Compacted mesoporous titania nanosheets anode for pseudocapacitance‐dominated, high‐rate, and high‐volumetric sodium‐ion storage

Surface‐redox pseudocapacitive nanomaterials show promise for fast‐charging energy storage. However, their high surface area usually leads to low density, which is not conducive to achieving both high volumetric capacity and high‐rate capability. Herein, we demonstrate that TiO2 nanosheets (meso‐TiO2‐NSs) with densely packed mesoporous are capable of fast pseudocapacitance‐dominated sodium‐ion storage, as well as high volumetric and gravimetric capacities. Through compressing treatment, the compaction density of meso‐TiO2‐NSs is up to ~1.6 g/cm3, combined with high surface area and high porosity with mesopore channels for rapid Na+ diffusion. The compacted meso‐TiO2‐NSs electrodes achieve high pseudocapacitance (93.6% of total charge at 1 mV/s), high‐rate capability (up to 10 A/g), and long‐term cycling stability (10,000 cycles). More importantly, the space‐efficiently packed structure enables high volumetric capacity. The thick‐film meso‐TiO2‐NSs anode with the mass loading of 10 mg/cm2 delivers a gravimetric capacity of 165 mAh/g and a volumetric capacity of 223 mAh/cm3 at 5 mA/cm2, much higher than those of commercial hard carbon anode (80 mAh/g and 86 mAh/cm3). This work highlights a pathway for designing a dense nanostructure that enables fast charge kinetics for high‐density sodium‐ion storage.

5][6][7] For grid applications, two systems are required. 8,9One is the middle-long-term energy storage system for peak-load shifting, which requires high energy density for storing energy during the day. 4,102][13][14][15] For such an application scenario, the abilities of fast charging and high-power delivery of SIBs are highly demanded, but it is full of challenges.The Na + superionic conductor (NASICON) type cathodes have shown excellent high-rate capability, 12,13 yet suitable anodes with comparable high-rate capability, long-term cycles, and decent energy density that remain largely unexplored.
7][18][19] The sodiation curve of the HC anode is delivered into two parts: slope region from Na + adsorption and plateau region from Na + intercalation and/or filling processes. 16Limited by the sluggish Na + diffusion into HC anodes, their rate performance is still unsatisfactory. 18Additionally, the HC with rich micropores leads to relatively low tapping and packing density; thus, its overall volumetric capacity still remains further improved. 17Titanium dioxides (TiO 2 ) are earth-abundant and low-cost.][22][23] Moreover, the TiO 2 anodes display pseudocapacitive sodium-ion storage behaviors, which are exceeding attractiveness for achieving high-rate performance. 21,22ecently, the insightful sodium-ion storage mechanism of anatase TiO 2 was revealed: a surface-dominated process in which only ~5 nm surface layers of TiO 2 (A) become amorphous and subsequent Ti 4+ /Ti 3+ redox reaction in the amorphous titanium oxide surface layer. 23uch surface redox sodiation exhibited capacitor-like kinetics and enables thick electrodes to retain high-rate properties.According to this finding, the nanoparticles within 10 nm are necessary to maximumly utilize the surface-redox reaction regions and achieve high gravimetric capacity.However, it faces huge challenges to overcome the low tapping density, self-aggregations, and difficulties in preparing thick-film electrodes of ultrafinesized TiO 2 nanoparticles/grains for applications.
5][26][27][28][29][30][31][32][33] Unfortunately, densely packed 2D laminates usually suffer from sluggish electrochemical kinetics due to their severe self-stacking, leading to undesirable diffusion of electrolytes and ions. 34,357][38][39][40][41] The incorporation of mesoporous frameworks in 2D materials is able to drastically enhance the permeability of dense laminate and afford rapid ion diffusion.And the close-packed porous structure allows improved density, thereby leading to desired charge storage performances.Designing 2D mesoporous titania materials shows promise in achieving high-dense and high-rate sodium-ion storage, but it is scanty and difficult in the controlled synthesis of compact porous structures. 36,42Furthermore, determining the relationships among the porosity, compaction density and kinetics limitations of dense TiO 2 electrode film is significant and needs to be deeply revealed.
In this study, we design compacted mesoporous TiO 2 nanosheets (meso-TiO 2 -NSs) as sodium-ion storage anode.The meso-TiO 2 -NSs, composed of monolayered mesopores, are densely packed into thick laminates.Through facile hydrothermal-induced self-assembly and compressing treatment, the density of these compacted nanosheets is tunable by controlling the compressing pressure and getting much higher.Simultaneously, the interconnected mesoporosities in the thick electrodes (a mass loading of 10 mg/cm 2 ) also afford a highly accessible surface area for effective access to electrolytes, giving rise to enhanced charge storage kinetics.Consequently, the compacted mesoporous nanosheets anode exhibits high gravimetric (165 mAh/g) and volumetric capacities (223 mAh/cm 3 ) at a high current density of 5 mA/cm 2 , which are much better than the commercial HC anodes (80 mAh/g and 86 mAh/cm 3 ).

