Aligned Copper-Zinc-Tin-Sulfide (CZTS) Nanorods ... - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport

Aligned Copper-Zinc-Tin-Sulfide (CZTS) Nanorods as Lithium-Ion Battery Anodes with High Specific Capacities Gerard Bree, Hugh Geaney, Killian Stokes, and Kevin M. Ryan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05386 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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The Journal of Physical Chemistry

Aligned Copper-Zinc-Tin-Sulfide (CZTS) Nanorods as Lithium-Ion Battery Anodes with High Specific Capacities Gerard Bree, Hugh Geaney, Killian Stokes, Kevin M. Ryan* Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland ABSTRACT: Highly aligned Copper Zinc Tin Sulfide (CZTS) nanorods (NRs) electrophoretically deposited directly on the current collector are tested for suitability as Li-ion battery (LIB) anodes in both half cell (HC) and full cell (FC) configurations. This facile fabrication process offers several advantages for high-performance nanostructured battery electrodes, notably the formation of a dense, conductive carbon and binder-free film maximizing active material content. High initial capacities of 1,611 mAh g-1 and 1,369 mAh g-1 are achieved for HC and FC, respectively. The capacity trends and degradation mechanisms for this combined alloying and conversion material are analyzed in detail using differential capacity plots (DCPs) and electrochemical impedance spectroscopy, and it is determined that an evolution in electrode resistance (instead of typical material pulverization/delamination) is the major driver of an initial capacity fade followed by a dramatic capacity recovery. Differences in capacity retention trends between HCs and FCs are highlighted, emphasizing the importance of extended testing in commercial-style setups for a complete material evaluation.

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INTRODUCTION The demand for rechargeable batteries with high gravimetric capacities is increasing dramatically year on year due to the expanded use of consumer electronics and electric vehicles. Conventional Li-ion technology is limited by the specific capacities of the materials used for the cathode (e.g. LCO, 135 mAh g-1)1 and anode (graphite, 372 mAh g-1)2. Anode materials that alloy or undergo conversion upon lithiation can demonstrate capacities an order of magnitude higher than graphite which stores Li by intercalation.2–5 Capacity decay over extended cycling has however remained a problem for high theoretical capacity materials.6–11 Alloying materials (such as Si, Sn and Ge) offer extremely high theoretical capacities (up to 3,579 mAh g-1 for Si), however they undergo large volume expansion (up to 400%12) upon lithiation, causing pulverization and delamination of material.2,13 Conversion-based materials typically undergo less expansion, but suffer from reduced electrode conductivity associated with the formation of lithium chalcogenides,14–17 as well as their partial solubility in typical electrolytes causing electrode mass loss.7, Metal sulfide materials (such as SnS/ZnS) are capable of combined alloying and conversion mode lithiation within single materials, while maintaining a low material cost, and offer a route towards overcoming the limitations of these individual processes.6,7 This synergistic combination has led to reduced voltage hysteresis, lower volume expansion and enhanced overall capacity.18–24 The associated multi-stage lithiation process typically involves the formation of Li2S followed by further alloying of Li with the metal. It has been proposed that the Li2S formed subsequently acts as a host matrix/buffer which aids the accommodation of volume expansion, offering enhanced electrode stability when compared to the alloying material alone.25 The incorporation of multiple elements (e.g. Cu2SnS3) can enhance this accommodation of volume expansion, in addition to providing greater conductivity.6,26 One material offering great potential is copper zinc tin sulfide (Cu2ZnSnS4 or CZTS); a low-cost, nontoxic material with applications in solar photovoltaics,27 catalysis28 and water splitting29. Its suitability 2 ACS Paragon Plus Environment

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for use as an electrode material in Li-ion batteries has been established,30–33 with measured HC capacities above 1,000 mAh g-1 for bulk film,34 nanostructured23 and thin-film35 morphologies. The lithiation reaction pathway has been described as follows18: Cu2ZnSnS4 + 8Li+ + 8e- -> 2Cu + Zn + Sn + 4Li2S

488 mAh g-1

Sn + 4.4Li+ + 4.4e- -> Li4.4Sn 268 mAh g-1 Zn + 1.5Li+ + 4.4e- -> Li1.5Zn 91 mAh g-1

(1)

(2) (3)

