Topological Structure‐Modulated Collagen Carbon as Two‐in‐One Energy Storage Configuration toward Ultrahigh Power and Energy Density

Efficient energy storage devices with suitable electrode materials, that integrate high power and high energy, are the crucial requisites of the renewable power source, which have unwrapped new possibilities in the sustainable development of energy and the environment. Herein, a facile collagen microstructure modulation strategy is proposed to construct a nitrogen/oxygen dual‐doped hierarchically porous carbon fiber with ultrahigh specific surface area (2788 m2 g−1) and large pore volume (4.56 cm3 g−1) via local microfibrous breakage/disassembly of natural structured proteins. Combining operando spectroscopy and density functional theory unveil that the dual‐heteroatom doping could effectively regulate the electronic structure of carbon atom framework with enhanced electric conductivity and electronegativity as well as decreased diffusion resistance in favor of rapid pseudocapacitive‐dominated Li+‐storage (353 mAh g−1 at 10 A g−1). Theoretical calculations reveal that the tailored micro−/mesoporous structures favor the rapid charge transfer and ion storage, synergistically realizing high capacity and superior rate performance for NPCF‐H cathode (75.0 mAh g−1 at 30 A g−1). The assembled device with NPCF‐H as both anode and cathode achieves extremely high energy density (200 Wh kg−1) with maximum power density (42 600 W kg−1) and ultralong lifespan (80% capacity retention over 10 000 cycles).

which well inherits the hierarchically fibrous microstructure even after the following high-temperature calcination.However, it exhibits unfavorable pore structure (1038 m 2 g −1 , 0.995 cm 3 g −1 ) and N-doping content (1.16 at %) due to low degree cross-linking interaction resulting from the inadequate utilization of the active sites within Col-Fs.Accordingly, exploring alternative approaches for simultaneously realizing both heteroatoms doping and pore regulation without multistep treatments is necessary.
In our case, we proposed a facile strategy by manipulating hydrolytic temperature to achieve the local microfibrous breakage/disassembly and thus increasing the exposed active positions in the Col-Fs, giving rise to a N/O dual-doped hierarchically porous carbon fiber (NPCF-H) with ultrahigh specific surface area (SSA) (2788 m 2 g −1 ) and large pore volume (4.56 cm 3 g −1 ) to serve as both anode and cathode for synchronously achieving extremely high power and energy density.Ex/in situ experiments combined with DFT calculations verify that the dual-heteroatom doping could effectively regulate the electronic structure of carbon atom framework with highly enhanced electric conductivity and decreased diffusion energy barrier toward Li + storage.Besides, the tailored micro/mesoporous structures simultaneously provide ample and available active storage sites together with fast charge transfer channels during the adsorption/desorption process.Benefiting from the optimized electrochemical performance of NPCF-H as both anode and cathode, the assembled lithium-ion capacitors (LICs) configuration achieves an extremely high energy density and power density as well as an impressive long-lasting lifespan.

