Lithium-ion batteries (LIBs) are considered to be one of the most important energy storage technologies. As the energy density of batteries increases, battery safety becomes even more critical if the energy is released unintentionally. Accidents related to fires and explosions of LIBs occur frequently worldwide. Some have caused serious threats to human life and health and have led to numerous product recalls by manufacturers. These incidents are reminders that safety is a prerequisite for batteries, and serious issues need to be resolved before the future application of high-energy battery systems. This Review aims to summarize the fundamentals of the origins of LIB safety issues and highlight recent key progress in materials design to improve LIB safety. We anticipate that this Review will inspire further improvement in battery safety, especially for emerging LIBs with high-energy density.

Materials for lithium-ion battery safety

The need for lighter, thinner, and smaller products makes lithium ion batteries popular power sources for applications such as mobile phones, laptop computers, digital cameras, electric vehicles, and hybrid electric vehicles. For high power applications, the development of high capacity and high voltage electrode materials is in progress. Battery performance and safety issues are also related to the properties of the electrolytes used. To improve the properties of the electrolytes, small amounts of other components, known as electrolyte additives, are incorporated. This paper reviews the recent progress in electrolyte additives used to improve performance and other properties, such as safety. This review classifies the additives based on their functions and their effects on specific electrode materials focusing on electrodes under current development. From anodes: carbonaceous electrodes, silicon, tin and Li4Ti5O12; from layered cathodes: LiCoO2, Li-rich and LiNiyMnyCo1−2yO2 (NMC); from spinel: LiMn2O4, and from olivine: LiFePO4 are selected. We believe that this approach will help readers easily identify and understand the additives suitable for their target materials.

Electrolyte additives for lithium ion battery electrodes: progress and perspectives

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex bra...

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation.

Progress of enhancing the safety of lithium ion battery from the electrolyte aspect

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such “organopolymerisation” reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

Copolymerization of e-caprolactone and methyl methacrylate with (1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phophoranylidenamino]-2Λ5,Λ5-catenadi(phosphazene) (t-BuP4) as the catalyst has been studied. Nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) measurements indicate that the cyclic ester and vinyl monomer form a random copolymer. Thermogravimetric analysis (TGA) measurements demonstrate that such a random copolymer exhibits a decomposition temperature higher than those of the corresponding homopolymers. The present study reveals a “hybrid copolymerization” i.e., a combination of vinyl polymerization and ring-opening polymerization, which makes the incorporation of cyclic and vinyl monomers into the same polymer chain possible. Hybrid copolymerization is expected to produce a number of new interesting polymers.

Hybrid Copolymerization of ε-Caprolactone and Methyl Methacrylate

Hyperbranched poly(e-caprolactone) (h-PCL) is synthesized via hybrid copolymerization of e-caprolactone (CL) with glycidyl methacrylate (GMA) as the branching agent and (1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phophoranylidenamino]-2Λ5,Λ5-catenadi(phosphazene) (t-BuP4) as the catalyst in one step. Proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FTIR) spectroscopy show that the polymerization regarding the double bonds and ring-opening take place simultaneously. Size exclusion chromatography measurements with light scattering and viscosity detection indicate that the resultant polymer has a hyperbranched structure.

One-step synthesis of hyperbranched biodegradable polymer

The effects of ethylene ethyl phosphate (EEP) as a multifunctional electrolyte additive on safety characteristics and electrochemical performance of lithium-ion batteries are investigated. Based on the flammability test, the self-extinguishing time of the electrolyte with 10% EEP is only less than half of the baseline electrolyte, which indicates that EEP is a highly efficient flame retardant for the electrolyte. During overcharging the LiNi1/3Co1/3Mn1/3O2/Li cells, incorporation of EEP into the electrolyte can postpone the sharp voltage rise. Therefore, EEP is an improver of safety characteristics of lithium-ion batteries, both in terms of flame resistance and overcharge protection. Furthermore, the EEP-containing electrolyte in the half-cells and full-cells both exhibit higher initial coulombic efficiency and cycling stability than the baseline electrolyte. It is concluded that EEP is a good film-formation additive not only for the graphite anode, but also for the LiNi1/3Co1/3Mn1/3O2 cathode. Therefore, EEP is proposed as a promising multifunctional electrolyte additive for lithium-ion batteries.

Ethylene ethyl phosphate as a multifunctional electrolyte additive for lithium-ion batteries

Ring-opening polymerization (ROP) of e-caprolactone (CL) using salicylic acid (SAA) as the organocatalyst and benzyl alcohol as the initiator in bulk at 80 °C successfully proceeded to give a narrowly distributed poly(e-caprolactone) (PCL). In addition, 2-hydroxyethyl methacrylate, propargyl alcohol, 6-azido-1-hexanol, and methoxy poly(ethylene glycol) were also used as functional initiators. The 1H NMR, SEC, and MALDI-TOF MS measurements of the PCL clearly indicate the presence of the initiator residue at the chain end, implying that the SAA-catalyzed ROP of CL was through the activated monomer mechanism. The kinetic experiments confirmed the controlled/living nature of the SAA-catalyzed ROP of CL. Furthermore, the block copolymerization of CL and δ-valerolactone successfully proceeded to give poly(e-caprolactone)-block-poly(δ-valerolactone). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1185–1192

Jinbao Xu

Papers

Hybrid Copolymerization of ε-Caprolactone and Methyl Methacrylate

One-step synthesis of hyperbranched biodegradable polymer

Ethylene ethyl phosphate as a multifunctional electrolyte additive for lithium-ion batteries

Controlled/living ring-opening polymerization of ε-caprolactone with salicylic acid as the organocatalyst

Metal-free controlled ring-opening polymerization of ε-caprolactone in bulk using tris(pentafluorophenyl)borane as a catalyst