| RESULTS AND DISCUSSION
The meso-TiO 2 -NSs are synthesized through a facile solution-processed self-assembly method, involving the selective tetrahydrofuran (THF) evaporation into the intermediate hydrogel of F127/TiO 2 composite monomicelles and subsequent solvothermal treatment at 100°C for 6 h (Supporting Information: Figure S1). 43otably, the primary mono-micelles in hydrogel possess a uniform diameter of ~12 nm, which is the basic assembly unit for the 2D mesoporous TiO 2 layers (Supporting Information: Figure S2).After the spatially confined micellar assembly in an autoclave, white precipitates composed of a layered morphology can be obtained (Supporting Information: Figure S3).At last, the white precipitates are annealed in nitrogen to remove the soft mono-micelles templates.The X-ray diffraction pattern indicates the anatase phase after annealing (Supporting Information: Figure S4A).In addition, the carbon content of 15.4 wt% is from the carbonized residual surfactant (Supporting Information: Figure S4B).Such remaining carbon scaffolds are able to impede nanocrystal growth and prevent framework collapse during annealing.
Field-emission scanning electron microscopy images reveal the meso-TiO 2 -NSs with an ultrathin thickness (Figure 1A and Supporting Information: Figure S5).The high-resolution transmission electron microscopy image (Figure 1B) clearly shows visualized mesopores, comprised of polycrystalline anatase nanocrystals (confirmed by the selected area electron diffraction [SAED] pattern, inset of Figure 1B).Close examination of atomic force microscopy (AFM) measurement shows an average thickness of ~4.9 nm for an individual nanosheet (Figure 1C).The nitrogen sorption isotherms (Figure 1D) show the typical type-IV hysteresis loop.The Brunauer-Emmett-Teller (BET) specific surface area (SSA) is calculated to be 141 m 2 /g.The pore size distribution curve further visualizes a centered mesopore size at ~3.9 nm (Figure 1E), demonstrating the well-defined mesoporous skeleton with in-plane monolayered mesopores.All the features of monolayered meso-TiO 2 -NSs are schematically displayed in Figure 1F.In comparison, the TiO 2 -NSs were prepared through annealing the precursors in the air, whose BET SSA is only 31 m 2 /g (Supporting Information: Figure S6).Commercial HC (Type-2 product; Kuraray Co.) was characterized for comparison as well (Supporting Information: Figure S7).The compaction density of meso-TiO 2 -NSs, TiO 2 -NSs, and HC is measured under gradually increased pressures by a high-precision servo motor and displacement sensor with an adaptive pressure regulation control system (PRCD2100; IEST).The compaction densities were collected under loading and unloading models (Figure 2A).As pressure rises, the compaction densities of meso-TiO 2 -NSs show a gradually increasing trend from 0.58 to 1.64 g/cm 3 at 300 MPa (unloading model).The TiO 2 -NSs show higher compaction density up to 2.45 g/cm 3 .Compared with TiO 2 samples, the commercial HC shows lower compaction density from 0.91 to 1.15 g/cm 3 under the same testing conditions.It is found that the rich porous meso-TiO 2 -NSs show high elasticity than that of the TiO 2 -NSs (Supporting Information: -Figure S8) after compress treatment, indicating the structural stability of the inner mesoporous materials.The meso-TiO 2 -NSs show well-retained high surface area (108 m 2 /g) and mesoporous structure (~3.8 nm) after high compression at 300 MPa (Figure 2B).After making electrodes with compression, the nanosheet morphologies are well-retained (Figure 2C).The above results further validate the stability of the mesoporous 2D structure.
The electrodes were then prepared by mixing with carbon additives and binders, followed by being compressed under different pressures (denoted as meso-TiO 2 -NSs-X, X represents compressed pressure).The mass loading of each electrode was finely controlled at 2.5 mg/ cm 2 .The thickness undergoes a gradual decline from 39.1 to 17.2 μm with the increase of compress pressures (Figure 2D-H).By comparing the selected samples at pressure of 0, 30, 110, 190, and 270 MPa, the increased compression pressure allows for a systematic increase of compaction density from 0.58 to 1.58 g/cm 3 and a decrease of porosity from 84.9% to 58.9% (Figure 2I). 44hese results validate that such 2D meso-TiO 2 -NSs allow for a tunable model system for further exploring their electrochemical response related to structural parameters.Such novel mesoporous nanosheets could be densely compacted without the collapse of porous structure and loss of high porosity (Figure 2C).Therefore, the Na + ions could pass across the densely stacked layers and intercalate into the TiO 2 rapidly, leading to high-rate performance (Figure 2J).
