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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
Comprehensive reference exploring innovative auxiliary materials as a variety of battery components to enhance battery performance, safety, and longevity
Functional Auxiliary Materials in Batteries: Synthesis, Properties, and Applications overviews the latest research on the applications of organic functional materials and low-dimensional structural materials as functional auxiliary materials in batteries. The book introduces the properties and preparation methods of these materials, summarizes the application mechanisms and conclusions, and puts forward novel insights and prospects towards more sustainable and environmentally friendly battery technologies.
The first five chapters of this book expand around the application of organic functional materials in batteries, including separators, binders, electrolytes, and functional additives. The last two chapters of this book expand around the application of low-dimensional structural materials in batteries, including conductive agents and functional additives.
Functional Auxiliary Materials in Batteries includes information on:
- Film forming, flame retardant, high voltage, and overcharge protection additives
- Adjusting factors in biopolymer materials such as molecular structure, composition, and morphology to precisely regulate and optimize battery performance
- Ionic liquids and single-ion conductors as a more secure and widely usable alternative to traditional organic electrolytes
- Self-healing materials, covering their positive effects on energy density, cost reduction, safety, and sustainability and their challenges including complexity and material compatibility
- Carbon-based materials that mitigate polysulfide shuttle effects and extend cycle life
Functional Auxiliary Materials in Batteries is an essential reference for new researchers seeking to quickly understand the progress of research in related fields. The book is also valuable for senior researchers seeking inspiration for innovation.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Application of Organic Functional Additives in Batteries
1.1 Introduction
1.2 Fluorinated Additives
1.3 Nitro Additive
1.4 Nitrile Additives
1.5 Phosphate Ester Additives
1.6 Sulfate Ester Additives
1.7 Conclusion and Outlook
References
2 Application of Biopolymers in Batteries
2.1 Introduction
2.2 Overview of Biopolymers
2.3 Application of Biopolymers in Binders
2.4 Application of Biopolymers in Electrolytes
2.5 Application of Biopolymers in Electrolyte Additives
2.6 Application of Biopolymers in Separators
2.7 Application of Biopolymers in Anode Functional Layers
2.8 Conclusion and Outlook
References
3A Application of Synthetic Polymers in Batteries: Carbon‐chain Polymers
3A.1 Introduction
3A.2 Overview of Synthetic Polymers Materials
3A.3 Application of Synthetic Polymers in Binders
3A.4 Application of Synthetic Polymers in Electrolytes
3A.5 Application of Synthetic Polymers in Battery Separators
3A.6 Application of Synthetic Polymers in Anodes
3A.7 Conclusions and Outlook
References
3B Application of Synthetic Polymers in Batteries: Hetero‐chain Polymers
3B.1 Introduction
3B.2 Overview of Synthetic Polymers Materials
3B.3 Application of Synthetic Polymers in Binders
3B.4 Application of Synthetic Polymers in Electrolytes
3B.5 Application of Synthetic Polymers in Battery Separators
3B.6 Conclusions and Outlook
References
4 Application of Nontraditional Organic Ionic Conductors in Batteries
4.1 Ionic Liquids
4.2 Application of ILs in Batteries
4.3 Single‐Ion Conductive
4.4 Application of Single‐Ion Conductive in Batteries
4.5 Conclusions and Outlook
References
5 Application of Self‐Healing Materials in Batteries
5.1 Introduction
5.2 Types of Self‐Healing Materials for Battery Applications
5.3 Applications of Self‐Healing Materials in Batteries
5.4 Conclusions and Outlook
References
6 Application of Low‐Dimensional Materials in Batteries
6.1 Introduction
6.2 Low‐Dimensional Composite Cathode Materials
6.3 Low‐Dimensional Composite Materials in Separators
6.4 Low‐Dimensional Composite Current Collectors
6.5 Low‐Dimensional Composite Anode Materials
6.6 Conclusion and Outlook
References
7 Applications of Porous Organic Framework Materials in Batteries
7.1 Introduction
7.2 Types of Porous Organic Framework Materials
7.3 Applications of Porous Organic Framework Materials in Batteries
7.4 Conclusion and Outlook
References
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Functions of fluorinated additives. (a) The dual‐layered film can...
Figure 1.2 Design concept of TMSP–FEC complex as a multifunctional electroly...
Figure 1.3 Schematic illustration of the reinforced mechanism of the LiTFA–L...
Figure 1.4 Multivalent electrolyte design realizes intrinsic high LiNO
3
solu...
Figure 1.5 Solutions for sacrificial NO
3
−
additives. (a) Schematic dia...
Figure 1.6 Functions of nitrile compounds. (a) Three transference mechanisms...
Figure 1.7 Solutions to improve the compatibility of nitrile compounds with ...
Figure 1.8 Functions of phosphate ester in electrolytes. (a) Schematic illus...
Figure 1.9 Solutions to optimize the compatibility of phosphate ester with a...
Figure 1.10 SEI and CEI forming properties of sulfate. (a) Schematics illust...
Chapter 2
Figure 2.1 Application diagram of biopolymers in battery.
Figure 2.2 Molecular structure of biopolymers mentioned in this chapter.
Figure 2.3 CS acts as a binder to limit electrode material expansion and iod...
Figure 2.4 Schematic illustrations of (a) binding role of lignin in wood, mi...
Figure 2.5 (a) Structure of traditional zinc button batteries and pouch batt...
Figure 2.6 (a) Synthesis route of L‐Li, the preparation method of L‐Li/PVDF ...
Figure 2.7 Schematic representation for the synthesis of ODGelMA hydrogel el...
Figure 2.8 Structural characterizations of the CNCs‐ZnSO
4
electrolyte. (a) S...
Figure 2.9 The structure and mechanical properties of the CNF membranes. (a)...
Figure 2.10 Construction of LbL‐(CS/SA)
4
film on Zn anodes. (a) Schematic il...
Chapter 3a
Figure 3A.1 Typical chemical structure of carbon‐chain polymers commonly use...
Figure 3A.2 (a) The advantages of PVA compared to PVDF. (b) The properties r...
Figure 3A.3 Expansion resistance and self‐healing mechanism diagram of PA co...
Figure 3A.4 Schematic of the microstructure of the PVDF electrolyte (a) and ...
Chapter 3b
Figure 3B.1 Typical chemical structure of hetero‐chain polymers commonly use...
Figure 3B.2 Preparation of in situ crosslinking binder. (a) Chemical interac...
Figure 3B.3 Structure and bonding mechanism of prepared PU‐based binder. (a)...
Figure 3B.4 Preparation mechanism of double cross‐linked poly(ionic liquid) ...
Figure 3B.5 Schematic illustration of PEO and PU sample at tension state wit...
Figure 3B.6 Schematic illustration of the functions of ZCNF for PEO‐based LM...
Chapter 4
Figure 4.1 Structures of some typical cations and anions of ILs: I: 1‐alkyl‐...
