What is MnO₂?

Structure & Properties | Tunnels | Applications | MnO₂ in Batteries | Limitations | Renewed Interest

---

MnO₂, also known as manganese dioxide, is an inorganic metal oxide compound. It has a ratio of one manganese atom for every two oxygen atoms. The oxidation state of manganese is most commonly 4+ to balance the 2- charge from the two oxygen atoms.

The properties of MnO₂ crystals depend on arrangement of manganese and oxygen atoms. There are a variety of MnO₂ crystal structures with different properties, making this an interesting material to use for a range of applications from energy storage to catalysis. Scientists are exploring the ability to combine interesting properties through controlled growth of MnO₂ with specific crystal structures.

Other names: Pyrolusite, dioxomanganese, hyperoxide of manganese, black oxide of manganese, manganic oxide

Metal Oxides

View Full Range

Structure and Properties

---

MnO₂ crystals exist as several different morphologies, making it a polymorph like many other metal oxides. All MnO₂ polymorphs are made up of interlinked MnO₆ octahedra as the building block.

The crystallographic arrangement of bonded manganese and oxygen atoms dictates the structural, thermodynamic, and kinetic properties of each MnO₂ polymorph. The morphology, specific surface area, pore size, intrinsic conductivity, crystal structure, interlayer bound water and crystal form of MnO₂ will affect its electrochemical performance. Their earth-abundant, environmentally friendly nature and low manufacturing cost makes them an attractive material in lots of areas of research including catalysis, energy storage and ion-sieving.

Polymorph	α-MnO2	β-MnO2	R-MnO2	γ-MnO2	δ-MnO2	λ-MnO2
Mineral Type / Structure	Hollandite	Pyrolusite (Rutile structure)	Ramsdellite	Distorted Boehmite (Intergrowth mixed microdomain)	Birnessite (Layered)	Defect spinel
Space Group	I4/m	P42/mnm	Pnma	-	C2/m	Fd3m
Tunneling	2 × 2	1 × 1	1 × 1	2 × 1 1 × 1	Layers	3D Pores
Thermal Stability (°C)	500	500	400	400	Interlayer – 200 Layer - 500	240

Tunnels

One-dimensional tunnels within some of the different MnO₂ polymorph structures (α-MnO₂, β-MnO₂, R-MnO₂, and γ-MnO₂) allow for the intercalation of different small ions. This structural property has attracted the attention of battery scientists. The relatively open structures, featuring either the more spacious 2 × 2 or 2 × 1 tunnel, generally offer a higher degree of metal ion diffusion due to the presence of multiple open channels along the tunnel or in the planes nonparallel to the tunnel direction. However, as the walls of these larger tunnels are more spread out, the oxide ions have fewer neighbors to bond with. This lower "bonding density" makes the structure more likely to collapse.

Thermal Stability

The thermal stability of MnO₂ polymorphs is explained in terms of the bonding environment of the oxide ions. If they are bonded to three manganese ions (β-MnO₂) they exist in more rigid stable networks whereas when only bonded to two manganese ions oxygens are weaker links in the chain. The thermal stabilities of MnO₂ polymorphs were experimentally shown to be β > α > R ≈ γ > δ ≈ λ below 650 °C. β and α-MnO₂ are the most stable and maintain their structures up to 500 °C making them highly attractive in preventing battery degradation. R and γ transform to β-MnO₂ at 400 °C therefore losing the attractive 2×1 tunnels. λ-MnO₂ undergoes the same transformation to β-MnO₂ at only 240 °C. δ-MnO₂ transforms into α-MnO₂ above 300 °C after losing its layered periodicity below 200 °C.

Particle Size

MnO₂ particle size directly influences material properties and can be selected for a given application. For example, when used in energy storage, the smaller the particle size the larger the probability of ions intercalating within the crystal. The diffusion distance of ions in the MnO₂ crystals is shortened, enhancing their ability to exploit the ion transport channels in the bulk phase.

