This is the peer reviewed version of the following article: Li J., Xu X., Luo Z., Zhang C., Zuo Y., Zhang T., Tang P., Infante-Carrió M.F., Arbiol J., Llorca J., Liu J., Cabot A.. Co–Sn Nanocrystalline Solid Solutions as Anode Materials in Lithium-Ion Batteries with High Pseudocapacitive Contribution. ChemSusChem, (2019). 12. : 1451 - . 10.1002/cssc.201802662, which has been published in final form at https://dx.doi.org/10.1002/cssc.201802662. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Co-Sn nanocrystalline solid solutions as anode materials in lithium-ion batteries with high pseudocapacitive contribution Junshan Li,[a,b,] Xijun Xu,[c,d,] Zhishan Luo,[a] Chaoqi Zhang,[a] Yong Zuo,[a] Ting Zhang,[e] Pengyi Tang,[e] Maria F. Infante-Carrió,[e] Jordi Arbiol,[e,f] Jordi Llorca,[g] Jun Liu,[c,d,*] Andreu Cabot[a,f,*] Abstract: Co-Sn solid-solution nanoparticles with the Sn crystal their relatively high abundance, low cost and high electrical structure and tuned metal ratios were synthesized by a facile one- conductivity. [7,9] pot solution-based procedure involving the initial reduction of a Sn In terms of stability, the huge lattice expansion and precursor and the posterior incorporation of Co within the Sn lattice. contraction of the anode material during cycling strongly reduces Such nanoparticles were used as anode materials for Li-ion batteries. the battery performance due to a loss of electrical connection by Among the different compositions tested, Co0.7Sn and Co0.9Sn electrode pulverization. In the case of Sn, the Sn/Li22Sn5 electrodes provided the highest capacities, with values above 1500 reaction has associated a 300% volume change, which mAh g-1 at a current density of 0.2 A g-1 after 220 cycles and up to inevitably leads to major structural rearrangements generally 800 mAh g-1 at 1.0 A g-1 after 400 cycles. Up to 81 % resulting in a loss of capacity.[10,11] Different strategies have been pseudocapacitance contribution at a sweep rate of 1.0 mV s-1 were proposed to tackle down this major issue. One main approach is measured for these electrodes, providing fast kinetics and long to alloy the active phase, Sn, with a second elements that durability. The excellent performance of Co-Sn alloy nanoparticle- buffers the volume changes.[12,13] In this direction, Sn-based based electrodes was associated to both the small size of the crystal alloys with Ni,[14–17] Co,[18–30] Fe,[31,32] Cu,[33,34] and Sb[35–37] have domains and their suitable composition, which buffered volume demonstrated superior cycling performance than bare Sn changes of Sn and contributed to a suitable electrode restructuration. anodes. Among the different Sn-based alloys tested, CoSn electrodes have shown particularly promising performances as anode materials for LIBs.[31] Co-Sn-C composites have been even used in commercial batteries, what has motivated a Introduction notable interest in this system.[38–42] Most previous works have focused on the intermetallic Co-Sn alloys: Co3Sn2, CoSn, CoSn2. Lithium-ion batteries (LIBs) have become the dominant energy Among these intermetallics, while some controversy remains, storage technology in portable applications. However, their CoSn2 has been considered as the optimum stoichiometry, due energy density, charging rate, and stability are still critical to its highest content of Sn. However, beyond intermetallic performance parameters that has plenty of room for phases, a range of Co-Sn solid solutions exist that are yet to be improvement by optimizing both anode and cathode materials.[1– explored. 4] At the anode, current commercial LIBs use graphite, which has Besides alloying the active material to improve cycling a relatively low maximum theoretical capacity (372 mAh g-1 to performance, the use of nanostructured electrodes can reduce form LiC6).[5] Alternatively, anode materials such as Sn, Ge and the overall stress generated on the material domains during Si provide platforms with potentially higher lithium storage lithiation, thus diminishing mechanical disintegration and capacities: Li22Sn5 (994 mAh g-1), Li22Ge5 (1600 mAh g-1), and improving stability. Furthermore, nanosized materials present Li22Si5 (3000 mAh g-1) respectively.[6–8] While Si is the most additional advantages, such as faster rate capabilities because abundant element and has the highest potential storage capacity, of the shorter Li-ion diffusion paths and a potentially huge Sn and Sn-based compounds are particularly appealing owing to pseudocapacitive contribution associated with their very high surface/volume ratio.