Figure 4.2 (a) Anion‐reinforced effect on the ionic conductivity and electro...
Figure 4.3 (a) Structure and composition of the IGEM. (b) Comparison between...
Figure 4.4 (a) Synthesis route of GPE; (b) Li plating mechanism in LE and F‐...
Figure 4.5 Synthesis route of dCOF‐NH
2
‐Xs.
Figure 4.6 Diagram of the procedure for UiO‐66‐IL based SSLMB [76]
Figure 4.7 Diagram of the fabrication (a), composition (b), and operation (c...
Figure 4.8 Structure and composition of anionic borate network polymer. (a) ...
Figure 4.9 (a) Diagram of SICPE membrane synthetic processes; (b) FTIR spect...
Chapter 5
Figure 5.1 Photograph of the PAM‐PEGMA‐IL ionogel and schematic illustration...
Figure 5.2 Schematic illustration of the synthesis of PAAm‐O‐B hydrogel elec...
Figure 5.3 (a) Fabrication of the PANa‐Fe
3+
electrolyte from acrylic aci...
Figure 5.4 Design and performance of full‐device autonomous self‐healing and...
Figure 5.5 (a) Supramolecular structure and dynamic hydrogen bonding for sel...
Figure 5.6 (a) Schematic of synthesizing poly(B‐GMA) by RAFT polymerization....
Figure 5.7 Schematic illustration of structures and functions of the self‐he...
Figure 5.8 (a) Schematic chemical structure of the SHP‐PEG binder. (b) Schem...
Figure 5.9 (a) Schematic of the self‐healing PDPP binder. (b) Self‐healabili...
Chapter 6
Figure 6.1 A typical charge/discharge profile for a Li–S battery.
Figure 6.2 (a) The design of N‐ACNT/G hybrids with graphene and aligned CNTs...
Figure 6.3 (a)
X
‐doped nanocarbon materials (
X
= N, O, F, B, P, S, and Cl) a...
Figure 6.4 (a) Synthesis procedure of the S@Mxe@PDA hybrid electrode.(b)...
Figure 6.5 (a) SEM image of the mesoC‐coating separator.(b) The flexibil...
Figure 6.6 Li deposition of the routine 2D Cu foil electrode and GF‐modified...
Figure 6.7 (a) The poling process of the BTO‐coated PE separator and the eff...
Figure 6.8 (a) 2D‐Al, (b) 2D‐GF and (c) 3D‐CNT current collectors for Li–S b...
Figure 6.9 SEM images of UGF and CNT‐UGF. Right is a schematic of the 3D int...
Chapter 7
Figure 7.1 Representative examples of the relationship between molecular con...
Figure 7.2 Schematic illustration of the synthetic route of NSM composite....
Figure 7.3 The illustration of significant enhancement of K
+
ion storage...
Figure 7.4 Simulated Na
+
ion diffusion within HOF‐DAT. (a) Schematic sho...
Figure 7.5 Schematic illustration for the configuration of cathode‐supported...
Figure 7.6 Design philosophy of the quasi‐solid electrolyte (a) Operation pe...
Figure 7.7 The left diagram shows a tri‐metallic (Fe, Ni, and Co) MOF‐74 str...
Figure 7.8 Schematic illustration of the synthesis of pyridine‐linked triazi...
Figure 7.9 The schematic diagram of a Li‐CO
2
battery.
Figure 7.10 Schematic illustration of the role and Li deposition behaviors w...
Figure 7.11 (a) Schematic synthesis of the sulfonated COFs, selective permea...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Functional Auxiliary Materials in Batteries
Synthesis, Properties, and Applications
Wei Hu
Author
Dr. Wei Hu
University of Science and Technology Beijing
No. 30 Xueyuan Road
Haidian District
Beijing 100083
China
Cover Image: © VectorMine/Adobe Stock
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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© 2025 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
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Print ISBN: 978‐3‐527‐35529‐7
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Preface
With the growing shortage of global energy and resources, the development of new energy materials has become the focus of worldwide attention. New energy materials through the transformation and utilization of traditional energy (such as solar energy, biomass energy, geothermal energy, wave energy, ocean current energy, and tidal energy, as well as the thermal cycle between the surface and deep layers of the ocean), as well as the development of new energy technologies, effectively solve the energy crisis problem, to achieve sustainable development. They can help to reduce environmental pollution, reduce greenhouse gas emissions, and protect the ecological environment by replacing traditional fossil fuels. For example, the use of renewable energy sources such as solar and wind reduces reliance on fossil fuels, thereby reducing air pollution and carbon emissions. The development of new energy technologies can effectively solve the problem of energy crisis to achieve sustainable development. The research and application of new energy materials require the cross and integration of multiple disciplines, which promotes the development of materials science and other basic disciplines. With the continuous progress of science and technology and the upgrading of industrial structures, the application of new energy materials will be more and more extensive, and the promoting role of economic development will be more and more obvious. The research and development of new materials will continue to promote the development of new energy technologies and provide strong support for solving global energy problems and environmental problems.
As a secondary energy source, electric energy is widely used because it is easy to convert into other forms of energy, such as heat, wind, potential energy, and kinetic energy. Furthermore, it also has the advantages of convenient transmission, easy production and use, clean, safe, and economical. These advantages make electric energy a very important and widely used form of new energy application in modern society. Batteries play a vital role in the supply, storage, conversion, and management of electrical energy; energy storage batteries and power batteries are two important types of batteries. Energy storage batteries are mainly used for the storage of electrical energy, widely used in solar power equipment and wind power equipment energy storage, as well as renewable energy storage; power batteries are mainly used for electric vehicles, electric bicycles, and other mobile equipment to provide power. The development and application of battery materials have greatly promoted the progress of human society, improved people's way of life, and laid a solid foundation for future scientific and technological development and environmental protection.
Batteries consist of positive and negative active materials and other auxiliary materials, which play a key role in the manufacture and performance of batteries. The active material is the core of the electrochemical reaction of the battery, responsible for the storage and release of energy. The auxiliary material provides the necessary structural support and transport pathway for the active material to ensure the smooth progress of the electrochemical reaction. In summary, the active material is the key to the electrochemical performance of the battery, and the auxiliary material provides the necessary structural and transport support for these active materials, which jointly determines the overall performance and application range of the battery.
