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All-vanadium aqueous rechargeable lithium- batteries: The green macro-battery

Cor Potgieter

The lead-acid battery, which South Africa has abundant wind application. The main cost factors and sources, and yet are the inherent low was invented in 1859 by French the country mainly relies on coal and the necessity of using an ion- as a power source. Renewable selective membrane. The energy physicist Gaston Planté, is the energy sources are cyclical. A cost- density is limited by the solubility of effective technology the vanadium compounds, which oldest type of rechargeable is required if these power sources are used as active materials. Water battery. These batteries are are to ever significantly contribute to typically comprises more than half the country’s energy needs. Due to of the weight of typical used in a variety of applications, its lower cost, the lead-acid battery (Rychcik and Skyllas-Kazacos, 1988). is still the most widely used battery The ion-selective membrane such as motor vehicles, boats for large-scale energy storage. It is separator comprises approximately environmentally unfriendly, however, 20% of the battery cost (Prifti et al., and standby power installations. and has poor deep discharge and 2012). Eliminating the water and the long-term cycling performance. The membrane therefore holds the key to Since the technology is all-vanadium aqueous lithium-ion high energy and cost-effectiveness. battery (VARLB) is a better alternative. 155 years old, there is a need for Surface-supported active exploring alternatives. The research path leading to the material tests VARLB started with investigations Aside from the global movement around increasing the energy density Initial experiments with ion exchange of the regular vanadium substrates and super towards greener technology, battery (VRB). Due to its long life substrates were conducted. The idea span, the VRB is a promising energy was to support the active of the the search for new technology storage technology for the large-scale VRB on a solid surface, rather than energy storage market. The VRB is a dissolving them. This immediately is driven by the need for a safer redox flow cell. removed the energy density limit placed by the solubility of the active rechargeable battery. The lead Redox flow cells differ from substances. This also makes an in lead-acid batteries can cause commercial voltaic cells in the ion-selective membrane separator sense that energy is stored in redundant, because the active lead poisoning and the battery’s charged electrolytes, not in the materials are immobilised on the cell’s . The reactants surfaces. These initial trials sulphuric acid is highly corrosive. and products all remain dissolved led to a battery that uses the lithium- throughout the charge and discharge ion battery storage mechanism. cycle. An ion-selective membrane is required to keep the solutions from Lithium-ion batteries mixing. The cell short-circuits if the solutions mix, which forms heat. In Lithium-ion batteries have the the VRB, vanadium-based half-cell highest energy density and longest reactions are used in both electrodes. life span of all the commercially This eliminates the common flow cell available battery technologies. problem of cross-contamination. The Organic electrolytes have classically mixing of the electrolytes inevitably been used to maximise the usable occurs due to the membranes window, and therefore leaking slightly. In VRBs, only the energy density of the battery. vanadium compounds are formed Organic electrolytes are required due when mixing occurs. These reactions to the extreme reactivity of lithium can easily be reversed by recharging with moisture and air, but they are the cell. Therefore, the toxic, flammable and expensive. theoretically has an infinite life span. The production costs of traditional lithium-ion batteries are inhibiting for Unfortunately, the high cost of the the large batteries required for load VRB has hampered its commercial levelling (Wessels et al., 2012).

FEATURES 55 INNOVATE 9 2014 ENERGY IRT

The costs are high because of the Aqueous electrolytes generally have Combining VO2 and LiV2O5 (Figure 1), use of expensive organic electrolytes, ion conductivities of approximately the expected cell reactions are as safety concerns with large-scale two orders of magnitude higher than follows: batteries and because moisture- and those of organic electrolytes, and are air-free controlled environments are non-toxic and non-flammable (reduction):