[43–46] This pseudocapacitive contribution is particularly appealing because it can significantly improve both [a] Catalonia Institute for Energy Research - IREC, Sant Adrià de the rate performance of LIBs and their stability. Besòs, Barcelona, 08930, Spain. *Prof. A. Cabot, acabot@irec.cat In the present work, we take advantage of the versatility of [b] Departament d’Electronica, Universitat de Barcelona, 08028 Barcelona, Spain colloidal synthesis methods to produce Co-Sn solid solution [c] Guangdong Provincial Key Laboratory of Advanced Energy Storage nanoparticles (NPs) with tuned Co:Sn ratios, from 0.3 to 1.3. Materials, School of Materials Science and Engineering, South After removing surface ligands, we use these NPs to test the China University of Technology, Guangzhou 510641, P. R. performance of Co-Sn solid solutions as anode materials for China.*Prof. J. Liu, msjliu@scut.edu.cn [d] SUNWODA-SCUT Joint Laboratory for Advanced Energy Storage LIBs, defining an optimal composition and demonstrating this Technology, South China University of Technology, Guangzhou system to be characterized by a high energy storage capacity, 510641, P. R. China with a high pseudocapacitive contribution and a notable stability. [e] Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain [f ] ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain [g] Institute of Energy Technologies, Department of Chemical Results and Discussion Engineering and Barcelona Research Center in Multiscale Science and Engineering. Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain Colloidal Co-Sn NPs were produced by the reduction of tin(II) acetate and cobalt(II) acetylacetonate with borane tert-  J. Li and X. Xu equally contributed to this work. butylamine (TBAB) at 180 °C, in a solution containing oleylamine Supporting information for this article is given via a link at the end of (OAm), oleic acid (OAc) and tri-n-octylphosphine (TOP, see the document. details in experimental section). Figures 1a-d show representative TEM micrographs and size distribution histograms of the quasi-spherical NPs produced. The average crystal structure was conserved, which is in contrast to the NP size was estimated to be 6 - 7 nm for all compositions except results obtained in previous works that make use of higher for the Sn-richest NPs, which had an average size of 10 nm. reaction, alloying or annealing temperatures to produce Co-Sn XRD analysis showed that, independently of the Co:Sn ratio, intermetallic alloys. NPs conserved the Sn crystal structure. However, XRD peaks XPS analysis (Figure S2) showed the Co:Sn ratio of the appeared shifted to lower angles, as it corresponds to the Co0.9Sn NPs to be 0.7, which pointed at a slightly Sn-rich introduction of a slightly larger atom, Co, within the Sn lattice. surface. We hypothesize that this Sn-rich surface was related to The formation of CoSn solid solutions was somehow surprising a slight oxidation of the NPs exposed to ambient conditions. We when taking into account that previous works reported the believe air exposure resulted in a slight restructuration of the formation of intermetallics, i.e. CoSn2, CoSn and Co3Sn2, when alloy due to the higher Sn affinity to oxygen that drove the co-reducing proper amounts of the two elements. We associate diffusion of Sn to the surface.[47] the differences in the products obtained between our synthesis Figure 2a shows STEM micrographs and EELS chemical protocol and previous works to the relatively low synthesis composition maps of the Co0.9Sn NPs. All Co0.9Sn NPs temperatures we used which prevented the crystallization of contained the two elements in similar ratios. Within each NP, Co independent Co NPs and Co-Sn intermetallic phases. 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. With the exposition to air, these NPs developed a Sn-rich surface associated to a differential reactivity of the two elements with oxygen.[49–51] 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. 