Introduction

Magnesium-ion battery is a new type of rechargeable battery with magnesium metal or alloy as the negative electrode, and magnesium salt as electrolyte. Magnesium is similar to lithium in chemical properties. Compared with Li, Mg is rich in resources and easy to purify. In addition, the characteristic of magnesium-ion as a divalent ion enables it to carry and store more charge, which explains the results of higher volume specific capacity (3832 mAh mL⁻¹)^[1].  Besides, magnesium-ion batteries are able to avoid dendrite growth problems, which is one of the major safety challenges of lithium-ion batteries. The growth of lithium dendrite will cause instability of the interface between electrodes and electrolyte, uninterrupted consumption of electrolyte, reduce the efficiency of batteries, and may even pierce the diaphragm leading to short connection inside lithium-ion batteries, resulting in combustion and explosion of batteries caused by thermal uncontrolled.

Figure 1. Lithium dendrite forming process (Figure by Hanru Liu).

The history of magnesium-ion batteries can be traced back to the 1960s. However, due to the large ion-radius and high polarization of magnesium ions, which lead to the instability of the battery reaction process and poor electrochemical performance, it has been difficult to achieve commercial applications. Until recent years, with the development of new materials and improvement of process, the performance of magnesium-ion batteries has been significantly improved. Overall, magnesium ion batteries have a very promising future and are expected to become the next generation of renewable batteries with high energy density and low cost.

Operating principle

Mg-ion batteries share a similar working principle with lithium-ion battery: During charging, magnesium ions are separated from the positive electrode and migrate to the negative electrode through the electrolyte under the driving of external voltage. At the same time, magnesium ion is embedded in the negative electrode material. Due to the balance of charge, it is required that an equal amount of electrons flow from the positive electrode to the negative electrode in the wire of the external circuit. The result of charging is that the negative electrode is in a magnesium-rich state, and the positive electrode is in a high energy state of magnesium-poor state. The flow of electrons in the external circuit forms a current to realize the conversion of chemical energy to electrical energy.

Figure 2. Operating principle (Figure by Hanru Liu).

Materials

Positive electrode

Magnesium ion battery positive electrode mainly consists of the active material, conductive additives, fluid collector and binder, which is the core part of the battery. Ideal positive electrode materials for rechargeable magnesium batteries should meet the requirements of large capacity, high voltage platform, good reversibility, high cycle efficiency, safety and stability, rich resources, and easy preparation. Currently, the research of positive electrode materials for magnesium secondary batteries mainly focuses on transition metal sulfides, transition metal oxides, polyanionic compounds, sulfur and sulfur compounds, organic compounds and composite materials. The collector must be corrosion resistant and stable, and will not react with other substances. At present, stainless steel foil is the common collector for magnesium ion batteries. Positive electrode materials are one of the key materials of Magnesium Ion Batteries (MIBs), which directly affect the operating voltage and the specific capacity of the batteries.  At present, the main types of positive electrode materials involved in the research are embedded & exfoliated positive electrode materials, conversion type positive electrode materials, organic positive electrode materials, etc.

Embedded & Exfoliated Positive Electrode Material

Generally speaking, the embedded and exfoliated material, also known as intercalation materials, can keep the structure stable during the cycle and achieve a stable cycle. They are the most widely studied positive electrode materials in magnesium ion batteries. These intercalated compounds are also considered as potential candidates for positive electrodes in magnesium ion battery systems due to their successful applications in lithium ion battery systems.

1.Chevrel phase compounds. The application of Chevrie phase-type positive electrode materials in MIBs is the earliest and most successful, and its general structure formula is Mo₆T₈ (T = S, Se and Te). Mo₆S₈ is one of the most successful positive electrode materials for magnesium batteries because of its fast magnesium insertion kinetics and excellent cycle stability^[2].

2. Layered compounds. Layered materials have unique two-dimensional channels, which provide abundant chemically active insertion sites, enabling them to rapidly embed/detach. The high electronic conductivity of layered compounds helps to accelerate the reaction kinetics and therefore has great potential for application in the field of energy storage batteries. Layered materials have good application prospects for magnesium ion batteries because of the weak van der Waals force between layers. Layered transition metal oxides (LTMOs) are considered to be one of the most promising positive electrode materials for magnesium ion batteries due to their high operating voltage, stable structure and low cost. V₂O₅ is the most representative layer transition metal oxide and has excellent electrochemical performance in batteries.

Figure 3. V₂O₅. Figure by Hanru Liu^[3]

3. Polyanionic compounds. Polyanionic compounds are three-dimensional network structures with strong covalent bonds on transition metals and polyanions. These materials are abundant in variety, high voltage, stable in structure, and have strong inductive effect between polyanions, which make them potential positive electrode materials for magnesium ion batteries. For instance, phosphates with NASICON structure may be the best candidate^[4].

