The lithium-air battery, Li-air for short, is a metal-air battery chemistry that uses the oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow. Originally proposed in the 1970s as a possible power source for electric vehicles, Li-air batteries recaptured scientific interest in the late 2000s due to advances in materials technology and an increasing demand for environmentally safe and oil-independent energy sources.
The major appeal of the Li-air battery is the extremely high energy density, a measure of the amount of energy a battery can store for a given volume, which comes far closer than the energy density of most other types of batteries to that of traditional gasoline powered engines. Li-air batteries gain this advantage in energy density since they use oxygen from the air instead of storing an oxidizer internally.
The technology is still in its infancy and will require significant research efforts in a variety of fields before a viable commercial implementation is developed.1 However, scientists and industry alike see potential in its development. Currently, four types of Li-air batteries are being pursued; aprotic,23 aqueous,4 solid state,5 and mixed aqueous/aprotic.6
In the mid-1990s, K.M. Abraham and co-workers demonstrated the first non-aqueous Li–air battery with the use of a Li negative electrode (anode), a porous carbon positive electrode (cathode), and a gel polymer electrolyte membrane that served as both the separator and ion-transporting medium.78 Oxygen from the atmosphere enters the pores of the carbon cathode to serve as the cathode active material. In the discharge of the Li–air battery, this oxygen is reduced and the products are stored in the pores of the carbon electrode. The Li ion conducting gel polymer electrolytes used to construct polymer Li–air cells included those based on polyacrylonitrile (PAN)9 and polyvinylidene fluoride (PVdF).10 The electrolyte can also be organic liquid, dry organic polymer or inorganic solid electrolytes.
Lithium-based batteries have received considerable attention over the past 40 years, especially since the introduction of the first commercial cells 20 years ago. Lithium-based batteries have shown high performance mostly due to the high specific energy densities intrinsic to lithium-based materials.11 Coincident with investigation of lithium battery technologies, metal-air batteries, specifically zinc, have received attention due to the very high energy densities associated with their design. The high theoretical specific energy densities for metal-air batteries are possible due to the use of atmospheric oxygen as the cathode, eliminating a traditional cathode structure. Recently, lithium-air batteries have been proposed as the next step in lithium battery architecture, due to the high specific energy density of lithium with respect to air (3840 mAh/g).12
Currently, many challenges prevent the realization of high-performance lithium-air batteries. Substantial difficulties are faced in preparing structures for the precipitation of lithium peroxide at the cathode. In addition, the cathode must be made electrically conductive. Currently, porous carbon electrodes are the material of choice, but pore clogging by lithium peroxide in aprotic systems must be balanced with the need for oxygen permeation. Several catalysts have been used to improve cathode performance, with MnO2 being the material of choice. The actual mechanism of improvement due to catalyst activity is not yet clear, but it is theorized it may alter the structure of the oxide deposits.1314 Significant challenges have also been faced at the pure lithium anode. Dendritic lithium deposits, long a problem in lithium-ion batteries, can eventually lead to decreased energy capacity. In addition, current electrolytes are often unstable with respect to the lithium anode, necessitating a need for new electrolytes or a redesigned electrolyte/anode interface. Further complicating the design of lithium-air batteries is degradation of battery materials by atmospheric contaminants, such as water vapor.2
Despite the need for innovation in key areas of research and design related to creating a fully functional, high-efficiency lithium-air battery, the applications of such a battery are far reaching. The significant increase in energy density afforded by a lithium-air battery opens opportunities for lithium based power storage that current lithium based batteries could not hope to fulfill.
