Li-rich Layered Oxide Cathode Research Articles

Heteroepitaxial oxygen-buffering interface enables a highly stable cobalt-free Li-rich layered oxide cathode

Abstract The oxygen redox process plays an essential role for the high charge-discharge capacity in Li-rich layered oxide (LLO) cathodes. The irreversible release of lattice oxygen may lead to surface reconstruction and cathode-electrolyte interfacial reactions, transition metal (TM) dissolution, as well as microcrack evolution, etc. during cycling that limit the commercial application of LLO cathodes. Herein, we propose the design of a heteroepitaxial Fluorite(CeO2)@Rocksalt@Layered interface with oxygen buffering effects in Cobalt-free Li1.2Mn0.53Ni0.27O2 through the incorporation of ceria. Experimental characterization and theoretical calculations reveal that the fluorite CeO2 nanolayer with oxygen vacancies suppresses the irreversible lattice oxygen loss and cathode-electrolyte interfacial reactions in LLOs. Moreover, the synergy involving the formed rocksalt interphase and Ce3+ doping in the bulk not only stabilizes the structural integrity, resulting in substantial enhancement of capacity/voltage retention, but also accelerates the electrochemical kinetics upon cycling. This finding may pave the path for utilizing the reversible oxygen redox process and designing new high capacity TM-oxide cathode materials.

In Situ Electrooxidation of Electrolyte Additive to Improve Capacity Retention in Li-Rich Layered Oxide Cathode, Li2RuO3

Conventional positive electrode materials for Li-ion technology utilize a Li+ intercalation mechanism charge-balanced by redox on transition metals within an oxide host lattice. Moving forward, multi-electron cathodes that gain capacity by contribution to the redox from the structural anions are of interest for the hybrid intercalation- and conversion-type chemistry during cycling. One such material, Li2RuO3, first studied by Goodenough and coworkers, exhibits high capacity but also a significant capacity loss after the first charge and long-term capacity fade. We suspect the capacity loss is due to structural distortions induced as the lattice accommodates the removal of Li+. Electrolyte additives with tailored chemistry can be an efficient and cost-effective way to stabilize structural changes and promote reversible cycling. Here, in situ, irreversibleelectrooxidation of soluble electrolyte additives at the cathode-electrolyte interface was investigated as a route to stabilizing the cathode surface. Electrooxidation causes dramatically improved capacity retention and stabilizes high-voltage oxidation processes in Li2RuO3. Through understanding how to stabilize structural distortions in Li-rich layered oxide cathodes, we hope to promote reversible cycling near theoretical capacity and target design principles for active electrolyte additives that enhance battery performance.

Local Redox Reaction of High Valence Manganese in Li2MnO3-Based Lithium Battery Cathodes

Summary Li2MnO3 endows Li-rich layered oxides with a superior energy density; however, its electron transfer mechanism is still unclear. Here, the electronic and thermodynamic behavior of LixMnO3 (x = 0.5–2) is determined using first-principles computations. The cationic redox, anionic redox, and local electron exchange in Mn-O polyhedron are revealed. An obvious electron donation inclination of manganese is demonstrated when the Li+ content is low. Through ab initio molecular dynamics, high valence Mn6+ and Mn7+ are generated in the form of MnO4 tetrahedrons with structural transformation toward a disordered structure, controlled by the Li+ content and dynamic energy barrier. Furthermore, a local redox reaction between Mn6+ or Mn7+ and their surrounding oxygen atoms is elucidated. Consequently, a comprehensive understanding regarding the electron transfer in LixMnO3 is provided. Our results bring insight regarding the electron transfer mechanism in Li-rich layered oxide cathode materials and encourage further reconsideration and investigation into its redox mechanism.

Structure and Interface Design Enable Stable Li-Rich Cathode.

