As clean energy systems continue to expand, large-scale energy storage has become essential for smoothing the intermittent and fluctuating output of solar and wind power. In this context, sodium-ion batteries (SIBs) offer a practical route for stationary storage owing to the low cost, wide availability, and lithium-like electrochemical characteristics of sodium^[1,2]. Consequently, SIBs demonstrate broad application prospects in smart grids, consumer electronics, and electric transportation. The electrochemical behavior of the anode directly affects the usable capacity, long-term stability, and high-rate response of SIBs. Its performance acts as a bottleneck restricting the industrialization process of SIBs. Anodes explored for SIBs can be broadly classified into carbonaceous materials^[3,4], alloying-type systems^[5,6], metal-based compounds^[7-9], and two-dimensional (2D) layered structures^[10,11]. However, the critical drawbacks of traditional carbon-based materials (low capacity), alloy-type materials (severe volume expansion), and metal compounds (poor conductivity) render them inadequate for the demands of high-performance SIB applications. Accordingly, designing advanced anodes and developing effective modification strategies have become key research directions in the SIB field^[12,13].

As typical two-dimensional transition-metal carbides, nitrides, and carbonitrides, MXenes possess layered frameworks, metallic conductivity, mechanical robustness, and chemically tunable surface groups^[14-17]. Since the first MXene was reported in 2011, this material family has been increasingly explored for SIB anodes^[18]. Despite these advantages, MXene-based electrodes still suffer from inherent structural drawbacks that limit their practical performance. Specifically, van der Waals attraction between neighboring MXene layers promotes restacking, which reduces accessible surface area and active-site exposure while limiting fast Na⁺ diffusion within the interlayer channels^[19-21]. Meanwhile, the limited diversity and low reactivity of surface terminations on MXenes lead to weak electrolyte compatibility and make the layered structure prone to collapse during repeated cycling^[22,23], thereby compromising prolonged electrochemical durability. To address these issues, researchers have developed various modification strategies^[24,25]. Among them, grafting modification is considered one of the most promising methods due to its ability to precisely regulate surface chemical properties, effectively inhibit sheet restacking, and introduce new active sites^[26-28]. Compared with other organic cross-linkers, terephthalaldehyde (TPAL) offers several structural advantages for regulating MXene interlayers. Aliphatic dialdehydes, such as glutaraldehyde, can react with amino groups but their flexible chains may not provide sufficient rigidity to stabilize the interlayer spacing. Monofunctional aromatic aldehydes can introduce aromatic units but cannot effectively bridge adjacent MXene sheets. Other aromatic dialdehydes, such as phthalaldehyde or isophthalaldehyde, may also form imine linkages, but their different aldehyde positions lead to less linear molecular geometries. In contrast, TPAL contains two para-positioned aldehyde groups and a rigid benzene backbone, enabling covalent coupling with aminated Ti₃C₂ while serving as a molecular pillar to stabilize the expanded interlayer spacing. Therefore, TPAL was selected as a representative aromatic dialdehyde cross-linker to construct the double-lock interlayer structure. In principle, TPAL grafting may provide additional active sites, reinforce Na⁺ adsorption, and stabilize the electrode/electrolyte contact during cycling. Nevertheless, TPAL-grafted MXene anodes for SIBs remain insufficiently explored, and both the grafting process and the Na-storage behavior require further clarification.

In this work, TPAL is covalently grafted within aminated Ti₃C₂ MXene interlayers to construct a Ti₃C₂-TPAL composite with a well-defined double-lock architecture. The double-lock effect is defined as the combination of covalent anchoring and rigid interlayer pillaring. The imine bonds formed between TPAL and aminated Ti₃C₂ serve as the first lock, chemically anchoring the MXene sheets and suppressing restacking or interlayer slippage. The rigid aromatic TPAL molecules serve as the second lock, acting as molecular pillars to enlarge and stabilize the Ti₃C₂ interlayer spacing. The improved oxidation resistance, interfacial charge redistribution, and enhanced Na⁺ transport kinetics are therefore discussed as consequences of this double-lock structure rather than as separate components of the double-lock effect. By combining in situ X-ray diffraction (XRD), Distribution of relaxation times (DRT) analysis, and simplified density functional theory (DFT) calculations, the role of TPAL in stabilizing the Ti₃C₂ interlayers and improving Na-storage kinetics is investigated from both experimental and theoretical perspectives. The coordinated regulation of the interlayer structure and Na-storage kinetics gives Ti₃C₂-TPAL favorable electrochemical performance in SIB half cells, including 57.0 mAh g^-1 at 10 A g^-1 and 54.3 mAh g^-1 after 2,500 cycles at 5 A g^-1. Pairing with a Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) cathode, this configuration reaches cathode-mass-normalized energy and power densities of 190.8 Wh kg^-1 and 9,040 W kg^-1, respectively. The molecular-grafting-induced double-lock architecture reported here offers a useful strategy for constructing advanced 2D-material-based anodes for SIBs.

