Boron Oxide Enhancing Stability of MoS₂ Anode Materials for Lithium-Ion Batteries

Abstract

Molybdenum disulfide (MoS₂) is the most well-known transition metal chalcogenide for lithium storage applications because of its simple preparation process, superior optical, physical, and electrical properties, and high stability. However, recent research has shown that bare MoS₂ nanosheet (NS) can be reformed to the bulk structure, and sulfur atoms can be dissolved in electrolytes or form polymeric structures, thereby preventing lithium insertion/desertion and reducing cycling performance. To enhance the electrochemical performance of the MoS₂ NSs, B₂O₃ nanoparticles were decorated on the surface of MoS₂ NSs via a sintering technique. The structure of B₂O₃ decorated MoS₂ changed slightly with the formation of a lattice spacing of ~7.37 Å. The characterization of materials confirmed the formation of B₂O₃ crystals at 30% weight percentage of H₃BO₃ starting materials. In particular, the MoS₂_B3 sample showed a stable capacity of ~500 mAh·g⁻¹ after the first cycle. The cycling test delivered a high reversible specific capacity of ~82% of the second cycle after 100 cycles. Furthermore, the rate performance also showed a remarkable recovery capacity of ~98%. These results suggest that the use of B₂O₃ decorations could be a viable method for improving the stability of anode materials in lithium storage applications.

1. Introduction

Low-dimensional layered structures of transition metal chalcogenides (TMCs) have attracted increased attention because of their superior properties, such as high conductivity, high stability, easy processing, and easy computing, in two-dimensional (2D) structures [1,2,3,4,5,6]. Therefore, various research has been undertaken to utilize TMCs in applications that traditionally used graphene materials [7,8,9,10]. Among them, MoS₂ is the most well-known TMC material. MoS₂ nanosheets (NSs) can be easily obtained through either top-down approaches, such as scotch tape, sonication, and chemical exfoliation, or bottom-up approaches, such as hydrothermal, chemical vapor deposition, and microwave-assisted methods. 2D MoS₂ NSs possess high conductivity, flexibility, and a large surface area, thereby making them potential candidates for anode materials in lithium storage applications. The MoS₂ NSs have a theoretical capacity of ~670 mAh·g⁻¹, which is twice that of graphite (~372 mAh∙g⁻¹) [11]. However, previous reports have shown that the MoS₂ NS anodes undergo fast degradation due to the dissolution of sulfur atoms and dislocation of MoS₂ nanosheets during the cycling process [12,13]. Moreover, the conversion reaction of MoS₂ to form Li₂S, the solid electrolyte interface (SEI) layer, and the degradation of the electrolyte resulted in the formation of a gel-like polymeric layer, which led to fast capacity fading [14]. Many attempts have been made to enhance the stability of MoS₂ NSs based on the use of graphene/carbon nanotube (CNT)/carbon cloth as skeletons, carbon coating layers, or the addition of foreign materials (such as TiO₂, MnO, Ag, and Sn) to prevent the restacking of MoS₂ and co-contribute to the electrochemical conversion reaction with lithium [11,14,15,16,17,18,19,20]. For example, Kong et al. demonstrated that MoS₂ nanoplates, with coverage of rolled-up graphene layers, form a core–shell MoS₂@graphitic nanotube, which showed a high rate performance and high capacity without using a binder [21]. Yoo et al. used CNTs as skeletons to grow MoS₂ via microwave irradiation [22]. The cylindrical-structured MoS₂ on CNTs exhibited advantageous electrochemical properties, such as high rate and high stability, as anode materials in lithium-ion batteries (LIBs). Ren et al. combined both graphene and CNTs as a frame structure for the decoration of MoS₂ nanoparticles (NPs), which delivered a high reversible capacity of ~600 mAh∙g⁻¹ for 200 cycles [23]. Qu et al. decorated Fe₂O₃ NPs on MoS₂ NSs via a hydrothermal method and sintering process, in which the anodes exhibited high-rate performances and a high reversible capacity of ~900 mAh∙g⁻¹ [24]. Zhao et al. prepared the composition MoO₃/MoS₂, which has core–sheath structure, via a sulfurization technique [25]. The MoO₃/MoS₂ core–sheath anodes exhibited a negative fading phenomenon and achieved a capacity of ~1500 mAh∙g⁻¹ after 150 cycles. Even though many attempts on improving the electrochemical performance of MoS₂ NS have been made, the mechanisms are still not clearly revealed and further improvement in stability is still needed to meet the requirements of practical applications.

