Concentrated Laminate Structure in Dense MXene Monoliths Promises High‐Capacity Sodium Storage

MXenes have great potential as fast‐charging anodes for sodium storage due to their excellent electrical conductivity, high pseudocapacitive charge storage, and large interlayer distance. The intercalation pseudocapacitance provided by the active sites within the laminate MXene nanosheets is generally the major contributor to their sodium‐storage capacity. Thus, it is highly preferred to construct porous materials with abundant laminate structures to overcome the ion‐diffusion limitation in MXene multilayer films and increase the accessible interlayer sites. Herein, the enhancement of laminate structures in a pre‐assembled Ti3C2Tx network is achieved, under the effects of interlayer slipping of MXene nanosheets during capillary densification, and finally obtained a dense monolith with both high density (2.37 g cm−3) and high porosity (87.3 m2 g−1). This MXene anode material delivers a high capacity of 185 mAh g−1 and a superior rate performance of 55 mAh g−1 (5 A g−1). With improvement of both density and gravimetric capacity, this monolith has a high volumetric capacity of up to 200 mAh cm−3 at 1 A g−1 even after 2000 cycles. Herein, new insights are provided into the design of high‐capacity MXene anodes for sodium‐ion batteries and control of different 2D materials in compact structures.


Introduction
Sodium-storage technologies are considered as a competitive complement to lithium-ion batteries due to the widely distributed, low-cost sodium resources and similar working mechanisms. [1,2]ost anode materials for sodium-ion batteries, including carbonaceous materials, alloying materials, and conversion materials, suffer from sluggish sodium ions bulk diffusion and reaction kinetics, thus resulting in inferior rate performance. [3,4]Xenes, a 2D family of transition metal carbides and nitrides, have shown great potential as fast-charging electrodes due to their excellent electrical conductivity, abundant terminated functional groups, and mechanical stability. [5,6][17][18] The multilayer Ti 3 C 2 T x obtained directly by chemical etching synthesis or the reassembled films derived from oriented Ti 3 C 2 T x nanosheets have many laminate structures.However, the stacking and aggregation problems due to van der Waals forces severely limit the ion transport, resulting in low-sodium-storage capacity. [13,14,19][27][28] This capillary evaporation method has also been used to increase the density of 3D Ti 3 C 2 T x monoliths for high-volumetric sodium-storage capacities, but without improvement in gravimetric capacities. [29]herefore, it remains a significant challenge to concentrate laminate structures in 3D Ti 3 C 2 T x to improve both gravimetric and volumetric capacities of sodium storage without sacrificing its high-rate capability.
Here, we have shown that due to the high in-plane Young's modulus, Ti 3 C 2 T x nanosheets exhibit an interlayer slipdominated behavior during evaporation-induced drying, which is different from much flexible reduced graphene oxide (rGO) nanosheets and is conducive to the formation of a laminate structure.Therefore, the laminate structures in a 3D Ti 3 C 2 T x network are concentrated driven by the capillary forces, and a highsodium-storage capacity is achieved without sacrificing the rate performance.The porous structure shortens the diffusion path to allow rapid intercalation of sodium ions, while the concentrated laminate structure provides more active sites to accommodate more sodium ions.The shrinkage of the Ti 3 C 2 T x network also results in an improved monolith density (from 0.0167 to 2.37 g cm À3 ).As a result, the dense yet porous MXene monolith shows a high gravimetric capacity of 185 mAh g À1 at 100 mA g À1 after 1000 cycles, and an excellent volumetric capacity of 200 mAh cm À3 at 1 A g À1 , which are significantly superior to the Ti 3 C 2 T x aerogel and film.From the kinetic analysis, the pseudocapacitive sodium storage of the dense MXene monolith is enhanced, ensuring high-rate capability.We hope that this work will provide new insights into the material design for high-capacity sodium storage and compact structure control of different 2D materials.