The preparation route of Ti₃C₂-TPAL is illustrated in Figure 1A. Ti₃C₂ MXene was first obtained by treating the Ti₃AlC₂ precursor with HF, which selectively extracted Al from the MAX structure [Supplementary Figure 1]. Subsequently, amino groups (-NH₂) were introduced via ammonium fluoride (NH₄F) grafting reaction to obtain the functionalized intermediate Ti₃C₂-NH₂. Finally, Ti₃C₂-TPAL composite was constructed by the covalent coupling of Ti₃C₂-NH₂ and TPAL via a dehydration condensation reaction. In this structure, TPAL provides two coupled structural functions: covalent imine anchoring and rigid interlayer pillaring. These two effects suppress sheet restacking and stabilize the expanded Ti₃C₂ galleries, thereby improving the accessibility of Na⁺ storage sites.

The FTIR spectrum in Figure 1B shows that, compared with pristine Ti₃C₂, Ti₃C₂-TPAL exhibits characteristic peaks assigned to C=N (1,693.5 cm^-1), C-N (1,301.2 cm^-1), and amino-related vibrations, -NH₂ (1,198.3 cm^-1)^[32], supporting the successful introduction of TPAL and the formation of imine-related bonding structures. To further distinguish chemical grafting from simple physical mixing, FTIR spectra of Ti₃C₂, Ti₃C₂-NH₂, and the Ti₃C₂-NH₂/TPAL physical mixture were added in Supplementary Figure 2. The physical mixture retains the amino-related vibration features of Ti₃C₂-NH₂, indicating that simple mixing does not lead to an obvious condensation reaction, further supporting the formation of imine-related bonding structures in Ti₃C₂-TPAL. XRD analysis was used to examine the structural evolution induced by TPAL grafting. The (002) reflection shifts from 8.9° for pristine Ti₃C₂ to 6.9° for Ti₃C₂-TPAL, confirming the enlargement of the average distance between layers, which creates more pathways for Na⁺ transport between MXene layers [Figure 1C].

Ti₃C₂-TPAL exhibits a zeta potential of -23.8 mV [Figure 1D], whereas that of Ti₃C₂ is only -7.3 mV, indicating that the grafting of organic molecules increases the electronegativity of Ti₃C₂-TPAL. In electrolytes, Na⁺ ions are usually accompanied by a solvation sheath (surrounded by solvent molecules). To intercalate into the electrode material, Na⁺ ions must first shed these solvent molecules (desolvation), which is typically an energy-consuming and slow process. A strongly negatively charged surface can directly undergo electrostatic adsorption with bare Na⁺ ions, aiding in stripping solvent molecules and thereby accelerating interfacial reaction kinetics. In other words, grafting TPAL not only acts as a “pillar” in the Ti₃C₂ interlayers to expand the spacing but also significantly increases the electronegativity of Ti₃C₂. Analogous to acceleration devices on a widened highway, the negatively charged functional groups within the expanded interlayer spacing synergistically promote rapid Na⁺ ions migration and strong adsorption.

