Emerging Nanotechnologies in Rechargeable Energy Storage Systems

Emerging Nanotechnologies in Rechargeable Energy Storage Systems addresses the technical state-of-the-art of nanotechnology for rechargeable energy storage systems. Materials characterization and device-modeling aspects are covered in detail, with additional sections devoted to the application of nanotechnology in batteries for electrical vehicles.

In the later part of the book, safety and regulatory issues are thoroughly discussed. Users will find a valuable source of information on the latest developments in nanotechnology in rechargeable energy storage systems. This book will be of great use to researchers and graduate students in the fields of nanotechnology, electrical energy storage, and those interested in materials and electrochemical cell development.

• Gives readers working in the rechargeable energy storage sector a greater awareness on how novel nanotechnology oriented methods can help them develop higher-performance batteries and supercapacitor systems
• Provides focused coverage of the development, process, characterization techniques, modeling, safety and applications of nanomaterials for rechargeable energy storage systems
• Presents readers with an informed choice in materials selection for rechargeable energy storage devices

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 6, 2017
• ISBN: 9780323429962

Book preview

Emerging Nanotechnologies in Rechargeable Energy Storage Systems

Edited by

Lide M. Rodriguez-Martinez

CIC Energigune, Miñano, Spain

Noshin Omar

Mobility, Logistic and Automotive Technology Research Center (MOBI), Department of Electrical Engineering and Energy Technology (ETEC), Vrije Universiteit Brussel, Brussels, Belgium

Table of Contents

Cover

Title page

Copyright

Contributors

Preface

Chapter One: Electrolytes for Li- and Na-Ion Batteries: Concepts, Candidates, and the Role of Nanotechnology

Abstract

1. Introduction and Electrolyte Concept

2. Liquid Electrolytes

3. Solid Electrolytes

4. Conclusions

Glossary

Chapter Two: Review of Nanotechnology for Anode Materials in Batteries

Abstract

1. A High-Performance Anode

2. Benefits of a Nanostructured Anode

3. Geometrical Aspects and Design of Nanostructured Anodes

4. Carbon-Based Anodes

5. Silicon-Based Anodes

6. Metal Alloy Anodes

7. Metal Oxide–Based Anodes

8. Metal Phosphide and Sulfide Anodes

9. Summary and Conclusions

Glossary

Chapter Three: Review of Nanotechnology for Cathode Materials in Batteries

Abstract

1. Introduction

2. Nanostructural Design and Synthesis of Cathode Materials for Lithium-Ion Batteries

3. Nanoscale Surface Modification on Cathode Materials for Lithium-Ion Batteries

4. Conclusions

Glossary

Chapter Four: Nanotechnology in Electrochemical Capacitors

Abstract

1. Introduction

2. Basic Principles and Classification of Electrochemical Capacitors

3. Parameters Governing Supercapacitor Performance

4. Nanotechnology in Electrical Double Layer Capacitors

5. Pseudocapacitive Materials

6. Conclusions and Perspectives

Glossary

Chapter Five: Characterization of Nanomaterials for Energy Storage

Abstract

1. Macro- and Microscale Characterization

2. Ex Situ, Postmortem Analysis Versus in Situ Electrochemistry

3. Structural Analysis

4. Chemical Analysis (Spectroscopic Techniques)

5. Nanoscale Characterization

6. Electron Microscopy

7. Improved Instrumentation and Inspirations for New Methods

8. Summary

Glossary

Chapter Six: Electrochemical–Thermal Characterization and Thermal Modeling for Batteries

Abstract

1. Introduction

2. Heat Generation in Lithium-Ion Batteries

3. Electrochemical–Calorimetric Measurements on Lithium-Ion Batteries

4. Thermal Modeling of Lithium-Ion Batteries

5. Simulations With COMSOL Multiphysics

6. Conclusions

Glossary

Chapter Seven: Life Cycle Assessment of Nanotechnology in Batteries for Electric Vehicles

Abstract

1. Introduction

2. Case Study: Use of Nanomaterials in Li-Ion Battery Anodes

3. Life Cycle Impact Assessment

4. Discussion and Conclusions

Glossary

Chapter Eight: Safety of Rechargeable Energy Storage Systems with a focus on Li-ion Technology

