SemiSolid Lithium Rechargeable Flow Battery

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Semi-Solid Lithium Rechargeable Flow Battery Mihai Duduta, Bryan Ho, Vanessa C. Wood, Pimpa Limthongkul, Victor E. Brunini, W. Craig Carter, Yet-Ming Chiang* Global energy and climate-change concerns have accelerated the electrification of vehicles, aided by advances in battery technology. It is now recognized that low-cost, scalable energy storage will also be key to continued growth of renewable energy technologies (wind and solar) and improved efficiency of the electric grid. While electrochemical energy storage remains attractive for its high energy density, simplicity, and reliability, existing battery technologies remain limited in their ability to meet many future storage needs. Here we propose and demonstrate a new storage concept, the semi-solid flow cell (SSFC), which combines the high energy density of rechargeable batteries with the flexible and scalable architecture of fuel cells and flow batteries. In contrast to previous flow batteries, energy is stored in suspensions of solid storage compounds to and from which charge transfer is accomplished via dilute yet percolating networks of nanoscale conductors. These novel electrochemical composites constitute flowable semi-solid ‘fuels’ that are here charged and discharged in prototype flow cells. Potential advantages of the SSFC approach include projected system-level energy densities that are more than ten times those of aqueous flow batteries, and the simplified low-cost manufacturing of large-scale storage systems compared to conventional lithiumion batteries. Demand for batteries of higher energy and power has driven several decades of research in electrochemical storage materials, resulting recently in significant improvements in the stored energy of cathodes and anodes.[1,2] However, most batteries have designs that have not departed substantially from Volta’s galvanic cell of 1800, and which accept an inherently poor utilization of the active materials.[3] Even the highest energy density lithium ion cells currently available, e.g., 2.8–2.9 Ah 18650 cells having >600 Wh L−1, have less than 50 vol% active material. The reduced energy density, along with higher cost, result because the high-energy-storage compounds are diluted by inactive and costly components necessary to extract power (e.g., currentcollector foils, tabs, separator film, liquid electrolyte, electrode binders and conductive additives, and external packaging). Further dilution of energy density, by about a factor of two, occurs between the cell and system level.[4] Electrode designs that minimize inactive material, bio- and self-assembly, and 3D architectures are new approaches that promise improved design efficiency but have yet to be fully realized.[5–9]

M. Duduta, B. Ho, Dr. V. C. Wood, Dr. P. Limthongkul, V. E. Brunini, Prof. W. C. Carter, Prof. Y.-M. Chiang Massachusetts Institute of Technology Cambridge, MA 02139, USA E-mail: [email protected]