Conductivity

The electrical conductivity range of MnO₂ is 10^-5 - 10^-6 S/cm, which is low for typical energy storage materials. Strategies for increasing the electrical conductivity include reducing structures to the nanoscale range and using conductive additives such as carbon nanotubes and graphene.

Variable Oxidation State

The electronic configuration of manganese ([Ar]3d⁵4s²), a transition metal with five unpaired electrons, means manganese oxide compounds exist with manganese in a variety of oxidation states (+4, +3, +2, 0). MnO₂ most commonly contains manganese in the 4+ oxidation state to balance the charge of two oxygen atoms (2-). Depending on the polymorph and things like defects such as missing oxygen atoms (oxygen vacancies), other valence states are observed. This rich redox chemistry compared with other d-block elements and the ability to form species with a high oxidizing potential gives MnO₂ distinctive physicochemical features which are attractive for catalysis, pollution reduction, and biological processes.

Specific Capacity

MnO₂ has a large theoretical specific capacity of 1370 F g⁻¹, making it attractive as an electrode material for energy storage applications. The specific capacitance of actual MnO₂ crystals is dependent on their physical and chemical properties as well as their environment such as electrolyte type. The capacitance value decreases for different crystal forms of MnO₂, which following is the order: α > δ > γ > λ > β. This is due to the difference in the size of spaces such as tunneling or interlayer spacing in the crystal structure of different dimensions. By controlling the morphology, surface area, pore size, oxygen vacancies, ion accessibility or even doping of manganese dioxide we can control specific capacity.

Applications

---

MnO₂ is a versatile material with many interesting properties making it suited to a variety of applications, including:

Catalysis

MnO₂ can shift between Mn⁴⁺ and Mn³⁺ during catalytic reactions, enabling it to repeatedly convert H₂O₂ into H₂O and O₂. MnO₂ nanosheets have also been used as artificial oxidase for the detection and quantification of glutathione (GSH).

Detecting/Absorbing Toxic Metals

MnO₂ can electrochemically detect toxic metal ions such as Zn(II), Pb(II), Cu(II) and Hg(II) and can remove them from water due to its aqueous stability and attractive sorption characteristics.

Ion-Sieving

MnO₂ is particularly good at sieving small positively charged ions such as lithium (Li⁺) via its tunnels. This is vital for the recycling of lithium from secondary resources like salt-lake brines and lithium-containing wastewater. They are often referred to as octahedral molecular sieves (OMS) due to the MnO₆ octahedral building blocks that make up the MnO₂ polymorphs.

Energy Storage

MnO₂ was first used in energy storage in the 1860s where it was added to the zinc-based Leclanché cell. This technology advanced to the widely used "Duracell" alkaline primary cells of 1.5 V Zn/MnO₂. The use of MnO₂ in batteries will be discussed further later on. Supercapacitor electrodes made from MnO₂ have also been investigated due to its large theoretical specific capacity. The observed capacity of MnO₂ based electrodes for supercapacitor applications can be tuned through morphology engineering, defect engineering and heterojunction engineering.

Manganese Dioxide in Batteries

---

MnO₂ has been widely used in batteries due to their structural tunnels and high theoretical specific capacity. The flexibility to arrange the MnO₆ building blocks within their lattice structures allows properties to be tuned for enhanced energy storage performance.

The key properties of the MnO₂ polymorphs that are considered for battery applications are:

• Volume Expansion
• Electrical Conductivity
• Maximum Voltage
• Ion Mobility
• Stability
• Capacity

Types of Battery Containing MnO₂

Due to the different tunnel sizes of MnO₂ depending on morphology, a range of different metal ions can intercalate and be stored within the crystal structure. This has resulted in the use of MnO₂ in a range of battery technologies, including:

