Activation Of Li2MnO3 Research Articles

Understanding Degradation Mechanisms of Layered Transition Metal Oxide Cathodes through Multiscale Characterization

Co has been extensively used in lithium-ion battery (LIB) cathodes with LiTMO2 structure (TM: transition metals) due to its high specific capacity and stability. However, the scarcity and costliness of Co have prompted researchers to explore alternatives utilizing more abundant and cost-effective elements like Mn and Ni. Despite these efforts towards Co reduction or elimination, there remains a critical knowledge gap concerning the specific functions of cobalt in cathode capacity and structural stability. In this study, we employed advanced characterization techniques including transmission electron microscopy (TEM) and X-ray diffraction (XRD) to investigate the roles of cobalt in Co-free Ni-rich cathodes, as well as to elucidate degradation mechanisms in Li- and Mn-rich cathodes.To understand the Co roles, Co-rich LiNi0.6Co0.4O2, (NC64) and Mn-substituted Co-free LiNi0.6Mn0.4O2 (NM64) were used [1]. TEM observations show that both NC64 and NM64 surface structures change to a rock-salt phase after prolonged cycling, suggesting surface phase transformation was independent of the Co and Mn content. However, the surface structures for NC64 and NM64 during the cycling exhibit clear difference. The subsurface structure of NC64 completely transited to a Co3O4-type spinel-like structure. Such an irreversible phase transition is related to oxygen release, which triggers TM migration to form a spinel-like phase. In contrast, the structure stability of NM64 remained a typical layered structure after 100 cycles, indicating the Co reduction and Mn substitution improved the stability of the structure and oxygen of Ni-rich cathodes. Electron energy-loss spectroscopy (EELS) line scans from surface to subsurface of NM64 sample shows the O K-edge pre-peak only decreased near the surface, and the L edges of Ni and Mn also consistently shift towards lower energy loss near the surface. These results suggest that oxygen release only occurred on surface areas of the NM64 particles. In contrast, oxygen released not only occurred on the surface, but also extended into bulk NC64, because the O K-edge pre-peak of NC64 exhibited a relatively low intensity in the whole scan area of around 40 nm. In conclusion, Co is detrimental to structural/ morphological stability and oxygen reversibility. Co reduction and Mn increase could greatly alleviate these challenges in Ni-rich cathodes.HRTEM observations indicate that Li- and Mn-rich cathodes consist of nanoscale Li2MnO3 and layered domains. To understand the voltage fade mechanism in Li- and Mn-rich cathodes [2], XRD, TEM, 3D rotation electron diffraction (3D-rED), and EELS were performed on delithiated samples to investigate lattice displacement and nanostructure evolution. XRD and TEM studies show that inhomogeneous electrochemical activities and structural difference between Li2MnO3 and layered domains result in nanoscale strain. The accumulated strain severely affects the structural stability of the composite Li- and Mn-rich cathode, which triggers the decomposition of Li2MnO3 domains, oxygen release and TM migration. The activation of Li2MnO3 and oxygen release in turn release the lattice strain at high voltages.

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.

Effect of Three-in-One Surface Modification of Spherical, Co-Free Li-Rich Cathode Material for Li-Ion Batteries (Li1.2Mn0.6Ni0.2O2) with Citric Acid

The electrochemical activation of Li2MnO3 domains in Li- and Mn-rich layered oxides (LRLO) is highly important, and can be tuned by surface modification of the active materials to improve their cycling performance. In this study, citric acid was employed as a combined organic acid, reducing agent, and carbon precursor in order to remove surface residues from the calcination process, implement an oxygen deficient layer on the surface of the primary LRLO particles, and cover their surface with a carbon-containing coating after a final annealing step. A broad selection of bulk and surface sensitive characterization methods was used to characterize the post-treated spherical particles, providing the evidence for successful creation of an oxygen deficient near-surface region, covered by carbon-containing deposits. Post-treated materials show enhanced electrochemical discharge capacities after progressive Li2MnO3 activation, reaching maximum capacities of 247 mAh g−1. Gassing measurements reveal the suppression of oxygen release during the first cycle, concomitant with an increased CO2 formation for the carbon-coated materials. The voltage profile analysis in combination with post-mortem characterization after 300 cycles provide insights into the aging of the treated materials, which underlines the importance of the relationship between structural changes during scalable post-treatment and the electrochemical performance of the powders.

