A novel concept of Semi-solid, Li Redox Flow Air (O2) Battery: a breakthrough towards high energy and power batteries

Electrochimica Acta 206 (2016) 291–300

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

A novel concept of Semi-solid, Li Redox Flow Air (O2) Battery: a breakthrough towards high energy and power batteries$ Irene Ruggeri1 , Catia Arbizzani1, Francesca Soavi1,* Department of Chemistry “Giacomo Ciamician”, Alma Mater Studiorum
Università di Bologna, via Selmi 2, 40126 Bologna, Italy

A R T I C L E I N F O

Article history: Received 29 January 2016 Received in revised form 30 March 2016 Accepted 24 April 2016 Available online 26 April 2016 Keywords: organic Li/O2 battery semi-solid catholyte redox flow battery Lithium Redox Flow Air (O2) Battery porous carbon

A B S T R A C T

Worldwide efforts are being devoted to promote an efficient use of renewable energy sources and sustainable electric transportation. High efficiency energy conversion systems like batteries, which store/ deliver high energy and power densities, are under development. While Li-ion batteries (LIBs) are the best performing batteries on the market and redox flow batteries are already used for stationary plants, a drastic step forward is needed to increase energy and power performance and decrease costs. Li/O2 batteries are considered the next generation due to significantly higher energy delivery than LIBs. We demonstrate a radically new battery concept: a non-aqueous Li/O2 battery that operates with a semisolid, flowable catholyte. The proof-of-concept is proven by a catholyte based on 2% wt. SuperP carbon dispersed in tetraethylene glycol dimethyl ether – lithium bis(trifluoromethane)sulfonimide. Oxygen redox reaction at the semi-solid catholyte is investigated by electrochemical, morphological and spectroscopic analyses. The perfomance of a semi-solid, flow Li/O2 battery prototype operating at high discharge rates (up to 4 mA cm
2) with high discharge capacity (>175 mAh cm
2), energy (>500 mWh cm
2) and power (>7 mW cm
2) is reported. The strategies to approach the challenging target of 1 kWh kg
1 and 2 kWh L
1 are also discussed. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Redox flow batteries (RFBs) and Li-ion batteries (LIBs) are energy storage/conversion systems that play a key role for high penetration of intermittent and decentralized renewable energy sources and of electric vehicles. The basic design of RFBs includes two liquid electrolytes with soluble redox couples (anolyte and catholyte) that flow through separate compartments where the redox processes take place. Energy and power are decoupled, being the former related to catholyte and anolyte volume and composition and the latter to the rate of the processes at the electrode/ electrolyte interfaces. For this unique advantage and the operational flexibility, RFBs are already used for large scale electrical energy storage. One of the most developed RFBs is the aqueous vanadium redox that operates at 1.6 V with an energy density ca.

$ The semi-solid flow Li/O2 catholyte concept is under patent by the authors and Alma Mater Studiorum – University of Bologna (102015000040796_31/07/2015_61. U2164.12.IT.41). * Corresponding author. Francesca Soavi, Department of Chemistry “Giacomo Ciamician” Alma Mater Studiorum
Università di Bologna, via Selmi 2, 40126 Bologna, Italy. Tel.: +39 0512099797; fax: +39 0512099365. E-mail address: [email protected] (F. Soavi). 1 ISE members.

http://dx.doi.org/10.1016/j.electacta.2016.04.139 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

40 Wh L
1 and a specific energy of 25 Wh kg
1 using a chloride supporting electrolyte [1,2]. The low RFB energy density, limited by the cell operation voltage and active materials concentration, excludes any application in the electric vehicle field where Li-ion batteries featuring up to 150–200 Wh kg
1 are considered the best candidates [3]. Much research efforts are being devoted to the increase of energy and power of RFBs, lithium batteries and LIBs with attention to costs, safety and reliability. As it concerns RFBs, the main strategies are the use of i) light, solid metal anodes, ii) O2 (air) based catholytes to reduce volume and weight, iii) organic electrolytes and active species to increase cell voltage above 2 V and to widen temperature operation, iv) semi-solid anolyte and/or catholyte to circumvent the active materials solubility issues of conventional RFBs [1,2,4–8]. The use of a metal anode, like Zn or Li, has been actively pursued in RFBs. While Zn
bromine RFBs are already on the market, the use of lithium is relatively new. It was early proposed by J. B. Goodenough and H. Zhou and the following works opened the research towards the challenging integration of lithium batteries and RFBs into Li-redox Flow Batteries (LRFBs) [1,2,4,9–13]. The recent advances in RFBs including those based on lithium have been excellently reviewed by G.G. Soloveichik [2] and Fig. 1

