Li-rich Layered Oxide Cathode Research Articles

New information about the cyclable capacity fading process of a pouch cell with Li-rich layered oxide cathodes.

Most studies investigate the cyclable capacity fading mechanism of Li-rich layered oxides (LLOs) from the microscopic structure level, lacking discussions about how the structure degradation influences the performance of the pouch cell precisely and quantitatively. Based on the analysis of the evolution of key parameters during the whole cycling period, a new transition-type fading mechanism is proposed. From the early-to-middle stage of the cycling period, polarization increases, most of which is interface-related, causing about 67% of the whole capacity loss. From the middle-to-late stage of the cycling period, active material losses turn out to be the dominating factor, inducing about 61% of the total capacity loss. Diffusion-related polarization, replacing the interface type, is responsible for most of the increased overpotential. Relative analysis confirms that during the early stage, the increase of the charge transfer resistance, induced by CEI (cathode electrolyte interface) growth and initial surface layered-structure degradation, is the main source of interface polarization. As the cycling evolves to the late stage, severe bulky structure degradation, including lattice-oxygen release, Li/Ni mixture and generation of a new spinel phase, turns out to be the major factor, causing further capacity fading.

Highly reversible transition metal migration in superstructure-free Li-rich oxide boosting voltage stability and redox symmetry

The further practical applications of Li-rich layered oxides are impeded by voltage decay and redox asymmetry, which are closely related to the structural degradation involving irreversible transition metal migration. It has been demonstrated that the superstructure ordering in O2-type materials can effectively suppress voltage decay and redox asymmetry. Herein, we elucidate that the absence of this superstructure ordering arrangement in a Ru-based O2-type oxide can still facilitate the highly reversible transition metal migration. We certify that Ru in superstructure-free O2-type structure can unlock a quite different migration path from Mn in mostly studied cases. The highly reversible migration of Ru helps the cathode maintain the structural robustness, thus realizing terrific capacity retention with neglectable voltage decay and inhibited oxygen redox asymmetry. We untie the knot that the absence of superstructure ordering fails to enable a high-performance Li-rich layered oxide cathode material with suppressed voltage decay and redox asymmetry.

A Li-rich layered oxide cathode with remarkable capacity and prolonged cycle life

Introducing a facile ion-exchange method coupled with low-temperature thermal treatment, we have developed a strategy to enhance the cycling performance of Lithium-rich manganese-based layered oxides (LLOs). This approach results in the formation of a thin spinel phase layer, which is covered by a fast-ion-conducting Li3PO4 phase on the surface of pristine 0.5Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2 secondary particles of aggregated original nanoparticles, and an increase in oxygen vacancies. After undergoing 200 cycles at a rate of 1 C, the resulting cathode exhibits a high capacity of 204.7 mAh/g with a retention rate of up to 94.4 %, which is significantly superior to that of the original materials. The long-term cycling stability of the modified cathode is also evident at higher rates such as 2, 5, and 8 C. The electrochemical analysis suggests that the surface modification and oxygen vacancies can enhance the Li+ diffusion coefficient, improve anion redox reversibility and increase the capacity contribution from spinel oxidation, thereby improving the electrochemical performance of the cathode during cycling.

Highly structural stability from small-sized Li2MnO3-like domains in Co-free Li-rich layered oxide cathodes

Highly structural stability from small-sized Li2MnO3-like domains in Co-free Li-rich layered oxide cathodes

Weak σ–π–σ interaction stabilizes oxygen redox towards high-performance Li–rich layered oxide cathodes

The aggregation of Li2MnO3–like domains in Li-rich layered oxides (LLOs) causes severe capacity/voltage fading, which seriously impedes their commercial applications. Here, we design Co–free LLO models with well–dispersed Li2MnO3–like domains (D–LNMO) and aggregated Li2MnO3–like domains (A–LNMO) to investigate the oxygen redox process and structural stability. It is found that low oxygen partial pressure can disperse Li2MnO3–like domains by forming stable ONiMn4+Mn3+Li3 coordination configurations so that D-LNMO is predominant. Moreover, a novel oxygen oxidation mechanism involving a weak σ–π–σ interaction where oxygen redox in OTM2MnLi3 (TM = Ni, Mn) configurations is triggered by O in Li-O-Li configurations is revealed. Specifically, the lattice oxygen at the interface of Li2MnO3–like domains and LiTMO2 domains can be activated, which is beyond conventional Li-O-Li configuration. Due to the abundance of interfacial lattice oxygen in D–LNMO, more lattice oxygen participates in charge compensation, thereby relieving the oxidation load of oxygen ions, suppressing lattice oxygen release, and delaying irreversible structural transformation. Consequently, D-LNMO possesses highly reversible oxygen redox and exceptional structural stability, exhibiting superior cycling stability of high capacity. The findings provide new perspectives and concepts for designing high-energy Li-rich cathodes.