Li-rich layered-oxide cathodes have the highest theoretical energy density among all the intercalated cathodes, which have attracted intense interests for high-energy Li-ion batteries. However, O3-structured layered-oxide cathodes suffer from a low initial Coulombic efficiency (CE), severe voltage fade, and poor cycling stability because of the continuous oxygen release, structural rearrangements due to irreversible transition-metal migration, and serious side reactions between the delithiated cathode and electrolyte. Herein, we report that these challenges are migrated by using a stable O2-structured Li1.2Ni0.13Co0.13Mn0.54O2 (O2-LR-NCM) and all-fluorinated electrolyte. The O2-LR-NCM can restrict the transition metals migrating into the Li layer, and the in situ formed fluorinated cathode-electrolyte interphase (CEI) on the surface of the O2-LR-NCM from the decomposition of all-fluorinated electrolyte during initial cycles effectively restrains the structure transition, suppresses the O2 release, and thereby safeguards the transition metal redox couples, enabling a highly reversible and stable oxygen redox reaction. O2-LR-NCM in all fluorinated electrolytes achieves a high initial CE of 99.82%, a cycling CE of >99.9%, a high reversible capacity of 278 mAh/g, and high capacity retention of 83.3% after 100 cycles. The synergic design of electrolyte and cathode structure represents a promising direction to stabilize high-energy cathodes.

Lithium bis(oxalate)borate additive in the electrolyte to improve Li-rich layered oxide cathode materials

Lithium bis(oxalate)borate electrolyte additive improves the performance of Li-rich layered oxide cathodes due to its preferential combination with the generated active oxygen to form a stable boron-enriched interfacial film.

Effect of Different Composition on Voltage Attenuation of Li-Rich Cathode Material for Lithium-Ion Batteries.

Li-rich layered oxide cathode materials have become one of the most promising cathode materials for high specific energy lithium-ion batteries owning to its high theoretical specific capacity, low cost, high operating voltage and environmental friendliness. Yet they suffer from severe capacity and voltage attenuation during prolong cycling, which blocks their commercial application. To clarify these causes, we synthesize Li1.5Mn0.55Ni0.4Co0.05O2.5 (Li1.2Mn0.44Ni0.32Co0.04O2) with high-nickel-content cathode material by a solid-sate complexation method, and it manifests a lot slower capacity and voltage attenuation during prolong cycling compared to Li1.5Mn0.66Ni0.17Co0.17O2.5 (Li1.2Mn0.54Ni0.13Co0.13O2) and Li1.5Mn0.65Ni0.25Co0.1O2.5 (Li1.2Mn0.52Ni0.2Co0.08O2) cathode materials. The capacity retention at 1 C after 100 cycles reaches to 87.5% and the voltage attenuation after 100 cycles is only 0.460 V. Combining X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), it indicates that increasing the nickel content not only stabilizes the structure but also alleviates the attenuation of capacity and voltage. Therefore, it provides a new idea for designing of Li-rich layered oxide cathode materials that suppress voltage and capacity attenuation.

Stabilizing Li-rich layered cathode materials by nanolayer-confined crystal growth for Li-ion batteries

Li-rich layered oxides have aroused much interest in high-energy cathode materials for Li-ion batteries (LIBs). However, they still come with significant challenges of appreciable voltage decay and hysteresis upon cycling. Microstructure and surface are considered to be crucial for the stabilization of high-capacity Li-rich layered oxide cathode. In this work, we report the controlled synthesis of unique Li-rich layered nanobelts through nanolayer-confined crystal growth. The unique one-dimensional Li1.2Mn0.54Co0.13Ni0.13AlxO2 nanostructures and reconstruction of spinel phase domains may not only offer fast Li-ion diffusion pathways, but inhibit the structural collapse and mitigate the voltage decay upon discharge/charge cycling. The cathode made of Li1.2Mn0.54Co0.13Ni0.13AlxO2 layered/spinel nanobelts exhibits enhanced rate capability and long-term cyclability. An initial discharge capacity of 283.5 mA h g−1 at 0.1 C and the capacity retention of 88.1% over 100 cycles at 1 C are achieved. This work may pave an avenue toward nanostructuring and surface engineering of electrode materials and practical applications of high-energy-density LIBs.

