Xin-Yan Liu

Ionic shield for polysulfides towards highly-stable lithium–sulfur batteries

Lithium–sulfur batteries attract great attention due to their high energy density, while their real applications are still hindered by the rapid capacity degradation. Despite great efforts devoted to solving the polysulfide shuttle between the cathode and anode electrodes, it remains a serious challenge to build highly-stable lithium–sulfur batteries. Herein we demonstrate a strategy of introducing an ion selective membrane to improve the stability and coulombic efficiency of lithium–sulfur batteries. The sulfonate-ended perfluoroalkyl ether groups on the ionic separators are connected by pores or channels that are around several nanometers in size. These SO3− groups-coated channels allow ion hopping of positively charged species (Li+) but reject hopping of negative ions, such as polysulfide anions (Sn2−) in this specific case due to the coulombic interactions. Consequently, this cation permselective membrane acts as an electrostatic shield for polysulfide anions, and confines the polysulfides on the cathode side. An ultra-low decay rate of 0.08% per cycle is achieved within the initial 500 cycles for the membrane developed in this work, which is less than half that of the routine membranes. Such an ion selective membrane is versatile for various electrodes and working conditions, which is promising for the construction of high performance batteries.

Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control

Self-healing capability helps biological systems to maintain their survivability and extend their lifespan. Similarly, self-healing is also beneficial to next-generation secondary batteries because high-capacity electrode materials, especially the cathodes such as oxygen or sulfur, suffer from shortened cycle lives resulting from irreversible and unstable phase transfer. Herein, by mimicking a biological self-healing process, fibrinolysis, we introduced an extrinsic healing agent, polysulfide, to enable the stable operation of sulfur microparticle (SMiP) cathodes. An optimized capacity (∼3.7 mAh cm-2) with almost no decay after 2000 cycles at a high sulfur loading of 5.6 mg(S) cm-2 was attained. The inert SMiP is activated by the solubilization effect of polysulfides whereas the unstable phase transfer is mediated by mitigated spatial heterogeneity of polysulfides, which induces uniform nucleation and growth of solid compounds. The comprehensive understanding of the healing process, as well as of the spatial heterogeneity, could further guide the design of novel healing agents (e.g., lithium iodine) toward high-performance rechargeable batteries.

N-Methyl-2-pyrrolidone-assisted solvothermal synthesis of nanosize orthorhombic lithium iron phosphate with improved Li-storage performance

Exploring an efficient and effective way for synthesis of lithium iron phosphate (LiFePO4) with good Li-storage performance is a good way to fully demonstrate their applications for Li-ion batteries. In this contribution, LiFePO4 nanoparticles were synthesized by a facile solvothermal process with water/N-methyl-2-pyrrolidone (NMP) solvent system at a moderate temperature of 180 °C. The product was determined as single-phase orthorhombic LiFePO4, and the presence of crystal growth inhibitor NMP was favourable for the formation of smaller-sized LiFePO4 particles with improved electrochemical properties. After a carbon coating process, the LiFePO4/C sample afforded a reversible capacity of 140 mA h g−1 at 0.5 C, 106 mA h g−1 at 5.0 C at room temperature, and 163 mA h g−1 at 0.5 C, 153 mA h g−1 at 5.0 C at the higher temperature of 60 °C, respectively. The long cycle test at 0.2 C showed that no noticeable capacity fading was observed. The present LiFePO4 obtained by the facile solvothermal process had good thermal and electrochemical stability, which were attributed to facile Li ion diffusion and a good electron transfer pathway in the solvothermal LiFePO4 product.