Toward high energy and high efficiency secondary lithium batteries

Thesis (Ph. D.)--University of Rochester. Materials Science Program, 2014.Energy storage systems play an important role nowadays. Developing batteries with high energy and long cycle life has been an important research part in scientific and engineering field. Lithium ion batteries and the recent rising lithium sulfur batteries demonstrate a huge potential to be the next generation energy storage devices and being substitutes for fossil fuels in electric cars. This dissertation focuses on the development of cathode materials for lithium ion batteries with advanced electrochemical performances, and then on the design of novel lithium sulfur systems which can deliver a capacity five times as much as lithium ion batteries offer. In the first part of the dissertation, advanced cathode materials for lithium-ion batteries were investigated in the aspects of material synthesis and performance test. Li- Mn-rich composite materials have high theoretical capacities (200 - 300 mAh/g). High energy composite material 0.5Li2MnO3·0.5LiMn0.5Ni0.5O2, or written as Li1.5Ni0.25Mn0.75O2.5, was synthesized through a polymer-assisted method and a coprecipitation method in a continuous stirred tank reactor (CSTR). The as-synthesized powder using the polymer-assisted method has a primary particle size in the range of 100-300 nm, and can reach a discharge capacity of around 230 mAh/g at the current density of 5 mA/g, and 170 mAh/g at 20 mA/g. The secondary particles of the composite material synthesized through co-precipitation method were spheres with diameters of around 10 μm, and has an initial capacity of 295 mAh/g. 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 powder was synthesized via a spray pyrolysis method. The as-prepared material was spheres with a high porosity. The first discharge capacity of the material was over 300 mAh/g. High voltage spinel cathode material has a high working potential and thus can generate a high energy. LiNi0.24Mn1.76O4 was prepared through a simple solid state method and the as-prepared material was tested between 3 - 4.8 V. The material has excellent rate capabilities and cycling stabilities. Nanofiber cathode materials were successfully produced using the electrospinning method. Through controlling a series of electrospinning parameters, LiFePO4 and high voltage spinel LiNi0.5Mn1.5O4 (LMNO) nanofibers with a diameter as thin as 50 - 100 nm were fabricated followed by a subsequent heat-treating procedure. The well-separated nanofiber precursors supress LiNi0.5Mn1.5O4 particles’ growth and aggregation during the heating procedure, and led to good performances of high capacities and excellent rate capabilities of the final LMNO nanofibers. At a current density of 27 mA g-1, the initial discharge capacity of the cell was 130 mAh g-1 (charge-discharge between 3.5 - 4.8 V) and 300 mAh g-1 (2.0 - 4.8 V). In the second part of the dissertation, lithium sulfur systems were investigated due to their high theoretical capacity and the use of abundant and safe sulfur cathode material. To understand the chemistry and problems within a lithium-sulfur cell, various techniques were applied to study the system including SEM, TEM, XRD, Raman spectroscopy and etc. It was identified that one of the main problems in a Li-S cell that hinders its achieving a high performance is the so-called “shuttle reactions”. It was resulted from the dissolution of lithium polysulfides into the electrolyte and migration to the lithium anode. Another issue was the difficulty of fully discharge a sulfur cathode due to the low conductivity of reduction product, lithium sulfide. Several approaches aiming to resolve the problems were designed to improve the cell’s coulombic efficiency, capacity and cycle life. The electrochemical performances of lithium sulfur batteries and electrochemistry mechanisms within the cell system were investigated in the following aspects: 1) Impact of liquid electrolytes (including carbonate-based electrolytes and ether-based electrolytes) 2) Impact of the LiNO3 additive as a shuttle inhibitor 3) Impact of the sulfur/carbon ratio in the electrode 4) Impact of different carbon forms in the electrode 5) Impact of impregnating sulfur in the carbon pores in the electrode 6) Impact of adding polysulfide adsorbents 7) Surface morphology change of the lithium anode and the sulfur cathode 8) Composition of lithium polysulfide solution and powder 9) Self-discharge phenomenon 10) Cell intermediate and interface characterization Last but not least, we developed a novel polysulfide-based electrolyte that prevents the performance degradation inherent to Li-S batteries by self-healing. By creating a dynamic equilibrium between the dissolution and precipitation of lithium polysulfides at the sulfur/electrolyte interface, the Li-S cells were capable of delivering a superior capacity (1450 mAh/g, sulfur), which along with the high coulombic efficiency and excellent cycle life make our cells among the best performing Li-S cells. In addition, the present technology eliminates the need for complicated and costly electrode preparation. The polysulfides in the electrolyte eliminate the need for traditional lithium salts