Inside the electrode: Looking at cycling products in Li/O2 batteries

This work investigates the impact of electrochemical reactions and products on discharge capacity and cycling stability with electrolytes based on two common solvents – tetraethylene glycol dimethyl ether (TEGDME) and dimethyl sulfoxide (DMSO). Although the DMSO-based electrolyte exhibits better initial electrochemical properties compared to that based on TEGDME, e.g., higher discharge capacity and potential, the use of TEGDME results in a significantly better cycling stability. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) investigations of the gas diffusion electrodes (GDE) after first discharge reveal a considerable difference in discharge product morphology. With DMSO as solvent one high-potential reduction process leads to the formation of crystalline lithium peroxide (Li2O2) particles on the cathode surface area. SEM imaging of GDE cross-sections depicts that the (non-crystalline) product film formation at lower potentials during discharge with the TEGDME-based electrolyte results in a GDE pore clogging close to the O2 inlet, so that gas transport is hindered and the discharge ends at an earlier point. The higher cycling stability with LiTFSI/TEGDME, however, is attributed to (i) the apparently complete recovery of the GDE active surface by recharge and (ii) different parasitic reactions resulting in the formation of side product particles rather than films.acceptedVersio

High capacity nanostructured Li2FexSiO4/C with Fe hyperstoichiometry for Li-ion batteries

Li2FeSiO4-based materials have attracted a great deal of interest as cathodes for lithium ion batteries for its excellent thermal stability and good cycling ability. Nanoporous Li2FeSiO4/C composites with varying Fe stoichiometry were synthesized by a PVA-assisted sol–gel method. Different gel formation processes were used to control morphology and pore size distribution. The pore size distribution was controlled by adjusting the parameters of the gel ageing process, the carbon content and phase purity was controlled by adjusting the starch content, and the amount of secondary phases was controlled by adjusting the Fe stoichiometry. The electrochemical properties of the Li2FeSiO4/C composite were assessed using coin cells at 24 °C, and the optimized material showed an initial discharge capacity of 163 mAh g−1 at a discharge rate of C/16 (C = 160 mA g−1) and a high capacity retention of 96% after 200 cycles at a discharge rate of 1 C.© 2013 Elsevier Ltd. This is the authors’ accepted and refereed manuscript to the article

Inside the electrode: Looking at cycling products in Li/O2 batteries

This work investigates the impact of electrochemical reactions and products on discharge capacity and cycling stability with electrolytes based on two common solvents – tetraethylene glycol dimethyl ether (TEGDME) and dimethyl sulfoxide (DMSO). Although the DMSO-based electrolyte exhibits better initial electrochemical properties compared to that based on TEGDME, e.g., higher discharge capacity and potential, the use of TEGDME results in a significantly better cycling stability. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) investigations of the gas diffusion electrodes (GDE) after first discharge reveal a considerable difference in discharge product morphology. With DMSO as solvent one high-potential reduction process leads to the formation of crystalline lithium peroxide (Li2O2) particles on the cathode surface area. SEM imaging of GDE cross-sections depicts that the (non-crystalline) product film formation at lower potentials during discharge with the TEGDME-based electrolyte results in a GDE pore clogging close to the O2 inlet, so that gas transport is hindered and the discharge ends at an earlier point. The higher cycling stability with LiTFSI/TEGDME, however, is attributed to (i) the apparently complete recovery of the GDE active surface by recharge and (ii) different parasitic reactions resulting in the formation of side product particles rather than films

The effect of addition of the redox mediator dimethylphenazine on the oxygen reaction in porous carbon electrodes for Li/O2 batteries

