Developing a-SiNx:H anodes for lithium ion batteries

Abstract

Lithium-ion batteries are the most widely used energy storage devices, and currently dominate the portable electronic device market and automotive industry, owing to their superior energy density and long cycle life. Currently, the market of lithium-ion batteries is dominated by graphite as a material for anode. This material has proven to be very stable and reliable. However, the capacity delivered by graphite is quite low (372 mAh.g−1). As a result, there is a need to explore alternative anode materials which can deliver a superior capacity, and thereby a higher energy density. A promising alternative to overcome this limited capacity issue is the use of silicon anodes, which can theoretically deliver a specific capacity of 3579 mAh.g−1. However, silicon is prone to volumetric change (almost 200-300% of its original volume) upon cycling. This leads to loss of active material due to pulverization and delamination, severely affecting its capacity retention and hence the battery cycle life. Of the several ways in which the capacity retention of silicon anodes can be improved, we have combined two approaches: alloying silicon with nitrogen and making this material porous. This work aims at investigating the influence of the chemical composition, porosity and mass loading of the SiNx anodes developed, on the electrochemical performance when used as an anode for lithiumion battery. The monolithic SiNx anodes used in this work were fabricated using Plasma Enhanced Chemical Vapor Deposition (PECVD) and were deposited directly on copper foil, which is the most widely used current collector material for batteries. By varying the relative flow rate of silane and ammonia (precursor gases), and by varying the deposition power, the composition of the anode, its porosity and mass loading can be varied. The resultant anodes were employed to assemble coin cell against lithium metal as the counter electrode. Compositional analysis revealed that the anodes deposited at different power and at a particular flow-rate ratio (flow of ammonia to the total flow of gas) lead to the same stoichiometry of SiNx. When tested as anode for lithium-ion battery, the material with composition SiN0.32 could deliver a specific capacity of 1567 mAh.g−1 at a current density of 75 mA.g−1, which compares to a C-rate of C/20. With an increased nitrogen content in SiNx a decrease in the specific capacity and better capacity retention was observed. Furthermore, this work directs that a good balance of porosity and areal mass loading of SiNx is crucial to achieve high performance lithium-ion battery anode.