Design And Preparation Of Advanced Anode Materials For Na-ion Batteries And Their Na-storage Properties And Mechanism

With the increasing demands for lithium-ion batteries （LIBs）, the limited terrestrial reservation lithium resources will because more and more expensive. In order to meet the growing demands, sodium-ion batteries （SIBs） have attracted great attentions due to the well distribution, abundance and low cost of sodium resources. Moreover, sodium and lithium are in the same main group, hence have similar chemical and physical properties. This makes the SIBs share the similar storage mechanism with LIBs. However, radius of sodium ion is 106 pm, which is larger than that of lithium ion （76 pm）, resulting in a slow electrochemical process including sodium ions and electrons transfer. Therefore, increasing the ion and electron conductivity is the key to achieve high-performance. This paper focuses on the design of structure and morphology, synthesis and electrochemical properties of the graphene, alloy and organic material.（1） Although graphene oxide （GO） has large interlayer spacing, it is still inappropriate to be used as anode for SIBs because of the existence of H-bondings between the layers and the ultralow electrical conductivity. Reducing GO should one effective strategy, but the process is still unscalable, high energy-consuming and expensive for practical applications. Here, it is for the first time to achieve the superior Na-storage properties of un-reduced GO material via a simple and scalable alkali-metal-ion (（Li+, Na+, K+） functionalized process. The various alkali metals ions have played different effects on morphology, porosity, disordered degree and electrical conductivity, which are crucial for Na-storage capabilities. Electrochemical tests demonstrated that sodium ion functionalized GO （GNa） has shown outstanding Na-storage performance in terms of high initial reversible capacity of 250 mAh/g, excellent rate capability （78 mAh/g at 2 A/g） and long-term cycle life （110 mAh/g after 600 cycles at 1 A/g）.（2） However, this graphene material has low capacity, in order to further increase the capacity, Sb with high theoretical capacity has been composited with graphene. Simple physical composite of graphene and the active material cannot achieve satisfactory results. Hence, Sb/graphene composite （Sb-O-G） was obtained by in situ reduction of GO and the precursor of Sb, and in which chemical bonds between Sb and graphene were in-situ generated. And in comparison to the composite without chemical bonds （Sb/G）, it is found that chemical bonds guarantee intimate contact between Sb and graphene, shorten ion and electron transport path and stabilize the structure. Therefore Sb-O-G composite shows superior sodium storage performance, for instance, a high reversible capacity （550 mAh/g）, good cycling performance （without capacity fading after 200 cycles） and excellent rate property （220 mAh/g at 12 A/g）. To further investigate its practical application, we used Sb-O-G as negative electrode and Na₃V₂ （PO₄）₃ as positive electrode to assemble full cells. At 0.2 C, the energy density is up to 160 Wh/kg and power density reach 7.1 kW/kg at 120 C. Moreover, the coin and soft-package type full cells have successfully powered LED bulbs and calculator.（3） Organic material will be one of the future development directions due to the abundant raw materials, renewable and so on. The main defect is the poor electrical conductivity, in this paper we improve the electrochemical properties of organic anode via using nanotechnology. Through a simple solvent-induced acid-base reaction, sodium terephthalate nanosheet （NS-Na₂TP） can be obtained. Electrochemical tests indicate that NS-Na₂TP shows higher capacity, higher rate capability and better cycle life than Na₂TP bulk （B-Na₂TP）. For instance, the charge capacity of NS-Na₂TP is 248 mAh/g at current density of 25 mA/g, much higher than that （199 mAh/g） of B-Na₂TP. Further, at high current density of 1250 mA/g, BS-Na₂TP exhibits a charge capacity 1.55 times that of B-Na₂TP. After 100 cycles, the charge capacity of BS-Na₂TP is 106 mAh/g, much higher than that （60 mAh/g） of B-Na₂TP. More importantly, through study the cyclic voltammetry curves, charge-discharge profiles and ex-situ Fourier Transform Infrared spectra, it excitingly found that improved sodium storage performance of NS-Na₂TP can be attributed to the new one-step desodiation mechanism and optimized ionic/electronic transfer pathways.