| Since the 1990s,the most widely used anode material for lithium-ion batteries is graphite anode material,but with the increasing demand for new mobile energy products and large-scale grid energy storage development,graphite anode material with a theoretical specific capacity of only 372 m Ah·g-1 is unable to meet the needs of high-performance alkali metal ion batteries.Therefore,it is imperative to develop new technologies and synthesize new materials.Tin oxide material has a much higher theoretical specific capacity(781 m Ah·g-1)than graphite anode,and also has excellent stability due to the mechanism of conversion reaction,.The cobalt-tin alloy has an even higher theoretical specific capacity(994 m Ah·g-1)and excellent electronic conductivity.However,the tin-based alloy is subject to a more serious volume expansion problem owing to the alloying reaction mechanism.This thesis intends to construct a series of alloy/oxide composites by complementing the advantages of alloy and oxide.On the one hand,the construction of cobalt-tin alloy/oxide composites can take advantage of both the stability of oxide and the high capacity and high electronic conductivity of alloy to make up for the defects of a single material and obtain the synergistic effects;on the other hand,through the asynchronous reaction process of alloy and oxide and the mutual sapce restriction effect,the attack from repeated volume expansion and contraction during charging and discharging can be effectively suppressed,thus obtaining excellent cycle stability on the basis of high capacity and good conductivity at the same time.However,the general synthesis method is difficult to obtain the metal in the reduced state and the oxide in different oxidation states simultaneously and to achieve uniform dispersion.Therefore,this paper attempts to introduce the molten electrolysis technique as the synthesis method of alloy/oxide composite anode materials.Unlike the conventional preparation methods for alloys using reducing atmospheres or reducing agents and for oxide materials using air/oxygen conditions,the molten electrolysis method can achieve alloy/oxide composites with precisely controllable composition,through the comprehensive regulation of voltage,current,molten electrolyte,electrolysis time and temperature,atmosphere and other conditions.Furthermore,the resulting composites can get a nano-scale uniform distribution of metal and oxide particles.（1）The molten reduction method used in metallurgy was introduced to synthesize anode materials for lithium-ion batteries and used as an electrochemical property probe.Compared with conventional methods such as solvothermal method and inert gas calcination,the molten electroreduction method has advantages such as simple synthesis,easy expansion of precursors,and molten salt etching.We first build the molten electroreduction device.A mixing salt with Li Cl and KCl in the molar ratio of 1:1 was chosen as the molten salt,which provides a mild environment suitable for the controllable synthesis of composite materials;a corundum crucible was chosen as the carrier for holding the molten salt,which is conducive to the reduction of impurities;a two-electrode system was chosen for the electrode,the anode is graphite rod and the cathode is Sn O2/Co2Sn O4,The mixed oxide is derived from high-temperature sintering of Sn O2/Co3O4,which retains the stable cubic structure of the original particles and good for relieving volume expansion;sintering the cathode material makes the texture harder and the surface smoother,both of which are conducive to electrical reduction.After the molten electroreduction device construction,precursor selection,and preliminary investigation of molten electroreduction,we successfully obtained cobalt-tin alloy/oxide composites,which possesses better cycling stability than the Sn O2/Co2Sn O4 as an anode material for lithium-ion batteries,thanks to the successful composite between alloy and oxide in molten electroreduction,where the different phases are domain-limited and supported by each other,mitigates the volume expansion and pulverization during cycling.（2）By regulating the three conditions of voltage,temperature and time duration in the molten electroreduction process,the effect on the composition of the composites is studied.By designing the gradient voltage,it is known that the higher the voltage,the faster the reduction rate of the composite material,and different voltages can regulate the composition and ratio of different substances in different time periods,and the composite material with proper composition and ratio of substances has more excellent electrochemical properties;by designing the gradient temperature,it is known that the higher the temperature,the more tin elements are lost in the molten reduction process,which brings a new impact on the composition of the composite material.Different from the impact of voltage regulation,the proper temperature is good for both reducing energy consumption and adjusting the composition of substances;by designing the gradient length,we can know that the longer the time,the less oxide phase and more alloy phase in the composite,which helps us to adjust the ratio between different phases and give full play to the performance of stability of oxide and high capacity of alloy in the composite.（3）In addition to controlling the"constant"reaction conditions,the molten electroreduction can also"flexibly"regulate the voltage and current,thus leading to changes in the new structure and composition of the material and obtaining cobalt-tin alloy/oxide composites with higher overall electrochemical properties.We have adopted a dynamic regulation strategy to guide the synthesis of cobalt-tin alloy/oxide composites with different compositions and morphologies.Under the dynamic modulation strategy with different potential switching,we found that the cyclic capacity decayed faster in all the five groups of comparison materials modulated in this way,presumably owing to the high content of one of the oxide phases or phases in the alloy,which led to the high volume expansion of the composites.However,the precursor oxide has been shown to have a high volume expansion,so the molten electroreduction synthesis of pure alloy phase composites was guided by a dynamic regulation strategy of gradient depressurization,and an alloy/oxide composite with a"rising"capacity was obtained,unlike the alloy/oxide composite with a fast decaying capacity.The alloy composites with a"rising capacity"trend,unlike the alloy/oxide composites with fast decreasing capacity,were obtained and later characterized in detail.This discovery helps to expand the idea of molten electroreduction synthesis,which has practical value and promise. |
