Journal Pre-proofs
Densified MoS2/Ti3C2 films with balanced porosity for ultrahigh volumetric
capacity sodium-ion battery
Kun Ma, Yuru Dong, Hao Jiang, Yanjie Hu, Petr Saha, Chunzhong Li
PII: S1385-8947(20)33603-2
DOI: https://doi.org/10.1016/j.cej.2020.127479
Reference: CEJ 127479
To appear in: Chemical Engineering Journal
Received Date: 21 August 2020
Revised Date: 9 October 2020
Accepted Date: 19 October 2020
Please cite this article as: K. Ma, Y. Dong, H. Jiang, Y. Hu, P. Saha, C. Li, Densified MoS2/Ti3C2 films with
balanced porosity for ultrahigh volumetric capacity sodium-ion battery, Chemical Engineering Journal (2020),
doi: https://doi.org/10.1016/j.cej.2020.127479
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© 2020 Published by Elsevier B.V.
Densified MoS2/Ti3C2 films with balanced porosity for ultrahigh volumetric capacity 
sodium-ion battery
Kun Maa, Yuru Donga, Hao Jiang*a, Yanjie Hua, Petr Sahab, Chunzhong Li*a
a Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering 
Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, 
East China University of Science and Technology, Shanghai 200237, China.
b Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. 
Bati 5678, 760 01 Zlin, Czech Republic.
E-mail: jianghao@ecust.edu.cn (Prof. H. Jiang), czli@ecust.edu.cn (Prof. C. Z. Li).
Abstract
Developing high volumetric energy density sodium-ion batteries (SIBs) is indispensable 
for catering to the miniaturization and flexibility of various consumer electronics. Herein, we 
have reported the flexible and compact MoS2/Ti3C2 hybrid films with balanced porosity, 
where the few-layered MoS2 nanosheets are parallelly intercalated into the Ti3C2 interlayer 
space in virtue of strong electrostatic effect and difference in their sizes. The hybrid films 
have been stabilized by the two-dimensional (2D) confinement effect and the Ti-S-Mo bonds 
with a high density of ~ 2.9 g cm-3. Furthermore, the dual 2D compounds intrinsically possess 
satisfied ions conductivity, and meanwhile give rapid electrons transfer after assembling such 
superstructure. When directly used as SIB anode, the MoS2/Ti3C2 hybrid films deliver an 
exceptional volumetric specific capacity of 1510 mAh cm-3 at 0.28 mA cm-2 and 650 mAh 
cm-3 at 14 mA cm-2. The specific capacity remains unchanged after 300 cycles at 1.4 mA 
cm-2. More significantly, the areal specific capacity shows a linear relationship with the 
increase of film thickness from 9.6 to 43.1 μm without sacrificing the volumetric capacity.
Keywords: MXene, compact film, MoS2, volumetric capacity, Na-ion battery
1. Introduction
Exploiting high volumetric energy density sodium-ion batteries (SIBs) is extremely 
urgent in view of the miniaturization and flexibility trend of various electronic gizmos, and 
prominent cost efficiency and superior safety of sodium compared with lithium [1-4]. 
Nevertheless, the larger volume of Na+ than Li+ and unsatisfied kinetics limit paces to develop 
versatile electrode materials featured by high-efficiency and reversible Na+ 
intercalation/de-intercalation [5,6]. As a new 2D material class, high-density MXenes with 
rich terminal groups and superior conductivity have a great potential for decent volumetric 
energy storage [7-9]. However, their pseudocapacitance-type sodium storage behavior usually 
results in an unsatisfactory output capacity (< 450 mAh cm-3) [10-12]. Of late, 2D metal 
chalcogenide materials (2D-MSs) have focused the continuous concerns on sodium storage 
and green energy because of their featured laminar structure, large nanospace and high 
theoretical volumetric capacity (e.g. 3216 mAh cm-3 for MoS2) [13-16]. In particular, the 
nano-sized 2D-MSs with the shortened electron/ion transport distance and highly exposed Na+ 
storage sites usually possess a higher practical capacity [17,18]. In this regard, constructing 
nano-sized 2D-MSs/MXenes hybrids is very conducive to improving the sodium storage 
capability of MXene-based materials.
