Alternative electrochemical energy storage: potassium-based dual-graphite batteries

K. Beltropa, S. Beukera, A. Heckmanna, M. Winter*ab and T. Placke*a
aUniversity of Münster, MEET Battery Research Center, Corrensstr. 46, 48149 Münster, Germany. E-mail: martin.winter@uni-muenster.de; tobias.placke@uni-muenster.de
bHelmholtz Institute Münster, IEK-12, Forschungszentrum Jülich, Corrensstr. 46, 48149 Münster, Germany. E-mail: m.winter@fz-juelich.de

Received 3rd June 2017 , Accepted 20th July 2017

First published on 20th July 2017


In this contribution, we report for the first time a novel potassium ion-based dual-graphite battery concept (K-DGB), applying graphite as the electrode material for both the anode and cathode. The presented dual-graphite cell utilizes a potassium ion containing, ionic liquid (IL)-based electrolyte, synergetically combining the extraordinary properties of potassium, graphite and ILs in terms of cost effectiveness, sustainability and safety. The IL electrolyte shows a very stable cycling performance in combination with the graphite anode at a so far not reported reversible capacity of ≈230 mA h g−1. A highly reversible capacity of >42 mA h g−1 (with respect to the graphite cathode) even at a current of 250 mA g−1, and a Coulombic efficiency (CE) exceeding 99% in a potential range from 3.4 V to 5.0 V vs. K/K+ represent the corner pillars of this innovative battery technology. The very promising electrochemical performance is further emphasized by a capacity retention of 95% after 1500 cycles. Furthermore, the electrochemical formation of a stage-1 potassium graphite intercalation compound (K-GIC) from an IL electrolyte, resulting in a stoichiometry of KC8 is presented in this work for the first time. The presented results shed new light on an alternative energy storage technology, especially in view of stationary (“grid”) energy storage by employing environmentally friendly, abundant and recyclable materials.



Broader context

The development of energy storage technologies is of prime importance to address the preservation of a consistent energy supply to cope with the world's increasing energy demands, which is considered as one of today's most challenging issues. The risk evaluation of the present energy economy identifies the increasing shortages of non-renewable (fossil) resources and the environmental impact of their combustion, including global warming and (air) pollution, as major challenges, thus giving rise to the development of renewable energies such as wind or solar power. A main strategy to tackle these sophisticated challenges is the integration of clean and highly efficient energy storage based on renewables into different energy sectors, particularly transportation and stationary storage. The most efficient and feasible solution for these storage applications clearly points to rechargeable batteries. Beside lithium ion batteries (LIBs) that dominate the field of high energy density applications, including portable electronic devices and automotive applications, it is important to develop alternative battery technologies, which may have advantages in terms of costs, lifetime, safety and sustainability. In this work, we introduce a novel dual-graphite technology based on potassium as a promising option for grid applications, which may display environmental, safety and cost benefits over the state-of-the-art LIBs.

