Large-scale Electrical Energy Storage Applications Research Articles

Insights into P-Type Iron- and Manganese-Based Layered Sodium Cathodes

Sodium-ion batteries are a low-cost alternative to lithium-ion batteries in large-scale electric energy storage applications due to the natural-abundant sodium resources and similar working principle to LIBs during cycling [1]. Until now, many different cathodes for SIBs have been studied, including layered transition metal oxides (NaxTMO2, x<1), Prussian blue-type compounds, polyanionic and organic compounds [2]. Among them, NaxTMO2 has been extensively reported, due to its layered structure, easy synthesis, promising electrochemical properties, and feasibility for commercial production [3]. According to the occupation of Na ions and oxygen stacking, NaxTMO2 can be classified into P2 and O3 types, where P2 structure contasins trigonal prismatic sites of sodium ions and ABBA oxygen stacking sequence and O3 structure represents octahedral sites of sodium ions and ABCABC oxygen stacking sequence [4]. At low potentials (<2.0V), there is a phase transition P2-P2’ connected with the manganese redox activity and the Jahn-Teller distortion [5]. Compared with O3-type structure, P2-type NaxTMO2 provides higher specific capacity and better structural stability due to the lower diffusion barriers through Na-ion prismatic sites [6].Iron- and manganese-based layered oxides for sodium-ion batteries have attracted renewed interest due to their low cost and environmental friendliness. However, the phase change at high voltage and the Jahn-Teller effect lead to shortening cycle life and poor rate capability. We design a structure optimization of the Na2/3Mn1/2Fe1/2O2 cathode through a partial substitution of Fe3+ to explore the mechanism of redox activity of P2-type Mn/Fe-based layered cathodes. We present the investigation of the effect of simultaneous cationic substitution on the electrochemical performance and structural stability of P2-Na2/3Mn1/2Fe1/2O2. The obtained material exhibits a solid solution mechanism during desodiation/resodiation process and delivers an initial discharge capacity of 168 mA h g-1 at 0.1C and a capacity retention of 80% after 50 cycles. The modified P2 composition has stable cycle performance in the potential window from 1.5V to 4.5V. The main focus here is to understand the fundamental electrochemical mechanism of this layered cathode material by exploring the redox processes of transition metal cations and oxygen anions upon cycling. In situ X-ray synchrotron radiation diffraction is used to explore the structural changes during cycling. In situ X-ray Absorption Near Edge Spectroscopy is used to elucidate the local electronic structure of Fe and Mn atoms as it evolves throughout this electrochemical process. 23Na solid-state nuclear magnetic resonance spectroscopy is used to investigate Na coordination environments during desodiation/resodiation. Ex situ soft X-ray absorption spectroscopy results reveal the participation of oxygen anions in the electrochemical reactions. Thus, we explore the redox reactions of transition metals and oxygen to explain the mechanism and the electrochemical performance for the P2-type Mn/Fe-based layered cathodes.

Shunt Current Analysis of Vanadium Redox Flow Battery System with Multi-Stack Connections

Vanadium redox flow battery (VRFB) systems with multiple stacks due to long lifetime, low self-discharge, and flexible design, are commonly used in large-scale electrical energy storage applications In a VRFB system, pumps deliver positive and negative electrolytes to each stack through a piping system including channels and manifolds. However, the electrolyte flowing between cells through channels and manifolds and the electrolyte flowing between stacks through pipes are electrically conductive. Shunt currents are generated due to the voltage difference between the cells and between the stacks, which reduce the energy efficiency of the battery system. In particular, under different load connections, the shunt currents of a multi-stack VRFB system have different distributions and cause different impairments to the system efficiency. Therefore, it is important to predict the shunt currents of the multi-stack system under different load connections before the actual construction of the system. In this paper, a multi-stack VRFB system with 120 cells was explored by circuit-based modeling to evaluate the shunt currents according to the stack configuration such as serial, parallel, and mixed connections.Then, the Coulomb efficiencies (CEs) were investigated with operating currents at 36 and 54A. The results show that shunt currents are generally more significant at the center cell of a stack than at other cells under these stack configurations and exhibit a positive correlation with the battery state of charge (SOC), i.e., larger shunt currents increase with SOC. In addition, the CEs of the system are higher at 54A compared to those at 36A due to smaller shunt current losses regardless of the stack configurations. Furthermore, regardless of the operating current levels, the multi-stack system connected by parallel loads achieves the highest CEs compared to the series and mixed connected systems due to the absence of shunt current losses in the piping system. Figure 1

