Degradation Reactions in SONY-Type Li-Ion Batteries
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ymers with typical cell materials [1]. The stability of optimum cell structures were studied and the fundamental material interactions characterized.
PVDF is often used as a binder in cathode and anode electrodes. The monomeric structure of the PVDF is given below. (CH 2- CF 2 ) (1) x
It is known that these polymers can undergo highly exothermic decomposition reactions (a dehydrodefluorination type reaction). In these battery systems, lithium hexafluorophosphate (LiPF 6) is used as the salt. The question of the reactivity and stability characteristics of PVDF in the presence of this salt and its decomposition products are not well understood. In the presence of the solvents typically used in these batteries, the exothermic nature of the reactions can lead to thermal runaway. The state of charge of the cell also greatly affects the thermal stability of the cell. Cells in the fully charged state exhibit lower thermal runaway temperatures and greater reactivity [2]. In this work cell materials corresponding to those used in the commercial SONY US 18650 lithium-ion cell were investigated for thermal and chemical stability under charge/discharge conditions. The "as received" electrodes were initially received as sheets of current collector, 143
Mat. Res. Soc. Symp. Proc. Vol. 575 ©2000 Materials Research Society
aluminum cathode and copper for of theLixCoO2 anode, with of activematerial materialand on LixC6 one or both sides. for ThetheSONY-type cells consist as thecoatings active cathode (MCMB 2528 carbon) as the active anode material. Sheets of anode material of 701am thickness (9.4mg/cm2) were prepared on 251am copper foil while the two-sided cathode material of 1401am total thickness (18.9 mg/cm2 per side) was prepared on 20 lam aluminum foil. KS-6 graphite (5 wt%) was added to the cathode oxide to increase conductivity. The anode films were prepared with 10 wt% PVDF as the binder using N-methyl pyrrolidinone (NMP) as a solvent during the coating process while the cathode films were prepared with 5 wt% PVDF. The electrolyte (EM Industries, Inc.) consisted of ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC) (1:1:2 by weight) with 1.0 M LiPF6 as the salt. The electrode reactions are: charge Cathode Anode
Overall
Lil.oCoO2
•ge
6 C + yLi+ + ye
Lil.0CoO2 + 6C
•
LilyCoO2 + yLi÷ + ye
(2)
•
LiyC6
(3)
•
LiyC6 +
Lil.yCoO2
(4)
The range of stability of the LixCoO2 crystal structure limits the charged state of the cathode to about x=0.5, giving a nominal composition of Li0.5CoO2 for the fully charged cathode [3,4]. The potential of the fully charged cathode referenced to Li/Li+ is 4.1 V while that in the discharged state (Lil.0CoO2) is 3.0 V. The corresponding potential of the fully charged anode (Lil.0C6) is 0.0 V and 3.0 V in the discharge state (Li0.0C6). EXPERIMENT Experimental Plan The thermal stability of the electrode components was investigated using Differential Scanning Calorimetry (DSC). Calorimetric techniques have been shown to be very useful in the characterization of t
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