Co-Sn nanocrystalline solid solutions as anode materials in lithium-ion batteries with high pseudocapacitive contribution

: Co-Sn solid-solution nanoparticles with the Sn crystal structure and tuned metal ratios were synthesized by a facile one-pot solution-based procedure involving the initial reduction of a Sn precursor and the posterior incorporation of Co within the Sn lattice. Such nanoparticles were used as anode materials for Li-ion batteries. Among the different compositions tested, Co 0.7 Sn and Co 0.9 Sn electrodes provided the highest capacities, with values above 1500 mAh g -1 at a current density of 0.2 A g -1 after 220 cycles and up to 800 mAh g -1 at 1.0 A g -1 after 400 cycles. Up to 81 % pseudocapacitance contribution at a sweep rate of 1.0 mV s -1 were measured for these electrodes, providing fast kinetics and long durability. The excellent performance of Co-Sn alloy nanoparticle-based electrodes was associated to both the small size of the crystal domains and their suitable composition, which buffered volume changes of Sn and contributed to a suitable electrode restructuration.


Introduction
Lithium-ion batteries (LIBs) have become the dominant energy storage technology in portable applications.However, their energy density, charging rate, and stability are still critical performance parameters that has plenty of room for improvement by optimizing both anode and cathode materials. [1- 4][8] While Si is the most abundant element and has the highest potential storage capacity, Sn and Sn-based compounds are particularly appealing owing to their relatively high abundance, low cost and high electrical conductivity. [7,9]n terms of stability, the huge lattice expansion and contraction of the anode material during cycling strongly reduces the battery performance due to a loss of electrical connection by electrode pulverization.In the case of Sn, the Sn/Li22Sn5 reaction has associated a 300% volume change, which inevitably leads to major structural rearrangements generally resulting in a loss of capacity. [10,11]Different strategies have been proposed to tackle down this major issue.One main approach is to alloy the active phase, Sn, with a second elements that buffers the volume changes. [12,13]In this direction, Sn-based alloys with Ni, [14][15][16][17] Co, [18][19][20][21][22][23][24][25][26][27][28][29][30] Fe, [31,32] Cu, [33,34] and Sb [35][36][37] have demonstrated superior cycling performance than bare Sn anodes.Among the different Sn-based alloys tested, CoSn electrodes have shown particularly promising performances as anode materials for LIBs. [31][40][41][42] Most previous works have focused on the intermetallic Co-Sn alloys: Co3Sn2, CoSn, CoSn2.Among these intermetallics, while some controversy remains, CoSn2 has been considered as the optimum stoichiometry, due to its highest content of Sn.However, beyond intermetallic phases, a range of Co-Sn solid solutions exist that are yet to be explored.
Besides alloying the active material to improve cycling performance, the use of nanostructured electrodes can reduce the overall stress generated on the material domains during lithiation, thus diminishing mechanical disintegration and improving stability.[45][46] This pseudocapacitive contribution is particularly appealing because it can significantly improve both the rate performance of LIBs and their stability.
In the present work, we take advantage of the versatility of colloidal synthesis methods to produce Co-Sn solid solution nanoparticles (NPs) with tuned Co:Sn ratios, from 0.3 to 1.3.After removing surface ligands, we use these NPs to test the performance of Co-Sn solid solutions as anode materials for LIBs, defining an optimal composition and demonstrating this system to be characterized by a high energy storage capacity, with a high pseudocapacitive contribution and a notable stability.histograms of the quasi-spherical NPs produced.The average NP size was estimated to be 6 -7 nm for all compositions except for the Sn-richest NPs, which had an average size of 10 nm.XRD analysis showed that, independently of the Co:Sn ratio, NPs conserved the Sn crystal structure.However, XRD peaks appeared shifted to lower angles, as it corresponds to the introduction of a slightly larger atom, Co, within the Sn lattice.The formation of CoSn solid solutions was somehow surprising when taking into account that previous works reported the formation of intermetallics, i.e.CoSn2, CoSn and Co3Sn2, when co-reducing proper amounts of the two elements.We associate the differences in the products obtained between our synthesis protocol and previous works to the relatively low synthesis temperatures we used which prevented the crystallization of independent Co NPs and Co-Sn intermetallic phases.EDX analysis showed the Co:Sn ratio in Co-Sn solidsolution NPs to be: 1.3, 0.9, 0.7 and 0.3 when produced from nominal Co:Sn precursor ratios of 2.0, 1.5, 1.0 and 0.5, respectively.The final Co-poor NP stoichiometry (with respect to the nominal) and the pink color of the supernatant obtained after NP precipitation revealed that some of the cobalt precursor remained unreacted after 1h at 180 °C.We also observed that the same reaction conditions but in the absence of Sn precursor did not result in the formation of Co NPs.On the contrary, the same reaction in the absence of Co resulted in the formation of Sn NPs.We believe that in the reaction conditions used, the Sn precursor was first reduced to nucleate Sn NPs.Taking advantage of the lower energy for heterogeneous growth over homogeneous nucleation, during the 1h period at 180 °C Co ions within the solution incorporated to the initial Sn nuclei upon reduction with TBAB.Through this synthesis mechanism, the Sn crystal structure was conserved, which is in contrast to the results obtained in previous works that make use of higher reaction, alloying or annealing temperatures to produce Co-Sn intermetallic alloys.
XPS analysis (Figure S2) showed the Co:Sn ratio of the Co0.9SnNPs to be 0.7, which pointed at a slightly Sn-rich surface.We hypothesize that this Sn-rich surface was related to a slight oxidation of the NPs exposed to ambient conditions.We believe air exposure resulted in a slight restructuration of the alloy due to the higher Sn affinity to oxygen that drove the diffusion of Sn to the surface. [47]igure 2a shows STEM micrographs and EELS chemical composition maps of the Co0.9SnNPs.All Co0.9Sn NPs contained the two elements in similar ratios.Within each NP, Co and Sn distributions were mostly homogeneous, but most NPs presented a Sn-rich shell, consistent with XPS analysis.HRTEM micrographs (Figure 2b) clearly displayed a core-shell structure of the NPs.From HRTEM analysis, the core crystal structure could be assigned to the Co2.9Sn2 orthorhombic phase (space group = Pnma) with a = 7.1450 Å, b = 5.2500 Å and c = 8.1730 Å, or to the Co3Sn2 hexagonal phase (space group = P63/mmc) with a = b = 4.1130 Å and c = 5.1850 Å (SI). [48]This result is in contradiction with XRD patterns and EDX and XPS analysis.We hypothesize that solid-solution NPs with the Co0.9Sn