Auxiliary materials in batteries mainly include separators, conductive agents, binders, current collectors, electrolytes and electrolyte additives, sealing materials, thermal insulation materials, protective layers, and encapsulating materials. The importance of auxiliary materials in batteries is reflected in the following aspects: (1) they can significantly improve the electrical conductivity and mechanical strength of the battery, thereby improving the overall performance and lifespan of the battery; (2) by enhancing the bonding force and stability between the electrode materials, they help to prevent the battery from short circuit and overheating during the charging and discharging processes, thus ensuring the safety of the battery; (3) they can increase the conductivity of the electrolyte, helping to form a stable solid‐electrolyte interface film and reduce the burning tendency of the electrolyte, thereby improving the rate performance, low‐temperature performance, and the cycle and thermal stability of the battery; (4) they protect the materials in the battery from adverse external factors through various forms, thereby improving the safety, reliability, and service lifespan of the battery. These auxiliary materials play a key role in the production process of batteries, and their performance and selection have an important impact on the performance, lifespan, and safety of batteries. Continuous research and innovation help to improve these auxiliary materials and improve battery performance and reliability. With the development of technology, it is expected that more new materials and auxiliary materials will be used for power batteries in the future.
This book focuses on functional auxiliary materials in batteries; although these components are not a direct source of battery energy, their presence and performance are critical to improving the overall performance of the battery. Organic functional materials and low‐dimensional structural materials as the common auxiliary material compositions are widely used in various components of batteries. The first five chapters of this book expand around the application of organic functional materials in batteries, they usually are used as separators, binders, electrolytes, and functional additives. The last two chapters of this book expand around the application of low‐dimensional structural materials in batteries, they mainly are used as conductive agents and functional additives. Specifically, the book is divided into the following main chapters:
Chapter 1, Application of Organic Functional Additives in Batteries: Electrolyte additives, as the “fine‐tuning agent” in the electrolyte system, can considerably improve the performance of both the electrolyte and the battery through the introduction of a small number of functional additives. There are many kinds of these additives, including film‐forming additives, flame retardant additives, high‐voltage additives, overcharge protection additives, and so on, which improve the performance of lithium batteries in different aspects, thus significantly improving the overall performance and safety of the battery.
Chapter 2, Application of Biopolymers in Batteries: Biopolymers and their derivative materials bring new development opportunities in batteries due to their unique structures and properties. These materials are of natural origin, have renewable and degradable properties, and show excellent energy storage and conversion potential. These materials have been successfully applied in various battery systems, such as lithium‐ion batteries and supercapacitors, through delicate chemical design and optimization, with remarkable results. Biopolymer materials are rich in designability and tunability, and by adjusting factors such as their molecular structure, composition, and morphology, they can precisely regulate and optimize battery performance. This provides a broad space and unlimited possibilities for the future development of the battery field.
Chapter 3, Application of Synthetic Polymers in Batteries: Synthetic polymer materials are widely used in batteries due to their diversity of structure and function. For example, they can be used as binders, electrolyte materials, separators, functional coatings, flame retardants, and active material carriers. Due to the specificity of the molecular structure, each polymer has its own unique physical and chemical properties. The advancement of polymer technology focuses on developing corresponding functions to serve the performance requirements of batteries through rational use of their properties. Through the functional composite of a variety of different polymers or the introduction of inorganic functional materials to modify polymers to achieve functional enrichment and integration, polymer materials can achieve diversified development in battery applications. These applications demonstrate the diversity and importance of synthetic polymers in battery technology, which can further optimize battery performance and drive the rapid development of battery technology through molecular design and materials engineering.
Chapter 4, Application of Nontraditional Organic Ionic Conductors in Batteries: Energy storage batteries play a crucial role in modern society, and the main challenge in this research area is how to efficiently transport ions. Traditional organic electrolytes provide good ionic conductivity; however, they also face some major challenges, especially security risks. Ionic liquids and single‐ion conductors are considered promising new organic ionic conductor candidates with higher security. Ionic liquids have outstanding physical and electrochemical properties, including high safety (nonvolatile, nonflammable), wide operating temperature range, wide electrochemical window, high ionic conductivity, and excellent compatibility with electrode materials, all of which are beneficial to battery performance. In addition to their excellent thermal and chemical stability, single‐ion conductors are characterized by their extremely high ion transport numbers, as they can selectively transfer only cations or anions. Their application as battery electrolytes or electrolyte additives has significantly improved the stability and safety of the batteries.
Chapter 5, Application of Self‐Healing Materials in Batteries: This chapter explores the use of self‐healing materials in batteries, emphasizing their potential to improve battery performance, safety, and lifespan. The chapter outlines the critical need for battery innovation, focusing on energy density, cost reduction, safety, and sustainability. Self‐healing materials, capable of autonomously repairing themselves, enhance battery life by addressing issues such as electrode cracking and internal damage during charge cycles. They also improve safety by preventing failures like fires or explosions. Despite their promise, challenges such as technological complexity, cost, and material compatibility must be overcome to facilitate large‐scale commercial applications, ensuring sustainable future energy solutions.
Chapter 6, Application of Low‐Dimensional Materials in Batteries: The integration of low‐dimensional composite materials in high energy density lithium‐metal batteries, such as lithium‐–sulfur batteries, significantly enhances performance across cathodes, separators, current collectors, and anodes. Carbon‐based materials like graphene and carbon nanotubes improve active material loading and interfacial stability, addressing volume expansion issues. Additionally, these materials can introduce lithiophilic sites to mitigate polysulfide shuttle effects, thereby extending cycle life. For inactive components, modified separators effectively block polysulfide migration while facilitating lithium‐ion transport. Furthermore, innovative designs in current collectors enhance conductivity and mechanical strength. Future research should prioritize optimizing structural design and material stability to boost energy density and overall battery efficiency.
Chapter 7, Applications of Porous Organic Framework Materials in Batteries: This chapter explores the application of porous organic framework materials in battery technology, covering metal‐organic frameworks (MOFs), covalent‐organic frameworks (COFs), and hydrogen‐bonded organic frameworks (HOFs). These materials, with their adjustable pore structures and superior chemical stability, exhibit excellent electrochemical performance, significantly enhancing the energy density and cycle life of lithium‐ion batteries, sodium‐ion batteries, and zinc‐air batteries. By optimizing pore design and functionalization, porous organic framework materials not only improve the performance of electrode materials but also serve as electrolyte additives and catalyst supports, driving the development of next‐generation, high‐efficiency, and safe energy storage technologies.
In each chapter, the commonly used functional auxiliary materials in batteries are systematically introduced, including their structure, properties, application progress, and roles, as well as corresponding mechanisms in different systems.
In summary, auxiliary materials in batteries play a crucial role in improving battery performance and safety, reducing costs, and promoting technological innovation. I hope this book will be a tool book for applying organic functional materials and low‐dimensional structural materials in batteries by providing a comprehensive summary of the progress in these fields. It not only introduces the properties and preparation methods of these materials but also summarizes the application mechanism and conclusions, and puts forward some insights and prospects. It can help new researchers quickly understand the progress of research in related fields, and also can help senior researchers summarize research experience and get inspiration for innovation. With the further development of battery technology and changes in market demand, the research and application of auxiliary materials will continue to deepen.
