Laurence J. Hardwick

Li-O2 and Li-S batteries with high energy storage

Li-ion batteries have transformed portable electronics and will play a key role in the electrification of transport. However, the highest energy storage possible for Li-ion batteries is insufficient for the long-term needs of society, for example, extended-range electric vehicles. To go beyond the horizon of Li-ion batteries is a formidable challenge; there are few options. Here we consider two: Li-air (O(2)) and Li-S. The energy that can be stored in Li-air (based on aqueous or non-aqueous electrolytes) and Li-S cells is compared with Li-ion; the operation of the cells is discussed, as are the significant hurdles that will have to be overcome if such batteries are to succeed. Fundamental scientific advances in understanding the reactions occurring in the cells as well as new materials are key to overcoming these obstacles. The potential benefits of Li-air and Li-S justify the continued research effort that will be needed.

Reactions in the Rechargeable Lithium–O2 Battery with Alkyl Carbonate Electrolytes

The nonaqueous rechargeable lithium-O(2) battery containing an alkyl carbonate electrolyte discharges by formation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, CO(2), and H(2)O at the cathode, due to electrolyte decomposition. Charging involves oxidation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li accompanied by CO(2) and H(2)O evolution. Mechanisms are proposed for the reactions on discharge and charge. The different pathways for discharge and charge are consistent with the widely observed voltage gap in Li-O(2) cells. Oxidation of C(3)H(6)(OCO(2)Li)(2) involves terminal carbonate groups leaving behind the OC(3)H(6)O moiety that reacts to form a thick gel on the Li anode. Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, and C(3)H(6)(OCO(2)Li)(2) accumulate in the cathode on cycling correlating with capacity fading and cell failure. The latter is compounded by continuous consumption of the electrolyte on each discharge.

The lithium-oxygen battery with ether-based electrolytes.

The rechargeable Li–air (O2) battery is receiving a great deal of interest because theoretically it can store significantly more energy than lithium ion batteries, thus potentially transforming energy storage. Since it was first described, a number of aspects of the Li–O2 battery with a non-aqueous electrolyte have been investigated. The electrolyte is recognized as one of the greatest challenges. To date, organic carbonate-based electrolytes (e.g. LiPF6 in propylene carbonate) have been widely used. However, recently, it has been shown that instead of O2 being reduced in the porous cathode to form Li2O2, as desired, discharge in organic carbonate electrolytes is associated with severe electrolyte decomposition. As a result it is very important to investigate other solvents in the search for a suitable electrolyte. In this regard much attention is now focused on electrolytes based on ethers (e.g. tetraglyme (tetraethylene glycol dimethyl ether)). Ethers are attractive for the Li–O2 battery because they are one of the few solvents that combine the following attributes: capable of operating with a lithium metal anode, stable to oxidation potentials in excess of 4.5 V versus Li/Li, safe, of low cost and, in the case of higher molecular weights, such as tetraglyme, they are of low volatility. Crucially, they are also anticipated to show greater stability towards reduced O2 species compared with organic carbonates. Herein we show that although the ethers are more stable than the organic carbonates, the Li2O2 that forms on the first discharge is accompanied by electrolyte decomposition, to give a mixture of Li2CO3, HCO2Li, CH3CO2Li, polyethers/ esters, CO2, and H2O. The extent of electrolyte degradation compared with Li2O2 formation on discharge appears to increase rapidly with cycling (that is, charging and discharging), such that after only 5 cycles there is little or no evidence of Li2O2 from powder X-ray diffraction. We show that the same decomposition products occur for linear chain lengths other than tetraglyme. In the case of cyclic ethers, such as 1,3dioxolane and 2-methyltetrahydrofuran (2-Me-THF), decomposition also occurs. For 1,3-dioxolane, decomposition forms polyethers/esters, Li2CO3, HCO2Li, and C2H4(OCO2Li)2, and for 2-Me-THF the main products are HCO2Li, CH3CO2Li; in both cases CO2 and H2O evolve. The results presented herein demonstrate that ether-based electrolytes are not suitable for rechargeable Li–O2 cells. A Li–O2 cell consisting of a lithium metal anode, an electrolyte, comprising 1m LiPF6 in tetraglyme, and a porous cathode (Super P/Kynar) was constructed as described in the Experimental Section. The cell was discharged in 1 atm O2 to 2 V. The porous cathode was then removed, washed with CH3CN, and examined by powder X-ray diffraction (PXRD) and FTIR. The results are presented in Figure 1 and Figure 2. The PXRD data demonstrate the presence of Li2O2, consistent with previous PXRD data for a Li–O2 cell with a tetraglyme electrolyte at the end of the first discharge. However, examination of the FTIR spectra, Figure 2, reveals that, in addition to Li2O2, other products form. Although the FTIR spectra provide clear evidence of electrolyte decom-

