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Solid-state batteries (SSBs) offer a fundamental solution to mitigate the safety and reliability issues of conventional lithium-ion batteries utilizing flammable liquid electrolytes, and enable the bipolar configuration of high-voltage and high-energy storage systems. However, the conventional layer-by-layer (LbL) stacking process using individual electrolyte and electrode layers suffers from poor electrolyte–electrode contacts and challenging process complications for manufacturing multi-layer bipolar SSBs. Herein, we report an electrode-to-electrode (EtE) monolithic integration without a free-standing solid electrolyte layer for high-voltage bipolar SSBs. Positive and negative electrodes seamlessly combined with a thin solid electrolyte are fabricated by the infusion of a plastic-crystal-based polymer electrolyte (PCPE) into porous electrodes with a subsequent ultraviolet-induced solidification process. The infused PCPE in the electrodes forms continuous Li+ conduction channels as well as intimate solid–solid interfaces. The thin PCPE film (∼10 μm) formed in situ on the top of the electrodes during the infusion process provides ultra-high Li+ conductance (∼3.1 S cm−2 at 45 oC) between the two electrodes. SSBs are constructed via direct integration of the PCPE-infused electrodes: a unit cell-type SSB with LiNi0.6Co0.2Mn0.2O2 (positive) and Li4Ti5O12 (negative) show superior capacity (∼160 mAh g−1), rate capability (98 mAh g−1 at 2 C), and stable cyclability (81% after 100 cycles) at 45 oC than the SSB fabricated by the conventional LbL stacking process. Moreover, a 10 V-class, bipolar SSB comprising five unit cells stacked in series is constructed via the EtE monolithic integration of multiple PCPE-infused bipolar electrodes, and its stable cycle performance is corroborated with a capacity retention of 84%. This work demonstrates that the suggested EtE integration process offers a promising strategy to addressing the interfacial contact issues of SSBs, thereby being utilized to realize scalable, low-cost, high-voltage bipolar SSBs.

Solid-state batteries (SSBs) offer a promising technical solution to meet the key requirements of future energy storage systems, i.e., safety and high energy density. However, the realization of SSBs is hindered by low electrode performance that results from poorly controlled solid-solid contacts. Herein, we propose an effective strategy for tailoring conductive networks and reaction interfaces via the viscosity-controlled infusion of a molten-state polymer electrolyte precursor (polymer ionic conductor, PIC) into a porous composite electrode (CE). A poly(ethylene glycol)-based PIC penetrates a three-dimensional pore network of the CE and transforms to a highly viscous, stable phase under battery operating conditions. The infused PIC enables the formation of percolating Li+ conduction pathways as well as intimate solid-solid reaction interfaces, which leads to the full utilization of the CE at high mass loadings. SSBs assembled with PIC-infused LiFePO4-CEs exhibit superior capacity (154 mAh g−1), rate-capability, and cycling stability than SSBs with unmodified CEs. Moreover, a 10 V-class, bipolar-pouch SSB fabricated using the PIC-infusion technology shows reversible charge–discharge operations without a considerable performance loss. This study demonstrates that the proposed microstructural engineering provides an effective approach to resolving the interfacial solid-solid contact issues of SSBs and can be used to fabricate safe, high-energy, long-cycling SSBs.

To demonstrate an aqueous sodium-polyiodide flow battery (SIFB) for the first time, sodium titanium phosphate (NaTi2(PO4)3) was utilized as the negative electrode and I−/I3− couple was adopted as the positive redox active species. SIFB not only shows advantages of cost-effectiveness and environmental friendliness based on earth-abundant elements of sodium and iodine, but also exhibits a high energy density due to the high solubility of sodium iodide (~12.3 M at room temperature). Although NaTi2(PO4)3 has several advantages such as high Na-ion conductivity and good structural stability, its use as a negative electrode is limited because of intrinsically low electrical conductivity. In this study, NaTi2(PO4)3 nanoparticles were embedded in double carbon networks comprising internal and external carbon layers. Individual effects by the internal and external carbon networks on the electrochemical performance of NaTi2(PO4)3 were clearly distinguished. With the simultaneous formation of the two carbon networks, a SIFB exhibited outstanding electrochemical performance (86.0 mAh•g−1 at a rate of 7 C) and a long cycle life (93.5% capacity retention after 100 cycles).

