Transport properties of mechanochemically synthesized copper (I) selenide for potential applications in energy conversion and storage

This work studied the thermal stability, electrical, and thermoelectrical properties of copper(I) selenide, Cu2Se synthesized by high-energy milling in a planetary ball mill. The phase composition was investigated by X-ray powder diffraction analysis and scanning electron microscopy. The conversion of the precursors during mechanochemical synthesis and the stability of the product was monitored by thermal analysis. The dependence of electrical properties on the product porosity was observed. For the densification of Cu2Se, the method of spark plasma sintering was applied to prepare suitable samples for thermoelectric characterization. High-temperature thermoelectric properties of synthetic Cu2Se were compared to its natural analogue-mineral berzelianite in terms of its potential application in energy conversion. Based on the results a relatively high figure-of-merit, ZT parameter (~ 1.15, T = 770 K) was obtained for undoped Cu2Se, prepared by rapid mechanochemical reaction (5 min). Cyclic voltammetry measurements of Na/NaClO4/Cu2Se cell implied that mechanochemically synthesized Cu2Se could be used as a promising intercalation electrode for sodium-ion batteries.


Introduction
Copper (I) selenide Cu 2 Se is an interesting p-type semiconductor for its numerous potential applications such as solar cells, thermoelectric converters, photodetectors, superionic materials, optical filters, photovoltaics, and ion batteries [1][2][3][4][5] due to its thermal stability, electrical and thermoelectric properties.Cu 2 Se exists even at room temperature in different crystallographic modifications, including orthorhombic, monoclinic, and cubic structures, depending on the preparation method.Lévy-Clément and co-workers have prepared cubic Cu 2 Se by chemical bath deposition on an inert Pt-substrate from a selenosulfite-containing bath at 75 °C.By electrochemical transformation, its orthorhombic phase at room temperature was achieved [6].A monoclinic structure was reported by Murray and Heyding [7], while Stevels and Jellinek documented an orthorhombic structure of Cu 2 Se [8].These copper (I) selenide investigations and references belong to the period of 70-90 years of the last century.Later in 2011-12, Gulay and co-authors reported the monoclinic phase at room temperature [9], and Liu and co-workers cubic anti-fluorite Cu 2 Se structure at 450 K [10].The Cu 2 Se preparation methods as a solvothermal [11], hydrothermal [12,13], microwave-assisted hydrothermal method [14,15], electrodeposition [16], chemical bath deposition [17], chemical synthesis reduction [18,19], thermal evaporation [20], magnetron sputtering [21], solid-state reaction with pulsed laser deposition [22], microwave heating [23], and classical melting, annealing or sintering [24][25][26] were used in the last 10 years.However, most of these methods of Cu 2 Se preparation needed expensive equipment or a complicated post-treatment process.For the first time in 2012, Yu and co-authors prepared tetragonal α-Cu 2 Se by subjecting the Cu and Se starting materials to high-energy milling in a Spex mixer mill for between about 10-30 h, and milling speed > 1000 rpm [27,28].Later in 2015, Gahtori et al. synthesized nanostructured Cu 2 Se during 50 h of milling with a speed of 400 rpm [29], and Butt et al. during 5 h of milling with a speed of 450 rpm in planetary ball mills [30].They used the milling technique to generate diverse nanoparticles from starting materials for subsequent consolidating of the product nanoparticles under pressure and temperature to examine thermoelectric performance.Recently, mechanochemical synthesis for the preparation of nanostructured orthorhombic modification of Cu 2 Se after 5 min of milling in a planetary ball mill was realized and appealed extensive attention due to the simple, one-pot, solvent-free and very fast synthesis [31].The electrical properties, thermoelectric performance, and thermal stability of such mechanochemically synthesized copper(I) selenide have not yet been investigated.
In this paper, just the abovementioned properties of mechanochemically synthesized Cu 2 Se were studied in order to obtain new and unique knowledge about the potential use of this advantageously very fast-prepared semiconductor material in the energy conversion and storage field.The high-temperature thermoelectric performance of natural Cu 2 Semineral berzelianite was also measured and compared to its synthetic analogue.The properties of synthetic Cu 2 Se were studied by several characterization techniques, evaluated, and compared with reported copper selenides prepared by more laborious and costly methods.

