Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1-2x-yTiy)O2 for Lithium Ion Batteries

Unraveling the Dynamic Correlations between Transition Metal Migration and the Oxygen Dimer Formation in the Highly Delithiated LixCoO2 Cathode

The voltage-window expansion can increase the practical capacity of Li_xCoO₂ cathodes, but it would lead to serious structural degradations and oxygen release induced by transition metal (TM) migration. Therefore, it is crucial to understand the dynamic correlations between the TM migration and the oxygen dimer formation. Here, machine-learning-potential-assisted molecular dynamics simulations combined with enhanced sampling techniques are performed to resolve the above question using a representative CoO₂ model. Our results show that the occurrence of the Co migration exhibits local characteristics. The formation of the Co vacancy cluster is necessary for the oxygen dimer generation. The introduction of the Ti dopant can significantly increase the kinetic barrier of the Co ion migration and thus effectively suppress the formation of the Co vacancy cluster. Our work reveals atomic-scale dynamic correlations between the TM migration and the oxygen sublattice's instability and provides insights about the dopant's promotion of the structural stability.

High Cycling Stability of the LiNi0.8 Co0.1 Mn0.1 O2 Cathode via Surface Modification with Polyimide/Multi-Walled Carbon Nanotubes Composite Coating

The Ni-rich LiNi_0.8 Co_0.10 Mn_0.1 O₂ (NCM811) cathode coated by combining with multi-walled carbon nanotubes (MWCNTs) and polyimide (PI) produces a PI3-NCM811 cathode, which markedly improves cycling stability and suppresses secondary crystal cracking. The initial discharge capacity of the PI3-NCM811 cathode is 199.6 mAh g^-1 between 2.8 and 4.3 V at 0.1 C @ 25 °C, which is slightly lower than that of NCM811 (201.1 mAh g^-1 ). The PI3-NCM811 and NCM811 cathodes keep 90.6% and 64.8% of their initial discharge capacity at 1 C between 2.8 and 4.3 V after 500 cycles, respectively. Furthermore, the difference (21.1%) in capacity retention rate between PI3-NCM811 and NCM811 under the condition of 2.8-4.5 V became smaller compared with the difference (25.8%) under the condition of 2.8-4.3 V. This better cyclic stability is mainly attributed to the toughness and elasticity of PI, which inhibits the secondary cracking, maintains the structural integrity of the cathode particles, and protects the particles from electrolyte damage during long-term cycling.

Machine-Learning Approach for Predicting the Discharging Capacities of Doped Lithium Nickel-Cobalt-Manganese Cathode Materials in Li-Ion Batteries

Understanding the governing dopant feature for cyclic discharge capacity is vital for the design and discovery of new doped lithium nickel-cobalt-manganese (NCM) oxide cathodes for lithium-ion battery applications. We herein apply six machine-learning regression algorithms to study the correlations of the structural, elemental features of 168 distinct doped NCM systems with their respective initial discharge capacity (IC) and 50th cycle discharge capacity (EC). First, a Pearson correlation coefficient study suggests that the lithium content ratio is highly correlated to both discharge capacity variables. Among all six regression algorithms, gradient boosting models have demonstrated the best prediction power for both IC and EC, with the root-mean-square errors calculated to be 16.66 mAhg-1 and 18.59 mAhg-1, respectively, against a hold-out test set. Furthermore, a game-theory-based variable-importance analysis reveals that doped NCM materials with higher lithium content, smaller dopant content, and lower-electronegativity atoms as the dopant are more likely to possess higher IC and EC. This study has demonstrated the exciting potentials of applying cutting-edge machine-learning techniques to accurately capture the complex structure-property relationship of doped NCM systems, and the models can be used as fast screening tools for new doped NCM structures with more superior electrochemical discharging properties.

Critical Role of Ti4+ in Stabilizing High-Voltage Redox Reactions in Li-Rich Layered Material

Li-rich layered oxide materials are considered promising candidates for high-capacity cathodes for battery applications and improving the reversibility of the anionic redox reaction is the key to exploiting the full capacity of these materials. However, permanent structural change of the electrode occurring upon electrochemical cycling results in capacity and voltage decay. In view of these factors, Ti⁴⁺ -substituted Li₂ IrO₃ (Li₂ Ir_0.75 Ti_0.25 O₃ ) is synthesized, which undergoes an oxygen redox reaction with suppressed voltage decay, yielding improved electrochemical performance and good capacity retention. It is shown that the increased bond covalency upon Ti⁴⁺ substitution results in structural stability, tuning the phase stability from O3 to O1' upon de-lithiation during charging compared with O3 to T3 and O1 for pristine Li₂ IrO₃ , thereby facilitating the oxidation of oxygen. This work unravels the role of Ti⁴⁺ in stabilizing the cathode framework, providing insight for a fundamental design approach for advanced Li-rich layered oxide battery materials.