Material Synthesis and Characterization
NPCF-H was fabricated as illustrated in Figure 1a.Col-Fs were firstly hydrolyzed at 90 °C with Ti 4+ -based metal salts and subsequently annealed at 900 °C under an inert atmosphere and acid etching.Notably, the longitudinal bundle-shaped Col-Fs (Figure 1b) are cracked into shorter length fibers (Figure 1c).Ti 4+ is transformed into TiO x after carbonization as confirmed by X-ray diffraction (XRD) pattern (Figure S1a, Supporting Information), which is uniformly dispersed on the surface of fibers (TiO x @NCF-H), as shown in Figure 1c 1 ,c 2 .After acid treatment, NPCF-H exhibits a highly dispersed fibrous morphology with an average diameter of 2-4 μm (Figure 1d,e).The inset in Figure 1e further reveals the alveolate structure formed by the removal of close-packing TiO x within the carbon fibers.For comparison, NPCF-L was prepared through the same procedure but under a relatively lower hydrolytic temperature of 40 °C (Figure S2, Supporting Information).Different from the monodispersed fibrous structure of TiO x @NCF-H, the bundle-liked fibrous morphology of Col-Fs is inherited after hydrolyzing, carbonization, and etching processes (Figure S3, Supporting Information).And the titanium oxide can be confirmed as titanium monoxide (Figure S1b, Supporting Information).Additionally, the morphology of NPCF-H was further characterized by high-resolution transmission electron microscopy (HRTEM) (Figure 1f-i).Figure 1f,g show the typical transmission electron microscope (TEM) images, where a fibrous morphology with a hierarchically porous structure of NPCF-H is visible.The nano-graphitic domains of 2-3 nm with an interlayer spacing of 0.42 nm were further observed by HRTEM (Figure 1h).The high-angle annular dark-field (HAADF) image (Figure 1i) and corresponding elemental mapping of NPCF-H also confirm the porous structure and the uniform distribution of C, O, and N elements.
The physicochemical structures of NPCF-H and NPCF-L were further investigated, as displayed in Figure 2.There are two broad peaks correspond to (002) and (101) crystal plane diffractions (PDF#65-6212) in the XRD patterns (Figure 2a).Obviously, the (002) peak of NPCF-H shifts to a lower diffraction angle compared with that of NPCF-L (Figure S4, Supporting Information), indicating the increased interlayer spacing. [23]According to Bragg's law, the interlayer distance of the NPCF-H is 0.41 nm (almost consistent with the result of TEM in Figure 1h), while that of NPCF-L is 0.39 nm.Raman spectra in Figure 2b reveal two typical characteristic peaks of carbon materials, the defectinduced D peak (1350 cm −1 ) and the graphitization-induced G peak (1580 cm −1 ). [24]The calculated I D /I G ratios of NPCF-H and NPCF-L are 1.70 and 1.61, respectively, suggesting more local defects and disorders in NPCF-H.The defective structure was further detected by electron paramagnetic resonance (EPR) (Figure 2c), in which NPCF-H exhibits the higher EPR signal due to more free electrons generated by more intrinsic deficiencies in carbon atom frameworks. [25]Subsequently, N 2 adsorption-desorption isothermal analyses at 77 K were performed to investigate the pore structures of NPCF-H and NPCF-L.The SSA and total pore volume of NPCF-H are calculated to be 2788 m 2 g −1 and 4.56 cm 3 g −1 , respectively (Figure 2d and Table S1, Supporting Information), much larger than those of NPCF-L (1815 m 2 g −1 and 2.04 cm 3 g −1 ), which can be attributed to the decentralization of the entangled Col-Fs template under higher hydrolytic temperature.Also, pore distribution and cumulative pore volume (Figure 2e,f) demonstrate that the two samples show similar hierarchical structures.Of particular note, NPCF-H exhibits a smaller micropore size range varying from 1.0 to 1.5 nm, and a more developed mesoporous structure.Such hierarchically porous structures of NPCF-H could offer abundant reservoirs and accelerate the transportation of ions (Figure 2g), exhibiting promising prospects as electrode materials for LICs.
The surface chemical states of NPCF-H and NPCF-L were further investigated by X-ray photoelectron spectroscopy (XPS).It can be seen that the O and N contents of NPCF-H are 6.1 and 2.4, respectively, higher than those of NPCF-L (Figure 2h).The higher doping level of O in NPCF-H is beneficial to the wettability of carbon, [26] demonstrated by the smaller contact angle in Figure S5 (Supporting Information).The N1s spectrum can be deconvolved into pyridinic N (N-6), pyrrolic N (N-5), and quaternary N (N-Q), respectively (Figure S6, Supporting Information). [27]Compared with NPCF-L, NPCF-H exhibits higher atomic ratios of N-6 and N-5 (Figure 2i), which can effectively introduce affluent active sites beneficial for surface-dominated charge storage. [28]