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compares the energy performance of different flow battery chemistries. The use of O2-catholyte in LRFBs is a valuable strategy to develop batteries outperforming conventional RFBs and not-flow Li/O2 batteries in terms of specific energy and rate response, respectively. This approach brought to a novel configuration of Li/ O2 battery, i.e. the Li-air flow battery (LAFB) [2,4,12–18]. The very high specific capacity of the Li anode and O2 cathode and the theoretical cell voltage of 3 V result in a practical specific energy of 500 Wh kg
1 that is 2–3 times higher than that of the best LIBs available today on the market [19]. Yet, despite much research effort devoted to Li/O2 (not flowing) battery development in the last decade, several scientific and technological issues still need to be addressed [19–24]. In conventional Li/O2 batteries, the O2 redox reaction (ORR) takes place at the solid electrode/electrolyte interface, the electrolyte at the cathode side can be organic or aqueous. The solid cathode is a carbon/catalyst layer that is deposited on the current collector and is fed by O2; the anode is typically Li metal foil. While understanding ORR mechanism Li/O2 batteries is very challenging and still under debate, it has generally accepted that the cathode reaction involves the formation on the cathode surface of superoxide, peroxide and oxide species of lithium, and of hydroxide in the case of aqueous Li/O2. These species are expected to be reoxidized to give O2 (g) during recharge. The low conductivity of the solid discharge products that are formed on the cathode surface affects cathode discharge capacity and determines high cathode recharge overpotentials [24–29]. In aprotic Li/O2 batteries, while cathode discharge potentials are ca. 2.5–2.7 V vs. Li+/Li, recharge potentials are typically higher than 3.5 V vs. Li+/Li. This makes the choice of electrolyte crucial. While carbonate-based electrolytes are not stable toward oxygen radical species and at the high potentials required for recharge, dimethyl sulfoxide, ionic liquids and solvent in salt tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes are considered good candidates [19–25,30–41]. TEGDME–based electrolytes with LiCF3SO3 or lithium bis(trifluoromethane)sulfonimide (LiTFSI) salts feature high boiling point, low volatility and can support highly reversible formation/reoxidation of Li2O2 [19,23,31,34]. However, the stability of such electrolytes in contact with the lithium metal anode is still under investigation [19,42,43]. A wide variety of materials like noble metals, carbon-supported transition metal systems and structured porous carbons are under study as electro-catalysts for ORR, as well as redox mediators have been proposed [44]. It is widely accepted that carbon electrodes of high specific pore volume can host large amounts of discharge reaction products and make it possible to overcome 3500 mAh g
1, along with fast oxygen diffusion [19,21,23,24,26,45]. Nanostructured carbons, like CNT, CNF, graphene, and N-doped carbons have been demonstrated to be superior to commercial carbons like SuperP and Vulcan XC-72, not only for their tailored porosity, which is beneficial for mass diffusion of the chemical species involved in ORR, but also for their specific catalytic effect on ORR [19,21,23,45]. Cathode passivation by discharge products is one of the most serious drawbacks of Li/O2 batteries [45], along with slow O2 mass transport which in the case of air breathing cells limits current densities to one order of magnitude lower than the values for commercial LIBs [15–19]. LAFBs could obviate these issues. In these batteries, the electrolyte (aqueous or organic) is fed with O2 and acts as O2 carrier flowing through the cell and transports ORR product away from the cathode current collector. The use of an O2–based catholyte that is continuously fed with O2, from air or from an external tank, renders cell discharge capacity less dependent on cathoyte volume with respect to conventional RFBs. This is beneficial to size and weight reduction. It has been reported that the theoretical specific energy of aqueous LAFB can

be as high as 400 Wh kg
1 while experimentally, power densities of 7.5 mW cm
2 at 5 mA cm
2 have been obtained which were limited by the use of the ceramic separator [17,18]. The ceramic separator also caused very high recharge overpotentials (>2 V). Better cyclability results have been achieved with an organic LAFB fed with O2-saturated ionic liquid electrolyte. This cell featured a discharge capacity up to 600 mAh g
1 of carbon electrode under discharge currents of 0.2 mA cm
2, with a recharge efficiency of 92% [15]. The flow-Li/O2 battery concept has also been recently exploited by adding soluble redox mediators which catalyze the O2 reduction and evolution in the flowable electrolyte [16]. Y. G. Zhu et al. reported about a flow Li/O2 battery with soluble redox catalysts that comprises a lithium metal anode separated from a carbon felt cathode by a membrane, and a gas diffusion tank connected to the cathodic compartment by a pump. LiTFSI – TEGDME electrolyte with soluble redox catalysts is circulated between the gas diffusion tank and the cell. During discharge, O2 flows into the gas diffusion tank, and is reduced to form Li2O2. This is then deposited on the porous matrix of the tank. The gas diffusion tank thus assists in preventing passivation and pore clogging of the cathode. The authors of the paper postulate that the capacity of the cell would be limited by the size of the gas diffusion tank. The highest discharge capacity featured by this system is 45 mAh (11 mAh cm
2 of carbon felt cathode) at 0.125 mA cm
2 [16]. The need to decouple energy from power and to decrease the amount of inactive components has triggered the research towards the development of semi-solid, fluidic electrodes (strategy iv). Electroactive particles are dispersed in the electrolyte thus, the concentration of active materials in the anolyte and catholyte is not related to their solubility and can be significantly increased. However, the development of semi-solid RFBs requires an efficient management of viscous slurries. The use of a semi-solid anolyte with Zn particles which was circulated in the aqueous Zn/air cell has been proposed in the 70s’ by the Compagnie Générale d'Electricité (CGE) [46]. Recently, it has been demonstrated that the use of suspensions of Li-ion battery active materials, like LiFePO4, LiCoO2, and Li4Ti5O12, and carbon particles as electric percolating network, is an effective strategy to boost up power performance of Lithium batteries [2,47–52]. The projected specific energy and energy density of Li-LiCoO2 semi-solid batteries are 130–250 Wh kg
1 and 40–500 Wh L
1, respectively [47]. The use of semi-solid, fluidic electrodes has also been exploited for Li/polysulfides and Na-ion batteries, as well as for flow supercapacitors [53–58]. A lithium battery with a semi-solid, but not flowable, O2-catholyte has also been investigated by T. Zhang et al. in ambient air [59]. The cell comprises a gel cathode and an inorganic solid Li+ conductor catholyte (LTAP) featuring ca. 60000 mAh g
1 at 200 mA g
1, where the values are normalized over cathode content (0.8 mg cm
2) [59]. However, cyclability at deep discharges is still an issue and the evolution of the chemical/ physical properties of the semi- solid electrodes over cycling has not been deeply investigated yet. These are very challenging tasks that require the development of spectroscopic and microscopic techniques that should be tailored to the analysis of heterogeneous samples, like the semisolid slurry, where active species and side products are present at low concentration. Here, we propose for the first time a new LAFB concept: a nonaqueous, semi-solid lithium redox flow air (O2) battery (SLRFAB) that combines the high energy density of the Li/O2 battery with the flexible and scalable architecture of RFBs. The cell operates with a flowable semi-solid catholyte that is pumped through the cell and a lithium metal anode. The catholyte is a suspension of conductive carbon in O2-saturated, non-aqueous electrolyte. The concept differs from previously reported Li/O2 batteries, LAFBs and semi-solid RFBs because i) a low cost, metal-free, semi-

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Fig. 1. Projected values of volumetric energy density and specific energy of different flow battery chemistries [1,6,17,18,46,47,50,55],the theoretical target of not-flow Li/ O2 battery, the practical response of the SLRFAB proposed in this work (2% wt. SP/ 0.5 cm-thick RVC semi-solid flow Li/O2) and the projected values for an optimized system with 20% wt. SP catholyte and a 0.25 cm-thick RVC current collector. When specific energy and energy density data were not simultaneously available, they were reciprocally calculated considering catholyte and anolyte density of ca. 1 kg L
1.