Abstract

1. Introduction

2. Hazards

3. Failure Scenarios

4. Risk Mitigation

5. Safety Tests

6. Conclusions and Outlook

Glossary

Chapter Nine: Application of the Energy Storage Systems

Abstract

1. Introduction: Energy Storage Systems and Their Application

2. Characterization of Storage Cells and Devices, Parameters, and Features

3. Overview of Storage Cells, Modules, and Systems

4. Applications That Use Storage Facilities

5. Conclusions

Glossary

Index

Copyright

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Contributors

P. Alaboina,     North Carolina A&T State University, Greensboro, NC, United States

E. Bekaert,     CIC Energigune, Miñano, Spain

A.M. Bittner,     The Foundation for Science, Bilbao; CIC nanoGUNE, Donostia-San Sebastián, Spain

P.N. Borza,     Transilvania University of Brasov, Brasov, Romania

L. Buannic,     CIC Energigune, Miñano, Spain

S.-J. Cho,     North Carolina A&T State University, Greensboro, NC, United States

F. di Persio,     European Commission, Joint Research Centre (JRC), Directorate for Energy, Transport and Climate, Energy Storage Unit, Petten, The Netherlands

E. Goikolea,     CIC energiGUNE, Miñano, Spain

S. Goutam,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

M. Hernandez,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

V. Koroteev,     Novosibirsk State University, Novosibirsk, Russia

A. Kriston,     European Commission, Joint Research Centre (JRC), Directorate for Energy, Transport and Climate, Energy Storage Unit, Petten, The Netherlands

U. Lassi,     University of Oulu, Research Unit of Sustainable Chemistry, Kokkola, Finland

N. Lebedeva,     European Commission, Joint Research Centre (JRC), Directorate for Energy, Transport and Climate, Energy Storage Unit, Petten, The Netherlands

B. Lei,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

A. Llordés

CIC Energigune, Miñano

The Basque Foundation for Science, Bilbao, Spain

A. Melcher,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

M. Messagie,     Mobility, Logistic and Automotive Technology Research Center (MOBI) Vrije Universiteit Brussel, Brussels, Belgium

R. Mysyk,     CIC energiGUNE, Miñano, Spain

L. Oliveira,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

N. Omar,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

A. Pfrang,     European Commission, Joint Research Centre (JRC), Directorate for Energy, Transport and Climate, Energy Storage Unit, Petten, The Netherlands

S. Rangaraju,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

M. Rohde,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

V. Ruiz,     European Commission, Joint Research Centre (JRC), Directorate for Energy, Transport and Climate, Energy Storage Unit, Petten, The Netherlands

J. Salminen,     Boliden Kokkola, Kokkola, Finland

J. Sanfelix

Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels

Vrije Universiteit Brussel

H.J. Seifert,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

M.-J. Uddin,     North Carolina A&T State University, Greensboro, NC, United States

P. Van Den Bossche

Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

Vrije Universiteit Brussel, Anderlecht, Belgium

J. Van Mierlo,     Mobility, Logistic and Automotive Technology Research Center (MOBI), Vrije Universiteit Brussel, Brussels, Belgium

W. Zhao,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

C. Ziebert,     Karlsruhe Institute of Technology, Institute of Applied Materials–Applied Materials Physics, Eggenstein-Leopoldshafen, Germany

Preface

The internal combustion engine (ICE), used for vehicular propulsion since the beginning of the automobile era, has come under pressure as a significant contributor to air pollution and greenhouse gas emissions linked to global climate change. These growing environmental concerns, as well as rising petroleum prices, have strained the global economy and spurred research into the development of various types of clean energy transportation systems using electrically propelled vehicles, such as battery electric vehicles (BEV), plug-in hybrid electric vehicles (PHEV), and hybrid electric vehicles (HEV).

Development, sizing, optimizing, and modeling of the energy storage system are among the main challenges for the development of the BEV, PHEV, and HEV.

In particular there is a strong interest in the emerging energy storage technologies, such as lithium–sulfur, solid state, zinc–air, lithium–air, and hybrid capacitors. Research is focusing on higher performances, durability, safety, lifetime, and cost, with specific accents according to the application, traction or stationary.

A lot of scientific research work has been performed to provide solutions for these challenges.

Emerging Nanotechnologies in Rechargeable Energy Storage Systems provides an extensive overview of the technology development process for traction and stationary applications from material to system level.

The initial chapters describe in detail the present technology progress from materials points of view and in particular for nanotechnologies.

The second half of the book covers the system-related topics, such as safety, thermal management, life cycle assessment and applications.