Reaction (1) is a conversion-type lithiation, whereas reactions (2) and (3) are alloying. The occurrence of an additional intercalation mechanism has also been proposed,23,30 and, though not widely studied, it suggests the possibility of achieving capacities greater than the above lithiation pathway indicates. The metastable wurtzite phase of CZTS (as opposed to the more common kesterite) is of particular interest as it allows a wide degree of nanocrystal shape control,36 in particular anisotropic growth, enabling the NR morphology.37,38 In addition, cationic sites in crystals of this phase are interchangeable,39 and therefore large variations in stoichiometry are achievable. This flexibility offers a potential route toward the development of electrodes with tuneable capacity and conductivity. Degradation brought about by volume expansion of anode materials can also be overcome by utilizing the material in a nanostructured form rather than bulk,7,30,40–45 thereby better facilitating mechanical strain and providing void space to accommodate expansion. Additionally, the use of nanomaterials enhances electron transfer rates and rate capability due to increased surface area and reduced diffusion lengths.7,46 Functional nanomaterials in colloidal form can be deposited by a number of low-cost facile methods suitable for battery fabrication. Electrophoretic deposition (EPD) is particularly effective as the current collector is immersed in a suspension of charged nanoparticles and they are driven to its surface by an applied electric field.47 This is a versatile method offering distinct advantages, notably enhanced layer conductivity48 and the ability to deposit onto 3-D surfaces.49 This method has been utilized suc-



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cessfully for the fabrication of a high performance copper sulfide nanoparticle based battery cathode (> 200 mAh g-1after 50 cycles).50 In this work, we demonstrate the viability of binder and conductive carbon-free wurtzite CZTS as a Liion battery anode material, utilising facile synthetic and deposition methods to achieve ordered assembly and close packing of NRs (with a tightly controlled size distribution) at the electrode surface. This multi-stage lithiation material combines fully reversible conversion and alloying modes for enhanced Li-storage capacity, and gravimetric capacities in excess of 1,900 mAh g-1 are observed despite the lack of additives. We examined the stability trends in detail, isolating and identifying fundamental degradation mechanisms. Variations between HC (cycled vs. Li) and FC (cycled vs. LCO) configurations are highlighted, confirming the necessity of FC testing in order to gain a complete view of a materials characteristics, particularly conversion mode materials. This study illustrates the promise of CZTS nanorod layers as a battery anode material, and of EPD as a method of fabricating dense, conductive films suitable for use in commercial devices. EXPERIMENTAL METHOD Chemicals. Copper (II) acetylacetonate (99.9 %), zinc acetate (99.99 %), tri-octylphosphine oxide (TOPO, 99 %), 1-octadecene (90 %), tert-dodecylmercaptan (t-ddt, 98.5 %), 1-dodecanethiol (1-ddt, 98 %), anhydrous hexane (95 %), ammonium sulfide (40 – 48 % in H2O) were purchased from Sigma Aldrich, tin (IV) acetate (98 %) from ACROS Organics, tetradecylphosphonic acid (TDPA, 99 %) from PCI Synthesis, LCO cathode tapes from NEI corporation and separator paper from Celgard. CZTS NR Synthesis. The NRs were synthesised according to a previously published method.37 Briefly, in a typical synthesis, 131 mg copper (II) acetylacetonate, 45.5 mg zinc acetate, 88.5 mg tin (IV) acetate and 676 mg TOPO were mixed in 5 mL 1-octadecene in a 3-neck flask. The flask was then evacuated for 30 mins using a Schlenk line, after which they were exposed to an argon atmosphere. The temperature was ramped up to 270 °C, during which a 1 mL mixture of 7:1 t-ddt:1-ddt was injected into the 4 ACS Paragon Plus Environment