Materials and methods
Mechanochemically synthesized Cu 2 Se was prepared by milling in a laboratory planetary ball mill Pulverisette 6 (Fritsch, Germany) according to the conditions: loading of the mill-50 balls (10 mm in diameter), the material of the milling chamber and balls-tungsten carbide, the volume of the milling chamber-250 ml, the mass of Cu powder-3.08g, the mass of Se powder-1.92g, ball-to-powder ratio 73:1, milling atmosphere-Ar, rotation speed-550 rpm, and milling time-5 min [31].
The X-ray diffraction analysis (XRD) was carried out using a D8 Advance diffractometer (Bruker, Germany) with Bragg-Brentano geometry and Cu Kα as a radiation source.The Diffracplus Eva tool and the ICDD-PDF 2 database were utilized for phase identification.
A standard four-point probe method with a test head (Ossila Ltd., UK), and an electrical source were used to study the electrical properties of Cu 2 Se [32,33].The preparation of Cu 2 Se tablets weighing 0.37 g was carried out by pressing with a laboratory hydraulic tablet press (Specac, USA), at a pressure of 1 and 2 t, without retention time, and at room temperature.The round pressed tablets had a diameter of 7.06 ± 0.01 mm with a density reported in the literature of 6.9-7.0 g.cm 3 .To obtain reproducible results, the probe tips were fixed in a constant position (the distance between them was 1.27 mm) and loaded with a constant contact force [34].
The thermoelectric properties of natural Cu 2 Se and its synthetic analogue were studied on 10 mm round pellets with a thickness of 4 mm sintered by the spark plasma sintering (SPS) in graphite matrices at 500 °C, under the pressure of 50 MPa with holding time 10 min.After SPS, the pellets were polished and cut into geometrically suitable pieces for further measurements.The electrical resistivity, ρ and the Seebeck coefficient, S were measured by the four-terminal static direct-current method, using an LSR-3 m (Linseis, Germany) at temperatures from 300 to750 K.For the determination of thermal diffusivity, α in the temperature range from 100 to 700 K the laser flash apparatus LFA 427 (Netzsch, Germany) was used.The specific heat capacity, c p was measured by a comparative method using Inconel-718 alloy as a reference.Subsequently, thermal conductivity, κ was calculated according to the formula = .cp ., where ρ is the density of the sample.

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(2024) 19:73 | https://doi.org/10.1186/s11671-024-04025-5Research Cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCLP) experiments were carried out with an MPG-200 potentiostat/galvanostat (BioLogic Science Instruments SAS, France) at room temperature.In an Ar-filled glove box, the Swagelok cells were prepared from a mixture of Cu 2 Se: polyvinylidene fluoride: carbon in an 8:1:1 ratio cast on etched Al-foil as a working electrode and a Li-sheet as a counter and reference electrodes.As an electrolyte, 1 mol NaClO 4 was used in a non-aqueous solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1.The cells were cycled between 1.5 and 2.5 V vs. Na + /Na and for GCLP charge-discharge measurements between 1.5 and 2.4 V vs. Na + /Na at a current density of 10 mA.

