Search

Many flexible electrode materials with good electrochemical properties show poor sustainability due to the complex stress field inside the electrodes. Here, an analytical model is established to describe the ratcheting behaviors of the flexible electrode during electrochemical cycle with the presence of bending deformation, and is verified by finite element simulation. Under the impact of bending stress, the evolution of plastic strain during cyclic deformation is discussed and the effects of material softening due to the diffusion of Li-ions are considered. It is found that the electrode material accumulates a certain amount of plastic strain during each electrochemical cycle even under small bending deformation. Therefore, the flexible electrode material continuously accumulates plastic strain during cycling and becomes ineffective after several cycle times. Further, a strain hardening model is developed to discuss the possibility of mitigating ratcheting behavior. The increased yield strength in the cycling is found to be effective in suppressing the accumulation of plastic strain. The present work gives the new explanation for the poor sustainability of flexible batteries and provides a theoretical basis for further improvement.

Six samples of α-MnO2 with different K ion contents are synthesized by using KMnO4 and Mn(CH3COO)2·4H2O with different pH values at room temperature. The samples are labeled as Mx where x denotes the pH value. The anomalous effect of K ion content on the electrochemical capacitance properties of amorphous MnO2 is investigated systematically. It is found that with the increase of the K ion content, the MnO2 nanospheres form a more dense aggregates, the surface area dramatically decreases from 273 (M2) to 14 m2 g−1 (M12), and pore volume decreases from 0.349 (M2) to 0.014 cm3 g−1 (M12). The specific capacitance (SC) is decreased by 17.6% from 157 (M2) to 131 F g−1 (M12). In addition, the SC values are greatly influenced by the molar ratio of K to Mn in the range of 0–0.4. And it is found that part of K ions in the MnO2 tunnels can be replaced by Na ions during electrochemical reaction. Finally, an appropriate explanation of the effect of K ion content on the SC values is provided.

A small amount of cations such as Li + , Na + , K + , or NH 4 + is required to stabilize the tunnels during the formation of MnO 2 . Here, the effect of the content of K ion on crystalline state and capacitance behavior of MnO 2 are investigated. It is found that with the content of K ion increasing, manganese dioxide is more likely to maintain amorphous, while adding Li + , Na + , or NH 4 + has no such effect. Adding K ion not only delays the conversion of manganese dioxide crystalline, but also delays the phase transformation temperature, which causes the big difference of specific capacitance value from 58 F g −1 for H2/400 to 124 F g −1 for K12/400. Furthermore, the morphology, special surface area, and pore data change dramatically from H2/400 to K12/400. These results have important implications for the characterization of MnO 2 in supercapacitor applications.

The co-existence of two mechanisms, the surface chemisorption mechanism (SCM) and the tunnel storage mechanism (TSM), for the charge storage of MnO 2 is investigated. Sample α-MnO 2 whose tunnels (2 × 2) are almost vacant is synthesized by chemical co-precipitation technique. To verify whether the charge storage of as-prepared α-MnO 2 involves the charges based on the SCM besides the TSM, Ba 2+ and K + are intercalated into the tunnels of α-MnO 2 by refluxing and samples Ba-MnO 2 and K-MnO 2 are obtained, respectively. By powder X-ray diffraction, ICP and the extraction test, it is confirmed that Ba 2+ and K + occupy the tunnels and are difficult to remove. By electrochemical analyses, we have found that the charge storage of Ba-MnO 2 and K-MnO 2 basically depends on the SCM, as Ba 2+ and K + stabilized in the tunnels in advance block the intercalation/deintercalation of cations from the electrolyte. Thus, it can be inferred that for α-MnO 2 two mechanisms contribute to the charge storage. The contributions of α-MnO 2 's mechanisms are estimated. The charges based on the TSM predominantly account for 81.2–89.9% of the total charges. The charges on the SCM may contribute to 7.9–16.8% and are not negligible. The charges from double-layer capacitance may be negligible, since it is only 1.1–4.1%.

Effect of the electrolyte particularities on manganese dioxide in the neutral electrolytes containing alkaline (Li+, Na+, or K+) and alkaline-earth (Mg2+, Ca2+, or Ba2+) metal cations for electrochemical capacitor is investigated by cyclic voltammetry and electrochemical impedance spectroscopy. A very high capacitance of 357.9 F g−1 is measured when Ca2+ ion is used as electrolyte cation. The results show that cations of the electrolyte instead of proton or anions are responsible for the pseudo-capacitive behavior of manganese dioxide. The capacitance of manganese dioxide is found to depend strongly on the electrolyte particularities, for example, pH value, cation and anion species, and salt concentrations. A logarithmic dependence of capacitance of MnO2 on cation activity is obtained for all alkaline and alkaline-earth metal cations. The capacitance of MnO2 can be doubled by replacing univalent cation with bivalent cation at a close cation activity.

Manganese dioxide has been synthesized by a new simple self-reacting microemulsion method. The synthesized MnO 2 has been found to be amorphous structure containing a moderate amount of water by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Particles in a spherical shape with about 4 nm in diameter have been observed by transmission electron microscopy. Cyclic voltammetic tests have been performed between −0.5 and 0.5 V versus Hg/Hg 2 SO 4 in 1 mol L −1 Na 2 SO 4 solution at sweep rates up to 50 mV s −1 . A specific capacitance value as high as 246.2 F g −1 was obtained, which was much higher than 146.5 F g −1 of MnO 2 prepared by chemical co-precipitation. After 600 cycles, only 6% decrease of specific capacitance was measured which indicated that such a material possesses good cycling property.