Locating Shunt Currents in a Multistack System of All-Vanadium Redox Flow Batteries

An all-vanadium redox flow battery (VRFB) system, with multiple stacks, is typically used for large-scale electrical energy storage applications. In a VRFB system, pumps deliver positive and negati...

NASICON-Structured Na2VTi(PO4)3@C for Symmetric Aqueous Rechargeable Na-Ion Batteries with Long Lifespan

Mixed transition-metal ions NASICON-structured compounds are enabled simultaneously as cathode and anode for aqueous Na-ion batteries (ANIBs), simplifying the fabrication process and reducing manufacturing costs in practical applications. In this study, a mixed V³⁺/Ti⁴⁺ NASICON-structured Na₂VTi(PO₄)₃@C composite acts as a bifunctional electrode material of ANIBs, delivering excellent sodium storage performance including high reversible capacity (cathode: 52.4 mAh g–¹ at 1C, anode: 59.8 mAh g–¹ at 1C, 1C = 62 mA g–¹) and remarkable rate capability (cathode: 44.8 mAh g–¹ at 20C, anode: 52.9 mAh g–¹ at 20C), as well as long cycle life. Meanwhile, ex situ X-ray photoelectron spectra (XPS) measurements reveal that V⁴⁺/V³⁺ and Ti⁴⁺/Ti³⁺ oxidation/reduction couples are redox-active sites of NVTP@C cathode and anode, respectively. When NVTP@C assembled into symmetric ANIBs, a full cell achieves ultrahigh stability of over 2500 cycles at 10C, showing broad prospects in large-scale electric energy storage applications.

Active Materials for Aqueous Zinc Ion Batteries: Synthesis, Crystal Structure, Morphology, and Electrochemistry.

Aqueous zinc ion batteries (ZIBs) are truly promising contenders for the future large-scale electrical energy storage applications due to their cost-effectiveness, environmental friendliness, intrinsic safety, and competitive gravimetric energy density. In light of this, massive research efforts have been devoted to the design and development of high-performance aqueous ZIBs; however, there are still obstacles to overcome before realizing their full potentials. Here, the current advances, existing limitations, along with the possible solutions in the pursuit of cathode materials with high voltage, fast kinetics, and long cycling stability are comprehensively covered and evaluated, together with an analysis of their structures, electrochemical performance, and zinc ion storage mechanisms. Key issues and research directions related to the design of highly reversible zinc anodes, the exploration of electrolytes satisfying both low cost and good performance, as well as the selection of compatible current collectors are also discussed, to guide the future design of aqueous ZIBs with a combination of high gravimetric energy density, good reversibility, and a long cycle life.

Efficient and Facile Electrochemical Process for the Production of High-Quality Lithium Hexafluorophosphate Electrolyte.

The global consumption for lithium hexafluorophosphate (LiPF6) has increased dramatically with the rapid growth of Li-ion batteries (LIBs) for large-scale electric energy storage applications. Conventional LiPF6 production has a high cost and high energy consumption due to complicated separation and purification processes. Here, based on the electrode materials of LiMn2O4 and polyaniline (PANI), we propose a facile electrochemical extraction/release process for LiPF6 electrolyte production. This new technology consists of two independent steps: a PF6-- and Li+-extracting step using a PANI/LixMn2O4 cell in aqueous solution (an ion extraction step) and a LiPF6 electrolyte production step from the charged LiMn2O4/PANI+PF6- cell in an organic electrolyte (an ion release step). This new process can effectively avoid the contamination of HF residue in the final product, providing a great possibility to create a facile, energy-efficient, and low-cost LiPF6 electrolyte production.