DOI: 10.1002/aenm.201100152

Adv. Energy Mater. 2011, XX, 1–6

Decoupling power components from energy-storage components so that stored energy can be scaled independently of power is a strategy for improving system-level energy density. Redox flow batteries have such a design, in which active materials are stored within external reservoirs and pumped into an ion-exchange/electron-extraction power stack.[10] As the system increases in capacity, its energy density may asymptotically approach that of the redox active solutions. Aqueous-chemistry flow batteries are of much current interest for stationary applications due to their scalability, relative safety, and potentially low cost. However, they currently use low energy density chemistries limited by electrolysis to ≈1.5 V cell voltage and have low ion concentrations (typically 1–2 M), yielding ≈40 Wh L−1 energy density for the fluids alone.[10] Furthermore, the large fluid volumes that must be pumped produce parasitic mechanical losses that detract significantly from round-trip efficiency. The flow-cell’s design advantages are therefore offset by the use of low-energy-density active materials. In a new system we call a semi-solid flow cell (SSFC, Figure 1), the inherent advantages of a flow architecture are retained while dramatically increasing energy density by using suspensions of energy-dense active materials in a liquid electrolyte.[11] This approach to flowable electrodes can produce more than 10 times the charge storage density of typical flow-battery solutions, due to the much greater energy density inherent to solidstorage compounds. For example, in molarity units the concentration of reversibly-stored lithium in lithium-ion cathodes such as LiCoO2, LiFePO4, LiNi0.5Mn1.5O4, and 0.3 Li2MnO3–0.7 LiMO2 (M = Mn, Co, Ni), and anodes such as Li4Ti5O12, graphite, and Si (assuming ≈1000 mAh g−1 reversible capacity), is 51.2, 22.8, 24.1, 39.2, 22.6, 21.4, and 87 M, respectively.[2] Assuming a solids content of 50% (up to 70 vol% solids have been achieved in flowable suspensions of other materials), the volumetric capacity of the semi-solid suspensions is 5–20 times greater (e.g., 10 to 40 M) than that of aqueous redox solutions (≈2 M). The semi-solid approach may be applied to aqueous chemistries, in which case the volumetric energy density is also 5–20 times greater since cell voltages remain limited by electrolyte hydrolysis to ≈1.5 V.[12,13] When applied to nonaqueous Li-ion chemistries, however, energy density is further increased by another factor of 1.5–3, in direct proportion to cell voltage. (Energy density is the product of volumetric charge capacity, e.g., in molarity or Ah l−1 units, and the cell voltage. Specific values for the systems studied are given later.) In this work, we demonstrate working prototype SSFCs using flowable suspensions having up to ≈12 M concentration. We show that in addition to energy density advantages, the SSFCs can operate at low flow rates with very low mechanical energy dissipation. The design flexibility inherent in the SSFC approach may enable new use-models for electrical storage, such as rapid refueling

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Figure 1. a) Scheme: semi-solid flow cell (SSFC) system using flowing lithium-ion cathode and anode suspensions could enable new models such as transportation ‘fuels’ tuned for power versus range, or cold versus warm climates, with flexible refueling and recycling options. b) Fluid semi-solid suspension containing LiCoO2 powder as the active material and Ketjen black as the dispersed conductive phase, dispersed in alkyl carbonate electrolyte. c) Laboratory cell using monolithic copper and aluminum current collectors, with lithium metal reference electrode, and fed by tubing using a peristaltic pump. d–f) Galvanostatic charge/discharge curves for semi-solid suspensions having 26 vol% LiCoO2 (LCO) dispersed in 1.3 M LiPF6 in an alkyl carbonate blend, 20 vol% LiNi0.5Mn1.5O4 (LNMO) dispersed in 1 M LiPF6 in ethylene carbonate:dimethyl carbonate (3:7), and 25 vol% Li4Ti5O12 (LTO) dispersed in 1 M LiPF6 in dimethyl carbonate.

of vehicles by fuel or fuel tank exchange, tuning of suspensions as needed for power, energy, and operating temperature, and extension of service life by renewing suspension chemistry or incorporating serviceable system components. Energy-Dense, Electrochemically Active Semi-Solid Suspensions: The proposed concept cannot work without effective charge transfer from the active material particles to the current collectors of the cell. Our approach to accomplishing this objective, illustrated in Figure 2, takes advantage of two limiting cases of particle aggregation behavior to produce novel, electrochemically active composites: 1) diffusion-limited cluster aggregation (DLCA) of conductive nanoparticles at low volume fraction (≈1 vol%) to form percolating conductor networks; and 2) volumetrically dense packing of micrometer-scale storage particles to maximize storage energy density. Interactions between nanoscale particles are typically dominated by colloidal rather than gravitational forces.[14] In solutions of high salt concentration such as ionically conductive electrolytes, surface charges are screened and attractive London–van der Waals attractions dominate, resulting in ‘hit and stick’ behavior that forms fractal particle networks by DLCA.[15] Testing numerous nanoscale carbons including various carbon blacks, vapor-grown carbon fibers (VGCF), and multiwall carbon nanotubes (MWNTs), we found that percolating networks form in typical nonaqueous lithium-conducting electrolytes at particle concentrations less than 1 vol%. Figure 2c shows an example for Ketjen black in