An appropriate amount of new spinel phase induced by control synthesis for the improvement of electrochemical performance of Li-rich layered oxide cathode material

Reconstructing a novel layered-spinel composite structure in Li-rich layered oxides (LLOs) is considered as an efficient method to improve its electrochemical performance, including the initial coulombic efficiency (ICE), rate and cycle performances. Herein, we report the preparation and characterization of the Li1+xMn0.54Ni0·13Co0·13O2 (x = 0.08, 0.14, 0.20 and 0.26) samples by a modified sol-gel route with an annealing process. X-ray diffraction (XRD) patterns in combination of their refinement results, Raman spectra, high resolution transmission electron microscopy (HRTEM), selected electron diffraction (SEAD) and electrochemical measurements of the materials indicate the presence of different amounts of novel cobalt-rich spinel phase (LiCoMnO4) within these materials, which increases with the decrease of relative lithium compositions. It is found that an appropriate amount of new spinel phase induced by the control synthesis process (using reasonable lithium usage and annealing treatment) contributes to the improvement of the electrochemical performance of LLO, specifically, the Li1·14Mn0·54Ni0·13Co0·13O2 (Li1.14) sample delivers a high initial discharge specific capacity and ICE of 303.0 mA h g−1 and 77.0%, respectively at 20 mA g−1, good rate capability (∼200 mA h g−1 at 1.0 C rate; 1.0 C = 200 mA g−1) and capacity retention capability (83.7% after 100 cycles at 1.0 C rate). The results presented herein offer evidence that the incorporation of the novel cobalt-rich spinel phase in LLOs by a simple synthesis route is a feasible way to improve its electrochemical performance.

Surface Ni-rich engineering towards highly stable Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials

Li-rich layered oxide cathode materials (LLOs) are regarded as promising next-generation cathode candidate in high-energy-density lithium ion batteries due to their high specific capacity over 250 mA h g−1. However, LLOs always suffer from a series of severe issues, such as rapid voltage fading, fast capacity decay and bad cycling stability. In this work, Li1.2Mn0.54Ni0.13Co0.13O2-Li1.2Mn0.44Ni0.32Co0.04O2 (LLO-111@111/811) hybrid layered-layered cathode is constructed via facilely increasing surface Ni content. Profiting from this special design, the prepared LLO-111@111/811 cathode exhibits a remarkable specific capacity of 249 mA h g−1 with a high capacity retention of 89.3% and a high discharge voltage of 3.57 V with a voltage retention of 83.0% after cycling 350 times at 0.5 C. As a result, the specific energy of LLO-111@111/811 cathode is 887 Wh Kg−1 at 0.5 C and it keeps as high as 658 Wh Kg−1 after 350 cycles. LLO-111@111/811 also exhibits an initial high capacity of 169 mA h g−1 at a high rate of 5 C and maintains a good capacity retention of 90.0% after 200 cycles. This strategy can successfully improve structural stability, suppress capacity decay and restrain voltage fading of LLOs, which is beneficial for their practical application.

Mechanism of Structural Change and the Trigger of Electrochemical Degradation of Li-Rich Layered Oxide Cathodes during Charge–Discharge Cycles

Li-rich layered oxides are promising candidates for next-generation cathode materials for Li-ion batteries because of their high capacity. However, a low cycle durability accompanied by structural ...

