Synthesis and Electrochemical Performances of SnS Anode and Sn-Ag-O etc. Thin Films Anodes for Lithium Ion Batteries
|Keywords||Lithium ion batteries Anode materials Nanomaterials SnS Carbon coating Carbon-aerogel Sn-based thin film anode Electrochemical performances|
As a potential alternative for commercial carbon anode in lithium ion batteries, Sn-based anode materials have attracted much attention for larger specific capacity and specific energy in recent years. Unfortunately, Sn-based anode materials have poor cyclability and lower first coulombic efficiency, which prevent Sn-based anode materials from practical usage. In fact, the characteristics of Sn-based anode have close relationship with the components, preparation methods, sizes and structures of materials. In this present work, SnS anode materials for lithium ion batteries were synthesized by ball-milling, micro-wave assist and chemical precipitation. Furthermore, SnS was coated or combined with carbon to increase its electrochemical performances. Sn-Ag-O and Ag-C thin films were deposited by magnetron sputtering as the anode for thin film lithium ion batteries. All as-prepared anodes were investigated and discussed on their structures, morphologies and electrochemical performances.Layered SnS, with particle sizes of 150-200 ran and 60-70 nm, were obtained respectively by milling the mixture of Sn powder and S powder in 250 ml ball-milling vessel for 50 h and 100 h. The average particle sizes of ball-milled SnS in 300 ml vessel for 50 h, 100 h and 250 h were 20-30 nm. The ex-situ XRD investigation of SnS revealed the lithiation/ delithiation mechanism of SnS during the first discharge/charge process. During discharge, SnS was decomposed into Sn and LixS. Then, lithium ions reacted with Sn to form Li-Sn alloy. In charge process, lithium ions extracted from Li-Sn alloy. In the following cycles, LixS phase played a role of buffer matrix without electrochemical activation toward lithium ions. For ball-milled SnS particles, the more the milling time was, the more capacity was and the better cyclability was. Lamellar SnS and nanosized SnS with sizes of less than 10 nm were respectively prepared by micro-assist and chemical precipitation. SnS nanorods can be obtained with addition of CTAB surfactant during chemical precipitation process. Generally, the electrochemical performances of SnS particles were better than lamellar SnS and SnS nanorods. And, SnS particles with smallest size showed the best electrochemical performance. It was believed that smaller particle size could decrease the distances for lithium ion diffusion, facilitate the contact between SnS and electrolyte and weaken the impact of volume change from Li-Sn alloy.Carbon-coated SnS were prepared via calcination of the mixture of SnS and PVA with weight ration of 1: 1 under 400℃、 500℃、 600℃、 700℃ and 800℃. The thickness of amorphous carbon coating calcinated at 700℃ was 4-5 nm. Carbon-coated SnS prepared at 400℃ showed the highest coulombic efficiency of 66.2%, while SnS without carbon-coating presented only 31.3%. Carbon-coated SnS prepared at 700℃ exhibited the best electrochemical performances and its rate capability was superior to that of uncoated SnS. The carbon coating could enhance the conductivity of electrode and buffer the volume change during discharge/charge. From the EIS test results, it was concluded that the carbon coating was in favor of charge transfer process kinetically and beneficial for lithiation/ delithiation.SnS/carbon aerogel nanocomposites were prepared by calcination of SnS/carbon gel with different amount of SnS at 650℃ under Ar gas protection. 72 wt.% SnS/carbon aerogel nanocomposites showed net-like structure with net pore-sizes of 20-30 nm and nanoscale SnS distributed in net structure uniformly. 48 wt.% SnS/carbon aerogel nanocomposites presented tree-like structure in which nanoscale SnS existed non-homogeneously. For 16 wt.% SnS/carbon aerogel nanocomposites, carbon aerogel exhibited regular sphere shape and nanoscale SnS presented outside of the spheres. With decreasing the amount of SnS, the first coulombic efficiency of SnS/carbon aerogel nanocomposites increased, but the cyclability decreased. The excellent cyclability of 72 wt.% SnS/carbon aerogel nanocomposites was dependent on the buffering effects of net-like carbon aerogel. Moreover, net-like structure could provide more diffusion paths for lithium ions in electrolyte.With increasing the depositing temperature, the XRD diffraction peaks of Sn and Sn-Ag alloy in the as-deposited thin films became more evident. While the first discharge capacity and coulombic efficiency gradually decreased. Under various depositing powers, Ag was always crystalline in the deposited thin films. The glass characteristics of Sn, SnO and Sn-Ag alloy were weakened with the decrease of Sn content. The discharge capacities and coulombic efficiency were in direct proportion with Sn content, but the cyclability deteriorated with increasing the amount of Sn. Depositing bias voltage showed no obvious effects on the structure and electrochemical performances of deposited thin films. Crystal Sn was obtained in Sn2.3AgO0.9 thin film after annealed at 200℃ and its electrochemical performances were improved. After annealed at 500℃, the thin film was mainly composed of crystal SnO2 and Ag with large grains, which lead to poor electrochemical performances. Multi-layers Ag/SnOx/Ag/SnOx/Ag thin film showed better cyclability after annealed at 200℃ due to the buffer effects of outermost Ag layer. The as-deposited Ag0.35C0.65 thin film was composed of amorphous carbon and crystal Ag. The Ag particles or clusters with about 10 nm diameter distributed homogeneously in amorphous carbon. The cycling capacity of Ag0.35C0.65 thin film descended rapidly in first several cycles and subsequently stabilized. The capacity was maintained at 1030 μAh/cm2·μm up to 100 cycles. The preferable conductivity and stable structures of Ag and carbon during discharge/charge process guaranteed outstanding electrochemical performances of Ag0.35C0.65 thin film.