Mixing MXenes with nano-sized 2D-MSs has been regarded as the most direct strategy 
for building 2D-MSs/MXene composites (Fig. 1a) [19-21]. For example, Zhang et al. 
prepared a SnS2/MXene nanocomposite by simply mixing few-layered SnS2 and exfoliated 
MXene nanosheets, exhibiting the enhanced electrochemical performance [19]. 
Unfortunately, the weak contact and disordered arrangement between two components 
tremendously hinder the electron/ion transport along with inferior rate capability and 
meanwhile cause the poor stability. Recently, intercalating 2D-MSs species into MXene 
interlayers has attracted keen interest owing to the unique confinement effect and elaborate 
coupling between components (Fig. 1b) [22-24]. For example, our group designed a 
MoS2-in-Ti3C2 hybrid where few-layered MoS2 nanocrystals vertically pillared Ti3C2 layers, 
showing a high gravimetric specific capacity and excellent structural stability [22]. 
Nevertheless, the vertical nanocrystals create overmuch pores and high specific surface area, 
which inevitably decrease the densification of composites and hence trigger an inferior 
volumetric performance. Summarized from the design concept of above nanostructures, we 
can infer that the plane-to-plane architecture with balanced porosity will endow electrode 
materials with the compact structure and the appropriate ion migration path (Fig. 1c), which is 
expected to simultaneously obtain high volumetric/gravimetric capacity and rapid reaction 
kinetics. However, it is still challenging to synthesize the plane-to-plane MSs/MXene based 
hybrids and achieve the trade-off between porosity and densification.
Herein, we have proposed the design of a flexible and dense MoS2/Ti3C2 hybrid films 
with balanced porosity, in which the few-layered MoS2 nanosheets are intercalated into the 
Ti3C2 interspaces parallelly with the help of strong electrostatic effect and difference in their 
sizes. The balanced compaction and porosity not only can significantly increase the tap 
density of film to ~ 2.9 g cm-3, but also guaranty complete electrolyte wetting and fast ion 
diffusion. Moreover, the films have been stabilized by the 2D confinement effect and the tight 
Ti-S-Mo bonds. Furthermore, such dual 2D components with large interlayer space 
intrinsically possess satisfied ions conductivity, and meanwhile assembled superstructures 
give rapid electrons transfer. When directly used as anodes for SIB, the MoS2/Ti3C2 hybrid 
films contribute exceptional volumetric capacity of 1510 mAh cm-3 at 0.28 mA cm-2 and 650 
mAh cm-3 even at 14 mA cm-2, which vastly outperform those of MoS2/Ti3C2 mixture films 
(770 mAh cm-3 at 0.28 mA cm-2/280 mAh cm-3 at 14 mA cm-2). Additionally, the hybrid films 
also show a prolonged life-span of 300 cycles at 1.4 mA cm-2. More impressively, the areal 
specific capacity can be linearly tuned up to 4.3 mAh cm-2 as the film thickness increases 
from 9.6 to 43.1 μm without sacrificing the volumetric capacity.
2. Experimental Section
2.1 Preparation of the PDDA modified Ti3C2 nanosheets and the exfoliated MoS2 
nanosheets
The exfoliated Ti3C2 (E-Ti3C2) nanosheets were firstly prepared by selectively etching 
Al in Ti3AlC2 and subsequent ultrasonic treatment. Specifically, 2 g of LiF powder (Energy 
Chemical) were dissolved in the mixture solution of HCl (16 mL) and deionized (DI) water (4 
mL). Then, 2 g of Ti3AlC2 (Forsman Technology Co., Ltd.) were poured into the above 
solution in batches before stirring for 24 h at 40 °C. Afterward, the solid residue was washed 
by DI water, then dispersed in DI water (400 mL) and treated with probe-ultrasonic using 
JYD-900L instrument for 10 min with a pulse of 8 s on and 2 s off in Ar atmosphere. 