The morphological changes induced by TPAL modification were further investigated by SEM and transmission electron microscopy (TEM). The results show that the Ti₃C₂-TPAL composite completely inherits the 3D layered framework of Ti₃C₂ [Figure 2A and B]. As evidenced by HRTEM images [Figure 2C and D], the local interlayer distance of Ti₃C₂-TPAL is approximately 1.41 nm, larger than that of pristine Ti₃C₂ (0.90 nm). Since HRTEM reflects only selected local regions, the average interlayer spacing was further evaluated from the XRD (002) peak using Bragg’s law. The (002) peak shifts from 8.9° for Ti₃C₂ to 6.9° for Ti₃C₂-TPAL, corresponding to calculated d-spacings of 0.993 and 1.280 nm, respectively. This decrease in the (002) diffraction angle confirms the enlarged average interlayer spacing after TPAL grafting. Additional control data show that Ti₃C₂-NH₂ has a (002) peak at 8.19° with a d-spacing of approximately 1.08 nm, whereas the Ti₃C₂-NH₂/TPAL physical mixture shows a (002) peak at 8.58° with a d-spacing of approximately 1.03 nm [Supplementary Figures 3 and 4], indicating that simple mechanical mixing with TPAL does not induce obvious interlayer expansion. These XRD-derived average spacings are consistent with the HRTEM observation, confirming the expanded interlayer structure after TPAL grafting. This widened spacing provides more accessible pathways for Na⁺ transport and interfacial storage, while reducing ion-diffusion resistance within the expanded Ti₃C₂ galleries. Additionally, SEM-energy-dispersive X-ray spectroscopy (EDX) elemental mapping [Figure 2E] confirms the uniform distribution of C, N, and Ti elements in Ti₃C₂-TPAL, indicating the homogeneous incorporation of N-containing amino/imine-related species after TPAL grafting. Based on EDX atomic-percentage analysis, the mass fraction of covalently grafted TPAL is estimated to be ~16.9 wt%, and full calculation details are retained in Supplementary Figure 5.

The surface composition, valence states, and bonding environment of the composite were further analyzed by XPS. After TPAL grafting, no pronounced shift was observed in the main Ti 2p or O 1s peaks, suggesting that the Ti-C framework and oxygen-containing surface terminations of Ti₃C₂ were largely preserved. The main XPS changes are reflected in the N 1s and C 1s regions, where the C=N and C-N components confirm the formation of imine-related bonding structures between TPAL and aminated Ti₃C₂. As shown in Figure 2F, N 1s deconvolution gives three fitted components located at 396.4, 398.9, and 400.9 eV^[33-35], corresponding to C=N, C-N, and -NH₂ configurations. These peaks reveal the formation of robust chemical bonds between the surface-aminated Ti₃C₂ and TPAL molecules. A further comparison of C 1s spectra [Figure 2G] shows that Ti₃C₂-TPAL exhibits characteristic peaks of π-π* transitions, C-N, and C=N at 283.7, 285.7, and 287.0 eV^[36,37], respectively, which are absent in pristine Ti₃C₂. This significant signal evolution further corroborates the introduction of TPAL.

The resistance of Ti₃C₂-TPAL against ambient oxidation was preliminarily evaluated by comparing the color, morphology, and Ti 2p XPS spectra of Ti₃C₂ and Ti₃C₂-TPAL after exposure to the same air-aging conditions. Air aging experiments indicate that Ti₃C₂-TPAL maintains its original black color and intact 3D layered microscopic skeleton after 15 days of environmental exposure [Figure 2H, Supplementary Figure 6]; in contrast, pristine Ti₃C₂ not only faded to grayish-black but also saw its layered structure collapse due to oxidation, making the accordion structure unrecognizable. The relative content of the Ti-O 2p_3/2 peak in the high-resolution Ti 2p spectrum serves as a key indicator for evaluating the oxidation degree of the Ti₃C₂ surface. Analysis shows that the Ti-O 2p_3/2 content of aged Ti₃C₂-TPAL only slightly increased from 7.7 at% to 7.8 at% [Figure 2I and J]. Conversely, this component in pristine Ti₃C₂ surged from 11.7 at% to 43.4 at%, confirming deep oxidative destruction of its crystal structure. This comparison indicates that TPAL grafting can mitigate the oxidation of Ti₃C₂ under the present ambient exposure conditions, which may help preserve surface-active sites during short-term storage. The porous characteristics of Ti₃C₂ and Ti₃C₂-TPAL were further evaluated by N₂ adsorption-desorption measurements [Supplementary Figure 7]. The BET surface area increases from 7.9354 m² g^-1 for pristine Ti₃C₂ to 37.3411 m² g^-1 for Ti₃C₂-TPAL, indicating that TPAL grafting improves the accessible surface area of the layered MXene structure. The corresponding pore-size distribution further shows a more pronounced pore-volume contribution in Ti₃C₂-TPAL, suggesting improved electrolyte-accessible channels after molecular grafting. These features can enhance electrolyte access and accelerate Na⁺ transport across the expanded MXene interlayers.

