Low-Strain KVPO₄F@C as Hyperstable Anode for Potassium-Ion Batteries

Abstract

Potassium-ion batteries with carbon-based materials and alloy materials as anodes possess pronounced potassium storage and cycling abilities, yet they suffer from harsh synthetic processes, low initial Coulombic efficiency, and limited structure stability. Herein, we first put forward potassium vanadium fluorophosphate (KVPO₄F) as a highly stable anode material for potassium-ion batteries (PIBs). KVPO₄F@C composite is successfully synthesized through hydrothermal method followed by a low-temperature roasting process. An amorphous carbon film is homogeneously wrapped on the surface of the KVPO₄F particles to improve electronic conductivity. As the anode for PIBs, the KVPO₄F@C presents a super-high discharge capacity of 242.32 mAh g⁻¹ and a superior cycle stability over 120 cycles at 100 mA·g⁻¹ with 93.1% capacity retention. Additionally, it maintains 92.9% of its initial capacity of 100 mA·g⁻¹ at 1000 mA·g⁻¹ after cycling 2100 times, which indicates an excellent long cycling performance. The in situ X-ray Diffraction and ex situ Transmission Electron Microscopy tests prove the minimal volume variation and stable structure of KVPO₄F@C. The designed KVPO₄F@C provides facilitative electronic conductivity and stable structure, successfully innovating an ultra-stable and high-performance anode material for potassium-ion batteries and other energy storage application fields.

1. Introduction

The vigorous development of science and technology brought unprecedented brilliancy to human society. The continued consumption of fossil fuels makes the demand for clean, renewable, and sustainable energy sources ever-growing. Due to the intermittency, volatility, and uneven spatial distribution of renewable energy such as water energy, wind energy, and solar energy, as well as the irreversible trend of vehicle electrification and the rapid increase of the energy storage market, electrochemical energy storage systems have ushered in significant development opportunities. Among the various electrical energy systems, secondary batteries are extensively investigated because they are environmentally friendly, convenient, recyclable, and have high safety [1,2,3,4,5]. Thereinto, as one of the most promising secondary battery systems, lithium-ion batteries (LIBs) own several advantages such as high energy density and power density, low self-discharge rate, and small size [6,7,8]. However, the battery manufacturing costs increase continually because the scarcity and excessive consumption of lithium resources [9,10,11]. It is necessary to design a new battery system with low cost, energy density, power density, and cycle stability comparable to lithium-ion batteries to make up for the shortage of lithium-ion batteries.

The reserves of sodium and potassium elements, both of which are alkali metals, are almost 1000 times that of lithium reserves in the Earth’s crust. Their prices are much lower than lithium and their properties are similar to lithium. Therefore, the material system and industrialization process of lithium-ion batteries can be learned and the application can be carried out in other alkali metal battery systems. Sodium-ion batteries have been widely examined and some research progress has been made. Potassium-ion batteries have as great research value and application potential as sodium-ion batteries. They work in a similar way to lithium-ion batteries by shuttling potassium ions between positive and negative electrodes. Meanwhile, compared with the standard hydrogen electrode potential, the redox potential of lithium, sodium, and potassium is −3.04 V, −2.71 V, and −2.93 V, respectively. Potassium is more negative than sodium and more similar to lithium. Potassium-ion batteries may provide a higher voltage than sodium-ion batteries, depending on if the voltage provided by the battery is equal to the potential difference between the positive and negative electrodes when other parameters are kept similar. Owing to natural abundance and low cost of potassium resources, potassium-ion batteries (PIBs) have recently captured growing interest as one of the most beneficial electric energy storage systems [12,13,14]. The really low redox potential (−2.94 V) of the K⁺/K couple contributes to the high work potential and energy density, which reveals a superior application prospect of PIBs for sustainable energy storage technology [15,16,17,18]. However, as one of the key components, the mainstream negative electrodes for PIBs suffer from serious problems, in terms of the pulverization phenomenon of graphite, the serious volume expansion of the metals and metal oxides/sulfides/phosphates, and low initial Coulombic efficiency of hard/soft carbon [19,20,21,22]. Therefore, it is of great significance to put forward an innovative electrode to meet the development needs of potassium-ion batteries.

