Li-rich Layered Oxide Research Articles

Semi-Metallic Superionic Layers Suppressing Voltage Fading of Li-Rich Layered Oxide Towards Superior-Stable Li-Ion Batteries.

Li-rich layered oxides (LRLOs) with greater specific capacity density are constrained by voltage attenuation and inferior rate performance because of irreversible oxygen release, metal dissolving and poor lithium-ion transport capacity. Herein, a simple surface modification is designed to solve the performance degradation and structural collapse of LRLOs. Combining experiments with density functional theory (DFT) calculations, a semi-metallic LiMn2 O4 -like structure (LMO) with spin-polarized conducting electrons, is introduced to the surface of the cathode restrains the activated surficial lattice oxygen ions by its stable oxygen vacancies. Additionally, Ni doping results in a fast-ion conductor Li0.8 Nb0.96 Ni0.2 O3 structure (LNO) with lowered lithium ions diffusion barrier, which is tightly conjugating to substrate and synergistically reinforces the Li diffusion path through the cathode-electrolyte interphase. Moreover, Mn dissolution is successfully relieved due to the decrease in Mn concentration in the coating layers. As a result, the modified material (LRLO@LMO@LNO) exhibits an ultra-high discharge capacity of 120.4 mAh g-1 even at 10 C with a very small discharge voltage attenuation of 313 mV after 600 cycles (0.52 mV per cycle) at 1 C. Undoubtedly, this method discloses a simple and effective approach to promote the practical utilization of high-energy-density.

Artificial Post-Cycled Structure Modulation Towards Highly Stable Li-Rich Layered Cathode.

High-capacity Li-rich layered oxides (LLOs) suffer from severe structure degradation due to the utilization of hybrid anion- and cation-redox activity. The native post-cycled structure, composed of progressively densified defective spinel layer (DSL) and intrinsic cations mixing, is deemed as the hindrance of the rapid and reversible de/intercalation of Li+ . Herein, the artificial post-cycled structure consisting of artificial DSL and inner cations mixing is in situ constructed, which would act as a shield against the irreversible oxygen emission and undesirable transition metal migration by suppressing anion redox activity and modulating cation mixing. Eventually, the modified DSL-2% Li-rich cathode demonstrates remarkable electrochemical properties with a high discharge capacity of 187 mAh g-1 after 500 cycles at 2 C, and improved voltage stability. Even under harsh operating conditions of 50 °C, DSL-2% can provide a high discharge capacity of 168 mAh g-1 after 250 cycles at 2 C, which is much higher than that of pristine LLO (92 mAh g-1 ). Furthermore, the artificial post-cycled structure provides a novel perspective on the role of native post-cycled structure in sustaining the lattice structure of the lithium-depleted region and also provides an insightful universal design principle for highly stable intercalated materials with anionic redox activity.

Anion-Cation Dual-Ion Multisite Doping Stabilizes the Crystal Structure of Li-Rich Layered Oxides.

Li-rich layered oxide (LLOs) cathode materials are gaining increasing attention as lithium-ion batteries (LIBs) pursue greater energy density. However, LLOs still suffer from severe capacity fading and voltage decay due to their unstable crystal structure. Hence, the anion-cation dual-ion multisite doping strategy based on Mg and S atoms is used to stabilize the crystal structures of LLOs. Mg substitutes Li atoms in the Li and transition-metal (TM) layers, while S atoms occupy tetrahedral interstitial sites and lattice O sites, all of which contribute to the crystal structure stability of LLOs. Theoretical calculations show that Mg/S dual-ion multisite doping successfully reduces the energy levels of the TM 3d-O 2p and isolated O 2p orbitals, which effectively stabilizes the lattice oxygen. Therefore, multisite-doped samples exhibit promising electrochemical performance. This strategy provides a new approach to enhance the crystal structure stability of LLOs for the design of high-energy-density Li-ion batteries.

Efficient improvement in electrochemical properties of high-voltage Li-rich Mn-based layered oxide cathode by addition of 1,3-divinyltetramethyldisiloxane to electrolyte

1,3-Divinyltetramethyldisiloxane (DTMS) is a multifunctional additive that is used to improves the cycling stability and capacity retention of Li-rich Mn-based layered oxide cathodes (LRMs). Cycling performance evaluations demonstrate that LRM/Li cells without the additive exhibit lower capacity retention. DTMS can significantly improve the capacity retention of LRM/Li cells from 42.8% to 75% at 0.5C. Theoretical calculations indicate that DTMS is preferentially oxidized on the LRM surface. Physical characterization results reveal that DTMS generates a layer on the LRM surface that is both less resistive and thinner than the LRM by reacting with HF/F− from the electrolyte. This DTMS-derived layer inhibits adverse reactions between the cathode and electrolyte, thus, effectively maintaining the cathode structure.

