Evaluation of properties and performance of nanoscopic materials in vanadium diboride/air batteries

Hohenadel

et al. 2014

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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...

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confidence: 87%

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confidence: 99%

The Net Discharge Mechanism of the VB₂/Air Battery

Hohenadel

et al. 2014

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“…For example, under the same load, a 2.5, 5, 10 or 30 mAh VB 2 /air cell discharged to 80%, 60%, 50% or 30 to 40% columbic efficiency respectively. 9,12,13 In this study, we provide a path to ameliorate this effect through advanced anode configurations with an improved conductive matrix. …”

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confidence: 99%

“…[11][12][13] One challenge to the implementation of VB 2 /air batteries is that only thin anode batteries (for example 10 mAh in a 1 cm diameter coin * Electrochemical Society Student Member.…”

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confidence: 99%

Higher Capacity, Improved Conductive Matrix VB₂/Air Batteries

Lefler

Parkey

et al. 2016

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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 ...

“…[13][14][15][16] The voltage and rate advantages of nanoscopic compared to macroscopic VB 2 anodes in VB 2 /air batteries has been characterized, providing 10 to 20% higher voltage as well as higher coulombic efficiency under low and moderate discharge loads. 15 While there have been few prior characterizations of the anodic properties of TiB 2 , the high conductivity of the material makes it a strong candidate for a potential anodic active material.…”

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confidence: 99%

High Energy Capacity TiB₂/VB₂Composite Metal Boride Air Battery

Lefler

Rhodes

et al. 2015

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Transition metal borides, such as VB 2 and TiB 2 , are studied as battery anode materials both individually and as a composite anode for an air cathode battery. The combination of VB 2 and TiB 2 is shown to enhance anodic battery performance. In alkaline media, VB 2 and TiB 2 anodically discharge to yield respectively 11 and 6 electrons per molecule with VB 2 intrinsic gravimetric charge capacity of 4,060 mAh/g and 2,314 mAh/g for TiB 2 , 3 to 5 fold higher than a conventional zinc anode. With an air cathode using external O 2 , these boride/air batteries discharge at ∼1 V, and exhibit unusually high, primary battery energy capacities. We show that the coulombic efficiency attained by the TiB 2 anode in boride/air batteries is not a strong function of the anode capacity per unit area (cm 2 ) of anode, while the efficiency of the VB 2 anode decreases with increasing anode capacity. The discharge of VB 2 shows a unique singular voltage plateau, indicating that the 11 electrons discharge at a similar potential. Two voltage plateaus are observed during the TiB 2 discharge indicating the possibility of a two-step anodic oxidation. The combination of VB 2 and TiB 2 exhibits evidence of a synergistic increase of the capacity and discharge voltage of boride/air batteries. The development of improved energy density battery systems is driven by emerging technological demands of longer operational time and lighter weight for medical, military and consumer electronic devices. A principal focus of battery research in the past decade has been the advancement of the energy capacity of rechargeable Li-ion batteries, which have a capacity of 100 to 200 Wh/kg. Despite this, the capacity of rechargeable batteries remains approximately 5-fold lower than that of primary batteries. The highest commercial primary battery energy capacity is exhibited by zinc/air batteries.1 Replacement of the air battery's zinc electrode by a higher capacity anode material has the potential to further increase the energy capacity. New materials, such as metal borides, have been studied due to their demonstrated high capacities as anodes for primary alkaline batteries. Of the large number of borides that have been investigated as anodes in alkaline media, TiB 2 and VB 2 exhibit the highest stability and demonstrated anodic capacity.2-9 Although Zn, TiB 2 , and VB 2 have similar formula weights (65.39, 69.49. 72.56 g/mol), the Zn oxidative discharge releases only two electrons, whereas TiB 2 and VB 2 are observed to discharge up to 6 and 11 electrons, respectively. While the conventional Zn anode has an intrinsic capacity of 820 mAh/g (2*FW/Faraday*mAh), TiB 2 and VB 2 have respective intrinsic capacities of 2,314 and 4,060 mAh/g. Coupled with an air cathode, vanadium diboride has been reported to have among the highest energy density of any primary battery (5,300 kWh/kg). 9 The 11 electron discharge reaction of the VB 2 /air battery is given by the following half cell and full cell reactions: It is interesting to note that VB 2 discharges each of its 11...