Li-rich Compounds Research Articles

Is There Hope for Ni-Based Disordered Rock Salt Cathode Materials?

Li-ion batteries achieved in the last 3 decades extremely high energy densities, making them the energy storage devices of choice for the electrification of our society. Despite this improvement, their energy density is still low as compared to gasoline, motivating further research towards materials with better performances. State of the art positive electrodes are nowadays layered materials with a ratio Li:M of 1:1, with M being one or multiple transition metals. LiNiO2 is a noteworthy example. Disordered rock salt (DRX) materials were instead thought to be electrochemically inactive until the development of a percolation theory,1 which demonstrated that lithium-rich materials (Li:M>1.1) with a cation disordered rock-salt structure (DRX) can achieve high reversible specific capacities. The general formula of these lithium-rich compounds can be written as Li1+y(MM′)1−yO2 where the Li content ranges usually from 1.05 to 1.33. With M is indicated a redox active specie (Mn, Ni, Fe, Cr, V) and M’ is typically a d0 element, which is redox inactive during charge and discharge and well accommodates the disordered structure.2 Materials with such structures have been shown to deliver extremely high specific capacities, resulting in equally high specific energies.3 In our recent work we investigated two DRX compounds based on Ni-redox. First, Li2NiO3, which is layered (with monoclinic distortion), but that decomposes to a disordered rock salt structure with cycling. The material displays interesting defect chemistry but fast capacity fading.4 More recently, we developed a class of Li-rich compounds with the chemical composition Li2yNiyTi2-3yO2 (y = 0.50, 0.55, 0.60, 0.67). These samples were prepared via solid-state reaction at three different temperatures (700 °C, 800 °C, 900 °C). By X-ray and neutron diffraction, we demonstrated a pure phase solid solution at high temperature, yet, by decreasing the synthesis temperature and increasing the Li content, phase separation occurs into a rock salt-type and a Li2TiO3 phase. The synthesis process has been also verified by DFT and by in situ X-ray diffraction. In all DRX materials synthesized, the electrochemical performances revealed a strong voltage hysteresis between charge and discharge and fast capacity fade.5,6 This raises then the question of whether any Ni-based DRX material can provide high capacity, high rate capability and stability, while avoiding the commonly encountered large hysteresis, or whether this can only be achieved in Mn-based compounds. In this presentation, by discussing our own work and recent literature reports, we aim at answering this question. Bibliography Urban, Alexander, Jinhyuk Lee, and Gerbrand Ceder. “The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes.” Advanced Energy Materials4, 13: 1400478.Chen, Dongchang, Juhyeon Ahn, and Guoying Chen. “An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes.” ACS Energy Letters, 2021, 1358–76.Clément et al., Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes, Energy & Environmental Science, 2020, 13 (2), 345-373Bianchini et al., From LiNiO2 to Li2NiO3: Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides, Chem. Mater. 2020, 32, 9211−9227Reitano, et al. Manuscript submitted, 2024.Li, Biao, Khagesh Kumar, Indrani Roy, Anatolii V. Morozov, Olga V. Emelyanova, Leiting Zhang, Tuncay Koç, et al. “Capturing Dynamic Ligand-to-Metal Charge Transfer with a Long-Lived Cationic Intermediate for Anionic Redox.” Nature Materials21, 10: 1165–74.

Synthesis, Structural and Electrochemical Investigation of Disordered Rock Salt Compounds Based on Ni and Ti

