The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.[1]
Specific energy | 40.104 MJ/kg (11,140 Wh/kg) theoretical |
---|---|
Energy density | ? J/m³ |
Specific power | 11,400 W/kg |
Nominal cell voltage | 2.91 V |
Pairing lithium and ambient oxygen can theoretically lead to electrochemical cells with the highest possible specific energy. Indeed, the theoretical specific energy of a non-aqueous Li–air battery, in the charged state with Li2O2 product and excluding the oxygen mass, is ~40.1 MJ/kg = 11.14 kWh/kg of lithium. This is comparable to the theoretical specific energy of gasoline, ~46.8 MJ/kg. In practice, Li–air batteries with a specific energy of ~6.12 MJ/kg = 1.7 kWh/kg of lithium at the cell level have been demonstrated. This is about 5 times greater than that of a commercial lithium-ion battery, and is sufficient to run a 2,000 kg electric vehicle for ~500 km (310 miles) on a single charge using 60 kg of lithium (i.e. 20.4 kWh/100 km). However, the practical power and cycle life of Li–air batteries need significant improvements before they can find a market niche.
Significant electrolyte advances are needed to develop a commercial implementation.[2] Four approaches are being considered: aprotic,[3][4][5] aqueous,[6] solid-state[7] and mixed aqueous–aprotic.[8]
A major market driver for batteries is the automotive sector. The energy density of gasoline is approximately 13 kW·h/kg, which corresponds to 1.7 kW·h/kg of energy provided to the wheels after losses. Theoretically, lithium–air can achieve 12 kW·h/kg (43.2 MJ/kg) excluding the oxygen mass. Accounting for the weight of the full battery pack (casing, air channels, lithium substrate), while lithium alone is very light, the energy density is considerably lower.[9]
History
Originally proposed in the 1970s as a possible power source for battery electric vehicles, and hybrid electric vehicles, Li–air batteries recaptured scientific interest late in the first decade of the 2000s due to advances in materials science.
Although the idea of a lithium–air battery was around long before 1996,[10][11][12][13] the risk-to-benefit ratio was perceived as too high to pursue. Indeed, both the negative (lithium metal) and the positive (air or oxygen) electrodes are the reasons why, respectively, rechargeable lithium-metal batteries failed to reach the market in the 1970s (the lithium-ion battery in a mobile device uses a LiC6-graphite compound on the negative electrode, not a lithium metal). Nevertheless, due to a perceived lack of other alternatives to high specific energy rechargeable batteries, and due to some initially promising results from academic labs,[10][11] both the number of patents and of free-domain publications related to lithium–oxygen (including Li–air) batteries began growing exponentially in 2006.[14][11] However, the technical difficulties facing such batteries, especially recharging times, nitrogen and water sensitivity, and [15] the intrinsic poor conductivity of the charged Li2O2 species are major challenges.[16]
Design and operation
In general lithium ions move between the anode and the cathode across the electrolyte. Under discharge, electrons follow the external circuit to do electric work and the lithium ions migrate to the cathode. During charge the lithium metal plates onto the anode, freeing O
2 at the cathode.[17] Both non-aqueous[18] (with Li2O2 or LiO2 as the discharge products) and aqueous (LiOH as the discharge product) Li-O2 batteries have been considered.[19][20] The aqueous battery requires a protective layer on the negative electrode to keep the Li metal from reacting with water.
