Stabilizing Li-Rich Layered Cathode Materials Using a LiCoMnO4 Spinel Nanolayer for Li-Ion Batteries

Lithium-rich cathodes have excess lithium in the transition metal layer and exhibit an extremely high specific capacity and good energy density. However, they still have some disadvantages. Here, we propose LiCoMnO₄, a new nanolayer coating material with a spinel structure, to modify the surface of lithium cathode oxide (Li_7/6Mn_1/2Ni_1/6Co_1/6O₂) with a layered structure. The designed cathode with nanolayer spinel coating delivers an excellent reversible capacity, outstanding rate capability, and superior cycling ability whilst exhibiting discharge capacities of 300, 275, 220, and 166 mAh g^-1 at rates of 0.1 C at 2.0-4.8 V formation and 0.1, 1, and 5 C, respectively, between 2.0 and 4.6 V. The cycling ability and voltage fading at a high operational voltage of 4.9 V were also investigated, with results showing that the nanolayer spinel coating can depress the surface of the lithium cathode oxide layer, leading to phase transformation that enhances the electrochemical performance.

A current collect-free Li1.2Ni0.13Co0.13Mn0.54O2flexible film for high-performance lithium-ion batteries

Due to the high demand for more convenient flexible devices, there are more requirements for higher performance of flexible batteries. The layered lithium-rich manganese-based Li_1.2Ni_0.13Co_0.13Mn_0.54O₂cathode material has the advantages of higher energy density, higher discharge capacity and environmentally friendly, so it can be used for high-performance flexible electrode cathode material. Its theoretical capacity can reach more than 250 mAh g^-1, which is higher than most cathode materials currently used in commercialization. Here we synthesize Li_1.2Ni_0.13Co_0.13Mn_0.54O₂(LNCM) cathode, and then use a simple method to make a current collect-free LNCM flexible film. This film has excellent flexibility and electrochemical performance. At 25 mA g^-1, its initial discharge capacity reaches 314.0 mAh g^-1. After 200 cycles of 500 mA g^-1, its capacity retention rate is 82.1%, the attenuation is about 0.08% per cycle. Moreover, by bending at any position of the flexible film, it can still remain intact, and the soft-packaged battery made by the flexible film can still be used under the bending condition and keep the brightness of the LED lamp unchanged. This shows that using Li_1.2Ni_0.13Co_0.13Mn_0.54O₂to make high-performance flexible electrodes is a simple and effective method, which is expected to be practically applied to flexible electronic devices.

Surface LiMn1.4Ni0.5Mo0.1O4 Coating and Bulk Mo Doping of Li-Rich Mn-Based Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with Enhanced Electrochemical Performance for Lithium-Ion Batteries

To improve the initial Coulombic efficiency, cycling stability, and rate performance of the Li-rich Mn-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode, the combination of LiMn_1.4Ni_0.5Mo_0.1O₄ coating with Mo doping has been successfully carried out by the sol-gel method and subsequent dip-dry process. This strategy buffers the electrodes from the corrosion of electrolyte and enhances the lattice parameter, which could inhibit the oxygen release and maintain the structural stability, thus improving the cycle stability and rate capability. After LiMn_1.4Ni_0.5Mo_0.1O₄ modification, the initial discharge capacity reaches 272.4 mAh g^-1 with a corresponding initial Coulombic efficiency (ICE) of 84.2% at 0.1C (1C = 250 mAh g^-1), far higher than those (221.5 mAh g^-1 and 68.9%) of the pristine sample. Besides, the capacity retention of the coated sample is enhanced by up to 66.8% after 200 cycles at 0.1C. Especially, the rate capability of the coated sample is 95.2 mAh g^-1 at 5C. XRD, SEM, TEM, XPS, and Raman spectroscopy are adopted to characterize the morphologies and structures of the samples. This coating strategy has been demonstrated to be an effective approach to construct high-performance energy storage devices.