Secondary Li–O2 batteries are promising due to their potentially high theoretical energy density. However, both the discharge (oxygen reduction reaction, ORR) and the recharge reaction (oxygen evolution reaction, OER) are associated with high irreversible losses, and multiple side reactions, depending on the electrolyte of choice. Addition of redox mediators is currently considered a promising route to combat the challenges of the highly irreversible ORR/OER. In this work, the effect of addition of the redox mediator 5,10-dimethylphenazine (DMPZ) on the capacity and reversibility of the oxygen reaction is investigated in porous carbon electrodes. The electrolytes are based on tetraethylene glycol dimethyl ether (TEGDME) as solvent, and either Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as salt, or a combination of LiTFSI and LiNO3 salt, alternatively dimethyl sulfoxide (DMSO) as solvent, with LiTFSI salt. The addition of DMPZ results in a significant improvement of the reversibility of the ORR/OER reactions for electrolytes based on LiTFSI in DMSO, and LITFSI + LiNO3 in TEGDME. This is attributed to a depression of the side reactions limiting the recharge reaction in these electrolytes. Post mortem analyses by XRD, SEM, as well as FIB-SEM investigations of cross sections, are used to characterize the products from the side reactions

Flame-made Lithium Transition Metal Orthosilicates

Li2MSiO4 (M = Fe, Mn, Co) compounds have since their discovery gained increased attention as alternative, inexpensive and inherently safe positive electrodes for Li-ion batteries. To meet the required performance for an electrode, sophisticated, complex and time-consuming synthesis measures are required at present. Here, we present a time-efficient and scalable aerosol combustion method with subsequent annealing, leading to nanoscale and carbon-coated Li2FeSiO4 and Li2Fe0.5Mn0.5SiO4. Using liquid-feed flame spray pyrolysis, we demonstrate synthesis of orthosilicate materials, with phase purities exceeding 95 wt. % according to Rietveld quantifications, in a relatively short time. The importance of the precursor concentration, in order to obtain loosely agglomerated nanoparticles, is discussed and the long-term performance is investigated. In the case of Li2FeSiO4, the optimised precursor concentration yielded particles of about 30 nm, which delivered an initial discharge capacity of up to 150 mAhg−1 at 60 °C and C/20. Furthermore, over 50% of the capacity is retained at a high rate of 5C, and long-term cycling showed outstanding capacity retention of over 90% after 300 cycles at a moderate rate of C/2. Li2Fe0.5Mn0.5SiO4 on the other hand, was shown to suffer from a severe capacity fade, and upon prolonged cycling the redox activity can be attributed solely to Fe

Performance and failure analysis of full cell lithium ion battery with LiNi0.8Co0.15Al0.05O2 and silicon electrodes

The influence of the lithium inventory on the performance and degradation mechanism of NCA||Si cells operating at a third of the theoretical silicon capacity is analysed. The lithium inventory was increased by electrochemical prelithiation to a value of 300 mAhg−1(Si). Full-cells were cycled at harsh conditions with a cut-off of 4.4 V to maximise the capacity. The higher lithium inventory resulted in an increased reversible capacity from 163 to 199 mAhg−1(NCA). The cycle-life was increased by 60% and reached 245 cycles. Three-electrode and post-mortem analyses revealed that the main reason for capacity fade is repeated SEI repair, consuming the lithium inventory. Differential capacity analysis revealed different degradation of silicon anodes cycled in half-cells compared to full-cells. No shifts in the alloying/dealloying peaks are present in full-cell geometry while changes are observed in half-cell geometry. This is expected to be caused by a limited alloying capacity in the full-cell and lithium consumption during cycling, alleviating material stresses. We conclude that the lithium consumption is the main factor causing capacity fade in NCA||Si cells. The decreasing degree of lithiation over cycling due to the lithium consumption is likely to be the reason for the absence of structural degradations of full-cell cycled silicon.publishedVersio

PVA-assisted combustion synthesis and characterization of porous nanocomposite Li2FeSiO4/C

Porous Li2FeSiO4/C nanocomposites were synthesized by a PVA-assisted combustion method, and phase pure Li2FeSiO4 was produced for samples prepared with 20 and 30 wt.% starch. The electrochemical properties of the Li2FeSiO4/C composite were assessed using coin cells. The PVA-assisted combustion method gave materials with surface areas up to 37.7 m2 g− 1 and initial discharge capacity of 135 mAh g− 1 at a discharge rate of C/16 (C = 160 mA g− 1). Both phase purity and discharge capacity were highly sensitive to the amount of carbon precursor used in the synthesis, and a nominal carbon content of 20% was found to give the best performance with respect to charge and discharge capacity

