Correlating Li/O2 cell capacity and product morphology with discharge current.

The discharge rate is critical to the performance of lithium/oxygen batteries: it impacts both cell capacity and discharge-phase morphology, and in so doing may also affect the efficiency of the oxygen-evolution reaction during recharging. First-discharge data from tens of Li/O2 cells discharged across four rates are analyzed statistically to inform these connections. In the practically significant superficial current-density range of 0.1 to 1 mA cm(-2), capacity is found to fall as a power law, with a Peukert's-law exponent of 1.6 ± 0.1. X-ray diffractometry confirms the dominant presence of crystalline Li2O2 in the discharged electrodes. A completely air-free sample-transfer technique was developed to implement scanning electron microscopy (SEM) of the discharge product. SEM imaging of electrodes with near-average capacities provides statistically significant measures of the shape and size variation of electrodeposited Li2O2 particles with respect to discharge current. At lower rates, typical "toroidal" particles are observed that are well approximated as cylindrical structures, whose average radii remain relatively constant as discharge rate increases, whereas their average heights decrease. At the highest rate studied, air-free SEM shows that particles take needle-like shapes rather than forming the nanosheets or compact films described elsewhere. Average particle volumes decrease with current while particle surface-to-volume ratios increase dramatically, supporting the notion that Li2O2 grows by a locally mass-transfer-limited nucleation and growth mechanism.

Electrochemistry of magnesium electrolytes in ionic liquids for secondary batteries.

The electrochemistry of Mg salts in room-temperature ionic liquids (ILs) was studied using plating/stripping voltammetry to assess the viability of IL solvents for applications in secondary Mg batteries. Borohydride (BH4(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (Tf2N(-)) salts of Mg were investigated. Three ILs were considered: l-n-butyl-3-methylimidazolium (BMIM)-Tf2N, N-methyl-N-propylpiperidinium (PP13)-Tf2N, and N,N-diethyl-N-methyl(2-methoxyethyl)ammonium (DEME(+)) tetrafluoroborate (BF4(-)). Salts and ILs were combined to produce binary solutions in which the anions were structurally similar or identical, if possible. Contrary to some prior reports, no salt/IL combination appeared to facilitate reversible Mg plating. In solutions containing BMIM(+), oxidative activity near 0.8 V vs Mg/Mg(2+) is likely associated with the BMIM cation, rather than Mg stripping. The absence of voltammetric signatures of Mg plating from ILs with Tf2N(-) and BF4(-) suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg(Tf2N)2/PP13-Tf2N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg(2+) dissociation.