Vanadium diphosphide as a negative electrode material for sodium secondary batteries
Introduction
Giant leaps in the augmentation of renewable energy sources, in particular, solar and wind, have brought to light the pivotal role of energy storage systems in addressing the intermittency of these alternative energy sources [1,2]. As a result, lithium-ion batteries (LIBs) have gained immense popularity owing to their exceptional energy and power densities, long lifespan and versatility in both large-scale and consumer applications [[3], [4], [5]]. However, scarcity of natural reserves and restrictive costs of LIB components [6] such as lithium, cobalt, and nickel have aroused skepticism in their sustainability, compelling researchers to explore alternative battery chemistries such as sodium-based technologies [[7], [8], [9], [10]]. It is this context, that interest in sodium-ion batteries (NIBs) has rekindled over the last decade [[10], [11], [12], [13], [14]], propelled by the great abundance of sodium reserves on terrestrial surfaces and in seawater [[15], [16], [17], [18], [19]]. Moreover, the low redox potential of Na+/Na (−2.71 V vs. standard hydrogen electrode) points towards the prospects of developing competitive NIBs with high energy densities [20,21].
Despite the propitious electrochemistry of NIBs, their advancement is greatly inhibited by incompatibility with conventional negative electrodes such as graphite due to their limited capabilities to facilitate sodium intercalation during battery operations. Even though recent studies have shown reversible Na+ ion intercalation into graphite induced by co-intercalation of solvent forming complex (Na+[(diglyme)2C20]‒) [22] that result in larger interlayer distances [23], the limited capacities achieved are inadequate for high energy density NIBs. As an alternative, hard carbons with randomly oriented turbostratic graphene domains, have shown considerable improvement, exhibiting higher reversible capacities and lower operating voltages than graphite [[24], [25], [26]]. A first cycle charge capacity of 430.5 mAh g−1 at a rate of 30 mAh g−1 was obtained from a biomass-derived hard carbon (HC) prepared by single-step pyrolysis of shaddock peel on account of their large interlayer distances and honeycomb morphology [27]. However, the safety concerns with hard carbons related to the possible sodium metal deposition due to the sodiation potential close to 0 V vs Na/Na+, have motivated researchers to explore other materials for negative electrodes for NIBs. Sodium titanates (Na2Ti3O7, Na2Ti6O13) are another prospective class of material providing low voltage for realizing high energy densities for NIBs [[28], [29], [30]].
The binary MyXz compounds consisting of the 3d transition metals (M) and main group element (X) have emerged as versatile negative electrode materials, showing different charge-discharge mechanisms in both LIBs and NIBs, depending on their combinations [31]. For instance, an early 3d transition metal oxide, TiO2, operates as a topotactic insertion-type electrode in LIBs [32] but can facilitate conversion-type mechanism in NIBs [33]. On the other hand, the later 3d transition metal oxide CuO, has been found to shows conversion mechanisms in both LIBs and NIBs [34].
Within this class of materials, metal phosphides have emerged as a promising electrode material candidates, showing high theoretical capacities in both weight and volume [[35], [36], [37]]. A tin phosphide electrode (Sn4P3) prepared by ball-milling showed a high capacity of 718 mAh g−1 at 100 mA g−1 with negligible capacity fading over 100 cycles of NIB operations. The remarkable performance was largely attributed to the confinement of Sn nanocrystallites in the amorphous phosphorus matrix [38]. Similarly, a copper phosphide carbon composite (CuP2/C) also prepared through high energy ball-milling, achieved a reversible capacity of 450 mAh g−1 with a high capacity retention of 95% after 100 cycles of NIB operations [39]. Ex-situ XRD analyses revealed the formation of Na3P and reversible formation of CuP2 after sodiation and desodiation, respectively. Due to their superior performance, numerous reports that comprehensively detail the electrochemical performance of phosphorus and phosphide in NIBs have been availed [40,41].
Vanadium phosphides (VxPy) are known to their diverse electrochemistry in secondary batteries [[42], [43], [44]]. For instance, V4P7 prepared by high-energy ball-milling (HEBM) shows topotactic insertion of both Na+ and Li+ insertion for NIBs [45] and LIBs [46], respectively. Likewise, the performance of a vanadium diphosphide (VP2) electrode prepared by HEBM and solid-state annealing was investigated for LIBs, yielding superior and showed high discharge capacities of 890 and 640 mAh g−1, respectively [44]. According to X-ray diffraction (XRD) analysis, an amorphous product was observed after lithiation of VP2 and the redox center at the anion site was proposed because of no change in V K-edge spectra in XAFS. Other VxPy species such as VP4, have been found to facilitate both conversion and intercalation reactions during LIB operation. The electrochemical mechanism was determined to entail an initial reaction which resulted in the formation of Li3P and VP, thereby allowing the subsequent insertion of Li into VP at low voltages [43]. As a result, a significantly higher discharge capacity of 1290 mAh g−1 was attained. It can be deduced from the aforementioned studies that increasing phosphorus content in VxPy induces conversion-based mechanism. However, in the reported case of VP4, huge volume fluctuations occurring during the conversion reaction led to capacity fading. Thus, intermediate phosphorus amounts in VxPy could be envisaged to exhibit stable cyclability. Although intensive studies and reports on this family of metal phosphides exist, information regarding the charge-discharge mechanism of VxPy is not yet well understood.