November 2024
Wei Hu
USTB, Beijing, China
1Application of Organic Functional Additives in Batteries
1.1 Introduction
Driven by energy transformation and environmental protection, battery technology has received unprecedented attention as key to energy storage and conversion. Lithium‐ion batteries (LIBs) are mainly composed of electrolytes, cathodes, and anodes, of which, for liquid electrolytes, separators are often used as supporting materials. As one of the critical materials for battery manufacturing, the electrolyte is mainly used to construct an ion transport channel between cathodes and anodes inside the battery and is a medium for lithium‐ion migration and charge transfer (CT) and known as the “blood” of LIBs.
However, with the continuous progress of battery technology, electrolyte performance requirements are also increasing. The traditional electrolyte system has been gradually challenged to meet the needs of modern high‐performance batteries, especially in improving energy density, prolonging cycle life, enhancing safety, and other challenges. To address these issues, research on battery electrolyte additives has emerged as a significant area for the advancement of battery science and technology.
Electrolyte additives, as the “fine‐tuning agent” in the electrolyte system, can considerably improve the performance of the electrolyte and the battery by introducing a small number of functional additives. Therefore, employing functional additives in the electrolyte is a crucial way to improve the performance of the battery. Additives used in electrolytes need to meet the following primary conditions: (i) soluble in organic electrolytes; (ii) no apparent side effects with other components of the battery; (iii) small dosage and remarkable effect; (iv) no toxicity or negligible toxicity; and (v) low price.
Currently, some widely used additives are fluorinated compounds, nitro compounds, nitrile compounds, phosphate ester compounds, and sulfate ester compounds. This chapter describes the functions of these compounds in batteries in detail and discusses the possible drawbacks of the additives and their solutions.
1.2 Fluorinated Additives
1.2.1 Functions of Fluorinated Additives
1.2.1.1 Improvement of Safety Performance
Fluorinated electrolytes, such as fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and methyl(2,2,2‐trifluoroethyl) carbonate (FEMC), have a relatively lower heat release and a higher onset and peak temperature compared to nonfluorinated carbonates, which results in improved safety performance. Zhang et al. [1] used differential scanning calorimetry (DSC) to evaluate the thermal stability of various commonly used electrolytes and summarized the solvent exothermic phase diagram. The results of the self‐extinguishing time (SET) tests have shown that fluorinated electrolytes such as bis(2,2,2‐trifluoroethyl) carbonate (TFEC) have better thermal stability as well as flame retardancy. In addition, Meng et al. [2] used fluorinated electrolytes, such as fluoromethyl 1,1,1,3,3,3‐hexafluoroisopropyl ether (HFE), to reduce flammability and improve the safety performance of liquid electrolytes.
1.2.1.2 SEI‐Forming Additives
Due to the strong electron absorption characteristics of the F functional group, the fluorinated electrolyte's lowest unoccupied molecular orbital (LUMO) is generally lower than that of the ordinary electrolyte, which means that the fluorinated electrolyte will preferentially react with the anode to generate LiF over other solvents [3]. The high surface energy and low diffusion barrier of LiF are conducive to promoting the rapid distribution of Li+ parallel to the interface to achieve uniform deposition of lithium, which is considered the most favorable inorganic component in the solid electrolyte interface (SEI) [4, 5]. In addition, LiF is a wide‐bandgap insulator that prevents electrons from tunneling through the SEI [6]. The formation of dense and uniform inorganic‐rich SEI film, on the one hand, can effectively inhibit the further reaction between the anode and the electrolyte [7, 8]. On the other hand, it can improve the reversibility of the anode to form a stable Li+ plating and stripping process and inhibit the growth of lithium dendrites [9].
Liao et al. [10] observed the presence of LiF in the SEI formed by FEC‐added electrolytes using X‐ray photoelectron spectroscopy (XPS) and confirmed that FEC with low LUMO has a preferential reduction on lithium metal. In addition to lithium metal anodes, Shin et al. [11] and Li et al. [12] found that FEC on the surface of commercial graphite anodes would also reduce to form a dense, uniform, and LiF‐rich SEI, thus enabling a stable graphite–electrolyte interface under low‐temperature cycling and conferring excellent cycling stability to the battery. Guo et al. [13] combined classical molecular dynamics (CMD) simulation and XPS analysis of the anode to demonstrate that FEC contributes to the formation of a stable and fluorine‐rich SEI and improves the reversibility of Na metal. In addition to the widely studied FEC, fluorinated carbonates such as difluoro‐substituted DFEC [14] and hexafluoro‐substituted TFEC [1] are also considered effective solvents for the construction of fluorinated SEIs. The decomposition of DFEC and TFEC can similarly produce the inorganic component LiF to form a dense and homogeneous fluorinated SEI.
The dense SEI formed from the fluorinated electrolyte can inhibit further reactions between the electrolyte and the anode. Nitrile compounds, such as succinonitrile (SN), have been widely used in electrolytes due to the benefits of high ionic conductivity and high oxidation stability, but –C≡N is highly reactive and easily reacts with lithium metal, so it is quite essential to impede the contact between –C≡N and lithium metal. The LiF‐rich protective layer formed by the degradation of FEC on the surface of lithium metal can effectively inhibit the side reaction between the cyano group and lithium metal, thus improving the cycle performance of the battery [15].
In addition to the decomposition of fluorinated additives to form LiF to change the composition of the SEI, some studies have also shown that the addition of fluorinated electrolytes will change the structure of the SEI. Li et al. [16] observed individual lithium metal atoms and their interfaces with SEI using cryo‐electron microscopy and found that the SEI nanostructure formed with FEC was ordered and multilayered: the inner layer was an amorphous polymer matrix, and the outer layer was Li2O with a large grain size (∼15 nm). In contrast, the SEI without FEC is a mosaic structure. The authors concluded that the SEI with multilayer nanostructures has better mechanical stability and is more favorable for a homogeneous lithium plating–stripping process. In contrast, the mosaic‐structured SEI has poor mechanical properties due to the random distribution of inorganic substances, resulting in possible fracture during cycling and the formation of dead lithium due to the uneven distribution of its grains. In addition, Aurbach et al. [6] suggested that the addition of fluorinated electrolytes, such as FEC, will form a highly cross‐linked polymer network on the lithium metal surface, which accommodates the volumetric changes in the contraction and expansion of the lithium metal for lithium deposition and dissolution during cycling.