Results and Discussion
A dense and porous MXene monolith (DPMM) with a concentrated laminate structure was prepared by a 3D assembly assisted by graphene oxide and evaporation-induced drying.As a control sample, a porous MXene monolith (PMM) was produced using freeze-drying instead of evaporation-induced drying.The pure MXene film was prepared by vacuum filtration of Ti 3 C 2 T x dispersions.The detailed synthesis procedure is presented in Supporting Information.The main structural differences among the three materials are schematically shown in Figure S1 (Supporting Information).The morphology of PMM and DPMM was characterized using scanning electron microscope, as shown in Figure 1a,b.DPMM exhibits a densely packed morphology different from the open macropore of PMM, resulting in a higher apparent density (2.37 g cm À3 ) than 16.7 mg cm À3 of PMM.The Ti 3 C 2 T x film exhibits a typical multilayer structure with slit pores, consisting of Ti 3 C 2 T x nanosheets stacked face to face (Figure S2, Supporting Information).
The pore structure and surface area of three materials were investigated by the N 2 adsorption/desorption method (Figure 1c).DPMM, PMM, and the Ti 3 C 2 T x film have Brunauer-Emmett-Teller specific surface areas of 87.3, 28.4, and 7.8 m 2 g À1 , respectively.Among them, DPMM has an IUPAC Type IV N 2 adsorption/desorption isotherms combined with an H2-type hysteresis loop. [30]That is, the obvious hysteresis loop appears after the micropore filling reaches equilibrium, representing the dominant micropores and mesopores.Pore size distribution curves give more detailed information on the pore structure (Figure S3, Supporting Information).In contrast, PMM exhibits a Type II isotherm together with some characteristics for Type IV, representing the dominance of macropores and the presence of a small amount of mesopores.While the Ti 3 C 2 T x film has a Type III isotherm combined with an H4-type hysteresis loop, indicating that it is mainly composed of slit pores.These results indicate that the direct evaporation of water shrinks the pores but retains the 3D interconnected porous character in DPMM.
XRD measurements and high-resolution TEM (HRTEM) were conducted to highlight the differences in the laminate structure of the three materials.Compared with the Ti 3 C 2 T x film, both DPMM and PMM exhibit downward shifted (0002) peaks and weakened (0002), (0004), and (0006) peaks, indicating an increased interlayer distance (from 11.0 to 13.6 Å) and fewer occurrences of stacking (Figure 1d).DPMM does not show a change in interlayer distance but has an increased crystallite size along the c-axis L c during the shrinkage process, as indicated by the reduced full width at half maximum of the (0002) peak.The average number of layers (N) of DPMM is calculated as 10 compared to the 7 of that of PMM according to the Bragg equation and Scherrer formula (detailed calculations are provided in Supporting Information).The same result can be clearly observed from the HRTEM images that DPMM has the thicker pore walls and more laminate structures than PMM (Figure 1e-f and S4, Supporting Information).Moreover, due to the enhanced overlapping and interlinking of Ti 3 C 2 T x nanosheets, DPMM has a higher electrical conductivity of %310 S cm À1 than that of PMM (%82 S cm À1 ) (Table S1, Supporting Information).All these results indicate that the water evaporation concentrates the laminate structure of 3D Ti 3 C 2 T x to the same extent as that of 2D Ti 3 C 2 T x without compromising the porous properties.All the structural parameters are summarized in Table S1 (Supporting Information).