Anode
Lithium metal is the typical anode choice. At the anode, electrochemical potential forces the lithium metal to release electrons via oxidation (without involving the cathodic oxygen). The half-reaction is:[21]
- Li ⇌ Li+ + e−
Lithium has high specific capacity (3,840 mAh/g) compared with other metal–air battery materials (820 mAh/g for Zinc, 2,965 mAh/g for aluminium).[22] Several issues affect such cells.The main challenge in anode development is preventing the anode from reacting with the electrolyte. Alternatives include new electrolyte materials or redesigning the interface between electrolyte and anode. Lithium anodes risk dendritic lithium deposits, decreasing energy capacity or triggering a short circuit.[23]The effects of pore size and pore size distribution remain poorly understood.[22]
Upon charging/discharging in aprotic cells, layers of lithium salts precipitate onto the anode, eventually covering it and creating a barrier between the lithium and electrolyte. This barrier initially prevents corrosion, but eventually inhibits the reaction kinetics between the anode and the electrolyte.[24] This chemical change of the solid–electrolyte interface (SEI) results in varying chemical composition across the surface, causing the current to vary accordingly. The uneven current distribution furthers branching dendrite growth and typically leads to a short circuit between the anode and cathode.[25]
In aqueous cells problems at the SEI stem from the high reactivity of lithium metal with water.[26]
Several approaches attempt to overcome these problems:
- Formation of a Li-ion protective layer using di- and triblock copolymer electrolytes.[27] According to Seeo, Inc.,[27] such electrolytes (e.g., polystyrene with the high Li-ion conductivity of a soft polymer segment, such as a poly(ethylene oxide (PEO) and Li-salt mixture) ) combine the mechanical stability of a hard polymer segment with the high ionic conductivity of the soft polymer–lithium-salt mixture. The hardness inhibits dendrite shorts via mechanical blocking.
- Li-ion conducting glass or glass-ceramic materials[7][28][29] are (generally) readily reduced by lithium metal, and therefore a thin film of a stable lithium conducting material, such as Li
3P or Li
3N, can be inserted between the ceramic and metal. This ceramic-based SEI inhibits the formation of dendrites and protects the lithium metal from atmospheric contamination.
Cathode
At the cathode during charge, oxygen donates electrons to the lithium via reduction. Mesoporous carbon has been used as a cathode substrate with metal catalysts[30] that enhance reduction kinetics and increase the cathode's specific capacity.[31] Manganese, cobalt, ruthenium, platinum, silver, or a mixture of cobalt and manganese are potential metal catalysts. Under some circumstances manganese-catalyzed cathodes performed best, with a specific capacity of 3,137 mA·H/g carbon and cobalt-catalyzed cathodes performed second best, with a specific capacity of 2414 mA·H/g carbon.[32] Based on the first pore-scale modeling of lithium–air batteries, the micro-structure of the cathode significantly affects battery capacity in both non-pore-blocking and pore-blocking regimes.[33]
Most Li–air battery limits are at the cathode, which is also the source of its potential advantages. Atmospheric oxygen must be present at the cathode, but contaminants such as water vapor can damage it.[4]Incomplete discharge due to blockage of the porous carbon cathode with discharge products such as lithium peroxide (in aprotic designs) is the most serious.
Catalysts have shown promise in creating preferential nucleation of Li
2O
2 over Li
2O, which is irreversible with respect to lithium.[34]
Li–air performance is limited by the efficiency of the reaction at the cathode, because most of the voltage drop occurs there.[22] Multiple chemistries have been assessed, distinguished by their electrolyte. This discussion focuses on aprotic and aqueous electrolytes as solid-state electrochemistry is poorly understood.
In a cell with an aprotic electrolyte lithium oxides are produced through reduction at the cathode:
- Li+ + e− +O
2 + * → LiO
2* - Li+ + e− +LiO
2* →Li
2O
2*
where "*" denotes a surface site on Li
2O
2 where growth proceeds, which is essentially a neutral Li vacancy in the Li
2O
2 surface.
Lithium oxides are insoluble in aprotic electrolytes, which leads to cathode clogging.[35]
A MnO
2 nanowire array cathode augmented by a genetically modified M13 bacteriophage virus offers two to three times the energy density of 2015-era lithium-ion batteries. The virus increased the size of the nanowire array, which is about 80 nm across. The resulting wires had a spiked surface. Spikes create more surface area to host reaction sites. The viral process creates a cross-linked 3D structure, rather than isolated wires, stabilizing the electrode. The viral process is water-based and takes place at room temperature.[36][37]
Electrolyte
Efforts in Li–air batteries have focused on four electrolytes: aqueous acidic, aqueous alkaline, non-aqueous protic, and aprotic.