Electrochemical Performance of Carbon Anode
Next, the electrochemical performances of NPCF-H and NPCF-L as anode were compared.Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) in a potential range of 0.01-3.0V (vs Li/Li + ) are implemented to explore the kinetics evaluation of the two samples.Typical CV curves of NPCF-H and NPCF-L for the initial three cycles at 0.1 mV s −1 are displayed in Figure S7a,b, Supporting Information.An irreversible reduction peak at about 0.7 V can be observed in the first Energy Environ.Mater.2024, 7, e12536 cathodic sweep for both NPCF-H and NPCF-L, while the larger irreversible area of NPCF-H ascribes to its larger SSA for the formation of solid electrolyte interface (SEI) films. [29,30]The subsequent coincident cycles suggest the good reversibility of NPCF-H.Figure S7c,d, Supporting Information exhibit the first three charge-discharge curves of NPCF-H and NPCF-L at 0.1 A g −1 , respectively.Similarly, there is a short-lived plateau at about 0.8 V for both NPCF-H and NPCF-L, relating to the SEI films and the size relationships follow the same principle in CV.Although the initial Coulombic efficiencies of NPCF-H and NPCF-L are 38.8% and 41.9%, respectively, which can be offset by using prelithiation technology. [31]The rate performance in Figure 3a exhibits that NPCF-H shows higher capacities than that of NPCF-L at every current density, the charge-discharge curves of NPCF-H are shown in Figure 3b.In specific, NPCF-H delivers high specific capacities of 1668 and 352.8 mAh g −1 at 0.1 and 10 A g −1 , respectively.As a comparison, NPCF-L just delivers 78.5 mAh g −1 at 10 A g −1 , corresponding to a capacity retention of 15.1% based on the specific capacity at 0.1 A g −1 (Figure S8, Supporting Information), lower than that of NPCF-H (21.2%).][34][35][36] Furthermore, NPCF-H exhibits stable cycling capability with a capacity retention of 99.0% at 0.2 A g −1 after 50 cycles (Figure 3d).Even under a high current density of 2 A g −1 , NPCF-H delivers a high specific capacity of 590.9 mAh g −1 over 500 repeated cycles as depicted in Figure 3e.[34][35][36] As such, the quantitative kinetic analysis according to the CV measurements at different scan rates was performed to detect the electrochemical behavior of the NPCF-H electrode.Figure 3f exhibits the peak current increases and expands to cover a higher potential range when the scan rates increase from 0.1 to 5 mV s −1 , indicating the surfacedominated characteristic. [37]Furthermore, the storage mechanism also can be revealed by the following equation: where a and b are adjustable parameters.Particularly, the electrochemical kinetics is dominated by the diffusion process when the value of b approaches 0.5, while it is closed to 1.0 corresponding to the surface-controlled.In the range of 0.5-1, the energy storage mechanism is determined by diffusion and capacitive process. [29]s for NPCF-H, the b-values of the reduction and oxidation process are 0.81 and 0.78, respectively (Figure 3g), which indicates the electrochemical kinetics of NPCF-H are co-controlled by diffusion and capacitive behaviors.Furthermore, the ratio of capacitive contributions can be calculated by the following equation. [28] where k 1 and k 2 are adjustable parameters, calculated by the slope of i(V)/v 1/2 versus v 1/2 .The k 1 v and k 2 v 1/2 represent the contributions of diffusion-dominated and surface capacitive processes, respectively.As shown in Figure 3h, the contribution of the capacitive-dominated process for NPCF-H at 2 mV s −1 occupies 66.96%, obviously outperforming the capacitive contribution of NPCF-L (Figure S9, Supporting Information), which is especially beneficial for the fast capacitive-dominated charge storage. [12,38,39]

