2. The Lithium Oxygen Batteries
2.4 The Li-O 2 battery
The Li-O2 battery (often called Li-air or lithium-air battery) is the battery consuming oxygen from the ambient atmosphere like other Zn-air batteries. In addition, The Li-O2 battery possesses a very high specific energy about 3700 mAh/g because lithium is the lightest metal, which is much larger than that of graphite or other commercially available anodes (of course including other metal air batteries). Thus it have been shed light on as a new energy generating system. Figure 2- 8 shows the theoretical and practical specific energy densities of many batteries including the Li- O2 battery.
Figure 2-8. The graph and table of theoretical and practical specific energy densities of many batteries.74
The common point that the Li-O2 and Zn-air battery both react with reduced oxygen. Thus generally, people believed that Zn-air and Li-air are the almost same or at least similar to each other. However, it is technically different between Li-O2 and Zn-air battery. Figure 2-9 shows the different reaction way of Zn air and Li-O2 battery.
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Figure 2-9. The reaction way of Zn air and Li-O2 battery. Zn air battery is one compartment cell meanwhile Li-O2 battery is two compartment cell.
And Li-O2 battery need to exclude incoming moisture and nitrogen from the atmosphere to the battery, so sadly speaking, making a rechargeable real lithium-“air” battery is so difficult (I would not say it is impossible) with today’s knowledge and technologies. Therefore, the researches have been usually investigated by the Li-O2 system so far using a pure oxygen gas.75 In the Li-O2 battery with a nonaqueous electrolyte (Li-O2 battery can be divided to four types by electrolyte type shown in Figure 2-10 but in this thesis, I focused on nonaqueous electrolyte system), reactions between reduced oxygen and lithium ions results in the formation of lithium peroxide (Li2O2) (or possibly lithium oxide (Li2O)) as the final discharge reaction product on or in the cathode pores or surfaces. Since the capacity of Li-O2 battery can be determined by total amount of Li that can be stored in Li2O2 or Li2O in the cathode, the capacity of Li-O2 battery is normalized by cathode weight and the porous materials are favored as the cathode.
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Figure 2-10. The four types of Li-O2 batteries. (a) nonaqeous, (b) aqeous, (c) hybrid, and (d) solid state electrolyte type.74
To achieve a truly rechargeable Li-O2 battery, reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) resulting in the formation and oxidation of Li2O2, respectively, are the key point. The nonaqueous Li-O2 battery was first reported by Abraham and Jiang in 1996.76 Since then, there have been vast efforts to improve the performance of the Li-O2
battery to enlarge the capacity and to extend cycle life. The reaction of Li-O2 battery looks simple as below.77
1. Li+ + O2 + e- → LiO2 (E0 = 3.0 V vs. Li/Li+) 2. 2Li+ + O2 + 2e- → Li2O2 (E0 = 3.1 V vs. Li/Li+) 3. 4Li+ + O2 + 4e- → 2Li2O (E0 = 2.9 V vs. Li/Li+)
The dissolved oxygen in the electrolyte reduces on the surface of a cathode via a one electron transfer process to a super oxide ion (O2 -). This reduced oxygen ion reacts with Li+ ions to form lithium superoxide (LiO2) as the intermediate reaction product. LiO2 can further chemically or electrochemically be converted to lithium peroxide (Li2O2) as the final discharge product. At the charge reaction, lithium peroxide (Li2O2) oxidizes as Li+ ions and O2 as below.
