Na0.44MnO2 Aqueous Sodium-Ion Batteries - Journal of The

Journal of The Electrochemical Society, 162 (6) A803-A808 (2015)

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Relating Electrolyte Concentration to Performance and Stability for NaTi2 (PO4 )3 /Na0.44 MnO2 Aqueous Sodium-Ion Batteries Wei Wu,a Sneha Shabhag,b Jiang Chang,a Ann Rutt,a and Jay F. Whitacrea,c,z a Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA b Aquion Energy, Pittsburgh, Pennsylvania 15201, USA c Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

In aqueous electrolyte batteries, the salt concentration of the electrolyte affects the ionic conductivity, which will further affect the rate performance of the battery as well as the diffusion of reactive species that can cause self-discharge, particularly in thick electrode devices. To this end, we explored the implications of device performance using Na0.44 MnO2 cathode material, NaTi2 (PO4 )3 , anode material in a NaClO4 -based aqueous solutions with molarities as high as 5. Experiments included physical property characterizations, cyclic voltammetry, and constant current charging/discharging methods. Preliminary results indicate that, in cells with thick electrodes (∼1mm), rate capability and electrode utilization increased significantly with higher molarity solutions: capacity at the 1.5 C rate increased 38% by increasing the salt concentration from 1 M to 5 M. At the same time the oxygen-related self-discharge phenomenon was diminished when using higher electrolyte molarities, though there was still measurable loss in capacity in in the electrodes. Irreversible capacity loss was observed to occur even in electrolytes with the lowest oxygen content, suggesting that self discharge and capacity loss are not necessarily causally related. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0121506jes] All rights reserved. Manuscript submitted January 12, 2015; revised manuscript received February 9, 2015. Published February 20, 2015.

Clean and renewable energy technologies will require low cost batteries. As such, aqueous electrolyte alkali-ion batteries are promising solutions for those applications where the constraints on energy density and weight are less rigid compared to mobile or portable applications. The use of an aqueous electrolyte offers several advantages, specifically for large-scale energy storage applications, compared with batteries that are based on organic electrolyte blends. Neutral pH aqueous electrolyte batteries are non-flammable, have fast internal ion transportation and have the promise of having a relatively lower manufacturing cost. However, the stability window of water limits the voltage of an aqueous cell. Researchers have found the practical stability window of aqueous electrolyte is wider than the theoretical limit due to the kinetic effect.1–5 This enables the usage of materials such as LiMn2 O4 whose operating potential exceeds the thermodynamic limit of pure water in the aqueous system.6–8 Since Dahn’s group first reported VO2 /LiMn2 O4 system with aqueous electrolyte in 1990s,9,10 many groups studied potential electrode materials which are viable in aqueous electrolyte: LiV3 O8 /LiCoO2 , LiV3 O8 /Li[Ni1/3 Co1/3 Mn1/3 ]O2 , TiP2 O7 /LiMn2 O4 , LiTi2 (PO4 )3 / LiMn2 O4 , LiTi2 (PO4 )3 /LiFePO4 .6,11–14 Their sodium analogs also show attractive performance: the activated carbon Na0.44 MnO2 hybrid system has demonstrated good cycle stability over 1000 cycles,15,16 NaTi2 (PO4 )3 /Na0.44 MnO2 system has demonstrated good rate capabilities.17 However, capacity fading of aqueous cells is often reported in the literature with different electrode material systems.3,13,17,18 Dissolved oxygen reacting with inserted Li/Na ions was believed to be one of the causes for capacity fading.3 In our study, we closely examined the influence of dissolved oxygen on anode stability. To make aqueous electrolyte batteries economically viable, relatively thick electrode structures must be used.5,19 Electrolyte with good ionic conductivity is then needed to support such thick format electrodes to enhance ion transport throughout the device, thereby reducing ionic polarization. To this end, it is of interest to probe the relationship between electrolyte salt content, ionic conductivity, and overall device stability. We elected to study the performance of an all-sodium battery device that used NaTi2 (PO4 )3 as the active anode material and Na0.44 MnO2 as the cathode material. To probe the importance of electrolyte salt content in an aqueous solution, NaClO4 was used with concentrations ranging from 0.1 M to 5 M. This salt was chosen because of its high solubility in water, though there are

