycle stability for HCBHAT-PH based electrodes.The capacity contribution of CNT was also considered for calculating the capacity of HCBHAT-PH@CNT even if the low capacity of 50 mAh g −1 for CNT at 0.5 A g −1 (Figure S15, Supporting Information).To further illustrate its excellent cycle stability, long-term cycle performance of HCBHAT-PH@CNT at higher current density (2.5 C) was performed.As provided in Figure 3e, HCBHAT-PH@CNT electrode delivers a high capacity of ≈200 mA h g −1 at initial 200 cycles, and a stable capacity of 120 mA h g −1 with nearly 100% coulombic efficiency (CE) over 1600 cycles.The capacity retention of HCBHAT-PH@CNT after 1600 cycles at 2.5 C is 60%.The capacity delay from the 200th cycle may be caused by the electrode partial deterioration after longterm cycling at such high current density.Among various HATbased cathode materials, the HCBHAT-PH@CNT electrode exhibited a good comprehensive performance (Table S5, Supporting Information).
To deeply understand the excellent electrochemical performance, the HCBHAT-PH@CNT electrode was further studied.The electrical conductivity of HCBHAT-PH@CNT (1.18 × 10 −3 S cm −1 ) was calculated from its current-voltage curve (Figure 3f), which is significantly higher than that of HCBHAT-PH (7.09 × 10 −8 S cm −1 , Figure S16, Supporting Information), illustrating the key role of CNT in boosting electrical conductivity for HCBHAT-PH@CNT composite. [35]This was further confirmed by the small charge-transfer impedance of HCBHAT@CNT electrode (14.5 Ω) after 60 cycles (Figure 3g).After 60 cycles, the Warburg coefficient () of the HCBHAT-PH@CNT electrode increased to 42.92 Ω s −1/2 , outperforming that of before cycling (29.39 Ω s −1/2 , Figure S17, Supporting Information) because the diffusion of Li + was hindered by the formed interface between electrode and electrolyte. [36]In addition, the small changes in charge transfer impedance and Warburg impedance before and after 60 cycles further indicate the stable electrode structure of HCBHAT-PH@CNT during cycling.Besides, a galvanostatic intermittent titration technique (GITT) was conducted to study lithium-ion diffusion rate (D Li+ ) of HCBHAT-PH@CNT electrode (Figure 3h).The calculated D Li+ for HCBHAT-PH@CNT electrode with different charge states varied from 10 −14 to 10 −8 cm 2 S −1 , exceeding that of conventional inorganic cathode materials (e.g., 10 −16 to 10 −14 cm 2 S −1 for LiFePO 4 ). [37]The excellent ion diffusion ability can be ascribed to spacious interior space and rich mesoporous structure of HCBHAT-PH@CNT.The low charge transfer resistance and high ion diffusion rate of HCBHAT-PH@CNT endow superior electrochemical performance in LIBs.Overall, benefiting from the high conductivity provided by CNT and porous structure of conjugated HCBHAT-PH polymers for high charge and ion transfer ability, insolubility for stable electrode structure as well as increased utilization of redox sites and electronic conductivity by in situ HCBHAT-PH coating on CNT, the HCBHAT-PH@CNT electrode exhibits excellent electrochemical performance.
To understand electrochemical kinetics of HCBHAT-PH@CNT electrode, its CV curves at different scan rates were tested.As shown in Figure 4a, the peak current is not proportional to the square root of the scanning rate, indicating that both Faraday and non-Faraday reactions occurred during the lithium-ion storage process.Generally, the relationship between the measured peak current (i) and the scanning rate (v) in the CV curves can be described according to Equation ( 1), [38] where the parameters (a and b) can be adjusted.The calculated values of b for oxidative peaks (O1 and O2) in Figure 4a are 0.998 and 0.968 (Figure 4b), while the calculated values of b for reduction peaks (R1 and R2) in Figure 4a are 0.832 and 0.984, respectively.According to the power law (Equation ( 1)), the value of b for the redox reaction limited by semi-infinite linear diffusion is 0.5, while that for the capacitive process is 1.0. [39]he calculated values of b for the HCBHAT-PH@CNT electrode are close to 1.0, suggesting that charge storage mechanism of the electrode is mainly controlled by the capacitance process, which is a fast surface-controlled pseudo-capacitance process. [40]he capacitance effect can be attributed to the good charge transport ability of HCBHAT-PH@CNT provided by CNT and the fast ion diffusion ability due to the porous structure, resulting in outstanding electrochemical performance of the HCBHAT-PH@CNT electrode.The capacitance contribution (k 1 v) and diffusion contribution (k 2 v 1/2 ) can be further analyzed based on Equation (2). [41]For the HCBHAT-PH@CNT electrode, the capacitance contribution at a low scanning rate of 0.2 mV s −1 is determined to 84.9%, while the capacitance contribution at 1.0 mV s −1 increases to 95.9% (Figures 4c; Figure S18, Supporting Information), indicating a dynamic and fast pseudo-capacitance process for lithium-ion storage in the HCBHAT-PH@CNT electrode.
