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Electrophoretic Deposition of Tin Sulfide Nanocubes as High‐Performance Lithium‐Ion Battery Anodes

Here we report the use of assemblies of SnS nanocubes as Lithium ion battery anodes. The particles are deposited in dense, conductive thin films with high gravimetric capacity using electrophoretic deposition, negating the requirement for binders or conductive additives. While SnS nanocube ensembles display both alloying and conversion modes, a significant benefit to capacity retention during long term cycling was observed by limiting the upper cutoff voltage to 1 V. In this alloying only regime that is more realistic for practical use, a discharge capacity of 552 mAhg was delivered with a loss of only 0.08 %/cycle observed over the 400 charge/discharge cycles. We further show that Li2S formation that occurs in the first lithiation acts as a buffer to the expansion and contraction though crucially this effect is optimized if this species is not cycled further (>1V). The SnS nanocube electrodes are tested in both half cell (HC) and full cell (FC) configurations and analyzed using ex-situ SEM and EIS analysis. Finally, the electrophoretic deposition of SnS nanocubes onto a 3D textured current collector is demonstrated to increase the mass loadings. Introduction The increasingly widespread usage of electric vehicles and the intermittency of renewable energy sources have driven a growing global requirement for more energy dense secondary batteries. This will necessitate the development of high-performance materials to replace the current commercially-used graphite anodes in Li-ion batteries (LIBs). Many Li alloying materials with higher theoretical gravimetric capacities than graphite (372 mAh/g), such as silicon (3,579 mAh/g), germanium (1,384 mAh/g) and tin (994 mAh/g), have been proposed; however problems with cost, scalability and/or long-term stability have thus far largely prevented commercialisation. Tin offers a significant enhancement of gravimetric capacity, combined with material safety and relative abundance. Furthermore, the slightly higher discharge voltage of tin (0.4 – 0.75 V) when compared with Si/Ge means that potential safety problems associated with electroplating of Li are avoided. [8] The major barrier preventing the widespread use of tin-based anodes is significant capacity loss experienced during extended cycling. This loss is largely attributed to the volume expansion and contraction during lithium insertion/de-insertion, causing pulverization/delamination and loss of electrical contact. Several approaches have been undertaken to overcome this deficiency, of which nanostructuring and the utilization of tin in sulfide form are discussed here. The use of nanostructured battery electrodes has been demonstrated to mitigate material pulverization through enhanced mechanical strain accommodation and the provision of void space to buffer expansion. Furthermore, the high surface area to volume ratio of nanostructures increases the number of electrochemically active sites while maintaining short Li diffusion lengths, enhancing rate capability and energy efficiency. Zero dimensional Sn nanocrystals have shown particular promise, combining the advantages of nanostructuring with relatively simple and scalable fabrication processes. [5] Typically, battery anodes consisting of films of nanoparticles are fabricated by the initial synthesis of nanoparticles followed by deposition onto a suitable current collector. A number of deposition methods are possible, including self-assembly and more notably EPD, in which nanoparticles dispersed in a solvent are driven to, and assembled at, the current collector by an applied electric field. Although not widely studied in nanostructured battery anode fabrication, this method is particularly advantageous in terms of enhanced film densification, conductivity and adhesion, while eliminating the need for binder and conductive additives which reduce active mass. Crucially, EPD also enables uniform deposition onto 3-D current collectors, which provide reduced Li diffusion lengths, enhanced conductivity and better expansion accommodation than their 2-D counterparts. The utilization of tin in sulfide form brings about a combined alloying and conversion lithiation process, involving the (usually poorly reversible, though not always) formation of Li2S during the first lithiation. This Li2S then acts as a chemically inactive amorphous “buffer” during subsequent cycling of the alloying stage. The presence of this buffer has been reported to reduce pulverization during expansion/contraction, [35,37–42] thereby increasing capacity retention during extended cycling. Indeed, the use of Sn in sulfide form has been demonstrated to significantly increase capacity retention during extended cycling. However, most reports perform extended cycling of tin sulfide over a wide voltage range e.g. 0 – 2.5/3 V vs Li/Li. 45] This typically achieves the highest possible initial capacity by including both processes, however given the irreversibility of the conversion process (Equation 1), the benefit of its inclusion is often short-lived or even limited to a single cycle. To our knowledge, no examinations into how cycling the Li2S material affects its role as a buffer have been performed. Tin can be synthesized in both monosulfide (SnS) and disulfide (SnS2) forms. [50,42] SnS maximizes the reversible portion of the capacity, and undergoes lithiation via the following two-step mechanism: SnS + 2 Li → Sn + Li2S (Eq 1) 353 mAh/g (Conversion, > 1 V vs. Li/Li) Sn + 4. 4 Li → Li4.4Sn (Eq 2) 778 mAh/g (Alloying, < 1 V vs. Li/Li) This results in a total Li storage capacity of 1,131 mAh/g, with 778 mAh/g generally considered reversible. An often overlooked concept is the varying energy densities that can be extracted from each process, given the wide voltage ranges over which they occur. Despite the significant contribution to gravimetric capacity that conversion processes can make, the characteristic high voltages (vs. Li/Li) reduce their contribution to energy density (when compared with low voltage alloying processes). Furthermore, typical Li-ion batteries output at a high and narrower 10.1002/celc.201900524 A cc ep te d M an us cr ip t ChemElectroChem This article is protected by copyright. All rights reserved.