4.Spinel structure positive electrode material. The general formula of spinel structure is MgT₂X₄ (T = Ti, V, Mn, etc; X = O, S, SE, etc.). It has the advantages of large capacity, high working voltage, and three-dimensional diffusion path is expected to increase the energy density of materials. For example, spinel MgMn₂O₄ is an negative electrode material with a theoretical capacity of 272 mAhg^-1 and is expected to be used in high capacity magnesium ion batteries in the future^[5].

Conversion Type Positive Electrode Materials

1. Transition metal sulfides and sulfur. If S is used as positive material for Mg-ion batteries and Mg-negative is combined, a two-electron conversion reaction occurs (Mg²⁺+ S+2e^- ↔ MgS) results in a theoretical energy density of up to 2500 WhL^-1[6]. However, compared with the Li-S system, the research of Mg-S batteries is still in the initial stage. The main challenge is to find an appropriate electrolyte system that is compatible with both sulfur positive electrode and magnesium negative electrodes^[7].

2. Transition metal oxides. Transition metal oxides, especially manganese oxides, are the most studied positive electrode materials for Mg-ion batteries due to their rich composition and crystal structure^[8]. Replacing sulfur with oxygen in the positive electrode material to increase the embedded voltage and theoretical capacity is expected to achieve higher energy density.

Organic Positive Electrode Materials

Organic materials have attracted increasing attention due to their richness, diversity, structural flexibility, and adjustability. Organic materials with oxidation-reduction activity can achieve rapid diffusion of Mg²⁺due to weak intermolecular forces. The development of organic positive electrode materials provides new opportunities for the development of magnesium ion positive electrode materials.

1. Carbonyl compounds. The chemical formula of carbonyl compounds is 2R-C=O (R=H, CH₂, benzene ring, etc.), which has the advantages of high theoretical capacity, flexible molecular structure, and abundant raw materials. Among them, benzoquinone based carbonyl compounds are a class of positive electrode materials with great application potential in MIBs^[9], such as DMBQ (dibromoanthraquinone), whose room temperature discharge capacity is higher than that of most inorganic positive electrode materials.

2. Organic free radicals^[10]. In addition to carbonyl compounds, organic free radicals with redox activity can also be used as positive electrode materials for MIBs, the general formula of which is N-3R (R = H, O, benzene ring, etc.). Poly(4-methylacrylic acid)-2,2,6,6-tetramethylpiperidine-1-nitroxyl radical ester (PTMA) was first used in lithium-ion batteries. Because of its fast electron transport, it is highly expected to solve the slow diffusion of Mg²⁺ ions.

3. Organic sulfides^[11]. Organic sulfides are n-type organic materials. The general formula is R-S-S-R (R=benzene ring, quinary ring, etc.). The mechanism of magnesium storage is based on the reversible fracture and formation of disulfide bonds (S-S). Up to now, organic sulfide positive electrodes are still limited by poor cycling performance, and their progress in MIBs is slow.

Negative electrode

Negative electrode materials require reversible deposition and dissolution of magnesium ions. Magnesium metal or activated carbon/carbon cloth is generally selected as the electrolyte, and the corresponding electrolyte is usually an ether electrolyte or magnesium perchlorate electrolyte. The uniform deposition of magnesium in the cycle makes it a good negative electrode material in itself. Current research is focused on how to avoid the formation of passive film on the surface of Mg negative electrodes in some traditional polar organic electrolytes or aqueous electrolytes, which results in Mg²⁺ being insulated by the passive film. Currently, the main research directions are nanostructured Mg and inserted negative electrode materials synthesized by alloy method. The thickness of passivation film can be effectively reduced by using 2.5 nm Mg negative electrode in MgO battery system. The alloy insert negative electrode materials mainly include Bismuth, Antimony, Tin and other alloy negative electrode materials. As a negative electrode, nano-cluster Mg₃Bi₂ can obtain 360 mAhg^-1 electrochemical performance in LiCl-APC electrolyte, and it remains stable during 200 cycles^[12]. Bi nanotubes evolve into interconnected nanopores during mg insertion and removal, exhibiting excellent cyclic stability and folding performance. This indicates that nanostructured negative electrode materials not only support volume expansion effectively, but also maintain effective electrical contact.

Electrolyte

Liquid Electrolyte

Liquid electrolyte is one of the most suitable electrolytes for current magnesium ion battery system. Compared with solid electrolytes, liquid electrolytes have higher ionic conductivity, better reversibility, better cycling performance, easier preparation and lower viscosity. The liquid electrolytes of magnesium ion battery system mainly include inorganic electrolyte, boron-based electrolyte, magnesium organic haloaluminate-based electrolyte, phenol or alcohol-based electrolyte and non-nuclear-philic electrolyte.

