A Novel Sodium-ion Rechargeable Battery Manickam

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ECS Transactions, 45 (29) 95-102 (2013) 10.1149/04529.0095ecst ©The Electrochemical Society A Novel Sodium-Ion Rechargeable Battery Manickam Minakshi and David Ralph School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, AUSTRALIA Energy storage is always an important issue and will be far more important in the future than at any time in the past. Storing large amounts of electricity cheaply, something that will be essential for making renewable energy the primary source, rather than just the supplemental source it is a current challenge. Such storage will make it practical to store energy from wind turbines and solar farms for later use. To achieve these, sodium-based energy storage systems have been identified as a key technology for the future as the lithium technology is more expensive. Existing sodium technologies work at high temperature, where molten sodium and molten sulphur are the anode and cathode respectively and they have never found widespread use. An alternative strategic approach used in this study is water-based sodium-ion intercalation cell (MnO2|NaOH|NaCo1/3Ni1/3Mn1/3PO4) for a promising low temperature energy storage device. Introduction The availability of efficient and low cost batteries is the key technology for storage and utilization of the energy generated by sustainable but intermittent processes. Efficient and low cost power storage is also the enabling technology for the development of portable electronic devices such as mobile phones and laptop computers. Rechargeable lithium batteries now dominate the market and represent the technology powering almost all portable electronic devices and their use in electric vehicles is under intense investigation (1-3). Lithium is the lightest metallic element with a very low standard reduction potential (−3.04 V vs SHE) enabling high voltage and high energy density. Moreover, the ionic radius of Li+ is small and its diffusion into and out of host electrode materials (intercalation) results in excellent reversibility. Despite these merits, serious concerns about the high cost and limited global reserves of lithium are held (4-5). The market success of lithium ion batteries is steeply increasing demand for lithium, making the metal more expensive and limiting market expansion for the technology. An alternate approach to the technology of large scale energy storage is required and is an objective of this work. Manganese and Iron are both much more abundant in the earth’s crust than Lithium and both these elements can be fabricated into electrode materials for large scale energy storage applications (6-7). Sodium also has a low standard reduction potential (−2.71 V vs SHE) and is much more abundant and much cheaper than Lithium. A battery technology based on abundant Mn, Fe and Na is therefore an appealing choice as an alternative to lithium technology (8-9). Although Sodium and Lithium share similar chemical properties the ionic volume of Na is 2.5 times larger than that of Li (10). Batteries based on reversible intercalation of Na require cathodes that can accommodate 95 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) the larger Na+ ions in the host matrix. In this report, the performance of manganese dioxide (MnO2) electrodes with electrolytes of LiOH, NaOH and KOH were first examined. Based on the results, a cell using a MnO2 cathode coupled with a sodium intercalated olivine phosphate (NaCo1/3Ni1/3Mn1/3PO4) anode using aqueous NaOH as the electrolyte was constructed and tested. The choice of the olivine phosphate as an electrode was inspired by our earlier studies on the lithium analogue (11-12). Sodium and sodium-ion batteries have evolved recently (8-9) and to the best of our knowledge, the performance of an aqueous cell relying on sodium ion intercalation has not been reported. During charge and discharge, Na+ ions from the synthesized olivine anode shuttle back and forth between the two electrodes. The initial results reported here have shown demonstrated the potential of these systems, particularly for stationary storage connected to renewable energy production where high energy/power density is less critical (13-15). Experimental Material Synthesis Electrolytic manganese dioxide (EMD, γ-MnO2, type IBA sample 32) used in this work was purchased from the Kerr McGee Chemical Corporation. Analytical grade potassium hydroxide (KOH), zinc sulphate heptahydrate (ZnSO4•7H2O), lithium hydroxide monohydrate (LiOH•H2O) and sodium hydroxide (NaOH) were purchased from Sigma Aldrich and used in this study. Zn foil (99.9%) was obtained from Advent research materials. Analytical grade acetate salts of sodium, lithium, cobalt, manganese and nickel as well as ammonium dihydrogen phosphate and polyvinylpyrrolidone (PVP) were purchased from Sigma Aldrich. Sol-gel synthesis of the olivine phase (LiCo1/3Mn1/3Ni1/3PO4) was achieved by dissolving stoichiometric amounts of lithium acetate, cobalt acetate, manganese acetate, nickel acetate and ammonium dihydrogen phosphate in water at 80°C. PVP was included as a chelating agent for the metal cations and the pH adjusted to 3.5 by adding nitric acid. The homogeneous mixture was heated and stirred forming a thick transparent gel which was dried at 110°C in hot air oven for 12 h. Furnace heating was carried out at 300oC for 8 h and at 600oC for 6 h in air with intermittent grinding. The LiCo1/3Mn1/3Ni1/3PO4 powder finally obtained was ground before further analysis and use in the electrochemical cell. The sodium analogue of the olivine phase was prepared in the same manner with sodium acetate substituted for the lithium acetate. Electrochemical characterization Electrodes were fashioned from the EMD material by mixing with 15 wt. % of carbon black (A-99, Asbury USA) and 10 wt. % of polyvinylidenedifluoride (PVDF, Sigma Aldrich) and pressing the mixture into a disc shape with a diameter and thickness of 12 and 0.5 mm respectively. These electrodes weighed 35 mg and contained 26 mg of MnO2 with a theoretical capacity of 308 mA h g−1 for a discharge of 1 mol (e−) per mol Mn. Electrodes of the olivine phases were constructed in the same manner. Electrochemical test cells were constructed with either MnO2 or olivine phase disk as the cathode and Zn metal or the olivine phase as the anode. A filter paper (Whatman filters 96 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) 12) was used as the separator and the electrolyte was a solution of K, Li or NaOH at a concentration of 7 M or saturated amount. The electrochemical cells were charged and discharged galvanostatically at 0.2 mA cm−2 (8 mA g−1) by using an 8 channel battery analyser (MTI Corp., USA) operated by a battery testing system (BTS). Experiments were terminated automatically if the voltage required for the constant current exceed narrow limits or after the passage of certain amount of charge. All electrochemical measurements were carried out at ambient temperature (25 ± 1o C). Results and Discussion Electrochemical behaviour of the cathodic MnO2 material The ‘zinc-manganese dioxide cell’ has come to dominate the primary battery market. Since 1866, when George Leclanché invented the galvanic cell, MnO2 has been the principal cathode constituent in dry cells. Today it is used in alkaline, zinc chloride, Leclanché, magnesium and aluminium primary batteries as well as alkaline secondary batteries (16). The Zn/MnO2 cells are now well developed and the MnO2 phase used in batteries is electrolytic manganese dioxide (EMD, γ-MnO2) commonly called ramsdellite. The EMD form of MnO2 exhibits the greatest battery activity (17) which arises from the ‘2×1 (empty) tunnels’ present in polyhedra-based crystals providing enhanced electrochemical activity compared to pyrolusite (NMD) which contain only ‘1×1 (empty) tunnels’ (18). Lithium or proton (H+) transport into the cathode phase during electrochemical discharge occurs much more readily within the 2×1 tunnels of ramsdellite than in the 1×1 tunnels of pyrolusite (reactions 1 and 2). MnO2 + Li+ + e− Š LiMnO2 [1] MnO2 + H+ + e− Š MnOOH [2] Zn| aqueous electrolyte |MnO2 cell To investigate the transport of electrolyte cations into the MnO2 structure a cell with a zinc anode, MnO2 cathode and an electrolyte containing 1 M ZnSO4 and either LiOH, NaOH or KOH was discharged and charged over a number of cycles at a constant current density (0.2 mA/cm2). The results for the first discharge–charge cycle of the 3 aqueous cells are shown in figure 1. The discharge characteristics for all three cells are similar demonstrating capacities between 140 and 240 mA h g−1 compared with a theoretical maximum 308 mA h g−1. However the charge cycle with KOH as electrolyte shows clearly that the cathodic reaction in the ‘KOH electrolyte’ cell is not reversible. With the other alkaline electrolytes LiOH and NaOH, the discharge reaction can be successfully reversed and the cell recharged. Interestingly, the cell using NaOH as electrolyte demonstrated a higher capacity than that using LiOH, 83 and 45 % of the theoretical maximum for the NaOH and LiOH electrolytes respectively. The cell with KOH as electrolyte showed a sharp 97 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) drop in discharge voltage from 1.2 to 1.0 V (Fig. 1). During the subsequent charge, the cell was found to be not rechargeable, supporting earlier observations (19, 20) where the intercalation of K+ into the ‘host’ MnO2 was found to be irreversible. The K+ ions transported into the cathode retard the usual protonation mechanism (21) for reversibility. 