Understanding Voltage Hysteresis for High-Energy-Density Li–S Batteries

Li–S batteries are promising candidates for next-generation energy storage technologies owing to their high theoretical capacity and low weight and the wide availability of S. The addition of Se to S is considered a rational design principle to regulate the polarization of Li–S cells intrinsically. Moreover, the electrochemical utilization of solid-state Li2–xS (0.0 ≤ x ≤ 1.0) provides sufficiently high theoretical specific capacity (836 mA h g–1) and long-term stability. However, solid-state Se-doped Li–S compounds during (de)­lithiation have not been studied in detail. Therefore, we performed combined experimental and theoretical studies to reveal the reduction of polarization by Se doping owing to multiple factors that were previously assumed to be negligible. Experimentally, the polarization reduction in Se-doped Li2S is dependent on the electronic, ionic, and thermodynamic properties of the Se dopant. Theoretically, Se doping simultaneously lowers the formation energy, bond symmetry of Li–S­(Se), energy required for structural changes, and electronic stability, resulting in the reduction of polarization. Our concrete understanding of the two types of Li–S electrodes can aid the design of advanced high-energy solid-state Li–S batteries

Importance of Chemical Distortion on the Hysteretic Oxygen Capacity in Li-Excess Layered Oxides

Nonhysteretic redox capacity is a critical factor in achieving high energy density without energy loss during cycling for rechargeable battery electrodes, which has been considered a major challenge in oxygen redox (OR) for Li-excess layered oxide cathodes for lithium-ion batteries (LIBs). Until recently, transition metal migration into the Li metal layer and the formation of O–O dimers have been considered major factors affecting hysteretic oxygen capacity. However, Li-excess layered oxides, particularly Ru oxides, exhibit peculiar voltage hysteresis that cannot be sufficiently described by only these factors. Therefore, this study aims to unlock the critical impeding factors in restraining the non-polarizing oxygen capacity of Li-excess layered oxides (herein, Li2RuO3) that exhibit reversible OR reactions. First, Li2RuO3 undergoes an increase in the chemical potential fluctuation as both the thermodynamic material instability and vacancy content increase. Second, the chemical compression of O–O bonds occurs at the early stage of the OR reaction (0.5 ≤ x ≤ 0.75) for Li1–xRu0.5O1.5, leading to flexible voltage hysteresis. Finally, in the range of 0.75 ≤ x ≤ 1.0, for Li1–xRu0.5O1.5, the formation of an O­(2p)–O­(2p)* antibonding state derived from the structural distortion of the RuO6 octahedron leads to the irreversibility of the OR reaction and enhanced voltage hysteresis. Consequently, our study unlocks the new decisive factor, namely, the structural distortion inducing the O­(2p)–O­(2p)* antibonding state, of the hysteretic oxygen capacity and provides insights into enabling the full potential of the OR reaction for Li-excess layered oxides for advanced LIBs

Synaptic Characteristics of Atomic Layer-Deposited Ferroelectric Lanthanum-Doped HfO₂ (La:HfO₂) and TaN-Based Artificial Synapses

Analog synaptic devices have made significant advances based on various electronic materials that can realize the biological synapse properties of neuromorphic computing. Ferroelectric (FE) HfO2-based materials with nonvolatile and low power consumption characteristics are being studied as promising materials for application to analog synaptic devices. The gradual reversal of FE multilevel polarization results in precise changes in the channel conductance and allows analogue synaptic weight updates. However, there have been few studies of FE synaptic devices doped with La, Y, and Gd. Furthermore, an investigation of interface quality is also crucial to enhance the remnant polarization (Pr), synaptic conductance linearity, and reliability characteristics. In this study, we demonstrate improved FE and artificial synaptic characteristics using an atomic layer-deposited (ALD) lanthanum-doped HfO2 (La:HfO2) and TaN electrode in the structure of an FE thin-film transistor (ITO/IGZO/La:HfO2/TaN), where indium–tin oxide (ITO) and indium–gallium–zinc oxide (IGZO) were used as source/drain and channel materials, respectively. Improved Pr and lower surface roughness were achieved by doped HfO2 and ALD TaN thin films. This synaptic transistor shows long-term potentiation and long-term depression with 200 levels of conductance states, high linearity (Ap, 0.97; Ad, 0.86), high Gmax/Gmin (∼6.1), and low cycle-to-cycle variability. In addition, a pattern recognition accuracy higher than 90% was achieved in an artificial neural network simulation