The book deals with key topics in the field of rechargeable energy storage technologies that are interesting for researchers, PhD students, master students, R&D centers, and battery system and application designers.

Lide M. Rodriguez-Martinez

Noshin Omar

Chapter One

Electrolytes for Li- and Na-Ion Batteries: Concepts, Candidates, and the Role of Nanotechnology

E. Bekaert*

L. Buannic*

U. Lassi**

A. Llordés*,†

J. Salminen‡

*    CIC Energigune, Miñano, Spain

**    University of Oulu, Research Unit of Sustainable Chemistry, Kokkola, Finland

†    The Basque Foundation for Science, Bilbao, Spain

‡    Boliden Kokkola, Kokkola, Finland

Abstract

This chapter focuses on electrolytes for application in rechargeable batteries, in particular for lithium and sodium ion technologies. Along with general concepts and technical requirements, the differences between liquid and solid electrolytes will be explained with their respective advantages and limitations. Common electrolytic solutions include various combinations of salts, solvents, and additives. Solid electrolyte materials range from polymer-based membranes, inorganic materials (including oxides and sulfides), to various combinations of both. A brief description of each family of electrolytes will be presented along with the role that nanotechnology has in improving their performances. In addition, the critical role played by the solid electrolyte interphase (SEI) layer, whether with a liquid or a solid electrolyte, will be addressed. Finally, special attention will be given to the challenging integration of solid electrolytes into high-performing solid-state battery devices.

Keywords

electrolytes

solid electrolyte interface

ionic liquids

polymer-based electrolytes

inorganic electrolytes

oxides

sulfides

glasses

composite solid electrolytes

solid-state batteries

Contents

1 Introduction and Electrolyte Concept

2 Liquid Electrolytes

2.1 Importance of the SEI layer

2.2 Additives: general

2.3 Electrode–electrolyte compatibility: SEI with ionic liquids

2.4 Use of nanotechnology in liquid electrolytes

3 Solid Electrolytes

3.1 Polymer-based electrolytes

3.2 Inorganic electrolytes

3.3 Composite solid electrolytes

3.4 Integration of solid electrolytes into all-solid-state battery devices

3.5 The promise of nanostructured electrolytes

4 Conclusions

References

1. Introduction and Electrolyte Concept

All electrochemical devices, such as batteries, capacitors, electrolytic cells, or fuel cells contain electrolytes, which is the ion transport media; its role is identical irrespective of the selected chemistry or device. The electrolyte is commonly sandwiched between two electrodes and is responsible for ionic charge transfer between them.

Good interfacial contact combined with chemical stability is needed between electrodes and electrolyte to ensure an effective ionic transfer and should be continuously evaluated for emerging materials. In fact, these electrified interfaces have been the center of interest since the rise of modern electrochemistry and continue to be in the lithium-based and beyond-lithium rechargeable battery technologies [1].

In batteries, the energy output depends on the chemical nature of the positive and negative electrodes (also called cathode and anode, respectively; this convention is only correct during the discharge process) [1]. Ideally, the electrolyte should not undergo chemical changes during battery operation, that is, when the faradaic processes take place within the electrodes. Therefore, regarding chemical reactivity, the electrolyte can be considered as an inert component of the battery requiring stability against both positive and negative surfaces. In the actual device, the electrochemical stability of the electrolyte is made in passivation (kinetic) rather than thermodynamic [2]. This is particularly important for rechargeable battery systems but is often a challenge due to the strong oxidizing and reducing properties of the cathode and anode, respectively. Indeed, the voltage window of operation for Li- and Na-ion batteries is given by the nature of the electrolyte and electrode materials and its chemical potential. Typically, liquid electrolytes have less stability than solids at high potentials, as their organic components can easily become oxidized (Fig. 1.1A).

Figure 1.1   (A) Relative energy diagram of electrode potentials and electrolyte energy gap in a typical Li-ion battery where μA, electrochemical potential values of anode; μc, electrochemical potential values; Eg, energy gap; Voc, open circuit voltage; HOMO, highest occupied molecular orbital; and LOMO, lowest unoccupied molecular orbital. (B) Schematic diagram of lithium intercalation/deintercalation. (Adapted from K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (10) (2004) 4303–4418 [2], C. Daniel, Materials and processing for lithium-ion batteries, JOM 60 (9) (2008) 43–48 [3], and E. Peled, D. Golodnitsky, G. Ardel, Advanced model for solid electrolyte interphase electrodes in liquid and polymer electrolytes, J. Electrochem. Soc. 144 (1997) L208– L210 [4]).

