Electrochemical Mechanism And Phase Transition Of Lithium Iron Phosphate Nanoparticles

Lithium-ion battery?LIB?plays an irreplaceable role in electric vehicles.The phosphate-olivine LiFePO₄?LFP?has become an extremely attractive and competitive commercial cathode because of its abuntant raw materials?low cost?being environmental friendly and well-demonstrated super cycle life?excellent safety performance and high rate performance.The dynamic phase transition of LiFePO₄during charge/discharge plays a decisive role in electrochemical performance.Understanding the phase transition mechanism of LiFePO₄ contributes to further optimize the electrochemical performance of LiFePO₄ cathode.In addition,the morphology of LiFePO4 also accounts in this subject to obtain LiFePO4 with reasonable crystal orientation for optimizing the electrochemical performance considering anisotropy of LiFePO₄ and the Li⁺1D diffusion characteristics.Based on the understanding of fast channels of Li⁺diffusion in LiFePO₄ crystal,ac orientation preferred LiFePO₄ nano particles?NPs?have been synthesized by ethylene glycol solvothermal method,with size distribution below 100 nm,which exhibits stable cycling performance and discharged 166.5 mAh·g^-1 at 0.1 C-rate,even86.5 mAh·g^-1 at 10 C-rate.LiFePO₄ with ac plane preferred has a higher Li diffusion and migration rate,thus resulting in superior electrochemical performance.The phase transition of LiFePO₄ porous electrode was investigated via Galvanostatic Intermittent Titration Technique?GITT?,then we discuss the phase transition in every depth and how C-rates affect phase transition of LiFePO₄.On this foundation,the phase tranisition connection between single-particle and porous electrode was established,as well as the correspondence between the phase transition and the response potential.Under tiny current,when Li⁺inserts,the porous electrode undergoes uniform discrete phase transition since LiFePO₄ nanoparticles were filled one by one,while when Li⁺extracts,it undergoes random nucleation phase transformation due to the non-linear concentration fluctuation.Under higher current,Li⁺concentraction in the electrolyte decreases significantly,thus forming a macroscopic"diffusion"interface along the concentration gradient,which may result in solid solution phase transition under the high C-rates.In addition,the open circuit voltage?OCV?,temperature and SOC can strictly meet Arrhenius equation,which provides an accurate calculation for activation energy of phase change and OCV under real conditions.In this paper,LiFePO₄ with solid solution phase transition path was prepared by chemical delithiation method to test and verify the fast kinetic response of the solid solution phase transition in LiFePO₄ NPs,which further confirms the nature of high rate capability of LiFePO4:LiFePO4 NPs undergoes solid solution phase transformation during high C-rates.On this basis,we also simulated the electric field distribution of LiFePO₄ during charge/discharge by applying external voltage on the LiFePO₄ power to explore the change of phase state in electric field and then established the relationship between current density and phase transition.It is found that the critical electric filed of new phase corresponds to 10 C-rate,and two stable and reversible new phases appear to be present at 17.5°and 33.5°,respectively.When electric field exceeds the threshold under ultra-high current densities?20 C and above?,the mid-phase exists but it's very complicated and unstable.Furthermore,the cirtical current density of phase transition was determined via differential capacity curve.Blow 2 C-rate,LiFePO₄ NPs undergoes concurrent phase transition,the two phases?LiFePO₄ and FePO₄?co-exist in single particles.Above 2 C-rate,LiFePO₄NPs will form a continuous diffusion interface,and there is unstable mid-pahse in a single particle.While higher than 10 C-rate,a stable and reversible solid solution phase appears in the overpotential gradient distribution electrode,and the phase separation is inhibited,thus the electrode material undergoes a single phase transition,further revealing the solid solution phase is the reason of high-rate performance in LiFePO₄NPs.