ii) The formation of lithium alloys
2.5 Sacrificial interlayers
As most materials are unstable in contact with lithium metal, the idea of using an unstable layer comes naturally and the number of possible materials is huge. In contrast to stable interlayers, these materials do not need to have a high ionic conductivity if the reaction products do. They can also be electronically conducting if the reaction products are electronically insulating and limit the decomposition reaction.
From a technical point of view, the application of a sacrificial interlayer does not require major considerations. As the material is supposed to react, it does not matter whether it is crystalline or amorphous and the deposition should not require high temperatures, enabling low–cost processing.
The decomposition products should act as a diffusion barrier for lithium (as neutral component Li0), thus, the ion–conducting reaction products must be electronically insulating. In the case of „LiPON“, this behavior occurs naturally. A similar behavior can also be observed for phases within the quasi–binary Li2S–P2S5 system , .
The critical aspect in terms of sacrificial interlayers is their thickness. If the interlayer is thicker than a critical thickness dcrit, parts of the interlayer remain between electrolyte and lithium anode unreacted. If the original interlayer does not have ionic conductivity, it will remain in the system as highly resistive interlayer, inhibiting the battery performance. If the interlayer is too thin, it may lead to incomplete local protection.
Finding metastable materials, which react in contact with lithium metal by forming only good lithium–ion conducting and electronically insulating compounds is the major challenge in the realization of sacrificial interlayers.
In order to exemplify the design of artificial interlayers, the properties of „LiPON“ will be discussed before deducing parameters that sacrificial interlayers need to possess to create a stable interphase.
2.6 „LiPON“ solid electrolyte
iPON” is the most widely used thin–film solid electrolyte. Since its first synthesis in the early 1990s –, it has gained an outstanding position in the field of thin–film batteries. The term „LiPON“ does not relate to the sum formula of the material but is an acronym for the elements that are included. „LiPON“ is amorphous, nitrogen–doped lithium phosphate. The structural backbone is lithium phosphate, which forms PO4–chains in which phosphorous is tetrahedrally coordinated by four oxygen atoms, and lithium ions are coordinated to the phosphate groups. „LiPON“ is usually formed by sputter deposition from a Li3PO4 target in reactive nitrogen atmosphere , , , –. During the deposition, oxygen of the phosphate group is partially substituted by nitrogen. The incorporation of nitrogen results in two different nitrogen species that are distinguishable via X–ray photoelectron spectroscopy (XPS). Nitrogen can form three single bonds to three adjacent phosphorous atoms (triply coordinated nitrogen, Nt) P−N<P
P or it can be present as doubly coordinated nitrogen (Nd) with one single bond and one double bond to two adjacent phosphorous atoms (P−N=P). The smaller the nitrogen content in
„LiPON“, the larger the fraction of P−N<P
P. If the nitrogen content increases, more P−N=P is formed. Although lots of research has been conducted to explain the ionic conductivity of
„LiPON“, the conduction mechanism remains yet unexplained. Due to its glassy nature, the prediction of conductivity mechanisms or reactions in contact with electrode materials are rather difficult. In literature, calculations are usually performed assuming different structures for different stoichiometry of the material , , . Howbeit, one general remark on the conductivity of the material can be made. The conductivity of „LiPON“ depends on the nitrogen content of the films and is typically in the range of 10–7 to 3∙10–6 S cm–1 , , –. The higher the nitrogen content of the films, the higher the ionic conductivity.
Although the ionic conductivity of „LiPON“ is up to three orders of magnitude lower than the conductivity of good solid electrolytes, its straightforward deposition at room
temperature has led it to become the most prominent thin–film solid electrolyte. In thin–
film separators the ionic conductivity is less important than in thick films ASSB cathode and anodes because the electrolyte thickness of thin separators is usually in the range of a few micrometers or less –. On this scale, even a low ionic conductivity contributes little to the overall cell resistance. „LiPON“ has also been used due to its excellent long–term stability in contact with lithium metal .
