Improved Na storage and Coulombic efficiency in TiP 2 O 7 @C microflowers for sodium ion batteries
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Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China 2 Shenzhen Research Institute of Shandong University, Shenzhen 518000, China 3 Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong Innovation Campus, North Wollongong New South Wales 2500, Australia 4 State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 15 July 2020 / Revised: 14 August 2020 / Accepted: 16 August 2020
ABSTRACT Ti-based anode materials in sodium ion batteries have attracted extensive interests due to its abundant resources, low toxicity, easy synthesis and long cycle life. However, low Coulombic efficiency and limited specific capacity affect their applications. Here, cubic-phase TiP2O7 is examined as anode materials, using in-situ/ex-situ characterization techniques. It is concluded that the redox reactions of Ti4+/Ti3+ and Ti3+/Ti0 consecutively occur during the discharge/charge processes, both of which are highly reversible. These reactions make the specific capacity of TiP2O7 even higher than the case of TiO2 that only contains a simple anion, O2−. Interestingly, Ti species participate only one of the redox reactions, due to the remarkable difference in local structures related to the sodiation process. The stable discharge/charge products in TiP2O7 reduce the side reactions and improve the Coulombic efficiency as compared to TiO2. These features make it a promising Ti-based anode for sodium ion batteries. Therefore, TiP2O7@C microflowers exhibit excellent electrochemical performances, ~ 109 mAh·g−1 after 10,000 cycles at 2 A·g−1, or 95.2 mAh·g−1 at 10 A·g−1. The results demonstrate new opportunities for advanced Ti-based anodes in sodium ion batteries.
KEYWORDS TiP2O7, electrochemical reactions, anodes, sodium ion batteries
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Introduction
Na-ion batteries (NIBs) as one of next-generation batteries attract extensive attention in the past years, due to the abundant reserves of Na in earth, the similar working principles and fabrication protocols to those of Li-ion batteries (LIBs), as well as the low-cost current collector for anodes [1, 2]. However, the relatively large ionic radius and heavy ionic mass of Na+ impede its diffusion in electrodes, greatly increasing the voltage hysteresis and reducing the round-trip energy efficiency [3, 4]. Meanwhile, the large ionic radius of Na+ also induces a giant volume change in the discharge/charge processes, easily leading to structure collapse and particle pulverization. Then, the effective electrical connection between active materials and current collectors is lost, resulting in a rapid decay of performances [5, 6]. To address these issues, the selection of electrode materials is very important, because the electrode materials based on intercalation reactions always exhibit the small volume cha
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