Preparation And Electrochemical Properties Of FeS₂ & NiS₂ Thin Films

FeS2 and NiS2 are promising candidates for cathode materials in lithium batteries or lithium ion batteries due to their high theoretical capacity, low cost (abundant mineral), non-toxicity and high high current densities. They have been used as cathode materials in thermal batteries and primary cells in the past. Because of the poor cycle performances of electrochemistry in room temperature, FeS2 and NiS2 have not used as cathode materials in second batteries. In this paper, Fe2O3 films and NiO films were prepared by sol-gel techniques, then the FeS2 and NiS2 films were synthesized on gold substrates after the metal oxide films being heat-treated in sealed quartz tubes in gaseous sulfur.The effects of technological conditions on the phase structures and the electrochemical performances of films have been studied. And the diffusion coefficient of Li+ion in FeS2 and NiS2 Materials have been determined by potential step method.After metal Au being heat-treated at 400-800℃in sealed quartz tubes in gaseous sulfur, the phase structure of Au has not been changed according to the result of XRD diffraction. And It is determined that the metal Au has excellent electrochemical stability in 1M LiPF6 of EC/DMC (1:1, v/v) solution in the range of 1.0-2.5 V (vs Li+/Li) potential by cyclic voltammetry and constant current charge-discharge techniques. So the Au is adequate substrate on which the FeS2 and NiS2 were prepared and the Au can be used as current collector for cathode of Li batteries in the range of 1.0-2.5V (vs Li+/Li) potential. The predominance area diagrams of 1gPS2-1gPO2 and 1gPS2-1gPSO2 for Fe-S-0 and Ni-S-0 were drawn by the improved component activity term method of base element. And the influences on the purity of pyrite by the residual oxygen impurity in the process of synthesis have been characterized according to these drawn predominance area diagrams. The predominance area diagrams show that the high-purity FeS2 and NiS2 can be prepared by heat-treatment of iron/ iron oxides and nickel/nickel oxide with sulfur in sealed quartz reactor. Iron oxide films and nickel oxide films were prepared on gold substrates by sol-gel method with Both Acetylacetone and Ferric chloride and chlorepoxy-propone dissolved in ethanol solution. The Iron oxide films and nickel oxide films were crack-free, transparent and uniform. These metal oxide film exhibit good bonding with the Au matrixs.The effects of sulfurzing temperature, sulfurizing time and the gaseous sulfur pressure on the phase structures and the electrochemical performances of films have been studied systematically. The experimental results show that the FeS2 films can be prepared by the F2O3 films being heat-treated at 350-800℃in 13atm gaseous sulfur for 20hours. The SEM determined that the grain size of the films will increase as the heat-treatment temperature increases. Especially after being heat-treated at 800℃, the grain sizes of the FeS2 films reach 8μm. While the FeS2 films is prepared at 500℃in 13atm gaseous sulfur for 20hours, the prepared FeS2 films exist the best electrochemical performances:the first discharge capacity is 900mAh g-1, and 30th discharge capacity is 410 mAh g-1。Meanwhile the experimental results show that the NiS2 films can be prepared by the NiO films being heat-treated at 400-600℃in 15atm gaseous sulfur for 20hours. After NiO films being heat-treated at 700-800℃for 20hours, the NiS2 phase structure films cannot be obtained and the nickel on the matrix will loss Owing to evaporation. While the NiS2 films is prepared at 600℃in 15atm gaseous sulfur for 20hours, the prepared NiS2 films exist the best electrochemical performances:the first discharge capacity is 880mAh g-1, and 30th discharge capacity is 400 mAh g-1.The open-circuit potential (OCV) of the fresh Li/FeS2,Li/NiS2 cell is 2.2-3. IV, which is larger than that of theory. This is caused by FeS2,NiS2 electrodes containing impurities of S,O.While the Li/FeS2 or Li/NiS2 cell is discharged to 1.0V wtith 0.4C current at the first discharging,100% of the FeS2 in cathode can be reduced to Fe+2Li2S and 100% of the NiS2 in cathode can be reduced to Ni+2Li2S. Subsequently at the first charging,100% of the reduced Fe can be oxidized to Li2FeS2 and 100% of the reduced Ni can be oxidized to Ni3S2.The diffusion coefficient have been determined by potential step method:the diffusion coefficientof Li+ion in is 1.4×10-16 cm2 S-1 and the diffusion coefficient of Li+ion in the prepared NiS2 film is 5.3×10-17 cm2 S-1. It is determined that the diffusion polarization of Li+ion into crystals of FeS2 or NiS2 film is one of the reasons that causes the discharging potential is low at the first discharging 1M LiPF6 EC/DMC (1:1, v/v) solution by cyclic voltammetry and constant current charge-discharge techniques.Moreover, electrochemical performance of the NiS2 film is better than that of the FeS2 film between the second and the 10th cycle at 0.4C dishcharging, but electrochemical performance of the NiS2 film is worse than that of the FeS2 film after the 10th cycle.