Surface Modification of the LiNi0.8Co0.1Mn0.1O2 Cathode Material by Coating with FePO4 with a Yolk-Shell Structure for Improved Electrochemical Performance

Coating with FePO₄ with the size of 20-30 nm on the surface of a LiNi_0.8Co_0.10Mn_0.1O₂ (NCM811) cathode produces an LFP3@NCM811 cathode via a sol-gel method, which markedly reduces secondary crystal cracking. A stable particle structure greatly improves the cycling stability of the LFP3@NCM811cathode, which retains 97% of its initial discharge capacity compared to NCM811 (78%) after 100 cycles at 2.7-4.5 V. Furthermore, it retains 86 and 63% of its initial discharge capacity after 400 cycles for LFP3@NCM811 and NCM811, respectively. The initial discharge capacity of the LFP3@NCM811 cathode is 218.8 mAh g^-1 at 0.1 C, and the discharge capacity of the LFP3@NCM811 cathode is achieved to be 151.4 mAh g^-1 at 5 C, which is 15 mAh g^-1 higher than that of the NCM811 cathode. These are due to the reduction of cation mixing for a certain amount of Fe²⁺/Fe³⁺ or PO₄^3- doped into the NCM811 surface, and the yolk-shell structure formed by coating with FePO₄ helps improve the electronic conductivity and accelerate the Li⁺ transport. The cycling stability is mainly due to the secondary cleavage inhibition, which maintains the structural integrity of the cathode particles during the long cycle process and protects the inside of the particle from harmful electrolytes.

Degradation Mechanisms and Mitigation Strategies of Nickel-Rich NMC-Based Lithium-Ion Batteries

Abstract

The demand for lithium-ion batteries (LIBs) with high mass-specific capacities, high rate capabilities and long-term cyclabilities is driving the research and development of LIBs with nickel-rich NMC (LiNixMnyCo1−x−yO2, $$x \geqslant 0.5$$x⩾0.5) cathodes and graphite (LixC6) anodes. Based on this, this review will summarize recently reported and widely recognized studies of the degradation mechanisms of Ni-rich NMC cathodes and graphite anodes. And with a broad collection of proposed mechanisms on both atomic and micrometer scales, this review can supplement previous degradation studies of Ni-rich NMC batteries. In addition, this review will categorize advanced mitigation strategies for both electrodes based on different modifications in which Ni-rich NMC cathode improvement strategies involve dopants, gradient layers, surface coatings, carbon matrixes and advanced synthesis methods, whereas graphite anode improvement strategies involve surface coatings, charge/discharge protocols and electrolyte volume estimations. Electrolyte components that can facilitate the stabilization of anodic solid electrolyte interfaces are also reviewed, and trade-offs between modification techniques as well as controversies are discussed for a deeper understanding of the mitigation strategies of Ni-rich NMC/graphite LIBs. Furthermore, this review will present various physical and electrochemical diagnostic tools that are vital in the elucidation of degradation mechanisms during operation to supplement future degradation studies. Finally, this review will summarize current research focuses and propose future research directions.

Graphic Abstract

The demand for lithium-ion batteries (LIBs) with high mass specific capacities, high rate capabilities and longterm cyclabilities is driving the research and development of LIBs with nickel-rich NMC (LiNixMnyCo1−x−yO2, x ≥ 0.5) cathodes and graphite (LixC6) anodes. Based on this, this review will summarize recently reported and widely recognized studies of the degradation mechanisms of Ni-rich NMC cathodes and graphite anodes. And with a broad collection of proposed mechanisms on both atomic and micrometer scales, this review can supplement previous degradation studies of Ni-rich NMC batteries. In addition, this review will categorize advanced mitigation strategies for both electrodes based on different modifications in which Ni-rich NMC cathode improvement strategies involve dopants, gradient layers, surface coatings, carbon matrixes and advanced synthesis methods, whereas graphite anode improvement strategies involve surface coatings, charge/discharge protocols and electrolyte volume estimations. Electrolyte components that can facilitate the stabilization of anodic solid-electrolyte interfaces (SEIs) are also reviewed and tradeoffs between modification techniques as well as controversies are discussed for a deeper understanding of the mitigation strategies of Ni-rich NMC/graphite LIBs. Furthermore, this review will present various physical and electrochemical diagnostic tools that are vital in the elucidation of degradation mechanisms during operation to supplement future degradation studies. Finally, this review will summarize current research focuses and propose future research directions.

