TiO2 Nanotube Layers Decorated with Al2O3/MoS2/Al2O3 as Anode for Li-ion Microbatteries with Enhanced Cycling Stability

TiO2 nanotube layers (TNTs) decorated with Al2O3/MoS2/Al2O3 are investigated as a negative electrode for 3D Li-ion microbatteries. Homogenous nanosheets decoration of MoS2, sandwiched between Al2O3 coatings within self-supporting TNTs was carried out using atomic layer deposition (ALD) process. The structure, morphology, and electrochemical performance of the Al2O3/MoS2/Al2O3-decorated TNTs were studied using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and chronopotentiometry. Al2O3/MoS2/Al2O3-decorated TNTs deliver an areal capacity almost three times higher than that obtained for MoS2-decorated TNTs and as-prepared TNTs after 100 cycles at 1C. Moreover, stable and high discharge capacity (414 µAh cm−2) has been obtained after 200 cycles even at very fast kinetics (3C).

To overcome these problems, surface modification of the TNT layers by coating, decorating, and doping with various materials have been extensively explored [6,[25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Because of the low volumetric expansion and high porosity, the surface modified TNT layers deliver high capacity, while keeping the mechanical stability of the nanostructured electrode. In our recent work, we showed, for the first time, TNT layers homogenously decorated with ultrathin MoS 2 nanosheets using atomic layer deposition (ALD) process that can be used as anode for 3D µLIBs [6]. The MoS 2 -decorated TNT layers deliver superior electrochemical performance in comparison to their pristine counterparts. However, the capacity fades continuously during cycling due to the formation of thick solid electrolyte interphase (SEI) on the surface of the electrode and the loss of active material [6].
In the present study, we report the remarkable electrochemical properties obtained for the reversible insertion of Li ions in Al 2 O 3 /MoS 2 /Al 2 O 3 -decorated TNT layers. The capacity fading is strongly attenuated by protecting the MoS 2 nanosheets with Al 2 O 3 sandwich coating, produced before and also after the MoS 2 ALD process. The 3D multilayers deliver excellent areal capacities with good stability up to 200 cycles even at very fast kinetics, making the Al 2 O 3 /MoS 2 /Al 2 O 3 -decorated TNT layers a potential candidate as a negative electrode for high performance µLIBs.

Synthesis of TNTs and ALD-Decorated TNTs
Self-organized TNT layers with a thickness of~20 µm and an inner diameter of~110 nm were produced via anodization of thin Ti foils (127 µm thick, Sigma-Aldrich) according to the previous published work [39]. In brief, the Ti foils were anodized in an ethylene glycol-based electrolyte containing NH 4 F (170 mm) and 1.5 vol % H 2 O at 60 V for 4 h. Prior to anodization the Ti foils were degreased by sonication in isopropanol and acetone for 60 s, respectively, and dried in air. The anodization setup consisted of a high-voltage potentiostat (PGU-200 V; Elektroniklabor GmbH) in a two-electrode configuration, with a Pt foil as a counter electrode and the Ti foil as a working electrode. After anodization, the TNT layers were sonicated in isopropanol for 5 min and dried in air. Before further use, the TNT layers were annealed in air in a muffle oven at 400 • C for 1 h to obtain crystalline anatase phase.
The coating of Al 2 O 3 on the TNT layers was prepared using trimethylaluminum (TMA, Strem, 99.999+%) and deionized water (18 MΩ) as aluminum and oxygen precursors, respectively [29,39]. Under these conditions, one ALD Al 2 O 3 growth cycle was defined by the following sequence: TMA pulse (500 ms)-TMA exposure (5 s)-N 2 purge (10 s)-H 2 O pulse (500 ms)-H 2 O exposure (5s)-N 2 purge (10 s). All processes were carried out at a temperature of 150 • C, using N 2 (99.9999%) as the carrier gas, at a flow rate of 400 sccm. The ALD process of 9 cycles Al 2 O 3 corresponds to a nominal thickness of 1 nm Al 2 O 3 , as shown in our previous work [29].

