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....

Deterioration of a Li-ion battery charged to an elevated voltage is investigated. Insertion of an insulating layer consisting of TiO 2 powders and PVDF binder between the cathode and the separator suppresses the self-discharge during a storage test at 60 • C. This layer prevents the carbonization of the separator by oxidative cathode and this effect is greater when the insulating layer is placed on the cathode than on the separator. Development of a Li-ion battery with higher energy density has been of great interest because of increasing power consumption of mobile applications such as notebook computers and smart phones with multiple functions. Demand for smaller size of such applications also requires higher energy density of Li-ion batteries. One way to increase the energy density of a Li-ion battery is to utilize higher amount of Li ions in the cathode material by elevating the charging voltage. [1][2][3][4][5] However, other unfavorable influences have to be considered at the same time when charging a battery to a higher voltage: It is known that the structure of layered cathode materials such as the most commonly used LiCoO 2 tends to become unstable when Li atoms are extracted to more than Li/M = 0.65. 6, 7 Higher charging voltage also means more oxidative atmosphere on the cathode side where the electrolyte and the separator tend to be more oxidized, 8,9 which may do harm to the performance of a battery.In this paper, the deterioration of the high temperature storage performance of a Li-ion battery at higher charging voltage is investigated. Effect of an insulating layer as the second layer between the cathode and the separator on the storage performance at 60 • C is studied and possible deterioration mechanism at higher charging voltage is discussed. ExperimentalPreparation of electrode.-The slurry for cathode was prepared by mixing LiCoO 2 , acetylene black (AB), and polyvinylidene fluoride (PVDF) in N-methyl pyrrolidinone (NMP) with a weight ratio of 95:2.5:2.5. An Al current corrector was coated with the slurry, dried in a flowing air at 90 • C and then pressed with a pressure-roll until pre-determined thickness.For the anode, graphite, carboxymethylcellulose sodium (CMC) and styrene-butadiene rubber (SBR) were mixed in an aqueous solution with a weight ratio of 98:1:1 to make anode slurry. A Cu current collector for anode was coated with the slurry, dried in a flowing air at 110 • C and pressed.Preparation of second layer between cathode and separator.-To make a slurry for the second layer, TiO 2 , AB and PVDF in a weight ratio of 95:0:5, 90:5:5, 0:95:5 or 0:0:100 were mixed in NMP. TiO 2 used in the experiments has rutile structure and its average particle diameter is 250 nm.Both sides of the pressed cathode were coated with the slurry of 1μm thickness for each side, or one side of a PE micro porous film of 16μm thickness was coated with the slurry to form a second layer of 1μm thickness after drying. In either case, the intermediate layer was dried in a flowing air at 90 • C. Electrochemical behav...

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