Advanced anode materials for potassium ion batteries : insertion chemistry and stability exploration

Abstract

Potassium ion batteries (PIBs) have emerged as attractive alternatives to the prevailing lithium ion batteries (LIBs) for large-scale energy storage. The natural abundance of potassium sources brings about potential benefits in the cost and sustainability. Several promising cathodes relying on non-toxic Mn and Fe elements are developed, further reducing the cost and increasing environmental benignity. Turning to the anodes, the graphite shows a theoretical capacity of merely 279 mAh g-1 for K ions storage, which is much lower than that in LIBs. Therefore, this thesis aims to design high-capacity anodes for PIBs through insertion chemistry exploration and electrode/electrolyte interface optimization. Comparison studies between alkali-metal ion storage are firstly conducted to provide insights into the effects of ionic carriers on the structural evolution of electrodes, which are currently somewhat lacking due to the absence of appropriate host materials. MoS2/carbon nanofiber is used as a model material to investigate Li/Na/K ions storage behavior via in/ex situ transmission electron microscopy (TEM). It is found the nanofiber shows a more significant volume expansion of 140% toward K ion uptake than Li (103%) and Na (123%) ions insertion. However, the layered MoS2 structure is largely preserved after K ion insertion, while tiny particles are observed in the cases of Li and Na because of structural collapse. The reason roots in the less electrovalent of K-S bond than Li and Na ones, which is beneficial for maintaining more layered species upon K ion incorporation. The detailed phase transition mechanisms of MoS2 upon K ion insertion are investigated by in situ X-ray diffraction (XRD) and theoretical simulation, confirming the intermediate phases of K0.5MoS2 and K1.0MoS2. These intercalated compounds effectively preserve the layered structure and maintain original morphology for the stable cycling of microsized MoS2 particles without carbon coating. Subsequent efforts are dedicated to stabilizing Sb anodes with a superior theoretical capacity of 660 mAh g-1. However, the severe particle pulverizations and solid electrolyte interphases (SEIs) fracture lead to poor electrochemical stabilities. With the assistance of newly developed ethylene glycol diethyl ether (EGDEE)-based electrolyte, the commercial Sb microparticles with a high reversible capacity of 573 mAh g-1 are stabilized over 180 cycles under 0.1 A g-1 in PIBs. The reason lies in the formation of the elastic SEIs on the particle surface, which effectively wraps the microsized Sb particles to accommodate repeated swelling-contracting and prevent copious electrolytes decompositions. Similar enhancement is observed in another ether-based electrolyte based on tetrahydrofuran (THF) solvent. The Sb electrode delivers a sustainable capacity of 600 mA h g-1 in 1 M KFSI/THF over 100 cycles under 0.1 A g-1. Moreover, such ether-based electrolytes can be extended to stabilize microsized Sn anode for sodium ion batteries (SIBs). Sn microparticles maintain a high capacity of 669 and 786 mAh g-1 after 100 cycles under 0.2 A g-1 for EGDEE and THF-based electrolytes, respectively. The atomic force microscopy (AFM) tests are conducted to unravel the mechanical origin of improving electrochemical performance. It reveals that robust SEIs with superb mechanical properties are constructed in ether-based electrolytes, significantly boosting the performance of the alloy anode. Phosphorus is another high-capacity anode with a theoretical capacity of 1154 mAh g-1 towards K ion storage. Black phosphorus-graphite (BP/G) composite with a high BP loading of 80 wt.% is synthesized and stabilized via the utilization of a localized high concentration electrolyte (LHCE), i.e., KFSI in trimethyl phosphate (TMP) with a fluorinated ether as the diluent. We reveal the benefits of high concentration electrolytes rely on the formation of inorganic component rich SEI, which effectively passivates the electrode from copious parasite reactions. Furthermore, the diluent increases the electrolyte's ionic conductivity for achieving attractive rate capability and homogenizes the element distribution in the SEI. The latter essentially improves SEI's maximum elastic deformation energy for accommodating the volume change, resulting in excellent cyclic performance. This work promotes the application of advanced PIBs by adopting high-capacity BP anodes, on the one hand. On the other hand, it unravels the beneficial roles of LHCE in building robust SEI for stabilizing alloy anodes.