Preparation And Optimization Of Fe₃O₄/C Composites For Lithium Storage

Since their commercialisation,lithium-ion batteries have taken over the market as a complementary power source for portable electronic devices on their own merits,and have greatly promoted the development of new energy vehicles.However,due to the low specific capacity(372 m Ah g^-1)of the currently used commercial graphite anodes,it is difficult to meet the development needs of diversified electronic devices in the future.Therefore,there is an urgent need to develop next-generation anode materials with high specific capacity,long cycle life and high rate performance.Ferroferric oxide（Fe₃O₄）based on the conversion reaction to store lithium,due to its high specific capacity,good safety performance,abundant natural resources,non-toxic and harmless and low cost,is considered to be an ideal negative electrode for high-performance lithium-ion batteries material.However,its low electrical conductivity and the defects of serious damage to the electrode structure due to volume expansion during cycling have hindered the commercialization of Fe₃O₄.To solve the above problems,scientists have done a lot of relevant research work.There are two main methods currently available.First,by adjusting the morphology and structure to adjust the lithium ion migration rate,so that Fe₃O₄ can reversibly store lithium.Second,carbon coating with Fe₃O₄ not only can improve its electrical conductivity but also mitigate its volume expansion effect occur during cycling.In this paper,through the design of two-dimensional（2D）and three-dimensional（3D）structures and simultaneous carbon coating,the introduction of polypyrrole-derived nitrogen-doped carbon（NC）,and the partial replacement of Fe³⁺in Fe₃O₄ by Ti⁴⁺with a larger radius to improve the lithium storage performance of Fe₃O₄.The main contents are as follows:1.Aiming at the poor conductivity of Fe₃O₄ and its serious cyclic volume expansion,we firstly prepared 2D Fe₃O₄/C composite materials through a two-step method of hydrothermal and calcination.The composite material combines the dual advantages of 2D structure and carbon coating.On the one hand,it improves conductivity,and on the other hand,it alleviates the volume expansion caused by the cycle process,thus improving the lithium storage performance.The electrochemical test results show that the 2D Fe₃O₄/C composite exhibits excellent lithium storage performance.2D Fe₃O₄/C composite delivers 1193.0 m Ah g^-1 at 300 m A g^-1 after 200cycles with an high initial Coulombic efficiency（79.05%）,and 364.1 m Ah g^-1 at 3000m A g^-1 after 800 cycles.However,it is still needed to improve its rate performance and long-cycle stability under high current density for practical application.However,the rate performance and long-terms stability of 2D Fe₃O₄/C composite at high current density are difficult to meet the needs of practical applications,so further improvements are needed.2.To improve its rate performance,3D flower-like Fe₃O₄/C composites have been further fabricated after optimized and the formation mechanism of 3D flower-like structure also has been investigated.Compared to 2D structure,3D structure not only can provide more active sites for lithium storage,enhance the mechanical stress and flexibility of Fe₃O₄/C,but also increase the bulk transport of ions and electrons.Electrochemical test results show that 3D Fe₃O₄/C composite delivers 1165.4 m Ah g^-1 at 277.2 m A g^-1 after 300 cycles,a high initial coulombic efficiency（80.0%）,and 440.4 m Ah g^-1 at the high current density of 4620 m A g^-1 after1000 cycles,exhibiting excellent rate performance and cycle performance.3.To enhance the long-cycle stability of 2D Fe₃O₄/C composites,1D NC derived from 1D PPy was firstly obtained via in-situ polymerization and followed by one-step calcination.Subsequently,Fe₃O₄/C/NC composites have been fabricated by calcination after hydrothermal.Compared with the 2D Fe₃O₄/C structure,the introduction of NC effectively forms a"link bridge"between nanosheets,increasing the bulk transport capacity of ions and electrons,thereby improving its dynamic performance.Fe₃O₄/C/NC composite delivers a high specific capacity of 828.6 m Ah g^-1 at a low current density of 300 m A g^-1 over 450 cycles,and 619.5 m Ah g^-1 even at3000 m A g^-1 over 1200 cycles,showing excellent long-cycle stability.4.To improve the rate performance and long-cycle performance of 2D Fe₃O₄/C composites at the same time,2D(Fe_2.5Ti_0.5)_1.04O₄/C/MXene composite（FOTC/MXene）has been successfully fabricated via a three-step method,including hydrothermal self-assembly,ultrasonic mixing,and high-temperature calcination.In the composite structure,the partial replacement of Fe³⁺in Fe₃O₄ by Ti⁴⁺with a larger radius,which not only increase the ion diffusion channel,but also the constructed bimetal oxide is more conducive to the insertion/extraction of ions,thus realizing high-performance lithium storage.Electrochemical test results show that 2D(Fe_2.5Ti_0.5)_1.04O₄/C/MXene delivers a high specific capacity of 757.2 m Ah g^-1 at 3000m A g^-1 over 800 cycles,and whose specific capacity can be obtained 452.5 m Ah g^-1even at 10000 m A g^-1 for another 1200 cycles.The results shows the excellent rate performance and long cycling stability of(Fe_2.5Ti_0.5)_1.04O₄/C/MXene.