Preparation And Investigation Of High Voltage Electrolyte For Lithium-ion Battery

Li-ion batteries have been widely used as power sources for portable devices including mobile phones, cameras and laptops due to their characteristics of high energy density, long lifetimes and environmental friendliness. With the rapid development of hybrid electric vehicles and electric vehicles, Li-ion batteries, the ideal power sources for electric cars, is facing unprecedented challenges, i.e., there is an urgent need to improve the safety and energy density of the Li-ion cells. One way to improve the energy density of Li-ion batteries is to increase the working potential of the cathode. However, a major challenge for the production implementation of the high voltage cathodes is the instability of currently used carbonated-based electrolytes at high voltages. The electrolyte suffers from severe oxidative decomposition on the cathode surface, causing a rapid capacity fading. In addition, HF produced by a reaction between trace water and LiPF6 promotes the dissolution of metal ions from the cathode, leading to destruction of the cathode material. These undesirable reactions scarify battery cycling life and safety. To solve these problems, a high voltage spinel cathode LiNi_0.5Mn_1.5O₄ was selected as research object. Various electrolyte additives and novel electrolyte systems were investigated to improve the cycling performance of LiNi_0.5Mn_1.5O₄. The compatibility of these electrolytes with graphite anode was also investigated.1?N-methylpyrrole?MPL? is a well-known monomer unit that used to modify the surface of electrode materials. When MPL was used as an electrolyte additive for Li/LiNi_0.5Mn_1.5O₄ cell, it was oxidized around 3.6 V, leading to the formation of SEI film on the surface of LiNi_0.5Mn_1.5O₄ electrode. Incorporation of 0.3 wt% MPL into the conventional carbonate-based electrolyte resulted in enhanced cycling performance of Li/LiNi_0.5Mn_1.5O₄ cell. The capacity retention of Li/LiNi_0.5Mn_1.5O₄ cell after 200 cycles was improved from 83.2% to 89.5% at room temperature?ca. 25 oC? and from 59.1% to 87.4% at elevated temperature?55 oC?. This enhanced performance can be ascribed to the conductive SEI film formed by in-situ electrochemical polymerization of MPL, which prevents direct contact of the LiNi_0.5Mn_1.5O₄ surface to electrolyte and thus effectively alleviates the undesirable electrolyte decomposition. MPL was able to reduce prior to electrolyte solvent and form SEI film on graphite surface due to its higher reduction potential than EC. The collective effects of this SEI film facilitated the intercalation and deintercalation of Li+, leading to improved performance of Li/graphite cell.2?A novel electrolyte additive, allyloxytrimethylsilane?AMSL?, was screened out through DFT calculation based on frontier molecular orbital energy and Li+ binding affinity. Theoretical calculation and experimental results revealed that AMSL additive is preferentially oxidized to form a SEI film composed of organic silicon-based species and ester moiety on the surface of Li Ni_0.5Mn_1.5O₄ cathode. Incorporation of AMSL effectively inhibited the formation of LiF?LixPFy?LixPOyFz in SEI film and then improved its conductivity. Moreover, the less resistive and high thermal stable SEI film formed by AMSL was responsible for the enhanced performance of Li/Li Ni_0.5Mn_1.5O₄ cell. The discharge capacity retention of the cell was improved from 73.1% to 80.2% after 500 cycles at 25 °C and 1 C, from 52.4% to 92.5% after 100 cycles at 55 °C and 0.5 C. This additive also showed good compatibility with graphite anode, delivering a slightly higher discharge capacity than the one without additive.3?The commonly used LiPF6 salt in conventional carbonate-based electrolyte is very susceptible to hydrolysis to produce HF and POF3. These protonic acids cause the structure damage and the formation of a resistive and unstable surface film on LiNi_0.5Mn_1.5O₄ cathode. As a result, the high-voltage cathode suffers a remarkable capacity fade in conventional electrolytes. Addition of 0.5 wt% N,N-diethyl-trimethylsilylamine?EMSA? into the electrolyte effectively eliminated H2 O and HF, alleviated transition metal ions dissolution from the cathode, and thus enhanced the cycling stability of Li/LiNi_0.5Mn_1.5O₄ cell. The discharge capacity retention of the cell was improved from 76.1% to 84.9% after 500 cycles at 25 °C and 1 C, from 59.1% to 84.2% after 200 cycles at 55 °C and 1 C. While H2 O and HF in the electrolyte were scavenged by EMSA, Li/graphite cell exhibited excellent cycling performance. Charge/discharge capacity of the graphite anode with EMSA was increased by about 8%.4?Obviously, proper electrolyte additives are able to dramatically enhance the cyclic performance of LiNi_0.5Mn_1.5O₄ cathode. The LiPF6 salt is unstable toward heat and moisture, carbonate solvents are highly flammable and easy to decompose. These disadvantages of the electrolyte need to be overcome to satisfy the demand of high voltage, high safety Li-ion batteries. We designed a nonflammable electrolyte with high electrochemical and thermal stability by using lithium difluoro?oxalato?borate?LiDFOB? as Li salt, sulfolane?SL? and propylene carbonate?PC? as main solvents. This sulfone-based electrolyte showed good compatibility with LiNi_0.5Mn_1.5O₄ cathode. The discharge capacity retention of Li/LiNi_0.5Mn_1.5O₄ cell was improved from 90.7% to 93.4% after 200 cycles at 25 °C and 0.5 C, from 78.6% to 88.2% after 100 cycles at 55 °C and 0.5 C. In addition, the compatibility of sulfone-based electrolyte toward graphite anode was remarkably improved by using fluoroethylene carbonate?FEC? as an electrolyte additive. The Li/graphite cell with FEC exhibited excellent cycling stability, delivering a charge capacity of 349.2 mAh g-1 initially and maintaining capacity retention of 99.1% after 100 cycles at 25 °C and 0.5 C. Based on the sulfone-based electrolyte, we also investigated the performance of mixture Li salt electrolyte and high concentration LiTFSI electrolyte. Unfortunately, these modified sulfone-based electrolyte extensively corroded the aluminum current collector, and this problem remained to be solved in the near future.