1. Green reagent. Green reagent is a nucleophilic reagent and is considered the first electrolyte to observe reversible deposition of magnesium. As early as the 1920s, it was found that magnesium could deposit reversibly in the green reagent electrolyte, which opened the door for the development of magnesium ion batteries. However, this reagent has many problems, such as narrow electrochemical window, easy oxidation, low ionic conductivity and poor stability. In addition, the green reagent is easy to oxidize and results in low negative electrode stability, so it is not considered suitable for the future development of magnesium batteries^[13].

2. Organic Magnesium Chloroaluminate Electrolyte. Compared with the Green reagent, the organic magnesium chloroaluminate electrolyte has a higher efficiency of magnesium dissolution and deposition, reaching almost 100% and higher negative electrode stability. The synthetic magnesium-aluminium chloride complex electrolyte (MACC) with inorganic salt magnesium chloride (MgCl₂) and aluminium chloride (AlCl₃) as reactants was the first reported electrolyte with better performance than Green reagent. The Coulomb efficiency of MACC electrolyte is 100%, with a broader electrochemical window (2-3V) and lower Mg deposition potential^[14].

3. Non-nucleophilic electrolyte (HMDSMgCl)^[15]
   

   . Non-nucleophilic electrolytes have the advantages of high negative electrode stability, high solubility and high Coulomb efficiency, which further widen the selection of positive electrode materials, make sulfur also be used as positive electrode materials, and open up a new system of Mg-S batteries. The compatibility of non-nucleophilic electrolyte with sulfur makes it a promising electrolyte for magnesium ion batteries and makes large capacity magnesium batteries possible. If commercialization can be achieved reasonably, great commercial benefits will be created.

4. Boron-based electrolyte. The development of boron-based electrolytes can be traced back to 1957. In recent years, researchers have found that boron-based electrolytes have good stability, corrosion resistance and reversibility of high deposition on different collecting fluids, which attracts people's attention. Therefore, boron-based electrolytes, especially Mg(BH₄)₂, are considered one of the most promising electrolytes^[16].

Solid Electrolyte

Solid electrolytes have the advantages of good safety, good mechanical properties, wide voltage window and high energy density. Electrolytes can be divided into inorganic solid electrolytes, organic solid electrolytes and organic-inorganic composite solid electrolytes according to their composition. At present, the research on Mg solid-state electrolytes is in the initial stage. The solid electrolytes used in magnesium solid-state batteries are basically divided into inorganic system (phosphate, borohydride, chalcogenide, metal-organic frame material), organic polymer system (adding magnesium salt, possibly inorganic filler) and organic-inorganic composite solid electrolyte.

1. Inorganic solid electrolyte. The inorganic solid electrolyte MgZr₄ (PO₄)₆ (MZP) synthesized by sol-gel method has a conductivity of 7.23*10-3 scm^-1 at 725 ℃. Mg (BH₄) (NH₂) electrolyte was synthesized by mixing ethylenediamine with Mg (BH₄) ₂. The conductivity at 70 ℃ is 6.00*10^-5 s/cm. Ternary spinel Se fossil (MgSc₂Se₄) was prepared by cold pressing as a solid electrolyte with ionic conductivity of about 1.00*10^-4 S cm^-1 at room temperature^[17].

2. Organic solid electrolytes. Organic solid electrolytes, also known as polymer solid electrolytes (SPE), are one of the most promising electrolytes due to their high safety and good mechanical properties. They are mainly composed of organic polymers and magnesium salts. Common organic polymer matrices and magnesium salts include polyoxyethylene (PEO), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (P (VDF-HFP), polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and polymethyl methacrylate (PMMA), Mg (ClO₄) ₂, magnesium trifluoromethylsulfonate (Mg (Tf) ₂), Mg (BH₄) ₂. In order to improve the conductivity of SPE, modification methods such as polymer copolymerization and plasticizer addition are often used^[18].

3. Organic-inorganic composite solid electrolyte. Organic-inorganic composite solid electrolyte is composed of polymer electrolyte and inorganic filler (MgO, Titanium dioxide, SiO₂, Al₂O₃). The presence of MgO nanoparticles in PVDF-HFP based polymer electrolytes increases the proportion of peristaltic segments of the polymer and promotes the transport of Mg²⁺. When MgO nanoparticles with 3% (mass fraction) are added, the conductivity can reach 8*10^-3 S/cm. Similarly, the composite polymer electrolyte PEO/Mg(BH₄)₂/MgO with MgO nanoparticles added has a 98% deposition/dissolution efficiency for Mg²⁺. The capacity of solid state batteries assembled with Mo₆S₈ as positive electrode is almost unchanged after 150 cycles at 100 ℃^[19].

