Effects of Upper Cutoff Potential on LaPO₄-Coated and Uncoated Li[Ni_0.42Mn_0.42Co_0.16]O₂/Graphite Pouch Cells

Abstract

Li[Ni0.42Mn0.42Co0.16]O2/graphite pouch cells, with and without a LaPO4 coating on the positive electrode active material, containing the additives prop-1-ene-1,3-sultone, 1,3,2-dioxathiolane-2,2-dioxide and tris-(trimethyl-silyl) phosphite were studied using charge-hold-discharge cycling experiments up to potentials between 4.4 and 4.5 V at 40°C. Simultaneous electrochemical impedance spectroscopy measurements were made during cycling. The effects of a 24 hour hold at the top of charge at upper cutoff voltages of 4.4, 4.425, 4.45, 4.475, and 4.5 V on the impedance and capacity retention of both LaPO4-coated and uncoated cells were investigated. All coated cells cycled above 4.4 V exhibited severe capacity fade and large impedance growth over 80 cycles. Uncoated cells cycled to 4.45 V and below exhibited small capacity fade and virtually no impedance growth. Although uncoated cells above 4.45 V were superior to coated cells, capacity fade and impedance growth were present. Impedance growth remains one of the biggest obstacles to high voltage NMC/graphite lithium-ion cells.