Conventional Li- and Na-ion batteries (commonly abbreviated LIB and SIB, respectively) are those containing a liquid electrolyte, which provides the ion transport medium between the cathode and anode through porous separator. The lithium or sodium ions exist as dissolved species Li+ or Na+ in the electrolyte and carry out charge transfer reactions at the electrode surfaces, namely oxidation or reduction in the cell reaction. The electrolyte between cathode and anode provides a sufficient source of lithium or sodium ions for the electrode reactions, while maintaining electrical insulation. Ion transport within the electrolyte is influenced by the solvation degree and by the presence of the counter ion species (anions for charge balance). When the battery is discharging the negative electrode release some of its lithium ions, which flow through the electrolyte to the positive electrode and vice versa during charging (Fig. 1.1B).

The constant demand for batteries with higher energy density leads to the development of more oxidizing/reducing electrode materials, continuously increasing the stability requirement of the electrolyte [2]. A specific surface chemistry is often crucial for kinetic stability of the electrode/electrolyte interfaces. The redox potential against some reference potential, are commonly used to quantify the redox potential of electrode materials. The range in volts between the oxidative and reductive decomposition limits of an electrolyte is employed to quantify the stability of an electrolyte, which is known as the electrochemical window. The redox processes occurring at each electrode should occur within this electrochemical window to allow for rechargeable battery operation. Electrochemical stability is not the only property that an electrolyte should meet. An ideal electrolyte should provide the following characteristics:

• High ionic conductivity to efficiently transfer ionic charges between electrodes

• Low electronic conductivity to prevent self-discharge and short circuiting

• Large electrochemical stability window to prevent its decomposition in the range of cell working potentials

• Chemically inert to the other cell components including separator, current collector, electrode substrates, and cell packaging

• High wettability of the electrodes surface

• Robustness in harsh conditions, such as extreme temperatures, mechanical and electrochemical abuses

• Environmentally friendly

To date, the most widespread electrolytes and the ones found in commercial batteries are electrolytes based on liquids [2]. However, solid electrolytes are now rapidly emerging as promising alternatives given their wider electrochemical window of stability [5,6]. In addition, unlike liquid electrolytes, solids are nonflammable and do not suffer from leakage, providing a safer option for large-scale application (e.g., electric vehicle) [7].

However, solid electrolytes have not yet been widely used in commercial batteries, owing to materials’ limitations, such as low ionic conductivity and poor wetting properties, as well as processing-related characteristics, that limit their integration and assembly into solid-state devices (Fig. 1.2). The high interfacial resistance at the solid–solid interface between the electrodes and electrolyte arises as a key scientific challenge that still needs to be solved for the successful implementation of these materials in commercial batteries (Table 1.1).

Figure 1.2   Schematic of an all-solid-state battery device and main challenges arising at the solid–solid interfaces. (Image credit: A. Llordés).

Table 1.1

Advantages and disadvantages of solid versus liquid electrolytes [8]

The replacement of traditional liquid electrolytes by solid electrolytes would expand the spread of rechargeable batteries. Not only will it provide a safer system—the flammable and toxic organic solvents will no longer be present—but it will also lead to substantial increase in energy density (2–3 greater than existing battery technologies) [9]. Indeed, the use of metallic anodes (Li metal or Na metal for LiB and NaB, respectively), providing the ultimate energy storage capacities, becomes possible as the solid electrolyte acts both as a separator and medium for ionic charge transfer. The solid electrolyte becomes a physical barrier, possibly preventing the growth of metallic dendrites from the anode to the cathode, a common limitation of current battery systems. The combination of ionic conductor medium and separator in a single component will also facilitate cell processing. Additionally, the absence of low vapor pressure solvents will allow for higher vacuum level during cell sealing, which will have a positive impact on the more resistive interfaces between electrodes and solid electrolyte. However, unlike its liquid counterpart, a solid electrolyte will not wet the electrode materials. It is therefore necessary to rethink the composition of the electrodes and incorporate a fraction of solid-state electrolyte in order to ensure good ionic transport within the electrodes and minimize interfacial resistance at the junction between electrodes and electrolytes.