Local structure adaptability through multi cations for oxygen redox accommodation in Li-Rich layered oxides

Stable lattice oxygen redox (l-OR) is the key enabler for achieving attainable high energy density in Li-rich layered oxide cathode materials for Li-ion batteries. However, the unique local structure response to oxygen redox in these materials, resulting in energy inefficiency and hysteresis, still remains elusive, preventing their potential applications. By combining the state-of-the-art neutron pair distribution function with crystal orbital overlap analysis, we directly observe the distinct local structure adaption originated from the potential O–O chemical bonds. The structure adaptability is optimized based on the nature of multi transition metals in our model compound Li1.2Ni0.13Mn0.54Co0.13O2, which accommodates the oxygen redox and at the same time preserves the global layered structure. These findings not only advance the understanding of l-OR, but also provide new perspectives in the rational design of high-energy-density cathode materials with reversible and stable l-OR.

Influence of Defect Reaction on Electrical/Electrochemical Behaviors of Oxide Electrodes for Li Batteries

Functional oxides are known to show interesting electrical properties by introduction of desired defect structures. Electrode performance in Li batteries is also highly dependent on its defect reaction through synthesis and/or charging-discharging cycles. For examples, the reversible capacity of layered cathode such as LiCoO2 is limited by its highly defective layered structure when state of charge greater than 50%. On the other hand, capacity of spinel or olivine cathodes shows higher delithiation rate due to better structural stability. In this study, the behavior of Li-rich layered oxide cathode and Li4Ti5O12 (LTO) are investigated and illustrated based on their defect reactions. For examples, the as-received zero-stain anode oxide Li4Ti5O12 shows conductivity as low as 10-9 S/cm and then causes high interface polarization and low rate capability. With proper processing under low pO2, the electron conduction and electrochemical properties of LTO was significantly improved. Such property enhancement may be well illustrated by defect reaction under low pO2 environment. In other words, lattice oxygen near surface of LTO tends to be removed under very low pO2. Consequently, positively charged oxygen vacancy is formed and then charge-compensated by the creation of two negatively charged electrons. These electrons are eventually associated with tetravalent Ti ions. The fast electron pathway is established by the coexistence of Ti+3 and Ti+4 in Ti cation sublattice. Furthermore, Li-rich layer-structured cathode formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) during 1st charging process was found to exhibit oxygen oxidation, vacancy formation. In the following discharging, the manganese reduction (or activation) occurred. As a result, a reversible capacity as high as 250 mAh/g was observed (Fig. 1(a)). Such redox reactions may be illustrated by defect chemistry. In other words, during 1st charging, electrons and oxygen was extracted from lattice oxygen with the formation of oxygen vacancies. In the next discharging process, electrons was accepted by cathode oxides accompanied by valence changing of Mn+4 to Mn+3. It is known that typical electrochemical reactions are highly dependent up movement of electrons and ions. With understanding of defect reaction during materials processing and charging/discharging, advanced electrode materials with much improved electrochemical and/or stability may be developed.

Modification of Electrochemical Behavior of Li-Rich Layered Oxide Cathode through Defect Consideration

Cathode materials formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) with layered structure received much attention due to their surprisingly high reversible capacity for Li-ion battery applications.[1,2] For better understanding of such high-capacity layered cathode, a systematic study on powder morphology, crystal structures and reaction mechanisms of Li1.2Mn0.54Ni0.13Co0.13O2 during electrochemical charge/discharge cycles were conducted. First, the capacity of Li-rich layered cathode powder was strongly affected by its particle size. To obtain various particle sizes of cathode powder, different powder synthesis routes have been adopted. By usng sol-gel process, particle size of Li1.2Mn0.54Ni0.13Co0.13O2 powder obtained is around 200 nm. On the other hand, Li1.2Mn0.54Ni0.13Co0.13O2 powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place. Although Li-rich layer-structured cathode material show a reversible capacity as high as 250 mAh/g at low rate, its electrochemical properties such as capacity loss at first cycle, rate capability and capacity fading still need to be examined and characterized. In this study, Li1.2Mn0.54Ni0.13Co0.13O2 were investigated. The TEM structural analysis shows that the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.

