This is the accepted version of the following article: Li J., Xu X., Luo Z., Zhang C., Yu X., Zuo Y., Zhang T., Tang P., Arbiol J., Llorca J., Liu J., Cabot A.. Compositionally tuned Ni x Sn alloys as anode materials for lithium-ion and sodium-ion batteries with a high pseudocapacitive contribution. Electrochimica Acta, (2019). 304. : 246 - . 10.1016/j.electacta.2019.02.098, which has been published in final form at https://dx.doi.org/10.1016/j.electacta.2019.02.098 © https://dx.doi.org/10.1016/j.electacta.2019.02.098. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Compositionally tuned NixSn alloys as anode materials for lithium-ion and sodium-ion batteries with a high pseudocapacitive contribution Junshan Li,a,b, Xijun Xu,c,d, Zhishan Luo,a Chaoqi Zhang,a Xiaoting Yu,a Yong Zuo,a Ting Zhang,e Pengyi Tang,e Jordi Arbiol,e,f Jordi Llorca,g Jun Liu,c,d,* Andreu Cabota,f,* a Catalonia Institute for Energy Research - IREC, Sant Adrià de Besòs, Barcelona, 08930, Spain b Departament d’Electronica, Universitat de Barcelona, 08028 Barcelona, Spain c Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, P. R. China d SUNWODA-SCUT Joint Laboratory for Advanced Energy Storage Technology, South China University of Technology, Guangzhou 510641, P. R. China 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 Engineering and Barcelona Research Center in Multiscale Science and Engineering. Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain ABSTRACT Nickel tin alloy nanoparticles (NPs) with tuned composition NixSn (0.6 ≤ x ≤ 1.9) were synthesized by a solution-based procedure and used as anode materials for Li-ion batteries (LIBs) and Na-ion batteries (SIBs). Among the compositions tested, Ni0.9Sn-based electrodes exhibited the best performance in both LIBs and SIBs. As LIB anodes, Ni0.9Sn-based electrodes delivered charge- discharge capacities of 980 mAh g-1 after 340 cycles at 0.2 A g-1 rate, which surpassed their maximum theoretical capacity considering that only Sn is lithiated. A kinetic characterization of the charge- discharge process demonstrated the electrode performance to be aided by a significant pseudocapacitive contribution that compensated for the loss of energy storage capacity associated to the solid-electrolyte interphase formation. This significant pseudocapacitive contribution, which not only translated into higher capacities but also longer durability, was associated to the small size of the crystal domains and the proper electrode composition. The performance of NixSn-based electrodes toward Na-ion storage was also characterized, reaching significant capacities above 200 mAh g-1 at 0.1 A g-1 but with a relatively fast fade over 120 continuous cycles. A relatively larger pseudocapacitive contribution was obtained in NixSn-based electrodes for SIBs when compared with LIBs, consistently with the lower contribution of the Na ion diffusion associated to its larger size. Keywords: colloidal bimetallic nanoparticles; nickel tin alloy; anode materials; lithium-ion batteries; sodium-ion batteries. 1 INTRODUCTION Lithium ion batteries (LIBs) have become essential in portable electronic applications. However, while widely spread in the market, current LIBs are far from being optimized electrochemical energy storage devices.[1–5] LIBs still suffer from moderate durabilities and current densities, which are in large part associated to the limitations of actual electrode materials. Additionally, the availability of lithium poses middle-long term limitations to this technology.[6,7] Thus, the development of improved electrode materials for LIBs and alternative battery technologies based on more abundant ions, such as sodium, is a worthwhile endeavor. Current commercial LIBs use graphite as anode material, what limits the theoretic maximum energy density to 375 mAh g-1.[8–11] On the other hand, sodium ion batteries (SIBs) cannot make use of graphite due to the insignificant Na-insertion in this material.[12–14] As an alternative anode material for both LIBs and SIBs, Sn and Sn-based alloys have been extensively studied due to their abundance, low toxicity and high energy density, 992 mAh g-1 for LIBs and 847 mAh g-1 for SIBs, corresponding to the formation of Li22Sn5 and Na15Sn4, respectively.[15–18] However, Sn undergoes huge volume changes during charge-discharge cycles that shorten its usage time.[19–24] This drawback can be partially overcome by reducing the size of the crystal domains in the electrode. The use of nanostructured electrodes provides additional advantages in terms of increasing rate capability, because of the shorter Li-ion diffusion paths, and increasing the pseudocapacitive contribution associated with the larger surface/volume ratios.[25–30] An additional strategy to improve stability is to alloy tin with non-active elements, reducing in this way the volume changes and potentially increasing the pseudocapacitive contribution. In this direction, bimetallic Sn-based allows such as Cu- Sn,[31–33] FeSn,[33–35] Co-Sn,[34–53] and Ni-Sn[17,33,54–56] have been tested as base materials for LIB and/or SIB electrodes with excellent results. Not considering pseudocapacitive effects, the main drawback of alloying Sn with non-active metals is the decrease of the maximum energy density potentially achieved with the amount on non-active metal introduced. Thus, the alloy composition needs to be finely and continuously tuned along the whole solid-solution range to find the optimal composition. However, most previous works have focused on studding the performance of intermetallic Sn-based alloys, with strongly constrained compositions. In the present work, we