Zheng Li’s research while affiliated with Massachusetts Institute of Technology and other places

Batteries which use dissolved redox-active species, such as redox flow batteries (RFBs), are often considered to be constrained in their operation and energy density by the solubility limit of the redox species. Here, we show that soluble redox active electrolytes can be reversibly cycled deeply into the precipitation regime, permitting higher effective concentrations, higher energy densities, and lower costs. Using aqueous sodium polysulfide negative electrolytes cycled in the nominal Na2S2 to Na2S4 capacity range as an example, we show that the effective solubility can be increased from 5M in the fully-dissolved state to as much as 10M using the precipitation strategy. Stable cycling was observed at 8M concentration over more than 1600h at room temperature. We also analyze the range of polysulfide electrochemical stability, and characterize the precipitate composition before and after aging. This enhanced effective concentration approach may be generalized to other redox chemistries that utilize solubilized reactants, and may be especially useful for long-duration storage applications where slow charge-discharge rates allow equilibration of precipitated species with the redox-active solution.

The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints.

In this Backstory, Yet-Ming Chiang and colleagues explain how their cost-focused approach led to the discovery of an affordable battery technology based on readily available materials. Their findings appear in the October 2017 issue of Joule.

Sulfur is an attractive reactant for such concepts due to its exceptionally low cost, high natural abundance, and high specific and volumetric capacity owing to its two-electron reaction. Taking the cost-per-capacity (e.g., in US$/Ah) as a metric, sulfur has the lowest cost of any known electrode-active compound with the exception of water and air. However, in order to take advantage of sulfur’s low-cost potential, all other components must also have low cost. Towards enabling ultralow cost grid storage, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide chemistry in conjunction with lithium or sodium as the working ion, and an air-breathing cathode. Four different laboratory cell constructions are used to test the half-cell and full-cell reactions, including a pumped air-breathing cell that exhibits stable room-temperature cycling over 960h with a lithium polysulfide anolyte and dissolved lithium sulfate catholyte. In this approach the solution energy density is 30-150 Wh/L, which exceeds current solution-based flow batteries, and the chemical cost of stored energy is exceptionally low, especially when using sodium polysulfide (~1 US$/kWh). Results of techno-economic modeling are also presented, which show that when projected to full system-level, this new approach has energy and power costs that are comparable to those of pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), but without their geographical and environmental constraints. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Virtually all intercalation compounds exhibit significant changes in unit cell volume as the working ion concentration varies. NaxFePO4 (0<x<1, NFP) olivine, of interest as a cathode for sodium-ion batteries, is a model for topotactic, high strain systems as it exhibits one of the largest discontinuous volume changes (~17% by volume) during its first-order transition between two otherwise isostructural phases. Using synchrotron radiation powder X-ray diffraction (PXD) and pair distribution function (PDF) analysis, we discover a new strain-accommodation mechanism wherein an amorphous phase forms to buffer the large lattice mismatch between primary phases. While the new phase has short-range order only, it is refined with a periodicity closing matching the a and b lattice parameters of one crystalline endmember phase, and the c lattice parameter of the other. Within this amorphous phase form very fine spherical nanocrystallites of the olivine phase. We suggest that amorphous phase formation is a strain-accommodation mechanism that may apply to systems with large transformation strains.

Redox flow batteries have the potential to provide low-cost energy storage to enable renewable energy technologies such as wind and solar to overcome their inherent intermittency and to improve the efficiency of electric grids. Conventional flow batteries are complex electromechanical systems designed to simultaneously control flow of redox active fluids and perform electrochemical functions. With the advent of redox active fluids with high capacity density, i.e., Faradaic capacity significantly exceeding the 1–2 M concentration equivalents typical of aqueous redox flow batteries, new flow battery designs become of interest. Here, we design and demonstrate a proof-of-concept prototype for a “gravity-induced flow cell” (GIFcell), representing one of a family of approaches to simpler, more robust, passively driven, lower-cost flow battery architectures. Such designs are particularly appropriate for semi-solid electrodes comprising suspensions of networked conductors and/or electroactive particles, due to their low energy dissipation during flow. Accordingly, we demonstrate the GIFcell using nonaqueous lithium polysulfide solutions containing a nanoscale carbon network in a half-flow-cell configuration and achieve round trip energy efficiency as high as 91%.