Li-ion batteries achieved in the last 3 decades extremely high energy densities, making them the energy storage devices of choice for the electrification of our society. Despite this improvement, their energy density is still low as compared to gasoline, motivating further research towards materials with better performances. State of the art positive electrodes are nowadays layered materials with a ratio Li:M of 1:1, with M being one or multiple transition metals. Disordered rock salt (DRX) materials without a layered structure were thought to be electrochemically inactive. With the development of the percolation theory, 1(Urban, Lee, and Ceder) it was demonstrated that lithium-rich materials (Li:M>1) with a cation disordered rock-salt structure (DRX) can achieve high reversible specific capacities. The general formula of these lithium-rich compounds can be written Li1+y(MM′)1−yO2 where the Li content ranges usually from 1.05 to 1.33, essential to promote Li percolation pathways. With M is indicated a redox active specie (Mn, Ni, Fe, Cr, V) and M’ is typically a d0 element, which is redox inactive during charge and discharge and therefore stabilizes the disordered structure and acting as charge compensator.2 (Chen, Ahn, and Chen)In this work a class of Li-rich compounds with the chemical composition Li2yNiyTi2-3yO2 has been investigated (y = 0.50, 0.55, 0.60, 0.67). These samples were prepared via solid-state reaction at three different temperatures (700 °C, 800 °C, 900 °C). X-ray diffraction has been used to determine the crystal structure of the samples, revealing a predominant rock salt-type phase. However, by decreasing the synthesis temperature and increasing the Li content, phase separation occurs into a rock salt-type and a Li2TiO3 phase. The synthesis has been also verified by DFT and by in situ X-ray diffraction. Neutron diffraction experiment have been carried out to fully understand the cationic distribution in the samples. Finally, the electrochemical performances of the samples have been investigated in Li half cells, revealing a strong voltage hysteresis between charge and discharge.3 (Li et al.) Bibliography Urban, Alexander, Jinhyuk Lee, and Gerbrand Ceder. “The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes.” Advanced Energy Materials 4, no. 13 (September 2014): 1400478. https://doi.org/10.1002/aenm.201400478.Chen, Dongchang, Juhyeon Ahn, and Guoying Chen. “An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes.” ACS Energy Letters, March 17, 2021, 1358–76. https://doi.org/10.1021/acsenergylett.1c00203.Li, Biao, Khagesh Kumar, Indrani Roy, Anatolii V. Morozov, Olga V. Emelyanova, Leiting Zhang, Tuncay Koç, et al. “Capturing Dynamic Ligand-to-Metal Charge Transfer with a Long-Lived Cationic Intermediate for Anionic Redox.” Nature Materials 21, no. 10 (October 2022): 1165–74. https://doi.org/10.1038/s41563-022-01278-2

Novel Topological Motifs and Superconductivity in Li-Cs System.

In this work, we determined the phase diagram and electronic properties of the Li-Cs system by using an evolutionary crystal structure prediction algorithm coupled with first-principles calculations. We found that Li-rich compounds are more easily formed in a wide range of pressures, while the only predicted Cs-rich compound LiCs3 is thermodynamically stable at pressures above 359 GPa. A topological analysis of crystal structures concludes that both Li6Cs and Li14Cs have a unique topology that has not been reported in existing intermetallics. Of particular interest is the fact that four Li-rich compounds (Li14Cs, Li8Cs, Li7Cs, and Li6Cs) are found to be superconductors with a high critical temperature (∼54 K for Li8Cs at 380 GPa), due to their peculiar structural topologies and notable charge transfer from Li to Cs atoms. Our results not only extend an in-depth understanding of the high-pressure behavior of intermetallic compounds but also provide a new route to design new superconductors.

Cycling mechanism of Li2MnO3: Li–CO2 batteries and commonality on oxygen redox in cathode materials

Summary Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.

Mixed ionic-electronic transport in the high-entropy (Co,Cu,Mg,Ni,Zn)1-xLixO oxides

A series of the high-entropy (Co,Cu,Mg,Ni,Zn)1-xLixO oxides with a lithium substitution level of x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 is evaluated in terms of the crystal structure, morphology and transport properties, with thorough studies aimed at elucidation of the nature of different contributions to the total electrical conductivity. It is found that cubic Fm-3m structure is preserved in the whole investigated series, with (Co,Cu,Mg,Ni,Zn)0.8Li0.2O composition showing a high internal strain, supporting to some degree one of the so-called core effects, anticipated for the high-entropy materials. For samples with Li content x > 0.20 the strain is relaxed by formation of the oxygen vacancies. As unambiguously evidenced by DC polarization experiments and measured impedance spectroscopy data with ionically-blocking Au and reversible Li electrodes used, the previously reported in the literature transition to the lithium superionic conductivity in the Li-rich compounds, up to σi ≈ 1–10⋅10−3 Scm−1, is more complex, with emergence of the electronic conduction as well, reaching similar magnitude for (Co,Cu,Mg,Ni,Zn)0.7Li0.3O. The observed behavior upon increase of lithium concentration (x) can be explained by a qualitative change of the nature of the electronic and ionic defects present in (Co,Cu,Mg,Ni,Zn)1-xLixO series, with initial oxidation of 3d metals (mainly Co), followed by possible formation of the interstitial lithium, and final emergence of the oxygen vacancies. Furthermore, the recorded electrochemical properties of (Co,Cu,Mg,Ni,Zn)0.7Li0.3O lithium cell electrode, suggesting presence of intercalation-like behavior at the initial stages of lithiation, confirm the proposed mixed ionic-electronic conductivity.

