Definition and working principle

A battery is an electrochemical device that allows conversion of the chemical energy into electrical energy. The most common classification of battery is dictated by their ability to be recharged or not. A primary battery is non rechargeable. After discharge, it is not possible to

Fluoride-ion batteries: introduction and state of the art

recharge it as the electrochemical reactions taking place are irreversible, and the battery is discarded. A secondary battery is rechargeable and several charge discharge cycles can be performed until significant loss of performance is experienced.

Strictly speaking, a battery is a package constituted of one or more electrochemical cells that are in series and/or in parallel. For instance, in a typical LIB used in laptops, the Li-ion cells are in series or parallel to modulate the battery’s output voltage and capacity, respectively.

Some batteries can be constituted of a single cell, such as the well-known AA alkaline batteries for instance. For this reason they are often referred as AA cells.

An electrochemical cell, or galvanic cell, is composed of two half cells, each composed of an electrode in contact with an electrolyte. The conversion of the energy from chemical to electrical is made possible through electrochemical reactions also called redox (contraction of reduction-oxidation) reactions taking place simultaneously at the cathode and anode. These chemical reactions take place in the bulk, or at the surface, of the electrodes which are in contact with the electrolyte, and generate electricity (electrons) that is directed in the external circuit, connected to the appliance. The electrolyte is an ionic conductor. It ensures the shuttling of ions from one electrode to the other during the discharge or charge processes, and must be electronically insulating to avoid short-circuits. In a LIB, the electrolyte ensures the movement of the lithium ion Li⁺. In a FIB, it is the fluoride ion F^- that is transported in the electrolyte. The concept of FIBs will be further detailed in the next section.

Another difference that will be of particular interest in this manuscript is the “state” of the battery, usually defined by the liquid state or solid state in which the electrolyte can be found.

On one hand, most of battery technologies use liquid electrolytes and are therefore generally referred as conventional batteries. On the other hand, solid-state batteries are composed of solid electrolytes. All-solid-state batteries, like the FIBs on which this manuscript focusses, are solely composed of solid materials for both the electrolyte and the electrodes. This particular solid configuration leads to some advantages, notably in terms of security, as the risk of leakage or short-circuit can be easily avoided. However, they present serious challenges like for instance the lower ionic conductivity of the solid electrolyte, or the large volumetric variations expected to be observed upon cycling.

Fluoride-ion batteries: introduction and state of the art

1.1.2.Working principle

The FIBs studied in this thesis are classified as all-solid-state batteries. For this reason, a FIB’s electrode contains solid electrolyte. (N. B.: The purpose of adding electrolyte in the electrode is addressed in 2.1.1.2. Such approach does not apply to microbatteries.)

One of the key components of the battery is the active material (AM) which allows producing electrical energy through redox reactions. In FIBs, the AM is either a metal (𝑀) or a metal fluoride (𝑀𝐹_𝑥) or even a mixture of both. They undergo redox reactions within the electrode to yield electrons (𝑒⁻) or fluoride ions (𝐹⁻) as presented in Figure I.1.

Upon discharge, the metal fluoride cathode 𝑀𝐹_𝑥 (positive electrode) undergoes reduction while the metal anode 𝑀^′ undergoes a simultaneous oxidation according to the reactions presented below.

Discharge

At the cathode : 𝑀𝐹_𝑥 + 𝑥𝑒⁻ → 𝑀 + 𝑥𝐹⁻ 1.

At the anode : 𝑀′ + 𝑥𝐹⁻ → 𝑀′𝐹_𝑥 + 𝑥𝑒⁻ 2.

Total : 𝑀′ + 𝑀𝐹_𝑥 → 𝑀′𝐹_𝑥+ 𝑀 3.

The fluoride ions flow through the electrolyte while a similar charge of electrons passes simultaneously through the external circuit. Upon charge, the reactions occur in the opposite direction. The previously reduced metal 𝑀 (positive electrode) is now the anode and undergoes oxidation to yield back the metal fluoride. Simultaneously, the previously oxidized 𝑀′𝐹_𝑥 is now the cathode and undergoes reduction to yield back its corresponding metal 𝑀^′.

Charge

At the anode : 𝑀 + 𝑥𝐹⁻ → 𝑀𝐹_𝑥+ 𝑥𝑒⁻ 4.

At the cathode : 𝑀′𝐹_𝑥 + 𝑥𝑒⁻ → 𝑀′ + 𝑥𝐹⁻ 5.

Total : 𝑀′𝐹_𝑥 + 𝑀 → 𝑀′ + 𝑀𝐹_𝑥 6.

Fluoride-ion batteries: introduction and state of the art

Figure I.1. Illustration of a fluoride-ion battery upon discharge (left) and charge (right).

The anode is always associated to the oxidation process while the cathode is always associated to the reduction process. In a secondary battery, during successive discharge and charge processes, each electrode is alternatively the anode and the cathode. To distinguish the electrodes, one must observe the potential of their respective redox reactions. The electrode with the higher potential will be called the positive electrode and the other electrode the negative. This concept will be further explained in section 1.2.1.

In the above equations (1→6), x indicates the number of fluoride ions exchanged per mole of metal ion and is consequently indicative of the oxidation state of the metal. These reactions are only applicable if the metal ions do not oxidize or reduce to intermediary oxidation states (i.e.

𝑀³⁺↔ 𝑀²⁺, 𝑀²⁺↔ 𝑀⁺etc). For example, in the case of FeF3 (Fe³⁺), one can imagine an intermediary FeF2 (Fe²⁺) step before the full reduction to Fe°, as in LIBs[1]. Equation 1 would then be written as follow:

At the cathode: 𝑀𝐹_𝑦+ 𝑥𝑒⁻ → 𝑀𝐹_𝑦−𝑥+ 𝑥𝐹⁻ ͹^. With 2 ≤ y ≤ 5 and 1 ≤ x ≤ 4, even if assembly of practical cells with MFy (y ≥ 4) could be extremely delicate, as MF4’s and MF5’s are in most cases metastable, powerful oxidizing agents, and liquid, gaseous or extremely volatile at room temperature and ordinary pressure.

Fluoride-ion batteries: introduction and state of the art