Anomalously high elastic modulus of a poly(ethylene oxide)-based composite electrolyte

Abstract

The practical use of lithium metal anodes in solid-state batteries requires a polymer membrane with high lithium-ion conductivity, thermal/electrochemical stability, and mechanical strength. The primary challenge is to effectively decouple the ionic conductivity and mechanical strength of the polymer electrolytes. We report a remarkably facile single step synthetic strategy based on in-situ crosslinking of poly(ethylene oxide) (xPEO) in the presence of a woven glass fiber (GF). Such a simple method yields composite polymer electrolytes (CPE) of anomalously high elastic modulus up to 2.5 GPa over a broad temperature range (20 °C – 245 °C) that has never been previously documented. An unsupervised machine learning algorithm, K-mean clustering analysis, was implemented on the hyperspectral Raman mapping at the xPEO/GF interface. Using such a unique means, we show for the first time that the promoted mechanical strength originates from xPEO and GF interactions through dynamic hydrogen and ionic bonding. High ionic conductivity is achieved by the addition plasticizer (e.g. tetraglyme), where trifluoromethanesulfonate anions are tethered to the xPEO matrix and Li⁺ cations are favorably transported through coordination with the plasticizer. Further, stringent galvanostatic cycling tests indicates the CPE can be stably cycled for >3000 h in a Li-metal symmetric cell at a moderate temperature (nearly 1500 Coulombs/cm² Li equivalents), outperforming most of the PEO-based electrolytes. The GF reinforced CPE reported here has multifunctional uses, such as solid electrolytes for all solid-state batteries and membranes for redox-flow batteries. Although the focus of this study is on lithium-based batteries, the results are equally promising for other alkali metal based batteries such as sodium and potassium.

Introduction

The sustainability of modern society rests on the development of renewable energy sources, such as solar, wind, and tidal power combined with cost effective energy storage, to store power during excess generation and supply during peak demand. [1,2] In this regard, the development of low-cost, scalable energy storage systems with adequate cycle-life and safety is critical. [3] Further, attaining high energy density without jeopardizing safety is also important for a number of applications such as electric vehicles and consumer electronics. Lithium and sodium based solid-state batteries promise higher energy density and provides inherently safe operation as they replace flammable liquid electrolytes with solid counterparts. [4], [5], [6] The lithium metal anode has almost 10 times higher theoretical capacity than its graphite counterpart, [7] and clearly can be one of the most promising disruptive technologies to advance electric-vehicles and large-scale grid storage. [8,9] However, there are several technical and scientific challenges that need to be addressed. Stable cycling of lithium metal requires a chemically and inherently stable solid-state separator with high ionic conductivity and mechanical strength. [10], [11], [12]

Poly (ethylene oxide) (PEO) is the most extensively studied polymer electrolyte material and is relatively stable against Li and Na metal. [13], [14], [15] But membranes composed of high molecular weight, linear PEO exhibit critically low ambient temperature ionic conductivity (~10⁻⁷ S/cm). [15,16] Previous work, including our own, showed that conductivity can be improved by using plasticizers and/or large anion lithium salts. [15,[17], [18], [19] However, these approaches inevitably lead to decreased mechanical rigidity (storage moduli, E’ < 10 MPa), [20], [21], [22] resulting in plastic flow and lithium dendrite growth during galvanostatic cycling. Furthermore, the low melting temperature (T_m) of PEO (~65 °C) inherently limits the temperature window over which these membranes can be used. Several strategies have been developed to improve the mechanical properties of PEO-based membranes. These include: 1) covalently binding a mechanically rigid phase (e.g. polystyrene) to the ion-conducting phase, [23,24] 2) embedding inorganic fillers into a polymer matrix, [25], [26], [27] 3) covalently bonding surface-modified inorganic particles to the polymer membrane, [28,29] 4) incorporating the polymer into an inorganic matrix, [30] and 5) crosslinking the PEO to increase its dimensional stability. [18] Theoretically, lithium dendrite growth would be suppressed if a homogeneous solid electrolyte could be used. [31] Despite a great deal of effort, the demonstrated mechanical rigidity in terms of the elastic modulus of all PEO-based electrolytes is so far still several orders of magnitude lower than Li metal (1.9 to 7.9 GPa). [3,[32], [33], [34]

Herein, we report a single-step synthesis route to fabricate a PEO based polymer membrane of exceptionally high storage modulus (up to 2.5 GPa) over a very broad temperature range (20 °C to 245 °C). The synthesis is enabled by in-situ crosslinking of PEO in the presence of a woven glass fiber (GF). Historically, GF woven and other fiber materials have been intensively explored as reinforcement methods for composite polymer electrolytes (CPEs). [35], [36], [37], [38], [39], [40], [41] However, as far as we are aware, few studies have reported a storage modulus of composite membranes at this anomalously high level. Although there seems to be a consensus in literature that the strengthened mechanical properties in the fiber reinforced composite membranes are ascribed to the stress transfer between fibers and polymer matrix, a proper understanding of its underlying science remains quite challenging to date. To address this challenge, we further implemented hyperspectral Raman mapping on the GF/polymer interfacial region. Unveiled by an unsupervised machine learning algorithm, K-mean clustering analysis of the Raman mapping, the exceptionally high mechanical strength is attributed to strong GF/polymer interactions, including hydrogen bonding and dynamic ionic bonding. Due to its superior mechanical strength, the CPE can tolerate high plasticizer loading, up to 33 wt%, with a resultant room temperature ionic conductivity up to 1.2 × 10⁻⁴ S/cm. The storage modulus of the plasticized CPE remains above 450 MPa, comparable to the performance of the state-of-the-art composite electrolytes of a sophisticated design. [42] Based on stringent stripe/plate cycling tests in a Li|membrane|Li symmetric cell, such GF reinforced polymer electrolytes allow for 1498 C/cm² equivalent Li striping/plating over the course of >4 months at 70 °C, with no observable Li dendrite growth. A proof-of-concept full cell test using a Li|CPE|LiFePO₄ (LFP) configuration shows that the cell can deliver >135 mAh/g capacity for 100 cycles at C/15, with the capacity loss <0.06% per cycle. The cell maintained a Columbic efficiency close to 1 over the course of >3 months at 75 °C, demonstrating its excellent thermal and electrochemical stability in harsh conditions. The combination of ultra-high mechanical strength, dimensional stability, high ionic conductivity, electrochemical stability, and supreme capability of Li dendrite resistance provides a new and scalable route of synthesizing composite membranes for a number of electrochemical energy storage systems where high energy density is essential.