Since the market launch of the lithium ion battery (LIB) in 1991, the triumphant progress of this battery technology has continued unbroken to this day.1–3 The growing demand of LIBs for portable consumer electronics and in particular for automotive applications have raised concerns about the limitation of the lithium resources (0.002 wt% of Li in the earth's crust; 20 mg kg−1 earth's crust; 0.18 mg l−1 seawater)4 and triggered numerous research studies on alternatives, primarily for large-scale stationary (“grid”) energy storage based on earth-abundant elements such as sodium (2.7 wt% of Na in the earth's crust; 28[thin space (1/6-em)]400 mg kg−1 earth's crust; 11[thin space (1/6-em)]000 mg l−1 seawater)4 and potassium (2.4 wt% of K in the earth's crust; 26[thin space (1/6-em)]000 mg kg−1 earth's crust; 380 mg l−1 seawater).4 In recent years, much effort has been made on sodium ion batteries (NIBs) to compete with the state of the art LIBs due to their believed lower costs.5–8 However, the poor ability of Na ions to intercalate into graphite, resulting in a very limited capacity (≈35 mA h g−1 by forming NaC64)9,10 as well as the higher redox potential for Na/Na+ (−2.71 V vs. standard hydrogen electrode (SHE)) compared to Li/Li+ (−3.04 V vs. SHE) still remain great challenges for this technology. Against this background, potassium ion batteries (KIBs) display a suitable alternative to NIBs due to the lower redox potential of −2.93 V vs. SHE for K/K+ compared to Na/Na+, suggesting higher cell voltages together with the comparatively high natural occurrence, low cost and non/lower-toxicity of potassium.11–15 Furthermore, the recently demonstrated exceptional cycling performance of carbonaceous anode materials in terms of reversible K+ intercalation/de-intercalation, including graphite,11–13 soft and hard carbon11,16,17 as well as graphene18 seems to pave a pathway to high-performance KIBs. The major challenge to come up with a battery based on K chemistry, similar to common LIBs, is rooted in the demand for a suitable high-capacity and high-voltage intercalation cathode host material. Several materials such as metal–organic frameworks (Prussian blue19 and its analogs20,21), layered materials22,23 as well as polyanionic compounds24–26 have been studied, showing promising results.

Alternatively, the unique redox-amphoteric character of graphite (4.6 eV)27 enables also a reversible accommodation and release of anions between graphene sheets at potentials above 4.5 V vs. Li/Li+, resulting in an anion-intercalating graphite cathode. The working principle of the simultaneous uptake and release of cations and anions into/from a graphite based anode and cathode, respectively, is visualized in Fig. 1A (charge process) and Fig. 1B (discharge process). The so called “dual-ion battery” (DIB) has attracted growing interest due to its exceptional electrochemical performance including high cell voltages and long-term cycling stability, together with good safety properties, low cost and environmental friendliness.28–32 The “dual-carbon” or “dual-graphite” battery has been presented by the pioneering studies of McCullough et al. and Carlin et al.33–36 Further achievements in the field of this battery concept have been reported by Seel and Dahn and by Ishihara et al.37–40 Against this background, the beneficial effect of a high voltage electrolyte based on a fluorinated solvent on the overall cell performance of a dual-graphite cell was shown by Read et al.41


image file: c7ee01535f-f1.tif
Fig. 1 (A and B) Working principle of the K-dual graphite battery (K-DGB). (C) Potential profiles of graphite/K metal half cells for the 1st, 2nd and 50th cycle at 10 mA g−1 (3 formation cycles) and 50 mA g−1 for the ongoing cycles. (D) CCC investigation at 50 mA g−1. (E) Potential profile of the 1st cycle at 27 mA g−1. (F) Ex situ XRD measurements of graphite electrodes according to the corresponding SOCs marked in part (E). Electrolyte: Pyr14TFSI + 0.3 M KTFSI + 2 wt% ES.

