Poly(ethylene oxide) (PEO) based materials are widely considered as promising candidates of polymer hosts in solid-state electrolytes for high energy density secondary lithium batteries. They have several specific advantages such as high safety, easy fabrication, low cost, high energy density, good electrochemical stability, and excellent compatibility with lithium salts. However, the typical linear PEO does not meet the production requirement because of its insufficient ionic conductivity due to the high crystallinity of the ethylene oxide (EO) chains, which can restrain the ionic transition due to the stiff structure especially at low temperature. Scientists have explored different approaches to reduce the crystallinity and hence to improve the ionic conductivity of PEO-based electrolytes, including blending, modifying and making PEO derivatives. This review is focused on surveying the recent developments and issues concerning PEO-based electrolytes for lithium-ion batteries.

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

The motivation for lithium battery development and a discussion of ion conducting polymers as separators begin this review, which includes a short history of polymer electrolyte research, a summary of the major parameters that determine lithium ion transport in polymer matrices, and consequences for solid polymer electrolyte development. Two major strategies for the application of ion conducting polymers as separators in lithium batteries are identified: One is the development of highly conductive materials via the crosslinking of mobile chains to form networks, which are then swollen by lithium salt solutions ("gel electrolytes"). The other is the construction of solid polymer electrolytes (SPEs) with supramolecular architectures, which intrinsically give rise to much enhanced mechanical strength. These materials as yet exhibit relatively common conductivity levels but may be applied as very thin films. Molecular composites based on poly(p-phenylene)- (PPP)-reinforced SPEs are a striking example of this direction. Neither strategy has as yet led to a "breakthrough" with respect to technical application, at least not for electrically powered vehicles. Before being used as separators, the gel electrolytes must be strengthened, while the molecularly reinforced solid polymer electrolytes must demonstrate improved conductivity.

Polymer Electrolytes for Lithium-Ion Batteries

Recent advances in all-solid-state rechargeable lithium batteries

In this review, state-of-the-art polymer electrolytes are discussed with respect to their electrochemical and physical properties for their application in lithium polymer batteries. We divide polymer electrolytes into the two large categories of solid polymer electrolytes and gel polymer electrolytes (GPE). The performance requirements and ion transfer mechanisms of polymer electrolytes are presented at first. Then, solid polymer electrolyte systems, including dry solid polymer electrolytes, polymer-in-salt systems (rubbery electrolytes), and single-ion conducting polymer electrolytes, are described systematically. Solid polymer electrolytes still suffer from poor ionic conductivity, which is lower than 10−5 S cm−1. In order to further improve the ionic conductivity, numerous new types of lithium salt have been studied and inorganic fillers have been incorporated into solid polymer electrolytes. In the section on gel polymer electrolytes, the types of plasticizer and preparation methods of GPEs are summarized. Although the ionic conductivity of GPEs can reach 10−3 S cm−1, their low mechanical strength and poor interfacial properties are obstacles to their practical application. Significant attention is paid to the incorporation of inorganic fillers into GPEs to improve their mechanical strength as well as their transport properties and electrochemical properties.

Polymer electrolytes for lithium polymer batteries

Solid-state electrolytes (SSEs) have emerged as high-priority materials for safe, energy-dense and reversible storage of electrochemical energy in batteries. In this Review, we assess recent progress in the design, synthesis and analysis of SSEs, and identify key failure modes, performance limitations and design concepts for creating SSEs to meet requirements for practical applications. We provide an overview of the development and characteristics of SSEs, followed by analysis of ion transport in the bulk and at interfaces based on different single-valent (Li+, Na+, K+) and multivalent (Mg2+, Zn2+, Ca2+, Al3+) cation carriers of contemporary interest. We analyse the progress in overcoming issues associated with the poor ionic conductivity and high interfacial resistance of inorganic SSEs and the poor oxidative stability and cation transference numbers of polymer SSEs. Perspectives are provided on the design requirements for future generations of SSEs, with a focus on the chemical, geometric, mechanical, electrochemical and interfacial transport features required to accelerate progress towards practical solid-state batteries in which metals are paired with energetic cathode chemistries, including Ni-rich and Li-rich intercalating materials, sustainable organic materials, S8, O2 and CO2. Solid-state batteries based on electrolytes with low or zero vapour pressure provide a promising path towards safe, energy-dense storage of electrical energy. In this Review, we consider the requirements and design rules for solid-state electrolytes based on inorganics, organic polymers and organic–inorganic hybrids.

Designing solid-state electrolytes for safe, energy-dense batteries

Ionic conductivities for poly(ethylene oxide) complexes with lithium salts of monobasic and dibasic acids and blends of poly(ethylene oxide) with lithium salts of anionic polymers

D.J Bannister

Papers

Ionic conductivities for poly(ethylene oxide) complexes with lithium salts of monobasic and dibasic acids and blends of poly(ethylene oxide) with lithium salts of anionic polymers

Ionic conductivities of poly(methoxy polyethylene glycol monomethacrylate) complexes with LiSO3CH3

Ion conductivity in polysiloxane comb polymers with ethylene glycol teeth