Qing
Shen
,
Yibo
He
and
Junjie
Wang
*
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, People's Republic of China. E-mail: wang.junjie@nwpu.edu.cn
First published on 18th April 2023
Potassium metal batteries (PMBs) are expected to become the most promising new generation of new type large-scale energy storage equipment owing to their abundant resources, low cost, and low operating potential. However, the direct use of metallic potassium (K), especially the “hostless” K metal, leads to easy growth of dendrites that pierce the separator, resulting in a short circuit of the battery. In this work, a simple and effective strategy to construct dendrite-free PMBs via plating metallic K into binary-metal (NiCo, NiFe, CoFe) nanoparticle-embedded two-dimensional (2D) N,O co-doped carbon (K–NiCo@NOGC, K–NiFe@NOGC, K–CoFe@NOGC) was proposed. The factors affecting the electrochemical performance of potassium metal cells were discussed from the aspects of experiment and theoretical calculation. Our fundamental studies reveal that potassium dendrite growth can be effectively inhibited on the substrate containing nitrogen-oxygen rich functional groups, as confirmed by low nucleation overpotential, high coulombic efficiency and enhanced adsorption energy. Among the three kinds of synthesized materials, NiCo@NOGC can serve as an ideal host for dendrite-free potassium deposition and exhibits outstanding stable cycling performance with high coulombic efficiency (CE) at a current density of 0.5 mA cm−2 (CE = 99.4% after 450 cycles). Moreover, the practicality and functionality of this anode are demonstrated by assembling K–NiCo@NOGC‖PB full batteries with a long cycle life of 500 cycles and a high-capacity retention of 80%.
However, it is extremely difficult to use K metal directly as an anode in reversible plating and stripping processes. Potassium metal is known to be highly reactive and will spontaneously react with the electrolyte to form a solid electrolyte interface (SEI) layer.18,19 In addition, due to the “hostless” nature of K-metal anodes, the SEI is unstable and can be extremely easily disrupted by a mechanical deformation during the process of K plating/stripping.20 Previous studies have found that the formation of a repeatedly unstable SEI during K plating/stripping can hardly withstand large electrode volume fluctuations, which further promotes the growth of K dendrites.21,22 Furthermore, non-uniform current flow and potassium ion flux “hostless” K metal anodes lead to inhomogeneous nucleation, which makes K morphology uncontrollable and causes the growth of K dendrites.23,24 With the growth of dendritic K, the protruding K dendrites can easily penetrate the separator, causing the cell to short circuit.25,26 These problems must be overcome in order to advance the research and application of K-metal batteries. While there are some reports of work on how to inhibit potassium dendrite growth, research in this area is still in its early stages.
Very recently, several effective protocols have been reported to reduce the depletion of K and inhibit dendrite growth, such as modifying the electrolyte composition,27,28 introducing an artificial solid-electrolyte interphase (ASEI),29 and substituting the metal anode with a liquid Na–K alloy.17,30 Nevertheless, electrolyte modification and ASEI cannot repress the huge volumetric change during K plating/stripping, and the employment of liquid K–Na alloy anodes may lead to serious safety hazards. Recently, potassium was hosted within 3D scaffolds to homogenize K+ influx and to minimize the volume change. In particular, the passivation of the potassium metal surface by 2D materials has attracted recent attention by enabling stable K electrode-position and suppressed dendrite formation.31-33 However, the direct use of 2D materials to protect the surface of K anodes remains challenging because such materials have poor affinity for potassium metal, leading to high nucleation barriers and uneven potassium deposition. Although, in recent studies, different groups have introduced 2D carbon frameworks containing lithiophilic metal nanoparticles (Co, Fe and Ni) to guide homogeneous initial Li deposition,34-37 such methods have not been reported for potassium metal batteries. In addition, much of the reported fabrication of metal-embedded carbon structures requires a complex top-down process, which is usually unsuitable for large-scale electrode production in the field of metal cells.38,39
Therefore, in this work we developed bimetal nanoparticle (NiCo, NiFe, CoFe) embedded 2D N,O-codoped graphited carbon (denoted as NiCo@NOGC, NiFe@NOGC, CoFe@NOGC) synthesized by a simple process of hydrothermal and annealing with uniformly distributed potassium-philic functional groups on the surface as a robust nucleation buffer layer of PMBs. Electrochemical measurements confirmed that the NiCo@NOGC coated copper collector exhibits excellent electrochemical performance with a low nucleation overpotential, small voltage hysteresis, and high coulombic efficiency (CE). Besides, the K–NiCo@NOGC composite anode exhibits better cycling stability and rate performance than pristine K in full battery systems with the Prussian blue (PB) cathode. This study revealed the following mechanisms: first, co-doping of N and O in the carbon matrix enhanced the affinity between K and the substrate, resulting in smooth nucleation of K as well as a uniform deposition process; Second, the 2D carbon network with a high surface provides sufficient space to suppress the volume change during K plating/stripping. Finally, both the 2D graphitized carbon network and the highly dispersed NiCo alloy nanoparticles can guide the uniform dispersion of electrons to avoid charge concentration, further restraining the growth of K dendrites and maintaining long-term cycling stability.