Direct Observation of Li-Ion Transport Heterogeneity Induced by Nanoscale Phase Separation in Li-rich Cathodes of Solid-State Batteries.

Li-rich layered oxide (LLO) cathode materials with high specific capacities could significantly enhance the energy density of all-solid-state lithium batteries (ASSLBs). However, the specific practical capacities of LLO materials in ASSLBs are extremely low due to poor initial activation. Here, scanning transmission electron microscopy with in situ differential phase contrast imaging was first used to study the initial activation mechanism of Li1.2 Ni0.13 Co0.13 Mn0.54 O2 . Li-ion transport heterogeneity was observed in LLO grains and across the LLO/Li6 PS5 Cl interface, due to the coexistence of the nanoscale Li2 MnO3 and LiNi1/3 Co1/3 Mn1/3 O2 phases. Consequently, the severely constrained activation of Li2 MnO3 during the first charging could be attributed to a nanoscale phase separation in LLO, hindering Li-ion transport through its particles, and causing high impedance in the Li2 MnO3 domain/Li6 PS5 Cl interface. This study could facilitate interface design of high-performance LLO-based ASSLBs.

Direct Observation of Li‐Ion Transport Heterogeneity Induced by Nanoscale Phase Separation in Li‐rich Cathodes of Solid‐State Batteries

AbstractLi‐rich layered oxide (LLO) cathode materials with high specific capacities could significantly enhance the energy density of all‐solid‐state lithium batteries (ASSLBs). However, the specific practical capacities of LLO materials in ASSLBs are extremely low due to poor initial activation. Here, scanning transmission electron microscopy with in situ differential phase contrast imaging was first used to study the initial activation mechanism of Li1.2Ni0.13Co0.13Mn0.54O2. Li‐ion transport heterogeneity was observed in LLO grains and across the LLO/Li6PS5Cl interface, due to the coexistence of the nanoscale Li2MnO3 and LiNi1/3Co1/3Mn1/3O2 phases. Consequently, the severely constrained activation of Li2MnO3 during the first charging could be attributed to a nanoscale phase separation in LLO, hindering Li‐ion transport through its particles, and causing high impedance in the Li2MnO3 domain/Li6PS5Cl interface. This study could facilitate interface design of high‐performance LLO‐based ASSLBs.

Synergistic-effect of high Ni content and Na dopant towards developing a highly stable Li-Rich cathode in Li-ion batteries

Developing a stable layered Li-rich oxide (LLO) is of utmost importance to realize electric vehicles (EVs), which offer driving ranges similar to internal combustion engine. In this study, the Ni concentration in the transition metal (TM) layer of the LLOs is increased gradually from 13% to 32% and the synergistic effect of doping Na is systematically analyzed. X-ray refinement studies of Ni excess LLOs reveal that increasing Ni content and doping Na increases the “c” lattice parameter and reduces the cation mixing. Electrochemical analysis of Ni excess cathode in Li-ion cell confirms that increasing the Ni content suppresses the Li2MnO3 activation and the layered to spinel structural transition in LLOs. Ni excess cathode (32% Ni) showed > 99% capacity retention (at 0.2C) compared to 84% retention for the electrode with 13% Ni content. Increasing the Ni content in TM layer negatively affected the rate capability. Doping Na in Li layer further enhances the electrochemical properties of these Ni excess electrodes with improved rate capability. The Na doped Ni excess cathode (24% Ni) furnishes discharge capacities and energy densities of 180 mAhg−1 and 613 WhKg−1 compared to 154 mAhg−1 and 471 WhKg−1 of undoped cathode with 13% Ni. The pillaring effect of inactive Na suppress the voltage fading upon cycling; Na doped Ni excess cathode (24% Ni) retained 93% of its initial voltage after 200 cycles, which is ∼ 0.35 V higher than the 13% Ni counterpart. In full cell configuration, Na doped Ni excess (20% and 24% Ni) electrodes exhibit enhanced energy densities of ∼ 100 WhKg−1than the undoped electrode with 13% Ni.