solid catholyte which operates without any solubilised catalyst or mediator is flowing through the cell; ii) oxygen redox reaction (ORR) can take place at the dispersed carbon particle surface thus limiting current collector passivation and making it possible to achieve high discharge capacity densities; iii) the carbon percolating network of the semi-solid component multiplies electrode reaction sites and area which is beneficial for high practical current densities; iv) ORR products are expected to be deposited on the surface of the catholyte particles and can be electrochemically reoxidized; this positively impacts on the reduction of the volume of any external tank planned to recover discharge products. The proof-of-concept of the SLRFAB is demonstrated by lowcost commercial components that have been already widely investigated for conventional (breathing) Li/O2 batteries. Indeed, our approach is a new strategy to improve battery performance by a novel cell design that can be exploited for the development of advanced-material cells. A catholyte based on SuperP carbon (SP) in TEGDME-LiTFSI saturated with O2 was used. SP and TEGDMELiTFSI were selected because they are considered a useful platform for further development of Li/O2 batteries components and design [19,21]. The results of the galvanostatic tests carried out at porous current collectors like carbon paper (CP) and reticulated vitreous carbon coated with SP carbon (RVCSP) in conventional and flowcells are reported and discussed. 2. Experimental The electrolyte was a solution of tetraethylene glycol dimethyl ether (Sigma-Aldrich, TEGDME, 99.0%): bis (trifluoromethane sulfonyl) imide lithium salt (Sigma-Aldrich, LiTFSI, 99.0%) 9:1 molar ratio, which was prepared and stored in glove-box (MBraun, Ar atmosphere, H2O and O2 <1 ppm). Before use, the water content of the solution was checked by Karl-Fisher titration (831 KF Coulometer, Metrohm) and resulted 160 ppm. The catholye was prepared in glove-box by adding to the electrolyte Super-P (SP, 2% w/w, Erachem Comilog N.V., BET 65.5 m2 g
1) which was dried under vacuum at 120  C overnight (B585 Buchi oven), before use.

293

The viscosity of the electrolyte and of the catholyte with 2% w/w and 5% w/w of SP was evaluated by a Couette flow in a concentric cylinder (Anton Paar Physica MCR301 rotational viscometer, method MCR301-SN827409). The shear rate ranged from 0 to 200 s
1 with a sweep rate of 0.2 s
1. The current collectors were carbon paper (CP, Spectracarb 2050, Spectracorp, thickness 40 mils, density 0.64 g cc
1) and Reticulated Vitreous Carbon (RVC, ERG Aerospace Corporation, foam, 3% nominal density, 100 PPI, 0.5 cm-thick, 53 mg cm
2) which were dried under vacuum at 120  C overnight before use. The RVC was coated with SP carbon powder by drop casting 95% SP – 5% polyvinylidene fluoride (PVdF, Kynar HSV900) in N-methyl pyrrolidone ink (28 mg of SP per mL), followed by heating at 60  C overnight. The label RVCSP indicates SP-coated RVC. The O2 redox reaction (ORR) was investigated by galvanostatic measurements in a conventional, thermostated, 5 mL cell at CP (0.45-0.5 cm2) and RVCSP (0.6 cm2) electrodes and in a semi-flow cell with RVCSP (A = 0.385 cm2) electrode. The reference electrode was a silver wire in 610
2 M AgTFSI in PYR14TFSI; the electrode potentials were checked vs. Li and are reported vs. the Li+/Li couple. The electrolyte and catholyte were saturated with O2 (g) (>99.999%, SIAD). The flow Li/O2 battery was assembled in dry box. A cross-shaped Teflon cell (BOLA) with 2 fiber glass separators (Whatman GF/F) and 4 stacked Lithium disks (0.534 g cm
3, 0.64 cm2  300 mm each disk) was used. The Li/separator/RVCSP stacking was sandwiched between two stainless steel cylinder current collectors (A = 0.64 cm2). Two cell fittings were used for the cell stacking and the additional two fittings, orthogonal to the formers, were employed to provide catholyte circulation in a close circuit. Only the RVCSP electrode intercepted the catholyte flow in order to limit the contact of the Lithium surface with the O2 dissolved in the catholyte. The only path for the catholyte flow was through the cathode current collector. Circulation of the catholyte through the semi-flow cell was obtained by a Watson-Marlow 120S/DV peristaltic pump operating at 200 rpm with a 4.8 mm diameter silicon tube, which permitted a flow of ca. 170 mL min
1. The catholyte (TEGDME:LiTFSI 9:1 mol, 2% wt. SP, 30 mL) was continuously fed with O2 gas. For basic studies, an additional reference electrode (610
2 M AgTFSI in PYR14TFSI) which intercepted the catholyte flow out of the cell was used. The electrochemical tests were performed by a Bio-Logic VSP multichannel potentiostat/galvanostat with electrochemical impedance spectroscopy (EIS) module. EIS was performed in the 1 kHz-100 mHz frequency range with 5 mV AC perturbation and by taking 10 points/decade. X-ray diffraction measurements (XRD) were performed by a PANalytical X'Pert PRO powder diffractometer equipped with a X'Celerator detector (CuKa radiation, l = 1.5406 Å, 40 mA, 40 kV), radiation source and Ni filter by continuous scanning mode (0.04 2u s
1 scan rate, 0.05 2u step size). Micro Raman measurements were performed by an HORIBA-XploRATMPLUS with a l= 532 nm laser. FTIR analyses were carried by a Bruker Optics Tensor 27 apparatus (2 cm
1 resolution). TEM and SEM images were obtained by using a Philips CM100 (accelerating voltage 80 kV) and a Zeiss EVO 50, respectively; TEGDME was used as dispersing media for the preparation of TEM catholyte samples. 3. Results and discussion 3.1. Preliminary characterization of the semi-solid catholyte A preliminary study was performed in a 5 mL conventional electrochemical cell where the catholyte (TEGDME:LiTFSI 9:1 mol, 2% wt. SP) was continuously fed with O2 and mechanically stirred to simulate cathode flow operating conditions. The cathodic current collector was CP. A lithium counter electrode (in large

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Fig. 2. Electrode potential profiles during galvanostatic tests at  0.25 mA cm
2 of CP (0.45 cm2) in conventional electrochemical cell with stirred, O2-saturated catholyte or electrolyte. a) Discharge test: CP (wine line, CP/catholyte) and lithium (black line) response over long term discharge in the catholyte and CP discharge in the electrolyte without SP (green dashed line, CP/electrolyte); b) Recharge test: charge profile of a fresh CP in 45 h-discharged catholyte (wine line, CP/catholyte45h-d), of the 45 hdischarged CP in fresh electrolyte (wine dashed line, CP45h-d/electrolyte), and of a fresh CP in a fresh catholyte (black line, CP/catholyte).

