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3 Spinel (LiM2O4; M = Mn, Ni)

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2  Lithium-ion battery overview19



the electrical conduction paths (Specific electrical conduction paths are supplied

between the particles and the current collector. This is done by means of carbon

black, a special carbon conductor.) This entails that the active material particles are

no longer electrically connected to the current collectors.

This aging process can become manifest at both the positive and the negative

electrodes. Further aging processes are discussed in detail in [6]. The lifetime of

the battery cells depends on the operating conditions, the materials applied, the

electrolyte composition, and the quality of the production process. It differs in relation to the application, the design of the lithium-ion battery cell, and the operating

conditions.



Bibliography

1. Ozawa K (2009) Lithium ion rechargeable batteries – materials, technology, and new applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany

2. Garche J (2009) Encyclopedia of electrochemical power sources, Vol. 6. Elsevier B. V.

3. Robert Bosch Battery Systems GmbH, Stuttgart, Germany

4. Reitzle A, Fetzer J, Fink H, Kern R (2011) Safety of lithium-ion batteries for automotive applications. AABC Europe, Mainz, Germany

5. Aifantis KE, Hackney SA, Kumar RV (2010) High energy density lithium batteries. WileyVCH Verlag GmbH & Co. KGaA, Weinheim, Germany

6. Garche J (2009) Encyclopedia of electrochemical power sources. Secondary batteries – lithium

rechargeable systems – lithium-ion: aging mechanisms, Vol. 5. Elsevier B. V.

7. Leuthner S, Kern R, Fetzer J, Klausner M (2011) Influence of automotive requirements on test

methods for lithium-ion batteries. Battery testing for electric mobility, Berlin, Germany



3



Materials and function

Kai Vuorilehto



Contents

3.1Introduction���������������������������������������������������������������������������������������������������������������������� 21

3.2 Traditional electrode materials ���������������������������������������������������������������������������������������� 21

3.3 Traditional inactive materials ������������������������������������������������������������������������������������������ 23

3.4 Alternatives for standard electrode materials ������������������������������������������������������������������ 24

3.5 Alternatives for standard inactive materials �������������������������������������������������������������������� 26

3.6Outlook���������������������������������������������������������������������������������������������������������������������������� 27

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3.1Introduction

Lithium-ion batteries are hi-tech devices made of complex and highly pure chemicals and other raw materials. This Chapter aims to give a comprehensive picture

of these materials and their functions. One might think that the lithium-ion battery

is lightweight due to the small mass of its main component, lithium. However, this

is not quite true: only 2 % of the battery mass is lithium, the rest being electrode

materials, electrolyte, and inactive structural components.



3.2



Traditional electrode materials



The basic structure of the lithium-ion battery has changed little since 1991, when

Sony brought the first version on the market. The main components of the l­ ithium-ion

battery are shown in Fig. 3.1.

K. Vuorilehto (*)

Aalto University Helsinki, Kemistintie 1, 02150 Espoo,

Finland

e-mail: kai.vuorilehto@helsinki.fi

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

R. Korthauer (ed.), Lithium-Ion Batteries: Basics and Applications,

https://doi.org/10.1007/978-3-662-53071-9_3



21



22



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Fig. 3.1  Components of a traditional lithium-ion battery during discharging (Courtesy of Antti

Rautiainen, Tampere University of Technology)



The positive electrode is often called “cathode” and the negative electrode

“anode”. These names represent reality only when discharging the battery. This

is different during the charging process, where the positive electrode works as

anode and the negative works as cathode. The misleading nomenclature stems from

lithium primary batteries that are never charged.

The traditional positive electrode material is lithiated cobalt oxide, LiCoO2.

It has a layered structure with alternating cobalt, oxygen, and lithium ion layers.

During charging, lithium leaves the crystal (deintercalation); during discharging,

it returns (intercalation). However, only 50 % of the lithium may be utilized. If

more than half of the lithium leaves the crystal, the structure may collapse and

liberate oxygen [1]. This can cause thermal runaway, as oxygen is able to burn the

electrolyte.

For complete discharging, the reaction at the positive electrode is:





2Li 0.5 CoO 2 + Li+ + e− → 2 LiCoO 2



Thus for one mole (7 g) of active lithium, two moles (189 g) of Li0.5CoO2 are needed

as host for lithium during discharge.

By far the most common negative electrode material is graphitic carbon. It has

carbon atoms in parallel graphene layers (Fig. 3.1, insert on the right). During



3  Materials and function23



charging, the lithium ions are intercalated into the graphite, between its layers.

During discharging, lithium leaves the graphite.

Unlike cobalt oxide, graphite is stable even without lithium, so it can be almost

completely discharged.

For complete discharging, the reaction at the negative electrode is:





LiC6 → Li+ + e− + 6 C



Thus for one mole (7 g) of active lithium, there are six moles (72 g) of carbon that

act as host for the lithium during charging.



3.3



Traditional inactive materials



The positive and negative electrode materials are powders that are applied as coatings on current collectors, resulting in composite electrodes. The positive current

collector is aluminum foil, typically 15 to 20 μm thick. Aluminum has a high conductivity and it is rather stable even at the high potential of the positive electrode.

The negative current collector is copper foil, typically 8 to 18 μm thick. Aluminum

would be lighter and cheaper but it cannot be used at the low potential of the negative electrode due to parasitic formation of a lithium/aluminum alloy.

For the coating process, a mixture of electrode material, binder, conducting additives, and solvent is prepared. The binder is needed to ensure good cohesion of the

electrode particles and sufficient adhesion to the current collector. Traditionally, polyvinylidene difluoride (PVDF) has been used as binder. PVDF forms hair-like structures that efficiently keep the coating together (Fig. 3.1, insert on the left). As PVDF is

not soluble in water, N-methyl pyrrolidone (NMP) is used as the solvent. It is vaporized during the drying of the composite electrode, and thus the finished cell does not

contain any NMP. Carbon black is used as conducting additive. The amount of additives is usually a trade secret. The order of magnitude is 1 to 5 % for carbon black,

2 to 8 % for PVDF. In energy optimized cells, the amount of additives is minimized as

additives do not store energy. In power optimized cells, good contact and conductivity

are more important, and the amount of additives can thus be higher. In research cells,

the amount of additives may be up to 10 % as extra volume and mass are ignored.

The void volume between the positive and the negative electrode as well as the

pores of the electrodes are filled with electrolyte. It is a solution of lithium salt in

a mixture of organic solvents. In commercial cells, lithium hexafluorophosphate,

LiPF6, is used as the lithium salt.

Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate

(EMC), and diethyl carbonate (DEC) are the most commonly used organic solvents.

Of these, EC is essential for cell stability as it protects the graphite surface [2].

However, at room temperature it is solid, preventing the use of pure EC. Typically a

ternary mixture of EC and two of the other carbonates is preferred.

To avoid direct contact and a short circuit between the positive and negative electrodes, a microporous membrane is used as a separator. As organic electrolytes have



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