Study On Preparation, Characterization And High Temperature Performance Of Polymerized Lithium Borate - Based Electrolytes

As one of the key components in lithium ion batteries, the electrolyte plays a vital role in separating the cathode and anode to avoid short circuit and providing the transmission of lithium ions. Therefore, the electrolyte performance has a significant influence on the cycling and security performance of the batteries. In our previous works, a polyborate lithium salt(PLTB) with superior thermal stability and corresponding gel polymer electrolyte with wide electrochemical stability and single ion conductor feature were prepared in our lab. Based on these attractive characteristics, the application of this gel polymer electrolyte in LiMn2O4 batteries was explored in this dissertation. It turned out that this GPE could significantly improve the capacity retention of the batteries especially at high temperature of 55℃, endowing it a very promising candidate for large-scale industrial applications. In addition, inspired by our previous work, we have synthesized a couple of alcohol based polyborate lithium salts named PLPB and PLDB by a one-step reaction in an aqueous solution which possessed good purity and superior thermal stability. The corresponding gel polymer electrolyte EC/DMC-PLPB@PVDF-HFP and EC/DMC-PLDB@PVDF-HFP exhibited wide electrochemical stability, acceptable ionic conductivity, the ability to passivate Al current collector, high lithium ion transference number as well as stable battery performance. It was worth noting that the PLPB, synthesized from the low-cost feedstock pentaerythrite, possessed the highest lithium ion concentration per repeating unit among the reported polymeric lithium borate salts in recent years, which could make it promising for large-scale applications. These are described as follows:(1) Based on the attractive characteristics of EC/DMC-PLTB@PVDF-HFP, the application of this gel polymer electrolyte in LiMn2O4 batteries was explored. It turned out that this GPE slightly outperform the conventional EC/DMC-LiPF6 electrolyte in LiMn2O4 batteries at room temperature with slightly higher capacity retention after 100 cycles at 0.5C. While in the case of 55℃, a significant improvement in capacity retention was observed where the GPE based batteries possessed a 77% capacity retention, which was much higher than the 43% for conventional electrolyte based LiMn2O4 batteries. In addition, the single ion gel polymer electrolyte also improved the rate capabilities of LiMn2O4 batteries. Several characterization methods were used to analysis the advantages for single ion gel polymer electrolyte. The change of the battery resistance and distance of redox peaks was less for single ion GPE based batteries, demonstrating that much less side reaction occurred owing to its better thermal and electrochemical stability. In addition, the SEM results showed that the LiMn2O4 electrode was less corroded since less Manganese element was detected with ICP-Ms method in the electrolyte after cycling. All those results demonstrated that the single ion GPE exhibited stable performance and could be adequate for applications in LiMn2O4 batteries.(2) A couple of polyborate lithium salts PLPB and PLDB were synthesized via a facile one-step reaction in an aqueous solution. The simplified preparation method as well as the only product besides water highly commit to green chemistry idea. The lithium salts synthesized were demonstrated to possess good purity as well as superior thermal stability with a decomposition temperature at 240℃. It was worth noting that the PLPB, synthesized from the low-cost feedstock pentaerythrite, possessed the highest lithium ion concentration per repeating unit among the reported polymeric lithium borate salts in recent years.(3)Inspired by the previously synthesized polyborate lithium salts, we prepared polymer electrolyte PLPB@PVDF-HFP and PLDB@PVDF-HFP. A small amount of EC/DMC was used as plasticizer to swell the dry membranes so as to improve the ionic conductivity. According to EDX mapping analysis, the B element was homogeneously distributed on the dry membranes, indicating uniformity of both membranes and good compatibility of these lithium borates with PVDF-HFP. It was shown in SEM that both of the membranes possessed relatively smooth surface and no porous structure could be seen on the surface, which could be of crucial importance for inhibiting the forming of lithium dendrites. By electrochemical characterization, the single-ion GPE exhibited wide electrochemical stability, acceptable ionic conductivity, the ability to passivate Al current collector, high lithium ion transference number as well stable battery performance both at room temperature and elevated temperatures. Specifically, the Li//PC-PLPB@PVDF-HFP//LiFePO4 battery operated quite well at an elevated temperature of 120℃, which could endow it very promising candidate for applications in aerospace, military and oil and gas exploration.