Hard carbon is the most promising anode material for sodium‐ion batteries and potassium‐ion batteries owing to its high stability, widespread availability, low‐cost, and excellent performance. ...Understanding the carrier‐ion storage mechanism is a prerequisite for developing high‐performance electrode materials; however, the underlying ion storage mechanism in hard carbon has been a topic of debate because of its complex structure. Herein, it is demonstrated that the Li+‐, Na+‐, and K+‐ion storage mechanisms in hard carbon are based on the adsorption of ions on the surface of active sites (e.g., defects, edges, and residual heteroatoms) in the sloping voltage region, followed by intercalation into the graphitic layers in the low‐voltage plateau region. At a low current density of 3 mA g–1, the graphitic layers of hard carbon are unlocked to permit Li+‐ion intercalation, resulting in a plateau region in the lithium‐ion batteries. To gain insights into the ion storage mechanism, experimental observations including various ex situ techniques, a constant‐current constant‐voltage method, and diffusivity measurements are correlated with the theoretical estimation of changes in carbon structures and insertion voltages during ion insertion obtained using the density functional theory.
Li+, Na+, and K+ ions have identical storage mechanisms in hard carbon–adsorption followed by intercalation. The sloping voltage capacity is attributed to the adsorption of the carrier ions on defect sites, edge sites, and the surface of micropores, whereas the low‐voltage plateau capacity is caused by the intercalation of the carrier ions into graphitic layers.
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•A thick and stable CEI layer is formed by addition of additives in electrolyte.•Stable CEI layer could effectively suppress side reactions on electrode surface.•Longer cyclic life of ...SIBs could be obtained by a simple and cost-effective way.
Although Na0.67Fe0.5Mn0.5O2 has attracted tremendous attentions as a cathode material for sodium-ion batteries (NIBs), undesirable side reactions at the interphase between the electrode and electrolyte have limited its wide utilization. An effective way to prevent the side reaction is to artificially induce a mechanically robust and chemically stable cathode electrolyte interphase (CEI). In this paper, functional additives of NaF and Na2CO3 were used to artificially form a thick and stable CEI layer, and their effects were deeply investigated. It was demonstrated that the functional additives in electrolytes could be partially decomposed during a charge process, then it could play a key role in forming a thick and stable interphase directly on the cathode surface. The newly formed CEI layer could sup- press the dissolution of transition metals into the electrolyte and prevent the deterioration of the solid interphase layer. As a result, the additives successfully prevent the capacity fading problem of Na0.67Fe0.5Mn0.5O2 during electrochemical cycling, which results in the improved cyclability compared to the bare Na0.67Fe0.5Mn0.5O2. We believe that the addition of functional additive is a simple and cost-effective way to artificially form a stable CEI layer on a cathode, therefore, this approach is expected to be widely applicable to other electrode materials that suffer from the unstable interfaces.
Hard carbon is a promising anode material for sodium ion batteries (NIBs). In this study, a two-step carbonization approach is developed to enhance the electrochemical performance of lignocellulose ...biomass-derived hard carbon. The first step comprises slow low-temperature pyrolysis of fir wood that produces an amorphous carbon in which hexagonal planes are embedded in the amorphous carbon region to some extent. The second step comprises high-temperature carbonization at 1300 °C, which yields a hard carbon with a high degree of graphitization, an increased layer-plane length, and a low micropore volume. Two-step carbonized hard carbon delivers a large reversible capacity of 276 mAh g−1 at 50 mA g−1 after 100 cycles and high rate capacities of 108 mAh g−1 at 1.0 A g−1 and 76.3 mAh g−1 at 2.5 A g−1. The low-voltage plateau capacity below 0.1 V is 194 mAh g−1. The results of these experiments indicate that the exceptional electrochemical performance of two-step carbonized hard carbon arises from the effective suppression of micropore formation and a good balance between the degree of graphitization and number of defect sites. High-voltage adsorption of Na+ ions in micropores inhibits Na+-ion diffusion into the graphitic region of micropore-enriched hard carbon.
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•Two-step carbonization of wood was developed to enhance low-voltage plateau capacity.•Low-voltage plateau capacity below 0.1 V of 194 mAh g−1 were achieved.•Stable capacity of 276 mAh g−1 for 100 cycles was achieved as an anode in NIBs.•Na+ ion adsorption in micropores inhibited Na+-ion diffusion into graphitic region.