Recently, lithium nickel cobalt manganese oxide (NMC) cathode materials have been effectively enhanced their stability performance by using boron compounds, such as cobalt boride (Co_xB) and B₂O₃, for surface modifications [26,27,28]. Yoon et al. revealed that Co_xB metallic glass in Ni-rich NMC can effectively enhance the stability of cathode materials via reactive wetting [26]. Li et al. utilized B₂O₃ as a surface-modification material to enhance the performance of the NMC111 cathode [27]. The use of B₂O₃ also resulted in graphene combined with a MoS₂ hierarchical structure, which improved the photo/electro properties of the graphene/MoS₂ composition for bio applications [29]. Riyanto et al. reported that a boron-doped graphene quantum structure with MoS₂ could deliver a high capacity of ~1000 mAh∙g⁻¹ [30]. However, the effect of B₂O₃ in lithium-ion batteries has not been investigated. B₂O₃ is a low-cost material with low environmental pollution and easy processing, and it plays an important role in many applications such as thermochemical energy storage, the addition of glass fibers, and the synthesis of boron compound materials such as BN [31,32]. B₂O₃ is believed to enhance the electrochemical properties of MoS₂ as it is conducted on 2D graphene materials.

In this study, we report the use of boron-oxide-nanoparticle-decorated MoS₂ NSs as anode materials in LIBs. The MoS₂ NSs were prepared using a chemical exfoliation method, and the decoration of B₂O₃ was carried out using a facile sintering technique. The results showed enhanced cycling stability in the MoS₂ anode when B₂O₃ formed a crystal structure, delivering a reversible capacity of ~500 mAh∙g⁻¹. These results suggest that the use of B₂O₃ can be a viable strategy for stabilizing anode materials for lithium storage applications.

2. Materials and Methods

2.1. Chemical Materials

Molybdenum (VI) sulfide (MoS₂, powder, 98%), boric acid (H₃BO₃, powder > 99.5%), solution of n-butyllithium in hexane (2.5 M), 1-methyl-2-pyrrolidone (NMP, anhydrous, 99.5%), and polyvinylidene fluoride (PVDF, MW 534,000) were purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA). Super-P amorphous carbon black (C, ~40 nm, 99.99%) was purchased from Alpha Aesar Inc. (Tewksbury, MA, USA).

2.2. Exfoliation of MoS₂ NSs

The exfoliation of the MoS₂ NSs was performed according to the method outlined in previous reports [16,33,34]. In brief, 1.0 g of MoS₂ powder and 3 mL of butyllithium/hexane were mixed in a 10-mL vessel (placed in a glove box) to prevent the self-heating of butyllithium. The 1.6 M butyllithium/hexane was prepared by diluting the delivered 2.5 M butyllithium/hexane solution into hexane solvent. The mixture was maintained for 2 days to form Li_xMoS₂. Li_xMoS₂ was then collected via centrifugation to remove the hexane and residual butyllithium. The obtained Li_xMoS₂ was added to 200 mL of deionized (DI) water and placed in a sonication bath for 2 h to exfoliate MoS₂. Finally, 1T-MoS₂ was washed with DI water four times to remove lithium ions and then freeze-dried using a Labconco freeze dryer (Labconco Corp., Kansas, MO, USA).

2.3. Preparation of Boron Oxide Decorated MoS₂ NS

For boron oxide decoration, the MoS₂ NSs were collected after washing four times with DI water. The amount of MoS₂ was determined by weighing the same amount of MoS₂ NS in the solution after freeze-drying. The boric acid to MoS₂ NS weight ratios were approximately 10, 20, and 30%. The mixtures were dispersed in DI water by sonication for 1 h, then freeze-dried, and sintered at 400 °C for 2 h in a tube furnace under Ar gas. The collected powder was denoted as MoS₂_B1, -B2, and -B3 with increasing amounts of boric acid (10, 20, and 30 wt%, respectively).

