Li2SO4 Electrolyte Research Articles

Unrivaled Combination of Surface Area and Pore Volume in Micelle-Templated Carbon for Supercapacitor Energy Storage

We created Immense Surface Area Carbons (ISACs) by a novel heat treatment that stabilized the micelle structure in a biological based precursor prior to high temperature combined activation – pyrolysis. While displaying the morphology akin to commercial activated carbon, ISACs contain an unparalleled combination of electrochemically active surface area and pore volume (up to 4051 m2 g-1, total pore volume 2.60 cm3 g-1, 76% small mesopores). The carbons also possess the benefit of being quite pure (combined O and N: 2.6 – 4.1 at.%), thus allowing for a capacitive response that is primarily EDLC. Tested at commercial mass loadings (~ 10 mg cm-2) ISACs demonstrate exceptional specific capacitance values throughout the entire relevant current density regime, with superior rate capability primarily due to the large fraction of mesopores. In the optimized ISAC, the specific capacitance (Cg) is 540 F g-1 at 0.2 A g-1, 409 F g-1 at 1 A g-1 and 226 F g-1 at a very high current density of 300 A g-1 (~0.15 second charge time). At intermediate and high currents such capacitance values have not been previously reported for any carbon. Tested with a stable 1.8 V window in a 1M Li2SO4 electrolyte, a symmetric supercapacitor cell yields a flat energy – power profile that is fully competitive with organic electrolyte systems: 29 Wh kg-1 at 442 W kg-1 and 17 Wh kg-1 at 3940 W kg-1. The cyclability of symmetric ISAC –cells is also exceptional due to the minimization of faradaic reactions on the carbon surface, with 80% capacitance retention at over 100,000 cycles in 1M Li2SO4 and 75,000 cycles in 6M KOH.

Two‐Dimensional Titanium Carbide MXene as a Capacitor‐Type Electrode for Rechargeable Aqueous Li‐Ion and Na‐Ion Capacitor Batteries

AbstractTo bridge the gap of the alkali‐metal‐ion (Li or Na) batteries and supercapacitors, hybrid devices called capacitor batteries are being designed, which are adaptive for large‐scale energy‐storage systems. Two‐dimensional transition‐metal carbide materials (MXene) with high electronic conductivities have become capable electrode materials for supercapacitors. Herein, Ti3C2Tx synthesized by selectively etching the Al layer from the Ti3AlC2 presents a remarkable electrochemical performance as an electrode material for the supercapacitor with Li2SO4 or Na2SO4 aqueous electrolyte. It displays a capacitance of 243 F g−1 at a low current density of 100 mA g−1 with a capacitance retention of 95 % in the Li2SO4 electrolyte, and a capacitance of 150 F g−1 at a low current density of 200 mA g−1 with a capacitance retention of 95 % in the Na2SO4 electrolyte, which shows its potential application for capacitor batteries. Moreover, the LiMn2O4//Ti3C2Tx Li‐ion capacitor battery and MnO2//Ti3C2Tx Na‐ion capacitor battery are designed and assembled for the first time. Such rechargeable aqueous capacitor batteries display capable capacities, remarkable rate abilities, and excellent cycling performances, which further demonstrates that Ti3C2Tx is an outstanding capacitor‐type electrode material for capacitor batteries.

Manganese-enriched electrochemistry of LiFePO4/RGO nanohybrid for aqueous energy storage

Manganese-doped lithium iron phosphate (LFMP) integrated with reduced graphene oxide (RGO) has been prepared via microwave-assisted synthesis and investigated as lithium-ion energy storage system in aqueous Li2SO4 electrolyte. The doping of the LFP was achieved with a low-cost commercial electrolytic manganese oxide (EMD) precursor using a microwave-assisted solvothermal technique. When compared to the undoped counterpart (LFP/RGO), obtained under similar experimental conditions, the LFMP/RGO nanohybrid showed an improved electrochemical performance. The LFMP/RGO gave a maximum areal capacitance of ca. 39.48 mF cm−2, power density of 70.3 mW cm−2 and energy density of 8 mWh cm−2 compared to the values for the pristine complex (LFP/RGO); ca. 16.85 mF cm−2, 54.4 mW cm−2 and 4.8 mWh cm−2. In addition, when the two types of electrochemical storage systems were subjected to voltage-holding (floating) experiment for 50 h, LFMP/RGO maintained 98% capacitance retention while LFP/G maintained 94% capacitance retention. The findings in this work prove that Mn-doping is capable of enhancing the electrochemical performance of the LFP material for energy storage.

Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors

Nitrogen-enriched graphene was fabricated via a facile strategy. Graphene oxide (GO) nanosheets and graphene oxide quantum dots (GQDs) were used as a structure-directing agent and in situ activating agent, respectively, after photoreduction under NH3 atmosphere. The combination of photoreduction and NH3 not only reduced GO and GQD composites (GO/GQDs) within a shorter duration but also doped a high level of nitrogen on the composites (NrGO/GQDs). The nitrogen content of NrGO/GQDs reached as high as 18.86 at% within 5min of irradiation. Benefiting from the nitrogen-enriched GO/GQDs hybrid structure, GQDs effectively prevent the agglomeration of GO sheets and increased the numbers of ion channels in the material. Meanwhile, the high levels of nitrogen improved electrical conductivity and strengthened the binding energy between GQD and GO sheets. Compared with reduced GO and low nitrogen-doped reduced GO, NrGO/GQD electrodes exhibited better electrochemical characteristics with a high specific capacitance of 344Fg−1 at a current density of 0.25Ag−1. Moreover, the NrGO/GQD electrodes exhibited 82% capacitance retention after 3000 cycles at a current density of 0.8Ag−1 in 6M KOH electrolyte. More importantly, the NrGO/GQD electrodes deliver a high energy density of 43Whkg−1 at a power density of 417Wkg−1 in 1M Li2SO4 electrolyte. The nitrogen-doped graphene and corresponding supercapacitor presented in this study are novel materials with potential applications in advanced energy storage systems.

Solvent-Free Mechanochemical Synthesis of Nitrogen-Doped Nanoporous Carbon for Electrochemical Energy Storage.

Nitrogen-doped nanoporous carbons were synthesized by a solvent-free mechanochemically induced one-pot synthesis. This facile approach involves the mechanochemical treatment and carbonization of three solid materials: potassium carbonate, urea, and lignin, which is a waste product from pulp industry. The resulting nitrogen-doped porous carbons offer a very high specific surface area up to 3000 m2 g-1 and large pore volume up to 2 cm3 g-1 . The mechanochemical reaction and the impact of activation and functionalization are investigated by nitrogen and water physisorption and high-resolution X-ray photoelectron spectroscopy (XPS). Our N-doped carbons are highly suitable for electrochemical energy storage as supercapacitor electrodes, showing high specific capacitances in aqueous 1 m Li2 SO4 electrolyte (177 F g-1 ), organic 1 m tetraethylammonium tetrafluoroborate in acetonitrile (147 F g-1 ), and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate; 192 F g-1 ). This new mechanochemical pathway synergistically combines attractive energy-storage ratings with a scalable, time-efficient, cost-effective, and environmentally favorable synthesis.

Comparative operando study of degradation mechanisms in carbon-based electrochemical capacitors with Li2SO4 and LiNO3 electrolytes

This paper reports on the operando study of degradation mechanisms in carbon-based electrochemical capacitors with Li2SO4 and LiNO3 electrolytes. Several electrochemical techniques, coupled with mass and Raman spectrometry have been implemented to recognize and describe the processes occurring at the carbon electrode/aqueous electrolyte interface. It has been shown that for the systems operating with mild, water-based electrolytes, the performance fade might have a different origin; dependently on the salt used for electrolyte preparation, carbon surface might be either oxidized or anion might be reduced. A detrimental effect of oxygen evolution at elevated voltages (up to 1.8 V) has also been observed. An in-depth study involving the implementation of the operando techniques also allowed the most optimal voltage for both systems to be determined, taking into account their long-term performance as well.

TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for Aqueous Lithium-Ion Batteries

This paper reports a facile sol-gel synthesis method to successfully prepare the TiP2O7/expanded graphite (EG) nanocomposite as an advanced anode material for aqueous lithium-ion batteries. The constructed TiP2O7 nanocomposites (50-100 nm) are in situ encapsulated in the pore and layer structure of expanded graphite with good conductivity and high specific surface area. As a consequence, the resulting TiP2O7/EG electrode exhibits a reversible capacity of 66 mAh g-1 at 0.1 A g-1 with an appropriate potential of -0.6 V before hydrogen evolution in aqueous electrolytes, and also demonstrates greatly enhanced cycling stability with 75% capacity retention after 1000 cycles at the current density of 0.5 A g-1. A full cell consisting of TiP2O7/EG anode, LiMn2O4 cathode, and 1 M Li2SO4 electrolyte delivers a specific energy of 60 Wh kg-1 calculated on the weight of both cathode and anode materials with an operational voltage of 1.4 V. It also exhibits superior rate capability and remarkable cycling performance with a capacity maintenance of 66% over 500 cycles at 0.2 A g-1 and 61% at 1 A g-1 over 2000 cycles.