2.0 LiOH NaOH Cell Potential / V 1.5 1.0 KOH 0.5 0.0 0 75 150 225 300 Cell Capacity / mAh g-1 375 450 . Figure 1. Discharge-charge curves illustrating the cyclability of three Zn|MnO2 aqueous cells with the electrolyte containing 1 M ZnSO4 and either LiOH, NaOH or KOH (saturated or 7 M). The reversibility for both the LiOH and NaOH cells (in Fig. 1) indicates that the redox mechanism is reversible with sodium and lithium intercalation. Although the shapes of the discharge curves are quite similar, the drop in voltage is slower for the NaOH cell compared with the LiOH counterpart. However, the mid discharge voltage is higher for LiOH cell. The NaOH cell (called the sodium-ion cell) is fully reversible with a discharge capacity of 225 mA h g−1 with a flat discharge voltage of 1.3 V. These results show that aqueous electrolytes with sodium ions can be reversibly intercalated into MnO2 cathodes. This concept of sodium intercalation with excellent capacity and reversibility motivated a search other potential cathodic materials suitable for aqueous rechargeable batteries. Electrochemical behaviour of olivine phosphate host The successful demonstration of intercalation of sodium into an oxide cathode prompted a closer evaluation of transition metal olivine phosphates like LiMPO4 and NaMPO4 (M = transition metal). The main advantages of olivine cathodes are high discharge rates, excellent capacity retention, high energy density and structural stability however their disadvantages include poor electronic conductivity and high redox voltage (high reduction potential). To overcome these issues, a novel transition metal substituted with the general stoichiometry (Li or Na)Co1/3Ni1/3Mn1/3PO4 were synthesized using a sol-gel technique (11-12). The behavior of this material with the aqueous LiOH and 98 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) NaOH electrolytes was of interest because of the potentially attractive features of low cost and benign recycle. The cells constructed and tested were Zn|(LiOH)| LiCo1/3Ni1/3Mn1/3PO4 and Zn|(NaOH)| NaCo1/3Ni1/3Mn1/3PO4 (figures 2 and 3 respectively). 2.0 LiCo1/3Ni1/3Mn1/3PO4 | LiOH| Zn Cell potential / V Charge 1.5 1.0 Discharge 0.5 1 25 0.0 0 20 40 60 80 -1 Cell capacity / mAh g 100 Figure 2. Discharge-charge curves illustrating the cyclability of LiCo1/3Ni1/3Mn1/3PO4 | LiOH| Zn cell containing saturated concentration of aqueous LiOH containing 1 mol L-1 of ZnSO4 as the electrolyte. Cycle numbers are indicated in the figure. With both these cells, a charge phase was conducted first followed by a discharge phase to complete the cycle. The unique behaviour shown in figure 2 is an increase in the specific capacity of the Li olivine phosphate cathode from an initial discharge capacity of 60 mA h g−1 increasing to 80 mA h g−1 over 25 charge-discharge cycles. This effect was ascribed to changes in the organic layer trapped in the electrode during synthesis (22-23). Figure 3 shows the behaviour of the analogous sodium olivine phosphate cell with NaOH electrolyte. During the charge process there was a gradual increase in potential to 1.7 V and no subsequent change as 80 mA h g−1 of charge was passed. During the subsequent discharge, the voltage initially decreased to 0.7 V and then gradually decreased further to 0.6 V as 80 mA h g−1 of charge was passed. This demonstrates that sodium ions can be reversible intercalated into the olivine phosphate ‘host’ cathode NaCo1/3Ni1/3Mn1/3PO4, to a capacity of at least 80 mA h g−1 however the cell polarization is quite high. The observed low voltage plateau (Fig. 3) is not ideal cathodic performance. The behavior of the olivine phosphate as an anode was investigated. The unique behaviour of increase in discharge capacity over cycling that observed for lithium olivine phosphate is not seen for the sodium analogue. 