2.4. Material Characterization

The structure of the materials was measured by X-ray diffraction (XRD) (D/MAX-2200 Rigaku Tokyo, Japan) over the 2θ range of 10–70°. The morphologies, sizes, and detailed structures of B₂O₃ decorated MoS₂ NS were analyzed using scanning electron microscopy (SEM) (Hitachi S4700, Tokyo, Japan) and transmission electron microscopy (TEM, TECNAI G2F30, FEI Corp., Hillsboro, OR, USA).

2.5. Electrochemical Measurements

To evaluate the electrochemical performance of the materials and their lithium storage capability, the materials were assembled as working electrodes in half-cell LIBs using a coin-type cell (CR 2032, Rotech Inc., Gwangju, Korea) with a lithium reference electrode. The active material was mixed with PVDF and carbon super P at a weight ratio of 70:15:15 in a NMP solution to form a slurry. The working electrode was prepared by casting the slurry on a copper electrode, using the doctor blading method, followed by drying in a vacuum oven at 70 °C for 24 h. The battery structures were assembled under Ar gas in a glovebox with positive pressure. The separator and electrolyte were polyethylene and 1 M LiPF₆ in ethylene carbonate/diethylene carbonate (EC: DEC = 1:1 by volume). The galvanostatic electrochemical charge–discharge performances of the cells were measured using a battery cycle tester (WBCS3000, WonAtech, Seocho-gu, Seoul, Korea) across the voltage range of 0.01–3.0 V versus Li/Li⁺. Cyclic voltammetry (CV) tests, across a voltage range of 0.01–3.0 V, and electrochemical impedance spectroscopy (EIS), over a frequency range of 100 kHz to 0.1 Hz, were performed using ZIVE MP1 (WonAtech, Seocho-gu, Seoul, Korea). All the specific capacities were calculated based on the weights of the active materials.

3. Results and Discussion

Figure 1 shows the XRD patterns of the MoS₂ NSs and MoS₂_B1, -B2, and -B3 samples synthesized with 10, 20, and 30 wt% boric acid. The MoS₂ NS exhibited a main peak at ~14.2°, indicating the main orientation of the (002) plane in the 2D structure, as per JCPDS #37-1492. The other weak peaks of MoS₂ indicated the presence of multiple layers of these materials. These results are consistent with MoS₂ NSs synthesized by various methods, such as hydrothermal or sonication methods [19,20,35]. The B₂O₃ at lower concentrations of 10% and 20% did not exhibit the peak of boric oxide, which can be due to the amorphous structures on the MoS₂ NS surface. When increasing the boric acid to 30 wt%, the crystallinity of B₂O₃ was observed. The structure of B₂O₃ matched the cubic structure of B₂O₃ in JCPDS card #06-0297 with a high lattice constant (a = 10.05 Å). This lattice constant was sufficiently high compared to the 0.76 Å of lithium ion. Therefore, B₂O₃ coverage on MoS₂ may not affect lithiation/delithiation. In addition, the XRD patterns of the MoS₂_B1, -B2 and B3 samples show a broad peak at ~12°. According to Bragg’s law, the lattice spacing can be calculated from the equation $d=\lambda /2sin\theta$, where $\lambda$ is the X-ray wavelength and $\theta$ is the diffraction angle. Therefore, the lattice spacing of this peak is ~7.37 Å, and this can be attributed to the expansion of the MoS₂ layers or the stacking of MoS₂ NSs with B₂O₃ NPs. This stacking layer had a large lithium-ion radius, thereby generating a facile path for the insertion/desertion of these ions.