composition and Sn structure were initially formed.51] Within the electron beam during HRTEM analysis, the core, having a higher Co content due to the diffusion of Sn to the surface, crystallized to an intermetallic Co3Sn2 phase with potential additional Sn segregation to the surface.Co-Sn solid solution NPs were explored as anode material in LIBs.Before testing their performance, the organic ligands used to control the growth of the NPs in solution were removed by treating them with a mixture of hydrazine and acetonitrile. [36,37]TIR analysis confirmed the effectiveness of the ligand removal step (Figure S4).LIB anodes were prepared by casting a nonaqueous slurry containing 80 wt% of active material, polyvinylidene (PVDF, 10 wt%) as a polymer binder, and Super P as conductive additive (10 wt%).After vacuum drying, all anodes had similar mass loading of the active materials (ca.0.79-1.36mg).All electrodes were tested under the same conditions, using coin type half-cells with metallic Li as counter electrodes (see details in experimental section).The electrochemical performance of Co-Sn NP-based electrodes was initially assessed through cyclic voltammetry (CV) with a scan rate of 0.1 mV s -1 in the potential window of 0-3.0 V (vs.Li + /Li).As shown in Figure 3, all CV cycles showed a similar trend, but the initial two cycles displayed more pronounced peaks than following ones at 1.31 V and 2.05 V in the forward scan and 0.65 V and 1.45 V in the reverse one.Differences were ascribed to the formation of the solid electrolyte interphase (SEI) layer during the first cycles and the reduction of the surface SnOx layer formed during NP manipulation and electrode preparation, in agreement with previous works. [52,53]The overlap of the 3rd and subsequent cycles indicated a reasonable stability of the electrode.The two obvious peaks around 0.65 and 0.05 V in the reverse scan were assigned to the reversible lithium insertion in the CoSn alloy to form Li4.4Sn. [54]During the anodic sweep, peaks at 0.52V were related to the extraction of Li ions from the electrode.
Qualitatively similar voltammograms were obtained for the four compositions tested.
Figures 4a-d display the first three charge-discharge cycles at a current density of 0.2 A g -1 of the electrodes containing Co-Sn NPs with different ratios.For all the compositions tested, small charging and discharging plateaus were observed at around 0.4 V and 1.6 V, respectively.The charge voltage at ca. 0.4 V was in good agreement with previous results. [29,55]Figure 4e-f shows the charge-discharge capacity and related efficiency over 400 cycles at a current density of 1.0 A g -1 (activated at 0.2 and 0.5 A g -1 for 10 cycles respectively).The low initial coulombic efficiency measured for all the electrodes, ca.50%, was associated with the SEI formation.During the first few cycles, the coulombic efficiency increased to ca. 98% and it was stabilized at this value for several hundreds of cycles.All compositions showed a similar trend, with an initial very fast decrease of the capacity, attributed to the SEI formation, a following slower loss of capacity, associated to a partial disintegration of the anode material, a capacity recover after a certain number of cycles and a moderate and sustained decrease of capacity at much larger cycle numbers.We hypothesize the reactivation to be in part associated to a rearrangement of the active material domains within the anode making a larger amount of electroactive material accessible to Li ions, although at the same time reducing electrical conductivity as observed from the EIS analysis below.This rearrangement of the active material could also provide larger surface areas and increase the pseudocapacitive contribution to the total energy storage capacity. [56]On the other hand, a restructuration of the active material at the atomic scale, and particularly its amorphisation, could facilitate lithium insertion.Co0.9Sn and Co0.7Sn electrodes showed the highest Li storage capacities among the compositions tested.For the Co0.9Sn electrode at 0.2 A g -1 , as shown in Figure S5, during the first cycle, the coulombic efficiency was just 55.7%, with a high discharge (869 mAh g -1 ) and charge capacity (1560 mAh g -1 ).A strong capacity loss was observed during the first cycles, down to charge and discharge capacities of 629 mAh g -1 and 647 mAh g -1 with 97.2% coulombic efficiency at the 24 th cycle.With continuous cycling, the coulombic efficiency remained stable and the capacity gradually increased up to charge and discharge capacities of 1534 mAh g -1 and 1555 mAh g -1 at the 220 th cycle.58][59][60] To evaluate the rate capability of the Co-Sn electrodes, galvanostatic cycling was performed at current rates between 0.05 to 4 A g -1 (Figure 5a). Figure 5b presents the corresponding charge-discharge profiles from 0.05 to 4.0 A g -1 .For Co0.9Sn, the electrode delivered a discharge capacity of 804, 702, 598, 532, 448, 365, 267 mAh g -1 at 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g -1 , respectively.
Figure S6 compares the EIS data obtained from electrodes with different Co-Sn compositions and from the Co0.9Sn electrode in the first and the 400th cycle, all at 1 A g -1 .From the Nyquist plots, not a straightforward dependence of the anodeelectrolyte charge transfer resistance on the Co-Sn alloy composition was observed.However, the lowest resistances were obtained for the optimum compositions Co0.7Sn and Co0.9Sn.With cycling, the electrode resistance increased and two semicircles evolved, one corresponding to the SEI layer impedance and the other to the charge-transfer impedance on the electrode-electrolyte interphase.In the low frequency region, slopes well above 1 for all compositions and both for the fresh and cycled samples indicated a significant capacitive behavior.
The kinetics of the Co-Sn electrodes was further investigated using CV at different scan rates, from 0.1 to 1 mV s -1 .Figure 5c presents the CV curves obtained from the Co0.9Sn electrode at the scan rates of 0.1, 0.2, 0.4, 0.7, 1.0 mV s -1 in the potential range 0-3.0 V vs Li + /Li.Three anodic peak were observed at 0.52, 1.31, 2.05 V, all of them increasing with the scan rate.
Two main charge-storage mechanisms determine the electrode storage capacity: i) a diffusion-controlled contribution associated to the Li22Sn5 alloy formation; and ii) a surfacerelated capacitive contribution known as the pseudocapacitive contribution. [61]The pseudocapacitive contribution is particularly attractive because it is a much faster and stable process, whereas the diffusion-controlled alloying is slower and generally provides relatively poor cycle life.
4] From the linear fit of the logarithmic plot of the current vs. scan rate (Figure 5d), b values of 0.80, 0.93 and 0.84 were calculated at 0.52, 1.31, 2.05 V, respectively.These values indicated a fast kinetics resulting from a pseudocapacitive effect.At each potential, the current density contribution at a given scan rate could be divided into two parts, a diffusioncontrolled (k1ν 1/2 ) and a capacitor-like fraction (k2ν):

