Electrolyte Distribution In A Non-aqueous Lithium-Air Battery And Its Impact On Battery Performance

In many emerging battery systems, lithium air batteries have wide application, whose theoretical capacity density is up to 3852 Ah/kg, and the theorecical energy density is up to 11400 Wh/kg at the thermodynamic equilibrium voltage of 2.96 V. However, the development of lithium air batteries still faces a series of challenging promble. In addition to the serious air electrode polarization, electrolyte decomposition, and poor cyclability, the actual discharge capacity of the battery is also far lower than its theoretical value.The area of triple phase boundaries of the lithium air battery affects the performance of the discharge capacity.Generally the more triple phase boundaries, the greater the capacity. But if used a given cathode material, electrolyte distribution on the positive electrode will be directly related to the triple phase boundary of batteries. Therefore, this paper focus on the characteristic of electrolyte distribution on air electrodes and its mechanism on cells, had carried out a series of studies. By adding different amounts of electrolyte(1 M LiTFSI/TMS) to the positive and negative of battery, this paper investigated the discharge capacity and the distribution characteristics on affinity air electrode, who hates or hydrophobics to electrolyte, then study the effect of different electrolyte injection methods on the electrolyte distribution and performance of the battery. The main works and conclusions in this paper are as follows:1. The amount of electrolyte is the most direct and critical impact factor affecting the electrolyte distribution on electrodes. Too much electrolyte is easy to drown electrode pores, which would greatly limit the oxygen transport within the electrodes. However too little, the volume of electrolyte is insufficient wetting the electrode surface, leading to a smaller solidliquid two-phase interface, and thus the battery reaction interface is small.This paper though a research that different amount of electrolyte added in batteries founded that a better amount of the electrolyte addition of the negative/positive side was30 ?L / 60 ?L, 10 ?L / 60 ?L and 40 ?L / 60 ?L, when KB, SP and CNT as air electrodes and with 2.5 mg loadings. At this time, the discharge capacity of the batteries was 7.1410 mAh, 6.8965 mAh and 5.066 mAh. Combined with EDS analysis, electrolyte tended to form a relatively broad distribution in KB and SP electrodes, while a distribution peak in CNT electrodes, it is mainly because of the mesoporous distribution and the electrolyte absorbing capacity of materials itself.2. In addition to the amount of electrolyte, the injection method of electrolyte is also related to the electrolyte distribution. Based on the above mentioned partially wetted, introducing some electrolyte into the inner of electrodes through simple methods of soaking the electrode or blending electrolyte and C together, for the purpose of utilization of the porous area near side of the carbon paper to increase the tri-phase reaction interface. It was founded that additional electrolyte injection into KB and SP electrodes easily flooded the mesoporous due to their strong aspiration capability, Thus it would affect the mass transfer of oxygen, leading to the decrease of the discharge capacity. Instead, CNT electrodes with a poor aspiration capability, the electrolyte is relatively difficult to wet pores, hence introducing a small amount of electrolyte to the electrode internal via blending tiny amount of electrolyte and the CNT powder could effectively optimize the electrolyte distribution, and the discharge capacity of batteries was increased from 5.066 mAh to 7.547 mAh when 0.143 mL/cm2 electrolyte introduced.3. Combined with SEM and XRD techniques, it could be found that the lithium air batteries generated circular annular Li2O2 during discharge, and the discharge product was insoluble in 1 M LiTFSI/TMS electrolyte system, so the Li2O2 particles deposited on the electrode surface, resulting in the impendence increase of batteries. With EDS analysis, it could be found a significant phenomenon of electrolyte redistribution during charging and discharging process:(1) due to the generation of solid Li2O2 some of the electrolyte is extruded carbon paper during discharging;(2) When the battery in charging, electrodes created a new channel because of the decomposition of Li2O2, thus hydrophilic KB and SP electrodes showed significant electrolyte reflux phenomenon under the action of capillary pressure, while CNT electrodes, with hydrophobic properties of electrodes themselves and a smaller aperture, are difficult to be re-entered by the electrolyte, so there is no reflux phenomenon.