Chemistry-mechanics-geometry coupling in positive electrode materials: a scale-bridging perspective for mitigating degradation in lithium-ion batteries through materials design

Abstract

Despite their rapid emergence as the dominant paradigm for electrochemical energy storage, the full promise of lithium-ion batteries is yet to be fully realized, partly because of challenges in adequately resolving common degradation mechanisms. Positive electrodes of Li-ion batteries store ions in interstitial sites based on redox reactions throughout their interior volume. However, variations in the local concentration of inserted Li-ions and inhomogeneous intercalation-induced structural transformations beget substantial stress. Such stress can accumulate and ultimately engender substantial delamination and transgranular/intergranular fracture in typically brittle oxide materials upon continuous electrochemical cycling. This perspective highlights the coupling between electrochemistry, mechanics, and geometry spanning key electrochemical processes: surface reaction, solid-state diffusion, and phase nucleation/transformation in intercalating positive electrodes. In particular, we highlight recent findings on tunable material design parameters that can be used to modulate the kinetics and thermodynamics of intercalation phenomena, spanning the range from atomistic and crystallographic materials design principles (based on alloying, polymorphism, and pre-intercalation) to emergent mesoscale structuring of electrode architectures (through control of crystallite dimensions and geometry, curvature, and external strain). This framework enables intercalation chemistry design principles to be mapped to degradation phenomena based on consideration of mechanics coupling across decades of length scales. Scale-bridging characterization and modeling, along with materials design, holds promise for deciphering mechanistic understanding, modulating multiphysics couplings, and devising actionable strategies to substantially modify intercalation phase diagrams in a manner that unlocks greater useable capacity and enables alleviation of chemo-mechanical degradation mechanisms.

Fig. 1. Schematic overview of chemistry–mechanics coupling in Li-ion batteries. The coupling between chemistry and mechanics is illustrated for, interfacial reactions, diffusion, and phase transformation processes in (A–C), respectively. A representative governing equation is provided below for each process and is accompanied by a simplified depiction of the terms linking mechanics (blue) with electrochemical phenomena (orange).

Fig. 2. Chemistry–mechanics coupling in intercalation-induced phase transformations. (A) Simplified depiction of a free energy landscape with a double well potential that favors phase separation. The crystal structures of the ε-Li_0.3V₂O₅ and δ-Li_0.9V₂O₅ phase of Li_xV₂O₅ phase diagram are depicted in (B) to illustrate the structural differences possible from lithiated phases separated by a miscibility gap. A microstructural evolution is modeled considering (C) 1% (D) 5% and (E) 20% volume changes between a Li-poor and Li-rich phase. At small volume changes (or lattice misfits) a two-phase separation motif is preferred. However, at larger volume changes (or lattice misfits) a solid-solution system is the energy minimizing microstructure. Panel (B) has been reproduced (adapted) with permission from Santos et al., Copyright (2020) Mater. Horiz.

Fig. 3. A diverse palette of levers by which fundamental chemistry–mechanics coupling mechanisms in cathode materials can be altered to control intercalation-phase transformations, mitigate stress accumulation, and prolong cycle life.

Fig. 4. The interplay between primary particle geometries and phase transformations. (A) Simplified phase diagram showing evolution of lithiation-induced Li_xV₂O₅ phases. Scanning electron microscope (SEM) images of (B) nanospheres, (C) nanowires, (D) nanoplates, and (E) micron-sized “bulk” platelets of orthorhombic α-V₂O₅; (F) discharge/charge profiles of α-V₂O₅ particles at C-rate of C/5 for the first cycle between 2.0–4.0 V; (G) calculated surface area of α-V₂O₅ powders by Brunauer–Emmett–Teller (BET) analyses. (H) Shows the crystal structures of α-V₂O₅, ε-Li_xV₂O₅, ε′-Li_xV₂O₅ and δ-Li_xV₂O₅ while contrasting the profoundly different phase evolution observed from bulk and nanosphere electrodes. (I) Modeling of phase separation across particles with different size. The smaller particles are lithiated at higher rates due to larger surface-area-to-volume ratios, (H) has been reproduced (adapted) with permission from Luo et al., Copyright (2022) Nat. Mater. Panel (I) has been reproduced (adapted) with permission from Zhao et al., Copyright (2017) RSC Adv.