Cobalt Free Structurally Integrated Positive Electrode Materials for Lithium-Ion Batteries

Introduction Rechargeable Lithium-ion batteries (LIBs) having high energy density have been widely used as energy storage device for electric vehicles (EVs) and hybrid-electric vehicles (HEVs). It is necessary to increase the energy density of LIBs in order to meet future energy requirements. Since energy density depends mainly on positive electrodes, exploration of new positive electrode materials with high specific capacities, good rate capabilities and cycling stabilities become crucial.In this context, positive electrodes, which are composites of Li-rich layered materials (high specific capacity) and spinel materials (high rate capability) attracted intense attention. Li-rich layered positive electrode materials deliver a capacity of ~250 mAh g-1 when charged above 4.5 V[1]. A large irreversible initial capacity is observed, which is attributed to the complete electrochemical activation of Li2MnO3 with the simultaneous removal of oxygen and lithium (“Li2O”) from the crystal lattice[1]. Further, this class of materials exhibits poor rate capability[2]. Spinel-type positive electrodes are marked by its high operating voltage (4.7 V), good cycling stability as well as high rate capability due to the fast Li+ diffusion through its three-dimensional structure, even though it offers a low discharge capacity (about 140 mAh g-1)[4]. Additionally, Fe substitution to the spinel positive electrode materials was found to be effective in improving thermal stability, structural stability as well as electrochemical performance[5]. Further, Fe is cost-effective and is earth abundant[3]. Results and Discussion In the present work, Fe–containing cobalt free structurally integrated (at molecular level) Li-rich layered-spinel positive electrode material was synthesized by sol-gel method. Four different annealing temperatures were selected to get final products. The structure and morphology of the materials were characterized by using X-ray diffraction (XRD) and Field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) studies showed that obtained composite materials contain spinel (space group Fd-3m) as well as Li-rich layered phases (space group C2/m). Mössbauer spectroscopy experiments revealed the coordination environment and oxidation state of the Fe present in the composite. Furthermore, the electrochemical performance was investigated within the voltage range 2.00 - 4.95 V vs. Li+/Li. The obtained cyclic voltammograms further confirmed the coexistence of Li-rich layered and spinel components in the composites. Selected materials delivered high discharge capacity, close to 200 mAh g-1 and ˃ 90% capacity retention after 50 cycles and will be discussed in detail. Acknowledgement: A.Bhaskar and H. Enale acknowledge the financial support from funding agency, Science and Engineering Board (SERB), New Delhi, India through ECR/2017/002556 program. D. Dixon acknowledges the financial support from funding agency, Science and Engineering Board (SERB), New Delhi, India through Ramanujan Fellowship, under the grant number SB/S2/ RJN-162/2017. A. Surendran is grateful to UGC New Delhi (Ref. no.:63/CSIR-UGC NET DEC. 2017) for the UGC SRF grant and Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India. The CIF, CSIR-CECRI is acknowledged for technical support with material characterization. A. Thottungal acknowledges the Council of Scientific and Industrial Research (CSIR) for a senior research fellowship.

