The hemp stems are pretreated to obtain the hemp stems powder as shown in Fig. 1a, and then carbonized to obtain the carbide as shown in Fig. 1b. As shown in Fig. 1c, d, the morphology of the UAC and AC-600 sample was characterized by SEM. Both the samples are amorphous carbon overall, no obvious macropore is observed. The role of the activator ZnCl₂ is to promote pore formation and dissolve tar and other by-products [28]. The image also indicates that AC-600 is a complex of a large number of sheet-like structures and slit-like interspace, which will provide more active sites. Figure 2a, b shows TEM patterns of UAC and AC-600. Compared with UAC, AC has more obvious pores than UAC, results in providing more active sites and thus increasing the specific capacity of the batteries. Figure 2c, d depicts high-resolution TEM spectra of UAC and AC-600. It can be seen that UAC has pores at high magnification and is primarily microporous. Compared to UAC, AC-600 has more pores and larger pore sizes, indicating that the material has an excellent activation effect. In general, the porosity of AC is attributed to the natural internal porous structure of the hemp stems and the good activation effect of the activator.

The X-ray diffraction patterns of UAC and AC are shown in Fig. 3a. A broad diffraction peak around 22° corresponds to the (002) reflection of the graphite structure, which distributed to the presence of continuous parallel graphite sheets in the material. The relatively weak peak at 44° corresponding to the crystal plane (100) is considered honeycomb structures formed by sp2 hybridization [30, 31]. Besides, no sharp peaks were observed on these two diffraction peaks, indicating that both samples exhibit the out-of-order structure of disordered carbon material.

The Raman spectra of AC and UAC are shown in Fig. 3b. The D-band represents the disordered carbon layer structure and defects in the carbon material, and the G-band signifies the vibration of sp2 hybridized carbon atoms in the graphite sheet structure. Usually, I_D/I_G is used to indicate the disorder degree of carbon. The I_D/I_G of two carbon materials is 1.15 and 1.17, indicating that both have high amorphousness, more edges, and other defects. These features will provide more active sites for the insertion of lithium ions, which are of great benefit to improving the reversible capacity of the electrodes.

The results of the surface area and pore size distribution of AC are shown in Fig. 4. The isotherm can be expressed as type I, indicating that the carbon material has plenty of micropores. The closed hysteresis loop of the adsorption-desorption isotherm can be classified as H4 type, indicating the presence of slit-like pores, which are formed by the accumulation of material debris particles. It delivers an excellent specific surface area that BET value is 589.54 m²/g. The pore size of AC is mainly distributed in the range of micropores that refers to pores of smaller than 2 nm, which is consistent with the results of the N₂ adsorption-desorption isotherm. The pore volume and average pore diameter of AC were 0.332 cm³/g and 2.250 nm, respectively. There are not only many micropores, but also mesopores in the material, providing more active sites, and facilitating the cycling insertion and extraction of lithium ions. The transfer speed of ions is improved, and the impedance of batteries reduces [13].

In order to investigate the electrochemical behavior of the porous material, the material was analyzed by cycle stability performance, rate performance, impedance, and cyclic voltammetry (CV) tested for the anode of lithium-ion batteries.

Figure 5a shows that the charge-discharge cycle performance of activated carbon by different activation temperatures at a rate of 0.2 °C, in which the blue line corresponds to the Coulombic efficiency of AC-600. It delivers a distinguished capability clearly that the specific capacity of AC-600 is 495.4 mAh/g, which is much higher than the graphite theoretical capacity. The first discharge specific capacity and charge specific capacity are 2469.7 mAh/g and 1168.1 mAh/g, respectively. The first cycle has poor coulomb efficiency (only about 36%), which is consistent with the common characteristics of lithium-ion batteries cycle performance [15, 20]. The huge capacitance loss of the first cycle is attributed to the irreversible consumption of a large amount of lithium ions by the solid electrolyte interface (SEI) film forming on the electrode surface due to the large specific surface area. Its CE is around 100%, which denotes that AC-600 has a small capacity loss rate. The charge and discharge curves of the first cycle to the 100th cycle of UAC and AC-600 are shown in Fig. 5b, c. Both the charge capacity and the discharge capacity are gradually stabilized with the increase of the number of cycles. It can be found that the coincidence state of 50th and 100th charge-discharge profiles are perfectly impressive, indicating that the material has good stability in cycle performance.

