Microporous pillared carbon prepared from graphite oxide silylated with methyltrichlorosilane for three times was tested for the electrode of electric double layer capacitor. The capacitance gradually increased as the applied potential increased and reached 74 F/g at an initial stage of cycling in 1 M triethylmethylammonium tetrafuluoroborate (TEMABF4) -propylene carbonate (PC) when it is used as a positive electrode, independent of the counter cations or existence of solvent molecules. This value indicated that two BF4− ions were accommodated in a micropore of it. On the other hand, only one ion per micropore was stored when larger ions such as TEMA+, bis(trifluoromethanesulfone) imide (TFSI−) and ethylmethylimmidazorium (EMI+) ions were used. These results indicated that two larger ions can not be accommodated in a micropore of pillared carbon.
Thermal pore stability of activated carbon materials and float charging durability of Li-ion capacitors (LICs) using heated activated carbons were evaluated. Silicon-carbide-derived carbon (SiC-CDC) was prepared via a chlorination/steam activation process, and its properties were compared to commercial activated carbon. After heat-treating both samples at 1400°C, the surface area of SiC-CDC decreased by only 10%, whereas that of the commercial activated carbon declined by 20%. The heat-treated SiC-CDC exhibited excellent durability as an LIC positive electrode. These results provide insights into the effect of preparation methods on the thermal stability and durability of carbon-based electrodes as LIC positive electrode.
The hybrid capacitor with an activated carbon positive electrode, a carbon negative electrode, and 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) electrolyte was constructed and investigated as a high-performance hybrid capacitor. Three different carbon materials, Ketjen Black (KB), acetylene black, and graphite, were employed as negative electrode material. The insertion/extraction reaction of BMI+ cations into the carbon negative electrodes was suggested from the strength change of X-ray diffraction peaks. The KB negative electrode showed the highest charge-discharge cycle performance due to the multiple effect of the large surface area and small graphitic crystals, which is flexible to stand for the large volumetric expansion with BMI+ cation insertion reaction.
We report the results of spatially-resolved non-destructive operando electrode reaction analysis for practical cylindrical 18650 battery cells by using a high-energy confocal X-ray diffraction (XRD). A combination of high-energy X-rays (72 keV) and a confocal XRD method, which extracts structural information in a limited area that satisfies a confocal condition, allows us to observe electrode reactions in a cylindrical battery cell in a non-destructive way, resolving the double-side-coating electrode structure.
We observed that electrode reactions were faster in the outer-part electrode than in the inner-part at the initial state reflecting intrinsic cell structure (position of current tab). For a battery cell deteriorated after 500 charge/discharge cycles, in contrast, electrode reactions were faster in the inner-part electrode than in the outer-part, suggesting that the outer-part is more deteriorated than the inner part. The results of characterization of disassembled electrodes show that the observed slow response of the outer-electrode of a 500-cycled cell is attributed to various factors increasing resistance such as cracks in cathode particles, formation of insulating surface oxide-phase, and anomalous growth of solid electrolyte interphases (SEIs). As shown here, the high-energy confocal XRD is effective for non-destructive analysis of electrode reactions.
For the development of lithium-ion batteries (LIBs) for electric vehicles, the reduction in the internal resistance of LIBs is strongly required. On the graphite negative electrodes, solid electrolyte interphase (SEI) is inevitably formed and causes partly the internal resistances. In addition, SEI covers the surface of graphite negative electrodes and affects the active sites for lithium-ion intercalation/deintercalation reactions. In this study, we investigated the influence of SEIs derived from vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene carbonate (EC) on the active sites for lithium-ion intercalation/deintercalation at highly oriented pyrolytic graphite (HOPG). We clarified the relation between the standard rate constant (k0) of [Ru(NH3)6]3+/2+ that is a parameter for the edge site of graphite and the interfacial lithium-ion transfer resistance (Rct) in various electrolyte solutions that deliver different SEIs. In the plots of k0 vs. Rct−1, there is a positive linear correlation between these two parameters, and the slopes increased in the order VC < EC < FEC. Additionally, the activation energy for the interfacial lithium-ion transfer remained unchanged despite the variation in SEIs. Based on these results, we conclude that SEIs affected the frequency factor of Arrhenius equation for the interfacial lithium-ion transfer on graphite.
Tin-nickel (Sn-Ni) alloy is a promising candidate as an anode for the lithium-ion capacitor (LIC) because it is superior in volumetric energy density compared with that of the graphite anode. However, its cycle durability requires improvement, even with a higher utilization ratio of the anode. The effect of lithium salts, LiPF6 and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is investigated for usage in the LIC in severe conditions (utilization ratio of the anode: 20%). The LIC with LiTFSI delivered its initial capacity up to ∼400 cycles, which is 4 times longer than the LIC with LiPF6. The reason for the capacity decay in the LiPF6 system is attributed to the narrowing of the potential range of the activated carbon cathode due to a widening potential range of the Sn-Ni alloy anode during operation. This widening is attributed to the loss of the active material due to peeling-off from the substrate. However, when LiTFSI is used, no such decay is observed. It is suggested that a polymer-like solid electrolyte interphase derived from TFSI− may suppress the loss of the active material. This finding can encourage the development of an Sn-based anode for LICs in combination with a mild operating condition and electrolyte additives.
To prolong durability of lithium-ion batteries, stability of solid electrolyte interphase (SEI) formed at a graphite negative electrode should be improved, but the correlation of the SEI stability with the graphite structure is still unclear. This study focused on co-intercalation of dimethoxyethane (DME) into SEI-covered graphitized carbon nanosphere (GCNS) to investigate SEI degradation behavior. In situ Raman spectroscopy revealed that both ethylene carbonate (EC)-derived and propylene carbonate (PC)-derived SEIs partly passivated the DME co-intercalation, but the PC-derived SEI degraded more rapidly than the EC-derived one. Additionally, the SEI at GCNS heat-treated at 2900°C had less stability than that at GCNS heat-treated at 2600°C, which is attributable to the graphite layer stacking and surface morphology.