Lithium plating in a commercial lithium-ion battery – A low-temperature aging study
Introduction
The lifetime of Li-ion batteries is crucial concerning their application as energy storage devices in mobile and stationary operation. There are various degradation processes in Li-ion batteries [1], [2], [3] which affect performance and durability. It is therefore very important to elucidate the underlying aging mechanisms in order to avoid or mitigate their occurrence. Graphite is the common active material for the negative electrode in Li-ion batteries and it mainly determines the overall aging behavior. The most important degradation mechanisms of the graphite electrode are continuous growth of the solid electrolyte interphase (SEI) and metallic lithium deposition [4].
Lithium plating is different from all other degradation processes in terms of its temperature dependence [5]. It is caused by low-temperature charging with high current and at a high state-of-charge (SOC). However, poor capacity balance can also lead to metallic lithium deposition at higher temperatures. Besides capacity loss and impedance rise lithium plating also presents a serious safety hazard. The metallic lithium can grow dendritically which may cause an internal short circuit of the cell [6], [7], [8]. The major aspects of lithium plating in Li-ion batteries are reviewed in Ref. [9], including deposition criteria and different modelling approaches. Intercalation of lithium-ions in the graphite particles and lithium plating on the particle surface are competing during low-temperature charging. High charge currents lead to charge transfer limitation at the particle/SEI interface. Lithium plating occurs when the graphite potential is reduced below 0 V vs. Li/Li+. However, plating might also be due to mass transport limitation. If the lithium-ion diffusion in the graphite particle is too slow, the particle surface becomes saturated with lithium-ions which consequently leads to lithium plating. These polarization effects are both aggravated with decreasing temperature.
There are a lot of aging studies on commercial Li-ion batteries in the literature which mainly address the performance degradation at ambient or high temperatures due to SEI growth and loss of graphite active material [10], [11], [12], [13]. However, the occurrence of lithium plating is only shown as a consequence of cycling in a narrow SOC range [14]. Others assume that lithium plating is the reason for fast performance degradation [15].
In this study lithium plating is forced by low-temperature charging in order to investigate the corresponding aging effects. It is important to note that all other degradation processes and side reactions, e.g. SEI growth, are assumed to be negligible at low temperatures according to the Arrhenius equation [5]. Therefore, lithium plating is the only significant aging process at these cycling conditions. However it must be considered that plated lithium metal induces further degradation due to additional surface film formation and electrochemical isolation of active material.
The investigated commercial Li-ion battery contains LiFePO4 (LFP) as active material of the positive electrode. It is known that LFP is highly durable due to negligible degradation processes [16]. Therefore, the observed aging effects are assumed to be entirely caused by lithium plating and its accompanied processes. LFP also exhibits a very flat voltage profile [17] which simplifies the interpretation of full cell voltage profiles. In detail, all voltage plateaus are due to the phase behavior of the graphite electrode. This is crucial for an unambiguous characterization of the aging behavior by nondestructive electrochemical methods [18], [19], [20].
Short-term effects of lithium plating are already known [21], [22] and allow for an indirect detection and quantification of this aging process [23]. The operating conditions for lithium plating in the investigated battery were also identified in our previous study [23]. These conditions are applied here in order to characterize the long-term effects of lithium plating on the battery performance. Furthermore, the reversibility of the plating losses is investigated by recovery cycling at higher temperatures. In addition to the electrochemical results, aged cells are opened in different aging states for quantitative information about the plating process. This includes determination of the lithium layer thickness and the mass of the plated lithium metal.
Section snippets
Commercial Li-ion battery and electrochemical test equipment
The investigated commercial Li-ion battery is a cylindrical 26650-type cell with 2.5 Ah rated capacity. Like mentioned before, the cell chemistry is based on a graphite (negative electrode) and LFP (positive electrode). This determines the voltage range of 2.0 V–3.6 V, i.e. discharge and charge cutoff voltage. The battery is allowed to be operated down to −30 °C, according to the data sheet.
Electrochemical measurements are performed on a BaSyTec CTS battery test system which is combined with a
Capacity loss
The capacity loss exhibits an unexpected behavior during long-term cycling. Fig. 1a shows the capacity retention curves for the investigated plating conditions. There is a change in curvature in the capacity retention which means that the capacity loss is reduced at progressed degradation states. This inflection point indicates a decrease of lithium plating as the only aging process at these operating conditions. Furthermore, it seems that the capacity retention approximates a constant value at
Conclusions
The presented study elucidates the degradation effects of lithium plating on the negative graphite electrode as the most severe aging process in Li-ion batteries during low-temperature cycling. The observed capacity retention behavior, i.e. decreasing capacity losses at higher cycle numbers, seems peculiar at first. However, nondestructive electrochemical investigations reveal that lithium plating leads to a significant loss of cyclable lithium. This leads to severe capacity fade and also
Acknowledgment
This work was financially supported by the Helmholtz-Institut Ulm (HIU) and the Zentrum für Sonnenenergie-und Wasserstoff-Forschung Baden-Württemberg (ZSW).
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