Physicochemical Screen Effect of Li Ions in Oxygen Redox Cathodes for Advanced Sodium-Ion Batteries

Unlike in lithium-ion batteries (LIBs), in sodium-ion batteries (SIBs), nonhysteretic oxygen redox (OR) reactions are observed in Li-excess Na-layered oxides. This necessitates an understanding of the reaction mechanism of an O3-type Li-excess Mn oxide, Na­[Li1/3Mn2/3]­O2, a novel OR material designed for advanced SIBs. It could establish the role of Li in triggering nonhysteretic oxygen capacities during (de)­sodiation. Three biphasic mechanisms were compared using first-principles calculations under the desodiation modes: (i) Na/vacancy ordering, (ii) Li migration in the NaO2 layer, and (iii) in-plane Mn migration. The migrated Li ions generated a “physicochemical screen” effect upon electrochemical OR reactions in the oxide cathode. Thermodynamic formation energies showed different biphasic pathways upon charging in Na1–x[Li2/6Mn4/6]­O2 (NLMO) under the three modes. O–O bond population indicated that biphasic-reaction paths -i and -iii were derived from generating inter/intralayer O–O dimers, and path-iii was triggered by the formation of a Mn–O2–Mn moiety. However, Li migration exhibited an ideal OR process (O2–/On–) without forming anionic dimers. The electronic structures of Mn­(3d) and O­(2p) revealed that Li migration pushed lattice-based O­(2p)-hole states to a high energy level, resulting in the chemical suppression of O2 molecule formation. Selectively decoupled oxygen ordering indicated that the oxygen species coordinated with two Mn (OMn2) derived from Li migration played an important role in nonhysteretic oxygen capacities during cycling. From these findings, we propose the “physicochemical screen” concept that physically suppresses interlayer O–O dimers and chemically hinders discretized O­(2p)–O­(2p) states formed by molecular O2. This could significantly impact the role of Li ions in Li-excess OR-layered oxides for SIBs

Intrinsic Origin of Nonhysteretic Oxygen Capacity in Conventional Na-Excess Layered Oxides

An intriguing redox chemistry via oxygen has emerged to achieve high-energy-density cathodes and has been intensively studied for practical use of anion-utilization oxides in A-ion batteries (A: Li or Na). However, in general, the oxygen redox disappears in the subsequent discharge with a large voltage hysteresis after the first charge process for A-excess layered oxides exhibiting anion redox. Unlike these hysteretic oxygen redox cathodes, the two Na-excess oxide models Na2IrO3 and Na2RuO3 unambiguously exhibit nonhysteretic oxygen capacities during the first cycle, with honeycomb-ordered superstructures. In this regard, the reaction mechanism in the two cathode models is elucidated to determine the origin of nonhysteretic oxygen capacities using first-principles calculations. First, the vacancy formation energies show that the thermodynamic instability in Na2IrO3 increases at a lower rate than that in Na2RuO3 upon charging. Second, considering that the strains of Ir–O and Ru–O bonding lengths are softened after the single-cation redox of Ru4+/Ru5+ and Ir4+/Ir5+, the contribution in the oxygen redox from x = 0.5 to 0.75 is larger in Na1–xRu0.5O1.5 than that in Na1–xIr0.5O1.5. Third, the charge variations indicate a dominant cation redox activity via Ir­(5d)–O­(2p) for x above 0.5 in Na1–xIr0.5O1.5. Its redox participation occurred with the oxygen redox, opposite to the behavior in Na1–xRu0.5O1.5. These three considerations imply that the chemical weakness of Ir­(5d)–O­(2p) leads to a more redox-active environment of Ir ions and reduces the oxygen redox activity, which triggers the nonhysteretic oxygen capacity during (de)­intercalation. This provides a comprehensive guideline for design of reversible oxygen redox capacities in oxide cathodes for advanced A-ion batteries