The effect of electrochemically inactive Ti substituted for Ru in Li2Ru1-Ti O3 on structure and electrochemical performance

Electrochemical inactive Ti is used as the substituted element to improve the electrochemical properties of layered Li 2 RuO 3 via stabilizing the crystal structure. The approach of substituting electrochemically active with inactive elements has widely been used to improve the electrochemical performance of Li-rich intercalation cathode materials. This especially is true for Li-rich compounds where almost all of the Li + ions are reversibly (de)intercalated during electrochemical cycling. The beneficial mechanism behind this substitution with electrochemically inactive elements is still not clear yet. Li 2 RuO 3 is chosen as basis for a model solid solution system to investigate the effect of electrochemically inactive elements owing to its high specific capacity of more than 300 mAh g −1 and the significant contribution of anion redox mechanism. Herein, Li 2 Ru 1- x Ti x O 3 solid solution series are synthesized and the effect of substituting with electrochemical inactive Ti for Ru on structure and electrochemical performance have been comprehensively investigated. The electrochemical performance is significantly improved, especially for Li 2 Ru 0.8 Ti 0.2 O 3 , and the capacity retention after 50 cycles increases from 81% to 90%, as compared to the end member Li 2 RuO 3 . Results of electrochemical impedance spectroscopy show that Ti substitution reduces the charge transfer impedance, which favors the Li + diffusion across the electrolyte–electrode interface and improves the electronic conductivity. For the first time, nuclear magnetic resonance was utilized to confirm that a small part of Ti ions exchange their position with Li ions in the Li layer. This research provides a better understanding of electrochemical inactive element substitution and strong insights for the functional design of the next generation of Li-rich cathode materials.

Computational framework for a systematic investigation of anionic redox process in Li-rich compounds

Anionic redox processes play a key role in determining the accessible capacity and cycle life of Li-rich cathode materials for batteries. We present a framework for investigating the anionic redox processes based on data readily available from standard DFT calculations. Our recipe includes a method of classifying different anionic species, counting the number of species present in the structure and a preconditioning scheme to promote anionic redox. The method is applied to a set of LixMnO3 (1 ≤ x ≤2) structures, with cationic disorder, to identify the evolution of anionic redox processes during cycling. Additionally, we investigate how different choices of exchange-correlation functionals affect the formation of anionic redox species. The preconditioning of the structures is shown to promote the formation of peroxo-like species. Furthermore, the choice of exchange-correlation functional has a large impact on the type of anionic redox species present, and thus care must be taken when considering localization in anionic species.

Lightning fast conduction

In the intensive search for higher performance Li-ion cathode materials the spotlight is firmly fixed on Li-rich compounds. Now, a strategy utilizing a fluorinated, disordered LiMn2O4 spinel structure shows how ultra-fast Li-ion diffusion and high energy density can be achieved with Mn and O redox.

In situ X-ray Raman spectroscopy and magnetic susceptibility study on the Li[Li0.15Mn1.85]O4 oxygen anion redox reaction.

Li-rich compounds have received significant attention as electrode materials for lithium-ion batteries (LIBs) because of their large rechargeable capacities (qrecha). We have demonstrated a novel reaction scheme of one of the Li-rich compounds, Li[Li0.15Mn1.85]O4, where Mn4+ ions are reduced to lower valence states such as Mn3+ and Mn2+ ions during charging at voltages above 5.0 V.