Thereafter, the supernatant that experienced centrifugation at 3000 rpm for 45 min was further 
centrifuged for another 45 min at 8000 rpm to collect large E-Ti3C2 precipitants. The small 
E-Ti3C2 can be realized by further treating the large Ti3C2 NSs dispersion with 
probe-ultrasonic for 30 min in Ar atmosphere. Finally, the PDDA (Adamas-beta, average 
molecular weight is 100 000-200 000) modified Ti3C2 (~1.0 mg mL-1) was obtained via 
re-dispersing the above precipitates into 0.05 wt% PDDA solution.
The exfoliated MoS2 (E-MoS2) nanosheets were gained based on the previous literature 
with a minor modification [25]. Specifically, the solid MoS2 powder (1 g, Sigma-Aldrich) was 
dispersed in 200 mL of sodium deoxycholate solution (2 mg mL−1) and then stirred for 60 
min. The mixed solution was treated by probe-ultrasonic for 8 h with a pulse of 6 s on and 4 s 
off. After centrifugation at 1500 rpm for 80 min, the supernatant was collected and further 
centrifuged at 6000 rpm for another 80 min. Afterwards, the sediments were washed with DI 
water thrice. Finally, the E-MoS2 (~1.0 mg mL-1) suspension was obtained by re-dispersing 
above precipitates into DI water.
2.2 Preparation of the MoS2/Ti3C2 hybrid films
100 mL of E-MoS2 dispersed solution were dropped into the 50 mL of PDDA modified 
Ti3C2 suspension under stirring, then standing for 1 h. Afterwards, the as-generated 
MoS2/Ti3C2 agglomerates settled to the bottom. Finally, the flexible and compact MoS2/Ti3C2 
hybrid films were obtained by vacuum filtering MoS2/Ti3C2 sediment with subsequent 
freeze-drying procedure. The thicknesses of films can be regulated by changing the amount of 
filtered solutions. As controls, the MoS2/Ti3C2 mixture films were prepared via directly 
filtering mixture solution of E-MoS2 and E-Ti3C2 free of PDDA decoration. Pure Ti3C2 films 
were also synthesized by filtering the E-Ti3C2 solution.
2.3 Characterization
The product X-ray diffraction (XRD) analyses were executed employing a Bruker D8 
Advance X-ray diffractometer with Cu Ka radiation. The structural morphology was 
investigated using field-emission scanning electron microscope (FESEM, Hitachi S-4800) and 
transmission electron microscope (TEM, JEOL-2100). The X-ray photoelectron spectroscopy 
(XPS, Bruker RFS 100/S spectrometer, 532 nm laser beam) spectra were characterized to 
acquired surface compositions and element valence of products. Nitrogen 
adsorption/desorption curves were taken on a Micromeritcs ASAP 2010 
Brunauer-Emmett-Teller (BET) measurement analyzer.
2.4 Electrochemical Measurements
Electrochemical performances were investigated by applying coin-type 2032 half cells, 
which were integrated in a glove box filled with argon. The resultant free-standing films were 
directly as working electrode. The counter electrode was Na foil, and the separator that we 
chose was Whatman GF/D. The 1.0 M NaPF6 in dimethoxyethane (DME) was applied as the 
electrolyte. Cyclic voladtammetry (CV) experiments were implement on an Autolab 
PGSTAT302N at different sweep speeds from 0.2 to 1.0 mV s-1. Galvanostatic 
charge/discharge results were recorded on a LANDCT2001A battery tester in the thermotank 
(25 °C). The galvanostatic intermittent titration technique (GITT) technology was also 
implemented on aforesaid battery tester with a test pulse of charging and discharging at 100 
mA g-1 for 15 min and break for 1 h.
3. Results and discussion
Fig. 1 The design concept and synthesis process of electrode materials for SIBs with high 
volumetric and gravimetric capacity.
