Insights into Formation and Li‐Storage Mechanisms of Hierarchical Accordion‐Shape Orthorhombic CuNb2O6 toward Lithium‐Ion Capacitors as an Anode‐Active Material

The orthorhombic CuNb2O6 (O–CNO) is established as a competitive anode for lithium‐ion capacitors (LICs) owing to its attractive compositional/structural merits. However, the high‐temperature synthesis (>900 °C) and controversial charge‐storage mechanism always limit its applications. Herein, we develop a low‐temperature strategy to fabricate a nano‐blocks‐constructed hierarchical accordional O–CNO framework by employing multilayered Nb2CTx as the niobium source. The intrinsic stress‐induced formation/transformation mechanism of the monoclinic CuNb2O6 to O–CNO is tentatively put forward. Furthermore, the integrated phase conversion and solid solution lithium‐storage mechanism is reasonably unveiled with comprehensive in(ex) situ characterizations. Thanks to its unique structural merits and lithium‐storage process, the resulted O–CNO anode is endowed with a large capacity of 150.3 mAh g−1 at 2.0 A g−1, along with long‐duration cycling behaviors. Furthermore, the constructed O–CNO‐based LICs exhibit a high energy (138.9 Wh kg−1) and power (4.0 kW kg−1) densities with a modest cycling stability (15.8% capacity degradation after 3000 consecutive cycles). More meaningfully, the in‐depth insights into the formation and charge‐storage process here can promote the extensive development of binary metal Nb‐based oxides for advanced LICs.


Introduction
With the rapid development of the global economy, the world's diminishing chemical energy like gas, coal, and oil turns out to be increasingly prominent, and a serious energy crisis is happening all over the world.[3] Therefore, the development of clean and sustainable new energy has become an important topic in the world.Accordingly, electrochemical energy storage devices (EESDs) are emerging, and used widely in daily life because of their convenience and less affected by environmental factors. [4,5]High energy density lithium-ion batteries (LIBs) and high-power supercapacitors (SCs) stand out from others because of their respective unique advantages.However, the sluggish Li + diffusion always renders the LIBs with low power delivery and short cycle life, [5] and the surface charge-storage nature makes the SCs modest energy density. [6]hus, they cannot give consideration to both high energy density and high power density at all when singly applied. [7]Therefore, it becomes the research focus in the nextgeneration energy storage field to develop an advanced electrochemical device that can make up for the shortcomings of LIBs and SCs to meet the demanding needs for electrochemical energy storage in the future.
Lithium-ion capacitors (LICs), as a representative hybrid cell, which smartly combine the cationic (de)intercalation anode and anion adsorption-desorption cathode, make full use of the respective advantage of SCs and LIBs to achieve higher power density than LIBs and larger energy densities than SCs meanwhile. [7,8]As a result, LICs have attracted more attentions than ever.9][10] Accordingly, it is still a critical challenge to improve the intrinsic rate properties of anode materials toward advanced LICs.
][13][14] Unfortunately, the slow electronic conductivity greatly limits its further practical development. [15]Very recently, the smart design of bimetallic Nb-based mixed oxides (i.e., M-Nb-O, M=Mo, [16,17] Cu, [15,18,19] W, [20][21][22][23] Mn, [24] Ni, [25,26] Al, [27] etc.) has been known as an effective yet simple avenue to improve the electronic conductivity of Nb 2 O 5 itself, along with even higher theoretical capacities owing to the richer redox chemistry originating from other active species for some cases.Among these binary candidates, the columbite structure M-Nb-O with the general expression of MNb 2 O 6 (M=Mn, [24] Co, [28] Ni, [25] Cu, [18] Zn, [29] etc.) stands out from others, due to their unique crystal structure and a single type of channel for lithium-ion (de)intercalation. [25][32] Unfortunately, two issues should be addressed well for the CNO electrode.One thing is the hightemperature synthesis, particularly for the case orthorhombic CuNb 2 O 6 (O-CNO), the annealing temperature of >900 °C is highly requisite, [33] resulting in higher energy consumption.][36] Typically, three views including the conversion of CNO into NbO, NbO 2 , and Cu, [34] the insertion of Li + into CNO forming new phases (Li 3 NbO 4 and Cu), [32] and the formation of Li x CuNb 2 O 6 (0 ≤ x ≤ 1.9) [36] are proposed over its first lithiation process.One especially notes that all the three are phenomenologically put forward with simple ex situ characterization technologies.Therefore, the exploration of facile and lower temperature synthesis methodologies, and unveiling the intrinsic lithium-storage mechanism via advanced in situ characterizations become of equal significance for the CNO anodes toward next-generation LICs.
With the detailed considerations in mind, we, in the contribution, first fabricated hierarchical accordion-shape monoclinic CNO (M-CNO) framework with multilayered Nb 2 CT x MXene as the precursor with a facile two-step avenue (i.e., solvothermal treatment and subsequent annealing at 800 °C), and the as-obtained M-CNO was transformed into the O-CNO just after ground.In this way, the higher temperature treatment (>1000 °C for 12 h) was avoided if the direct conversion from precursor to the O-CNO was conduct.Furthermore, the authentic lithium-storage mechanism of the O-CNO anode was rationally proposed by combining in(ex) situ techniques for the first time.The newly formed LiNb 3 O 8 and NbO 2 over the initial lithiation are the electroactive phases for the reversible lithium storage of the O-CNO, and the appearing Cu acts as a conductive network to improve additional fast pathway for electron transport.Benefitting from its unique lithiumstorage process and structural merits, the resulted O-CNO anode showed with rapid electronic and ionic transport capability, and delivered highrate capacities and long-duration cycling properties.When assembled with activated carbon (AC) cathode, a high energy density of 138.9 Wh kg −1 was provided by the O-CNO-based LICs at the power density of 200 W kg −1 , along with outstanding cycling stability.