３０ Li2O2 → 2Li+ + O2 + 2e-
However, a real cell not works simply as like the reactions. We need to consider the reactions between Li+ and reduced oxygen, the solubility of gaseous oxygen in the electrolyte, lithium dendrite formation, Li2CO3 formation and CO2 gas evolution during cycling leading to increase the capacity, decrease the overpotential and improve the cycle ability. These have been considered as major research topics for Li-O2 cell. Thus there have been extensively studies about catalysts and electrolytes to solve those problems.78
The cathode
The cathode material for the Li-O2 battery is usually a porous carbon which can store discharge products more. So the cell capacity is mainly limited by the cathode properties such as surface area, conductivity, pore volume, and pore size distribution. Basically the oxygen solubility and the diffusion coefficient of nonaqueous electrolytes are relatively small factors. (compared to factors from cathode) So the kinetics of the reactions rate, the amount of formed Li2O2 are strongly determined by cathode. Most of the cathodes used for Li-O2 cells are made of carbon, binder, and possibly catalyst so far.79The most common technique making a cathode for Li-O2 battery is just mixing active materials (usually carbon) and catalysts with binder using a solvent to make a slurry, then cast the slurry onto a metal mesh or foam or carbon paper as a current collector. At the early stage of the Li-O2 battery (around ~ 2010), several studies have been mainly focused on carbon and catalysts to increase the discharge capacity or to optimize the properties of the carbon cathode.33c, 80 Most of these studies were aiming at improving the formulation of the cathode to provide more space to store higher amounts of Li2O2. However, after many studies have been reported, the surface area is not the only parameter affecting the cell performance. Many variable factors are connected to each other such as pore size distribution of carbon (PSD), pore volume, porosity, the electrode thickness, the loading density, cathode formulation, etc. affecting the discharge capacity and performance of the Li-O2 battery.48a, 81
After then, catalysts often used as a component of the cathode have been extensively studied because it is believed that catalysts can improve the kinetics of ORR and OER reactions. They can alleviate the charge overpotential in the cell. The effect of different catalysts including many metal oxides such as MnO2, Co3O4, precious metals such as Au, Pt or composite, and nonprecious alloys on the performance of Li-O2 cells have been studied.78a, 82 After such many efforts to improve the performance to solve the problems from the cathode, the discharge capacity and kinetics of the reactions have been improved indeed by catalysts. However, the cycle ability is
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hampered by the carbon material itself. It is reported that Li2O2 can react with carbon cathode, 63a,
83 forming Li2CO3 and evolving CO2 gas. Recently, other designs like a binder-free cathode and carbon-free cathode have been reported.55, 57, 84
The Anode
The Li-O2 battery use a lithium metal directly. (sometimes, further treated metal used) So the formation and growth of dendrites on lithium metal has been the most common issue. In addition, basically lithium is one of the strongest reducing agent so that the contact between lithium and the aprotic electrolytes results in partial decomposition of the electrolyte on the lithium and formation of a surface layer so called the solid electrolyte interphase layer (SEI). The SEI layer can protect the electrolyte from further decomposition by the negative electrode. So to suppress the dendrite formation and electrolyte decomposition, additives and solid electrolytes have been studied.85
The electrolyte
Lithium superoxide (LiO2) and lithium peroxide (Li2O2) are very reactive. So the most of the known electrolytes are unstable in the Li-O2 battery. Especially carbonate based electrolytes can be easily decompose during the cell cycling because of super oxide radical (O2-).10a, 86 Several different mechanisms have been proposed for the decomposition of carbonate based electrolytes.
It has been believed that the super oxide radical (O2-) reacts via nucleophilic substitution with the C of the carbonyl group carbonate based electrolyte.87 It has also been suggested that the super oxide radical attacks the ethereal carbon of the electrolytes. Several studies have recently reported that ether based electrolytes are relatively strong to the super oxide radical.32c, 88 However, further studies revealed that ether based electrolytes also cannot be avoided from degradation during the cell cycling.89 Beside the instability of electrolyte solvents, degradation of lithium salts is also one of the main issues. Several lithium salts such as LiPF6, LiTFSI, LiBF4, LiClO4 and LiCF3SO3 etc are decomposed during the cycling due to the reaction with Li2O2.90 Although already many different types of electrolytes and further improved electrolytes such as aprotic organic electrolytes, polymer electrolytes, ionic liquids, etc. have been investigated, the degradation of the electrolyte solvents and salts in the Li-O2 battery is still one of the major challenges which should be overcome and elucidated.
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I mentioned the problems of carbon cathode and the decomposition or degradation of electrolytes and salts in Li-O2 battery above. Therefore in this thesis, I tested carbon- and binder- free cathode for Li-O2 battery.
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