| Lithium iron phosphate（LiFePO4）has always occupied an irreplaceable position in lithium-ion battery cathode materials due its good cycle performance,high theoretical specific capacity（～170 m Ah/g）,good thermal stability.LiFePO4 is one of the mainstream cathode materials for commercial power lithium-ion batteries.At the same time,its composition is environmental friendly and free from rare earth contributing to its low cost,it is regarded as one of the earliest commercial cathode materials.However,low electronic conductivity(10-9 S/cm)and low lithium ion diffusion coefficient(10-3～10-6 cm2/s)result in poor rate performance,which severely restricts its use in high-power lithium-ion batteries,and at the same time limits its development in solid-state lithium-ion batteries.Aiming at the problems above low conductivity of LiFePO4,the effects of different sintering process parameters on the performance of LiFePO4 cathode material are studied in this thesis.In order to overcome detrimental interdiffusion between the layers（e.g.anode,electrolyte and cathode）rapid（e.g.heating rate 100℃/min）processing techniques like Spark Plasma Sintering（SPS）or low temperature processing techniques,such as Cold Sintering Process（CSP）have been considered.We report on novel cold sintering techniques（e.g.Cold Sintering Process（CSP）and Cold Isostatic Sintering（CIS））to enable fabrication of dense LiFePO4（LFP）cathodes.In this thesis,high-purity lithium iron phosphate（LiFePO4）cathode powder was used as the raw material,the effects of different processing settings and solvents（H2O,0.1M LiOH,ethanol,dimethyl sulfoxide（DMSO））on the sintering of LiFePO4ware studied.So first,under the pressure of 400 MPa,the effect of the same volume fraction and different solutions on the performance of the LFP cathode was studied.Second,the same cold sintering process in the reference as a control was compared.Our analysis,rationalizes the complex mechano-chemical interaction between liquid and particle（i.e.congruent vs incongruent dissolution）during the transient liquid phase sintering.XRD,SEM and FTIR confirmed that crystalline and well-distributed LiFePO4 particles were sintered by CIS under 400MPa for 1h with 10 vol.%DMSO.In addition,the electrochemical impedance（EIS）results showed that the materials prepared using cold isostatic under 400 MPa with 10 vol.%of DMSO can reach ionic and electronic conductivities as high as 1.9×10-3 S/cm and 1.2×10-4 S/cm.The latter showed better electrochemical performance with the high specific capacity（～122m Ah/g and～238 m Ah/cm3）and the better capacity retention than the cell without CIS at 1C.In addition,a good rate performance was observed during the 1 C,2C,5C cycle test on the sintered LFP liquid cells.During cold isostatic sintering,DMSO enhanced dissolution of ions prevented incongruent dissolution and hydrolysis without the formation of undesired second phases which acts as a barrier for ionic conduction.Compared with traditional firing,a part of the energy can be used to manufacture the LFP cathode without heating.Furthermore,the liquid half-cells were assembled using commercial LFP films followed by CIS.It is anticipated that the even pressure distribution produced generated by an isostatic sintering technique offers an industrially scalable and reliable processing route to assemble an operational solid battery with remarkably improved performance.In addition,considering the possible electrochemical effects and lithium ion migration/non-blocking effects in spark plasma sintering（SPS）,we propose a comparison of spark plasma sintering to LiFePO4 using Alternating Current（AC）and Direct Current（DC）in the sintering process.Using impedance spectroscopy,ICP-AES and XPS verified the hypothesis that DC quantified Liion migration.During DC SPS sintering,lithium ion concentration gradients were observed in the vicinity of the cathode and anode.This electrode effect is caused by a lowering of the activation energy for lithium ion migration,which leads to the fracture of the LFP specimen.The spark plasma sintering of LFP has electrochemical properties.In a reducing atmosphere,the material will undergo a partial reduction process under the action of a DC pulse electric field.This partial reduction changes the electrical properties of the material and stimulates electronic conductivity.The use of an AC field seems effective to inhibit undesired Liion migration and achieve high ionic conductivity as～4.5×10-3 S/cm which exceeds by one order of magnitude samples processed under a DC field.In addition,the high ionic and electronic conductivity and completeness of the LFP pellet using AC could provide reliability for the assembly of solid-state batteries.The comparison between DC and AC suggest that rapid heating effect end electrode effect needs to be considered in the DC sintering process. |
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