Synthesis And Performance Of Inorganic Solid-state Electrolytes For Lithium Secondary Batteries

Currently organic liquids electrolytes have certain safety issues caused by theinflammability, explosiveness and leakiness. All solid state lithium ion battreies usedinorganic solid electrolytes offer a fundamental solution to the safety issues of conventionallithium ion battreies containing organic electrolytes. They have lots of advantages such asexcellent reversibility and cycle performance, low self-discharge rate, and the inhibitoryeffect on side reaction, and have been used in microelectronics system and all kinds ofportable electronic devices. The key is development of solid electrolytes with high ionicconductivity, low electronic conductivity, and wide electrochemical stability window, goodthermal and chemical and good compatibility with electrode material. A lot of researcheshave been done to develop solid electrolytes with excellent performance; however, it isdiffcult to obtain the high ionic conductivity and good stability. Therefore, alternative solidelectrolytes with good thermal, chemical, electrochemical stability and high ionicconductivity in thin film are imperative. This paper is focused on the development of novelsolid electrolytes system including oxide and oxysulfide electrolytes and the investigationof relationship between local structure and performance. The main research progresses areas followed:(1) A novel electrolyte thin film has been prepared by radio-frequency magnetronsputtering using a Li–B–P–O target in a pure N2atmosphere at various temperatures. Thinfilm has been prepared by radio-frequency magnetron sputtering using a Li–B–P–O targetin a pure N2atmosphere at various temperatures. Analyses suggest that nitrogenincorporation leads to a more complicated cross-linked structure which brings about morelithium ion transport channels in the network; on the other hand, thin film deposited athigher temperature shows the mixture of crystal and amorphous structure, which is morebeneficial to the improvement on lithium ion motility in the network. The thin films exhibitan ionic conductivity of3.5×10-6S/cm and a wide electrochemical stability window, as wellas outstanding chemical durability; they can be used as electrolytes in all solid state thinfilm lithium ion batteries.(2) A novel crystalline Li–Ti–P–S–O electrolytes are synthesized by a solid statereaction. The reaction mixture was cold-pressed into small planchet with a small quantity ofoxygen in particle clearance, and then was reacted by solid state process. The results show that the material has an oxysulfide structure which increases the crosslinking density andpreventing the reaction between sulphur and oxygen and moisture in the environment. Thelithium ionic conductivity of crystalline Li–Ti–P–S–O electrolytes is10-7～10-8S/cm, andthe electrochemical performance didn’t change a lot after being exposed to the oxygwnfrom the ambient air. However, the large content of titanium in sample results in theoxidation-reduction reaction between Ti4+and metal lithium, which means the electronicconduction is the majority in the conduction process. It indicates crystalline Li–Ti–P–S–Oelectrolytes are suitable to be the electrode material in lithium ion batteries.(3) A novel amorphous Li–Ti–P–S–O electrolytes are synthesized by a high-energyball milling. The reaction during milling resulted in the oxysulfide structure. Theamorphous structure resulted in isotropical conductance of the material, and the highlydisordered network structure was helpful for the migration of lithium ionic. Besides, theionic conductivity of amorphous electrolytes is higher than that of oxysulfide crystallineelectrolytes (9.8×10-6S/cm), that is because the integrity of electrolytes particles as well.The material is a good lithium ion conductor and can be used as solid-state electrolyte inlithium ion batteries.(4) A novel Li–Ti–P–S–O glass-ceramic electrolytes are synthesized by a two-stepmethod involving mechanical milling and the following low-temperature heat treatment.The glass-ceramic structure formed after the two-step reaction resulted in the highestlithium ionic conductivity of4.16×10-5S/cm in Li–Ti–P–S–O electrolytes, which is anorder of magnitude higher than that of amorphous electrolytes, and two or three orders ofmagnitude higher than that of crystalline electrolytes. The work accomplishes the doping ofthe element of sub group Ⅳ in sulfide electrolytes system for the first time and broadensthe family of inorganic solid-state electrolytes.