Controllable Synthesis,Energy Storage Performance And Mechanism Of Alkaline Earth Metal Vanadate Electrode Materials

The development of emerging metal-ion batteries,such as Na-ion batteries and Ca-ion batteries,is very important for alleviating the shortage of lithium resource and overcoming the bottlenecks of Li-ion battery technology.The absence of high-performance electrode materials is a main challenge for the development of new energy storage devices.This thesis focuses on alkaline earth metal vanadates,and systematic investigations from materials synthesis,characterization,formation process,electrochemical performance to energy storage mechanism are performed.We study the application potential of several new alkaline earth metal vanadates as electrodes for Na-ion batteries,Ca-ion batteries as well as Li-ion batteries.We reveal two different optimizing mechanisms of the alkaline earth metal ions in Na-ion battery anode and Ca-ion battery cathode,respectively,which lay some foundations for the exploitation of high-performance electrode materials and the development of emerging metal-ion batteries.The details are as follows:?1?Based on hydrothermal synthesis technique combined with other methods,several new alkaline earth metal vanadates are synthesized,including CaV4O9nanowires,CaV₄O₉ microflowers,SrV₄O₉ microflowers,Mg_0.25V₂O₅·H₂O microplates,Mg_0.35V₂O₅·H₂O nanobelts,Ca_0.44V₂O₅·H₂O nanobelts and Sr_0.32V₂O₅·0.7H₂O nanobelts.The detailed characterizations and analyses are carried out on their crystal phases,morphologies,valence states,elements distributions and formation mechanisms.?2?The Na-storage performance and mechanism of CaV4O9 nanowires for Na-ion battery anode are systematically investigated.It is found that CaV₄O₉nanowires exhibit small volume change?<10%?and an optimizing effect from in situ formed CaO,which results in a superior cycling?up to 1600 cycles?and rate performance with an applicable reversible capacity(>300 mAh g^-1).The specific sodium storage mechanism is demonstrated through in situ/ex situ characterizations and theoretical calculation.The Na⁺insertion results in the in situ conversion from CaV₄O₉ to NaVO₂ and CaO nanograins.The CaO nanograins are electrochemically inactive,which will not participate in the reaction.However,it will effectively inhibit the agglomeration of the active components,and then preserve the high reversibility of the subsequent reaction.?3?Nanosheet-assembled compact CaV₄O₉ microflowers are constructed,and realized both high areal capacity and stable cycling performance at high mass loadings.The compact microflower structure leads to an increased tap density of the electrode materials.Meanwhile,the assembled nanosheets maintain the nano-effects of the active materials for favorable electrochemical reactions.When used as Li-ion battery anodes,a high areal capcity of².5 mAh cm^-2 at a high mass loading of 4.4mg cm^-2 is obtained,and stable cyclings over 400 cycles with areal capacity over 1.5mAh cm^-2 is demonstrated.When used as Na-ion battery anodes,the superior electrochemical performance was also observed at high mass loadings.?4?The electrochemical performance and mechanism of Mg_0.25V₂O₅·H₂O microplates as Ca-ion battery cathode are systematically investigated.It is demonstrated that the reversible specific capacity can reach¹20 mAh g^-1.Based on in situ/ex situ characterizations,it is found that Mg_0.25V₂O₅·H₂O is an ultrastable Ca-ion battery cathode.The layer spacing variation during Ca²⁺ions insertion/extraction is demonstrated to be only 0.09?,which results in the excellent cycling stability.?5?The Ca-storage performance and mechanism of Mg_0.35V₂O₅·H₂O,Ca_0.44V₂O₅·H₂O and Sr_0.32V₂O₅·0.7H₂O nanobelts are systematically compared.It is found that the different alkaline earth metal ions in the layers have important impacts on the structure stability and cycling performance.We demonstrated that Mg²⁺ions in the interlayer results in the best structure stability and the best cycling stability.