In view of the low raw material price and high security, triclinic LiVPO₄F and NaVPO₄F, the framework materials based on phosphate polyanion, have been widely investigated as cathode materials because of their high operating voltage and excellent thermal stability [23,24,25,26]. Interestingly, there is a special insertion reaction at around 1.8 V associated with the V²⁺/V³⁺ redox couple, indicating the potential of lithium or sodium vanadium fluorophosphate as an anode material [7,19,27]. There are many studies describing the existence and the key role of fluorine in the work of KVPO₄F materials and focus on the improvement of electrochemical performance [27,28,29]. The synthesis method of KVPO₄F materials mainly includes carbothermal reduction method, sol-gel method, hydrothermal method, ion exchange method, and so on [28,29,30,31]. The research ideas of material modification that can be used for reference mainly focus on the substitution of beneficial elements, conductive substances, and coating modification of fast ion conductors [32,33,34,35]. Hence, we can speculate that potassium vanadium fluorophosphate (KVPO₄F) possesses a bright possibility to be a potential anode material for potassium-ion batteries [28,29,30,31]. However, there are few papers putting forward the key effect about potassium ions storage of KVPO₄F at low operating voltages [32,33,34,35]. In addition, owing to the low electronic conductivity (i.e., 1.84 × 10⁻⁵ S·m⁻¹) and the unstable electrode/electrolyte interface, KVPO₄F as an electrode material suffers an inferior cycling and rate performance [36,37,38,39]. According to the modification examples of polyanionic materials in lithium-ion batteries and sodium-ion batteries fields, in situ substitution of elements can change the internal structure of materials, improve the ion transport of materials, and make the material structure more stable by using lattice defects such as negative/cation vacancy [39]. The coating of conductive carbon material and fast ion conductor can increase the interface stability between the electrode particle interface and electrolyte, which is a very effective modification method [40]. Therefore, KVPO₄F as anode for PIBs with superior electrochemical performance is urgently needed to be explored.

Herein, we design the KVPO₄F@C composite via hydrothermal method assisted by a facile sintering process for the first time [20,40,41]. In this composite, the homogeneous carbon layer coating on the surface of the KVPO₄F particle is formed in situ as a conductive network for improved electronic conductivity. The KVPO₄F primary particles uniquely stack to form micron-sized secondary particles during hydrothermal process. The KVPO₄F@C as anode material is fully investigated. Due to the superior electronic conductivity and structural flexibility, KVPO₄F@C as anode shows a discharge capacity of 242.32 mAh·g⁻¹ and a superior cycle stability over 120 cycles at 100 mA·g⁻¹ with a 93.1% capacity retention. In addition, after cycling 2100 times at 1000 mA·g⁻¹, KVPO₄F@C reveals the specific charge capacity of 100 mA·g⁻¹ and maintains a 92.9% capacity retention, which indicates an excellent long-life cycling performance. Thus, a KVPO₄F@C composite is a highly ideal anode material for developing the ultra-stable potassium-ion batteries.