To confirm the morphologies of the MoS₂ NSs and their B₂O₃ decorations, the materials were subjected to SEM and TEM measurements, as shown in Figure 2. As seen in Figure 2a, the MoS₂ NSs were exfoliated from the bulk material to nanosheets with a wide size ranging from 200 nm to a few micrometers. The size diversity is due to the strong reaction of intercalated lithium between MoS₂ layers and DI water, which broke the NSs into smaller structures and the random shape of the bulk materials. This result is consistent with previous reports of MoS₂ NSs prepared using the liquid exfoliation method [16,34,36]. Moreover, the MoS₂ NSs with low amounts of B₂O₃ (10 and 20 wt% of boric acid) show a surface with tiny spots or blurred surface on the MoS₂ NS, which are the amorphous structure B₂O₃ NP decorations, as illustrated in Figure 2b,c. The MoS₂_B2 sample had larger B₂O₃ particles on its surface. When the B₂O₃ increased to 30 wt%, the SEM image in Figure 2d reveals B₂O₃ NPs with sizes in the range of ~10–20 nm. The crystallinity of B₂O₃ depends on the amount of boric acid, which could be due to the large surface area of MoS₂ NS. At low concentration, the sintering of low amount of boric acid on MoS₂ created imperfect lattices, leading to the low crystalline structure or amorphous structure of B₂O₃. On the other hand, when the concentration of boric acid was high enough (>30 wt%), the complete lattices of B₂O₃ NPs formed, indicating the high crystalline structure of B₂O₃ NPs. Therefore, it is suggested that a low amount of B₂O₃ only forms an amorphous structure and a high amount of B₂O₃ (>30 wt%) is sufficient to form a crystalline structure on the surface of MoS₂.

TEM measurements were conducted to further reveal the structure of the MoS₂ NSs and B₂O₃ NPs on the MoS₂. Figure 2e shows a high-resolution TEM (HRTEM) image of the MoS₂ NSs. The surface image clearly shows a lattice plane spacing of approximately 0.264 nm, which corresponds to the (101) plane of MoS₂. Thus, MoS₂ NSs with high crystallinity were obtained. However, in the MoS₂_B1 samples, the MoS₂ NSs were hindered by a blurred surface, which indicated the amorphous structure of B₂O₃, as illustrated in Figure 2f. The blurred surface area increased in MoS₂_B2 owing to the increasing amount of B₂O₃ amorphous structure, as shown in Figure 2g. In addition, crystalline B₂O₃ was observed in the MoS₂_B3 samples (Figure 2h). The lattice spacing was measured as 0.211 nm, which corresponds to the d-spacing of the B₂O₃ crystal. These results strongly indicated the presence of well-decorated B₂O₃ NPs on the MoS₂ NS surface.

The electrochemical properties of B₂O₃-decorated MoS₂ were recorded by CV tests at a low scanning rate of 0.1 mV∙s⁻¹, in the range of 0.0–3.0 V (vs. Li/Li⁺) (Figure 3). The reaction at the anode can be expressed by the following equation:

For lithiation:

$${\mathrm{MoS}}_2+{\mathrm{xLi}}^{+}+{\mathrm{xe}}^{-}\to {\mathrm{Li}}_{\mathrm{x}}{\mathrm{MoS}}_2$$

(1)

$${\mathrm{Li}}_{\mathrm{x}}{\mathrm{MoS}}_2+\left(4-\mathrm{x}\right){\mathrm{Li}}^{+}+(4-\mathrm{x}){\mathrm{e}}^{-}\to \mathrm{Mo}+2{\mathrm{Li}}_2\mathrm{S}$$

(2)

For delithiation:

$$\mathrm{Mo}\to {\mathrm{Mo}}^{4+}+4{\mathrm{e}}^{–}$$

(3)

$${\mathrm{Li}}_2\mathrm{S}\to \mathrm{S}+2{\mathrm{Li}}^{+}+2{\mathrm{e}}^{–}$$

(4)

Finally, the solid electrolyte reaction at first cycles:

$${\mathrm{Li}}^{+}+{\mathrm{e}}^{-}+\mathrm{electrolyte}\to \mathrm{SEI}$$

(5)