Based on the outstanding properties of fluoride, some researchers have also used fluoride to modify the lithium metal to improve the performance of the battery. Yan et al. [17] constructed a double‐layer film structure, with an outer layer dominated by the organic constituents (ROCO2Li and ROLi) and an inner layer dominated by the inorganic constituents (LiF and Li2CO3) on the Li metal surface through the spontaneous reaction of lithium metal in FEC, as shown in Figure 1.1a. The double‐layer membrane structure achieves columnar deposition of lithium by pre‐forming uniform deposition nucleation sites. Through the characterization of the morphology of lithium metal after cycling, it is found that the surface of the pretreated lithium metal was smooth and dense, and the lithium deposition was uniform, which inhibited the formation of lithium dendrites. Kim et al. [3] pretreated metal fluoride (MxFy) onto lithium metal at a low annealing temperature, and MxFy reacted and decomposed with Li to form metallic M nanoparticles and robust LiF inorganic compounds. The metal M nanoparticles formed a uniform Li–M alloy phase with the Li metal, which can promote the formation of a uniform LiF interphase layer and the uniform diffusion of Li, as shown in Figure 1.1b.
Figure 1.1 Functions of fluorinated additives. (a) The dual‐layered film can regulate the uniform deposition of Li ions during repeated charge/discharge cycles and protect the Li metal anode without dendrite formation.
Source: Reproduced from Yan et al. [17]/with permission of Wiley‐VCH Verlag GmbH & Co. KGaA.
(b) Schematic illustration of in vitro interphase evolution employing the conversion reaction of metal fluorides (top) and Li plating/stripping of Li–M alloy with the LiF outermost layer (bottom).
Source: Kim et al. [3]/American Chemical Society/CC By NC‐ND 4.0.
(c) The Li+ solvation structures and corresponding desolvation energies of EC + DMC (left) and 45% IF (right) electrolytes.
Source: Reproduced from Liu et al. [18]/with permission from Royal Society of Chemistry.
However, at the same time, Kim et al. [3] also proposed that the inorganic‐rich SEI formed by organic electrolytes with FEC additives on the surface of Li metal is mainly composed of Li2O rather than LiF and retains an inhomogeneous mosaic‐type structure. Tao et al. [19] quantitatively researched the evolution of inactive lithium in lithium‐free anode batteries with different electrolytes using mass spectrometry titration and nuclear magnetic resonance (NMR) spectroscopy techniques and found that FEC itself could inhibit the formation of dead lithium metal, but the correlation between LiF formed by the reduction of FEC and dead lithium metal or SEI was weak. Therefore, the assertion that FEC can form a dense and uniform LiF‐rich SEI layer needs to be further verified.
1.2.1.3 High Oxidation Stability
Similarly, due to the electron‐withdrawing property of the F functional group, the highest occupied molecular orbital (HOMO) of the fluorinated electrolyte is low, indicating high oxidation stability, which can be matched with the high‐voltage cathode [20]. Wang et al. [21] proposed a perfluorinated electrolyte FEC/FEMC through potentiostatic testing of graphite||lithium battery and found that the fluorinated electrolyte showed lower leakage current, which reflects the high‐voltage stability of fluorinated electrolyte. The composition and morphology of the cathode electrolyte interface (CEI) layer formed on the recycled graphite revealed that the CEI formed by the fluorinated electrolyte contained less C=O, ROCO2Li, LiF, and LixPOyFz, suggesting that the reaction between the electrolyte and the graphite cathode was reduced due to the high oxidation stability of the fluorinated electrolyte. It was also observed by transmission electron microscope (TEM) that the fluorinated electrolyte formed a thinner and more uniform CEI.
Although fluorinated electrolytes have relatively low HOMO and are generally challenging to undergo oxidation, some researchers have found that the addition of fluorinated electrolytes may also participate in the formation of CEI. Lu et al. [22] simultaneously introduced 1H,1H,5H‐perfluoropentyl 1,1,2,2‐tetrafluoroethylether (F‐EAE) and FEC into the organic electrolyte of the LiNi0.5Mn1.5O4 (LNMO)‐based battery. The electrochemical floatation tests showed that the addition of fluorinated electrolytes significantly reduced the leakage current and improved the oxidation stability of the electrolyte. TEM also revealed that a uniform and thin passivation layer was formed on the surface of the LNMO cathode with the fluorinated electrolyte. The synergistic effect of the two fluorinated electrolytes formed a modification of the cathode, which could prevent the direct contact between the electrolyte and the active cathode particles and further inhibit the dissolution of the transition metal ions. The XPS results showed that the CEI of the fluorinated electrolyte contained a higher content of F and P substances, and the antioxidant capacity was improved, thus increasing the reversible capacity and the cyclic stability performance of the LNMO cathode. Guo et al. [13] found that the CEI formed by the electrolyte with FEC was thinner and denser. It was also found that this CEI was enriched with NaF by XPS, indicating that FEC was also involved in the formation of CEI, thereby protecting the cathode interface. Nagarajan et al. [23] investigated the composition of CEI at different depths by energy‐tunable synchrotron‐based hard X‐ray photoelectron spectroscopy, and it was also found that the addition of FEC to the carbonate electrolyte was also beneficial in improving the film‐forming ability of the cathode.
1.2.1.4 Promotion of the Formation of Anion‐Rich Solvation Structure
Compared with the widely used carbonate electrolytes and ether electrolytes, fluorinated electrolytes have a weaker solvation ability. Moreover, the stronger electron‐withdrawing ability of fluorine atoms can contribute to the distribution of negative charges, lowering the lattice energy of the salt and facilitating the dissolution of lithium salts in the solvent [24]. Su et al. [25] systematically explored the solvation ability of different electrolytes and found that fluorinated electrolytes have lower solvation ability and are less favorable for coordination with Li+ compared to their nonfluorinated counterparts, but weak solvation can induce more anions to participate in the solvated structure. Zhang et al. [1] compared the solvation ability of ethyl methyl carbonate (EMC), FEC, and TFEC with Li+, and the results showed that the binding energy with Li+ decreased sequentially with the increase of the degree of fluorination, indicating that TFEC is a weakly solvating solvent and rarely participated in the solvation structure of Li+. With the introduction of TFEC, the interaction of Li+ with the solvent is weakened, but the interaction with the anion is enhanced, again proving that the fluorinated electrolyte can promote the formation of an anion‐rich solvation structure. Liu et al. [18] also proposed that the ester‐based electrolyte exhibited a weak solvation structure with a low coordination number at low temperatures, and the FEC was free and hardly coordinated with Li+, as shown in Figure 1.1c.
In addition, some researchers have suggested that the reason why FEC can decompose to form LiF‐rich SEI is also related to the participation in solvation structure. Chen et al. [26] suggested that FEC was selected as the internal solvation complex, thus forming the fluorinated SEI. Su et al. [27] systematically explored the solvation pattern of the SEI‐forming agent, FEC, in the electrolyte system and found that the Li+ was solvated by at least one FEC molecule on average to ensure the formation of stable SEI. If the solvation number of FEC is <1, other organic electrolyte molecules coordinated with Li+ would decompose to form unfavorable SEI. Only when the solvation number of FEC is ≥ 1, almost all lithium complexes can be preferentially reduced during the formation process to construct fluorinated SEI.