take advantage of the versatility of colloidal synthesis method to produce nanoparticles (NPs) of a range of Ni-Sn solid solutions with Ni:Sn ratios from 0.6 to 1.9. After removing surface ligands, we use these NPs to test the performance Ni-Sn solid solutions as anode materials for LIBs and SIBs. 2 EXPERIMENTAL Chemicals: Nickel(II) acetylacetonate (Ni(acac)2·xH2O (x∼2), 95%, Sigma-Aldrich), tin(II) acetate (Sn(oac)2, 95%, Fluka), oleic acid (OAc, Sigma-Aldrich), oleylamine (OAm, 80-90%, TCI), tri-n- octylphosphine (TOP, 97%, Strem), borane tert-butylamine complex (TBAB, 97%, Sigma-Aldrich), TIMCAL Graphite & Carbon Super P ( Super P, KJ group), polyvinylidene fluoride (PVDF, KJ group), N-methy1-2-pyrrolidone (NMP, 99%, Aladdin), hydrazine monohydrate (N2H4 64-65%, reagent grade, 98%, Sigma-Aldrich) and acetonitrile (CH3CN, extra dry, Fisher) were used as received without any further purification. Chloroform, acetone and ethanol were of analytical grade and purchased from various sources. An argon-filled glove-box was used for storing and dealing with sensitive chemicals. Colloidal Synthesis of NiSn NPs: All the syntheses were carried out using standard airless techniques using a vacuum/dry argon gas Schlenk line. Ni-Sn NPs were prepared following the scaled-up version of a protocol we previously detailed.[57] Briefly, 20 mL OAm, 1.0 mL OAc, 0.9 mmol Ni(acac)2·xH2O and 0.6 mmol Sn(oac)2 were loaded into a 50 mL three-neck flask containing a magnetic stirring bar. The reaction was strongly stirred and degassed under vacuum at 80 °C for 2 hours to remove water, air, and low-boiling point impurities. Then, a gentle flow of argon was introduced, 5 mL of TOP were injected and the reaction flask was heated to 180 °C at 5 °C/min. Meanwhile, a reducing solution was prepared by dissolving 5 mmol TBAB in 5 mL OAm through sonication for 30 min, and subsequently degassed this mixture for 1 hour at ambient temperature. This reducing solution was injected into the reaction flask containing the Ni and Sn precursor at 180 °C. Upon injection, a visible color change, from deep green to black, was immediately observed. The reaction was maintained at 180 °C for 1 hour, followed by a rapid cool down to room temperature using a water bath. The content of the reaction mixture was transferred to two centrifuge tubes, followed by centrifuging at 9000 rpm for 3 min after introducing acetone as non-solvent. The precipitate was suspended in chloroform and centrifuged again after adding additional acetone. This process was repeated twice. Finally, the NPs were suspended in 10 mL chloroform. Ligand removal: The native organic ligands were removed from the NP surface according to a previously published report.[28,58] Briefly, 25 mL acetonitrile and 0.8 mL hydrazine hydrate was introduced into a vial containing the precipitated NPs. The mixture was stirred for 4 hours at room temperature and then collected by centrifuging at low speed. The product was further washed with acetonitrile and centrifuged at 2000 rpm for another 3 times. NPs were collected and stored in inert air atmosphere after drying under vacuum at room temperature. Characterization: Powder x-ray diffraction (XRD) was measured on a Bruker AXS D8 Advance x- ray diffractometer with Cu K radiation (λ = 1.5106 Å) operating at 40 kV and 40 mA. Scanning electron microscopy (SEM) analyses were performed on a ZEISS Auriga SEM with an energy 3 dispersive X-ray spectroscopy (EDS) detector at 20 kV. Transmission electron microscopy (TEM) analyses were carried out on a ZEISS LIBRA 120, operating at 120 kV, using a 200 mesh Carbon- coated grid from Ted-Pella as substrate. 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. For the ICP, 5 mg sample dessolved in 10 ml freshly prepared Agua Regia. 0.5 ml were taken into 24.5 ml MilliQ water. X-ray photoelectron spectroscopy (XPS) was done on a SPECS system equipped with an Al anode XR50 source operating at 150 mW and a Phoibos 150 MCD-9 detector. The pressure in the analysis chamber was kept below 10-7 Pa. The area analyzed was about 2 mm x 2 mm. The pass energy of the hemispherical analyzer was set at 25 eV and the energy step was maintained at 1.0 eV. Data processing was performed with the Casa XPS program (Casa Software Ltd., UK). Binding energies were shifted according to the reference C 1s peak that was located at 284.8 eV. The Fourier transform infrared spectrometer (FTIR) data were recorded on an Alpha Bruker spectrometer. Electrochemical measurements: Ni-Sn NPs (80 wt%) were mixed with Super P (10 wt%), PVDF (10 wt%) and NMP. The obtained slurry was bladed onto a copper foil and dried at 80 ºC for 24 h in a vacuum oven. Working electrodes were obtained by cutting the printed foil into circular disk with a diameter of 12.0 mm. The mass loading of active materials was estimated to be 0.7-1.2 mg cm−2. To test the performance of electrodes based on Ni-Sn NPs, half cells were assembled in the glove box (H2O and O2 < 0.1 ppm) using Celgard2400 as separator. As electrolyte for LIBs, a 1 M LiPF6 solution in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) with 5 vol% fluoroethylene carbonate (FEC) as additive was used. For SIBs, 1 M NaClO4 in propylene carbonate (PC)/ EC (1:1 in volume) with 5 vol% FEC was used as the electrolyte. Galvanostatic charge-discharge were measured by a battery test system (CT2001A, LAND) with cutoff potentials from 0.01 V to 3.0 V. Cyclic voltammograms (CV) were obtained using an electrochemical workstation (Gamry Interface 1000) in the voltage range of 0–3.0 V at scan rates from 0.1 mV s-1 to 1 mV s-1. 4 RESULTS AND DISCUSSION NixSn NPs with tuned composition (0.6