Optimizing the potential of intercalation on anode for long-cycle 420 Wh/kg Li-ion batteries

Utilization of novel cathode and anode materials is expected to achieve ultrahigh energy density of Li-ion batteries, but it usually suffers from a large capacity fade in full cells. Here we propose a strategy towards optimizing reversibility of intercalation/deintercalation in anode via regulating the negative/positive capacity ratio (N/P) ratios in full cells with Li-rich layered oxide cathode and SiOx@graphite anode. Compared with high N/P ratio, the cell with an optimal N/P ratio of 0.90 displays the best cycling performance. Some comprehensive characterizations such as in-situ pressure and electrochemical measurements reveal that the lower N/P ratio could reduce the polarization on this composite anode to maximize the utilization of the graphite component and minimize the irreversible pressure growth. On the basis of unraveled failure mechanisms, a 21 Ah multilayer pouch cell with low N/P ratio delivers the energy density up to 420 Wh kg−1 based on total mass of the cell and the capacity retention retains 82% after 300 cycles at 0.2 C. This work highlights the strong coupling between cell parameters and electrochemical performances, providing a new insight towards the dominant factor for irreversible Li-ion behavior during cycling besides the structural degradation of materials for high-energy-density Li-ion batteries.

A Li-rich layered oxide cathode with negligible voltage decay

A Li-rich layered oxide cathode with negligible voltage decay

In situ cathode-electrolyte interphase enables high cycling stability of Co-free Li-rich layered cathodes

Despite the extensive research in Li-rich layered oxides (LLOs), which are promising candidates for high-energy density cathodes, their cycle life still cannot meet the real-world application requirements. The poor cycle performance arises from the electrolyte decomposition at high voltage, resulting in damage and subsequent surface-initiated conversion of the cathode from layered to spinel phase. This problem is even more challenging for Co-free LLO cathodes. Here, we report a one-pot synthesis of in situ carbonate-coated nanostructured Co-free LLO (Li2CO3@LLO) through a polyol-assisted method. This inorganic coating suppresses oxygen release, provides good Li–ion transport, and protects the cathode from adverse reactions with the electrolyte. The obtained material exhibits excellent long-term stability, with 76% capacity retention after 1000 cycles at a 0.2 C rate without any Co addition, demonstrating a path forward for using LLOs as a next-generation Li–ion battery cathode.

Understanding the Activation of Anionic Redox Chemistry in Ti4+-Substituted Li2MnO3 as a Cathode Material for Li-Ion Batteries

Layered Li-rich oxides, demonstrating both cationic and anionic redox chemistry being used as positive electrodes for Li-ion batteries, have raised interest due to their high specific discharge capacities exceeding 250 mAh/g. However, irreversible structural transformations triggered by anionic redox chemistry result in pronounced voltage fade (i.e., lowering the specific energy by a gradual decay of discharge potential) upon extended galvanostatic cycling. Activating or suppressing oxygen anionic redox through structural stabilization induced by redox-inactive cation substitution is a well-known strategy. However, less emphasis has been put on the correlation between substitution degree and the activation/suppression of the anionic redox. In this work, Ti4+-substituted Li2MnO3 was synthesized via a facile solution-gel method. Ti4+ is selected as a dopant as it contains no partially filled d-orbitals. Our study revealed that the layered “honeycomb-ordered” C2/m structure is preserved when increasing the Ti content to x = 0.2 in the Li2Mn1–xTixO3 solid solution, as shown by electron diffraction and aberration-corrected scanning transmission electron microscopy. Galvanostatic cycling hints at a delayed oxygen release, due to an improved reversibility of the anionic redox, during the first 10 charge–discharge cycles for the x = 0.2 composition compared to the parent material (x = 0), followed by pronounced oxygen redox activity afterward. The latter originates from a low activation energy barrier toward O–O dimer formation and Mn migration in Li2Mn0.8Ti0.2O3, as deduced from first-principles molecular dynamics (MD) simulations for the “charged” state. Upon lowering the Ti substitution to x = 0.05, the structural stability was drastically improved based on our MD analysis, stressing the importance of carefully optimizing the substitution degree to achieve the best electrochemical performance.

