Research On The Mechanism Of Interactions Between Key Materials In LiFePO₄Batteries

Lithium-ion phosphate battery is one of the most promising candidate power sourcesfor electrical vehicles with the outstanding advantages such as high safety, long cycle lifeand environmentally friendly. However, the cycle life must be further improved and therapid capacity fade problem at high temperature must be solved before its automotiveapplication, which need intensive study on the capacity fade behavior and relatedmechanism in lithium-ion phosphate battery. Iron dissolution and lithium consumption aretwo of the main interactions which lead to capacity fade, hence becoming the main contentof the study.In this thesis, commercial LiFePO₄cathode material was used to study irondissolution behavior in electrolytes. Up to five factors involving temperature, time,solvents, lithium salts and the states of charge of the cathode were used to comprehensivelyevaluate the behavior and mechanisms of iron dissolution. The results suggest evaluateelevated temperature, longer electrolyte residence time and high sate of charge of thecathode can increase iron dissolution, while the kinds of electrolyte solvents and lithiumsalts have smaller influence.Capacity fade behavior of18650style LiFePO₄battery under cycle and steady statewas studied respectively. Batteries cycled at different temperatures showed different cycleperformances. Rapid capacity fade was observed at60℃and the cycle life shortenedabout55%compared with the cell cycled at25℃，and all the capacity fade curvesdelivered linear decreasing trend during most of the test time. On standing condition, thecell charged to100%state of charge suffered severe capacity fade, with the decrease of thecharging status, the capacity fade slowed down. Based on the analysis on the capacity fade of the LiFePO₄batteries, we found that thecapacity of the cathode maintained, while a part of lithium inventory was lost during thefirst de-intercalation process, and the lost amount of lithium inventory in the cathodescorrelated well with the capacity fade of the LiFePO₄battery. A quantitative study of thelithium content in the negative electrode was carried out by ICP, and it was amazing to findthat the lithium content in the negative electrode correlated well with the lithium inventoryloss of the cathode. The consistent relationship between capacity fade of the cell, thelithium inventory lose in the cathode and lithium consumptions in the anode fully showsthat the loss of recyclable lithium which was consumed in the anode is the main cause ofthe capacity fade in LiFePO₄battery.