1.2.1.5 Reduction of Desolvation Barrier
The desolvation energy of lithium is also reduced due to the weak solvation of the fluorinated electrolyte. Therefore, the addition of fluorinated electrolytes is expected to reduce the desolvation energy and accelerate the transport kinetics of Li+ in SEI [28]. Zhang et al. [20] showed that for the electrolyte, from nonfluorinated to perfluorinated electrolyte, the binding energy of solvent molecules to Li+ decreased, leading to a lower coordination number. The perfluorinated electrolyte optimized the Li+‐dipole structure and accelerated the desolvation process of solvated Li+, which resulted in the generation of SEI with low transport resistance during the plating/stripping process.
1.2.2 Synergies of Fluoroethylene Carbonate with Other Compounds
As the most widely used fluorinated electrolyte, FEC has many advantages. Furthermore, FEC may also have synergistic effects when used with other electrolytes to improve the electrochemical performance of the battery.
1.2.2.1 Fluoroethylene Carbonate and Other Fluorinated Electrolytes
Both FEC and DFEC are fluorinated electrolytes with excellent lithium anode stability. DFEC further reduces the solvation ability of carbonyl oxygen due to the two strong electron‐withdrawing fluorine atoms located on both sides of the carbonate [29], so the solvation energy of DFEC is even lower compared to FEC. As DFEC contains two fluorine atoms, its LUMO is also further reduced, which preferentially undergoes reduction on lithium metal over FEC. Aurbach et al. [6] found that in high‐voltage Li||NMC batteries, in the presence of only FEC, the oxidative decomposition products of the electrolyte diffused from the cathode and ultimately to the lithium metal anode, which produced a thicker and more resistive surface film. However, in the presence of both FEC and DFEC, the DFEC with lower LUMO decomposed and passivated on the Li anode, and then, the FEC acted as a healing agent to continuously “repair” the SEI on the Li anode in the subsequent cycles, which reduced the consumption of FEC.
The bis(trifluoroacetyl)amine (BTFA) molecule has lower LUMO with more fluorine atoms than FEC. Therefore, BTFA will preferentially decompose over FEC to form fluorinated SEI. Wang et al. [30] designed an in situ generation of an atomically rooted SEI (R‐SEI) based on the synergistic interaction of BTFA and FEC. The results of ab initio molecular dynamics (AIMD) simulations showed that in the presence of only FEC, the F atoms generated from the C–F bonds in FEC preferred to stay on the surface of Na (110); in the presence of only BTFA, the F atoms within the molecule were released into the Na interior within 200 fs. However, in the presence of both BTFA and FEC, BTFA induced the F atoms of FEC to enter the inner layer of Na, forming a vertical fluorine concentration gradient. The XPS results showed that the content of inorganic species in R‐SEI continued to increase with the sputtering depth, while the content of organic species continued to decrease, suggesting that the solution containing both BTFA and FEC realized a large number of F atoms implanted from the outer layer to the inner layer, forming a multilayer SEI, which was conducive to the improvement of the cycling stability performance of batteries.
1.2.2.2 Fluoroethylene Carbonate and Lewis Base
FEC is a Lewis acid that can accept electron pairs from a Lewis base in the electrolyte environment to form a Lewis acid–base complex. The complex not only can retain its respective functions but also have some synergistic effects. Yang et al. [31] introduced Lewis acid, FEC, and Lewis base, tris(trimethylsilyl) phosphite (TMSP), in carbonate electrolyte, where FEC reacted with TMSP by in situ complexation to form a TMSP–FEC complex. TMSP can be used as an impurity scavenger and a CEI‐forming additive, while FEC is an SEI‐forming additive, and the two synergistically formed an inorganic–organic composite (F/P/Si‐rich SEI) and a highly stable CEI. TMSP–FEC complex effectively protected the cathode and the anode and improved the comprehensive performance of the battery, as shown in Figure 1.2.
Figure 1.2 Design concept of TMSP–FEC complex as a multifunctional electrolyte additive for lithium metal batteries (LMBs).
Source: Reproduced from Yang et al. [31]/with permission of Elsevier.
1.2.2.3 Fluoroethylene Carbonate and Glyme
FEC is thought to reduce to form LiF‐rich SEI during the cycling process, but since its structure is similar to that of vinyl carbonate, it is expected to produce some carbon‐rich organic substances, such as ROCO2Li. However, too much carbon‐rich organic substance may cover up the LiF‐rich SEI formed earlier. As a high donor number (DN) solvent, Glyme has a solubilizing effect on substances rich in C–C–O [32]. Therefore, a certain amount of diethylene glycol dimethyl ether (G2) can dissolve undesired carbon‐rich substances on SEI, thereby increasing the content of inorganic compounds in SEI and stabilizing the composition of SEI. Biswal et al. [33] introduced both FEC and G2 into a carbonate‐based electrolyte and demonstrated using XPS that the addition of G2 reduced the contents of Li2CO3 and C–C–O in SEI, decreasing the activation energy of SEI and CT and increasing the SEI diffusivity and exchange current. In summary, the synergistic effect of FEC and G2 can form a stable fluorinated SEI rich in polyene networks, promote the migration of Li+ in SEI, and facilitate the uniform deposition of Li+.
1.2.3 Drawbacks of Fluoroethylene Carbonate
1.2.3.1 Generation of HF Gas
Under the catalysis of Lewis acid PF6− or high temperature, FEC is susceptible to dehydrofluorination to produce hydrogen fluoride (HF). On the one hand, the by‐product HF will destroy the SEI film, and in the battery with silicon anode, it will corrode silicon particles [34]; on the other hand, HF will lead to the formation of a thicker CEI and catalyze the dissolution of Mn in the case of ternary cathode [11], in addition to corroding the collector aluminum foil [35]. Thus, the generation of HF gas causes irreversible damage to both the anode and cathode, ultimately resulting in a degradation of the cycling performance of the battery.
1.2.3.2 Increase of Impedance and Loss of Impedance
Some researchers have argued that although FEC facilitates the formation of SEI at the anode, it forms a thicker CEI film on the surface of the cathode, which increases the resistance and capacity loss of the battery. Yang et al. [35] found that the initial capacity of Li||LiCoO2 (LCO) batteries was significantly reduced after the addition of the FEC additive, which attributed to the excessive decomposition of FEC and resulted in the formation of a thick interfacial layer on the surface of the cathode that hindered the lithium‐ion transport.
1.2.3.3 Incompatibility with Other Electrodes
FEC is widely used on lithium metal and silicon anodes, but FEC cannot form stable SEI on graphite anodes. Shen et al. [36] studied the SEI formed on graphite anodes in different electrolyte systems and found that the SEI formed with the addition of FEC was thicker and denser than that formed with the addition of ethylene carbonate (EC) under the same conditions. Xia et al. [37] investigated the compatibility of FEC‐based electrolytes with graphite anodes for the first time. However, the FEC additive was unable to form a protective SEI on the graphite surface because the introduction of F atoms lowered the LUMO of FEC, resulting in a higher reduction potential of FEC than its fluorine‐free counterpart EC. However, lithium bis(oxalate)borate (LiBOB) and lithium difluoro(oxalato) borate (LiDFOB), as SEI‐forming additives introduced to the electrolyte, could effectively inhibit the reduction of FEC, forming a thin and robust SEI on the graphite anode.