The fabrication procedure of the compact MoS2/Ti3C2 hybrid films is illustrated in Fig. 
1. Specifically, the exfoliated Ti3C2 nanosheets (E-Ti3C2 NSs) and exfoliated MoS2 
nanosheets (E-MoS2 NSs) were firstly prepared by different delaminated process, whose 
morphology features are shown in Fig. S1. The E-MoS2 NSs possess a size of several 
micrometers and a few-layer lateral feature, while E-MoS2 NSs also have the few-layer 
structure with a 200 nm size. The big size difference between E-MoS2 and E-Ti3C2 NSs 
guarantees the feasibility of balancing porosity and densification of the MoS2/Ti3C2 
superstructures. The detailed demonstration and discussion will be executed later. Then, the 
sodium deoxycholate (SDC) surfactant coated E-MoS2 NSs solution with a zeta potential of 
-28 mV (Fig. S2) was dropped into the positively PDDA-modified Ti3C2 NSs suspension (59 
mV). Afterwards, the MoS2/Ti3C2 hybrids generated and settled down to the bottom (right of 
Fig. S3) because of strong electrostatic interaction. Finally, the flexible and compact 
MoS2/Ti3C2 hybrid films can be obtained by filtering above precipitation, in which the 
few-layered MoS2 NSs were parallelly intercalated into Ti3C2 interlayers. As a control, the 
MoS2/Ti3C2 mixture film was synthesized via simple physical mixture of MoS2 and Ti3C2 free 
of PDDA decoration. The MoS2 content in the mixture film is 65% according to the 
inductively coupled plasma result.
Fig. 2 (a) Photos, (b) cross-section SEM images (inset: enlarged corresponding SEM images), 
and (c-f) SEM-EDS mapping of the flexible and compact MoS2/Ti3C2 hybrid films. (g) TEM 
image of the MoS2/Ti3C2 hybrid films (inset: enlarged corresponding HRTEM image).
As revealed in Fig. 2a and Fig. S4, the MoS2/Ti3C2 hybrid films exhibit the prominent 
flexibility and mechanical strength with a high and stable conductivity of ~47 S cm-1 during 
repeated 500 bending cycles, which is much higher than ~14 S cm-1 of the control sample. 
Further, the cross-section morphologies of the flexible MoS2/Ti3C2 hybrid films are observed 
in Fig. 2b. Such films demonstrate a regular plane-to-plane dense structure with more 
moderate pores than pure Ti3C2 films (Fig. S5), which can facilitate the electrolyte infiltration 
and ion diffusion. In addition, the homogeneous element distribution (Fig. 2c-f) also proves 
the uniform and orderly assembly of the MoS2 and Ti3C2 NSs in the MoS2/Ti3C2 hybrid films. 
Interestingly, even when the thickness is amplified to ~43 μm, the hybrid films still exhibit a 
high compactness while the MoS2/Ti3C2 mixture films show a loose morphology with the 
random arrangement of nanosheets and excess pores owing to electrostatic repulsion (Fig. 
S6). Specifically, even after ultrasonic treatment for 30 min, the MoS2/Ti3C2 hybrid films 
keep an intact compact morphology profiting from the strong electrostatic interaction and 
chemical interface coupling between two components. However, the MoS2 NSs in 
MoS2/Ti3C2 mixture films randomly distribute on Ti3C2 surface with obvious folds resulting 
from the weak combination between E-MoS2 and E-Ti3C2 NSs (Fig. S7). To further reveal the 
importance of the difference in nanosheet sizes in assembling the compact MoS2/Ti3C2 
superstructures. The small Ti3C2 (S-Ti3C2) NSs with a size of ~ 200 nm are first prepared and 
shown in Fig. S8a. Then, the loose and rigid MoS2/S-Ti3C2 hybrid films with a low density of 
∼1.5 g cm-3 can be obtained via compositing MoS2 NSs and PDDA modified S-Ti3C2 NSs 
(Fig. S8c-d). The specific interpretation can be visualized by the schematic in Fig. S9. 