2. Liquid Electrolytes

Liquid electrolytes are commonly used in batteries and supercapacitors due to their low surface tension and viscosity, and high wetting property [10]. These characteristics provide optimum contact with the materials composing the electrodes, leading to low interfacial resistance [11]. Some storage technologies as Li-S or metal batteries need special requirements in terms of electrolyte due to the high chemical reactivity of battery components as polysulfide or metallic lithium [12,13]. Moreover the electrolyte design also requires considering other factors as the poor electrochemical kinetics for some components and the selection has to consider and raise the electronic and ionic path.

A typical liquid electrolyte is usually a solution of a Li or Na salt dissolved in an organic solvent mixture. In rechargeable devices, water or organic compounds are used as solvents and are referred to as aqueous and nonaqueous electrolytes, respectively. They are straightforward to prepare and homogenize due to the fast ionic diffusion in liquids, which makes them easily scalable for large-scale energy storage applications. The organic solvents include propylene carbonate (PC), ethylene carbonate (EC), diethylene carbonate (DEC), dimethylene carbonate (DMC), and their mixtures. In Li-ion batteries, the most typical salt is lithium hexafluorophosphate, LiPF6, but LiBF4, LiAsF6, LIBOB, LiTFSI, LiFSI have also been commonly used [14]. The latter two, LiTFSI and LiFSI, belong to newer generation of salts providing better chemical stability. For Na-based batteries, equivalent salts have been investigated including NaClO4, NaBF4, NaPF6, NaTf, NaTFSI, and NaFSI [15]. Organic solvent-based electrolytes are flammable and susceptible to thermal runaway phenomenon. A big problem is that EC, one of the most volatile solvents, is also used as an additive to form a stable SEI layer [16]. Nonvolatile room temperature molten salts, known as ionic liquids, have also been investigated intensively within recent years as candidates for lithium-ion battery electrolyte components.

Many studied battery systems have more or less problems with electrolyte interactions. The organic solvents of the liquid electrolyte are required to efficiently dissolve the selected Li salts. Their dissolution power is such that it can also lead to dissolution of the cathode or anode materials. There are many examples in different battery chemistries. Powerful organic solvents can dissolve sulfur and polysulfides, for example, in well-studied lithium sulfur (Li-S) battery systems [17]. The dissolved sulfur species can migrate through the cell causing power fade and capacity losses. That problem is severe hindrance in development of this promising technology [17,18]. Ionic liquids can also dissolve lithium metal oxide cathode materials. At the electrode interface, the electrolyte should not react continuously but only provide a small protective layer called solid electrolyte interface (SEI).

Sodium batteries have been tested with similar mixtures of organic solvents and Na-based salts. However, this technology has not yet reached commercialization because the use of common organic solvents, such as PC, EC, DEC, or DMC has not been adequate to enable long lifetime for sodium batteries [19].

Chemical stability is one of the most important properties for the electrolyte especially in terms of safety and performance. The consumption of the electrolyte due to irreversible reactions on electrode/electrolyte interface and chemical instability can lead to continuous capacity fading, loss in power rate and increase of cell internal pressure and generation of gaseous products that can induce explosion of the device [20–23]. Moreover, the electrolyte decomposition might be highly exothermic in some aggressive cases and may lead to a thermal runaway of the battery. Liquid electrolytes require a complex thermal management system [24]. Moreover, the electrolyte decomposition might be highly exothermic in some aggressive cases and may lead to a thermal runaway of the battery. Liquid electrolyte requires a more complex thermal management system [24].

Extensive effort is ongoing to enhance the reliability and safety of battery electrolytes. Different approaches are being studied:

1. Additives to build up stable SEI and/or increase the thermal stability [25].

2. Redox shuttles to protect from overcharge in particular for Li-S systems [26].

3. Shut-down separators to prevent thermal runaway [27].

4. Other type of salts (as imidazolium, LiNO3, LiBH4, LiB3H8, LiCB11H12) to reduce toxicity [28,29].

5. Use of solid electrolytes (ceramic, polymer, or composites).

2.1. Importance of the SEI layer

Electron movement through the cell can be prevented if the potential is higher than the energy level of the highest occupied molecular orbital (HOMO) of the electrolyte to prevent the oxidation of electrolyte on the anode (Fig. 1.1). Similarly, the potential of the cathode has to be lower than the energy level of the lowest unoccupied molecular orbital (LUMO) of the electrolyte to prevent reduction on the cathode. As a result, the electrode potentials of a battery are limited by the HOMO–LUMO gap of the electrolyte. This cell potential limitation can be partly prevented by kinetic control, that is, keeping the anode potential too low to be stable [30]. During the first charge of the battery, the electrolyte is reduced on the anode, but the decomposition products formed during the reaction form a passivating layer, called solid-electrolyte interphase (SEI) [31,32]. SEI layer is formed around the anode and it prevents any further reduction occurring during the following cycles. If the SEI layer is damaged due to side reactions, many detrimental effects occur for the battery performance. These include electrolyte decomposition, rapid heating and pressure rise in the cell, larger deposition of insulating solids, decrease of energy efficiency, and increased resistivity [20–22,33].

