In this study we have investigated the electrochemical properties of hollow silicon nanospheres encapsulated with a thin carbon shell, HSi@C, as a potential candidate for lithiumion battery anodes. Hollow Si nanospheres are formed using a templating method which is followed by carbon coating via carbonization of a pyrrole precursor to form HSi@C. The synthesis conditions and the resulting structure of HSi@C have been studied in detail to obtain the target design of hollow Si nanospheres encapsulated with a carbon shell. The HSi@C obtained exhibits much better electrochemical cycle stability than both micro-and nano-size silicon anodes and deliver a stable specific capacity of 700 mA h g-1 after 100 cycles at a current density of 2 A g-1 and 800 mA h/g after 120 cycles at a current density of 1 A g-1. The superior performance of HSi@C is attributed to the synergistic combination of the nanostructured material, the enhanced conductivity, and the presence of the central void space for Si expansion with little or no change in the volume of the entire HSi@C particle. This study is the first detailed investigation of the synthesis conditions to attain the desired structure of a hollow Si core with a conductive carbon shell. This study also offers guidelines to further enhance the specific capacity of HSi@C anodes in the future.

We introduce a new concept of hybrid Na-based flow batteries (HNFBs) with a molten Na alloy anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane for grid-scale energy storage. Such HNFBs can operate at ambient temperature, allow catholytes to have multiple electron transfer redox reactions per active ion, offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na alloy anode, and enable the use of both aqueous and non-aqueous catholytes. Further, the molten Na alloy anode permits the decoupled design of power and energy since a large volume of the molten Na alloy can be used with a limited ion-exchange membrane size. In this proof-of-concept study, the feasibility of multi-electron transfer redox reactions per active ion and multiple active ions for catholytes has been demonstrated. The critical barriers to mature this new HNFBs have also been explored.

In this study, we have investigated the key factors dictating the cyclic performance of a new type of hybrid sodium-based flow batteries (HNFBs) that can operate at room temperature with high cell voltages (>3 V), multiple electron transfer redox reactions per active ion, and decoupled design of power and energy. HNFBs are composed of a molten Na-Cs alloy anode, flowing aqueous catholyte, and a Na-β″-Al2O3 solid electrolyte as the separator. The surface functionalization of graphite felt electrodes for the flowing aqueous catholyte has been studied for its effectiveness in enhancing V(2+)/V(3+), V(3+)/V(4+), and V(4+)/V(5+) redox couples. The V(4+)/V(5+) redox reaction has been further investigated at different cell operation temperatures for its cyclic stability and how the properties of the solid electrolyte membrane play a role in cycling. These fundamental understandings provide guidelines for improving the cyclic performance and stability of HNFBs with aqueous catholytes. We show that the HNFB with aqueous V-ion catholyte can reach high storage capacity (∼70% of the theoretical capacity) with good Coulombic efficiency (90% ± 1% in 2-30 cycles) and cyclic performance (>99% capacity retention for 30 cycles). It demonstrates, for the first time, the potential of high capacity HNFBs with aqueous catholytes, good capacity retention and long cycling life. This is also the first demonstration that Na-β″-Al2O3 solid electrolyte can be used with aqueous electrolyte at near room temperature for more than 30 cycles.

In this study, a new mechanism for the reduction of vanadyl acetylacetonate, VO(acac) , to vanadium acetylacetonate, V(acac) , is introduced. V(acac) has been studied for use in redox flow batteries (RFBs) for some time; however, contamination by moisture leads to the formation of VO(acac) . In previous work, once this transformation occurs, it is no longer reversible because there is a requirement for extreme low potentials for the reduction to occur. Here, we propose that, in the presence of excess acetylacetone (Hacac) and free protons (H ), the reduction can take place between 2.25 and 1.5 V versus Na/Na via a one-electron-transfer reduction. This reduction can take place in situ during discharge in a novel hybrid Na-based flow battery (HNFB) with a molten Na-Cs alloy as the anode. The in situ recovery of V(acac) during discharge is shown to allow the Coulombic efficiency of the HNFB to be ≈100 % with little or no capacity decay over cycles. In addition, utilizing two-electron-transfer redox reactions (i.e., V /V and V /V redox couples) per V ion to increase the energy density of RFBs becomes possible owing to the in situ recovery of V(acac) during discharge. The concept of in situ recovery of material can lead to more advances in maintaining the cycle life of RFBs in the future.