Here, we report for the first time on an innovative cross-over of KIBs and DIBs resulting in a novel battery technology named as a potassium-based dual-graphite battery (K-DGB). A commercially available synthetic flake-type graphite (KS6L; Imerys Carbon & Graphite) serves as the active material for both the anode and the cathode, in combination with a potassium containing ionic liquid (IL) electrolyte, namely N-butyl-N-methyl bis(trifluoromethanesulfonyl) imide (Pyr14TFSI; Solvionic; purity: 99.9%) + 0.3 M potassium bis(trifluoromethanesulfonyl) imide (KTFSI; Solvionic; purity: 99.5%) + 2 wt% ethylene sulfite (ES; Sigma Aldrich; purity: 99%) as an electrolyte additive. ES was used as an SEI forming electrolyte additive to prevent possible co-intercalation of Pyr14+ cations into the graphite anode and to increase the overall cell performance.31,42 The cycling of a K-DGB applying the baseline electrolyte without ES is exemplarily shown and discussed in the ESI (Fig. S2). Further detailed graphite characteristics as well as experimental details for the electrode preparation and electrochemical analysis can be found in the ESI. In order to verify the reversible intercalation/de-intercalation of K+ ions into the graphitic anode from an IL-based electrolyte, constant current cycling (CCC) investigations of K metal/KS6 anode half cells at 10 mA g−1 (3 formation cycles) and at 50 mA g−1 for the ongoing cycles, between 0.01 V and 1.5 V vs. K/K+ were conducted, and the corresponding potential profiles (1st, 2nd and 50th cycle) as well as the cycling performance are depicted in Fig. 1C and D, respectively. Furthermore, to gain a deeper understanding of how the staging behavior of K+ ions into graphite changes with cycling, dQ/dV plots for selected cycles are depicted in Fig. S1 (ESI). The potassiation of graphite in the first cycle can be divided in three distinct potential regions. The potential slope starting from 1.25 V to 0.4 V vs. K/K+ is mainly attributed to an initial reductive electrolyte decomposition, accompanied by solid electrolyte interphase (SEI) formation on graphite, resulting in a charge capacity that exceeds the theoretical capacity for potassium intercalation into graphite in the first cycle, which is similar to the behavior in LIBs and also reported elsewhere.43–45 This behavior is vanished in the 2nd and the ongoing cycles, indicating an effective SEI. We would like to emphasize the great enhancement of the first cycle CE (82%) by using the IL-based electrolyte with ES as an SEI additive, compared to the reported first cycle CE of 57%11 and 66%45 in organic carbonate-based electrolytes. However, the applied current rate as well as the composition, mass loading and porosity of the electrode will have a large impact on the first cycle CE and has to be taken into account when comparing CE values. The second sloping potential region from 0.4 V to 0.2 V vs. K/K+ (Fig. 1C) is attributed to the formation of a stage-2 phase (KC24) followed by a broad intercalation plateau, representing the stage-2 (KC24) into stage-1 (KC8) phase transition as further supported by XRD measurements (cf. Fig. 1E and F). The potassiation/de-potassiation into/from graphite displays a relatively low voltage hysteresis, which, in turn results in a high energy efficiency (≈92–93% at 50 mA g−1), that is close to the one for lithium intercalation/de-intercalation into/from graphite.46 Furthermore, a very stable cycling performance with an average discharge capacity of ≈230 mA h g−1 and average CE exceeding 99% could be achieved.

Ex situ XRD measurements for selected states of charge (SOCs) taken from the first cycle from CCC measurements at 27 mA g−1 (≈0.1C) were conducted to further study the intercalation/de-intercalation of K+ ions into graphite (Fig. 1E and F). To enable a better comparison between the potential profile and the XRD pattern, representative colored and numbered dots (Fig. 1E) can be correlated with the same-colored XRD scans (Fig. 1F).

The initial scans of the KS6L graphite electrode at 0.9 V and 0.5 V vs. K/K+ (points 1 and 2) show the (002) reflection of pristine graphite at 2θ = 26.55°, indicating high crystallinity of the used graphite and negligible K+-intercalation into graphite at these potentials. Upon charging, the (002) graphite reflection vanished (cf. point 3) and starts to split up into four new reflections, arising at 2θ = 16.79°; 20.41°; 30.99° and 33.72°, respectively when reaching a charge potential of 0.2 V vs. K/K+ (point 4), indicating the formation of a two phase region comprising of both the stage-2 K-GIC (KC24) and the stage-1 K-GIC (KC8). Further potassiation to the charge end-point potential of 0.01 V vs. K/K+ (point 6) leads to an intensity decrease of the characteristic stage-2 XRD reflections at 2θ = 20.41° and 31.05° and to the subsequent intensity increase of the stage-1 reflections at 2θ = 16.77° and 33.68°. The change in the electrode color from black (pristine graphite) to golden color (potassiated graphite; KC8) further evidenced the formation of a stage 1 K-GIC. However, the golden color vanished during XRD measurements, indicating a distinct self-discharge (a limited stability of the stage 1 K-GIC), which is presumably responsible for the missing pure stage-1 phase at 0.01 V vs. K/K+ in the XRD measurement. During de-potassiation to a discharge end-point potential of 1.5 V vs. K/K+ (point 10), the reverse process via stage formation takes place, resulting in a fully recovered graphite structure and revealing the high reversibility of K+ ion intercalation/de-intercalation into/from graphite when applying an IL-based electrolyte. The K+ ion intercalation between the graphene layers of graphite is accompanied by an increase of the interlayer distance from the initial 3.35 Å (pristine graphite) to 5.28 Å (potassiated graphite), leading to a corresponding volume expansion of 57.6%. The presented results are in very good agreement with the already reported results for K+ ion intercalation/de-intercalation into/from graphite using organic solvent-based electrolytes.11,14 To the best of our knowledge, the reversible potassiation/depotassiaton of graphite in a K/graphite cell from an IL-based electrolyte, forming a stage-1, KC8-GIC, confirmed by ex situ XRD, is reported herein for the first time.

