The aging of lithium-ion batteries is a complex chemical reaction process. From a mechanistic perspective, it mainly refers to irreversible reactions that occur at the contact surfaces between the positive and negative electrodes, electrolyte, and electrolyte electrodes of lithium-ion batteries. These reactions lead to capacity decay and impedance increase in lithium-ion batteries, which are collectively referred to as aging. In general, the aging mechanism of lithium-ion batteries can be divided into three categories: the reaction between active materials and electrolytes at the electrode interface (also commonly summarized as the generation of intermediate phases in solid electrolytes), the self degradation of active material structures during cycling, and the aging of non active components.
(1) The reaction between active materials and electrolytes at the electrode interface
Because the electrolyte of lithium-ion batteries is liquid, there is a solid/liquid interface between the positive and negative electrode materials and the electrolyte. At this interface, the active material undergoes a chemical reaction with the electrolyte, producing insoluble products that adhere to the electrode surface, forming a new phase interface, These insoluble products are referred to as the intermediate phase of solid electrolytes. The intermediate phase of a solid electrolyte, like a separator, is non-conductive but allows ions to pass through. It is composed of various compounds, mainly related to the composition of the electrolyte and additives.
In a lithium-ion battery with graphite as the negative electrode and a mixture of ethylene carbonate solvent and lithium hexafluorophosphate as the electrolyte, the main components of the intermediate phase of the initially formed solid electrolyte on the negative electrode surface are ethylene dicarbonate and lithium fluoride, which are the reduction products of ethylene carbonate and lithium hexafluorophosphate. Subsequently, lithium ethylene dicarbonate may undergo further decomposition, producing some insoluble components in the electrolyte, soluble components in the electrolyte, and a small amount of gas. The insoluble components include lithium alkoxide, lithium fluorophosphate, polyethylene oxide, lithium carboxylate, lithium carbonate, lithium oxide, and lithium fluoride, while the soluble components include ethers, low polyethylene oxide, and fluorophosphate, Gases include carbon dioxide and ethylene. The decomposition reaction of lithium ethylene dicarbonate causes a more porous phase transition in the solid electrolyte, leading to further reduction of the electrolyte and the generation of lithium ethylene dicarbonate and lithium fluoride. Repeated decomposition and reduction reactions will lead to thickening of the intermediate phase in the solid electrolyte. However, over time, the decomposition reaction will decrease, The thickness of the intermediate phase in the final solid electrolyte will tend to stabilize.
The formation mechanism of the intermediate phase in the positive electrode solid electrolyte is different from that in the negative electrode, and compared to the negative electrode, the formation of the intermediate phase in the positive electrode solid electrolyte is rarely discussed as an important mechanism leading to the aging of lithium-ion batteries. At present, there is no unified opinion on the formation mechanism of the intermediate phase in positive electrode solid electrolytes. However, in a recently published publication, Markevich et al. found that the content of lithium fluoride in the intermediate phase of solid electrolytes formed at the positive electrode is much lower than that at the negative electrode, It is pointed out that although the intermediate phase of solid electrolyte itself can lead to an increase in electrode impedance, it also has the function of protecting the electrode from electrolyte erosion and preventing large-scale electrolyte oxidation.
(2) Self degradation of active material structures in cycles
The self degradation of the active material structure during cycling is another major cause of aging in lithium-ion batteries, mainly including the corrosion and dissolution of electrode active materials, electrode structure decay, and electrode damage caused by particle breakage. Taking ternary lithium-ion batteries as an example, the corrosion dissolution of active materials refers to the dissolution of transition metal elements such as manganese and nickel in the positive electrode into the electrolyte, followed by deposition at the negative electrode, which can also cause the dissolution and regeneration of the intermediate phase in the solid electrolyte of the negative electrode. The decay of electrode structure is due to the detachment of lithium ions from the positive electrode material during cycling, which can lead to the instability of the positive electrode material structure. The transition metal elements in the material are easily mixed and discharged, causing the material to transform from a layered structure to a spinel right structure, and ultimately into a rock salt structure, resulting in the loss of the positive electrode material. Particle breakage is often caused by high current in lithium-ion batteries, increased mechanical stress in porous electrodes, fragmentation of active particles, and lattice changes, which can also lead to a reduction in lithium insertion positions in the active material.
(3) Aging of inactive components
The non active components of lithium-ion batteries mainly include electrode adhesives and positive and negative current collectors. As the number of cycles in lithium-ion batteries increases and time passes, the electrode adhesives may decompose, while the positive and negative current collectors may corrode, leading to an increase in internal resistance of lithium-ion batteries. However, the aging of non active components is generally not considered the main cause of aging in lithium-ion batteries.