Abstract:
Lithium-ion batteries have rapidly become important components of many devices thatrequire portable power, be it a CD player, implanted medical device, or hybrid-electric automobile. The battery has three main parts – a cathode where reduction occurs (usually a highly oxidized metal oxide), an organic electrolyte to conduct ions, and an anode where oxidation occurs (usually lithium metal or graphite). In many of these devices, the anode is the limiting factor in either device performance or device cost. Present anodes work well, but have many safety issues – notably overcharge problems or lifetime problems related to lithium's unique electrochemistry. Alternative anode materials, typically Sn- or Sb-based alloys, have several problems, including high irreversible capacity, which is the amount of lithium inserted on the first charge that does not return to the anode on discharge. A significant amount of this "lost lithium" is involved in side reactions on the anode surface between the charged anode, e.g. Li3Sb, and the organic electrolyte forming insoluble lithium salts and complexes. The purpose of this study was to attempt to lower the irreversible capacity by adding silver to the electrode surface, thereby altering the interface between the electrode and the electrolyte.Silver coatings were deposited on anodes using four different methods: (1) silver salt reduction by aldehydes, (2) in-situ decomposition of silver salts in the electrolyte, (3) a surface coating of an insoluble silver salt, and (4) a physical mixture of silver metal and the active material. Each method was found to produce different results. The silver coatings via the aldehyde reduction (Tollens Reaction) improved the performance of the cells containing synthetic (GDR) graphite, while cells using a Cu6Sn5 alloy were less pronounced, although it seemed to increase rate capability. The in-situ deposition method decreased the irreversible capacity for the Cu6Sn5 anode, while the ball-milled physical mixture made the irreversible capacity worse, probably due to the oxidation of the underlying electrode materials. Overall, silver coatings look very promising, but more work needs to be done yet.
