The electrochemical discharge of VB 2 is a unique process that involves the multiple electron per molecule oxidation of the tetravalent transition metal ion, V (+4 → +5), and each of the two borons 2×B (−2 → +3), corresponding to a net 11 electron discharge mechanism of the VB 2 /air cell as described by the overall cell reaction: VB 2 + 11/4O 2 → B 2 O 3 + 1 2 V 2 O 5 . However, in the presence of alkaline electrolytes, the discharge products include alkali salts associated with vanadaic and boric acid. In this study, we used FTIR, XRD, and coulombic efficiency measurements to probe the discharge products of high capacity cells and isolate KVO 3 as the principal vanadium discharge product. Additionally, we show that K 2 B 4 O 7 is the probable borate product. From FTIR analysis in KOH electrolyte, it is evident that the alkaline VB 2 /air discharge reaction is: VB 2 + 11/4O 2 + 2KOH → 1 2 K 2 B 4 O 7 + KVO 3 + H 2 O. XPS shows that the surface structure of nanoscopic VB 2 is very different from macroscopic VB 2 , which may contribute to the improved electrochemical properties of the nanoscopic material. The understanding of the discharge process and factors affecting performance contribute to furthering the development of extremely high capacity VB 2 /air batteries that utilize multi-electron processes. Metallic zinc has been used as an anode material in the majority of aqueous primary systems due to zinc metal's high two-electron oxidation capacity and effective discharge. The zinc-carbon battery, known as the Leclanché cell, was first introduced in the 19 th century as a low-cost solution for early energy storage needs. The zinc cell, which produced approximately 65 Wh kg −1 , was ideal only for low-rate discharges.1,2 Until the development of the zinc/alkaline/manganese dioxide battery and the zinc/air cell, there was little improvement in primary batteries. The alkaline Zn/MnO 2 cell has since dominated primary electrochemical storage, providing 145 Wh kg −1 . Although more expensive than the zinc-carbon battery, the alkaline cell improved performance by increasing energy densities and power capabilities. The zinc/air battery, using external O 2 as the battery active cathode reactant further improves the energy density of primary battery systems. It would be useful for electronic devices to have even higher energy storage densities than that available with zinc/air batteries.3 Most metal/air batteries to date have been unsuccessful in reaching the high-energy densities that are made possible by multi-electron oxidations, due to material passivation or chemical instabilities. 4 To provide high energy density cells, there has been an effort to develop high-capacity multi-electron per molecule charge storage processes.5-21 Vanadium diboride (VB 2 ) undergoes a multiple electron oxidation process, which to its completion involves an extraordinary 11 electron per molecule oxidation, including oxidation of the tetravalent transition metal ion, V(+4 → +5), and each of the two borons 2xB(−2 → +3). VB 2 has an intri...

Transition metal borides, such as VB 2 , have been investigated as alternative, higher capacity anode materials. The VB 2 high capacity is due to the capability to undergo a 4060 mAh/g formula weight multiple electron (11 e − ) alkaline oxidative discharge at a singular discharge potential plateau. With a comparable formula weight (10% higher) to zinc, VB 2 has an intrinsic gravimetric capacity five fold higher than the 2 e − oxidation of the widely used zinc alkaline anode. One challenge to the implementation of VB 2 /air batteries is that resistive oxide products impede the discharge depth, and only thin anode batteries (for example 10 mAh in a 1 cm diameter cell) had been demonstrated to discharge effectively. This study demonstrates that (i) smaller particle size (nano-VB 2 , as opposed to macroscopic VB 2 ) helps to alleviate this effect and (ii) a stacked anode compartment configurations improve the anode conductive matrix significantly, resulting in an increase in the coulombic efficiency of high capacity, thicker anodes in VB 2 /air batteries. Combined, these effects provide a 50% relative increase in the coulombic efficiency (from 50% to 75% at an 0.4 V discharge cutoff) of a 30 mAh coin cell, and increase the coulombic efficiency of the 100 mAh cell to 50%. Higher energy density portable power is needed for consumer electronic, medical, and military devices, and drives the need for increased energy density batteries. Zinc-air batteries are primary batteries with the highest commercial energy capacity. They provide a practical capacity of up to 1,756 Wh/L, which is 5-fold higher than that of rechargeable Li-ion batteries and 10-fold higher than conventional alkaline primary (zinc anode/manganese dioxide cathode) batteries. 1,2The capacity of zinc is limited by its oxidative discharge, which releases two electrons per zinc, leading to an intrinsic capacity of 820 mAh/g (=2 * FW/Faraday * mAh). Transition metal borides have been investigated as alternative, potentially higher capacity anode materials due to their ability to undergo oxidative discharge processes that release multiple electrons per molecule. The highest capacity of these materials, vanadium diboride, has an alkaline capacity per VB 2 (FW 72.561 g/mol) intrinsic capacity of 4060 mAh/g (charge/FW), approximately five times that of zinc. [3][4][5][6][7][8][9] As with the zinc anode, the borides can chemically react to generate hydrogen, and this causes a parasitic loss of battery capacity. In 2007, we noted that a zirconia overlayer impedes this parasitic reaction and promotes the battery discharge reaction.3,8 VB 2 coupled with an air cathode results in a battery material that is among the highest energy density of any primary battery (5,300 kWh/kg). 8 The VB 2 anode discharges all 11 of its electrons at a singular voltage plateau, in accordance with: [8][9][10] Anode :The thermodynamic (intrinsic battery) potential of Equation 3 is 1.55 V. [8][9][10] The experimental VB 2 /air battery is observed to discharge at a fraction of this intrinsic ...