Evolution and expansion of Li concentration gradient during charge-discharge cycling

To improve the performance of Li-ion batteries (LIBs), it is essential to understand the behaviour of Li ions during charge-discharge cycling. However, the analytical techniques for observing the Li ions are limited. Here, we present the complementary use of scanning transmission electron microscopy and atom probe tomography at identical locations to demonstrate that the evolution of the local Li composition and the corresponding structural changes at the atomic scale cause the capacity degradation of Li(Ni_0.80Co_0.15Mn_0.05)O₂ (NCM), an LIB cathode. Using these two techniques, we show that a Li concentration gradient evolves during cycling, and the depth of the gradient expands proportionally with the number of cycles. We further suggest that the capacity to accommodate Li ions is determined by the degree of structural disordering. Our findings provide direct evidence of the behaviour of Li ions during cycling and thus the origin of the capacity decay in LIBs.

Thermal Stability Enhancement through Structure Modification on the Microsized Crystalline Grain Surface of Lithium-Rich Layered Oxides

Lithium-rich layered oxides have been considered as the most promising candidate for offering a high specific capacity and energy density for lithium-ion batteries. However, their practical applications are still suffered by the cycle instability and also closely related thermal stability. Here, microsized crystalline grains with good dispersion of lithium-rich layered oxides are prepared by a molten-salt method, while a spinel structure is also introduced on a grain surface by following chemical oxidation and annealing process, and their thermal performance with different cutoff voltages during the charge process is systematically studied using differential scanning calorimetry method. Results have shown that thermal stability of microsized crystalline grains is better than that of spherical secondary agglomerates, the spinel structure introduction on the grain surface of microsized crystalline grains can contribute obviously to their thermal stability, in which the onset temperature of the exothermic peak has been increased by 103 °C, and the thermal release value can be reduced as much as about 40% when the battery was charged to 4.8 V. Furthermore, the electrochemical performance, especially cycle stability under a high temperature, has also been enhanced for spinel-modified microsized crystalline grains. This work not only develops the microsized crystalline grains with good dispersion of lithium-rich layered oxides, confirming the advantages of these materials compared to spherical secondary agglomerates, but also reveals the method to improve their thermal stability by grain surface structure modification, opening the way to optimize the comprehensive performance of electrode materials for batteries.

Integrated Surface Functionalization of Li-Rich Cathode Materials for Li-Ion Batteries

As candidates for high-energy density cathodes, lithium-rich (Li-rich) layered materials have attracted wide interest for next-generation Li-ion batteries. In this work, surface functionalization of a typical Li-rich material Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ is optimized by fluorine (F)-doped Li₂SnO₃ coating layer and electrochemical performances are also enhanced accordingly. The results demonstrate that F-doped Li₂SnO₃-modified material exhibits the highest capacity retention (73% after 200 cycles), with approximately 1.2, 1.4, and 1.5 times of discharge capacity for Li₂SnO₃ surface-modified, F-doped, and pristine electrodes, respectively. To reveal the fundamental enhancement mechanism, intensive surface Li⁺ diffusion kinetics, postmortem structural characteristics, and aging tests are performed for four sample systems. The results show that the integrated coating layer plays an important role in addressing interface compatibility, not only limited in stabilizing the bulk structure and suppressing side reactions, synergistically contributing to the performance enhancement for the active electrodes. These findings not only pave the way to commercial application of the Li-rich material but also shed new light on surface modification in batteries and other energy storage fields.

Crystal structural design of exposed planes: express channels, high-rate capability cathodes for lithium-ion batteries

Developing high-performance lithium ion batteries (LIBs) requires optimization of every battery component. Currently, the main problems lie in the mismatch of electrode capacities, especially the excessively low capacity of cathodes compared with that of anodes. Due to the anisotropy of the crystal structure, different crystal planes play different roles in the transmission of lithium ions. Among these, the {010} facets of layered-structure materials, the (110) planes of spinel cathodes and the (010) planes of olivine cathodes can provide open surface structures, which furnish express channels for the rapid and efficient transmission of lithium ions, leading to enhanced rate performance. However, due to the high-energy surfaces of these crystal planes, they tend to disappear in the synthetic process, forming thermodynamic equilibrium products dominated by low-energy and electrochemically-inactive planes. From the structure design of the material itself, preparing functional materials with specific morphologies and crystal structures is considered to be the most effective way to improve the cyclability and rate performance of LIB cathodes. In this review, we highlight the latest developments in selectively exposing the crystal planes of LIB cathode materials. The synthetic method, the corresponding electrochemical performance, especially the rate capability, and the growth mechanism have been systematically summarized for layered-structure cathodes of LiCoO2, LiNixCoyMn1-x-yO2 and Li2MnO3·LiMO2, spinel cathodes of LiMn2O4 and LiNi0.5Mn1.5O4, and olivine cathodes of LiFePO4. This in-depth discussion and understanding is beneficial for the rational design of well-performing LIB cathodes and can provide direction and perspectives for future work.

