Fast Ionic Diffusion-Enabled Nanoflake Electrode by Spontaneous Electrochemical Pre-Intercalation for High-Performance Supercapacitor

Abstract

Layered intercalation compounds NaxMnO2 (x = 0.7 and 0.91) nanoflakes have been prepared directly through wet electrochemical process with Na+ ions intercalated into MnO2 interlayers spontaneously. The as-prepared NaxMnO2 nanoflake based supercapacitors exhibit faster ionic diffusion with enhanced redox peaks, tenfold-higher energy densities up to 110 Wh·kg−1 and higher capacitances over 1000 F·g−1 in aqueous sodium system compared with traditional MnO2 supercapacitors. Due to the free-standing electrode structure and suitable crystal structure, NaxMnO2 nanoflake electrodes also maintain outstanding electrochemical stability with capacitance retention up to 99.9% after 1000 cycles. Besides, pre-intercalation effect is further studied to explain this enhanced electrochemical performance. This study indicates that the suitable pre-intercalation is effective to improve the diffusion of electrolyte cations and other electrochemical performance for layered oxides, and suggests that the as-obtained nanoflakes are promising materials to achieve the hybridization of both high energy and power density for advanced supercapacitors.

Biologically derived melanin electrodes in aqueous sodium-ion energy storage devices

“…These data confirm that the full cell system is anode-limited (79). Full cells composed of λ-MnO 2 cathodes and Na + -loaded melanin anodes exhibit a specific capacity of 7-16 mAhg −1 (normalized by anode mass) over a potential range of 1.0 V. These specific capacities are comparable to other more exotic anode materials used previously in sodium-ion charge storage materials, and are significantly lower than the best-performing materials studied for traditional battery applications (29,80). However, the envisioned biomedical applications that will be enabled by melanin-based energy storage materials have modest specific capacity requirements that are achievable with the current demonstration.…”

“…Furthermore, organic electrodes can be prepared using biologically derived materials or biomass toward the goal of achieving sustainable energy storage material production (1,20). Carbonization of naturally derived materials can produce highly porous materials that exhibit suitable performance for use in primary batteries and supercapacitors (28)(29)(30). Anodes have been fabricated using biopolymers including polysaccharides, polypeptides, and cellulosic derivatives (31,32).…”

Charge Storage in Cation Incorporated α-MnO2

“…The favorable incorporation of larger cations into the 2×2 tunnels of α-MnO 2 is corroborated by the natural existence of cryptomelane (KMn 8 O 16 ) and the spontaneous incorporation of Na + into α-MnO 2 as observed experimentally. 36 These interstitial cations also all induce charge-switching transitions that occur within the SPW, and should therefore all contribute to charge storage in α-MnO 2 .…”

“…The interstitial and substitutional mechanisms we identify as leading to charge storage are supported by the large observed capacity of cryptomelane (KMn 8 O 16 ) 39,40 and the enhanced capacity of α-MnO 2 when pre-intercalated with large amounts of Na + . 36 In the limit of non-interacting defects and thin-film α-MnO 2 , the interstitial cation mechanism shown in Figure 4a will result in 1.5 electrons transferred per Mn-center (See SI Section H), with a small additional contribution from the substitutional cation mechanism, also shown in Figure 4a. This accounts for the 1.1 electrons per Mn center observed experimentally for thin-film α-MnO 2 .…”

Sodium Charge Storage in Thin Films of MnO2Derived by Electrochemical Oxidation of MnO Atomic Layer Deposition Films

“…The CV peak that appears at ∼0.5 V versus Ag/AgCl upon oxidation beyond 2e − /Mn agrees with previous measurements of manganese oxide preintercalated with sodium. 41 This CV peak corresponds to the charge storage of cations in Mn vacancies. 27 The appearance of this peak suggests that further oxidation leads to Mn vacancies that contribute additional capacity.…”