As shown in Figure 3a, the bare MoS₂ materials show a cathodic peak in the first cycle at ~0.76 V, which is the lithiation process to form Li_xMoS₂ and the deep lithiation to form Mo and Li₂S, as shown in Equations (1) and (2). The peak between 0.1–0.5 V could be due to the formation of the SEI layer (5). These results are consistent with previous reports on 1T MoS₂ in the first CV cycle [15,16]. From the second cycle, redox couple peaks were recorded at 1.05/1.72 V and 1.87/2.38 V, which are the reactions in Equations (2) and (3); and Equations (1) and (4), respectively. The CV curves of anodes MoS₂_B1 and B2 were similar. In these two anodes, the first cycle shows cathodic peaks at ~0.96 and 0.41 V, which correspond to the reactions (1) and (2), respectively. The anodic peaks were located at ~1.7 and 2.3 V, which correspond to the reactions (3) and (4), respectively. SEI layer formation was recorded together with the peak of reaction (2) at ~0.41 V. It is noted that the bare MoS₂ NSs and MoS₂_B1 and -B2 electrodes show the strong redox couple peaks at 1.87/2.38 V vs. Li⁺/Li (corresponding to (1) and (4) reactions) and the weak redox couple peaks at ~1.05/1.72 V vs. Li⁺/Li (corresponding to (2) and (3) reactions). This emphasizes the hard oxidation of Mo to Mo⁴⁺, thus leading to the degradation of the cycling stability. In contrast, for MoS₂_B3 electrode, the formation of B₂O₃ crystals was significantly effective in improving the electrochemical properties of the MoS₂ NSs. In the first cycle, the cathodic scan showed two peaks at 0.78 and 0.20 V vs. Li⁺/Li, which corresponds to the lithium insertion into MoS₂ (Equation (1)) and the deep insertion of Li into MoS₂/formation of SEI layer (Equations (2) and (5)). The peaks of the MoS₂_B3 anode were positioned at lower potential compared to those of MoS₂_B1 and -B2 electrodes, which were at ~0.9 and 0.4 V vs. Li⁺/Li. This peak shift could be due to the formation of B₂O₃ crystalline introducing a different interface to the electrolyte in comparison to the amorphous B₂O₃, which leads to the harder diffusion of Li in the first cycle. From the second cycle, the redox couple peaks were recorded at 0.82/1.73 V and 1.77/2.42 V, which correspond to the reactions (2) and (3); and Equations (1) and (4), respectively. The third cycle showed a similar curve to the second cycle, indicating the stable electrochemical reaction after the first cycle. Furthermore, the relative intensity of Mo’s oxidation peak located at ~1.73 V for the MoS₂_B3 anode (Equation (3)) was significantly enhanced in comparison to those of MoS₂_B1 and -B2 and bare MoS₂ NSs anodes. It is noted that the insertion of Li in MoS₂ at high potential is relative to the formation of a gel-like polymeric SEI layer due to the S dissolution in electrolyte [37]. The MoS₂_B1, -B2, and bare MoS₂ NSs anodes show a high cathodic peak at ~1.87 V after three cycles, which is higher than that located at 1.77 V of MoS₂_B3 anode, indicating the higher amount of S was dissolved in electrolyte. Therefore, the MoS₂_B3 anode has high amount of recovered MoS₂ NS, resulting in the high oxidation peak intensity of Mo to Mo⁴⁺. This could be due to the stability of crystalline B₂O₃ allowing the insert/desertion of Li ions [38]. Moreover, the sulfur atoms have high electron affinity, thus, they could not pass through the B₂O₃ lattice [39]. It indicates that the crystalline B₂O₃ effectively protected the MoS₂ layer, preventing the loss of S atoms and the formation of gel-like polymeric SEI layer.

To further observe the effect of B₂O₃ on the MoS₂ materials, the initial voltage profiles of B₂O₃ decorated samples are shown in Figure 4. The first three cycles of MoS₂_B1, -B2, and MoS₂ NSs seem to be unstable, showing a clear change from the first to the second and third cycles. The voltage plateau of the first discharge curve was slightly reduced from the MoS₂ NSs to the MoS₂_B1, -B2, and -B3 samples, where two plateaus at 1.1/0.51 to 1.1/0.51, 1.1/0.50, and 1.1/0.48, respectively, are shown. This indicates that the B₂O₃ crystals in the MoS₂_B3 samples changed the lithium insertion potential. In the second and third cycles, the voltage plateaus were similar for the MoS₂_B3 electrode, thereby indicating stable electrochemical properties from the second cycle. In addition, the initial discharge capacity of these anodes was high, but it reduced after each cycle owing to the formation of the SEI layer and degradation behavior in lithium ion batteries, such as cracks, sulfur dispersion in the electrolyte, and dendrite growth [40]. The initial discharge capacities for the bare MoS₂ NSs and MoS₂_B1, -B2, and -B3 were 747.1, 717.7, 638.1, and 717.2 mAh∙g⁻¹, respectively. The difference in the initial discharge capacities also depended on the formation of the SEI layer and the binding of B₂O₃ to MoS₂. The B₂O₃ was reported as a low lithium ion storage capability [41]. Therefore, the increased amount of B₂O₃ in MoS₂ led to the decreased charge/discharge capacities of MoS₂ anode materials. In particular, the charge/discharge capacities of the MoS₂_B3 anode in the third cycle were 505.2/475.0 mAh∙g⁻¹.

