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).

1. Introduction

Nowadays, microelectrochemical systems are key devices for providing power for micro/nanoelectromechanical devices (M/NEMS) in the fields of bio/medical engineering, aerospace, and intelligent sensors [1,2,3]. The microelectrochemical systems can be classified based on their power source as rechargeable Li-ion microbatteries (µLIBs) [4,5,6], microsupercapacitors [7], microfuel cells [8], and microthermoelectric batteries [9]. The two main requirements for selecting power sources for M/NEMS devices are high energy/power densities and long lifetime [10,11]. Planar 2D µLIBs energy and power densities have an intrinsically inverse correlation, i.e., microbatteries with thick electrodes deliver a high-energy and a low-power density, while the reverse is true for thin electrodes [12]. Hence, the development of 3D µLIBs forms a viable alternative to planar 2D µLIBs to overcome the tradeoff between power and energy [13,14]. Nanomaterials such as nanopillars, nanorods, nanowires, and nanotubes are widely explored as potential electrode materials for 3D µLIBs due to their short ion diffusion distances, high aspect ratio, and small foot print [15,16,17,18].

Self-supported TiO2 nanotube (TNT) layers have been extensively explored as anodes for 2D/3D µLIBs due to their unique one-dimensional architecture, high self-ordering degree, short Li+ diffusion distance, fast electron transport, safety (high lithiation potential ~1.7 V vs. Li/Li+), low self-discharge rate, and nontoxic nature [18,19,20,21,22]. However, their low theoretical capacity (168 mAh g−1) and poor electronic conductivity pose a major obstacle for practical application [20,23,24].

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 MoS2 nanosheets using atomic layer deposition (ALD) process that can be used as anode for 3D µLIBs [6]. The MoS2-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 Al2O3/MoS2/Al2O3-decorated TNT layers. The capacity fading is strongly attenuated by protecting the MoS2 nanosheets with Al2O3 sandwich coating, produced before and also after the MoS2 ALD process. The 3D multilayers deliver excellent areal capacities with good stability up to 200 cycles even at very fast kinetics, making the Al2O3/MoS2/Al2O3-decorated TNT layers a potential candidate as a negative electrode for high performance µLIBs.

2. Materials and Methods

2.1. 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 NH4F (170 mm) and 1.5 vol % H2O 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 samples were coated using atomic layer deposition (ALD) (Beneq TFS-200) with 15 cycles MoS2 (henceforth named as MoS2-TNTs) or with a three-layer coating consisting of 9 cycles Al2O3—15 cycles MoS2—9 cycles Al2O3 (henceforth referred as Al2O3/MoS2/Al2O3-TNTs). The coating of MoS2 was carried out as described in our previous work with bis(t-butylimido)bis(dimethylamino) molybdenum (Strem, 98%) and hydrogen sulfide (99.5%) as molybdenum and sulphur precursors, respectively [6]. The MoS2 was deposited within the TNT layers by applying 15 ALD cycles at a temperature of 275 °C with N2 (99.9999%) as carrier gas at a flow rate of 500 standard cubic centimeters per min (sccm). The molybdenum precursor was heated up to 75 °C to increase its vapor pressure. Under these deposition conditions, one growth ALD cycle was defined by the following sequence: Bis(t-butylimido)bis(dimethylamino) molybdenum pulse (4 s)—Bis(t-butylimido)bis (dimethylamino) molybdenum exposure (45 s)—N2 purge (90 s)—H2S pulse (2.5 s)—H2S exposure (45 s)—N2 purge (90 s).

The coating of Al2O3 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 Al2O3 growth cycle was defined by the following sequence: TMA pulse (500 ms)—TMA exposure (5 s)—N2 purge (10 s)—H2O pulse (500 ms)—H2O exposure (5s)—N2 purge (10 s). All processes were carried out at a temperature of 150 °C, using N2 (99.9999%) as the carrier gas, at a flow rate of 400 sccm. The ALD process of 9 cycles Al2O3 corresponds to a nominal thickness of 1 nm Al2O3, as shown in our previous work [29].

2.2. 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 mm2 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 MoS2 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).

2.3. 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, MoS2-TNTs, or Al2O3/MoS2/Al2O3-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 LiPF6 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 cm2 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 cm2), the obtained capacities are given in areal capacities (mAh cm−2).

