Studies On Preparation, Modification And Electrochemical Performance Of Bismuth-based Cathode Materials For Lithium Ion Batteries

Chemical conversion reaction, which was gradually developed in recent years, is anew mechanism of lithium storage for lithium-ion batteries. Due to the breakthroughon the structural constraints of traditional intercalation compounds, the materialsbased on conversion reaction can occur to a multi-electron transfer reaction in thecharge-discharge processes, thus obtaining excellent theoretical capacity, whichprovides a novel approach to realize high energy density for cathode materials.However, in the age of pursuiting more compact electronic products, the cathodematerials possessing both a high gravimetric energy density and volumetric energydensity become a hot direction. Because the bismuth-based cathode materials canyield a high capacity via chemical conversion reaction, the two bismuth-basedcompounds of BiF₃and BiPO₄are selected as the research object in this paper. Thepreparation processes of the two compounds are discussed, and the correspondingmodifications of materials are also performed. The main results are as follows:（1） BiF₃was studied as the cathode material based on conversion reaction. Firstly,BiF₃was directly prepared via liquid precipitation method, the effects of carbonconductive agent such as graphite, acetylene black, and Super P carbon black onelectrochemical performance of BiF₃were discussed. The results showed that theBiF₃/C composite with acetylene black had the best electrochemical performance. Itsinitial discharge capacity was230.7mAh g^-1at a rate of30mA g^-1, and after15cyclesthe discharge capacity was still kept to62.1mAh g^-1. Especially, at a rate of450mAg^-1, its discharge capacity was still183.2mAh g^-1. The rate capacity retention was ashigh as72.7%. Besides, the composite showed the lowest degree of polarization andcharge transfer resistance （Rct=98.75）.（2） The BiF₃/C/AlPO₄composites were prepared by solid state milling process, andthe influences of amorphous AlPO₄content on SEI film and the electrochemicalperformance of BiF₃/C/AlPO₄composites have been examined systematically. Theresults showed that the BiF₃/C/AlPO₄composite with15.0wt.%AlPO₄had the bestelectrochemical performance. At a rate of30mA g^-1, it exhibited the highest initialdischarge capacity of263.6mAh g^-1and the lowest irreversible capacity of45.1mAhg^-1. After5cycles, its capacity retention percentage was still as high as67.3%. Even ata high rate of900mA g^-1, the discharge capacity was still as high as225.5mAh g^-1,which exhibited the rate capacity retention of84.0%. When the cell was discharged to1.8V, in the absence of AlPO₄, SEI film would be formed on the Bi0nanometal surfaces. And it has been totally absent with increasing the amount of AlPO₄to15.0wt.%.（3） For the first time, the feasibility of metal phosphates as the cathode materialsbased on conversion reaction has been explored. Firstly, the hexagonal BiPO₄wasdirectly synthesized via liquid precipitation method. Subsequently, low and hightemperature phase were obtained by thermal treatments over the hexagonal phase atdifferent high temperatures. The effects of phase transformation on electrochemicalproperties of BiPO₄also have been examined in detail, and the charge-dischargemechanism of BiPO₄was explored. The results showed that the charge-dischargeprocesses of BiPO₄were based on the mechanism of conversion reaction, namely, itwas converted into metallic Bi0and Li3PO₄during lithiation, undergoing reconversionupon delithiation into BiPO₄. Besides, the hexagonal BiPO₄showed the bestelectrochemical performance. At a rate of26.45mA g^-1, it exhibited an initialdischarge capacity of252.0mAh g^-1, a reversible capacity of188.3mAh g^-1, and thefirst discharge plateaus of2.37V. After40cycles, its specific capacity was still kept to77.4mAh g^-1. When the current density was increased to529.04mA g^-1, it stillrevealed a specific capacity of236.7mAh g^-1. The rate capacity retention was as highas91.4%.