Yuh-Chieh Lin

Insights into the Performance Limits of the Li7P3S11 Superionic Conductor: A Combined First-Principles and Experimental Study

The Li7P3S11 glass-ceramic is a promising superionic conductor electrolyte (SCE) with an extremely high Li(+) conductivity that exceeds that of even traditional organic electrolytes. In this work, we present a combined computational and experimental investigation of the material performance limitations in terms of its phase and electrochemical stability, and Li(+) conductivity. We find that Li7P3S11 is metastable at 0 K but becomes stable at above 630 K (∼360 °C) when vibrational entropy contributions are accounted for, in agreement with differential scanning calorimetry measurements. Both scanning electron microscopy and the calculated Wulff shape show that Li7P3S11 tends to form relatively isotropic crystals. In terms of electrochemical stability, first-principles calculations predict that, unlike the LiCoO2 cathode, the olivine LiFePO4 and spinel LiMn2O4 cathodes are likely to form stable passivation interfaces with the Li7P3S11 SCE. This finding underscores the importance of considering multicomponent integration in developing an all-solid-state architecture. To probe the fundamental limit of its bulk Li(+) conductivity, a comparison of conventional cold-press sintered versus spark-plasma sintering (SPS) Li7P3S11 was done in conjunction with ab initio molecular dynamics (AIMD) simulations. Though the measured diffusion activation barriers are in excellent agreement, the AIMD-predicted room-temperature Li(+) conductivity of 57 mS cm(-1) is much higher than the experimental values. The optimized SPS sample exhibits a room-temperature Li(+) conductivity of 11.6 mS cm(-1), significantly higher than that of the cold-pressed sample (1.3 mS cm(-1)) due to the reduction of grain boundary resistance by densification. We conclude that grain boundary conductivity is limiting the overall Li(+) conductivity in Li7P3S11, and further optimization of overall conductivities should be possible. Finally, we show that Li(+) motions in this material are highly collective, and the flexing of the P2S7 ditetrahedra facilitates fast Li(+) diffusion.

Uniform second Li ion intercalation in solid state ϵ-LiVOPO4

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K–edge absorption spectroscopy and density functional theory.

Comparison of the polymorphs of VOPO4 as multi-electron cathodes for rechargeable alkali-ion batteries

Multi-electron polyanion cathodes offer the potential for achieving both high voltage and high capacity in rechargeable alkali-ion batteries. Among the few materials known to exhibit multi-electron cycling, the polymorphs of VOPO4, which operate on the V3+–V4+–V5+ redox couples, are particularly promising due to the high gravimetric capacities that have been achieved and the high voltage of the V4+/5+ couple. In this work, we performed a systematic first principles investigation, supported by careful electrochemical characterization and published experimental data, of the relative thermodynamic stability, voltage, band gap, and diffusion kinetics for alkali intercalation into the β, e and αI polymorphs of VOPO4. We find that all VOPO4 polymorphs remain reasonably stable with the insertion of one alkali ion per V, but are significantly destabilized with the insertion of two alkali ions per V. The voltages for Na insertion are ∼0.33–0.69 V lower than those for Li insertion. We find that the αI polymorph is predicted to have higher Li+ migration barriers and larger band gaps than the β and e polymorphs, which account for the relatively worse electrochemical cycling performance observed. On the other hand, only the αI polymorph exhibits reasonably low barriers for Na+ migration compared to the β and e polymorphs, which are consistent with observed electrochemical performances reported thus far in the literature. We also show that differences in the voltage, kinetics and rate capability of these different polymorphs for Li and Na insertion can be traced back to their fundamentally different VO6/VO5–PO4 frameworks.

Correction to Insights into the Performance Limits of the Li7P3S11 Superionic Conductor: A Combined First-Principles and Experimental Study

Author(s): Chu, Iek-Heng; Nguyen, Han; Hy, Sunny; Lin, Yuh-Chieh; Wang, Zhenbin; Xu, Zihan; Deng, Zhi; Meng, Ying Shirley; Ong, Shyue Ping