Oxygen-modified multiwalled carbon nanotubes: physicochemical properties and capacitor functionality

Summary Multiwalled carbon nanotubes (MWCNTs) have found numerous applications in energy conversion systems. The current work focused on the introduction of oxygen moieties onto the walls of MWCNTs by five different reagents and investigating the associated physicochemical properties. Oxygen-containing groups were introduced onto MWCNTs using an ultrasound water-bath treatment with HNO3, HCl, H2O2 or HCl/HNO3 solution. Physicochemical properties were characterised by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman, thermal gravimetric analysis, textural characteristics, cyclic voltammetry and electrochemical impedance spectroscopy. The study focus was mainly on linking the physicochemical properties of oxygen-functionalised MWCNTs and suitability in electrochemical capacitors using group one sulfates. From the Fourier transform infrared spectroscopy KBr pellet protocol, peaks at 3400, 2370 and 1170 cm−1 suggest oxygen-containing functionalities on MWCNTs. HNO3 treatment introduced highest oxygen-containing moieties and achieved highest specific capacitance in Li2SO4 and Na2SO4 electrolytes of 36.200 F g−1 (77 times better than pristine) and 45.100 F g−1 (2.5 times enhancement), respectively. For K2SO4, it was 33.600 F g−1 (4.9 times better) with HNO3/HCl-treated samples. Oxygen-functionalised MWCNTs displayed both pseudo and electrochemical double-layer mechanism of enhanced charge storage and cycle stability in group one sulfates electrolytes. The dominating charge storage mechanism was pseudo, and Na2SO4 was the best electrolyte amongst the three group one sulfates investigated. Copyright © 2017 John Wiley & Sons, Ltd.

Hydrothermal synthesis of pure LiMn2O4 from nanostructured MnO2 precursors for aqueous hybrid supercapacitors

Pure LiMn2O4 samples with high crystallinity (LMO-1# and LMO-2#) were successfully synthesized by a facile hydrothermal method using δ-MnO2 nanoflowers and α-MnO2 nanowires as the precursors. The as-prepared samples were analyzed by XRD, SEM, and Brunauer-Emmett-Teller (BET), and their capacitive properties were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge test. Two LiMn2O4 samples showed good capacitive behavior in aqueous hybrid supercapacitors. AC//LMO-1# and AC//LMO-2# delivered the initial specific capacitance of 45.4 and 40.7 F g−1 in 1 M Li2SO4 electrolyte at a current density of 200 mA g−1 in the potential range of 0∼1.5 V, respectively. After 1000 cycles, the capacitance retention was 97.6% for AC//LMO-1# and 93.7% for AC//LMO-2#. Obviously, LMO-1# from δ-MnO2 nanoflowers exhibited higher specific capacitance and better cycling performance than LMO-2#, so LMO-1# was more suitable as the positive electrode material in hybrid supercapacitors.

Nitrogen and Sulfur Co-Doped 2D Titanium Carbides for Enhanced Electrochemical Performance

Two-dimensional (2D) transition metal carbides (MXenes) have been recently attracted as promising electrode materials for energy storage applications. Herein, we report a high-capacity supercapacitor material based on nitrogen and sulfur co-doped titanium carbides (NS-Ti3C2), which is prepared by a simple thiourea-assisted carbonization of large-scale Ti3C2 nanosheets. Importantly, results indicate that the 2D Ti3C2 treated with thiourea-assisted carbonization results in change in its microstructure, specific surface area, and mesopores. And, it also has a crucial effect on co-doping content and position of N and S, which can improve the accessibility to active sites. The carbonization treated NS-Ti3C2 nanosheets exhibit a high specific capacitance of 175 F g−1 at 2 mV s−1 in 1 M Li2SO4 electrolyte solution, and a prominent cycling stability (90.1% of the initial capacitance after 5000 cycles). The significant improvement in electrochemical performance of the NS-Ti3C2 can be attributed to larger surface areas and more mesopores, actually, the doping of N and S can enhance the intercalation of ion electrolyte, provide more abundant electrical charge, and thus contribute to the pseudocapacitance. Hence, this new type of nitrogen and sulfur co-doped Ti3C2 could be a promising electrode material for high-performance supercapacitors.