The rate discharge performance of the as-prepared materials at current densities of 0.2 C–5 °C is shown in Fig. 5d. The AC-600 exhibits good rate ability with average discharge capacities of 522.6 mAh/g, 295.6 mAh/g, 205.4 mAh/g, 142.9 mAh/g, and 65.2 mAh/g at current densities of 0.2 °C, 0.5 °C, 1 °C, 2 °C, and 5 °C, separately. The initial performance of the AC-600 is higher, and the capacity drops significantly at larger magnifications, but when the discharge rate is restored to 0.2 °C, the performance of AC-600 can still be restored to a higher reversible capacity of 416.3 mAh/g. Conversely, the initial capacity of UAC is lower, but the capacity decreases less at large rates. The UAC exhibits average discharge capacities of 313.3 mAh/g, 255.7 mAh/g, 227.1 mAh/g, 209.2 mAh/g, 181.7 mAh/g, and 323.5 mAh/g at same current densities as AC-600. Although it has a lower specific capacity than AC-600, it exhibits good capacity retention. This phenomenon can be attributed to the large specific surface area of AC-600 caused by the activation process, so that the specific surface area in contact with lithium ions increases. As the electrochemical cycle progress proceeds, large side reactions consume a large amount of lithium ions and are irreversible, resulting in a decrease in capacity.

In order to further confirm the origin of the good performance of AC-600 and also to identify the possible reasons for performance fading, the TEM spectrum of the spent electrode material after cycling was measured. As shown in Fig. 6, partial surface of the AC-600 is actually broken after cycling, exposing the internal porous structure. This may be attributed to the excessive activation effect that occurs on the surface of the carbon material. Partial surface damage and SEI re-formation occur during cyclic insertion-extraction of lithium ions.

The impedance spectrum of the samples was tested to reveal the kinetics of the electrodes during ion transport, as shown in Fig. 7a, b. The high-frequency semicircle corresponds to the contact resistance. The semicircle of the intermediate frequency region is attributed to the charge transfer impedance at the electrode/electrolyte interface. The oblique line at an angle of about 45° to the real axis corresponds to the lithium-ion diffusion process in the carbon electrode [32]. No obvious semicircle is observed in the impedance spectra of UAC due to the large resistance of UAC. Conversely, the impedance map of AC-600 exhibits a relatively obvious semicircle. This is attributed to the large pore distribution inside the activated sample, which promotes the transport of lithium ions and accelerates the timely embedding and escaping of ions in the anode material. The initial 3 cycles of cyclic voltampere (CV) curves at a scan rate of 0.1 mV/s between 0.01 and 3.0 V are displayed in Fig. 7c, d. In the reduction process of the first circle, there is a sharp peak around 0.7 V and a weak peak around 1.35 V. For two samples, the cathodic peak at 1.35 V indicated that an irreversible reaction has begun between electrode and electrolyte [18]. The peak around 0.7 V is due to the decomposition of the electrolyte on the electrode surface and the formation of the solid electrolyte interface (SEI) film. These peaks disappeared in the subsequent second and third cycles, indicating that the above reactions in the first cycle are irreversible. In the first cycle, the lithium deintercalation process occurs at anodic peak around 0.25 V, which is consistent with many reported carbon substances [8, 18]. The difference is that the lithium deintercalation process of UAC is faster at low corresponding voltages while the reaction of AC-600 is flatter at the whole process. In the case excluding that UAC is hardly a mesoporous or macroporous structure, it can be reasonably concluded that the surface pores of the UAC are more combined with lithium ions, resulting in faster lithium removal of UAC during charging. Both AC-600 and UAC have a tendency to gradually coincide with the subsequent second and third cycles, and the second and third circles are substantially completely coincident in the figure, indicating that the electrode material has good stability.