At present, the rapid development of global electric vehicles makes the demand of high energy density battery increasingly strong. In view of this, major developed countries in the world, such as the United States and Japan, have set the development goal of developing secondary lithium battery with energy density up to 500 Wh/kg. The working principle of the classical lithium battery based on ternary positive electrode material system is based on the transition metal (TM) related cationic REDOX reaction. However, due to the limitation of cationic active capacity, the capacity of this kind of battery is difficult to reach 500 Wh/kg, while the anion REDOX activity related to oxygen element is expected to greatly improve the energy density of the battery. It has become the hot frontier of secondary battery system research.
The research team of Haoshen Zhou, Professor of Nanjing University, has designed and developed a stable, large-capacity positive electrode material system based on anion REDOX activity, which is applied to lithium metal (Li) soft package full battery. Through the reversible conversion between lithium oxide (Li2O) and lithium peroxide (Li2O2), the performance of battery devices is significantly improved. The energy density of more than 500 Wh/kg was obtained for the first time, and the energy density of more than 400 Wh/kg could still be obtained after more than 100 stable cycles. More importantly, the nickel content of the battery is only 1.59%(mass fraction), which is far lower than the traditional high nickel ternary positive electrode, and the cost is lower and more suitable for large-scale production. It plays an important role in promoting the development of battery industry and electric vehicles. Related studies show that in the design process of positive materials for Li metal batteries, the reversible conversion between Li2O and Li2O2 can provide high energy density for the positive system, not limited to the traditional transition metal REDOX capacity. Theoretical calculation shows that the theoretical energy density of Li2O/Li2O2 conversion is as high as 2565 Wh/kg. In order to improve the "safe" charging depth without oxygen precipitation, researchers also prepared matching nickel-based carbon alloy conductive catalytic framework (Ni-CAC) to cover Li2O to form Li2O@Ni-CAC electrode system. On the one hand, it helps to improve the battery performance. On the other hand, it is beneficial to reduce the use of nickel metal and reduce the cost. Then, Li2O@Ni-CAC and Li were used as positive and negative electrodes and ether solvents as electrolytes to assemble a complete soft-pack battery, and electrochemical performance test was carried out. In situ gas phase mass spectrometry and Raman spectroscopy experiments show that there is an obvious conversion process from Li2O to Li2O2 during the charging process, and the reversible "safe" charging depth of Li2O@Ni-CAC electrode system is 750 mAh/g(based on the mass of Li2O active substance). Once the value exceeds this value, irreversible oxygen precipitation will occur. Then control the charge depth is not more than 750 mAh/g, charge and discharge cycle test, found that the battery can be stable reversible cycle more than 100 times, and the energy density is up to 950 Wh/kg(taking into account the quality of all electrode materials, but not including the soft shell); However, after taking into account the soft-pack shell, the battery still achieves an energy density of more than 500 Wh/kg, reaching 513.5Wh /kg, which is the soft-pack battery with the highest energy density reported in literature at present (after taking into account the quality of the entire soft-pack battery), and the output energy density is still higher than 400 Wh/kg after more than 100 stable cycles. It shows excellent cyclic stability.
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