A study of the thin film battery electrolyte lithium phosphorus oxynitride deposited by an ion beam assisted process

Thin film Li-ion batteries are currently the subject of a world-wide research effort because of their many potential applications as portable energy sources. One of the key elements of these batteries is the electrolyte. Since it was first produced in the early 1990's, the preferred solid state thin film Li-ion electrolyte is lithium phosphorus oxynitride (LiPON), which is normally grown by means of reactive rf sputtering of a Li₃PO ₄ target in an N₂ atmosphere. Solid electrolytes such as LiPON have several advantages compared to the liquid electrolytes normally used in bulk batteries. Solid electrolytes avoid leakage and have excellent charge-discharge cycling properties. Furthermore, sputtered-deposited LiPON proved to be stable versus Li+/Li from 0 to +5.5 V, which exceeded the stability window of any of the liquid electrolytes.; In this work we present a general study of the properties of LiPON thin films deposited by an alternative process: ion beam assisted deposition (IBAD). In this process Li₃PO₄ is vacuum thermally evaporated and the condensing film is simultaneously bombarded with nitrogen ions which incorporate to form LiPON. Because of its application as an electrolyte and because of a previous study in which we showed that tensile stress led to cracking of the LiPON films and subsequently to shorting of the battery devices, the emphasis of the study was placed on improving the electrochemical properties of the films and on reducing their residual stress. Additional effort was aimed at learning about the structure and the composition of our films. It has been shown that IBAD LiPON thin films are undoubtedly capable of high quality performance as the electrolyte in Li-ion thin film batteries. Their ionic conductivity is almost as high, and their electronic conductivity as low, as those of the sputtered films. Their major advantages when compared to sputtered LiPON films are: (i) a higher deposition rate; (ii) a lower concentration of reduced-phosphorus in IBAD films which makes them less hazardous and particularly more stable against relatively high oxidizing potentials (>8 V); and (iii) this higher decomposition voltage which allows faster charging of batteries by safely permitting higher overpotentials. Regarding the residual stress, it has been shown that it is mostly of thermal origin, which indicates that it can be greatly reduced by cooling the samples during the deposition process and/or by reducing the sources of heating energy.