Electrodes for Li-ion Batteries (eBook)

Materials, Mechanisms and Performance

Laurence Croguennec, Laure Monconduit, R mi Dedryv re (Autoren)

eBook Download: EPUB

2015
Wiley (Verlag)
978-1-119-00738-8 (ISBN)

Infos

The electrochemical energy storage is a means to conserve electrical energy in chemical form. This form of storage benefits from the fact that these two energies share the same vector, the electron. This advantage allows us to limit the losses related to the conversion of energy from one form to another. The RS2E focuses its research on rechargeable electrochemical devices (or electrochemical storage) batteries and supercapacitors. The materials used in the electrodes are key components of lithium-ion batteries. Their nature depend battery performance in terms of mass and volume capacity, energy density, power, durability, safety, etc. This book deals with current and future positive and negative electrode materials covering aspects related to research new and better materials for future applications (related to renewable energy storage and transportation in particular), bringing light on the mechanisms of operation, aging and failure.

Autor

Laurence Croguennec, Université de Bordeaux, France. Laure Monconduit, Universite Montpellier 2, France. Rémi Dedryvère, Université de Pau et des Pays de l'Adour, France.

Inhalt

ACKNOWLEDGMENTS vii

PREFACE ix

INTRODUCTION xi

CHAPTER 1. NEGATIVE ELECTRODES 1

1.1. Preamble 1

1.2. Classic materials: insertion mechanism 3

1.2.1. Graphitic carbon 3

1.2.2. Titanium oxides 7

1.3. Toward other materials and other mechanisms 13

1.3.1. Silicon 14

1.3.2. Other block p elements 19

1.4. Summary on negative electrodes 27

CHAPTER 2. POSITIVE ELECTRODES 29

2.1. Preamble 29

2.2. Layered transition metal oxides as positive electrode materials for Li-ion batteries: from LiCoO2 to Li1+xM1-xO2 30

2.2.1. The layered oxide LiCoO2: the starting point 31

2.2.2. From LiNiO2, initially explored as an alternative to LiCoO2, to the commercialization of LiNi0.80Co0.15Al0.05O2 (NCA) and LiNi1/3Mn1/3Co1/3O2 (NMC) 34

2.2.3. Electrode/electrolyte interfaces and aging phenomena in layered oxides 40

2.2.4. High-capacity Li-rich layered oxides 43

2.3. Alternatives to layered oxides 49

2.3.1. Materials with spinel structure: from LiMn2O4 to LiNi1/2Mn3/2O4 50

2.3.2. The olivine phase LiFePO4: a small revolution 57

CONCLUSION 63

BIBLIOGRAPHY 65

INDEX 81

Textauszug

Introduction

Toward efficient Li-ion batteries

In its most classic structure, a lithium-ion (Li-ion) battery contains a negative electrode made of carbon graphite, a positive electrode made of a layered oxide LiMO2 (M transition metal, e.g. LiCoO2) and a polypropylene separator soaked in an electrolyte made of a lithium salt (e.g. LiPF6) dissolved in a mixture of alkyl carbonate organic solvents (e.g. ethylene carbonate–dimethyl carbonate (EC–DMC)). The reversible electrochemical process is as follows:

At the positive electrode: LiMO2 Li(1-x)MO2 + x Li+ + x e-, 0 < x < 0.5

At the negative electrode: C6 + y Li+ + y e- LiyC6, 0 < y < 1

It is on this LiCoO2//graphite combination that the success of the Li-ion accumulators is built, a success which has enabled the tremendous growth in portable electronics that has completely revolutionized our society. The challenge for scientific research in this domain is precisely to distance itself from this classic schema in order to respond to new requirements for future applications targeted at developing new positive and negative electrode materials.

It is necessary to note that positive electrodes are often called “cathodes” in scientific literature in this domain, just as negative electrodes are often called “anodes". Although this term should only be used when the battery is discharging and not charging (these are rechargeable accumulators), we will sometimes use the terms “cathode” and “anode” in this book for the sake of simplicity. The electrodes are made up of a large majority of electrochemically active materials (between 70 and 95%), but also of polymer binder and potentially a conductive additive. In the following, we will only focus on active materials.

The main prerequisites for determining the choice of active materials for positive electrodes (cathode) and negative electrodes (anode) in a Li-ion battery are summarized in Table I.1.

Table I.1. The conditions that constitutive active materials (AM) of positive and negative electrodes should meet in order to create a Li-ion battery

Negative electrode AM

Positive electrode AM

AM should have a reversible reaction with Li, with the minimum possible structural changes on insertion/extraction

The insertion/extraction of Li should be carried out as rapidly as possible to achieve high power densities

AM should insert/react with a maximum of Li to reach a high capacity

Weak potential close to that of Li+/Li to obtain maximum tension in the Li-ion battery

High potential compared to that of Li+/Li to obtain maximum tension in the Li-ion battery

AM should have good compatibility with the electrolyte

AM should have good electronic and ionic conductivity

AM should be abundant, synthesis should be simple and not expensive

AM should be thermally and chemically stable and non-toxic

The gravimetric or volumetric energy density (Wh/kg or Wh/L) is a major criterion for evaluating a battery’s performances. This being dependent on the product of the capacity × potential difference of the two electrodes, by simplifying the problem it is possible to seek the materials displaying the highest gravimetric capacities (mAh/g) or volumetric capacities (mAh/cm3) possible, with the highest possible potential for the positive and the lowest possible potential for the negative. In reality, the problem is more complex; for example, for the negative electrode, a potential a little higher than that of graphite facilitates an increase in safety. However, given the enormous difference in gravimetric capacity observed between active materials of the two electrodes (favoring the negative), an improvement in the capacity of the positives represents a more important gain.