The difference in the laminate structure and pore structure of the three anodes will certainly lead to distinct sodium-storage properties (Table S2, Supporting information).The cyclic voltammetry (CV) profiles and galvanostatic charge-discharge profiles were tested within a potential window of 0.005-3 V to study the sodium-storage behaviors (Figure 2a-b and Figure S5, Supporting Information).For all three anodes, a large and broad irreversible peak around 1.2 V is observed in the first cycle, which is attributed to the formation of the solid electrolyte interface layer and irreversible reactions between sodium ions and various surface functional groups (-OH and -F) on Ti 3 C 2 T x nanosheets. [31]Apart from that mentioned previously, the CV profiles of the three anodes are approximately rectangular with no obvious redox peaks.Quasi-linear galvanostatic charge-discharge curves with no apparent plateaus are observed, indicating the rapid pseudocapacitive storage of sodium ions during the intercalation and deintercalation process, consistent with previous studies on the sodium-storage mechanism of MXenes. [15,32]he PMM with less laminate structure and Ti 3 C 2 T x films with over-stacking structures have capacities of 122 and 142 mAh g À1 at a current density of 0.1 A g À1 , respectively.However, DPMM shows high reversible sodium-storage capacity of 151 mAh g À1 , which is higher than the value reported in the previous work, where the densification of the MXene monolith is mainly achieved by the shrinkage of the rGO nanosheets. [29]The introduction of flexible rGO nanosheets at high dosages inevitably leads to the curling of nanosheets during the capillary shrinkage process, and thus to difficulties in maintaining the laminate structure of the MXene-rGO composite network.In contrast, in a MXene network, the Ti 3 C 2 T x nanosheets with high in-plane Young's modulus will exhibit interlayer slip-dominated and out-of-plane curling-inhibited behaviors, thus inducing a concentrated laminate structure, and a high-sodium-storage capacity.In addition, electrochemical impedance spectroscopy results suggest that DPMM has the lowest charge-transfer resistance (R ct ) after 18 cycles, as evidenced by the smallest semicircle in the high-frequency range of the Nyquist plots, indicating an excellent reaction kinetics (Figure 2c).
Thanks to the concentrated laminate structure in the porous network, DPMM also shows excellent rate performance, with the highest specific capacities exhibited at all current densities, compared to PMM and Ti 3 C 2 T x film (Figure 2d).Even at a high current density up to 5 A g À1 , DPMM has an appreciable capacity of 55 mAh g À1 .When the current density returns to 0.1 A g À1 , DPMM recovers to a capacity of 160 mAh g À1 , indicating good reversibility of intercalation behavior.Although PMM has similar capacity retention (33%) to DPMM at high current densities, it always has a lower capacity.In sharp contrast, the Ti 3 C 2 T x film delivers the worst rate performance of 17 mAh g À1 at 5 A g À1 , possibly due to the limited diffusion of sodium ions in the stacked layers.In addition, DPMM has superior long-term cycling stability compared to PMM and Ti 3 C 2 T x film (Figure 2e).Specifically, after 1000 cycles at 100 mA g À1 , DPMM has a high capacity of 185 mAh g À1 with a retention of 116%, which is significantly better than the capacity of 133 mAh g À1 and a retention of 109% for PMM.The gradual increase in the capacity of DPMM is due to the activation of the internal active sites of the Ti 3 C 2 T x nanosheets during cycling. [33]In contrast, the slow ion diffusion in the densely stacked layers and the unavailable active sites during cycling cause the Ti 3 C 2 T x film to have a capacity of only 56 mAh g À1 and a retention of 39%.
