A multicompositional particulate LiNi0.9Co0.05Mn0.05O2 cathode in which LiNi0.94Co0.038Mn0.022O2 at the particle center is encapsulated by a 1.5 µm thick concentration gradient (CG) shell with the ...outermost surface composition LiNi0.841Co0.077Mn0.082O2 is synthesized using a differential coprecipitation process. The microscale compositional partitioning at the particle level combined with the radial texturing of the refined primary particles in the CG shell layer protracts the detrimental H2 → H3 phase transition, causing sharp changes in the unit cell dimensions. This protraction, confirmed by in situ X‐ray diffraction and transmission electron microscopy, allows effective dissipation of the internal strain generated upon the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability as compared to those of the conventional single‐composition LiNi0.9Co0.05Mn0.05O2 cathodes. The compositionally partitioned cathode delivers a discharge capacity of 229 mAh g−1 and exhibits capacity retention of 88% after 1000 cycles in a pouch‐type full cell (compared to 68% for the conventional cathode). Thus, the proposed cathode material provides an opportunity for the rational design and development of a wide range of multifunctional cathodes, especially for Ni‐rich LiNixCoyMn1‐x‐yO2 cathodes, by compositionally partitioning the cathode particles and thus optimizing the microstructural response to the internal strain produced in the deeply charged state.
A multicompositional cathode in which the particle center is encapsulated concentration gradient (CG) shell is synthesized. The radial texturing of the refined primary particles in the CG shell protracts the detrimental H2 → H3 phase transition. This protraction allows effective dissipation of the internal strain from the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability.
Ni-rich LiNi x Co y Mn1–x–y O2 cathodes (x = 0.6, 0.8, 0.9, and 0.95) were tested to characterize the capacity fading mechanism of extremely rich Ni compositions. Increasing the Ni fraction in the ...cathode delivered a higher discharge capacity (192.9 mA h g–1 for LiNi0.6Co0.2Mn0.2O2 versus 235.0 mA h g–1 for LiNi0.95Co0.025Mn0.025O2); however, the cycling stability was substantially reduced. LiNi0.6Co0.2Mn0.2O2 and LiNi0.8Co0.1Mn0.1O2 retained more than 95% of their respective initial capacities after 100 cycles, while the capacity retention of LiNi0.9Co0.05Mn0.05O2 and LiNi0.95Co0.025Mn0.025O2 was limited to 85% during the same cycling period. The relatively inferior cycling stability of LiNi x Co y Mn1–x–y O2 with x > 0.8 is attributed to the phase transition near the charge-end, causing an abrupt anisotropic shrinkage (or expansion during discharge), which was suppressed for compositions of x < 0.8. Residual stress stemming from the phase transition destabilized the internal microcracks and allowed the microcracks to propagate to the surface, providing channels for electrolyte penetration and subsequent degradation of the exposed internal surfaces formed by the microcracks. Further developments in particle morphology are required to dissipate the intrinsic lattice strain, stabilize the surface, and modify the composition to attain a satisfactory long-term cycling stability, and hence battery life.
A new class of layered cathodes, LiNixCoyB1−x−yO2 (NCB), is synthesized. The proposed NCB cathodes have a unique microstructure in which elongated primary particles are tightly packed into spherical ...secondary particles. The cathodes also exhibit a strong crystallographic texture in which the a–b layer planes are aligned along the radial direction, facilitating Li migration. The microstructure, which effectively suppresses the formation of microcracks, improves the cycling stability of the NCB cathodes. The NCB cathode with 1.5 mol% B delivers a discharge capacity of 234 mAh g−1 at 0.1 C and retains 91.2% of its initial capacity after 100 cycles (compared to values of 229 mAh g−1 at 0.1 C and 78.8% for pristine LiNi0.9Co0.1O2). This study shows the importance of controlling the microstructure to obtain the required cycling stability, especially for Ni‐rich layered cathodes, where the main cause of capacity fading is related to mechanical strain in their charged state.
A new class of layered cathodes, LiNixCoyB1−x−yO2 (NCB), is proposed. NCB cathodes have a highly oriented microstructure in which the primary particle widths can be controlled by varying the boron fraction in the composition. The unique microstructural configuration of the NCB cathodes effectively suppress the formation of microcracks and significantly improve the cycling stability compared to a conventional cathode.
The electrochemical and structural stabilities of a conventional LiNi0.90Co0.045Mn0.045Al0.01O2 (NCMA90) cathode and a core–shell with concentration gradient cathode (CSG‐NCMA90) are evaluated by ...cycling the cathodes at different depths of discharge (DoDs). The CSG‐NCMA90 cathode consists of fine, elongated primary particles that are radially aligned from the center of a spherical secondary particle. This unique microstructure effectively suppresses microcrack formation and propagation in the highly charged state. Moreover, microstructural analysis through transmission electron microscopy reveals that the thin elongated primary particles, largely featuring (001) facets on their lateral sides, are tolerant of electrolyte attack, thus suppressing surface degradation. In a full cell, these microstructural features enable the CSG‐NCMA90 cathode to retain 90.7% of its initial capacity after 1000 cycles at 100% DoD. Unlike conventional Ni‐rich layered cathodes whose capacity should be restricted to ≈60–80% to ensure their long service life, the proposed CSG‐NCMA90 cathode can be cycled at full capacity, thus facilitating higher electrochemical performance and realizing the development of economical Li‐ion batteries.