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alkyl carbonate electrolyte (1.3 M LiPF6 salt), viewed by wet scanning electron microscopy (SEM). Into this structure we added micrometer-scale particles of electrode-active cathodes (e.g., LiCoO2, LiFePO4, LiNi0.5Mn1.5O4) and anodes (e.g., graphite, Li4Ti5O12). We found that the nanoscale conductor network serves a second critical function beyond ‘wiring’ the active material for charge transfer: it stabilizes the larger particles from settling out of suspension. Figure 2d shows a wet SEM image of the LiCoO2 particle distribution in such a suspension, while Figure 2e shows an X-ray tomography showing short-range clustering of the LiCoO2 particles. Evidence for physically and electronically percolating networks was found through rheological and transport measurements. Figure 2a shows that 0.3–0.6 vol% Ketjen black produces strong shear-thinning behavior indicative of network formation, whether the suspension contains Ketjen black alone or LiCoO2 at 22.4 or 40 vol%. In fact, the suspensions containing both solid phases have several-fold higher viscosity than either suspension alone, indicating complex interactions (to be elucidated elsewhere). A conductivity cell designed to operate with stationary or flowing liquids was used to obtain Nyquist plots such as in Figure 2b. Since flow rates up to 10 mL min−1 (1.6 mm cylindrical channel) produced less than a factor of 2 change in conductivity in most cases (see Supporting Information (SI)), we use results obtained under nonflowing conditions to illustrate composition effects for several suspensions.