Probing and suppressing voltage fade of Li-rich Li1.2Ni0.13Co0.13Mn0.54O2 cathode material for lithium-ion battery

The voltage fade of Li-rich layered oxide cathode material Li1.2Ni0.13Co0.13Mn0.54O2 (LNCM) heavily hinders its application in Li-ion batteries. Herein, we revisit the origin of the voltage fade of LNCM and propose a solution to suppress this effect. It is demonstrated that the voltage fade of the LNCM cathode comes from the structural change of the crystal from layer structure to spinel structure, involving all the cations rearrangement. Such rearrangement of all the cations in LNCM is due to the high degree of delithiation and oxygen release at high cutoff voltage of 4.8 V. It is also evidenced that nickel and cobalt change from low valence to high valence at the discharged state, which not only inhibits the transport of lithium ions, but also leads to the loss of high voltage platforms. In particular, the cation rearrangement of Li/Mn causes valence change from Mn4+ to Mn3+, resulting in the decrease of discharge voltage platform at the cutoff voltage of 4.8 V much higher than at 4.6 V. The lower charge cutoff voltage can be a solution to suppress the voltage fade of LNCM cathode materials and have a good stability of LNCM cathode materials without compromising the battery performance.

Effect of the Cooling Process on the Structure and Charge/Discharge Cycling Performance of Li2MnO3 -LiNi0.5Mn0.5O2 -LiNi1/3Co1/3Mn1/3O2 Li-Rich Solid-Solution Layered Oxide Cathode Materials for Li-Ion Battery

The effect of the cooling process on charging/discharging capacities of Li2MnO3- LiNi1/2Mn1/2O2-LiNi1/3Mn1/3Co1/3O2 Li-rich solid-solution layered oxide (LLO) cathode materials for lithium ion battery was examined by applying two types of the cooling processes of quenching the calcinated LLO samples with nitrogen liquid and slow cooling of the LLO samples in the furnace at the controlled decreasing rate of the temperature to thirteen LLO samples having different compositions. The results of the comparison between discharging capacities observed with the LLO samples prepared with two types of the cooling processes indicated that the cooling process for the LLO samples to exhibit high discharge capacity was not limited to either one. Which process was effective for the LLO samples to exhibit high discharge capacity depended on the composition of the LLO samples. In the composition of the Li[Li0.20Mn0.58Ni0.18Co0.04]O2 sample (Li2MnO3 (60%) -LiNi0.5Mn0.5O2 (30%) - LiNi1/3Co1/3Mn1/3O2 (10%)) which exhibited the best cathode performance among the LLOs examined, the effect of quenching was maximum and as the composition of the LLOs went away from the composition of the Li[Li0.20Mn0.58Ni0.18Co0.04]O2 sample, the effect of quenching became smaller and slow cooling process was superior to the quenching process. The structural difference of the LLO samples prepared with two types of the cooling process was made connection with the cathode performance observed. Figure 1

Improvement of the Cycling Stability of Li-Rich Layered Mn-Based Oxide Cathodes Modified by Nanoscale LaPO4 Coating

Surface modification is usually an effective strategy to improve the cycling stability and rate capability of the Li-rich layered oxide cathode materials. Herein, the high-crystallinity LaPO4 with good ionic conductivity was homogeneously deposited on the surface of Li-rich layered oxide by the slow formation of LaPO4 nanoparticles because of chelating effect between citric acid and La3+ as well as the using of appropriate phosphorus source. The surface structure and electrochemical properties of Li-rich Mn-based materials were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and galvanostatic charge/discharge tests. The results indicate that LaPO4 nanoparticles are homogeneously coated on the surface of Li-rich layered Mn-based oxide, and the modification of LaPO4 with appropriate nanoscale thickness can obviously promote the cycling stability and rate capability of cathode material. Especially, the material modified by 2 wt % LaP...

