Elsevier

Energy Storage Materials

Volume 43, December 2021, Pages 305-316
Energy Storage Materials

Synergistic voltage and electrolyte mediation improves sodiation kinetics in µ-Sn alloy-anodes

https://doi.org/10.1016/j.ensm.2021.09.014Get rights and content

Highlights

  • Sn electrodes in pure PC electrolyte perform best in an optimal potential window.

  • Lower charge cut-off voltage promotes a 'mat' type thick passivation layer.

  • Higher charge cut-off voltage rapidly degrades the battery performance.

  • Addition of FEC electrolytes stabilizes both Na metal and Sn electrode.

Abstract

Alloying electrodes, such as tin (Sn), are promising candidates for sodium-ion batteries because of their high specific capacity, electronic conductivity, and low sodium insertion voltage. However, sizeable volumetric change and electrode-electrolyte interface evolution in Sn preclude prolonged performance. The electrochemical potential window, compounded by the choice of electrolyte and additive combination, plays a critical role in the interface instability, which yet remains unresolved. This study, based on a comprehensive set of electrochemical, microscopy, and spectroscopic analyses, sheds light into the interface instability and reveals that the use of fluoroethylene carbonate additives in carbonate-based electrolytes can dramatically improve the interface stability of such alloying anodes. Electrochemical and morphological analyses show that without the additive, a higher end-of-charge voltage can cause breakdown and reformation of an unstable passivating layer, leading to rapid electrochemical performance decay. A novel three-electrode-based analytics reveals that superior interphase stability with higher microstructural integrity of the Sn electrode can alleviate the detriments from the upper cut-off voltage restrictions. Addressing the hitherto unresolved role of the electrochemical potential window, this study comprehensively examines and elucidates the causality of interfacial instability and the underpinnings of electrochemical complexations in sodium-alloying anodes.

Introduction

While the lithium-ion batteries (LIBs) are the current de facto franchise for appeasing the fast-growing exigency for portable energy storage devices, de jure, other chemistries such as sodium-ion batteries (SIBs) are gaining a positive momentum towards establishing a promising alternative in large‐scale grid energy storage because of their ubiquitousness, and striking similarities with LIB chemistry. The relevant thermodynamic, kinetic, and transport properties fare advantageously or disadvantageously for the SIBs due to the wider ionic size, the less convenient redox potential of Na+, and different bonding characteristics. This entails constraint into Na intercalation and leads to a restricted selection of electrode materials. For instance, graphite, a popular anode in LIBs, is an outcast in SIBS due to the fundamental discrepancy between Na+ ionic radius of ≈1.02 nm and graphite interlayer distance of ≈0.334 nm. Past studies have focused on morphological tailoring or formulating 'non-graphitizing' carbons to either bypass or cleave the bottleneck. [1], [2], [3], [4] Nevertheless, most of these studies, along with many others, consummated that these carbonaceous materials only possess specific charge-discharge capacities around 200–300 mAh/g with limited rate capability and poor cycling stability, even with the complex fabrication process and expensive electrolyte additives [5], [6], [7], [8], [9], [10], [11], [12].

Due to the limited capacity of carbonaceous material, alloy-based electrodes (e.g., Si, Sn, etc.) are quite popular because of their higher theoretical specific capacity. Past studies have identified volume expansion with higher shear stress and cracks, disconnection/pulverization of active material as one of the root causes of electrochemical instability, which makes Sn undesirable for prolonged use [13], [14], [15], [16]. Usage of active bi/tri-metallic alloys Sn-Sb [17,18], Sn-Bi-Sb [19], Sn-P [20], [21], [22], and active- electrochemically inert alloys such as Sn-Co [23], Sn-Ni [24], Sn-Fe [25] have been one of the commonly used approaches to elevate cycling stability besides high specific capacity. Utilizing composite Sn electrode structures by introducing new physio-chemical properties to improve cycling capability increases the material/production cost, which can be eased by using elemental micro-Sn particles in a tailored electrochemical condition.

In addition to the electrode material, the design of a stable electrolyte composition is essential as the upgrowth of the electrode-electrolyte interface during the electrochemical reaction process dominates the electrode integrity and reversibility of battery cycling by preventing parasitic side reactions. Understanding the physio-chemical properties and rational design of the electrode-electrolyte interface is a long-pursued task for the battery community [26], [27], [28]. Given the vacuousness of pure Sn electrode response and microstructural changes of electrode-electrolyte interface layer during stable and metastable phase change events, cycling performances are often vaguely described. Although previous studies have shown the dependency of the phase sequence with terminal cut-off voltages, the possibility for cut-off voltage optimization for robust electrode-electrolyte interface remains unrecognized [29,30].

This paper studied the evolution of the electrode-electrolyte interface during different electrochemical conditions with different charge-discharge protocols. Carbonate-based electrolytes (e.g., EC, PC, DMC, EMC, DEC, etc.) in the presence of additives (FEC, VC) are standard for exerting efficient and reversible performance in LIBs [31], [32], [33], [34]. By leveraging LIB's knowledge, before seeking an ideal electrolyte for SIBs, it is imperative to understand how carbonates react with anode material for effective passivation [28]. Thus, carbonate-based electrolytes, PC and PC: FEC, were picked as the electrolytes for these sets of experiments. Deterioration mechanism of Sn-based sodium-ion battery and microstructural change during charge-discharge cycle tests in restricted terminal voltages was investigated thoroughly in this work. The impact of upper and lower cut-off limits on kinetic hindrance caused by slower ionic diffusion and charge transfer resistance has been discussed in detail. Comprehensively, this study addresses the following research gaps: a) the reversibility of oxidation and reduction peaks of NaxSn for different charge cut-off voltages, b) the morphological and structural change of Sn skeletons; and c) the reversibility of electrode reactions and the evolution of electrode surface in different electrolyte cocktails.

Section snippets

Electrochemical and morphological analysis of Sn anodes with PC electrolyte

Electrode potential is one of the most powerful tools in electrochemistry controlled by experimentalists to change the position of thermodynamics equilibria and reaction rates (kinetics). During the sodiation-desodiation process, electrode potential varies through the multi-step reactions in the Sn anode. To better grasp equilibrium phase transition behavior, the battery is discharged at a rate slow enough for Na to form an alloy, adopting a distinct crystal structure during intermediate steps

Conclusion

In this study, using Sn microparticles in sodium-ion batteries as an exemplar system, we reveal the electrochemical properties of Sn electrode, Na metal, and interfacial instability as a function of the operational potential window. This study has led to several critical points of understanding.

  • (1)

    It is found that in pure PC electrolyte (without electrolyte additives), operation at a lower end-of-charge voltage (0.7 V and 0.8 V) promotes a 'mat' type thick passivation layer, which is easily

Experimental setup

Electrode and Electrolyte Preparation: Tin electrodes with 70 wt.% Sn (10 μm, Sigma Aldrich), 11 wt.% Carboxymethyl Cellulose (CMC) binder (Sigma Aldrich), and 19 wt.% carbon black (Super C65-TIMCAL) are prepared in the conventional slurry method using DI water and ethanol as solvent. Electrolytes were prepared in a controlled moisture environment (H2O < 0.1 ppm). Sodium salts (NaClO4) were dried in a vacuum oven for 12 h at 100 °C. Then the dried sodium salts were dissolved in Polycarbonate

CRediT authorship contribution statement

Susmita Sarkar: Conceptualization, Data curation, Methodology, Formal analysis, Writing – original draft. Partha P. Mukherjee: Conceptualization, Funding acquisition, Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Financial support from the National Science Foundation (Award Number:1805656) is gratefully acknowledged.

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