Completeness of mechanochemical synthesis and thermal stability of the Cu 2 Se
Copper (I) selenide was prepared by mechanochemical synthesis with a milling time of 5 min, which was confirmed by the XRD pattern in Fig. 1.Unreacted elemental precursors Cu and Se were not present in the product Cu 2 Se with orthorhombic structure (ICDD PDF 47-1448), as shown by their reference patterns.The sample milled for 0.5 min contained, in addition to unreacted precursors, a Cu 3 Se 2 phase (ICDD PDF 65-1656) as an intermediate product of the mechanochemical synthesis of Cu 2 Se which gradually disappeared with increasing milling time.The kinetics of this mechanochemical reaction, together with the proposed mechanism and refined structure of the Cu 2 Se product, has recently been described in detail and published by [31].
The unmilled and milled 2Cu/Se mixtures for 0.5 and 5 min were subjected to thermal analysis to monitor the conversion of precursors during mechanochemical synthesis DTA curves of 2Cu/Se mixtures with the various times of milling are shown in Fig. 2.
Only one thermal effect was visible in the DTA curve of the unmilled mixture of the precursors.The endothermic peak at 215 °C corresponds to the melting of hexagonal selenium as described in our earlier research of mechanochemically synthesized bismuth selenides [35].The reduction of this DTA signal was more pronounced after 0.5 min of milling.The traces of the melting of unreacted selenium can be observed.A slight sign of an exotherm at ~ 127 °C is probably related to the transformation of a very small amount of low-temperature (LT) Cu 2 Se synthesized by milling alongside the predominant intermediate Cu 3 Se 2 phase (see Fig. 1).According to Gulay and co-authors, the LT modification of Cu 2 Se is transformed to the high-temperature (HT) modification above ~ 400 K [9].After 5 min of milling, no peaks and thus no thermal transformations were detected in the DTA curve, which means that HT modification of Cu 2 Se was prepared by mechanochemical synthesis with complete conversion of precursors to the final product and agreed with the XRD pattern in Fig. 1.The negligible indirectness of the DTA curve was due to the residual moisture in the product.In addition, it was clear from thermal measurements that mechanochemically synthesized Cu 2 Se was stable up to 300 °C.

The study of the electrical properties of synthesized Cu 2 Se
As the representative engineering parameter, the porosity of potential electrode materials is important.The porosity of Cu 2 Se was varied by changing the pressure of the tablet press from 1 to 2 t (see Table 1).When the pressure of 2 t was applied the porosity decreased by 9%.Therefore, the electrical properties of mechanochemically synthesized Cu 2 Se product depending on the porosity were also experimentally investigated.Namely, sheet resistance, resistivity, and conductivity were derived from the measurement data population consisting of 200 values for each Cu 2 Se product with different porosity.The resulting values are in Table 1.
The conductivity of Cu 2 Se increased 4.4-fold due to a decrease in the porosity of the material by applying a pressure of 2 t in preparation of the tablets.The value was in agreement with the electronic conductivity of the Cu 2 Se sample conventionally sintered at 973 K for 20 h [36].Analogously, the resistivity of Cu 2 Se decreased 4.1 times and the sheet resistance 3.7 times.The reason was the contacts between nanoparticles, which formed better conductive networks [37].
The SEM observations of the Cu 2 Se products obtained by applying the pressures of 1 and 2 t in Fig. 3 also clearly documented the decrease in porosity of Cu 2 Se/2 t due to a reduction in pore size and number.Due to the higher pressure, the grain boundaries are reduced, thereby the grain boundary scattering of carriers is also reduced and the conductivity is improved.