3. 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 MoS2 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 MoS2 led to a decoration of the TNT layers with small MoS2 sheets [6].

Figure 1 shows a STEM-HAADF image of the edge of TNT decorated with 9 cycles Al2O3—15 cycles MoS2—9 cycles Al2O3 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 MoS2 nanosheets appear smaller [6]. This can be explained by the different chemical nature of the surfaces that MoS2 was deposited on: herein, the 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.

Figure 1

STEM-HAADF image in high magnification and the STEM EDX elemental maps showing the distribution of Mo, S, and Al on the surface of the TiO2 nanotube layers (TNTs).

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 Mo4+ belonging to the MoS2 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 (SO42−) (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 S2− 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 SO42− (sulfate) [46], possibly due to the interaction of sulfur with the water used in Al2O3 synthesis.

Figure 2

(a) X-ray photoelectron spectroscopy (XPS) survey spectra, (b) Mo 3d high resolution spectra and (c) S 2p high resolution spectra for TNT layers decorated with 15 cycles MoS2 and with 9 cycles Al2O3—15 cycles MoS2—9 cycles Al2O3.

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. TiO

Figure 3

Cyclic voltammograms of (a) as-prepared TNTs, (b) MoS2-TNTs and (c) Al2O3/MoS2/Al2O3-TNTs recorded at a scan rate of 1 mV s−1

In comparison to as-prepared TNTs, the CV curves show additional peaks for the MoS2-TNTs (Figure 3b) and Al2O3/MoS2/Al2O3-TNTs (Figure 3c). These peaks are attributed to the multistep reaction of Li+ with MoS2. During the first discharge (lithiation), the two cathodic peaks at 1.25–1.75 V and 0.5 V vs. Li/Li+ are attributed to phase transformation of MoS2 in to LixMoS2 and the subsequent complete reduction of Mo4+ to Mo0 and Li2S, respectively, according to Equations (2) and (3) [49,50]. Upon the charge (delithiation) process, the shallow peak at 1.9 vs. Li/Li+ associated with retrieval of LixMoS2 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 Li2S to S according to Equations (3) and (4), respectively [49,50]. This phenomenon is more pronounced for MoS2-TNTs because of the absence of the protective Al2O3-coating layer. MoS Li Li

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].

Figure 4

Galvanostatic charge/discharge profiles of (a) as-prepared TNTs, (b) MoS2-TNTs, and (c) Al2O3/MoS2/Al2O3-TNTs at 1C.

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.

Figure 5

Long-term cycling tests of as-prepared TNTs, MoS2-TNTs, and Al2O3/MoS2/Al2O3-TNTs: (a) at 1C for 100 cycles, and (b) the corresponding coulombic efficiency vs. cycle number and (c) at 3C for 200 cycles.

Table 1

Comparison of the electrochemical performance of as-prepared and atomic layer deposition (ALD)-decorated TNTs with TNTs coated with various materials.

Working Electrode	First Discharge Capacity (µAh cm−2) at C-Rate	Discharge Capacity after (n) Cycle (µAh cm−2)	Coulombic Efficiency (%) after (n) Cycles
as-prepared TNTs	1C-652	222 (100)	~98% (100)
	3C-952	130 (200)	~98% (200)
MoS2-TNTs	1C-1286	220 (100)	~98% (100)
	3C-1520	195 (200)	~98% (200)
Al2O3/MoS2/Al2O3-TNTs	1C-729	640 (100)	>99% (100)
	3C-887	414 (200)	>99% (200)
SnO2@TNTs [55]	2C-469.8	113 (50)	>94%(50)
Co3O4@TNTs [56]	1C-200	103 (25)	NA
TNTs@Fe2O3 [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 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).

Figure 6

Cross sectional SEM images of (a) as-prepared TNTs, (b) MoS2-TNTs, and (c) Al2O3/MoS2/Al2O3-TNTs after 200 cycles at 3C. Solid electrolyte interphase (SEI) layer thickness and surface roughness is indicated by a white line and red arrows. (d) High magnification STEM HAADF image and the STEM-EDX elemental maps showing the distribution of Mo, S, and Al on the surface of the TNT for Al2O3/MoS2/Al2O3-TNTs.

4. 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.