In this study, we use in-situ transmission electron microcopy (TEM) to investigate the thermal decomposition that occurs at the surface of charged LixNiyMnzCo1-y-zO2 (NMC) cathode materials of ...different composition (with y, z=0.8, 0.1 and 0.6, 0.2 and 0.4, 0.3), after they have been charged to their practical upper limit voltage (4.3V). By heating these materials inside the TEM, we are able to directly characterize near surface changes in both their electronic structure (using electron energy loss spectroscopy) and crystal structure and morphology (using electron diffraction and bright-field imaging). The most Ni-rich material (y, z = 0.8, 0.1) is found to be thermally unstable at significantly lower temperatures than the other compositions – this is manifested by changes in both the electronic structure and the onset of phase transitions at temperatures as low as 100°C. Electron energy loss spectroscopy indicates that the thermally induced reduction of Ni ions drives these changes, and that this is exacerbated by the presence of an additional redox reaction that occurs at 4.2V in the y, z = 0.8, 0.1 material. Exploration of individual particles shows that there are substantial variations in the onset temperatures and overall extent of these changes. Of the compositions studied, the composition of y, z = 0.6, 0.2 has the optimal combination of high energy density and reasonable thermal stability. The observations herein demonstrate that real time electron microscopy provide direct insight into the changes that occur in cathode materials with temperature, allowing optimization of different alloy concentrations to maximize overall performance.
Lithium-ion capacitors are considered highly promising as a hybrid-type energy storage system and are suitable for large-scale energy storage applications because of their superior power and energy ...density as well as prolonged cycle life. In this study, we developed an activated carbon (AC)-based electrode with excellent capacitive performance using salacca peel, a native Indonesian fruit, as the carbon precursor. The AC was synthesized via hydrothermal treatment of salacca peel with cerium (III) chloride (CeCl
3
) as the catalyst, followed by microwave-assisted chemical activation; the obtained sample was denoted as AC–S–CE. The addition of CeCl
3
during the hydrothermal carbonization facilitated the formation of micropores in the AC; this resulted in a considerably greater surface area (1264.4 m
2
g
–1
) and a more defective graphitic structure than that of AC synthesized in the absence of CeCl
3
(AC–S, 988.9 m
2
g
–1
) and of commercially available AC (742.8 m
2
g
–1
). In terms of being an LIC cathode, all the ACs exhibited a non-faradaic charge–discharge mechanism. AC–S–CE exhibited a higher capacitance of 90.6 F g
–1
at 0.05 A g
–1
and improved cycling performance compared with those of AC–S and commercially available AC.
Graphic abstract
Rechargeable lithium–oxygen (Li–O2) batteries have higher theoretical energy densities than today’s lithium-ion batteries and are consequently considered to be an attractive energy storage technology ...to enable long-range electric vehicles. The main constituents comprising a cathode of a lithium–oxygen (Li–O2) battery, such as carbon and binders, suffer from irreversible decomposition, leading to significant performance degradation. Here, carbon- and binder-free cathodes based on nonprecious metal oxides are designed and fabricated for Li–O2 batteries. A novel structure of the oxide-only cathode having a high porosity and a large surface area is proposed that consists of numerous one-dimensional nanoneedle arrays decorated with thin nanoflakes. These oxide-only cathodes with the tailored architecture show high specific capacities and remarkably reduced charge potentials (in comparison with a carbon-only cathode) as well as excellent cyclability (250 cycles).
A facile, template-free route using supercritical methanol for the preparation of hierarchical mesoporous Li4Ti5O12 spinel microspheres in a very short reaction time is introduced. Nanosized, primary ...Li4Ti5O12 particles (5–10 nm) are loosely aggregated and form micron-sized, secondary mesoporous spheres (0.2–2.0 μm) with a pore size of 2–10 nm. Subsequent calcination of the as-synthesized Li4Ti5O12 at a low temperature of 600 °C results in excellent long-term cyclability and high rate performance. The discharge capacity after 400 cycles at 1 C is 134.9 mAh g−1 (77.6% of the initial discharge capacity), and the discharge capacity at 10 C is 108.5 mAh g−1. The formation mechanism of hierarchical mesoporous microspheres in supercritical methanol is discussed.
► Hierarchical mesoporous Li4Ti5O12 microsphere synthesized in supercritical methanol. ► Fast synthesis with no template or surfactant used. ► Li4Ti5O12 shows excellent long-term cyclability and high-rate capability. ► Much better performance than that from supercritical water and solid-state methods.