Fig. 5. Applications of high-dimensional imaging techniques to study the role of particle geometries in battery performance. (A) Schematic representation of a deep-learned algorithm developed to perform instance segmentation of a polydispersion of nanoparticles whose geometry and composition have been mapped by X-ray hyperspectral imaging. Reprinted (adapted) with permission from Santos et al., Copyright (2022) Patterns. (B) Visualization of a composite NMC electrode and its primary particles, resulting from X-ray holotomography measurements. The correlation of 15 attributes in illustrated by the circular plot in (C) The “+” and “−” symbols describe a positive or negative correlation between attributes, respectively. Panels (B) and (C) were reprinted (adapted) with permission from Li et al. Copyright (2022) Science.

Fig. 6. Contrasting patterns of phase multiphasic lithiation in electrode materials. (A) In a diffusion-limited regime, wherein bulk diffusion cannot match the rate of surface reactions, a shrinking-core model is observed. (B) In a nucleation-limited regime, lithiation into a layered material with anisotropic diffusion manifests in intercalation waves stemming from limited nucleation points. (C) Striated phase-separation pattern deduced from STXM measurements of a mechanically bent V₂O₅ nanowire. (D) A translation of the lithiation heterogeneities shown in (C) to von Mises stress maps via finite element analysis reveals bending-induced control of the resulting stress gradients experienced by the nanowire during delithiation. (E) A complementary phase-field model, modeled after experimental observations, shows the evolution of phase separation patterns as a function of an increasing interfacial parameter, κ, which penalizes the formation of sharp phase boundaries (F) shows a stress-driven twinned microstructure in a LMO sample imaged by transmission electron microscopy. (G) Phase-field simulations for LiFePO₄ show how coherency stress destabilizes a uniform delithiation front and induces filamentary growth patterns. Panels (A) and (B) have been reproduced (adapted) with permission from Fraggedakis et al. Copyright (2020) Energy Environ. Sci. (C–E) were reproduced (adapted) with permission from Santos et al., Copyright (2020) Mater. Horiz. (F) has been reproduced (adapted) with permission from Erichsen et al., Copyright (2020) ACS Appl. Energy Mater. (G) has been reproduced (adapted) with permission from Yang et al., Copyright (2020). J. Mater. Chem. A.

Fig. 7. Strain-driven alteration of phase transformations. (A) Illustration of introducing strain in a thin-film cathode material via epitaxial mismatch with an incommensurate substrate. (B) 3D plot of the free energy landscape between the α- and ε-phase of V₂O₅. The effect of film strain is reflected through a modification of the energy barrier height between the two phases. (B) Has been reprinted (adapted) with permission from Zhang et al., Copyright (2021) J. Mech. Phys. Solids. (C) Cyclic voltammograms of a strained (red and blue), and unstrained (black) V₂O₅ thin film, demonstrating a strain-induced shift in intercalation potentials. (C) Has been reprinted (adapted) with permission from Muralidharan et al. Copyright (2017) ACS Nano.

Fig. 8. Lithiation across interconnected particles and reaction rate-induced changes in phase separation patterns. (A) Schematic depiction of lithiation across interconnected particles. The smaller particle becomes preferentially lithiated due to a higher surface-area-to-volume ratio whilst depleting the larger interconnected particle. (B) and (C) show the evolution of average lithium concentration in a pair of interconnected particles during comparably slow (10⁻⁷ mol m⁻²) and fast (10⁻⁴ mol m⁻²) reaction rates (c^(p)_s), respectively, on the electrode/electrolyte interface. Here, the rapid reaction suppresses phase separation. A similar current-induced transition from particle-by-particle to concurrent (de)lithiation is observed for LiFePO₄, when the discharge rate is changed from 1C (D) to 20C (E). Portions of panel (A) have been reproduced (adapted) with permission from de Jesus et al., Copyright (2017) J. Mater. Chem. A. Panels (B) and (C) have been reproduced (adapted) with permission from Zhao et al., Copyright (2017) RSC Adv. (D) and (E) have been reproduced (adapted) with permission from Li et al., Copyright (2014) Nat. Mater.

