To improve the safety performance of Li-ion batteries using LiCoO 2 cathodes without sacrificing battery capacity, a new electrode concept using a multilayered cathode as an alternative to a mixture of different kinds of powder is introduced. Results from a double-layered cathode consisting of a LiCoO 2 main layer with a LiFePO 4 sublayer on top of an aluminum current collector are presented. The mechanism of how the LiFePO 4 layer improves the tolerance of the batteries against overcharging is discussed. © 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2709503͔ All rights reserved. The safety of Li-ion batteries has become of great concern with increasing power consumption of applications ranging from mobile phones and notebook computers to hybrid electric vehicles ͑HEVs͒ and power tools. Research on anode, 1,2 cathode active mass, 3-5 and flame retardant electrolyte 6-8 have improved tolerance of Li-ion batteries against abuse or hazardous conditions such as overcharging, internal short circuit, and high-temperature storage. However, most of these improvements compromise the cell performance such as the overall capacity and/or rate capability.Safety of Li-ion batteries is evaluated under various hazardous conditions such as heating, crushing, overcharging, and nail penetration. Among them, overcharging a battery is one of the most hazardous situations, where both heat generation and short circuit occur simultaneously. Thus, our aim is to improve the tolerance of batteries against overcharging.There are some signals, such as abnormal voltage, heat generation, gas generation, smoke, and ignition that can be observed when a battery is overcharged. It is necessary to stop the charging process before, for example, smoke or fire generation, where the situation becomes fatal. Currently, there are some safety mechanisms in commercialized Li-ion batteries that detect such signals as voltage or temperature change. For example, a current shutdown function of polyethylene ͑PE͒ separator is triggered by the heat generation inside Li-ion batteries.9,10 However, the shutdown of the separator strongly depends on the melting temperature of PE so that the battery may reach more than 100°C when the shutdown function operates. In such a case, there is a chance of thermal runaway of the charged cathode and the battery is not safe. In addition, some electrolyte additives can protect a battery from overcharging according to the battery voltage.11,12 Even though these additives are often effective in minute amounts, the function of these additives is likely to deteriorate as the battery temperature rises.This study proposes a multilayered cathode structure to improve the safety of Li-ion batteries for future applications with more severe requirement of safety such as HEVs. Chosen in this work is a double-layered cathode consisting of an outer main layer and a sublayer on an aluminum ͑Al͒ current collector as shown in Fig. 1. Once the battery is overcharged, the sublayer acts as an ohmic barrier and shuts the charging current down....

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