In this work we investigated an energy-efficient bio-templated route to synthesize nano-structured FePO4 for sodium-based batteries. Self-assembled M13 viruses and single wall carbon nanotubes (SWCNTs) have been used as a template to grow amorphous FePO4 nanoparticles at room temperature (the active composite is denoted as Bio-FePO4-CNT) to enhance the electronic conductivity of the active material. Preliminary tests demonstrate a discharge capacity as high as 166 mAh/g at C/10 rate, corresponding to composition Na0.9FePO4, which along with higher C-rate tests show this material to have the highest capacity and power performance reported for amorphous FePO4 electrodes to date.

Electroanalytical, structural, and microanalytical techniques are combined to demonstrate the first clear and convincing case of electrochemical storage by Al-ion intercalation in a Prussian blue analog host. In addition, a high power asymmetric capacitor using Al-ion intercalation is demonstrated. Al, Fe, and Cu elemental maps of intercalated Al0.18 K0.02Cu[Fe(CN)6]0.7·3.7H2O obtained by transmission electron microscopy (TEM) energy-dispersive X-ray spectroscopy (EDX) area scan are shown.

We recently demonstrated that the high solubility of polysulfides in nonaqueous electrolytes can be exploited to make a high energy density lithium-sulfur flow battery with low storage cost 1–3 . Here we used a new approach, whereby percolating networks of nanoscale conductor particles (in this case carbon black) are incorporated within the electrode forming an embedded current collector which distributes electrochemical activity throughout the volume of the flow electrode, rather than being confined to the surfaces of stationary current collectors. Compared to the traditional approach, this architecture enables significantly higher capacity (~1200 mAh / g S) and reversibility by allowing cycling of polysulfide solutions (2.0-2.5V) into the precipitation regime (~2V), where discharge proceeds via precipitation of insoluble Li 2 S, as shown in Figure 1. The majority of the available capacity of this battery lies in this precipitation regime. Therefore, understanding the kinetics of the precipitation process is essential to improving the reversibility and rate capability of lithium-sulfur flow batteries. Here, we will discuss the nucleation and growth kinetics of the precipitates on various conductive substrates, including various carbon surfaces and conducting oxides. Moreover, we will discuss the effects of cycling conditions and substrate on the morphology of the precipitates, examples of which are shown in Figure 2. The precipitation of lithium sulfide has long been a challenge for lithium-sulfur batteries due to its low electronic conductivity 4,5 , so controlling the morphology of precipitates can reduce losses due to Ohmic polarization. For a given depth of discharge, higher electrode level energy densities (Wh/L) may be obtained by increasing the sulfur concentration in the polysulfide solution. However, as sulfur concentration—and therefore electrode capacity—increase, the current needed to cycle at a fixed C-Rate increases as well. Therefore, it is important to understand the concentration-dependent transport and kinetic properties of lithium polysulfide solutions so that the rate-limiting mechanisms may be appropriately addressed. We have undertaken a systematic effort to characterize the ionic conductivity and exchange current density of lithium polysulfide solutions in various solvents as a function of sulfur concentration and solution composition (i.e. n in Li 2 S n ). Figure 3 shows the concentration-dependent ionic conductivity of Li 2 S 8 solutions in selected non-aqueous solvents. Acknowledgements: This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Figure captions: Figure 1. Fivefold higher reversible capacity for the nanoscale conductor suspension (1.5 vol% carbon black) compared to a traditional carbon fiber current collector when galvanostatically cycling in a non-flowing cell with a lithium anode. Figure 2. Scanning electron microscope image of lithium sulfide precipitates on a network of multi-walled carbon nanotubes after first discharge of a stationary lithium polysulfide cell at C/4 rate. On the circled nanotube, lithium sulfide particles merge together to form a conformal coating. Figure 3. Ionic conductivity of lithium polysulfide solutions (Li 2 S 8 ) in selected nonaqueous solvents at varying sulfur concentrations from 1-8M. 1. Fan, F. Y. et al. Polysulfide Flow Batteries Enabled by Percolating Nanoscale Conductor Networks. Nano Lett. (2014). doi:10.1021/nl500740t 2. Demir-Cakan, R. et al. Li--S batteries: simple approaches for superior performance. Energy {&} Environ. Sci. 6, 176 (2012). 3. Yang, Y., Zheng, G. & Cui, Y. A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy {&} Environ. Sci. 6, 1552 (2013). 4. Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J.-M. Li – O 2 and Li – S batteries with high energy storage. Nat. Mater. 11, 19–30 (2012). 5. Manthiram, A., Fu, Y. & Su, Y.-S. Challenges and prospects of lithium--sulfur batteries. Acc. Chem. Res. 46, 1125–1134 (2013).