A new chemical synthesis for vanadium sulfide as high performance cathode

Since 1990s, rechargeable Li-ion batteries have been widely used in consumer electronics such as cell phones, global positioning systems (GPS), personnel digital assistants (PDA), digital cameras, and laptop computers. Recently Li-ion batteries received considerable attention as a major power source for electric vehicles. However, significant technical challenges still exist for widely deploying Li-ion batteries in electric vehicles. For instance, the energy density of Li-ion batteries is not high enough to support a long-distance commute. The Li-ion batteries used for the Nissan Leaf and Chevy Volt only can support 50 – 100 miles per charge. The cost of Li-ion battery packs in electric vehicles is still high. The battery pack for the Chevy Volt costs about $8,000, and the larger one in the Nissan Leaf costs about $12,000. To address these problems, new Li-ion battery electrode materials with high energy density and low cost should be developed. Among Li-ion battery cathode materials, vanadium pentoxide, V2O5, is one of the earliest oxides studied as a cathode for Li-ion batteries because of its low cost, abundance, easy synthesis, and high energy density. However, its practical reversible capacity has been limited due to its irreversible structural change when Li insertion is more than x = 1. Tremendous efforts have been made over the last twenty years to improve the phase reversibility of LixV2O5 (e.g., 0 ≤ x ≤ 2) because of vanadium pentoxides\u27 potential use as high capacity cathodes in Li-ion batteries. In this thesis, a new strategy was studied to develop vanadium pentoxide cathode materials with improved phase reversibility. The first study is to synthesize vanadium oxide cathodes via a new chemical route – creating a phase transformation from the vanadium sulfide to oxide. The β-Na0.33V2O5 was prepared via a new method of chemical synthesis, involving the chemical transformation of NaVS2 via heat-treatment at 600 °C in atmospheric air. The β-Na 0.33V2O5 particles were well crystalized and rod-shaped, measuring 7–15 &mgr;m long and 1–3 &mgr;m wide with the formation of the crystal defects on the surface of the particles. In contrast to previous reports contained in the literature, Na ions were extracted, without any structural collapse, from the β -Na0.33V2O5 structure and replaced with Li ions during cycling of the cell in the voltage range, 1.5 V to 4.5 V. This eventually resulted in a fully reversible Li intercalation into the LixV2O5 structure when 0.0 ≤ x ≤ 2.0. The second study is to apply the synthesis method to LiVS2 for the synthesis of β\u27-LixV2O5 for use as a high performance cathode. The synthesis method is based on the heat treatment of the pure LiVS2 in atmospheric air. By employing this method of synthesis, well-crystalized, rod-shaped β\u27-LixV 2O5 particles 20 – 30 &mgr;m in length and 3 – 6 &mgr;m in width were obtained. Moreover, the surface of β\u27-Li xV2O5 particles was found to be coated by an amorphous vanadium oxysulfide film (~20 nm in thickness). In contrast to a low temperature vanadium pentoxide phase (LixV2O5), the electrochemical intercalation of lithium into the β\u27-LixV2O 5 was fully reversible where 0.0 \u3c x \u3c 2.0, and it delivered a capacity of 310 mAh/g at a current rate of 0.07 C between 1.5 V and 4 V. Good capacity retention of more than 88% was also observed after 50 cycles even at a higher current rate of 2 C. The third study is the investigation of NaVS2 as a cathode intercalation material for sodium ion batteries. We have shown that reversible electrochemical deintercalation of x ~ 1.0 Na per formula unit of Na xVS2, corresponding to a capacity of ~200 mAh/g, is possible. And a stable capacity of ~120 mAh/g after 30 cycles was observed. These studies show that the new chemical synthesis route for creating a phase transformation from the vanadium sulfide to oxide by heat treatment in air is a promising method for preparing vanadium oxide cathode material with high reversibility. Although this sample shows a relatively low voltage range compared with other cathodes such as LiCoO2 (3.8 V) and LiFePO 4 (3.4 V), the large capacity of this sample is quite attractive in terms of increasing energy density in Li-ion batteries. Also, NaVS 2 could be a promising cathode material for sodium ion batteries