amount and separated by the cathode compartment via a porous frit) and an Ag-based reference electrode were used. Fig. 2a compares the CP response in the stirred O2 saturated electrolyte without (CP/electrolyte) and with (CP/catholyte) dispersed SP particles under galvanostatic discharge at 0.25 mA cm
2. The CP/electrolyte and CP/catholyte open circuit potentials before discharge are similar, i.e. 2.90  0.03 V vs. Li+/Li. At 0.25 mA cm
2, the CP/catholyte potential is 2.7 V vs. Li+/Li (Fig. S2) and 300 mV higher than that of the CP/electrolyte system. This is due to the presence of the carbon percolating network of the catholyte that enhances the reaction surface area and, thus, reduces the electric ohmic losses. More importantly, for the first time, Fig. 2a demonstrates that the substitution of the conventional electrolyte with the semi-solid catholyte dramatically increases the discharge capacity of the Li/O2 cathode. Indeed, when SP is added to the electrolyte, the discharge time increases from 4 h (1 mAh cm
2) up to 11 days (66 mAh cm
2), with the discharge being limited, in the latter case, by lithium consumption as demonstrated by the fast rising of lithium potential. Replacing the lithium counter electrode with a fresh one made it possible to extend the discharge over 19 days, to reach the discharge capacity value of 114 mAh cm
2 and to achieve the areal energy density of 300 mWh cm
2. These are unprecedented high values for Li/O2 batteries. After 19 days the CP potential dropped down to 2 V vs Li+/ Li, presumably because of the formation of a passivation layer over its surface. Indeed, by substituting the used CP with a fresh one we could extend O2 reduction in the same catholyte over additional 10 days, after which the discharge was intentionally cut-off for diagnostic measurements. The 29-day discharge corresponded to an impressive, high discharge capacity of 175 mAh cm
2 and an

areal energy density of 490 mWh cm
2. We intentionally cut the discharge after 29 days so that the specific capacity of total SP dispersed in the catholyte (0.1 g) was limited at ca 800 mAh g
1. We also tested an SP coated CP electrode (1.6 mg cm
2 SP) in SPfree, stirred electrolyte. In this case the discharge capacity was improved with respect to the CP/electrolyte case, but it lasted only 9 h at 0.25 mA cm
2, corresponding to 2.25 mAh cm
2 and 1400 mAh g
1 of SP. These values well compare with the data reported in literature for SP-based solid cathodes tested in conventional Li/O2 batteries. Indeed, the typical loading of SP on current collector ranges between 1 and 10 mg cm
2, and the discharge capacity of SP electrodes at low current density (<0.1 mA cm
2, <70 mA g
1) is ca. 3500 mAh g
1 and 3–30 mAh cm
2, a value which is expected to decrease with the increase of the current [14,15,21,29,33,45]. Hence, to the best of our knowledge, the capacity density that we achieved by the use of the flowable semi-solid catholyte is one order of magnitude higher than the values reported in literature for not-flow Li/O2 batteries with the same components. In order to understand to which extent the CP current collector and the flowing semi-solid catholyte are involved in the discharge process and in the formation of related products, we performed the following test. In a new conventional electrochemical cell, we discharged a CP/catholyte system over 45 h. Afterwards, we recharged the used CP in a fresh electrolyte (CP45h-d/electrolyte) and we recharged the discharged semi-solid catholyte with a fresh CP current collector (CP/catholyte45h-d). The charge profiles are reported in Fig. 2b along with the response of a fresh CP/catholyte system before discharge (CP/catholyte). The recharge profile of the CP/catholyte45h-d features a plateau at 3.5 V vs Li+/Li which could be attributed to reoxidation of the discharge products which were previously formed in the catholyte. The sharp increase of the potential profile after ca. 25 h clearly indicates the end of the recharge process with an efficiency of 55%. The 3.5 V plateau is absent in the case of CP45h-d/electrolyte and of the fresh CP in fresh catholyte (CP/catholyte) system. This indicates that i) the plateau is not related to CP or electrolyte anodic decomposition and that ii) the previous 45 h-discharge mainly involved the semi-solid phase of the flowable electrode. Fig. 3 reports the TEM images of the catholyte after 45 h discharge and 25 h recharge (45/25-catholyte, Fig. 3a), and after 700 h discharge (700-catholyte, Fig. 3b). The catholyte is made of 40–50 nm carbon particles that are well connected to give the percolating network (TEM image of the fresh catholyte is reported in Fig. S3). Discharge causes the formation of solid products that are both deposited on the SP carbon particles and dispersed within the carbon network. These products feature different morphology depending on the depth of discharge. In the case of 45/25catholyte, an amorphous film is covering part of the carbon conducting network (Fig. 3a). More interestingly, small needles (up to 5 nm length, <1 nm thick) are present on the SP particles and completely surround them in some cases. This causes the disconnection of the coated grains from the percolating network. Flake-shaped moieties (<50 nm) are also present and not connected to the carbon (Fig. 3a). The amount of these flakes is higher in 700-catholyte, as well as the SP coating is more evident. Fig. 3b evidences “core-shell” structures, where the shell is made of small (1–2 nm) needles and the core is carbon. The needles grow to give connected “sea urchin”-like structures. The nature of the discharge products in the catholyte was investigated by FTIR, micro Raman, and XRD analyses. The chemical/physical characterization of semi-solid electrodes is very challenging because it requires analytical techniques that have to be tailored to heterogeneous samples where active species and side products are present at very low concentration. In our system the liquid electrolyte solution (solvent and 0.5 M lithium salt) and

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295

Fig. 3. TEM images of the catholyte after 45 h discharge and 25 h charge (3a) and after 700 h discharge (3b) at 0.25 mA cm
2 in conventional, O2-satured catholyte, stirred electrochemical cell.