=
/ + To distinguish the fraction of the current arising from Li + insertion and that from a capacitive process at each specific potential, k1 and k2 were determined by plotting i(V)/ν 1/2 vs. ν 1/2 . [44,62]Figure 5e shows the CV profiles at 0.4 mV s -1 and compares the capacitive current (blue shaded region) with that for the total measured current (red curve) for the Co0.9Sn electrode.The relative pseudocapacitive contributions at sweep rates of 0.1, 0.2, 0.4, 0.7 and 1.0 mV s -1 were 55%, 59%, 65%, 73% and 81%, respectively (Figures 4f and S7).For comparison, the pseudocapacitive study of Co0.3Sn electrode is presented in Figure S8, the calculated contributions at sweep rates of 0.1-1.0 mV s -1 were lower than that of Co0.9Sn.These results clearly suggest that the pseudocapacitive charge-storage amount does occupy a high portion of the whole energy storage capacity, which is associated to the small size of the Co-Sn NPs used and their Sn-rich and oxidized surface.To estimate the practical application of Co-Sn NP-based anodes, they were tested in the range of 0.01-1.5 V vs. Li/Li + .As shown in Figures 6a-b and S9, over continuous chargedischarge cycles, Co0.7Sn and Co0.9Sn electrodes show the highest capacity and stability, stabilizing at 360 mAh g -1 at 0.5 A g -1 .This value is comparable to theoretical capacities of graphene-based electrodes.The rate capability of the Co0.9Sn electrode is also shown in Figure 6c-d.Specifically, the electrode delivered discharge capacities of 520, 453, 421, 388, 336, 253 mAh g -1 at 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g -1 , respectively.Additionally, this electrode delivered a stable charge-discharge capacity at 0.1 A g -1 after continuous 60 cycles at variable charging rate.