The electrochemical performances were measured by the assembly of the half cell (2032-type coin cell) using sodium disk as counter/reference electrode.Figure 3A shows the discharge and charge curves of meso-TiO 2 -NSs, TiO 2 -NSs, and HC anodes in the initial two cycles.The HC anode shows the typical slope-plateau curves with a reversible capacity of 295 mAh/g. 16The TiO 2 -NSs display a slope from 3.0 to 0.2 V, and then a long plateau from 0.2 to 0.01 V, corresponding to an irreversible crystalline to amorphous transformation. 23Then, the subsequent charging and discharging processes show a much-curved slope, demonstrating a typical pseudocapacitive behavior.Differently, the meso-TiO 2 -NSs anode shows a continuous sodiation reaction accompanied by the slope-like curve, owing to its ultrafine crystalline sizes.The initial columbic efficiencies of meso-TiO 2 -NSs and TiO 2 -NSs are over 70% (Supporting Information: Table S1).The reversible capacity of meso-TiO 2 -NSs is higher than that of TiO 2 -NSs (222 vs. 164 mAh/g).Although the HC anode displays higher gravimetric capacity at low currents, the meso-TiO 2 -NSs anode shows better rate capability than the TiO 2 -NSs and HC anodes at high currents (Figure 3B).
The porosity plays a pivotal impact on the reaction kinetics, especially at high rates (Figure 3C).At the high specific currents of 2, 4, and 10 A/g, the capacities of meso-TiO 2 -NSs increase along with the compression of porosity from 85% to 65%, but the values further decrease when the porosity downs to 61% and 56%.Similar trends are observed in the TiO 2 -NSs.The meso-TiO 2 -NSs anode shows the best rate capacity by compressed under 110 MPa (porosity = 65%, ρ = 1.35), while it is 30 MPa for that of TiO 2 -NSs (porosity = 67%, ρ = 1.25).It is found that both two electrodes exhibit the best rate capability at a porosity of 65%, but the gravimetric capacity of meso-TiO 2 -NSs is much higher, indicating that the high surface area and interconnected porosity of mesoporous structure provide rich surface redox sites.HC anodes show the same trends as well (Supporting Information: Figure S9), while the porosity of ~36% and ρ = 1.08 is best for the high-rate performance.
The relationship of volumetric capacity and compaction density for meso-TiO 2 -NSs (Figure 3D), TiO 2 -NSs (Figure 3E), and HC (Figure 3F) anodes at various specific currents are parallelly compared.With increased compress pressures, the compaction densities and volumetric capacities increase simultaneously.However, after over-compression, the porosities of electrodes are largely reduced and then limit the Na + diffusion, leading to decreased gravimetric capacities (Figure 3D-F).Owing to the high compaction densities of meso-TiO 2 -NSs and TiO 2 -NSs electrodes, they deliver comparable volumetric capacities to HC electrodes at 0.1 A/g.Additionally, the meso-TiO 2 -NSs anode shows higher volumetric capacities than those of TiO 2 -NSs and HC anodes when the currents increase to over 4 A/g.Among the samples, the meso-TiO 2 -NSs-110 MPa (porosity = 65%, ρ = 1.35) shows the best balance of volumetric and gravimetric capacities and high-rate capability.The galvanostatic charge-discharge curves of meso-TiO 2 -NSs-110 MPa display the small voltage hysteresis varies at high specific currents (Figure 3G).In addition, the compressed mesoporous-TiO 2 -NSs anodes deliver stable 10,000 ultralong cycles, demonstrating superior stability (Figure 3H).
To reveal the effect of compress and porosity on the electrochemical charge storage kinetics, cyclic voltammetry (CV) and electrochemical impedance spectroscopy were employed.The CV curves of meso-TiO 2 -NSs anodes show the typical pseudocapacitive behavior, much broad cathodic/anodic peaks at 0.82/0.70V (Figure 4A), corresponding to the redox reaction of Ti 4+ /Ti 3+ . 21,22The well-overlapped CV curves indicate the high compress treatment has minor interference with the microstructure.With the increase in sweep rates, the shifts of redox peaks are negligible (Supporting Information: Figure S10).According to the b value analysis (Supporting Information: Figure S11), 45 the fitted results show that all the meso-TiO 2 -NSs electrodes are consistent with ~1 (Figure 4B), indicating a surfacecontrol dominated charge storage process.After being compressed at 270 MPa, the b value slightly decreases, 1 take the percentage of 93.6% for meso-TiO 2 -NSs-110 MPa at 1 mV/s (Figure 4C), and the other ones with different porosities show over 93% as well (Supporting Information: Figure S10F), which are higher than that of 88.7% for TiO 2 -NSs (Supporting Information: Figure S12).These results highlight the advantages of mesoporous structure with high electrode-electrolyte contact area for the rapid charge storage processes.