We introduce a new concept of hybrid Na-based flow batteries (HNFBs) with sodium-based stationary anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane for grid-scale energy storage [1]. Such HNFBs can operate at ambient temperature; allow catholytes to have multiple electron transfer redox reactions per active ion; offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na anode. Further, the Na anode gives high voltage (> 3 V) for this hybrid flow battery. In this work, the feasibility of using the aqueous catholyte in HNFBs to utilize multi-electron transfer redox reactions per active ion and multiple active ions [2] for catholytes has been evaluated and demonstrated. The catholyte chemistry can be the same as or similar to that of traditional RFBs. For example, the reactions in both the anolyte and catholyte in all vanadium RFBs can potentially be used in the catholyte of HNFBs, as shown by reactions (1) to (3) below. Cathode:  VO2+ + H2O ↔ VO2 + + e-             Eo = +1.0 V vs. SHE                   (1)                        V3+ + H2O ↔ VO2+ + 2H+ + e-             Eo = +0.34 V vs. SHE        (2)                 V2+ ↔ V3+ + e-                                     Eo = -0.26 V vs. SHE           (3) Anode:      Na ↔ Na+ + e−                                    Eo = -2.7 V vs.SHE              (4) Based on these redox reactions of the catholyte and assuming that the catholyte contains 2.5M active V ions, one can obtain a theoretical specific energy of 483.7 Wh/kg, which is 18 times the specific energy provided by conventional all vanadium RFBs (~25 Wh/kg) [3]. The feasibility of using the aqueous catholyte in HNFBs is evaluated using VOSO4 (V4+) solution. The CV test (Figure 1a) shows the stable electrochemical potential window for the aqueous catholyte solution and the redox potential of V ions. Addition of BiCl3 in V aqueous electrolytes has been found an effective approach to improve the reversibility of V ion redox reactions [4]. The A, B, C redox peaks are attributed to the V4+/V5+, V3+/V4+, and V2+/V3+ redox reaction couples, respectively, while the strong D peaks are generated by the redox reaction of Bi3+/Bi0. Figure 1a also shows that the O2 evolution reaction (OER) and H2 evolution reaction (HER). Based on that and further considering the overpotential in the cell, the practical threshold can be 2.2 – 4.0 V vs. Na+/Na. The charge/discharge behavior of the HNFB with aqueous catholyte and a flat β”-Al2O3 membrane are examined and shown in Figure 1b. Three plateaus can be observed on the 1st cycle of both discharge and charge curves, corresponding to the redox reactions of V(IV)/V(III), Bi3+(III)/Bi(0), and V(III)/V(II) as labelled in the figure. Therefore, for the first time the concept of multiple electron transfer redox reactions per active ion in aqueous electrolytes (2 redox per V active ion) has been validated. Moreover, the result shows multiple electron transfer obtained by two active species, i.e. V and Bi ions. It should be noted the low capacity for each redox reaction shown in Figure 1b is mainly attributed to the insufficient mass transportation in the catholyte due to weak stirring and thick, porous graphite felt electrode making the mass transportation inside the felt slow and difficult. The investigations on improving the capacity are undergoing. Reference [1]           L. Shaw, J. Shamie, United States Patent, in application, 2014. [2]           W. Wang, L. Li, Z. Nie, B. Chen, Q. Luo, Y. Shao, X. Wei, F. Chen, G.-G. Xia, Z. Yang, Journal of Power Sources 2012, 216, 99. [3]           W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater. 2013, 23, 970. [4]           B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang, Nano Lett. 2013, 13, 1330. Figure 1. (a) CV curves of aqueous electrolytes with and without BiCl3 in half cells at a scan rate of 10 mV/s in which a graphite felt, Ag/AgCl, and Pt wire were used as the working, reference, and counter electrodes, respectively. (b) The charge/discharge profile of a full cell with the 0.01M VOSO4 -0.05M Na2SO4 -1.5M HCl -0.002M BiCl3aqueous solution as the catholyte. Figure 1