The highly reversible uptake and release of TFSI-anions into/from a graphitic cathode from an IL-based electrolyte was first reported by Placke et al.28 The transition from the lithium-based dual-ion battery (Li-DIB) to the potassium-based DIB (K-DIB) applying the specific metals (Li vs. K) as anode materials, results in slightly lower discharge capacities and CEs for the K-DIB (cf. Fig. 2). A compilation of a systematic electrochemical investigation of the K-DIB, including potential profiles for anion intercalation/de-intercalation into graphite, differential capacity profiles (dQ/dV) for selected cycles, taken from the CCC investigations at 50 mA g−1 between 3.4 V and 5 V vs. K/K+ (Fig. 2C), as well as the cycling and rate performance of the K-DIB are depicted in Fig. 2A–D, respectively. According to our previously reported results, the intercalation/de-intercalation behavior of TFSI-anions into/from graphite in the K-DIB is very comparable to that in Li-DIBs.30–32 The onset potential for the first anion uptake is strongly affected by a kinetic hindrance, due to the initial “widening” of the graphene sheet interlayer distance against the attracting van der Waals forces and, therefore, shifted to higher values compared to the ongoing cycles (cf. Fig. 2A and B). In addition, the anion uptake is accompanied by an interlayer distance increase of the graphene sheets from the initial ≈0.335 nm to ≈0.821 nm, which corresponds to a total volume expansion of ≈140%.47 A stable cycling performance with a discharge capacity of ≈45 mA h g−1 and a CE exceeding 97% were obtained (cf., Fig. 2C). However, only a minor effect on the discharge capacity by increasing the current rate from the initial 5 mA g−1 (0.1C) to 250 mA g−1 (5C) was observed, indicating a promising rate capability of the system (cf., Fig. 2D). Furthermore, the increased C-rate has a beneficial effect on the CE of the system, as the time during cycling at highly oxidative potentials, causing parasitic reactions at the cathode surface, i.e. electrolyte degradation, is greatly reduced. At a specific current of 250 mA g−1 a CE of >99% was achieved. The evaluation of the suitability of both K+- and TFSI-anion intercalation/de-intercalation into/from a graphitic anode and cathode, respectively applying an IL-based electrolyte, was effectively proven in K metal based cells and paves a pathway to the final combination of two graphite electrodes in a dual-graphite battery (K-DGB) full cell. In Fig. 3, a detailed electrochemical investigation of the K-DGB is presented, confirming this novel battery technology to be highly promising. The anode and cathode potential profiles vs. K/K+ for the charge and discharge processes of the 1st, 2nd and 50th cycle taken from the CCC investigation are depicted in Fig. 3A (see the detailed comments in the ESI). Corresponding to the K-DIB, the charge/discharge rate for the first three cycles was set to 10 mA g−1, while the following cycles were performed at 50 mA g−1. As a consequence of SEI formation on the graphite anode and of the above discussed “activation” process, due to the first anion intercalation into the graphitic cathode, as well as of oxidative electrolyte decomposition, it is understandable that the first cycle strongly differs from the ongoing cycles. The first cycle CE of 64.5% for the K-DGB is reduced compared to the K-DIB (79.1%), due to the above mentioned parasitic reactions at the graphite anode. However, the CE increased rapidly to 81.3% in the second, and to 96.5% in the 50th cycle, respectively. The cycling performance of 3 equivalently composed K-DGB cells including the standard deviation is illustrated in Fig. 3B. The system exhibits a very stable cycling performance with a reversible capacity of ≈47 mA h g−1 (with respect to the cathode capacity) and a CE exceeding 96%, thus, reproducing the good cycling performance of the K-DIB (cf. Fig. 2). In addition, the subsequent current density increase from 5, 25, 50, 100 to 250 mA g−1 does not negatively influence the reversible capacity of the K-DIB, indicating very good reversibility and rate capability (cf. Fig. 3C). The promising cell performance is further supported by the excellent long-term cycling performance of the K-DGB cell (Fig. 3D), showing an average reversible capacity of 42 mA h g−1 at 250 mA g−1 together with a CE in the range of 98–99%. The K-DIB delivers an impressive capacity retention of 95% after 1500 cycles (Fig. 3D). Even though this work features a first proof of concept for this novel technology, further systematic studies related to the stability and/or a possible aging of the graphite anode, improvements in the electrolyte and an optimization of the anode/cathode capacity balancing are necessary to further improve the K-DGB technology.


