Exploring Electrochemical Energy Storage of SnO2, Sn3N4, and SnOxNy through Ald 

Tuesday, 3 October 2017: 08:40
Chesapeake L (Gaylord National Resort and Convention Center)
D. M. Stewart, A. Pearse (University of Maryland), K. Gregorczyk (University of Maryland, College Park), and G. W. Rubloff (Institute for Systems Research)
As conversion electrodes in lithium ion batteries, Sn and SnO2 have been well studied in the literature, while the properties of Sn3N4 and SnOxNy have yet to be been explored in any detail. To study the differences in the electrochemical performance of SnO2, Sn3N4, and SnOxNy, an ALD process was developed which allows for a controlled N/O ratio. This process uses tetrakis(dimethylamido)tin (TDMAS) as a precursor with remote nitrogen plasma and H2O. As with many nitride ALD processes, obtaining Sn3N4 films with low O and C contamination is challenging due to the greater reactivity of Sn with O, and incorporation of precursor ligands. Optimized process parameters for the pulse times and substrate temperature were obtained for the nitride process, which were incorporated into the oxynitride process. SnOxNy films were grown by periodically pulsing water into the reactor after several cycles of the nitride process. By varying the number of nitride cycles between water pulses, the relative concentration of O and N in the film can be controlled between 0% N and 95% N.

The Li2O matrix formed on conversion of SnO2 is expected to be more chemically stable than the Li3N matrix formed from Sn3N4, but at the cost of lower ionic conductivity. Solid-state half cells were constructed using SnO2, Sn3N4, and SnOxNy thin films versus thermally evaporated Li with ALD LiPON as the electrolyte. The electrochemical properties of the anodes were tested by cyclic voltammetry and galvanostatic cycling. Only a single narrow conversion reaction was observed, regardless of film composition, but the voltage at which the reaction occurred shifted as a function of the N/O ratio. At voltages below 0.4 V vs Li/Li+ the half cells shorted, possibly due to mechanical breakdown of the LiPON layer from significant volume expansion of the anodes during the alloying reaction with Li. However, the stability of the cells was improved by varying the composition and thickness of the anodes. Galvanostatic intermittent titration and impedance spectroscopy were used to analyze the ionic conductivity of the anodes before and after the initial conversion reaction and as the N/O ratio was varied. The high energy storage capacity of SnOxNy electrodes, which can be deposited conformally by ALD, makes them attractive materials for applications in thin film, solid state devices, where the particle aggregation and delamination effects are limited by the physical constraints of the device.