Whether the choice of anode and cathode materials for lithium-ion batteries affects safety

The internal composition of Lithium Battery is mainly positive electrode | electrolyte | diaphragm | electrolyte | negative electrode, on this basis, the electrode ear welding, packaging and other steps finally form a complete cell. After the initial charge and discharge of the battery cell, the chemical component capacity and exhaust and other steps, it can be used in the factory. The first step in this process is the selection of materials. The main factors affecting the safety of the material are its intrinsic orbital energy, crystal structure and material properties.

Positive electrode material

The main role of the positive active material in the battery is to contribute to the specific capacity and specific energy, and its intrinsic electrode potential has a certain impact on safety. For example, in recent years, China has widely used the low-voltage material LiFePO4 (lithium iron phosphate) as a positive electrode material for power batteries in transportation vehicles (such as hybrid electric vehicle HEV, electric vehicle EV) and energy storage devices (such as Uninterruptible Power Supply UPS). However, the safety advantages of LiFePO4 in many materials actually come at the expense of energy density, which means that the battery life of its users (such as EV, UPS) will be limited. Ternary materials such as NMC(LiNixMnyCo1-x-yO2) have excellent energy density performance, but as an ideal cathode material for power batteries, the safety issue has not been fully resolved. To study the thermal behaviour of cathode materials, researchers have done a lot of work and found that the intrinsic electrode potential and crystal structure are the main factors affecting its safety, such as whether the electrode potential μC and the highest occupied orbital HOMO of the electrochemical window of the electrolyte are perfectly matched, and whether multiple lithium ions can pass smoothly through the lattice at the same time. The safety performance of positive active materials can be improved by the choice of material type and element doping.

Negative electrode material

The influence of the negative active material on the safety performance is mainly due to the relationship between its intrinsic orbital energy and the configuration of the electrolyte LUMO and HOMO. In the process of fast charging, the speed of lithium ion through the SEI (Solid Electrolyte Interface) film may be slower than the deposition rate of lithium in the negative electrode, and the lithium branch crystals will grow continuously with the charge and discharge cycle, which may lead to internal short circuit and ignite the combustible electrolyte thermal runaway, limiting the safety of the negative electrode in the fast charging process. Only when the difference between the negative electromotive force of the lithium alloy with carbon material as a buffer layer and the electromotive force of lithium is less than -0.7Ev, i.e. μA<μLi0.7eV, can it be guaranteed that the deposition of lithium will not cause a short circuit. For safety reasons, the power battery should use a negative electrode material with an electromotive force of less than 1.0eV (relative to Li+/Li0) to achieve safe fast charging or to control the charging voltage well below the deposition potential of lithium. Li4Ti5O12 has safety advantages in fast charging and fast discharging due to its electromotive force of 1.5eV (relative to Li+/Li0), which is lower than the LUMO of the electrolyte. There is also a negative material, Ti0.9Nb0.1Nb2O7, which can be rapidly charged and discharged for more than 30 weeks at a voltage of 1.3≤V≤1.6V (relative to Li+/Li0) and has a specific capacity of 300mAhg1, which is higher than LTO. During the discharge process, since there is no competition between the speed of lithium ions through the SEI film and the deposition on the negative electrode, the fast discharge process is safe.