Improved Electrochemical Performance of Lithium Air Batteries with N-Methyl-2-Pyrrolidone Based Composite Polymer Electrolytes

Abstract

Lithium air batteries are able to have higher energy density because of the lighter cathode and the fact that oxygen is freely available in the environment. This causes a great interest on these type of batteries. However, the performance of Li-air batteries is affected by many factors such as type of catalysts and electrolyte composition. The stability of electrolytes is a significant limitation for cycle life performance in Li-air batteries. In recent years, most efforts have focused on the development of stable electrolytes to improve cycle life and capacity of rechargeable Li-air batteries. Therefore, stability of the polymer composite electrolytes against the reduced oxygen species generated during the discharge process was investigated. Since Li2O2 is generated at the cathode surface during cycling, an investigation of the thermal stability, conductivity and resistance of electrolytes with Li2O2 was conducted. The other challenge of the stability of Li-air batteries is degradation of lithium anode. For this reason, great efforts of this study focused on improving lifetime and recycleability through, obtaining stable electrolyte with polymeric and inorganic additives. For this aim, 1 M LiPF6 and LiBF4 salts dissolved in N-methyl-2-pyrrolidone (NMP) aprotic solvent and selected amount of polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO), which is mostly used as binders for air electrodes in Li-air batteries and Al2O3 nanoparticles added into the electrolyte to improve thermal stability, provide highly reversible Li-O2 reaction during charging/discharging and prevent clogging of the porous structure of the cathode. In order to investigate electrochemical performance of the produced nanocomposite electrolytes, ECC-Air test cell was assembled in Ar-filled glove box. The lithium foil was used as anode, Sigracet 24BC gas diffusion layer (GDL) was used as the cathode and prepared nanocomposite solutions were used as the electrolyte. The anode and cathode was separated with a glass fiber separator. Differential scanning calorimetry (DSC) and derivative thermagravimetry (TG) measurements were used for thermal stability studies for the electrolytes. The cells were cyclically tested using 0.1 mA/cm2 current density over a voltage range of 2.15–4.1 V. Electrochemical impedance spectroscopy (EIS) measurements was carried out to investigate the effect of the PVDF, PeO and Al2O3 additives on the resistivity of the electrolyte. After the electrochemical cycling test, the surface of the cathode (GDL) was characterized using environmental scanning electron microscopy, X-ray diffraction patterns and Raman spectroscopy to investigate deposited lithium compounds such as LiO2, Li2O2 and Li2CO3 on the cathode during cycling test.