The choice of electrolyte is deemed a crucial factor dictating not only the electrochemical performance but also the safety of the battery [47]. Among the commonly used electrolytes, organic electrolytes have turned out to be highly flammable and volatile, causing hazardous fires in some batteries [9,48]. As such, ionic liquid electrolytes (ILs), have emerged as safe electrolyte alternatives for both LIBs and NIBs owing to their high thermal stability, low flammability and negligible volatility [[49], [50], [51], [52], [53], [54]]. Furthermore, their wide electrochemical windows make them compatible with a variety of negative and positive electrodes in a wide temperature range [55,56], displaying high performances that in some cases even surpass organic electrolytes [[57], [58], [59], [60], [61], [62]]. Besides, the adoption of IL in NIBs can facilitate stable operations at intermediate temperatures which activate the electrochemical capabilities of certain inactive materials, enabling them to yield unprecedentedly high rate capability [[63], [64], [65], [66]]. The prospect of intermediate-temperature operations also unlock opportunities to take advantage of waste heat from automobiles and large-scale applications.
The ability of IL to induce the formation of uniform and stable solid electrolyte interface (SEI) layer in certain negative electrodes has been known to prompt huge improvements in cyclability and rate capability among NIBs [[67], [68], [69]]. For example, a FeTiO3–C composite negative electrode used alongside Na[FSA]–[C3C1pyrr][FSA] (20 : 80 in mol) (FSA− = bis(fluorosulfonyl)amide and C3C1pyrr+ = N-methyl-N-propylpyrrolidinium) IL, showed excellent cyclability marked by a capacity retention of 113% after 2000 cycles at 90 °C [70]. In a comparative study between IL and organic electrolyte, red phosphorus-acetylene black (AB) composite prepared by ball-milling exhibited better cyclability during IL operations [71]. This was ascribed to the differences in the SEI components formed by the two electrolytes, with IL producing a more robust SEI as was revealed by hard X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. Similarly, the SEI layer on HC using 1 M Na[FSA]–[C3C1pyrr][FSA] IL and 1 M Na[ClO4]–EC: DEC (1:1 v/v) (EC = ethylene carbonate and DEC = diethyl carbonate) was systematically investigated at 25 °C using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) [72]. The HC/HC symmetric cell showed significantly lower charge transfer resistance in the case of IL compared to organic electrolyte owing to a thinner SEI layer in IL electrolyte. In another case, during an investigation of vanadium phosphide-phosphorus composite (V4P7–5P) negative electrode utilizing 1 M Na[FSA]–[C3C1pyrr][FSA] IL at 25 and 90 °C, a stable cyclability was observed in the course of 100 cycles [73]. However, severe capacity degradation occurred when the electrode was operated in 1 M Na[PF6]–EC:DEC (1:1 v/v) organic electrolyte, as was reflected by increasing impedance as cycling progressed.
In this study, the electrochemical performance of VP2 is investigated using 1 M Na[FSA]–[C3C1pyrr][FSA] IL for NIBs at 25 and 90 °C. The electrode material was prepared by a one-step HEBM at 850 rpm for 20 h and further characterized by XRD, XPS and transmission electron microscopy (TEM). The performance was thereafter validated using EIS and scanning electron microscopy (SEM) upon cycling. The charge-discharge mechanism is ascertained using ex-situ XRD, XPS, and X-ray absorption fine structure (XAFS) and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopic analysis.
Section snippets
Experimental
All the air-sensitive materials were handled in an Ar-filled glove box (H2O < 1 ppm, O2 < 1 ppm). Vanadium (Kojundo Chemical Lab, purity 99.9%) and phosphorus (Wako Pure Chemical Industries, purity 98%) powders were ball-milled using a planetary ball mill (Pulverisette 7 Fritsch) for 20 h at 850 rpm under Ar atmosphere in a 1:2 molar ratio to obtain VP2 powder. A 20 cm3 grinding bowl with zirconia balls (with diameters of 3 mm) was used. The ball-milling parameters were optimized by changing
Results and discussions
The VP2 sample was prepared via a one-step HEBM of stoichiometric amounts of vanadium and red phosphorus powder at the milling speed of 850 rpm for 20 h using a 50:1 ball-to-powder weight ratio. To determine the optimal conditions for the synthesis of VP2, XRD patterns of the compounds formed under varying milling times and speeds at a constant ball-to-powder ratio of 50:1 (w/w) were taken as shown in Fig. S1, respectively. The patterns (Fig. S1a) indicate that ball milling at 400 rpm yields VP
Conclusions
In this study, we report the electrochemical performance and reaction mechanism of VP2 as a negative electrode material for NIBs using the Na[FSA]–[C3C1pyrr][FSA] IL electrolyte. The VP2 active material was prepared via a one-step high energy ball-milling process 850 rpm for 20 h. XRD, SEM, TEM, and XPS analyses were used to characterize the VP2 as a polycrystalline material with a uniform distribution of V and P throughout the crystallite. Galvanostatic tests performed on the electrode in the
Previously rejected submission: response to editor/reviewer comments
This paper was not rejected previously.
CRediT authorship contribution statement
Shubham Kaushik: Validation, Investigation, Formal analysis, Visualization, Writing - original draft. Kazuhiko Matsumoto: Resources, Validation, Data curation, Conceptualization, Methodology, Writing - review & editing, Supervision. Yuki Orikasa: Validation, Investigation, Writing - review & editing. Misaki Katayama: Validation, Investigation. Yasuhiro Inada: Validation, Investigation. Yuta Sato: Validation, Investigation, Writing - review & editing. Kazuma Gotoh: Validation, Investigation,
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.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgements
This study was supported by the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT) program Elements Strategy Initiative to Form Core Research Center (JPMXP0112101003).
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