To further understand the lithium-ion storage mechanism, the ex situ FT-IR spectra of HCBHAT-PH electrode under different discharge and charge states were recorded to monitor the composition change of the electrode.As given in Figure 4d,e, the peak intensity of C═O stretching vibration at 1727 and 1670 cm −1 as well as the C═N stretching vibration at 1540 cm −1 gradually decreases as the discharge processes (from point A to C), revealing that these C═O and C═N groups are redox sites for Li + storage.Particularly, after charging to 4.0 V (vs Li/Li + ), the peak intensity of these C═O and C═N groups almost completely recovered (from point D to F), revealing the reversible Li + ion release during the charge process.The lithiation/delithiation mechanism was further proved by ex situ XPS (Figure 4f,g).The N 1s high-resolution XPS spectrum of the initial HCBHAT-PH electrode displays two peaks at 399.43 and 400.45 eV, ascribing to the C═N bond in the HAT structure and the C─N bond in the imide groups, respectively. [42,43]The O 1s high-resolution XPS spectrum of the initial HCBHAT-PH electrode has only one peak at 532.38 attributed to the X═O bond (X═C and O). [36]During discharge to 1.2 V, the relative peak intensity of the C═N bond decreases, while that of the C─N bond increases gradually.In ad-dition, a peak belonging to the C─O bond appeared at 533.29 eV in the O 1s XPS spectrum, and its relative intensity also increased during the discharge process and decreased with charge process.When charged to 4.0 V, the relative intensities of all peaks return to their initial states.C 1s high-resolution XPS analysis also shows the same induction (Figure S19, see Note S1 for details, Supporting Information).These results revealed the reversible switching between C═X and C─X (X═O or N) in the HCBHAT-PH electrode during charge-discharge cycling.Ex-situ FT-IR and XPS analysis of HCBHAT-PH electrode during discharging and charging process suggest that the C═O and C═N groups are redox sites for Li + storage.
To further understand the structure evolution and energy change for the multiple lithiation process in the HCBHAT-PH, DFT calculation was performed to simulate the discharge process of HCBHAT-PH. [44,45]The repetitive unit of HCBHAT-PH structure was used for this simulation (Figure S20, Supporting Information).The electrostatic potential (ESP) map in Figure 5a shows that the electron-rich region is mainly located at near the N and O atoms in the original HCBHAT-PH, indicating that lithium ions may be stored on the N and O atoms.This is consistent with the results of XPS and FT-IR analysis in Figures 4e-g.Additionally, differential charge analysis indicates that the charge was transferred from N/O atoms to Li + after Li + adsorption on the electrode (Figure S21, Supporting Information), confirming the strong interaction between Li + and C═O/C═N redox sites.The calculated projected density of states (PDOS) of lithiated HCBHAT-PH shows the clear overlap between Li + and N/O atoms (Figure 5b), which further confirms the strong Li + bonding and feasible electron transfer in HCBHAT-PH electrode.
The electrochemical lithium storage process was also simulated by DFT calculation.In principle, the theoretical number of lithium ion storage in repetitive unit of HCBHAT-PH is 9 (Figure 5c).Typically, the HAT part (marked as position A) can receive six Li + , and the imide part (marked as position B) can receive three Li + .To obtain the lithiation sequence of these redox sites, three lithiation models (represented as SiteA+3Li, SiteA+6Li and SiteB+3Li) were first established, and the corresponding absorption energies for each Li + on these models were calculated.According to the calculation results (Figure 5c), Li + first tends to insert into site A, where one Li + is bonded with two N atoms to form SiteA+3Li.The Li + ions in the SiteB+3Li model are adsorbed by the O of the imide group, and the binding strength of Li + is lower than that of Site A+3Li.In the SiteA+6Li model, two Li + are adsorbed by two N atom bonds, and the Li + binding strength is significantly lower than that of SiteA+3Li and SiteB+3Li.The adsorption energy diagram exhibited that the lithiation process can be divided into three steps: HCBHAT-PH+3Li, HCBHAT-PH+6Li, and HCBHAT-PH+9Li (Figures 5d,e).Additionally, the total energy of the above lithium storage route is also calculated.The total energy of the initial HCBHAT-PH electrode was calculated to −6.326 × 10 4 eV.The energies of HCBHAT-PH +3Li (site A+3Li) and HCBHAT-PH +6Li (Site A+3Li and Site B+3Li) with two chelating sites lithiated decrease to −6.388 × 10 4 and −6.450 × 10 4 eV, respectively.These two processes corresponded to the discharge process of HCBHAT-PH electrode in the potential range of 4.0-2.5 V and 2.5-1.8V versus Li/Li + , respectively, which are related to the two pairs of redox peaks at 1.99/1.81and 3.05/2.61V versus Li/Li + in the CV curve of HCBHAT-PH electrode.Finally, after three Li + were further bonded at site A, the energy continuously decreased to −6.511 × 10 4 eV, corresponding to the discharge process in the potential range of 1.8-1.2V versus Li/Li + .However, in the range of 2.5-1.2V versus Li/Li + , its CV curve only showed one pair of redox peaks, which may be related to the overlap of the last two lithiation/delithiation steps.In this discharge process (Figure S22, Supporting Information), only one C═O group in each imide moiety reacts with Li-ion because deep discharge process (the reaction of two C═O groups with Li-ion) will damage the structure, leading to the irreversible process. [46]