A stable SEI layer is therefore a necessity for a safe battery performance, especially because the most commonly used anode material, graphite, has a potential that is too low (1.1 V vs. Li) for any organic solvent-based electrolyte to operate without a SEI layer. Most cathode materials are within the electrochemical window (∼4 V vs. Li) of the organic solvent-based electrolytes. Thus cathode operation does not require an SEI layer in the same way the anode does. SEI layer composition depends on the identity of the lithium salt and the solvent in the electrolyte.

Improving safety of a conventional organic solvent-based electrolyte system can be done by improving the stability of the SEI layer that is formed on the surface of the anode as a result of decomposition products of electrolyte reduction. This interface prevents the fresh electrolyte contacting with the anode, slows down the reaction rate, and thus enables the use of lithium at the anode. The same phenomenon occurs for lithiated carbon anode. Stable SEI layer is therefore a prerequisite for a good battery performance and it also determines the battery voltage limit. Chemical modification of the SEI layer and additives, such as catechol carbonate, alkyl sulfones, alkyl phosphates or phosphazenes [34,35], which might also act as flame retardants, are used to improve stability and also increase the battery safety for lithium-ion batteries with organic solvent electrolytes [36].

Despite these efforts, liquid electrolytes still present severe drawbacks. In this frame, ionic liquids (ILs) are predicted to play a major role as the result of their hydrophobicity and intrinsic safety [37]. ILs could be used as a protective layer of lithium anode against moisture, for example. On the other hand, ILs exhibit a very high viscosity compared to standard organic solvents. Consequently, in order to obtain acceptable conductivity level, proper mixtures with ILs and salts has to be made [38]. Further, low viscosity cations and, especially, anions need to be researched [39]. Last but not least and despite the fact that ILs are good solvents for ionic materials, they present a high reactivity versus metallic lithium. They are mostly used as additives in classical electrolytes due to their high cost [40].

2.2. Additives: general

Additives have been extensively studied to address some of the limitations of liquid electrolytes. The use of additives can enhance cell safety and chemical stability resulting in improved lifetime, better performance, and durability. Additives can prevent or hinder the flammability of organic electrolytes, shut down battery operation under abuse conditions, and protect from overcharge or increase the cell overcharge tolerance. Similar to electrolytes, additives should not be toxic and need to be inexpensive. Overall, additives have an effect on improving Li-ion battery safety performance but can also have a negative impact on the electrochemical properties, and a stable SEI layer requires additives, such as EC, in the electrolyte [36]. Batteries in general must be designed to be as safe as possible even under difficult conditions.

2.2.1. Electrolyte additives used in Li-ion batteries

Use of electrolyte additives is one of the most economical and effective methods for the improvement of battery performances. According to their functions, the electrolyte additives can be divided into five categories: (1) SEI forming improver, (2) cathode protection agent, (3) salt stabilizer, (4) safety protection agent, and (5) other agents, such as solvation enhancer molecules and wetting agents. Usually, the amount of additive in the liquid electrolyte is less than 5 wt.% and is sufficient to significantly improve the battery cycle life [34].

2.2.1.1. Additives for SEI forming improver

SEI layer formation around the anode prevents any further reduction occurring during the following cycles. The main components of SEI are the decomposition products of electrolyte solvents and salts. SEI layer formed before the intercalation of lithium ions is unstable and abundant with inorganic compounds. The SEI formation can be facilitated by chemical modification, that is, by chemical coating an organic film layer onto the graphite surface via an electrochemical reduction of additives. Additives for SEI forming improver can be divided into reduction-type and reaction-type. Reduction-type additives (including either polymerizable monomer or reducing agent) assist and facilitate the formation of a stable SEI layer. Reaction-type additives act as scavengers of radical anions, or complexes with the decomposition products to form more stable SEI components. These additives are not reduced during cell operation [34].