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The electrochemical activity of the semisolid suspensions provides a more critical functional test. Figure 1d–f show galvanostatic charge/discharge curves for flowable suspensions containing LiCoO2, LiNi0.5Mn1.5O4, and Li4Ti5O12 as active materials, measured in nonflowing half-cell configuration (each versus a Li-metal electrode). The LiCoO2 semi-solid suspension (26 vol%), when galvanostatically cycled at C/3.2 rate (room temperature), exhibited ≈115 mAh g−1 capacity; at C/20 rate, the capacity reached 140 mAh g−1 (not shown). A 20 vol% suspension of the high-voltage spinel LiNi0.5Mn1.5O4 exhibited the expected capacity (120 mAh g−1) and voltage (4.7 V average) and showed stable behavior over 30 cycles at C/5 rate. Similarly, the Li4Ti5O12 spinel exhibited the expected insertion voltage (1.55 V) and specific capacity (≈170 mAh g−1). Note the modest polarization (voltage hysteresis) in Figure 1d–f. Thus each semi-solid electrode exhibits the electrochemical activity of a conFigure 2. a) Viscosity versus shear rate for suspensions of nanoparticulate carbon (Ketjen ventional solid lithium ion battery electrode. black) and LiCoO2 (LCO) in alkyl carbonate electrolyte. The suspensions show shear-thinning The generality of this behavior is further behavior consistent with the presence of Ketjen networks partially disrupted by shear stress. shown in the SI for a nonflowing lithium ion b) Nyquist plots for the different suspensions and their components. The high frequency intercept on the real axis provides the ionic conductivity, and is the same for the pure alkyl car- cell using a nanoscale olivine as the cathode bonate electrolyte and the 0.6% Ketjen suspension in the same electrolyte (Z = 55 Ω intercept and Li4Ti5O12 as the anode. For semi-solid anodes, a major concern is corresponds to 22 mS cm−1 ionic conductivity since cell configuration factor = 1.2 cm−1). At higher solids fractions the ionic conductivity decreases, e.g., the slurry containing 22.4% LCO the likely deleterious effects of solid-electrolyte and 0.6% Ketjen has 6 mS cm−1 ionic conductivity (Z = 200 Ω). The 22.4% Li4Ti5O12 (LTO) + interface (SEI) formation on electron trans0.6% Ketjen suspension uses dioxolane solvent (1 M LiPF6) and has lower ionic conductivity of port. The SEI is formed upon reduction of − 1 1 mS cm (Z = 750 Ω). The electronic conductivity, extrapolated from low frequency data, is alkyl carbonate solvents at potentials (relative about 102 lower than the ionic conductivity, being 0.06 and 0.01 mS cm−1, for the LCO + Ketjen + and LTO + Ketjen suspensions, respectively. c) Wet-cell SEM images of Ketjen black in alkyl to Li/Li ) of ≈0.8 V or less, and is not a concarbonate electrolyte show extended percolating networks formed by diffusion-limited cluster cern for Li4Ti5O12 due to its higher potential aggregation, whereas in (d) a suspension of 22.4 vol% LCO and 0.6 vol% Ketjen in the same (1.55 V versus Li/Li+). For graphite anodes electrolyte shows uniform distribution of LCO particles. e) A 3D reconstruction of a 10 vol% (e.g., MCMB) having ≈0.15 V potential versus LCO and 0.6 vol% Ketjen suspension obtained using X-ray tomography shows clusters of LCO Li/Li+, decreasing electronic conductivity due particles without apparent long-range percolation. to SEI formation could be readily detected in our tests. We found that by decorating the In Figure 2b, the high frequency intercept on the real axis proMCMB graphite with metallic Cu using electroless deposition, vides the ionic conductivity, and is the same for the pure alkyl a substantial degree of electronic percolation could be maincarbonate electrolyte and its mixture with 0.6% Ketjen black; tained, and we therefore do not rule out the use of low-voltage the Z = 55 Ω intercept yields 22 mS cm−1 ionic conductivity anodes in our approach. Nonetheless, shifting the potential since the cell configuration factor is 1.2 cm−1. At higher solids window upwards by using a higher-voltage cathode and anode fraction the ionic conductivity decreases: the slurry containing together, such as LiNi0.5Mn1.5O4 with Li4Ti5O12 (producing a 22.4% LiCoO2 and 0.6% Ketjen in alkyl carbonate electrolyte 3.2 V cell) is effective for maintaining high energy density while exhibited 6 mS cm−1 ionic conductivity (Z = 200 Ω). The 22.4% minimizing SEI. Li4Ti5O12, 0.6% Ketjen suspension was prepared in a lower conContinuous and Intermittent Flow Cell Tests versus Li/Li+: ductivity electrolyte consisting of 1 M LiPF6 in dioxolane; the Semi-solid suspensions that were confirmed to be electrochemintercept at Z = 750 Ω indicates 1 mS cm−1 ionic conductivity. ically active in nonflowing cells were subsequently tested under For these particular suspensions, the electronic conductivity of flowing conditions. Half-cells in which cathode suspensions the suspension, extrapolated from the low frequency data, is were cycled against a fixed Li-metal negative electrode were about 102 lower than the ionic conductivity, and is expected to be tested under two pumping modes: 1) a continuously circurate-limiting during electrochemical cycling. For the LiCoO2 + lating mode in which the suspension is only partially charged/ Ketjen (in alkyl carbonate electrolyte) and Li4Ti5O12 + Ketjen (in discharged during its residence time in the flow channel; and dioxolane electrolyte) suspensions, the electronic conductivity 2) an intermittent mode where a single cell volume of semiis 0.06 and 0.01 mS cm−1, respectively. These variations can be solid is pumped into the cell, completely charged or discharged attributed to differences in aggregation behavior when either under static conditions, then displaced by a new volume of electrolytes or active materials differ. fresh semi-sold. (Conventional flow cells operate in the first




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Figure 3. Semi-solid half-flow-cell test: multistep galvanostatic charge/ discharge of LiCoO2 suspension (22.4 vol% solids (11.5 M), 0.7 vol% Ketjen, 1.3 M LiPF6 in a blend of alkyl carbonates) flowing at 20.3 mL min−1, separated from stationary Li metal negative electrode by microporous separator film.

mode: circulating using one anode and one cathode reservoir, the state-of-charge (SOC) of the fluids’ entirety is continuously increased/decreased.) A typical single flow channel had 0.85 cm2 active membrane area, being 80 mm long and 1.6 mm wide and having depths of 1.4 to 3.2 mm (i.e., distance between the separator and the bottom of the channel). C-rates that are reported below refer to the capacity in a single cell volume, not the system-level C-rate, which depends on the relative capacities of the stack and the storage tanks. We used microporous polymer separators (Celgard 2500 and Tonen) developed for lithium-ion batteries that have a mean pore size of