the solid carbon matrix masked the signals of the discharge products that might be detected by spectroscopy, surface analyses and XRD. The FTIR spectra of the catholyte collected after 700 h discharge was similar to those of the pristine catholyte thus supporting electrolyte stability over long-time discharge (Fig. S4a). XRD did not evince new crystalline phases after 700 h discharge (Fig. S5a) because, as shown by TEM, the discharge products were nanometric and highly dispersed in the semi-solid phase (Fig. 3a– 3b). Only micro Raman suggested the formation of peroxide species during discharge (Fig. S4b) and this is in agreement with literature. Indeed, the semi-solid, flowable cathode design here proposed is novel but its component chemistry is analogous to what reported in Refs. [29,33–36,60]. The chemistry of ORR at carbon electrodes, like SP, in TEGDME-based electrolytes has been deeply investigated by XRD, FTIR, Raman spectroscopy, gas chromatography, SEM, TEM and TOF-SIMS studies. While at present there is still debate about the nature of the ORR products, it is generally accepted that Li2O2 is the main discharge product at the SP carbon surface in TEGDME. Li2O2 is reoxidized to give O2 and lithium ions during recharge. The formation of carbonate-based species due to carbon and/or electrolyte oxidation has also been observed [21,23,29,33–38,60–65]. Hence, we tentatively assumed

that the needles detected by TEM are Li2O2 crystals (Fig. 3a–3b). The morphology of such particles is very different from the large (micrometric) toroidal aggregates that are observed after the discharge of conventional Li/O2 batteries based on TEGDME-LiTFSI and SP [66]. Presumably, our dynamic conditions and the very large surface area of the reaction sites favor nucleation of Li2O2 over growth (0.1 g of SP with ca. 64 m2 g
1 is dispersed in 5 mL of catholyte). We detected Li2O2 on the CP current collector surface only after 19 days discharge (Fig. S5b). This layer caused the increase of the electrode impedance which is discussed in the ESI section (Fig. S6) and that explains the potential drop after 19 days (Fig. 2a). The results reported above suggest that ORR discharge products are mainly formed on the surface of the solid phase of the catholyte and/or mechanically removed away from the electrode or SP particles by the vigorous catholyte flow. While the formation of ORR products on CP current collector cannot be completely avoided, it is significantly alleviated by the use of SP-based catholyte. This would permit to reach long Li/O2 battery operation. We can tentatively describe the mechanism of ORR in the catholyte by the following equations:

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O2(sol) + 2Li+ + SP + 2 e
! SP/Li2O2(s)

(1a)

O2(sol) + 2Li+ + SP + 2 e
! SP + Li2O2(sol)

(1b)

SP/Li2O2(s) ! O2(sol) + 2Li+ + SP + 2 e


(2a)

Li2O2(sol) + SP ! O2(sol) + 2Li+ + SP + 2 e


(2b)

The SP percolating network brings electrons in the catholyte where dissolved O2 and Li+ are present. ORR can take place at the SP surface and ORR reduction products, namely Li2O2, grow on the dispersed SP particles (Eq. (1a)) and/or are dissolved/dispersed in the catholyte (Eq. (1b)). During recharge, Li2O2 which was previously formed on the SP surface is reoxidized to O2 and Li+ which are dissolved back in the catholyte (Eq. (2a)). The SP percolating network ensures the electric contact needed to reoxidize also Li2O2 dissolved/dispersed in the liquid phase of the catholyte (Eq. (2b)). At lower extent, Li2O2(sol) is also formed on the CP current collector and, eventually, is mechanically removed and brought into the catholyte bulk. In our system, the concentration of the redox active species, i.e. O2, is not limiting the discharge. The catholyte is continuously fed (saturated) with O2 and it acts as a carrier of both the electroactive species (O2) and redox sites (SP) that flow through the porous current collector. This permits to circumvent the low solubility of O2 in TEGDME when LiTFSI is present (4.43 mM in TEGDME [67] and 0.26 mM in 1 M LiTFSI-TEGDME [68]). As an indication, the capacity for the complete reduction of the O2 dissolved in 5 mL of a saturated 1 M LiTFSI-TEGDME solution that is not continuously fed with O2 would be only of 0.2 mAh (a 2-electron reduction process is considered). According to Eq. (1a), an SLRFAB working with a cathode current collector which is not affected by the ORR products and with excess Li would feature a discharge capacity determined by the passivation of the catholyte SP. As an example, the capacity of a cell working with 10 mL of catholyte (density ca. 1 g cm
3) with 2% of SP would be 700 mAh (3500 mAh g
1 of SP [21,33,45]). If the electrode geometric area is 0.45 cm2 and the current density is 0.25 mA cm
2, the full catholyte discharge would last more than 7 months. This would downplay the long cyclability and rechargeability requirements. On the other hand, this also makes lab-scale cyclability studies at full catholyte discharge/ charge not practical. Therefore, we performed cycling tests at 0.25 mA cm
2 by limiting the discharge time to 24 h and the recharge catholyte potential to 4 V vs. Li+/Li. The 10-day cyclability test demonstrates the good semi-solid catholyte performance and the results are reported in Fig. 4. The cycled capacity of 6 mAh cm
2 is low for our system, but is comparable with values reported for conventional cells [19,29,33–36,38,40,41,59,60]. Being partial, the

Fig. 4. CP (0.5 cm2) potential profile during 24h-galvanostatic discharge/recharge cycles (6 mAh cm
2) in conventional electrochemical cell with stirred, O2-saturated catholyte at 0.25 mA cm
2.