2. Materials and Methods

Synthesis: The KVPO₄F@C composite was synthesized via a simple hydrothermal reaction route assisted by a low-temperature pre-roasting and sintering process. These are the detailed steps: firstly, potassium fluoride KF (Metal level), vanadium pentoxide V₂O₅ (A.R., 99%), diammonium hydrogen phosphate NH₄H₂PO₄ (A.R., 99%), and oxalic acid dihydrate H₂C₂O₄·2H₂O (A.R., 99.5%) (with 20% excess of the stoichiometric amounts) were dissolved in deionized water to obtain an orange-yellow solution. H₂C₂O₄·2H₂O acted as the reductant and carbon source. Then, the solution was transferred to a 200 mL Teflon-lined autoclave and kept at 200 °C for 24 h. After naturally cooling down, the black precipitate was filtered and washed separately with distilled water and ethanol three times, followed by drying under vacuum oven at 60 °C for 12 h. The obtained powder was applied as precursor. Finally, the as-prepared precursor was annealed at 350 °C for 2 h for pre-roasting, and then roasted at 680 °C for 6 h in a tube furnace, the roasting process is under argon atmosphere. Both heating and cooling rates are 2 °C·min⁻¹. The as-obtained black sample was named as KVPO₄F@C.

Material characterizations: The micro-morphologies of KVPO₄F@C were conducted by the Field Emission Scanning Electron Microscope (FESEM, Hitachi S-4800, 20 kV). The microstructure of the synthesized KVPO₄F@C was conducted by the Transmission Electron Microscopy (TEM, Titan G2 60-300). The Powder X-ray Diffraction (XRD, Rint-2000, Rigaku, Cu Kα) was employed to confirm the crystal structures of KVPO₄F@C. The Energy Dispersive Spectrometer (EDS) mapping analysis was employed to confirm the elemental distributions of KVPO₄F@C. Carbon–sulfur analyzer equipment (Eltar, Germany) was employed to confirm the carbon content in the samples. The in situ XRD (Bruker AXS, D-76187, Karlsruhe, Germany) and ex situ TEM and XPS (XPS, Thermo Scientific K-Alpha) were carried out to investigate the structure and morphology changes of KVPO₄F@C anode during cycling. The electrodes, obtained by disassembling the half-cells in the glove box, need to be washed with the DME (Dimethoxyethane) solution several times and dried in vacuum prior to the observations. The inductively coupled plasma atomic emission spectrometry (ICPAES, IRIS intrepid XSP, Thermo Electron Corporation) was employed to ascertain the loss mass content of elemental V dissolved in cycled cells.

Electrochemical tests: The electrochemical tests with KVPO₄F@C as anode for PIBs were conducted by assembling the CR2032 coin-type cells. The detailed steps of the working electrode fabrication were as follows: KVPO₄F@C (active material), polyvinylidene fluoride (binder), and acetylene black (conductive agent) with a ratio of 8:1:1 were fully mixed and ground for 0.5 h, then, the mixture was decanted into a small bottle with appropriate N-methyl pyrrolidinone (NMP) solution and stirred for 12 h to form slurry. The slurry was spread on the smooth copper foil (current collector), and then dried at 120 °C for 6 h in a vacuum oven. It was punched into rounded pieces and the working electrodes were acquired with an area of 1.13 cm². The average weight of the active material on each piece weighs about 1.6~2.0 mg. We assembled the Coin-type cells in a dry Ar-filled glove box. The counter electrode was rounded potassium metal pieces, and the cellulose paper was employed as the separator. The electrolyte was 3 M KFSI (Potassium difluorosulfonimide) in DME (Dimethoxyethane) solvent. The electrochemical tests were conducted via an automatic galvanostatic battery testing system by NEWARE, as the battery circler has a potential range of 0.01–3.0 V. Cyclic voltammetry (CV) of KVPO₄F@C as anode was tested with the CHI660A electrochemical analyzer. We carried out the CV tests within the voltage range from 0.01 to 3.0 V at the scanning rate of 0.1 mV s⁻¹·h.

In order to investigate the structural stability mechanism and the loss of vanadium from KVPO₄F@C during electrochemical process, we disassembled the cells with KVPO₄F@C as anode after cycling at 3.0 V after the first time and 100 times. The pole pieces were immersed in DME solution several times in 0.5 h for further investigation of the morphology and property changes. In order to investigate the lost vanadium, the cycled cells were disassembled and dissolved in the washing solution. Then, the washing liquid was selected and dissolved in hydrochloric acid, and the content of vanadium was determined by ICP equipment. By comparison, we can confirm the loss rate of vanadium in electrolyte.