Carbon nanotubes (CNTs) have been considered as a potential anode material for next generation Lithium-ion batteries (LIBs) due to their high conductivity, flexibility, surface area, and lithium-ion insertion ability. However, the low mass loading and bulk density of carbon nanomaterials hinder their use in large-scale energy storage because their high specific capacity may not scale up linearly with the thickness of the electrode. To address this issue, a novel three-dimensional (3D) architecture is rationally designed by stacking layers of free-standing CNTs with the increased areal density to 34.9 mg cm −2 , which is around three-times higher than that of the state-of-the-art graphitic anodes. Furthermore, a thermal compression process renders the bulk density of the multi-stacked 3D CNTs to be increased by 1.85 g cm −3 , which yields an excellent volumetric capacity of 465 mAh cm −3 at 0.5C. Our proposed strategy involving the stacking of 3D CNT based layers and post-thermal compression provides a powerful platform for the utilization of carbon nanomaterials in the advanced LIB technology.

Titanium-based oxides have been considered promising anode materials for rechargeable sodium (Na)-ion batteries (NIBs); however, the slow reaction kinetics of TiO2 due to its low electrical conductivity remains a critical problem to be addressed for its practical application in NIBs. Here, we report one-dimensional (1D) TiO2–C composite nanofibers for use as an anode for NIBs, which were fabricated via an electrospinning technique combined with optimized heat-treatment. The TiO2–C composite nanofibers are composed of tiny anatase TiO2 nanocrystals surrounded by and/or embedded in the conducting carbon matrix. When tested in an electrochemical Na cell, the TiO2–C nanofiber electrode retained a high charge capacity of 140 mA h g−1, even after 100 cycles at a current density of 60 mA g−1, and it exhibited excellent rate-capability (105 mA h g−1 at 3 A g−1), proving its feasibility as an NIB anode. The superior Na storage performance of the nanofiber anode is mainly attributed to the unique composite architecture, that is, a large number of active sites per volume due to the high aspect ratios and short lengths for Na transport through 1D nanofibers and TiO2 nanocrystals as well as highly conducting electron paths.

An important component in lithium–air batteries is the oxygen electrode, which facilitates the oxygen reduction reaction during the discharge process and the oxidation reaction of Li 2 O 2 during the charge process. The performance of the oxygen electrode strongly depends upon the activity of the materials employed. Herein, we report the catalytic activity of the graphene/zirconium doped ceria (ZDC) nano-blend prepared by mixing graphene, which is prepared using the modified Hummers method followed by H 2 reduction, and ZDC, chemically prepared. With merely 10% loading of ZDC on the graphene, Li-O 2 cells showed a threefold increase in their discharge capacities. The well exfoliated layered structure of graphene and the mesoporous structure of ZDC facilitated the diffusion of the electrolyte and oxygen in the inner electrode to enhance the catalytic efficiency and also offered ample volume for the accumulation of discharge product. The synergistic effect of the fast kinetics of electron transport provided by the graphene support and the high electro-catalytic activity provided by the ZDC resulted in the excellent performance of the oxygen electrode in the Li-O 2 batteries.

There is a great deal of current interest in the development of rechargeable sodium (Na)-ion batteries (SIBs) for low-cost, large-scale stationary energy storage systems. For the commercial success of this technology, significant progress should be made in developing robust anode (negative electrode) materials with high capacity and long cycle life. Sn-P compounds are considered promising anode materials that have considerable potential to meet the required performance of SIBs, and they have been typically prepared by high-energy mechanical milling. Here, we report Sn-P-based anodes synthesised through solvothermal transformation of Sn metal and their electrochemical Na storage properties. The temperature and time period used for solvothermal treatment play a crucial role in determining the phase, microstructure, and composition of the Sn-P compound and thus its electrochemical performance. The Sn-P compound prepared under an optimised solvothermal condition shows excellent electrochemical performance as an SIB anode, as evidenced by a high reversible capacity of ~560 mAh g−1 at a current density of 100 mA g−1 and cycling stability for 100 cycles. The solvothermal route provides an effective approach to synthesising Sn-P anodes with controlled phases and compositions, thus tailoring their Na storage behaviour.