Materials Characterization
The morphology and chemical composition of the fresh and cycled electrodes were characterized by a field emission electron microscope (FE-SEM JEOL JSM 7500F, JEOL, Tokyo, Japan) and a transmission electron microscope (Titan Themis 60-300, Thermo Fisher Scientific, Eindhoven, Netherlands) operated at 300 keV and equipped with a high angle annular dark field detector for scanning transmission electron microscopy (STEM-HAADF) and Super-X energy dispersive X-ray (EDX) spectrometer with 4 × 30 mm 2 windowless silicon drift detectors. All the EDX elemental maps are shown in net intensities, which represent the count intensities according to the background corrected and fitted model performed by Velox 2.9 software. Cross section views were obtained from mechanical bended TNTs. Dimensions of the layers were measured and statistically evaluated using proprietary Nanomeasure software.
The surface chemical state of MoS 2 was monitored by X-ray photoelectron spectroscopy (XPS) (ESCA2SR, Scienta-Omicron, Taunusstein, Germany) using a monochromatic Al Kα (1486.7 eV) X-ray source operated with 250W and 12.5kV. The binding energy scale was referenced to adventitious carbon (284.8 eV).

Electrochemical Characterization
The electrochemical performance tests were performed using standard two-electrode Swagelok cells that were assembled in a glovebox filled with high purity argon (Ar). The half-cells consist of as-prepared TNTs, MoS 2 -TNTs, or Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs as the working electrode and Li foil (1 mm in thickness and 9 mm in diameter) as the reference electrode. The two electrodes were separated by a Whatman glass microfiber soaked in organic liquid electrolyte solution (0.35 mL) composed of 1m LiPF 6 dissolved in a 1:1 vol.% mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC).
The electrochemical performance tests (cyclic voltammetry, CV, galvanostatic charge−discharge) were performed using a VMP3 potentiostat (Bio Logic, France). The CV curves were recorded in a potential window of 0.01-3 V at a scan rate of 1 mV s −1 . Galvanostatic tests were performed at multiple C-rate in the potential window of 0.01-3 V. The current was applied based on TNTs assuming a porosity of 70.5%. The porosity calculation is based on the amount of the TNTs per cm 2 and should be noted that it is only an estimated value (see supplementary materials for the calculations). C/n means the battery is fully charged or discharged up to its total storage capacity in n hours (for this work 1C = 340 µA cm −2 ). As the surface area of the as-prepared and ALD-decorated TNTs are macroscopic (0.82 cm 2 ), the obtained capacities are given in areal capacities (mAh cm −2 ).

Results and Discussion
The highly ordered TNT layers were 20 µm thick, and the nanotubes had an inner diameter of 110 nm resulting in an aspect ratio of 180, as shown in our previous publication [6]. As the amount of MoS 2 decorated on the TNT layers by 15 ALD cycles is very low, it was not possible to visualize it by using SEM. However, as proved previously by STEM-EDX, already 2 ALD cycles of MoS 2 led to a decoration of the TNT layers with small MoS 2 sheets [6]. Figure 1 shows a STEM-HAADF image of the edge of TNT decorated with 9 cycles Al 2 O 3 -15 cycles MoS 2 -9 cycles Al 2 O 3 and the corresponding EDX maps (see Figure S1a for the EDX spectrum). These maps reveal a homogenous distribution of Mo and S as well as of Al on the TNT wall. In comparison with our previous publication, the MoS 2 nanosheets appear smaller [6]. This can be explained by the different chemical nature of the surfaces that MoS 2 was deposited on: herein, the MoS 2 was deposited on the Al 2 O 3 layer, while in our previous publication the MoS 2 was deposited directly on the TNT walls [6]. The initial ALD growth of MoS 2 is different on different surfaces, and, thus, MoS 2 nanosheets observed herein are smaller than if they are directly grown on TiO 2 . Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 12 MoS2 was deposited on the Al2O3 layer, while in our previous publication the MoS2 was deposited directly on the TNT walls [6]. The initial ALD growth of MoS2 is different on different surfaces, and, thus, MoS2 nanosheets observed herein are smaller than if they are directly grown on TiO2. XPS survey spectra of TNT layers decorated with 15 cycles MoS2 and with 9 cycles Al2O3-15 cycles MoS2-9 cycles Al2O3 are shown in Figure 2a. For 15 cycles MoS2 sample, Ti 2p and O 1s signals stem from the underlying TNT layer. In the case of the sandwich sample it is observed that the intensity of the O 1s signal increases and the Ti 2p decreases, due to the presence of the Al2O3 layers; therefore most of the O 1s comes from the Al2O3. The C species detected on both TNT layers are related to adventitious carbon. Figure 2b shows the corresponding Mo 3d high-resolution spectra (HR) along with the S 2s signal. As can be seen, the HR signals on both samples are relatively broad. This can be explained by the very thin MoS2 decoration as on the TiO2/MoS2 interface, as well as on the Al2O3/MoS2 some Mo-O bonds might be built. When higher ALD MoS2 cycle numbers were applied (results not shown), the signals became narrower due to thicker MoS2 nanosheet decorations, and the XPS spectra showed pure MoS2 [6]. Considering this, Mo 3d HR spectra of both samples show their corresponding spin-orbit Mo 3d5/2/Mo 3d3/2 and were deconvoluted into three doublets. The first one (red), centered at ~229.0/232.1 eV, is assigned to Mo 4+ belonging to the MoS2  Figure 2b shows the corresponding Mo 3d high-resolution spectra (HR) along with the S 2s signal. As can be seen, the HR signals on both samples are relatively broad. This can be explained by the very thin MoS 2 decoration as on the TiO 2 /MoS 2 interface, as well as on the Al 2 O 3 /MoS 2 some Mo-O bonds might be built. When higher ALD MoS 2 cycle numbers were applied (results not shown), the signals became narrower due to thicker MoS 2 nanosheet decorations, and the XPS spectra showed pure MoS 2 [6]. Considering this, Mo 3d HR spectra of both samples show their corresponding spin-orbit Mo 3d 5/2 /Mo 3d 3/2 and were deconvoluted into three doublets. The first one (red), centered at~229.0/232.1 eV, is assigned to Mo 4+ belonging to the MoS 2 lattice [40,41]. The second one (blue), located at~229.9/233.0 eV, is attributed to Mo bonded with oxygen to form MoO 2 [42]. The last doublet (orange) at~232.5/235.6 eV corresponds to MoO 3 [43,44]. It is notable that in the sandwich sample the signals corresponding to MoS 2 decrease, while molybdenum oxide signals increase. This could be due to the interaction of MoS 2 with the water used as a precursor for the synthesis of Al 2 O 3 . Besides, S 2s peaks of the 15 cycles MoS 2 sample, centered at~226.6 (MoS 2 ) (dark cyan) and 229.5 eV (SH-thiol groups) (purple), respectively, and S 2s peaks of the sandwich sample, located at 226.8 (MoS 2 ) (dark cyan) and 234.2 (SO 4 2− ) (green), respectively, agree well with the chemical species observed in S 2p.
In Figure 2c, the deconvoluted HR S 2p spectra of both samples confirm the presence of MoS 2 with the doublet S2p 3/2 /S2p 1/2 (dark cyan), centered at~161.9/163.1 eV, which corresponds to the S 2− state from the MoS 2 lattice [45]. However, each sample presented two different additional chemical species. 15 cycles MoS 2 sample show another doublet (purple) at~163.6/164.8 eV attributed to SH that remained on the surface after the MoS 2 deposition [46]. The sandwich sample displayed its doublet (green) at 167.8/169.0 eV, assigned to SO 4 2− (sulfate) [46], possibly due to the interaction of sulfur with the water used in Al 2 O 3 synthesis.
Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 12 lattice [40,41]. The second one (blue), located at ~229.9/233.0 eV, is attributed to Mo bonded with oxygen to form MoO2 [42]. The last doublet (orange) at ~232.5/235.6 eV corresponds to MoO3 [43,44]. It is notable that in the sandwich sample the signals corresponding to MoS2 decrease, while molybdenum oxide signals increase. This could be due to the interaction of MoS2 with the water used as a precursor for the synthesis of Al2O3. Besides, S 2s peaks of the 15 cycles MoS2 sample, centered at ~226.6 (MoS2) (dark cyan) and 229.5 eV (SH -thiol groups) (purple), respectively, and S 2s peaks of the sandwich sample, located at 226.8 (MoS2) (dark cyan) and 234.2 (SO4 2− ) (green), respectively, agree well with the chemical species observed in S 2p. In Figure 2c, the deconvoluted HR S 2p spectra of both samples confirm the presence of MoS2 with the doublet S2p3/2/S2p1/2 (dark cyan), centered at ~161.9/163.1 eV, which corresponds to the S 2− state from the MoS2 lattice [45]. However, each sample presented two different additional chemical species. 15 cycles MoS2 sample show another doublet (purple) at ~163.6/164.8 eV attributed to SH that remained on the surface after the MoS2 deposition [46]. The sandwich sample displayed its doublet (green) at ~167.8/169.0 eV, assigned to SO4 2− (sulfate) [46], possibly due to the interaction of sulfur with the water used in Al2O3 synthesis. Figure 3a-c shows the cyclic voltammetry curves obtained for as-prepared TNTs, MoS2-TNTs and Al2O3/MoS2/Al2O3-TNTs recorded at a scan rate of 1 mV s −1 in the potential window of 0.01-3 V vs. Li/Li + . All the CV curves obtained exhibit a cathodic peak at 1.7 V vs. Li/Li + and anodic peak at 2.2 V vs. Li/Li + associated to the reversible insertion/extraction of Li + into/from anatase according to Equation (1) [5,18,47,48]. However, the first insertion peak for Al2O3/MoS2/Al2O3-TNTs is shallow and shifts to the lower potential because of the Al2O3 insulating coating which slows down the Li-diffusion [29]. This behavior is not observed in the subsequent cycles due to the formation of a conductive Al-O-Li phase.   [5,18,47,48]. However, the first insertion peak for Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs is shallow and shifts to the lower potential because of the Al 2 O 3 insulating coating which slows down the Li-diffusion [29]. This behavior is not observed in the subsequent cycles due to the formation of a conductive Al-O-Li phase.
LixMoS2 + (4 -x) Li + (4 -x) e ⇌ Mo + 2Li2S, Li2S ⇌ 2Li + + S + 2e -, Compared to the CV curves of as-prepared TNTs and Al2O3/MoS2/Al2O3-TNTs, the MoS2-TNTs shows broader peaks and larger surface area under the CV curve. This is attributed to the MoS2-decoration contributing to the total capacity and modification of the electrode structure. However, the peak intensity and area under the CV curve diminish with cycling. In our previous work, we reported that electrochemical performance of MoS2-TNTs is affected by the dissolution of S combined to the formation and growth of a SEI layer [6]. In contrast, reversible and stable CV curves are obtained for Al2O3/MoS2/Al2O3-TNTs owing to the ALD-deposited Al2O3 thin layers. The surface modification results in the improved stability of the electrode by limiting the S dissolution and the growth of the SEI layer through the formation of a stable Al-O-Li composite [29]. The electrochemical performance was evaluated through the examination of the charge/discharge profiles obtained by galvanostatic cycling tests. Figure 4a-c, shows the galvanostatic charge/discharge profiles for as-prepared TNTs, MoS2-TNTs, and Al2O3/MoS2/Al2O3-TNTs at a current density of 340 µA cm −2 (1C) in the potential window of 0.01-3 V vs. Li/Li + . The charge/discharge profiles are in agreement with the electrochemical behaviors observed from the CV plots. For as-prepared TNTs and MoS2-TNTs, the obtained capacity fades with cycle number unlike to the Al2O3/MoS2/Al2O3-TNTs. This is attributed to the beneficial effects of the Al2O3-coating on the TNTs, which are in agreement with works reported in the literature [25,29,51,52].  (2) and (3) [49,50]. Upon the charge (delithiation) process, the shallow peak at 1.9 vs. Li/Li + associated with retrieval of Li x MoS 2 from Mo is dwarfed by the broader and more prominent peak at 2-2.75 V vs. Li/Li +, which correspond to the oxidation of Li 2 S to S according to Equations (3) and (4), respectively [49,50]. This phenomenon is more pronounced for MoS 2 -TNTs because of the absence of the protective Al 2 O 3 -coating layer.
