Lithium plating

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collector into the electrolyte and ions into the current collector. At low current densities even for an almost completely blocking interface only a few charge carriers of either species remain at the interface. The higher the current density, the higher the accumulation of charge carriers at the interface. From a certain “threshold” concentration, electrons and ions will combine by forming lithium metal. In the case of a negligible electronic conductivity in one phase, even negligible current densities are sufficient to cause lithium plating.

If the transference numbers for the lithium ions and the electrons are the same in both phases, no lithium plating will occur. As soon as ^tLi+,Phase 1

t_{e–,Phase 1}

≠

^tLi+,Phase 2

t_{e–,Phase 2}

,

lithium plating can occur as fractions of the charge carriers will remain at the interface. At this point only the difference in the transference numbers of each species determines when lithium plating starts. The higher the current density and the bigger the difference of the transference numbers, the sooner lithium plating occurs. In the ideal case, when lithium plating is a desired process, the ionic conductivity in the current collector should be insignificant, like the electronic conductivity in the electrolyte. If these requirements are fulfilled, even small current densities and charge carrier concentrations at the interface will result in lithium plating. Similar thoughts on transport properties of interfaces in solid electrolytes have already been made in literature a few decades ago [114], [115].

Especially in terms of the directed application in lithium–free batteries, lithium plating is an interesting phenomenon [96], [98], [104], [118], [119]. As depicted in Figure 7, the anode current collector in a lithium–free battery is directly positioned on the electrolyte without inserting an anode layer. The lithium anode is formed in situ during the first charging of the battery by removing the lithium ions from the cathode and depositing them between the current collector and the electrolyte. As all the lithium in this kind of battery is stored in the cathode, no excess lithium is present. Therefore, measures must be taken to prevent lithium loss during the first charging due to SEI formation or side reactions, as well as the further consumption of lithium during each cycle needs to be prevented. To ensure a reversible cycling behavior, the lithium deposition on the current collector must take place homogeneously.

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When artificial interlayers are applied to stabilize the anode|electrolyte contact, lithium plating can also be a cause of degradation in lithium batteries. As lithium plating solely depends on the transport properties of two adjacent phases, it can occur even between two electrolyte layers if they have different electronic and ionic partial conductivities and if the current densities are sufficiently high. This makes the point that lithium plating – although it is a kinetic phenomenon – can have an influence on the thermodynamics in lithium batteries. Figure 8 schematically shows the potentials in a lithium ion battery consisting of a lithium anode, a cathode and two electrolytes with different transport properties before and after lithium plating. Before lithium plating, there is a potential gradient in both the electrolytes and the potential decay in each phase depends on the conductivities in both phases (equation 14). If unwanted lithium plating occurs due to different transport properties, one can assume that a second lithium anode is formed at the interface between electrolyte 1 and electrolyte 2. This results in a change of the potential gradient in both electrolytes. Due to the formation of a second lithium anode phase, the potential decay across

Figure 7: Lithium plating between a copper foil (red) and a solid electrolyte.

Different transport numbers of Li⁺ and e^– in a solid electrolyte and an adjacent phase lead to the precipitation of lithium metal at the interphase when charge carriers cannot cross the phase boundary.

The higher the difference in the transport numbers, the lower the current that is needed to initiate lithium plating. Plating of lithium leads to the deformation of at least one of the phases at the interface.

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electrolyte 1 vanishes as the potential of lithium at both sides of the electrolyte is equivalent.

This means there is no driving force for the ionic motion in electrolyte 1 anymore.

Concurrently, the overall potential difference between anode and cathode then needs to be reduced across electrolyte 2. This results in a larger potential gradient and a larger driving force for the ionic motion between the cathode and the newly formed lithium anode. To Figure 8: Schematic depiction of the potentials in a lithium ion battery before and after lithium plating between two electrolytes with different transport properties.

Lithium plating at the interphase leads to the formation of a phase with a high lithium potential. Between the newly formed lithium and the cathode is a large potential gradient which increases the driving force for lithium plating. After the formation of lithium there is no potential gradient between the anode|electrolyte 1 interface and the electrolyte 1|lithium interface, which means there is no driving force for ion migration. Only if the plated lithium is fully removed, lithium from the anode is involved in the charge transfer process again.

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prevent unwanted lithium plating (and subsequent dendrite formation) in composite electrolytes, it must be ensured that electrons and ions at the interface cannot form lithium metal.

Taking these findings into account, it is questionable whether the long–term electrochemical (thermodynamic) stability, as demanded by Liu and co–workers can be improved by applying metal interlayers [91]. A thermodynamically unstable interface will decompose, as long as the decomposition is not kinetically hindered. As long as both electrons and Li ions can get in contact with the electrolyte, the interlayer will not serve as a diffusion barrier, and a decomposition has to take place. Germanium, like silicon, is able to form an alloy with lithium and it can be assumed that lithium will diffuse upon cycling into the Ge layer to form an alloy [92]. If lithium gets in contact with the electrolyte again, or if the potential vs. Li⁺/Li gets low enough, the decomposition reaction will continue. The rate of alloy formation depends on the diffusion coefficient of lithium in germanium (which is determined as the reduced diffusion coefficient of the lithium ions and electrons). Haro et al. used a thin Ge layer to increase the lithium uptake in Si nanotubes as battery anodes [119]. They suggest that this result is governed by the good electronic conductivity of germanium. Germanium transports electrons that are necessary to form lithium metal. The material can therefore not function as a protective layer to stop the reaction between lithium and the electrolyte. Metal interlayers can only be able to stop an interfacial reaction on the short–term scale but the reaction will still occur after longer/elongated cycling times and the protective effect will only be due to kinetic effects but not due to thermodynamic stabilization.

The purpose of the examined interlayers in this thesis is to prevent the electrolyte decomposition in contact with lithium metal. As discussed above, to fulfill this purpose, these interlayers need to have a good ionic but negligible electronic conductivity. That means the artificial interlayer will most likely have different transport properties than the unstable solid electrolyte. In this case it must be ensured that the interlayer is not too thick. Otherwise, lithium plating between the electrolyte and the interlayer may occur. Then the electrolyte can still react with newly formed lithium metal and the interlayer will not fulfill its purpose.

In general, materials are rare, that are thermodynamically stable in contact with lithium metal, and therefore, this work presents the concept of sacrificial interlayers.

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Im Dokument Sacrificial interlayers for all-solid-state batteries (Seite 54-59)