Simultaneous modulation of cathode/anode and electrolyte interfaces via a nitrile additive for high-energy-density lithium-metal batteries

Nickel-rich layered oxides have great potential for commercial development applications, so it is critical to address their stability over long cycles. Ensuring long-term cycle stability relies heavily on the stability of the interface between the electrode and electrolyte in Li|LiNi0.8Co0.1Mn0.1O2 (NCM811) batteries. In this work, a denser, more stable and thinner nickel-rich cathode/electrolyte interface was constructed by electrolyte engineering with succinonitrile (SN) as an additive. The increase of organic compound content in the formed Ni-rich cathode/electrolyte interface can fully release the stress and strain generated during repetitive charge-discharge processes, and significantly reduce the irreversible phase transition during the nickel-rich cathode charge-discharge processes. Additionally, this interface impedes the breakdown of electrolytes and the dissolution of transition metals. Furthermore, the addition of SN additives also forms a more stable lithium metal anode/electrolyte interface. Notably, batteries containing SN additives (0.5, 1.0 and 1.5 wt%) show excellent electrochemical performance compared to base electrolytes. Particularly, the improvement is most significant with an SN addition of 1.0 wt%. After 250 cycles at 1C rate, the capacity retention rate of the battery improved by 32.8%. Thus, this work provides a new perspective for simultaneously constructing a stable interface of nickel-rich cathode and lithium metal anode with a high energy density in lithium metal batteries.

Side Reactions/Changes in Lithium-Ion Batteries: Mechanisms and Strategies for Creating Safer and Better Batteries

Lithium-ion batteries (LIBs), in which lithium ions function as charge carriers, are considered the most competitive energy storage devices due to their high energy and power density. However, battery materials, especially with high capacity undergo side reactions and changes that result in capacity decay and safety issues. A deep understanding of the reactions that cause changes in the battery's internal components and the mechanisms of those reactions is needed to build safer and better batteries. This review focuses on the processes of battery failures, with voltage and temperature as the underlying factors. Voltage-induced failures result from anode interfacial reactions, current collector corrosion, cathode interfacial reactions, overcharge, and over-discharge, while temperature-induced failure mechanisms include SEI decomposition, separator damage, and interfacial reactions between electrodes and electrolytes. The review also presents protective strategies for controlling these reactions. As a result, the reader is offered a comprehensive overview of the safety features and failure mechanisms of various LIB components.

A Facile Potential Hold Method for Fostering an Inorganic Solid-Electrolyte Interphase for Anode-Free Lithium-Metal Batteries

Anode-free lithium batteries are regarded as an ultimate form of high-energy-density lithium-ion batteries. Unfortunately, irreversible lithium loss during cycling plays a major role in degrading the overall cell performance in the anode-free configuration. To alleviate the deterioration, building a robust solid-electrolyte interface on an anode current collector is an indispensable requirement. Here, we present a facile in-situ electrochemical method of a potential hold during the first charge to guide more salt-derived (less solvent-derived) decomposition on the anode interface. We show the distinctive decomposition potential of lithium salts and ether/carbonate solvents, where the Li-solvation structures with salt contact-ion-pairs preferentially decompose to form LiF-rich and less organic components, leading to enhanced lithium Coulombic efficiency in Li||Cu cells as well as mitigating the capacity fade of Cu||LiFePO₄ and Cu||LiNi_0.8 Mn_0.1 Co_0.1 O₂ cells.

Thermal-Responsive and Fire-Resistant Materials for High-Safety Lithium-Ion Batteries

As one of the most efficient electrochemical energy storage devices, the energy density of lithium-ion batteries (LIBs) has been extensively improved in the past several decades. However, with increased energy density, the safety risk of LIBs becomes higher too. The frequently occurred battery accidents worldwide remind us that safeness is a crucial requirement for LIBs, especially in environments with high safety concerns like airplanes and military platforms. It is generally recognized that the catastrophic thermal runaway (TR) event is the major cause of LIBs related accidents. Tremendous efforts have been devoted to coping with the TR concerns in LIBs, and thus enhance battery safety. This review first gives an introduction to the fundamentals of LIBs and the origins of safety issues. Then, the authors summarize the recent advances to improve the safety of LIBs with a unique focus on thermal-responsive and fire-resistant materials. Finally, a perspective is proposed to guide future research directions in this field. It is anticipated this review will stimulate inspiration and arouse extensive studies on further improvement in battery safety.

Tale of Three Phosphate Additives for Stabilizing NCM811/Graphite Pouch Cells: Significance of Molecular Structure-Reactivity in Dictating Interphases and Cell Performance

Electrolyte additives have been extensively used as an economical approach to improve Li-ion battery (LIB) performances; however, their selection has been conducted on an Edisonian trial-and-error basis, with little knowledge about the relationship between their molecular structure and reactivity as well as the electrochemical performance. In this work, a series of phosphate additives with systematic structural variation were introduced with the purpose of revealing the significance of additive structure in building a robust interphase and electrochemical property in LIBs. By comparing the interphases formed by tripropyl phosphate (TPPC1), triallyl phosphate (TPPC2), and tripropargyl phosphate (TPPC3) containing alkane, alkene, and alkyne functionalities, respectively, theoretical calculations and comprehensive characterizations reveal that TPPC3 and TPPC2 exhibit more reactivity than TPPC1, and both can preferentially decompose both reductively and oxidatively, forming dense and protective interphases on both the cathode and anode, but they lead to different long-term cycling behaviors at 55 °C. We herein correlate the electrochemical performance of the high energy Li-ion cells to the molecular structure of these additives, and it is found that the effectiveness of TPPC1, TPPC2, and TPPC3 in preventing gas generation, suppressing interfacial resistance growth, and improving cycling stability can be described as TPPC3 > TPPC2 > TPPC1, i.e., the most unsaturated additive TPPC3 is the most effective additive among them. The established correlation between structure-reactivity and interphase-performance will doubtlessly construct the principle foundation for the rational design of new electrolyte components for future battery chemistry.

