Letter www.acsami.org
Para-Conjugated Dicarboxylates with Extended Aromatic Skeletons as the Highly Advanced Organic Anodes for K‑Ion Battery Chao Li,† Qijiu Deng,† Haochen Tan,† Chuan Wang,† Cong Fan,*,† Jingfang Pei,† Bei Cao,*,‡ Zhihong Wang,† and Jingze Li*,† †
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China ‡ State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, Department of Chemistry, The University of Hong Kong, Hong Kong 999077, P. R. China S Supporting Information *
ABSTRACT: A new family of the para-conjugated dicarboxylates embedding in biphenyl skeletons was exploited as the highly advanced organic anodes for K-ion battery. Two members of this family, namely potassium 1,1′-biphenyl-4,4′-dicarboxylate (K2BPDC) and potassium 4,4′E-stilbenedicarboxylate (K2SBDC), were selectively studied and their detailed redox behaviors in K-ion battery were also clearly unveiled. Both K2BPDC and K2SBDC could exhibit very clear and highly reversible twoelectron redox mechanism in K-ion battery, as well as higher potassiation potentials (above 0.3 V vs K+/K) when compared to the inorganic anodes of carbon materials recently reported. Meanwhile, the satisfactory specific and rate capacities could be realized for K2BPDC and K2SBDC. For example, the K2BPDC anode could realize the stable rate capacities of 165/143/135/ 99 mAh g−1 under the high current densities of 100/200/500/1000 mA g−1, respectively, after its electronic conductivity was improved by mixing a very small amount of graphene. More impressively, the average specific capacities of ∼75 mAh g−1 could be maintained for the K2BPDC anode for 3000 cycles under the high current density of 1 A g−1. KEYWORDS: organic anodes, para-conjugated dicarboxylates, extended aromatic skeletons, graphene, K-ion battery
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of K+ ion (1.38 Å vs 0.76/1.00 Å for Li+/Na+ ion) could seriously impede the further development of KIBs based on the conventional intercalation/deintercalation mechanism. Moreover, it is worth noting that the exploitation of anode materials for KIBs has priority to their cathode counterparts, since the proper anode materials can directly and effectively avoid the risk of producing highly reactive K metal during charging/ discharging processes. Currently, several inorganic cathodes8 such as KTi2(PO4)3,9 K0.25V2O5,10 K0.3MnO2,6 Prussian blue and its analogues,11−13 and very few inorganic anodes (e.g., carbon materials14−16 and K2Ti80717) have been reported for KIBs. Regretfully, the specific capacities and rate performance for these inorganic anodes in KIBs were unsatisfactory, along with the poor cell cyclability and low inserting K-ion potential (vs K/K+). Advantageously, organic compounds are gradually attracting more and more attentions as the highly promising electrode materials.18,19 Not only pure organic materials are structure contrivable, composed by abundant elements, and almost
he rechargeable Li-ion batteries (LIBs) widely commercialized in portable electronics and/or electric vehicles gradually meet the fatal challenge that the global resources of Li element will probably be running out. Moreover, the LIBs based on the conventional inorganic electrodes are approaching the ceiling of energy density for the limited choices of intercalation cathodes and thus hardly meet the urgently increasing need for energy-storage requirement, particularly under the price consideration for large-scale electrical grid application.1 Alternatively, most attentions are shifted back to the second alkali element of Na-ion batteries (NIBs) because of the low-cost Na minerals (∼2.74% of Na vs ∼ 0.0065% of Li in the earth’s crust) and the similar electrochemical properties. Tremendous and significant progresses have been reported in NIBs nowadays.2−4 Interestingly, K-ion batteries (KIBs) of the same alkali group have largely not been considered up to now,5 despite the fact that they exhibit in principle similar advantages with NIBs but the standard redox potential of K/K+ couple (−2.94 V vs SHE) is much lower than Na/Na+ (−2.71 V) and closer to Li/Li+ (−3.04 V) couple.6 Remarkably, the experimental measurements have shown that the redox potential of K/K+ couple in nonaqueous electrolytes could be the most negative one among all alkali-metal analogues.7 However, the apparently large radius © 2017 American Chemical Society
Received: June 22, 2017 Accepted: August 4, 2017 Published: August 4, 2017 27414
DOI: 10.1021/acsami.7b08974 ACS Appl. Mater. Interfaces 2017, 9, 27414−27420
Letter
ACS Applied Materials & Interfaces
Figure 1. Chemical structures of potassium 1,1′-biphenyl-4,4′-dicarboxylate (K2BPDC) and potassium 4,4′-E-stilbenedicarboxylate (K2SBDC), and their proposed redox mechanism in K-ion battery.