The study of the thermoelectric properties of synthesized and natural Cu 2 Se
The results of the Rietveld analysis showed that nanostructured synthetic Cu 2 Se prepared by high-energy ball milling crystallized in an HT orthorhombic phase [31].After SPS, which was applied for densification of the samples for measurement of thermoelectric properties, the phase composition and crystal structure of natural (mineral berzelianite) and synthetic Cu 2 Se were unchanged which was confirmed by XRD patterns in Fig. 4.
However, the superionic α->β phase transition was reflected in the transport and thermolectric properties measurements (Fig. 5a-c) for both the synthetic and the mineral samples.For the synthetic sample was the phase transition well visible from the fluctuations of the electrical resistivity ρ, the Seebeck coefficient S and the thermal conductivity κ at T ~ 400 K.Much less pronounced changes in these properties can be observed for the natural sample, consisting of a mixture of polymorphs and contaminated with calcite mineral (CaCO 3 ), at temperatures around 350 K.The positive   values of S hinted that holes represented the major charge carrier for both studied samples.This fact was confirmed also by a positive sign of the Hall coefficient, R H values observed for the synthetic sample.The concentration of the free carriers for this sample around 300 K started at 4.7 × 10 20 cm −3 and increased up to 8.2 × 10 20 cm −3 at 570 K with a steep downward fluctuation in the area below the phase transition, i.e. between 360 and 410 K (see Fig. 6).Such high free carrier concentration, the linear increase of S and ρ (except the fluctuations in the phase change region) classified both samples as degenerate semiconductors.The slopes of the temperature dependence of Hall mobility µ H = f(T) outside of the phase change region indicated in both the pure α-phase (T < 360 K) and the pure β-phase (T > 410 K) showed a dominance of acoustic scattering mechanism.Inside the region, one can observe dramatically decreased mobility with an extremely large temperature dependence, which was according to [38,39] fully consistent with the ideal characteristic of electron critical scattering.The values of the dimensionless thermoelectric figure-of-merit (ZT) of the synthetic sample surpassed more than three times these for the natural sample (see Fig. 5d).It was due to the almost twice higher values of thermal conductivity of the natural samples, which can be caused by the presence of a quite high concentration of calcite impurity phase in the mineral sample (see Fig. 4) and mainly by a lower Cu stoichiometry of the sample, which is given approximately as Cu 1.8 Se.Just the difference in the stoichiometries of both samples explained also the observed changes between ρ and S values of both samples.The maximal value of the ZT parameter, i.e.ZT = 1.15 at 770 K, was well comparable with the previously reported ones for Cu 2 Se composition [27,29].One of the highlights of this work was the preparation of high-quality Cu 2 Se material, synthesized in a very high yield (100%) within a very short reaction time (5 min).
Table 2 compares the ZT value of mechanochemically synthesized Cu 2 Se with published ZT values and preparation conditions of Cu 2 Se synthesized by various and combined techniques.It is obvious that the highest ZT values were achieved just by the Cu 2 Se products synthesized by milling-mechanical alloying, in comparison with techniques that are also multi-step and especially more temperature-demanding.Our mechanochemical synthesis process was the fastest among all the mentioned techniques, one-step, one-pot and performed at room temperature with the ZT value of > 1.In addition, the main advantage of such synthesis is easy scaling up using the industrial vibratory mills, e.g.ESM 324-1 ks (Siebtechnik, Germany), capable of producing up to tons of material per year.Further research and testing is still needed for verification.