An Al‐doped concentration gradient Ni‐rich cathode, LiNi0.9Co0.045Mn0.045Al0.01O2 (CSG‐NCMA90), is proposed. The CSG‐NCMA90 cathode is composed of radially oriented rod‐shaped primary particles. The unique microstructure of the CSG‐NCMA90 cathode effectively suppresses the formation of microcracks, and rod‐shaped primary particles largely featuring protective (001) facets are tolerant of electrolyte attack. The optimized microstructure significantly improves the cycling stability compared to a conventional cathode.
Because electric vehicles (EVs) are used intermittently with long resting periods in the fully charged state before driving, calendar aging behavior is an important criterion for the application of ...Li‐ion batteries used in EVs. In this work, Ni‐rich LiNixCoyMn1−x−yO2 (x = 0.8 and 0.9) cathode materials with high energy densities, but low cycling stabilities are investigated to characterize their microstructural degradation during accelerated calendar aging. Although the particles seem to maintain their crystal structures and morphologies, the microcracks which develop during calendar aging remain even in the fully discharged state. An NiO‐like phase rock‐salt structure of tens of nanometers in thickness accumulates on the surfaces of the primary particles through parasitic reactions with the electrolyte. In addition, the passive layer of this rock‐salt structure near the microcracks is gradually exfoliated from the primary particles, exposing fresh surfaces containing Ni4+ to the electrolyte. Interestingly, the interior primary particles near the microcracks have deteriorated more severely than the outer particles. The microstructural degradation is worsened with increasing Ni contents in the cathode materials, directly affecting electrochemical performances such as the reversible capacities and voltage profiles.
Microstructural degradation of Ni‐rich LiNixCoyMn1–x–yO2 cathode materials during calendar aging is reported. Because of parasitic reactions, microcracks develop and remain across the entire secondary particle, even after discharging. An NiO‐like phase rock‐salt structure accumulates and exfoliates from the main particles, continuously exposing fresh surfaces. This phenomenon reflects in the electrochemical performance including the discharge capacity and the voltage profile.
Substituting W for Al in the Ni‐rich cathode LiNi0.885Co0.10Al0.015O2 (NCA89) produces LiNi0.9Co0.09W0.01O2 (NCW90) with markedly reduced primary particle size. Particle size refinement considerably ...improves the cathode's cycling stability such that the NCW90 cathode retains 92% of its initial capacity after 1000 cycles (compared to 63% for NCA89), while the cathode produces a high initial discharge capacity of 231.2 mAh g−1 (at 0.1 C). Thus, the proposed NCW90 can deliver high energy density and a long battery lifetime simultaneously, unlike other Ni‐rich layered oxide cathodes. This unprecedented cycling stability is mainly attributed to a series of interparticular microfractures that absorb the anisotropic lattice strain caused by a deleterious phase transition near the charge end, thereby improving the cathode's resistance to fracture. Microcrack suppression preserves the mechanical integrity of the cathode particles during cycling and protects the particle interior from detrimental electrolyte attack. The proposed NCW90 cathode provides an improved material from which a new series of Ni‐rich layered cathode can be developed for next‐generation electric vehicles.
Substituting W for Al in the Ni‐rich cathode LiNi0.885Co0.10Al0.015O2 (NCA89) produces LiNi0.9Co0.09W0.01O2 (NCW90) with markedly reduced primary particle size. Particle size refinement provides improved cycling stability such that the NCW90 cathode retains 92% of its initial capacity after 1000 cycles (compared to 63% for NCA89). Thus, the NCW90 cathode represents a new series of Ni‐rich layered cathodes for next‐generation electric vehicles.
Dominance of various scattering mechanisms in determination of the carrier mobility is examined for silicon (Si) nanowires of sub-10-nm cross-sections. With a focus on
p
-type channels, the ...steady-state hole mobility is studied with multi-subband Monte Carlo simulations to consider quantum effects in nanoscale channels. Electronic structures of gate-all-around nanowires are described with a 6-band
k
·
p
model. Channel bandstructures and electrostatics under gate biases are determined self-consistently with Schrödinger-Poisson simulations. Modeling results not only indicate that the hole mobility is severely degraded as channels have smaller cross-sections and are inverted more strongly but also confirm that the surface roughness scattering degrades the mobility more severely than the phonon scattering does. The surface roughness scattering affects carrier transport more strongly in narrower channels, showing ∼90 % dominance in determination of the mobility. At the same channel population, 110 channels suffer from the surface roughness scattering more severely than 100 channels do, due to the stronger corner effect and larger population of carriers residing near channel surfaces. With a sound theoretical framework coupled to the spatial distribution of channel carriers, this work may present a useful guideline for understanding hole transport in ultra-narrow Si nanowires.