Stabilizing Li-rich layered oxide cathode interface by using silicon-based electrolyte additive

In this study, we investigate the efficacy of 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (ViD4) as an electrolyte additive to enhance the electrochemical stability of Li-rich (LRO)/Li cells. The LRO/Li cell in the 1 vol% ViD4 electrolyte displays a mere 27.9 % capacity loss after 100 cycles at 0.5C (1C = 300 mAh−1), in comparison with the 66 % observed in the baseline electrolyte. Theoretical calculations reveal that ViD4 possesses a lower calculated oxidation potential than the electrolyte solution, signifying its preferential oxidation propensity. Physical characterization results demonstrate the formation of a uniform ViD4-derived film spanning 2–3 nm on the LRO cathode surface. This film enhances the stability of the cathode/electrolyte interface and safeguards the structural integrity of LRO. Moreover, ViD4 acts as a scavenger for hydrogen fluoride (HF), which is a decomposition product of LiPF6. Theoretical calculations verify the feasibility of ViD4 in effectively eliminating HF.

Manipulated Fluoro‐Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization‐Induced Capacity Loss in Li‐Rich Layered Cathode

AbstractElectrolyte engineering is a fascinating choice to improve the performance of Li‐rich layered oxide cathodes (LRLO) for high‐energy lithium‐ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro‐ether. By benefit of the decomposition of fluoro‐ether under nucleophilic O‐related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro‐ether acts as “sutures”, ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V‐class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high‐energy anionic redox related cathode systems.

Manipulated Fluoro-Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization-Induced Capacity Loss in Li-Rich Layered Cathode.

Electrolyte engineering is a fascinating choice to improve the performance of Li-rich layered oxide cathodes (LRLO) for high-energy lithium-ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro-ether. By benefit of the decomposition of fluoro-ether under nucleophilic O-related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro-ether acts as "sutures", ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V-class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high-energy anionic redox related cathode systems.

Ultrafast synthesis of cation/anion co-doped Li-rich layered oxide cathodes with Li+/Ni2+ mixing structural defects

An ultrafast nickel foil-based thermal shock method is reported to synthesize cation/anion co-doped Li-rich layered oxide with Li+/Ni2+ mixing in seconds.

Building oxygen-rich vacancy coating for fast Li-ion diffusion kinetics and stable cycling of lithium-rich layered oxide cathode at wide-temperature

As a cathode with a high energy density for Li-ion batteries, Li-rich layered oxide (LLO) cathodes suffer from short lifespans and poor rate performance, mainly due to their irreversible lattice oxygen evolution leading to severe structural degradation and oxygen release. Herein, we propose the use of Zn-doped SnO2 coated with LLO cathode to tackle these issues. Compared to the traditional method of coating oxides on the surface of lithium-rich cathodes, Zn doping of SnO2 coating can artificially create abundant oxygen vacancies for LLO cathode, which can provide a buffer boundary for active oxygen generated by lithium-rich cathode during circulation, effectively promoting the reversible reaction of oxygen redox couple and stabilizing lattice oxygen. The experimental results show that the surface of the LLO cathode's Zn-doped SnO2 is rich in oxygen vacancies could significantly boost the kinetics of lithium ion transfer, and effectively inhibit layer-to-spinel phase transition, and lattice oxygen release. By taking advantage of the Zn-doped SnO2 coating, the capacity stability of LLO cathode is raised from 58.1 % to 86.7 % at 1C after 500 cycles and also obtains outstanding rate performance and cycling durability at wide-temperature (−10 to 55 °C).

