Fundamentals of Lithium-Ion Cells

Lithium-ion cells are differentiated in primary and secondary cells. While primary cells often allow for higher energy density, secondary cells have a crucial advantage: their ability to be recharged. Two electrodes, electronically separated by a thin polymer layer, are connected ionically by an electrolyte as illustrated in Figure 3.1. An electrode consists of an active material, coated on a thin, current collecting metal foil. The active material has the ability to intercalate lithium-ions, meaning the reversible insertion of lithium-ions.

In an ideal scenario, the active material acts as a host structure without interfering with the intercalated lithium-ion, and its physical and electrochemical properties remain unchanged during this process.

While discharging the cell, lithium-ions are de-intercalated from the anode (negative electrode). While the positively charged lithium-ions move through the electrolyte, the corresponding electrons are forced via an external circuit and an attached load. At the cathode (positive electrode), lithium-ions are intercalated and recombine with the electrons. In electrochemistry, the procedure is described as a redox reaction, where a reduction occurs at the cathode and the respective oxidation occurs at the anode. During the charging process, where an external charger is connected and energy is added to the system, the process is reversed.

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charge discharge e

-Current collector Current collector

Li⁺

Cathode Anode

Figure 3.1 – Working principle of a lithium-ion cell 3.1.1 Components in Lithium-Ion Cells

Lithium-ion cells consist of various components which are optimised for the particular purpose of application, for example high energy density, high power capability, or long lifetime. The most important ones will be briefly discussed in the following.

Anode Materials

Lithium metal is the most suitable anode material for lithium-ion cells due to its low voltage and high energy density, but is currently not used in secondary lithium-ion cells due to major safety concerns. During the charging process, lithium is not equally deposited on the electrode, but rather tends to form irreversible dendrites (small, multi-branching, tree-like structures). Beside the induced capacity fade due to the loss of cyclable lithium, after numerous charging and discharging processes, the size of the dendrite can become so large that it penetrates the separator, leading to a short circuit of the cell and in a worst case scenario to a thermal runaway of the cell. A safer and the most common anode material used in commercial lithium-ion cells is graphite, a crystalline form of carbon which allows the intercalation of up to one lithium ion per six carbon atoms (LiC₆) (Dahn, 1991). During the first charging process, a surface layer – denoted as solid electrolyte interphase (SEI) – is formed on the anode, consuming lithium and electrolyte to some extent, but also passivating the electrode to minimise further side reactions.

Cathode Materials

Depending on the requirements regarding energy density, power capability, price, and cycle stability, a large variation of cathode materials is used in lithium-ion cells. The first commercially available cell, introduced in 1991 by Sony, contained lithium cobalt oxide

3.1. FUNDAMENTALS OF LITHIUM-ION CELLS 39 (LiCoO₂) as cathode material. Extensive research efforts led to the reduction of cost by replacing the expensive and toxic cobalt (Co) partially with other transition metals such as nickel (Ni), manganese (Mn) and aluminium (Al).

Focussing on automotive application, the field of possible cathode materials can be narrowed down to LiNi_0.8Co_0.15Al_0.05O₂ (NCA) (Madhavi, Rao, Chowdari, & Li, 2001) and LiNi_xMn_yCo_zO₂ (NMC) (Pan et al., 2013). Various combinations of NMCXYZ are available, denoted as, for example, NMC111, NMC442, and NMC622, where the ratio of the used metals has been varied. Other materials like LiFePO₄(LFP) or LiMn₂O₄(LMO) play currently only a minor role in automotive applications due to their low voltage (Padhi, Nanjundaswamy, & Goodenough, 1997) or low cycle stability (Choi & Manthiram, 2006).

Separator

Separators in lithium-ion cells have to fulfil a large number of requirements. Beside a low price, high mechanical as well as chemical stability in various media over a large voltage range is required. A high porosity in combination with a small pore size enables fast lithium diffusion through the layer. In wound cells like cylindrical or prismatic cells, most commonly polymer separators are used. Stacked cells also allow the use of glass fibre or ceramic coated polymer separators, which are usually not as flexible as their polymer-only counterparts, but offer high mechanical stability and easier handling during production.

Electrolyte

Non-aqueous electrolytes are used for lithium-ion cells, containing a combination of various organic solvents and a conducting salt. The solvent is, depending on the application and the used active materials in the cell, a mixture of different carbonates (e. g. ethylene carbonate (EC), dimethyl carbonate (DMC)). The conducting salt in commercial cells is lithium hexafluoride (LiPF₆).

Various additives enhancing the cell performance are reported in literature. A special focus is set on the graphite surface layer, as a smooth and homogeneous passivation of the anode is a crucial requirement for a long cycle life. Furthermore, a stabilising effect on the electrolyte for high and very low voltages is desired, as the electrolyte is only electrochemically stable within a certain potential range, usually ranging from 0.4 V to 4.8 V (Xu, Ding, & Jow, 1999).

3.1.2 Battery Degradation

During operation but also when the cell is stored under open circuit conditions (i. e. not charged or discharged), a continuous loss of power capability and energy density can be observed in lithium-ion cells, commonly denoted as ageing of the battery (Barré et al., 2013; Broussely et al., 2005; Vetter et al., 2005).

The power capability loss originates from an increased overpotential due to higher impedance. The energy density loss is caused by capacity fade and impedance rise. The

energy density shall be used to quantify the ageing effects in the further course of this work.

The cause for the capacity fade is divided into two effects. The first one is loss of active material, where the active material of the electrodes loses electrical contact to the current collector or is consumed by parasitic chemical side reactions. As a consequence, it loses its capability to reversibly intercalate lithium-ions. The second possibility is the loss of lithium inventory. During operation, the electrochemical stability window of the electrolyte is frequently violated. As described in Section 3.1.1, this leads to the formation of a surface layer denoted as SEI during the first cycles, which is supposed to insulate the anode and prevent further reaction. Due to the mechanical stress during cycling, the layer is constantly damaged, which leads to further side reactions, consuming lithium, which is actually supposed to shuttle between the two electrodes. This process is accelerated by elevated temperatures (Safari & Delacourt, 2011). The increasing thickness of the layer leads to a higher impedance of the cell, as the intercalation of the lithium-ions is impeded. This is reflected by a larger potential drop during the discharging process or an increased potential rise when charging.

The described ageing mechanisms occur both during cycle and calendar ageing.

However, the ratio of the effects may vary for the two ageing modes.

The inactive components of a cell as separator (Peabody & Arnold, 2011) and current collector (Braithwaite et al., 1999) also undergo certain degradation, but their effect on the overall performance of the cell is rather small.