An In-Depth Understanding of Mechanically Disordered Structure for Less Hysteretic Fast Charging

Disordered structures have garnered global attention due to their potential to enhance redox kinetics, increase capacity, and achieve high energy density. Various approaches have been explored, for example, Li_1.211Mo_0.467Cr_0.3O₂ (LMCO) transitions into a disordered rock-salt structure after several charge-discharge process, resulting in increased capacity and different operating voltage profiles.¹ Additionally, cation-disordered Li-excess cathode materials can enable oxygen redox, providing extra capacity with high energy density.² Contrary to the research on how disordered structure with specific chemistries or chemical treatment can affect battery performance, the study for disordered structure formed by mechanical stress with extrinsic properties is insufficient. Lithium-sulfur batteries, which undergo nearly 80% volumetric change during charge/discharge cycles, are ideal for investigating the effects of mechanical stress-induced distortion. By adjusting the pore size of the host material, mechanical stress can be applied to lithium sulfide (Li₂S), creating disordered structures. This allows for a direct comparison between strained and unstrained Li₂S, providing valuable insights into the impact of mechanical stress on structural stability and battery performance.<br/><br/>Based on this sulfide model, we hypothesize that mechanical compression can facilitate hysteresis-less fast charging based on three key components: electro-chemo-mechanics, phase transition kinetics, and ionic kinetics. First, a new domain called electro-chemo-mechanics was introduced with the concept of "electrochemical stiffness" (ρ=ΔV/ΔQ), which relates the voltage to the state of charge (SoC), inspired by the stress–strain relationship in mechanics. Mechanical compression lowers the operating voltage, reducing the initial electrochemical stiffness, and making Li-ion extraction and insertion easier, thus mitigating hysteresis during charging and discharging process. Second, compression destabilizes the unstrained phase, lowering the kinetic barrier and resulting in faster, hysteresis-less phase transitions, as strained Li₂S structures exhibit reduced barriers and lower electrochemical potentials compared to unstrained structures. Third, we examine ionic kinetics, showing that compression breaks the symmetric migration pathways, creating new paths with lower barriers for Li ions, resulting in significantly higher ion mobility and enhanced diffusion rates in strained Li₂S models. These hypotheses were validated through a data-driven approach, combining DFT calculations and experimental analyses. Our findings present a novel method for achieving hysteresis-less fast charging based on various multi-physical perspectives, along with a new domain, electro-chemo-mechanics.<br/><br/>1 Lee, Jinhyuk, et al. "Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries." science 343.6170 (2014): 519-522.<br/>2 Seo, Dong-Hwa, et al. "The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials." Nature chemistry 8.7 (2016): 692-697.