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flask at 155 °C. The solution immediately turned from dark green to clear yellow after the injection. The reaction was allowed to proceed for 30 mins after injection, before being naturally cooled to 100 °C and dispensed into a vial. To achieve Sn-rich rods, the precursor masses were altered to 111 mg copper (II) acetylacetonate and 95.1 mg tin (IV) acetate, while the others were kept constant. Electrophoretic Deposition. The NRs were then washed and deposited according to a modified version of a method previously published.51 Briefly, 50 mg tetradecylphosphonic acid (TDPA) was added to a 1 ml extract of the as-synthesised product, and sonicated for 5 mins. 1 ml isopropanol was added and the mixture was vortexed for approx. 5 s. The NRs were then isolated by centrifugation at 4,000 rpm for 5 mins before being dried under a stream of argon and redispersed in 1 ml anhydrous hexane. The deposition bath was created by adding 0.2 ml of this to 5 ml anhydrous hexane. Two Cu foil plates held approx. 5 mm apart were immersed in the bath and a voltage of 300 V was applied using a high voltage power supply (TECHNIX SR-5-F-300). Deposition on the positive electrode was observed and film thickness could be controlled by varying the total immersion time. All battery electrodes utilised 0.1 0.2 mg/cm2 active material. Ligand Removal. The resultant films typically included significant organic content in the form of the TDPA ligands. Removal of this material reduces inactive material and enhances film conductivity.48,52 This treatment was performed by immersing the electrodes in a 20mM solution of ammonium sulfide in methanol for 30 s, followed by rinsing in methanol. Typically, an 8x8 mm electrode gained approx. 1525 µg during this treatment, due to formation of copper sulfide from the copper foil. Characterization. Scanning electron microscopy (SEM) was performed with a Hitachi SU-70 system equipped with an Oxford Instruments EDS detector, X-Ray diffraction (XRD) with a PANalytical X’Pert PRO MPD instrument with a Cu Kα radiation source (λ = 1.5418 A) with a 1-D X’celerator strip detector. Raman spectroscopy was carried out using a Horiba Labram 300 spectrometer system equipped with a 633nm laser. 5 ACS Paragon Plus Environment


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Electrochemical Measurements. The electrochemical performance of the electrodes was evaluated by assembly of a Swagelok-type two-electrode cell in an Ar-filled glovebox. The electrodes were examined against either elemental Li (in a HC configuration) or LCO (in an FC configuration), with an electrolyte soaked separator in between. LCO cathodes consisted of a 0.64 cm2 electrode on Al foil with a capacity of 555 µAh (implying a P:N ratio of 8.5, based on theoretical anode capacity). The electrolyte used for all tests was a 1 M solution of LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v, Sigma Aldrich) with 3 % vinylene carbonate (Sigma Aldrich) as an additive. The specific currents and capacities were determined based on the mass of NRs determined prior to ligand removal and thus represents a small (510%) overestimation of the actual mass of NRs. RESULTS AND DISCUSSION An overview of the battery fabrication process is shown in the schematic in Figure 1. Briefly, monodisperse CZTS NRs (approx. 25 x 8 nm, Figure 2a) were synthesised utilising a hot-injection method published previously.37 The NRs were subjected to a ligand exchange to TDPA, after which they were dispersed in a 5 ml hexane bath and deposited onto Cu foil by


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Figure 1. Process schematic for the hot-injection synthesis and electrophoretic deposition of CZTS NRs, followed by their incorporation into a LIB

Figure 2. (a) TEM image of NRs. (b) Cross Sectional SEM image of an aligned film of NRs deposited by EPD. (c) XRD and (d) Raman spectra obtained from a film of NRs. electrophoretic deposition to form the electrode. While in dispersion, the NRs assembled (through dipole-dipole interactions51) into discs up to 1 µm in diameter (consisting of > 10,000 individual rods). These superstructures were then driven to the positive electrode by the applied electric field. Films of approx. 300 - 400 nm in thickness were formed in which the NRs were aligned preferentially along the z axis (Figure 2b). EDS analysis indicated a stoichiometry of Cu1.92Zn0.87Sn0.98S4, while the obtained XRD and Raman spectra confirmed the presence of single phase wurtzite (Figure 2c & d). In order to improve conductivity and reduce inactive mass, the film was subjected to a chemical-based ligand removal step involving immersion in a solution of ammonium sulfide48,53 after deposition. 7 ACS Paragon Plus Environment