Thermodynamics of graphite intercalation binary alloys of Li-Na, Na-K, and Li-K from van der Waals density functionals

Graphite may store lithium or potassium, but not sodium, in its interlayer space under ambient conditions. It is, however, unclear whether binary alkali alloys of Li-Na, Li-K, and Na-K may substitute pure Li or K to form binary alkali alloy-graphite intercalation compounds. We investigate thermodynamics of the binary alloy-graphite intercalation compounds using density functional theory with van der Waals density functionals. We find Li-rich co-intercalation compounds and K-rich ones are associated with negative formation energies, and the Na-K alloy has the broadest domain of co-intercalation (approximately up to 36% Na). Because of convexity of the formation-energy functions, these compounds are metastable and tend to decompose even when formation energies are negative. Na metal is among the decomposition products. Binary Li-K alloys in graphite form segregated phases of LiC6 and KC8, and this allows one to fabricate Li-K mixed-ion batteries using graphite anodes, whereas Li-Na and Na-K alloys are thermodynamically unfavorable. The study highlights the importance of convexity of formation-energy functions in thermodynamics of alloy-graphite intercalation compounds.

Anionic Redox in Li-Rich Rocksalt Oxides Studied Via X-Ray Photoelectron Spectroscopy

Layered LiMO2 compounds can be considered as a progressive substitution of excess Li+ for M3+ in the [MO2] layers, with a 1:1 Li:M ratio. A derivative from this type of compound corresponds to a material with the general formula of Li(Li1/3M2/3)O2. This has been alternatively described with the formula Li2MO3 (where M has been reported as Mn, Ru, Ti, Zr, Sn, Pt), and often referred to as a “Li-rich” rocksalt oxide (with Li/M > 1 and space group C2/m). Li-rich rocksalt oxides are of technological importance as cathode materials in rechargeable Li-ion batteries. Recently, the possibility of using oxygen as a redox centre in this kind of oxides has been established in the design of cathode materials with enhanced capacities (> 200 mAh/g). [1-3] Among the numerous compounds with Li-rich Li2MO3 structure type, Li2MnO3 is the most studied so far; the combination with other transition metals such as Co and Ni can help to improve the capacity, rate capability and reduce the capacity fading. In this work, a study of a series of Li-rich compounds with rock salt structure was synthesised with a general formula of Li1+δ(Ni1-x-yCoxMny)1-δO2 and electrochemically cycled versus lithium metal. An increase in the specific capacity compared with materials of the same family such as layered LiNi1/3Co1/3Mn1/3O2 was observed, in line with numerous reports by research groups worldwide. In this presentation, X-ray photoemission spectroscopy of O1s taken at different stages of the load curve of the Li1+δ(Ni1-x-yCoxMny)1-δO2 electrode material will be presented. The experimental evidence pertaining to the oxidation of O2- driving to the formation of peroxo-like species (O2)n− in the material and charge compensating the extraction of Li+ ions by the removal of electrons from lattice oxygen; that allows this material to have more capacity than expected will be critically discussed.

Stabilization of O-O Bonds by d0 Cations in Li4+ xNi1- xWO6 (0 ≤ x ≤ 0.25) Rock Salt Oxides as the Origin of Large Voltage Hysteresis.

Multinary lithium oxides with the rock salt structure are of technological importance as cathode materials in rechargeable lithium ion batteries. Current state-of-the-art cathodes such as LiNi1/3Mn1/3Co1/3O2 rely on redox cycling of earth-abundant transition-metal cations to provide charge capacity. Recently, the possibility of using the oxide anion as a redox center in Li-rich rock salt oxides has been established as a new paradigm in the design of cathode materials with enhanced capacities (>200 mAh/g). To increase the lithium content and access electrons from oxygen-derived states, these materials typically require transition metals in high oxidation states, which can be easily achieved using d0 cations. However, Li-rich rock salt oxides with high valent d0 cations such as Nb5+ and Mo6+ show strikingly high voltage hysteresis between charge and discharge, the origin of which is uninvestigated. In this work, we study a series of Li-rich compounds, Li4+xNi1–xWO6 (0 ≤ x ≤ 0.25) adopting two new and distinct cation-ordered variants of the rock salt structure. The Li4.15Ni0.85WO6 (x = 0.15) phase has a large reversible capacity of 200 mAh/g, without accessing the Ni3+/Ni4+ redox couple, implying that more than two-thirds of the capacity is due to anionic redox, with good cyclability. The presence of the 5d0 W6+ cation affords extensive (>2 V) voltage hysteresis associated with the anionic redox. We present experimental evidence for the formation of strongly stabilized localized O–O single bonds that explain the energy penalty required to reduce the material upon discharge. The high valent d0 cation associates localized anion–anion bonding with the anion redox capacity.

