Chapter 2

Fundamentals in Electrochemistry and Hydrometallurgy

Alexandre Chagnes     PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, Paris, France     Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS 3459, France

Abstract

The development of processes for recycling spent lithium-ion batteries (LiBs) requires having a good knowledge of the battery components, skills in electrochemistry and physicochemistry applied to LiBs, and obviously, thorough backgrounds in separation science and chemical engineering. On the other hand, the engineers involved in the development of the next generations of LiBs should integrate very early special considerations to make recycling easier and economic as well as to minimize the environmental impacts of the batteries during its whole life cycle (eco-design). Therefore, engineers must have a very good view of the recycling processes and the associated physicochemistry. This chapter provides fundamental skills in electrochemistry and physicochemistry to help the engineers to develop LiBs and processes for recycling spent LiBs. This chapter gives prerequisites to the reader on physicochemistry and electrochemistry applied to LiBs. After giving a short view of the battery components in the first part of this chapter, the second part brings the backgrounds to understand the physicochemical and electrochemical phenomena taking place in LiBs. In the third part of this chapter, the global flow sheet for recycling LiBs is presented and a special attention is paid on the physicochemistry involved in hydrometallurgical processes, and especially solvent extraction.

Keywords

Electrochemistry; Fundmentals; Hydrometallurgy; Lithium-ion batteries; Recycling

1. Fundamentals in Lithium-Ion Batteries

1.1. Principle and Definition

A battery contains several electrochemical cells that reversibly convert chemical energy into electrical energy from a spontaneous redox reaction in which electron transfer is forced to take place through a wire (Figure 2.1). An electrochemical cell contains four main components: cathode, anode, electrolyte, and separator. Ions move between the anode and the cathode at which an oxidation reaction (electrons transfer from the electrode to the electrolyte) and a reduction reaction (electrons transfer from the electrolyte to the electrode) occur, respectively. More specifically, a lithium-ion battery (LiB) is a rechargeable battery that contains several cells in which lithium ions move between the anode and the cathode. The cathode is commonly a lithium metal oxide material, which emits lithium ions to the anode during charging and receives lithium ions during discharging, whereas the anode, i.e., graphite in most LiBs, receives lithium ions from the cathode during charging and emits lithium ions during discharging (Figure 2.2).
Figure 2.1 Charge-discharge in rechargeable batteries.
Figure 2.2 Charge-discharge in lithium-ion batteries. The electrolyte contains a high-grade lithium salt (LiPF6, LiBF4, etc.) dissolved in a dipolar aprotic organic solvent such as a mixture of alkyl carbonates (for instance, ethylene carbonate and dimethyl carbonate). The separator is a microporous polymer membrane allowing lithium ions to pass through the pores and prevents short-circuits between the cathode and the anode. More information about the electrodes, the electrolytes, and the separator used in LiBs will be given in the next chapters. The electrochemical cell is then an element of the battery, which delivers between 3 and 4 V depending on the lithium-ion technology. These cells are plugged together in parallel to make a block, which delivers the same voltage but higher capacity than the electrochemical cells. The blocks can be plugged in series to make a battery, which delivers higher voltage (typically, 12 V in the case of LiBs) and the batteries can be plugged together in series and/or parallel to make a pack of batteries that exhibits higher voltage and energy than that delivered by the block. Four quantities are used to define the performance of a battery: Specific energy density (by weight or by volume in Wh/kg or Wh/L, respectively), which represents the amount of energy stored in the battery by unit of mass or volume. Power to weight ratio in W/kg, which represents the electrical energy provided by one kilogram of battery per second. Capacity in ampere-hour (Ah) is the amount of current provided by a battery before its complete discharge. The capacity is denoted Cn or C/n with n the number of hours for a complete discharge of the battery. Cycling ability, which represents the number of charge-discharge cycles that can be achieved without any loss of performance (drop of capacity). The nature of the electrolyte and the electrodes material has a strong influence on both energy density and cycling ability as explained below. In particular, it is crucial to maintain as low as possible the internal resistance in a LiB throughout its life, especially if the battery is dedicated to applications requiring a high charge rate like it is the case for electric vehicles. The electric power P is equal to the product of the voltage by the current delivered by the battery (P = VI). Therefore, the electric power is all the greater as the voltage is high, i.e., the battery resistance is low as shown in the following equation:

=Voc-RbI

(2.1)

where Voc denotes the open-circuit voltage (which only depends on the electrodes material) and Rb is the internal battery resistance. The internal battery resistance (Rb) can be expressed as follows:

b=Rel+Rin(N)+Rin(P)+Rc(N)+Rc(P)

(2.2)

where Rel, Rin(P), Rin(N), Rc(P), and Rc(N) denote the electrical resistance, the interfacial resistance at positive (P) and negative (N) electrodes, and the resistance of the current collector at positive (P) and negative electrodes (N), respectively. The electrical resistance depends on the width between the positive and the negative electrodes (L), the geometric area of the electrodes (A), and the ionic conductivity of the electrolyte (?):

el=L/(?A)

(2.3)

Equation (2.3) shows that the electrical resistance can be lowered by decreasing the width between the electrodes and by increasing the ionic conductivity of the electrolyte and the geometric area of the electrodes. Nevertheless, the area of the electrodes should not be too large as the interfacial resistance (Rin) is proportional to A/Asp, where Asp denotes the interfacial area at the electrode/electrolyte interface (which can be assimilated to the specific surface of the electrode). On the other side, the interfacial resistance can be lowered by increasing the specific surface of the electrode, i.e., by using porous electrodes with nanoparticles provided that a good electrical contact occurs between the nanoparticles. The resistance of the collector (Rc) mainly depends on the conductivity of the material used as current collector, the electrodes material, and the geometric surface of the collector. The latter must be as high as possible and the width of the electrode must be as thin as possible:

c=lAse+1dsm

(2.4)

where l is the mean free path of an electron through the electrode width, sm is the electrical conductivity of the collector, se is the electronic conductivity of the electrode, and d is an empirical parameter. Then, the internal resistance of a battery can be decreased by optimizing the geometry of the battery, by using porous electrodes, and by increasing the ionic conductivity of the electrolyte. Nevertheless, the performances of LiBs do not only depend on the internal battery resistance. For instance, the longevity and the charge rate of a battery are governed by the nature of the electrode materials (diffusion coefficient of lithium ions into the host material, resistance of the material against large volume variation, etc.) and the electrode/electrolyte interface that results from the reactivity of the electrode material toward the electrolyte. The optimization of the electrolyte properties, the investigation of the electrochemical phenomena that takes place at the electrode, and the electrode/electrolyte interface require fundamental knowledge in solution chemistry and electrochemistry. These prerequisites are given in the following part of this chapter and will be useful all along this book.

1.2. Physicochemistry

1.2.1. Viscosity

The viscosity of a fluid is a measure of its resistance to a gradual deformation by shear stress or tensile stress. The Poiseuille's law defines the relationship in absence of turbulence between the resistance and the viscosity for uniform fluids, i.e., Newtonian fluids. In the case of a laminar flow into a cylinder, the velocity of flow varies from zero at the walls to a maximum along the centerline of the cylinder. If the fluid is split into thin layers that slide smoothly over each over, the...