High Conductivity LiFePO[sub 4]∕C Composite Thin Films with Ti Underlayers Deposited by Radio Frequency Sputtering

Abstract

LiFePO 4 /C thin films have been deposited on Ti-coated Si and stainless-steel substrates by radio frequency magnetron sputtering. Negative electric bias ͑0,−50,−80 V͒ and postdeposition vacuum annealing ͑150-650°C͒ was applied to modify the film properties. Suitable substrate bias resulted in films with finer grains and pure olivine phase. The films deposited with Ti underlayers exhibited enhanced crystallization, grain sizes, and conductivity, as compared with films deposited without Ti underlayers. Composition depth profiles showed interdiffusion at the film/underlayer interfaces after annealing. Charge-discharge and cyclic voltammetry curves revealed the different electrochemical characteristics of the films. The films with Ti underlayers displayed lower initial capacity than those without underlayers due to heavy doping of Ti. However, the cycling stability is greatly improved, which was attributed to the enhanced adhesion property of the Ti underlayers to the substrates and films.

Influence of titanium nitride interlayer on the morphology, structure and electrochemical performance of magnetron-sputtered lithium iron phosphate thin films

“…As a consequence, a layer with LieTi composition forms at the interface which can be observed by the bump in the LiTi þ signal. The observed Ti interdiffusion agrees well with Chiu et al [12].…”

“…Such a heat treatment can intensify interdiffusion as shown by Chiu et al for LFP thin films on substrates with a titanium interlayer [12]. The effect of interdiffusion between substrate and LFP film was also observed by Legrand et al on MgO substrates [20].…”

“…1. Chiu et al also reported a Li 3 Fe 2 (PO 4 ) 3 phase in their XRD pattern of LFP thin-film which was deposited on Ti [12]. The reflection peaks from the 160 nm thick LFP thin film deposited on TiN interlayer can either be attributed to the LFP thin film or to the substrate.…”

Low Temperature Deposition of Polycrystalline Lithium Transition Metal Oxide Thin Films for Micro Batteries

“…The conventional cathode materials, such as LiCoO 2 , LiMn 2 O 4 , and LiFePO 4 , of lithium/lithium ion batteries are also strong candidates for the cathodes of micro batteries. The minimization of such materials is best achieved by thin film technologies, such as sol-gel (1-3) and sputtering (4)(5)(6) techniques. The thin film cathode materials are characterized by three main categories (7)(8)(9)(10): layered oxides, which are composed of hexagonal close packed oxygen atom networks with lithium and transition metal ions on alternating (111) planes, such as LiCoO 2 , LiNiO 2 and LiCo x Ni 1-x O 2 ; spinels, such as LiMn 2 O 4 , composed of three dimensional octahedral and tetrahedral frameworks; polyanion compounds, such as LiFePO 4 , composed of tetrahedral anion mainframes.…”

“…There have been substantial amount of papers regarding preparation of these thin film cathodes. However, most of them showed that post or in-situ high temperature annealing is inevitable in order to fabricate well-crystallized thin film cathodes (1)(2)(3)(4)(5)(6)(7)(8). Although amorphous or nano-crystalline thin films were also used as cathode materials (11), they generally exhibit low specific capacity and sloppy discharge plateau, compared with the polycrystalline counterparts.…”