(Invited) Materials Design of High-Energy Cation/Anion Redox in Li-Rich Cathode Materials: Li5FeO4, Li4Mn2O5, and Li2MO3

The energy density of many lithium metal oxide battery cathodes is limited by redox reactions associated with the transition metal alone. Anionic redox reactions are affording new opportunities to increase this energy density. Here, we present results from first-principles density functional theory (DFT) calculations aimed at understanding the combined cation/anion redox of Li-rich materials, and designing new materials that will enable high-capacity, reversible cycling with minimal oxygen evolution. We illustrate this approach on three Li-rich compounds: Li5FeO4, Li4Mn2O5, and Li2MnO3. Li5FeO4: Initial removal of Li is accompanied by Fe migration to form a disordered rocksalt structure. A local Li6-O coordination of oxygen, identified by DFT calculations, raises the O 2p band and enables reversible O-/O2- redox behavior, previously unknown in this material. This insight leads to the prediction and subsequent experimental demonstration of anionic and cationic redox reactions with good reversibility and without any obvious O2 gas release. Li4Mn2O5: We study the recently-reported, high-capacity, disordered rocksalt-type Li4Mn2O5 compound and also determine the ground state ordered structure of Li4Mn2O5 via a DFT-based enumeration method. DFT calculations show that the delithiation process occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: i) Mn3+ ➝ Mn4+ (LixMn2O5, 4 > x > 2), ii) O2− ➝ O1− (2 > x > 1), and iii) Mn4+ ➝ Mn5+ (1 > x > 0) concomitant with Mn migration from the original octahedral site to the adjacent tetrahedral site. Finally, we predict that alloying with M = V and Cr in Li4(Mn,M)2O5 would produce new stable compounds with substantially improved electrochemical properties. Li2 MO3: We catalog the family of Li2 MO3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li2MIO3–Li2MIIO3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density lithium-ion batteries.

(Invited) Understanding the Oxygen Release at High Potential for Transition Metal Oxides

The design of new materials is a pivotal question for the development of the next generation of electrochemical energy storage and conversion devices. Recently, triggering the redox activity of oxygen ions in transition metal oxides used as positive electrodes for Li-ion batteries was proposed as a promising strategy to develop Li-rich compounds with reversible capacity exceeding those of classical insertion layered compounds. Nevertheless, the science at play must be rationalized in order to control this phenomenon and assess the potential applications of such Li-rich electrodes. Indeed, following this discovery, fundamental questions remain. Among them, the exact nature of the oxygen species formed upon oxidation is critical since it governs not only its redox potential but also the discharge behavior for which sluggish kinetics and hysteresis were observed. Using recent examples, the different types of oxygen species will be discussed and their electrochemical behavior compared. Moreover, while early measurements suggested gaseous oxygen to be produced for these Li-rich at potentials above 4.3 V vs. Li+/Li, recent observations point toward oxygen evolution within a potential range that can span from 3.5 to 4.5 V vs. Li+/Li. This observation asks for 1) the development of new analytical tools in order to accurately detect and quantify these gaseous species and 2) to rationalize the standard potential for the evolution of lattice oxygen from the transition metal oxides. These two points will be discussed and strategies to circumvent this detrimental phenomenon will be proposed.

Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li-Sn Compounds at Ambient and Moderately High Pressure.

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li-Sn batteries. It can, however, be overcome with the help of void space engineering by using a LixSn phase as the prelithiated anode, where an optimal value for x is desired. Currently, Li4.25Sn is known as the most lithiated Li-Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (x > 4.25) Li-Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li-Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li-Sn system, in general, but also in improving our understanding of the reaction mechanism in Li-Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li-Sn compounds, our study reveals the existence of five unreported stoichiometries (Li8Sn3, Li3Sn1, Li4Sn1, Li5Sn1, and Li7Sn1) and their associated crystal structures at ambient and high pressure. Although Li8Sn3 has been identified as one of the most stable Li-Sn compound in the entire pressure range (1 atm-20 GPa) with R3̅m symmetry, the Li-rich compounds like Li3Sn1-P2/m, Li4Sn1-R3̅m, Li5Sn1-C2/m, and Li7Sn1-C2/m are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li5Sn1 and Li7Sn1 opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li17Sn4.

