Conspectus
Lithium-ion batteries (LIBs) are nearly ubiquitous
energy storage
solutions, powering devices ranging from consumer electronics to electric
vehicles. To advance these applications, current LIB research efforts
are directed toward improving energy and power densities, cyclic lifetimes,
charging speeds, and safety. These parameters are intrinsically tied
to properties of the active electrode materials, such as the redox
mechanism, chemical composition, and crystal structure. One particularly
challenging issue is that the active electrode materials that possess
higher theoretical energy densities are generally more susceptible
to degradation during cycling. A notable example is the family of
layered multicomponent transition metal oxides, which is the incumbent
class of active LIB cathode materials for electric vehicles. To increase
their theoretical capacities, the transition metal fraction in these
materials is trending toward higher Ni content. However, Ni-rich chemistries
suffer from electrochemical, crystallographic, and mechanical degradation
that increase in severity with increasing Ni content. Furthermore,
alternative high-energy cathode materials, including overlithiated
layered oxides and disordered rock salt materials, present additional
stability challenges that must be overcome before they can be realistically
incorporated into LIB technology.
The chemomechanical degradation
in high-energy LIB cathode materials
occurs at multiple length scales. Point defects, such as antisite
defects or vacancies, are commonly generated during electrochemical
cycling and can contribute to the loss of cyclable active material.
At both the primary and secondary particle level, electrochemical
cycling also induces significant volumetric changes and state-of-charge
heterogeneity, generating regions of high stress and strain that are
precursors to mechanical fracture. Finally, at the electrode level,
nonuniform charge transfer reactions throughout the electrode can
lead to locally overcharged regions that become sites of enhanced
degradation. To address these issues, active cathode material design
and electrode engineering are being heavily pursued to accelerate
improvements in LIB energy density. To consolidate the current understanding
of chemomechanical degradation and provide guidance on mitigation
strategies, a comprehensive overview of degradation mechanisms across
multiple length scales is critically needed.
In this Account,
we first outline the origins of chemomechanical
degradation for high-energy LIB cathodes, including layered oxides,
overlithiated layered oxides, and disordered rock salt structures.
Specifically, we delineate the thermodynamic and kinetic origins of
defect generation at the atomic level and then progress to the kinetic
origins of broader degradation mechanisms at the particle level and
electrode level. Next, we discuss strategies for minimizing chemomechanical
degradation in high-energy LIB cathodes at multiple length scales.
Finally, we provide a forward-looking perspective on how to ...