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. EDX analysis showed the Co:Sn ratio in Co-Sn solid- Figure 2. a) STEM and EELS compositional maps of Co0.9Sn NPs. b) HRTEM solution NPs to be: 1.3, 0.9, 0.7 and 0.3 when produced from micrograph of Co0.9Sn NPs exposed to atmosphere and displaying a core-shell nominal Co:Sn precursor ratios of 2.0, 1.5, 1.0 and 0.5, structure. respectively. The final Co-poor NP stoichiometry (with respect to Co-Sn solid solution NPs were explored as anode material the nominal) and the pink color of the supernatant obtained after in LIBs. Before testing their performance, the organic ligands NP precipitation revealed that some of the cobalt precursor used to control the growth of the NPs in solution were removed remained unreacted after 1h at 180 °C. We also observed that by treating them with a mixture of hydrazine and acetonitrile.[36,37] the same reaction conditions but in the absence of Sn precursor FTIR analysis confirmed the effectiveness of the ligand removal did not result in the formation of Co NPs. On the contrary, the step (Figure S4). LIB anodes were prepared by casting a non- same reaction in the absence of Co resulted in the formation of aqueous slurry containing 80 wt% of active material, Sn NPs. We believe that in the reaction conditions used, the Sn polyvinylidene (PVDF, 10 wt%) as a polymer binder, and Super precursor was first reduced to nucleate Sn NPs. Taking P as conductive additive (10 wt%). After vacuum drying, all advantage of the lower energy for heterogeneous growth over anodes had similar mass loading of the active materials (ca. homogeneous nucleation, during the 1h period at 180 °C Co 0.79-1.36 mg). All electrodes were tested under the same ions within the solution incorporated to the initial Sn nuclei upon conditions, using coin type half-cells with metallic Li as counter reduction with TBAB. Through this synthesis mechanism, the Sn electrodes (see details in experimental section). Qualitatively similar voltammograms were obtained for the four 0.2 compositions tested. Figures 4a-d display the first three charge-discharge Current (mA) 0.0 cycles at a current density of 0.2 A g-1 of the electrodes containing Co-Sn NPs with different ratios. For all the -0.2 2nd cycle compositions tested, small charging and discharging plateaus were observed at around 0.4 V and 1.6 V, respectively. The -0.4 4th cycle 1st cycle charge voltage at ca. 0.4 V was in good agreement with 3rd cycle previous results.[29,55] Figure 4e-f shows the charge-discharge -0.6 capacity and related efficiency over 400 cycles at a current 0.0 0.5 1.0 1.5 2.0 2.5 3.0 density of 1.0 A g-1 (activated at 0.2 and 0.5 A g-1 for 10 cycles + Voltage (V vs. Li /Li) respectively). The low initial coulombic efficiency measured for Figure 3. Initial cyclic voltammograms obtained from the Co0.9Sn electrode in all the electrodes, ca. 50%, was associated with the SEI the voltage window 0-3.0 V vs. Li+/Li at 0.1 mV s-1. formation. During the first few cycles, the coulombic efficiency increased to ca. 98% and it was stabilized at this value for The electrochemical performance of Co-Sn NP-based several hundreds of cycles. All compositions showed a similar electrodes was initially assessed through cyclic voltammetry trend, with an initial very fast decrease of the capacity, attributed (CV) with a scan rate of 0.1 mV s-1 in the potential window of 0- to the SEI formation, a following slower loss of capacity, 3.0 V (vs. Li+/Li). As shown in Figure 3, all CV cycles showed a associated to a partial disintegration of the anode material, a similar trend, but the initial two cycles displayed more capacity recover after a certain number of cycles and a pronounced peaks than following ones at 1.31 V and 2.05 V in moderate and sustained decrease of capacity at much larger the forward scan and 0.65 V and 1.45 V in the reverse one. cycle numbers. We hypothesize the reactivation to be in part Differences were ascribed to the formation of the solid associated to a rearrangement of the active material domains electrolyte interphase (SEI) layer during the first cycles and the within the anode making a larger amount of electroactive reduction of the surface SnOx layer formed during NP material accessible to Li ions, although at the same time manipulation and electrode preparation, in agreement with reducing electrical conductivity as observed from the EIS previous works.[52,53] The overlap of the 3rd and subsequent analysis below. This rearrangement of the active material could cycles indicated a reasonable stability of the electrode. The two also provide larger surface areas and increase the obvious peaks around 0.65 and 0.05 V in the reverse scan were pseudocapacitive contribution to the total energy storage assigned to the reversible lithium insertion in the CoSn alloy to capacity.[56] On the other hand, a restructuration of the active form Li4.4Sn.