Figure 2. CV multiple curves of (a) K2BPDC and (b) K2SBDC in K-ion half cells. Scan rate was 0.1 mV s−1 and voltage window was 0.1−2.5 V.
0.55 V (vs K+/K), respectively. After their particle dispersion and electronic conductivity were simply improved, both the rate capacities and cell cyclability could be satisfactorily realized. For instance, the modified K2BPDC anode could achieve the average specific capacity of ∼75 mAh g−1 for 3000 cycles under the high current density of 1 A g−1. The chemical structures of potassium 1,1′-biphenyl-4,4′dicarboxylate (K2BPDC) and potassium 4,4′-E-stilbenedicarboxylate (K2SBDC) were depicted in Figure 1. Their synthesis could feasibly follow the similar procedures reported by our group,24 where the neutralization reaction between the acid precursors of 1,1′-biphenyl-4,4′-dicarboxylic acid (H2BPDC) or 4,4′-E-stilbenedicarboxylic acid (H2SBDC) and the base of KOH could afford K2BPDC and K2SBDC with satisfactory yields above 80%. See synthetic details in the Supporting Informaion. The infrared (IR) spectra of the as-obtained K2BPDC and K2SBDC were shown in Figure S1a, b. Combined the experimental results with the simulation data calculated (Figure S1c, d), the bands located at around 1399/1403 cm−1 for K2BPDC/K2SBDC were assigned to νs (COO−) vibrations, whereas the bands residing at 1583/1588 and 1539/1547 cm−1 were assigned to their νas (COO−) vibrations coupled with the benzene skeleton vibrations, respectively.25 At the same time, the broaden peaks in the range of 2400 to 3200 cm−1 belonging to the stretching vibrations of O−H bond in H2BPDC and H2SBDC disappeared. Furthermore, the purity and chemical structures of K2BPDC and K2SBDC were confirmed by 1H NMR spectra (Figure S2), where the distinct H signals from benzene ring (δ = 7.85/7.68 ppm or δ = 7.76/7.56 ppm) and alkene (δ = 7.25 ppm) were clearly detected but no H signals
without concern for production cost, but also both LIBs and NIBs based on organic electrodes have achieved comparatively high gravimetric energy density and cell cyclability to their inorganic counterparts.20,21 Much more importantly, unlike the inorganic solid/crystal formed by covalent and/or ionic bond, the solid/crystal state of organic materials (small molecules or polymers) is mainly interacted by van der Waals forces, which strongly indicates that organic electrodes could provide more void room and low energy barrier for accommodating most metal ions without the size concern. Truly, our group and Chen’s group independently and initially unveiled that the typical representative of potassium terephthalate (K2TP) belonging to the family of para-aromatic dicarboxylates could be the highly efficient organic anode for KIBs, and satisfactory results were realized.22,23 Despite these encouraging advancements, it is remarkable that the families of organic anodes suitable for KIBs are currently very scarce. Meanwhile, the rate capacities and cell cyclability of organic anodes in KIBs still need improving. Herein, we initially report a new family of organic anodes for KIBs based on the para-conjugated dicarboxylates of aromatic biphenyl skeleton. Compared to K2TP of only one phenyl ring, the biphenyl skeleton with larger π conjugation should be more advantageous for intermolecular electron transportation. Two typical members of this family, namely potassium 1,1′-biphenyl4,4′-dicarboxylate (K2BPDC) and potassium 4,4′-E-stilbenedicarboxylate (K2SBDC), were selectively and fully studied. Both K2BPDC and K2SBDC could exhibit very clear and highly reversible two-electron redox mechanism in KIBs. Meanwhile, they could possess the high potassiation potentials of 0.35 and 27415
DOI: 10.1021/acsami.7b08974 ACS Appl. Mater. Interfaces 2017, 9, 27414−27420
Letter
ACS Applied Materials & Interfaces
Figure 3. Optimized molecular structures, frontier orbital distributions and the related energy levels calculated for organic BPDC2− and SBDC2− anions, as well as for their reduced states.