Abuse-Tolerant Electrolytes for Lithium-Ion Batteries

Safety issues currently limit the development of advanced lithium-ion batteries (LIBs) and this is exacerbated when they are misused or abused. The addition of small amounts of fillers or additives into common liquid electrolytes can greatly improve resistance to abuse without impairing electrochemical performance. This review discusses the recent progress in such abuse-tolerant electrolytes. It covers electrolytes with shear thickening properties for tolerating mechanical abuse, electrolytes with redox shuttle additives for suppressing electrochemical abuse, and electrolytes with flame-retardant additives for resisting thermal abuse. It aims to provide insights into the functioning of such electrolytes and the understanding of electrolyte composition-property relationship. Future perspectives, challenges, and opportunities towards practical applications are also presented.

Regulation of Cathode-Electrolyte Interphase via Electrolyte Additives in Lithium Ion Batteries

As the power supply of the prosperous new energy products, advanced lithium ion batteries (LIBs) are widely applied to portable energy equipment and large-scale energy storage systems. To broaden the applicable range, considerable endeavours have been devoted towards improving the energy and power density of LIBs. However, the side reaction caused by the close contact between the electrode (particularly the cathode) and the electrolyte leads to capacity decay and structural degradation, which is a tricky problem to be solved. In order to overcome this obstacle, the researchers focused their attention on electrolyte additives. By adding additives to the electrolyte, the construction of a stable cathode-electrolyte interphase (CEI) between the cathode and the electrolyte has been proven to competently elevate the overall electrochemical performance of LIBs. However, how to choose electrolyte additives that match different cathode systems ideally to achieve stable CEI layer construction and high-performance LIBs is still in the stage of repeated experiments and exploration. This article specifically introduces the working mechanism of diverse electrolyte additives for forming a stable CEI layer and summarizes the latest research progress in the application of electrolyte additives for LIBs with diverse cathode materials. Finally, we tentatively set forth recommendations on the screening and customization of ideal additives required for the construction of robust CEI layer in LIBs. We believe this minireview will have a certain reference value for the design and construction of stable CEI layer to realize desirable performance of LIBs.

Flexible, Self-Healing, and Fire-Resistant Polymer Electrolytes Fabricated via Photopolymerization for All-Solid-State Lithium Metal Batteries

The cyclophosphazene-based self-healing polymer electrolytes (CPSHPE) is designed and fabricated via the copolymerization of hexa(4-ethyl acrylate phenoxy) cyclotriphosphazene (HCP), (2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate) (UPyMA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) under UV irradiation. The cross-linking structure formed by HCP could effectively enhance the mechanical strength of the polymer electrolyte, and the cyclotriphosphazene as the core is able to improve the flame-retardant properties. Benefiting from the phenyl groups in HCP and the cross-linking structure, the CPSHPE shows high thermal stability (up to 300 °C). On the other hand, the supramolecular network fabricated by the dynamic ureido-pyrimidinone (UPy) dimers endows the polymer electrolyte with good self-healing capability and is expected to improve the reliability of polymer lithium batteries. Moreover, the cells were fabricated with LiFePO₄ (LFP), CPSHPE, and Li anodes show good reversible specific capacity. The CPSHPE could be a promising candidate as the multifunctional polymer electrolyte to improve the safety performance of lithium metal batteries.

Fluorinated Cyclic Phosphorus(III)-Based Electrolyte Additives for High Voltage Application in Lithium-Ion Batteries: Impact of Structure-Reactivity Relationships on CEI Formation and Cell Performance

Two selected and designed fluorinated cyclic phosphorus(III)-based compounds, namely 2-(2,2,3,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane (PFPOEPi) and 2-(2,2,3,3,3-pentafluoro-propoxy)-4-(trifluormethyl)-1,3,2-dioxaphospholane (PFPOEPi-1CF₃), were synthesized and comprehensively characterized for high voltage application in lithium-ion batteries (LIBs). Cyclic voltammetry (CV) and constant current cycling were conducted, followed by post mortem analysis of the NMC111 electrode surface via scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). To support and complement obtained experimental results, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were performed. Theoretical and experimental findings show that the considered phospholane molecule class enables high voltage LIB application by sacrificial decomposition on the cathode surface and involvement in the formation of a cathode electrode interphase (CEI) via polymerization reaction. In addition, obtained results point out that the introduction of the CF₃ group has a significant influence on the formation and dynamics of the CEI as well as on the overall cell performance, as the cell impedance as well as the thickness of the CEI is increased compared to the cells containing PFPOEPi, which results in a decreased cycling performance. This systematic approach allows researchers to understand the structure-reactivity relationship of the newly synthesized compounds and helps to further tailor the vital physicochemical properties of functional electrolyte additives relevant for high voltage LIB application.

Materials for lithium-ion battery safety

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.

Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li⁺/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.