The synthesis method of the PPDC samples used in this study is as follows. First, phosphoric acid (10 mL, 87%) was diluted into a 10% concentration phosphoric acid solution. The flour was dissolved in 50 mL, 10% phosphoric acid solution to form a homogeneous gel-like substance. The flour was purchased from a ordinary vegetable market. Then, the gel was reacted in a reactor at 180°C for 12 h. The obtained samples after the reaction were dried and heated to 600, 700, and 800°Cunder argon flow at a heating rate of 3°C min⁻¹ for 3 h. Finally, the calcined samples were washed with 10% HCl aqueous solution, filtered, and washed with distilled water several times until neutral; the porous carbon with different heating temperatures were dried at 60°C for 12 h and marked as PPDC-600, PPDC- 700 and PPDC-800. Using cheap flour as raw material, PPDC with hierarchical porous structure and phosphorus element doping was designed as an anode material for k-ion half-cells. The high specific surface area, hierarchical porous structure, and phosphorus doping of PPDC materials can facilitate ion/electron transport during charge and discharge.

Raman spectra were tested with a 488 nm laser (Jobin-Yvon Lab RAM HR-800) and thermal gravimetric analysis (TGA) was performed with a TG-209F1. N₂ adsorption/desorption isotherm was carried out specific surface area and porosity distribution measurement with an Autosorb IQ Gas Sorption System at 77 K. The morphologies and chemical structures of the carbonaceous material were characterized by SEM (Zeiss SIGMA).

Active materials, conductive carbon and carboxymethyl cellulose with mass ratio of 8:1:1 were dispersed in a mixed solution (1 mL) of ethanol and H₂O, and ball-milled for 30 min. Then, the obtained slurries were painted on the Cu foil and dried at vacuum environment at 60°C for 12 h. The working electrode with an average mass loading of each electrode about 0.9 mg cm⁻² is a disc with diameter of 12 mm. Potassium metal and glass fiber film were used as the anode electrode and the separator, respectively. The 5 M KFSI dissolved in ethylene carbonate/dimethyl carbonate mixture (EC/DMC by 1:1 vol.) was used as electrolyte. The coin-type cells (2032) were assembled in a MB-Labstar (1,200/780) glove box (Munich, Germany) under Ar atmosphere. The concentrations of moisture and oxygen were maintained below 0.5 ppm. The CT2001A battery test system (LANDTE Co., China) and a CHI660E electrochemical station (CHI instrument Co., Shanghai, China) were used to test the electrochemical performance.

The micromorphology of obtained PPDC-600, PPDC-700 and PPDC-800 were investigated by scanning electron microscope (SEM) images and SEM-mapping (Figure 1). As shown in Figures 1A,B, the PPDC-600 exhibited typical porous structure with rough surface and conductive network. This structure beneficial to achieve fast ion/electron transportation path during charge and discharge process. Figure 1C is element mapping of C, O and P for PPDC-600. Obviously, the C, O and P element evenly distribute in the carbon skeleton. Figure 1 d displays the element content statistics of C, O and P in PPDC-600. What is noteworthy is that the P doping is conductive to provide more active sites for K⁺ storage and further enhance the electrical conductivity, thus offering excellent battery performance. Figure 1E-l demonstrate similar porous structure and uniform distribution of C, O and P in PPDC-600, PPDC-700 and PPDC-800. The difference is the element contents of C, O and P due to different calcination temperatures.

Raman spectroscopy measurements were used to characterize the structures of PPDC-600, PPDC-700, PPDC-800 (Figures 2A–C). Apparently, two obvious Raman peaks for the three samples appearing at about 1,350 cm⁻¹ and 1,600 cm⁻¹ are assigned to the D band of amorphous carbon and G band of graphitic carbon, respectively. Obviously, the values of I_D:I_G ratio of the PPDC gradually decrease as the temperature rises, indicating the increasing degree of graphitization, the reductive ordered carbon atoms and active sites (Zeng et al., 2014; Yao et al., 2021; Zhou et al., 2021). All in all, temperature is a double-edged sword. The PPDC-700 were prepared at an appropriate temperature of 700°C, thus displayed better K+ storage performance than PPDC-600 and PPDC-800. Nitrogen adsorption and desorption isotherms and pore size distribution were analyzed to investigate the microporous and mesoporous structures of the PPDC. As displayed in Figures 2D–F, specific surface areas of the PPDC-600, PPDC-700, PPDC-800 are calculated to be 765, 796, and 806 m² g⁻¹ Figures 2G–I exhibit that the pores mainly locates between 2 nm and 10 nm, indicating the existence of mesoporous. The large specific surface area and hierarchically porous structure are beneficial to effectively accommodate potassium ion storage and alleviate volume expansion. (Song et al., 2017; Zhang et al., 2019b; Zhou et al., 2020).

