A Comparative Study on the K-ion Storage Behavior of Commercial Carbons

Abstract

Potassium-ion battery, a key analog of lithium-ion battery, is attracting enormous attentions owing to the abundant reserves and low cost of potassium salts, and the electrochemically reversible insertion/extraction of the K-ion within the commercial graphite inspires a research spotlight in searching and designing suitable carbon electrode materials. Herein, five commercially available carbons are selected as the anode material, and the K-ion storage capability is comparably evaluated from various aspects, including reversible capacity, cyclability, coulombic efficiency, and rate capability. This work may boost the development of potassium-ion batteries from a viewpoint of practical applications.

1. Introduction

The global energy crisis and carbon neutrality target impose higher requirements on the industrial production and affect our daily life. Therefore, green energy sources are increasingly developed and employed, including solar energy, wind energy, tide energy, etc. However, these energies sources are confronted with intrinsic power fluctuations induced by the vagaries of climate, thus demanding energy storage devices with high power density, high energy density, durable cyclability, and cost effectiveness [1,2,3,4]. Consequently, to tackle the resource deficiency of current lithium-ion batteries (LIBs), especially lithium and cobalt, novel and resource-abundant battery systems need to be urgently exploited [5]. As analogs of LIBs, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are attracting enormous attention [6,7,8,9]. Additionally, in the year 2021, the first commercial SIBs were launched by Contemporary Amperex Technology Co., Limited (CATL) with hard carbons as the anode and layered oxides as the cathode [10]. As for PIBs, the reversibly electrochemical K-ion intercalation/extraction in the current commercial graphite anode of LIBs opens another window for lowering the price and simplifying the commercial production period [11,12,13,14,15,16,17].

Since Ji’s group’s first reported graphite as the anode for PIBs, various modification strategies have been employed to improve the rate capability and cyclability, such as the interlayer engineering and pore regulating strategies [18,19]. In particular, substantial efforts have been made in regulating the electrolyte component, and remarkably improved cyclability was achieved [20,21,22,23]. Moreover, graphitic carbons with a hollow structure, which can effectively alleviate the volume variation and permit sufficient electrolyte permeation, were also reported as promising anode materials for PIBs [24,25,26,27]. The success of hard carbons in SIBs was also extended to PIBs [28,29,30,31,32,33]. Specifically, hard carbons with heteroatom doping and porous structure exhibited superior K-ion storage capability, including high reversible capacity, excellent rate performance, and cyclability [34,35,36,37,38]. Simultaneously, soft carbon was also reported as a feasible anode material for PIBs with well-balanced low-voltage capacity and rate capability [39,40,41]. Despite carbons, various anode materials with high reversible capacities were also reported, especially the conversion-type (FeS₂, [42] CoS₂, [43] etc.) and alloying-type (Sb, [44,45] P, [46,47] Bi, [48,49] Sb₂S₃, [50,51] SnS₂, [52,53,54] Sn₄P₃, [55,56,57,58] etc.) materials. However, the development of these materials is still at the stage of fundamental research, and solutions to the large volume expansion, high voltage hysteresis, and structural re-alignment issues are still not mature [59,60,61], demanding the in-depth research and more efforts in the future.

As depicted in Figure 1, among the reported anode materials of PIBs, carbons account for a large part and continue to increase year by year [62,63]. However, most studies focus on material design for a better electrochemical performance; evaluations of the commercially available carbons are still absent, and research on them may be closer to practical applications. Herein, we choose five commercial carbons, including natural graphite (NG), artificial graphite (AG), hard carbon (HC), soft carbon (SC), and activated carbon (AC), as the anode materials for PIBs. In order to construct a correlation between the microstructure of the current commercial carbons and their corresponding K-ion storage capabilities, a comparative study on reversible capacity, cyclability, coulombic efficiency, and rate capability was conducted. Additionally, this work may provide a guideline for selecting proper anode materials of PIBs.

2. Materials and Methods

2.1. Materials

All carbons used in this work are commercially available. Natural Graphite (NG, No. 918), Artificial Graphite (AG, No. ACP-S360), Hard Carbon (HC, No. BHC-400), Soft Carbon (SC, No. BSC-3), and Activated Carbon (AC, No. SAC-18) were all purchased from Shenzhen Kejing Star Technology Company and directly used without any further purification or treatment.

2.2. Materials’ Characterization

The morphology and microstructure were characterized by FESEM (HITACHI SU8010). The phase structure was determined through XRD (D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 10° min⁻¹. Raman spectra were collected on a Renishaw inVia confocal Raman microscope using a 532 nm laser as the excitation source.

2.3. Electrochemical Measurements

The working electrodes were prepared by mixing the active materials (NG, AG, HC, SC, or AC), Super P (conductive additive), and PVDF (polyvinylidene fluoride, binder) with a mass ratio of 8:1:1. It needs to be noted that a 3 wt.%PVDF solution dissolved in NMP (N-Methyl pyrrolidone) solvent was employed instead of the direct use of the powder to achieve a better dispersion. The as-prepared slurry was subsequently coated onto the copper foil followed by a vacuum-drying process at 90 ℃ for 10 h to evaporate the NMP solvent. The dried electrodes were then cut into circulars with a diameter of 12 mm. Half cells (CR2032 coin cell) were assembled in an Ar-filled glove box (MBRAUN-UNILab pro, H₂O < 0.1 ppm, O₂ < 0.1 ppm), with metallic potassium as the counter electrode, a Whatman GF/C glass fiber as the separator, and a 0.8 M KPF₆ solution dissolved in EC/DEC (1:1 by volume) as the electrolyte (the volume of electrolyte for each coin cell was ~200 µL). The galvanostatic charge/discharge tests were performed using a LAND-CT2001A multichannel galvanostat (Wuhan, China) at room temperature.

