Silicon Nitride Coated Silicon Thin Films As Anodes for Li-Ion Batteries
40 nm silicon thin films were deposited by PECVD on copper foil using silane as precursor. The nitride was formed by addition of ammonia to the gas flow in the late stages of the deposition, and different stoichiometries were obtained by changing the ratio of these gases. By varying the flow rate of ammonia, coatings with four different compositions were made; pure silicon (A), stoichiometric Si3N4(D) and two intermediate compositions (B and C). Three electrodes were punched from each of the resulting films and mounted in coin cells with lithium metal counter electrodes and cycled at a current rate of C/3 for 150 cycles. During cycling, all the cells exhibited an initial increase in capacity, peaking at close-to-theoretical capacity within 10-40 cycles before beginning a slow decline. The average charge capacity for each series after 50, 100 and 150 cycles is presented in the figure, showing that the capacity retention increases with increasing nitrogen content for series A, B and C, and then decreases for series D. This indicates that a nitrogen coating has a positive effect on the deterioration mechanisms of the electrode, and that this effect increases with increasing nitrogen content of the coating, as long as a stoichiometric nitride is not formed.
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