Lithium batteries are inefficient for the first time. Which solution is strong?

The first effect of an annoying battery is low, and it is filled with 10 amps and 9 amps; the second annoying cycle life is poor, and less than 500 is finished; the three annoying battery voltage is low, and 4.2V is over. In view of the problem of poor cycle life of lithium-ion batteries, research work has been done a lot.

Measures for electrode materials, measures for electrolytes, and measures for electrode structures have been relatively mature, and the life of lithium-ion batteries has been greatly improved.

The high-voltage electrode materials developed for low battery voltages, as well as the supporting electrolytes, have gradually matured and are being gradually applied.

Only the first low efficiency problem has not been solved well. There is no mature solution at present. At present, there is inert lithium powder developed by 3M Company. Although it can be used as a supplemental negative lithium source, it is poor in safety (there is dust explosion). The risk), high cost (high material cost, high equipment cost) is difficult to obtain a wide range of applications in the short term, and the positive lithium supplement seems to be a possible choice. The positive lithium supplement only needs to add lithium oxide to the positive electrode. The cost is low, the original process is not changed, and metal lithium is not involved, so the safety is greatly improved. 　

Taking a typical LiCoO2/C full battery as an example, during the first charging process, the potential of graphite gradually decreases with the embedding of Li+. When the potential is lower than the stable potential of the electrolyte, the electrolyte will be reduced on the surface of the graphite negative electrode. Decomposition, consuming a portion of lithium, resulting in an irreversible capacity of about 10%. This capacity loss will be more pronounced when the negative electrode is replaced by a hard carbon, silicon or the like having a higher irreversible capacity. 　

Li5FeO4 is an ideal source of positive electrode lithium with a specific capacity of 867 mAh/g. Theoretically, 5 Li+ can be supplied per mole of Li5FeO4. By mixing a certain amount of Li5FeO4 into the conventional positive electrode material, the lithium ion battery can be significantly improved. First efficiency and energy density.

Xin Su et al. conducted research on Li5FeO4 as a positive lithium source. They used LiCoO2 (the coulombic efficiency was 98%), the negative electrode used hard carbon (the first coulombic efficiency was only about 80%), and Li5FeO4 was synthesized by the solid phase method. During the first charge, the LFO material can release at least 4 Li+, equivalent to a specific capacity of 700 mAh/g or more, as shown in the following reaction equation:

In this process, most of the lithium ions cannot be reversibly embedded in LiFeO2, but these lithium ions can be used to offset the irreversible capacity loss caused by the negative electrode. Therefore, in use, we only need to add a small amount of Li5FeO4 to the positive electrode material.

In the experiment, it was found that by adding only 7% of LFO material to the positive electrode, the positive charge capacity of the positive electrode can reach 233 mAh/g, while the initial discharge capacity is only 160 mAh/g, and the 7% content of LFO provides an additional 31% of Li+. Lithium ions eventually enter the negative electrode material, making up for the first time the negative electrode is inefficient. 　

Therefore, when a graphite material (about 10% irreversible capacity) is used as the negative electrode material, the content of LFO in the positive electrode can be appropriately lowered. 　

It can be known from the calculation that when the capacity ratio of the positive and negative electrodes is 1:1, the remaining capacity of the positive electrode is actually only 129 mAh/g (charge voltage is 2.7-4.3 V) due to the large irreversible capacity of the hard carbon. After the addition of LFO to the positive electrode, since the lithium of the LFO supplements the lithium lost during the first charging, the residual reversible capacity of the positive electrode reaches 159 mAh/g, which means that the entire battery has an energy density increase of about 10%.

Since 7% of LFO can provide an additional 31% of lithium ions, the amount of LFO added can be appropriately reduced to just satisfy the irreversible capacity of the negative electrode (for example, about 10% of graphite and about 25% of Si material), so the energy of the battery Density can be further improved. 　

At the same time, the study also found that the LFO addition not only improved the first efficiency of the battery, but also provided the additional Li to significantly improve the cycle performance of the battery. The 50-cycle capacity retention rate increased from 90% to 95% (LCO/hard carbon). The energy spectrum analysis and X-ray diffraction of the negative electrode of the battery after long-term circulation show that the LiFeO2 material produced by the release of lithium ions from the LFO material remains in the positive electrode, and there is no risk of the Fe element being precipitated again in the negative electrode after the Fe element is dissolved. 　　

Li5FeO4 material is a safe, reliable and efficient cathode source for lithium addition. Its cost is relatively low, it can release a large amount of lithium ions during the first charge, and the product after releasing lithium ions is extremely low in activity, and lithium re-intercalation or Dissolved, it is a very promising source of positive electrode lithium. With the application of high capacity, high irreversible capacity anode materials such as silicon anodes, the demand for cathode lithium-filled materials will further expand.