3. Results and Discussion

The morphology and structure of commercial carbons were first evaluated. As depicted in Figure 2, all commercial carbons exhibited a uniform morphology of particles, with an average diameter of 5~10 μm, which is also in accordance with the current commercial graphite anode for LIBs. Moreover, a uniform morphology can ensure the data consistency of batteries, suitable for this comparative study on various carbons.

The crystal structure of carbons proved a key parameter affecting the electrochemical performance. As shown in Figure 2, the XRD patterns of all five carbons delivered quite different profiles. Specifically, NG and AG exhibited the typical structure of graphite, delivering standard peaks located at 26.5°, 42.3°, 44.5°, and 54.8°, corresponding to (002), (100), (101), and (004) crystal planes, respectively. As for HC, SC, and AC, all peaks were in a broad shape, especially the characteristic (002) peak, which indicates a low crystallinity with a relatively amorphous or short-range order structure rather than the typical long-range order structure of graphite. Moreover, the (002) peaks of HC, SC, and AC shifted slightly toward a lower degree, correlating to an enlarged interlayer spacing. The Raman spectra further confirmed the above results. NG and AG exhibited the characteristic D band, G band, and 2D band with a low I_D/I_G value of 0.2~0.3, indicating a characteristic sp² carbon structure with barely any defects. Meanwhile, HC, SC, and AC all exhibited a high I_D/I_G value exceeding 1.0, consistent with the low crystallinity structure. In general, the carbons with high crystallinity are expected to deliver a low discharge potential in PIBs, which indicates a higher energy density in full cell battery, while the opposite always benefits the power density.

With the potassium tablet as the counter electrode and a conventional 0.8M KPF₆ in EC/DEC (1:1 by volume) as the electrolyte, the electrochemical performances of all five carbons were well evaluated. As shown in Figure 3a,b, all carbons exhibited a relatively stable cyclability. Specifically, after 50 cycles at 50 mA g⁻¹, NG, AG, HC, SC, and AC maintained 71.2%, 62.9%, 82.9%, 79.8%, and 60.8% of the initial capacity (the second cycle). Moreover, NG and HC delivered a relatively high reversible capacity of over 250 mAh g⁻¹. Moreover, a huge gap between the specific capacity of the first and second cycle can be noticed, which should be ascribed to the relatively low initial coulombic efficiency (ICE), a key indicator of the full cell battery assembly. As depicted in Figure 3c, the ICEs of NG, AG, HC, SC, and AC were ~38.84%, 52.51%, 53.15, 49.69%, and 15.60%, respectively. Apparently, AC delivered the lowest ICE value, which should be due to its high specific surface area that consumed enormous K-ions and electrolyte to form a solid electrolyte interphase (SEI) layer on the surface. Despite ICE, the coulombic efficiencies in the subsequently cycles were also critical. As summarized in Figure 3d, the average coulombic efficiencies (ACE) of NG, AG, HC, and SC were all in the range of 94~98% (from the 2nd to the 50th), while the ACE of AC was merely ~89%. However, under an ideal condition, an ACE of over 99.5% can ensure a ~80% capacity retention after 50 cycles; thus, the current ACE was far beyond the criteria of the commercial application. Moreover, energy density and energy efficiency are key parameters when evaluating a battery system, and in the case of PIBs anode, emphasis should be put on the potential and corresponding hysteresis. As profiled in Figure 3e,f, all carbons except AC delivered a larger portion of capacity under 0.5 V, and NG showed the lowest median discharge/charge potential as well as the lowest potential hysteresis. With the development of electric vehicles, the demands for fast charging capability are proposed, which is in accordance with the power density or rate capability of a battery system. Herein, NG and HC with superior overall performance were selected for the rate capability evaluation. As shown in Figure 3g, NG delivered a relatively high reversible capacity of 285.7 mAh g⁻¹, 263.5 mAh g⁻¹, 222.5 mAh g⁻¹, 108.5 mAh g⁻¹, 58.1 mAh g⁻¹, and 27.3 mAh g⁻¹ at 0.02 C, 0.05C, 0.1C, 0.2C, 0.5C, and 1.0C, respectively. Meanwhile, HC delivered a reversible capacity of 243.2 mAh g⁻¹, 180.3 mAh g⁻¹, 132.3 mAh g⁻¹, 82.0 mAh g⁻¹, 38.8 mAh g⁻¹, 16.4 mAh g⁻¹ at 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1.0C, respectively. It is obvious that both NG and HC exhibited a relatively high reversible capacity at rates below 0.1C and a rapid deterioration at 0.2C. As concluded in Figure 3h, the capacity dropped sharply at high rates, which limits the practical prospect and requires more efforts on the structural design of carbons and the molecular optimization of the electrolyte.

4. Conclusions

In summary, we selected and evaluated the electrochemical K-ion storage capabilities of five commercially available carbons, including natural graphite, artificial graphite, hard carbon, soft carbon, and activated carbon. Comparable and comprehensive studies were conducted on reversible capacity, coulombic efficiency, cyclability, and potential hysteresis. As concluded in Figure 4, natural graphite and hard carbon delivered a superior overall performance among the five carbons, while activated carbon could only serve as a good conductive additive rather than an anode material. Moreover, the low initial coulombic efficiency as well as the rate capability remain a common issue among all five selected carbons. Therefore, more efforts should be contributed to the structural modification of commercial carbons and composition design of the electrolyte to boost the commercial progress of potassium-ion batteries.