Research Progress of MnO₂/rGO as Cathode Materials for Zinc Ion Batteries

Limited lithium resources and potential safety/environmental issues restrict the further development of lithium-ion batteries (LIBs). There exists an urgent need to develop new green batteries. Compared with organic electrolytes, aqueous electrolytes are environmentally friendly, inexpensive, and have a high ionic conductivity. Among them, zinc ion batteries (ZIBs) using aqueous electrolytes become a focus of recent research due to their low manufacturing cost, safety and environmental friendliness, as well as their unique properties, such as high abundance of metallic zinc, suitable redox potential (–0.76 V vs. SHE), and high theoretical capacity (823 mA·h·g^–1). So far, the cathode materials suitable for storing zinc ions as ZIBs are manganese-based oxides, vanadium-based oxides, Prussian blue analogs, and organic compounds. Among these materials, vanadium oxides have relatively low redox potentials, Prussian blue analogs have insufficient theoretical specific capacity, and organic materials have poor electrical conductivity, resulting in a poor overall battery performance. In contrast, manganese oxides, with abundant resources, high operating voltage (i.e., 1.0–1.9 V vs. Zn/Zn²⁺), and high theoretical specific capacity (i.e., 308 mA·h·g^–1 for single-molecule reaction, and 616 mA·h·g^–1 for bimolecular reaction), become the most promising cathode materials for development. Meanwhile, manganese oxide faces short board problems such as low intrinsic electronic conductivity and electrode dissolution during charging and discharging, which hinder its further development. Reduced graphene oxide (rGO) has some advantages as a carbon material due to its ultra-high theoretical surface area (2630 m²·g^–1), high electrical conductivity and porosity. Therefore, MnO₂/graphene nanocomposites are prepared using rGO as a conductive substrate in combination with manganese oxides with a high theoretical specific capacity, solving the problem of poor durability performance of manganese oxides and achieving a high specific surface area as well as optimizing the electrical properties.

Focusing on the research progress of MnO₂/rGO nanocomposites in the anode of zinc ion batteries (ZIBs),this review firstly analyzes the structural characteristics of different crystallines MnO₂ (δ, α, γ) to explore the most suitable MnO₂ crystalline for ZIBs. The lattice constant of each unit is 2.3 Å through the connection shared by octahedral [MnO₆] edge and corner sharing connection. The 2D lamellar structure δ-MnO₂ has a relatively wide interlayer spacing (7 Å), and the large distances can accommodate moderate impurity cations. The tunnel structure of α-MnO₂ has large dimensions (i.e., 4.6 Å × 4.6 Å), which can accommodate most of the metal ions and water molecules, and the γ-MnO₂ consists of randomly arranged of 1 × 1 (2.3 Å× 2.3 Å) and 1 × 2 (2.3 Å × 4.6 Å) tunnels, which are characterized by hollow, porous, and 1D structures. This review also analyzes the composite mode and performance of different crystallines of MnO₂/rGO to reveal the MnO₂/rGO preparation mode that is most beneficial to the cycling stability of the batteries, explore the common law in the energy storage mechanism, and look forward to the direction of the development of the high-performance ZIBs cathode materials and optimization strategies.

1) α-MnO₂ has superior st ability and specific capacity. It is attributed to the double-chain structure of α-MnO₂, with a larger average cross-sectional area of tunneling, which is favorable for ion storage. The lamellar structure of δ-MnO₂ is prone to structural deterioration during cycling, leading to the irreversible growth of new phases, and the crystal structure of γ-MnO₂ consists of a random arrangement of 1 × 1 and 1 × 2 tunneling structures, which is unstable and prone to the transformation of the crystalline structure, restricting the insertion/extraction of Zn²⁺.

2) The in-situ-constructed α-MnO₂/rGO belongs to the optimum cathode materials for zinc ion batteries. The in-situ growth method enables the α-MnO₂ arrays to be anchored in the rGO surface layer (110). The Mn atoms on the crystalline surface and the rGO substrate are bonded by Mn-O-C covalent bonds, and the interface is in atomic-level contact, which reduces the charge transfer resistance, promotes the coordination adsorption of Zn²⁺, decreases the activation energy of extraction and blunts the active sites on the surface of rGO, and inhibits the occurrence of side reaction such as a hydrogen precipitation reaction.

3) The energy storage mechanism of MnO₂/rGO cathode material is mainly based on H⁺/Zn²⁺ insertion/extraction, supplemented by other energy storage mechanisms. In the process of H⁺ extraction, the exposure of surface active sites and short diffusion paths are crucial, so the specific surface area and interfacial proton affinity of rGO need to be optimized as a priority. Zn²⁺ extraction is limited by the ionic size and volumetric strain, so it is necessary to strengthen the conductive network and the structural stability and to design a suitable pore structure.

To further enhance the electrochemical performance of zinc ion batteries, the MnO₂/rGO composites still need to be further investigated from the following two aspects in the future:

1) Although the rGO composite strategy improves the performance of MnO₂ to a certain extent, the electrochemical performance of MnO₂/rGO composites can be further enhanced to meet the demands of high-efficiency and long-life zinc ion batteries if combined with a variety of strategies, such as pre-insertion of guest ions, defect engineering, and nanomodulation.

2) MnO₂/rGO cathode materials still face a problem of MnO₂ dissolution, leading to the loss of active material and reducing the battery capacity. Common solutions are to add Mn²⁺ salts to the electrolyte or to use solid-state electrolytes, which are costly and unfavorable for mass production applications. Therefore, exploring low-cost MnO₂ dissolution inhibition strategies will become an important aspect in zinc ion battery research, while the development of flexible, wearable, and high-security electronic devices will be the future development of zinc ion batteries.