Abstract
In order to develop long-lifespan batteries, it is of utmost importance to identify the relevant aging mechanisms and their relation to operating conditions. The capacity loss in a lithium-ion battery originates from (i) a loss of active electrode material and (ii) a loss of active lithium. The focus of this work is the capacity loss caused by lithium loss, which is irreversibly bound to the solid electrolyte interface (SEI) on the graphite surface. During operation, the particle surface suffers from dilation, which causes the SEI to break and then be rebuilt, continuously. The surface dilation is expected to correspond with the well-known graphite staging mechanism. Therefore, a high-power 2.6 Ah graphite/LiNiCoAlO2 cell (Sony US18650VTC5) is cycled at different, well-defined state-of-charge (SOC) ranges, covering the different graphite stages. An open circuit voltage model is applied to quantify the loss mechanisms (i) and (ii). The results show that the lithium loss is the dominant cause of capacity fade under the applied conditions. They experimentally prove the important influence of the graphite stages on the lifetime of a battery. Cycling the cell at SOCs slightly above graphite Stage II results in a high active lithium loss and hence in a high capacity fade.
Highlights
The investigation of degradation mechanisms in lithium-ion batteries has proven essential for increasing the service life and extending operation
Capacity fade is caused by a loss of active electrode material: For example, if the cathode material becomes unstable at high potentials, it can no longer store lithium [1,2]
One would expect the loss of active lithium, active lithium loss (ALL), as well as the loss of active anode material, active material loss of the anode (AMLA), to be greatest for the cell cycled in the SOC range 45–65% which includes Stage II at an SOC of 55%
Summary
The investigation of degradation mechanisms in lithium-ion batteries has proven essential for increasing the service life and extending operation. Full understanding of these mechanisms enables actions to reduce and mitigate degradation. A longer service life of the batteries used leads to less frequent replacement in the application. This reduces resource consumption and contributes to greater sustainability. Capacity fade is caused by a loss of active electrode material (loss of storage medium): For example, if the cathode material becomes unstable at high potentials, it can no longer store lithium [1,2].
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