www.advenergymat.de FULL PAPER 1801781 (1 of 11) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Enhanced Cycling Stability of Macroporous Bulk Antimony- Based Sodium-Ion Battery Anodes Enabled through Active/ Inactive Composites Olivia Ruiz, Mark Cochrane, Manni Li, Yan Yan, Ke Ma, Jintao Fu, Zeyu Wang, Sarah H. Tolbert, Vivek B. Shenoy, and Eric Detsi* DOI: 10.1002/aenm.201801781 resources used in commercial LIBs. [1–3] The desire to keep pace with a growing market has led to intensive research of alternative rechargeable battery technolo- gies that utilize earth-abundant elements, such as sodium- and magnesium-ion batteries (SIBs and MIBs). [2,4–10] While progress towards practical MIBs is hin- dered by the lack of suitable electrolytes that are compatible with Mg metal, [10–14] a major challenge with SIBs corresponds to the fact that achieving energy densi- ties equal to, or exceeding LIBs requires high-capacity anode materials such as phosphorus (2596 mAh g -1 for Na 3 P), tin (847 mAh g -1 for Na 3 Sn), antimony (660 mAh g -1 for Na 3 Sb), etc. [15,16] Among these candidate materials, Sb is particularly promising because of its exceptionally fast kinetics during Na insertion. [17–20] As with common high- capacity anode materials, phase trans- formations associated with alloying reactions of Sb with Na give rise to large mechanical stresses and huge volume expansions of up to 400% during full sodiation. [19–26] In turn, these expansions break the electrical contact between the active materials (i.e., Sb grains) and other electrode components including the binder, conductive additives, and current collector. [27–32] This ultimately leads to battery failure after limited cycling. A common strategy to Engineering strategies based on “nanostructuring” and “active/inactive composites” are commonly used separately to improve the performance of alkali-ion battery electrodes. Here, these two strategies are merged to further enhance the performance of alloy-type alkali-ion battery anodes. Specifically, macroporous antimony (Sb)/magnesium fluoride (MgF 2 ) active/inactive composite material is used as a high-performance Na-ion battery anode. The porous Sb phase with pore size in the sub-micrometer range acts as the electrochemically active component and the electrochemically inactive dense MgF 2 phase acts as a mechanical buffer. Na-ion battery anodes made of porous Sb/MgF 2 active/inactive composites are reversibly sodiated for over 300 cycles, delivering a capacity of ≈551 mAh g -1 after 300 cycles at a C-rate of C/2. This performance is remarkable because the porous Sb/MgF 2 composite is not made of mesoporous structures. Furthermore, the cycling longevity of this porous Sb/MgF 2 composite outperforms the common nano- structured Sb-based Na-ion battery anode materials. This good performance is attributed to the “porous active/inactive” configuration, where the dense inactive mechanical buffer phase absorbs part of the phase transformation- induced stresses, while porosity in the active phase helps to accommodate the phase transformation induced volume expansions and electrolyte transfer into the bulk of this composite. Sodium-Ion Batteries O. Ruiz, M. Cochrane, M. Li, K. Ma, J. Fu, Z. Wang, Prof. V. B. Shenoy, Prof. E. Detsi Department of Materials Science and Engineering University of Pennsylvania Philadelphia, PA 19104-6272, USA E-mail: detsi@seas.upenn.edu M. Li School of Materials Science and Engineering Harbin Institute of Technology Harbin 150001, China Dr. Y. Yan, Prof. S. H. Tolbert Department of Chemistry and Biochemistry University of California Los Angeles Los Angeles, CA 90095-1569, USA Z. Wang State Key Laboratory of Advanced Welding and Joining Harbin Institute of Technology Harbin 150001, China Prof. S. H. Tolbert The California NanoSystems Institute University of California Los Angeles Los Angeles, CA 90095, USA Prof. E. Detsi Vagelos Institute for Energy Science and Technology (VIEST) Philadelphia, PA 19104, USA 1. Introduction The rechargeable battery industry is currently dominated by lithium-ion battery (LIB) technology, however there is growing concern for the limited availability of raw lithium and cobalt Adv. Energy Mater. 2018, 1801781