Superior potassium-ion storage properties by engineering pseudocapacitive sulfur/nitrogen-containing species within three-dimensional flower-like hard carbon architectures

Rechargeable non-aqueous potassium-ion batteries have been regarded as one of ideal candidates for large-scale electric energy storage applications. However, the achievement of high reversible capacity and rate capability is still a great challenge. In this work, an extraordinary S-/N-/O- multielement-doped three-dimensional (3D) flower-like hard carbon architecture is firstly proposed as promising anode material for low-cost potassium-ion batteries. The 3D flower-like architecture not only provides abundant exposed surface-active sites but acts as highly conductive interconnected network for electron transport. More encouragingly, the introduction of highly reactive -N-Cx-S- species has been revealed to show highly reversible pseudo-capacitive charge storage behavior, inherently enlarging the slope reversible capacity to the highest value of 423 mAh g−1 at low current density of 0.05 A g−1. As well, benefitted from the structural and compositional advantages, an unprecedented rate capability (251 mAh g−1 at 1.0 A g−1) and stable cycling stability (362 mAh g−1 after 300 cycles at 0.5 A g−1) has been obtained, which outperforms most of carbonaceous materials for K-ion storage. Our present work not only provide new understandings on surface/conversion-synergistic driven K-ion storage mechanisms but offer effective material engineering strategies for improving the properties of potassium-ion batteries.

Numerical modelling and in-depth analysis of multi-stack vanadium flow battery module incorporating transport delay

The flow battery module comprised of multi-stack is commonly constructed for use in large-scale electrical energy storage applications. In such a multi-stack module, the transport delay associated with electrolyte flow in the piping systems inevitably exists that can impose a significant impact on module design and operation performance. In this paper, a complete dynamic model incorporating transport delay is developed for the multi-stack vanadium flow battery module. Based on the model, the module performance and capacity utilization are comprehensively analyzed for different module designs and various operational conditions. Simulation results demonstrate that the transport delay can cause uneven concentration distributions along the piping and lead to a poor stack voltage uniformity in the module, which can subsequently result in premature voltage cut-off in operation and a degraded capacity utilization. Meanwhile, the analyses also prove that the transport delay and its negative effect on the module can be effectively reduced by optimizing the electrolyte feeding mode in addition to adopting a high variable flow rate and a small pipe radius. Such an in-depth simulation analysis considering the transport delay not only offers a cost-effective way to analyze a multi-stack flow battery system, but also provides a deep insight into design and optimization of the large-scale flow battery module that can allow both high system efficiency and superior capacity utilization to be achieved.

Multi-electron reaction materials for sodium-based batteries

Sodium-based rechargeable batteries are very promising energy storage and conversion systems owing to their wide availability and the low cost of Na resources, which is beneficial to large-scale electric energy storage applications in future. In the context of attempting to achieve high-energy densities and low cost, multi-electron reaction materials for both cathodes and anodes are attracting significant attention due to high specific capacities involved. Here, we present a brief review on recently reported multi-electron reaction materials for sodium-based batteries. We mostly concentrate on true multi-electron reactions that involve individually valence changes greater than one per redox center, but in addition include materials in the discussion, which undergo multi-electron processes per formula unit. The theoretical gravimetric and volumetric (expanded state) capacities are studied for a broad range of examples. Then, the practically achievable volumetric energy density and specific energy of Na cells with hard carbon, sodium (Na), and phosphorus (P) anodes are compared. For this purpose, various data are recalculated and referred to the same basis cell. The results show the potential superiority of the cells using multi-electron reaction materials and provide an intuitive understanding of the practically achievable energy densities in future Na-based rechargeable batteries. However, these multi-electron reaction materials are facing several key challenges, which are preventing their high-performance in current cells. In order to overcome them, general strategies from particle design to electrolyte modification are reviewed and several examples in both cathode and anode materials using such strategies are studied. Finally, future trends and perspectives for achieving promising Na-based batteries with better performance are discussed.