[54] During the anodic sweep, peaks at 0.52V were Co1.3Sn Co0.9Sn Co0.7Sn Co0.3Sn a 3.5 3.5 c 3.5 3.5 2nd cycle b 2nd cycle 2nd cycle d 2nd cycle 3.0 3.0 3.0 3.0 3rd cycle 3rd cycle 3rd cycle 1st cycle 3rd cycle 1st cycle 1st cycle 1st cycle 2.5 2.5 2.5 2.5 Voltage (V) Voltage (V) Voltage (V) Voltage (V) 2.0 2.0 2.0 2.0 1.5 1.5 1.5 1.5 0.2 A g-1 0.2 A g-1 0.2 A g-1 0.2 A g-1 1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5 0.0 0.0 0.0 0.0 0 400 800 1200 1600 0 400 800 1200 1600 0 400 800 1200 1600 0 400 800 1200 1600 Capacity (mAh g-1) Capacity (mAh g-1) Capacity (mAh g-1) Capacity (mAh g-1) e 1500 f 1500 g 1500 h 1500 100 100 100 100 Capacity (mAh g-1) 1200 1200 1200 1200 Capacity (mAh g-1) Capacity (mAh g-1) Capacity (mAh g-1) 80 80 80 80 Effciency (%) Efficiency (%) Efficiency (%) Efficiency (%) 900 900 0.2A g-1 900 900 60 1.0 A g-1 60 -1 1.0 A g-1 60 -1 Charge 60 0.2 A g-1 0.5 A g-1 0.2 A g 0.2 A g Discharge 600 600 600 0.5 A g-1 600 -1 40 40 40 0.5 A g 40 1.0 A g-1 300 Charge 20 300 20 300 20 300 20 Charge Charge Discharge Discharge Discharge 0 0 0 0 0 0 0 0 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 Cycle number Cycle number Cycle number Cycle number related to the extraction of Li ions from the electrode. material at the atomic scale, and particularly its amorphisation, could facilitate lithium insertion. 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. Co0.9Sn and Co0.7Sn electrodes showed the highest Li storage strong capacity loss was observed during the first cycles, down capacities among the compositions tested. For the Co0.9Sn to charge and discharge capacities of 629 mAh g-1 and 647 electrode at 0.2 A g-1, as shown in Figure S5, during the first mAh g-1 with 97.2% coulombic efficiency at the 24th cycle. With cycle, the coulombic efficiency was just 55.7%, with a high continuous cycling, the coulombic efficiency remained stable discharge (869 mAh g-1) and charge capacity (1560 mAh g-1). A and the capacity gradually increased up to charge and discharge capacities of 1534 mAh g-1 and 1555 mAh g-1 at the 220th cycle. The kinetics of the Co-Sn electrodes was further The extraordinarily high capacities obtained, above the investigated using CV at different scan rates, from 0.1 to 1 theoretical maximum not only for Co-Sn alloys, but also for pure mV s-1. Figure 5c presents the CV curves obtained from the Sn, may be attributed to the ultra-small particle size of the active Co0.9Sn electrode at the scan rates of 0.1, 0.2, 0.4, 0.7, 1.0 anode material, which provided additional active sites for Li-ion mV s-1 in the potential range 0-3.0 V vs Li+/Li. Three anodic peak storage and a larger density of diffusion channels for Li ions to were observed at 0.52, 1.31, 2.05 V, all of them increasing with access all the active material.[56–60] the scan rate. To evaluate the rate capability of the Co-Sn electrodes, Two main charge-storage mechanisms determine the galvanostatic cycling was performed at current rates between electrode storage capacity: i) a diffusion-controlled contribution 0.05 to 4 A g-1 (Figure 5a). Figure 5b presents the corresponding associated to the Li22Sn5 alloy formation; and ii) a surface- charge-discharge profiles from 0.05 to 4.0 A g-1. For Co0.9Sn, the related capacitive contribution known as the pseudocapacitive electrode delivered a discharge capacity of 804, 702, 598, 532, contribution.[61] The pseudocapacitive contribution is particularly 448, 365, 267 mAh g-1 at 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g-1, attractive because it is a much faster and stable process, respectively. whereas the diffusion-controlled alloying is slower and generally Figure S6 compares the EIS data obtained from electrodes provides relatively poor cycle life. with different Co-Sn compositions and from the Co0.9Sn Generally, the relationship between the measured current electrode in the first and the 400th cycle, all at 1 A g-1. From the (i) and the scan rate (ν) can be expressed as: Nyquist plots, not a straightforward dependence of the anode- = electrolyte charge transfer resistance on the Co-Sn alloy According to previous reports, a diffusion controlled composition was observed. However, the lowest resistances process is characterized by a scan rate dependence with b = 0.5, were obtained for the optimum compositions Co0.7Sn and whereas an ideal capacitive behavior translates into b = 1.