3. Results and Discussion

Figure 1a depicts a schematic illustration of synthetic steps for the synthesis of the designed KVPO₄F@C composite. The prepared raw materials were mixed in deionized water with continuous stirring; then the mixed raw material agglomerated into a micro-sized spherical precursor after hydrothermal reaction. The obtained precursor shows hexagonal shape; it transformed into secondary particles through the low-temperature roasting process. Figure 1b shows the XRD pattern of the KVPO₄F@C composite; the high-intensity characteristic peaks of KVPO₄F were detected, indicating that the synthesis method has successfully produced the crystalline KVPO₄F, which exhibits the high-intensity characteristic peaks of KVPO₄F without any peaks from impurities found, indicating that the crystalline KVPO₄F with a high purity was successfully produced by the simple synthesis method. In addition, at around 26.5°, there is no distinctive peak associated with pyrolytic carbon found. It can be put down that the carbon phase in the composite is in an amorphous state. The carbon content in KVPO₄F@C is about 1.1 wt.% from the carbon–sulfur (C–S) analysis. This is a very small mass ratio of carbon element in composite, which places less interference on the capacity of KVPO₄F@C. In addition, the existence of pyrolytic carbon can hinder the fluorine loss during the roasting action, which effectively avoids the impurities under the high temperatrue treatment. The KVPO₄F@C composite has better electronic conductivity because of the amorphous carbon layer coating on the surface of particles. Figure 1c,d and Figure S1 show the SEM images of KVPO₄F@C particles, it is obvious that the micron-sized secondary particles and hexagonal primary particles are formed by the hydrothermal reaction, and the particles in the size distribution are micro-size since the large particles are aggregated from the smaller nano-size ones.

In order to further study the micro-morphologies of KVPO₄F@C, TEM, HRTEM, and EDS mappings tests were carried out. As shown in Figure 2a and Figure S2, the TEM image of KVPO₄F@C shows the regular outline and the reunion action from primary particles to secondary ones. In addition, potassium vanadium fluoride phosphate material has a large size distribution at micron scale, and the internal structure of the synthesized material is compact, which indicates the high compaction density of KVPO₄F@C. There is residual carbon on the surface of the KVPO₄F particles, which is left after the reduction reaction during the roasting process. The existence of pyrolytic carbon can effectively increase the electronic conductivity of synthetic KVPO₄F@C composite material, which makes it a beneficial material to apply as the electrode for battery systems. Figure 2b exhibits the enlargement of a single KVPO₄F@C particle, which illustrates a hexagonal micro-sized structure. The hexagonal structure is formed by the hydrothermal reaction. The shape of the particle is similar to a brick, which is conducive to scattered stacking, forming a stable architecture. From the HRTEM image (Figure 2c), it is obvious that the KVPO₄F crystals are identified, whose surfaces are wrapped by the amorphous carbon layer. The designed core/shell structure of KVPO₄F@C can give rise to a stable electrode surface. The stable electrode surface hinders the corrosion reaction at the electrode/electrolyte surface, constructing a comfortable charge and discharge cycling environment. EDS mappings of KVPO₄F@C are presented in Figure 2d; it is obvious that K, F, P, V, and O elements distribute a similar outline to the bulk nanoparticle, indicating the KVPO₄F core. In addition, there is C element found partly surrounding the inner particle. Such a large hexagonal structure can effectively boost the tap density. From the EDS and TEM results, it is obvious that amorphous pyrolytic carbon is wrapped on the surfaces of KVPO₄F@C particles, which can effectively connect multiple KVPO₄F@C particles. The amorphous carbon coating layer is formed by stacking flexible carbon film nanosheets during the roasting progress. In a nutshell, this special route achieved the fabrication of KVPO₄F@C composite through roasting precursor from the hydrothermal process to build a special structure.

