  Lithium-ion batteries can last many years but sometimes exhibit rapid,
nonlinear degradation that severely limits battery lifetime. In this work, we
review prior work on "knees" in lithium-ion battery aging trajectories. We
first review definitions for knees and three classes of "internal state
trajectories" (termed snowball, hidden, and threshold trajectories) that can
cause a knee. We then discuss six knee "pathways", including lithium plating,
electrode saturation, resistance growth, electrolyte and additive depletion,
percolation-limited connectivity, and mechanical deformation -- some of which
have internal state trajectories with signals that are electrochemically
undetectable. We also identify key design and usage sensitivities for knees.
Finally, we discuss challenges and opportunities for knee modeling and
prediction. Our findings illustrate the complexity and subtlety of lithium-ion
battery degradation and can aid both academic and industrial efforts to improve
battery lifetime.