Physicochemical and Structural Characterizations
A diagram of the synthetic procedure of O-CNO samples is schematically shown in Scheme 1. Specifically, the Nb 2 AlC powder (Figure S1a, Supporting Information) was etched using lithium fluoride and hydrochloric acid to obtain the accordion-like multilayered Nb 2 CT x MXene (Figure S1b, Supporting Information), which is fully authenticated by the X-ray diffraction (XRD) data (Figure S1c, Supporting Information).Afterward, the resultant Nb 2 CT x is transformed to M-CNO via solvothermal treatment and subsequent calcination in air for 6 h at 800 °C.All the reflections, as shown in Figure 1a, are well indexed as the monoclinic phase (JCPDS no.83-0369) of a space group of P21/c (14), where the distorted NbO 6 and CuO 6 octahedra are linked by the common edge sharing, forming the orderly repeated Cu-Nb-Nb-Cu-Nb-Nb-Cu zigzag chains (Figure 1b).The XRD pattern with a Rietveld refinement shows a small R factor (R wp = 5.66% and R p = 4.05%), which also confirms the formation of phase-pure M-CNO.More interestingly, the conversion of M-CNO into O-CNO (JCPDS no.81-0269) with a space group of Pbcn (60) unexpectedly occurs just after ground manually for about 15 min.It can be evidenced by the integration of (131) and (131) diffraction peaks of M-CNO into a single diffraction peak (311) as observed in Figure 1c.The prominent difference from M-CNO is the CuO 6 octahedra.Specifically, the monoclinic phase possesses the symmetric octahedral, while the orthorhombic owns the distorted octahedral, leading to even larger twodimensional (2D) channels for fast Li + transportation (Figure 1d).Thus, the O-CNO is deemed as an ideal high-rate capability anode candidate for LICs. [32]It should assume that the phase transition phenomenon here is a classic stress-induced process, which can be clarified from a crystallographic point of view.Generally, supposing that the oxygen layers located above and below the copper layer glided along [101] (in P2 1 /c) in the opposite direction, and/or the chains of CuO 6 octahedra are stretched simultaneously in their direction of propagation (Figure 1e), the crystalline transformation, that is, a displacive process, will probably take place, [37] along with a 0.83% volume reduction in the unit cell.It is worthy of mentioning that the inverse process from O-CNO to M-CNO cannot be realized with our experimental efforts, indicating the less stable feature of the M-CNO at room temperature, and complete irreversibility of the phase conversion here.
To better understand the advantage of stress-induced phase transition, the thermally induced phase transformation, which is a very common avenue for this, was comprehensively explored using the P-CNO (i.e., hydrothermal product) as the starting material.As illustrated in Figure 1f, when the annealing temperature is below 900 °C, the sample always tends to form the monoclinic even if the annealing duration is extended to 12 h.With the temperature increasing to 1000 °C and being kept for 6 h, the monoclinic and orthorhombic systems still coexist.With the extension of holding time at 1000 °C, the monoclinic gradually changes to orthorhombic system, and only the duration is up to 12 h, the phasepure O-CNO can be successfully obtained.As schematically summarized in Figure 1g, our strategy (i.e., thermally and stress-induced phase transition) here circumvents the high calcination temperature (1000 °C) and long duration (12 h) to fabricate the O-CNO, hugely reducing energy consumption, which favors for its practical application.More interestingly, such thermally or stress-induced phase transition can be effectively restrained by partially substituting Cu by other species like Ni (Figure S2, Supporting Information), Zn (Figure S3, Supporting Information), or Co (Figure S4, Supporting Information).
The panoramic field-emission scanning electron microscopy (FESEM) image of the M-CNO (Figure 2a) visualizes that the threedimensional (3D) M-CNO inherits well the open accordion-like structure of the Nb 2 CT x , and is constructed with many crystalline nanoparticles (≈50-200 nm in size) assembled nanosheet subunits (Figure 2b-d).The high-resolution transmission electron microscopy (HRTEM) characterization (Figure 2d), corresponding to the white rectangle region in Figure 2c, clearly illustrates discernable fringes of a lattice spacing of 0.365 nm, going well with the (031) plane of the M-CNO, as estimated from the intensity profile (the inset in Figure 2d).Corresponding energy dispersive X-ray spectroscopy (EDX) mapping observations evidence that the elements of Cu, Nb, and O uniformly are distributed in the M-CNO sample (Figure 2e).Furthermore, the open accordion-like framework is well retained for the case of O-CNO (Figure 2f).As examined from TEM (Figure 2g) and HRTEM (Figure 2h) images, the well-defined interplanar distances of 0.36 and 0.365 nm, corresponding to the (311) and (310) crystalline planes of the O-CNO, respectively, are very clear.Relatively clear selected area electron diffraction pattern (SAED), as seen in Figure 2j, manifests a series of diffraction rings, which can be indexed the (310)/(311) planes of O-CNO, verifying its polycrystalline characteristics.
To further study specific chemical constitution of the prepared O-CNO, X-ray photoelectron spectroscopy (XPS) were applied.The survey spectrum (Figure 3a) evidence that the elemental Nb, Cu, and O coexist with an approximate stoichiometric proportion of 1:2:6, indicative of the pure-phase O-CNO.Based on the deconvolution of the Cu 2p core-level spectra (Figure 3b), the energy separation of Cu 2p 3/2 (934.4 eV) and Cu 2p 1/2 (954.3eV) is calculated as 19.9 eV, which suggests the Cu(II) in the O-CNO. [15,18,32]The Nb 3d 3/2 and 3d 5/2 signals are located at 208.5 and 206.8 eV, respectively, confirming the existence of Nb 5+ species (Figure 3c). [11,38]The O 1s core-level spectrum (Figure S5, Supporting Information) is fitted as the metal-O-bonded lattice oxygen (Cu-O or Nb-O, 529.8 eV) and surface-bonded chemi-or physisorbed oxygen species (531.8 eV). [29]The peaks (the blue) appearing below 400 cm −1 , as  shown in Raman spectrum (Figure 3d), are ascribed to the Cu-O and/or Nb-O stretching couple with O-Nb-O bending, [32] while those located at around 638, 530, and 478 cm −1 correspond to the stretching vibration modes of the Nb-O bond. [39]Besides, the vibration peak in purple at 900 cm −1 is reasonably related to the symmetrical Nb=O bridging bond in the O-CNO. [24]Figure 3e profiles the electronic band structure of O-CNO using density functional theory (DFT) calculations with Hubbard U, as presented in Figure 3e.Typically, the O-CNO with a net magnetic moment of 4.0 μ B shows different spin-up and spin-down contributions at the unique symmetric points in the Brillouin region. [40]The intermediate bands in the spindown channel near the Fermi level divide the band gap of O-CNO into two sub-band gaps.One is 0.64 eV at B, and the other is 1.77 eV along the B → B 2 region.It is the partially occupied bands near the Fermi level that indeed contribute to the unique conductivity of p-type O-CNO, and are more conducive to the electron transition from valence band to conduction band, when compared with the indirect-band gap semiconductors like NaNbO 3 (≈3.4eV) and T-Nb 2 O 5 (≈3.2 eV). [38]While M-CNO has a direct band gap (0.64 eV) at Γ and an indirect one (2.00 eV) along Γ → C region (Figure S6, Supporting Information). [40]The smaller band gap implies the better electronic conductivity of the O-CNO itself.