Optimized activation of Li2MnO3 effectively boosting rate capability of xLi2MnO3∙(1-x)LiMO2 cathode

Poor rate capability and voltage decaying have been challenging for high energy/power density Li-rich Mn-based oxide (LMO) cathodes, restricting their application to a certain extent. Herein, a trace Bi-doping is introduced to mediate the release of O2 and ameliorate the transmission of electrons. As a result of appropriate Bi-doping, voltage fading is mitigated and cyclability is considerably enhanced. However, the common drawbacks between pristine and modified materials revealed the residual issues of cathode. That is, the slow kinetic Li2MnO3 phase is account for the poor rate capability of LMO and further restricted the capacity contribution of LMO due to the sandwiched position of LiMO2 phase in the two phase system. As further activation on the Li2MnO3 phase exhibited enhanced cyclability, this research can be a guideline for optimized LMO via focusing on the properties of Li2MnO3 component.

Microstructure-Controlled Li-Rich Mn-Based Cathodes by a Gas–Solid Interface Reaction for Tackling the Continuous Activation of Li2MnO3

Li-rich Mn-based cathodes have attracted much attention due to their high capacity stemming from anion redox above 4.5 V. However, the continuous activation of Li2MnO3 in Li-rich Mn-based materials, which correlates with O2 release and TM migration, is usually unfavorable to structural stability. Herein, based on a gas-solid interface reaction, we tackle this continuous activation phenomenon by restricting the capacity release of Li2MnO3 via NH4HCO3 treatment in the Li1.2Ni0.36Mn0.44O2 cathode. After modification, oxygen vacancies associated with the spinel phase are introduced on the surface. The 4 mol % NH4HCO3-modified material's capacity starts at 182 mAh g-1 at 1 C instead of increasing from 173 to 186 mAh g-1 for 25 cycles in the pristine material. Meanwhile, it also exhibits an excellent capacity retention of 93.24% after 200 cycles (at 1 C), with a small voltage decay rate of 1.19 mV cycle-1.

Accelerating the activation of Li2MnO3 in Li-rich high-Mn cathodes to improve its electrochemical performance.

Li-rich high-Mn oxides, xLi2MnO3·(1 - x)LiMO2 (x ≥ 0.5, M = Co, Ni, Mn…), have attracted extensive research interest due to their high specific capacity and low cost. However, slow Li2MnO3 activation and poor cycling stability have affected their electrochemical performance. Herein, to solve these problems, morphology regulation and LiAlF4 coating strategies have been synergistically applied to a Li-rich high-Mn material Li1.7Mn0.8Co0.1Ni0.1O2.7 (HM-811). This dual-strategy successfully promotes the activation process of the Li2MnO3 phase and thus improves the electrochemical performance of HM-811. Theoretical computation indicates that the LiAlF4 layer has a lower Li+ migration barrier than the HM-811 matrix, so it could boost the diffusion of Li+ ions and promote the activation of the Li2MnO3 phase. Benefiting from the morphology regulation and LiAlF4 coating, the HM-811 cathode shows a high initial charge capacity of >300 mA h g-1. In addition, the modified HM-811 could deliver superior electrochemical performance even at a low temperature of -20 °C. This work provides a new approach for developing high performance cathode materials for next-generation Li-ion batteries.

Deciphering the effects of hexagonal and monoclinic structure distribution on the properties of Li-rich layered oxides.

Uniform distribution of Li2MnO3 and LiMO2 components in a Co-free Li-rich layered oxide is achieved by treating precursors with NH3·H2O, which expands the lattice parameter and promotes the activation of Li2MnO3, resulting in excellent electrochemical performance. What's more, it also contributes to the storage stability of Li-rich layered oxides.

