Correlation between electrochemical-calorimetric characteristics and structural changes upon lithiation/delithiation in lithium-ion cells.

摘要

An accelerated rate calorimeter (ARC) in combination with a battery cycler and a precision multimeter was used to measure the heat dissipation from, and heat accumulated in, commercially available lithium-ion cells (Sony US 18650, and Panasonic CGR 18650) during cycling under various operating parameters. An integral energy balance was used to determine the total heat generated in the test cells during cycling. DC current interruption technique was used to determine the time-dependent area-specific impedance, ⟨ASIt⟩, of the cells which was well correlated to steeply increased heat dissipation rate at the end of discharge. The reversible (entropic) heat effect was found to be exothermic during discharge and endothermic during charge. Using four different methods, values were obtained for the entropy of reaction (DeltaS) during discharge of the Sony Li-ion cell. By extrapolating to zero rate, the reversible entropy of faradaic reaction for this cell was found to be --37 +/- 3 J K-1 per g mole of Li within the temperature range (35 to 55°C).;A phase diagram for LixCoO2 was determined from potential measurements as a function of lithium concentration (x) in the host electrode matrix. The diagram shows clear evidence for the temperature-dependent phase transition between hexagonal and monoclinic structures in LixCoO 2 during electrochemical calorimetric cycles under various cell operation temperatures. This phase transition was correlated to the transient heat effect changing from exothermic to endothermic heat generation, and reverse, observed in measurements on cells containing Li0.5CoO2.;Some comparative studies have been conducted for electrochemical characteristics of Li/LixCoO2 and Li/LixNi0.8Co 0.2O2 2016 size coin cells as a function of x. The order-disorder transition was indicated in the differential chronopotentiograms by a minimum dx/dV at ca. x = 0.5 in LixCoO2 and ca. x = 0.3 in and LixNi0.8Co0.2O2. The chemical diffusion coefficient of Li (DLi+) and other kinetic properties for lithium intercalation reaction into LixCoO 2 and LixNi0.8Co0.2O2 were measured by galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). DLi+ reached a peak rate, which was assigned to phase transitions in lithium host electrode materials. The temperature dependence of the order-disorder phase transition around Li 0.5CoO2 indicated by reduced DLi+ at higher temperature. The impedance spectrum of Li/LixCoO 2 and Li/LixNi0.8Co0.2O2 cells showed the SOC dependence of kinetic parameters of Li intercalation/deintercalation reaction and solid electrolyte interface (SEI) in Li/LixCoO 2 and Li/LixNi0.8Co0.2O2 Systems.;The structural characteristics of LixCoO2 was studied with in situ X-ray diffraction (XRD) method as Li was electrochemically extracted.