(Invited) Novel Ti4O7 /MnO2 Pseudocapacitive Composite for High Energy Density and Stability Supercapacitor

Titanium dioxide (TiO2)-rutile has been considered as one of the promising candidates in supercapacitors due to its low cost, electrochemical stability, natural abundance, high surface area, low toxicity, and easy preparation in defined nanoscale dimensions. However, its specific capacitance is substantially lower than those of other pseudocapacitive metal oxides. In this study, we have demonstrated that the specific capacitance of TiO2-rutile can be dramatically improved by increasing oxygen deficiency. After phase transition to Magnéli phase, Ti4O7-electrode possesses a high specific capacitance of 168 F/g at 2 mV/sec, which is 10 times that of pristine TiO2-rutile. To further improve the specific capacitance, Ti4O7 is modified with 7 wt% MnO2, showing 345.5 F/g in an aqueous Li2SO4 electrolyte between 1 V and -1 V versus Ag/AgCl/saturated KCl without the problem of Mn ion dissolution. Furthermore, the composite electrode exhibits a high specific capacitance of 225 F/g even at a high scan rate of 400 mV/sec since Ti4O7 shows high conductivity. In addition, the Ti4O7/MnO2 supercapacitor exhibits 81 % capacitance retention with cycling up to 5000 cycles under the large potential range (1.6V). In view of the energy storage capacity, this new composite electrode holds great promise for next-generation high-energy supercapacitors. Figure 1

Electrochemical supercapacitor studies of porous MnO2 nanoparticles in neutral electrolytes

In this study, porous MnO2 nanoparticles (sample A and sample B) with higher active surface area were synthesized using sonochemical and soft template methods. To determine the crystalline phase, the samples were characterized to study their microstructure, chemical composition, and physical properties. X-ray diffraction results showed that both the samples were amorphous. Microstructure study confirmed that the sample A is spherical, existing with rod-shaped morphology whereas sample B shows flake-like morphology. The Brunauer–Emmett–Teller results showed the value obtained for sample B to be 1559 m2 g−1, which is effectively high when compared to that of sample A. The electrochemical capacitor behavior of the prepared nanoparticles was investigated in 0.1 M Li2SO4 and Na2SO4 electrolytes. The cyclic voltammogram result showed that both the sample electrodes behave as an ideal capacitor in both electrolytes. The charge–discharge test result indicated that the highest specific capacitance value of 280 F g−1 was obtained for sample B electrode in Na2SO4 electrolyte with good capacity retention of 92.31% after 500 cycles. The electrochemical impedance spectroscopy measurements confirm that sample B electrode has a lower Rct value in Na2SO4 electrolyte when compared to that in Li2SO4 electrolyte.

Direct growth of Fe3O4-MoO2 hybrid nanofilm anode with enhanced electrochemical performance in neutral aqueous electrolyte

To enhance the electrochemical energy storage performance of supercapacitors (SCs), the current researches are general directed towards the cathode materials. However, the anode materials are relatively less studied. In the present work, Fe3O4-MoO2 (FO-MO) hybrid nano thin film directly grown on Ti substrate is investigated, which is used as high-performance anode material for SCs in Li2SO4 electrolyte with the comparison to pristine Fe3O4 nanorod array. The areal capacitance of FO-MO hybrid electrode was initially found to be 65.0mFcm−2 at 2mVs−1 and continuously increased to 260.0% after 50 cycles of activation. The capacitance values were considerably comparable or higher than many reported thin-film iron oxide-based anodes in neutral electrolyte. With the protection of MoO2 shell, the FO-MO electrode developed in this study also exhibited excellent cyclic stability (increased to 230.8% after 1000 cycles). This work presents a promising way to improve the electrochemical performance of iron oxide-based anodes for SCs.