A major driving force in lithium-air battery development is the demand for advanced battery technology for the automotive sector. The energy density of gasoline is approximately 13 kWh/kg, which corresponds to 1.7 kWh/kg of energy provided to the wheels when accounting for losses. The theoretical energy density of the lithium-air battery is 12 kWh/kg (43.2 MJ/kg) excluding the oxygen mass. It has been theorized that a practical energy density of 1.7 kWh/kg at the wheels of an automobile could be realized when accounting for over-potentials, other cell components, battery pack ancillaries, and the much higher efficiency of electric motors.15
In a nearer future, proponents of the technology expect lithium-air batteries to replace the lithium-ion batteries currently powering portable electronic devices. Lithium-air batteries have the potential to have 5-15 times the energy density of current lithium-ion batteries. Thus even the most conservative estimates indicate that a modern-day lithium-ion battery may someday be replaced by a lithium-air battery 1/5 the size or a lithium-air battery with a capacity 5 times as large. Whether lithium-air batteries lead to reduced battery sizes or longer lasting batteries, the potential for a vast reduction in price or a vast increase in vehicle range is an attractive consequence of developing such battery technology.16
Although the electrochemical details vary by battery design (and consequently electrolyte type), in general, lithium is oxidized at the anode forming lithium ions and electrons. The electrons follow the external circuit to do electric work and the lithium ions migrate across the electrolyte to reduce oxygen at the cathode. When an externally applied potential is greater than the standard potential for the discharge reaction, lithium metal is plated out on the anode, and O2 is generated at the cathode.17
Lithium metal is the current choice of anode material for Li-air batteries. At the anode, electrochemical potential forces the lithium metal to give off electrons as per the oxidation. The half reaction is given below:18
- Li ↔ Li+ + e-
- Li ↔ Li+ + e-
Lithium has high specific capacity (3840 mAh/g) compared with other metal-air battery materials (820 mAh/g for Zinc, 2965 mAh/g for aluminium) making it an excellent choice for an anode material.11 However, there are some issues associated with metallic lithium as the anode. Upon charging/discharging in aprotic cells, a multilayer deposition of lithium salts creates a mass diffusion barrier between the lithium and electrolyte which initially prevents further corrosion of the lithium metal but eventually inhibits the reaction kinetics between the anode and the electrolyte.19 This chemical heterogeneity of the solid-electrolyte interface (SEI) results in morphologically heterogeneous structure prone to non-uniform current distributions. Uneven current distributions further the dendrite growth and typically leads to a short between the anode and cathode.20 Also, in aqueous cells problems at the SEI stem from the high reactivity of lithium metal with water.21
Several approaches have been taken to overcome problems at the SEI associated with lithium metal anodes.
- 1. Formation of a Li-ion conductive artificial protective layer using novel di- and triblock copolymer electrolytes.22
- According to Seeo, Inc.22 The electrolytes made from di- and triblock copolymer (e.g. polystyrene with the high Li-ion conductivity of a soft polymer segment, such as a poly(ethylene oxide PEO/ Li-salt mixture) ) combine the mechanical stability of a hard polymer segment, which will inhibit dendrite shorts via mechanical blocking, with the high ionic conductivity of the soft polymer/lithium salt mixture.
- 2. Use of a Li-ion conducting glass or glass-ceramic material.23245
- Li-ion conducting ceramic materials are (generally) readily reduced by lithium metal, and therefore a thin film of a lithium stable conducting material, such as Li3P or Li3N, could be inserted between the ceramic and metal. This ceramic based artificial SEI would not only inhibit the formation of dendrites, but would also protect the lithium metal from atmospheric contamination.
At the cathode, reduction occurs by the recombination of lithium ions with oxygen. Currently, mesoporous carbon has been used as a cathode material with metal catalysts.25 Metal catalysts incorporated into the carbon electrode enhance the oxygen reduction kinetics and increase the specific capacity of the cathode.16 Manganese, cobalt, ruthenium, platinum, silver, or a mixture of cobalt and manganese are currently used as metal catalysts. A study by Abraham et al. found that manganese catalyzed cathodes performed best, with a specific capacity of 3137 mAh/g carbon, and cobalt catalyzed cathodes performed second best, with a specific capacity of 2414 mAh/g carbon.7
The Li-air cell performance is limited by the efficiency of reaction at the cathode because most of the cell voltage drop occurs at the cathode.11 Thus, improvement of cathode in a Li-air battery is essential for overall Li-air cell performance enhancement. Currently, there exist multiple battery chemistry (for more see battery designs) delineated by electrolyte choice, so the exact electrochemical reaction at the cathode varies between Li-air batteries. The discussion below is focused on aprotic and aqueous electrolytes as the exact electrochemistry taking place in solid-state electrolytes is not well understood.