- + required for manufacturing. (Luo et al., 2010). The fabrication V2O5 + e + Li LiV2O5...... E0= 0.33 V costs are much lower for ARLBs, (oxidation):⇌ Aqueous rechargeable lithium- because no environmental control - + 2Li0.5VO2 2VO2 + e + Li ...... E0= -0.42 V ion batteries is required during manufacturing Cell: ⇌ to limit air and moisture levels. This V O + 2Li VO LiV O + 2VO ...... E = 0.75 V Aqueous rechargeable lithium- makes the ARLB ideally suited for the 2 5 0.5 2 2 5 2 0 ion batteries (ARLBs) are less large-scale energy storage market, ⇌ The expected specific capacity of a dangerous replacements for which requires batteries with a low symmetrical vanadium oxide ARLB standard lithium-ion batteries and cost, high safety characteristics and a is in the range of 120 mAhg-1. The are inexpensive. The stable voltage long life. specific energy density using a window of aqueous electrolytes cell voltage of 0.75 V is therefore (without considering electrode All-vanadium oxide ARLB 90 Whkg-1, which is almost double overpotentials) is 1.23 V, compared that of the lead-acid battery. to the normal hydrogen electrode The all-vanadium oxide ARLB is a (NHE), and approximately 3 V for special ARLB. It is based solely on Ongoing research organic electrolytes. Figure 1 pairs vanadium compounds, just like the stability range of water with the the VRB. A symmetrical vanadium So far, tested experimental cells reaction potential of a few materials oxide ARLB combines the strengths have reached about a third of the that have been tested in lithium-ion of the regular VRB and other non- expected theoretical energy density. batteries. Combining a suitable half- symmetrical ARLBs. This battery Cell resistance is still a problem, but cell pair in the stability range of water requires no ion-selective membrane the optimisation of the electrode creates an ARLB. and has an inherently high energy formulation and manufacturing density, because the solubility of process is underway to improve this. The left side of Figure 1 shows the the reactants does not limit the The use of foam as current O /H evolution potential versus the energy density. Cross-contamination 2 2 collector is an exciting prospect, and normal hydrogen electrode (NHE) for of soluble vanadium compounds this super-conductive material may a different pH in 1 M Li SO aqueous should not cause any irreversible side 2 4 solve the cell resistance problem. solution. The right side shows the reactions. Despite the wide interest in A research team led by Prof Ncholu lithium-ion intercalation potential of the VRB and the extensive studies on Manyala, an associate professor in various electrode materials versus lithium-ion intercalation compounds, the Department of Physics at the NHE and Li/Li+. AC: activated carbon, no publications could be found on University of Pretoria, is the world NASICON: materials with NASICON an ARLB based solely on vanadium leader in graphene foam produced structure (Luo et al., 2010). compounds. by chemical vapour deposition, and will collaborate on the VARLB 1.5 4.5 project.

LiCoO2 LiNiO 2 References 1.0 4.0

O LIFePO Luo, J-Y, Cui, W-J, He, P & Xia, Y-Y. 2010. 2 evolution 4 Raising the cycling stability of aqueous 0.5 LiMn2O4 3.5 Potential (V versus Li lithium-ion batteries by eliminating oxygen in the electrolyte. Nature NASICON LiV2O5 Chemistry 2:760–765. 0.0 3.0 Prifti, H, Parasuraman, A, Winardi, S, AC Lim, TM & Skyllas-Kazacos, M. 2012.

VO2 Membranes for redox -0.5 H -2.5 2 evolution applications. Membranes 2:275–306. + /Li) Rychcik, M & Skyllas-Kazacos, M. 1988.

Potential (V versus NHE) LiV O Characteristics of a new all-vanadium -1.0 3 5 -2.0 LiNbO5 y-FeOOH redox flow battery.Journal of Power Sources 22:59–67. Wessels, CD, McDowell, MT, Peddada, SV, -1.5 1.5 Pasta, M, Huggins, RA & Cui, Y. 2012. 0 7 14 Tunable reaction potentials in open pH framework nanoparticle battery electrodes for grid-scale energy  Figure 1: The half-cell reduction potential map of some lithium-ion battery storage. ACS Nano 6:1688–1694. materials.

FEATURES 56 INNOVATE 9 2014