{010} oriented Li1.2Mn0.54Co0.13Ni0.13O2@C nanoplate with high initial columbic efficiency

Lithium rich Mn-based oxide with high capacity and low cost has been viewed as a promising cathode for lithium ion batteries with high energy density. However, there still exists the drawback of the low initial columbic efficiency that caused by the irreversible electrochemical activation of Li2MnO3. Here, {010}-oriented Li1.2Mn0.54Co0.13Ni0.13O2@C nanoplate is prepared by using active carbon to absorb the ethylene glycol solution containing Li+, Co2+, Mn2+ and Ni2+ after the calcination at 850 °C. Ethylene glycol contributes to the formation of the exposed {010} planes, the nanoplate morphology and oxygen vacancies (OVs). The as-prepared material presents a reversible capacity of 276 mA h g−1 and high initial columbic efficiency of 92% at 25 mA g−1, superior rate capability and excellent cycling stability. The high initial columbic efficiency is mainly due to OVs with the function to decrease the irreversible charge capacity related with oxygen release over 4.5 V. And the rate capability is improved by the exposed {010} planes for the enhanced Li+ diffusion and the coating carbon for the excellent electron conductivity. Our work provides a facile method for exposing {010} active planes in lithium rich Mn-based oxide with high initial columbic efficiency.

Roles of Mn and Ni in Li-rich Mn-Ni-Fe oxide cathodes

Cobalt free, lithium-rich manganese, nickel, and iron composite oxides (nanoscale mixture of monoclinic, Li2MnO3, and rhombohedral, LiMO2, phases) are promising cathodes as they show attractive electrochemical performance in initial cycles and can reduce the cost and toxicity of lithium ion battery. However, capacity and voltage fading on cycling are the downsides. To better understand the electrochemical performance and degradation mechanism, a series of Li1.2Mn0.40+xNi0.30-xFe0.10O2 materials with x = 0, 0.05, 0.10, and 0.15 was synthesized and systematically studied. The results of this study suggest that two phase composition is beneficial for superior electrochemical performance. The series showed first discharge capacity at 0.1C of 267, 204, 226, and 277 mAhg−1 for x = 0, 0.05, 0.10, and 0.15 respectively, with capacity retention after 100 cycles at 0.3C of 57 %, 57 %, 94 % and 67 %. For the first time we report EXAFS data analysis at all three K-edges: Mn, Ni and Fe before and after extended cycling. Ni reduction to Ni2+ after cycling and voltage fading increased with Ni fraction in the series. There is no trend of Mn reduction (Li2MnO3 activation) on cycling with Mn content indicating composition selection is crucially important for improved performance. The information on changes in electronic states and atomic distances, which allows proposing comprehensive electrochemical (role of Li2MnO3 phase) and degradation (Li+/Ni2+ exchange, voltage fading, and capacity fading) mechanisms.

Electrochemical Activation of Li2MnO3 Electrodes at 0 °C and Its Impact on the Subsequent Performance at Higher Temperatures.

This work continues our systematic study of Li- and Mn- rich cathodes for lithium-ion batteries. We chose Li2MnO3 as a model electrode material with the aim of correlating the improved electrochemical characteristics of these cathodes initially activated at 0 °C with the structural evolution of Li2MnO3, oxygen loss, formation of per-oxo like species (O22−) and the surface chemistry. It was established that performing a few initial charge/discharge (activation) cycles of Li2MnO3 at 0 °C resulted in increased discharge capacity and higher capacity retention, and decreased and substantially stabilized the voltage hysteresis upon subsequent cycling at 30 °C or at 45 °C. In contrast to the activation of Li2MnO3 at these higher temperatures, Li2MnO3 underwent step-by-step activation at 0 °C, providing a stepwise traversing of the voltage plateau at >4.5 V during initial cycling. Importantly, these findings agree well with our previous studies on the activation at 0 °C of 0.35Li2MnO3·0.65Li[Mn0.45Ni0.35Co0.20]O2 materials. The stability of the interface developed at 0 °C can be ascribed to the reduced interactions of the per-oxo-like species formed and the oxygen released from Li2MnO3 with solvents in ethylene carbonate–methyl-ethyl carbonate/LiPF6 solutions. Our TEM studies revealed that typically, upon initial cycling both at 0 °C and 30 °C, Li2MnO3 underwent partial structural layered-to-spinel (Li2Mn2O4) transition.