The XRD results of NiCo@NOGC, NiFe@NOGC and CoFe@NOGC are shown in Fig. 2(a) and S2.†Fig. 2(a) indicates three significant diffraction peaks at 26°, 44.38°, and 51.68°, which can be attributed to the (002) crystal planes of graphitic carbon (PDF#12-212), and (111) and (200) crystal planes of metallic Ni (PDF#01-1258) or metallic Co (PDF#01-1255), respectively. These results further confirmed the formation of NiCo alloy nanoparticles in the N, O co-doped graphitized carbon structure. Furthermore, the two strong diffraction peaks of CoFe@NOGC at 44.62° and 65.24° can be indexed to the (110) and (200) crystal planes of the CoFe alloy (PDF#00-44-1433) (Fig. S2(a)†), while NiFe@NOGC exhibits diffraction peaks at 43.1° and 50.5° corresponding to the (111) and (200) planes of the NiFe alloy (PDF#00-3-1207) (Fig. S2(b)†). These results also provide evidence for the formation of CoFe and NiFe alloy nanoparticles in the corresponding composite. However, it can be noted that no strong carbon signal was detected in NiFe@NOGC and CoFe@NOGC, which is different from that of NiCo@NOGC (Fig. 2(a)). This phenomenon indicates that NiCo@NOGC has the highest graphitization degree among these three synthesized composites.
Furthermore, the Raman spectroscopy results of NiCo@NOGC (Fig. 2(b)), NiFe@NOGC and CoFe@NOGC (Fig. S3(a)†) show two typical peaks situated at 1350 and 1580 cm−1, which correspond to graphitic carbon (G band) and the defect of amorphous carbon (D band), respectively. It is worth mentioning that G and D bands are related to high-frequency E2g first-order graphitic crystallites and the defect-induced carbon structure, respectively.15 The peak intensity ratios of bands G and D (IG/ID) of these composites were calculated to be 1.12 (NiCo@NOGC) (Fig. 2(b)), 1.07 (NiFe@NOGC) (Fig. S3(a)†), and 1.01 (CoFe@NOGC) (Fig. S3(a)†), which indicates again that the graphitization degree of NiCo@NOGC is higher than those of NiFe@NOGC and CoFe@NOGC. This indicates that the synergistic effect of metallic cobalt and nickel can promote the graphitization of carbon, thereby improving the electrochemical performance of the material. The porosities and specific surface areas of synthesized NiCo@NOGC, NiFe@NOGC, and CoFe@NOGC were measured from nitrogen adsorption/desorption isotherms (Fig. 2(d)–(e), S3(c) and (d)†). The BET surface areas of NiCo@NOGC, NiFe@NOGC, and CoFe@NOGC were estimated to be 392.9, 341.4, and 259.3 m2 g−1. The enlarged surface area would facilitate the immersion of the electrolyte, and the unique porous structure effectively enhances the reaction kinetics via providing a large number of interfaces between electrolytes and NiCo@NOGC and reducing the ion/electron transport distance, thereby enhancing its electrochemical performance.24
XPS measurement was conducted to detect the chemical states and electronic structures of the surfaces of synthesized samples. XPS proved the existence of elements C, O, and N in the NiCo@NOGC sample (Fig. 2(f)). Based on XPS results, the atomic contents of O and N in NiCo@NOGC were estimated as 17.27 and 4.93%, respectively. While the low atomic contents of Co and Ni in NiCo@NOGC are about 0.43 and 0.41%, respectively. This can be attributed to that most Co and Ni are encapsulated into graphitic carbon layers, and the low Co and Ni content tested by XPS is only the surface content. However, according to the results of Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) measurement, the exact contents of metals in the compositions are confirmed: the contents of Ni and Co in NiCo@NOGC are 3.69 and 4.28 wt%, respectively; the contents of Ni and Fe in NiFe@NOGC are 18.15 and 9.45 wt% and Co and Fe in CoFe@NOGC are 11.3 and 15.68 wt%, respectively (Table S3†). And the fitting XPS of Ni and Co is indicated in Fig S4.† And Fig. S5 and S6† indicate the XPS spectra of NiFe@NOGC and CoFe@NOGC composites. The detailed atomic contents of N and O of NiCo@NOGC, NiFe@NOGC, and CoFe@NOGC are listed in Table S1.† The atomic nitrogen and oxygen contents of NiCo@NOGC are higher than those of NiFe@NOGC, and CoFe@NOGC. It was confirmed by the subsequent experimental measurements and DFT calculations that high contents of nitrogen and oxygen can effectively improve the adsorption of metal K on the carbon matrix.