Enhanced electrochemical performance of TiO2 nanotube array electrodes by controlling the introduction of substoichiometric titanium oxides

Although anodized titania nanotubes (TNTs) possess unique advantages as an electrode material, the poor electrical conductivity limits their practical application. We herein report a facile route to enhance electrical conductivity and improve electrochemical behavior by controlling the introduction of substoichiometric titanium oxides (TinO2n-1), which can facilitate charge propagation in TNTs and improve kinetics of ions and electron transport in the electrode. Specific capacitance is as high as 3.77 mF cm−2, which is about 47 times higher than that of the untreated samples (0.08 mF cm−2). Galvanostatic charge-discharge results show TNT arrays-based electrode stable capacitance behavior with excellent capacitance retention even after 1000 continuous charge-discharge cycles at a current density of 50 μA cm−2 in Li2SO4 electrolyte. The ease of synthesis and the superior electrochemical performance suggest a promising application for the TNT arrays-based material in energy storage field.

Towards New Micro-Scale Supercapacitors: Pseudocapacitance of Hydrous Ruthenium Oxide Investigated By Multi-Scale Time-Resolved Electrochemical Methods

In recent years, the use of metal oxide thin films with nanoscale porosity for diverse electrochemical applications, e.g. energy storage, EC devices, etc. has become a subject of growing interest. 1 Among these materials, ruthenium oxide is a particular metal oxide which can offer a type of electrochemical capacitance termed pseudocapacitance which also involves faradaic charge-transfer reactions. RuOx. nH2O stands as an electroactive material with very high surface area which prosecute the fast reversible redox transitions in the potential window of charge and discharge. The amorphous and hydrous form exhibits noticable pseudocapacitance higher than its crystalline counterpart. 1, 2 For the improvement of the performance of metal oxide based electrodes, main attention has been attributed to their nanostructuration with various morphologies due to the unique properties and functionalities that can effectively be exploited in electrochemical devices. Among various synthesis methods such as sol-gel, spray pyrolysis and hydrothermal synthesis, electrochemical synthesis remains to be a fast, simple, low-cost and low-temperature technique which may also lead to the formation of homogenous nanoscale films with desirable qualities. Moreover, the incorporation of surfactant molecules in the electrogenerated film results in the production of thin nanostructured films by using potential-controlled self-assembly of surfactant-inorganic aggregates at solid-liquid interfaces 3 . The resulted nanostructures, with small sizes, and large surface area to volume ratios, are expected to facilitate the ion intercalation/extraction process. These improvements are attributed to a facilitated transfer and short diffusion length for ions transport, a high electrode/electrolyte contact area, and a better stress/strain management of the material during ion intercalation/adsorption. Despite the morphological information obtained by some in situ or ex situ characterization techniques such as X-ray photoelectron spectroscopy, small and wide-angle X-ray scattering, none of these methods alone provide the information on the exact identification of the transferred ionic species, their dynamics of transfer at the interfaces, as well as the role of electrolyte composition and the effect of ions solvation on the different electrochemical phenomena. Classical EQCM (Electrochemical Quartz Crystal Microbalance) was largely used to overcome these problems but its performances are not enough to extract pertinent results. Therefore, here, an alternative characterization tool was proposed to overcome the limitations of these classical EQCM. Specifically, the pseudocapacitive behavior related to the cation transfer in hydrous RuOx thin films was investigated by coupled time resolved characterization methods A c-electrogravimetry, consisting of in-situ coupling of electrochemical impedance spectroscopy (EIS) and fast quartz crystal microbalance (QCM), was used here: it measures simultaneously the electrochemical impedance, ΔE/ΔI (ω), and the mass/potential transfer functions (Δm/ΔE (ω)) It provides the access to the relevant information on the kinetics of species transferred at the solid/solution interfaces, and their transport in the bulk of the materials, the nature of these species as well as their relative concentration within the material. This coupling dominates over the limitations of QCM technique and has the ability to deconvolute the global mass variations provided by QCM measurements. Specifically, it detects the contribution of the charged or uncharged species and to identify anionic, cationic, and the free solvent contributions during various redox processes. 4,5 In the present work, this approach is exploited to study the ion transfer behaviour of amorphous hydrous RuOx in both mostly compact and rather porous structures. An electrochemical pathway was chosen, so-called surfactant-assisted electrodeposition is used for their preparation. In addition, we particularly focus on the amorphous materials rather than crystalline ones to emphasize its role in the electrochemical performance of such films. The electrochemical performance and ion intercalation/adsorption mechanisms were studied in aqueous H2SO4, Li2SO4 and Na2SO4 electrolytes by electrogravimetric methods (EQCM, and ac−electrogravimetry). Special attention is attributed to understand the nature of the ions involved in the charge compensation, solvation and the role of the electrolytes and the dynamic information of ions transfer at the electrode/electrolyte interfaces, ac−electrogravimetry as a gravimetric probe is used to study the complex H+ or Li+ intercalation/extraction mechanisms and to extract subtleties unreachable with classical tools.