Conclusion
In summary, we have developed a new hexaazatriphenylenebased polyimide cathode material with high-density pyrazine N and carbonyl groups.Benefiting from its insolubility for stable electrode structure, porous structure for rapid ion transport, and conjugated structure/complex with CNT for quick charge transport, the resultant HCBHAT-PH@CNT electrode exhibited excellent electrochemical performance with high stable capacity, high rate capability, and long-term stability.Combined experimental with DFT calculations indicated that both edge pyrazine N and carbonyl groups in conjugated polymers were served as redox sites for high-capacity lithium storage.Additionally, the uniform growth of HCBHAT-PH polyimide on the CNT surface and pseudocapacitance mechanism provided quick kinetics and high-rate performance.This work opens up new ventures for the development of high-performance organic cathode materials with dense redox sites for lithium storage.

Experimental Section
The Experimental Section was carefully described in Supporting Information.

Figure 1 .
Figure 1.Schematic illustration for synthesis of HCBHAT-PH@CNT and corresponding energy storage process.

Figure 3 .
Figure 3. Electrochemical performance of HCBHAT-PH@CNT electrode.a) CV curves of the HCBHAT-PH@CNT electrode at the potential range of 1.2-4.0V for initial five cycles (scan rate: 0.2 mV s −1 ).b) Charge and discharge profiles and c) rate capability of HCBHAT-PH@CNT electrode at various current densities.d) Cycling performance of HCBHAT-PH@CNT, HCBHAT, and HCBHAT-PH over 200 cycles at 1.25 C. e) Long-term cycle stability of HCBHAT-PH@CNT at 2.5 C. f) Current-voltage curve for HCBHAT-PH@CNT at 298 K. g) Nyquist plots of HCBHAT-PH@CNT electrode before and after 60 cycles.h) GITT curve and diffusion coefficient of HCBHAT-PH@CNT electrode.

Figure 4 .
Figure 4. a) CV profiles of HCBHAT-PH@CNT electrode at various scan rates.O1/O2 and R1/R2 represent the oxidation and reduction peaks, respectively.b) The b-values determination of catholic and anodic current peaks.c) Capacitive-contribution ratio of HCBHAT-PH@CNT electrode at different scan rates.d) Discharge-charge profile of HCBHAT-PH electrode at 0.5 C in the voltage range of 1.2-4.0V versus Li/Li + in the first cycle.The selected points A-F represent the pristine, discharged to 1.8, 1.2 V versus Li/Li + and charged to 2.0, 3.0, and 4.0 V versus Li/Li + , respectively.e) Ex situ FT-IR spectra of HCBHAT-PH electrode at different states.f) Ex situ N 1s and g) O 1s XPS spectra of HCBHAT-PH electrode at different states.

Figure 5 .
Figure 5. a) The ESP map of HCBHAT-PH.b) The PDOS of O, N, and Li in lithiated HCBHAT-PH.c) The adsorption energies for per Li + at different sites on the HCBHAT-PH.d) The lithiation pathway obtained from simulations.The left axis shows the redox potential versus Li + /Li, and the right axis shows the total energy of various HCBHAT-PH structures.e) Structural evolution during the lithiation procedure.