Due to unique structure advantages, the N-HPC samples were tested as anode for K storage.
Figure 3a shows the CV curves for the N-HPC-1.5 between 0.01 − 3.0 V (
vs. K/K
+) at 0.1 mV s
‒1. A wide peak at 0.638 V is shown in the 1
st discharge cycle but disappears afterwards, arising from the resolving of the electrolyte as well as the construction of the solid electrolyte interphase (SEI) layers [
16]. The broad anodic peak at 0.475 V together with the sharp cathodic peak at 0.02 V is attributed to the intercalation/deintercalation of potassium ions. The nearly overlapping of CV curves in the 2
nd and 3
rd cycles, suggesting the good reversibility of the K
+ storage process. In the first CV cycle of N-HPC-0.5, the reaction peak forming the SEI layer is not clear (Fig. S6a). Due to the existence of ZnO in N-HPC-2.5, the CV curve (Fig. S6b) shows some more impurity peaks.
Figure 3b displays the first three galvanostatic charge-discharge (GCD) profiles of the N-HPC-1.5 at 0.05 A g
‒1. The corresponding initial Columbic efficiency (ICE) is determined to be 20.1% based on the primary charge and discharge capacities (215.9 and 1028.1 mAh g
‒1). The ICE could be attributed to irreversible electrolyte decomposition and the formation of the SEI films. Compared with the other two samples, N-HPC-1.5 has a higher initial capacity and reversible capacity (Fig. S6c, d). As shown in
Fig. 3c, the N-HPC-1.5 anode displays superior capacity retentions at various current densities. The N-HPC-1.5 anode delivers the discharge capacity of 236.4, 217.4, 195.4, 164.5, 141.5, and 117.6 mAh g
‒1 at current densities of 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 A g
‒1, respectively, which outperform those of N-HPC-0.5 and N-HPC-2.5 electrode. Surprisingly, the capacity is recovered to 250.1 mAh g
‒1 when the current density goes back again to 0.05 A g
‒1. The excellent capacity of N-HPC-1.5 is due to the multi-effects of 3D hierarchical porous architecture, large layer interval, and high nitrogen doping content. Such rate performance is also comparable to many other carbonaceous anodes reported in literatures (
Fig. 3d), such as activated carbon-2 (AC-2) [
32], self-supporting sub-micro carbon fibers wrapped in carbon nanotubes (SMCF@CNTs) [
33], nitrogen-doped carbon microsphere (NCS) [
34], polynanocrystalline graphite (PG) [
35], hierarchical porous carbon (HPC) [
17], and oak-based hard-carbon (OHC1100) [
36].
Figure 3e displays the outstanding long-term cycling stability of the N-HPC anode. Amazingly, the N-HPC-1.5 anode shows a lifespan over 800 cycles with a reversible capacity of 105.0 mAh g
‒1 at 1.0 A g
‒1 and a capacity retention rate of ~ 77.3%, resulting in an extremely low decay rate of only 0.03% per cycle.