Obviously, the reduced size inevitably increases the surface energy of materials, which 
aggravates the tendency of self-aggregation and counteracts the electrostatic effect between 
two components, thus resulting in random combination of the nanosheets. The TEM images 
of Fig. 2g further provide the microstructure detail of the MoS2/Ti3C2 hybrid films, where 
few-layered and small-sized MoS2 NSs are evenly and parallelly distributed on the surface of 
micron-sized Ti3C2 substrates. This plane-to-plane design concept achieves a highly compact 
combination of MoS2 and Ti3C2, which can contribute to high volumetric capacity.
Fig. 3 (a) XRD patterns of the Ti3C2 films, the MoS2/Ti3C2 mixture films and the MoS2/Ti3C2 
hybrid films. (b) Nitrogen adsorption-desorption isotherms (the inset is the corresponding 
pore structure distribution). (c) High-resolution Ti 2p spectra and (d) Mo 3d spectra of the 
MoS2/Ti3C2 mixture films and the MoS2/Ti3C2 hybrid films.
The crystal structure of the as-prepared samples was revealed by X-ray powder 
diffraction (XRD). As demonstrated in Fig. 3a, the (002) peak of Ti3C2 is absent in both 
MoS2/Ti3C2 hybrid films and MoS2/Ti3C2 mixture films, suggesting the single-layer structure 
of Ti3C2 NSs [26]. Furthermore, the weakening of the MoS2 (002) peak in MoS2/Ti3C2 hybrid 
film can be proved by the intensity ratio of (002)/(103) peaks. The smaller ratio of the 
MoS2/Ti3C2 hybrid film than MoS2/Ti3C2 mixture film indicates that the self-agglomeration of 
MoS2 in the hybrid film has been effectively suppressed [27]. Notably, because of the ordered 
and compact assembly, the MoS2/Ti3C2 hybrid films show a slightly reduced specific surface 
area compared to MoS2/Ti3C2 mixture films (Fig. 3b). Meanwhile, the MoS2/Ti3C2 hybrid 
films have a richer and more balanced pore distribution dominated by 10-50 nm mesopores 
while the pores of MoS2/Ti3C2 mixture films are mainly distributed over 100 nm (inset of Fig. 
3b). The balanced porosity can significantly improve the material tap density and meantime 
guaranty complete electrolyte wetting and fast ion diffusion, which can propel a high 
volumetric capacity and rapid reaction kinetics. The surface electronic states and chemical 
structure of the films were further researched through employing X-ray photoelectron 
spectroscopy (XPS) analyses. The Ti 2p spectra for MoS2/Ti3C2 hybrid films and MoS2/Ti3C2 
mixture films are shown in Fig. 3c. It is notable that two small peaks at 457.4/462.8 eV 
belonging to Ti-S-Mo bond [28], are observed in the MoS2/Ti3C2 hybrid films while 
disappearing in the MoS2/Ti3C2 mixture films. Meantime, a 0.3 eV shift to low binding 
energy of both Mo 3d and S 2p peak positions can be seen in the MoS2/Ti3C2 hybrid films 
compared with the MoS2/Ti3C2 mixture films (Fig. 3d and Fig. S10), further corroborating the 
strong interface coupling [22,29]. Such strong covalent bonds not only effectively mitigate the 
nanosheet self-agglomeration to stabilize the hybrid films, but also can be applied as the 
electron bridge at the MoS2/Ti3C2 heterointerfaces.
Fig. 4 (a) Thickness versus areal loading plot for the MoS2/Ti3C2 hybrid films. (b) Rate 
performance of the MoS2/Ti3C2 mixture films and the MoS2/Ti3C2 hybrid films at 2.8 mg 
cm-2. (c) The long-term cycle at 1.4 mA cm-2 of the MoS2/Ti3C2 hybrid films at 2.8 mg cm-2. 