Electrochemical Evaluation
Electrochemical behaviors of the O-CNO are initially performed in the half-cell configurations.Figure 4a displayed the first cyclic voltammetry (CV) curve within the potential range from 0.01 to 3.0 V (vs Li/Li + ) at a scanning rate of 0.1 mV s −1 .Notably, numerous peaks discerned in the first cathodic process feature the involved complex electrochemical reactions.Typically, the reduction peak located near 2.35 V (P 1 ) and disappearing completely during subsequent cycles (Figure 4b) is probably related to the irreversible conversion of O-CNO decomposed into LiNb 3 O 8 and NbO 2 , as well as the formation of Cu nanoparticles. [32]The intense peaks at 1.7 V (P 2 ) and 1.2 V (P 4 ), accompanied with other three moderate ones at 0.8 V (P 6 ), 0.65 V (P 7 ), and 0.25 V (P 8 ) correspond to the reduction of Cu 2+ into Cu and the formation of Li 2 O, which follows the growth of solid electrolyte interface (SEI) film. [41]Besides, the prominent peak at 1.3 V (P 3 ) and a broad peak at 1.0 V (P 5 ) are assigned to the valence variation from Nb(V) to Nb(IV), along with the partial reduction of Nb(IV) to Nb(IIV). [42,43]During the subsequent CV cycles (Figure 4b), two pronounced peaks at 1.5/ 1.75 V and 1.0/1.2V are detected, indicating the reversible redox behavior of the LiNb 3 O 8 phase. [42]In addition, other peaks at 1.3/ 1.6 V are observed, which should be ascribed to the lithiation/delithiation processes of NbO 2 . [44]urthermore, in(ex) situ XRD measurements were used to verify the above electrochemical reaction process as analyzed above, and clarify the inherent lithium storage mechanism of O-CNO anode, as well as the phase conversion and structural changes during (dis)charging process.Figure 5a shows the in situ XRD patterns from 10 to 50°over the first original discharge/charge cycles at 0.1 A g −1 with the potential window from 0.01 to 3.0 V (vs Li/Li + ), and a sweep rate of 7°min −1 is set for this.As seen in Figure 5a, an appreciable variation is evident, highlighting the indeed occurrence of phase transitions with different parameters over the discharging/charging process.With the beginning of the discharge process, the peaks of O-CNO at 2θ = 12.5, 24.3, 24.7, 30.3, 35, and 36.5°,corresponding to (200), ( 111), (310), (311), (002), and (021) planes, respectively, gradually diminish, revealing the crystal structure of O-CNO lattice progressively collapses (Figure 5b).Meanwhile, some weak and broad peaks arise at 2θ = 12.2, 26, and 33.5°, and the original peaks at 24 and 35.5°steadily become wider (Figure 5a,c), indicative of the formation of some new phases.More distinctly, during the further (de)lithiation process, the reflections at 12.2 and 26°only show a slight shift to smaller 2θ values, then to large diffraction angles by degrees during the charging process, and are finally back to original positions.It is the appearance of lattice breathing process here that features the classic Li + -(de) intercalation mechanism in the newly formed phases.Note that the peaks at 12.2 and 24°are well indexed to the (002) and (004) planes of LiNb 3 O 8 , while the peaks at 26, 35.5, and 33.5°are well corresponded to the (400), (222) planes of NbO 2 , and (111) plane of Li 2 O.Besides this, the original peak at 2θ = 43.3°isgradually replaced by a wide and distinct peak, which is well indexed to the (111) plane of Cu.Further FESEM/HRTEM observations of the O-CNO electrode at fully discharged states (Figure S7, Supporting Information) also can validate the uniform distribution of the nano Cu, besides the LiNb 3 O 8 and NbO 2 .It is worthy of mentioning that the generated Cu phase cannot participate in the subsequent electrochemical reaction, but will hugely enhance the electronic conductivity of electrodes.Of note that even if the phase transition arises over the initial lithiation process, the accordion-like hierarchical skeleton is well retained (Figure S7, Supporting Information).Moreover, the same phase transition process with Li + intercalation takes place in the M-CNO anode as well (Figure S8, Supporting Information).
The involved electrochemical process in the first (de)lithiation cycle is further solidly supported by the ex situ XRD data (Figure 5d).Obviously, for the O-CNO electrode, the LiNb 3 O 8 and NbO 2 phase are formed after the first cycle, which is in line with the in situ XRD analysis above.During the second cycle, no new reflections appear besides the periodic variation of those at 12.2 and 26°corresponding to the (002, LiNb 3 O 8 ) and (400, NbO 2 ) planes, respectively, which proves the intercalation reaction involved lithium storage mechanism of both the LiNb 3 O 8 and NbO 2 .The partialenlarged contour images (Figure 5d) show that only a reversible shift by 0.22 and 0.18°F Energy Environ.Mater.2024, 7, e12583 correspond to the 12.2 and 26°diffraction peaks can be determined during the (de)lithiation states, which means that the lithium intercalation only causes 1.9% and 0.65% widening between the (002) planes and (400) planes.This minimal change suggests that just small volume expansion occurs in the LiNb 3 O 8 and NbO 2 phases.In summary, the reasonable electrochemical lithium-storage mechanism of O-CNO anode is proposed by the following equations: characterization. [45]Moreover, with the DFT nudged elastic band (DFT-NEB) calculations, the diffusion barriers along the surface and the b direction of the as-formed LiNb 3 O 8 are estimated as 0.17 and 0.70 eV, respectively (Figure 6b), which is still comparable to those of T-Nb 2 O 5 (≈0.47 eV), [46] NiNb 2 O 6 (≈0.46 eV), [25] and TiO 2 (≈0.496 eV) [47] for rapid Li + storage/release.Besides this, the LiNb 3 O 8 is endowed with a narrow band gap of ≈3.1 eV (Figure 6c), similar to other previously reported intercalated anodes including the aforementioned T-Nb 2 O 5 and NaNbO 3 , [38] and Li 4 Ti 5 O 12 (≈3.14eV), [25] ensuring its convenient electronic transport.As for the tetragonal NbO 2 with a space group I4 1/a (a = b = 13.70 Å, c = 5.987 Å, α = β = γ = 90°, V = 1123.7 Å3 ), a rutile