Multifunctional surface structure with enhanced electron and ion conductivities for superior electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes

Co-free Li-rich Mn-based layered oxides (LRMLOs) are attractive cathode materials for lithium-ion batteries due to their low cost, high energy density and environmental friendliness. However, they also suffer from the drawbacks of low initial Coulombic efficiency, rapid voltage fading and capacity decay, and poor rate performance as the general LRMLO cathode, which prevent their practical applications. In the present work, a multilayer surface coating composed of Li2B4O7/B2O3 and a spinel heterostructure rich-in oxygen vacancies are in-situ formed at the surface of the Co-free LRMLO particles by a facile thermal reduction treatment with NH4B5O8. The multilayer surface improves greatly the electron and ion conductivities, and plays significantly synergistic roles on suppressing oxygen release, reducing the dissolution of Mn and Ni elements, retarding the structure transformation from layered to spinel, and preventing the side reaction of the active material with electrolyte during cycling for the cathodes. As a result, the overall electrochemical properties of the cathodes are significantly improved. With an optimized addition of 10 wt% NH4B5O8, the initial Coulombic efficiency of the cathode is as high as 90.0%, and a capacity of 275 mAh g-1 is obtained at 0.1 C (1 C = 200 mA g-1) with the capacity retention of 98.5% after 100 cycles. A capacity of 194 mAh g-1 is maintained after 300 cycles at 1 C, with the capacity retention of 92.8%. A capacity of 117 mAh g-1 is obtained at a high rate of 20 C. In addition, the voltage fading during cycling is also greatly inhibited. The structure evolution of the cathode material during cycling is systematically investigated and the mechanism for the improved electrochemical properties is proposed. The present work provides a new surface structure design strategy and the method for enhancing the overall electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes.

Machine learning-based evaluation of functional characteristics of Li-rich layered oxide cathode materials using the data of XPS and XRD spectra

Li-ion batteries are the most wide-spread electrochemical energy storage systems. Cathode materials are a major focus because they are considered the limiting element of overall battery performance. Li-rich layered oxides are among the actively investigated cathode materials designated for using in Li-ion batteries of new generation and approaching to the imposed high requirements of high energy and power densities. However, the inherent complexity of their structure resulting in the well-known problems with the structural stability has hindered the widespread use of Li-rich layered oxides as the cathode materials. In this study, we have used machine learning (ML) methods to obtain the models able to predict the electrochemical and structural characteristics of these materials. The data from XPS and XRD spectra were used as the additional descriptors to evaluate the target functional characteristics: initial discharge capacity, coulombic efficiency and capacity fade. One of the capacities of ML to impute the missing data were exploited to solve the problem of incomplete experimental data. The obtained models have shown the reasonable predictive performance. The analysis of the experimental data and the results of modeling have feed to further considerations concerning the complexity of the multifold processes that determine the target functional characteristics of these materials.

Guaiacol as an Organic Superoxide Dismutase Mimics for Anti-ageing a Ru-based Li-rich Layered Oxide Cathode.

High-capacity Li-rich layered oxides using oxygen redox as well as transition metal redox suffer from its structural instability due to lattice oxygen escaped from its structure during oxygen redox and the following electrolyte decomposition by the reactive oxygen species. Herein, we rescued a Li-rich layered oxide based on 4d transition metal by employing an organic superoxide dismutase mimics as a homogeneous electrolyte additive. Guaiacol scavenged superoxide radicals via dismutation or disproportionation to convert two superoxide molecules to peroxide and dioxygen after absorbing lithium superoxide on its partially negative oxygen of methoxy and hydroxyl groups. Additionally, guaiacol was decomposed to form a thin and stable cathode-electrolyte interphase (CEI) layer, endowing the cathode with the interfacial stability.

Stabilized Li-Rich Layered Oxide Cathode by a Spontaneously Formed Yb and Oxygen-Vacancy Rich Layer on the Surface.

Li-rich layered oxides (LLOs) are among the most promising cathode materials with high theoretical specific capacity (>250 mAh g-1 ). However, capacity decay and voltage hysteresis due tostructural degradation during cycling impede the commercial application of LLOs. Surface engineering and element doping are two methods widely applied tomitigate the structural degradation. Here, it is found that trace amount lanthanide element Yb doping can spontaneously form a surficial Yb-rich layer with high density of oxygen vacancy on the LLO-0.3% Yb (Li1.2 Mn0.54 Co0.13-x Ybx Ni0.13 O2 where x = 0.003) cathodes, which mitigating lattice oxygen loss and the non-preferred layered-to-spinel-to-rock salt tri-phase transition. Meanwhile, there are also some Yb ions doped into the lattice of LLO, which enhance the binding energy with oxygen and stabilize the lattice in grain interior during cycling. The dual effects of Yb doping greatly mitigate the structure degradation during cycling, and facilitate fast diffusion of lithium ions. As a result, the LLO-0.3% Yb sample achieves significantly improved cycling stability, with a capacity retention of 84.69% after 100 cycles at 0.2 C and 84.3% after 200 cycles at 1 C. These finding shighlight the promising rare element doping strategy that can have both surface engineering and doping effects in preparing LLO cathodes with high stability.

