Rational Fabrication And Performance Of Anode Materials For Li/Na Ion Batteries

Graphite as commercial LIBs anode has been widely used, however, its low capacity impedes the development of novel LIBs with high energy density and power density. Thus, searching for new electrode materials is kind of important. Among the next-generation anodes, metal oxides are particularly attractive candidates, since they are low cost and good capacity. Unfortunately, their performances are seriously suffered from their intrinsic drawback, namely, low conductivity and large volume changes, resulting in the low capacity and poor rate capability. Hence, fabrication of rational nanostructure and introduction of conductive carbon can dispose of those issues. Recently, the rising price of lithium resource has spawned the evolution of roomtemperature Sodium-ion batteries?SIBs?. Metal oxides as SIBs anode show low capacity, since Na₂O during the discharged process suffers from the poor kinetics. Based on the different storage energy mechanism, carbon and alloys materials can display better performance, and also have some problems. The high conductivity enables carbon material with the cycle stability and excellent rate performance, but its specific capacity is relatively low. Although alloy-based materials realize a high capacity, the huge volume expansion during cycling causes a fast capacity decay. In this work, stimulated by material internal structure and external modification, rational design and constructing composite by simple hydrothermal synthesis, CVD and sol-gel method are utilized to prepare LIBs or SIBs electrodes with high performance. Combined with a series of characterized methods, material structural effects on the electrochemical properties have been in-depth evaluated.Firstly, in order to dispose of the CoO volume expansion upon cycling, a simple hydrothermal with calcination was used to prepare the porous CoO nanosheets with controllable pore size. CoO nanosheets will be broken, if the pore size is too small not to endow the electrode with enough space to accommodate the volume changes derived from the largely formed Li₂O, which results in rapid capacity decay. On the other hand, when the pore of CoO nanosheets is too large, nanonets will be bending upon cycling, which reduces the probability of electron and ion recombination rate, thus affecting the large-current performance and cycle stability. The material pore size is therefore appropriate, those pores supply enough space to buffer volume expansion so as to achieve the structural integrity of the nanonets for the excellent performance. In this scenario, when CoO has the optimal pore size, it displayed as high as 700 mAh g^-1 at 1 A g^-1 for 200 cycles.Secondly, a simple hydrothermal synthesis and CVD had been used to modify Fe₂O₃ and obtain Fe₃O₄@C to boost its conductivity. Different from conventional methods, CVD is simple, uniform coating layer, and even expandable production. Importantly, the monodisperse nanoparticles provide a structural basis so as to achieve uniform coating carbon. Fe₃O₄@C achieved a highly capacity of 1000 mAh g^-1 at 100 mA g^-1 after 60 cycles; even at 1 A g^-1, its capacity is as high as 800 mAh g^-1.Finally, a simple sol-gel route was used to synthesize Sb nanoparticles uniformlydecorated on porous rich N carbon nanosheet to enhance the capacity of carbon nanosheets as SIBs anode. The incorporation of N not only boosts the absorptivity of Sb but also improves the conductivity and pseudocapacitance of carbonaceous material for higher capacity. The uniformly dispersion of Sb effectively mitigates the volume change, improves the utilizated rate of materials and increases the cycle stability. Moreover, carbon materials works as the electron conductor, and a porous structure can decrease the length of ions diffusion pathways, which helps enhance the rate capability. Thus, the composite exhibits high reversible capacity, superior rate performance and long cycling stability. At 0.05 A g^-1, the capacity of composite is up to 325 mAh g^-1. Even at a large current of 10 A g^-1, a good reversible capacity of 142 mAh g^-1 was still obtainable. Rational design enables electrode material with a stable cycling performance, says, 220 mAh g^-1 at 2.0 A g^-1 after 180 cycles; the Coulombic efficiency is close to 100%, uncloaking composite with the highly reversibility.This work had deeply studied controllable pore nanosheets, CVD coating carbon and nanoparticles modified N rich carbon. The ideas of design can broaden horizons of preparing electrode materials. The subtle synthesis and fabricated approaches can widen their applicability to other areas such as electrocatalytic and lithium-oxygen batteries.