Design And Performance Study Of Borate Glass-based Lithium-ion Battery Anode Materials

Oxide glass is considered as a potential anode material for lithium-ion batteries because of its unique disorder network structure and no grain boundary,which is conducive to the diffusion and storage of Li⁺ions,as well as keeping structural stability of the anode materials during charging/discharging process.Despite of a great deal of researches on the glass anode,both the reversible capacity and rate capability of the oxide glass anodes are still far from the requirement for the next generation of advanced Li Bs.For this reason,in this work,Fe₂O₃-B₂O₃（Fe B）glass is chosen as as the research object.Firstly,the effect of the glass composition on the structure,thermodynamics and electrochemical properties of Fe₂O₃-B₂O₃（Fe B）glass are systematically studied.The glass composition with the best electrochemical properties are obtained.Secondly,considering the structural heterogeneity of the Fe B glasses,we have tuned both the glass compositions and microstructures of Fe B glass by means of acid corrosion and heat treatment,respectively,in order to improve their electrochemical properties.The main research contents and conclusions of this thesis are as follows:（1）x Fe₂O₃-（100-x）B₂O₃（x=20,40,60,90 mol%）（Fe B）glass were prepared.The effect of compositon on the phase,microstructure and thermodynamic properties were studied.The electrochemical properties of Fe B glass anodes were studied in detail.It is found that when x≥40mol%,Fe₂O₃ and Fe₃O₄ crystals are precipitated spontaneously in the glass matrix with the increase of x.According to the Mossbauer’s results,the ratio of Fe²⁺/Fe³⁺increases with the increase of Fe₂O₃ content,which significantly improves the electronic conductivity of Fe B glass-ceramics.When x increases to a certain value（i.e.,90 mol%）,the precipitated crystals are mainly Fe₃O₄due to the relatively high melting temperature which is benefit to the formation of Fe²⁺ions.When used as the anode materials of Li-ion batteries,the electrochemical properties of Fe B glasses,including the reversible capacity,electrical conductivity and rate prtfotmsnce,are highly improved with the increase of x.The reversible capacity of 90Fe B anode reached 505 m A h g^-1 at 1000m A g^-1 current density at the end of 1000 charging/discharging cycles with good cycling stability.This is mainly due to the unique microstructure of 90Fe B,in which the Fe₃O₄nanocrystals with grid structure are evenly dispersed in the glass matrix,providing more active sites for Li⁺-ions storage and enhancing the transport dynamics of lithium ions.（2）According to the results of content（1）,it is known that when x≥40mol%,Fe B glass ceramics will be formed.Therefore,in this section,we chose x=10mol%,20mol%,and 30mol%to prepare10 Fe B,20 Fe B,and 30Fe B glass samples.DSC results show that10Fe B glass exhibits significant tructural heterogeneity expressed by two glass transition peaks and the lower one corresponding to the B₂O₃ glass phase.The effect of nitric acid treatment on the structure and properties of 10Fe B,20Fe B,and 30Fe B glass was investigated.Combining the results of DSC and XRF,it can be concluded that the B₂O₃in glass is mainly corroded by nitric acid,thereby,reducing the thermal stability of glass.The acid treated glass samples exhibit significantly improved electrochemical performances compared with the as-received glass.In particular,the specific capacities of the acid treated samples are nearly double of those of 10Fe B and 20Fe B.Taking10Fe B as an example,its capacity is 136.7 m Ah g^-1 after 1000 charging/discharging cycles at1000 m A g^-1,while the capacity of the 10Fe B-H anode can reach 270.8 m Ah g^-1 under the same conditions.（3）We prepared 10Fe B glass ceramics by heat treating the as-received sample at the two crystallization temperatures,i.e.,946 K and 1018 K,for different times according to the DSC results.It was found that the heat treatment always leads to the simultaneous precipitation of both Fe₂O（BO₃）and Fe₃（BO₃）O₂ crystals since the two crystallization temperatures are close enough（two consecutive peaks in the DSC curve）.The capacity of 10Fe B glass ceramics was improved compared to that of 10Fe B glass.In particular,the 10Fe B-1018K30min based anode showed a reversible capacity of 533.5 m Ah g^-1 at the end of 1000 cycles at a current density of 1000 m A g^-1.The enhanced capacity is attributed to the crystals in the galss matrix which reduces the charge transfer internal resistance of the anode material and provides more active reaction sites for the storge of Li⁺ions.The research in this thesis paves a way for the design and development of high-performance glass anodes for lithium-ion batteries.