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The electrochemical characteristics of the electrodes were determined by cycling in a HC configuration vs. Li, utilising an electrolyte consisting of a 1 M solution of LiPF6 in a 1:1 EC/DEC with 3% vinylene carbonate as an additive. The cyclic voltammograms (CVs) of cycles 1, 2 & 5 (Figure 3) exhibited several anodic and cathodic peaks, corresponding to the various lithiation and delithiation processes occurring during the cycle. These processes could be separated into two types; namely conversion, involving the formation and deformation of sulfides, and alloying, involving the formation and deformation of LiSn and Li-Zn alloys. The CV (Cycle 1) shows cathodic peaks at 1.7 V, 1.15 V, 0.7 V and ~0.01 V vs. Li/Li+. Those in the 1 – 2 V range can be attributed to equation (1) (i.e. the formation of Li2S and Cu, Zn & Sn metals).30,34 The sub 1V cathodic interactions consist of the formation of Li-Sn/Zn alloys (Equations 2 & 3), as well as the formation of a solid-electrolyte interface (SEI) layer, known to form in this voltage range.5,54,55 The 4 anodic peaks visible in the 1st cycle delithiation (at 0.5 V, 1.4 V, 1.75 V and 2.3 V) suggest that the quaternary CZTS material does not itself reform, but is instead replaced by individual metal sulfides. Typically, Cu2-xS related processes occur > 1.5V,50,56,57 whereas those for ZnS and SnSx occur at voltages below this range.58,59 The 4 anodic peaks observed then likely correspond to the de-alloying of Zn and Sn (broad peak), the formation of ZnS and SnSx, and the formation of Cu2-xS, respectively. In subsequent cycles, a more complex behaviour was observed, in which the large CZTS cathodic peak at 1.15 V was not present, and was instead replaced by several peaks at 2.15 V, 1.4 V and 1.25 V (likely associated with the formation of elemental Cu, Sn and Zn, respectively, along with Li2S). This type of behaviour (a large change in CV characteristics from cycle 1 to cycle 2) has been observed for CZTS and similar materials previously,20,30,34 and confirms the major compositional changes that occur during the first lithiation/delithiation cycle. The identification of electrochemical processes in the CZTS system can be further confirmed by contrast with pure Cu2-xS, obtained by exposing a piece of bare Cu foil to the ammonium sulfide (ligand removal) treatment. A comparison of DCPs (Supporting Information, Figure S1) confirm that CZTS peaks in the sub 1.4 V region are indeed associated with Zn 8 ACS Paragon Plus Environment

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and Sn species. The presence of higher voltage peaks for both materials suggested that they are associated with the lithiation and delithiation of Cu2-xS.

Figure 3: CZTS cyclic voltammograms (sweep rate = 0.5 mV/s) for cycles 1, 2 & 5 of a CZTS-Li HC. Cycle 1 peaks are labelled with corresponding reactions



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Figure 4. Gravimetric and volumetric discharge (delithiation) capacities (a) and coulombic efficiencies (b) of CZTS-Li cells cycled at 3 specific currents. Volumetric capacities were calculated assuming hexagonally close packed NR arrays60 and a bulk density of 4.56 g cm-3 (see SI for more information).61,62 Inset in (b) is a schematic showing the structure of the HC. (c) Voltage profiles and (d) DCPs of select cycles of cell cycled at 200 mA g-1 In order to assess the suitability of this material for use as an anode in Li-ion batteries, CZTS electrodes were cycled in a HC from 0 to 2.5 V at specific currents corresponding to 100, 200 and 500 mA g-1. Figure 4a shows the discharge capacity of these cells as a function of cycle number. The initial capacities varied from 898 mAh g-1 to 1,611 mAh g-1. It is likely that this large variation, in addition to the greater-than-theoretical capacities, were caused by (a) initial SEI formation and electrolyte decomposition,14,63 and (b) the contribution from the copper sulfide film formed on the surface of the current collector by the ligand removal step. The contribution to measured capacity made by this film was previously discovered to be initially quite considerable, however this was not maintained under continued 10 ACS Paragon Plus Environment

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cycling50 (this was confirmed in this study by galvanostatic cycling of a copper sulfide film alone, see Figures S2 & S3). While the use of Cu as the current collector adds more complexity to the system due to the formation of this film, it is commercially preferred64 due to its high conductivity, stability within anode voltage ranges (up to 3 V vs. Li/Li+ 65) and may also have a beneficial effect in that it retards the dissolution of polysulfide materials.66 Capacities above theoretical predictions have been obtained previously for CZTS18,23 and similar materials.20 The long-term stability of the CZTS films when cycled was heavily dependent on specific current (note that no conductive carbon or binder was added to these electrodes). By cycling the electrodes for an extended time, three distinct phases of cell performance were observed, namely; (a) an initial rapid capacity fade, followed by (b) a recovery, and (c) a capacity stabilization/further fade period. The initial drop in capacity was less severe at lower currents (23 % for 100 mA g-1 compared with 54 % for 500 mA g-1). A maximum capacity of 1,947 mAh g-1 was recorded at cycle 150 for the electrode cycled at 100 mA g1

