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Heteroatom doping can effectively improve the electrochemical activity of carbon materials. In this paper, three kinds of fluorine-doped biomass carbon are synthesized using industrial hemp core as the precursor and Poly tetra fluoroethylene (PTFE), Diethylaminosulphur trifluoride (DAST) and Hydrofluoric acid (HF) as fluorinating agents, respectively. Among them, the PTFE-treated biomass carbon (PTFE-CHEMP) is wrapped by fluorine-containing nanotubes, which has the most defects, the most F doping amount and the optimum pore size, and is beneficial to the adsorption of K+. When used as the anode of potassium ion batteries (PIBs), the PTFE-CHEMP electrode can provide an average reversible capacity of 369.6 mAh g−1 (200 mA g−1) in 500 cycles. Even at a high current of 2000 mA g−1, it can still provide an excellent rate capability of 229.3 mAh g−1. Kinetic analysis verify that its excellent rate performance comes from the K+ storage mechanism dominated by surface-driven behavior. In addition, the constant current intermittent titration technique (GITT) shows that it still has the storage capacity for K+ at high potentials. This work not only provides a reference for subsequent research on fluorine-doped biomass carbon, but also provides a potential solution for the production of low-cost, high-capacity PIBs anodes.

A unique nanostructure electrode consisting of RuO 2 nanoparticles with ultra-fine diameter (1.9 nm) anchored on the surface of graphene nanosheets (GNS) and carbon nanotube (CNT) is prepared as a binder-free supercapacitor electrode through two-step electrochemical routes. At first, free-standing GNS and CNT (GC) are directly deposited on the surface of carbon fiber cloth (CFC) to form a cross-linked composite via a cathodic electrophoretic deposition method. Safranin, a kind of cationic organic dye, is introduced in this stage as a new-type dispersant to disperse GNS and CNT in water to form a suspension solution. After the following electro-deposition process, the as-prepared GC composite is uniform covered with RuO 2 nanoparticles. Benefiting from the combined advantages of GNS, CNT and ultra-fine RuO 2 nanoparticles in such a binder-free structure, the hybrid electrode exhibits a high specific capacitance up to 480.3 F g −1 (based on the total mass of GNS, CNT and RuO 2 ) and remarkable cycling stability (89.4% capacitance retention after 10000 cycles). Furthermore, the assembled symmetric supercapacitor exhibits a high energy density of 30.9 Wh kg −1 and power density of 14000 W kg −1 with excellent stability performance (92.7% capacitance retention after 10000 cycles). Thus, the remarkable performance of the resultant RuO 2 electrode has provided a rational design strategy for developing supercapacitors with high energy density.

Carbon materials, especially graphene nanosheets (GNS) and/or multi-walled carbon nanotube (MWNT), have been widely used as electrode materials for supercapacitor due to their advantages of higher specific surface area and electronic conductivity, but the relatively low specific capacitance thus results in low energy density hindering their large applications. On the contrary, MnO2 exhibits higher energy density but poor electrical conductivity. In order to obtain high performance supercapacitor electrode, here, combining the advantages of these materials, we have designed a facile two-step strategy to prepare 3D MnO2-GNS-MWNT-Ni foam (MnO2-GM-Ni) electrode. First, GNS and MWNT is wrapped on the surface of Ni foam (GM-Ni) via a “dip & dry” method by using an organic dye as a co-dispersant. Then, by using this 3D GM-Ni as substrate, MnO2 nanoflakes are in-situ supporting on the surface of GNS and MWNT through a hydrothermal reaction. The specific capacitances of MnO2-GM-Ni electrode reach as high as 470.5 F g−1 at 1 A g−1. Furthermore, we have successfully fabricated an asymmetric supercapacitor with MnO2-GM-Ni and GM-Ni as the positive and negative electrodes, respectively. The MnO2-GM-Ni//GM-Ni asymmetric supercapacitor exhibits a maximum energy density of 35.3 Wh kg−1 at a power density of 426 W kg−1 and also a favorable cycling performance that 83.8% capacitance retention after 5000 cycles. These results show manageable and high-performance which offer promising future for practical applications.

Potassium-ion batteries (PIBs) have huge advantages in terms of price and resource abundance, and are ideal substitutes for lithium-ion batteries (LIBs), but there is currently little research on their high-temperature fields. In this work, the sulfur-doped biomass carbon (CHP/S) is synthesized from biomass hemp stalks as the carbon source and sulfur powder as the sulfur source, with a high sulfur content of 18.6%. Sulfur mainly exist in the form of bonded sulfur (S-C, S-O) and have quite favorable stability. As a PIBs anode, it exhibits a specific capacity of 589 mAh g−1 (30 mA g−1) at 60 °C, which is 1.5 times the capacity at 25 °C. Electrochemical analysis and kinetic analysis indicate that the increased capacity mainly comes from the “arousal effect” of sulfur under high temperature conditions, and an increased temperature has a positive effect on the absorption and desorption performance of the material. In addition, the full cell assembled with Perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) cathode shows greater capacity (~80 mAh g−1) at 60 °C than at 25 °C. Unfortunately, the electrolyte decomposition problem caused by high temperature is easily dangerous. The result is of great significance to the research of high-temperature PIBs.

As a mainstream energy storage device, lithium-ion batteries have always been the focus of scientific researchers. As the most promising anode material, lithium metal has the advantages of high capacity and low reduction potential. Unfortunately, lithium dendrites can cause serious safety hazards during cycling. In this study, biomass carbon with a large number of straight holes was embedded with nickel particles as the anode framework. Nickel induces lithium to complete nucleation and growth in the pores and inhibits dendrite growth. Solid electrolyte interface (SEI) film growth, dead lithium, and electrode breakage all occur within the holes to a limited extent. Benefited from this internal reaction, the electrode obtains better electrochemical and cycling performances. The nickel-modified electrode was stably cycled for 1370 h at a current density of 5 mA/cm2 with a capacity of 2 mAh/cm2. Moreover, more massive capacity lithium storage (5 mAh/cm2) was tried, and the cycle was stable for 630 h. Using the same biomass carbon matrix, the pores were filled with sulfur powder as the positive electrode. After 100 periods of the full cell operation, the capacity retention rate is 85%.

M1/3Ni1/3Mn2/3O2 (M = Mg or Zn) is sucessfully synthesize from Na2/3Ni1/3Mn2/3O2 via a electrochemical conversion method. The structure and morphology of the as-prepared material are systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS), respectively. The nano-sized morphology of M1/3Ni1/3Mn2/3O2 is beneficial for the reduction of diffusion length and transfer acceleration of cations from the surface into the bulk interiors. XRD patterns reveal that crystal structure is maintained after the reversible Mg2+ or Zn2+ intercalation/deintercalation. Its performance for Mg2+ and Zn2+ intercalation/deintercalation in aquesous electrolytes is evaluated by cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. Mg1/3Ni1/3Mn2/3O2 exhibits an initial capacity of 170.5 mAh g−1 in 1 mol L−1 Mg(NO3)2, Zn1/3Ni1/3Mn2/3O2 displays capacity of 113.7 mAh g−1 in 1 mol L−1 ZnSO4. Mg1/3Ni1/3Mn2/3O2 shows a good cycling performance and maintains 52.7 mAh g−1 at 1.0C after 200 cycles in 1.0 mol L−1 MgSO4.