Cycle performances of KVPO₄F@C as anode material for potassium-ion batteries are fully tested. The initial three consecutive cyclic voltammetry (CV) curves at 0.1 mV·s⁻¹ are depicted in Figure 3a. There are two broad characteristic peaks located at around 1.03 V and 0.81 V in the initial cycle ascribed to the formation of solid electrolyte interface (SEI), which builds a stable interface between electrode and electrolyte. Two pairs of reduction/oxidation peaks are located at around 0.81/1.0 V and 1.02/1.26 V during the first whole (de)intercalation process, which associates with the insertion/extraction of K⁺ ions into/out of the crystal lattice of KVPO₄F@C anode. The CV curves in the next two cycles display the similar two pairs of reduction and oxidation peaks, which are located at 1.00 V and 1.26 V, respectively. The same outline of CV curves is because of the stable solid electrolyte interface (SEI) formed during the first cycling process. The constant polarization represents KVPO₄F@C anode possessing a stable cyclic environment after first cycling. Figure 3b shows the initial charge/discharge profiles of KVPO₄F@C at 100 mA·g⁻¹, which possess the reversible specific charge capacity of 242.32 mAh·g⁻¹ corresponding to the initial Coulombic efficiency of 50.6%. This working plateau is higher than the plateau of traditional graphite, which can avoid the dendritic effect of potassium metal. The cycling performances of KVPO₄F@C are displayed in Figure S3. The KVPO₄F@C anode achieves an initial specific charge capacity of 242.32 mAh·g⁻¹ at 100 mA·g⁻¹, and it still maintains a capacity as high as 225.59 mAh·g⁻¹ even after being cycled 120 times, equivalent to a high-capacity retention of 93.1%. The rate performance of KVPO₄F@C at various current densities is shown in Figure 3c. It is worth noting that the designed KVPO₄F@C exhibits much higher capacity at high current densities of 4000 mA·g⁻¹. Specifically, the KVPO₄F@C anode reveals the superior reversible capacities about 242.32, 195.23, 175.60, 160.78, 104.34, and 92.42 mAh·g⁻¹ when charged at the increasing current densities of 100, 200, 500, 1000, 2000 and 4000 mA·g⁻¹ 20 times, respectively. In addition, the charge capacity could recover to its original value when the current density returns to 100 mA·g⁻¹, which indicates a superior recoverability rate of KVPO₄F@C. The charge–discharge curves of KVPO₄F@C at different current densities are shown in Figure S4. The long-term cycling performance of this special KVPO₄F@C is revealed. As displayed in Figure 3d and Figure S5, the KVPO₄F@C exhibits an excellent cycling performance, achieving a specific charge capacity of 160.24 mAh·g⁻¹ with a capacity retention of about 92.9% after cycling 2100 times at the high current density of 1 A·g⁻¹. The super specific capacity, excellent rate property, and outstanding long-term cycling performance together with high-capacity retention demonstrate that the designed KVPO₄F@C is an ideal choice as the anode material for potassium-ion batteries.

It is well known that potassium-ion battery anode materials suffer poor cycling performance below room temperature. Anode materials for potassium-ion batteries react with organic solvents in the electrolyte, especially at low temperatures, resulting in rapid capacity decay. Therefore, it is urgent to improve the electrochemical performance at low temperatures in practical applications. To evaluate the electrochemical performance of KVPO₄F@C at lower than room temperature, charge–discharge tests under 20 °C were conducted at 100 mAh·g⁻¹ 100 times. In Figure S6a, the initial discharge specific capacity of KVPO₄F@C (227.2 mAh·g⁻¹) at 20 °C is similar to that of KVPO₄F@C at 25 °C (225.6 mAh·g⁻¹), and both show similar charge capacities (247.2 mAh·g⁻¹ at 20 °C and 242.3 mAh·g⁻¹ at 25 °C). As a result, KVPO₄F@C reveals 91.9% initial Coulombic efficiency at 20 °C, close to that of KVPO₄F@C at 25 °C (93.1%). In addition, with the pyrolytic carbon coating, the cycle performance at low temperature remains stable. As shown in Figure S6b, KVPO₄F@C maintains a discharge capacity of 213.6 mAh·g⁻¹ with 94.1% capacity retention after cycling 100 times, indicating the superior comprehensive electrochemical performance of KVPO₄F@C.