Targeted Surface Doping with Reversible Local Environment Improves Oxygen Stability at the Electrochemical Interfaces of Nickel-Rich Cathode Materials

Elemental doping represents a prominent strategy to improve interfacial chemistry in battery materials. Manipulating the dopant spatial distribution and understanding the dynamic evolution of the dopants at the atomic scale can inform better design of the doping chemistry for batteries. In this work, we create a targeted hierarchical distribution of Ti⁴⁺, a popular doping element for oxide cathode materials, in LiNi_0.8Mn_0.1Co_0.1O₂ primary particles. We apply multiscale synchrotron/electron spectroscopy and imaging techniques as well as theoretical calculations to investigate the dynamic evolution of the doping chemical environment. The Ti⁴⁺ dopant is fully incorporated into the TMO₆ octahedral coordination and is targeted to be enriched at the surface. Ti⁴⁺ in the TMO₆ octahedral coordination increases the TM-O bond length and reduces the covalency between (Ni, Mn, Co) and O. The excellent reversibility of Ti⁴⁺ chemical environment gives rise to superior oxygen reversibility at the cathode-electrolyte interphase and in the bulk particles, leading to improved stability in capacity, energy, and voltage. Our work directly probes the chemical environment of doping elements and helps rationalize the doping strategy for high-voltage layered cathodes.

Improving the Electrochemical Performance and Structural Stability of the LiNi0.8Co0.15Al0.05O2 Cathode Material at High-Voltage Charging through Ti Substitution

LiNi_0.8Co_0.15Al_0.05O₂ (NCA) has been proven to be a good cathode material for lithium-ion batteries (LIBs), especially in electric vehicle applications. However, further elevating energy density of NCA is very challenging. Increasing the charging voltage of NCA is an effective method, but its structural instability remains a problem. In this work, we revealed that titanium substitution could improve cycle stability of NCA under high cutoff voltage significantly. Titanium ions with a relatively larger ion radius could modify the oxygen lattice and change the local coordination environment of NCA, leading to decreased cation migration, better kinetic and thermodynamic properties, and improved structural stability. As a result, the Ti-substituted NCA cathode exhibits impressive reversible capacity (198 mA h g^-1 at 0.1C) with considerable cycle stability under a cutoff voltage up to 4.7 V. It is also revealed that Ti could suppress oxygen release in the high-voltage region, benefitting cycle and thermal stabilities. This work provides valuable insight into the design of high-voltage layered cathode materials for high-energy-density LIBs.

Molecular Electrostatic Potential: A New Tool to Predict the Lithiation Process of Organic Battery Materials

This work is pioneering to introduce molecular electrostatic potential (MESP) to investigate the interaction between lithium ions and organic electrode molecules. The electrostatic potential on the van der Waals surface of the electrode molecule is calculated, and then the coordinates and relative values of the local minima of MESP can be correlated to the Li binding sites and sequence on an organic small molecule, respectively. This suggests a gradual lithiation process. Similar calculations are extended to polymers and even organic crystals. The operation process of MESP for these systems is explained in detail. Through providing accurate and visualizable lithium binding sites, MESP can give precise prediction of the lithiated structures and reaction mechanism of organic electrode materials. It will become a new theoretical tool for determining the feasibility of organic electrode materials for alkali metal ion batteries.

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.

Electrochemical Characteristics of Layered Transition Metal Oxide Cathode Materials for Lithium Ion Batteries: Surface, Bulk Behavior, and Thermal Properties

Layered lithium transition metal oxides, in particular, NMCs (LiNi_xCo_yMn_zO₂) represent a family of prominent lithium ion battery cathode materials with the potential to increase energy densities and lifetime, reduce costs, and improve safety for electric vehicles and grid storage. Our work has focused on various strategies to improve performance and to understand the limitations to these strategies, which include altering compositions, utilizing cation substitutions, and charging to higher than usual potentials in cells. Understanding the effects of these strategies on surface and bulk behavior and correlating structure-performance relationships advance our understanding of NMC materials. This also provides information relevant to the efficacy of various approaches toward ensuring reliable operation of these materials in batteries intended for demanding traction and grid storage applications. In this Account, we start by comparing NMCs to the isostructural LiCoO₂ cathode, which is widely used in consumer batteries. Effects of changing the metal content (Ni, Mn, Co) upon structure and performance of NMCs are briefly discussed. Our early work on the effects of partial substitution of Al, Fe, and Ti for Co on the electrochemical and bulk structural properties is then covered. The original aim of this work was to reduce the Co content (and thus the raw materials cost) and to determine the effect of the substitutions on the electrochemical and bulk structural properties. More recently, we have turned to the application of synchrotron and advanced microscopy techniques to understand both bulk and surface characteristics of the NMCs. Via nanoscale-to-macroscale spectroscopy and atomically resolved imaging techniques, we were able to determine that the surfaces of NMC undergo heterogeneous reconstruction from a layered structure to rock salt under a variety of conditions. Interestingly, formation of rock salt also occurs under abuse conditions. The surface structural and chemical changes affect the charge distribution, the charge compensation mechanisms, and ultimately, the battery performance. Surface reconstruction, cathode/electrolyte interface layer formation, and oxygen loss are intimately related, making it difficult to disentangle the effects of each of these phenomena. They are driven by the different redox activities of Ni and O on the surface and in the bulk; there is a greater tendency for charge compensation to occur on oxygen anions at particle surfaces rather than on Ni, whereas the Ni in the bulk is more redox active than on the surface. Finally, our latest research efforts are directed toward understanding the thermal properties of NMCs, which is highly relevant to their safety in operating cells.