(d) Comparison of gravimetric and areal capacities of recently reported anode materials for 
SIBs. (e) Cycling stability at 1.4 mA cm-2 of the MoS2/Ti3C2 hybrid films with different areal 
loading. (f) Volumetric and areal capacities at 1.4 mA cm-2 of the MoS2/Ti3C2 hybrid films 
versus thickness. (g-i) SEM images of the MoS2/Ti3C2 hybrid films at 12.6 mg cm-2 for 
different sodiation states.
The sodium storage capabilities of the MoS2/Ti3C2 hybrid films with different MoS2 
content were firstly examined in coin-type 2032 cells (Fig. S11). The MoS2/Ti3C2 hybrid 
films can be straightway applied as work electrode free of conductive agent and binder. The 
as-optimized films with 67 wt% MoS2 content show the highest discharging capacity at every 
current density compared to the MoS2/Ti3C2 film with MoS2 content as low as 50 wt% 
(L-MoS2/Ti3C2) and the MoS2/Ti3C2 film with high MoS2 content of 74 wt% (H-MoS2/Ti3C2). 
Impressively, the areal loading of the optimized MoS2/Ti3C2 hybrid films can be linearly 
regulated from 2.8 to 12.6 mg cm-2 with the thickness range of 9.6 to 43.1 μm only by 
controlling volume of the filtered solutions (Fig. 4a). The volumetric density of the hybrid 
films is then calculated to be ∼2.9 g cm-3, which is significantly higher than 1.8 g cm-3 of the 
MoS2/Ti3C2 mixture films and most of electrode materials in the reported literatures [30-33]. 
Then, the capacity retention of the MoS2/Ti3C2 hybrid films at 2.8 mg cm-2 was evaluated at 
areal current densities from 0.28 mA cm-2 to 14 mA cm-2 (Fig. 4b). The MoS2/Ti3C2 hybrid 
films deliver an exceptional volumetric capacity of 1510 mAh cm-3 with an initial Coulombic 
efficiency of ~ 68.4% at 0.28 mA cm-2 corresponding to a fascinating gravimetric specific 
capacity of 540 mAh g-1 (Fig. S12), which is far superior than the MoS2/Ti3C2 mixture films 
(770 mAh cm-3/420 mAh g-1). Inspiringly, an outstanding capacity of 650 mAh cm-3 is still 
gained even at the high current of 14 mA cm-2. As measured again at 0.28 mA cm-2, the 
MoS2/Ti3C2 hybrid films can maintain an average specific capacity of 1450 mAh cm-3. 
Moreover, the films also control a decent cycling capability. The discharge capacity of 1360 
mAh cm-3 is achieved with only 2% capacity attenuation after prolonged 100 cycles at 0.56 
mA cm-2. The superior performance can be attributed to the decent ion/electron migration and 
robust structure, which is caused by the large intrinsic interlayers of 2D components, tight 
chemical interface interaction, and unique 2D confinement effect. As displayed in Fig. 4c, the 
MoS2/Ti3C2 hybrid films still contribute a specific capacity of 970 mAh cm-3 even after 500 
cycles at 1.4 mA cm-2. For all we know, such excellent volumetric capacity is almost the best 
report for free-standing electrode materials for SIBs [8,31,33-37]. An overall comparison is 
shown in Fig. 4d. To further clarify effect of the areal loading on electrochemical properties, 
the capacity retention of MoS2/Ti3C2 hybrid films with various thicknesses are also 
investigated. As provided in Fig. 4e, all the MoS2/Ti3C2 hybrid films do not show visible 
capacity loss after 100 cycles until the areal loading is above 10.5 mg cm-2. The volumetric 
and areal capacities versus thicknesses were further demonstrated in Fig. 4f. As the increase 
of thickness, the volumetric capacity is almost constant at 1.4 mA cm-2 with the linear 
increasing of areal capacity from 1.1 to 4.3 mAh cm-2. Further, the cross-section 
morphologies of the MoS2/Ti3C2 hybrid films with a maximum areal loading of 12.6 mg cm-2 
at disparate charging/discharging states are also shown in the Fig. 4g-i. After the first full 
sodiation, there is a visible expansion from 43.1 to 57.8 μm in the hybrid film because of the 
Na+ intercalation and inevitable swelling effect caused by electrolyte infiltration. 
