Article pubs.acs.org/JPCC
Degradation of NASICON-Type Materials in Contact with Lithium Metal: Formation of Mixed Conducting Interphases (MCI) on Solid Electrolytes Pascal Hartmann,†,§ Thomas Leichtweiss,† Martin R. Busche,† Meike Schneider,‡ Marisa Reich,‡ Joachim Sann,† Philipp Adelhelm,† and Jürgen Janek*,†,§ †
Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 58, 35392 Gießen, Germany SCHOTT AG, Corporate Research and Technology Development, Hattenbergstraße 10, 55014 Mainz, Germany § BELLA, Institut für Nanotechnologie, Karlsruher Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡
S Supporting Information *
ABSTRACT: We report on the transport properties of lithium ion conducting glass ceramics represented by the general composition 5+ 4+ Li1+x−yAl3+ x My M2−x−y(PO4)3 with NASICON-type structure and their stability in contact with lithium metal. In particular, solid electrolyte phases with M = Ge, M = Ti, Ge, and M = Ti, Ta were investigated. AC impedance spectroscopy and DC polarization measurements were applied to determine the conductivity as a function of temperature, and to extract the partial electronic conductivity. The maximum total conductivity at room temperature was found to be about 4 × 10−4 S/cm for the solely Ge containing sample. We demonstrate that the combination of vacuum-based lithium thin film deposition and X-ray photoelectron spectroscopy (XPS) is well suited to study the reactivity of the solid electrolyte membranes in contact with lithium. As a major result, we show that none of the materials investigated is stable in contact with lithium metal, and we discuss the reactive interaction between solid electrolytes and Li metal in terms of the formation of a mixed (ionic/electronic) conducting interphase (MCI) following the well-known SEI concept in liquid electrolytes.
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INTRODUCTION In current generations of rechargeable lithium ion batteries (LIBs), liquid or gel-polymer electrolytes with aprotic solvents are applied rather than solid electrolytes. The reasons are twofold: First, the slow transport of ions in most storage materials requires the use of highly interpenetrating electrode networks of the lithium ion storing phase, the electronconducting carbon additive, and the lithium ion conducting electrolyte with typical dimensions of the different phases on the nano- and microscale. In conventional electrodes, these networks are easily formed by filling the porous solid electrode composite with liquid electrolyte. Second, liquid electrolytes typically show a higher conductivity than solid electrolytes. Interestingly, using liquid aprotic electrolytes, the formation of the so-called solid electrolyte interphase (SEI) on the negative electrode is one of the key processes for the reliable functionality of LIBs. The SEI is thought to be a solid multiphase lithium ion-conducting film with a thickness in the nanometer range preventing continuous decomposition of the aprotic solvent during cycling and which in the case of graphite also protects against cointercalation of solvent molecules and exfoliation.1,2 In fact, today, suitable additives in the liquid electrolyte of LIBs support the formation of the SEI.3 Therefore it is no surprise that continuous effort is spent to develop © 2013 American Chemical Society
lithium conducting glasses, ceramics, polymers, or composites of those as electrolyte materials with high stability, high conductivity, and high transference number for Li ions, hoping that these might later be used to build a stable “artificial SEI” or an ion-selective membrane in batteries. In fact, solid electrolytes are already successfully employed in all-solid-state lithium batteries (SSLB) produced in thin film technology;4,5 first commercial products have recently become available.6,7 In these SSLBs, the solid electrolyte film is in direct contact with lithium metal and the positive electrode. In the future, solid electrolytes may also become necessary in lithium/sulfur batteries as a physical barrier to prevent the socalled polysulfide redox shuttle mechanism:8,9During cycling, lithium polysulfide (Li2Sx, 2 < x < 8) intermediates are formed at the positive electrode, which dissolve in the liquid electrolyte. Without physical barrier, the soluble polysulfides are able to diffuse toward the negative electrode (metallic lithium), where they chemically react, leading to an internal short-circuit.10,11 The same concept of such “hybrid cells”, i.e., cells including both solid and liquid electrolytes, can be employed in the Received: May 24, 2013 Revised: September 16, 2013 Published: October 3, 2013 21064
dx.doi.org/10.1021/jp4051275 | J. Phys. Chem. C 2013, 117, 21064−21074
The Journal of Physical Chemistry C
Article
isostructural to LiGe2(PO4)3 and LiTi2(PO4)3. SiO2 (Cristobalite) is observed as impurity phase (