Phase Separation and d Electronic Orbitals on Cyclic Degradation in Li–Mn–O Compounds: First-Principles Multiscale Modeling and Experimental Observations

A combined study involving experiments and multiscale computational approaches is conducted to propose a theoretical solution for the suppression of the Jahn–Teller distortion which causes severe cyclic degradation. As-synthesized pristine and Al-doped Mn spinel compounds are the focus to understand the mechanism of the cyclic degradation in terms of the Jahn–Teller distortion, and the electrochemical performance of the Al-doped sample shows enhanced cyclic performance compared with that of the pristine one. Considering the electronic structures of the two systems using first-principles calculations, the pristine spinel suffers entirely from the Jahn–Teller distortion by Mn³⁺, indicating an anisotropic electronic structure, but the Al-doped spinel exhibits an isotropic electronic structure, which means the suppressed Jahn–Teller distortion. A multiscale phase field model in nanodomain shows that the phase separation of the pristine spinel occurs to inactive Li₀Mn₂O₄ (i.e., fully delithiated) gradually during cycles. In contrast, the Al-doped spinel does not show phase separation to an inactive phase. This explains why the Al-doped spinel maintains the capacity of the first charge during the subsequent cycles. On the basis of the mechanistic understanding of the origins and mechanism of the suppression of the Jahn–Teller distortion, fundamental insight for making tremendous cuts in the cyclic degradation could be provided for the Li–Mn–O compounds of Li-ion batteries

Deciphering Enhanced Solid-State Kinetics of Li–S Batteries via Te Doping

Owing to their high gravimetric energy, low cost, and wide availability of required materials, Li–S batteries (LSBs) are considered as a promising next-generation energy storage technology. However, the sluggish redox kinetics and dissolution of lithium polysulfides during the electrochemical reactions are key problems to overcome. The improvement of the long-term cycle life of LSBs solely by converting insoluble solid-state electrolyte-soluble lithium polysulfides (LiPSs) (Li2Sx, where 1 ≤ x ≤ 2, 836 mAh g–1) is an ingenious method, but solid-state LiPS conversion has sluggish redox kinetics owing to the intrinsically low electrical conductivity of solid-state LiPS compounds (Li2S and Li2S2). This study applied Te doping to S cathodes and conducted experimental and theoretical analyses on the Te-doped solid-state LiPSs to investigate the effect of Te on the redox kinetics of the solid-state LiPS conversions for high-performance LSBs. The qualitative and quantitative electrochemical characterization demonstrated that Te induced an increase in the kinetics. Furthermore, the enhanced kinetics were explained at the atomic scale by the theoretical thermodynamics and chemomechanics investigations. The design of high-performance LSBs will benefit the strong understanding of Te-doped S electrodes in solid-state conversion

Determining Factors in Triggering Hysteretic Oxygen Capacities in Lithium-Excess Sodium Layered Oxides

Oxygen redox (OR) reactions in sodium layered oxide cathodes have been studied intensively to harness their full potential in achieving high energy density for sodium-ion batteries (SIBs). However, OR triggers a large hysteretic voltage during discharge after the first charge process for OR-based oxides, and its intrinsic origin is unclear. Therefore, in this study, an in-depth reinvestigation on the fundamentals of the reaction mechanism in Na­[Li1/3Mn2/3]­O2 with a Mn/Li ratio (R) of 2 was performed to determine the factors that polarize the OR activity and to provide design rules leading to nonhysteretic oxygen capacity using first-principles calculations. Based on thermodynamic energies, the O2–/O22– and O2–/On– conditions reveal the monophasic (0.0 ≤ x ≤ 4/6) and biphasic (4/6 ≤ x ≤ 1.0) reactions in Na1–x[Li2/6Mn4/6]­O2, but each stability at x = 5/6 is observed differently. The O–O bond population elucidates that the formation of an interlayer O–O dimer is a critical factor in triggering hysteretic oxygen capacity, whereas that in a mixed layer provides nonhysteretic oxygen capacity after the first charge. In addition, the migration of Li into the 4h site in the Na metallic layer contributes less to the occurrence of voltage hysteresis because of the suppression of the interlayer O–O dimer. These results are clearly elucidated using the combined-phase mixing enthalpies and chemical potentials during the biphasic reaction. To compare the Mn oxide with R = 2, Na1–x[Li1/6Mn5/6]­O2 tuned with R = 5 was investigated using the same procedure, and all the impeding factors in restraining the nonhysteretic OR were not observed. Herein, we suggest two strategies based on three types of OR models: (i) exploiting the migration of Li ions for the suppression of the interlayer O–O dimer and (ii) modulating the Mn/Li ratio for controlling the OR participation, which provides an exciting direction for nonhysteretic oxygen capacities for SIBs and lithium-ion batteries

Critical Role of Titanium in O3-Type Layered Cathode Materials for Sodium-Ion Batteries

Recently, the substitution of inactive elements has been reported as a promising strategy for improving the structural stability and electrochemical performance of layered cathode materials for sodium-ion batteries (SIBs). In this regard, we investigated the positive effects of inactive Ti substitution into O3-type NaFe0.25Ni0.25Mn0.5O2 based on first-principles calculations and electrochemical experiments. After Ti substitution, Na­[Ti0.03(Fe0.25Ni0.25Mn0.5)0.97]­O2 exhibits improved capacity retention and rate capability compared with Ti-free NaFe0.25Ni0.25Mn0.5O2. Such an improvement is primarily attributed to the enhanced structural stability and lowered activation energy for Na+ migration, which is induced by Ti substitution in the host structure. Based on first-principles calculations of the average net charges and partial densities of states, we suggest that Ti substitution effectively enhances the binding between transition metals and oxygen by increasing the oxygen electron density, which in turn lowers the energy barrier of Na+ migration, leading to a notable enhancement in the rate capability of Na­[Ti0.03(Fe0.25Ni0.25Mn0.5)0.97]­O2. Compared with other inactive elements (e.g., Al and Mg), Ti is a more suitable substituent for improving the electrochemical properties of layered cathode materials because of its large total charge variation contributing to capacity. The results of this study provide practical guidelines for developing highly reliable layered cathode materials for SIBs

Analog Synaptic Transistor with Al-Doped HfO₂ Ferroelectric Thin Film

Neuromorphic computing has garnered significant attention because it can overcome the limitations of the current von-Neumann computing system. Analog synaptic devices are essential for realizing hardware-based artificial neuromorphic devices; however, only a few systematic studies in terms of both synaptic materials and device structures have been conducted so far, and thus, further research is required in this direction. In this study, we demonstrate the synaptic characteristics of a ferroelectric material-based thin-film transistor (FeTFT) that uses partial switching of ferroelectric polarization to implement analog conductance modulation. For a ferroelectric material, an aluminum-doped hafnium oxide (Al-doped HfO2) thin film was prepared by atomic layer deposition. As an analog synaptic device, our FeTFT successfully emulated short-term plasticity and long-term plasticity characteristics, such as paired-pulse facilitation and spike timing-dependent plasticity. In addition, we obtained potentiation/depression weight updates with high linearity, an on/off ratio, and low cycle-to-cycle variation by adjusting the amplitude and number of input pulses. In the simulation trained with optimized potentiation/depression conditions, we achieved a pattern recognition accuracy of approximately 90% for the Modified National Institute of Standard and Technology (MNIST) handwritten data set. Our results indicated that ferroelectric transistors can be used as an alternative artificial synapse