In situ X-ray diffraction and electrochemical impedance spectroscopy of a nanoporous Li2FeSiO4/C cathode during the initial charge/discharge cycle of a Li-ion battery

Understanding of the structural evolution of the cathode during the charge/discharge processes is crucial to describe the Li insertion/de-insertion mechanisms in a Li-ion battery. An in situ XRD cell has been specially fabricated to study a nanostructured electrode using a standard laboratory diffractometer. This cell was used to investigate phase transformations of a nanoporous Li2FeSiO4/C cathode in the initial charge/discharge cycle by in situ XRD as well as analyzing the full Li-ion battery by electrochemical impedance spectroscopy (EIS). The battery was operated in chronocoulometric mode for the in situ XRD and galvanostatic intermittent titration technique (GITT) mode for the EIS. Coexistence of two different polymorphs, P21/n and Pmn21 of Li2FeSiO4, was observed in the in situ XRD patterns. The amount of P21/n phase, which was the only phase present before cycling, decreased while the amount of Pmn21 phase increased during the first cycle. In the fully discharged state the Pmn21 phase appeared as the main phase. An inductive loop was observed in the impedance spectra which is believed to arise from the formation of a concentration cell (Li|P21/n||Pmn21|Li) from which current flows in opposition to the Li being intercalated/de-intercalated into and out of the Li2−xFeSiO4 electrode

High capacity Mg batteries based on surface-controlled electrochemical reactions

Mg batteries are one of several new battery technologies expected to partially substitute lithium-based batteries in the future due to the lower cost and higher safety. However, the development of Mg batteries has been greatly hindered by the sluggish Mg migration kinetics in the solid state. Here, we exploit a high performance cathode for Mg battery based on a tailored nanocomposite, synthesized by in-situ growth of nanocrystalline Mn3O4 on graphene substrates, which provides high reversible capacities (~ 220 mA h g−1 at 15.4 mA g−1 and ~ 80 mA h g−1 at 1.54 A g−1), good rate performance (high reversibility at various current rates), and excellent cycling stability (no capacity decay after 700 hundred cycles). The magnesiation mechanism in our cell system has been identified as a combination of capacitive processes and diffusion-controlled reactions involving electrolyte solvents. Characterization is performed by ex-situ transmission electron microscopy (TEM)/scanning TEM (STEM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) in addition to quantitative kinetics analysis. Exploiting the high-performance capacitive-type electrodes, where the specific capacity is limited by the kinetics of surface processes and not by bulk Mg ion diffusion governing the properties of conventional intercalation-type electrodes, could reveal a new approach to developing commercially viable Mg batteries.acceptedVersio

Comparing electrochemical perormance of transition metal silicate cathodes and chevrel phase Mo6S8 in the analogous rechargeable Mg-ion battery system

Polyanion based silicate materials, MgMSiO4 (M = Fe, Mn, Co), previously reported to be promising cathode materials for Mg-ion batteries, have been re-examined. Both the sol-gel and molten salt methods are employed to synthesize MgMSiO4 composites. Mo6S8 is synthesized by a molten salt method combined with Cu leaching and investigated in the equivalent electrochemical system as a bench mark. Electrochemical measurements for Mo6S8 performed using the 2nd generation electrolyte show similar results to those reported in literature. Electrochemical performance of the silicate materials on the other hand, do not show the promising results previously reported. A thorough study of these published results are presented here, and compared to the current experimental data on the same material system. It appears that there are certain inconsistencies in the published results which cannot be explained. To further corroborate the present experimental results, atomic-scale calculations from first principles are performed, demonstrating that diffusion barriers are very high for Mg diffusion in MgMSiO4. In conclusion, MgMSiO4 (M = Fe, Mn, Co) olivine materials do not seem to be such good candidates for cathode materials in Mg-ion batteries as previously reported.acceptedVersio