Study On Key Influencing Factors Of Non - Aqueous Electrolyte Lithium Primary Battery

The lithium-air battery is a new green energy technology. Because the battery cathode of lithium-air batteries is oxygen derived from the air which does not need to be stored in the battery, it is called "breathing battery". In addition, lithium-air batteries have become one of the most promising directions of development due to its outstanding performance in terms of energy density. Currently, researches on lithium-air batteries focus on improving battery specific capacity, the development of the secondary battery and discharge mechanism. For primary batteries, the capacity has increased dramatically, but there is still room for improvement. However, the researchers have not yet reached consensus on charging and discharging mechanisms for the battery. Especially, there is a difference in the charging process, while the discharge process undergoes a corresponding change in different electrolyte systems. In this paper, a lithium-air battery with a high specific capacity of is developed based on the selection of the electrolyte composition, electrode components, membranes, air filtration membrane, and the corresponding air cell structure design. Based on the research, the discharge mechanism of the battery system was discussed through testing of discharge products and discharged electrode morphology and electrochemical impedance spectroscopy. Simultaneously, the key factors affecting the battery performance are analyzed using molecular dynamics calculations, and a discharge model was established and described.The configuration of the lithium cell was determined through the establishment of cell structure, electrolyte composition, cell structural material selection and air electrode design. The electrolyte was 1 mol·L-1 LiOTf in the solvent of a mixture of propylene carbonate and ethylene carbonate (the ratios of the volume:VPC/VEC=1). The battery separator material and air filtration membrane silicone oil were made out of glass fiber filter membrane and polydimethylsiloxane (PDMS) film, respectively. A 2025 button cell was assembled for tests. With the battery exposed to air, discharging was carried out at a constant current of 0.1mA ·cm-2 to cut-off voltage of 2V at room temperature. A battery specific capacity of more than 6000mAh·g-1carbon+binder is observed.The battery discharge products were analyzed using XRD, FTIR and Raman, which indicate that the main discharging products are lithium carbonate (Li2CO3), lithium oxalate (LiC2O4) and lithium acetate (CH3COOLi). Cyclic voltammetry shows that the characteristic conversion peaks of lithium superoxide occur at about 2.1V and 2.3 V. Therefore, according to PC/EC characteristics of alky1 carbonate solvent, we presented the attack path for nucleophilic attack group, and explained the discharge mechanism of the battery system.During the discharge process, the battery discharge termination mechanism is described based on studying AC impedance spectra of the various stages of the battery and air electrode scanning electron microscopy (SEM) topography maps. Because PC and EC are good nucleophilic attack groups, nucleophilic attack reaction was favored in the presence of the lithium ions, electrons and oxygen. This leads to the decomposition of solvents and formation of discharge products. However, the formation of different products in different discharge potentials will result in the air electrode surface covered by alternate layers of different discharge products. The change of the battery internal impedance during the discharge process is then caused by the different non-conductive discharge products. At the beginning of discharge, the concentration of oxygen in electrolyte was lower than that in the equilibrium state, indicative of depletion of oxygen dissolved into the electrodes. Partial discharge products were deposited on the surface of the carbon material owing to serious electrode polarization. In the subsequent process, the concentration of oxygen attained a new equilibrium, and discharge product showed a stable deposition process. Furthermore, the active sites in bare electrode were gradually covered by discharge products which lowered the battery voltage. In stead, an alternate deposition of the discharge products showed that the interface was unstable, resulting in pullout and breakage of discharge products. Thus, the newly-formed active sites were exposed. This is the reason why discharge was sustained. One possible reason for the final termination of discharge process was that the depletion of electrolyte caused the completion of discharge, and another was that the electrode surface was completely covered by discharge products. The latter gave rise to the active sites inaccessible to the electrolyte.The transmission microscopy of the electrode morphology for that of single-walled carbon nanotube material before and after porous electrode carbon material discharge was compared. Porosity of the carbon material formed by a porous carbon electrode was analyzed. We believe that there is a close relationship between the discharge capacity and the air electrode composition in the discharge process. The greater porosity of the air electrode carbon material and surface area, the higher the discharge capacity of the prepared air electrode. The lithium-air battery discharge reaction takes place in large-sized mesoporous carbon material and a large hole, while the discharge product does not appear in the microscopic hole and hole of a smaller size (2-30nm). Discharge products adhere to the carbon surface, indicating that the surface of the carbon material contributes to the lithium-air battery discharge capacity.Currently, the important factor affecting the air battery discharge is the diffusion of oxygen. The major obstacle to the use of molecular dynamics modeling software for oxygen diffusion coefficient study are "air filtration membrane" and "electrolyte" which are barriers to be overcome for oxygen’s entry into a battery. It is found that the diffusion coefficient for oxygen in PDMS membrane material is lower than that in the electrolyte during lithium-air battery operation:DPDMS= 1.6255 ×10cm-2·s-1, DPC= 2.0425 ×10cm2·s-1 and DEC= 2.251 × 10-5cm2·s-1. This means the oxygen transport is controlled by the membrane material. A high oxygen diffusion coefficient of the film material is the key to improving battery performance because the lithium-air battery is controlled by the diffusion process.In addition, the battery discharge process showed that the discharge efficiency of a lithium-air battery is related with the supply of oxygen. The higher the concentration of oxygen at the electrode reaction sites, the lower over-potential in the discharge processes and the higher the current efficiency of the battery.Based on the study, we developed a configuration for lithium-air battery with a high capacity, and described discharge mechanism for the battery. The discharge process of the battery was analyzed using modeling. This haslaid a foundation for a better battery.