Lithium Storage Mechanism
To further reveal the optimized mechanism of the improved electrochemical kinetics, in situ Raman coupled with density functional theory (DFT) calculations were implemented to detect the Li + storage property of hierarchical pore carbon structure with N, O co-doping.
Raman spectroscopy was first applied to real-time monitor the structural evolution of carbon materials during the electrochemical process using NPCF-H as working electrode and lithium foil as reference electrode (Figure 4a-c and Figure S10, Supporting Information).
During the discharge process, the position, intensity, and shape of D and G bands are deeply correlated with the adsorption and intercalation of Li + (Figure 4a,b). [40]As the potential decreases from OCV to 0.01 V, the D band gradually shrinks ascribing to that adsorbed Li + occupied the active sites to limit the breathing motion of sp 2 atoms in the rings at edge planes, [41] and the G-band frequency (ω G ) displays a blue shift from 1582 to 1520 cm −1 (Figure 4c) resulting from the insertion of Li + . [42,43]Moreover, the G band undergoes a pronounced decrease and finally fades into the signal noise in the potential range of 0.3 to 0.01 V due to the weakening of resonance caused by Li + intercalation.It is worth noting that there are no characteristic split peaks of G-band for NPCF-H compared with graphite intercalation compounds (GIC), [44,45] which suggests Li + inserted randomly into the nano-graphitic domains of NPCF-H (Figure 4d).The weak interaction of Li + with NPCF-H is inclined to facilitate the fast Li + storage kinetics compared with the staged GIClike phase.For the charge process, both the D and G bands gradually show a positive shift along with increased intensity, indicating a reversible Li + storage mechanism of NPCF-H.DFT calculations (Figure 4e-k) were further implemented to detect the functional mechanism of N and O heteroatoms and their effects on Li + storage.The adsorption energy (ΔE a ) for GC (graphite structure), NGC (graphite structure with C vacancy and N atom doping), and NOGC (graphite structure with C vacancy and N and O atoms doping) were calculated (Figure S11, Supporting Information).Compared with GC (−1.98 eV) and NGC (−2.58 eV), the lowest adsorption energy (−3.55 eV) of NOGC indicates that the doping of N and O atoms greatly improves the Li + adsorption ability, which can promote the electrochemical performance of LIBs. [29]To a certain extent, it explains the large specific capacity of NPCF-H. [46]Electron density difference as shown in Figure 4e-g reveals the transfer of charge from the adsorbed Li to the nearest adjacent atom.Besides, it can be explicitly observed that the charge depletion of the Li atom and the charge accumulation around the N or O dopants in the NGC and NOGC models due to the enhanced adsorption ability and electronegativity of NPCF-H, which would favor the capacitance performance of Li + -storage.Figure 4h-j further unravel the lowest diffusion energy barrier of Li + within NOGC (0.072 eV) compared with that of NGC (0.086 eV) and GC (0.103 eV).These results demonstrate that the introduction of the N and O dopants provides a rapid electron transfer pathway, which accelerates the electrochemical kinetics of the battery-type anode.Moreover, the NOGC exhibits the highest density of states (DOS) around the Fermi level (Figure 4k), indicating a metallic bandgap and wonderful electronic conductivity. [47]The good electrical conductivity of NOGC with rapid electron transfer would also be an explanation for the large capacitive contribution of NPCF-H.In all, the N and O doping can effectively tune the electronic structure of carbon atoms and promote the adsorption and diffusion capability of Li + , achieving the favorable Li + -storage ability of NPCF-H.

Electrochemical Performance of Carbon Cathode
Subsequently, the electrochemical performances of NPCF-H as cathode were investigated in half-cell configurations between 2.0 and 4.5 V (vs.Li/Li + ). Figure 5a displays the CV curves of NPCF-H and AC with a high specific surface area of 2000 m 2 g −1 (Figure S12, Supporting Information) at a scan rate of 1 mV s −1 .Both NPCF-H and AC exhibit a quasi-rectangular shape, suggesting electrochemical double-layer capacitance (EDLC) and subordinate pseudo-capacitance due to the adsorption/desorption of PF 6 − within the porous structures. [12,36]The CV curves of AC gradually deviate from the rectangular shape (Figure S13, Supporting Information) along with the increase of scan rates.Significantly, NPCF-H still retains a decent rectangular shape without severe deviation even at a high scan rate of 40 mV s −1 (Figure 5b), indicating the smaller electrode polarization and excellent rate capability, the corresponded GCD curves (Figure S14, Supporting Information) also confirm the capacitance mechanism of NPCF-H electrode.
Notably, the NPCF-H (Figure 5c) delivers larger capacities than AC and NPCF-L (Figure S15, Supporting Information) at all current densities, which still retains a high capacity of 75.0 mAh g −1 at a high current density of 30 A g −1 .Besides, NPCF-H also displays excellent cycling capability with a capacity of 95.9 mAh g −1 at 1 A g −1 after 2000 cycles (Figure 5d).Even cycling at a higher current density of 5 A g −1 , NPCF-H exhibits negligible capacity loss over 5000 cycles with the overlapped linear charge-discharge curves (Figure 5e).The excellent electrochemical performances are highly comparable to that of many previously reported conventional carbon materials (Figure 5f and Table S3, Supporting Information), [12,32,35,36,[48][49][50][51] and thus it can be affirmed that the NPCF-H is a promising cathode material for LICs.

