Chapter 2
Aging mechanism
Several phenomena contribute in damaging of lithium ion cell, their understanding allows to build up a proper mathematical models and experimental tests.
This study aims to deepen the knowledge on capacity fade, so the phenomena which cause this type of damaging are examined in the next section.
A schematic summary of all the possible degradation mechanism is reported in
gure 2.1.
Figure 2.1: Schematic summary of degradation mechanism in lithium ion cell [4]
can be stored in the electrodes, the fewer the potential which can be reached at the end of the charge.
The lithium consumption is mainly due to side reactions which occur in the electrolyte, such us decomposition reaction, lithium plating and especially surface lm formation (SEI growth). Moreover lithium ions could be trapped in electrically isolated particles and no longer available for cycling.
Loss of anode active material. Cracks on the particles of the active material could lead to a reduction of the material available for lithium insertion and loss of electrical contact between the particle and the binder or the current collector.
Loss of cathode active material. Structural disordering and crack could re-duce the active material available for lithium insertion.
In these study we particularly focus on anode damaging, meant as reduction of electrical contact with the current collector and cracks propagation.
An interesting summary of the cause/eect relations of degradation mechanism is reported in gure 2.2.
Figure 2.2: Lithium-ion anode ageingcauses, eects, and in uences [4]
2.1.1 Change in electrode/electrolyte interface and SEI for-mation and growth
Changes at the electrode/electrolyte interface due to side reactions between anode and electrolyte are considered by many researchers to be the major source for
ageing. In gure 2.3 are summarized the main phenomena which lead to changes at the anode/electrolyte interface.
Figure 2.3: Lithium-ion anode ageingcauses, eects, and in uences [7]
Graphite anodes operate at voltage values outside the electrochemical stability window of the electrolyte components, as shown in gure 2.4. For this reason the electrolyte decomposition takes place at the electrode/electrolyte interface when the electrode is in the charged state.
The decomposition products, the so called SEI layer, covers the electrode's sur-face. The SEI is mainly formed during the rst cycles and protects the electrodes from further degradation, but causes also a capacity loss since it consumes lithium ion, and the loss of cyclable lithium is considered the most important reason to capacity loss.
The irregular growth of the SEI layer could be caused by cracks in graphite layer, since their presence allows the exposition of fresh active material to electrolyte.
Several researches [22, 16] demonstrated that lithium intercalation causes crack in graphite particles. On the contrary no researches describes a possible mechanical failure, such as cracks, due to vibration load. It is supposed that if vibration load enhances cracks propagation, how might be expected, also this source of damage could induce a capacity fade.
The properties of the SEI layer are not comparable to the one of a solid electrolyte, and other charged species (anions, electrons, solved cations) and neutral species (solvent and impurities) still diuse through the SEI in the later stages of cycling, causing anode corrosion (capacity fade) and electrolyte decomposition throughout the entire battery life. However this eects occur at a lower extent and at lower rate, compared to the rst cycle. [7]
On a long time scale the SEI penetrates into pores of the electrode, resulting in a decrease of the accessible active surface area of the electrode.
The SEI growth can leads to a gradual reduction of the contact between the par-ticles of the active material, causing an increase in the cell impedance.
The increase in electrode impedance is considered to be caused by the growth of the SEI as well as by changes of the SEI in composition and morphology. Indeed even if the SEI formation takes place just in the rst cycles, the SEI conversion proceeds also during further cycling and storage.
Figure 2.4: Electrochemical potential of some common active materials related to electrochemical window of the electrolyte (HOMO-LUMO)
It is widely accepted that high temperature aects the morphology and com-position of the SEI layer. Indeed the negative impact of elevated temperature on cell ageing is mainly related to the degradation of the SEI layer which starts to break down and to dissolve. Great in uence of the temperature is registered when the cell are stored at elevated temperature even below 60°C.
Even low temperatures result in dierent changes. It emphasizes the slow lithium ions diusion into carbon and/or in the electrolyte. Moreover it can cause metallic lithium plating and lithium dendrite growth.
Finally the interaction of the cathode with anode has to be taken into consid-eration. In fact the transport of soluble SEI products between anode and cathode and possible chemical redox reactions at the respective electrodes could occur.
However the main eect of the cathode on the anode properties is transition metal dissolution. In fact transition metal ions (such as Mn2+) in LiMn2O4 spinel can be incorporated into the anode SEI, leading to accelerated cell ageing.
In [16] the loss of active material from the cathode was observed after the vibration test.
2.1.2 Damaging of the active material
The damage of active material is considered one of the main contribution to the capacity fade, mainly for three reasons: it reduces the active site for lithium ions intercalation [4], it reduce the electric contact and enhance the SEI production.
For what concern the rst reason is clear that a reduction of the speci c active area decreases the hosting capability of lithium ions in the electrode, decreasing the capacity which can be stored in the cell.
The particle damaging can reduce the electric contact between the particles and the binder, reducing the electric transfer capability, moreover it can reduce the contact between the particle and the current collector, decreasing the amount of electrons transferred to the external circuit. This phenomenon is detected by cell impedance rise.
Therefore the crack formation creates new free surface within the graphite par-ticles exposed to the electrolyte, this leads to the formation of a SEI layers on the new free surface, as in the rst cycles of the cell. The high rate of SEI formation on the new surface cause a sharp increase of the lithium consumption, no more available for cycling.