The Impact of Upper Cut-Off Voltage on the Cycling Performance of Li-Ion Cells with Positive Electrodes Having Various Nickel Contents

Abstract

The charge-discharge cycling performance of pouch cells with single crystal LiNi0.5Mn0.3Co0.2O2 (SC532), LiNi0.8Mn0.1Co0.1O2 (SC811) and a prototype polycrystalline Co-free core–shell material with an average 94% Ni content (Ni94) were compared in this work. Two upper cut-off voltages (UCVs) per cell type were chosen to either include or exclude the remnant of the “H2-H3 phase transition” region, if present, of each positive electrode material. The core–shell Ni94 shows comparable performance to the SC532 and better performance than the SC811 only at 20 °C and 4.04 V UCV. In other testing conditions, the SC532 has the best performance followed by the SC811. The cross-section SEM images of the fresh Ni94 electrode show microcracks from electrode calendaring which is detrimental to its cycling performance as the exposed Ni-rich core has a high reactivity with the electrolyte which induces large impedance increase. The Ni94 material shows quite poor capacity retention and large impedance growth when charged to 4.18 V, through the large volume change associated with the “H2–H3 remnant,” but acceptable capacity retention when only charged to 4.04 V that avoids this large volume change.