Oxygen reactions in a non-aqueous Li+ electrolyte.

Oxygen (O2) reduction is one of the most studied reactions in chemistry.1 Widely investigated in aqueous media, O2 reduction in non-aqueous solvents, such as CH3CN, has been studied for several decades.2–7 Today, O2 reduction in non-aqueous Li+ electrolytes is receiving considerable attention because it is the reaction on which operation of the Li–air (O2) battery depends.8–29 The Li–O2 battery is generating a great deal of interest because theoretically its high energy density could transform energy storage.8, 9 As a result, it is crucial to understand the O2 reaction mechanisms in non-aqueous Li+ electrolytes. Important progress has been made using electrochemical measurements including recently by Laoire et al.29 No less than five different mechanisms for O2 reduction in Li+ electrolytes have been proposed over the last 40 years based on electrochemical measurements alone.25–29 The value of using spectroelectrochemical methods is that they can identify directly the species involved in the reaction. Here we present in situ spectroscopic data that provide direct evidence that LiO2 is indeed an intermediate on O2 reduction, which then disproportionates to the final product Li2O2. Spectroscopic studies of Li2O2 oxidation demonstrate that LiO2 is not an intermediate on oxidation, that is, oxidation does not follow the reverse pathway to reduction.

Lithium Intercalation into Mesoporous Anatase with an Ordered 3D Pore Structure

There is a great deal of interest in TiO2 nanoparticles, nanowires and nanotubes due to their potential advantages (safety, rate) as anodes replacing graphite in a new generation of rechargeable lithium batteries. Here we report the synthesis of mesoporous anatase with an ordered 3D pore structure, using a hard template, and investigate its properties as a lithium intercalation host. It exhibits a hierarchical pore structure. Despite being composed of micrometer sized particles, the ordered mesoporous morphology inside the particles results in a high Li storage capacity and high rates of intercalation, with the material exhibiting an energy density between 30 and 200% higher than the best high rate performance reported to date for any titanate (6 nm nanoparticle anatase). It has been proposed that the reason nanoparticles such as anatase and LiFePO4 exhibit facile Li insertion is the ability of such particles to transform spontaneously for one phase to the other, i.e., a particle is either phase A or B but not both. The micrometer sized mesoporous particles cannot do so but still show facile intercalation. This is related to the ease with which the strain of transforming between the anatase (phase A) and the orthorhombic Li0.59TiO2 structures (phase B) is accommodated within the thin (6.5 nm) walls on intercalation. Mesoporous anatase with an ordered 3D pore structure was synthesized using the silica KIT-6 as a hard template (see experimental section). The ordered pore structure is evident in the TEM data (Figure 1A,B) and replicates that of the KIT-6 hard template with space group Ia3d. An a0 lattice parameter for the mesostructure of 23.3 nm was extracted from the data. The mesoporous structure is preserved throughout as demonstrated by examining many particles. The walls (6.5 nm) are composed of anatase crystallites. A lattice spacing of 0.350 nm was observed in HRTEM (Figure 1B), in good agreement with the d-spacing of 0.352 nm associated with the (101) direction of anatase (ICDD 00-0015062). The low and wide-angle PXRD data are shown in Figure 2. The low-angle diffraction patterns exhibit one relatively sharp peak below 18, which could be indexed as the 211 reflection in the Ia3d space group, corresponding to an a0 lattice parameter of 23.5 nm in good agreement with the TEM data. The broad peaks in the wide-angle PXRD for the as-prepared mesoporous material are in good agreement with those for anatase nanoparticles, AK-1 (Bayer) (Figure 2A). The mesoporous anatase peak widths are greater than those of the nanoparticles in accord with the walls being thinner than the diameter of the nanoparticles (15 nm). The mesostructures were investigated further by N2 sorption measurements. Typical type IV isotherms exhibiting H2 hysteresis were observed (Supporting Information, Figure S1a), consistent with the mesoporosity evident in the TEM and low-angle PXRD data. BJH pore size distributions exhibit at least three peaks, demonstrating a hierarchical pore structure. Awell resolved narrow peak centered at 5 nm, a peak at approximately 11 nm and a third broad peak at ca. 50 nm (Figure S1b). The first peak corresponds to the mesopores observed by TEM in Figure 1. The second arises because KIT-6 has two interpenetrating sets of pores connected by microporous bridges. In regions where the bridges are complete, both pores will be filled and the replica TiO2 is composed of the 5 nm pores. Where the bridges are incomplete and only one set of KIT-6 pores are filled, the replica exhibits 11 nm pores (Figure S2a). Such a phenomenon had been discussed previously for other KIT-6 templated materials. The third peak corresponds to interparticle voids. The 11 Figure 1. TEM and HRTEM data for ordered mesoporous anatase: A,B) as-prepared; C,D) after 1000 cycles (12000 mAg ).