To evaluate the stability of the anode materials, cycling tests were performed at a current rate of 0.1 A∙g⁻¹ for 100 cycles, as illustrated in Figure 5a–d. Detailed comparison of specific capacities of as-prepared anode materials are also shown in

Table 1. Comparison of specific capacities of bare MoS₂ NS and B₂O₃ decorated MoS₂ anodes.

Anode	Initial Capacity (mAh g−1)	Current Rate (A∙g−1)	Capacity after 100 Cycles (mAh∙g−1)
Bare MoS2	747.1	0.1	109
MoS2_B1	717.7	0.1	125
MoS2_B2	638.1	0.1	140
MoS2_B3	717.2	0.1	451

. The MoS₂ NSs showed stability for ~20 cycles, and its capacity was subsequently dramatically reduced and maintained at only ~109 mAh∙g⁻¹ at the 100th cycle (Figure 5a). The addition of B₂O₃ also resulted in very fast degradation, and the remaining capacity was ~125 mAh∙g⁻¹ and ~140 mAh∙g⁻¹ at the 100^th cycle in the MoS₂_B1 and B2 anodes, respectively (Figure 5b,c). The MoS₂_B1 and B2 showed the enhancement of lattice spacing of MoS₂, facilitating the insertion/desertion of Li ions. However, the amorphous B₂O₃ could not prevent the loss of S atoms. Therefore, MoS₂_B1 and B2 anodes exhibited inferior stability to the pure MoS₂ NS. In contrast, the crystalline B₂O₃ in the MoS₂_B3 electrode showed a high capacity in the first cycle, and it demonstrated prolonged cycling stability for 100 cycles. As shown in Figure 5d, the capacity of MoS₂_B3 at the 100th cycle was ~451 mAh∙g⁻¹, which was 86.2% of the second cycle (~510 mAh∙g⁻¹) and ~62.9% of the first cycle. Therefore, it can be concluded that the enhancement of the redox reaction with the B₂O₃ crystals was effective in improving the stability of the MoS₂ NSs.

The protective role of B₂O₃ was further confirmed via ex-situ XPS spectra, as shown in Figure 6. Both bare MoS₂ NS and MoS₂_B3 anodes were compared at the initial state and at 3.0 V 40 cycles. The initial state of MoS₂ and B₂O₃ decorated samples presented the same conditions of Mo⁴⁺ and S²⁻. However, after 40 cycles, bare MoS₂ NSs showed significant change in Mo 4f peak. Mo⁴⁺ peak intensity reduced, and the Mo⁵⁺ and Mo⁶⁺ peaks appeared. Moreover, in S 2p spectrum, the S 2p peak split to S²⁻ peak at ~162 eV and a S*²⁻ peak at ~163.6 eV, which might be related to the formation of polymeric SEI layer due to the S dissolution or the unrecoverable Li₂S [42,43], indicating unstable MoS₂ NS anode. On the other hand, the MoS₂_B3 anode show a better stability, where the main peak of Mo 4f assigned to Mo4+ was maintained with partial Mo⁶⁺ peaks. It is noted that the Mo⁶⁺ peak might appear due to the sample preparation method as pointed out in previous reports [44,45,46]. The S 2p peak of MoS₂_B3 showed a small change with S*²⁻ peak, which might be due to a partial loss of S to polymeric layer or unrecoverable Li₂S. These results indicate that B₂O₃ layer efficiently protected MoS₂ layer, preventing the loss of S to electrolyte. The small amount of S loss can be further improved after optimizing the B₂O₃ layers.