The Na-storage properties of Ti₃C₂-TPAL were examined in SIB half-cells over 0.01-2.5 V. The first three CV curves collected at 0.1 mV s^-1 are shown in Figure 3A. In the first CV curve, the reduction current below 1.5 V can be mainly attributed to SEI formation, electrolyte decomposition, and irreversible surface/interfacial reactions associated with Na⁺ storage. Figure 3B shows that Ti₃C₂-TPAL delivers 385.8 mAh g^-1 during the first discharge and 153.9 mAh g^-1 during the subsequent charge at 0.1 A g^-1. The relatively low initial Coulombic efficiency is a common issue for MXene-based sodium-storage anodes, mainly due to their abundant surface terminations, large exposed surface area, and defect-rich layered structure. This irreversible capacity loss can be attributed to SEI formation, electrolyte decomposition, irreversible reactions involving surface functional groups, and partial Na⁺ trapping at defects or interlayer sites. Therefore, Ti₃C₂-TPAL should be regarded as a MXene-based anode with improved high-rate stability and long-term cycling durability, while its initial Coulombic efficiency still requires further optimization. The nearly identical CV and charge-discharge profiles from the second to third cycle reflect good reaction reversibility. During cycling at 0.1 A g^-1, Ti₃C₂-TPAL maintains 88.1 mAh g^-1 after 200 cycles [Supplementary Figure 8], whereas pristine Ti₃C₂ only delivers 55.7 mAh g^-1. The higher capacity retention supports the role of TPAL grafting in reinforcing the electrode framework and improving cycling stability.

Rate performance tests show that Ti₃C₂-TPAL delivers excellent high-rate capacities of 101.9, 91.1, 81.7, 74.9, 67.6, and 57.0 mAh g^-1 at 0.2, 0.5, 1, 2, 5, and 10 A g^-1 [Figure 3C], respectively, far exceeding those of the Ti₃C₂ anode. After the current density returns to 0.2 A g^-1, the capacity recovers to 90.8 mAh g^-1, indicating excellent rate and reversibility for Ti₃C₂-TPAL. Even after 2,500 cycles under a rate of 5 A g^-1, a capacity of 54.3 mAh g^-1 is preserved [Figure 3D], demonstrating the excellent long-cycling performance. For comparison, the long-term cycling performances of pristine Ti₃C₂ and Ti₃C₂-NH₂ were further tested under the same current density and are provided in Supplementary Figure 9. Both control samples show lower capacities than Ti₃C₂-TPAL after long-term cycling, indicating that TPAL grafting provides additional structural stabilization beyond pristine Ti₃C₂ and the aminated intermediate. The rate capability and long-term cycling stability of Ti₃C₂-TPAL compare favorably with those of recently reported MXene-based anodes. [Figure 3E, Supplementary Table 1].

Electrochemical kinetic analysis further elucidates the impact of the imine-bond-based covalent anchoring between TPAL and Ti₃C₂ on optimizing battery performance. The CV curves exhibit similar shapes at scan rates from 0.2 to 1.1 mV s^-1 [Supplementary Figure 10]. Despite minor peak shifts, the consistent shapes confirm a stable electrochemical mechanism. Compared to Ti₃C₂, the CV curves of Ti₃C₂-TPAL exhibit reduced polarization effects and enhanced reversibility. Furthermore, based on the relationship between the response current (i) and the scan rate (v)^[38]:

i = av^b (4)

The b value is commonly used to distinguish diffusion-limited and surface-capacitive behaviors, with values approaching 1.0 indicating a stronger capacitive response. As shown in Figure 3F, Ti₃C₂-TPAL exhibits b values of 0.99 and 0.78 for peak 1 and peak 2, respectively, which exceed those of pristine Ti₃C₂ (0.78 and 0.76). This comparison reveals that Na⁺ storage in Ti₃C₂-TPAL is mainly governed by surface-controlled pseudocapacitive processes. The pseudocapacitive-dominated Ti₃C₂-TPAL electrode enables fast Na⁺ uptake and release, thereby supporting its favorable high-rate response. The capacity fractions from capacitive and diffusion-limited processes were further quantified using^[38]:

i = k₁v + k₂v^1/2 (5)