Impressively, even after 100 cycles, the compact structure of hybrid film is still 
well-maintained with a moderate thickness increase of ~17%, verifying a high structural 
stability. The Ti 2p spectrum after cycles also confirms the existence of Ti-S-Mo covalent 
bonds (Fig. S13), indicating the stable chemical interface coupling in the MoS2/Ti3C2 hybrid 
films. As displayed in Fig. S14, due to the swelling effect and the weak combination between 
nanosheets, the thickness of the MoS2/Ti3C2 mixture films expands from 52.0 to 208.2 μm 
with the obvious cracks and aggregates.
Fig. 5 (a,b) The initial four CV curves of the MoS2/Ti3C2 mixture films and the MoS2/Ti3C2 
hybrid films at 0.2 mV s-1, respectively. (c) Normalized capacitive and diffusion-controlled 
capacities contribution at different sweep rates of the MoS2/Ti3C2 hybrid films. (d) 
Electrochemical impedance spectra, (e) linear relationship of the peak current (ip) versus the 
square root of the scan rate (v1/2), and (f) Na-ion diffusion coefficient under discharge and 
charge states of the MoS2/Ti3C2 mixture films and the MoS2/Ti3C2 hybrid films.
To gain an insight into the essence of such admirable sodium storage performance, the 
first cyclic voltammograms (CV) in 0.01-3 V at 0.2 mV s-1 of two products were summarized 
in Fig. S15. In the first scanning, two products reveal the similar outline, which indicates that 
both have the same electrochemical reaction process. In particular, the MoS2/Ti3C2 hybrid 
electrode possesses stronger current intensity, representing higher specific capacity. Notably, 
during the following three laps (Fig. 5a,b), the MoS2/Ti3C2 hybrid electrode reveals two 
visible and stable peaks at 1.36/1.91 V belonging to the generation and decomposition of 
Na2S, respectively [38,39]. In contrast, these two peaks in the MoS2/Ti3C2 mixture electrode 
continue to weaken until disappear in the fourth circle. That comparison shows that the 
MoS2/Ti3C2 hybrid films control a good reversibility, which is mainly because of the strong 
chemical bonding and unique 2D confinement effect in the MoS2/Ti3C2 hybrid films. The 
reaction kinetics of the MoS2/Ti3C2 hybrid electrode was further investigated by the CV 
results at various test rates from 0.2 to 1.0 mV s-1 (Fig. S16a). The detailed charge storage 
process can be qualitatively analyzed through a logarithmic equation of log i = b log v + log a 
[40], where a and b are variable value, v is scan rate and i represents measured current. The b 
value of 0.5 or 1.0 usually means diffusion-dominated reaction or surface-controlled reaction, 
respectively. Fig. S16b exhibits the MoS2/Ti3C2 hybrid electrode has the high b values for 
both the anode (0.88) and cathode (0.91), indicating the electrochemical process is principally 
surface-controlled, which corresponds to fast kinetics of capacitive storage. The capacitive 
ratios are calculated by splitting the measured current i into capacitive-dominated process 
(k1v) and the diffusion-dominated process (k2v1/2) based on following equation [22,41]:
i = k1v + k2v1/2
By assuming  and  constants, we can determine the contribution ratios of capacitive 𝑘1 𝑘2
and diffusion-controlled capacities. As exhibited in Fig. 5c, the capacitive ratio gradually 
enlarges upon the increasing of scanning rate from 0.2 to 1.0 mV s-1. The capacitive reaction 
almost regulates the entire sodium storage with the 73.5% proportion at 1.0 mV s-1 (Fig. S17). 
That high pseudocapacitance contribution can ensure more and faster Na+ storages. 
Furthermore, the electrochemical impedance spectra were executed and provided in Fig. 5d. 
