Aging mechanism in Li ion cells and calendar life predictions

doi:10.1016/S0378-7753(01)00722-4

Journal of Power Sources

Volumes 97–98, July 2001, Pages 13-21

https://doi.org/10.1016/S0378-7753(01)00722-4 Get rights and content

Abstract

In this work, the long term calendar life of lithium ion cells for satellite and standby applications has been studied in experiments where the capacity evolution is tracked as a function of storage temperature. Cells containing either LiCoO₂ and LiNi_xM_yO₂ positives coupled with a graphite negative were float charged at 3.8 or 3.9 V. This study focused on losses at the negative electrode and the data were fit to a model which involved a rate-determining step governed by electronic conductivity of the solid electrolyte interphase (SEI) layer, following Arrhenius’ law as a function of temperature. When nickel-based positives are used, a “lithium reserve” exists on the negative and this property enhances the calendar life for long life applications.

Introduction

Lithium ion batteries in a short time span have captured the commercial markets for powering high-end electronics applications such as portable phones, camcorders, computers, etc. This was a natural outcome due to demonstrated excellent energy density and cycle life for these applications where the typical expected product life is probably 3–4 years. Other industrial applications, like standby power sources or satellites require very long life, typically more than 10 years, in difficult environmental conditions. Outstanding cycle life at small DOD has now been demonstrated [1], [2]. However, the ability of Li ion cells to operate over very long time domains must still be demonstrated. Calendar life prediction for such cells must take into account possible aging reactions, with or without cycling, and almost no data to date has been reported. In order to evaluate the calendar life, different cell designs were tested in our laboratories over a period of time; the results are described and interpreted herein.

Section snippets

Experimental

High temperature was adopted as an expedient method to accelerate reaction rates within the cells so that cell-aging extrapolations could be made. Electrolyte reactions with electrode materials are the most probable parasitic reactions; therefore, a priori the most severe storage condition is the fully charged stated. This is particularly true for materials like lithiated metal oxides and lithiated amorphous or poorly crystallized carbons, which have an energy content that has strong voltage

MP prismatic

Cells have been tested following a cross matrix involving four temperatures (15, 30, 40 and 60°C), and three voltages 3.8, 3.9 and 4.0 V. Three cells were tested for each condition. The cells are checked every month, after a 5 h charge at a constant current of 1.5 A (C/3), followed by a constant voltage step at 4.1 V to obtain the nominal capacity. Diagnostic discharge was made at 30°C using 0.94 A (C/5) to a 2.7 V cut-off. Fig. 1 shows, for example, the capacity evolution during float under 3.9 V at

Discussion

The most common hypothesis for irreversible capacity loss during storage of a rechargeable cell involves side reactions between the electrolyte and the active materials. Oxidation on the positive, or reduction on the negative may occur, leading to capacity reduction. If these reactions occur simultaneously, the capacity loss may be reversible, as the equivalent part of the lithium lost on the negative electrode is re-inserted as Li⁺ ions in the positive material. Indeed, reversible capacity

Conclusions

From this study, it can be concluded that the lithium oxidation on the negative electrode is a major side reaction affecting the cell capacity on storage at high temperature. When graphite is used for the negative electrode, this phenomenon is not greatly influenced by cell voltage, since the negative electrode voltage varies little as a function of state of charge. As a cell ages, the production of insoluble species on the graphite modifies the SEI properties, leading to a reduction of the

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