Here, k₁v and k₂v^1/2 correspond to the surface-capacitive and diffusion-limited components, respectively. As presented in Figure 3G, the surface-capacitive fraction of Ti₃C₂-TPAL is 77.8% at 0.2 mV s^-1 and rises to 90.1% as the scan rate reaches 1.1 mV s^-1. This value is higher than that of pristine Ti₃C₂ under the same scan rate (84.6%), indicating faster surface-dominated Na⁺ storage kinetics after TPAL grafting. These kinetic results indicate that the Na-storage behavior of Ti₃C₂-TPAL is mainly governed by surface-controlled pseudocapacitive processes rather than typical bulk intercalation. Therefore, the capacity contribution is attributed primarily to fast Na⁺ adsorption/desorption at accessible surfaces and interfaces, assisted by limited Na⁺ insertion within the expanded Ti₃C₂ galleries. GITT tests show that Ti₃C₂-TPAL possesses higher diffusion coefficients during both charge and discharge processes [Figure 3H, Supplementary Figure 11]. Electrochemical kinetic test results indicate that grafting TPAL molecules increases reactive active sites and significantly boosts reaction kinetics. Functioning as rigid pillars, TPAL expands the Ti₃C₂ interlayer spacing to facilitate ion transport and minimize diffusion resistance. This synergistic effect effectively promotes the rapid and stable storage of Na⁺ ions.

In situ XRD and SEM were further utilized to verify the structural stability of Ti₃C₂-TPAL. Interestingly, the (002) characteristic peak of Ti₃C₂-TPAL remains stable at 6.9° during the charge-discharge process [Figure 4A], with almost no shift observed. In contrast, the (002) reflection of pristine Ti₃C₂ shifts from 8.9° to 8.3° during discharge [Supplementary Figure 12]. After the subsequent charge process, this peak only recovers to 8.4° rather than returning to its initial position, suggesting incomplete structural recovery and irreversible lattice distortion during cycling. This breathing phenomenon of Ti₃C₂ has been widely reported in many research works^[39,40].

After prolonged cycling, Ti₃C₂-TPAL preserves a smooth and compact electrode surface comparable to the pristine state [Figure 4B, Supplementary Figure 13]. The electrode thickness changes moderately from 12 to 16 μm [Figure 4C], corresponding to a thickness increase of 33.3%. The moderate thickness increase of Ti₃C₂-TPAL after cycling may be mainly associated with SEI formation. By comparison, the cycled pristine Ti₃C₂ electrode shows pronounced surface cracking [Figure 4D], with its thickness increasing from 14 to 23 μm [Figure 4E], corresponding to a thickness expansion of 64.3%. The continuous expansion and contraction of Ti₃C₂ interlayer spacing are considered the main cause of Ti₃C₂ agglomeration or electrode cracks, which subsequently lead to cycling performance degradation.

DRT analysis allowed for resolving the EIS spectra into distinguishable relaxation processes [Supplementary Figures 14 and 15]. DRT analysis was used as a complementary method to separate overlapping relaxation processes in the EIS spectra. Because the extracted DRT peaks can be influenced by regularization and data quality, the peak assignment is discussed cautiously and mainly used for comparative analysis between Ti₃C₂ and Ti₃C₂-TPAL under the same processing conditions. Within the DRT spectrum, distinct peaks are assigned to specific physicochemical processes. According to the commonly used interpretation of DRT spectra for battery electrodes, the relaxation peaks in the 0.01-0.1 s region are tentatively assigned to interfacial charge-transfer-related processes. This assignment should be regarded as a comparative interpretation rather than an exclusive identification, because DRT peak positions can be affected by data quality, regularization parameters, and overlapping electrochemical processes. The relaxation signals appearing at τ values above 0.1 s can be assigned to electrolyte-phase mass-transport processes.

