ETH Zurich team shows vanadate-borate glasses as inexpensive high-capacity cathodes for Li-ion batteries

19 November 2014

A team from ETH Zurich in Switzerland has demonstrated the use of vanadate-borate glasses (Li₂O-B₂O₃-V₂O₅, referred to as V₂O5-LiBO₂) as high-capacity cathode materials for rechargeable Li-ion batteries for the first time. The composite electrodes with reduced graphite oxide (RGO) deliver specific energies around 1,000 Wh/kg and retain high specific capacities in the range of ~ 300 mAh/g for the first 100 cycles.

Vanadium oxide (vanadate)-based materials are attractive cathode alternatives due to the many oxidation state switches of vanadium, resulting in a high theoretical specific capacity. However, irreversible phase transformations and/or vanadium dissolution starting from the first discharge cycle result in significant capacity losses. In their open access paper published in Nature’s Scientific Reports, the ETH Zurich team says that these problems can be circumvented if amorphous or glassy vanadium oxide phases are used.

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Irreversible phase transformations and volume work leading to amorphization as well as to loss of low valence state metal ions into the electrolyte accompany most high capacity materials during cycling. To tackle these problems, we have chosen borate-based glasses of V₂O₅ in order to explore vitreous redox-active systems pursuing the goal of utilizing many oxidation states of vanadium to the highest possible extent and fixing the vanadate group by a network former to enhance cycling stability.

Fabrication of the materials is simple and cost-efficient; mixture of 80 wt-% V₂O₅ and 20 wt-% LiBO₂ is melted at 900 °C. Subsequent quenching to room temperature produces the glass material. To fabricate the V₂O₅ – LiBO₂ glass electrode, they manually mixed the active material (70 wt-%), conductive carbon (20 wt-%) and PVDF binders (10 wt-%).

Upon testing, they obtained a first discharge capacity of 327 mAh/g; the cell was charged with a capacity of 308 mAh/g in the subsequent cycle. This finding shows that the huge capacity loss associated with irreversible phase transformation of crystalline V₂O₅ materials is largely circumvented by the V₂O₅ – LiBO₂ glass, they said.

However, the discharge capacity drastically drops to 125 mAh/g at the 35^th cycle, when the rate is increased to 400 mA/g, and recovers to 260 mAh/g at the 45^th cycle when the rate is 50 mA/g. The researchers suggested that the limited capacities at high rates are probably caused by poor kinetics of the glassy material.

To improve cycling properties with higher charge/discharge capacities, they fabricated a composite electrode of the V₂O₅ – LiBO₂ glass with reduced graphite oxide (RGO).

They found that the first discharge capacity was raised to ~ 405 mAh/g for the composite electrode. A high capacity of ~ 390 mAh/g was reached in the subsequent charge, showing that the RGO/V₂O₅ – LiBO₂ glass composite also does not suffer from the large irreversible capacity loss associated with the phase transformations. This initial charge capacity was largely preserved in the range of ~ 300 mAh/g within the first 100 cycles.

The capacity delivered at the highest rate (400 mA/g) was more than doubled compared to the amount obtained for the glass electrode without RGO.

(Top) the rate capability of the RGO/V₂O₅ – LiBO₂ glass composite within 1.5–4.0 V at 50, 100, 200 and 400 mA/g rates (at room temperature). (Bottom) discharge capacity vs. cycle within 1.5–4.0 V at 50 and 100 mA/g rates. Afyon et al. Click to enlarge.

For practical battery applications, the overall cycling stability may still be improved via better cell and electrode engineering, the exploration of different protective coatings and more stable electrolyte systems. Nevertheless, the results obtained for vanadate – borate glasses are very encouraging and may trigger further studies for similar glass systems that could encompass the practical use of glassy materials as next generation electrode materials for rechargeable Li-ion batteries.

Resources

• Semih Afyon, Frank Krumeich, Christian Mensing, Andreas Borgschulte Reinhard Nesper (2014) “New High Capacity Cathode Materials for Rechargeable Li-ion Batteries: Vanadate-Borate Glasses” Scientific Reports 4, Article number: 7113 doi: 10.1038/srep07113