2.2.1.2. Additives for SEI morphology modifier

The presence of inorganic components, such as Li2CO3 and LiF, can lead to SEI instability [21]. Therefore, many boron-based anion receptors have been developed to dissolve LiF [41]. The most well-known receptor is tris(pentafluorophenyl)borane (TPFPB), which is sufficient to improve cycleability and capacity retention of cells containing LiPF6- or LiBF4-based electrolytes. Alkali metal salts have also been considered for improvement of SEI formation. In Ref. [42], it was observed that the SEI formed in the presence of sodium ions (NaClO4) is more uniform and has less resistance than without sodium ions. Pretreatment of graphite materials using insoluble sodium salts in organic electrolytes (such as Na2CO3 or LiCl) has also been investigated to improve SEI formation [43]. On the other hand, opposite effects of potassium ions have been observed with different electrolytes [44].

2.2.1.3. Additives for cathode protection

From the electrolyte viewpoint, the performance of cathode materials is dependent on two factors, the presence of water or acidic impurities and the irreversible oxidation of electrolyte solvent [45]. Additives for cathode protection are used for this purpose. In the first proposed mechanism, the electrolyte solvents are chemically oxidized (oxygen released from the cathode) to generate H2O and CO2. The produced H2O can hydrolyze LiPF6, forming acidic products (such as HF), which dissolve cathode materials. Additives for cathode protection, such as amine molecules can form complexes with PF5 thus reducing the reactivity and acidity of PF5, and preventing the dissolution of cathode. The other mechanism of these additives is to form a protective film on the cathode surface. This occurs when additive molecules combine with the dissolved metal ions to form insoluble products, which effectively cover the surface to prevent dissolution. Basically, the combination of these two mechanisms seems to be the best solution to protect the cathode [34].

2.2.1.4. Salt stabilizer additives

that show Lewis acid behavior. Thermal and chemical instability of LiPF6 electrolyte is affected by the following factors: (1) high equilibrium constant of LiPF6 decomposition and (2) high reactivity of the resulting PF5 gas formation. As a result of these reactions, the deposition of solid LiF in the SEI layer is increased leading to higher resistivity (or interfacial impedance) and gas generation inside the battery. Therefore, additives for LiPF6 salt stabilizer are used to tackle these problems [34]. Some of the problems related to Lewis acid type anions and resulting corrosive effects can be avoided by using lithium salts with chemically more stable anions TFSI– and FSI–, for example.

2.2.1.5. Additives for safety protection

Safety protection additives are used for overcharge protection and as fire-retardant additives. The overcharge protection additives are classified as redox shuttle and shutdown additives. The former protects the cell overcharge reversibly, while the latter terminates the cell operation permanently. During the overcharge, shuttle molecules are oxidized reversibly at the positive electrode. The oxidized species then diffuse to the negative electrode where they are reduced back to neutral molecules. The maximum current for shuttle additive depends on the concentration of the shuttle molecules in the electrolyte, the diffusion constant of the shuttle molecules, and the number of charges carried by the shuttle species [46]. Shuttle additives should meet several requirements as presented in Table 1.2. Several candidates for redox shuttle additives have been tested, but most of them do not meet these requirements. The first shuttle additives included compounds, such as metallocenes and dihydrophenazine derivatives [46] with a small redox potential range (2.8–3.5 V), and were therefore only suitable for low-voltage lithium batteries. Recently, anisole compounds have been studied due to their promisingly high redox potential and good solubility in the lithium battery electrolytes [47]. Shutdown additives for overcharge protection were recently introduced by Dantsin et al. [48]. Majority of these shutdown additives belong to the aromatic family.

Table 1.2

Typical electrolyte additives used in lithium-ion batteries

Additional safety concerns are related to the high flammability of the organic liquid electrolyte. Fire-retardant additives have been developed to face this problem. The mechanisms of these additives are physical, by building isolating layers to stop the combustion process, or chemical, by terminating radical chain reactions [30].

2.2.1.6. Other types of additives

Other additives include ionic solvation enhancers and viscosity diluters. Ionic solvators, such as crown compounds, promote the solubility of Li salts and consequently increase ionic conductivity. Wetting agents can be used to improve the wetting of the separator by the electrolyte. Sometimes the liquid electrolyte cannot wet the nonpolar polyolefin separator efficiently, particularly in the presence of propylene carbonate or ethylene carbonate, which are commonly added to improve thermal tolerance and performances at elevated temperatures. Different types of molecules have been tested including ionic and nonionic surfactants.