structure consisting of NbO 6 octahedron along the c-axis is evidence, forming the strings through the edge-sharing, consequently, the 2D ion diffusion channel is built along the c-axis between the strings which connect by the corner-sharing.More impressively, the NbO 2 possesses a band gap as small as ≈0.45 eV, as derived from Figure 6d, highlighting its superb electron conductivity.Additionally, the higher theoretical capacity (≈429 mAh g −1 ) and lower (de)intercalation platform (≈1.3 V) both further make NbO 2 more competitive with other Nb-based intercalation-type anodes. [43]he selected charge/discharge plots (0.1 A g −1 ) of the O-CNO anode are exhibited in Figure 7a.An irreversible phase transitioninduced small platform emerges at 2.35 V in the first discharging plot, and then diminishes in the following discharging process. [32]And the one at ≈1.25 V is attributed to the reduction transition from the Nb 5+ to Nb 4+ /Nb 3+ , [11] which is consistent with the aforementioned CV data (Figure 4a).Moreover, during the first cycle, partial irreversible  (Figure S9, Supporting Information) electrodes from 0.05 to 2.0 A g −1 .Obviously, even though the O-CNO electrode displays a smaller capacity of ≈245.6 mAh g −1 at a small current density of 0.1 A g −1 , compared to T-Nb 2 O 5 (≈300.0mAh g −1 ), it delivers a capacity as large as ≈150.3mAh g −1 at 2.0 A g −1 , even higher than two times that of T-Nb 2 O 5 (≈57.0 mAh g −1 ).Thus, a modest capacity decay of ≈38.8% can be obtained as the current density increases by 40 times for the O-CNO, whereas ≈81.0% for the case of T-Nb 2 O 5 .][50][51] The high-rate characteristic is undoubtedly bound up with the unique energy storage mechanism of O-CNO itself, which can solve the dynamic difference between cathode and anode in LICs.Besides, the O-CNO shows competitive electrochemical stability in comparison with its counterpart of T-Nb 2 O 5 both at 0.1 (Figure S10, Supporting Information) and 1.0 A g −1 (Figure 7d).Typically, after 500 charge-discharge cycles at 0.1 A g −1 , a competitive capacity of 460.7 mAh g −1 remained reversibly for the O-CNO, along with a maintained capacity of ≈150.4 mAh g −1 after 2000 consecutive cycles under a high current density of 1 A g −1 .Furthermore, the more centralized voltage profiles upon cycling further validates sturdy electrochemical charge-discharge stability of O-CNO (Figure S10b, Supporting Information), in contrast with T-Nb 2 O 5 (Figure S10c, Supporting Information).It is particularly noted that an initial increase in capacity with cycling occurs in both O-CNO and T-Nb 2 O 5 , which maybe originates from the gradual amorphous of electrode materials along with the gradual penetration of electrolyte into the electrodes, resulting in the exposure of more active sites and partial reversible reaction of SEI film. [32,38]he kinetic properties of electrochemical process the obtained O-CNO and T-Nb 2 O 5 were further evaluated by CV test from 0.2 to 1.0 mV s −1 .As the sweep rate increases, the curves of the two anodes show similar wide peaks along with modest polarization (Figure 7e; Figure S11, Supporting Information).The smaller peak gap of O-CNO (Figure 7e) reveals its even fast kinetics for lithium storage than T-Nb 2 O 5 (Figure S11a, Supporting Information).Typically, the relationship between electrochemical response peak current (i) and scanning rate (v) can be obtained by the following equation: where a and b are both variable parameters.The b value derived from the equation is closely related to the kinetics information of electrochemical reactions.Generally, when b = 0.5, it is a typical solid diffusion controlled Faraday process; and for the case of b = 1, it is a pure capacitance process (non-Faraday process). [38]he b values of cathodic and anodic peaks, as shown in Figure 7f, are calculated as 0.86 and 0.73, respectively, suggesting the capacitive behavior with a fast lithium (de)intercalation process for the O-CNO.The detailed contributions from the surface capacitive behavior and diffusion-controlled process can be generally distinguished by the equation: where the k 1 v and k 2 v 1/2 represent the surface capacitive behavior part, and the diffusion-controlled process contribution, respectively. [22]Notably, the capacitive behavior contribution (blue region) is calculated as ≈89.4% of the O-CNO at 0.8 mV s −1 , as presented in Figure 7g.For comparison, the b values of T-Nb 2 O 5 are 0.75 and 0.77 for the anodic and cathodic process, respectively (Figure S11b, Supporting Information), and just a capacitive behavior contribution of 84.3% is obtained (Figure S11c, Supporting Information).What is more attractive is that the capacitive behavior contribution is as high as 92.1% for the O-CNO when the scan rate of 1.0 mV s −1 is applied, as presented in Figure 7h.The lithium-ion diffusion coefficient (D Li ) of O-CNO and T-Nb 2 O 5 electrodes during the first two cycles are further explored by galvanostatic intermittent titration technique (GITT) (Figure S11d,e, Supporting Information).The D Li values can be determined according to the following formula: [11,52] where τ, n m , V m , S, L, ΔE s , and ΔE t are the relation time (600 s), number of moles of O-CNO, molar volume of O-CNO, electrodeelectrolyte contact area, electrode thickness, potential change induced by the current pulse, and the potential variation of constant current (dis)charging, respectively (Figure S11f, Supporting Information).As can be seen from Figure 7i, the D Li are somewhat different during the first and second discharge of O-CNO, which may be ascribed to its unique electrochemical process, as discussed above (Figure 5).During the first discharge cycle, the D Li of O-CNO changes significantly from ≈3 × 10 −14 to ≈4 × 10 −10 cm 2 s −1 .In particular, the lower D Li values can be detected at 2.4 V (≈3 × 10 −14 cm 2 s −1 ) and 1.3 V (≈8 × 10 −14 cm 2 s −1 ), where more reduction reactions take place (Figure 4).More interestingly, over the second discharge, the stable D Li are discerned between ≈2 × 10 −11 and ≈3 × 10

