Overcharge behavior of metal oxide-coated cathode materials

Abstract

Commercial lithium-ion (Li-ion) cells usually operate at a maximum temperature of 60°C, and protective devices are used to control the cell-operating voltage. These protective devices consist of a positive temperature coefficient (PTC) material and protective circuits that block overcharging above 4.35 V, overdischarging below 3V, and the overcurrent above 1 C. Despite this protection, many accidents (fire and explosion) associated with the cell itself or malfaction of the protective devices have been reported.1 Such accidents are the result of the thermal runaway of the cell. Thermal runaway occurs when heat generation exceeds heat dissipation because the rate of heat generation increases exponentially with the increasing cell temperature, while the rate of heat transfer to a cool environment increases only linearly.2 Hence, controlling the degree of heat generation is critical to prevent thermal runaway, which is due mainly to the violent exothermic reaction of Lix CoO2 with the flammable electrolyte, resulting in oxygen evolution from the cathode.3-7 In order to minimize the reaction, previous studies have focused on reducing the flammable nature of the electrolytes by adding phosphorus-based additives and cosolvents to the electrolytes or redox shuttle additives.8-11 However, Cho et al. reported a more fundamental approach to improving the thermal stability of the Lix CoO2 cathode. They coated the cathode with AlPO4 nanoparticles prepared from water.12 The AlPO4 coating improved the thermal stability of the cathode, as opposed to using either Al2O3 or ZrO2 coatings derived using the sol-gel method.13-15 Despite this, some metal oxides have been reported to avoid the unwanted surface reaction and protect the bulk. 16-26. © 2009 Springer Science+Business Media, LLC.