[62–64] a 1600 b 3.5 c 2.4 100 4.0 2.0 1.0 0.5 0.2 0.1 0.05 Peak 1 3.0 1.6 Peak 2 Peak 3 Capacity (mAh g-1) 1200 A g-1 80 Current (mA) 2.5 0.8 Efficiency (%) Voltage (V) 0.05 A g-1 2.0 0.0 0.1 60 800 0.1 0.2 1.5 -0.8 0.5 40 -1 1.0 0.1 mV s 2.0 1.0 -1.6 400 4.0 Charge 20 -1 0.5 -2.4 1.0 mV s Discharge 0 0 0.0 -3.2 0 20 40 60 80 0 200 400 600 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 + Cycle number Capacity (mAh g-1) Voltage (V vs. Li /Li) 0.2 1.0 Diffusion controlled d e f Peak 1 Capacitive 0.0 b=0.80 0.5 Current (mA) Log i (A g ) -0.2 100% -1 Peak 2 0.0 80% -0.4 b=0.93 Contribution Peak 3 b=0.84 -0.5 60% -0.6 81% 73% -1 0.4 mV s 65% 40% 59% 55% -1.0 -0.8 logi = b⋅logν + loga 20% -1.0 -1.5 0% -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1 + 0.2 0.4 0.6 0.8 1 Log ν (mV s ) Voltage (V vs. Li /Li) Scan rate (mV s-1) Co0.9Sn. With cycling, the electrode resistance increased and From the linear fit of the logarithmic plot of the current vs. scan two semicircles evolved, one corresponding to the SEI layer rate (Figure 5d), b values of 0.80, 0.93 and 0.84 were calculated impedance and the other to the charge-transfer impedance on at 0.52, 1.31, 2.05 V, respectively. These values indicated a fast the electrode-electrolyte interphase. In the low frequency region, kinetics resulting from a pseudocapacitive effect. slopes well above 1 for all compositions and both for the fresh and cycled samples indicated a significant capacitive behavior. Figure 5. Li-ion storage performance of the Co0.9Sn electrode: a) Charge-discharge curves at rates: 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 0.1 A g-1. b) Rate performance at 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g-1. c) CV curve at the scan rates of 0.1, 0.2, 0.4, 0.7, 1.0 mV s-1. d) Logarithmic dependence between peak current density and scan rate at the anodic peaks 0.52, 1.31, 2.05 V. e) Capacitive contribution to the total current contribution at 0.4 mV s-1. f) Normalize capacitive and diffusion-controlled contribution at the scan rates of 0.1, 0.2, 0.4, 0.7, 1.0 mV s-1. At each potential, the current density contribution at a ν1/2.[44,62] Figure 5e shows the CV profiles at 0.4 mV s-1 and given scan rate could be divided into two parts, a diffusion- compares the capacitive current (blue shaded region) with that controlled (k1ν1/2) and a capacitor-like fraction (k2ν): for the total measured current (red curve) for the Co0.9Sn = / + electrode. The relative pseudocapacitive contributions at sweep To distinguish the fraction of the current arising from Li+ rates of 0.1, 0.2, 0.4, 0.7 and 1.0 mV s-1 were 55%, 59%, 65%, insertion and that from a capacitive process at each specific 73% and 81%, respectively (Figures 4f and S7). For comparison, potential, k1 and k2 were determined by plotting i(V)/ν1/2 vs. the pseudocapacitive study of Co0.3Sn electrode is presented in Figure S8, the calculated contributions at sweep rates of 0.1-1.0 solid solutions were tested as anode materials in LIBs on a half- mV s-1 were lower than that of Co0.9Sn. These results clearly cell battery system. Among the different compositions tested, suggest that the pseudocapacitive charge-storage amount does Co0.9Sn and Co0.7Sn NPs provided the best performance, with a occupy a high portion of the whole energy storage capacity, charge-discharge capacity above 1500 mAh g-1 at a current which is associated to the small size of the Co-Sn NPs used and density of 0.2 A g-1 after 220 cycles and up to 800 mAh g-1 at 1.0 their Sn-rich and oxidized surface. 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 360 100 1.5 mAh g-1 at 0.5 A g-1. These values were larger than that of a 1500 b 80 graphite currently used in commercial devices and larger than ) -1 1200 the theoretical maximum for Co-Sn alloys and even for pure Sn. Efficiency(%) Capacity(mAh g 1.0 Voltage (V) 60 1st cycle 900 2nd cycle Through the kinetic analysis of Co0.9Sn NPs by the CV 3rd cycle 600 0.1 A g -1 -1 40 0.5 4th cycle measurement, we found these charge-discharge capacities to 0.5 A g 300 20 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 0 0 0.0 0 20 40 60 80 100 120 0 150 300 450 -1 600 750 the particles. Cycle number Capacity (mAh g ) 1.5 c 1500 100 d Experimental Section ) 0.1 -1 80 Capacity(mAh g Efficiency(%) 1200 0.2 1.0 Voltage (V) 0.5 900 60 1.0 Unit: A g -1 2.0 Colloidal synthesis of Sn and Co-Sn NPs: Syntheses were carried out 600 0.1 0.2 0.1 40 0.5 4.0 using standard air-free techniques. All the reagents and solvent were 0.5 1.0 -1 2.0 4.0 20 Unit: A g analytical grade and used without further purification. In a typical 300 