Lithium Battery Electrode Materials: Research Progress on MoS₂ Improving Conductivity

As one of the highest energy density commercial rechargeable battery systems, the electrochemical performance of electrode materials in lithium-ion batteries directly determines the energy density, cycle life, and rate capability. The theoretical specific capacity of graphite anodes is only 372 mAh/g, which is increasingly inadequate for the growing demands of electric vehicles and large-scale energy storage. Molybdenum disulfide (MoS₂) is considered a promising candidate for next-generation lithium-ion battery anodes due to its high theoretical specific capacity (approximately 670 mAh/g, about 1.8 times that of graphite), unique two-dimensional layered structure, and tunable bandgap width. Starting from the lithium storage mechanism of MoS₂, this article reviews the research progress of MoS₂ as a lithium battery electrode material and the strategies for improving its electrical conductivity.

Lithium Storage Mechanism of MoS₂ as an Anode Material

The lithium storage process of MoS₂ is closely related to its crystal structure. MoS₂ belongs to the hexagonal crystal system with the space group P6₃/mmc. Within the layer, it features an S-Mo-S sandwich structure, with an interlayer spacing of approximately 0.615 nm, and the layers are connected by weak van der Waals forces. This structure allows Li⁺ to intercalate between the MoS₂ layers and undergo conversion reactions at certain potentials.

According to the reaction potential, the lithium storage process of MoS₂ can be divided into two stages:

Intercalation reaction stage (potential ≥ 1.0 V vs. Li/Li⁺):

$$MoS_2 + xLi^+ + xe^- ightarrow Li_xMoS_2$$

This process corresponds to Li⁺ intercalation between MoS₂ layers to form Li_xMoS₂, where MoS₂ maintains the 2H phase structure. The theoretical upper limit of lithium intercalation is x ≈ 1, contributing a specific capacity of approximately 167 mAh/g.

Conversion reaction stage (potential < 1.0 V vs. Li/Li⁺):

$$Li_xMoS_2 + (4-x)Li^+ + (4-x)e^- ightarrow Mo + 2Li_2S$$

The conversion reaction produces nano-Mo metal and Li₂S, contributing the remaining specific capacity of approximately 503 mAh/g. The theoretical specific capacity of fully lithiated MoS₂ is 670 mAh/g, about 1.8 times that of conventional graphite anodes.

However, in practical applications, MoS₂ faces two main challenges: first, the intrinsic conductivity of 2H-phase MoS₂ is relatively low (on the order of 10⁻⁴ S/cm), which limits electron transport; second, MoS₂ undergoes significant volume change (approximately 103%) during charge-discharge cycles, easily causing electrode pulverization, active material detachment, and repeated SEI film growth, leading to rapid capacity decay.

Main Strategies for Improving the Conductivity of MoS₂

To address the insufficient conductivity of MoS₂, current research focuses on the following four directions:

1. Nanostructure Design

Preparing MoS₂ into nanosheets, nanoflowers, nanotubes, and other structures can significantly shorten the Li⁺ diffusion path and increase the specific surface area. For example, single-layer MoS₂ nanosheets have a thickness of approximately 0.7 nm, a specific surface area of up to 80–120 m²/g, and the Li⁺ diffusion distance is reduced by 1–2 orders of magnitude compared with bulk materials.

Tested by the BET method (GB/T 19587-2017), the specific surface area of mesoporous MoS₂ nanoflowers is usually between 60–100 m²/g, much higher than the 5–10 m²/g of bulk MoS₂. A high specific surface area facilitates electrolyte infiltration and active site exposure, but it also intensifies SEI film formation, which requires a balance in structural design.

2. Carbon-Based Composite Materials

Combining MoS₂ with carbon materials such as graphene, carbon nanotubes, and porous carbon is currently the most common conductivity enhancement approach. The carbon framework not only provides continuous electron transport channels but also buffers the volume change of MoS₂.

Typical cases include:

• MoS₂/graphene composites: specific capacity maintained at 900–1100 mAh/g after 100 cycles at 0.1 A/g current density
• MoS₂/carbon nanotube composites: capacity retention of approximately 80% after 500 cycles at 1 A/g
• MoS₂/porous carbon composites: specific capacity of 300–400 mAh/g maintained even at the high rate of 5 A/g

According to the battery cycle test method specified in GB/T 18287-2013, the capacity retention rate of the above composite materials after 300 cycles at 1C rate is usually between 75% and 90%.