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E-mail: [email protected]

other electrolyte salts that may also be attractive for environmental reasons. Experimental Synthesis of electrode materials.— Na0.44 MnO2 was synthesized via a method previously reported in literature by mixing Na2 CO3 with MnO in a 0.25:1 ratio and ball milled (Spex 8000 Mixer Mill) via a SiC crucible for 90 minutes.15 The precursor was fired at ∼750◦ C for 10 hours with heating and cooling ramp rate of 5◦ C/min in air. NaTi2 (PO4 )3 was synthesized via a solid-state method previously described in literature.18 Stoichiometric precursor materials to synthesize NaTi2 (PO4 )3 were mixed with 5 wt% graphite and ball milled in SPEX SamplePrep 8000D MIXER/MILL for 10 hours. The ball milled precursors were then fired at ∼950◦ C for 10 hours under an Argon working gas with heating ramp rate of 10◦ C/min. Materials property characterization.— X-ray diffraction data has been collected via an X’Pert diffractometer with the PIXcel detector equipped with a Cu Kα radiation (λ = 1.54056 Å), the scan ranged from 2θ = 10◦ to 60◦ with step size 0.05◦ . Surface morphologies of both cathode and anode materials were observed via Philips XL30 Scanning Electron Microscope. Electrochemical characterization.— Cyclic voltammetry (CV) and constant current charging/discharging tests were completed using a Bio Logic VMP3 Multi-Channel Potential/Electrochemical Impedance Spectrometer. For the cyclic voltammetry studies, an Hg/Hg2 SO4 electrode (MSE, Koslow Scientific Company, 0.634 V vs. S.H.E) was used as reference electrode. A piece of platinum foil rinsed in ethanol and placed in an ultrasonic bath for 20 minutes was used as the counter electrode. Working electrodes for CV composed of 80% active materials, 10% carbon black and 10% Polytetrafluoroethylene (PTFE, Shamrock Fluoro FG) were pressed onto a stainless steel mesh into a thin layer with an area around 0.5 cm2 via a manual hydraulic press (Carver, Inc.) at 8 metric ton per square centimeter. Electrodes for GCPL test were made with the same composition. Cathode materials were pressed into a 19 mm pellet with thickness of ∼950 μm and mass loading 100 mg/cm2 while anode materials were pressed into 13 mm diameter pellet with thickness of ∼530 μm and mass loading 113 mg/cm2 . The mass ratio of cathode to anode is set to 4:1 in order to make this setup an anode limited system. The capacity ratio is 3:2 based on active materials.

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Journal of The Electrochemical Society, 162 (6) A803-A808 (2015) Results and Discussion

Physical properties.— X-ray diffraction experiments were performed to verify the crystal structure of as-synthesized materials. Fig. 1 shows that diffraction patterns of both cathode and anode materials synthesized via solid-state methods have good agreement with the reference patterns. Some traceable amount of Na0.7 MnO2 was observed in Fig. 1a. Fig. 2a and 2b show the needle-like morphology of Na0.44 MnO2 which was consistent with literatures.15,16,20 Fig. 2c and 2d show the surface morphology of carbon coated NaTi2 (PO4 )3 where there is a thin layer of graphite covering the particles of NaTi2 (PO4 )3 which was introduced during precursor preparations. Our group has reported the effect of the intimate carbon on the electrochemical performance of NaTi2 (PO4 )3 .18 Electrochemical performance.— The ionic conductivity of electrolyte with different concentrations has been measured at 25◦ C (WTW Cond 3210). Fig. 3 has shown that ionic conductivity increases with increasing concentration before 5 M and non-linear correlation after 5 M. The highest ionic conductivity is at 5 M NaClO4 : 180 mS/cm. The non-linear behavior after 5 M is due to the effect of increased static electrical force between ions when the distance be-

Figure 1. XRD patterns of a) Na0.44 MnO2 and b) reference pattern ICDD #27-0750, c) NaTi2 (PO4 )3 with graphite and d) reference pattern ICDD #33-1296.

Figure 2. SEM images of a,b) Na0.44 MnO2 ; c,d) NaTi2 (PO4 )3 with graphite.

tween them decreases; the static force starts to impede the movement of ions which results in lower ionic conductivity compared to diluted solutions.21 The ionic conductivities of 0.1 M, 1 M and 5 M NaNO3 ; 0.05 M, 0.5 M and 2 M Na2 SO4 has also been measured. These data show that the ionic conductivity values observed were not strongly related to the anion species employed. To understand the effect of electrolyte concentrations on the electrochemical performance of NaTi2 (PO4 )3 /Na0.44 MnO2 system, we’ve conducted cyclic voltammetry experiments on both cathode and anode materials in aqueous electrolyte with different concentrations of NaClO4 : 0.1 M, 1 M and 5 M. The scan rate was 0.1 mV/s, which is intended for better observations of all electrochemical processes. Fig. 4a shows the cyclic voltammetry data of both cathode and anode materials in 5 M NaClO4 electrolyte. Na0.44 MnO2 shows three typical redox pairs located at −0.30 V, −0.07 V and 0.15 V (versus MSE). NaTi2 (PO4 )3 exhibits one pair of Na-ion insertion/extraction peaks at −1.29 V and −1.09 V (versus MSE) with insertion of Na-ion before the onset of significant hydrogen evolution. The cyclic voltammetry results agree well with previous reports and the insertion and extraction potentials in aqueous electrolyte are consistent with that observed in the organic electrolyte.15–18,22,23 Fig. 4b and 4c show the cyclic voltammetry results of identical electrodes in electrolyte with different concentrations: 0.1 M, 1 M and 5 M.