In addition to graphite anodes, for some high‐voltage cathodes, FEC may likewise be detrimental to the long‐term stable performance of batteries. Aktekin et al. [38] investigated the effect of the FEC additive on LNMO‐Li4Ti5O12 (LTO) cells. The XPS results indicated that with the increase in the FEC content of the electrolyte, the thickness of the formed CEI increased, and the content of organic substances containing C–C–O also increased, contrary to previous results of the formation of LiF‐rich SEI. Cycling and rate performance tests also showed that the addition of FEC did not improve the discharge capacity and the cycling stability of the battery but rather negatively affected the long‐term stability of the battery system with a high‐voltage LNMO cathode.
1.2.3.4 Recycling Issues
Due to their extreme persistence and challenge in biodegrading, halogenated organic pollutants pose a major threat to human health, the ozone layer, and ecological safety when released into the environment. Fluorine is one of the halogen elements with the most environmental impact among them. Therefore, fluorinated additives are detrimental to the environment in the long run. Consequently, it is imperative to both strictly recycle and dispose the used batteries with fluorinated electrolytes and to design and develop new types of fluorine‐free electrolytes that will be more conducive to environmental safety and battery recycling [24].
1.3 Nitro Additive
1.3.1 Functions of Nitro (NO3−)
1.3.1.1 Participation in Solvation and Desolvation Structures
LiNO3 is a lithium salt with a high DN (22.2 kcal mol−1) [39]. The binding energy of NO3− to Li+ is usually higher than that of conventional lithium salt anions as well as solvent molecules [10]. Therefore, when LiNO3 is dissolved into the electrolyte, NO3− is able to expel the solvent molecules from the solvation sheath and preferentially participates in the lithium‐ion inner solvation structure, forming an anion‐rich solvation environment [40–43]. Zhu et al. [44] confirmed that the addition of NO3− altered the solvation structure of Li+ through Raman spectra, confirming a decrease in the number of both vinylene carbonate (VC) and dimethyl carbonate( DMC) in the solvation structure, as well as a change in the intensity of the peaks in the infrared spectrum. In addition, Wahyudi et al. [45] found that the addition of NO3− shifted the peaks of 7Li NMR toward more positive values, demonstrating the shielding of the electron cloud around Li+, indicating a larger solvated cluster scale, as well as the entry of the electron‐donating anion, NO3−, into the solvation sheath of Li+.
Furthermore, NO3− enhances the interaction of TFSI− with Li+. In the presence of NO3−, the number of free TFSI− in the electrolyte decreases due to an increase in the number of TFSI− participating in the Li+ solvation process. Wahyudi et al. [45] detected the presence of the TFSI− anion aggregated ion pairs at 747 cm−1 in Raman spectroscopy, indicating that NO3− enhances the TFSI–Li+ interaction. Fu et al. [46] also demonstrated using Fourier transform infrared (FTIR) and Raman spectra that the addition of NO3− caused the peaks to be blueshifted, suggesting that TFSI− existed more in the contact ion‐pair (CIP) or aggregated state.
High‐concentration electrolyte (HCE) is considered to preferentially compete with solvent molecules due to the increase in salt concentration, forming an anion‐rich solvation structure [47]. However, as the salt concentration in HCE increases, the electrolyte becomes more viscous. To improve the overall performance of the electrolyte, diluents such as hydroflurane need to be added to reduce the viscosity to form localized high‐concentration electrolytes (LHCEs). However, the aforementioned measures undoubtedly increase the cost of electrolytes [48, 49]. In contrast, the application of NO3− additives not only enables NO3− itself to participate in the solvation of Li+ preferentially but also promotes the interaction between TFSI− and Li+, thus achieving an anion‐rich solvation environment while ensuring low cost and hardly changing the viscosity of the electrolyte, which is a favorable pathway to realize anion‐derived interfacial chemistry.
Additionally, NO3− not only participates in the formation of solvation structure but also modulates the distance between the Li+–solvent–anionic complex and the electrode surface to regulate the properties of the electrolyte and stability of the electrode, thus in turn affecting the thermodynamic and kinetic properties of the Li+–solvent–anionic complex during desolvation process at the electrode interface [42]. Anion‐rich solvation structures can lower the desolvation energy barrier and promote the Li+ desolvation behavior [41]. Stuckenberg et al. [50] demonstrated that higher oxidation currents with LiNO3 in cyclic voltammetry (CV) tests imply faster kinetics of the lithium electrodeposition/dissolution process, that is, the Li+ desolvation behavior is effectively enhanced. The additive can change the solvation structure and interface model to promote the desolvation of Li+. The “distance” between the Li+–solvent–anionic complex and the electrode surface is also a crucial aspect that affects the stability of the electrolyte and electrode.
1.3.1.2 Formation of Inorganic‐Rich SEI
Solvation structures are considered to be precursors of SEI, so anion‐rich solvation structures have a greater tendency to form anion‐derived SEI. However, it is generally believed that the solvent‐rich solvation structures are more likely to generate organic‐rich SEI, and such organic substances tend to have low ionic conductivity and mechanical properties, which is detrimental to the stability of SEI. The anion‐rich solvation structures are more likely to generate inorganic‐rich SEI with superior ionic conductivity and mechanical properties to form a stable SEI [51].
LiNO3 with low LUMO can preferentially undergo a reduction reaction at the anode over solvent molecules [52]. Reduction reactions tend to go through two processes [53]:
Eight electrons from lithium metal are required for complete decomposition, and the reduction decomposition of NO3− anion can lead to more LiNxOy components in the anode interface [54, 55]. Ma et al. [52] confirmed the occurrence of the aforementioned reactions by XPS characterization of SEI, where Li3N, LiNxOy, and Li2O were observed simultaneously, generating inorganic‐rich and robust SEI to achieve better protection of lithium electrodes. Thus, an inorganic‐rich SEI layer was formed by the reduction decomposition of LiNO3 with low LUMO. Since NO3− enhances the interaction of TFSI− with Li+, more TFSI− is involved in the solvation structure and ultimately decomposed on the SEI to produce the favorable inorganic component LiF. Zhang et al. [56] found that the content of Li3N and LiF in the SEI was increased after the introduction of LiNO3 using XPS, indicating that LiNO3 may promote the formation of the interface layer rich in Li3N–LiF. It was further verified that the introduction of NO3− also promoted the decomposition of TFSI−. Additionally, the authors simulated the structural configuration changes near the Li anode by AIMD to elucidate the potential mechanism of LiF formation. It was found that the bond lengths of both C–S and C–F bonds were elongated from 0 to 20 ps after the addition of LiNO3 compared to those without LiNO3, which reduced the energy required for bond breaking and accelerated the decomposition of lithium bis(trifluoromethanesulphonyl)imide (LiTFSI). It was demonstrated that LiNO3 led to the breakage of C–S and C–F bonds of LiTFSI molecules, inducing a large amount of LiF generation in the SEI.
