The capacity of the battery is an important parameter, and its value is determined by the capacity of the positive electrode material, the negative electrode material capacity, the negative electrode-positive electrode capacity ratio, and the electrode potential. In the battery design process, these need to be carefully considered. This is the author's study. Note for battery design.
Figure 5.1 shows the half-cell capacity of a lithium metal oxide cathode material for lithium. When the battery is charged, the positive electrode material removes lithium atoms and undergoes crystal structure changes. The theoretical capacity of the electrode material assumes that all lithium ions in the material participate in the electrochemical reaction. The capacity that can be provided, that is, the lithium atoms in the positive electrode material are fully discharged during charging, and actually the lithium ion deintercalation coefficient is less than 1, the actual material gram capacity = lithium ion extraction coefficient × theoretical capacity, such as the theoretical capacity of lithium cobalt oxide 274mAh/ g. The actual capacity is 140 mAh/g, and the lithium ion extraction coefficient is about 0.5. The gray part of the figure shows no part of the capacity involved in the electrochemical reaction. In addition, even if the extracted lithium atoms are still small, they cannot be returned. To the initial structure, this part can not be returned to the initial structure of the capacity is the irreversible capacity of the cathode material. This value is related to many variables, such as the type of metal elements, the ratio of the atomic radius of lithium and metal elements, particle size, etc. In general, The first irreversible capacity of LiCoO2 is 3–5 mAh/g, and that of LiNiO2 material is 20–30 mAh/g. One or two charge/discharge cycles are experienced. Coulomb efficiency close to 100%.
Figure 5-2 is a schematic diagram of the half-cell capacity of a carbon-based negative electrode material on a lithium plate. A graphite negative electrode material reacts with lithium to produce LiC6 with a theoretical capacity of 372 mAh/g, and actually reacts to generate LixC6 (x<1) , 石墨负极实际客容量一般360 mAh/g, 图中灰色部分即没有参与电化学反应的容量部分. 石墨负极的首次不可逆容量主要是由于电解液在负极表面形成SEI膜消耗锂离子造成的, 导致部分锂离子嵌入负极材料之后无法再次脱出返回金属锂电极. 这个不可逆容量与材料结晶度, 结构, 比表面积和颗粒粒径等相关. 商业化的石墨负极不可逆容量一般为20-30 mAh/g. 两个充电/放电周期后, 库仑效率也是接近100%.
For a full battery, both positive and negative materials have an initial irreversible capacity. The battery capacity can be illustrated by the schematic shown in Figure 5-3. During the initial charge, the supplied lithium is removed from the positive electrode material, and part of it is consumed to form an SEI layer on the negative electrode surface. In the initial irreversible reaction, during the subsequent discharge process, the battery capacity will appear in two cases depending on the difference between the positive and negative irreversible capacities. It is assumed that the irreversible keg capacity of the positive electrode material is Fc and the weight of the living material is Mc; negative electrode irreversible gram volume For Fa, the live matter weight is Ma. When Fc*Mc < Fa*Ma, 即正极材料的不可逆容量小于负极不可逆容量时, 放电后负极返回到正极的锂不足以填充正极的容量, 正极部分容量无法得到充足的锂供应, 电池容量受到负极材料限制. 相反, 当Fc*Mc > Fa*Ma, ie when the irreversible capacity of the positive electrode material is greater than the irreversible capacity of the negative electrode, the lithium supplied from the negative electrode after discharge is sufficient, but the irreversible capacity of the positive electrode is high, the reversible capacity of the positive electrode is limited, and part of lithium remains on the negative electrode side, and lithium precipitation may occur. Therefore, the design of the battery capacity is limited by the initial irreversible properties of the electrode material.
As shown in Figure 5-4, the voltage of the battery is the potential difference between the positive and negative electrodes. The voltage of the battery needs to be designed based on the open-circuit voltage of the positive and negative electrodes. It is necessary to comprehensively consider various conditions such as charge and discharge temperature and depth of discharge. With the same voltage, the inherent electrochemical behavior of the positive electrode and negative electrode may also be different. The charge balance of the battery is not only affected by the potential of the electrode, but also by the ratio of the positive and negative electrodes in the battery.
The potential balance in the battery is schematically shown in Figures 5-5 and 5-6. Figure 5-5 shows the potential balance change process of the battery when the initial irreversible capacity of the positive electrode increases. Figure 5-6 shows the initial value of the negative electrode. When the irreversible capacity increases, the potential balance of the battery changes. This battery design adjustment process can be achieved by adjusting the capacity ratio of the positive and negative electrodes, which is equivalent to adding excess lithium in the positive or negative electrode to offset the irreversible capacity of the negative or positive electrode. This design adjustment of the potential balance is closely related to the battery capacity, voltage and safety characteristics and must be carefully considered.
In battery capacity design, an important criterion is that the negative electrode must have a greater reversible capacity than the positive electrode. Although the negative electrode capacity is smaller, the battery may have some advantages, such as a large battery capacity, but lithium may appear at the negative electrode during charging. Dendritic deposits on the surface cause safety problems. As shown in Figure 5-7, if the initial capacity ratio of the negative electrode to the positive electrode is set to 1, the so-called N/P ratio (initial negative electrode capacity/initial positive electrode capacity), assuming positive and negative electrodes The electrodes have the same initial irreversible capacity, and the battery capacity is also 80 mAh. Even with a larger capacity positive electrode, the battery capacity is limited to a smaller capacity range. On the other hand, if the negative electrode is used in a large irreversible Capacity and negative electrode, when the initial capacity ratio of the positive electrode is 1.5, the capacity of the battery is reduced to 70 mA. This means that the N/P ratio needs to be properly adjusted to avoid this effect.
The N/P ratio also affects the cycle life of the battery. The attenuation of the capacity may be due to the reaction between the positive electrode, negative electrode, electrolyte, and separator. When the constant N/P ratio is 1.1, the initial irreversible capacity of the negative electrode is assumed to be greater than that of the positive electrode. Figure 5-8 and Figure 5-9 illustrate the effects of cathode degradation and anode degradation on the cycle life and safety of the battery, respectively. If we assume that the irreversible reaction of the cathode for every 100 cycles will result in a decrease in capacity at 10 mA, the results As shown in Figure 5-8. Initially, when the battery capacity was 78 mA, even after the positive electrode degraded for 100 cycles, the N/P ratio was 1.1, and the actual battery capacity was 88 mAh. After 200 cycles, The N/P ratio is lower than 1. Lithium begins to deposit on the negative electrode and the battery capacity is still 88 mAh. However, the safety of the lithium battery is seriously threatened.
As shown in Fig. 5-9, the irreversible reaction of the negative electrode resulted in a decrease in capacity of 10 mA per 100 cycles. For an N/P ratio of 1.1, it was assumed that the initial irreversible capacities of the positive and negative electrodes were 10 and 22 mA, respectively. The initial battery capacity was 78 mAh. After 100 cycles, the positive attenuation caused a capacity of 68 mAh. After 200 cycles, the N/P ratio was greater than 1.1, and the battery capacity dropped to 58 mAh. There is no problem with the safety, but the battery capacity gradually wears off.