Figure 3. Ionic conductivities of aqueous Na2 SO4 , NaNO3 and NaClO4 electrolyte with different molarities at 25◦ C.


Figure 4. Cyclic voltammetry result of a) NaTi2 (PO4 )3 and Na0.44 MnO2 in 5 M NaClO4 at scan rate 0.1 mV/s; b) Na0.44 MnO2 in 0.1 M, 1 M and 5 M NaClO4 at 0.1 mV/s rate; c) NaTi2 (PO4 )3 in 0.1 M, 1 M and 5 M NaClO4 at 0.1 mV/s rate.

At higher concentrations, the redox peaks of both samples are sharper and closer which indicate faster kinetics. This is consistent with the ionic conductivity difference shown in Fig. 3. The equilibrium potentials for both cathode and anode are shifting to more positive values with increasing concentration, as expected by the

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Figure 5. Relationship between the electrode potential and the specific capacity calculated by integrating the first anodic current peak of cyclic voltammetry for samples with different mass loadings (Sample A: 20 mg/cm2 and Sample B: 40 mg/cm2 ) in 0.1 M, 1 M and 5 M NaClO4 .

Nernst Equation. Similar phenomenon has been observed when testing electrode materials in both organic electrolyte and aqueous electrolyte where the ionic conductivity difference is more profound.23 Fig. 5 shows the relationship between the specific capacity and electrode potential by integrating the first anodic peak current of cyclic voltammetry from initial potential to equilibrium potential of NaTi2 (PO4 )3 . To study the importance of electrode thickness, electrodes of two different loadings were tested. Sample A has

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Figure 6. Rate performances of 2 electrode cells with different electrolyte concentrations: 0.1 M, 1 M and 5 M. The cathode contains 100 mg/cm2 Na0.44 MnO2 with thickness of 950 μm while the anode contains 113 mg/cm2 carbon coated NaTi2 (PO4 )3 with thickness of 530 μm.

mass loading of 20 mg/cm2 and sample B has mass loading of 40 mg/cm2 . Both samples were tested in 0.1 M, 1 M and 5 M NaClO4 . The potential difference between samples with high mass loading and low mass loading becomes smaller in electrolyte with higher salt concentrations, showing the quantitative impact of higher salt concentration electrolytes in facilitating ion transport in thicker electrodes. The specific capacity calculated in 5 M NaClO4 is around 107 mAh/g which is approximately 80% of theoretical capacity of NaTi2 (PO4 )3 , which has been made and documented elsewhere.13,18,23 Fig. 6 shows the rate performance of a Na0.44 MnO2 /NaTi2 (PO4 )3 thick electrode full cell in electrolyte with different concentrations. The specific capacity value is calculated with respect to active mass of anode materials due to the anode-limited design used. Fig. 7 shows the discharge profiles of NaTi2 (PO4 )3 /Na0.44 MnO2 full cell in electrolyte with different concentrations. The three batches have similar capacity values at 0.1 C rate while at higher rates the capacity is significantly lower for the cell with a lower salt concentration. When cycled at 1.5 C rate the cell with 0.1 M NaClO4 retained only 13.3% of the initial capacity at 0.1 C rate while the cell with 1 M and 5 M NaClO4 retained 37.8% and 54.8%. This result is in good agreement with the ionic conductivity measurement and cyclic voltammetry results. Within a certain range, higher electrolyte concentration enables faster kinetics. These data suggest that the electrolyte concentration could significantly affect the rate performance. Additionally, the columbic efficiencies of these cells are over 95% after first few cycles under constant current charging/discharging test without rest between each cycle. The magnitude of this effect will be greater for thicker, denser electrode structures. The impact of dissolved oxygen.— The self-discharge phenomenon of intercalation materials in aqueous electrolyte has been reported by many groups.3,9,24 Luo et al. has reported the reaction between intercalated Li and O2 dissolved in electrolyte which could be one of the reasons for the poor cycle performance of aqueous electrolyte cells.3 Analogous to this Li system, the reaction between intercalated Na with oxygen and water is shown in Equation 1.3,9,10 Na(intercalated) + (1/4)O2 + (1/2)H2 O ⇔ Na+ + OH−

