Challenges Considering the Degradation of Cell Components in Commercial Lithium-Ion Cells: A Review and Evaluation of Present Systems

Owing to the high energy and power density of lithium-ion cells (1200 Wh kg-1 and 200 Wh kg-1) and due to their compact design, they are used as energy storage devices in many contemporary mobile applications such as telecommunication systems, notebooks and domestic appliances. Meanwhile their application is not limited only to consumer electronics, they are also standard in hybrid electric (HEVs) and electric vehicles (EVs). However, the profitable application of lithium-ion cells in the automobile industry requires lower costs, lower safety risks, a higher specific energy density and a longer lifetime under everyday conditions. All these aspects are directly or indirectly related to the degradation of the materials in a lithium-ion cell. One possibility for reducing the costs is a second life application of the cells after their usage in (H)EVs. In order to enable this, the safety risks at the end of life of a cell operated in a vehicle have to be reliably predicted. This requires a fundamental knowledge about underlying material degradations during operation. The safety risk of a lithium-ion cell increases during operation because the voltage windows in which the electrodes are cycled shift, resulting in a higher possibility that at least one electrode is operated in a meta- or unstable state. Furthermore, higher impedances due to material degradations lead to increasing heat generation and therefore to an increase in the risk of failure. Higher energy densities can be achieved by raising the end of charge voltage of a cell, causing additional safety risks because many cathode materials tend to decompose at high voltages. Another possibility for achieving higher energy densities is to use nickel-rich or lithium-excess cathode materials, since cathodes are currently limiting the capacity of lithium-ion cells. But these systems show a poor cycling stability (a higher degradation rate). The lifetime of a lithium-ion cell is limited by the degradation of the individual cell components. Although the degradation of materials is the key consideration in achieving lower costs, a higher safety standard, higher energy densities and a longer lifetime, the degradation of the individual cell components in dependence on the operation conditions has hardly been investigated and is poorly understood. The present work reviews known material degradations in commercial lithium-ion cells, shows a way to analyze such degradations in dependence on the operation conditions and describes how these degradation processes lead to observed performance drops.