catholyte discharges reported in Fig. 4 kept part of the SP particles from passivation. The conductivity of the percolating network was partially preserved so that overpotentials did not significantly increase over cycling. At deep catholyte discharge, an extensive, though not homogeneous, deposition of Li2O2 on SP is expected which might decrease the conductivity of the carbon network and increase polarizations. Using a catholyte excess with respect to the amount required to balance lithium areal capacity could be a strategy to match good cycling performance and deep cell discharge. 3.2. The SLRFAB prototype The next step was to demonstrate the outstanding performance of the semi-fluidic cathode even in flow Li/O2 battery configuration. A macroporous current collector for easy and fast flow of the catholyte through the cathodic compartment, which is important when high discharge rates and viscous catholyte are considered, was required. We thus used the RVC current collector, and we coated it with SP in order to improve the electrode surface area and the electronic contact with the SP particles dispersed in the catholyte (RVCSP electrode, SEM image reported in Fig. S7a–b). The enhancement of the current collector’s specific surface area, which is achieved by the RVC carbon coating with SP, provided a further reduction of the ohmic loss and, consequently, a noticeable increase in electrode potential during discharge, which reached 2.85 V vs. Li+/Li at 0.25 mA cm
2 (Fig. S2). EIS also indicated that electrode impedance of RVCSP was dramatically reduced with respect to CP (Inset of Fig. S6 and related comments). RVCSP was used to assemble the SLRFAB schematized in Fig. 5a and b. Fig. 5c reports the image of the lab-scale prototype under operation. The cell core is made up of a lithium metal anode and the cathodic RVCSP porous current collector, which are separated by a fibre glass membrane. The catholyte is fed through the cell by a peristaltic pump while being continuously enriched with O2 (g). For basic studies, an additional reference electrode which intercepted the catholyte flow was used. The exceptionally high discharge capacity of the semi-solid cathode enabled long-time polarization tests on the prototype and investigation of the cell’s power response. The tests consisted of the repetition of 1h-discharge steps at current densities from 0.05 mA cm
2 up to 4 mA cm
2. The electrode potential cut-off were 2 V vs. Li+/Li and 1 V vs. Li+/Li for the cathode and the anode, respectively. The pump rate was 200 rpm, which corresponded to a catholyte flow of ca. 170 mL min
1. The results are reported in Fig. 6. Fig. 6a shows the electrode potentials and cell voltage trends during the consecutive 1h-discharge steps. Cell voltage (Vcell) was 2.9 V at 0.05 mA cm
2 and total discharge capacity after the subsequent discharge steps was ca. 50 mAh cm
2. Cell voltage dropped after 40 h at currents >2.5 mA cm
2. Notably, the cathode featured a high potential (Vcath) of 2.90 V vs Li+/Li at 0.05 mA cm
2, which decreased by only 10% when the current was increased up to 3 mA cm
2. This indicates that cell performance is not affected by the cathode and catholyte. Rather, cell performance at the longest times and highest currents was affected by the lithium anode. Indeed, lithium overpotential reached 1 V after 40 h discharge and currents >2.5 mA cm
2, a situation that may be related to lithium depletion. Lithium depletion is avoidable by periodically recharging the cell. Cell rechargeability was, therefore, studied at first by consecutive, 1h-recharge steps at low currents, from 0.9 mA cm
2 to 0.05 mA cm
2, that were performed after the previous discharge sequence (Fig. 6b). The recharge cell voltages increased with current density and ranged between 3.05 V and 3.85 V. This leads to an exceptionally low cell recharge overvoltage of 150 mV at

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Fig. 5. The SLRFAB cell scheme (a) and (b) and the prototype under operation (c). The prototype featured RVCSP current collector (0.385 cm2, 7 mg cm
2 of SP).

Fig. 6. SLRFAB performance. (a-c) Trends of the potentials of RVCSP cathode (Vcath) and Lithium anode and of cell voltage (Vcell) during the following galvanostatic tests: (a) 1h-discharge steps from 0.05 mA cm
2 up to 4 mA cm
2; (b) 1h-recharge steps from 0.9 mA cm
2 to 0.05 mA cm
2; (c) 1h-discharge and recharge steps from 0.25 mA cm
2 up to 4 mA cm
2; (d) cell and cathode discharge polarization curves and power vs. current plots.

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0.05 mA cm
2, which increases up to 950 mV at 0.9 mA cm
2.1 At the highest current, Vcath is 3.3 V vs Li+/Li, which corresponds to a low cathode recharge overpotential of 400 mV.2 Cell rechargeability was further investigated by repetition of 16 cycles performed by 1h-discharge and recharge steps at current densities varying from 0.25 mA cm
2 up to 4 mA cm
2. Discharge and recharge were limited by cathode potential cut-off of 2 V vs. Li+/Li and 3.7 V vs Li+/Li, respectively. The recharge cut-off was set to prevent electrolyte oxidative degradation. The results are reported in Fig. 6c. Reducing the steps and the periodical recharging of the cell helped to circumvent lithium depletion phenomena. This approach also made it possible to achieve 4 mA cm
2. Fig. 6d shows the discharge polarization curves of the prototype in terms of Vcell, cell power (Pcell = i x Vcell), Vcath and cathode power (Pcath = i x Vcath). To the best of our knowledge, this is the first time that polarization curves of a non-aqueous Li/O2 battery are reported. The SLRFAB design promotes ORR kinetics and, particularly, the mass transport of the species involved during ORR while achieving high Pcath of ca 10 mW cm
2 and Pcell of 7.5 mW cm
2. Cell discharge voltage is shaped by cathode and anode potential losses. The cathode potential does not significantly change up to 3 mA cm
2. For higher currents, diffusion limitation causes a 20% cathode potential drop and Pcath reaches its maximum value. Note that the Vcath decrease could be limited by optimizing catholyte flow rate. On the other hand, the lithium anode discharge overpotential is notable and mainly contributes to cell voltage drop even for currents lower than 3 mA cm
2. Several researchers have reported the formation of a stable solid electrolyte interface (SEI) on Li metal and low Li stripping/plating overpotentials in TEGDME – LiTFSI [35,36]. However, these findings refer to current densities lower than those that we explored. The Nyquist plots reported in Fig. S8 demonstrate that lithium anode is the electrode that mainly contributes to total cell impedance and to cell impedance increase over cycling. The Nyquist plot of the lithium anode is dominated by a high frequency semicircle, which is ascribed to SEI, with a real axis intercept of 100 V cm2, which increases up to 170 V cm2 after cycling. These data have to be compared with the initial and final cell impedances of 120 V cm2 and 180 V cm2. Lithium SEI could be improved by optimizing cell assembly. Indeed, lithium was not coated by any protective layer and was separated from the cathode only by two fiber glass separators that are permeable to the electrolyte. Oxygen dissolved in the electrolyte might diffuse through the separators to the anodic compartment thus undermining lithium cyclability. The RVCSP impedance slightly worsens over cycling because diffusion-related processes become more sluggish, as in the case of CP electrodes tested in conventional stirred cells (cfr. Fig. S6). The SEM images of the RVCSP electrode and the catholyte, collected before and after cycling, are reported in Fig. S7. The images suggest that SP particles agglomerate and passivate upon cell operation. The cycled RVCSP and, mainly, the cycled catholyte display macropores that may be formed during the high-rate recharge steps after O2 evolution and/or carbon oxidation. Indeed, the XRD of the recharged RVCSP electrode after cycling shows that Li2O2 is absent, as expected, but that a layer of Li2CO3, which is produced by carbon oxidation and/or TEGDME degradation, is present (Fig. S9a). The oxidative environment at the SP or RVCSP surfaces may

