Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type NaxNi0.22Co0.11Mn0.66O2NaTi2(PO4)3

Redox flow batteries (RFB) are promising technologies for energy storage due to the long life, low cost, high round-trip efficiency and independent scalability of energy and power capabilities. Semi-solid flow batteries (SSFBs) are a special class of RFB, in which anolyte and catholyte consist of flowable suspensions of solid active materials rather than dissolved redox species. Thus, the concentration of active redox centres in the anolyte and catholyte of the SSFB can be significantly increased. Using intercalation type active materials such as those typically used in Li-ion batteries (LIBs), e.g. Li4Ti5O12, LiCoO2 or LiNi0.5Mn1.5O4, the energy densities can reach up to 300–500 W h L , which is more than 10 times higher than that of all-vanadium RFBs (40 W h L ). Compared to LIBs, SSFBs present several advantages: (I) power and energy can be scaled independently, (II) the amount of inactive materials such as current collectors or housing is decreased, and (III) the manufacturing processes become simpler and more cost-effective. Sodium-ion batteries (SIBs) attracted increasing attention in the past few years since sodium is abundant, inexpensive, and does not alloy with aluminium which allows for cheaper current collectors. Energy densities of ca. 200 W h kg 1 have been proposed to be achievable. Even more importantly, while sodium intercalation compounds do not necessarily exhibit similar performance like their lithium counterparts, sodium does offers an even larger variety of crystal chemistries than lithium. As such, SIB technology is still considered to be in its infancy, and new active materials are developed. A key aspect in both LIB and SIB is the formation of the so called solid electrolyte interphase on negative (SEI) and positive (CEI) electrodes operated outside the electrochemical stability window due to reductive or oxidative decomposition of the carbonate based electrolyte. In the case of SSFBs, the formation of these passivating films has a specific detrimental effect since it hinders the electrical connection between the current collector and single particles dispersed in the electrolyte. In consequence, Duduta et al. employed Li4Ti5O12 as negative electrode material since it operates above the reduction potential of carbonate electrolytes to construct the first proof of principle of nonaqueous Li-ion SSFB. For SIBs, however, the search for ‘‘SEI-free’’ negative electrodes is more difficult, since the intercalation of sodium into the analogues of Li4Ti5O12 or TiO2 does not operate within the stability window of the electrolyte. The NASICON material NaTi2(PO4)3 (NaTP), however, does operate at a very flat potential plateau located at around ca. 2.1 V vs. Na/Na, which is well above the stability limit of typical electrolytes. As it can be seen from Fig. 1a, a charge capacity of 125 mA h g 1 can be utilized when cycled as solid film electrode versus metallic sodium in a three-electrode Swagelok cell, which is very close to the theoretical value of 133 mA h g . The positive electrode material should likewise operate at high potentials just below the potential of electrolyte oxidation. As shown in Fig. 1b, P2-type NaxNi0.22Co0.11Mn0.66O2 (NaNCM) has been demonstrated to store reversibly ca. 130 mA h g 1 in the range 4.3–2.1 V with excellent cyclibility. Detailed structural characterization of NaTP and NaNCM is given in the ESI.† On the base of the electrochemical performances of these materials, we selected NaTP and NaNCM as negative and positive electrode a Catalonia Institute for Energy Research, Jardins de les Dones de Negre, e1, 08930 Sant Adria de Besos, Barcelona, Spain. E-mail: edgar.ventosa@rub.de b Helmholtz Institute Ulm (HIU) Electrochemistry I, Helmholtzstrasse 11, 89081 Ulm, Germany c Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany d Institute of Physical Chemistry, University of Muenster, Corrensstrasse 28/30, Muenster, 48149, Germany e Analytical Chemistry – Center for Electrochemical Sciences (CES), Ruhr-University Bochum, Universitätsstr. 150, 44780 Bochum, Germany f Departament d’Electronica, Facultat de Fisica, Universitat de Barcelona, Martı́ i Franques 1, 08028 Barcelona, Spain † Electronic supplementary information (ESI) available: Experimental details, XRD patterns, SEM images. See DOI: 10.1039/c4cc09597a Received 1st December 2014, Accepted 22nd December 2014

Park. [27]60 mmol of Ti(IV) isopropoxide was slowly dissolved in a mixture of 240 mL 30% H 2 O 2 as oxidant and 90 mL 25% NH 3 ammonia as complexing agent to yield a clear yellow solution.120 mmol citric acid monohydrate, 90 mmol ammonium dihydrogen phosphate and 15 mmol sodium carbonate were subsequently added.Adjusting the pH value to 6-7 using nitric acid resulted in heat generation and foaming while the solution turned orange.120 mmol ethylene glycol was added as crosslinker.Raising the temperature to 80 °C resulted in the formation of a viscous orange gel and a further increase of the temperature resulted in the exothermic formation of a sticky brown resin indicating crosslinking of citrate and ethylene glycol.The resin was pyrolysed in air at 350 °C for 2 h, reground and calcined again at 800 °C (5 °C/min) in air for 8 h.The resulting white, crystallized NaTP powder was carbon-coated to increase conductivity by heat treatment with 10 wt.-% of glucose in Ar-flow for 6 h at 700 °C using a tubular Quartz oven.The resulting black powder contained ca. 2 wt.-% elemental carbon as determined by elemental analysis.

