Filler addition of polyethylene oxide solid polymer electrolyte and polymer single ion conductor

Filler addition of polyethylene oxide solid polymer electrolyte and polymer single ion conductor

1. Add filler

Usually adding fillers to solid PEO polymer electrolyte will improve its comprehensive performance, mainly due to the following reasons: the addition of fillers will reduce the crystallinity of PEO polymers, especially for PEO polymers with higher crystallinity at room temperature, which can increase the amorphous phase region to facilitate the migration of Li+; adding fillers to PEO polymer electrolytes can also form fast Li+ channels near the fillers; at the same time, it can also increase the mechanical properties of the polymer electrolytes, such as Young’s modulus and interface stability.

There are two main types of fillers that have been widely studied: one is inert fillers, including ceramic oxides (such as Al2O3, TiO2, SiO2, etc.), ferroelectric ceramic materials (such as BaTiO3, etc.), super acidic oxides (such as acid ZrO2, succinonitrile, etc.), clay (such as montmorillonite, etc.).

Another type of filler is an inorganic fast lithium ion conductor. For example, researcher Hu Liangbing from the University of Maryland blended lithium copper complex tantalum oxide (LLZTO) nanowires with PEO to obtain a solid polymer electrolyte with high room temperature ionic conductivity. However, it should be noted that the large-scale and large-area preparation of LLZTO nanowires with good crystallinity is still a key issue that needs to be solved urgently. Researcher Guo Xiangxin explained in detail the effects of LLZTO addition and nanoparticle size on the electrochemical and battery performance of solid polymer electrolytes by mixing PEO and LLZTO nanoparticles in a certain proportion, combined with the percolation threshold theory. And found that the particle size is 43nm, the room temperature ionic conductivity of LLZTO and solid composite polymer electrolyte can reach 2.1×10-4S/cm, but the alkalinity of inorganic materials should be paid attention to the influence of the stability of PEO materials when the above materials are combined.

Filler addition of polyethylene oxide solid polymer electrolyte and polymer single ion conductor
Figure 1 – The influence of LLZTO volume fraction and particle size on the ionic conductivity of polymer electrolytes

2. Polymer single ion conductor

The traditional anion and cation combination type of lithium salt must be dissociated by the lithium salt to form free lithium ions before the lithium ion can be transported. It usually has a low lithium ion migration number, which seriously affects the electrochemical performance of the battery. In comparison, there is almost no migration of anions in the polymer single-ion conductor electrolyte, and the migration number of lithium ions is relatively high, close to 1, which is considered to be an ideal choice for solving the concentration polarization of lithium batteries.

In 1995, Armand proposed that the use of a strong electron withdrawing group fluorinated alkyl group instead of ordinary hydrocarbon group can promote the dissociation of lithium ions. Figure 2 shows the synthesis process of poly[(4-styrenesulfonyl)(fluorosulfonyl)imide] (LiPSFS). The polymer electrolyte prepared by blending LiPSFSI and PEO exhibits high ionic conductivity, the lithium ion migration number is 0.9, and has a relatively stable electrochemical stability window at 4.5V.

Filler addition of polyethylene oxide solid polymer electrolyte and polymer single ion conductor
Figure 2 – LiPSFSI synthesis step diagram

The polymer single ion conductor is usually directly introduced into the ion conducting region in the monomer structure of the polyanion, generally achieved by copolymerizing an anionic lithium salt monomer without an ion-conducting structure with a polymer containing an ion-conducting structure (such as PEO) or an organic small molecule monomer (such as an acrylic monomer containing an EO unit).

As shown in Figure 3, Michel Armand et al. prepared a triblock copolymer single ion conductor (LiPSTFSI-b-PEO-b-LiPSTFSI), where LiPSTFSI is used to provide Li+; the EO segment in PEO is used to increase the flexibility of the main chain and provide a migration path for Li+, and the LIPSTESI structural units are separated by PEO, which is easy to form microphase separation, which helps to improve the mechanical properties of the polyanionic salt. In addition, when the mass fraction of LiPSTFSI is 20% (EOLi is about 30 at this time), the ionic conductivity is the largest, and it is 1.3×10-5S/cm at 60°C. This is already at a higher level in single-ion conductor polymer electrolytes, and block copolymer electrolytes have higher ionic conductivity and better mechanical properties. The most commonly used is the di-block or tri-block structure, in which the lithium salt-polymer acts as a block to provide channels for ion conduction. The advantage of this type of polymer electrolyte is that it is composed of two amorphous blocks with low glass transition temperature, which can maintain the good mechanical properties of the electrolyte without sacrificing the fluidity of the chain segment. This is also conducive to improving the ionic conductivity of the electrolyte and suppressing the generation of lithium dendrites.

Filler addition of polyethylene oxide solid polymer electrolyte and polymer single ion conductor
Figure 3 – Structure diagram of triblock copolymer single ion conductor LiPSTFSI-b-PEO-b-LiPSTFSI