Composite-Structure Material Design for High-Energy Lithium Storage

High-energy storage devices are in demand for the rapid development of modern society. Until now, many kinds of energy storage devices, such as lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), and so on, have been developed in the past 30 years. However, most of the commercially exploited and studied active electrode materials of these energy storage devices possess a single phase with low reversible capacity or unsatisfied cycle stability. Continuous and extensive research efforts are made to develop alternative materials with a higher specific energy density and long cycle life by element doping or surface modification. A novel strategy of forming composite-structure electrode materials by introducing structure units has attracted great attention in recent years. Herein, based on previous publications on these composite-structure materials, some important scientific points focusing on the design of composite-structure materials for better electrochemical performances reveal the distinction of composite structures based on average and local structure analysis methods, and an understanding of the relationship between these interior composite structures and their electrochemical performances is discussed thoroughly. The lithiation/delithiation mechanism and the remaining challenges and perspectives for composite-structure electrode materials are also elaborated.

Synthesis Method for Long Cycle Life Lithium-Ion Cathode Material: Nickel-Rich Core-Shell LiNi0.8Co0.1Mn0.1O2

High-nickel materials with core-shell structures, whose bulk is rich in nickel content and the outer shell is rich in manganese content, have been demonstrated to improve cycle stability. The high-nickel cathode material LiNi_0.8Co_0.1Mn_0.1O₂ is a very promising material for lithium-ion batteries; however, its low rate performance and especially cycle performance currently hamper further commercialization. This study presents a new synthesis method to prepare this core-shell material (LiNi_0.8Co_0.1Mn_0.1O₂@ x[Li-Mn-O], x = 0.01, 0.03, 0.06). Electrochemical data show that LiNi_0.8Co_0.1Mn_0.1O₂@ x[Li-Mn-O] ( x = 0.03, CS-0.03) exhibits the best high-rate performance, cycle stability, and thermal stability. The initial discharge capacity of the core-shell sample CS-0.03 is 118 mAh g^-1, which is almost the same as the discharge capacity of pristine LiNi_0.8Mn_0.1Co_0.1O₂ (117 mAh g^-1) at the rate of 10 C in the voltage range of 3.0-4.3 V. Notably the capacity decay of CS-0.03 is 18.4% after 200 cycles compared to 27% decay in capacity of the pristine sample. Furthermore, CS-0.03 exhibits better thermal cycling stability. The capacity retention of the CS-0.03 sample reached 65.1% which is over 1.3 times than that of the pristine one, whose capacity retention is 49.2% after 105 cycles (55 °C). Evidently, the core-shell structured CS-0.03 sample has excellent cycle stability and this synthesis method can be applied to other cathode materials.

Unravelling the Structure and Electrochemical Performance of Li-Cr-Mn-O Cathodes: From Spinel to Layered

To explore a new series of cathode materials with high electrochemical performance, the spinel-layered (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0, 0.25, 0.5, 0.75, and 1) composites are synthesized with the sol-gel method. X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Raman spectra reveal that the structure of the (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] cathode materials evolves from spinel to hybrid spinel-layered and layered structures with the increase of the Li concentration. Test results reveal that the structure and electrochemical performance of (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0.25, 0.5 and 0.75) composites have the characteristics of both spinel ( x = 0) and Li-rich layered phases ( x = 1). In particular, x = 0.5 and 0.75 electrodes exhibit relatively high capacity retention and rate capability, which is mainly ascribed to the synergistic effect of the spinel and Li-rich layered phases, the 3D Li-ion diffusion channels of the spinel phase, and the low charge-transfer resistance ( R_ct) and Warburg diffusion impedance ( W_o).

