Research On Mechanisms And Solutions Of The Capacity Fading Of Lithium Manganate-graphitebatteries

Lithium ion battery based on LiMn2O4-graphite system is one of the mostpromising power sources for the electric vehicles in many respects, such as low cost,thermal safety, and excellent rate capability. Nevertheless, capacity of LiMn2O4-graphitecells fades fairly at room temperature with cycling, and this fading issignificantlyaccelerated at elevated temperature, which inhibits its practical applications. In thiswork, we study the deposition process of manganesedissolved from cathode and thecorrelation between the manganesedepositionprocess on anode and capacity fading oflithium-ion batteries. Based on that, we study the cycle performanceimprovementmethods via controlling the dissolution-deposition of manganese.(1) Deposition process of manganese on anodes-ion exchange model. We are ableto demonstrate clearly by X-ray absorption fine structure spectroscopy along with X-rayphotoelectron spectroscopy that, surprisingly, manganese deposits on the graphiteanodeas Mn(II)-O. And the Mn deposition can be observed on anode with Li insertionpotential higher than Mn deposition potential, which proofs the limitation of theelectrochemical reduction or chemical reduction model of the Mn deposition as reportedin previous papers. Therefore, we raise the ion-exchange model, which is the ionexchange reaction between Mn2+and mobile Li+in the SEI,which blocks the lithium diffusionpath, and thus, leads to the increase of impedance of graphite anode, and hence the capacity fadeof the battery.(2) Improve the cycle performance by suppressing thedissolution-depositionprocess by surficial modification. The surface-coated andsurface-doped LiMn2O4are prepared by sol-gel and atomic layer deposition methodsrespectively. Both the surface doping and surface coating of LiMn2O4significantlyimproved the cycle and impedance performance of LiMn2O4electrodes under elevatedtemperature, but the surface doping shows higher capacity and lower impedance due tothe faster charge transportation in the cationic doped layer. The reason for the improvedstability of the coated electrodes is associated to the fact that the surface coating layerssuppress the dissolution and deposition of Mn in the LiMn2O4-graphite lithium ionbatteries. (3) Improve the cycle performance by adjusting the deposition reaction ofmanganese on anode by electrolyte additives. The electrolyte additives fluoroethylenecarbonate (FEC) and vinylene carbonate (VC) effectively improve the cycleperformance of LiMn2O4-graphite batteries at elevated temperature, but not by simplysuppressing the deposition or ion-exchange reaction of Mn on anodes. The in-situimpedance measurements indicate that VC and FEC are able to weaken the impact ofMn deposition on the lithium ion transport of SEI, which sheds some light on how toimprove the cell performance.