Conclusions
In conclusion, Co-Sn solid-solution NPs with average size in the 6-10 nm range were synthesized via a simple one pot colloidalbased approach.The CoxSn NP composition was adjusted, 1.3 ≤ x ≤ 0.3, by tuning the ratio of the initial precursors.The low synthesis temperature favoured the nucleation of Sn NPs and the subsequent inclusion of Co to the Sn lattice, forming a solid solution with the Sn crystal phase, instead of an intermetallic compound.The same strategy could be used to produce a much more extended range of Co-Sn compositions.Co-Sn NPs presented a Sn-rich surface after exposure to air.These Co-Sn solid solutions were tested as anode materials in LIBs on a halfcell battery system.Among the different compositions tested, Co0.9Sn and Co0.7Sn NPs provided the best performance, with a charge-discharge capacity above 1500 mAh g -1 at a current density of 0.2 A g -1 after 220 cycles and up to 800 mAh g -1 at 1.0 A g -1 after 400 cycles in the range 0-3.0 V.When testing in the range 0-1.5 V, these two electrode delivered an average of 360 mAh g -1 at 0.5 A g -1 .These values were larger than that of graphite currently used in commercial devices and larger than the theoretical maximum for Co-Sn alloys and even for pure Sn.Through the kinetic analysis of Co0.9Sn NPs by the CV measurement, we found these charge-discharge capacities to include a very large pseudocapacitive contribution, up to 81% at a sweep rate of 1 mV s -1 , which we related to the small size of the particles.