materials,15 which could be very benefited to reduce the risk of producing K metal by technological control. Meanwhile, these reduction potentials observed were also expectably higher than the reduction value (∼0.32 V) for K2TP due to their extended π conjugation.22 To get deeper insights about the electronic structures for organic BPDC2− and SBDC2− anions, we subsequently performed the density function theory (DFT) calculations by using Gaussian09 program. The WB97XD functional and the method of 6-311+G* were selectively employed to optimize the structures and calculate the corresponding redox potentials for organic BPDC2− and SBDC2− anions (see details in the Supporting Information). The obtained potential data were 0.70 V for BPDC2−/BPDC3− couple (vs K/K+) and 1.11 V for SBDC2−/SBDC3− couple (vs K/K+). The optimized molecular structures, frontier orbital distributions and the related energy levels calculated for organic BPDC2− and SBDC2− anions, as well as for their reduced states, were depicted in Figure 3. As expected, the molecular structure of BPDC2− anion could be significantly altered due to the formation of CC double bond in C2−C3 position during the reduction process, as confirmed by the decreased dihedral angle (C1−C2−C3-C4) from BPDC2− (−40.3°) to BPDC4− (0.0°). On the other hand, due to the effective π conjugation, the molecular plane of SBDC2− could remain nearly unchanged during reduction process with the only change of bond length. Notably, the deep HOMO (highest occupied molecular orbital) levels (cal. −8.06 and −7.75 eV) for BPDC2− and SBDC2− indicated that they were difficult to lose electrons (be oxidized) even they were negatively charged; Meanwhile, the relatively lower LUMO (lowest unoccupied molecular orbital) level of SBDC2− (cal. −0.09 eV) than that (cal. 0.51 eV) of BPDC2− could manifest that less driving force was needed for its reduction (injecting electrons), which was in accordance with the above CV results. Furthermore, it was concluded from the DFT calculations that maximum two-electron acceptance could be realized in organic BPDC2− and SBDC2− anions and the radical intermediates (BPDC3− and SBDC3−) could be electrochemically stable, as proved by their globally distributed SOMO (single occupied molecular orbital) levels. Subsequently, the galvanostatic discharge−charge experiments and rate performance of K2BPDC and K2SBDC in K-ion half cells were carried out, where the composited anodes were
from the residual COOH group were observed, indicating the complete conversion of H2BPDC and H2SBDC. In their X-ray diffraction patterns (Figure S3), the selected characteristic peaks for K2BPDC (2θ = 6.14, 15.53, and 27.84°) and K2SBDC (2θ = 5.51, 27.75, and 33.37°) were respectively reported. Since the growth of their single crystal was unsatisfactory even tremendous efforts were paid, the information about their lattice planes regarding these typical peaks was currently unavailable. Both K2BPDC and K2SBDC exhibited very impressive thermal stability, as confirmed by the high thermal decomposition temperatures (Td, corresponding to 5% weight loss in the thermogravimetric analysis) of 578 °C for K2BPDC and 542 °C for K2SBDC (Figure S4), which were significantly higher than their acid analogues of H2BPDC (300 °C) and H2SBDC (363 °C). The morphology images collected by scanning electron microscope (SEM) for the newly prepared K2BPDC and K2SBDC were shown in Figure S5, where they exhibited the relatively uniform shape of thin slices in the size range of micrometer scale (>1 μm). The tap density of K2BPDC or K2SBDC was calculated to be around 0.7 g/cm3, which was lower than the inorganic and carbonaceous anode materials (e.g., ∼ 1 g/cm3 for graphite) and thus could lead to the less volumetric energy density. As shown in Figure 2, the multiscanned cyclic voltammograms (CVs) of K2BPDC and K2SBDC were initially carried out to inspect their electrochemical behaviors in K-ion half cells (see details in SI). In the first cathodic scan (Figure 2a), K2BPDC displayed complicated reduction behaviors, where the board potential around 0.5 V (vs K+/K) was attributed to the irreversible formation of solid-electrolyte interface (SEI) and the potential located at 0.2 V was the reduction of BPDC2− anion.22 Gradually, the clear and stable redox couple belonging to the organic BPDC2− anion appeared after the first scan, where the cathodic peak was located at ∼0.35 V and the back reversible anodic peak was at ∼0.97 V. Similarly, the irreversible formation of SEI for K2SBDC was observed at ∼0.63 V in the first cathodic scan but then the clear redox behaviors from the organic SBDC2− anion expectably emerged during the second scan (Figure 2b), and the reversible cathodic and anodic peaks stabilized at ∼0.55 and ∼1.12 V, respectively. Notably, the reduction potentials (∼0.35 and ∼0.55 V) for K2BPDC and K2SBDC were significantly higher than the potassiation potentials (
