1. Structural changes in cathode materials
Cathode materials are a crucial component of lithium-ion batteries. When lithium ions are extracted from the cathode, the metal elements must be oxidized to a high oxidation state to maintain the material's electrical neutrality, resulting in a compositional shift. This shift can easily lead to phase transfer and changes in bulk structure. Phase transitions in electrode materials can cause changes in lattice parameters and lattice mismatch. The resulting induced stress causes grain breakage and crack propagation, leading to mechanical damage to the material's structure and consequently, a decline in the electrochemical performance of the lithium-ion battery.
2. Structure of negative electrode material
Commonly used negative electrode materials for commercial lithium-ion batteries include carbon materials and lithium titanate. This article analyzes graphite as a typical negative electrode. The capacity decay of lithium-ion batteries first occurs during the formation stage, when an SEI (Sediment Injection) forms on the surface of the negative electrode, consuming some lithium ions.
With the use of lithium-ion batteries, changes in the graphite structure can also lead to a decrease in battery capacity. Studies have found that although the carbon material retains the morphology and structure of graphite after cycling, the half-width at half-maximum (WHM) of its (002) crystal plane increases, resulting in a smaller grain size along the c-axis. This change in crystal structure causes cracks in the carbon material, which in turn damages the SEI film on the negative electrode surface and promotes its repair. The excessive growth of the SEI film consumes active lithium, thus causing irreversible capacity decay in lithium-ion batteries.
3. Oxidative decomposition of electrolyte
The properties of the electrolyte significantly affect the specific capacity, lifespan, rate charge/discharge performance, operating temperature range, and safety performance of lithium-ion batteries. The electrolyte mainly consists of three parts: solvent, electrolyte, and additives. The decomposition of both the solvent and the electrolyte leads to a loss of battery capacity. Electrolyte decomposition and side reactions are important factors in lithium-ion battery capacity decay. Regardless of the positive and negative electrode materials or manufacturing process used, with the cycling of lithium-ion batteries, electrolyte decomposition and interfacial reactions with the positive and negative electrode materials will cause capacity decay.
4. Fast charging and discharging
During fast charging, the excessive current density leads to severe polarization of the negative electrode, making lithium-ion battery deposits more pronounced. This causes the copper foil at the boundary between the copper foil and the carbon-based active material to become brittle and prone to cracking. The spontaneous winding of the battery cell is constrained by a fixed space, preventing the copper foil from stretching freely and creating pressure. Under this pressure, existing cracks spread and grow, and due to insufficient space for expansion, the copper foil breaks.
5. Long-term deep charge and discharge
From an internal structural perspective, discharging would cause two problems: first, excessive evaporation of the electrolyte; and second, excessive reaction of the negative electrode in the lithium-ion battery would alter its dielectric film, leading to a decrease in intercalation/deintercalation capability and resulting in permanent capacity loss.
Charging is crucial, especially considering voltage stability and the potential for increased grid voltage late at night. Chargers that have already stopped charging may resume charging when the voltage rises, leading to overcharging of the battery. This can cause changes in the positive electrode material structure, capacity loss, decomposition, oxygen release, and violent oxidation reactions with the electrolyte, potentially resulting in combustion and explosion. Additionally, the electrolyte's organic solvents/electrolyte lithium salts may decompose. Over-discharge of lithium on the negative electrode can lead to the dissolution of the copper current collector and the formation of copper dendrites on the positive electrode.