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The oxygen solubility in aqueous electrolyte as a function of concentration at 25◦ C was measured and is shown in Table I. Fig. 8 shows the measurement of the open circuit potential of Na3-x Ti2 (PO4 )3 in a Swagelok cell with/without a rubber cap sealing the top. The cell without a rubber cap sealing the cap apparently has a

Figure 7. Discharge profiles of 2 electrode cells with different electrolyte concentrations: 0.1 M, 1 M and 5 M at different discharge rate.

faster self-discharge rate: the potential of Na3-x Ti2 (PO4 )3 would only sustain less than 10 hours while the potential of the cell with rubber cap lasts 5 times longer at equilibrium potential. Although the cell with rubber cap is not perfectly sealed, the difference between the hours each cell lasted at equilibrium confirmed that it is the access to new oxygen instead of oxygen dissolved in electrolyte that would


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Table I. Oxygen solubility in NaClO4 solution with different concentrations. Salt Concentration (molar/L)

Oxygen Solubility (mg/L)

0 0.1 1 2 3 5 6 7 8 9 10

8.2 7.8 7.7 7.5 6.6 5.3 4.2 3.4 2.1 2.4 3.1

significantly affect the self-discharge. It is worth mentioning that during the test, we constantly monitored the electrolyte level to make sure the electrodes were always submerged in electrolyte. Fig. 9 shows the self-discharge time for same anode materials in electrolyte with different molar concentrations of NaClO4 in an open system: 1 M, 3 M and 5 M. It obviously shows the sample in electrolyte with higher molarities retain the equilibrium potential longer. This result is consistent with the oxygen solubility shown in Table I. Fig. 9b shows the time the electrode can sustain at equilibrium potential at different molarities: the sample in 5 M NaClO4 can last over 10 hours while the one in 1 M NaClO4 can only last 2 hours. Electrolyte with higher salt concentration has relatively lower oxygen concentration initially. During the open circuit potential period the oxygen diffuses toward the electrode and the self-discharge reaction happens. The slower the diffusion of oxygen to the electrode surface, the longer the materials will hold at equilibrium potential. This observation further confirms our conclusion that the access to oxygen causes the self-discharge in this aqueous system. Given the volume of electrolyte contained within the full volume of the coin cell (∼1.4 cm3 ) and considering the maximum possible oxygen solubility in aqueous solution at the different salt concentrations, we can estimate that the cell contains approximately 3.6 × 10−7 mole of oxygen. According to in Equation 1, this amount of oxygen would consume 1.4 × 10−6 moles of intercalated Na which is about 7 × 10−7 mole of Na3 Ti2 (PO4 )3 . Therefore, the total mass of NaTi2 (PO4 )3 that could react with dissolved oxygen when charged is 0.28 mg; this is an extremely small value compared with the total active anode mass in our coin cell: ∼110 mg. In a practical case, the amount of oxygen

Figure 9. a) OCV of charged anode in electrolyte with different salt concentrations in three electrodes setup; b) durations at equilibrium potential in different salt concentrations.

in a coin cell is even lower considering the electrolyte won’t take up the total volume of coin cell and the oxygen dissolution is lower with higher salt concentrations. These data suggest that the self-discharge caused by dissolved oxygen should be extremely minimal and should have nearly no impact on cycle life when thick format electrodes were utilized since the oxygen will be almost immediately consumed given there is no access to new oxygen. In addition, increasing salt concentration helps reduce the amount of dissolved oxygen, which could further reduce the influence of oxygen. Figs. 8 and 9 also suggest that the oxygen-related self-discharge effect can be minimized in a high molarity electrolyte and sealed environment. However, we still observed approximately 5% capacity fade after 30 cycles in Fig. 6 given the sealed cell with limited cycling potential window for these three cells. In addition, the columbic efficiency is over 95%. These results suggest that oxygen-related selfdischarge is not the primary cause for capacity loss in aqueous systems. We have been actively conducting experiments to understand the true nature of capacity loss for this aqueous electrolyte system. Conclusions The effect of electrolyte salt concentrations and the presence of oxygen dissolved in electrolyte on the electrochemical performance and stability of NaTi2 (PO4 )3 /Na0.44 MnO2 sodium-ion battery system was examined. As expected, rate capability increased with higher salt concentration electrolyte up to approximately 5 M, above which the benefits were minimal. The molarity of the electrolyte affected the rate of dissolved oxygen content present which in turn impacted the self-discharge rate in unsealed test vessels. Analysis shows that, however, thick format electrodes packaged in devices with minimal free electrolyte should suffer virtually no self discharge due to dissolved oxygen. Significant capacity fade was observed even in these oxygen starved cells with high molarity electrolyte, a finding that suggests that there is not a strong correlation between the oxygen mediated self-discharge and loss of materials function. Concurrently, highly concentrated electrolyte can potentially raise challenging issues such as corrosion, especially at more extreme electrochemical potentials. Elucidating the effect of concentrations of electrolyte on the corrosion issues will be a topic of future study. Acknowledgment

Figure 8. OCV of charged anode in non-sealed and sealed three electrode cell.