Fig. 2(g) shows that the spectrum of C 1s for NiCo@NOGC was fitted to four corresponding peaks at 288.7, 286.0, 284.2, and 284.7 eV, which belong to doping functional groups (CO and C–O/N), C–C and CC.42,43 These data demonstrate that NiCo@NOGC was efficiently doped with oxygen and nitrogen, which agrees well with the elemental mapping results. The N 1s spectrum in Fig. 2(h) shows that five peaks of pyridinic N (398.5 eV), pyrrolic N (400.3 eV), graphitic N (401.0 eV), oxidized N (404.5 eV), and metal-bonded N (N–Co, 398.8 eV)34,44 exist in NiCo@NOGC. Previous studies proved that pyridine and pyrrole nitrogen atoms are both lithium-philic and can be helpful to reduce nucleation overpotentials.45 Therefore, it is highly expected that the synthesized NiCo@NOGC possesses K-philic properties to promote K nucleation. Furthermore, the O 1s spectra for NiCo@NOGC (Fig. 2(i)) can be decomposed to four peaks located at 533.0, 531.4, 532.2, and 533.9 eV, which correspond to O states for C–O, O–H, CO, and O–Ni, respectively.46,47 The contents of CO, C–O and O–H were estimated as 0.46, 0.17and 0.15, respectively, by calculating their area integral. This indicates that the surface-bound oxygen is mainly in the forms of CO and C–OH. This result can be directly supported by the FTIR measurement (Fig. 1(e) and S3(b)†) that the characteristic absorptions of carbonyl (CO) and C–OH were clearly observed in NiCo@NOGC. In addition, the measured contents of each component of N, O functional groups by XPS analysis are listed in Table S2†. These results all show that NiCo@NOGC with rich nitrogen-oxygen functional groups was successfully synthesized.
To further elucidate the influence of oxygen-containing functional groups and different types of nitrogen doping on K plating, adsorption energy and charge density transfer between K atoms and different sites on NiCo@NOGC were explored by performing DFT calculations. The adsorption of K atoms on the surfaces of Cu, graphene (Gr) and the graphene/NiCo heterostructure (C–NiCo) was also studied for comparison. The calculated adsorption energies of the K atom on different surfaces are shown in Fig. 3(b) and S7(a).† The calculations of charge density difference for the adsorption of K atoms on graphene with different functional groups (Fig. S7(c)–(h)†) revealed that not only the adsorption of the K atom on graphene can be enhanced by the introduction of functional groups, but also a net gain of charge can be observed in the K atom, indicating the transfer of charge from the adsorbed K to the nearest adjacent atom. The charge depletion of the absorbed K atom and the charge accumulation around the nitrogen, oxygen-containing group were explicitly observed, suggesting a stronger K-adsorption of nitrogen, oxygen-containing functional groups on the surface. Therefore, it is highly expected to achieve a low nucleation barrier and thus a uniform deposition of K metal.
