Synthesis And Discharge Mechanism Of Composite Vanadium Oxides Cathode Materials For Thermal Cell

Vanadium oxides as cathode material of molten salt thermal battery have higher initial voltage, theoretic capacity and thermal stability than those of disulfide, but with inferior stabilization of discharge voltage, lower available capacity. Although some developments have been acquired in recent years, there are still many questions to further investigate, such as synthesis mechanism on vanadium oxides, intrinsic relationship between structure and performances, discharge mechanism. In this dissertation, a series of +4/+5 and mixed valence (lithium) vanadium oxides including Li_1+xV₃O₈,γ-LiV₂O₅,VO₂ and their composite materials LVO(lithiated vanadium oxide),VOC(vanadium-oxide-carbon) were synthesized by heat reaction, molten salt flux and soft chemistry method, respectively. The study on synthesized mechanism, physical and electrochemical performances, and discharge mechanism are emphased. The approaches to further improve the electrochemical performances are discussed. The main results are as follows:(1) Li_1+xV₃O₈ cathode material was synthesised by a reformative solid reaction. From room temperature to 550℃, the formula of electrical conductivity and temperature is lnσ= 1.505 - 2.141×10³ /T . Activated energy of electrical conductivity is 0185eV, and thermal stability can get to 620℃. Doping with molybdenum can increase the voltage about 0.3 V and specific capacity near 25%. The chemical lithiation can greatly improve the stability of voltage by eliminating the voltage pinnacle at the beginning of discharge.(2)Pure phase VO₂ was prepared by a late-model soft-chemistry and one-step process. The electrical conductivity of VO₂ quickly increases along with the semiconductor-metal transition at 68℃, and the energy gap at low temperature is about 0.1 eV. The open-circuit voltage of VO₂ cathode of a single cell is 2.6V vs. Li-B anode. The initial voltage is 2.52V when the current density equals 100mA·cm^-2. The specific capacity of VO₂ obtained by one-step process is 758Asg^-1 at cut-off voltage of 1.5 V. Fining the grain can markedly enhance voltage and capacity. (3)A new route to synthesis singleγ-LiV₂O₅ phase is put forward. The influences of reactive temperature, time, lithium resource, lithium content on resultant phase is studied. The equation of electrical conductivity ofγ-LiV₂O₅ and temperature is lnσ= 0.15 -1.74×10³/T, and the energy gap is about 0.35eV. Thermal stability is up to 620℃. The open-circuit voltage ofγ-LiV₂O₅ single cell is also 2.6V vs. Li-B anode, and the initial voltage and specific capacity at current density 100 mA·cm^-2(cut-off voltage 1.8V) are 2.35V, 410 As·g^-1, respectively.(4)The model of discharge mechanism of vanadium oxides is firstly established based on the results from analyzing the phases of the discharge leftover and the Li₂-Li₂O-V₂O₅-VO₂ high temperature diagram,. The primary reasons of the lower available specific capacity of vanadium oxides consist in the lower electrical conductivity of the discharge produces Li₃VO₄, which aggravates the polarization. The technologies including adding the conductivity reagent, compounding technologies and modifying physical morphology of the grain are brought forward to increase the capacity utilization of cathode materials.(5)A nucleation-growth model including deoxidization shearing and assembling oxygen shearing is first brought forward, which can be used to character the nucleation and growth process of Li_xV₂O₅ and VO₂(B) phase in the course of lithiation of mixed valence (+4,+5) vanadium oxides. A new idea to further improve the material performances by an approach, namely, two (or more) phases cooperated-discharge and complementary performances is expatiated, which is validated by the test facts that the electrochemical performances ofγ-LiV₂O₅/VO₂ composite materials are well than those of two single materials.(6)The influencing mechanism of the reactive conditions on the resultant phase, morphology and electrochemical performances are investigated. The active composition of VOC material are confirmed. The function mechanism of the chloride eutectic molten salt and carbonelement during the preparation and discharge courses are further definited. The mechanism of the reaction is put forward. VOC material as cathode materilas of thermal battery can present higher voltage and stable discharge curve. The highest open-circuit voltage is 3.16V and the voltage flat locates at 3.05V. The specific capacity of VOC achieves to the most (about 580As·g^-1) at the current density of 100mA·cm^-2.Through the study of this paper, the synthesis technology of involved vanadium oxides is further developped, and the selectivity of cathode materials for the middle or long life thermal battery is enhanced. The application and related theory of vanadium oxides for thermal battery are supplemented. It is of theoretical significance and engineering application values to the application, development and systematization of cathode materials of vanadium oxides.