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In this study, we have demonstrated that boron doping of Ni-rich LiNixCoyAl1−x−yO2 dramatically alters the microstructure of the material. LiNi0.885Co0.1Al0.015O2 is composed of large ...equiaxed primary particles, whereas a boron-doped LiNi0.878Co0.097Al0.015B0.01O2 cathode consists of elongated particles that are highly oriented to produce a strong, crystallographic texture. Boron reduces the surface energy of the (0 0 3) planes, resulting in a preferential growth mode that maximizes the (0 0 3) facet. This microstructure modification greatly improves the cycling stability; the LiNi0.878Co0.097Al0.015B0.01O2 cathode maintains a remarkable 83% of the initial capacity after 1000 cycles even when it is cycled at 100% depth of discharge. By contrast, the LiNi0.885Co0.1Al0.015O2 cathode retains only 49% of its initial capacity. The superior cycling stability clearly indicates the importance of the particle microstructure (i.e., particle size, particle shape, and crystallographic orientation) in mitigating the abrupt internal strain caused by phase transitions in the deeply charged state, which occurs in all Ni-rich layered cathodes. Microstructure engineering by surface energy modification, when combined with protective coatings and composition modification, may provide a long-sought method of harnessing the high capacity of Ni-rich layered cathodes without sacrificing the cycling stability.
Detailed analysis of the microstructural changes during lithiation of a full‐concentration‐gradient (FCG) cathode with an average composition of LiNi0.75Co0.10Mn0.15O2 is performed starting from its ...hydroxide precursor, FCG Ni0.75Co0.10Mn0.15(OH)2 prior to lithiation. Transmission electron microscopy (TEM) reveals that a unique rod‐shaped primary particle morphology and radial crystallographic texture are present in the prelithiation stage. In addition, TEM detected a two‐phase structure consisting of MnOOH and Ni(OH)2, and crystallographic twins of MnOOH on the Mn‐rich precursor surface. The formation of numerous twins is driven by the lattice mismatch between MnOOH and Ni(OH)2. Furthermore, the twins persist in the lithiated cathode; however, their density decrease with increasing lithiation temperature. Cation disordering, which influences cathode performance, is observed to continuously decrease with increasing lithiation temperature with a minimum observed at 790 °C. Consequently, lithiation at 790 °C (for 10 h) produced optimal discharge capacity and cycling stability. Above 790 °C, an increase in cation disordering and excessive coarsening of the primary particles lead to the deterioration of electrochemical properties. The twins in the FCG cathode precursor may promote the optimal primary particle morphology by retarding the random coalescence of primary particles during lithiation, effectively preserving both the morphology and crystallographic texture of the precursor.
Crystallographic twins form by the precipitation of MnOOH in full‐concentration‐gradient precursors, Ni0.75Co0.10Mn0.15(OH)2, for Li‐ion batteries. The twins persist in cathodes through lithiation, but with varying density depending on the temperature of lithiation. The twin density influences the size and crystallographic orientation of primary particles in cathodes, which unequivocally affect their electrochemical properties.
A spherical O3‐type NaNi0.5Mn0.5O2 cathode, composed of compactly‐packed nanosized primary particles, is synthesized by the coprecipitation method to examine its capacity fading mechanism. The ...electrochemical performance cycled at different upper cut‐off voltages demonstrate that the P3′ to O3′ phase transition above 3.6 V is primarily responsible for the loss of the structural stability of the O3‐type NaNi0.5Mn0.5O2 cathode. The capacity retention is greatly improved by avoiding the P3′ to O3′ phase transition, and 94.2% and 90.7% of the initial capacities (108.9 mAh g−1 at 3.35 V and 125.4 mAh g−1 at 3.58 V) are retained after 100 cycles. During cycling at 4.0 V, rapid capacity fading (75.5% of 147.5 mAh g−1 after 100 cycles) is observed. The poor Na+ ion intercalation stability is directly attributed to the extent of microcracks caused by the abrupt change in the lattice structure. Microcracks traversing the entire secondary particle compromise the mechanical integrity of the cathode and accelerate electrolyte infiltration into the particle interior, causing the subsequent degradation of the exposed internal surfaces. Thus, suppressing microcracks in secondary particles is one of the key challenges for improving the cycling stability of hierarchical structured O3‐type NaNi0.5Mn0.5O2 cathodes.
The capacity retention of the O3‐type NaNi0.5Mn0.5O2 cathode is strongly dependent on the extent of microcracking within the secondary particles resulting from the anisotropic volume change during charge/discharge cycling caused by the P3′–O3′ phase transition occurring above 3.6 V. The microcracks allow the penetration of the electrolyte into the particle interior, resulting in chemical damage via electrolyte attack.
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