Insertion of an Insulating Layer between Cathode and Separator for Improving Storage Characteristics of Li-Ion Batteries

Abstract

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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