Zr/Ti trace Co-doping induced disordered structure to enhance the cycling stability of Li-rich Mn-based layered oxide cathodes

Due to the ultrahigh energy density, Li-rich Mn-based layered oxide cathodes have attracted great attentions. However, the irreversible structural transformation usually leads to long-cycled capacity decay, which severely restricts their practical applications. Here, a Zr/Ti trace co-doping strategy is proposed to stabilize the structure and obtain excellent electrochemical performance by constructing bulk phase disorder and surface Zr-enriched structures. The designed Li1.2Mn0.53Ni0.13Co0.13Zr0.005Ti0.005O2 (ZT-LRM) not only shows a higher specific capacity but also achieves an excellent long-cycle performance. The capacity retention of ZT-LRM is 69 % after 500 cycles at 5C (1250 mA g−1), which is much higher than that (34 %) of the Li1.2Mn0.54Ni0.13Co0.13O2 cathode. Neutron powder diffraction and transmission electron microscopy are used to demonstrate the disordered structure in the particles and the line scanning proves the surface Zr enrichment. In-situ X-ray diffraction verifies the more stable structural evolutions of the ZT-LRM during cycling. This simple modification method provides a reference for commercial applications of Li-rich Mn-based layered oxide cathodes.

Li2So4 Coating of Co-Free, Li-Rich Layered Oxide Cathodes through a Carbonate Coating Method for Improving Cycle Performance

In the continued evolution toward high-performance lithium-ion batteries, cobalt has presented itself as a major obstacle due to its price and accessibility. Thus, Co-Free, Li-rich Layered Oxide Cathodes (CF-LLC) have become a promising cathode candidate for their exclusion of Cobalt and high theoretical capacity. Nevertheless, CF-LLC suffers from LLC issues such as phase transformation and voltage fade. Due to high overpotential from sluggish kinetics, the oxygen plateau of CF-LLC occurs at a higher-than-normal potential, leading to a shortened first cycle charge and lower discharge capacity. To mitigate this, we implemented a sulfate coating of the carbonate precursor; which upon lithiation, presents a Li2SO4 coated LLC (S-CF-LLC). The Li-sulfate coating prevents agglomeration of the primary particles during lithiation, leading to a larger surface area and thus, faster kinetics. The coating also prevents increased degradation from the increased surface area by acting as a protective barrier. The S-CF-LLC was successfully synthesized using a hydrothermal synthesis method with a wet chemical coating step and the synthesis was confirmed using XRD, SEM, XPS, and HR-TEM techniques. Galvanostatic cycling showed that the first cycle improved from 236.1mAhg-1 to 263.5mAhg-1 after coating, owing to the improved kinetics. GITT analysis of the increased diffusion coefficient further confirmed the overall improved kinetics. Moreover, the efficacy of the coating was shown through cycle data after 100 cycles, which showed good coulombic efficiency with a high average of >99%, good capacity retention, and good voltage fade per cycle improvement. Overall, the successfully synthesized S-CF-LLC opens up the possibility of practical cobalt-free high-performance batteries.

Tuning Li2MnO3-Like Domain Size and Surface Structure Enables Highly Stabilized Li-Rich Layered Oxide Cathodes.

Severe capacity/voltage fading still poses substantial obstacles in the commercial applications of Li-rich layered oxides, which stems from the aggregation of Li2MnO3-like domains and unstable surface structure. Here, we report highly stabilized Co-free Li1.2Ni0.2Mn0.6O2 with uniformly dispersed Li2MnO3-like domains and a protective rock-salt structure shell by reducing the oxygen partial pressure during high-temperature calcination. Experimental characterizations and DFT calculations reveal that the uniformly dispersed and small-sized Li2MnO3-like domains suppress the peroxidation of lattice oxygen, enabling highly reversible oxygen redox and excellent structural stability. Moreover, the induced rock-salt structure shell significantly restrains lattice oxygen release, TM dissolution, and interfacial side reactions, thereby improving the interfacial stability and facilitating Li+ diffusion. Consequently, the obtained Li1.2Ni0.2Mn0.6O2 which was calcinated under an oxygen partial pressure of 0.1% (LNMO-0.1) delivers a high reversible capacity of 276.5 mAh g-1 at 0.1 C with superior cycling performance (a capacity retention rate of 85.4% after 300 cycles with a small voltage fading rate of 0.76 mV cycle-1) and excellent thermal stability. This work links the synthesis conditions with the domain structure and electrochemical performance of Li-rich cathode materials, providing some insights for designing high-performance Li-rich cathodes.

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.