Full Length ArticleStable cathode material enabled by Mg2+ intercalation layered potassium vanadate for high rate and long life potassium ion batteries
Graphical abstract
Introduction
In many energy storage technologies, lithium-ion batteries are favored due to their high specific energy, great cycle stability and high energy conversion rate [1]. In recent years, due to lithium-ion batteries being successful, an increasing number of scholars have concentrated on sodium and potassium for their comparable chemical properties to lithium, namely sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) [2]. Because of the various advantages of PIBs such as: K+ can intercalate into carbon to form KC8, which means a considerable capacity of 279 mAh g−1; The stock of K resources is abundant and K/K+ have a low standard reduction potential [3], recently, the study of PIBs electrode materials has thus been the subject and the research on the anode has indeed made significant progress [4], [5], [6]. Reversible potassium storage is possible in a variety of anode materials including carbonaceous, organic, transition metal chalcogenides and metal alloys [7], [8], [9]. They all have a specific capacity of over 250 mAh g−1 [10]. However, because of high operating voltage and large size of K+, cathode materials typically have a much lower specific capacity than anode materials [11]. The advancement of full cells is also impeded by deficiencies in specific capacity, limited rate capability, and significant capacity fading [12]. Besides, the voltage of cathode materials determines the upper limit of the voltage of a full cell, which further has a substantial impact on the cell's energy density. In view of this, research on cathode materials is urgently required.
In general, the cathode material contributes the most active K+ in a full cell, which means that it is essential to provide a sturdy structure for accommodating large size of K+. To search for suitable cathode materials that is capable of maintaining the stable intercalation and deintercalation of K+. A lot of materials have been investigated, mainly including layered transition metal oxides, metal hexacyanometalates, polyanionic compounds, and organic compounds [13]. Among various cathode materials, layered transition metal oxides are considered to be a hopeful electrode material due to their satisfactory energy density and large two-dimensional K+ diffusion path [14]. And vanadium-based metal oxides are ideal cathode materials for PIBs due to their wide interlayers, multiple vanadium oxidation states, diverse structure and abundant reserves [15], [16]. However, due to its large size, the intercalation of K+ into layered transition metal oxides is still hindered, which causes many layered transition metal oxides with low capacity and slow reaction kinetics. To this end, numerous methods were developed to improve the electrochemical performance. For instance, modulating the K+ contents, phase-engineered, element substitution, single-crystal growth strategy and regulation of electrolyte salts are resultful strategy to improve their stability and reversible capacity [17], [18], [19], [20], [21], [22], [23]. Nonetheless, whether it is controlling K+ contents or substituting elements, layered metal oxide still has inevitable capacity decay during cycling. Because the repeatedly intercalation and deintercalation of K+ throughout the charging and discharging processes will lead the layered structure unstable and eventually affect electrochemical performance. For enhancing the stability of layered structure, pre-intercalation of other metal ions is considered to be one of the keys to solving the above issues [24], [25]. Generally, the introduced heterogeneous metal ions are able to act as struts between the layered structures, thereby mitigating the drastic volume fluctuations caused by K+ intercalation and deintercalation.
Herein, Mg2+ pre-intercalated K0.5V2O5 (denoted as Mg-KVO) is reported to overcome the limitations of PIBs cathode on cycling decay and poor rate performance. Due to its electrochemical inertness, the intercalated Mg2+ plays an irreplaceable role in providing stable structural stability, great electronic conductivity and fast ion mobility [26]. According to electrochemical tests, the Mg-KVO is capable of providing stable specific capacity over 60 mAh g−1 at 10 mA g−1. At the same time, it exhibits a long cycle life with high capacity retention of 91.5% after 1000 cycles at 1000 mA g−1. This is competitive in comparison with previous studies as PIBs cathode material, such as K0.5V2O5, K0.45MnO2 and K0.3MnO2 with capacity retention of 86%, 72% and 63% after 100 cycles at 100 mA g−1 [27], [28]. XRD patterns indicate that the introduction of Mg2+, which accounts for only 1/600 of K+, preserves the original crystal structure of the material. The ex-situ X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show the reversible intercalation/deintercalation of K+ during charge and discharge, accompanied by changes in the valence state of vanadium. Moreover, Mg-KVO also exhibits quite stable electrochemical performance in full cells constructed with Mg-KVO cathode and a multi-porous hard carbon (MPHC) anode. In addition to a specific capacity of 52.5 mAh g−1 at 20 mA g−1, it is reversibly maintained at 40.6 mAh g−1 over 100 cycles at 50 mA g−1, with 93% capacity retention. This work offers a practical idea to develop layered vanadium oxides and optimize the electrochemical performance as PIBs cathodes.
Section snippets
Material prepared
Analytical grade reagents were available commercially. First, 2 mmol V2O5 (Macklin, 99%) and 1.5 mmol H2C2O4 (Macklin, 98%) were dispersed in 60 mL of deionized water in a beaker. Then, 20 mmol KCl (Macklin, GR, 95%) and 2 mmol MgCl2·6H2O (Macklin, GR, 99%) were added with constant magnetic stirring for 30 min until getting a uniform orange-transparent solution. In the next step, the mixture was transferred to Teflon-lined sealed autoclave and kept at 200 °C for 24 h. Subsequently, anhydrous
Results and discussion
Schematic diagrams of the preparation procedure of Mg-KVO are exhibited in Fig. 1a and Fig. S1. In this work, V2O5 acted as precursor was first dissolved in deionized water with reducing agent H2C2O4. Subsequently, KCl and MgCl2·6H2O were added to the solution as potassium and magnesium precursors, respectively. The product was prepared after the following hydrothermal reaction. During this process, VO5 square pyramids convert to VO6 octahedra under the action of the reducing solution. Then the
Conclusion
In conclusion, the electrochemical performance of Mg-KVO is greatly enhanced by the intercalation of Mg2+, which gives rise to its enhanced electronic conductivity and stable structural stability. Efficient cycling stability with 91.5% capacity retention over 1000 cycles at 1000 mA g−1 was achieved, which significantly shows the key role of Mg2+ in maintaining crystal structure stability during cycling. From the ex-situ XRD and XPS characterizations, the reversible change of vanadium oxide
CRediT authorship contribution statement
Haoxiang Lin: Writing – original draft, Writing – review & editing. Yuanji Wu : Writing - review & editing. Hongyan Li: Supervision, Funding acquisition, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (22209057), the Guangdong Basic and Applied Basic Research Foundation (2021A1515010362), and the Guangzhou Basic and Applied Basic Research Foundation (202102020995), and also supported by the Open Fund of Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications (2020B121201005).
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