In a cell with an aqueous electrolyte the reduction at the cathode can also produce lithium hydroxide:
Aqueous
An aqueous Li–air battery consists of a lithium metal anode, an aqueous electrolyte and a porous carbon cathode. The aqueous electrolyte combines lithium salts dissolved in water. It avoids the issue of cathode clogging because the reaction products are water-soluble.[6] The aqueous design 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 and electrolyte. Commonly, a lithium-conducting ceramic or glass is used, but conductivity are generally low (on the order of 10−3 S/cm at ambient temperatures).[26]
Acidic electrolyte
- 2Li + ½ O
2 + 2H+ → 2Li++ H
2O
A conjugate base is involved in the reaction. The theoretical maximal Li–air cell specific energy and energy density are 1,400 W·h/kg and 1,680 W·h/L, respectively.[9]
Alkaline aqueous electrolyte
- 2Li + ½ O
2 + H
2O → 2LiOH
Water molecules are involved in the redox reactions at the air cathode. The theoretical maximal Li–air cell specific energy and energy density are 1,300 W·h/kg and 1,520 W·h/L, respectively.[9]
New cathode materials must account for the accommodation of substantial amounts of LiO
2, Li
2O
2 and/or LiOH without causing the cathode pores to block and employ suitable catalysts to make the electrochemical reactions energetically practical.
- Dual pore system materials offer the most promising energy capacity.[38]
- The first pore system serves as an oxidation product store.
- The second pore system serves as oxygen transport.
Aprotic
Non-aqueous Li–air batteries were demonstrated first.[10] They usually use mixed ethylene carbonate+ propylene carbonate solvents with LiPF6 or Li bis-sulfonimide salts like conventional Li-ion batteries, however, with a gelled rather than liquid electrolyte.[20] The voltage difference upon constant current charge and discharge is usually between 1.3 and 1.8 V (with an OCP of ca. 4.2 V) even at such ridiculously low currents as 0.01–0.5 mA/cm2 and 50–500 mA/g of C on the positive electrode (see Figure 2),[19][18][39] However, the carbonate solvents evaporate and get oxidized due to a high overvoltage upon charge.[40] Other solvents, such as end-capped glymes, DMSO, dimethylacetamide, and ionic liquids, have been considered.[19][20] The carbon cathode gets oxidized above +3.5 V v Li during charge, forming Li2CO3, which leads to an irreversible capacity loss.[20]
Most efforts involved aprotic materials, which consist of a lithium metal anode, a liquid organic electrolyte and a porous carbon cathode.[3] The electrolyte can be made of any organic liquid able to solvate lithium salts such as LiPF
6, LiAsF
6, LiN(SO
2CF
3)
2, and LiSO
3CF
3, but typically consisted of carbonates, ethers and esters.[3][17] The carbon cathode is usually made of a high-surface-area carbon material with a nanostructured metal oxide catalyst (commonly MnO
2 or Mn
3O
4). A major advantage is the spontaneous formation of a barrier between anode and electrolyte (analogous to the barrier formed between electrolyte and carbon–lithium anodes in conventional Li-ion batteries) that protects the lithium metal from further reaction with the electrolyte. Although rechargeable,[9] the Li
2O
2 produced at the cathode is generally insoluble in the organic electrolyte, leading to buildup along the cathode/electrolyte interface. This makes cathodes in aprotic batteries prone to clogging and volume expansion that progressively reduces conductivity and degrades battery performance.[6][26][41] Another issue is that organic electrolytes are flammable and can ignite if the cell is damaged.[7]
Although most studies agree that Li
2O
2 is the final discharge product of non-aqueous Li-O2 batteries, considerable evidence that its formation does not proceed as a direct 2-electron electro-reduction to peroxide O2−
2 (which is the common pathway for O2 reduction in water on carbon) but rather via a one–electron reduction to superoxide O−
2, followed by its disproportionation: