Study On Preparation And Electrochemical Properties Of Lithium-rich Compound Cathode Prelithiation Additive For Lithium Ion Batteries

With the increasing requirement for battery endurance of electric vehicles and portable electronics,high-energy-density lithium-ion batteries have become a hot topic of scientific research.The most effective way to improve the energy density of lithium-ion batteries is to develop cathode or anode materials with high specific capacity.At present,the development of cathode materials is in the bottleneck period,and the research of anode materials is in the rising stage.There are various materials with high specific capacity sothat the anode materials have become a breakthrough to improve the energy density of lithium-ion batteries.However,most of the anode materials will be unable to be applied because of the formation of solid electrolyte interphase（SEI）film on the surface with high initial capacity loss,especially for anode with high specific capacity.The method of adding additional active lithium to the batteries whichiscalledreplenishment technology is one of the most promising strategies to solve the initial capacity loss of the anode.The method of adding active lithium to thecathode which is called the technology of cathode prelithiation is relatively simple,practical,and has attracted extensive attention.In this thesis,weselect Li₅Fe O₄and Li₂Se as cathode prelithiation additive for research.The specific work is as follows.Li₅Fe O₄ synthesized by high-temperature solid-phase method has the disadvantages of large particle size,long synthesis time and low purity,which limit its application.In this paper,the molten salt method is used for the first time,using the mixed lithium salt of Li OH and Li NO₃ as lithium source and molten salt,and nano iron oxide as an iron source.The conditions affecting the electrochemical performance of Li₅Fe O₄ and the best synthesis process conditions are explored by orthogonal experiment.The results show that the element ratio of iron to lithium and tablet pressure are the two parameters that have the greatest influence on the electrochemical performance of Li₅Fe O₄.Under the optimum process conditions,When charged to 4.5 V,it has a high prelithiation capacity of 672.8 m Ah g^-1.Li₅Fe O₄ with a mass fraction of 2.8%was coated on the surface of Li Fe PO₄.Andthe effect of this addition method on Li Fe PO₄ is studied.The research shows that Li₅Fe O₄ after lithium removal will improve the cycle performance of Li Fe PO₄,but influence the rate performance to a certain extent.Li Fe PO₄ coated with 2.8%Li₅Fe O₄ and graphite are assembled into full cells to explore its Li-compensation effect.The research shows that the reversible capacity of Li Fe PO₄（Li₅Fe O₄）/graphite is increased by 9.2%compared with the without prelithiation inthefirstcycle,and its specific capacity is increased by 7.1%after 100 cycles.Most lithium supplement additives produce gas after lithium removal,which has a negative impact on battery performance.In this paper,we explore the use of lithium selenide,which has high theoretical specific capacity and does not produce gas,as a cathode prelithiation additive.Li₂Se is prepared by chemical lithium of Se powder with lithium naphthalene reagent and solid-state sintering.The research shows that Li₂Se has a high prelithiation capacity of 548.0 m Ah g^-1,a low discharge specific capacity(69.1 m Ah g^-1),a high irreversible capacity,and a suitable potential in the voltage range of 1-4.3 V.It can be used as a cathode prelithiation material.Li₂Se with a mass fraction of 7%is added to the NCM811 to study the effect of Li₂Se on the performance of NCM811 half cells.The research shows that the lithium removal product of Li₂Se will affect the capacity release at the beginning cycles of NCM811,and has no adverse effect on the performance of long cycles.It is shown that the de-lithium product of Li₂Se affects the capacity release of NCM811 at the beginning of the cycle and has less effect on the performance of long cycles.This was analyzed by electrochemical impedance spectroscopy,which is due to the fact that the de-lithium product of Li₂Se increases the impedance of electron transfer.In the subsequent cycles,the product undergoes a shuttle effect and the impedance of the cell drops to normal levels.Li₂Se with a mass fraction of 7%was added to NCM811 and assembled with silicon carbon into a full celltoexplore its Li-compensation effect.The research shows that Li₂Se can not only compensate the irreversible capacity loss of silicon carbon in the first cycle,but also improve the cycle performance of the whole battery.The reversible capacity of NCM811/silicon carbon is increased in the first cycle,which is 5.8%higher than that of the cells without prelithiation,and 37.1%after 150 cycles.