1 The recharge cell overvoltage was calculated by Vcell,rech-Vcell, OCV, where Vcell, rech is the voltage of the cell during recharge at given current and Vcell, OCV is the cell voltage during rest (2.9 V). 2 The recharge cathode overpotential was calculated by Vcath,rech
Vcath, OCV, where Vcath, rech is the cathode potential during recharge at given current and Vcath, OCV is the cathode potential during rest (2.9 V vs. Li+/Li).

promote the formation of Li2CO3 on the SP and RVCSP carbon particles. However, after 25 h cycling, FTIR spectroscopy did not give evidence of decomposition of the cycled, flowing semi-solid catholyte (Fig. S9b). 4. Practical and projected performance of SLRFAB The core of the SLRFAB comprises a lithium metal anode, which is limiting the battery areal discharge capacity. The cell is continuously fed with O2, therefore O2 solubility is not affecting capacity. Our prototype featured 4 lithium metal disks (ca. 65 mg cm2, 3830 mAh g
1) and if we assume a 20% excess Li, the cell discharge capacity is 195 mAh cm
2. The minimum amount of SP carbon (3500 mAh g
1) required to balance this capacity value is 55 mg cm
2. The practical areal energy density delivered by the flow cell at an average cell voltage of 2.8 V is 545 mWh cm
2. The maximum specific energy, based only on lithium and SP solid components weights, is 4.6 kWh kg
1 and the practical specific energy normalized over the bulk catholyte with 2 wt% SP (2.75 g cm
2, density ca. 1 g cm
3) is 195 Wh kg
1. The corresponding energy density is ca. 190 Wh L
1. The porous current collector RVCSP (60 mg cm
2 including the SP layer, 0.5 cm thick) is playing a key role because it permits catholyte flow thorough the cell core and it reduces the practical specific energy and energy density values to ca. 190 Wh kg
1 and 160 Wh L
1. The practical energy performance of our prototype can be significantly improved by the increase of SP content in the catholyte and by reducing the volume of the porous current collector. An SP percentage of 5% is feasible (see SI, Fig. S1) as well as the RVCSP thickness can be halved, and these simultaneous approaches would raise the practical specific energy and energy density values up to 450 Wh kg
1 and 370 Wh L
1. The SP content increase in the catholyte is challenging because it will affect viscosity and flow dynamics, but it will be the main strategy to design a battery that outperforms any other Li/O2 battery and RFB reported up to now. Indeed, the projected performance of a cell with 20% wt SP catholyte and 0.25 cm-thick RVCSP will be 1.4 kWh kg
1 and 2.4 kWh L
1. The highest practical power obtained with our prototype is 7.5 mW cm
2 (Fig. 6d). Taking into account the real prototype composition, the specific power results 62 W kg
1 (on solid lithium and SP components) and 2.7 W kg
1 (normalized over the catholyte content with 2% SP). Noticeably, power performance does not depend on lithium and SP contents, because it is related only to the areal current density and cell voltage. This means that the most effective strategy to boost up specific power and power density would be the decrease of active (and inactive) component amounts. It can be calculated that a cell with 100 mm-thick lithium anode (5.4 mg cm
2) would require only 4.6 mg cm
2 of SP (230 mg cm
2 of 2%SP-catholyte) and feature 750 W kg
1 (on solid components) and 20 W kg
1 (2% SP catholyte and RVCSP included). The projected performance of a cell with 20%wt SP catholyte and 0.25 cm-thick RVCSP will be 80 W kg
1 and 385 W L
1. In our system lithium is limiting power. Improvement of Li/electrolyte interface properties is expected to decrease anode overpotentials with benefit on rate performance. The use of O2 impermeable separators will also improve lithium cyclability. The rate response of the cathode compartment is affected by the rate of transport of O2 from the gas phase to the electrolyte solution and of the dissolved O2 to the cathode current collector. Therefore, optimization of the O2 feeding rate (from an O2 tank or air to the electrolyte) and of the catholyte flow will also have a positive impact on power.

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5. Conclusions

Aknowledgments

We demonstrate a novel concept of semi-solid flow Li/O2 battery that combines the high energy density of Li/O2 batteries with the flexible and scalable architecture of RFBs. In our system, ORR mainly takes place at the carbon particles that are dispersed in the catholyte and are fed through the porous current collector and removed away from the electrode. This process alleviates the fast clogging of the cathode current collector that is caused by the deposition of ORR products, in turn determining the end of discharge. The result is a dramatic increase of discharge capacity density (areal) up to values never before reported for Li/O2 cathodes (175 mAh cm
2), thereby making the need for long cyclability less critical. The discharge capacity normalized to the electrode area is one order of magnitude higher than the values featured by the best Li/O2 cells reported up to now, including systems with both semi-solid, stationary catholytes and cells with flowable electrolytes with soluble catalysts/mediators [16,34,59]. The recharge overpotentials have also been significantly decreased (<1 V) with respect to aqueous and hybrid LRFBs. Furthermore, our concept presents the following advantages: i) expensive catalysts or mediators are not present; ii) the SP percolating network permits to avoid or at least reduce the volume of any external tank planned for the recovery of the ORR discharge products; iii) the discharge capacity depends only on the O2 concentration, which is kept constant by continuous bubbling, and on SP content in the catholyte. Notably, the semi-solid flow Li/O2 cathode features high operating potential at high discharge rates, up to 4 mA cm
2, which is of interest for practical applications. The prototype featured up to 500 mWh cm
2 (after Li substitution) and 7.5 mW cm
2, a performance that can be further enhanced. In fact, power and energy capabilities of the SLRFAB can be independently scaled, the energy being determined by the volume of the catholyte and the power by electrode size and catholyte flow rate. Our work even points to the need for developing a highperformance anode for fully exploiting the exceptionally high rate and capacity response of the semi-solid flowable cathode. Lithium metal is limiting both the areal capacity density and the cell response at high discharge currents. On the other hand, the SP percentage content in the catholyte and the porous current collectors are limiting the specific and volumetric values of energy and power. Based on experimental results, the projected practical battery performance can triplicate that of the best performing LIBs. Optimization of cathodic compartment in terms of carbon content and porous current collector design, would permit to develop a SLRFAB featuring up to 1 kWh kg
1 and 2 kWh L
1. Furthermore, the proof-of-concept is demonstrated by low-cost commercial materials, and advances are expected by the tailored design of the catholyte and anode components. Indeed, cost-effective cathodic materials outperforming commercial carbons in terms of specific capacity and stability are under study. O2 reduction and evolution catalysts and mediators that decrease cell overpotentials could further improve the system performance. Recently, it has been shown that carbon free materials are the most stable systems but their modest areal capacity has to be improved [21]. In addition, novel electrolytes, like ionic liquids, could permit to improve cycling performance. However, while novel materials with impressive gravimetric capacity (up to 8000 mAh g
1) are claimed, their performance rated to electrode area are still too low to be of interest to power real world devices [19]. Exploiting the semi-solid catholyte concept along with the use of these new materials may represent a valuable strategy for a new upsurge of Li/O2 battery performance.