Fig. 3(b) shows that the calculated adsorption energies of the K atom and Cu (−0.97 eV) and Gr (0.18 eV) are higher than that on the NiCo surface (−1.57 eV), indicating that NiCo is more potassiophilic than Cu and graphene is an anode material. In addition, the adsorption energy between the K atom and C–NiCo heterostructure (Eb = −0.62 eV) is enhanced compared with Gr (0.18 eV) (Fig. S7(b)†). More importantly, with the introduction of COOH* (–2.29 eV), O* (–1.86 eV), and OH* (–1.24 eV) functional groups, the binding energies between the K atom and C–NiCo surfaces were enhanced (Fig. 3(c)–(e)). Hence, one can conclude that all oxygen-containing functional groups (–COOH*, –O*and –OH*) can enhance the potassium affinity of the carbon surface and are conducive to the uniform distribution of potassium ions. For nitrogen-containing functional groups, some previous studies have suggested that both pyridinic and pyrrolic nitrogen contribute to lowering the nucleation overpotential and smoothing Li plating because they are lithiophilic.34,48 We also obtain a similar result that the binding energy between the K atom and PyN and PrN modified graphene was enhanced, indicating that pyrrolic and pyridinic nitrogen atoms can promote the deposition of K atoms. It is notable that NiCo nanoparticles of the synthesized NiCo@NOGC are uniformly embedded in graphite carbon layers surrounded by nitrogen atoms in this work (as depicted in Fig. 1(d)). Therefore, the NiCo atoms embedded in N-doping graphitic carbon layers can be directly involved in the bonding process of potassium. To prove the above analysis, the binding energies of K with NiCo@NOGC surfaces modified by GrN-NiCo (Fig. 3(f)), PyN-NiCo (Fig. 3(g)), PrN-NiCo (Fig. 3(h)) functional groups were calculated to be −1.40 eV, −2.83 eV and −2.56 eV, respectively. One can see that the binding interactions between the K atom and N-doped graphene were significantly enhanced by the involvement of NiCo and further improved the potassiophilicity.
Furthermore, Bader charge analysis and charge density difference caculations were conducted to reveal the charge transfer during K nucleation at doping sites of NiCo@NOGC (Fig. S8†). One can see that there is no significant charge transfer between the carbon matrix and NiCo alloy during the adsorption of the K atom on the modified C–NiCo surfaces with different functional groups. However, there is a significant charge transfer from the adsorbed K atom to the N and O dopants, indicating strong chemical interactions. Furthermore, all N and O atoms form σ bonds with adjacent carbon atoms and withdraw electrons from carbon atoms due to their greater electronegativity than carbon. In addition, both N and O atoms can participate in the delocalized π system of graphene, which further enhances their negative charge state. Therefore, the abundant N, O-containing functional groups on the surface of NiCo@NOGC generate strong potassium affinity, leading to uniform K deposition. These results are also consistent with the charge analysis of the potassiophilic heteroatom-doped carbon material reported by Zhang et al. and Xu et al.49,50 The combined results of these experiments and calculations indicate that these introduced oxygen groups and nitrogen doping enhance the K-philicity of the NiCo@NOGC surface and favour the electrochemical activity of NiCo@NOGC.
Coulombic efficiency (CE), defined as the ratio of stripping to plating capacity per cycle, is an important metric for evaluating the reversibility of K metal anodes. Therefore, to test the reversibility of each synthesized K metal anode during the repeated process of plating/stripping, the CEs on NiCo@NOGC, NiFe@NOGC, CoFe@NOGC and Cu asymmetrical cells were tested under the conditions of a current density of 0.5 mA cm−2 and an areal capacity of 0.5 mA h cm−2 (Fig. 4(e)). The Cu asymmetrical cell only lasts about 30 cycles before being shorted. Owing to the large specific surface area and abundant potassium adsorption sites, the NiFe@NOGC, CoFe@NOGC, and NiCo@NOGC electrodes can facilitate uniform current density and provide space for deposition of K during cycling. Therefore, compared with the Cu asymmetrical cell, the NiFe@NOGC and CoFe@NOGC asymmetrical cells provide 92.0% and 96.0% stable CE after nearly 60 and 90 cycles, respectively. Remarkably, one can see that the NiCo@NOGC asymmetrical cell exhibits a higher CE of 99.4% at 450 cycles and the full voltage–time curve of the NiCo@NOGC asymmetrical cell is shown in Fig. S11.†Fig. 4(f) shows that the coulombic efficiency continues to increase with the increase of the number of cycles. Moreover, the NiCo@NOGC matrix also exhibits high and stable average CEs of 93.1% and 91.5% at high current densities of 1.0 and 2.0 mA cm−2, respectively (Fig. S12†). A high CE is beneficial to the formation of a stable and homogeneous SEI on the electrode. In addition, X-ray photoelectron spectroscopy (XPS) measurements were performed to examine the chemical information of SEI layers on the cycled Cu–K and NiCo@NOGC-K anodes (Fig. S13†). The relative amount of carbon species is higher in the SEI on the cycled Cu–K (38.6%) than that on the cycled NiCo@NOGC-K (12.7%). Because the organic species is based on carbon, the higher C content indicates more organic components in the SEI, which is attributed to the continuous reaction between the K metal and the electrolyte. Therefore, such a stable and uniform SEI on the surface of the NiCo@NOGC-K composite electrode is beneficial to fast K ion migration and guarantees reversible K plating/stripping. Furthermore, the superiority of the K–NiCo@NOGC symmetrical cell was also demonstrated by electrochemical impedance spectroscopy (EIS) analysis. We know that the high-frequency semicircle radius in the Nyquist plots is related to the charge transfer resistance and the SEI resistance.51,52 Before cycling, the interfacial resistance of the K–NiCo@NOGC symmetric cell is the smallest compared with the K–CoFe@NOGC, K–NiCo@NOGC and K–Cu symmetric cells (Fig. S14†). More importantly, the interfacial resistance of the NiCo@NOGC symmetric cell decreases with increasing cycle times, further indicating a more stable SEI with less exposure of potassium to the electrolyte.