Understanding the effect of an in situ generated and integrated spinel phase on a layered Li-rich cathode material using a non-stoichiometric strategy

Recently, spinel-layered integrated Li-rich cathode materials have attracted great interest due to the large enhancement of their electrochemical performances. However, the modification mechanism and the effect of the integrated spinel phase on Li-rich layered cathode materials are still not very clear. Herein, we have successfully synthesized the spinel-layered integrated Li-rich cathode material using a facile non-stoichiometric strategy (NS-LNCMO). The rate capability (84 mA h g^-1vs. 28 mA h g^-1, 10 C), cycling stability (92.4% vs. 80.5%, 0.2 C), low temperature electrochemical capability (96.5 mA h g^-1vs. 59 mA h g^-1, -20 °C), initial coulomb efficiency (92% vs. 79%) and voltage fading (2.77 V vs. 3.02 V, 200 cycles@1 C) of spinel-layered integrated Li-rich cathode materials have been significantly improved compared with a pure Li-rich phase cathode. Some new insights into the effect of the integrated spinel phase on a layered Li-rich cathode have been proposed through a comparison of the structure evolution of the integrated and Li-rich only materials before and after cycling. The Li-ion diffusion coefficient of NS-LNCMO has been enlarged by about 3 times and almost does not change even after 100 cycles indicating an enhanced structure stability. The integration of the spinel phase not only enhances the structure stability of the layered Li-rich phase during charging-discharging but also expands the interslab spacing of the Li-ion diffusion layer, and elongates TM-O covalent bond lengths, which lowers the activation barrier of Li⁺-transportation, and alleviates the structure strain during the cycling procedure.

Novel MOF shell-derived surface modification of Li-rich layered oxide cathode for enhanced lithium storage

Li-rich layered oxide materials have attracted increasing attention because of their high specific capacity (>250 mAh g^-1). However, these materials typically suffer from poor cycling stability and low rate performance. Herein, we propose a facile and novel metal-organic-framework (MOF) shell-derived surface modification strategy to construct NiCo nanodots decorated (∼5 nm in diameter) carbon-confined Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles (LLO@C&NiCo). The MOF shell is firstly formed on the surface of as-prepared Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles via low-pressure vapor superassembly and then is in situ converted to the NiCo nanodots decorated carbon shell after subsequent controlled pyrolysis. The obtained LLO@C&NiCo cathode exhibits enhanced cycling and rate capability with a capacity retention of 95% after 100 cycles at 0.4 C and a high capacity of 159 mAh g^-1 at 5 C, respectively, compared with those of LLO (75% and 105 mAh g^-1). The electrochemical impedance spectroscopy and selected area electron diffraction analyses after cycling demonstrate that the thin C&NiCo shell can endow LLO with high electronic conductivity and structural stability, indicating the undesired formation of the spinel phase initiated from the particle surface is efficiently suppressed. Therefore, this presented strategy may open a new avenue on the design of high-performance electrode materials for energy storage.

Remarkably Improved Electrochemical Performance of Li- and Mn-Rich Cathodes upon Substitution of Mn with Ni

Li- and Mn-rich transition-metal oxides of layered structure are promising cathodes for Li-ion batteries because of their high capacity values, ≥250 mAh g^-1. These cathodes suffer from capacity fading and discharge voltage decay upon prolonged cycling to potential higher than 4.5 V. Most of these Li- and Mn-rich cathodes contain Ni in a 2+ oxidation state. The fine details of the composition of these materials may be critically important in determining their performance. In the present study, we used Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ as the reference cathode composition in which Mn ions are substituted by Ni ions so that their average oxidation state in Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ could change from 2+ to 3+. Upon substitution of Mn with Ni, the specific capacity decreases but, in turn, an impressive stability was gained, about 95% capacity retention after 150 cycles, compared to 77% capacity retention for Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes when cycled at a C/5 rate. Also, a higher average discharge voltage of 3.7 V is obtained for Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ cathodes, which decreases to 3.5 V after 150 cycles, while the voltage fading of cathodes comprising the reference material is more pronounced. The Li_1.2Ni_0.27Mn_0.4Co_0.13O₂ cathodes also demonstrate higher rate capability compared to the reference Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. These results clearly indicate the importance of the fine composition of cathode materials containing the five elements Li, Mn, Ni, Co, and O. The present study should encourage rigorous optimization efforts related to the fine composition of these cathode materials, before external means such as doping and coating are applied.