To investigate in-depth knowledge about K ions storage mechanism of KVPO₄F@C as anode, the morphologies transformation and structural evolution of the tested electrodes after specific cycling were significantly explored. As exhibited in Figure 4a,b, the cycling curves at 100 mA·g⁻¹ and the corresponding in situ XRD analysis were tested to achieve the structural evolution of KVPO₄F@C in the initial two cycles. It is obvious the KVPO₄F@C crystalline structure has changed during the K⁺ ions insertion/extraction process. During the charging process, the dominant peak located at 32.49° of KVPO₄F@C disappeared, and it reappeared after a whole cycling process. During the deeply embedded K⁺ ion process, there exists a very significant peak migration as a fresh phase appears at about 31.46°, which is associated with the V²⁺/V³⁺ redox couple. The results reveal that the K⁺ ions reversibly diffuse into/out of the KVPO₄F crystal lattice with admirable structural stability. As well, the characteristic peak migrates from 31.46° back to 32.49° during the K⁺ extraction process. Therefore, although the K ions insertion reaction is accompanied by a little volume expansion, KVPO₄F@C still keeps the superior reversibility. This phenomenon indicates that during the incorporation of K⁺ into the KVPO₄F crystal lattice, there is a low degree of structural arrangement. The TEM and HRTEM images of KVPO₄F@C anode discharged to 0.01 V after 100 cycles are shown in Figure 4c–e and Figure S7. The complete micro-sized particles possessing a regular hexagonal structure are observed (Figure 4c,d), which has the same diameter compared with the pristine KVPO₄F@C, indicating that KVPO₄F@C electrode shows no obvious volume expansion during the K⁺ ions insertion process. The HRTEM image in Figure 4e and Figure S8 further verifies the relationship between the evolution of KVPO₄F microstructure and potassium-ions storage. The HRTEM image in Figure S8 shows a complete and clear KVPO₄F lattice with nano-sized carbon thin film coating on the surface of the particle. It is obvious that the overall structure of KVPO₄F@C can be well maintained after 100 times of the fully discharge cycling process, further demonstrating excellent structural stability. According to the test data, the loss of vanadium element in KVPO₄F@C electrode is about 0.18 wt% after cycling 100 times. The pyrolytic carbon coating layer acts as a physical protection barrier to suppress harmful side reactions and enhance the chemical stability of KVPO₄F cores in KVPO₄F@C, resulting in a stable cycle performance.

4. Conclusions

In summary, the designed KVPO₄F@C composite was successfully synthesized via a simple hydrothermal reaction process followed by a one-step low-temperature annealing process. In this composite, the huge KVPO₄F particles were wrapped by the nano-sized amorphous carbon layer. KVPO₄F@C as anode for PIBs provides superior electrochemical performance, including an excellent rate property, a high K⁺ ions storage voltage, and an ultra-stable long-cycle performance. Concretely, it delivers a reversible specific capacity of 242.32 mAh·g⁻¹ with the initial Coulombic efficiency of about 50.6% and maintains a high capacity retention of about 93.1% at 100 mA·g⁻¹ after cycling 100 times. In addition, ultralong cycle life at 1000 mA·g⁻¹ is excavated, which possesses a high charge capacity of 160.24 mAh·g⁻¹ with ~92.9% retained capacity after 2100 cycles. This work demonstrates a great impetus of utilizing KVPO₄F@C composite as an admirable anode for potassium-ion batteries and innovates a route for designing high-performance electrodes for other energy storage and conversion systems.