synthesis of Co-Sn NPs with nominal composition Co:Sn = 3:2, 0.6 mmol 0 0 12 24 36 48 60 0 0.0 0 100 200 300 400 500 600 cobalt(II) acetylacetonate (Co(acac)2, 99%, Sigma-Aldrich) and 0.4 mmol -1 Cycle number Capacity (mAh g ) tin(II) acetate (Sn(OAc)2, 95%, Fluka) were added into a 50 mL three- neck round-bottomed flask. Subsequently, 20 mL of oleylamine (OAm, Figure 6. Li-ion storage performance of the Co0.9Sn electrode in the range 0- 80-90%, TCI) and 1.0 mL of oleic acid (OAc, Sigma-Aldrich) were loaded 1.5 V vs. Li/Li+: a) Charge-discharge curves at rates: 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 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 0.1 A g-1. b) Rate performance at 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g-1. c) Charge- degassed under vacuum for 2 hours and then backfilled with Ar. Then, 5 discharge capacity and related efficiency over 130 cycles at a current density mL of tri-n-octylphosphine (TOP, 97%, Strem) were injected, and of 0.5 A g-1: activated at 0.1 A g-1 for 10 cycles. d) Initial charge-discharge afterward the solution was heated up to 180 °C at 5 °C min-1. Right after curves at 0.1 A g-1. reaching 180 °C, 5 mL of a degassed OAm solution containing 5 mmol of borane tert-butylamine (TBAB, 97%, Sigma-Aldrich) was injected. Upon To estimate the practical application of Co-Sn NP-based injection of this reducing complex, the solution became black, but the anodes, they were tested in the range of 0.01-1.5 V vs. Li/Li+. As reaction mixture was maintained at 180 °C for 1 additional hour to allow shown in Figures 6a-b and S9, over continuous charge- 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 discharge cycles, Co0.7Sn and Co0.9Sn electrodes show the min using a water bath. NPs were collected by centrifugation after adding highest capacity and stability, stabilizing at 360 mAh g-1 at 0.5 A an excess of acetone. The precipitate was dispersed in chloroform and g-1. This value is comparable to theoretical capacities of centrifuged a second time with an excess of acetone. This washing graphene-based electrodes. The rate capability of the Co0.9Sn process was repeated for three times. Finally, NPs were stored in electrode is also shown in Figure 6c-d. Specifically, the chloroform. 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. Ligand removal: 25 mL of acetonitrile containing 0.8 mL hydrazine Additionally, this electrode delivered a stable charge-discharge hydrated were added into a vial containing about 100 mg of precipitated capacity at 0.1 A g-1 after continuous 60 cycles at variable NPs. The solution was strongly stirred for 4 hours and centrifuged. The charging rate. 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. Conclusions Characterization: X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Advance X-ray diffractometer In conclusion, Co-Sn solid-solution NPs with average size in the with Cu K radiation (λ = 1.5106 Å) operating at 40 kV and 40 mA. 6-10 nm range were synthesized via a simple one pot colloidal- Transmission electron microscopy (TEM) analyses were performed on a based approach. The CoxSn NP composition was adjusted, 1.3 ZEISS LIBRA 120, operating at 120 kV. High-resolution TEM (HRTEM) ≤ x ≤ 0.3, by tuning the ratio of the initial precursors. The low 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 synthesis temperature favoured the nucleation of Sn NPs and resolution of 0.19 nm. High angle annular dark-field (HAADF) STEM was the subsequent inclusion of Co to the Sn lattice, forming a solid combined with electron energy loss spectroscopy (EELS) in the Tecnai solution with the Sn crystal phase, instead of an intermetallic microscope by using a GATAN QUANTUM filter. Composition analysis compound. The same strategy could be used to produce a much was carried out using a ZEISS Auriga Scanning electron microscopy more extended range of Co-Sn compositions. Co-Sn NPs (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) presented a Sn-rich surface after exposure to air. These Co-Sn 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. and to MINECO/FEDER grant ENE2015-63969-R. TZ has Fourier transform infrared spectrometer (FTIR) data was recorded on an received funding from the CSC-UAB PhD scholarship program. Alpha Bruker spectrometer. Part of the present work has been performed in the framework of Electrochemical measurements: To evaluate the intrinsic Universitat Autònoma de Barcelona Materials Science PhD electrochemical performance of Co-Sn NPs, the working electrode was program. 