Precursor pre-oxidation enables highly exposed plane {010} for high-rate Li-rich layered oxide cathode materials

Precursor synthesis technology is a key factor to improve the electrochemical properties of Li-rich layered oxide (LLO) cathode materials. However, the effect of precursor preparation strategies on the morphology and electrochemical properties of cathode materials has not been clearly elucidated. In this work, an LLO cathode material (denoted as LMNC-300) with highly exposed {010} planes and a lower amount of surface residual lithium is reported via pre-oxidation of a precursor at 300 °C. This LMNC-300 cathode material exhibits an initial Coulombic efficiency of 91.7% with a discharge capacity of 277.2 mA h g−1 at 0.1 C, an excellent high-rate capability with a discharge capacity of 175.1 mA h g−1 even at 5.0 C and a good cycling performance with a capacity retention of 92.6% after 200 cycles at 0.5 C. In addition, a pouch cell consisted of the LMNC-300 cathode and a commercial graphite anode presented good cycling stability with a capacity retention of 81.6% after 500 cycles at 2.0 C. The outstanding electrochemical performance can be ascribed to an accelerated Li+ diffusion dynamics. Therefore, the method reported in this work will be significant to understanding the effects of preparation strategies of transition metal precursors on the electrochemical performance of Li-rich layered cathode materials for high-energy density Li-ion batteries.

Defects in Li-rich manganese-based layered oxide: A first-principles study

Lithium-rich manganese-based layered oxides are of great interest as cathode materials for lithium ion batteries due to their high energy density. The voltage decay and capacity fading during prolonged charge/discharge cycling are the key obstacles for their practical usage. In this work, using density functional theory, we investigated the origin of the Ni surface segregation by calculating the defect formation energies of antisite defects, including Ni cation substituting a Li cation [Formula: see text] and pairs of Ni cation swapping with Li cation ([Formula: see text]–[Formula: see text]) in [Formula: see text], [Formula: see text] and [Formula: see text] to represent the Mn-rich, Mn–Ni equivalence and Ni-rich conditions, respectively. Results show that [Formula: see text]–[Formula: see text] defect is of energy favorable in Li-rich and Mn-rich layered oxide cathode. Al-doping is beneficial for improving the structure stability and preventing the surface segregation, thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes. This work provides deep insights for the design of layered cathode materials with both high capacity and voltage stability during cycling.

Synthesis, microstructure, and electrochemical performance of Li-rich layered oxide cathode materials for Li-ion batteries

Li-rich layered oxides Li1.2Mn0.54Ni0.13Co0.13O2 were synthesized by modified Pechini method using various compositions of the reaction mixture. Difference in the electrochemical performance of cathodes on their basis is explained by different morphology and microstructure of the materials. The porous hierarchical structure favors a better electrochemical performance. The presence of defects, including crystal twins, in the samples is considered to be a major reason that leads to their poor cyclability and rate capability.

Stable heteroepitaxial interface of Li-rich layered oxide cathodes with enhanced lithium storage

Lithium and oxygen activities can have substantial influences on the kinetics of ion and electron transport and the structural integrity of Li-rich layered oxide (LLO) cathodes, since reversible oxygen redox is ascribed to the extra capacity beyond the theoretical capacity from transition metal redox at high voltages. Herein, we demonstrate a liquid-solid interfacial reaction to generate a heteroepitaxial interface with tunable Li/O activities on LLOs using molten boric acid. The experimental and theoretical analyses indicate that the atomic scale interface is comprised of a disordered rock salt structure containing substantial Li/O vacancies along the layered structure, associated with a segregation tendency of Ni and Co. The formation of this heteroepitaxial interface with Li/O vacancies improves the ionic/electronic conduction and electrochemical/structural stability, leading to a high discharge capacity of 283 mA h g-1 with initial Coulombic efficiency of 91.7% (0.1 C, 2.0–4.7 V vs. Li+/Li), excellent rate performance (246 and 159.7 mAh g-1 at 1 C and 10 C, respectively) and enhanced cyclic performance with a capacity retention of 92% after 100 cycles. The findings highlight the importance of a well-engineered interface for the design of high performance layered cathode materials for Li storage.