Compared to the CV curves of as-prepared TNTs and Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs, the MoS 2 -TNTs shows broader peaks and larger surface area under the CV curve. This is attributed to the MoS 2 -decoration contributing to the total capacity and modification of the electrode structure. However, the peak intensity and area under the CV curve diminish with cycling. In our previous work, we reported that electrochemical performance of MoS 2 -TNTs is affected by the dissolution of S combined to the formation and growth of a SEI layer [6]. In contrast, reversible and stable CV curves are obtained for Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs owing to the ALD-deposited Al 2 O 3 thin layers. The surface modification results in the improved stability of the electrode by limiting the S dissolution and the growth of the SEI layer through the formation of a stable Al-O-Li composite [29].
The electrochemical performance was evaluated through the examination of the charge/discharge profiles obtained by galvanostatic cycling tests. Figure 4a-c, shows the galvanostatic charge/discharge profiles for as-prepared TNTs, MoS 2 -TNTs, and Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs at a current density of 340 µA cm −2 (1C) in the potential window of 0.01-3 V vs. Li/Li + . The charge/discharge profiles are in agreement with the electrochemical behaviors observed from the CV plots. For as-prepared TNTs and MoS 2 -TNTs, the obtained capacity fades with cycle number unlike to the Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs. This is attributed to the beneficial effects of the Al 2 O 3 -coating on the TNTs, which are in agreement with works reported in the literature [25,29,51,52].   Figure 5a shows the discharge capacity vs. cycle number for as-prepared TNTs, MoS2-TNTs, and Al2O3/MoS2/Al2O3-TNTs cycled at 1C. The first cycle delivers a discharge capacity of 652 µAh cm −2 , 1286 µAh cm −2 , and 729 µAh cm −2 for the as-prepared TNTs, MoS2-TNTs, and Al2O3/MoS2/Al2O3-TNTs, respectively. The higher capacity obtained for the decorated-TNT electrodes are attributed to the contribution of MoS2 coating. The irreversible capacity observed after the first cycle is attributed to the side reactions of Li + with water molecule traces and the structural defects of the TNTs, and additionally, the dissolution of S and the formation of the SEI layer in the case of MoS2-TNTs [6,53,54]. It is clearly apparent that the Al2O3/MoS2/Al2O3-TNTs have superior cyclability than as-prepared TNTs and MoS2-TNTs with a reversible capacity of 640 µAh cm −2 obtained, whereas only 222 µAh cm −2 and 220 µAh cm −2 was retained after 100 cycles for the as-prepared TNTs and MoS2-TNTs, respectively. It is remarkable that the areal capacities increase with the number of cycles. This is attributed to the formation of microcracks as the result of Li + reaction with MoS2, which expose additional pore channels. In addition, the presence of Al2O3 decoration bestows the TNT electrodes with enhanced chemical properties. Figure 5b shows the coulombic efficiency (CE) at 1C for 100 cycles. The CE obtained for Al2O3/MoS2/Al2O3-TNTs at the first cycle was 62% and reached more than 99% just after three cycles. In comparison, as-prepared TNTs and MoS2-TNTs have a first cycle CE of 64% and 74% and reaching the 98% only after 15 and 85 cycles, respectively. These values indicate relatively more stable SEI formation on the surface of the Al2O3-coated electrode even after long-term cycling. It is remarkable that the beneficial effect of the Al2O3 coating is also evidenced at very fast kinetics (3 C) over 200 cycles as shown in Figure 5c. Indeed, the Al2O3/MoS2/Al2O3-TNT electrode is able to maintain a capacity of 414 µAh cm −2 , whereas the as-prepared TNTs and MoS2-TNTs retain only 130 µAh cm −2 and 195 µAh cm −2 , respectively. The main electrochemical results of the as-prepared and ALD-decorated TNTs in comparison with literature are shown in Table 1.   [6,53,54]. It is clearly apparent that the Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs have superior cyclability than as-prepared TNTs and MoS 2 -TNTs with a reversible capacity of 640 µAh cm −2 obtained, whereas only 222 µAh cm −2 and 220 µAh cm −2 was retained after 100 cycles for the as-prepared TNTs and MoS 2 -TNTs, respectively. It is remarkable that the areal capacities increase with the number of cycles. This is attributed to the formation of microcracks as the result of Li + reaction with MoS 2 , which expose additional pore channels. In addition, the presence of Al 2 O 3 decoration bestows the TNT electrodes with enhanced chemical properties. Figure 5b shows the coulombic efficiency (CE) at 1C for 100 cycles. The CE obtained for Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs at the first cycle was 62% and reached more than 99% just after three cycles. In comparison, as-prepared TNTs and MoS 2 -TNTs have a first cycle CE of 64% and 74% and reaching the 98% only after 15 and 85 cycles, respectively. These values indicate relatively more stable SEI formation on the surface of the Al 2 O 3 -coated electrode even after long-term cycling. It is remarkable that the beneficial effect of the Al 2 O 3 coating is also evidenced at very fast kinetics (3 C) over 200 cycles as shown in Figure 5c. Indeed, the Al 2 O 3 /MoS 2 /Al 2 O 3 -TNT electrode is able to maintain a capacity of 414 µAh cm −2 , whereas the as-prepared TNTs and MoS 2 -TNTs retain only 130 µAh cm −2 and 195 µAh cm −2 , respectively. The main electrochemical results of the as-prepared and ALD-decorated TNTs in comparison with literature are shown in Table 1.  Post-mortem analysis was carried out to provide further evidence for the positive contribution of the Al2O3 decoration on the electrochemical properties. Figure 6a-c shows the SEM images of the as-prepared TNTs, MoS2-TNTs, and Al2O3/MoS2/Al2O3-TNTs after 200 charge/discharge cycles at 3C, respectively. A very thick (ca. 6 µm) and rough SEI layer has been grown on MoS2-TNTs (Figure 6b) in comparison to as-prepared TNTs that is around 2 µm thick (Figure 6a). Similar behavior was observed from our previous work on MoS2-coated TNTs [6]. In contrary, the SEI formed on Al2O3/MoS2/Al2O3-TNTs is much thinner (ca. 1 µm) and smoother (Figure 6c) confirming the benefit of Al2O3 coatings. This effect is further evidenced by STEM-EDX elemental maps given in Figure 6d showing the homogenous distribution of Mo, S, and Al on the TNT walls after electrochemical tests (see Figure S1b for the EDX spectrum).   [57] 100 mA cm −2 -570 680 (50) 100% (50) Post-mortem analysis was carried out to provide further evidence for the positive contribution of the Al 2 O 3 decoration on the electrochemical properties. Figure 6a-c shows the SEM images of the as-prepared TNTs, MoS 2 -TNTs, and Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs after 200 charge/discharge cycles at 3C, respectively. A very thick (ca. 6 µm) and rough SEI layer has been grown on MoS 2 -TNTs ( Figure 6b) in comparison to as-prepared TNTs that is around 2 µm thick (Figure 6a). Similar behavior was observed from our previous work on MoS 2 -coated TNTs [6]. In contrary, the SEI formed on Al 2 O 3 /MoS 2 /Al 2 O 3 -TNTs is much thinner (ca. 1 µm) and smoother (Figure 6c) confirming the benefit of Al 2 O 3 coatings. This effect is further evidenced by STEM-EDX elemental maps given in Figure 6d showing the homogenous distribution of Mo, S, and Al on the TNT walls after electrochemical tests (see Figure S1b for the EDX spectrum).

Conclusions
In this work, enhanced electrochemical performance of TNT was achieved by decorating the surface with nanosheets of MoS2, sandwiched between Al2O3 coatings. ALD technique was used to homogenously deposit the MoS2 nanosheets and the Al2O3 layers on the self-supporting TNT layers. The excellent capacity and stability of Al2O3/MoS2/Al2O3-decorated TNT is attributed to the mechanical and structural stability imported by Al2O3 decoration. The Al2O3 limits the formation and growth of SEI layer and loss of active material during cycling. As a result, the Al2O3/MoS2/Al2O3-decorated TNT deliver an areal capacity almost three times higher than that obtained for MoS2-decorated TNT and as-prepared TNTs after 100 cycles at 1C.