Clearly, the MoS2/Ti3C2 hybrid films possess the smaller charge transfer resistance (198 Ω vs. 
322 Ω of MoS2/Ti3C2 mixture films) with a larger slope at low frequencies, demonstrating the 
better electron migration kinetics and improved Na+ transfer rates, which guarantee the high 
pseudocapacitance proportion. Electrochemical impedance spectra of the MoS2/Ti3C2 hybrid 
films with different areal loading are also shown into Fig. S18. The detailed comparison of 
charge and ion transfer ability of the different MoS2/Ti3C2 hybrid films and the MoS2/Ti3C2 
mixture film has also been summarized in Table S1. Based on CV results of Fig. S16a and 
Fig. S19a, the Na+ diffusion kinetics of two products are further evaluated using the 
Randles-Sevcik equation [42]. The MoS2/Ti3C2 hybrid films display more larger slope values 
than the MoS2/Ti3C2 mixture films (Fig. 5e and Fig. S19b), suggesting the MoS2/Ti3C2 hybrid 
films manage the faster ion diffusion reaction. The galvanostatic intermittent titration 
technique (GITT) are also used to quantify Na ion diffusivity (Fig. S20). As provided in Fig. 
5f, the  values for the MoS2/Ti3C2 hybrid films at various sodiation/desodiation stages 𝐷𝑁𝑎 +
spread over a range of ~ 10-9-10-10 cm s-1, which are greatly higher than the MoS2/Ti3C2 
mixture films and other anode materials for SIBs [43,44]. In consequence, it is very 
persuasive that such high conversion reversibility and rapid electron/ion transfer dynamics 
account for excellent electrochemical properties of the MoS2/Ti3C2 hybrid films.
4. Conclusions
In conclusion, we have designed a flexible and compact MoS2/Ti3C2 films with balanced 
porosity, in which the few-layered MoS2 nanosheets are parallelly intercalated into Ti3C2 
interlayer space due to strong electrostatic effect and difference in nanosheet sizes. The 
optimized porosity and densification endow the hybrid films with a high density of ~ 2.9 g 
cm-3 and the appropriate ion migration paths. Besides, the hybrid films have been stabilized 
by the unusual 2D confinement effect and tight Ti-S-Mo bonds. More importantly, the dual 
2D components with large interlayer distance intrinsically possesses satisfied ions 
conductivity, and meanwhile delivers rapid electron transfer after assembling superstructures. 
These peculiarities can guarantee the high volumetric sodium-ion storage of the MoS2/Ti3C2 
compact films. When directly applied as SIB anode, the MoS2/Ti3C2 hybrid films show a 
superhigh volumetric capacity of 1510 mAh cm-3 at 0.28 mA cm-2 and 650 mAh cm-3 at 14 
mA cm-2, which are far superior to 770 mAh cm-3 at 0.28 mA cm-2 and 280 mAh cm-3 at 14 
mA cm-2 of MoS2/Ti3C2 mixture films. Even after 300 cycles at 1.4 mA cm-2, the compact 
films still contribute a specific capacity of 1080 mAh cm-3 with a 96% retention. 
Impressively, the areal specific capacity can be linearly tuned from 1.1 to 4.3 mAh cm-2 
without obvious volumetric capacity recession. This proposal provides a new insight on 
engineering the microstructure of 2D materials to achieve high volumetric energy storage.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21975074 
and 21838003), the Basic Research Program of Shanghai (17JC1402300), the Shanghai 
Scientific and Technological Innovation Project (18JC1410500), and the Fundamental 
Research Funds for the Central Universities (222201718002).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://www.elsevier.com/
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Graphic Abstract
[45] Research Highlights
1. The flexible MoS2/Ti3C2 compact films with balanced porosity are reported.
2. The dense films are stabilized by the 2D confinement effect and the Ti-S-Mo bonds.
3. The superstructures with dual 2D components possess satisfied ion/electron conductivity.
4. The MoS2/Ti3C2 films show a high volumetric capacity with superior rate retention.
[46]