Structural characterization.
For NaNCM, X-ray powder diffraction (XRD) was performed using the Cu K α radiation on the Bruker D8 Advance diffractometer (Germany) in the 2θ range from 10° to 90°.The Rietveld refinement of structures was performed using the TOPAS software.For C/NaTi 2 (PO 4 ) 3 , a Bruker ACS Advance diffractometer (Cu K α , 2θ from 10° to 60°) was used.Elemental analysis of NaTi2(PO4)3 was performed in the central analysis facilities of the Ruhr-University Bochum using an AAS 6 vario (Analytik Jena) for Na and Ti determination and an vario EL (Elementar Hanau) for determination of carbon content.The Na:Ti was found to be 1.05:1.
The stoichiometry of several samples of the P2-NaNCM were checked via Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) with a Spectro ARCOS ICP-OES (Spectro Analytical Instruments, Kleve, Germany) instrument with axial plasma viewing in some previous works.The ratio of the transition metals was in accordance with previous works and confirmed the formula Na x Ni 0.22 Co 0.11 Mn 0.66 O 2 , whereas the Na content slightly varied in the range of 0.40 ≤ x ≤ 0.46. 23,26ectrode preparation The solid electrodes were prepared by mixing the active material, the conductive additive (Super C65, SFG6, Timcal) and the binder (PVdF, Solef S5130, Solvay) in NMP at a weight ratio of 85:10:5 (NaNCM:C65:PVdF) and 80:7:5:8 (NaTP:C65:SFG6:PVdF) The mixture was cast onto aluminum foil using Doctor-Blade technique.The solvent was evaporated at 80 °C, disc electrodes were cut (12 mm diameter), pressed (NaNCM) and then dried at 120 °C under vacuum overnight.The electrode loading was 2.5 -2.75 mg cm -2 for NaNCM and 2.76 mg cm -2 for NaTP.
The suspensions were prepared by mixing active material and conductive additive in 0.5 M NaPF 6 in EC:DMC (Sigma-Aldrich) by magnetic stirring overnight.For the positive fluid electrode, 1.25 g of NaNCM and 0.12 g of Ketjenblack EC-600 (AkzoNovel) were mixed in 6 mL of electrolyte.For the negative fluid electrode, 1.25 g of NaTP and 0.10 g of Ketjenblack EC:-600 were mixed in 6 mL of electrolyte.Less amount of carbon additive was added in the negative electrode suspension since NaTP was already carbon-coated.This journal is © The Royal Society of Chemistry 2012 Cell assembly and electrochemical characterization.
For the evaluation of the solid electrode, three-electrode Swagelok cell was assembled in Ar-filled glovebox (O 2 < 2 ppm and H 2 O < 1 ppm).Sodium metal was used as counter and reference electrodes.
Whatman GF/D glass fiber filters served as separators.The electrolyte was 1 M NaPF 6 (Sigma-Aldrich) in propylene carbonate (UBE) for NaNCM and 1 M NaClO 4 (Sigma-Aldrich) in propylene carbonate Between each element, several gaskets must be introduced to avoid slurry leakages.Finally, the whole system is closed by metallic end-plates to give consistency to the system.Figure S3

(
Merck) for the NaTP.Electrochemical measurements were performed with a Maccor series 4000 battery tester (USA) for the NaNCM and, for the NaTP, a Bio-Logic VMP-3 (Bio Logic SAS, Claix, France) at charge rate of 0.1 C, being 1 C equivalent to 123 mA g -1 and 117 mAh g -1 for NaNCM and NaTP, respectively.Potential window was 4.3 -2.1 V and 2.5 -1.5 V vs. Na metal for NaNCM and NaTP, respectively For the evaluation of the fluid electrodes, a filter-press cell (FigureS1) was used.The cell was assembled inside an Ar-filled glove-box (O 2 and H 2 O < 1 ppm).The filter press-cell configuration consists of several independent elements sandwiched (e.g.current collector, gaskets, channel frames) to define reaction zones (negative suspension and positive suspension compartments) separated by a Celgard 2500 film.
illustrates the three electrode configuration cell used in this study.The semi-solid suspension containing solvents (alkyl carbonates), conductive salt (NaPF 6 ), conductive agent (carbon black) and electroactive particles is pumped through a Teflon plate, an Ethylene Propylene Diene Monomer (EPDM) gasket, and the positive (titanium plate) or negative (titanium plate) current collector until reaching the channel frame.The channel frame consists of a 0.5 mm thick Teflon frame, sandwiched by two 0.5 mm thick gaskets made of expanded Teflon (ePTFE).Together they form a 75 mm long x 4 mm wide x 1 mm (approximately 0.3 mL slurry and 3 cm 2 ) deep channel.The working electrode zone is limited by the separator (Celgard