Suppressed voltage decay and improved electrochemical performance by coating LiAl5O8 on the surface of Li1.2Mn0.54Ni0.13Co0.13O2

LiAl5O8-coated Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials were synthesized by using the co-precipitation method along with a facile sol-gel coating process. Homogeneous and 3–5 nm thin LiAl5O8 film was formed on the LMNC surface. After the coating modification, discharge specific capacity, the initial Coulombic efficiency, and rate performance were improved to some extent. It was particularly important to note that the voltage decay of the battery after the charge and discharge cycle was well suppressed. Specifically, the 3 wt% LiAl5O8-coated electrode can deliver the initial discharge capacity of 243.5 mAh g−1 with a low irreversible capacity loss of 51.1 mAh g−1 and the initial Coulombic efficiency reached the maximum of 82.63% at 0.1 C rate. Although the 1 wt% LiAl5O8-coated electrode was cycled through 100 cycles at a rate of 0.5 C, the capacity retention rate was still up to 95.8%, while that of the LiAl5O8-free one was only 82.8%. The stable and thin LiAl5O8 coating layer protected the electrode structure from HF corrosion and other side reactions and reduced the oxygen loss (in the form of Li2O) by suppressing the initial activation of Li2MnO3.

Reactive template synthesis of Li1.2Mn0.54Ni0.13Co0.13O2 nanorod cathode for Li-ion batteries: Influence of temperature over structural and electrochemical properties

Common preparation methods of manganese-based Li-rich layered oxides include co-precipitation, combustion, spray pyrolysis, and molten salt synthesis. Herein, we present a facile new reactive-templating route to prepare manganese-based lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 using β−MnO2 nanorod, which plays a dual role as reactive template as well as Mn-source. Rietveld refinement and electron diffraction patterns confirm the formation of phase pure, highly crystalline Li1.2Mn0.54Ni0.13Co0.13O2 with two integrated layered components of 0.5(Li2MnO3).0.5(LiMn1/3Ni1/3Co1/3O2). Electron microscopy studies reveal the formation of anisotropic rod-like crystals of 0.8–1.0 μm in length and ∼200 nm in thickness. The Li1.2Mn0.54Ni0.13Co0.13O2 nanorod as cathode for Li-ion battery, delivers an impressive reversible capacity of 223 mAh.g−1 at 0.1C rate after 150 cycles and 161 mAh.g−1 at 1 C rate after 300 cycles. Rapid structural transition from layered to spinel-like phase at high temperature (55 °C) leads to gradual decay in discharge capacity upon cycling, whereas low Li-ion diffusivity, cell resistance, and high viscosity hampers the performance at low temperature (5 °C). Impedance spectroscopy along with (dis)charge differential plots corroborate that activation of Li2MnO3 and Li-ion de(intercalation) into the MnO2 phase is very facile at high temperature (55 °C), which leads to high specific capacity, high coulombic efficiency, and high-power capability.

Investigation of Li1.17Ni0.20Mn0.53Co0.10O2 as an Interesting Li‐ and Mn‐Rich Layered Oxide Cathode Material through Electrochemistry, Microscopy, and In Situ Electrochemical Dilatometry

AbstractDespite their high capacity, Li‐ and Mn‐rich layered oxide cathode materials suffer from capacity fading during prolonged cycling when cycled to potentials higher than 4.5 V vs. Li+/Li. In the work reported herein, we have synthesized the Li‐ and Mn‐rich Li1.17Ni0.20Mn0.53Co0.10O2 cathode material by using a sol‐gel method. The composition and structure are analyzed by ICP, XRD, SEM, and TEM. During testing in half‐cells between 2.5 and 4.6 V vs. Li+/Li at 20 mA g−1, the initial capacity of about 165 mAh g−1 increases during cycling, reaching about 200 mAh g−1 after 30 cycles. The capacity then slowly fades, reaching 188 mAh g−1 after 120 cycles. Also, a high average discharge potential of 3.8 V is obtained, which decreases to 3.6 V after 120 cycles. This study leads to new insights about the effect of Ni concentration on the stability of the Li‐ and Mn‐ rich cathode materials. The in situ electrochemical dilatometry (ECD) study demonstrates an irreversible process during the 1st cycle, which can be related to the activation of Li2MnO3. During subsequent cycles, the electrodes’ thickness does not change, which reflects a morphological stabilization, in contrast to the continuous electrochemical instability of these materials upon cycling.