Electrochemical Evaluation of the O-CNO//AC Hybrid Device
The electrochemical properties of the hybrid LICs were constructed by O-CNO used as the anode electrode coupling with commercial activated carbon (AC), as schematically illustrated in Figure 8a.The AC cathode presents a typical rectangular CV curve, and exhibits a specific capacity of ≈50 mAh g −1 at 1 A g −1 , along with a remarkable cycle lifespan (Figure S12, Supporting Information).Before assembled into a full cell, the O-CNO anode is particularly pre-lithiated, and the prelithiated O-CNO anode and AC cathode were examined at the mass ratio of 1:3 to balance the capacity difference between the two.Figure 8b presents the CV curves of the AC//O-CNO cell at different scan rates from 5 to 50 mV s −1 .The unique quasi-rectangular shape can be observed indicating the synergy effect of rapid adsorption/desorption of PF 6 À at positive electrode and fast Li + (de)insertion at the negative.
Energy Environ.Mater.2024, 7, e12583 Moreover, with the increase in sweep rate, the shape of the CV curves still well exhibits similar shape without serious distortion, suggesting the high electrochemical reversibility and rate ability of the LICs.The galvanostatic charging/discharging profiles show a quasi-triangular shape, as demonstrated in Figure 8c, revealing a good capacitive behavior with high electrochemical reversibility.Moreover, the specific capacitances of the AC//O-CNO LICs are calculated to be ≈69.5, ≈55.5, ≈34.2, ≈22.2, and ≈14.5 F g −1 at the current density of 0.1, 0.2, 0.5, 1.0, and 2.0 A g −1 , respectively.The self-discharge is a parameter, which describes the spontaneous attenuation of voltage and energy of charged LICs.As detected from the Figure 8d, an extremely low self-discharge rate of only ≈22.3% can be obtained after kept for 22 h.In addition, the O-CNO//AC device obtains a small leakage current of 0.028 mA within 5 h (Figure 8e), which is obviously better than and/or comparable to other reported devices (Table S1, Supporting Information). [53,54]igure 8f presents the typical Ragone plot of the O-CNO//AC cell.[57][58] More attractively, about 84.2% capacitance retention, corresponding to a capacitance degradation of 0.0052% per cycle, is achieved after 3000 consecutive cycles at 1.0 A g −1 , along with the almost 100% CE values over cycling, indicating the good cycling stability of the hybrid cells (Figure 8g).

