What is a ternary lithium battery?
In nature, lithium is the lightest metal with the smallest atomic mass. Its atomic weight is 6.94 g/mol and p=0.53 g/cm3. Lithium is a kind of material with high specific energy because of its active chemical properties and easy to lose electrons and be oxidized to Li+. Therefore, the standard electrode potential is the most negative, which is -3.045V, and the electrochemical equivalent is the smallest, which is 0.26g/Ah. Ternary lithium batteries are lithium secondary batteries with nickel, cobalt and manganese oxide as cathode materials. It combines the good cycle performance of lithium cobalt oxide, the high specific capacity of lithium nickel oxide, the high safety and low cost of lithium manganate, and synthesizes nickel cobalt manganese oxide synergistically by means of molecular mixing, doping, coating and surface modification. Lithium ion rechargeable battery is widely studied and applied at present.
Lifetime of Ternary Lithium Batteries
The so-called lithium battery life refers to that after a period of time, the capacity of the battery decreases to 70% of the nominal capacity (room temperature 25 C, standard atmospheric pressure, and the capacity of the battery discharged at 0.2C), which can be regarded as the end of life. The cycle life of lithium batteries is usually calculated by the number of cycles in which lithium batteries are fully charged and discharged. In the process of using lithium batteries, irreversible electrochemical reactions will occur inside the batteries, which will lead to capacity degradation, such as the decomposition of electrolyte, inactivation of active materials, the collapse of positive and negative structures, which will lead to the reduction of lithium ion embedding and de-embedding, and so on. Experiments show that higher discharge rate will lead to faster capacity decay. If the discharge current is low, the battery voltage will approach the equilibrium voltage, which can release more energy.
The theoretical life of ternary lithium batteries is about 800 cycles, which is moderate in commercial rechargeable lithium batteries. Lithium iron phosphate is about 2,000 cycles, while lithium titanate is said to be able to reach 10,000 cycles. At present, the mainstream battery manufacturers promise more than 500 times in the specifications of ternary batteries (charging and discharging under standard conditions). However, after the batteries are packed into batteries, the cycle life of the batteries is about 400 times due to the consistency problem, mainly because the voltage and internal resistance can not be exactly the same. The manufacturer recommends that SOC use window be 10%~90%. Deep charge and discharge is not recommended. Otherwise, it will cause irreversible damage to the structure of positive and negative electrodes of batteries. If calculated by shallow charge and shallow discharge, the cycle life will be at least 1000 times. In addition, if lithium batteries are discharged regularly at high power and high temperature, the battery life will drop dramatically to less than 200 times.
Advantages and disadvantages of ternary lithium batteries
Ternary lithium battery is a battery with excellent comprehensive performance because of its balanced capacity and safety. The main functions and advantages and disadvantages of the three metal elements are as follows:
Co3+: Reduce the mixed cation occupancy, stabilize the layered structure of the material, reduce the impedance value, improve the conductivity, improve the cycle and rate performance.
Ni2+: It can increase the capacity of materials (increase the bulk energy density of materials). Because of the similar radius of Li and Ni, excessive Ni will also lead to lithium-nickel mixing because of dislocation with Li. The higher the concentration of nickel ion in lithium layer, the more difficult it will be to detach lithium in the layered structure, resulting in poor electrochemical performance.
Mn4+: It can not only reduce the cost of materials, but also improve the safety and stability of materials. However, excessive Mn content can easily lead to spinel phase and destroy the layered structure, resulting in reduced capacity and cyclic attenuation.
High energy density is the greatest advantage of ternary lithium batteries. Voltage platform is an important indicator of energy density of lithium batteries. It determines the basic efficiency and cost of batteries. The higher the voltage platform, the larger the specific capacity. Therefore, the batteries with the same volume, weight and even the same safe time can last for the batteries with higher voltage platform. It takes longer. The discharge voltage plateau of lithium tribasic battery is 3.7V, lithium iron phosphate is 3.2V, and lithium titanate is only 2.3V. Therefore, from the point of view of energy density, lithium tribasic battery has absolute advantages over lithium iron phosphate, lithium manganate or lithium titanate.
Poor safety and short cycle life are the main shortcomings of ternary lithium batteries, especially their safety performance, which has been a major factor limiting their large-scale configuration and large-scale integrated applications. A large number of field measurements show that it is difficult for large capacity ternary batteries to pass safety tests such as needling and overcharging, which is also the reason why manganese elements are often introduced into large capacity batteries and even mixed lithium manganate is used together. The cycle life of 500 cycles of lithium batteries is below the medium level, so the main application area of ternary lithium batteries is consumer electronic products such as 3C digital.