Applications

Magnesium ion batteries have the advantages of high energy density and low cost, and can be partly replaced by lithium batteries in the field of new energy electric vehicles when the technology is mature. Unfortunately, magnesium ion batteries are still facing many challenges, and it will be difficult for them to be used in the commercial field for some time to come. However, some researchers have improved the conductivity of magnesium ions at room temperature, which brings magnesium ion batteries closer to commercial use. Using MIL-101's MOF as a framework, the team encapsulated magnesium ions in its nanopores and generated a MOF-based electrolyte in which magnesium ions were loosely packed and allowed to migrate. In order to improve the ionic conductivity, they introduced it into acetonitrile steam and produced considerable conductivity data^[20].

Figure 4. The Mg²⁺ conductor consisting of a MOF. (License: CC BY^[20])

Other types of Mg batteries

Magnesium Primary Battery

Magnesium primary batteries were invented in the 1920s. Magnesium manganese dry batteries were designed and assembled using magnesium as negative material, magnesium perchlorate as electrolyte, and manganese dioxide as positive material. Magnesium manganese dry batteries have been used in military radio transmitters and telephones for many times due to their good temperature adaptability (-20 ℃——60 ℃)) and long storage life. Later, it was gradually replaced by new batteries and gradually exited the market.

However, nano-materials bring hope to primary magnesium batteries. Using nano-manganese dioxide-based positive electrode materials, the discharge capacity has been significantly improved, which makes this direction valuable for exploration^[21]

Magnesium Seawater Battery

Magnesium seawater batteries have obvious advantages and unique principles. Magnesium seawater batteries began to appear in the 1940s, usually referring to the chemical power supply that operates in the marine environment and uses sea water as the electrolyte, which is characterized by the fact that no additional electrolyte is required to carry. Magnesium seawater batteries usually use active metals or alloys as negative electrode, metal chlorides (CuCl, AgCl, PbCl₂), hydrogen peroxide or oxygen dissolved in seawater as positive electrode active material, and seawater as electrolyte solution, which is connected to the load to form a loop and provides power by the continuous dissolution of metals. During discharge, sea water is injected into the battery system as electrolyte to activate the magnesium negative electrode to release electrons. At the positive electrode, the active material accepts electrons in the form of a reduction reaction from which the overall battery reaction is established. However, it has also been shown that magnesium alloy can also be developed as a negative electrode material for seawater activated batteries, and Mg-3%Ga-2%Hg alloy shows good electrochemical performance in experiments.^[22]
Magnesium seawater batteries have obvious advantages. Different types of batteries have different structures and principles, but some or all of them use sea water as electrolyte. Therefore, it is not necessary to carry electrolytes to simplify the structure, reduce the quality and increase the energy density per unit. At the same time, to some extent, the polarization of the reactant on the electrodes is eliminated, so that the discharge performance of the electrodes is stable and the efficiency of the electrodes is improved. These characteristics make the magnesium seawater batteries play an important role in marine exploration, resource utilization, military defense and other fields.

Magnesium Air Battery

Magnesium air battery mainly consists of magnesium alloy as negative electrode, oxygen in air as positive electrode, and electrolyte. In magnesium air battery systems, neutral salt solutions are usually used as electrolytes. Air positive electrode is synthesized by roll and cold pressing of catalytic layer, collector layer and hydrophobic diffusion layer. Magnesium alloy is oxidized on the negative electrode during the discharge of Magnesium Air Battery. On the positive electrode, air penetrates through the hydrophobic diffusion layer of the air positive electrode and reaches the surface of the negative electrode. Oxygen is reduced at the solid-liquid interface^[23]

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Magnesium air batteries have many advantages:

1. Convenient to carry: Magnesium and magnesium alloy have a small density. Magnesium metal fuel cell is not only high in quantity but also light in weight, and is very suitable for portable power supply, field operation power supply, etc.
2. Safe to use: Magnesium air batteries react with neutral saline as electrolyte. The reactants are metal magnesium alloy, water and oxygen. Most of the products are magnesium hydroxide precipitates. The reactants and products are non-toxic and non-polluting. The recovered magnesium hydroxide can be regenerated into magnesium ingots for recycling after burning and reducing.
3. Wide range of applications: energy-consuming products such as medium-power lighting, communication equipment, small household appliances, fixed outdoor lighting, emergency backup power carried by vehicles or small vessels, etc., which can effectively reduce the cost of power use, solve the problem of power shortage in areas and stabilize the current of emergency backup power.
4. Rechargeable: Magnesium air batteries can be recharged by replacing the Magnesium plate after the power is exhausted, so they can be designed as "mechanical rechargeable" secondary batteries. It is known as a new energy with the most development and application prospects in the future.

References