In a cell with an aprotic electrolyte lithium oxides are produced through reduction at the cathode:
- Li+ + e- + O2 + * → LiO2*
- Li+ + e- +LiO2* → Li2O2*
- Where a "*" denotes a surface site on Li2O2 where the growth proceeds which is essentially a neutral Li vacancy in the Li2O2 surface.
In a cell with an aqueous electrolyte the reduction at the cathode can also produce lithium hydroxide:
- Acidic electrolyte
- 2Li + ½ O2 + 2H+ → 2Li++ H2O
- A conjugate base is involved in the reaction. The theoretical maximal Li-air cell specific energy and Li-air cell energy density is 1400 Wh/kg and 1680 Wh/l respectively.15
- Alkaline aqueous electrolyte
- 2Li + ½ O2 + H2O → 2LiOH
- Water molecules are involved in the redox reactions at the air cathode. The theoretical maximal Li-air cell specific energy and Li-air cell energy density is 1300 Wh/kg and 1520 Wh/l respectively15
The development of new cathode materials must account for the accommodation of substantial amounts of LiO2, Li2O2, and/or LiOH without causing a blockage of the cathode pores and find suitable catalysts to make the electrochemical reactions energetically practical.
- As an example, dual pore system materials are the most promising in terms of energy capacity.27
- The first pore system of the material serves as an oxidation product storage
- The second pore system of the material serves as oxygen transport.
Efforts in Li-air batteries have focused on four different chemical designs. All the designs have distinct advantages and significant associated technological challenges. It remains to be seen which design will become the standard for Li-air batteries of tomorrow.
Most worldwide effort on Li-air batteries has focused on the aprotic battery design. The aprotic design consists of a lithium metal anode, a liquid organic electrolyte, and a porous carbon cathode. Electrolytes can be made of any organic capable of solvating lithium salts (LiPF6, LiAsF6, LiN(SO2CF3)2, and LiSO3CF3), but have typically consisted of carbonates, ethers, and esters.17 The carbon cathode is usually made of a high surface area carbon material with a nanosized metal oxide catalyst (commonly MnO2 or Mn3O4). A major design advantage of the aprotic battery is the spontaneous formation of a barrier between the anode and electrolyte (much like the barrier formed between electrolyte and carbon-lithium anodes in conventional Li-ion batteries) which protects the lithium metal from further reaction with the electrolyte. Practically, the aprotic battery design draws interest as it has been shown to be rechargeable.15 However, it has major drawbacks in that Li2O2 produced at the cathode is generally insoluble in the organic electrolyte leading to build up along the cathode/electrolyte interface. This makes cathodes in aprotic batteries prone to clogging and volume expansion which reduces conductivity and degrades battery performance over time.421
In 2012, researchers announced a design that employed dimethyl sulfoxide as the electrolyte and gold nanoparticles as the cathode. They claimed to have achieved 100 charge cycles with a 5% capacity loss.28
The aqueous Li-air battery consists of a lithium metal anode, an aqueous electrolyte, and a porous carbon cathode. The aqueous electrolyte is simply a combination of lithium salts dissolved in water. The aqueous Li-air battery avoids the issue of cathode clogging experienced in aprotic batteries because the reaction products are water soluble, which allows aqueous Li-air batteries to maintain performance over time.4 The aqueous design also has a higher practical discharge potential than its aprotic counterpart. However, lithium metal reacts violently with water and thus the aqueous design requires a solid electrolyte interface between the lithium metal and aqueous electrolyte. Commonly, a lithium-conducting ceramic or glass is used, but conductivities are generally low (on the order of 10-3 S/cm at ambient temperatures).21
The aqueous/aprotic or mixed Li-air battery design is an attempt to unite advantages of both the aprotic and aqueous battery designs. Although there exist multiple hybrid designs, the common feature of these designs is a separated two-part (one part aqueous and one part aprotic) electrolyte connected by a lithium-conducting membrane. A lithium metal anode is in contact with the aprotic side of the electrolyte while the porous cathode is in contact with the aqueous side. A lithium-conducting ceramic is typically employed as the membrane joining the two electrolytes.415
