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:
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 aluminum) making it an excellent choice for an anode material. 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. 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.Also, in aqueous cells problems at the SEI stem from the high reactivity of lithium metal with water.
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.
According to Seeo, Inc. 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.
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.
Cathode and electrolyte
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. Metal catalysts incorporated into the carbon electrode enhance the oxygen reduction kinetics and increase the specific capacity of the cathode. 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.
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. 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.
It should be noted that lithium oxides are insoluble in aprotic electrolytes which leads to the cathode clogging.
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.
· 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 respectively
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.
· The first pore system of the material serves as an oxidation product storage
· The second pore system of the material serves as oxygen transport.
· Li-air battery designs
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.
Schematic of aprotic type Li-Air battery design
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 organicelectrolyte, 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. 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. 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.
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.
Schematic of aqueous type Li-Air battery design
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. 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).