Experimental Section
Colloidal synthesis of Sn and Co-Sn NPs: Syntheses were carried out using standard air-free techniques.All the reagents and solvent were analytical grade and used without further purification.In a typical synthesis of Co-Sn NPs with nominal composition Co:Sn = 3:2, 0.6 mmol cobalt(II) acetylacetonate (Co(acac)2, 99%, Sigma-Aldrich) and 0.4 mmol tin(II) acetate (Sn(OAc)2, 95%, Fluka) were added into a 50 mL threeneck round-bottomed flask.Subsequently, 20 mL of oleylamine (OAm, 80-90%, TCI) and 1.0 mL of oleic acid (OAc, Sigma-Aldrich) were loaded along with a magnetic bar in a three-neck flask connected with a thermometer, condenser and septum.The flask was heated to 80 °C and degassed under vacuum for 2 hours and then backfilled with Ar.Then, 5 mL of tri-n-octylphosphine (TOP, 97%, Strem) were injected, and afterward the solution was heated up to 180 °C at 5 °C min -1 .Right after reaching 180 °C, 5 mL of a degassed OAm solution containing 5 mmol of borane tert-butylamine (TBAB, 97%, Sigma-Aldrich) was injected.Upon injection of this reducing complex, the solution became black, but the reaction mixture was maintained at 180 °C for 1 additional hour to allow the NPs to grow.After 1 h reaction, the heating mantle was removed and the solution was cooled down to room temperature in approximately 3 min using a water bath.NPs were collected by centrifugation after adding an excess of acetone.The precipitate was dispersed in chloroform and centrifuged a second time with an excess of acetone.This washing process was repeated for three times.Finally, NPs were stored in chloroform.
Ligand removal: 25 mL of acetonitrile containing 0.8 mL hydrazine hydrated were added into a vial containing about 100 mg of precipitated NPs.The solution was strongly stirred for 4 hours and centrifuged.The NPs were further washed with acetonitrile 3 more times, followed by vacuum-drying at room temperature.The product was kept in an Ar-filled glove box.
Characterization: X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Advance X-ray diffractometer with Cu K radiation (λ = 1.5106Å) operating at 40 kV and 40 mA.Transmission electron microscopy (TEM) analyses were performed on a ZEISS LIBRA 120, operating at 120 kV.High-resolution TEM (HRTEM) and scanning TEM (STEM) studies were carried out using a field emission gun FEI Tecnai F20 microscope at 200 kV with a point-to-point resolution of 0.19 nm.High angle annular dark-field (HAADF) STEM was combined with electron energy loss spectroscopy (EELS) in the Tecnai microscope by using a GATAN QUANTUM filter.Composition analysis was carried out using a ZEISS Auriga Scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) detector operated at 20 kV.X-ray photoelectron spectroscopy (XPS) measurements were carried out in normal emission using an Al anode XR50 source operating at 150 mW and a Phoibos 150 MCD-9 detector.Fourier transform infrared spectrometer (FTIR) data was recorded on an Alpha Bruker spectrometer.

Electrochemical measurements:
To evaluate the intrinsic electrochemical performance of Co-Sn NPs, the working electrode was prepared by mixing dried NPs, Super P and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10 in an appropriate amount of N-methy1-2-pyrrolidone (NMP) to obtain a slurry.Then, the mixture was coated onto a Cu foil.Then, it was dried in a vacuum oven at 80 °C for 24 h.Subsequently, the foil was cut into disks with a diameter of 12 mm.The typical mass loading of active materials was estimated to be 0.7-1.2mg cm −2 .To test the performance of electrodes based on Co-Sn NPs, half cells were assembled in the glove box (H2O and O2 < 0.1 ppm) using Celgard2400 as separator.As electrolyte, a 1 M LiPF6 solution in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) with 5 wt% fluoroethylene carbonate (FEC) as additive was used.Galvanostatic charge-discharge were measured by a battery test system (CT2001A, LAND) with cutoff potentials from 0.01 V to 1.5 and 3.0 V. Cyclic voltammetry (CV) curves were performed by an electrochemical workstation (Gamry Interface 1000) in the voltage range from 0-3.0 V and the scan rate from 0.1 mV s -1 to 1 mV s -1 .

Figure 1 .
Figure 1.a-d) TEM micrographs of Co-Sn NPs with different compositions, as obtained from EDX and displayed in each image.e) Size distribution histograms of the Co-Sn NPs; f) XRD patterns of the NPs with different compositions.Sn and different Co-Sn intermetallic XRD patterns are shown as reference.

Figure 2 .
Figure 2. a) STEM and EELS compositional maps of Co0.9Sn NPs.b) HRTEM micrograph of Co0.9Sn NPs exposed to atmosphere and displaying a core-shell structure.

Figure 4 .
Figure 4. a-d) Initial charge-discharge curves at 0.2 A g -1 for the different electrode compositions as displayed on the top of each graph.e-h) Charge-discharge capacity and related efficiency over 400 cycles at a current density of 1.0 A g -1 : activated at 0.2 and 0.5 A g -1 for 10 cycles each.For the Co1.3Sn electrode only data at 0.2 A g -1 is shown.