. The estimated volumetric capacity at this point (4,264 mAh cm-3) in particular compares very favour-

ably with that of graphite (330 – 430 mAh cm-3).5 Given that this value is higher than the initial capacity, the observed initial decay was likely not related to material delamination or dissolution. Similar trends in stability have been noted previously for CZTS3335 and other sulfide materials,63,67,68 with the increase/recovery of capacity often attributed to an “activation” of the material,33,35,69 in addition to being correlated with a decrease in electrode impedance.68 Some level of initial capacity decay in the first few cycles was expected during the formation of the SEI layer (the intensity of the sub 0.5 V anodic current reduces rapidly as seen in Figure 3) as this largely irreversible process is typically selfterminating.54 The development of a strong and stable SEI layer can increase the mechanical stability of an electrode and retard parasitic electrode-electrolyte interactions,54 however it also irreversibly consumes Li and therefore leads to a low initial coulombic efficiency (CE, approx. 66-72 % for this materi-



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al, Figure 4b). The CEs quickly increased however, typically reaching > 95 % after 15 cycles and remaining in that region. By examination of voltage profiles and DCPs (Figures 4c, 4d & S4), it was determined that a majority of the rapid initial capacity fade was related to a shift in the position of conversion anodic peaks to higher voltages outside the measurement range (> 2.5 V). In fact, the contribution made to overall capacity from the lower voltage (< 1 V) alloying processes jumps from 18% initially to 27% at cycle 12. The peak shift was observed most clearly in a cell cycled over the wider 0 – 3 V vs. Li/Li+ range (Figure S5). Peak shifts are typically attributed to metal/metal interactions18 and/or increases in overpotential brought about by resistance changes70 as the electrode material is cycled. It is reasonable to assume that the peaks observed at higher voltages are particularly sensitive to electrode resistance changes because Cu, Zn and Sn are largely in sulfide form in this region (as opposed to metallic at lower voltages). This implies a consequential increase in electrical resistance of several orders of magnitude.71 The electrical conductivity of an electrode can vary significantly with the level of lithiation, with conversion-type materials typically exhibiting higher conductivities when fully lithiated.72,73 Additionally, a re-ordering of the electrode, coupled with the gradual formation of a resistive SEI layer, can be expected to have dramatic implications for the charge transfer resistance at the electrode. Unlike the DCP peaks associated with the conversion processes, it is notable that the alloying peak at approx. 0.5 V did not undergo any shift in position during cycling (Figure 4d), in addition to exhibiting a remarkably stable peak intensity. This suggests a significant variation in the robustness and stability of the delithiation processes, which will be examined further in a follow-up study. These resistance changes were examined in more detail by performing electrochemical impedance spectroscopy (EIS) measurements on delithiated and lithiated CZTS electrodes at points during galvanostatic cycling of a HC at 200 mA g-1. Results are shown in Figure 5. The obtained EIS spectra consisted of 3 elements; the high frequency intercept being the series resistance (Rs) of the cell, the high/mid frequen12 ACS Paragon Plus Environment

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cy semi-circle representing the charge-transfer resistance (Rct) at the electrode/electrolyte interface (typically including a contribution from the SEI layer), and the line at low frequencies corresponding to the Warburg impedance relating to diffusion of Li+.18,34 Significant variations in impedance were observed when comparing delithiated (Figure 5a) and lithiated (Figure 5b) electrodes, with Rct far lower in the lithiated state (< 1 kΩ lithiated vs. > 2 kΩ delithiated). Additionally, the delithiated Rct increased dramatically in the first 10 cycles (Figure 5c), and fell thereafter, which matches well with the shifts in high voltage peak position observed in the DCPs in Figure 4d. This heavily suggests that the observed capacity trends were impedance related. An impedance-related capacity decay would also explain why the decay was more severe for the electrode cycled at the highest current (since the level of overpotential is a product of impedance and cycling current). The decrease in delithiated impedance observed after 10 cycles is likely related to morphological evolution. Additionally it suggested some degree of irreversibility in the desulfidation of SnSx45,74,75 and ZnS76 (meaning that a greater proportion remained in metallic form even when the electrode was delithiated). Rs did not vary with state of charge i.e. lithiated or delithiated (Figure 5d), this was expected as it is not related to internal cell processes; however a gradual increase with cycling was observed, likely related to corrosion of the copper current collector.65 Nevertheless the Rs values was insignificant compared with the overall resistance of the cell.



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Figure 5. Impedance spectra of a CZTS-Li HC measured at several points during galvanostatic cycling in (a) a fully delithiated state, and (b) a fully lithiated state. Inset in (b) is the equivalent circuit of the cell. The extracted values for the charge transfer resistance Rct and series resistance Rs are shown in (c) and (d), respectively. Rct was calculated by fitting a semi-circle over the high-to-medium frequency points, and Rs was taken from the point at which the curves intercept the x-axis in the high frequency region (extrapolating if necessary). Note that it has been reported that Li foil can make a significant contribution to measured impedance in HCs.77 Through performing EIS measurements on symmetric “LiLi” cells (see Figure S9), a Li foil impedance value of approximately 400 Ω was estimated.