The study of the electrochemical properties of synthesized Cu 2 Se as a cathode for sodium-ion battery
In Fig. 7a) are presented the cyclic voltammetry (CV) curves of mechanochemically synthesized Cu 2 Se electrode-cathode at the scan rate of 0.5 mV s −1 from 1.5 to 2.5 V.As can be seen, the electrode showed quasi-reversible sodium ion insertion/extraction ability.During these five reduction processes, two obvious cathodic peaks at about 1.6 and 1.8 V were observed.For the corresponding oxidation processes, two anodic peaks were observed at about 1.8 and 2.1 V.The cathodic peak at about 1.8 V and anodic peak at 2.1 eV were in agreement with the measurement for the Cu 2 Se electrode fabricated by a single-step postselenized method exposing the surface of the copper grid to selenide vapour in a vacuum chamber at 400 °C according to [44].A voltage polarization of around 0.3 V between the charge and discharge profiles is suitable for practical application.
The cyclic performance of the Cu 2 Se electrode together with the reversibility of Na intercalation was tested by GCLP measurements.In Fig. 7b) the first 5, 50th, and 100th charge-discharge voltage profiles of the Na/NaClO 4 /Cu 2 Se battery cycled between 1.5 and 2.4 V at a constant current of 10 mA are shown.From these measurements, an even lower polarization value of 0.1 V was confirmed, since the plateau recorded during discharge was around 1.95 V and the charging plateau was around 2.05 V.These plateaus are related to Na-ions intercalation into Cu 2 Se and its electrochemical reaction.The first results implied that mechanochemically synthesized Cu 2 Se could be used as a promising intercalation electrode for sodium-ion batteries (SIBs).Ex-situ XRD analysis of discharged Cu 2 Se cathode was performed to identify the products of electrochemical reaction with sodium reported by [44]. Figure 8a) showed patterns of mechanochemically prepared Cu 2 Se cathode (as-prepared) and after discharging to 1.5 V.By comparing these patterns, it was clear that there was a total conversion of Cu 2 Se and its diffraction peaks disappeared after discharging.Two aluminium peaks come from the Al-foil substrate.However, the peaks of Na 2 Se and Cu have not appeared as products of the electrochemical reaction between Cu 2 Se and Na + , probably due to the formation of nanoparticles, which could not be identified using the XRD technique.In addition, unprominent Cu 2 Se 3 (ICDD PDF 65-1656) peaks belonging to the electrochemical reaction by-product appeared, which is known as a low-temperature phase in the Cu-Se system stable at room temperature [45].This by-product arising during the discharge process can be transformed from the metastable β-Cu 2 Se through the Cu-ion migration with high mobility in the Se-sublattice [46].
Lower magnification SEM image with element mapping of the discharged Cu 2 Se cathode in Fig. 8b) demonstrated the presence of Na a Se which probably corresponded to the Na 2 Se product of the electrochemical reaction.The presence of copper can correspond to elemental Cu but also to Cu 2 Se 3 a by-product of the electrochemical reaction which was also indicated by the XRD pattern of the discharged cathode in Fig. 8a).

Conclusion
In this work, the properties of mechanochemically prepared Cu 2 Se, which have the potential to be applied in practice, were investigated for the first time.The thermal analysis confirmed the complete conversion of the precursors of the mechanochemical reaction Cu and Se to the Cu 2 Se product after only 5 min of milling and its thermal stability.The reduction of its porosity due to press pressure had a positive impact on the increase in conductivity and the decrease in sheet resistance and resistivity, which is an interesting technological parameter for the fabrication of electrodes from the semiconductor synthesized in this way.Competitive values of the ZT parameter 1.15 at 770 K were achieved by the rapid mechanochemical reaction for the synthetic Cu 2 Se sample.The first results of electrochemical measurements of the Na/NaClO 4 /Cu 2 Se cell indicated that the mechanochemically synthesized Cu 2 Se has a promising potential for use as an intercalation electrode in SIBs.

Fig. 1
Fig. 1 XRD patterns of 2Cu/ Se mixtures milled for 0.5 and 5 min; reference patterns of Cu and Se precursors, Cu 3 Se 2 and Cu 2 Se

Fig. 3 Fig. 4 Fig. 5
Fig. 3 SEM images of Cu 2 Se products after applying various pressure of the tablet press: a 1 t, b 2 t

Fig. 6
Fig. 6 Free carrier concentration p (red circles) and Hall mobility µ H (blue circles) as a function of temperature T for the synthetic Cu 2 Se sample.The red dashed line serves only as a guide to the eye.Blue dashed lines represent Hall mobility slopes for the pure α-phase (T < 360 K) and the pure β-phase (T > 410 K).The violet dashed line represents fit according to a critical power law with Tc = 410 K and the critical exponent r = 0.25

Fig. 7 a
Fig. 7 a CV curves of the Na/NaClO 4 /Cu 2 Se cell between 1.5 to 2.5 V vs. Na + /Na at the scan rate of 0.5 mVs −1 ; b Typical charge-discharge voltage profiles of the initial 100 cycles of the Na/NaClO 4 /Cu 2 Se battery at a constant current rate of 10 mA between 1.5 and 2.4 V

Table 1
Electrical properties of Cu 2 Se products with different porosity and thickness depending on the pressure of the tablet press

Table 2
Comparison of ZT values of Cu 2 Se synthesized by various methods and conditions * RT-room temperature