Carbon-coated mesoporous LiTi2(PO4)3 nanocrystals with superior performance for lithium-ion batteries

Carbon-coated LiTi2(PO4)3 (LTP/C) nano- and/or microcrystals have been subjected to extensive research due to their potential application in aqueous rechargeable lithium-ion batteries (ARLBs) with enhanced electrochemical performance. In this paper, mesoporous LTP/C nanocrystals have been prepared by annealing the precursor obtained through a simple room-temperature co-precipitation method. The mesoporous LTP/C nanocrystals have been evaluated in the role of anode materials for lithium-ion batteries (LIBs) with organic and water electrolyte, respectively. After 1000 cycles at 5 and 20C, the specific discharge capacity of the LTP/C electrode with organic electrolyte retains 116.9 and 80.4mAhg−1, with capacity retention of 94.4% and 80.4%, respectively. After 1000 cycles at 1.37C, the specific discharge capacity of the LTP/C‖LiMn2O4 pouch cells with 1molL−1 Li2SO4 electrolyte retains 20.1mAhg−1, presenting a discharge voltage plateau around 1.21V and a capacity retention of 71.5%. The enhanced electrochemical performance of the mesoporous LTP/C nanocrystals makes them a promising candidate functioning as anode materials for LIBs and hybrid supercapacitors.

NaTi2(PO4)3 Nanoparticles Embedded in Carbon Matrix as Long-Lived Anode for Aqueous Lithium Ion Battery

NaTi2(PO4)3 nanoparticles with the average size of 10–20 nm have been embedded in carbon matrix by a novel two-step approach and the composite was employed as an anode for aqueous lithium ion battery (ALIB). An ALIB consisting of a commercial LiMn2O4 cathode, NaTi2(PO4)3/C anode and Li2SO4 electrolyte has been firstly constructed. The cell delivers an average platform voltage of 1.6 V and excellent rate capability with reversible capacities of 115 mAh g−1 and 83 mAh g−1 (based on the mass of NaTi2(PO4)3) at 30 mA g−1 and 1500 mA g−1, respectively. It is interesting to note that no obvious capacity fading is observed after 300 cycles at 150 mA g−1. Even after an ultra-long cycling test up to 3600 cycles at 1500 mA g−1, 95% of the initial capacity is maintained, indicating the superior cycling stability for as-prepared NaTi2(PO4)3/C composite. The Li ion intercalation/deintercalation mechanism of NaTi2(PO4)3 anode has also been investigated. The outstanding electrochemical performance and low cost enable the NaTi2(PO4)3/C a very promising anode candidate for ALIBs.

Preparation and capacitive properties of lithium manganese oxide intercalation compound

Lithium manganese oxide intercalation compound (Li0.7MnO2) supported on titanium nitride nanotube array (TiN NTA) was applied as cathode electrode material for lithium-ion supercapacitor application. Li0.7MnO2/TiN NTA was fabricated through electrochemical deposition and simultaneous intercalation process using TiN NTA as a substrate, Mn(CH3COO)2 as manganese source, and Li2SO4 as lithium source. The morphology and microstructure of the Li0.7MnO2/TiN NTA were characterized by scanning electron microscopy and X-ray diffraction analysis. The electrochemical performance of the Li0.7MnO2/TiN NTA was investigated by electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge measurements. Li0.7MnO2/TiN NTA exhibited higher capacitive performance in Li2SO4 electrolyte solution rather than that in Na2SO4 electrolyte solution, which was due to the different intercalation effects of lithium-ion and sodium-ion. The specific capacitance was improved from 503.3 F g−1 for MnO2/TiN NTA to 595.0 F g−1 for Li0.7MnO2/TiN NTA at a current density of 2 A g−1 in 1.0 M Li2SO4 electrolyte solution, which was due to the intercalation of lithium-ion for Li0.7MnO2. Li0.7MnO2/TiN NTA also kept 90.4 % capacity retention after 1000 cycles, presenting a good cycling stability. An all-solid-state lithium-ion supercapacitor was fabricated and showed an energy density of 82.5 Wh kg−1 and a power density of 10.0 kW kg−1.