A rechargeable lithium–oxygen (Li–O2) battery is considered as a promising technology for electrochemical energy storage systems because its theoretical energy density is much higher than those of state-of-the-art Li-ion batteries. The cathode (positive electrode) for Li–O2 batteries is made of carbon and polymeric binders; however, these constituents undergo parasitic decomposition reactions during battery operation, which in turn causes considerable performance degradation. Therefore, the rational design of the cathode is necessary for building robust and high-performance Li–O2 batteries. Here, a binder-free carbon nanotube (CNT) electrode surface-modified by atomic layer deposition (ALD) of dual acting RuO2 as an inhibitor–promoter is proposed for rechargeable Li–O2 batteries. RuO2 nanoparticles formed directly on the binder-free CNT electrode by ALD play a dual role to inhibit carbon decomposition and to promote Li2O2 decomposition. The binder-free RuO2/CNT cathode with the unique architecture shows outstanding electrochemical performance as characterized by small voltage gaps (∼0.9 V) as well as excellent cyclability without any signs of capacity decay over 80 cycles.

The mechanism transition in the oxygen reduction reaction at the Pt-dispersed carbon (Pt/C) electrode was investigated in an oxygen-saturated 0.5 M H2SO4 solution. The reaction was monitored by acquiring data for Pt loading, Nafion content and temperature by analyses of the rotating disk voltammograms and potentiostatic current transients (PCTs). From the shape of the cathodic PCTs and the dependence of the initial current density on the potential drop, it is suggested that oxygen reduction at the Pt/C electrode is controlled by the charge transfer at the electrode surface mixed with the oxygen diffusion in the solution below the value of the potential drop, ΔEtr, needed for the occurrence of the mechanism transition, whereas oxygen reduction is purely governed by the oxygen diffusion in the solution above ΔEtr. In particular, it was noted that the value of ΔEtr remained nearly constant irrespective of the Pt loading and Nafion content. On the other hand, the value of ΔEtr decreased as temperature increased, which is ascribed to the fact that the contribution of the Cottrell current enhanced by temperature rise to the fall in ΔEtr is overwhelmed by that contribution of the Butler–Volmer current increased. Consequently, it is concluded that it strongly depends upon the extrinsic parameters such as Pt loading, Nafion content and temperature as well as the intrinsic parameters such as rate constant for interfacial reaction and oxygen diffusivity in the solution, which mechanism of the overall oxygen reduction reaction is operative.

The mechanism transition of lithium transport through a Li 1− δ Mn 2 O 4 composite electrode caused by the surface-modification and temperature variation was investigated using the galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and the potentiostatic current transient technique. From the analyses of the ac-impedance spectra, experimentally measured from unmodified Li 1− δ Mn 2 O 4 and surface-modified Li 1− δ Mn 2 O 4 with MgO composite electrodes, the internal cell resistance of the MgO-modified Li 1− δ Mn 2 O 4 electrode was determined to be much smaller in value than that of the unmodified electrode over the whole potential range. Moreover, from the analysis of the anodic current transients measured on the MgO-modified Li 1− δ Mn 2 O 4 electrode, it was found that the cell-impedance-controlled constraint at the electrode surface is changed to a diffusion-controlled constraint, which is characterised by a large potential step and simultaneously by a small amount of lithium transferred during lithium transport. This strongly suggests that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport through the MgO-modified Li 1− δ Mn 2 O 4 electrode. Furthermore, from the temperature dependence of the internal cell resistance and diffusion resistance in the unmodified Li 1− δ Mn 2 O 4 composite electrode measured by GITT and EIS, it was concluded that which mechanism of lithium transport will be operative strongly depends on the diffusion resistance as well as on the internal cell resistance.

Lithium transport through the partially inactive fractal Li 1 − δ Mn 2 O 4 film electrode under the cell-impedance-controlled constraint was theoretically investigated by using the kinetic Monte Carlo method based upon random walk approach. Under the cell-impedance-controlled constraint, all the potentiostatic current transients calculated from the totally active and partially inactive fractal electrodes hardly exhibited the generalised Cottrell behaviour and they were significantly affected in shape by the interfacial charge-transfer kinetics. In the case of the linear sweep voltammogram determined from the totally active and partially inactive fractal electrodes, all the power dependence of the peak current on the scan rate above the characteristic scan rate deviated from the generalised Randles–Sevcik behaviour. From the analyses of the current transients and the linear sweep voltammograms simulated with various values of the simulation parameters, it was further recognised that the cell-impedance-controlled lithium transport through the partially inactive fractal Li 1 − δ Mn 2 O 4 film electrode strongly deviates from the generalised diffusion-controlled transport behaviour of the electrode with the totally active surface, which is attributed to the impeded interfacial charge-transfer kinetics governed by the surface inhomogeneities including the fractal dimension of the surface and the surface coverage by active sites and by the kinetic parameters including the internal cell resistance.