The charge-transfer time constant of Ti₃C₂-TPAL remains nearly unchanged at 0.020-0.025 s during charge and discharge [Figure 4F and G], which is lower than the 0.029-0.032 s of pristine Ti₃C₂ [Figure 4H and I]. This result confirms that the introduction of TPAL molecules lowers the charge transfer barrier, as schematically illustrated in Figure 4J and K. Furthermore, the DRT intensity plot can further reveal the stability of the electrode over long-term cycling. Compared to Ti₃C₂, the DRT intensity of Ti₃C₂-TPAL varies less during cycling [Supplementary Figure 15], highlighting the electrochemical stability of Ti₃C₂-TPAL. This not only corroborates the anchoring effect of TPAL on Ti₃C₂ sheets but also effectively inhibits structural accumulation and collapse. The stability of TPAL molecules during long-term cycling was also considered. Although partial cleavage of imine-related bonds cannot be completely excluded during repeated sodiation/desodiation, the stable long-term cycling behavior, nearly unchanged (002) peak position during in situ XRD measurements, reduced electrode deformation after cycling, and relatively stable DRT response suggest that the TPAL-assisted interlayer structure is largely maintained during cycling. Possible local bond cleavage or interfacial side reactions may still occur and should be further clarified by post-cycling spectroscopic analysis in future work. The Ti₃C₂-TPAL electrode demonstrates a unique “double-lock” effect: Based on the above structural and electrochemical evidence, the double-lock effect of TPAL can be assigned to covalent anchoring and interlayer pillaring. The covalent imine linkages restrict the restacking and slippage of Ti₃C₂ sheets, while the rigid aromatic TPAL pillars stabilize the enlarged interlayer spacing. This assignment is supported by the chemical-bonding evidence from FTIR/XPS, the enlarged interlayer spacing from XRD and HRTEM, the nearly unchanged (002) peak position during in situ XRD measurements, and the reduced electrode deformation after cycling. Therefore, the double-lock architecture contributes to the improved structural integrity and high-rate sodium-storage kinetics of Ti₃C₂-TPAL.

To provide trend-level theoretical insight into the possible influence of TPAL grafting on Na adsorption and migration, simplified DFT calculations were performed. It should be noted that the DFT models used here are simplified representations of Ti₃C₂ and Ti₃C₂-TPAL. Real Ti₃C₂T_x MXenes contain mixed surface terminations, such as -O, -OH, and -F, which may affect Na adsorption, diffusion barriers, electronic structure, and interlayer spacing. Therefore, the DFT results are used as auxiliary evidence rather than a complete description of the real Ti₃C₂T_x surface chemistry. Theoretical calculations indicate significant negative charge regions on the TPAL molecule surface [Figure 5A]. This feature enhances the electrostatic adsorption of Na⁺ ions, thereby suggesting potential active sites for Na⁺ storage within the Ti₃C₂-TPAL composite. Zeta potential test results further validate this theoretical prediction. The charge-density-difference maps are illustrated in Figure 5B and C. In these maps, electron-rich regions are shown in yellow, while electron-deficient regions are displayed in blue. Compared with the nearly uniform charge distribution on pristine Ti₃C₂, Ti₃C₂-TPAL exhibits more evident interfacial charge redistribution, suggesting modified local electronic interactions after TPAL grafting. The charge-density-difference maps suggest that TPAL grafting induces local interfacial charge redistribution rather than a uniform charge distribution. This interfacial polarization may contribute to improved Na⁺ affinity and charge-transfer behavior.

In addition, density of states analysis indicates [Supplementary Figure 16] that Ti₃C₂-TPAL presents a continuous density of states at the Fermi level [Figure 5D]. The absence of a significant band gap indicates excellent electronic conductivity, ensuring superior electron transport efficiency and providing a guarantee for the optimization of Na⁺ ions storage kinetics. Adsorption energy calculation results [Figure 5E, Supplementary Figure 17] indicate that the Na⁺ ions adsorption energy of Ti₃C₂-TPAL is -2.62 eV, which is significantly lower than the -2.04 eV of Ti₃C₂. Lower adsorption energy implies a stronger and more stable adsorption effect of the material surface on Na⁺ ions, which can supply sufficient ion sources for the Na⁺ ions storage process. Na⁺ ions diffusion barrier calculation [Figure 5F, Supplementary Figure 18] shows that the Na⁺ ions diffusion barrier of Ti₃C₂-TPAL is only 0.32 eV, representing a reduction of ~49% compared to that of Ti₃C₂ (0.63 eV). The lower diffusion barrier can effectively reduce energy loss during Na⁺ ions diffusion, further confirming its excellent Na⁺ ions diffusion kinetics. The more negative Na adsorption energy of Ti₃C₂-TPAL indicates enhanced Na affinity and improved interfacial Na capture compared with pristine Ti₃C₂. However, stronger Na adsorption is not necessarily always beneficial, because excessively strong Na binding may restrict Na⁺ migration or induce irreversible Na trapping. Therefore, the adsorption energy should be considered together with the Na diffusion barrier. In the present simplified model, Ti₃C₂-TPAL shows both stronger Na adsorption and a lower diffusion barrier than Ti₃C₂, suggesting that TPAL-related interfacial regulation may improve Na⁺ capture without severely hindering Na⁺ transport. The Ti₃C₂-TPAL composite effectively promotes the improvement of electrochemical kinetics during the Na⁺ ions storage process through multi-dimensional synergistic mechanisms, including constructing abundant electronegative sites, abundant built-in electric fields, excellent electronic conductivity, and strengthening Na⁺ ions adsorption while weakening its diffusion barrier.