Conclusion
In summary, the unique nano-blocks-constructed hierarchical accordional O-CNO framework was successfully constructed by a facile via a low-temperature synthetic methodology, toward high-performance LICs as the anode material.The stress-induced transformation from M-CNO to O-CNO was tentatively put forward from a crystallographic point of view, along with the heteroatoms-suppressed phase transition.With the systematic in(ex) situ physicochemical/structural characterizations and electrochemical methods, the special lithium-storage mechanism of O-CNO including electrochemical conversion of O-CNO into electroactive LiNb 3 O 8 /NbO 2 and Cu, and electrochemical (de)lithiation involved solid solution process of intercalation-type LiNb 3 O 8 /NbO 2 is reasonably proposed.Thanks to the fast (de)intercalation kinetics of the newly generated LiNb 3 O 8 /NbO 2 with high ionic/electronic conductivities, and extra well-distributed Cu electronic transport network, the obtained O-CNO electrode displayed even better rate capability with a capacity of 150.3 mAh g −1 at 2.0 A g −1 , and superb long-duration cycling capability with a well-retained capacity of 150.4 mAh g −1 over 2000 consecutive cycles at 1.0 A g −1 .Besides, a high energy density of 138.9 W kg −1 at 200.0 W kg −1 with a desirable cycling stability (84.2% after 3000 cycles) was achieved by the AC//O-CNO LICs.More meaningfully, the work here provides a unique perspective for in-depth understanding and rational design of next-generation anodes for advanced LICs.