High-voltage positive electrode materials for lithium-ion batteries

The ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts on high-voltage positive electrode materials over the past decade.

High-Capacity Layered-Spinel Cathodes for Li-Ion Batteries

Li and Mn-rich layered oxides with the general structure x Li2 MnO3 ⋅(1-x) LiMO2 (M=Ni, Mn, Co) are promising cathode materials for Li-ion batteries because of their high specific capacity, which may be greater than 250 mA h g(-1) . However, these materials suffer from high first-cycle irreversible capacity, gradual capacity fading, limited rate capability and discharge voltage decay upon cycling, which prevent their commercialization. The decrease in average discharge voltage is a major issue, which is ascribed to a structural layered-to-spinel transformation upon cycling of these oxide cathodes in wide potential ranges with an upper limit higher than 4.5 V and a lower limit below 3 V versus Li. By using four elements systems (Li, Mn, Ni, O) with appropriate stoichiometry, it is possible to prepare high capacity composite cathode materials that contain LiMn1.5 Ni0.5 O4 and Lix Mny Niz O2 components. The Li and Mn-rich layered-spinel cathode materials studied herein exhibit a high specific capacity (≥200 mA h g(-1) ) with good capacity retention upon cycling in a wide potential domain (2.4-4.9 V). The effect of constituent phases on their electrochemical performance, such as specific capacity, cycling stability, average discharge voltage, and rate capability, are explored here. This family of materials can provide high specific capacity, high rate capability, and promising cycle life. Using Co-free cathode materials is also an obvious advantage of these systems.

New insights into the modification mechanism of Li-rich Li1.2Mn0.6Ni0.2O2 coated by Li2ZrO3

The synergetic modification mechanism in Li₂ZrO₃-coated Li-rich Li_1.2Mn_0.6Ni_0.2O₂via a synchronous lithiation strategy has been proposed.

High performance LiNi0.5Mn1.5O4 cathode material with a bi-functional coating for lithium ion batteries

LiPO₃/LiNi_0.5Mn_1.5O₄ exhibited superior cyclability and rate performance as a result of a bi-functional coating layer of LiPO₃.

Surface Modification of Li1.2Ni0.13Mn0.54Co0.13O2 by Hydrazine Vapor as Cathode Material for Lithium-Ion Batteries

An artificial interface is successfully prepared on the surface of the layered lithium-rich cathode material Li1.2Ni0.13Mn0.54Co0.12O2 via treating it with hydrazine vapor, followed by an annealing process. The inductively coupled plasma-atomic emission spectrometry (ICP) results indicate that lithium ions are leached out from the surface of Li1.2Ni0.13Mn0.54Co0.12O2 by the hydrazine vapor. A lithium-deficiency-driven transformation from layered to spinel at the particle surface happens in the annealing process, which is proved by the results of X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). It is also found that the content of the spinel phase increases at higher annealing temperature, and an internal structural evolution from Li1-xM2O4-type spinel to M3O4-type spinel takes place simultaneously. Compared to the pristine Li1.2Ni0.13Mn0.54Co0.12O2, the surface-modified sample annealed at 300 °C delivers a larger initial discharge capacity of 295.6 mA h g(-1) with a Coulombic efficiency of 89.5% and a better rate performance (191.7 mA h g(-1) at 400 mA g(-1)).

Improving the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 at 4.6 V cutoff potential by surface coating with Li2TiO3 for lithium-ion batteries

The Li₂TiO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ (LTO@NCM) cathode materials are synthesized via an in situ coprecipitation method to improve the electrochemical performance of NCM.