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

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

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

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

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