No abstract

No abstract

The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge–discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.

We describe a V2O5 polysulfide anion barrier for a Li–S battery containing a Li metal anode, an organic solvent-based electrolyte, and a nanostructured carbon–sulfur composite cathode that cycles without degradation from dissolution of soluble polysulfide anions. Degradation is prevented by the micrometer-scale V2O5 layer that physically isolates the anode and cathode electrolytes, thus blocking diffusion of dissolved polysulfide anions while permitting solid-state transport of Li+ cations. This divided cell architecture eliminates the interaction of soluble polysulfide with the Li anode. Mechanical integrity of the thin V2O5 layer is achieved by depositing the layer on a commercial polymeric battery separator. In addition, the isolated cathode electrolyte is optimized by the intentional addition of Li2S8, which suppresses redistribution of sulfur within the nanostructured carbon–sulfur composite cathode. A 5 mA h pouch cell of this design has been cycled >300 times over ∼1 year without noticeable degradation at capacities of 800 mA h g-sulfur–1.