• Lithium-ion batteries (LIBs): MnO₂ is typically a cathode material in rechargeable LIBs. During the discharge cycle, lithium ions intercalate (insert themselves) into the crystal structure of the manganese dioxide.
• Calcium-ion batteries (CIBs): δ-MnO₂ as the cathode material in CIBs has the highest electrochemical performance due to higher Ca-ion diffusivity.
• Sodium-ion batteries (SIBs): MnO₂ loaded on graphene aerogel scaffold as cathode in a SIB exhibited excellent performance with power densities of > 70 mW cm⁻².
• Aqueous zinc-ion batteries (AZIBs): MnO₂ is the most widely studied cathode material. The two-electron reaction capable of manganese (Mn²⁺/Mn⁴⁺) results in high working voltage and theoretical capacity (616 mAh g⁻¹). It pairs naturally with zinc anodes in water-based electrolytes, making the battery inherently non-flammable. The working mechanism for these batteries is very complicated.
• Redox flow batteries: MnO₂ is a temporary solid phase created during charging where soluble Mn²⁺ in the electrolyte are oxidized and deposited on the electrode surface. The battery capacity is totally independent from the battery basic unit (cathode, anode and separator). Very high theoretical voltage and energy density.

Limitations of MnO₂-based Batteries

The limitations of each polymorph in terms of their practicality for use in rechargeable lithium-ion batteries are:

• α-phase: Li-ion transport at reduced capacity and low structural stability during reversible Li-ion intercalation.
• β-phase: Narrower 1×1 tunnels so lower Li-ion transport and largest cell volume expansion (%) upon lithiation, which leads to mechanical failure during operation.
• R-phase: Low electronic conductivity, higher activation energy for Li-ion diffusion through 2×1 tunnels compared to 2×2.
• γ-phase: Large cell volume expansion (%) upon lithiation.
• δ-phase: Whilst it has the highest Li-ion storage capacity, it suffers greatly from low thermodynamic stability, electronic conductivity and slow Li-ion transport kinetics.
• λ-phase: Slow Li-ion transport kinetics.

During battery discharge irreversible structural phase transformation can occur due to the Jahn-Teller effect (crystal lattice distorts). During discharge, Mn⁴⁺ is reduced to Mn³⁺ which is electronically unstable, so it undergoes a disproportionation reaction, essentially splitting into Mn⁴⁺ and soluble Mn²⁺. The gradual dissolution and re-deposition of Mn²⁺ will lead to cathode active materials reduction, structural collapse, and the rapid decay of battery capacity.

Why the Renewed Interest of MnO₂ in Battery Research?

---

The renewed interest in the use of MnO₂ in batteries is driven by a shift away from expensive, hard-to-source materials like cobalt and nickel. While MnO₂ has been the staple of disposable primary alkaline batteries for decades, it is now being reinvestigated for next-generation rechargeable systems. This is down to four trends in battery research:

Beyond-Lithium

MnO₂ has emerged as a top candidate for newer chemistries such as aqueous zinc-ion batteries (AZIBs), calcium-ion batteries (CIBs) and sodium-ion batteries (SIBs) as the world seeks alternatives to lithium-ion batteries (LIBs) due to cost and lithium scarcity.

Structural Tuning

As scientists are developing methods for controlling the nanostructure and tunnel architecture of different MnO₂ polymorphs, crucial properties such as stability and ion transport are selectively enhanced. This has broadened the applicability of MnO₂ in energy storage applications.

Solving the Polysulfide Shuttle in Li-S

Lithium-sulfur (Li-S) batteries have incredible theoretical energy density but suffer from the "shuttle effect," where sulfur species dissolve and ruin the battery. MnO₂ nanosheets are being used as a pollutant trap as their surface is highly polar and chemically binds to the polysulfides, locking them in place and increasing the battery's lifespan.

Sustainability

The geopolitical and ethical issues surrounding cobalt extraction have forced manufacturers to find other solutions.

• Abundance: Manganese is the 12th most abundant element in the Earth's crust.
• Cost: It is orders of magnitude cheaper than cobalt or nickel.
• Recyclability: MnO₂ is much easier to process and recycle safely compared to the complex chemistries found in high-end EV batteries.

Metal Oxides

View Full Range

Learn More

---