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Non-Faradaic Li Migration and Chemical Coordination Across Solid-State Battery Interfaces Forrest S Gittleson, and Farid El Gabaly Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03498 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017
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Nano Letters
Non-Faradaic Li+ Migration and Chemical Coordination Across Solid-State Battery Interfaces Forrest S. Gittleson*, Farid El Gabaly* Sandia National Laboratories, 7011 East Ave., Livermore, CA 94550, USA
ABSTRACT Efficient and reversible charge transfer is essential to realize high performance solid-state batteries. Efforts to enhance charge transfer at critical electrode-electrolyte interfaces have proven successful, yet interfacial chemistry and its impact on cell function remains poorly understood. Using X-ray photoelectron spectroscopy combined with electrochemical techniques, we elucidate chemical coordination near the LiCoO2-LIPON interface, providing experimental validation of space-charge separation. Space-charge layers, defined by local enrichment and depletion of charges, have previously been theorized and modeled, but the unique chemistry of solid-state battery interfaces is now revealed. Here we highlight the non-Faradaic migration of Li+ ions from the electrode to the electrolyte which reduces reversible cathodic capacity by ~15%. Inserting a thin, ion-conducting LiNbO3 interlayer between the electrode and electrolyte, however, can reduce space-charge separation, mitigate the loss of Li+ from LiCoO2, and return cathodic capacity to its theoretical value. This work illustrates the importance of interfacial chemistry in understanding and improving solid-state batteries.
KEYWORDS: Li-ion battery, space-charge layer, thin-film, pulsed laser deposition, solid electrolyte
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MAIN TEXT Solid-state batteries (SSBs) promise improvements in reliability, safety and integration over similar devices that contain organic liquid electrolytes. SSBs, however, suffer from high charge transfer resistances which limit their power and energy density.1–3 Modification of critical electrode-solid electrolyte interfaces can enhance cell performance and material stability, but an incomplete understanding of phenomena governing charge transfer in these regions remains an obstacle to further improvement.
Two mechanisms are commonly cited to explain the resistance inherent in SSB interfaces. The first involves the formation of a “space-charge layer” in which a chemical potential difference between two contacting species results in the separation of charge-enriched and charge-depleted regions on opposite sides of an interface.4,5 While energy level diagrams of SSBs rationalize space-charge separation,6,7 there is minimal supporting data on space-charge layer chemistry. Thick interphase regions (200-300 nm) have been observed to result from SSB cycling,8,9 but their origin and relation to space-charge separation is unclear. Another potential contributor to charge transfer resistance involves a “mutual diffusion layer” in which both ions and electrons are mobile. Such interlayers, which result from the intermixing of species across functional interfaces, make the separation of ions and electrons during cell cycling inefficient. Instances of mutual diffusion regions have been observed at LiCoO2-Li7La3Zr2O12, LiCoO2-Li3PS4, and LiCoO2-(Li2S-P2S5) interfaces.10–13
The insertion of functional interlayers between electrodes and electrolytes is a popular strategy to improve charge transfer. For liquid electrolyte systems, these interlayers may reduce electrode and electrolyte degradation and improve kinetics.14–18 In solid-state electrolyte systems, 2 ACS Paragon Plus Environment
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Nano Letters
interlayers may improve the rate capability and stability of cells with highly reactive electrolytes.19–22 Unfortunately, there is little evidence that presents a mechanistic understanding of how these interlayers function. Modeling studies suggest that stable interlayers may minimize space-charge separation and prevent species intermixing,5,11,23 but these reports are unsupported by experimental results.
In this letter, we elucidate charge transfer phenomena by probing chemical coordination at the interface between two well studied materials: a LiCoO2 cathode and a LIPO(N) electrolyte. Using X-ray photoelectron spectroscopy (XPS), we show compelling evidence that the formation of space-charge layers is spontaneous and that charge segregation may be exacerbated by cell cycling. The loss of Li+ from the electrode and accumulation in the electrolyte is highlighted as a critical mechanism that reduces the reversible capacity. We also demonstrate that modification of the LiCoO2-LIPON interface by inserting a thin, ion-conducting LiNbO3 interlayer can reduce space-charge separation and mitigate the loss of Li+. A simple adjustment of the deposition temperature for LiNbO3 serves as a cautionary example that while interface engineering is promising, unintended impacts are also possible. Our findings show that identifying the distinct chemistry of electrode-electrolyte interfaces is vital to improving SSB performance.
RESULTS AND DISCUSSION While the local stoichiometry of SSB interfaces undoubtedly affects cell function, previous studies inadequately explain the influence of contacting films (e.g. an electrolyte) on electrode chemistry. For greater insight, we employed pulsed laser deposition (PLD) and XPS to observe changes in the chemical coordination of a LiCoO2 (LCO) cathode during the growth of a Li3PO4
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(LIPO) solid electrolyte overlayer. The PLD technique involves the ablation of a target material with a laser pulse and its subsequent stoichiometric deposition through a vapor plume onto a substrate. This method offers the ability to closely control the deposition rate to