Zhu et al. [44] also succeeded in forming a lithium‐indium alloy on the lithium metal anode surface with the introduction of In(NO3)3 additive to change the composition of SEI and improve the electrochemical performance of the battery. Kim et al. [57] introduced both AgNO3 and LiNO3 as electrolyte additives to construct SEI with a lithophilic inner layer and a compositionally regulated outer layer on the lithium metal surface sequentially according to their LUMO energy levels. AgNO3 preferentially deposited on the lithium metal surface to form Ag and Ag2O due to the lower LUMO energy level (−3.185 eV) to form an inner layer of Ag‐based SEI, which significantly reduced the overpotential of the full battery Li||NCM84.
Due to the high ionic conductivity of the decomposition products of NO3−, the transport of Li+ in SEI and the diffusion kinetics are improved [58]. Inorganic components with high ionic conductivity can effectively reduce lithium nucleation overpotential, leading to larger grain size, which will grow laterally at low density. This growth mode is conducive to reducing the formation of “dead lithium” during lithium plating/stripping, inducing uniform lithium deposition and inhibiting the growth of lithium dendrites [52, 59, 60]. Liu et al. [61] found that lithium deposition on the Cu surface without LiNO3 was loose dendrite, while lithium deposition with LiNO3 was in the form of dense lumps with Li particles growing along the planar direction, indicating that NO3− can induce uniform lithium deposition behavior. The inorganic‐rich SEI with outstanding ionic conductivity and mechanical strength can reduce the interface impedance, improve the interface contact performance, ensure the stability of the SEI [10, 62], and inhibit the decomposition of solvent molecules [46, 63].
1.3.1.3 CEI‐Forming Additives
LiNO3 can also decompose on the cathode surface to form the CEI to ensure the stability of the cathode and the electrolytes. Fu et al. [46] determined that the addition of LiNO3 resulted in the appearance of small oxidation peaks at approximately 5.2 V using linear sweep voltammetry (LSV), which corresponded to the decomposition of LiNO3. XPS analysis of the CEI showed that the electrolyte with LiNO3 formed a CEI film with a higher F content compared to the electrolyte without LiNO3. In the subsequent static leakage current test, the steady‐state oxidative decomposition current of the electrolyte with LiNO3 was lower than that of the electrolyte without LiNO3, which demonstrated that the CEI film with LiNO3 could inhibit the decomposition of the electrolyte on the cathode surface of the NMC811 and reduce the accumulation of high‐resistance decomposition products. The CEI membrane impedance (RCEI) and charge transfer impedance (Rct) of the electrolyte with LiNO3 were lower, indicating that LiNO3 accelerated CT kinetics.
Zhu et al. [44] also found that the addition of LiNO3 altered the components of CEI and inhibited the further decomposition of the subsequent electrolyte using XPS. Fang et al. [41] conducted XPS and TEM to clarify the chemical composition and structure of the CEI layer and observed the presence of LiNxOy, a fast lithium‐ion conductor, in the CEI layer. Similar to the SEI, the inorganic‐rich CEI layer also significantly promoted the interfacial ion diffusion behavior. In addition, only a slight M–O signal was detected in the experimental group with the addition of LiNO3, whereas a stronger M–O signal peak appeared in the control group, indicating that the LiNO3 additive can not only inhibit the decomposition of the electrolyte but also inhibit the dissolution of the transition metal by hindering the vicious reaction between the solvent and the cathode to ensure the structural stability of the cathode.
LiNO3 also contributes to the stability of the cathode interface through the formation of an electric double layer (EDL). Wen et al. [64] found that NO3− exhibited a distinct voltage response effect, that is, it would be enriched at the cathode interface once the cathode was charged, thus forming Li+‐enriched, thermodynamically favorable EDL with solvent molecules well‐coordinated (Figure 1.3). The EDL dramatically accelerated the interfacial reaction kinetics and significantly improved the thermodynamic compatibility between carbonate electrolytes and high‐voltage LiTiMnO (LTMO) cathodes.
Figure 1.3 Schematic illustration of the reinforced mechanism of the LiTFA–LiNO3 dual‐salt additive on conventional carbonate electrolyte.
Source: Reproduced from Wen et al. [64]/with permission of Wiley‐VCH Verlag GmbH & Co. KGaA.
1.3.1.4 Functions in Lithium–Sulfur Batteries
NO3− is also an indispensable additive in lithium‐sulfur batteries. It was found that LiNO3 could catalyze the conversion of soluble lithium polysulfide (LiPS) to slightly soluble sulfur as a redox intermediate on the cathode near the end of the charging process (>2.5 V vs. Li/Li+), which inhibited the generation and deposition of polysulfide (PS) and prevented it from dissolving into the electrolyte [39, 45, 65]. Meanwhile, LiNO3 will have a coupling reaction with LiPS to generate a dense passivation layer of LiNxOy and LiSxOy on the surface of the lithium metal anode, inhibiting the side reaction of LiPS on lithium metal, reducing the formation of dendrites from PS shuttles, and accelerating the redox kinetics [66].
Kim et al. [67] discovered that NO3− can inhibit PS agglomeration through strong coordination with Li+. The authors investigated the low‐temperature discharge behavior and found that the second discharge plateau of the electrolyte without LiNO3 disappeared, indicating that the low‐order PS formed before the second voltage plateau could not be further reduced to solid Li2S. The researchers regarded PS aggregation as the conversion block from LiPSs to Li2S. In contrast, the electrolyte with LiNO3 showed excellent discharge behavior, and the second voltage plateau of the electrolyte became longer with the increase in LiNO3 content. It suggests that the electrolyte with a high DN value of LiNO3 can promote the conversion of LiPS to Li2S at low temperatures and reduce electrode passivation. Furthermore, the authors demonstrated by density flooding theory (DFT) calculations and molecular dynamics (MD) simulations that anionic NO3− with high DN can inhibit PS agglomeration and promote redox kinetics at low temperatures by strongly coordinating with Li+, thus improving the low‐temperature performance of the electrolyte.
1.3.1.5 Stabilization of Water Molecules
Zhang et al. [68] found that LiNO3 can stabilize water molecules through strong hydrogen bonding interactions and inhibit the hydrolysis of PF6− anions, thus suppressing the formation of highly corrosive HF. Through MD simulations, the authors discovered strong interactions between H2O and LiPF6, including O···Li+