Orthorhombic Lithium Titanium Phosphate As an Anode Material for Li-Ion Rechargeable Batteries

The wide voltage window and the reasonable ionic conductivity of the organic electrolytes presently adopted in the Li-ion secondary batteries allow high power output and energy density. However, the thermal instability and comparatively high production costs of these organic electrolytes have become a major hindrance to the significant scale-up of Li-ion batteries. Aqueous electrolytes could considerably reduce the production cost of the lithium-ion batteries and completely eliminate the fire hazard at the expense of reduced energy density. On the other hand, the restricted voltage window of the aqueous electrolytes allows only non-conventional sets of anode and cathode electrode materials to meet the proper working voltage of ~1.2 V in the full-cell configuration [1]. This requirement is met by using a non-carbonaceous host such as rhombohedral LiTi2(PO4)3 (RLTP) as an anode material with a redox potential of 2.5 V for the Ti4+/Ti3+ couple against Li. A number of studies on aqueous rechargeable Li-ion batteries using RLTP as an anode have been published along with the development of RLTP. For example, an LiMn2O4//LiTi2(PO4)3 cell with 1 M Li2SO4 as an aqueous electrolyte showed an initial capacity of 40 mAh g-1, and LiMn0.05Ni0.05Fe0.9PO4//LiTi2(PO4)3 with saturated Li2SO4 as an aqueous electrolyte showed an initial capacity of 87 mAh g-1 [2,3]. However, regardless of the nature of the cathode materials, the large initial irreversibility and the gradual capacity fade of RLTP have hindered its application as an anode for aqueous Li-ion batteries. Attempts to improve the electrochemical properties of RLTP by nano-sizing [4] and controlling the oxygen vacancy [5] have recently been reported. Meanwhile, the use of Li-rich lithium titanium phosphate phases instead of current RLTP has not been described yet. We consider that the use of Li-rich phases of Li1+xTi2(PO4)3 could be expected to reduce the initial irreversibility and improve the cycle life at the expense of somewhat lowered capacity. Our preliminary attempts to synthesize Li-rich single-phases of Li1+xTi2(PO4)3 (0≤x≤2) revealed that this Li-rich phase could easily be incorporated with the well-known rhombohedral LiTi2(PO4)3 contaminated with some other stable phases in the compositional space Li-Ti-P-O, such as LiTiPO5, Li4Ti5O12, LiTi2(PO4)3, and TiO2, depending on the synthesis temperature and the stoichiometry of the starting materials [6]. Therefore, our primary goal was to synthesize a single lithium-rich phase, which exists in a space group system different from that of RLTP (R-3c). A systematic approach to stabilize isotypic mixed-valent structures was previously pioneered by M. Catti. In that work, it was suggested that mixed-valent superstructures of Li1+xInxTi2-x(PO4)3 could exist when x < 0.5 or x > 1.0 [7], which is in good agreement with our preliminary data. Therefore, the smallest amount of lithium, x=1.5, was chosen to isolate the single phase of mixed-valent orthorhombic Li1.5Ti2(PO4)3. In this preliminary study, we tried to clarify how electrochemical properties could vary in the seemingly similar compounds LiTi2(PO4)3 and Li1.5Ti2(PO4)3. The electrochemical properties of these materials were evaluated in conventional half cells to measure the interfacial resistance and Li+ ionic diffusivity in these structures. In addition, their feasibilities as anodes for aqueous lithium-ion cells were tested in LiFePO4//LixTi2(PO4)3 full-cell configurations using 1M Li2SO4 electrolytes.

Urea-assisted hydrothermal synthesis of manganese dioxides with various morphologies for hybrid supercapacitors

Two MnO2 samples (MO-1 with thorn spheres and MO-2 with long nanowires) were synthesized by adding urea into KMnO4–H2SO4 reaction system at various hydrothermal temperatures. Urea could decrease the reaction temperature, and its hydrolysis could influence the morphology of MnO2. The as-prepared samples were analyzed by XRD and SEM. Their capacitive properties were investigated by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge/discharge test. The initial specific capacitance values of AC//MO-1 and AC//MO-2 hybrid capacitors were 24.99 and 27.58 F g−1 in 1 M Li2SO4 electrolyte, respectively. After 1000 cycles, the capacitance retention was 91.40% for AC//MO-1 and 96.93% for AC//MO-2. Therefore, AC//MO-2 exhibited better capacitive properties than AC//MO-1.