Professor Marina Mastragostino, Francesca De Giorgio and Dr. Simone Monaco are thanked for useful discussion of the design of the SLRFAB concept. The authors are grateful to Dr. Maria Roberta Randi (University of Bologna), Dr. Davide Fabris (Horiba) and Dr. Andrea Fiorani (Alma Mater Studiorum –Università di Bologna), and Dr. Micaela Pasquini (Alma Mater Studiorum –Università di Bologna) for TEM, micro Raman and rheological measurements. Dr. Josianne Lefebvre (École Polytechnique Montreal) and Dr. Kateryna Artyushkova (University of New Mexico) are acknowledged for the fruitful discussions about the characterization of the semi-solid catholyte by X-ray spectroscopies. The work was funded by Alma Mater Studiorum –Università di Bologna (RFO, Ricerca Fondamentale Orientata). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.04.139. References [1] Q. Huang, Q. Wang, ChemPlusChem 80 (2015) 312–322. [2] G.L. Soloveichik, Chem. Rev. 115 (2015) 11533–11558. [3] (a) R.M. Darling, K.G. Gallagher, J.A. Kowalski, S. Ha, F.R. Brushett, Energy Environ. Sci. 7 (2014) 3459–3477; (b) V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243–3262. [4] P. Leung, X. Li, C. Ponce de Leòn, L. Berlouis, C.T.J. Low, F.C. Walsh, RSC Adv. 2 (2012) 10125–10156. [5] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater. 23 (2013) 970– 986. [6] K. Gong, Q. Fang, S. Gu, S.F. Yau Li, Y. Yan, Energy Environ. Sci. 8 (2015) 3515– 3530. [7] Y. Ding, Y. Zhao, G. Yu, Nano Lett. 15 (2015) 4108–4113. [8] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M.J. Saleem, Electrochem. Soc. 158 (2011) R55–R79. [9] P.A. Malachesky, R.J. Bellows, H. Einstein, P. Grimes, E. Kantner, K. Newby, A. Young, 10.4271/820177, (1982). [10] M.R. Mohamed, S.M. Sharkh, F.C. Walsh, 10.1109/VPPC.2009.5289801, (2009). [11] Y. Lu, J.B. Goodenough, J. Mater. Chem. 21 (2011) 10113–10117. [12] Y. Wang, P. He, H. Zhou, Adv. Energy Mater. 2 (2012) 770–779. [13] Y. Zhao, Y. Ding, Y. Li, L. Peng, H.R. Byron, J.B. Goodenough, G. Yu, Chem. Soc. Rev. 44 (2015) 7968–7996. [14] J. Huang, A. Faghri, Electrochim. Acta 174 (2015) 908–918. [15] S. Monaco, F. Soavi, M. Mastragostino, J. Phys. Chem. Lett. 4 (2013) 1379–1382. [16] Y.-G. Zhu, C. Jia, J. Yang, F. Pan, Q. Huang, Q. Wang, Chem. Commun. 51 (2015) 9451–9454. [17] X.J. Chen, A. Shellikeri, Q. Wu, J.P. Zheng, M. Hendrickson, E.J. Plichta, J. Electrochem. Soc. 160 (2013) 1619–1623. [18] P. He, Y. Wang, H. Zhou, Electrochem. Commun. 12 (2010) 1686–1689. [19] L. Grande, E. Paillard, J. Hassoun, J.B. Park, Y.J. Lee, Y.K. Sun, S. Passerini, B. Scrosati, Adv. Mater. 27 (2015) 784–800. [20] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nature Mater. 11 (2012) 19–29. [21] M.D. Bhatt, H. Geaney, M. Nolan, C. O’Dwyer, Phys. Chem. Chem. Phys. 16 (2014) 12093–12130. [22] K.M. Abraham, J. Phys. Chem. Lett. 6 (2015) 830–844. [23] J. Lu, L. Li, J.-B. Park, J.-K. Sun, F. Wu, K. Amine, Chem. Rev. 114 (2014) 5611– 5640. [24] Y.-C. Lu, B.M. Gallant, D.G. Kwabi, J.R. Harding, R.R. Mitchell, M.S. Whittingham, Y. Shao-Horn, En. Env. Sci. 6 (2013) 750–768. [25] L. Johnson, C. Li, Z. Liu, Y. Chen, S.A. Freunberger, P.C. Ashok, B.B. Praveen, K. Dholakia, J.-M. Tarascon, B.G. Bruce, Nature Chem. 6 (2014) 1091–1099. [26] K.-H. Xue, E. McTurk, L. Johnson, P.G. Bruce, A.A. Franco, J. Electrochem. Soc. 162 (2015) 614–621. [27] C.J. Allen, J. Hwang, R. Kautz, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham, J. Phys. Chem. C 116 (39) (2012) 20755–20764. [28] C.P. Andersen, H. Hu, G. Qiu, V. Kalra, Y. Sun, Electrochem. Soc. 162 (2015) 1135– 1145. [29] B.D. Adams, C. Radtke, R. Black, K. Trudeaw, L.F. Nazar, Energy Environ. Sci. 6 (2013) 1772–1778. [30] M. Balaish, A. Kraytsterg, Y. Ein-Eli, Y. Phys. Chem. Chem. Phys. 16 (2014) 2801– 2822. [31] R. Younesi, G.M. Veith, P. Johansson, K. Edström, T. Vegge, Energy Environ. Sci. 8 (2015) 1905–1922.

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