Pressure-Induced Stable Li5P for High-Performance Lithium-Ion Batteries

Black phosphorus, the result of white P under high pressure, has received much attention as a promising anode material for Li-ion batteries (LIBs). However, the final product of lithiation, P63/mmc Li3P, is not satisfactory due to its poor conductivity. In this article we explore the high-pressure phase diagram of the Li–P system through first-principles swarm-intelligence structural search and present two hitherto unknown stable Li-rich compounds, Fm-3m Li3P at 4.2 GPa and P6/mmm Li5P at 10.3 GPa. Metallic Li5P exhibits interesting structural features, including graphene-like Li layers and P-centered octadecahedrons, where P is 14-fold coordinated with Li. Interestingly, both compounds exhibit good dynamical and thermal stability properties at ambient pressure, and the theoretical capacity of P6/mmm Li5P reaches 4326 mAhg–1, the highest among the already known Li–P compounds. Additionally, their mechanical properties are also favorable for electrode materials. Our work represents a significant step towar...

Structure Evolution from Layered to Spinel during Synthetic Control and Cycling Process of Fe-Containing Li-Rich Cathode Materials for Lithium-Ion Batteries.

As promising cathode materials for lithium-ion batteries (LIBs), Fe-containing Li-rich compounds of Li1+xFe0.1Ni0.15Mn0.55Oy (0 ≤ x ≤ 0.3 and 1.9 ≤ y ≤ 2.05) have been successfully synthesized by calcining the spherical precursors with appropriate amounts of lithium carbonate. The structures, morphologies, and chemical states of these compounds are characterized to better understand the corresponding electrochemical performances. With an increase of lithium content, Li1+xFe0.1Ni0.15Mn0.55Oy evolves from a complex layered-spinel structure to a layered structure. The lithium content also affects the average size and adhesion of the primary particles. At 0.1 C, sample x = 0.1 shows the highest first charge/discharge specific capacities (338.7 and 254.3 mA h g–1), the highest first Coulombic efficiency (75.1%), the lowest first irreversible capacity loss (84.4 mA h g–1), the highest reversible discharge specific capacity, and good rate capability. Notably, voltage fading can be alleviated through the adjustment of structural features. Such superior electrochemical performances of sample x = 0.1 are ascribed to the hierarchical micro-/nanostructure, the harmonious existence of complex layered-spinel phase, and the low charge-transfer resistance. An integral view of structure evolution from layered to spinel during synthetic control and cycling process is provided to broaden the performance scope of Li–Fe–Ni–Mn–O cathodes for LIBs.

Lithium Malonatoborate Additives Enabled Long Cycling Stability of 5 V Lithium Metal and Lithium Ion Batteries

