Design of Pyrophosphate Cathode Materials for High-Performance Rechargeable Na-Ion Batteries

Abstract

Na2MP2O7-NPs/C) is presented. Na2MP2O7-NPs/C prepared via a citric acid-assisted sol-gel method, followed by a post heat treatment, ball milling and re-heat treatment process (HT→BM→HT). From this synthetic process, uniform-size Na2MP2O7 (M = Fe, Co, Ni) nanoparticles under 50 nm which are homogeneously embedded in carbon can be obtained. In Fe-based pyrophosphate, Na2FeP2O7-NPs/C exhibits not only reversible capacity near the theoretical value (97 mA h g-1) over the voltage range of 2.0–4.0 V (vs. Na/Na+). Moreover, they display superior rate capability of 77, 70, 66 and 65 mA h g-1 even at high rates of 10, 20, 30 and 60 C, respectively. Equally notable is the exceptional long-term cyclability at high rates. At the rate of 10 and 60 C, capacity retention at 10000 cycles is over 80%, respectively. Meanwhile, electrochemical performance of Fe-based pyrophosphate is improved by compositional change. Compared to Na2FeP2O7, more electrochemical Fe2+/Fe3+ redox reaction can occur in Na3.12Fe2.44(P2O7)2 when charging/discharging, which anticipates in higher practical capacity and energy density than Na2FeP2O7. So, for improving the energy density of Fe-based pyrophosphate, therefore, Na3.12Fe2.44(P2O7)4 nanoparticles embedded in carbon (Na3.12Fe2.44(P2O7)4/C nanocomposite

Na3.12Fe2.44(P2O7)4-NPs/C) is synthesized for the same synthetic process. Indeed, Specific capacity of Na3.12Fe2.44(P2O7)2-NPs/C is measured to be higher than that of Na2FeP2O7-NPs/C. Furthermore, reduced graphene oxide-supported Na3.12Fe2.44(P2O7)4-NPs/C (Na3.12Fe2.44(P2O7)4-NPs/C/rGO) composite is also synthesized. The addition of small amount of rGO in Na3.12Fe2.44(P2O7)4-NPs/C exhibits not only a high reversible electrochemical reaction, but also a superior rate capability (92, 85 and 78 mA h g-1 at 2, 5 and 10 C), and long-term cyclic stability (capacity retention of 70% over 5000 cycle at 10 C). This low-cost and high-performance composite can be a promising cathode material in NIBs. Co-based pyrophosphate is studied. Among the three distinct polymorphs in Na2CoP2O7, it is known that triclinic polymorph (rose) exhibits similar electrochemical behavior with Na2FeP2O7, but significantly higher operating voltage than Na2FeP2O7 and even other NIB cathode materials. However, it is hard to obtain rose phase in ordinary synthetic process. Fine controls can make it possible to synthesize rose phase. So, several synthesis conditions that can obtain the rose phase are discussed in detail. Rose phase can be obtained in stoichiometric compound annealed at low temperature under 500 °C. Also, With the increasing of Co element and decreasing of Na element in non-stoichiometric Na2-2xCo1+xP2O7 compounds, metastable rose phase can be obtained stably at higher annealing temperature over 500 °C compared to stoichiometric compound. Na2-2xCo1+xP2O7/C nanocomposites (Na2-2xCo1+xP2O7-NPs/C) are synthesized for the same synthetic process with Fe-based pyrophosphate nanocomposites. In contrast to micro-sized bulk Na2-2xCo1+xP2O7, metastable rose becomes to major products both stoichiometric and non-stoichiometric Na2-2xCo1+xP2O7-NPs/C (0 ≤ x ≤ 0.2) heat-treated at 600 °C. All three rose phase Na2Co1P2O7-NPs/C, Na1.8Co1.1P2O7-NPs/C and Na1.6Co1.2P2O7-NPs/C exhibit the better electrochemical performance than B- Na1.8Co1.1P2O7/C. In particular, rose phase Na1.8Co1.1P2O7-NPs/C exhibits a good cyclic stability with the reversible capacity more than 80 mA h g-1. This high-performance Na2-2xCo1+xP2O7-NPs/C can be a promising high-voltage cathode material in NIBs. Finally, Na2FeP2O7-NPs/C uniformly loaded on the surface of flexible porous carbon cloth (Na2FeP2O7-NPs/C@PCC) electrode is fabricated by dip-coating process, This flexible electrode without any conductive agents and polymeric binders also exhibit excellent rate capability and long-term cyclability at high rate of 10 C (56 mA h g-1 after 2000 cycles). We show high-performance free-standing Na2FeP2O7-NPs/C@PCC electrodes for possible application in flexible NIBs. Overall, this thesis focuses on the synthesis of Na2MP2O7/C nanocomposites and evaluation of their electrochemical performance for high-performance NIB cathode material through nano-engineering and compositional engineering. Also, their application to free-standing electrode combined with the porous carbon cloth for possible flexible NIBs is investigated. Furthermore, synthetic strategy in this thesis would open the preparation of other sodium metal pyrophosphate nanocomposites, and will provide a new breakthrough to solve the faced problems in rechargeable NIBs and other energy storage devices.

Ever-increasing demands for energy production and energy storage, environmental-friendly energy storage systems have become important issues at the present time. Rechargeable batteries are one of the most attractive electrochemical energy storage systems to store both conventional energy from fossil fuels, nuclear reactions, and renewable energy from solar, wind, etc. Among the rechargeable batteries, rechargeable LIBs have been developed in recent years as power sources in energy storage because of their high power and energy density, little memory effect together with long operating life and good environmental compatibility. In addition, recent advances in LIB technologies toward increasing the gravimetric and volumetric energy densities enable LIB to applicate in the large-scale electrical energy storage such as electrical vehicles (EVs) or grid-scale energy storage systems (EESs). However, limitation of the availability of high-cost Li resources resulting from the lack of reserves and their uneven distribution have led to a search for alternative battery system. As a post LIBs, rechargeable Na-ion batteries (NIBs) are considered as one of the potential candidates for large-scale energy storage systems because of the abundance and low cost of sodium resources, and similar electrochemical behavior of Na-ion with the Li-ion for intercalation in the cathode. So, the development of high-performance cathode materials is needed for their NIB application. While there exist many challenges in the fabrication of cathodes, one of the polyanionic framework, Na2MP2O7 (M = transition metal), has been in the spotlight as a potential cathode material in NIBs because of its thermal, chemical stability and good electrochemical performance (good rate capability, cyclability in Na2FeP2O7, and high operating voltage in Na2CoP2O7). In this thesis, Na2MP2O7 (M = Fe, Co) cathode materials are designed for high-performance NIBs through both nano-engineering and compositional engineering. Also, free-standing electrode is fabricated by incorporating this high-performance cathode material with flexible electrode for high-performance flexible electrode. For this, first, general synthetic process for synthesizing Na2MP2O7 nanoparticles embedded in carbon (Na2MP2O7/C nanocomposite