| Nowadays,with the continuous development of the social productivity,human production activities have huge demand for new clean energy such as solar energy.The current urgent task is to effectively solve the problem of energy storage.Lithium-ion batteries with high specific energy become the preferred energy storage devices compared to conventional lead-acid batteries.As an important member of cathode materials for lithium ion battery,lithium iron phosphate cathode materials are highly favored by the market because of their high safety,environmental friendliness,and low price.However,in order to effectively overcome the intrinsic low conductivity and the slow rate of lithium ion diffusion in the material,the most common method including in-situ carbon coating,transition metal doping as well as particle size nanocrystallization have been utilized to improve the electrochemical performance of material.Of course,that using self-assembled and template method to assemble nanoparticles into micron-sized secondary particles is also a fairly novel approach.In addition,the direct electrode surface modification is also an effective method to improve the electrochemical performance of material.As we all known that the interface environment between the electrode and the electrolyte plays an important role in the release of material capacity and the stability of the entire battery performance.On the one hand,hydrofluoric acid in the electrolyte can strongly attack the surface of electrode,causing the dissolution of transition metal in the material during the charging and discharging of battery,especially at high temperatures.On the other hand,once work voltage exceeds a certain range the electrolyte would undergo severe redox decomposition reaction.When leading to the loss of active lithium,the generated decomposition product with electrical insulation properties would increase the electron transfer resistance of electrode surface.These factors can cause battery material demonstrating poor electrochemical performance.The direct electrode surface modification is like building an insulating layer,effectively preventing the adverse reaction between the surface of the electrode and the electrolyte,ensuring good cycle stability of battery during charge and discharge process.Furthermore,it is worth mentioning that the lithium ions in lithium iron phosphate escape from the material surface which need to overcome a larger barrier than that diffusion in the bulk.Selecting the appropriate material to modify the surface of the lithium iron phosphate electrode can effectively reduce the barrier and increase the lithium ion diffusion coefficient.Based on the simple,direct,and effective thinking,we focus on using ultrasonic spray technology and magnetron sputtering technology to modify the surface of lithium iron phosphate electrode,and develop material electrochemical performance.The improvement mechanism and lithium ion kinetic behavior have been systematically investigated.In addition,the modification of the surface of high-nickel ternary material was also performed,and its intrinsic mechanism of action was studied.The specific work is as follows:Firstly,based on the ultrasonic spray technique,the evenly distribution of nanoscale silicon on the lithium iron phosphate pole piece was finished via a simple and novel surface modification device of electrodes which we designed and fabricated independently.The experimental results indicate that,the surface modification of nano-sized Si presents better cyclicity at high charge/discharge rate,especially at elevated temperature.Moreover,Si modified electrode displays more complete surface topography and good material crystallinity in comparion with pure one.Through transmission electron microscopy,it can be found that the main factor for improving the electrochemical performance is attributed to the silicon particles having a typical core-shell structure with crystalline silicon as the core and amorphous silicon as the shell,and the dangling bonds of amorphous silicon shell can effectively adsorb hydrogen ion in the electrolyte which slows down the attack of hydrofluoric acid for the surface of pole piece.Secondly,in order to further study the role of nano-silicon in the process of charge and discharge,the phase changes of LiFePO4@C and LiFePO4@C/Si electrodes in Li+intercalation/deintercalation are systematically studied by XRD,Raman and EIS,respectively.The experimental results indicate that LiFePO4@C/Si has faster charged and discharged velocity than LiFePO4@C.Moreover,it is found that,LiFePO4@C/Si possesses larger diffusion coefficient and less activation energy than LiFePO4@C,which is consistent with the increase of charge depth,the enhancing charge and discharge velocity.And it is observed that there exists the best silicon content in enhancing the electrochemical performances of LiFePO4@C through EIS measurement.At last,it is suggested that constant-voltage charge is indispensable for a fully delithiation of the LiFePO4 material.Then,based on the protective and promoting lithium ion diffusion rate properties of nano-silicon,Si modified LFP electrodes successfully prepared with the technique of ultrasonic spray,combined with the anodic graphite electrodes,have been assembled in commercial 18650 cylindrical batteries.Under high temperature,the 18650 cells with nano-silicon modification show good cycling performance.After a series of phase characterization and analysis,it was found that the cathode with Si modification remains better LiFePO4 phase,less Li+ loss and integrity of structure.In addition,the battery with modification contains better graphite carbon structure,less deposited Fe2O3.More importantly,it keeps thinner thickness of SEI film.In conclusion,Si modification implements suppression of degeneration of LIBs at high temperature.Next,the compound with metallic copper as the core,and oxidation composite consisting of cuprous copper and cupric copper as the shell was evenly dispersed on the surface of LiFePO4 electrode via a facile magnetron sputtering technique.The effect of modification amount on the electrochemical performance of lithium iron phosphate was investigated,and its improvement mechanism was also explored.The results show that LiFePO4 with appropriate compound decoration demonstrates more excellent rate performance and cyclic stability under high current density.It attributed to strengthen of electric contact between active materials due to the existence of metallic copper in the compound.Based on a series of characteristic technology,it could be found that LiFePO4 decorated by compound remains intact crystal structure after undergoes many cycles at high current density.It means that cupric oxide and cuprous oxide formed on the copper surface offer a significant support for serving as a physical barrier.Moreover,superabundant compound decoration on the surface of electrode can deteriorate the electrochemical performance of battery.It is attributed to the excess copper surface exposure which intensifies the oxidation decomposition of electrolyte,causing lots of capacity consumption.Afterwards,the copper oxides were sputtered on the LiNio.6Coo.2Mno.2O2 pole piece by magnetron sputtering.The purpose is to set up a buffer layer between the active particles and the electrolyte for improving the interface environmental.Electrochemical test results show that the copper oxide deposition does not significantly improve the material's rate performance under room temperature.When the battery is under high temperature conditions,the protective effect of copper oxide is gradually highlighted and the material exhibits better cycle stability.The reason is that under the conditions of high voltage and high temperature test,the side reactions between the electrolyte and the electrode become increasingly fierce.However,the presence of copper oxide can effectively slow down the side reactions.Ultimately,the lithium iron phosphate thin films with aluminum foil as substrate were successfully prepared by magnetron sputtering.The work functions of the three thin film samples of LiFePO4,LixFePO4(LixFePO4 represents lithium iron phosphate thin film charged to 4.2V)and FePO4 were obtained by Kelvin Probe Force Microscopy(KPFM)measurement.It is found that the surface work function of LixFePO4 is about in the middle of two work functions of LiFePO4 and FePO4.The two phase coexistence delithiation model of lithium iron phosphate might be employed to explain this phenomenon. |
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