99 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) Cell Potential / V 2.0 Charge 1 5 1.5 1.0 0.5 Discharge 5 1 0.0 0 20 40 60 Cell Capacity / mAh g-1 80 Figure 3. Discharge-charge curves illustrating the cyclability of NaCo1/3Ni1/3Mn1/3PO4 | NaOH| Zn cell containing 7 M concentration of aqueous NaOH containing 1 mol L-1 of ZnSO4 as the electrolyte. Cycle numbers are indicated in the figure. Based on the above results, a cell containing an olivine phosphate (NaCo1/3Ni1/3Mn1/3PO4) anode, coupled with a MnO2 cathode containing 7 M NaOH as the electrolyte was constructed. In this cell, Na+ ions are propelled from the anode to the cathode and then from the cathode back to the anode under the respective discharge and charge phases of the cycle. The voltage profile observed during discharge (Fig. 4) was a smooth downward potential curve during movement of Na+ from the olivine to become intercalated in the MnO2 host. The negative electrode is oxidised and sodium is released into the electrolyte while the positive electrode intercalates sodium and undergoes reduction from Mn4+ to Mn3+. The discharge capacity was found to be 250 mA h g−1 compared with a theoretical maximum of 308 mA h g−1. During the subsequent charge phase, manganese undergoes oxidation from Mn3+ to Mn+4 and the cell is restored by the transfer of Na+ back to the olivine phosphate. It is interesting to note that de-intercalation of sodium during the subsequent charge phase does not result in conversion to a spinel or any other phase that affect the structural stability. During 25 cycles, the capacity of this cell was maintained at 91% of the theoretical maximum (data not shown). This indicates the structural stability of both electrodes as the sodium is diffused into and out of each electrode. Overall, the performance of NaOH cell still lags behind that of LiOH cell in terms of voltage and energy density, however this study shows that sodium-ion intercalation is feasible as an energy storage mechanism. The sodium ion system would have significant advantages because its low cost materials could be used at a much larger scale than a system based on lithium. Provided that energy density is a less critical requirement, for example in 100 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) load-leveling for the electrical grid, the prospects for batteries composed of this type of cell have potential use in stationary energy storage. 2.0 MnO2 |NaOH| NaCo1/3Ni1/3Mn1/3PO4 Cell Potential / V 1.5 1.0 Charge 0.5 0.0 Discharge -0.5 0 75 150 225 300 375 Cell Capacity / mAh / g 450 Figure 4. Discharge-charge curves illustrating the cyclability of MnO2|NaOH|NaCo1/3Ni1/3Mn1/3PO4 cell containing 7 M concentration of aqueous NaOH containing 1 mol L-1 of ZnSO4 as the electrolyte. Conclusions The proposed sodium-ion battery develops an important new family of energy storage devices based on an affordable, globally available element, sodium. The innovative science in this study involves reversible aqueous sodium electrochemistry at low temperature (against the available relatively high temperature, at which Na is molten). The sodium energy storage technology will offer immediate advantages over existing primary battery technologies in terms of high energy density, cost, safety and environmental considerations. This research paves the way for an alternative energy technology by removing global reliance on fossil fuels and replacing them with sustainable energy storage technology like sodium-ion intercalation cell (MnO2|NaOH|NaCo1/3Ni1/3Mn1/3PO4) that reduces our impact on the environment. This low voltage but larger energy storage (250 mAh g−1) aqueous intercalation cells have potential in applications with land-based power requirements, such as electric grid stabilization. In such applications, portability is not an issue, so the benefits of high voltage or low weight are not prevalent. 101 Downloaded on 2016-05-17 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 45 (29) 95-102 (2013) Acknowledgements The author (M. M) wishes to acknowledge the Australian Research Council (ARC). This research was supported under ARC’s Discovery Projects funding scheme (DP1092543). The views expressed herein are those of the authors and are not necessarily those of the Australian Research Council. References 1. 2. 3. 4. 5. 6. J.M. Tarascon and M. Armand, Nature, 414, 359 (2001). Y. Nishi, J. Power Sources, 100, 101 (2001). B. Scrosati, Electrochim. Acta, 45, 2461 (2000). 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