Cover Feature: Boosting the Electrochemical Performance of Lithium‐Rich Cathodes by Oxygen Vacancy Engineering (Batteries & Supercaps 7/2023)

The Cover Feature shows the Li-rich layered oxide, as promising cathode materials for powering electric vehicle (EVs) applications due to their high energy density. Post-annealing strategy is developed to pre-introduce oxygen vacancies (OVs) into Li1.2Mn0.457Ni0.229Co0.114O2 cathode to modify the local Mn coordination environments, enhance structural stability, and suppress oxygen release. The presence of oxygen vacancies through thermal expansion diminishes the phase transitions in cathode materials during the heating process. More information can be found in the Research Article by H. Gao, G. Wang and co-workers.

Correlating Rate-Dependent Transition Metal Dissolution between Structure Degradation in Li-Rich Layered Oxides.

Understanding the mechanism of the rate-dependent electrochemical performance degradation in cathodes is crucial to developing fast charging/discharging cathodes for Li-ion batteries. Here, taking Li-rich layered oxide Li1.2 Ni0.13 Co0.13 Mn0.54 O2 as the model cathode, the mechanisms of performance degradation at low and high rates are comparatively investigated from two aspects, the transition metal (TM) dissolution and the structure change. Quantitative analyses combining spatial-resolved synchrotron X-ray fluorescence (XRF) imaging, synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques reveal that low-rate cycling leads to gradient TM dissolution and severe bulk structure degradation within the individual secondary particles, and especially the latter causes lots of microcracks within secondary particles, and becomes the main reason for the fast capacity and voltage decay. In contrast, high-rate cycling leads to more TM dissolution than low-rate cycling, which concentrates at the particle surface and directly induces the more severe surface structure degradation to the electrochemically inactive rock-salt phase, eventually causing a faster capacity and voltage decay than low-rate cycling. These findings highlight the protection of the surface structure for developing fast charging/discharging cathodes for Li-ion batteries.

Origin of the Seriously Limited Anionic Redox Reaction of Li-Rich Cathodes in Sulfide All-Solid-State Batteries.

Li-rich layered oxide (LLO) cathode materials with mixed cationic and anionic redox reactions display much higher specific capacity than other traditional layered oxide materials. However, the practical specific capacity of LLO during the first cycle in sulfide all-solid-state lithium-ion batteries (ASSLBs) is extremely low. Herein, the capacity contribution of each redox reaction in LLO during the first charging process is qualitatively and quantitatively analyzed by comprehensive electrochemical and structural measurements. The results demonstrate that the cationic redox of the LiTMO2 (TM = Ni, Co, Mn) phase is almost complete, while the anionic redox of the Li2MnO3 phase is seriously limited due to the sluggish transport kinetics and severe LLO/Li6PS5Cl interface reaction at high voltage. Therefore, the poor intrinsic conductivity and interface stability during the anionic redox jointly restrict the capacity release or delithiation/lithiation degree of LLO during the first cycle in sulfide ASSLBs. This study reveals the origin of the seriously limited anionic redox reaction in LLO, providing valuable guidance for the bulk and interface design of high-energy-density ASSLBs.

Multi-scale design of three-dimensional highly-conductive spherical Li-rich layered manganese oxide toward long-term stable cyclability

The improvement of the cyclability and voltage decay of Li-rich layered manganese oxides (LLMOs) is crucial for developing Li-rich-based high energy density Li-ion batteries. However, the electron/ion transport mismatch and constant corrosion induced by the electrolyte remain huge challenges. In this work, we design a 3D highly conductive spherical structure using COMSOL simulations and synthesize a lithium silicate and graphene cocoated LLMO with high tap density by using liquid polysiloxane as the silicon source. Their synergistic effects reduce the activation energy in the lithiation/delithiation process, decrease the polarization, improve the thermal stability, and hinder the dissolution of transition metal ions in the material, thereby enhancing the electrochemical performance of the composite. After 200 cycles, the discharge capacity of the cocoated material is 212.9 mAh g−1 (91.3% capacity retention), the discharge specific energy reaches 713.5 Wh kg−1, and the discharge middle voltage increases by 0.1 V relative to the pristine sample after 200 cycles. The 3D structural design and multiscale modifications described herein greatly facilitate the commercialization of lithium-rich layered manganese oxides as cathode materials for next-generation Li-ion batteries with high energy density.

