Lithium–air battery
[1] 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.
However, the practical power and cycle life of Li–air batteries need significant improvements before they can find a market niche.
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.
Under discharge, electrons follow the external circuit to do electric work and the lithium ions migrate to the cathode.
[19][20] The aqueous battery requires a protective layer on the negative electrode to keep the Li metal from reacting with water.
At the anode, electrochemical potential forces the lithium metal to release electrons via oxidation (without involving the cathodic oxygen).
The half-reaction is:[21] 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).
The uneven current distribution furthers branching dendrite growth and typically leads to a short circuit between the anode and cathode.
[26] Several approaches attempt to overcome these problems: At the cathode during charge, oxygen donates electrons to the lithium via reduction.
[35] A MnO2 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.
[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.
4.2 V) even at such 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.
[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 LiPF6, LiAsF6, LiN(SO2CF3)2, and LiSO3CF3, but typically consisted of carbonates, ethers and esters.
[7] Although most studies agree that Li2O2 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: Traditionally, superoxide (O−2) was considered as a dangerous intermediate in aprotic oxygen batteries due to its high nucleophilicity, basicity and redox potential[19][18] However, reports[42][43] suggest that LiO2 is both an intermediate during the discharge to peroxide (Li2O2) and can be used as the final discharge product, potentially with an improved cycle life albeit with a lower specific energy (a little heavier battery weight).
Conventional porous carbon air electrodes are unable to provide mAh/g and mAh/cm2 capacities and discharge rates at the magnitudes required for really high energy density batteries for EV applications.
"[44] The capacity (in mAh/cm2) and the cycle life of non-aqueous Li-O2 batteries is limited by the deposition of insoluble and poorly electronically conducting LiOx phases upon discharge.
[46] Ceramic solid electrolytes (CSEs) of the NASICON family (e.g., Li1−xAxM2−x(PO4)3 with A ∈ [Al, Sc, Y] and M ∈ [Ti, Ge]) has been studied.
In contrast, solid polymer electrolytes (SPEs) can provide a higher conductivity at the expense of a faster crossover of water and of other small molecules that are reactive toward metallic Li.
The result offered energy efficiency of 93 percent (voltage gap of .2) and cycled more than 2,000 times with little impact on output.
[7] Current solid-state Li–air batteries use a lithium anode, a ceramic, glass, or glass-ceramic electrolyte, and a porous carbon cathode.
The ionic conductivity of current lithium fast ion conductors is lower than liquid electrolyte alternatives.
[55][56] In addition to the blockage of electron flow via the formation of an insulating product, cycling Li-air batteries results in the clogging of pores meant for oxygen diffusion.
[62] Li–air cells are of interest for electric vehicles, because of their high theoretical specific and volumetric energy density, comparable to petrol.
It offered higher energy density than conventional Li-ion batteries, cost less and avoided toxic byproducts.
[63][64] The solar cell used a mesh made from microscopic rods of titanium dioxide to allow the needed oxygen to pass through.