The developments of advanced Li-ion batteries (LIBs) not only depend on the advances in electrode chemistries but also rely on the improvement of the functional electrolyte system (salt, solvent, and additive) for high ionic conductivity and controlled solid electrolyte interphase (SEI) formation capability 1 To increase the energy density of LIBs, high voltage cathode materials (>4.5 V vs. Li/Li+) such as the Li-rich compounds, olivine-type LiNiPO4, and LiCoPO4, as well as spinel-type LiCoMnO4, and LiNi0.5Mn1.5O4 (LNMO) 2 have been intensively investigated. Particularly, LNMO has attracted more attention because of its fast lithium conduction through the three-dimensional channels and its low cost and environmentally benign. Unfortunately, it causes extensive oxidation of the electrolyte because of its high operation voltage, resulting in thickening of the passivation layer and large irreversible capacity loss. 3 To mitigate severe electrolyte oxidation and improve long cycling performance of LNMO based batteries, different additives have been evaluated. 4 In addition to organic molecules, lithium salts such as lithium bis(oxalato)borate (LiBOB), 5 and lithium difluoro(oxalato)-borate (LiDFOB) 6 have also been evaluated and showed improved long cycling stability. Even though LiBOB and LiDFOB have been extensively studied as additives, the resulting SEI layer and passivation layer generally suffers from high resistance, possibly due to two carbonyl groups being too close to each other. In an effort to reduce the resistance of the SEI layers by increasing the flexibility and yet at the same time enhance the electron delocalization of the anions, we have synthesized a series of fluorinated lithium bis(malonato) borate (LiBMB) salts (Scheme 1), 7-9 the close analog of LiBOB.10 Besides structurally more flexible, the two hydrogens on the C-2 position of the BMB anion can be substituted by different functional groups, and the electrochemical properties of the salts and their SEI and passivation characteristics can be tuned and optimized. Indeed, improvement has been observed by replacing the acidic hydrogen in lithium bis(2-fluoromalonatoborate) (LiBFMB) with a methyl group in lithium bis(2-methyl-2-fluoromalonato) borate (LiBMFMB) (Scheme 1). 7, 11 Unfortunately, the ionic conductivities of both LiBFMB and LiBMFMB based electrolytes are lower than that of LiPF6. To improve the ionic conductivity of these malonatoborate salts, we have further synthesized lithium difluoro-2-alkyl-2-fluoromalonatoborate salts (LiDFMFMB, LiDFEFMB, and LiDFPFMB where alkyl group is methyl, ethyl and propyl, respectively) whose ionic conductivities are close to that of LiPF6 in the carbonate mixture of EC-ethylene carbonate (EMC) (1-2, by wt.). 9 Electrochemical floating test showed that these half salts are less stable than LiPF6 above 4.9 V vs Li/Li+, 9 which inspired us to use them as additives in LNMO based batteries. Indeed, these additives exhibited high coulombic efficiency and improved long cycle stability in LNMO||NG full cells, thus providing a cost-effective way to improve the performance of high voltage LNMO batteries. In addition, the additives also exhibit better passivation of the aluminum current collector.

High Rate Performance Spinel-Layered Based Li-Rich Compounds As Cathode Materials for Next Generation Lithium-Ion Batteries Application

A quest to hunt for reliable, cost-effective and high rate performing materials constitute the hot topic in applications for lithium-ion batteries (LiBs) in hybrid/plug-in electric vehicles. In order to foresee these objectives, herein, we have attempted to synthesis spinel-layered Li-rich Li-Mn-M-O based compound as cathode material for lithium-ion batteries applications via simple, low temperature - solvothermal method. Both as-obtained and annealed samples were characterized for their phase, morphology using x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. When electrochemically tested against Li+/Li0 between 2.0 – 4.8V, the annealed Li-rich spinel-layered exhibited excellent specific capacity and rate capability even at high current density, making it as a probable next generation cathode material for LiBs application.

Switching the Structure Type upon Ag Substitution: Synthesis and Crystal as well as Electronic Structures of Li12AgGe4.

Li-rich compounds of metals and semimetals are interesting candidates for anode materials for rechargeable batteries. The investigation of the Li-rich part of the Li-Ag-Ge phase diagram led to the discovery of the new compound Li12AgGe4, which represents the Li-richest phase in the ternary phase system. The phase-pure compound is synthesized by high-temperature reaction of Li with stoichiometric amounts of premelted reguli of Ag and Ge. The structure was determined by single-crystal X-ray diffraction. Li12AgGe4 crystallizes in the Li13Si4 structure type in the space group Pbam (no. 55) with lattice parameters of a = 8.0420(2) Å, b = 15.1061(4) Å, and c = 4.4867(1) Å and exhibits the unique Zintl anion [AgGe2](7-)-iso(valence) electronic to the CO2 molecule-and Ge2 dumbbells. Li12AgGe4 adopts the atom packing of the lighter homologue Li13Si4 and not that of Li13Ge4 by the selective substitution of one out of seven Li positions by Ag. The calculation of the electronic structure indicates metallic property and the presence of strong covalent bonds between Ag and Ge in the linear triatomic Ge-Ag-Ge unit as well as π character between the Ge atoms of the dumbbells. The Ag-Ge bond order of the linear AgGe2 unit reaches its maximum at EF of Li12AgGe4 with full occupancy of all atomic positions (in contrast to the related Li12Ag1-xSi4), indicating that the formation of covalent Ag-Ge bonds is the driving force for the formation of the structure type.