Dual-site Doping to Enhance Oxygen Redox and Structural Stability of Li-rich Layered Oxides

Cobalt-free Li-rich layered oxides (LLOs) such as Li1.2Mn0.6Ni0.2O2 have attracted extensive attention owing to their high specific capacity and low cost. Nonetheless, numerous problems such as continuous voltage fading and capacity decay have become stumbling blocks in its commercial application. In this study, we propose an effective dual-site doping strategy by choosing Mo as the cation and F as the anion to enhance the capacity and cycling performance. The research demonstrates that the cycling stability of LLOs enhances with the doping ratio of Mo, and their capacity increases with the doping ratio of F. It is because Mo as a pillar enhances the structural stability and F doping is conducive to the activation of Li2MnO3. What's more, dual-site doping also promotes the diffusion of Li+ and reduces the internal resistance of the electrode. Due to these improvements, the 5F3M sample still maintains a discharge capacity of 190.98 mAh g-1 after 100 cycles at 200 mA g-1, which is much higher than 165.29 mAh g-1 of the Pristine sample. This discovery provides a new way to develop advanced layered oxide cathodes for both Na- and Li-ion batteries.

Stabilized Anionic Redox by Rational Structural Design from Surface to Bulk for Long-Life Fast-Charging Li-Rich Oxide Cathodes.

On account of high capacity and high voltage resulting from anionic redox, Li-rich layered oxides (LLOs) have become the most promising cathode candidate for the next-generation high-energy-density lithium-ion batteries (LIBs). Unfortunately, the participation of oxygen anion in charge compensation causes lattice oxygen evolution and accompanying structural degradation, voltage decay, capacity attenuation, low initial columbic efficiency, poor kinetics, and other problems. To resolve these challenges, a rational structural design strategy from surface to bulk by a facile pretreatment method for LLOs is provided to stabilize oxygen redox. On the surface, an integrated structure is constructed to suppress oxygen release, electrolyte attack, and consequent transition metals dissolution, accelerate lithium ions transport on the cathode-electrolyte interface, and alleviate the undesired phase transformation. While in the bulk, B doping into Li and Mn layer tetrahedron is introduced to increase the formation energy of O vacancy and decrease the lithium ions immigration barrier energy, bringing about the high stability of surrounding lattice oxygen and outstanding ions transport ability. Benefiting from the specific structure, the designed material with the enhanced structural integrity and stabilized anionic redox performs an excellent electrochemical performance and fast-charging property..

Voltage Decay of Li‐Rich Layered Oxides: Mechanism, Modification Strategies, and Perspectives (Adv. Funct. Mater. 25/2023)

Voltage Decay In article number 2213260, Zhaoping Liu, Quanbing Liu, Bao Qiu, and co-workers review the mechanism of voltage decay generation in Li-rich layered oxides, which is caused by a combination of phase transition, microscopic defects, and oxygen redox. The review also provides critical insight into the root of voltage decay, as well as predictions for potential solutions.

Computational Understandings of Cation Configuration-Dependent Redox Activity and Oxygen Dimerization in Lithium-Rich Manganese-Based Layered Cathodes

Lithium-rich manganese-based layered oxides have emerged as a fresh paradigm for developing advanced cathode materials with high energy density for next-generation lithium-ion batteries. Understanding lattice oxygen dimerization is quite essential for the optimal design of lithium-rich manganese-based cathode materials. Herein, based on density functional theory (DFT) calculations, a local Ni-honeycomb Li–Ni–Mn cation configuration for the Li1.22Ni0.22Mn0.56O2 cathode was carefully examined, which may coexist with the well-known local Li-honeycomb structure in experimentally synthesized Li1.2Ni0.2Mn0.6O2 samples. The local Li–Ni–Mn cation configurations have significant impacts on oxygen redox activity, transition metal atom migration, and oxygen dimerization in the charging process of LixNi0.22Mn0.56O2. It is found that there is no correlation between high lattice oxygen redox activity and easy oxygen dimerization, such as Li-honeycomb structures simultaneously exhibiting higher oxygen redox activities and higher activation energy barriers for prohibiting oxygen dimerization than Ni-honeycomb structures. The structural regulations of the local Li–Ni–Mn cation configuration by avoiding the local Ni-honeycomb structures to inhibit Mn migration and ease lattice oxygen dimerization and by making full use of the local Li-honeycomb structures would maximize performance of Li-rich Mn-based layered oxides. Such fresh insights provide us a fresh strategy to optimally design the local honeycomb structure for high-performance Li-rich Mn-based cathode materials.

