On discharge, the Li-O2 battery can form a Li2O2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li2O2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the LiO2 intermediate involved in the formation of Li2O2. However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO2 or Li2O2. Here we demonstrate that introduction of the additive 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li2O2 in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li2O2 on the cathode surface, which would otherwise lead to Li2O2 film growth and premature cell death. It also halves the overpotential during discharge, increases the capacity 80- to 100-fold and enables rates >1 mA cmareal(-2) for cathodes with capacities of >4 mAh cmareal(-2). The DBBQ additive operates by a new mechanism that avoids the reactive LiO2 intermediate in solution.

There is considerable interest in multivalent cation batteries, such as those based on magnesium, calcium or aluminium. Most attention has focused on magnesium. In all cases the metal anode represents a significant challenge. Recent work has shown that calcium can be plated and stripped, but only at elevated temperatures, 75 to 100 °C, with small capacities, typically 0.165 mAh cm, and accompanied by significant side reactions. Here we demonstrate that calcium can be plated and stripped at room temperature with capacities of 1 mAh cm at a rate of 1 mA cm, with low polarization (∼100 mV) and in excess of 50 cycles. The dominant product is calcium, accompanied by a small amount of CaH that forms by reaction between the deposited calcium and the electrolyte, Ca(BH) in tetrahydrofuran (THF). This occurs in preference to the reactions which take place in most electrolyte solutions forming CaCO, Ca(OH) and calcium alkoxides, and normally terminate the electrochemistry. The CaH protects the calcium metal at open circuit. Although this work does not solve all the problems of calcium as an anode in calcium-ion batteries, it does demonstrate that significant quantities of calcium can be plated and stripped at room temperature with low polarization.

Supercapacitors have unique advantages over lithium ion batteries in high power delivery and long cycling life, and are emerging as attractive electrochemical energy storage devices for future electrical vehicle application. [1][2][3][4] However, supercapacitors deliver an unsatisfactory energy density. Intensive efforts have been devoted to the enhancement of their energy density to make it comparable to that of rechargeable batteries. Among the supercapacitor electrode materials, pseudocapacitive transition-metal oxides and electronically conducting polymers based on faradic redox charge storage have attracted signifi cant attention because of their higher energy density than those of electrochemical double-layer capacitive carbon materials. [ 5 , 6 ] Currently, the most investigated pseudocapacitive materials are always directed towards the cathode material, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] whereas there are only a few reports on anode materials due to the unsatisfactory capacitive performance. [23][24][25][26][27][28] Among the various transition-metal oxides, V 2 O 5 possesses the unique advantages of high energy density [ 29 ] and wide potential window arising from its various vanadium oxidation states (V-II), [ 27 ] which render it a promising candidate as an anode material for supercapacitors. However, its poor electronic conductivity and high dissolution in liquid electrolyte are detrimental to high-rate and long-term cycling performance in electrochemical devices. The combination of V 2 O 5 with carbon nanotubes has been demonstrated to be an effective strategy to improve electronic transport, [ 10 , 30-32 ] but this kind of composite cannot prevent vanadium dissolution. In this work, polypyrrole (PPy)-known to be an electronic conductive polymer [ 33 , 34 ] -is grown uniformly on the surface of V 2 O 5 nanoribbon by using anionic dodecylbenzenesulfonate (DBS − ) as a surfactant. The obtained core-shell-structured PPy@V 2 O 5 nanocomposites is expected to resolve the above two problems simultaneously utilizing the electronic conductivity and polymeric coating effect of PPy ( Figure 1 a). [ 35 , 36 ] Electrochemical results demonstrate that our prepared PPy@V 2 O 5 nanocomposite exhibits excellent cycling and rate behavior, and is a promising candidate as an anode material for supercapacitors.The process for the growth of PPy on V 2 O 5 surface is schematically shown in Figure 1 b. The virgin V 2 O 5 nanoribbons were prepared by hydrothermal treatment of NH 4 VO 3 and poly(ethylene oxide)block -poly(propylene oxide)blockpoly(ethylene oxide) copolymer in acid solution at 120 ° C. [ 37 ] For the fabrication of the PPy@V 2 O 5 nanocomposite, V 2 O 5 nanoribbons were fi rst ultrasonically dispersed in water assisted by anionic surfactant dodecylbenzenesulfonate (DBS − ), with the hydrophilic head towards water phase. After addition of pyrrole, pyrrole monomer is supposed to adsorb on the V 2 O 5 surface via electrostatic interaction between anionic DBS − and protonated pyrro...

At the cathode of a Li-O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the electrode surface, problems overcome by forming/decomposing Li2O2 from solution. A Li-O2 cell is described that decouples completely the electrochemistry at the cathode surface from Li2O2 formation/decomposition. Mediators on discharge (2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ]) and charge (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO]) transfer electrons between the cathode surface and Li2O2. The cell cycles with a capacity of 2 mAh cm -2 areal at 1 mA cm -2 areal with low polarisation on charge/discharge, indicating that dual mediators combined with a true gas diffusion electrode could deliver 40 mAh cm -2 areal at rates >> 1 mA cm -2 areal. Arguably, the most important advantage of dual mediators is they avoid instability at the carbon cathode. Carbon is the most attractive material for the porous cathode in Li-O2 cells, but is too reactive degrading to Li2CO3. By forming/decomposing Li2O2 in solution and not in intimate contact with the carbon, by avoiding high charge potentials and because only mediators transfer electrons at the carbon surface, carbon instability is avoided (< 0.008 % carbon decomposition per cycle compared with 0.12 % without mediators), addressing one of the biggest barriers to the progress of Li-O2 cells.