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- Keywords: Colloidal; Bimetallic Nanoparticles; tin cobalt; 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 Anode; Lithium-ion Batteries 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 0.7-1.2 [1] C. M. Park, J. H. Kim, H. Kim, H. J. Sohn, Chem. Soc. Rev. 2010, 39, 3115–3141. mg cm−2. To test the performance of electrodes based on Co-Sn NPs, [2] M. N. Obrovac, V. L. Chevrier, Chem. Rev. 2014, 114, 11444– half cells were assembled in the glove box (H2O and O2 < 0.1 ppm) using 11502. Celgard2400 as separator. As electrolyte, a 1 M LiPF6 solution in [3] R. Marom, S. F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, J. Mater. ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) with 5 Chem. 2011, 21, 9938–9954. [4] P. G. Bruce, B. Scrosati, J. M. Tarascon, Angew. Chemie - Int. Ed. wt% fluoroethylene carbonate (FEC) as additive was used. Galvanostatic 2008, 47, 2930–2946. charge-discharge were measured by a battery test system (CT2001A, [5] L. X. Ding, G. R. Li, Z. L. Wang, Z. Q. Liu, H. Liu, Y. X. Tong, Chem. LAND) with cutoff potentials from 0.01 V to 1.5 and 3.0 V. Cyclic - A Eur. J. 2012, 18, 8386–8391. voltammetry (CV) curves were performed by an electrochemical [6] H. Tian, F. Xin, X. Wang, W. He, W. Han, J. Mater. 2015, 1, 153– workstation (Gamry Interface 1000) in the voltage range from 0–3.0 V 169. [7] B. Wang, B. Luo, X. Li, L. Zhi, Mater. Today 2012, 15, 544–552. and the scan rate from 0.1 mV s-1 to 1 mV s-1. [8] C. K. Chan, R. N. Patel, M. J. O’Connell, B. A. Korgel, Y. Cui, ACS Nano 2010, 4, 1443–1450. [9] H. Ying, W. Q. Han, Adv. Sci. 2017, 4, 1700298. [10] N. Nitta, F. Wu, J. T. Lee, G. Yushin, Mater. Today 2015, 18, 252– Author contributions 264. [11] J. B. Goodenough, K. S. Park, J. Am. Chem. Soc. 2013, 135, 1167– 1176. [12] M. R. Palacín, Chem. Soc. Rev. 2009, 38, 2565–2575. [13] M. F. Oszajca, M. I. Bodnarchuk, M. V. Kovalenko, Chem. Mater. The manuscript was prepared through the contribution of all 2014, 26, 5422–5432. authors. A. Cabot and J. Liu conceived and guided the project, [14] D. wei ZHANG, C. ge YANG, J. DAI, J. wu WEN, L. WANG, C. hua CHEN, Trans. Nonferrous Met. Soc. China (English Ed. 2009, 19, and supervised the work. J. Li designed the experiments, 1489–1493. produced the nanoparticles, conducted XRD, TEM, SEM-EDS [15] J. Liu, Y. Wen, P. A. Van Aken, J. Maier, Y. Yu, Nano Lett. 2014, 14, and FT-IR characterization, and wrote the first draft of the 6387–6392. [16] X. Fan, P. Dou, A. Jiang, D. Ma, X. Xu, ACS Appl. Mater. Interfaces manuscript. Z. Luo, C. Zhang and Y. Zuo significantly 2014, 6, 22282–22288. contributed to the results discussion. X. Xu performed the [17] M. Uysal, T. Cetinkaya, A. Alp, H. Akbulut, J. Alloys Compd. 2015, 645, 235–242. electrochemical measurements. T. Zhang, P. Tang, M. F. [18] N. Tamura, Y. Kato, A. Mikami, M. Kamino, S. Matsuta, S. Fujitani, J. Infante-Carrió and J. Arbiol performed structural and Electrochem. Soc. 2006, 153, A1626. [19] Z. Du, S. Zhang, J. Phys. Chem. C 2011, 115, 23603–23609. compositional NCs characterization by means of HRTEM and [20] C. M. Ionica-Bousquet, P. E. Lippens, L. Aldon, J. Olivier-Fourcade, EELS, and discussed the results. J. Llorca measured and J. C. Jumas, Chem. Mater. 2006, 18, 6442–6447. discussed XPS data. The manuscript was corrected and [21] G. O. Park, J. Yoon, J. K. Shon, Y. S. Choi, J. G. Won, S. Bin Park, K. H. Kim, H. Kim, W. S. Yoon, J. M. Kim, Adv. Funct. Mater. 2016, improved by all authors. 26, 2800–2808. [22] B. O. Jang, S. H. Park, W. J. Lee, J. Alloys Compd. 2013, 574, 325– 330. [23] J. R. González, F. Nacimiento, R. Alcántara, G. F. Ortiz, J. L. Tirado, Acknowledgements CrystEngComm 2013, 15, 9196–9202. [24] R. Gnanamuthu, Y. N. Jo, C. W. Lee, J. Alloys Compd. 2013, 564, 95–99. This work was supported by the European Regional [25] G. Ferrara, C. Arbizzani, L. Damen, M. Guidotti, M. Lazzari, F. G. Vergottini, R. Inguanta, S. Piazza, C. Sunseri, M. Mastragostino, J. Development Funds and by the Spanish Ministerio de Economía Power Sources 2012, 211, 103–107. [26] J. He, H. Zhao, J. Wang, J. Wang, J. Chen, J. Alloys Compd. 2010, y Competitividad through the project SEHTOP (ENE2016- 508, 629–635. 77798-C4-3-R). J. Li thanks the China Scholarship Council for [27] J. Shin, W.-H. Ryu, K.-S. Park, I.