Organic materials for rechargeable sodium-ion batteries

Rechargeable sodium-ion batteries (SIBs) have attracted great attention for large-scale electric energy storage applications and smart grid owing to the abundance of Na resources and comparable performance with lithium-ion batteries. The use of organic electrode materials enables a sodium storage system with high energy/power density, metal-free, environmental friendliness, flexibility, lightweight, and cost-effectiveness. More importantly, the structural diversity and the ease of functionalizing organic molecules allows straightforward controllability on the redox properties and thus on the battery performances. Despite the development of organic SIBs is still in its infant state, they have drawn dramatically growing attentions and shown great promises throughout research works. In this review, we summarize the research efforts to push forward the electrochemical performance of the organic SIBs. We explore the efforts made on molecular design and electrode design, and the combination of both forms the basis to regulate the electrochemical properties. Moreover, we summarize the strategies for reducing the solubility of the organic electrode materials in light of the importance of this issue. The crucial differences between organic lithium-ion batteries and organic SIBs are presented. Finally, future challenges and opportunities of further developing organic electrode materials are discussed.

Polydopamine: A Promising Organic Cathode for Rechargeable Batteries

Organic electrodes have been promising candidate for large-scale electrical energy storage applications thanks to several advantages, such as low cost, high theoretical capacity, and chemical diversities. However, their high solubility in organic electrolytes and poor electrical conductivity have limited their cycling stability as well as rate capability. A promising strategy to address these issues is to polymerize organic molecules onto conductive carbon substrates.1 In composite structure, the covalent bonds between the monomers can prevent the dissolution of molecules, while conductive carbon can support the fast redox reactions in the polymer.1 Tremendous efforts have been devoted in developing nanocomposite by polymerizing carbonyl compounds on various nanocarbons such as graphene or carbon nanotubes.1 The electrochemical performance of the composites strongly dependents on their microstructures, which indicates that the interface between the redox-active polymer and nanocarbon support should be optimized for maximizing the electrochemical performance of the composite electrodes.1 In our recent study, we investigated the electrochemical properties of polydopamine by both computational and experimental methods. The density functional theory (DFT) calculations show that polydopamine can interact with both electrolyte ions (cations and anions) at high voltage region of 2.5~4.1 V vs. Li, and the interactions are also confirmed by cyclic voltammetry measurements.1 Comparing with the complicated polymerization process for most polymer cathode materials, dopamine can be spontaneously polymerized in weak alkaline solutions.1 We fabricate the free-standing and flexible hybrid electrodes by spontaneously coating polydopamine onto few-walled carbon nanotubes.1 The hybrid electrodes delivered a gravimetric capacity of ~133 mAh/g in Li-cells utilizing double layer capacitance from FWNTs and multiple redox-reactions from polydopamine.1 In addition, for further improving the performance of the composite electrodes, we utilized reduced graphene oxide as a redox-active and conductive substrate for the growth of polydopamine. During the hydrothermal treatment, dopamine can reduce and functionalize the graphene oxide simultaneously, which results in a 3D structured functionalized graphene hydrogel. The functionalized graphene shows a significantly enhanced capacity up to 200 mAh/g. Reference: 1. T. Liu, K. C. Kim, B. Lee, Z. Chen, S. Noda, S. S. Jang and S. W. Lee, Energy & Environmental Science, 2016. DOI: 10.1039/C6EE02641A

Scalable Template-Free Synthesis of Na2Ti3O7/Na2Ti6O13 Nanorods with Composition Tunable for Synergistic Performance in Sodium-Ion Batteries