A novel boron-doping approach for long-term cycling life of Li-rich layered cathode

The commercial application of Li-rich Mn-based layered oxides (LLOs) is hindered due to their instability of lattice oxygen and dissolution of manganese. Herein, an effective and large-scale production co-precipitation boron-doping strategy was proposed to improve the performance of Li1.15Mn0.53Ni0.22Co0.1O2. We found that the boron introduced in the co-precipitation process will be uniformly distributed in the bulk and the resulting boron-doped Li1.15Mn0.53Ni0.22Co0.1O2 shows excellent electrochemical properties with low doping amount. Multi-dimensional analysis shows that boron doping significantly improves the structural stability of materials. After 1200 cycles at 1C, boron-doped sample can deliver 147 mAhg−1 with an excellent retention of 80.1%, which is much higher than that of the pristine sample of 44% after 700 cycles. Moreover, the discharge voltage decay of the boron-doped samples is only 0.54 mV per cycle (1.39 mV for pristine). It is worth mentioning that the pouch-cell full battery cycle performance of doped samples is also excellent. The doped cathode//graphite shows a superior capacity retention of 83.6% after 500 cycles without gas generation, while the pristine//graphite only hold the 46.6% of initial capacity. The dQ/dV curves and the SEM/EDS of electrode analysis verified higher reversibility of lattice oxygen, less manganese dissolution and more stable microstructure, which greatly improved the cycling life after boron doping.

Tuning Local Structural Configurations to Improve Oxygen-Redox Reversibility of Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are regarded as one of the most desirable cathode materials due to their high specific capacity. Nevertheless, the irreversible oxygen release associated with low oxygen stability prevents their widespread application. Herein, an improved oxygen redox reversibility was achieved by constructing Ni2+–O2––Ni2+ configurations. Superconducting Quantum Interference Device (SQUID) magnetometry measurements are used to track the evolution of the Ni2+–O2––Ni2+ configuration during the electrochemical process. The strongest 180° superexchange interaction in the Ni2+–O2––Ni2+ configuration, derived from the inevitable Li/Ni mixing in LLOs, regulates the local structure to form the ferrimagnetic (FiM) structural units. Consequently, the FiM structural units prevent the irreversible oxygen release and endow LLOs with high initial Coulombic efficiency (ICE). This work emphasizes the importance of the Ni2+–O2––Ni2+ configuration for LLOs with high reversible capacity and proposes a synthesis approach to modulate the amount of FiM structural units.

Synthesis and electrochemical performance of La2CuO4 as a promising coating material for high voltage Li-rich layered oxide cathodes

The structural transformations, oxygen releasing and side reactions with electrolytes on the surface are considered as the main causes of the performance degradation of Li-rich layered oxides (LROs) cathodes in Li-ion batteries. Thus, stabilizing the surfaces of LROs is the key to realize their practical application in high energy density Li-ion batteries. Surface coating is regarded as one of the most effective strategies for high voltage cathodes. The ideal coating materials should prevent cathodes from electrolyte corrosion and possess both electronic and Li-ionic conductivities simultaneously. However, commonly reported coating materials are unable to balance these functions well. Herein, a new type of coating material, La2CuO4 was introduced to mitigate the surface issues of LROs for the first time, due to its superb electronic conductivity (26–35 mS⋅cm−1) and lithium-ionic diffusion coefficient (10−12–10−13 cm2⋅s−1). After coating with the La2CuO4, the capacity retention of Li1.2Ni0.54Co0.13Mn0.13O2 cathode was increased to 85.9% (compared to 79.3% of uncoated cathode) after 150 cycles in the voltage range of 2.0–4.8 V. In addition, only negligible degradations on the deliverable capacity and rate capability were observed.

Surface-Diluted LiMn6 Superstructure Units Utilizing PO4 3- Confined Ni-Doping Sites to Stabilize Li-Rich Layered Oxides.

Serious capacity and voltage degradation of Li-rich layered oxides (LLOs) caused by severe interfacial side reactions (ISR), structural instability, and transition metal (TM) dissolution during charge/discharge need to be urgently resolved. Here, it is proposed for the inaugural time that the confinement effect of PO4 3- dilutes the LiMn6 superstructure units on the surface of LLOs, while deriving a stable interface with phosphate compounds and spinel species. Combining theoretical calculations, diffraction, spectroscopy, and micrography, an in-depth investigation of the mechanism is performed. The results show that the modified LLO exhibits excellent anionic/cationic redox reversibility and ultra-high cycling stability. The capacity retention is increased from 72.4% to 95.4%, and the voltage decay is suppressed from 2.48 to 1.29mV cycle-1 after 300 cycles at 1 C. It also has stable long cycling performance, with capacity retention improved from 40.2% to 81.9% after 500 cycles at 2 C. The excellent electrochemical performance is attributed to the diluted superstructure units on the surface of LLO inhibiting the TM migration in the intralayer and interlayer. Moreover, the stable interfacial layers alleviate the occurrence of ISR and TM dissolution. Therefore, this strategy can give some important insights into the development of highly stable LLOs.