Stress Evolution in Lithium-ion Composite Electrodes during Electrochemical Cycling and Resulting Internal Pressures on the Cell Casing

Abstract: Composite cathode coatings made of a high energy density layered oxide (Li1.2Ni0.15Mn0.55Co0.1O2, theoretical capacity ~377 mAh/g), polyvinylidene fluoride (PVdF) binder, and electron-conduction additives, were bonded to an elastic substrate. An electrochemical cell, built by pairing the cathode with a capacity-matched graphite anode, was electrochemically cycled and the real-time average stress evolution in the cathode coating was measured using a substrate-curvature technique. Features in the stress evolution profile showed correlations with phase changes in the oxide, thus yielding data complementary to in situ XRD studies on this material. The stress evolution showed a complex variation with lithium concentration suggesting that the volume changes associated with phase transformations in the oxide are not monotonically varying functions of lithium concentration. The peak tensile stress in the cathode during oxide delithiation was approximately 1.5 MPa and the peak compressive stress during oxide lithiation was about 6 MPa. Stress evolution in the anode coating was also measured separately using the same technique. The measured stresses are used to estimate the internal pressures that develop in a cylindrical lithium-ion cell with jelly-roll electrodes.