image file: c7ee01535f-f2.tif
Fig. 2 (A) Potential profiles of graphite (cathode)/K metal (anode) dual-ion cells for selected cycles between 3.4 V and 5.0 V vs. K/K+ at a charge/discharge current of 50 mA g−1. (B) Differential capacity (dQ/dV) profiles for selected cycles taken from the CCC investigation in part (A). (C) CCC investigation at 50 mA g−1 and (D) rate performance at different specific currents. Electrolyte: Pyr14TFSI + 0.3 M KTFSI + 2 wt% ES.

image file: c7ee01535f-f3.tif
Fig. 3 (A) Cathode (black) and anode (red) potential profiles of graphite/graphite cells at 10 and 50 mA g−1. (B) CCC investigation at 50 mA g−1 after 3 formation cycles at 5 mA g−1. (C) Rate performance at different specific currents and (D) long-term cycling performance at 250 mA g−1. Electrolyte: Pyr14TFSI + 0.3 M KTFSI + 2 wt% ES.

Conclusions

In summary, we report for the first time, novel K-DIB and K-DGB technologies, applying low-cost, and environmentally friendly graphite as both the anode and cathode in combination with an IL-based electrolyte to further improve the safety and sustainability of K-DGBs and K-DIBs. To the best of our knowledge, the electrochemical formation of a stage-1 K-GIC from the IL-based electrolyte, confirmed by ex situ XRD measurements is also reported herein for the first time. The combination of potassium and the dual-ion technology results in a high operating cell voltage (>4.5 V) with a discharge capacity exceeding ≈42 mA h g−1 (with respect to the cathode capacity) and a CE > 99% depending on the specific current. Furthermore, an excellent long-term cycling performance with a capacity retention of 95% after 1500 cycles was achieved. We hope that this work will result in further inventions addressing alternative battery technologies, based on abundant, low-cost, less toxic and safe active and inactive materials. Further systematic studies are needed to gain more insights into this novel storage technology.

Acknowledgements

The authors wish to thank the Federal Ministry of Education and Research (BMBF) for funding this work in the projects “Insider” (03EK3031A) and “BenchBatt” (03XP0047A). Furthermore, we thank Andre Bar for the graphical support.

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Footnote

Electronic supplementary information (ESI) available: Detailed experimental procedures and further electrochemical results. See DOI: 10.1039/c7ee01535f

This journal is © The Royal Society of Chemistry 2017