Sodium-ion batteries are regarded as one of the potential candidates to substitute lithium ion batteries in large-scale electric energy storage applications in the near future. Several forms of sodium titanates including Na2Ti3O7, Na2Ti6O13 and Na4Ti5O12 have recently been applied as anode in sodium ion batteries. Here, we present an energy-efficient solid-state synthesis, via the addition of carbon, to mass-produce a series of uniform and single-crystalline Na2Ti3O7/Na2Ti6O13 nanorods with tunable composition as anode materials for sodium-ion batteries.1 Due to the extra local heat generation and CO2/CO release from carbon oxidation, the carbon added in the synthesis provides an alternative low-temperature route for the Na2Ti3O7 formation at 450 °C, a reaction temperature much lower than that of conventional solid-state methods (750–1000 °C), and Na2CO3 is regenerated to be recycled in the synthesis. The high theoretical capacity of Na2Ti3O7 and low volume expansion of Na2Ti6O13 upon charge–discharge are synergistically exploited to achieve high electrochemical performance and stability.

Scalable modified Solid-State Synthesis of Na2Ti3O7 Nanorods As Anodes for Sodium-Ion Batteries

Sodium-ion batteries are regarded as one of the potential candidates to substitute lithium ion batteries in large-scale electric energy storage applications in the near future. [1] Several forms of sodium titanates including Na2Ti3O7, Na2Ti6O13 and Na4Ti5O12have been recently applied as anode in sodium ion batteries. [2-4] In this study, single-crystalline Na2Ti3O7 nanorods are successfully fabricated by solid-state method with the addition of some solid additives. The electrochemical performance of Na2Ti3O7 nanorods is compared to conventionally synthesized Na2Ti3O7 irregular particles. Na2Ti3O7 nanorods outperform Na2Ti3O7 irregular particles at all current rates in the range of 0.1 to 5 C. Without surface modification, the nanorods can still manifest a reversible discharge capacity of 70 mAhg-1at a high current rate of 5 C. The enhanced electrochemical performance can be attributed to the high surface area of the nanorods as well as efficient ion transport induced in highly uniform one-dimensional nanostructure. References H. Pan, Y.-S. Hu and L. Chen, Energy & Environmental Science, 2013, 6, 2338-2360.P. Senguttuvan, G. Rousse, V. Seznec, J.-M. Tarascon and M. R. Palacín, Chemistry of Materials, 2011, 23, 4109-4111.A. Rudola, K. Saravanan, S. Devaraj, H. Gong and P. Balaya, Chemical Communications, 2013, 49, 7451-7453.P. J. P. Naeyaert, M. Avdeev, N. Sharma, H. B. Yahia and C. D. Ling, Chemistry of Materials, 2014, 26, 7067-7072. Acknowledgements This project is financially supported by Research Grants Council of Hong Kong (GRF 17300014), HKU Small Project Funding (201409176115), HKU Strategic Research Theme on Clean Energy, and University Development Fund for Initiative for Clean Energy and Environment. Figure 1

Saltwater as the energy source for low-cost, safe rechargeable batteries

Cheap, familiar saltwater (NaCl solution) was utilized to build low-cost, safe rechargeable batteries for large-scale electrical energy storage applications.

Ambient temperature sodium-sulfur batteries.

Ambient- or room-temperature sodium-sulfur batteries (RT Na-S) are gaining much attention as a low-cost option for large-scale electrical energy storage applications. However, their adoption is hampered by severe challenges. This concept paper summarizes first the operating principles, history, recent progress, and challenges of RT Na-S battery technology, and then suggests future directions towards enhancing performance in order for it to be a viable technology.

A phosphorus/N-doped carbon nanofiber composite as an anode material for sodium-ion batteries

Sodium-ion batteries (SIBs) have been attracting intensive attention at present as the most promising alternative to lithium-ion batteries in large-scale electrical energy storage applications, due to the low-cost and natural abundance of sodium.