The solid-state battery design eliminates problems at the anode/cathode interfaces associated with using a liquid electrolyte. It is also attractive from a safety standpoint as organic solvents, currently used in lithium-ion batteries (and employed in the aprotic Li-air battery design), are flammable and at high temperatures the use of an organic electrolyte can lead to rupture and ignition of the battery.5 Current solid-state Li-air batteries use a lithium anode, a ceramic, glass, or glass-ceramic electrolyte, and a porous carbon cathode. The anode and cathode are typically separated from the electrolyte by polymer-ceramic composites which enhance charge transfer at the anode and electrochemically couple the cathode to the electrolyte. The polymer-ceramic composites serve to reduce the overall impedance of the solid-state Li-air battery. The main drawback of the solid-state battery design is the low conductivity of most glass-ceramic electrolytes. Lithium aluminum germanium phosphate has been found to be an effective electrolyte, but the ionic conductivity of current lithium fast ion conductors are still lower than liquid electrolyte alternatives.12
There are many challenges facing the design of Li-air batteries, which currently limits their use to the laboratory. One of the largest challenges lies in keeping the battery protected from the environment. Atmospheric oxygen must be present at the cathode, but the cathode can be degraded by humidity.
Most of the current limitations in Li-air battery development are seen at the cathode. One of the problems seen is incomplete discharge due to blockage of the porous carbon cathode with discharge product. The effect of pore size and pore size distribution is still poorly understood.11 Production of a cathode with high a pore size and ability to hold a large amount of Li2O2 is essential to Li-air battery development. Catalysts have shown promise in creating preferential nucleation of Li2O2 over Li2O, which is irreversible with respect to lithium.29
The current anode of choice in Li-air batteries is metallic Li, as Li offers the highest energy density. Due to the reactive nature of Li, the main challenge in anode development is preventing the anode from reacting. New interfacial materials or solid-state electrolytes will prevent anode degradation. Another area of concern when using metallic lithium anodes is dendrite formation, which will lead to a short circuit within the battery.
In current cell designs, the charge overpotential is much higher than the discharge overpotential. The presence of a significant charge overpotential indicates secondary reactions, besides recharging, are occurring.30 As a result, the electrical efficiency is only around 65%.11 There is some indication that catalysts, such as MnO2, Co, Pt, and Au can reduce the overpotentials, but the effect is still poorly understood.29 In addition, significant drops in cell capacity with increasing discharge rates have been observed by many researchers. The decrease in cell capacity is attributed to kinetic charge transfer limitations.11 Since the anodic reaction occurs very quickly, the charge transfer limitations are thought to occur at the cathode. Again, electrocatalysts could improve the charge transfer rate.
Long term battery operation requires chemical stability of all the components of the cell. Current cell designs show poor resistance to oxidation by the reaction products and intermediates. Many aqueous electrolytes are also volatile, and can be lost over time.11 One of the largest barriers to fully operable commercial cells is the development of effective environmental interfaces. Atmospheric oxygen is intrinsically required for cell operation, but the cell must be shielded from the environment, as water vapor can rapidly degrade the system.2
The primary application for Li-air battery development is in automotive. The high specific energy densities and volumetric energy densities required for next-generation hybrid and electric vehicles are beyond current battery designs. Li-air batteries are attractive for any application where weight is a primary concern, such as in mobile devices. Flow batteries, such as the vanadium redox battery may offer better performance for applications such as off-grid power storage.31
- Lithium-ion battery
- Lithium iron phosphate battery
- Lithium-ion polymer battery
- Nanowire battery
- Power-to-weight ratio
- Zinc–air battery
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