Battery fast charging is a key breakthrough to solve the convenience of electric vehicle charging. The breakthrough of battery fast charging technology will improve the user experience of end products. Battery fast charging technology has become the core competitiveness of power battery companies in the world to participate in future market competition, and is rapidly iteratively innovating. However, from the perspective of battery performance, battery fast charging is difficult to be compatible with battery life and energy density.
At the same time, frequent fast charging will lead to rapid decline in charging acceptance, energy density, and power capacity. It is generally considered that battery fast charging is a problem of battery reaction kinetics, involving lithium ion transport in the electrolyte and lithium ion diffusion in the solid-state electrode, and other properties are mainly related to material degradation caused by fast charging. This paper mainly discusses the performance degradation of lithium-ion batteries under fast charging conditions from the perspective of battery materials, and proposes some solutions for improving the lithium-ion battery fast charging performance in the future.
1. Graphite anode
A large number of studies have proved that metal lithium precipitation is the main reason for the capacity fading and power loss of lithium-ion batteries. Theoretically speaking, lithium metal in the anode is formed, because the coulombic efficiency of lithium metal is low and metal lithium is prone to side reactions with the electrolyte solvent. Both of these reactions will deplete the active Li+ that can be deintercalated between the cathode and anode materials in the battery, and consume a large amount of electrolyte, resulting in a large amount of gas generation inside the battery and resulting in uncontrolled SEI growth.
However, in fact, the battery fast charging does not always trigger lithium evolution, because in a normal lithium-ion battery, the impedance of the graphite anode is much lower than that of the cathode. A number of studies have shown that the polarization caused by normal battery fast charging is not enough to drive the potential of the graphite anode to the potential of metal lithium deposition.
Even if there is no precipitation of metal lithium, the battery fast charging will still accelerate the capacity decay and impedance growth of the battery. Related studies have found that the failure of the SEI on the graphite surface is one of the main reasons for this phenomenon. The disruption of the SEI also leads to the co-intercalation of solvated Li+ ions. The study proposes two solutions:
- First, reduce the particle size of graphite, which is conducive to the formation of stable SEI by reducing the particle size of the anode, thereby reducing the capacity fading of lithium-ion batteries during fast charging;
- Second, the use of electrolyte additives that can participate in the formation of stable SEI, through the addition of additives, the SEI film has good mechanical stress to further stabilize the performance of lithium-ion batteries in fast charging.
2. Cathode materials
Compared with graphite anode materials, layered cathode materials have higher impedance. It is generally believed that the cathode has slow reaction kinetics and high interface/phase transfer resistance is considered to be the main factor limiting the battery fast charging. According to practical experience, the reaction kinetics can be enhanced by reducing the particle size of cathode materials. In actual production, layered cathode materials exist in the form of spherical secondary particles, which are formed by agglomeration of many small block single crystals.
The low electronic and ionic conductivities between single crystal primary particles in these spherical particles limit the reaction kinetics of cathode materials. Fragmentation of large secondary particles into small single-crystal materials can enhance the rate capability of layered cathodes. It is worth noting that a large number of studies have shown that microcracks in the secondary particle structure during cycling are another important reason for the performance degradation of layered cathode materials, but microcracks are not the direct cause of performance degradation.
Instead, the immediate root cause is oxygen loss at high state-of-charge (SOC), which can be mitigated by limiting the SOC to relatively low levels during charging. Oxygen loss can be reduced in this regard by limiting the SOC to 75%, and can be achieved simply by reducing the charge cut-off voltage. On the other hand, the residual alkaline substances will significantly increase the Rsl of the cathode, and the elimination or reduction of residual alkali on the surface of the cathode material is another effective strategy to achieve lithium-ion battery fast charging.
In addition, studies in graphite/NCA batteries have shown that most of the capacity lost during battery fast charging can be recovered by in situ replenishment of Li+ ions. It shows that the battery fast charging will not destroy the crystal structure of NCA. Instead, it is the loss of active lithium ions that causes the capacity fade. In addition to the lithium precipitation and the reaction of the precipitated lithium on the surface and the electrolyte, the loss of active Li+ ions mainly comes from two sources:
- First, the reformation of the SEI of the graphite anode;
- Second, the oxygen loss of the layered cathode material.
The electrolyte in lithium-ion batteries mainly refers to the role of transporting lithium ions. The three factors reflected in the battery AC impedance spectrum that affect the battery impedance are mainly Rb, Rsl and charge transfer resistance (Rct), which affect the rate capability of the lithium-ion battery. These three impedances are all related to the electrolyte. Among them, Rb is mainly related to the movement of lithium ions in the liquid phase, but it only accounts for a small part of the total impedance in the impedance spectrum.
In other words, the transport of Li+ ions is unlikely to dominate the battery fast-charging limit. At high rates, the impedance of Li-ion batteries is determined by Rsl and Rct, especially Rct is affected not only by Rb and Rsl, but also by the desolvation and solvation activation energy of Li+ ions at the electrolyte-electrode interface/interphase. Therefore, the electrolyte can improve the battery fast charging performance through two strategies:
- The first is to develop electrolytes with low activation energies for the solvation and desolvation of Li+ ions;
- The second is to explore electrolyte additives that can participate in the formation of a robust and highly conductive SEI on the surface of graphite anodes and cathodes.
In short, the lithium-ion battery fast charging faces two challenges including:
- The SEI of the anode fails, resulting in exfoliation of graphite and a large consumption of solvent in the electrolyte;
- Slow electrode reaction kinetics, layered interfacial resistance, and high activation energy of solvation and desolvation of Li + are related.
Strategies that can be implemented to improve battery fast charging performance from the perspective of battery materials include:
- Reduce the particle size of graphite to stabilize SEI;
- Single crystallization of cathode materials to enhance cathode reaction kinetics;
- Develop electrolytes with low solvation and desolvation activation energies, and develop new additives to enhance SEI film resistance to mechanical stress.