Fig. 9. Leveraging geometry to improve cathode performance. A schematic depiction of a sol–gel process utilized to fabricate a templated LCO cathode using a wood template and a V₂O₅ porous architecture using a colloidal crystal template is shown in (A) and (B) respectively. A comparison of tortuosity and porosity of the templated LCO-1, templated LCO-2, and control LCO architectures is shown in (C). (D) Comparison of Li-ion and electron conductivities between the templated LCO-1 and LCO-2 cathodes against the control LCO. The normalized equilibrium concentration map shown in (E) shows that lithium redistributes homogenously across the continuously curved V₂O₅ 3D architecture. (F) Stress map of the Li_xV₂O₅ inverse opal structure derived from a finite element simulation based on the spatial lithiation pattern as imaged by X-ray spectromicroscopy. Panels (C) and (D) have been reprinted (adapted) with permission from Lu et al., Copyright (2018) Adv. Mater. Panels (E) and (F) have been reprinted (adapted) with permission from Andrews et al., Copyright (2020) Matter.

Fig. 10. Simulating crack propagation in active electrode materials. (A) Evolution of fracture and phase segregation during delithiation of a cylindrical particle. Reprinted (adapted) with permission from Xu et al. Copyright (2016) GAMM-Mitteilungen (B) lithium concentration and damage distribution in NMC particles with different numbers of primary particles. Reprinted (adapted) with permission from Bai et al. Copyright (2021) Int. J. Solids Struct. (C) Concentration profile and damage parameters for a composite cathode system in an ASSB. Reprinted (adapted) with permission from Rezaei et al. Copyright (2021) J. Mech. Phys. Solids.

Fig. 11. Improved electrochemical performance of pre-intercalated electrodes. (A) Capacity retention from bilayered δ-M_xV₂O₅ (M = Li, Na, K, Mg, Ca) electrode materials. Reproduced (adapted) with permission from Clites et al., Copyright (2018) Energy Storage Mater. (B) Schematic illustration of Mg_0.3V₂O₅·1.1H₂O. Here, pre-intercalated Mg²⁺ reinforces the layered structure, whereas the lattice water solvates Mg-ions and enables fast cation diffusion. Reproduced (adapted) with permission from Xu et al., Copyright (2019) Chem.

Fig. 12. Contrasting intercalation phenomena in V₂O₅ polymorphs. (A) Orthorhombic α-V₂O₅, represents the thermodynamic sink of the vanadium oxide phase diagram. (B–D) show the structures for the metastable ζ-V₂O₅, γ′-V₂O₅, and λ-V₂O₅ polymorphs, respectively, which have been stabilized by topochemical de-intercalation of native cations from ternary vanadium oxide M_xV₂O₅ bronzes. In α-V₂O₅, increasing lithiation brings about distortive structural transformations, which often coexist at the single-particle level as shown in (E). In sharp contrast, ζ-V₂O₅ accommodates homogenous lithiation as shown in (F) through Li-ion reordering along the 1D tunnels. By alleviating distortive phase transformations, ζ-V₂O₅ shows excellent capacity retention as shown in (G). (E–G) were reproduced (adapted) with permission from Luo et al., Copyright (2022) Proc. Natl. Acad. Sci.

Fig. 13. Electronic and atomistic structure origins of dopant-induced modifications in electrode materials. (A) Schematic illustration of the formation of strained phase boundary between two incommensurate phases. Here, substitutional doping lowers the elastic misfit, therefore reducing the coherency strain between the two phases upon phase boundary formation. Doping strategies fundamentally modify the thermodynamic landscape of the system in several ways. (B) Schematically depicts a change in the relative stability of two phases (ΔG), a modification to the activation energy barrier for the phase transformation (ΔG_a) and narrowing of the miscibility gap. (C) and (D) Contrast the changes in lattice constants and cell volumes, respectively, during delithiation in the biphasic regime of undoped and Mg-doped LNMO. Mg-doping in LNMO reduces the elastic misfit between the low- and high-lithiated phases. (E) Shows the density of states in α-V₂O₅ – like several electrodes based on transition metal oxides, the combination of narrow 3d bands and strong electron correlation engenders the formation of polarons as depicted in the upper panel. The coupling of polarons with nearby Li-ions introduces additional diffusion barriers. (F) Sn-doping into V₂O₅ stabilizes two polarons at different V centers. The stabilization of polaronic states before lithiation destabilizes additional polaron formation from the subsequently inserted Li-ion. (C) and (D) Were reproduced (adapted) with permission from Kang et al., Copyright (2021) J. Mater. Chem. A. (F) was reproduced (adapted) with permission from Suthirakun et al., Copyright (2018) J. Phys. Chem. C.

Fig. 14. A flowchart of the theoretical framework used to analyze and design structural transformation pathways in intercalation compounds. This framework has been applied to an open-source database (Materials Project) with over n > 5000 pairs of intercalation compounds.