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With the continuous upgrading of positive and negative electrode materials and the continuous optimization of battery structure, the specific energy of lithium-ion batteries has been greatly improved. Currently, with the support of high nickel ternary positive electrode/silicon carbon negative electrode, the specific energy of lithium-ion batteries has reached about 300Wh/kg, achieving the 2020 target. However, a specific energy of 300Wh/kg is almost the limit of the existing system, and further improvement in specific energy can only be achieved by replacing new material systems. From the current technological development, the most likely choice for the positive electrode is lithium rich materials, while the negative electrode is mainly made of metallic Li. The specific capacity of lithium rich materials can reach over 250mAh/g, far higher than current ternary materials, and can achieve the goal of 400Wh/kg specific energy. However, lithium rich materials face continuous voltage plateau degradation during cycling, which not only reduces the specific energy of the battery, but also affects the normal operation of the battery management system BMS.

In early research, it was generally believed that the voltage plateau decay of lithium rich materials was mainly due to the transition of the material from a layered structure to a spinel structure. However, recently EnyuanHu (first author) and XiqianYu (corresponding author) from Brookhaven National Laboratory discovered through advanced detection techniques that the valence states of transition metal elements in lithium rich materials continue to decrease during cycling, such as the transformation of Co element from the initial Co3+/4+to Co2+/3+, and Mn element to Mn3+/Mn4+. These transformations directly lead to the continuous decay of the voltage plateau of lithium rich materials. At the same time, the loss of O during cycling can cause structural defects and form very large pores inside the lithium rich material particles, which will... Further reduce the voltage plateau of lithium rich materials. The author believes that lithium rich surface coatings and modifications can effectively reduce the release of O, thereby suppressing voltage decay during the cycling process of lithium rich materials.

In the experiment, EnyuanHu used a typical lithium rich material Li1.2Ni0.15Co0.1Mn0.55O2 as the research object. The charge discharge curve and dQ/dV curve of the material after different cycles are shown in the following figure. It can be clearly seen from the figure that as the number of cycles increases, the voltage level of the lithium rich material shows a significant decay trend.

In order to analyze the mechanism of voltage decay in lithium rich materials during cycling, EnyuanHu used XAS tool to analyze the changes in the valence states of Ni, Co, Mn, and O elements in the material after the 1st, 25th, 46th, and 83rd cycles (as shown in the figure below). It can be seen from the figure that the valence states of Ni, Co, and Mn transition metal elements show a significant downward trend with the increase of cycling times. The changes in O atoms mainly occur in the front edge region. As can be observed from the figure below, with the increase of cycle times, the intensity of the front edge peak of O atoms shows a significant weakening trend, indicating a decrease in the bond energy between transition metal elements and O elements in the bulk phase.

Through semi quantitative analysis of the XAS data mentioned above, 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 can be seen from the figure that O and Ni supplied the main capacity at the first cycle, reaching 128mAh/g and 94mAh/g, respectively. However, as the cycle progressed, the capacity provided by O and Ni elements rapidly decreased. At 83 cycles, the capacity provided by O element was only 50mAh/g, and the capacity provided by Ni element also decreased to 66mAh/g. However, the capacity contributed by Mn and Co elements increases with the increase of cycle times. For example, during the first discharge, Mn and Co provided capacities of 14mAh/g and 26mAh/g, respectively. However, as the cycle reached 83 times, their capacities increased to 66mAh/g and 53mAh/g, respectively.

From the above analysis, it is not difficult to see that the increased capacity of Mn and Co elements in the cycling of lithium rich materials compensates for the loss of capacity of Ni and O elements, resulting in little change in the overall capacity of lithium rich materials. However, the components of these capacities have undergone earth shaking changes, shifting from the oxidation-reduction reaction of O and Ni to the oxidation-reduction reaction of Mn and Co, which will significantly alter the voltage characteristics of lithium rich materials. This can also be explained by the Fermi level diagram. At the beginning, the Fermi level of the lithium rich material is only slightly higher than that of Ni2+/Ni3+, so the potential difference between the lithium rich material and the metallic Li is relatively high. However, as the cycle progresses, the O on the surface of the lithium rich material undergoes reduction and precipitation, resulting in a decrease in the valence state of transition metal elements. The surface Ni element is first reduced, forming an inactive rock salt structure on the surface of the material, which reduces the capacity provided by Ni element. The reduction of Mn and Co elements leads to the occurrence of Mn3+/Mn4+and Co2+/Co3+, respectively, resulting in a significant increase in Fermi level and a decrease in open circuit voltage.

As mentioned above, the surface of lithium rich materials in lithium-ion batteries is very unstable during cycling. In order to analyze the structural changes on the surface of lithium rich materials during cycling, EnyuanHu used soft X-ray absorption for analysis. From the OK edge plot, it can be seen that the intensity of the front edge peak continues to decrease with the increase of cycling times. There may be two reasons for this phenomenon. One is that the surface layer structure of lithium rich materials decays from a layered structure to a rock salt structure. The second reason is that an inert layer containing Li2CO3, Li2O, LiOH, RCO2Li, and R (OCO2Li) 2 is formed at the electrode interface of lithium rich materials due to electrolyte decomposition. CK edge analysis also shows that... It was found that the content of Li2CO3 in the surface layer of the lithium rich material electrode significantly increased during cycling, which also supports the previous analysis.

EnyuanHu found through ADF-STEM imaging technology that after 15 cycles, a considerable number of large pores appeared inside the lithium rich material particles, which do not exist in fresh materials. According to calculations, the volume occupied by these large pores reached 1.5-5.2%, which means that the lithium rich material may lose up to 9% of O in 15 cycles. In order to further confirm the reasons for the formation of large pores mentioned above, the author used STEM-EELS to observe the particles of lithium rich materials and found that a thick layer of spinel/rock salt structure could be 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 cycling process.

EnyuanHu's work indicates that the main reason for the voltage decay of lithium rich materials during cycling is not the transition from layered structure to rock salt and spinel structure, but the continuous decrease in transition metal valence states during cycling. As the number of cycles increases, the lithium rich material will continuously lose O, causing the surface Ni element to be first reduced to form a rock salt structure, losing activity. At the same time, the reaction valence states of Mn and Co continue to decrease, resulting in a continuous decrease in the voltage plateau of the lithium rich material. The author believes that in response to this phenomenon, surface coating and surface modification treatment can be used to reduce O loss during cycling and suppress the voltage plateau decay of lithium rich materials.

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