Self-Assembly Of Copper And Silicon Nanoparticles For Lithium-Ion Battery Anodes

Lithium-ion batteries?LIBs?are used as the power sources of laptops,cellphones,cameras,etc,owing to their outstanding characters such as high energy density,no memory effect,and high work potential.Nevertheless,high capacity and charge rate are still desired to satisfy their wide applications.Current collectors and active electrode materials are important elements in LIBs and ultimately determine the cycling performance.The traditional flat current collectors cannot tightly contact the electrode materials especially during the charge-discharge?C-D?processes because of the volume change of the electrode materials,heat generation,etc,leading to the detachment of the electrode materials.Silicon?Si?shows higher theoretically gravimetric and volumetric capacities than the commercial graphite anodes,but its capacity fades rapidly resulting from the huge volume change of 300–400%and the subsequent smash,coalescence,detachment from the currrent collectors and poor electrical contact.A lot of special hierarchical structures have been prepared by self-assembly of nanoparticles,which can preserve the high surface area and effectively adjust the structural features by controlling the assembly process.Herein,we prepared a three-dimensional?3D?porous copper as anode current collectors by electrical field-induced self-assembly of copper nanoparticles?Cu NPs?,to enhance the physical and electrical contact with the electrode materials and increase the battery capacity.We also fabricated polyurethane?PU?/Cu/Si nanocomposite film anodes with high stretchability and electrical conductivity by layer-by-layer?LBL?assembly of PU,Cu NPs and Si NPs through their electrostatic attraction effects to accommodate the Si volume change upon lithiation/delithiation processes and maintain the physical and electrical contact,and thus to improve the battery performance.We synthesized 2–10 nm Cu NPs in water at 45 ^oC without inert gas protection by chemical reduction of copper chloride by L-ascorbic acid with cetyltrimethylammonium bromide?CTAB?as ligand.The size of the Cu NPs decreased from 9.6 nm to 3.3 nm when the concentration of the CTAB solution increased to 0.01 M,and the size decreased from 4.6 nm to 2.3 nm with the increase of the L-ascorbic acid conenctration from 0.1 M to 0.4 M.Because the CTAB and L-ascorbic acid adhered on the Cu NP surfaces,the Cu NP aqueous solutions exhibited high chemical stability and dispersity under ambient atmosphere.We also synthesized 8.0–14.0 nm Cu NPs in water at 60 ^oC with hydrazine and mercaptosuccinic acid as reductant and stabilizer,respectively.The Cu NPs after redispersion in water self-assembled into chains with zeta potential of–47 mV.When the content of the hydrazine increased from 3.5 g to 7.0 g,the size of the Cu NPs decreased from 13.5 nm to 9.0 nm and the wavelength at the UV-vis absorbance peak decreased from 570 nm to 560 nm.We also prepared a 3D porous Cu by electrical field-induced self-asembly of the Cu NPs by adjusting the concentration of acetic acid and CTAB as additives,potential/current densities,assembly time,etc.The porous Cu with a dendritic structure had uniform pores of?5?m with an average pore size of 200 nm.The dendrites with an average length of 700 nm were composed of 40 nm clusters,while the clusters were constituted of 3.5 nm Cu NPs that interconnected closely with crystal lattices.The“cluster–dendrite”growth mechanism of the self-assembly process for the porous Cu was also elegantly described.3D porous Cu anodes were facilely fabricated by the traditional slurry coating method using the 3D porous Cu as current collector.The porous Cu electrodes with graphite loadings of17.0,11.5,7.0 and 3.0 mg cm^–2 exhibited higher specific capacities of 42.4,69.2,164.9 and242.0 mAh g^–1 with higher capacity retention rates of 24.8,36.2,62.2 and 82.9%than the traditional flat Cu electrodes(capacities of 23.6,50.1,126.4 and 182.2 mAh g^–1,and retention rates of 18.2,24.8,46.7 and 56.8%,repectively)after 120 C-D cycles at 0.25 C rate,respectively.The porous Cu electrodes also preserved a higher total capacity of 1138?Ah cm^–2 than the reported porous Cu anodes(?345?Ah cm^–2)prepared by chemical vapor deposition,electrodeposition,magnetic sputtering,etc.[?PU/Si??PU/Cu?_m]_N nanocomposite films were prepared by LBL assembly of PU,Cu NPs and Si NPs through their electrostatic attraction.The LBL composite films with the Si NPs encapsulated by the interconnected Cu NPs showed metal luster,and many micro-nano-sized pores formed in the films.The color of the films changed from yellow to dark brown and the electroconductivity increased from 0.4 to 12.0 S cm^–1,when increasing the assembly cycle?m?of the?PU/Cu?bilayer from 1 to 4.The nanocomposite films also displayed a high ultimate tensile strength of 10.7 MPa with a high ultimate tensile strain of 110.8%.[?PU/Cu-Si?_m?PU/Cu?]_N nanocomposite films were also prepared via another LBL assembly route.The electroconductivity of the nanocomposite films increased with the decrease of the m value and reached 8.0 S cm^–1.The nanocomposite films also had high stretchability with a high ultimate tensile strength of 8.2 MPa and a high ultimate tensile strain of 125.1%.The[?PU/Si??PU/Cu?_m]_N nanocomposite film electrodes showed low average capacity decay rates of 0.16–0.17%per cycle in 300 C-D cycles at 1.0 C rate and high single and average Coulombic efficiencies of 96.6–100.0%and 99.0–99.4%,respectively.The nanocomposite film electrodes also showed a high discharge capacity of 1284 mAh g^–1 at the first cycle,and high capacities of 1252,820,735 and 687 mAh g^–1 at the second,100^th,200^th and 300^th cycle,respectively,giving corresponding capacity decay rates of 0.36%,0.12%and 0.07%per cycle for the first,second and third hundred C-D cycle,respectively.Moreover,the capacity retention rates of the film electrodes were 29–43%when the C-D rate increased from 0.1 C to2.0 C,and then 66–80%when the C-D rate returned to 0.1 C.The[?PU/Cu-Si?_m?PU/Cu?]_N nanocomposite film electrodes also exhibited low average capacity fading rates of 0.17–0.19%per cycle in 300 C-D cycles at 1.0 C rate and high single and average Coulombic efficiencies of 97.1–101.0%and 99.2–99.8%,respectively.The nanocomposite film electrodes also exhibited high discharge capacities of 1152,1111,690,601 and 574 mAh g^–1 at the first,second,100^th,200^th and 300^th cycle,respectively,indicating corresponding capacity fading rates of 0.40%,0.13%and 0.04%per cycle for the first,second and third hundred C-D cycle,respectively.Furthermore,the nanocomposite film electrodes displayed higher capacity retention rates of 25–30%than the Si NP electrodes prepared by the slurry coating method?2–8%?when increasing the C-D rate from 0.1 C to 2.0 C,and retained64–73%when the C-D rate returned to 0.1 C.