To explain the excellent sodium-storage performance especially the reversible specific capacity and rate performance after the enhancement of laminate structures in 3D Ti 3 C 2 T x monoliths, CV and the galvanostatic intermittent titration technique (GITT) tests were conducted to analyze the kinetics.The CV profiles of three electrodes at all scan rates (0.3-5 mV s À1 ) are mainly composed of a pair of broad cathodic/anodic peaks at around 0.6/ 0.9 V, corresponding to the process of deintercalation/intercalation of sodium ions in Ti 3 C 2 T x (Figure 3a and S6a-b, Supporting Information).DPMM has the smallest polarization at all scan rates, indicating the best intercalation reaction kinetics.The b values at the cathodic/anodic peaks used as an indicator of the extent to which the capacity is contributed by capacitive behavior are calculated using Equation ( 1) where the measured peak current (i p ) follows a power-law relationship with the scan rate (ν), and both a and b are tunable parameters, with the b value determined by the slope of the log(i p ) versus log(ν) curve.The b value of 0.5 indicates the diffusion-controlled mechanism, while the b value of 1.0 indicates the capacitive contribution process. [19]As shown in Figure 3b and S6c-d, Supporting Information, DPMM has b values (0.86 and 0.95) close to those of Ti 3 C 2 T x film (0.88 and 0.94) and different from PMM (0.78 and 0.89).These results indicate that, in contrast to PMM, the laminate structures make the sodium-storage capacity of DPMM and Ti 3 C 2 T x film be contributed more by capacitive behavior.Furthermore, the degree of capacitive contribution is quantified by the analytical method proposed by Dunn et al. as in Equation ( 2) where k 1 ν and k 2 ν 1 2 are the energy-storage contributions from the capacitive and diffusion-controlled processing, respectively. [23]s shown by the masked area in Figure 3c, the capacitive contribution of DPMM is 63% at a scan rate of 0.3 mV s À1 .With the increase of scan rates, the capacitive contribution of DPMM gradually increases to reach its highest value (89% at 5 mV s À1 ), which is similar to that of Ti 3 C 2 T x film (Figure 3d and S6e, Supporting Information).While for PMM, the capacitive contribution is the smallest at each scan rate, indicating that the absence of the laminate structure leads to a decrease in the pseudocapacitance (Figure S6f, Supporting Information).
The GITT was performed to study the diffusion kinetics of sodium ions throughout discharging and charging process (Figure 3e).The apparent diffusion coefficients of sodium ions ðD Na þ Þ of three anodes were calculated according to Equation (3) where τ is the pulse duration, m m and M m are the mass and molar mass of the active material, V m is the molar volume, and S is the surface area of the electrode. [34]ΔE s and ΔE τ can be obtained from the GITT curves.As shown in Figure 3f, the sodium-ion-diffusion coefficients of the three electrodes all show a decrease in the sodiation and desodiation processes, indicating that the interlayer diffusion of sodium ions has to overcome the gradually increasing electrostatic repulsion and diffusion barrier.
Compared to Ti 3 C 2 T x film, DPMM has a better diffusion rate throughout sodium ions intercalation and deintercalation process, because the 3D network provides continuous transport channels for ions and electrons, which is beneficial for high mass loading applications. [35]Although PMM has the highest diffusion coefficients due to the large pore space, the sacrificial laminate structure results in the lowest capacitive contribution.It is therefore concluded that the key to achieving a higher intercalation pseudocapacitance is to concentrate the laminate structure as much as possible based on the 3D network, resulting in a high-sodium-storage capacity and superior rate performance.
As discussed earlier, the dense Ti 3 C 2 T x monolith derived from the shrinkage of 3D porous structure has a concentrated laminate structure (Figure 4a).To further verify the feasibility of this strategy to improve the sodium-storage capacity, the Ti 3 C 2 T x clay (the clay was obtained in a similar way to the suspension, but without the fragmentation and delamination) with more laminate structures was used to replace the Ti 3 C 2 T x suspension, and to prepare multilayer dense and porous MXene monolith (referred to as ML-DPMM).The TEM image (Figure S7, Supporting Information) and the stronger peak intensity of the (0002) peak in the XRD patterns (Figure S8, Supporting Information) confirmed that ML-DPMM has a concentrated laminate structure, in contrast to DPMM.The specific surface area decreases slightly due to the increase in the number of Ti 3 C 2 T x layers (Figure S9, Supporting Information).The results of electrochemical tests show that ML-DPMM provides higher-sodiumstorage capacities of 212 mAh g À1 at a current density 0.01 A g À1 than DPMM (Figure 4b and S10, Supporting Information), demonstrating the importance of accessible laminate structures for high-sodium-storage capacity.
