The internal components of a lithium-ion battery mainly consist of a positive electrode, an electrolyte, a separator, another electrolyte, and a negative electrode. Further steps, such as welding the tabs and wrapping the battery in outer packaging, ultimately form a complete cell. After initial charging and discharging, capacity testing, and venting, the cell is ready for shipment. The first step in this process is material selection. Factors affecting material safety primarily include its intrinsic orbital energy, crystal structure, and material properties.
cathode materials
The primary role of positive electrode active materials in batteries is to contribute specific capacity and specific energy, while their intrinsic electrode potential has a certain impact on safety. For example, in recent years, China has widely used low-voltage material LiFePO4 (lithium iron phosphate) as a positive electrode material for power batteries in transportation vehicles (such as hybrid electric vehicles (HEVs) and electric vehicles (EVs)) and energy storage devices (such as uninterruptible power supplies (UPSs). However, the safety advantages exhibited by LiFePO4 among many materials come at the cost of energy density, meaning it limits the driving range of its users (such as EVs and UPSs). While ternary materials such as NMC (LiNixMnyCo1-x-yO2) exhibit excellent energy density, their safety as ideal positive electrode materials for power batteries remains a concern. To study the thermal behavior of cathode materials, researchers have done a lot of work and found that intrinsic electrode potential and crystal structure are the main factors affecting their safety, such as whether the electrode potential μC and the highest occupied orbital (HOMO) of the electrolyte electrochemical window are perfectly matched, and whether multiple lithium ions can pass smoothly through the lattice at the same time. The safety performance of cathode active materials can be enhanced by selecting the type of material and doping with elements.
Anode material
The impact of negative electrode active materials on safety performance mainly stems from their intrinsic orbital energy and the configuration relationship between the electrolyte's LUMO and HOMO. During fast charging, the rate at which lithium ions pass through the SEI (solid electrolyte interface) film may be slower than the rate at which lithium deposits on the negative electrode. Lithium branching can continue to grow with charge-discharge cycles, potentially leading to internal short circuits and igniting the flammable electrolyte, causing thermal runaway. This characteristic limits the safety of the negative electrode during fast charging. Only when the difference between the electromotive force (EMF) of the lithium alloy with a carbon-containing buffer layer and the EMF of lithium is less than -0.7 Ev (i.e., μA < μLi0.7 eV) can it be guaranteed that lithium deposition will not cause a short circuit. For safety reasons, power batteries should use negative electrode materials with an EMF less than 1.0 eV (relative to Li+/Li0) to achieve safe fast charging or to control the charging voltage well below the lithium deposition potential. Li4Ti5O12 has a safety advantage in fast charging and fast discharging because its EMF is 1.5 eV (relative to Li+/Li0), which is lower than the electrolyte's LUMO. Another anode material, Ti0.9Nb0.1Nb2O7, can perform rapid charge-discharge cycles for over 30 cycles at a voltage of 1.3≤V≤1.6V (relative to Li+/Li0) and possesses a specific capacity of 300mAh g1, higher than LTO. During discharge, because there is no competition between the rate of lithium ions passing through the SEI film and their deposition on the anode, the fast discharge process is safe.