The electrochemical performance of as-prepared three samples (PPDC-600, PPDC-700 and PPDC-800) were investigated to explore the influence of carbonization temperature. Figure 3A and Supplementary Figure S1 are the charge–discharge voltage profiles for selected cycles of PPDC anodes at a current density 100 mA g⁻¹, demonstrating similar battery behavior of the PPDC-600, PPDC-700 and PPDC-800 electrodes. As shown in Figure 3B and Supplementary Figure S2, the cyclic performances of PPDC electrodes were further tested at 100 mA g⁻¹. The Coulombic efficiency experiences a jitter at about 18th cycle, and dive in the last cycle, due to sudden changes in ambient temperature. The PPDC-700 delivers a initial discharge capacities of 565 mA h g⁻¹ and charge capacities of 376 mA h g⁻¹ and maintains a higher reversible capacity of 310 mA h g⁻¹ after continuous 50 cycles, indicating that the PPDC-700 displays higher reversible capacity and better cyclic stability. The initial large discharge capacity phenomenon is attributed to the SEI formed on the anode. The higher reversible capacity of the PPDC-700 was also proved by the larger peak area of the PPDC-700 than that of the PPDC-600 and PPDC-800 at 1 mV s⁻¹ (Supplementary Figure S3). Figure 3C presents that the essentially unchanged shape of the charge/discharge curves of the PPDC-700 anode at increased current densities, suggesting the prominent reaction kinetics of K-ions intercalation/deintercalation. Figure 3D describes the rate performances of the PPDC-600, PPDC-700 and PPDC-800 at different current densities form 100 mA g⁻¹ to 2 A g^-1. The PPDC-700 electrode shows the higher rate capacities of 377、321、248、198、182、136、93 mA h g⁻¹ at 0.1, 0.2, 0.3,0.4, 0.5, 1.0, and 2.0 A g⁻¹, respectively. The high invertible capacity of 377 mA h g⁻¹ is still obtained when the current density returns to 100 mA g⁻¹ Figure 3C exhibits the long-cycle of PPDC-700 electrode at 1,000 mA g⁻¹. After 1,000 cycles, the PPDC-700 always keeps a specific capacity as high as 197 mA h g⁻¹ while maintaining close to 100% Coulombic efficiency. It is proved that the hierarchically porous structure, and appropriate graphitization degree and P-doping content of the PPDC-700 electrode provide fast ion/electron transportation and effectively relieve the mechanical stress during the cycle to enhance the battery performance.

CV curves was measured at scan rates of 0.1–10 mV s⁻¹ in a voltage range from 0.01 to 3 V to analyze the kinetic mechanism of the PPDC-600, PPDC-700 and PPDC-800 electrodes (Figure 4). Figures 4A–C display similar the changing trend of peak shape for the PPDC-600, PPDC-700 and PPDC-800 electrodes. Figures 4D–F manifest the separation of the capacitive contribution (inner region) from the total capacity (outer region). The capacitive capacities of the PPDC-600, PPDC-700 and PPDC-800 electrodes account for 39.5%, 48.7% and 50.1% at a low scan rate of 1 mV s⁻¹, indicating the charge storage behavior is dominated by the ionic diffusion and surface reaction process. As presented in Figures 4G–I, with the scan rate rising to 0.1, 0.5, 1, 2, 5 and 10 mV s⁻¹, the fraction of capacitive capacity for the PPDC-700 increases to 31.8%, 39.9%, 48.7%, 52.5%, 77.1% and 98.9%, respectively. As for the PPDC-600, the fraction of capacitive capacity increases to 38.4%, 45.6%, 52.4%, 59.7%, 75.6%, and 96.8%, respectively. For the PPDC-800, the fraction of capacitive capacity increases to 35.8%, 44.4%, 50.1%, 57.2%, 73.9% and 96.4%, respectively (Supplementary Figure S4). This phenomenon further confirmed that PPDC-700 anode for PIBs with superior electrochemical reaction kinetics, thus provides higher reversible capacity, better cyclic stability and rate performance.

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