3. Phase Engineering (1T/2H Phase Control)

MoS₂ exists in multiple crystalline phases, with the 2H phase being the thermodynamically stable phase (semiconductor type) and the 1T phase being the metastable phase (metallic type). The conductivity of 1T-phase MoS₂ is approximately 10⁵ times higher than that of the 2H phase, so introducing the 1T phase through phase engineering can significantly improve electron transport.

Main preparation methods include:

• Alkali metal intercalation exfoliation: 2H phase is converted to 1T phase through n-butyllithium intercalation
• Hydrothermal/solvothermal method: controlling reaction temperature (180–220°C) and time (12–48 h) can yield 1T/2H mixed phases
• Doping-induced method: introduction of Re, Mn, and other dopant atoms can stabilize the 1T phase

The conductivity of 1T/2H mixed-phase MoS₂ typically reaches 1–10 S/cm, which is 4–5 orders of magnitude higher than that of the pure 2H phase.

4. Heteroatom Doping

Partial substitution of S atoms in MoS₂ with heteroatoms such as Se, N, and P can alter its electronic structure and introduce more active sites. N-doped MoS₂ can achieve a reversible specific capacity of 800–1000 mAh/g at 0.5 A/g current density, an improvement of approximately 30% over undoped MoS₂.

Electrode Preparation Process and Electrochemical Performance

The preparation process of MoS₂ anodes is similar to that of conventional graphite anodes, mainly including slurry preparation, coating, drying, calendaring, and slitting. However, the specific characteristics of MoS₂ materials require attention to the following points:

Typical electrochemical performance range (half-cell test, 1C = 670 mA/g):

• 0.1C initial discharge specific capacity: 800–1200 mAh/g
• 0.1C initial Coulombic efficiency: 75%–88%
• 1C cycle 100 times capacity retention: 80%–95%
• 5C high rate specific capacity: 200–400 mAh/g

According to GB/T 31486-2015 "Electrical Performance Requirements and Test Methods for Traction Batteries for Electric Vehicles", in a full battery, the capacity retention of MoS₂ anodes at -20°C is about 55%–65% of room temperature, and the capacity retention after 200 cycles at high temperature (55°C) is about 75%–85%.

Industrialization Challenges and Development Directions

Although MoS₂ has shown excellent lithium storage performance at the laboratory level, industrialization still faces the following challenges:

1. Low initial Coulombic efficiency: The initial Coulombic efficiency of MoS₂ is usually 75%–88%, lower than the 90%–95% of graphite, requiring compensation through pre-lithiation or artificial SEI film technology

2. Voltage hysteresis: The discharge plateau of MoS₂ (approximately 1.0–2.0 V) is higher than that of graphite (approximately 0.1 V). Although this is beneficial for safety, it reduces the full-cell energy density

3. Cycle life: Compared with the mature level of graphite anodes maintaining 80% capacity after more than 1000 cycles, MoS₂ composite materials typically have 70%–85% capacity retention after 500–1000 cycles

4. Cost control: The preparation cost of high-quality single-layer MoS₂ nanosheets or 1T-phase MoS₂ is relatively high, requiring the development of scalable production processes

Future research directions will focus on: controllable preparation of 1T-phase-enriched MoS₂, development of solid electrolyte matching technology, construction of full-cell system optimization, and improvement of initial efficiency through pre-lithiation technology. As the technology continues to mature, MoS₂ is expected to achieve industrial application in specific scenarios within 3–5 years, particularly in power tools, drones, and some electric vehicle fields that require high rate performance.

Selection and Testing Recommendations

For evaluating the application potential of MoS₂ as a lithium battery anode material, the following test procedures are recommended:

1. Physical property testing: XRD to confirm crystal phase (2H/1T), SEM/TEM to observe morphology, BET to test specific surface area, four-probe to test conductivity

2. Half-cell test: assemble CR2032 coin cells to test specific capacity, initial efficiency, rate capability, and cycle performance in the voltage window of 0.01–3.0 V

3. Full-cell test: match with NCM523 or NCM811 cathodes to test 1C cycle 500 times capacity retention and -20°C/55°C high-low temperature performance

4. Safety test: conduct nail penetration, extrusion, and overcharge tests in accordance with GB 38031-2020

MoS₂-based anode materials represent one of the active research directions in the current lithium battery field and have important reference value for battery designs pursuing high energy density or high power density. It is recommended to comprehensively evaluate the electrochemical performance, process feasibility, and cost competitiveness of MoS₂ composite materials in combination with the specific application scenario (power, energy storage, 3C digital) requirements.

Tags: Lithium Battery | Electrode Material | Molybdenum Disulfide | Anode Material | Conductivity | Nanostructure | Carbon Composite | 1T Phase | GB/T 31486 | Lithium Storage Mechanism