Theoretical Analysis of PF 6 − Ion
DFT calculations based on single-wall carbon nanotube (CNT), which was chosen as a proof-of-concept, were conducted to reveal the optimal microporous size for adsorption behaviors of PF 6 À .The size of fullydesolvated PF 6 À is around 0.4 nm.Additionally, an adequate range for balancing the gravity and repulsion between desolvated PF 6 À and carbon should be considered to form a stable structure. [52]Therefore, different pore sizes varying from 0.8 to 1.5 nm were chosen to explore the suitable pore structure for desolvated PF 6 À (Figure 6a). Figure 6b exhibits the variation tendency of adsorption energy and diffusion coefficient with enlargement of pore size.It can be seen that the corresponding ΔE a rapidly increases from −7.30 to −4.08 eV when the pore size increases from 0.8 to 1.0 nm, but with no significant improvement as the pore size is up to 1.5 nm, suggesting that 1.0 nm is a favorable aperture regarding thermodynamic reversibility.Furthermore, the diffusion coefficients of Li + within CNT with various pore sizes calculated by the molecular dynamics (MD) simulation are 8.74 × 10 −11 (0.8 nm), 1.90 × 10 −7 (1.0 nm) and 1.75 × 10 −9 m 2 s −1 (1.5 nm), respectively.The maximum diffusion coefficient in the case of 1.0 nm further indicates that it is an effective aperture beneficial for the ions transfer of capacitive cathode.Accordingly, NPCF-H, with a more favorable size for storage of PF 6 À (Figure 2e), performs a larger specific capacity compared with NPCF-L.
Next, the capacity contribution of hierarchical pore structures at different current densities is calculated to clarify the function of meso−/macroporous structure for capacitive cathode. [53]The detailed calculation process is provided in the experimental section and Figure S16, Supporting Information.As exhibited in Figure 6c, the ratio of C micro /C ext is a lessening trend accompanied by the increase of current density, where C micro and C ext are capacity per unit micropore surface area (<2 nm) and per unit external surface area (>2 nm), respectively.This tendency indicates that micropore-dominated capacity is limited by ions diffusion on the microporous surface, while the meso−/macropores are inclined to act as dominated charge storage reservoirs and charge transfer channels at high current density.Ex situ potential-dependent electrochemical impedance spectroscopies (EIS) of NPCF-H and NPCF-L cathodes were monitored to analyze the charge transport dynamics (Figure 6d,e).It can be seen that the charge transfer resistance (R ct ) of NPCF-H is lower than that of NPCF-L during the whole charge and discharge process, which demonstrates the fast charge-transfer kinetics resulting from the developed meso/macroporous structures. [54]Furthermore, the recovered depressed semicircles during the charge process indicate reversible ion kinetics transformation behaviors. [55]As was mentioned above, the hierarchical structure can offer a great number of active storage sites and fast charge transfer channels (Figure 6f), which is a good explanation for the excellent electrochemical performance of NPCF-H cathode.

