Monoanion-regulated high-voltage nitrile-based solid electrolyte with compatible lithium inertness

Abstract

High-voltage plastic solid electrolytes (PSEs) have emerged as appealing candidates for energy-dense Li-metal batteries, but their inherent instabilities toward reductive Li anodes pose a hurdle to practical application. Herein, we report the monoanion-regulated design of a lithium-inert, high-voltage PSEs, demonstrated with a nitrile-decorative PSEs (CN-PSEs) driven by BF₂C₂O₄⁻ monoanion. The monoanion-induced shielding layer helps to smooth lithium deposition, suppress the plastic matrix depletion, and thus retain robust Li/CN-PSEs interface even after cycling up to 1600 h in Li/Li cell, while the high-polarity nitrile group can support ion conduction and high-voltage tolerance up to 5.25 V. Benefit from the synergy, all-solid-state batteries with CN-PSEs show the superior cycling (300 cycles at 25 °C). This effective strategy provides a novel design principle for high-voltage PSEs to address the poor reductive stability.

Introduction

Rechargeable Li-metal batteries are ubiquitously recognized as the promising technology with high energy density. [1], [2], [3], [4] However, challenges like Li dendrites growth and severe interfacial side reactions can cause cell short circuit and eventually safety tragedies. [5], [6], [7] An emerging direction is in solid electrolytes (SEs), which endow batteries with improved safety, by replacing flammable and volatile liquid electrolytes. [8], [9], [10], [11], [12], [13] To date, a variety of SEs materials have been developed, [14], [15], [16], [17], [18], [19] of which plastic SEs (PSEs, combination of lithium salt and plastic matrix without any liquid solvents) are of extensive interest. Owing to its soft nature, PSEs can help preserve integrated and durable interface with electrode materials, eventually leading to extended cycling. [20], [21], [22] The most-investigated system of poly (ethylene oxide) (PEO) owns good film-forming capability and acceptable stability toward Li anodes, but the poor oxidative stability compromises the energy density. [23], [24] Cathode materials engineering and electrolytes hybrid are routine approaches to address the ill-famed compatibility of PEO PSE toward high-voltage cathodes, which have made PEO PSEs a huge leap for their application in high-energy-density batteries. [25], [26], [27], [28] For example, the in-situ cathode/PSEs interface has been constructed to push PEO PSEs to 4.2 V LiCoO₂/Li batteries. [29]

Instead of confronting the challenges within the restriction of oxidative instability, we shift the spotlight to other high-voltage-resistant PSEs. In this regard, the potential of nitrile-decorative PSEs (CN-PSEs) as the promising materials becomes evident. Indeed, the electron withdrawing nitrile group (-CN) with the strong bond energy (~854 KJ mol⁻¹) has tightly bound electrons in the highest occupied molecular orbital (HOMO), [30] which will render electrolytes enhanced oxidation-tolerant capability. [31] Previous reports on 5 V-class -CN functionalized liquid electrolytes have also established the significance. [32], [33] However, because of their insufficient energy gap between the lowest unoccupied molecular orbital (LUMO) and HOMO, sole high-voltage-tolerant PSEs cannot withstand reductive Li anodes. There is no exception for CN-PSEs with the crucially poor stability against Li metal. To tackle the problem, several approaches have emerged; examples of which mainly include: a) introducing another reduction-tolerant SEs to Li anodes; [25,34] b) employing interface-trimmed Li anodes. [35] The above designs have circumvented this problem to some extent. However, the key factors limiting CN-PSEs/Li interface compatibility have not been revealed. Thus, it is still a toughing task to achieve compatible Li-inertness in high-voltage CN-PSEs.

Lithium salt, the core component of PSEs, has been shown to modify interphase chemistry, thereby affecting certain functions of PSEs. [36], [37] Dual-salts or more complex blended-salts system win monosalt system in the common PEO PSEs, by virtue of balanced ionic conductivity, interface compatibility, etc. [38], [39], [40] However, diverse salt-matrix combinations in turn induce complex chemistries and ambiguous correlations among multiple factors, which complicate precise regulation of electrolytes. Despite the monosalt systems can simplify the problem, there have been few successful cases. For example, in spite of the relatively high ionic conductivity, the common LiN(SO₂CF₃)₂ (LiTFSI) monosalt based PEO PSEs show dendritic-lithium deposition and ultimately limited cycling lifetime. [41] Therefore, it is highly challenging to create a monosalt PSEs in feature of high ionic conductivity (>10⁻⁴ S cm⁻¹), wide electrochemical window (>4.5 V) and compatible interface. If such a PSEs design can be achieved, it would bring about monosalt PSEs based all-solid-state Li-metal batteries with high energy density and safety.

The magic LiBF₂C₂O₄ (LiODFB) salt can bring hope to monosalt PSEs by reshaping interface, smoothing Li deposition, and thereby effectively minimizing the side reactions. [42] But, the relatively low solubility of LiODFB limits its usage as sole mainsalt in PSEs. Herein, benefit from the high-polarity, plastic succinonitrile (SCN) matrix, the drawback of limited solubility can be made up. Synergistically, unexpected lithium inertness can be achieved in high-voltage CN-PSEs by ODFB⁻ monoanion-regulated design (Fig. 1a). The robust organic-inorganic ODFB⁻-derived shielding layer enables compatible CN-PSEs/Li interface. Eventually, not only does this monosalt CN-PSEs has high ionic conductivity (~10⁻³ S cm⁻¹ at 30°C) and wide electrochemical window up to 5.25 V, but it also demonstrates enhanced reduction-tolerant capability against Li anodes, and thus enabling long-term cycling to 1600 h at 1.0 mA cm⁻² in Li symmetric cell. Moreover, the assembled all-solid-state battery exhibits excellent cycle performance at room temperature.