In addition to concentrating the laminate structures in the 3D MXene monolith, the evaporation-induced drying strategy can also bring a high material density and thus the volumetric capacity of the electrode.As shown in Figure S11 and S12, Supporting Information, DPMM anode has an obvious volume shrinkage and a high electrode density of 1.44 g cm À3 .Therefore, a volumetric capacity of up to 200 mAh cm À3 (at 1 A g À1 ) is obtained even after 2000 cycles (Figure 4c), which is much higher than that of PMM and Ti 3 C 2 T x film anodes (Figure S13, Supporting Information).Compared with the previously reported MXene and carbon anodes for sodium storage, such as Ti 3 C 2 T x /CNT paper, [19] hollow Ti 3 C 2 T x spheres, [21] multilayer Ti 2 C 2 T x , [32] porous Ti 3 C 2 T x , [23] biomass-based carbon, [36] and 3D folded graphene, [37] DPMM anode shows both high volumetric performance and excellent long-term cycling performance (Figure 4d).It is therefore concluded that the evaporationinduced drying of MXene hydrogel not only increases the density of MXene monolith, but also concentrates laminate structures in 3D network to provide more active sites for the intercalation pseudocapacitance of sodium ions without compromising the ion-diffusion rates, thus the obtained dense MXene monolith has improved volumetric performance.

Conclusion
In summary, the 3D dense MXene monolith with a concentrated laminate structure achieves both high-sodium-storage capacity and excellent cycling stability.The concentrated laminate structure, induced by interlayer slipping of the nanosheets during evaporation-induced drying, exposes more active sites for sodium storage.The 3D network ensures fast surface pseudocapacitive response behavior, and the fast ion diffusion.As a result, DPMM exhibits a good gravimetric capacity of 185 mAh g À1 at 100 mA g À1 , with a capacity retention rate of 116% after 1000 cycles.In addition, DPMM has an excellent volumetric capacity of 200 mAh cm À3 at 1 A g À1 .When Ti 3 C 2 T x clay was used to fabricate the dense monolith, the sodium-storage capacity was further improved.Therefore, we believe that this work may provide a new strategy to achieve high-capacity sodium storage in MXene.Meanwhile, we hope that this work will also provide new insights into the precise control of different 2D materials in compact structures.

Figure 2 .
Figure 2. a) Cyclic voltammetry (CV) profiles of DPMM at a scanning rate of 0.1 mV s À1 .b) Galvanostatic charge-discharge profiles of DPMM, PMM, and Ti 3 C 2 T x film at 0.1 A g À1 .c) Electrochemical impedance spectroscopy plots of DPMM, PMM, and Ti 3 C 2 T x film after 18 cycles.d) Rate performance from 0.1-5 A g À1 , and e) cycling performance at 0.1 A g À1 of DPMM, PMM, and Ti 3 C 2 T x film.Before the cycling tests, a preliminary activation was performed at a low current density of 50 mA g À1 for 10 cycles.

Figure 3 .
Figure 3. a) Scan rate-dependent CV profiles (from 0.3 to 5 mV s À1 ) of DPMM.b) Relationship between log(i) and log(v) of DPMM.c) CV profiles of DPMM measured at 0.3 mV s À1 , depicting capacitive contribution indicated as masked region.d) Bar-graph depicting capacitive and diffusion controlled current contribution ratio of DPMM when scan rates change from 0.3 to 5 mV s À1 .e) Galvanostatic intermittent titration technique (GITT) curves of three electrodes using a pulse current of 50 mA g À1 for 10 min accompanied by a rest interval of 100 min between 3.0 and 0.005 V. f ) Sodium ions diffusion coefficients calculated from GITT curves.

Figure 4 .
Figure 4. a) Schematic diagram of concentrated laminate structures in DPMM.b) Galvanostatic charge-discharge profiles of DPMM and multilayer (ML)-DPMM at the fifth cycle.c) The long-term cycling of DPMM at 1 A g À1 .d) Volumetric capacities of DPMM (this work), Ti 3 C 2 T x /CNT paper, carbon, porous Ti 3 C 2 T x , hollow Ti 3 C 2 T x spheres, multilayer Ti 2 CT x , and 3D folded graphene.