Electrochemical Performance and Application of the Dual-Carbon Device
Based on the feasibility of employing NPCF-H as both anode and cathode, NPCF-H can serve as a two-in-one carbon electrode Energy Environ.Mater.2024, 7, e12536 configuration to assemble LICs, in which NPCF-H was used as a cathode coupled with prelithiated NPCF-H as anode and 1.0 M LiPF 6 electrolyte.Figure 7a shows the schematic diagram of the integrated LICs.During the charge process, Li + ions are absorbed on the carbon layers or the porous surface with O and N functional groups and defects or intercalated into the nano-graphitic domains, while PF 6 À ions move to the surface of the NPCF-H cathode, while the discharge process is reversed.In principle, the prelithiation technique is necessary to mitigate the initial lithium loss and achieve the excellent electrochemical performance of LICs by adjusting the optimal operating potential of the anode. [56]The optimal prelithiated point is determined to be 0.1 V by comparing the electrochemical performances of LICs with NPCF-H anode prelithiated to different potentials.(Figure S17, Supporting Information).Besides, the LIC is assembled with a 1:3 weight ratio of the anode and cathode due to the delivered higher specific capacitance (Figure S18, Supporting Information).And the optimum working voltage is regarded as 0.05-4.2V, which can be explained by the deviated CV curves from the quasi-rectangular shape and inferior cycling performance when the potential window is up to 4.5 V (Figure S19, Supporting Information).
The electrochemical performances of the optimal LICs are investigated.As shown in Figure 7b, the CV curves at different scan rates from 5 to 40 mV s −1 present the quasi-rectangular shape without obvious distortion resulting from the faradic reaction, demonstrating an ideal capacitive behavior, corresponding to the linear GCD curves in Figure 7c, [46] that is in favor of high reversibility and favorable rate performance.Figure 7d exhibits the cycling performance at 2 A g −1 , it is admirable that the symmetric LIC is capable to maintain a high capacity retention of 80% even after 10 000 charge/discharge cycles.Encouragingly, the symmetric LICs in this work can achieve an extremely high energy density of 200 Wh kg −1 with a maximum power density of 42 600 W kg −1 , sufficiently comparable to that of many previously reported LICs (Figure 7e and Table S4, Supporting Information), [32,35,36,49,57,58] which also bridges the gap between secondary Energy Environ.Mater.2024, 7, e12536 batteries such as lithium-ion batteries and supercapacitors.Furthermore, a practical pouch cell was further assembled with a size of 5 cm × 9 cm to light up a simple logo consisting of 60 light-emitting diodes (LEDs) in parallel for over 6 h (Figure 7f).The pouch cell can also drive a miniature windmill (0.075 W) for operating over 54 min and a windmill (3 W) for 40 s (Figure 7g), indicating the potential application as a promising energy storage device.

Conclusion
In summary, we have reported an N, O co-doped carbon material through topological structure modulation of local microfibrous breakage/disassembly of natural collagen fibers.Ex/in situ experiments combined with DFT calculations clarify that the dual-heteroatom doping and pore structure regulation could effectively provide valid active storage sites and promote electron transfer and ion diffusion capability.Consequently, the pseudocapacitive-dominated NPCF-H anode with good electric conductivity and electronegativity exhibits high specific capacity (1668 mAh g −1 at 0.1 A g −1 ) and outstanding rate performance (353 mAh g −1 at 10 A g −1 ).Moreover, the favorable micro/ mesoporous structure of NPCF-H cathode can offer affluent active sites for PF 6 À adsorption/desorption, enhancing the specific capacity (128.3 mAh g −1 at 0.1 A g −1 ) and promoting the rate performance (75.0 mAh g −1 at 30 A g −1 ).More importantly, an advanced energy storage device was assembled with the NPCF-H as two-in-one carbon electrodes, which can achieve an extremely high energy density of 200 Wh kg −1 with a maximum power density of 42 600 W kg −1 as well as an impressive capacity retention of 80% after 10 000 cycles.Our works provide insights into the construction principle of suitable electrode materials for efficient energy storage devices.

