PEO Based Polymer-Ceramic Hybrid Solid Electrolytes: a Review
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Feng et al. Nano Convergence (2021) 8:2 https://doi.org/10.1186/s40580-020-00252-5 REVIEW Open Access PEO based polymer-ceramic hybrid solid electrolytes: a review Jingnan Feng1,2, Li Wang2, Yijun Chen1, Peiyu Wang1, Hanrui Zhang1 and Xiangming He2* Abstract Compared with traditional lead-acid batteries, nickel–cadmium batteries and nickel-hydrogen batteries, lithium- ion batteries (LIBs) are much more environmentally friendly and much higher energy density. Besides, LIBs own the characteristics of no memory efect, high charging and discharging rate, long cycle life and high energy conversion rate. Therefore, LIBs have been widely considered as the most promising power source for mobile devices. Commonly used LIBs contain carbonate based liquid electrolytes. Such electrolytes own high ionic conductivity and excellent wetting ability. However, the use of highly fammable and volatile organic solvents in them may lead to problems like leakage, thermo runaway and parasitic interface reactions, which limit their application. Solid polymer electrolytes (SPEs) can solve these problems, while they also bring new challenges such as poor interfacial contact with electrodes and low ionic conductivity at room temperature. Many approaches have been tried to solve these problems. This arti- cle is divided into three parts to introduce polyethylene oxide (PEO) based polymer-ceramic hybrid solid electrolyte, which is one of the most efcient way to improve the performance of SPEs. The frst part focuses on polymer-lithium salt (LiX) matrices, including their ionic conduction mechanism and impact factors for their ionic conductivity. In the second part, the infuence of both active and passive ceramic fllers on SPEs are reviewed. In the third part, composite SPEs’ preparation methods, including solvent casting and thermocompression, are introduced and compared. Finally, we propose fve key points on how to make composite SPEs with high ionic conductivity for reference. Keywords: Solid electrolyte, Polymer electrolyte, Polyethylene oxide, Ceramic, Nanoparticle 1 Introduction trace back to 1834. However, the real threshold of stud- Lithium ion batteries (LIBs) are widely used in phones, ies on solid-state electrolytes is generally believed to be computers and other mobile devices owing to their high 1960s when Takahashi et al. [5] found the silver ionic −2 specifc energy, good capability, good cycle performance conductivity of Ag3SI (about 10 S/cm at ambient tem- and environmentally friendly property [1–3]. Although perature). Tese ceramic materials own excellent ionic traditional carbonate based liquid electrolytes have high conductivity, while they have fatal defects that their rigid ionic conductivity under normal temperature, the organic and brittle bodies will lead to bad contact with electrodes solvent contained has the potential danger of leakage and and bring great difculties to processing. Terefore, combustion, which may cause severe safety issues. Solid- the focus of research gradually shifted from inorganic state electrolytes, both inorganic solid electrolytes and materials to organic materials which own good fexibil- solid polymer electrolytes, can overcome these short- ity and processability. In 1973, Fenton et al. [6] reported ages [4]. Te research on solid-state ionic conductors can the transport of ions in polyethylene oxide (PEO)-alkali metal salts complexation, which started a new chapter of researches on solid polymer electrolytes (SPEs). After *Correspondence: [email protected] that, Armand et al. [7] reported that the ionic conduc- 2 Institute of Nuclear and New Energy Technology, Tsinghua University, tivity of such complexations could reach 10 −5 S/cm in Beijing 100084, China Full list of author information is available at the end of the article temperature range between 40 and 60 °C, indicating that © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Feng et al. Nano Convergence (2021) 8:2 Page 2 of 12 SPEs might be used under room temperature. Since then, these particles [15–18]. Consequently, polymer-ceramics the research on SPEs has become the most popular part hybrid become one of the most efective way to improve in related felds. the performance of SPEs. Compared with traditional liquid electrolytes, SPEs can not only alleviate the danger of fammability and pos- 2 Polymer‑LiX matrices sible side reactions with electrodes, but also retain the Polymers used in SPEs include polyacrylonitrile (PAN) excellent adhesion and flm-forming properties of poly- [19], PS [12, 13], polymethyl methacrylate (PMMA) mers. Moreover, the solid-state electrolyte membranes [20], polyvinylidene fuoride (PVDF) [21], polyvinyl are supposed to suppress the growing of lithium den- pyrrolidone (PVP) [22], etc. Compared with the other drite, which can further ensure the safety of the batter- polymers, PEO has many advantages due to its special ies during the charging and discharging process. Tese structure. PEO owns strong electron donating ether oxy- characteristics determine that SPEs have a promising gen (EO) groups, soft macromolecular backbone, good future. However, SPEs have not yet reached the practi- thermal stability and mechanical properties. As a con- cal accessibility due to their huge interfacial resistance sequence, PEO has become the most frequently studied and low bulk conductivity (10 −7 S/cm) at ambient tem- polymer base for SPEs. Te chemical formula of PEO is perature [8, 9]. To solve these problems, polymer/liq- H(OCH2CH2)nOH, as showed in Fig. 1b. It is water-sol- uid hybrid [10], polymer/polymer hybrid and polymer/ uble and in semi-crystalline state at room temperature. ceramic hybrid were developed [11]. Te polymer/liquid Usually, PEO is the name for the polymer with a molecu- hybrid is to form a gel-type polymer electrolyte by add- lar weight (Mn) greater than 20,000. When the molecular ing a small amount of liquid plasticizer, such as ethylene weight is less than 20,000, it is called polyethylene glycol carbonate (EC), propylene carbonate (PC) and dimethyl (PEG). carbonate (DMC) into the electrolyte. In such hybrid, the In SPEs, both anions and cations participate in the pro- liquid phase plays an important role in wetting, reduc- cess of ionic conduction. However, due to the mecha- ing the interfacial resistance and directly increasing the nism of redox reaction, only the charges carried by Li+ ionic conductivity. Nevertheless, such improvement is at are valid. Te number of evaluating the contribution of the expense of all-solid-state properties of SPEs, which Li+ to the whole ionic conductivity is named transference may reduce the mechanical strength of the electrolytes number (T ). Tis number is normally below 0.5 in Li+ and cause similar problems to liquid electrolytes. Te SPEs [10]. In the contrary, anions are fxed to the polymer polymer/polymer hybrid is formed by combining difer- skeleton through covalent bonds in polyelectrolytes. In ent kinds of polymers so that the hybrid can obtain the this way, polyelectrolytes are single ion-conducting elec- advantages of each one. As an example, the ionic conduc- trolytes and their T is close to 1. However, because of Li+ tivity of polystyrene (PS)-PEO-PS [12, 13] hybrid system the insufcient dissociation of Li +, the ionic conductiv- −4 can reach 2.3 × 10 S/cm at 60 °C, which is better than ity of polyelectrolytes is much lower than that of normal the performance of single PEO-based SPEs. But the ionic SPEs [23]. So, this article mainly focuses on normal SPEs conductivity is still not high enough for daily use under instead of polyelectrolytes. normal temperature. Te polymer/ceramic hybrid is to Te conductive mechanism of PEO-lithium salt (LiX) add inorganic fllers into the electrolytes. Tese fllers can matrices is shown in Fig. 1a. Te whole process can be be divided to two categories due to their own nature. Te summarized by the combination and fracture of EO-Li frst are passive fllers for there are no Li ions in them- bonds. First, the strong electron donating group EO will selves and thus they do not directly participate into the complex with the charge carrier Li +. Tere are about 5 transport process of Li+. Tey can improve the ionic con- EO to match with 1 Li+ [24, 25]. Ten the conduction of ductivity of SPEs mainly due to the complicated structure Li+ is completed through the segmental motion of PEO of polymer-passive fller interfaces, which results in the molecular chains [26]. In this way, Li+ can be transported suppression of recrystallization of PEO, more free-Li+ on a single chain or between diferent chains. Since the and fast ion transport channels. Te fllers in second molecular chain movement is restricted to amorphous kind are active, which contain Li ions in their bodies. regions, the transport of Li + in PEO is also limited to As a result, Li + can transport through PEO body, PEO- these regions, which means the crystallization zone active fller interfaces and active fllers’ bodies. Te ionic won’t exhibit ionic conductivity. Terefore, the ionic con- conductivity of the whole SPEs get enhanced in this way. ductivity of PEO largely depends on the proportion of Besides, the mechanical strength can get better by doping amorphous regions.