Monoanion-regulated high-voltage nitrile-based solid electrolyte with compatible lithium inertness
Graphical abstract
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.
Section snippets
Results and discussion
The essence of incompatible CN-PSEs/Li interface is the high reactivity between -CN and Li anodes. The high dipole moment, originating from the large electronegativity difference between N and C atom, determines the reactivity of the -CN. The Li alkali metal, as a strong Lewis base, which gives rise to nucleophilic attack, will trigger an polymerization reaction on the -CN and yield compounds with conjugated C=N bonds. [43], [44] Here, density functional theory (DFT) simulations were assisted
Conclusion
In summary, we successfully demonstrated a promising monoanion-regulated design for high-voltage CN-PSEs. The core change from lithium-sensitivity to lithium-inertness can be achieved in monosalt high-voltage CN-PSEs which endows ASS Li-metal batteries with greatly enhanced lifetime. These appealing results suggest that the salt anion-induced chemical difference mainly is responsible for the stability of CN-PSEs. A well-designed ODFB−anion-derived interface between CN-PSEs and Li metal can
Electrolytes synthesis
Succinonitrile (SCN, 99%), acetonitrile (AN, anhydrous, 99.8%), Lithium bis((trifluoromethyl)sulfonyl) azanide (LiTFSI, 99.95%) and lithium oxalyldifluoro borate (LiODFB, 99.95%) were supplied without further purification from Sigma-Aldrich. ODFB-PCSE and TFSI-PCSE were prepared by dissolving the pre-weighed salt into melted SCN. And the detailed preparation process is as follows: First, SCN was melted and stirred at 60 °C to form a transparent liquid with good fluidity. Next, SCN was doped by
Author contributions
Q. Hou, H. Wang. and K. Y. Xie designed this research. Y. Q. Qi, Z. Y. Ren and K. Zhang carried out the characterization and data processing. C. Shen performed computational simulations. Q. Hou, F. Z. Zhao and K. Y. Xie edited the paper.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgments
The authors acknowledge the financial support provided by the National Natural Science Foundation of China (51974256 and 52034011), the Outstanding Young Scholars of Shaanxi (2019JC-12), the National Natural Science Foundation of Shaanxi (2019JLZ-01) and the Fundamental Research Funds for the Central Universities (20GH020137 and 3102019JC005).
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