Direct Detection of Discharge Products in Lithium–Oxygen Batteries by Solid‐State NMR Spectroscopy

A closer look: Solid-state (7) Li and (17) O NMR spectroscopy is a valuable tool in the characterization of products formed in the lithium-oxygen battery, a necessary step in the development of a viable cell. Since lithium peroxide, the desired discharge product, has a unique (17) O NMR signature, it can be clearly identified.

An Investigation of the Effect of Graphite Degradation on the Irreversible Capacity in Lithium-ion Cells

An Investigation of the Effect of Graphite Degradation on the Irreversible Capacity in Lithium-ion Cells Laurence J. Hardwick * , Marek Marcinek a* , Leanne Beer, John B. Kerr*, Robert Kostecki b,* Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA a Electrochemical Society Active Member Present address: The Warsaw University of Technology, Noakowskiego 3, 00-664, Warsaw, Poland b Corresponding author: r_kostecki@lbl.gov, tel: (1) 510 486 6002, fax: (1) 510 486 7303

Charge storage mechanism of activated manganese oxide composites for pseudocapacitors

Manganese oxides can undergo an electrochemical activation step that leads to greater capacitances, of which the structural change and mechanism remains poorly understood. Herein we present a wide-ranging study on a manganese oxide synthesised by annealing manganese(II) acetate precursor to 300 °C, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Based on powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron and ex situ Raman microscopy, the as prepared manganese oxide was characterised as hausmannite-Mn3O4 with a minor portion of MnO2. The activation process of converting as-prepared hausmannite-Mn3O4 into amorphous MnO2 (with localised birnessite structure) by electrochemical cycling in 0.5 M Na2SO4 was examined. After activation, the activated MnOx exhibited capacitive performance of 174 F g−1 at a mass loading of 0.71 mg cm−2. The charge storage mechanism is proposed as the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, since the disordered birnessite-MnO2 does not provide an ordered layer structure for cations and/or protons to intercalate.

Solvent-Mediated Control of the Electrochemical Discharge Products of Non-Aqueous Sodium–Oxygen Electrochemistry

Abstract The reduction of dioxygen in the presence of sodium cations can be tuned to give either sodium superoxide or sodium peroxide discharge products at the electrode surface. Control of the mechanistic direction of these processes may enhance the ability to tailor the energy density of sodium–oxygen batteries (NaO2: 1071 Wh kg−1 and Na2O2: 1505 Wh kg−1). Through spectroelectrochemical analysis of a range of non‐aqueous solvents, we describe the dependence of these processes on the electrolyte solvent and subsequent interactions formed between Na+ and O2 −. The solvents ability to form and remove [Na+‐O2 −]ads based on Gutmann donor number influences the final discharge product and mechanism of the cell. Utilizing surface‐enhanced Raman spectroscopy and electrochemical techniques, we demonstrate an analysis of the response of Na‐O2 cell chemistry with sulfoxide, amide, ether, and nitrile electrolyte solvents.

Three-dimensional protonic conductivity in porous organic cage solids

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10−3 S cm−1 at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.