Nyquist plots of the meso-TiO 2 -NSs electrodes (Figure 4D) show a semi-circle at high frequencies and a straight line at low frequencies with a phase angle of 79°(close to 90°). 46The first intercept at the Re(Z) axis is the equivalent series resistance (R s ), and the semi-circle corresponds to the interfacial resistance (R f ) and charge transfer resistance (R ct ). 47According to the equivalent circuit model (inset of Figure 4D), the fitting results (Figure 4E) show the changes in resistance along with the compressing pressure.The R s is almost consistent with the pressures, indicating the sufficient filling of the electrolyte in the mesoporous electrode.The R f continuously decreases along with increased pressure, corresponding to the reduced formation of solid electrolyte interface layers.The R ct decreases from 3.4 (at 0 MPa) to 2.5 Ω•cm 2 (at 110 MPa) and then increases to 2.7 Ω•cm 2 (at 270 MPa).According to 48 the ion diffusion kinetics are further compared by fitting the plots of Re(Z) versus ω −1/2 at low frequencies (Figure 4F).The meso-TiO 2 -NSs-110 MPa shows the lowest slope value (k) among the five anodes, indicating the much enhanced D Na + by an appropriate compress treatment.Hence, the kinetics analysis reveals that the reduction of R f and R ct , and the increase of conductivity and diffusivity are the key factors that enhance the rate capability.
To meet the requirements of practical applications, it is significant to investigate the electrochemical performance at high mass loading.Figure 5A shows the chargedischarge curves of the meso-TiO 2 -NSs and HC at different current densities (1, 5, and 10 mA/cm 2 ) when the thick-film electrode has a high single-side mass loading of 10 mg/cm 2 .The thick-film meso-TiO 2 -NSs and HC anodes display similar charge-discharge curves to thin-film (Figure 3A) at different current densities.However, the plateau region of the HC anode disappears at current densities over 5 mA/cm 2 , leading to muchdecayed capacity at high rates.
Figure 5B summarizes the comparison of gravimetric and volumetric capacities of thick-film meso-TiO 2 -NSs and HC anodes as the function of areal current densities.The gravimetric capacity of meso-TiO 2 -NSs anode is lower than that of HC anode (202 vs. 273 mAh/g) at Sciences and Technologies of Energy Materials of Fujian Province, Grant/Award Number: HRTP-2022-19; Zhejiang Provincial Natural Science Foundation, Grant/Award Number: LR21E020003 K E Y W O R D S high volumetric capacity, mesoporous materials, sodium-ion batteries, titanium dioxide 1 | INTRODUCTION

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I G U R E 2 (A) The compacted density versus pressure plots of meso-TiO 2 -NSs, TiO 2 -NSs and commercial HC.(B) Nitrogen sorption isotherms and pore size distribution curve (inset of B) of meso-TiO 2 -NSs after compressed at 300 MPa.(C) SEM image of meso-TiO 2 -NSs electrode after compressed at 270 MPa.(D-H) SEM images of meso-TiO 2 -NSs electrode after different compressed: (D) 0 MPa, (E) 30 MPa, (F) 110 MPa, (G) 190 MPa, and (H) 270 MPa; scale bar = 20 μm.(I) The comparison of compacted density and porosity of meso-TiO 2 -NSs after being compressed at various pressure.(J) Schematic of the compacted meso-TiO 2 -NSs electrode with rapid Na + diffusion in the dense film, which is beneficial for combining high gravimetric and volumetric capacities, and high-rate capability.HC, hard carbon; meso-TiO 2 -NSs, mesoporous TiO 2 nanosheets; SEM, scanning electron microscopy.

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I G U R E 5 Thick-film properties of meso-TiO 2 -NSs versus HC anodes.(A) Galvanostatic charge-discharge curves, (B) gravimetric/ volumetric capacity versus current density plots, and (C) the relationship of areal capacity and mass loading of meso-TiO 2 -NSs and HC anodes, at the mass loading of 10 mg/cm 2 .CV curves of thick-film meso-TiO 2 -NSs (D) and HC anodes (E) at various sweep rates.(F) Gravimetric/volumetric energy density (with SHE hypothetical cathode) of thick-film meso-TiO 2 -NSs and HC anodes at different current densities.CV, cyclic voltammetry; HC, hard carbon; meso-TiO 2 -NSs, mesoporous TiO 2 nanosheets; SHE, standard hydrogen electrode.