(301c) High Performance Pillared V2O5 and MnO2 Cathodes for Lithium Ion Batteries

Lithium ion batteries (LIBs) are used extensively in portable electronics because of their high volumetric energy densities. Beyond this, LIBs are becoming more and more popular for aerospace and electric vehicle applications [1-3]. Layered lithium metal oxides, such as LiCoO₂, are commonly used as the cathodes given their high capacities and energy densities. However, the poor mechanical and thermal properties of these materials restrict their application under some challenging use conditions. For example, in military vehicle applications, the battery will be exposed to a wide operating temperature range. This is very challenging for LiCoO₂, as oxygen liberation will happen at ~200°C [4]. Also, when LIBs are charged and discharged, repeated lithium ion insertion and extraction leads to mechanical fatigue and deformation of the lattice structure [5], which causes capacity fade and increased thermal runaway hazards.

To address challenges associated with the layered oxide materials and improve their thermal and structural stabilities, as well as their rate capabilities and cyclabilities, modifications of the nanoscale structure have been carried out. Our approach is to intercalate nanoscale pillars between the layers to provide support [6-7], thus leading to a more robust structure. Vanadium pentoxide (V₂O₅) is a promising cathode material for LIBs, however it suffers from poor structural stability and limited rate capability [8]. Our research demonstrates significant improvements in the thermal and electrochemical properties of V₂O₅ xerogels via nanopillaring with Al₁₃ Keggin ions.

X-ray diffraction (XRD) and transmission electron microscopy (SEM) showed that the interlayer spacing of V₂O₅ xerogels (V₂O₅G) increased ~15% through intercalation of Al₁₃ Keggin ions, which can provide a more favorable pathway for Li⁺ diffusion. Aluminum contents as high as 9 wt% were achieved. In addition, thermal gravimetric analysis (TGA) illustrated that the pillared materials possessed better thermal stability. The V₂O₅-Al₁₃ was stable up to 400 °C compared to 350 °C for V₂O₅G.

Fundamental electrochemical properties of the V₂O₅G and V₂O₅-Al₁₃ were characterized using cyclic voltammetry. V₂O₅-Al₁₃ displayed improved redox activity supported by a prominent increase in current density relative to the untreated gel. To characterize the cycling performance, rate capability tests were conducted to determine capacity retention and capacities at high rate for V₂O₅G and V₂O₅-Al₁₃. V₂O₅G could only retain 36% of the initial capacity when switching from low rate (C/10) to high rate (C/2) discharge, while V₂O₅-Al₁₃ was able to retain 60% of the initial capacity. When returning to low rate (C/10), V₂O₅-Al₁₃ also showed better performance holding 80~95% of its original capacity. V₂O₅G, however, leveled off at 60%. As long-term stability at high rates of discharge is a challenge for LIBs [9], cycling at C/2 was also carried out to investigate their long-term cyclability. The results clearly show that V₂O₅G faded quickly from 75 to 52 mAh/g (based on active material) during its first 20 cycles, while V₂O₅-Al₁₃ started at a much higher capacity of 145 mAh/g and maintained 86% of this capacity after 50 cycles. The V₂O₅-Al₁₃ material also maintained 75% of its initial capacity after 100 cycles, demonstrating much better performance than V₂O₅G.

Given the positive results achieved by nanostructuring V₂O₅G, we also pillared manganese dioxide (MnO₂). Because of its low cost, non-toxicity, environmental friendliness and relative abundance [11-13], MnO₂ is particularly attractive for use as a cathode material. Two morphologies of layered MnO₂ have been successfully synthesized and characterization methods were used to defined their structural and compositional properties. The first discharge capacity for the Al₁₃ Keggin ion pillared MnO₂ (MnO-Al₁₃) was up to 265 mAh/g. These and other results from pillaring oxides will be presented in this paper.

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