The SEM images of Cu, NiCo@NOGC, NiFe@NOGC, and CoFe@NOGC electrodes before and after K plating are shown in Fig. 5 and S16.† Compared with the original Cu foil electrode (Fig. 5(a)), the surface of the Cu foil electrode after plating at a discharge capacity of 2 mA h cm−2 is covered with a few rod-like K nucleation particles (Fig. 5(b)). After plating at a capacity of 4 mA h cm−2, it becomes a porous structure with growth of dendrites (Fig. 5(c)). This porous electrode produces an inhomogeneous K flux, leading to inhomogeneous K plating and dendrite formation in subsequent cycles.53-55 As a result, large K protrusions can be observed on the bare Cu electrode after 10 cycles of plating/stripping (Fig. 5(d)). In addition, morphology evolution of NiFe@NOGC and CoFe@NOGC electrodes is shown in Fig. S16.† Compared with the initial stage of K plating (Fig. S16(a) and (e)†), the surfaces of the NiFe@NOGC and CoFe@NOGC electrodes exhibit rod-like structures after plating at a capacity of 2 mA h cm−2 (Fig. S16(b) and (f)†), and with K deposition increased at a capacity of 4 mA h cm−2 (Fig. S16(c) and (g)†), the surface of the NiFe@NOGC and CoFe@NOGC electrodes became relatively flat but not dense. Meanwhile, after 10 cycles of K plating/stripping (Fig. S16(d) and (h)†), the surfaces of NiFe@NOGC and CoFe@NOGC electrodes also show a relatively flat but not dense surface with some small potassium nanoparticles. In sharp contrast, both at the initial stage of K plating (Fig. 5(e)) and at the total capacity of 2 mA h cm−2 (Fig. 5(f)) and 4 mA h cm−2 (Fig. 5(g)) for successive K deposition, the NiCo@NOGC electrode shows a relatively smooth surface without any protruding rods. In particular, these smooth and flat potassium deposits are mainly accommodated in the pores around the NiCo particles. All these results further suggest that surface functional groups can guide the nucleation of K for uniform distribution, thus avoiding the formation of K dendrites at NiCo@NOGC electrodes. Furthermore, after plating/stripping of 10 cycles (Fig. 5(h)), the NiCo@NOGC electrode still indicates a smooth surface without any dendrite shape because of the suppressed volume change, which is beneficial to guarantee a stable and uniform SEI. The TEM and HRTEM measurements and elemental mapping of the NiCo@NOGC electrode were further carried out after plating/stripping for 10 cycles at 0.5 mA cm−2 at an areal capacity of 0.5 mA h cm−2. As shown in Fig. S17,† the morphology of NiCo nanoparticles was not changed and the nanoparticles are still uniformly dispersed in N- and O-rich 2D graphited carbon layers. Moreover, the average size of NiCo alloy nanoparticles after plating/stripping for 10 cycles (9.03 nm) was slightly changed significantly compared to that before cycling (8.8 nm), which further validates the high structural stability of NiCo@NOGC during cycling. The corresponding mapping exhibits a uniform element distribution, with K metal first nucleating at highly potassiophilic sites and embedded in the carbon matrix. These results demonstrated that the NiCo@NOGC electrode can achieve uniform K plating/stripping without the formation of potassium dendrites, leading to highly stable electrodes and interfaces in working cells. Fig. 5(i) schematically illustrates the mechanism of K deposition on NiCo@NOGC without dendrites after repeated cycling. This is of great significance for improving the cycle performance of potassium metal batteries.
Footnote |
† Electronic supplementary information (ESI) available: Experimental section, XRD patterns, SEM and TEM figures, XPS spectra, electrochemical results and EIS spectra (Fig. S1–S16). See DOI: https://doi.org/10.1039/d3ta00904a |
This journal is © The Royal Society of Chemistry 2023 |