Preparation And Properties Of New-type Polymer Electrolytes Of High Performance For Lithium Ion Batteries

With the development of the world economy and the improvement of people’s living in the21st century, the contradictions between energy supply and energy demand are going more and more intensive. In order to deal with the increasingly serious energy crisis and more stringent environmental protection requirements, the concerns are concentrating attention once again on national initiatives to reevaluate utilization of alternative energy sources and replacement of the internal combustion engine with a wireless electric motor. Secondary battery or rechargeable battery draws a widely public attention for it provides the portability of stored chemical energy with the ability to deliver this energy as electrical energy with a high conversion efficiency and no gaseous exhaust. Among the batteries, lithium ion batteries (LIBs), with the advantages of high energy density, high output voltage, high output power, low self discharge, wide work temperature, no memory effect and environment friendly, have gained an unprecedented importance in the last several decades as the energy storage of portable devices such as cell phones, digital carema, laptop computers and so on. Furthermore, lithium ion batteries are also promising power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), pure electric vehicles (PEVs) and storage green sources representing with wind, solar, tide energy, geothermal energy in smart grid. Liquid electrolytes for lithium ion batteries present potential safety problems due to the existence of large amount of organic solvents which are easy to get on fire leading to sometime fire and explosion because of misusage, short circuits or local overheating. The quest for safer and more reliable electrolyte systems is therefore urgent, and solid electrolytes are promising candidates in this regard. If solid electrolytes can replace the organic electrolyte, not only safety will be greatly improved but also other advantages such as no liquid leakage and long cycling life will be achieved. Li+ion conductors (ILICs) and solid polymer electrolytes (SPEs) are difficult to be used in the large-capacity lithium ion batteries for their defects. So it is necessary to find gel polymer electrolytes (GPEs) of high performance to improve the safty and reliability of large-capacity lithium ion batteries.The dissertation includes three major ports. Firstly, the design and preparation of novel single-ion conducting polymer electrolytes based on borate for lithium ionbatteries. Secondly, the prepation and research of novel trilayer gel membrane based on electrospun poly(vinylidene fluoride) and polyborate for lithium ion batteries. And thirdly, the research of gel composite membrane with low price and high performance.1. The design and preparation of novel single-ion conducting polymer electrolytes based on borate for lithium ion batteriesThe PEs are composed with polymers and electrolyte lithilum salts dissolved into polymers. Ion conduction takes place in the amorphous phase of polymers. The cation transport is assisted by segmental motion of the polymer chains. The counter anions in the PEs have weak interactions with the polymers, hence transport more easily. The major drawbacks of dual ion conduction in traditional PEs are the low Li+ion transference number (0.2-0.3). In the same time, the mobility of anions results in polarization and reduces the lithium ion transference number. Therefore it is necessary to design single-ion conducting polymer electrolytes to guarantee the stability of anions to gain good performance for lithium ion batteries.To date, two approaches have been reported to reduce the mobility of anions. The first one deals with introduction of interacting sites that preferentially interact with the anions. The other is to anchor anions to the polymer backbone,which is a common method to achieve single-ion conducting PEs. Lithium chelatoborates as electrolyte salts have been widely studied and its aim is to substitute LiPF6which is unstable at elevated temperature and sensitive with trace amount water. Among the borates, lithium bis(oxalate)borate (LiBOB) are the promising candidates used as electrolyte salts in lithium ion batteries which is not only the low production cost, wide potential window, good thermal stability but also they can form effective solid electrolyte interface (SEI) to stabilize the graphitic anode structure. LiBOB has the structure based on an oxalate-chelated borate and present good electrochemical performance. So we think that lithium chelatoborate-based polymer electrolytes whose monomers are like LiBOB may be an alternative single ion conducting PEs to promote the applications of lithium rechargeable battery and we designed the structures of the single ion conducting PEs like LiBOB.Based on the design, two novel single-ion conducting polymer electrolytes, LiPVAOB (lithium polyvinyl alcohol oxalate borate) and LiPAAOB (lithium polyacrylic acid oxalate borate) were prepared with polymer and lithium chelatoborates. The obtained membranes were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA). After adding an assistant (PC, propylene carbonate), the as-prepared two electrolytes present single ion conductive behavior and the ionic conductivity at ambient temperature can be up to6.11×10-3mS cm-1(LiPVAOB) and2.03×10-3mS cm-1(LiPAAOB). The electrochemical window of the PEs can be stable up to7.0V (vs. Li+/Li), which is of great attraction for5V lithium ion batteries with high energy density.2. The prepation and research of novel trilayer gel membrane based on electrospun poly(vinylidene fluoride) and polyborate for lithium ion batteries.The single-ion conducting polymer electrolytes, LiPAAOB and LiPVAOB whose monomers are structurally similar to LiBOB, present good single ion conductive behaviors, but the ionic conductivity at ambient temperature is very low (<10-5S cm-1at r.t.) and mechancial strength are also poor. The drawbacks prevent them from practical applications in lithium ion batteries. To further improve the safeties, mechanical properties and electrochemical performances of LiPAAOB and LiPVAOB, electrospun PVDF was used to achieve composite membranes of trilayer structure just like sandwich, PVDF-LiPVAOB and PVDF-LiPVAOB, which have similar structures of PP/PE/PP separators of Celgard. The ionic conductivity of the as-prepared gel membranes saturated with1mol L-1LiPF6electrolyte at ambient temperature can be up to0.26mS cm-1(PVDF-LiPVAOB) and0.35mS cm-1(PVDF-LiPAAOB), higher than that of the corresponding well-used commercial separator (Celgard2730),0.21mS cm-1. Moreover, the lithium ion transferences in the gel membranes at room temperature were improved greatly. The values can up to0.58, which are much higher than that of Celgard2730,0.27. Furthermore, the trilayer membranes show good self-extinguishing property after ignition. The absorbed solvents in the gel composite membranes are difficult to evaporate, which can be up to85℃,20℃higher than that for commercial separator (65℃). Their electrochemical performances of the GPEs are evaluated by using LiFePO4cathode. The obtained results suggest that the gel-type composite membranes show great attraction to the large-capacity battery systems requiring high safety and low cost.3. The research of gel composite membrane with low price and high performance. The application of the investigated gel PEs in large-scale systems is currently limited because of poor mechanical strength and high cost. For solving the problems, two novel composite membranes were prepared based on nonwoven fabrics with PVDF (NWF-PVDF) and glass fiber mats with PVDF (GFM-PVDF) exhibiting high safety (self-extinguishing), good mechanical property and low cost. The ionic conductivity of the as-prepared gel membranes saturated with1mol L-1LiPF6electrolyte at ambient temperature can be up to0.30mS cm-1(PVDF-NWF) and1.13mS cm-1(PVDF-GFM), higher than that of the corresponding well-used commercial separator (Celgard2730),0.21mS cm-1. Moreover, the lithium ion transferences in the gel membranes at room temperature were improved greatly. The values can up to0.43for PVDF-NWF and0.56for PVDF-GFM, which are much higher than that of Celgard2730,0.27. Furthermore, the absorbed solvents in the gel composite membranes are difficult to evaporate, which can be up to85℃,20℃higher than that for the commercial separator (65℃). Their electrochemical performances of the GPEs are evaluated by using LiFePO4cathode. The obtained results suggest that the gel-type composite membranes show great attraction to the large-capacity battery systems requiring high safety and low cost.In a word, single-ion conducting polymer electrolyte and gel electrolytes have been investigated to improve the safety and reliability of lithium ion batteries, and some of them present great promise for practical application.