What Is Battery Capacity Degradation Mechanism?
Change in Material Structure
The currently most widely used cathode materials mainly include the hexagonal layered structure of LiMO2 (where M=Co, Ni, Mn), the spinel structure of LiMn2O4, and the olivine structure of LiFePO4. Regardless of the structure, when lithium ions deintercalate from the cathode, to maintain the electrical state in the material, the metal element is inevitably oxidized to a higher valence state, which is accompanied by a phase transition process. Phase transitions often lead to phase shifts, so as lithium ions continuously intercalate and deintercalate in the material, the phase change continues, and in the long run, it will pose a threat to the crystal stability. Compared with the anode, the unequal reversible capacity caused by phase shifts and changes in the bulk structure of the cathode material has a significant impact on battery life. Graphite has a layered structure. When it is several layers thick, lithium ions intercalate into the interlayers during battery charging and combine with electrons transported from the external circuit to form lithiated graphite, and the interlayer spacing increases at this time; during discharge, lithium ions leave the graphite interlayers and release electrons to the external circuit, undergoing a deintercalation and oxidation reaction, and the interlayer spacing decreases at this time.
Dissolution of Active Material
The dissolution of cathode material refers to the process where the active material gradually decreases due to corrosion in the electrolyte. The dissolution of cathode material at high temperatures is one of the reasons for battery capacity decay, especially having a greater impact on the cycle performance and storage performance of batteries at high temperatures. The dissolution of transition metals under certain conditions is a problem that exists in all LiMO2 cathode materials. The main reasons why the dissolution of active material leads to the deterioration of battery performance are: $\textcircled{1}$ The dissolution of metal elements directly leads to the reduction of active material, causing battery capacity loss; $\textcircled{2}$ The dissolution of cathode material causes the degradation of the material structure and the formation of chemically inactive substances on the surface of the particles, which hinders the transport of lithium ions in the electrode material; $\textcircled{3}$ The solvated metal ions contained in the electrolyte migrate to the anode in the electrolyte and deposit on the anode surface in the form of metal or salt under low potential, and these deposits inevitably affect the stability and thickness of the SEI film on the anode surface, leading to increased electrode surface polarization and increased battery internal resistance. Therefore, the effect of active material dissolution on the electrolyte does not only come from dissolution but also from more adverse effects brought about by the dissolution of transition metals.
Consumption of Lithium Ions
In the design of lithium-ion batteries, the capacity of the battery is generally slightly more than that of the cathode, and the recyclable lithium ions are also provided by the cathode. Therefore, the reversible intercalation and deintercalation of lithium ions between the cathode and anode determine the battery capacity. During the first charge and discharge process, an SEI film is formed on the anode surface. The main components of this passivation film are various inorganic substances such as Li2CO3, LiF, Li2O, LiOH and various organic components such as ROCO2Li, ROLi, and (ROCO2)2Li. Thus, some lithium ions are consumed, and this capacity loss is irreversible. The performance of the anode is highly related to the morphology and stability of the SEI film, and the ability to form a stable SEI film on the anode surface has a non-negligible impact on battery performance. The formation of the SEI film consumes the limited lithium ions in the battery. If the SEI film is continuously damaged during the cycle, then the oxidation reaction at the anode/electrolyte interface will continuously occur to form a new SEI film. This process consumes the limited lithium ions provided by the cathode in the system, and the reduction of active lithium ions leads to capacity decay. The reduction of lithium ions in the electrolyte leads to a decrease in the conductivity of the electrolyte, and the loss of lithium ions in the cathode material causes an imbalance between the two electrodes of the battery.
Increase in Internal Resistance
During the long-term cycling of the battery, the increase in internal resistance is also an important reason for capacity decay. There are many reasons for the increase in internal resistance, mainly from two aspects: $\textcircled{1}$ The oxidation reaction occurring at the electrode/electrolyte interface in the electrolyte leads to an increase in the surface film resistance of the electrode, and the instability of the anode SEI film, continuously forming new surface films during the cycle, etc., all increase polarization and battery internal resistance; $\textcircled{2}$ The dissolution of metal ions in the cathode into the electrolyte, and the dissolved ionized metal ions migrate to the anode through the electrolyte and deposit on the anode surface in the form of metal or salt, resulting in increased electrode polarization. In addition, research has also proven that the corrosion of the current collector can also lead to an increase in internal resistance, but this effect is relatively small under the premise of pretreatment of the current collector. The increase in internal resistance leads to a decrease in energy density and capacity, especially for the anode, the reaction occurring at the electrode/electrolyte interface is the main reason for anode aging.
