Synthesis of Cation and Water Free Cryptomelane Type OMS-2 Cathode Materials: The Impact of Tunnel Water on Electrochemi
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Synthesis of Cation and Water Free Cryptomelane Type OMS-2 Cathode Materials: The Impact of Tunnel Water on Electrochemistry Altug S. Poyraz1, Jianping Huang2, Bingjie Zhang2, Amy C. Marschilok2,3*, Kenneth J. Takeuchi2,3*, and Esther S. Takeuchi1,2,3* 1
Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA. Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA. 3 Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, USA. 2
*corresponding authors: (ACM) [email protected], (KJT) [email protected], (EST) [email protected] ABSTRACT Cryptomelane type manganese dioxides (α-MnO2, OMS-2) are interesting potential cathode materials due to the ability of their one dimensional (1D) tunnels to reversibly host various cations including Li+ and an accessible stable 3+/4+ redox couple. Here, we synthesized metal cation free OMS-2 materials where the tunnels were occupied by only water and hydronium ions. Water was subsequently removed from the tunnels. Cation free OMS-2 and Dry-OMS-2 were used as cathodes in Li based batteries to investigate the role of tunnel water on their electrochemistry. The initial discharge capacity was higher for Dry-OMS-2 (252 mAh/g) compared to OMS-2 (194 mAh/g), however, after 100 cycles Dry-OMS-2 and OMS-2 delivered 137 mAh/g and 134 mAh/g, respectively. Li+ ion diffusion was more facile for Dry-OMS as evidenced by rate capability, at 400 mA/g. Dry-OMS-2 delivered 135mAh/g whereas OMS-2 delivered ~115 mAh/g. This first report of the impact of tunnel water on the electrochemistry of OMS-2 type materials demonstrates that the presence of tunnel water in OMS-2 type materials negatively impacts the electrochemistry. INTRODUCTION Naturally occurring cryptomelane type manganese dioxides (α-MnO2) are interesting insertion electrodes for rechargeable batteries due to their high ion exchange capacity and easily accessible Mn3+/4+ active redox couple.1-4 Synthetic tunnel structured α-MnO2 is also known as octahedral molecular sieve-2 (OMS-2).4, 5 OMS-2 has 2x2 (0.46 x 0.46 nm) tunnels in the preferential growth c-axis to form one dimensional (1D) ion diffusion paths. 1D tunnels of OMS2 can reversibly host numerous mono (i.e. Li+ and Na+) and divalent (i.e. Mg2+ and Zn2+) cations. Therefore OMS-2 materials have been investigated as cathodes in rechargeable Li+, Na+, Mg2+, and Zn2+ batteries.3, 6-10 As a cathode in rechargeable Li batteries, OMS-2 has a theoretical capacity of 300mAh/g and specific energy exceeding 500Wh/kg. 2x2 tunnel structured OMS-2 are often synthesized using aqueous solution based redox reactions. The tunnels are formed with the assistance of tunnel stabilizing and charge balancing cations such as K+, Li+, Ag+, H3O+, and NH4+.2, 4, 5, 11, 12 In addition to tunnel stabilizing cations, the tunnels of OMS-2 materials are also populated with structural (tunnel) water due to the high capillary condensation in micropores (0.46 x 0.46nm).9, 13 The effect of tunnel o
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