The electrical properties of the anode materials were evaluated using EIS measurements, as shown in Figure 7a. The equivalent circuit (modified Randle’s model) contained series resistance (R_S), charge-transfer resistance (R_CT), SEI layer resistance (R_SEI), a diffusion Warburg impedance element, and constant phase elements (CPE1 and CPE2). The extracted R_CTs of the MoS₂ NS and MoS₂_B1, -B2, and -B3 samples were 150.3, 118.4, 118.2, and 148.6 Ω, respectively. The addition of boron oxide did not significantly affect the resistance of the anode material. MoS₂_B1 and -B2 showed reduced resistance. Then, the resistance increased in MoS₂_B3 when B₂O₃ formed crystallinity owing to the low conductivity of B₂O₃. However, this resistance was still lower than that of the bare MoS₂. As a 2D layered structure material, the conductivity of MoS₂ decreases when the number of layers is reduced. In addition, the presence of B₂O₃ NPs may prevent the restacking of MoS₂ NSs and the NS material from forming a bulk structure, thereby enhancing the conductivity of the anode material. The rate performance of MoS₂_B3 is shown in Figure 7b. An increase in the current rate led to a decrease in capacity. At 1.0 A∙g⁻¹, the capacity was maintained at ~155 mAh∙g⁻¹. Nevertheless, the MoS₂_B3 electrode can be recovered to almost 98% when decreasing the current rate to 0.1 A∙g⁻¹, thus illustrating a highly reversible rate performance.

The recent works on the modification of MoS₂ are shown in

Table 2. Comparison of the electrochemical performance of the modified MoS₂ anodes in lithium-ion batteries.

Anode Material	Current Density (A∙g−1)	Initial Discharge Capacity (mAh∙g−1)	Cycle Number	Specific Capacity (mAh∙g−1)	References
Ag decorated MoS2	0.1	~900	100	510	[16]
TiO2 decorated MoS2	0.1	827	100	604	[17]
SiCN-MoS2	~0.1	~726	20	445.6	[47]
Carbon coated MoS2	0.1	1419	50	837	[48]
MoS2 on CNT	0.025	1200	50	650	[49]
B2O3 on MoS2	0.1	717	100	451	This work

. The initial discharge capacities were high above 800 or even 1200 or 1400 mAh∙g⁻¹. This might be due to the contribution to the capacity of the modified materials. From our method, B₂O₃ does not mainly contribute to the capacity, but effectively protects the MoS₂ layer, which maintains the highly stable capacity. Moreover, the sintering method and the utilization of the boric acid are cost-effective ways. Therefore, it can be readily scale-up to the industrial size. We also believe that the use of carbon-based and co-active materials can further improve the electrochemical performance of the materials presented in this study for lithium ion storage application.

4. Conclusions

In summary, B₂O₃ NP-decorated MoS₂ NSs were successfully fabricated via a facile chemical exfoliation and sintering process. The XRD, SEM, and TEM measurements confirmed that the crystalline B₂O₃ could be formed at high boric acid content of over 30 wt%. The presence of B₂O₃ created the lattice spacing of ~7.37 Å in MoS₂ NS. Crystal B₂O₃ formed with a lattice spacing of ~2.58 Å, improving the redox reaction in the conversion of MoS₂ during the cycling process. The high intensity of Mo oxidation peak and the lower potential of lithium insertion into MoS₂ indicated B₂O₃ layer played a role as a protective layer, preventing the dissolution of S atoms into electrolyte. The bare MoS2 material and amorphous B₂O₃ in MoS₂_B1 and -B2 anodes showed fast degradation after 20–40 cycles due to the loss of sulfur into the electrolyte. Meanwhile, the MoS₂_B3 electrode with protectable crystalline B₂O₃ layer demonstrated a stable capacity of ~500 mAh∙g⁻¹ and a high-capacity retention of ~86.2% after 100 cycles. These results suggest that B₂O₃ NP decorations on anode materials could be a potential approach for high-stability anodes in lithium storage applications.