TIN NANONEEDLES FOR LI-ION BATTERIES: GROWTH MECHANISMS, THICKNESS, AND PHASE CONTROL
Owen, Craig Daniel
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One-dimensional tin nanostructures (tin nanoneedles) have been shown to be an attractive alternative to traditional graphite electrodes due to low fabrication cost, a demonstrated reversible nanostructure, and a capacity of up to 850 mAh/g (~3x graphite). The nanoneedles are synthesized using a template-free facile electrodeposition process without the use of electrolyte additives or surfactants, and the growth is substrate independent due to the formation of an interfacial tin film on the substrate surface prior to nucleation. After nucleation, nanoneedle growth is dependent on obtaining 3D spherical diffusion conditions, and requires a critical Nernst diffusion layer thickness (mass transport rate). Electrodeposited nanoneedle density is mass transport controlled, and can altered by the variation of solution agitation, tin concentration, temperature, and cathodic current density. The interfacial tin film formed before nanoneedle nucleation/growth causes the overall electrochemical performance of the tin nanoneedle electrode in Li-ion half-cells to mimic that of a thick tin film – rapid capacity fade with macroscale cracking/pulverization of the electrode. Reducing the interfacial tin thickness through changing deposition conditions from galvanostatic to potentiostatic improves cycling stability to 80% capacity by ~38%. When potentiostatic deposition is coupled with Li-Sn alloy phase control, significant cycling stability improvements (from 15 cycles to >60 or >100 cycles) of ~430% or ~750% are obtained depending on the Li-Sn phases cycled. These increases however come with a tradeoff of lower overall electrode capacity due to selective Li-Sn alloy formation. Improved stability is obtained by avoiding or minimizing the large density transition observed between LiSn and Li2.33Sn which is identified as the cause of electrode degradation. By controlling the electrode cutoff potentials, phase control during cycling is obtained which is shown to be an effective method to improve tin nanoneedle electrode cycling stability. This phase control method also opens the possibility of improved cycling stability in other tin film and tin powder-based negative electrodes for Li-ion batteries.