To systematically evaluate the electrochemical performance of Ti₃C₂-TPAL in practical applications, it was matched with NFPP cathode to construct a SIBs full battery system (Ti₃C₂-TPAL||NFPP, Figure 6A). Based on the half-cell voltage profiles of NFPP and Ti₃C₂-TPAL, the Ti₃C₂-TPAL||NFPP full cell was operated within 0.04-3.65 V [Supplementary Figure 19]. The full-cell capacity obtained in this work, together with energy and power densities, was normalized to the active material of the NFPP cathode. Under 5 A g^-1, the initial charge and subsequent discharge capacities are 105.7 and 84.6 mAh g^-1, respectively, corresponding to an initial Coulombic efficiency of 80.0% [Figure 6B]. Furthermore, the full battery also presents excellent rate performance. At current densities of 0.2, 0.5, 1, 2, 5, and 10 A g^-1 [Figure 6C], the discharge specific capacities are 90.8, 88.4, 84.2, 75.2, 52.3, and 24.5 mAh g^-1, respectively. When the current density recovers to 0.2 A g^-1, the specific capacity can still reach 88.8 mAh g^-1. Correspondingly, the Ti₃C₂-TPAL||NFPP full cell delivers a maximum energy density of 190.8 Wh kg^-1 and a power density of 9,040 W kg^-1 based on the NFPP cathode active mass, demonstrating competitive energy-power output among reported MXene-based full cells [Figure 6D, Supplementary Table 2]. Even after 2,500 cycles at 5 A g^-1, the cell still provides 42.2 mAh g^-1 and maintains nearly 100% Coulombic efficiency [Figure 6E], confirming its durable full-cell operation. The long-term cycling stability of the Ti₃C₂-TPAL||NFPP full cell can be attributed to the stabilized Ti₃C₂-TPAL anode structure and the stable Na⁺ supply from the NFPP cathode. The TPAL-induced covalent anchoring and interlayer pillaring help suppress MXene restacking and mitigate structural deformation during repeated cycling, thereby maintaining stable Na-storage kinetics. A light-emitting diode (LED) was successfully powered by the assembled full cell [Inset of Figure 6E]. High-performance SIBs show significant advantages in smart grids, electric vehicles, and the energy storage sector. SIBs stabilize grid fluctuations and enhance renewable energy integration with cost advantages. Their fast-charging capability suits them for microcars and buses, while their safety profile supports wearable electronics. Ultimately, SIBs applications in smart grids and storage will facilitate “dual carbon” goals through technological diversification.

In summary, Ti₃C₂-TPAL was constructed through a molecular grafting strategy based on a well-defined double-lock architecture. The first lock originates from covalent imine anchoring between TPAL and aminated Ti₃C₂, which suppresses sheet restacking and interlayer slippage. The second lock arises from rigid aromatic TPAL pillaring, which enlarges and stabilizes the Ti₃C₂ interlayer spacing. These coupled structural effects contribute to the enhanced electrochemical performance of Ti₃C₂-TPAL, which delivers 57.0 mAh g^-1 at 10 A g^-1 and maintains 54.3 mAh g^-1 after 2,500 cycles at 5 A g^-1, showing competitive high-rate stability among reported MXene-based sodium-storage anodes. In situ XRD and DRT results indicate that TPAL grafting helps stabilize the Ti₃C₂ interlayers and reduce charge-transfer resistance. Simplified DFT calculations further provide trend-level support that TPAL-related interfacial regulation may enhance Na⁺ affinity and lower the Na migration barrier. Moreover, the assembled Ti₃C₂-TPAL||NFPP full battery demonstrates a cathode-mass-normalized energy density of 190.8 Wh kg^-1 and a power density of 9,040 W kg^-1. Overall, this work demonstrates that TPAL grafting can improve the interlayer stability, ambient oxidation resistance, and high-rate cycling durability of Ti₃C₂-based anodes. Rather than fundamentally solving all limitations of MXene anodes, this molecular-grafting strategy provides a useful approach for regulating MXene interlayers and enhancing high-rate sodium-storage kinetics.