Revealing of the Activation Pathway and Cathode Electrolyte Interphase Evolution of Li-Rich 0.5Li2MnO3·0.5LiNi0.3Co0.3Mn0.4O2 Cathode by in Situ Electrochemical Quartz Crystal Microbalance.

The first-cycle behavior of layered Li-rich oxides, including Li2MnO3 activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li2MnO3 activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi2MnO3·(1- x) LiNi0.3Co0.3Mn0.4O2 ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structures can significantly affect the activation pathway of Li1.2Ni0.12Co0.12Mn0.56O2, leading to an extra-high capacity. It is demonstrated that Li2MnO3 activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li2MnO3 governed by chemical decomposition (Li2O evolution). CEI evolution is closely related to Li+ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries.

Surface thermodynamic stability of Li-rich Li2MnO3: Effect of defective graphene

Li-rich Mn-based cathode materials used in Li-ion batteries show ultra-high capacity due to the favorable oxygen redox behavior induced by activation of Li2MnO3. However, structural degradation associated with oxygen loss and transition metal migration occurs due to excessive oxygen redox on the surface, which is the main origin of the low coulombic efficiency, capacity fade, and voltage decay, which limit the widespread application of this high-capacity material. In this study, the underlying mechanism of tuning the surface thermodynamic stability of the Li2MnO3 model material was systematically investigated. The results showed that a defective graphene coating can effectively stabilize surface oxygen by modification of the potential energy surface, while reducing Mn migration and increasing the diffusivity of Li ions. Theoretical calculations predicted an improvement in the electrochemical performance, which was confirmed by experimental results. These findings may provide a novel strategy for increasing the surface thermodynamic stability of electrode materials.

Oxygen vacancies in CeO2 surface coating to improve the activation of layered Li1.2Mn0.54Ni0.13Co0.13O2 cathode material for Li-ion batteries

This work reports the surface coating of Li1.2Mn0.54Ni0.13Co0.13O2 cathode material with nano-CeO2 by a versatile hydrothermal method. Thus, obtained nano-CeO2-coated Li1.2Mn0.54Ni0.13Co0.13O2 material was characterized by XRD, SEM, and TEM. It is revealed that the synthesized nano-CeO2 material has rich oxygen vacancies, and a spinel-phase layer is formed on the surface of host material. The electrochemical testing results show that Li1.2Mn0.54Ni0.13Co0.13O2 with 4 wt% CeO2 coating (denoted as C3) has good rate capability and enhanced cyclic stability, enhanced initial discharge capacity of 298.5 mA h g−1 (0.05 C) compared to 281.9 mAh g−1, and excellent initial coulombic efficiency of 86.94% compared to 77.28% for the pristine one in the potential range 2.0–4.8 V (vs. Li/Li+). It is worth noting that this modified strategy greatly reduces the irreversible capacity loss (ICR) of the first cycle of active materials, the ICR of the C3 (44.8 mAh g−1) is markedly lower than pristine material (82.9 mAh g−1) at the current density of 12.5 mA g−1 (0.05 C). Such improvements are mainly ascribed to the oxygen vacancies in nano-CeO2 coating layer, which are responsible for the promoted activation of Li2MnO3. Moreover, the formation of the spinel structure is beneficial to stabilize the crystal lattice of the bulk material and facilitate Li+ diffusion by unique 3D transport channels.