-D. Kim, ACS Nano 2013, 7, 7330– 7341. scholarship support. This project was supported by the National [28] J. Zhang, Y. Xia, J. Electrochem. Soc. 2006, 153, A1466. Natural Science Foundation of China (no. 51771076), the “1000 [29] S. Wang, M. He, M. Walter, F. Krumeich, K. V. Kravchyk, M. V. Kovalenko, Nanoscale 2018, 10, 6827–6831. plan” from Chinese Government, and the Project of Public [30] M. Walter, S. Doswald, F. Krumeich, M. He, R. Widmer, N. P. Stadie, Interest Research and Capacity Building of Guangdong Province M. V. Kovalenko, Nanoscale 2018, 10, 3777–3783. [31] X. L. Wang, W. Q. Han, J. Chen, J. Graetz, ACS Appl. Mater. (no. 2017A010104004). TZ, PYT, MFIC and JA acknowledge Interfaces 2010, 2, 1548–1551. funding from Generalitat de Catalunya 2017 SGR 327 and the [32] L. O. Vogt, C. Villevieille, J. Mater. Chem. A 2017, 5, 3865–3874. [33] W. X. Lei, Y. Pan, Y. C. Zhou, W. Zhou, M. L. Peng, Z. S. Ma, RSC Spanish MINECO coordinated projects between IREC and ICN2 Adv. 2014, 4, 3233–3237. VALPEC and subprojects RESOL and ANAPHASE (ENE2017- [34] Y. M. Lin, P. R. Abel, A. Gupta, J. B. Goodenough, A. Heller, C. B. Mullins, ACS Appl. Mater. Interfaces 2013, 5, 8273–8277. 85087-C3). ICN2 acknowledges support from the Severo Ochoa [35] Z. Liu, T. Song, U. Paik, J. Mater. Chem. A 2018, 6, 8159–8193. [36] M. Walter, S. Doswald, M. V. Kovalenko, J. Mater. Chem. A 2016, 4, Programme (MINECO, Grant no. SEV-2013-0295) and is funded 7053–7059. by the CERCA Programme / Generalitat de Catalunya. JL is a [37] M. He, M. Walter, K. V. Kravchyk, R. Erni, R. Widmer, M. V. Kovalenko, Nanoscale 2015, 7, 455–459. Serra Húnter Fellow and is grateful to ICREA Academia program [38] J. Zhu, D. Wang, T. Liu, C. Guo, Electrochim. Acta 2014, 125, 347– Y. Ji, D. Wang, et al., Adv. Mater. 2016, 28, 4747–4754. 353. [52] C. H. Cui, X. J. Liu, H. H. Li, M. R. Gao, H. W. Liang, H. Bin Yao, S. [39] X. Liu, J. Xie, H. Zhao, P. Lv, K. Wang, Z. Feng, K. ͆wierczek, Solid H. Yu, ChemCatChem 2012, 4, 1560–1563. State Ionics 2015, 269, 86–92. [53] N. Mahmood, C. Zhang, F. Liu, J. Zhu, Y. Hou, ACS Nano 2013, 7, [40] F. Nacimiento, P. Lavela, J. L. Tirado, J. M. Jiménez-Mateos, J. 10307–10318. Solid State Electrochem. 2012, 16, 953–962. [54] C. Lupu, J. G. Mao, J. W. Rabalais, A. M. Guloy, J. W. Richardson, [41] S. Il Lee, S. Yoon, C. M. Park, J. M. Lee, H. Kim, D. Im, S. G. Doo, Inorg. Chem. 2003, 42, 3765–3771. H. J. Sohn, Electrochim. Acta 2008, 54, 364–369. [55] K. Kravchyk, L. Protesescu, M. I. Bodnarchuk, F. Krumeich, M. [42] P. Chen, L. Guo, Y. Wang, J. Power Sources 2013, 222, 526–532. Yarema, M. Walter, C. Guntlin, M. V. Kovalenko, J. Am. Chem. Soc. [43] H.-S. Kim, J. B. Cook, S. H. Tolbert, B. Dunn, J. Electrochem. Soc. 2013, 135, 4199–4202. 2015, 162, A5083–A5090. [56] R. Hu, Y. Ouyang, T. Liang, H. Wang, J. Liu, J. Chen, C. Yang, L. [44] V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 2014, 7, Yang, M. Zhu, Adv. Mater. 2017, 29, 1605006. 1597–1614. [57] Y. He, A. Li, C. Dong, C. Li, L. Xu, Chem. - A Eur. J. 2017, 23, [45] T. Brezesinski, J. Wang, S. H. Tolbert, B. Dunn, Nat. Mater. 2010, 9, 13724–13733. 146–151. [58] J. Qin, X. Zhang, N. Zhao, C. Shi, E. Liu, J. Li, C. He, RSC Adv. [46] L. Yang, X. Li, S. He, G. Du, X. Yu, J. Liu, Q. Gao, R. Hu, M. Zhu, J. 2014, 4, 49247–49256. Mater. Chem. A 2016, 4, 10842–10849. [59] J. Qin, D. Liu, X. Zhang, N. Zhao, C. Shi, E. Z. Liu, F. He, L. Ma, Q. [47] J. Li, Z. Luo, F. He, Y. Zuo, C. Zhang, J. Liu, X. Yu, R. Du, T. Zhang, Li, J. Li, et al., Nanoscale 2017, 9, 15856–15864. M. F. Infante-Carrió, et al., J. Mater. Chem. A 2018, DOI [60] Z. Wang, D. Wang, S. Luo, S. Bao, Y. Liu, X. Qi, C. He, C. Shi, N. 10.1039/C8TA08242A. Zhao, New J. Chem. 2016, 41, 393–402. [48] H. Fjellvåg, A. Kjekshus, R. Stomberg, R. Zingales, I. Vikholm, F. [61] C. Chen, Y. Wen, X. Hu, X. Ji, M. Yan, L. Mai, P. Hu, B. Shan, Y. Urso, J. Weidlein, R. A. Zingaro, Acta Chem. Scand. 1986, 40a, 23– Huang, Nat. Commun. 2015, 6, 6929. 30. [62] T.-C. Liu, J. Electrochem. Soc. 1998, 145, 1882. [49] M. He, L. Protesescu, R. Caputo, F. Krumeich, M. V. Kovalenko, [63] X. Xu, S. Ji, M. Gu, J. Liu, ACS Appl. Mater. Interfaces 2015, 7, Chem. Mater. 2015, 27, 635–647. 20957–20964. [50] J. Li, Z. Luo, Y. Zuo, J. Liu, T. Zhang, P. Tang, J. Arbiol, J. Llorca, A. [64] X. Xu, J. Liu, J. Liu, L. Ouyang, R. Hu, H. Wang, L. Yang, M. Zhu, Cabot, Appl. Catal. B Environ. 2018, 234, 10–18. Adv. Funct. Mater. 2018, 28, 1707573. [51] Y. Liu, X. Liu, Q. Feng, D. He, L. Zhang, C. Lian, R. Shen, G. Zhao, TOC graph Colloidal Co-Sn nanoparticles as the application of lithium ion batteries.