As a general rule, additives should not worsen cell performance. Using an additive to address a specific issue can be beneficial but long term cycling should be performed to evaluate the durable benefits of additives over time.

2.3. Electrode–electrolyte compatibility: SEI with ionic liquids

Ionic liquids are molten organic salts of which several cation and anion combinations have shown to be potential electrolytes. Most of these ionic liquids are based on imidazolium cations [51]. Ionic liquids are liquids at room temperature. They have high ionic conductivity, are nonflammable, nontoxic, and have a wider electrochemical window than traditional organic solvent-based electrolytes. Due to the low vapor pressure, ionic liquids can be used in applications, such as Li/air batteries [52]. Most of the studied ionic liquids are not chemically stable below the voltage of 1.1 V, and therefore do not form a stable SEI layer and cannot be used with carbon or lithium anode. To remediate these limitations, they are usually mixed with organic solvents in order to form a protective SEI layer or used in combination with solid inorganic or polymer electrolytes. Recently, ionic liquids containing pyrrolidinium cation and bis(fluorosulfonyl)imide (FSI) anion showed high cyclability with Li/LiCoO2 cell without any additive [53].

2.4. Use of nanotechnology in liquid electrolytes

Battery performance is governed by several properties and complex interactions. Electrolytes used in rechargeable batteries must be chemically stable but reactive enough to form the SEI layer. These side reactions should not proceed beyond providing the protective layer. Any additional solid deposits on the electrodes contribute to increase interfacial impedance and overall resistance of the cell. All additives including nanomaterials must not contribute into harmful and detrimental processes during battery operations. New cell chemistries should be carefully tested and put into long cycling and storage tests with variable conditions to find out if any real improvements were made. It often turns out that promising cell chemistry works well for a short period of time and fails catastrophically before meeting expectations required for commercialization [54].

The search for improved properties in battery chemistries include, for example:

1. large electrochemical stability

2. high thermal stability

3. wide operating voltage range

4. wide operating temperature range

5. low vapor pressure

6. high conductivity

7. high capacity

8. long storage life

9. long cycle life

10. overall safety and abuse tolerance

11. low cost

Many of the previously mentioned points can be addressed by the choice of electrolyte and chemistries used. Ionic liquids, for example, could handle most of the points mentioned previously, except for cost, and in many cases, low temperature performance. The addition of solid powders of Al2O3, TiO2, and ZrO2, especially in nanoparticulate form, into liquid electrolytes can improve conductivity [45] in some cases. This is due to changes in balance between free ions and ion pairs due to altered physicochemical interactions. Within volume fraction range of 0.2–0.5% the conductivity was doubled. This approach is also investigated in the case of solid polymer electrolytes (Section 3.1). Nanomaterial additives can enhance the charge transfer properties in the electrolyte–electrode interface and in the electrode as well as tune properties in solid polymer or other combination electrolytes [45]. The favorable effects directly in liquid electrolytes are very limited but in the whole cell system, especially in the electrodes, there are more possibilities.

3. Solid Electrolytes

Battery safety can be improved by replacing the organic solvent-based liquid electrolyte with a nonflammable alternative, such as solid electrolytes. In addition to improved safety operation of the battery, those alternative electrolytes may provide additional beneficial properties, such as higher stable operational voltage (>4.5 V), too high for traditional organic solvents. They also offer compatibility with alternative battery chemistries (Li/O2 and Li/S), that have very high theoretical capacity and in which conversion chemistry is applied instead of intercalation chemistry [55]. Solid electrolytes also allow the use of some cathode materials, which have poor cycleability with liquid electrolyte (such as sulfur, which dissolves in organic solvents) [56].

Solid electrolytes regroup two main categories of candidates: polymer-based membranes and inorganic-based materials. While polymer-based electrolytes provide interesting mechanical properties due to the flexible nature of the polymer matrix, they often offer ionic conductivities 2 or 3 orders of magnitude below the ones of the more rigid inorganic materials [57]. In order to combine the advantages of both families, new concepts are currently being designed including the preparation of polymer–ceramic composites and polymer–ceramic hybrids (chemically bonded). Each category will now be discussed.

3.1. Polymer-based electrolytes

Polymer-based electrolytes consist of a chemical salt dispersed in a polymer matrix. The salt, once dissociated, can allow transport of charges between electrodes. However, as the salt is composed of one cation (Li+ or Na+) and one anion, the charge transfer will be assumed by both positive and negative entities, with possible