Lithium (Li)-air batteries, with their large theoretical specific energy (<11,000 Wh g−1), are one of the most promising energy-storage technologies, attracting much attention and several studies, with potential applications in transportation, portable devices and grid energy storage. This large theoretic specific energy is possible mainly because (i) the Li-oxygen (O2) couple exhibits a large potential (˜3.1V) vs Li/Li+ and (ii) Li metal has a high theoretical specific capacity (3862 mAh g−1).
The Li-air battery is often comprised of a Li metal anode, an electrolyte soaked separator, and a carbon air-cathode. During the discharge process, O2 dissolves into the electrolyte from the air cathode side which is open to the atmosphere and gets reduced on the carbon surface which is wetted by electrolyte, forming lithium peroxide (Li2O2) and lithium oxide (Li2O) as discharge products. Li2O formation is undesirable since it is irreversible during the charge process. This reaction continues until any one of the active components, namely Li metal, dissolved O2 and active carbon sites in the porous carbon network are exhausted. The overall reactions involved during the discharge/charge processes in rechargeable li-air battery using aprotic organic electrolytes is as shown below:
                    2        ⁢        Li            +              O        2              ↔                  Li        2            ⁢                        O                      2            ⁢                                                                ⁡                  (                      lithium            ⁢                                                  ⁢            peroxide                    )                                        2        ⁢        Li            +                        1          2                ⁢                                  ⁢                  O          2                      ↔                  Li        2            ⁢      O      ⁢                          ⁢              (                  lithium          ⁢                                          ⁢          oxide                )            
At an early stage in their development, Li-air batteries have to overcome many challenges before they can be applied to practical applications. They are significantly affected by several factors, like the electrolyte salts, cathode pore volume and loading, catalysts, the effect of lithium anode plating/stripping on the solid electrolyte interface (SEI) due to volume changes, O2 partial pressure, and other factors. Clearly, to improve the efficiency and capacity of metal-air and nonmetal-air batteries, deep insight is necessary into the reaction mechanisms of Li-air batteries during discharge and charge processes.
Nuclear magnetic resonance (NMR) is a physical phenomenon exploiting the magnetic properties of certain nuclei and used to study the physical and chemical properties of materials in a process called NMR spectroscopy. It has proven to be an effective analysis technique for Li-ion batteries and supercapacitors; it can provide invaluable information about chemical and structural changes in energy storage devices. Especially, in-situ 7Li NMR technique for Li-ion batteries and supercapacitors, has enabled to investigate, in real time, the chemical and structural changes that arise in electrode materials during cycling. Letellier, M.; Chevallier, F.; Beguin, F.; Frackowiak, E.; Rouzaud, J.-N., J. Phys. Chem. Solids 2004, 65, 245-251; and H. Wang, T. K.-J. Köster, N. M. Trease, J. Segalini, P-L. Taberna, P. Simon, Y. Gogotsi and C. P. Grey, J. Am. Chem. Soc., 2011, 133 (48), pp 19270-19273. These previous studies have proven the effectiveness of in-situ NMR technique in helping us understand the functioning of electrochemical energy storage devices, elaborating on the conditions of their failure and identifying new potential materials for use in batteries and supercapacitors.
NMR has been used previously to investigate Li-Air batteries. J. Xiao, J. Hu, D. Wang, D. Hu, W. Xu, G. L. Graff, Z. Nie, J. Liu, J. G. Zhang, J. Power Sources, 196 (2011), 5674-5678; M. Leskes, N. E. Drewett, L. J. Hardwick, P. G. Bruce, G. R. Goward, C. P. Grey, Angew. Chem. Int. Ed. 2012, 51, 8560-8563; L. A. Huff, J. L. Rapp, L. Zhu, A. A. Gewirth, J. Power Sources, 235 (2013), 87-94; and M. Leskes, A. J. Moore, G. R. Goward, and C. P. Grey, J. Phys. Chem. C 2013, 117, 26929-26939. These were ex-situ studies; which were performed pre- and post-cycling after disassembling the cell, which may invite many undesirable complications including the possibility of additional discharging and the effects of evaporation of solvents upon disassembly and the resulting accumulation of salts in the porous carbon network.