Utilizing Co2+/Co3+ Redox Couple in P2-Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium-Ion Batteries

Developing sodium-ion batteries for large-scale energy storage applications is facing big challenges of the lack of high-performance cathode materials. Here, a series of new cathode materials Na0.66Co x Mn0.66-x Ti0.34O2 for sodium-ion batteries are designed and synthesized aiming to reduce transition metal-ion ordering, charge ordering, as well as Na+ and vacancy ordering. An interesting structure change of Na0.66Co x Mn0.66-x Ti0.34O2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na0.66Co0.22Mn0.44Ti0.34O2 with a P2-type layered structure delivers a reversible capacity of 120 mAh g-1 at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g-1 can still be obtained, indicating a promising rate capability. The low valence Co2+ substitution results in the formation of average Mn3.7+ valence state in Na0.66Co0.22Mn0.44Ti0.34O2, effectively suppressing the Mn3+-induced Jahn-Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na0.66Co0.22Mn0.44Ti0.34O2 during charge/discharge is contributed by Co2.2+/Co3+ and Mn3.3+/Mn4+ redox couples. This is the first time that the highly reversible Co2+/Co3+ redox couple is observed in P2-layered cathodes for sodium-ion batteries. This finding may open new approaches to design advanced intercalation-type cathode materials.

Holy Grails in Chemistry: Investigating and Understanding Fast Electron/Cation Coupled Transport within Inorganic Ionic Matrices

Typically, power and energy are competing concepts in electrochemical energy storage, where one can be optimized only at the expense of the other. However, the specialized and diverse needs of new applications exceed the functional boundaries of existing battery chemistries, where both high power and high energy content are critical. The needed battery paradigms may not be realized by optimization of previous electrochemical energy storage technologies but rather require new basic science breakthroughs involving new materials chemistry. Here we propose that fundamental understanding of electron/cation coupled transport within inorganic ionic matrices is a holy grail that can potentially transform the energy storage landscape.

Persistent State-of-Charge Heterogeneity in Relaxed, Partially Charged Li1- x Ni1/3 Co1/3 Mn1/3 O2 Secondary Particles

Ex situ transmission X-ray microscopy reveals micrometer-scale state-of-charge heterogeneity in solid-solution Li1- x Ni1/3 Co1/3 Mn1/3 O2 secondary particles even after extensive relaxation. The heterogeneity generates overcharged domains at the cutoff voltage, which may accelerate capacity fading and increase impedance with extended cycling. It is proposed that optimized secondary structures can minimize the state-of-charge heterogeneity by mitigating the buildup of nonuniform internal stresses associated with volume changes during charge.

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.

Synthesis and characterization of LiNi0.48Co0.18Mn0.3Mg0.02Ti0.02O2 as a cathode material for lithium ion batteries

XRD patterns (a) and Rietveld refinement results (b and c) of LiNi_0.5Co_0.2Mn_0.3O₂ and LiNi_0.48Co_0.18Mn_0.3Mg_0.02Ti_0.02O₂.

A quinary layer transition metal oxide of NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2 as a high-rate-capability and long-cycle-life cathode material for rechargeable sodium ion batteries

A well-crystallized single-phase quinary layer transition metal oxide of NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2 was successfully synthesized. It exhibited excellent cycle performance and high rate capability as a cathode material for sodium-ion batteries.

Tailoring the surface properties of LiNi0.4Mn0.4Co0.2O2by titanium substitution for improved high voltage cycling performance

LiNi_0.4Mn_0.4Co_0.2O₂cathodes with and without Ti substitution were cycled to equivalent degrees of Li removal and transition metal oxidation states on electrode surfaces were probed using X-ray absorption spectroscopy.