Research On Accelerated Testing Method Of Performance Degradation For Li-ion Battery Materials

The lithium-ion battery owns a long cycle life. It would take a very long cycleusing the traditional method to examine the reliability of the battery, while using theaccelerated test methods can shorten the experimental cycles and reduce the cost ofexperiments. The current researchs on accelerated testing method of performancedegradation for li-ion battery materials are summarized in this thesis. Besides, theeffect of current and temperature on acceleration of the degradation under the30%DOD cycle for the battery materials is studied.Via controlling the charge or discharge time of the coin cell system and batterysystem, the30%depth of discharge is guaranteed. By using the electrochemical tests,the accelerated currents of the coin cell are determinated:0.6C,1.2C,2.4C and3.6C.accelerated temperatures are:25°C,35°C,45°C and55°C. The accelerated currentof the battery are0.6C,1.2C,1.8C and3C. Accelerated temperature are:25℃,35℃and45℃.By using the charge-discharge test, AC impedance test, XRD and SEM analysismeans, study the effect of current on acceleration of the degradation of the batterymaterials under the30%DOD cycle. The results show that the current has a very goodacceleration effect on the performance degradation of the battery materials. Both theanode and cathode materials follow the same degradation law along with the30%DOD cycle increasing: charging voltage platform increases, the discharge voltageplatform、charging and discharing time reduce. Reversibility of the battery materialsbecome worse. The impedance increases continuously. Based on the research on thevariation of the electrochemical properties and structural of both coin cell and batterysystem, the degradation of the positive LiCoO2is due to the LiCoO2crystal cellstretching deformation occuring along the c axis as the30%DOD cycle progresses.This leads to structure changes, while the bond between the battery material and thecurrent collector become weaker under the cycles. The degradation of the negativecarbon material(MCMB/graphite) is mainly contributed to the weaker bondingbetween the current collector and materials, as well as the intercalation/deintercalationof Li+in the graphite hexagonal grid structure causing the expansion and contractioneffects, the graphite layer stripping and materials collapsing under the long-term cycles.The current accelerates the degradation factors of the positive and negative materials.In addition, the SEI film that is generated at accelerated currents is loose. This leads tothe loss of lithium-ion because of the electrolyte and lithium embedding together, aswell as transfer speed of the lithium-ion is much larger than that within the graphite solid phase at high current, causing some lithium remain in the graphite surface. To acertain extent, all of these reasons accelerate the degradation of the battery. Overall,the degradation mechanisms of the materials at both the accelerated current and thenormal current are same. By mathematical simulation, the unified degradation modelof the battery at different currents under30%DOD cycle, y=0.19Cexp(-t/B)+0.79C,C-Initial reversible capacity，y-battery capacity，t-test time，B-current accelerateddegradation factor. B=6840.54485exp(-I/0.78416)+218.31952, I-testing current. TakeB into the degradation model of the battery to get the current accelerated capacitydegradation model.By charge-discharge tests and AC impedance tests, study the performancevariation of the coin cell under the temperature accelerated experiment. The resultsshow that the accelerated temperatures have an accelerated effect on the performancedegradathon of the cathode material LiCoO2.55℃owns the best accelerated effect，the degradation rate at55℃is18.6times of the one at25℃, the degradation rate at35℃is4.2times of the one at25℃,45℃is2.8times of the one at25℃.Temperature does not accelerate the performance degradation of the anodeMCMB material. Because the elevated temperatures reduce the impedance of the ofMCMB coin cell, ohmic polarization reduces at the same time, the electrochemicalactivity is improved, and the performance of the MCMB at the coin cell system isimproved.