The present article is concerned with the overview of the chemically/surface modified cubic spinel as a cathode electrode far lithium ion secondary batteries. Firstly, this article presented a comprehensive survey of the cubic spinel structure and its correlated electrochemical behaviour of . Subsequently, the various kinds of the chemically/surface modified and their electrochemical characteristics were discussed in detail. Finally, this article reviewed our recent research works published on the mechanism of lithium transport through the chemically/surface modified electrode from the kinetic view point by the analyses of the experimental potentiostatic current transients and ac-impedance spectra.

Lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1 M solution of LiClO4 in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li1−δMn2O4 single-layer and LiCoO2/Li1−δMn2O4 bilayer film specimens, the internal cell resistance of the LiCoO2/Li1−δMn2O4 bilayer film electrode was determined to be smaller in value than that of the Li1−δMn2O4 single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO2 surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO2/Li1−δMn2O4 bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li1−δMn2O4 single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.

Lithium transport through the fractal LiMn2O4 film electrode under the cell-impedance-controlled constraint was investigated by employing ac-impedance spectroscopy, potentiostatic current transient technique and linear sweep voltammetry. For this purpose, the flat and fractal LiMn2O4 film electrodes were prepared on the as-deposited Pt/polished Al2O3 substrate and the surface modified Pt/unpolished Al2O3 substrate, respectively. From the analysis of the ac-impedance spectra obtained from the flat and fractal electrodes, it is found that the apparent self-similar fractal dimension reduces the charge-transfer resistance, and the phase angle of the diffusion impedance under the semi-infinite diffusion condition positively deviates in absolute value from 45° with increasing fractal dimension. All the potentiostatic current transients experimentally measured from the flat and fractal LiMn2O4 electrodes showed non-generalised Cottrell behaviour which resulted from the cell-impedance-controlled constraint during lithium transport. In the case of linear sweep voltammogram theoretically calculated and experimentally measured from the flat and fractal LiMn2O4 electrodes, the power dependence of the peak current on the scan rate hardly exhibited the generalised Randles–Sevcik behaviour. From the analyses of the potentiostatic current transients and the linear sweep voltammograms, furthermore, it is experimentally confirmed that the internal cell resistance mainly determining the cell-impedance-controlled lithium transport strongly depends upon the fractal dimension as well as such external parameters as the applied potential step and the amount of lithium transferred during lithium transport.

The effect of pore structure on anomalous behaviour of the lithium intercalation into porous V 2 O 5 film electrode has been investigated in terms of fractal geometry by employing ac-impedance spectroscopy combined with N 2 gas adsorption method and atomic force microscopy (AFM). For this purpose, porous V 2 O 5 film electrodes with different pore structures were prepared by the polymer surfactant templating method. From the analysis of N 2 gas adsorption isotherms and the triangulation analysis of AFM images, it was found that porous V 2 O 5 surfaces exhibited self-similar scaling properties with different fractal dimensions depending upon amount of the polymer surfactant in solution and the spatial cut-off ranges. All the ac-impedance spectra measured on porous V 2 O 5 film electrodes showed the non-ideal behaviour of the charge-transfer reaction and the diffusion reaction, which resulted from the interfacial capacitance dispersion and the frequency dispersion of the diffusion impedance, respectively. From the comparison between the surface fractal dimensions by using N 2 gas adsorption method and AFM, and the analysis of ac-impedance spectra by employing a constant phase element (CPE), it is experimentally confirmed that the lithium intercalation into porous V 2 O 5 film electrode is crucially influenced by the pore surface irregularity and the film surface irregularity.