The power density (W/kg or W/L) is also an important criterion, since the batteries will be subject to peaks in electricity production (charge) or consumption (discharge) for some future applications, such as storage of renewable forms of energy. In this case, it is the considerations of kinetics that are important. The insertion/extraction of the lithium into the material, which is directly linked to the active material’s electronic and ionic conductivity, should be as rapid as possible. There also the problem is more complex because the kinetically limiting stage can be situated at the level of the interface between the active material and the electrolyte, as we will see next.

Economic and environmental considerations will be added to these criteria. It is important to take these into account in order to plan a large-scale development in applications such as transport (electric vehicles) and storage of renewable energies.

The active material’s chemical or electrochemical compatibility with the electrolyte is just as important a criterion as the previous ones, and the notion of an interface between the active material and the electrolyte (generally liquid) is indissociable from the electrodes’ performance in Li-ion batteries. All the Li-ion batteries currently available on the market can, in fact, only function due to the formation of electrode/electrolyte interfaces that are stable over time (at the negative electrodes, in particular). Figure I.1 shows a better understanding of this problem. The two electrode materials, positive and negative, are solids characterized by their Fermi levels EF+ and EF-. The positive electrode, whose potential is highest, corresponds to the material whose Fermi level is lowest in energy, since it is that which will accept the electrons coming from the negative electrode when the battery is discharged. Conversely, the negative electrode corresponds to the material whose Fermi level is highest in energy. The energy and potential scales are thus inverted.

Figure I.1. Representation of the energy and potential of the electrodes and the electrolyte in a Li-ion battery, making apparent the necessity of passivating the negative electrode’s surface. (Adapted from [GOO 10] with permission. Copyright 2010 American Chemical Society)

The two electrode materials are in direct contact with the liquid electrolyte, an environment made up of molecular species, characterized by their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Adding an electron to the electrolyte’s LUMO results in the reduction of the latter, whereas removing an electron from its HOMO results in its oxidation. So long as the positive electrode material’s Fermi level is situated above the electrolyte’s HOMO level, no electron transfer will occur from the electrolyte to the positive electrode, and the electrolyte remains electrochemically stable since it does not oxidize continually on contact with the electrode. This remains theoretically true for positive electrode materials whose potential does not exceed approximately 4.5 V versus Li+/Li, which is the case for the usual materials, such as LiCoO2.

On the contrary, so long as the Fermi level of the negative electrode material remains below the LUMO level of the electrolyte, no electron transfer can occur from the negative electrode to the electrolyte, and the latter will remain electrochemically stable since it does not continually reduce on contact with the electrode. Unfortunately, once lithiated (which corresponds to the battery’s charged state) the majority of negative electrode materials currently available or under study have a Fermi level situated above the electrolyte’s LUMO level, which is to say that their potential lies below the electrolyte’s reduction potential (estimated at approximately 0.8–1 V vs. Li+/Li). The electrolyte therefore reduces on contact with the negative electrode, which is clearly a major problem.

Only the formation of a passivation layer at the negative electrode’s surface can shift the electrolyte’s reduction limit by providing a kinetic stability. This layer electronically isolates the material from contact with the electrolyte to block the reduction process, while still allowing Li+ ions to diffuse and ensure the battery can function. The layer formed should thus have electronic insulating properties and ionic conducting properties. It is thought that its main formation mechanism (although not its only one) consists of reducing the molecular moieties of the electrolyte (solvents in particular) at the negative electrode’s surface when the battery is first charged, leading to deposition of reduced species at the surface of the electrode. It should be noted that its formation therefore consumes lithium. This passivation layer, which was first described by Peled in 1979 for a metallic lithium electrode under the term SEI (solid electrolyte interphase) [PEL 79], is today considered to be a major factor enabling Li-ion batteries to function. Instability of the SEI is equal to a loss in performance and premature aging of the battery. The search for new negative electrode materials cannot therefore be carried out without an in-depth study into the formation and stability of the SEI at their surface upon cycling.

Let us now examine the different negative and positive electrode materials, both...

Produkt-Details

• Erscheint lt. Verlag: 2.6.2015
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Schlagworte: Alternative • batteries • Classic • Commercialization • electrode • Electrodes • electrolyte • Energie • Energy • Hydrogen, Batteries & Fuel Cells • initially • Insertion • Introduction • Ix • licoo • LiIon • Lini • Materials • Metal • Negative • oxide licoo • oxides • Point • Positive • Transition • Wasserstoff, Batterien u. Brennstoffzellen • XM • xo
• ISBN-10: 1-119-00738-0 / 1119007380
• ISBN-13: 978-1-119-00738-8 / 9781119007388