Overcharge investigation of lithium-ion polymer batteries
Introduction
Rechargeable lithium-ion batteries (LIB) have been popularly accepted in a wide variety of applications including mobile phones, blue teeth, notebook computers, (hybrid) electric vehicles, etc. due to their high energy density, long cycle life and other unique properties. The lately developed lithium-ion polymer batteries (LIPB), because of their higher energy density and safety than the traditional lithium-ion batteries with liquid electrolyte, are expected to share more of the battery market. In spite of the great success in development and market, battery safety remains the main concern of the consumers and the fabricators. This concern becomes more severe when the battery works at high temperatures, high-rate charge and discharge, extreme overcharge and other abusive operations [1]. The more energy is stored, the more hazardous will be the energy storage system potentially.
Overcharge performance is an important feature of a battery. Some thermal analyses of LIB materials have been carried out in order to understand the overcharge mechanism [2], [3], [4]. Some authors used accelerated rate calorimetry (ARC) while others applied differential scanning calorimetry (DSC), trying to learn more about the reasons for the thermal runaway [5], [6], [7]. Tobishima and Yamaki [1], [8] reported the overcharge reaction while Leising et al. conducted systematic studies on the overcharge of LIB cells [9], [10]. Most of these studies are emphasizing the importance of the overcharge performances of the battery materials.
Systematic investigations are rare on the overcharge performances and solutions to the overcharge-induced failure for the newly born lithium-ion polymer batteries. This paper will investigate the thermal performances of Bellcore-type lithium-ion polymer batteries during overcharge and propose some solutions to the thermal runaway from the point of views of battery designing and operation.
Section snippets
Experimental
The stacked lithium-ion polymer batteries with a capacity of 650 mAh were products of Dongguan Amperex Electronics Technology Co., Ltd. (ATL). Cathode films were prepared by mixing LiCoO2 and carbon black in a solution of poly(vinyl difluoride)-hexafluoropropylene (PVDF-HFP) binder and dibutyl phthalate (DBP) plasticizer dissolved in acetone. The mixture was cast on a Mylar film to prepare the cathode film. Then the cathode film and Al grid were stacked together by hot lamination to prepare the
Results and discussion
The theoretical specific capacity of LiCoO2 is 274 mAh g−1. The coulombic efficiency in the first cycle of an LIPB is ca. 92%. This means that the capacity loss of LiCoO2 is roughly 11 mAh g−1 considering that the actually available (reversible) capacity of LiCoO2 is 137 mAh g−1 when the battery is charged to 4.2 V. Therefore, the actual value of x in LixCoO2 at full discharge (3.0 V) after the initial charge is ((137 × 0.92 + 137)/274) = 0.96. As the coulombic efficiencies for most commercial cells and
Conclusions
The overcharge performances of Bellcore lithium polymer batteries have been studied with various overcharge modes. By correlating the battery temperature and maximum voltage with the lithium concentration in the cathode, it is found that x = 0.16 is a critical point for the overcharge safety of the batteries. LixCoO2 will have an exothermic chemical reaction with the electrolyte after x = 0.16. If x can be kept above 0.16 or the reaction can be slowed down between x = 0.16 and 0, the battery
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