Lithium intercalation into Li[Ti5/3Li1/3]O4 film electrode in the two-phase coexistence was investigated from the thermodynamic and kinetic viewpoints. The electrode potential versus lithium content curve of the film electrode was theoretically calculated in consideration of the interactions between lithium ions based upon a lattice gas model with the Monte Carlo simulation. According to the model, it was proposed that a wide potential plateau indicating the coexistence of a Li-poor phase α and a Li-rich phase β is due to the repulsive interactions between lithium ions. From the analysis of the ac-impedance spectra in the coexistence of two phases α and β, it was confirmed that the fraction of α phase at the electrode surface and that fraction of β phase continuously decreases and increases, respectively, with increasing lithium content. The analysis of the current transient led to the conclusion that transport of lithium ions subjected to the repulsive interactions within the electrode is governed by the cell-impedance-controlled constraint at the electrode surface during the phase transformation between α and β phases.

Herein, we report the catalytic activities of porous nano-crystalline oxides with surface active sites synthesized by a wet chemical route. Zr doped ceria (ZDC) has been tested as active oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts in air electrodes for Li–O2 batteries. A ZDC-based air electrode exhibits a higher discharge capacity than that of a bare carbon-based air electrode. ZDC loaded in carbon air electrodes delivers a discharge capacity of 8435 mA h g−1. The higher discharge voltages of Li–O2 cells with ZDC, instead of bare carbon, could have originated from the higher oxygen reduction activity of ZDC. These oxides show a lower potential for Li2O2 oxidation than pure carbon. A significant increase in the kinetics for Li2O2 oxidation indicates the influence of the ZDC catalyst on the oxygen evolution reaction. The ORR and OER properties of the catalyst are explained in terms of the high fraction of active defect sites on its surface.

Lithium transport through the fractal Li i-δ Mn 2 O 4 film electrode with partially inactive surface under the cell-impedance-controlled constraint was investigated by employing ac-impedance spectroscopy, potentiostatic current transient (PCT) technique and linear sweep voltammetry (LSV). For this purpose, the totally active and partially inactive Li 1-δ Mn 2 O 4 film electrodes were obtained by surface modification of Li 1-δ Mn 2 O 4 film with polyvinylidene fluoride (PVDF) binder. Based upon the perimeter-area relation, the fractal dimension d F of the partially inactive surface was determined. From the analysis of the ac-impedance spectra obtained from the totally active and partially inactive fractal electrodes, it was found that the charge-transfer kinetics strongly depends upon the surface coverage by active sites θ. All the potentiostatic current transients did not exhibit the generalized Cottrell behavior even before the characteristic time t ch and all the power dependence of the peak current on the potential scan rate deviated from the generalized Randles-Sevcik behavior even above the characteristic scan rate v ch in the linear sweep voltammograms. In particular, as either θ or d F falls in value, the value of t ch is prolonged and at the same time the value of v ch is reduced as well. From the analyses of the potentiostatic current transients and the linear sweep voltammograms in terms of t ch and v ch , it was experimentally confirmed that the partial inactivity of the surface plays a significant role in the interfacial charge-transfer reaction and subsequent diffusion reaction during the whole lithium intercalation and deintercalation processes.

Sodium (Na)-ion batteries (NIBs) have attracted significant interest as an alternative chemistry to lithium (Li)-ion batteries for large-scale stationary energy storage systems. Discovering high-performance anode materials is a great challenge for the commercial success of NIB technology. Transition metal oxides with tailored nanoarchitectures have been considered as promising anodes for NIBs due to their high capacity. Here, we demonstrate the fabrication of a nanostructured oxide-only electrode, i.e., carbon- and binder-free NiCo2O4 nanoneedle array (NCO-NNA), and its feasibility as an anode for NIBs. Furthermore, we provide an in-depth experimental study of the Na storage reaction (sodiation and desodiation) in NCO-NNA. The NCO-NNA electrode is fabricated on a conducting substrate by a hydrothermal method with subsequent heat treatment. When tested in an electrochemical Na half-cell, the NCO-NNA electrode exhibits excellent Na storage capability: a charge capacity as high as 400 mAh g(-1) is achieved at a current density of 50 mA g(-1). It also shows a greatly improved cycle life (~215 mAh g(-1) after 50 cycles) in comparison to a conventional powder-type electrode (~30 mAh g(-1)). However, the Na storage performance is still inferior to that of Li, which is mainly due to sluggish kinetics of sodiation-desodiation accompanied by severe volume change.