In order to further demonstrate the viability of this material and fabrication technique for use in commercial batteries, a FC was constructed utilising CZTS as anode and LCO as cathode. This also enabled a study of the differences between HCs and FCs when evaluating anode materials. The FC was cycled at 14 ACS Paragon Plus Environment

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200 mA/g from 3.9 V to 1.4 V (equivalent to 0 V to 2.5 V in a HC). As can be seen in Figure 6a, an extremely high initial discharge capacity was observed (1,368 mAh g-1), which quickly decayed in subsequent cycles, similar to the trends observed in the initial cycles of the HC cycled at the same specific current. Interestingly, however, the expected “recovery” of performance was delayed and far less dramatic in the FC. A maximum capacity of 604 mAh g-1 was achieved after 474 cycles. It is important to note that while the capacity did not recover to its initial value in the same way as the HC, this still represents a significant capacity advantage over graphite. Figure 6b shows DCPs for this cell, with the cycles plotted the same as those in the equivalent HC in Figure 4d. Again the initial decay in performance largely occurs in the conversion portion of the voltage range (1.4 < V < 3). Two anode delithiation peaks at 1.6 - 1.7 V was observed in cycle 2; however they had disappeared by cycle 12, (similar to those in Figure 4d). In the case of the FC, however, no re-emergence of these peaks was observed, and so the capacity did not recover. This could be explained by the impact of LCO in the FC configuration. Here, the use of LCO, as opposed to an effectively mono-voltage Li foil in the HC, meant that the cathode voltage shifted upward, both (a) as it is delithiated during a cycle,5,78 and (b) as Li is consumed by irreversible reactions (such as SEI formation, irreversible Li2S formation, Li trapping79 etc., see SI for further analysis on irreversible Li consumption). This cathode voltage shift acted to reduce the potential over which the CZTS anode was cycled, leading to a greater reduction in stabilised capacity for the FC. This stark difference in behaviour between identical electrodes when cycled in HC and FC configurations demonstrates the importance of testing materials in both manners in order to assess their potential fully. This appears particularly true in the case of resistance-sensitive conversion materials.



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Figure 6. (a) Gravimetric capacity (anode basis), and coulombic efficiency of CZTS anode cycled at 200 mA g-1 in a FC configuration utilizing an LCO cathode. Inset in (a) is a schematic showing the FC structure. An initial FC energy density of 58.4 Wh/kg was observed (based on combined anode/cathode mass, note that the P:N ratio has not been optimized). The sinusoidal pattern observed in the latter cycles of the 3.9 – 1.4 V cell in Figure 6a is likely related to variations in temperature of the cell while undergoing testing (see Figure S10 for further analysis). (b) DCPs for select cycles of FC. CONCLUSIONS In this work, we fabricated a binderless and carbonless battery electrode consisting of a nanostructured CZTS (a combined alloying and conversion material). By utilising the facile and scalable EPD method, dense films consisting of aligned NRs with high gravimetric capacity (1,363 mAh g-1 after 100 cycles at 200 mA g-1 in a HC) were formed. The stability of the electrodes under cycling was heavily dependent on the applied current, and distinct phases of capacity decay and recovery could be discerned. DCPs reveal significant capacity deterioration capacity of conversion delithiation processes in the higher potential (> 1.5 V vs Li/Li+) range, largely due to increases in overpotential brought about by increasing electrode resistance (as confirmed by EIS measurement). In the case of a HC, this decay was quickly reversed, however the recovery was far slower and less dramatic in the case of a FC. Despite this, an FC 16 ACS Paragon Plus Environment

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capacity of 604 mAh g-1 was observed after 474 cycles. The large variation in capacity retention characteristics of HCs and FCs clearly demonstrates the need to perform extended cycling in both configurations in order to achieve a complete material evaluation. ASSOCIATED CONTENT Supporting Information. DCPs comparing (a) CZTS and Cu2-xS, (b) cells with and without ligand removal, long term cycling of Cu2-xS film alone, stability data for cell cycled from 0 – 3 V, ex-situ SEM images of long-term cycling, examination of effect of cell temperature on FC capacity. AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements The authors would like to acknowledge Science Foundation Ireland (SFI) Grant No. 11-PI-1148 for funding this research. REFERENCES (1)

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