Experimental Section
Raw Materials: Collagen Fibers (Col-Fs), one of the most affluent collagenenriched animal skin wastes, have been applied as raw materials in the traditional leather-manufacturing industry.Col-Fs display a hierarchically interwoven fibrous NPCF-H anode was achieved by directly contacting with the lithium foils in an organic electrolyte for 8 h.Electrochemical Performance Measurements: GCD measurements were employed by a battery test system (LAND CT-2001A) within the potential range of 0.01-3 V (vs.Li/Li + ) for anode, 2-4.5 V (vs.Li/Li + ) for cathode and 0.05-4.2V for LICs.The current density of half-cell was calculated based on the mass of active materials.For the LIC device, it was calculated based on the total mass of both anode and cathode.The energy density and power density of LIC were calculated by using the following equations: where i and t correspond to the discharge current (A) and time (s), respectively, V max and V min represent the voltages (V) at the initial and end of the discharge process, and m is the total mass of active materials including anode and cathode (kg).

Figure 1 .
Figure 1.Schematic illustration and morphological structure characterizations.a) Schematic illustration of the synthesis procedure of NPCF-H.FESEM images of b) Col-Fs, c 1 , c 2 ) TiO x @NCF-H and d, e) NPCF-H.f, g, h) TEM images and HRTEM images of NPCF-H.i) HAADF-STEM image and corresponding EDX elemental mappings of NPCF-H sample.

Figure 2 .
Figure 2. Characterizations of NPCF-H and NPCF-L.a) XRD pattern.b) Raman spectra.c) EPR spectra.BET analysis d) Nitrogen adsorption and desorption isotherms, e) corresponding pore size distribution, the insert is themicropore size distribution, and f) cumulative pore volume.g) Schematic illustration of NPCF-H with suitable pore size distribution and substantial accommodation for ion.h) XPS spectra survey of C, N, and O, and i) the atomic ratios of N-5, N-6 and N-Q.Characterizations of NPCF-H and NPCF-L.

Figure 3 .
Figure 3. Electrochemical characterizations of NPCF-H as anode.a) Rate performance of NPCF-H and NPCF-L from 0.1 to 10 A g −1 .b) Charge/discharge profiles of NPCF-H from 0.1 to 10 A g −1 .c) The electrochemical performance of NPCF-H anode compared with previously reported carbon-based anodes.Cycling performance of NPCF-H at d) 0.2 A g −1 and e) 2 A g −1 .f) CV curves of NPCF-H electrode at different scan rates of 0.1-5 mV s −1 .g) CV curve of NPCF-H at 2 mV s −1 , the insert is plots of lg(v) versus lg(i) calculated from CV curves; h) Plot of capacitive charge contribution to total charge at different scan rates.

Figure 4 .
Figure 4. Mechanism analyses for Li + -storage of NPCF-H.a) GCD curves.b) In situ Raman spectra.c) Evolution of the G band. d) Schematic illustration of Li + -storage for NPCF-H.The calculated electron density differences of Li-ion adsorbed in the e) GC, f) NGC, and g) NOGC.Diffusion barrier energy of Li + in h) GC, i) NGC, j) NOGC, and k) corresponding DOS.

Figure 5 .
Figure 5. Electrochemical characterizations of NPCF-H as cathode.a) CV curves of NPCF-H and AC at 1 mV s −1 .b) CV curves of NPCF-H at different scan rates from 1 to 40 mV s −1 .c) Rate capability of NPCF-H and AC at 0.1-30 A g −1 .d) Cycling performance of NPCF-H and AC at 1 A g −1 .e) Long cycling performance of NPCF-H at the current density of 5 A g −1 , the insert shows GCD curves at the initial stage and after 1000, 3000, 5000 cycles at 5 A g −1 .f) Comparison the electrochemical performance between NPCF-H and previously reported cathodes.Characterizations of NPCF-H as cathode.

Figure 6 .
Figure 6.Theoretical simulation configurations with different pore sizes.a) Front views and top views of CNT-0.8,CNT-1.0, and CNT-1.5.b) Variation tendency of ΔE a and diffusion coefficients as the enlargement of pore size.c) The ratio of C micro /C ext .d, e) Ex situ EIS spectra of NPCF-H and NPCF-L cathodes at various charge and discharge states.f) Schematic illustration of ions storage and diffusion mechanism for micro/mesopore-dominated NPCF-H cathode.