Experimental Section
Synthesis of multilayered Nb 2 CT x : Commonly, 4.0 g of Nb 2 AlC (11 Technology Co., Ltd) was hydrothermally treated with a mixed solution of HCl (9 M, 80 mL) and LiF (6.0 g) in a Teflon-lined autoclave for 36 h at a temperature of 180 °C.After naturally cooled to room temperature, the suspension was obtained by centrifuged at 7500 g.Finally, the suspension was washed with deionized (DI) water several times and dried at 60 °C in vacuum to obtain the multilayered Nb 2 CT x powder.
Synthesis of M-CNO, O-CNO, and T-Nb 2 O 5 : In a typically experiment, 50 mg of Nb 2 CT x and 50 mg of CuCl 2 Á2H 2 O were added into absolute ethanol solution (20 mL).After transferred into a Teflon-lined autoclave, the resultant solution was kept for 16 h at 180 °C.The resulting slurry was then washed with DI water and ethanol before drying in vacuum.The obtained powder (denoted as P-CNO) was sintered for 6 h at 800 °C in air with a ramp of 5 °C min −1 .After cooled to RT naturally, the M-CNO sample was obtained accordingly.The O-CNO was obtained by hand grinding M-CNO in a mortar for about 15 min or increasing annealing temperature and prolonging holding time.For comparison, pure Nb 2 CT x MXene was calcined for 2 h at 800 °C in air to obtain the T-Nb 2 O 5 sample.
DFT calculations: All calculations were carried out by performing Vienna AB initio simulation package (VASP) calculations with the projector augmented wave (PAW) potentials and generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) method.Moreover, the plane wave cutoff energy was 450 eV and a 2 × 2 × 1 sheet k-point mesh was carried out, and the energy (10 −4 eV) and force (0.05 eV Å−1 ) convergence criteria was used for ionic relaxations.
Electrochemical measurement: Typically, the active material (O-CNO or AC), carbon black, and polyvinylidene fluoride with a mass ratio of 70:20:10 were well mixed to prepare the working electrode slurry by grinding about 15 min.Especially, the M-CNO electrode slurry was prepared just by stirring, and other conditions were kept the same as the O-CNO.The O-CNO electrode with a mass loading about 1.2 mg cm −2 .The CR2032 coin cell were assembled using commercial electrolyte (1 M LiPF 6 in the ethylene carbonate/diethyl carbonate with a volume ratio of 1:1), and porous polypropylene film in a glove box filled with pure argon gas (MBRAUN UNI-LAB PRO, both H 2 O and O 2 contents <0.1 ppm).For half cells, the lithium foil was used as counter electrode and evaluated within a potential window from 0.01 to 3.0 V (vs Li/Li + ).It was worth noting that in the process of making the electrode, the M-CNO will be transformed into O-CNO during the mixing electroactive with CB and PVDF by grinding.The in situ XRD measurements were conducted with an in situ matrix using an X-ray transparent beryllium window, and the mass load of in situ battery is about 3.5 mg cm −2 .For the LICs, the commercial activated carbon (AC) was used as the cathode coupling with O-CNO as anode.The O-CNO was in direct contact with metallic lithium for pre-lithiation under additional pressure for 6 h to minimize the irreversible capacity loss.The weight ratio of cathode and anode was designed as 3:1 to balance the capacity difference between the two electrodes.The total mass loading of the electroactive materials in full devices was around 4.3 mg cm −2 .
The electrochemical workstation system of IVIUM was carried out for cyclic voltammetry (CV) measurements.The galvanostatic charge/discharge profiles were performed using the Land battery testing system (CT2001A).The energy density (E) and powder density (P) of the full device were obtained by the equations: [22] P ¼ ΔV Â i=m (1) where V max and V min were the beginning and end of discharge voltage (V), i was the discharge current (A), t was the discharge time (s), and m was set as the total mass of active materials in both electrodes (kg).