The authors thank for the funding and support from Carnegie Mellon University, The Department of Energy and Aquion Energy.

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Journal of The Electrochemical Society, 162 (6) A803-A808 (2015) References

1. C. Wessells, R. A. Huggins, and Y. Cui, Journal of Power Sources, 196, 2884 (2011). 2. R. Ruffo, C. Wessells, R. A. Huggins, and Y. Cui, Electrochemistry Communications, 11, 247 (2009). 3. J.-Y. Luo, W.-J. Cui, P. He, and Y.-Y. Xia, Nat Chem, 2, 760 (2010). 4. C. Wessells, R. Ruffo, R. A. Huggins, and Y. Cui, Electrochemical and Solid-State Letters, 13, A59 (2010). 5. J. F. Whitacre, S. Shanbhag, A. Mohamed, A. Polonsky, K. Carlisle, J. Gulakowski, W. Wu, C. Smith, L. Cooney, D. Blackwood, J. C. Dandrea, and C. Truchot, Energy Technology 3, 20 (2015). 6. J. Y. Luo and Y. Y. Xia, Advanced Functional Materials, 17, 3877 (2007). 7. Y.-g. Wang, J.-y. Luo, C.-x. Wang, and Y.-y. Xia, Journal of The Electrochemical Society, 153, A1425 (2006). 8. W. Tang, Y. Hou, F. Wang, L. Liu, Y. Wu, and K. Zhu, Nano Letters, 13, 2036 (2013). 9. W. Li and J. R. Dahn, Journal of The Electrochemical Society, 142, 1742 (1995). 10. W. Li, W. R. McKinnon, and J. R. Dahn, Journal of The Electrochemical Society, 141, 2310 (1994). 11. G. J. Wang, L. J. Fu, B. Wang, N. H. Zhao, Y. P. Wu, and R. Holze, Journal of Applied Electrochemistry, 38, 579 (2007). 12. J. K¨ohler, Electrochimica Acta, 46, 59 (2000). 13. H. Wang, K. Huang, Y. Zeng, S. Yang, and L. Chen, Electrochimica Acta, 52, 3280 (2007).

14. X.-H. Liu, T. Saito, T. Doi, S. Okada, and J.-i. Yamaki, Journal of Power Sources, 189, 706 (2009). 15. J. F. Whitacre, A. Tevar, and S. Sharma, Electrochemistry Communications, 12, 463 (2010). 16. A. D. Tevar and J. F. Whitacre, Journal of The Electrochemical Society, 157, A870 (2010). 17. Z. Li, D. Young, K. Xiang, W. C. Carter, and Y.-M. Chiang, Advanced Energy Materials, 3, 290 (2013). 18. W. Wu, J. Yan, A. Wise, A. Rutt, and J. F. Whitacre, Journal of The Electrochemical Society, 161, A561 (2014). 19. J. F. Whitacre, T. Wiley, S. Shanbhag, Y. Wenzhuo, A. Mohamed, S. E. Chun, E. Weber, D. Blackwood, E. Lynch-Bell, J. Gulakowski, C. Smith, and D. Humphreys, Journal of Power Sources, 213, 255 (2012). 20. Y. Cao, L. Xiao, W. Wang, D. Choi, Z. Nie, J. Yu, L. V. Saraf, Z. Yang, and J. Liu, Advanced Materials, 23, 3155 (2011). 21. A. H. H. Car Hamann and Wolf Vielstich, Electrochemistry, Second Completely Revised and Updated Edition, WILEY-VCH VerlagGmbH & Co. KGaA, Weinheim (2007). 22. W. Wu, A. Mohamed, and J. F. Whitacre, Journal of The Electrochemical Society, 160, A497 (2013). 23. S. I. Park, I. Gocheva, S. Okada, and J.-i. Yamaki, Journal of The Electrochemical Society, 158, A1067 (2011). 24. C. Wessells, F. La Mantia, H. Deshazer, R. A. Huggins, and Y. Cui, Journal of The Electrochemical Society, 158, A352 (2011).