Comparison of the Thermal Stability of FeOF Cathode in Na-Ion Batteries and Li-Ion Batteries

Recently ambient temperature sodium-ion batteries have drawn great attention particularly in large-scale electric energy storage applications for renewable energy and smart grid due to the low cost and abundant sodium resources. Up to now, lots of cathode and anode materials as well as electrolytes have been proposed for Na-ion batteries. However, little research about the safety issue of Na-ion batteries has been carried out. It is well known that Li-ion batteries have thermal risk under abuse or severe environment. Because Na-ion batteries have similar working mechanism as Li-ion batteries and sodium shows even higher reactivity against air, the study on thermal stability of Na-ion batteries is indispensable for its practical application.Iron oxyfluoride (FeOF) is a kind of iron based conversion type cathode material, which had been proposed as a cathode material for Li-ion batteries and obtained a large practical capacity due to conversion reactions [1, 2]. In this study, FeOF was also applied as the cathode material for Na-ion batteries. After electrochemical pretreatment in Na-ion or Li-ion batteries, the charged/discharged FeOF cathodes were mixed with corresponding electrolytes and thermally analyzed. By varying the ratio of cathode/electrolyte in the mixture, the heat generation mechanisms of FeOF cathodes in Na-ion and Li-ion batteries were discussed, and the thermal stability of Na-ion and Li-ion batteries were compared.FeOF was synthesized by a reaction of 1:2.33 molar ratio of Fe2O3 (Sigma-Aldrich) and FeF3 (Soekawa Chemicals Co., Ltd.) at a temperature over 1000 oC in a roll-quenching machine (Harddays Co. Ltd). The synthesized FeOF had been indexed as a tetragonal structure with P42/mnm space group. The obtained FeOF flake was ground with acetylene black using a planetary ball milling machine at a weight ratio of 70:25. 5 wt.% PVdF and polyacrylate binders were added in the cathodes for Li-ion batteries and Na-ion batteries, respectively. The electrochemical measurements were carried out with 2032 coin-type two-electrode cells. In Li-ion batteries, Li foil was used as a counter electrode and 1 mol/cm3 LiPF6/EC-DMC or 1 mol/cm3 LiClO4/PC as an electrolyte. In Na-ion batteries, Na foil was used as a counter electrode and 1 mol/cm3 NaClO4/PC as an electrolyte. The cells were cycled at constant current densities of 10 mA/g (0.02 mA/cm2). For thermal analysis, the cycled cathodes were taken out from the disassembled cells, soaked in PC or DMC, rinsed by DMC, and then vacuum dried. Finally, the cathode powder was packed into a stainless-steel pan together with some amount of corresponding electrolyte. During TG-DSC analysis, the hermetically sealed pan was heat up to 500 oC with a heating rate of 5 oC/min.Figure 1 shows the DSC curves of the mixtures of given amount of lithiated or sodiated FeOF cathode and 1 μl corresponding PC-based electrolytes. In both Li- and Na-ions batteries, charged FeOF cathodes showed large exothermic heat with an onset temperature of about 100 oC. This was attributed to reactions between expanded FeOF at the charged state and the electrolyte. Moreover, intercalated Na ions were found to induce an exothermic heat peaked at about 450 oC, while this peak was not observed in Li-ion batteries. Detailed discussion will be presented at the conference.

Room-temperature stationary sodium-ion batteries for large-scale electric energy storage

Room-temperature stationary sodium-ion batteries have attracted great attention particularly in large-scale electric energy storage applications for renewable energy and smart grid because of the huge abundant sodium resources and low cost. In this article, a variety of electrode materials including cathodes and anodes as well as electrolytes for room-temperature stationary sodium-ion batteries are briefly reviewed. We compare the difference in storage behavior between Na and Li in their analogous electrodes and summarize the sodium storage mechanisms in the available electrode materials. This review also includes some new results from our group and our thoughts on developing new materials. Some perspectives and directions on designing better materials for practical applications are pointed out based on knowledge from the literature and our experience. Through this extensive literature review, the search for suitable electrode and electrolyte materials for stationary sodium-ion batteries is still challenging. However, after intensive research efforts, we believe that low-cost, long-life and room-temperature sodium-ion batteries would be promising for applications in large-scale energy storage system in the near future.