Scheme 1 .
Scheme 1. Schematic illustration for synthetic process of the O-CNO.

Figure 1 .
Figure 1.a) Rietveld-refined XRD pattern and b) crystallographic structure of M-CNO.c) Rietveld-refined XRD pattern and d) crystallographic structure of O-CNO.e) Cu/Nb-O 6 octahedra of O-CNO, projected onto the ac plane, black points showing the atom positions before transition to O-CNO.f) XRD patterns of products obtained at various annealing temperatures and durations.g) Summary of transformation conditions to yield M-CNO and O-CNO.

Figure 2 .
Figure 2. a, b) FESEM, c) TEM, and d) HRTEM and corresponding intensity profiles indicating the measured interlayer of (031) plane and e) EDX mapping images of M-CNO.f) FESEM, g) TEM image, h,i) HRTEM image, and corresponding intensity profiles indicating the measured interlayer, and j) SAED pattern of O-CNO.

Figure 5 .
Figure 5. a) In situ XRD contour images and corresponding voltage curves during the first galvanostatic charge/discharge processes of the O-CNO electrode at 0.1 A g −1 , along with partial enlarged XRD patterns within the 2θ range from b) 23 to 27°, 28 to 33°, 34 to 40°, and c) 11 to 14°, 25 to 28°, and 42.5 to 44°.d) Ex situ XRD patterns and corresponding voltage curves during the initial two galvanostatic discharge/charge processes of the O-CNO electrode at 0.05 A g −1 .

Li y NbO 2
An interpretive scheme for the lithium-intercalation mechanism of the O-CNO anode materials during the charge/discharge process is illustrated in Figure 6a.As the lithium ions enter the anode materials during the discharge process, the structure of O-CNO collapses and pulverizes.The initial O-CNO is transformed into Cu, which is located in the electrochemically active insertion hosts of LiNb 3 O 8 and NbO 2 .Typically, the structure of LiNb 3 O 8 with a space group P21/a (a = 15.26Å, b = 5.03 Å, c = 7.46 Å, β = 107.34°) is formed by the parallel arrangement of edge-sharing NbO 6 and Nb/LiO 6 octahedrons along the a-axis to form a layer.The layers are connected in the direction perpendicular to the c-axis by the way of corner-sharing, forming a 1 × 1 zigzag channel along the b-axis where Li + can easily shuttle, which also corresponds to the peak shift in the in situ XRD

Figure 6 .
Figure 6.a) Structural transformation of O-CNO during the charge/discharge process, b) energy barrier of Li + diffusion in the LiNb 3 O 8 , and calculated band structures of c) LiNb 3 O 8 and d) NbO 2 .

Figure 7 .
Figure 7. a) Selected galvanostatic charge-discharge plots at 0.1 A g −1 of O-CNO.b) Rate behaviors, c) comparison of rate properties of O-CNO with those of other retrieved Nb-based oxide anodes, and cycling properties at d) 1.0 A g −1 of O-CNO and T-Nb 2 O 5 .e) CV curves, f) log(i) versus log(v) plots, and g) pseudocapacitive (blue) and battery-type (red) contributions at 0.8 mV s −1 of O-CNO.h) Normalized contribution ratios of the pseudocapacitive at different scanning rates and i) variation of D Li values at various discharge/charge states of the O-CNO and T-Nb 2 O 5 electrodes.
−10 cm 2 s −1 .Such phenomenon should be ascribed to the collapse of the original O-CNO structure and the stable transmission of lithium ions in the newly generated LiNb 3 O 8 and NbO 2 after the first cycle.By contrast, the T-Nb 2 O 5 shows relatively lower D Li values between ≈2 × 10 −15 and ≈2 × 10 −12 cm 2 s −1 .The improved Li + diffusion kinetics of the O-CNO anode can effectively alleviate the dynamic imbalance between the two involved electrodes of LICs, enormously favoring the enhancement in comprehensive electrochemical properties.

Figure 8 .
Figure 8. Characterization and electrochemical evaluation of the assembled AC//O-CNO device: a) Schematic illustration for the LICs.b) CV curves, c) charge-discharge plots at different current densities, d) self-discharge curve, e) leakage current profile, f) Ragone plot compared with those of other reported LICs, and g) long-duration cycling properties at 1.0 A g −1 .