However, the long–term stability of „LiPON“ is questionable. Recent findings carried out by researchers from TU Darmstadt have shown that „LiPON“ is not stable against lithium metal . Like Wenzel et al. they carried out an in situ XPS experiment. They measured the signals of a „LiPON“ thin–film and subsequently deposited a small amount of lithium on top of the film. From the changes of the measured binding energies they concluded that a reaction between lithium and the electrolyte layer took place. They suggested a reaction for two different specific „LiPON“ phases, i.e. Li4P2O7 and Li6P3O9N. Li acted as a reducing agent and lead to a decomposition of these two phases. In the case of the nitrogen–free phases, the following reaction took place:
Li4P2O7 + 8 Li → Li3PO4 + Li3P + 3 Li2O (15) The nitrogen–free sample decomposed into lithium phosphate and the binary compounds lithium phosphide and lithium oxide. For the nitrogen–containing sample they predicted the following reaction:
2 Li6P3O9N + 12 Li → Li3PO4 + Li4P2O7 + Li3N + 7 Li2O (16) In this case, the Li4P2O7 could further react according to equation 15 and in addition to the binary compounds Li3PO4 was found again. The contact with lithium lead to a decrease of the amount of network forming phases and the formation of simpler and mostly binary phases. These results are supported by the theoretical computations of Sicolo et al. . By means of Density Functional Theory (DFT) they calculated the defect formation energies for
„LiPON“ in contact with lithium metal. They found out that at the interface of lithium and
„LiPON“, neutral interstitial lithium defects are formed which cause a disruption of the electrolyte network. The Fermi level at the interface between lithium and „LiPON“ is located at the upper region of the band gap of „LiPON“ and close to the conduction band minimum . Electrons can easily access the conduction band and therefore induce a reaction.
The driving force for the decomposition and the question of the nature of the interfacial reaction has also been discussed by Albe et al. . For the DFT studies they examined a
„LiPON“ with the stoichiometry of Li5P4O8N3. They examined two different reaction paths:
Li5P4O8N3 + 24 Li → Li3PO4 + 3 Li3P + 3 Li3N + 4 Li2O (17) and
Li5P4O8N3 + 32 Li → 4 Li3P + 3 Li3N + 8 Li2O. (18) They calculated that the complete reduction of „LiPON“ and the formation of the binary compounds alone had a formation energy of –24.16 eV and was therefore thermo–
dynamically favored over the partial reduction (equation 17; –18.45 eV).
The Albe team also reported that the lithiation energy gets more negative upon lithiating
„LiPON“, therefore the lithiation gets even more favored when lithium is inserted. Hence, they could show theoretically as well as experimentally that the thermodynamically favored decomposition into the binary compounds does not take place. Instead, Li3PO4 is formed and remains at the interface. They suggested that some sort of kinetic barrier prevented the decomposition of Li3PO4.
Although these recent findings have proven the interfacial instability of „LiPON“, the reaction does not lead to a decomposition of the entire electrolyte. Otherwise, there would not be batteries containing „LiPON“ that run for more than 1000 cycles. The reason for the long cycle life of these batteries is that the reaction products Li3PO4, Li2O, Li3N, and Li3P have in parts a high ionic conductivity (Li3P ~ 10–4 S/cm , Li3N ~ 10–4 – 10–3 S/cm, –), but a negligible electronic conductivity. The formed interphase acts indeed as a diffusion barrier. The reaction rate ceases and the formed interphase has a thickness of only a few nanometers. The cycling data proves that such an interphase is able to effectively protect the lithium|electrolyte interface and an application of such an interphase in ASSBs should help to increase their stability and efficiency. A further discussion of the electrochemical examinations on “LiPON” thin–films is provided in chapter 4.1.4 of this thesis.
For the aforementioned reasons „LiPON“ is examined as a model system in this work.
Although the work of Schwöbel, Albe and Sicolo has already shed light on the reaction
between „LiPON“ and lithium, their results are still lacking detailed information on the structure and thickness of the SEI. Does the reaction lead to a multilayer–system consisting of different layers of Li3PO4, Li3P, Li2O and Li3N or is the SEI a homogeneous mixture of all these components? Does the stoichiometry of the electrolyte influence the SEI formation?
How thick is a natural passivating interface and how thick can sacrificial interlayers be?
From detailed examinations of the Li|“LiPON“ interface these questions will be answered and guidelines for the creation of artificial protection layers will be derived.