Detailed explanation of the four major problems in the negative electrode of lithium batteries

1. Where is the negative electrode critical? Where is the important body of the negative electrode of lithium battery? The output voltage of a lithium battery is equal to the difference between the positive electrode voltage and the negative electrode voltage. Therefore, the de-intercalation lithium voltage of the negative electrode of the lithium battery determines the output voltage of the battery (at least half). The lower the negative electrode operating voltage, the higher the battery operating voltage . The specific capacity of the battery is determined by the specific capacity of the positive electrode and the specific capacity of the negative electrode. The specific capacity of the negative electrode determines at least half of the specific capacity of the battery in a sense. The available capacity of the battery in practice is also related to the degree of skew of the negative electrode delithiation voltage channel. The flatter the delithiation voltage channel, the higher the available capacity of the negative electrode and the higher the specific capacity of the battery. Assuming that the capacity of the positive electrode is PmAh/g and the capacity of the negative electrode is QmAh/g, the theoretical specific capacity x of the battery is satisfactory. The following formula 2. The concession between mass ratio energy and voltage Strictly speaking, this concession does not include lithium metal. The operating voltage of lithium metal is 0V, the theoretical capacity is 3862mAh/g, and the practical capacity is determined by the utilization rate of active materials. Why give in? Generally speaking, a fundamental feature of current negative electrode candidate materials (excluding metal lithium) is that the larger the capacity of active electrode materials, the higher the delithiation voltage channel (perhaps average). For example, the average delithiation potential of graphite-based carbon materials The average delithiation potential of the Sn negative electrode is 0.5V, and the theoretical capacity is 990mAh/g; the average delithiation potential of the Si negative electrode is 0.45V, and the theoretical capacity is 4200mAh/g. The actual available capacity of Sn and Si is not confirmed yet, and it is determined by the operating conditions. Battery energy density and specific characteristics are determined by the product of average operating voltage (voltage difference between positive and negative electrodes) and mass specific capacity (perhaps volume specific capacity). When Si or Sn is used to replace carbon-based materials, can the specific capacity increase of the battery be Compensating for the drop in battery operating voltage is an important factor to be considered. A simple example shows that assuming the positive electrode is lithium iron phosphate, its operating voltage is calculated as 3.45V, and the capacity is calculated as 160mAh/g; when matched with graphite carbon (calculated as 0.15V, 350mAh/g practical capacity), 1g phosphoric acid Lithium iron matches 0.457g graphite carbon, and the theoretical specific energy of the full battery is (3.45-0.15)V*160mAh/1.457gu003d362.3mWh/g. The 1g lithium iron phosphate is matched with the Si negative electrode. The Si negative electrode is calculated according to 0.45V and 4200mAh/g. The specific energy of the matched full battery is (3.45-0.45)V*160mAh/1.038gu003d462mWh/g. Obviously, after the use of Si anode instead of graphite carbon anode, its capacity increase can compensate for the impact of the drop in battery voltage caused by the increase in its delithiation voltage. Of course, this is a theoretical consideration, because the actual capacity of the Si anode cannot reach its theoretical value. Assuming that the actual capacity of the Si negative electrode is p, it is necessary to satisfy the condition that the specific energy of the whole battery is greater than 362.3mWh/g. The satisfactory connection formula is of course. The above formula is still a little simplified. The connection between the Si negative electrode voltage and capacity is not considered, but Can explain the purpose of our current comment. It can be calculated that p must be at least 492.5mAh/g. In other words, the practical capacity of the Si anode must be greater than 492.5mAh/g to ensure that the quality of the battery is not worse (compared to the lithium iron phosphate/graphite carbon system) . The development of high-capacity C-Si composite anode materials can also learn from the above comments. Roughly speaking, the capacity of the Si part of the C-Si composite anode should not be less than 492.5mAh/g during practical use, otherwise it will have no meaning. . This is the compromise relationship between the increase in the negative electrode delithiation voltage and the increase in the reversible capacity. It is precisely because of the concessional relationship between these two parameters that lithium titanate and nitride are ignored here, because the voltage channels of the two are indeed high, which is too high to be tolerable. 3. The high lithiation state in the composite negative electrode material currently seems to be very unlikely to replace the graphite carbon negative electrode with Si, Sn, etc. alone; the compromise is the use of Si/C or Sn/C composite negative electrode, in a broad sense The one is the M/C composite negative electrode. One of the biggest problems of the M/C composite anode is the existence of a high lithiation state. High lithiation state is inevitable, this is because the average delithiation voltage of M is higher than that of graphite carbon. The high lithiation state is for M, and the high lithiation state of M will cause some unexpected problems. The newly prepared M/C composite product is in the lithium-free state, from the lithium-free state to the highly lithiation state of M. With the drastic change in volume, does it still maintain the initial micro-interconnection of M and C after lithiation? , The cycle stability is very important; in addition, there is also the problem of severe and low coulombic efficiency for the first charge and discharge. Perhaps this can be handled by pre-cycle, but the pre-cycle method will be accompanied by the development of new battery manufacturing processes. 4. Misunderstanding of negative electrode research. Regarding the lithium battery, its voltage cannot be operated to 0V; its specific characteristic is confirmed by the voltage difference between the positive and negative electrodes and the capacity of the positive and negative electrodes; this inherently determines that some voltage channels are not significant, perhaps even Obviously, the oxide that is higher than 1.0V or even needs to be operated from 3.0V to 0.0V, there is no possibility of practical use. Therefore, some current studies on the behavior of deintercalating lithium from oxides with the above characteristics have no meaning. For example, some studies on the behavior of deintercalating lithium from copper oxide and iron oxide have no meaning. Disclaimer: Some pictures and contents of articles published on this site are from the Internet. If there is any infringement, please contact to delete.