Lithium-ion battery has developed to the present day. Under the continuous upgrading of positive and negative materials and the continuous optimization of battery structure, the specific energy of the battery has been greatly improved. Currently, the high-nickel ternary positive electrode/silicon-carbon negative electrode is blessed. Under the lithium ion battery, the specific energy has reached 300Wh/kg, and the target of 2020 has been achieved. However, the specific energy of 300Wh/kg is almost the limit of the existing system. Continue to increase the specific energy and only replace the new material system. From the current technological development, the most likely choice for the positive electrode is lithium-rich material, and the negative electrode is mainly metal Li. The specific capacity of lithium-rich material can reach more than 250mAh/g, which is much higher than the current ternary material. Achieving the target of 400Wh/kg specific energy, however, the lithium-rich material faces a continuous voltage platform decay during the cycle, which not only causes the specific energy of the battery to decrease, but also affects the normal operation of the battery management system BMS.
In the early studies, it was generally believed that the voltage platform degradation of lithium-rich materials was mainly due to the transition of materials from lamellar structure to spinel structure, but recently Enyuan Hu (first author) and Xiqian Yu of Brookhaven National Laboratory (Corresponding author) et al. found through advanced detection techniques that the valence state of transition metal elements in lithium-rich materials in circulation continues to decrease, such as the Co element from the original Co. 3+/4+Change to Co 2+/3+, Mn element is also converted to Mn 3+/Mn 4+These changes directly lead to the continuous decay of the lithium-rich material voltage platform, while the O loss during the cycle causes structural defects and forms very large pores inside the lithium-rich material particles, which further reduces the voltage of the lithium-rich material. Platform. The author believes that lithium-rich surface coating and modification can effectively reduce the release of O, thereby inhibiting the voltage decay during the circulation of lithium-rich materials.
In the test, Enyuan Hu used a typical lithium-rich material Li. 1.2Ni 0.15Co 0.1Mn 0.55O2As the research object, the charge-discharge curve and the dQ/dV curve of the material after different cycles are as shown in the figure below. It can be clearly seen from the figure that as the number of cycles increases, the voltage platform of the lithium-rich material shows a significant decline. Down trend.
In order to analyze the mechanism of voltage decay of lithium-rich materials in the cycle, Enyuan Hu used XAS tools to analyze the valence of Ni, Co, Mn and O in the material after the first, 25, 46, and 83 cycles of lithium-rich materials. The trend of the state (as shown in the figure below), it can be seen from the figure that the valence states of the three transition metal elements Ni, Co, Mn show a significant downward trend with the increase of the number of cycles. The change of O atom mainly occurs in In the front side area, it can be noticed from the following figure that as the number of cycles increases, the intensity of the front edge of the O atom shows a significant weakening trend, which indicates that the bond between the transition metal element and the O element in the bulk phase is reduced. .
Through semi-quantitative analysis of the above XAS data, EnyuanHu obtained the contribution of different elements in the lithium-rich material to the overall capacity of the material at 1, 2, 25, 46 and 83 cycles (as shown in Figure a below). It was observed that O and Ni supplied the main capacity at the first cycle, reaching 128 mAh/g and 94 mAh/g, respectively. However, as the cycle progressed, the capacity provided by the O and Ni elements decreased rapidly, and at 83 cycles, the O element was provided. The capacity is only 50mAh/g, and the capacity provided by Ni element is also reduced to 66mAh/g. However, the capacity contributed by Mn and Co elements increases with the number of cycles, such as the capacity provided by Mn at the first discharge. 14mAh/g and 26mAh/g, respectively, but with the cycle to 83 times, the capacity of the two increased to 66mAh / g and 53mAh / g.
From the above analysis, it is easy to see that the increased capacity of the Mn and Co elements in the lithium-rich material compensates for the loss of Ni and O elements, so that the overall capacity of the lithium-rich material does not change much, but the components of these capacities However, there have been earth-shaking changes. From the redox reaction of O and Ni to Mn, the redox reaction of Co can significantly change the voltage characteristics of lithium-rich materials. This can also be explained from the Fermi level map. At the time, the Fermi level of lithium-rich materials is only slightly higher than Ni 2+/ Ni 3+Therefore, the potential difference between the lithium-rich material and the metal Li is relatively high, but as the cycle progresses, the surface O of the lithium-rich material undergoes reduction and precipitation, thereby causing a decrease in the valence state of the transition metal element, and a Ni element in the surface layer. Will be reduced first, forming a layer of inactive rock salt structure on the surface of the material, resulting in a decrease in the capacity provided by Ni. The principle of Mn and Co also causes Mn to occur separately. 3+/Mn 4+ And Co 2+/Co 3+Thus, the Fermi level is significantly increased, resulting in a decrease in the open circuit voltage.
Above we mentioned that the surface of the lithium-rich battery in the lithium-rich battery is very unstable. In order to analyze the structural changes of the surface of the lithium-rich material during the cycle, Enyuan Hu analyzed the soft X-ray absorption from the O K-edge diagram. It can be seen that the intensity of the front peak of the edge decreases continuously with the increase of the number of cycles, and there may be two reasons for this phenomenon. One is that the surface layer structure of the lithium-rich material decays from the layer structure to the rock salt structure. The two reasons are that the lithium-rich material electrode interface forms a layer containing Li due to electrolyte decomposition. 2CO 3, Li 2O, LiOH, RCO 2Li and R (OCO 2Li)2Inert layer, C K-edge analysis also found Li in the surface layer of lithium-rich material electrode 2CO 3The content is significantly increased in the cycle, which also supports the previous analysis.
Through the ADF-STEM imaging technology Enyuan Hu, after 15 cycles, a considerable number of large pores appeared in the lithium-rich material particles, and these large pores did not exist in the fresh material, according to the calculation of these large pores. The volume reaches 1.5-5.2%, which means that the lithium-rich material may lose up to 9% of O in 15 cycles. To further confirm the above-mentioned macroporous formation, the authors used STEM-EELS to carry out the lithium-rich material particles. Observed, it was found that a thick layer of spinel/rock salt structure was observed on the pore walls of the open pores on the surface of the particles, indicating that the formation of these pores is closely related to the O loss during the cycle.
The work of Enyuan Hu shows that the main cause of the voltage decay of lithium-rich materials during the cycle is not the transition of the layered structure to the rock salt and spinel structure, but the continuous decrease of the valence state of the transition metal during the cycle. Increasingly, the lithium-rich material will continue to lose O, resulting in the surface Ni element being first reduced to form a rock salt structure, losing activity, and the valence of Mn and Co continually decreasing, resulting in the continuous reduction of the voltage platform of lithium-rich materials. In response to this phenomenon, the author believes that the surface loss and surface modification treatment can reduce the O loss during the cycle and inhibit the voltage platform degradation of the lithium-rich material.