Imperial College London

Dr A.K. Ola Hekselman

Faculty of EngineeringDepartment of Materials

Visiting Researcher
 
 
 
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Contact

 

o.hekselman Website

 
 
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Location

 

2.22Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

5 results found

Pesci FM, Bertei A, Brugge RH, Emge SP, Hekselman AKO, Marbella LE, Grey CP, Aguadero Aet al., 2020, Establishing ultra-low activation energies for lithium transport in garnet electrolytes., ACS Applied Materials and Interfaces, Vol: 12, Pages: 32086-32816, ISSN: 1944-8244

Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and the inherent inhomogeneous current distribution due to the different ion dynamics at grains, grain boundaries and interfaces. In this study we use a combination of electrochemical impedance spectroscopy, distribution of relaxation times analysis and solid state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary and interfacial processes play in the ionic transport and electrochemical performance of garnet based cells. Variable temperature impedance analysis reveals the lowest activation energy (Ea) for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way towards targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices.

Journal article

Pesci FM, Brugge RH, Hekselman AKO, Cavallaro A, Chater RJ, Aguadero Aet al., 2018, Elucidating the role of dopants in the critical current density for dendrite formation in garnet electrolytes, Journal of Materials Chemistry A, Vol: 6, Pages: 19817-19827, ISSN: 2050-7488

Garnet-type solid electrolytes have attracted great interest in solid state battery research thanks to their high ionic conductivity at room temperature (10−3 S cm−1) and their electrochemical stability against lithium metal anodes. However, the formation of lithium dendrites following charge/discharge limits their applicability and commercialisation. Although widely investigated, no clear explanation of dendrite formation has been previously reported. In this work, we employ cubic Al- and Ga-doped Li7La3Zr2O12, which represent two of the solid electrolytes with higher technological importance, to investigate the formation and chemical composition of dendrites. For the first time, this study elucidates the role that the dopants play in determining the critical current density for dendrite formation and highlights the importance of controlling the dopant distribution in the garnet structure. We use a combination of techniques including Secondary Electron Microscopy and Secondary Ion Mass Spectrometry in order to analyse the microstructure and chemical composition of dendrites in Li7La3Zr2O12. We show that, following electrochemical cycling, Li6.55Ga0.15La3Zr2O12 systematically displays a critical current density 60% higher than Li6.55Al0.15La3Zr2O12. Chemical analysis revealed that in Li6.55Al0.15La3Zr2O12 the dendritic features are composed of a mixture of Al and Li species, whereas in Li6.55Ga0.15La3Zr2O12 they are uniquely composed of Li. We also show that only in pristine Li6.55Al0.15La3Zr2O12, the dopant segregates at the grain boundaries suggesting that local chemical inhomogeneity can have a fundamental role in the nucleation and propagation of dendrites.

Journal article

Brugge R, Hekselman A, Cavallaro A, Pesci F, Chater R, Kilner J, Aguadero Aet al., 2018, Garnet electrolytes for solid state batteries: visualization of moisture-induced chemical degradation and revealing its impact on the Li-ion dynamics, Chemistry of Materials, Vol: 30, Pages: 3704-3713, ISSN: 0897-4756

In this work, we reveal the impact of moisture-induced chemical degradation and proton–lithium exchange on the Li-ion dynamics in the bulk and the grain boundaries and at the interface with lithium metal in highly Li-conducting garnet electrolytes. A direct correlation between chemical changes as measured by depth-resolved secondary ion mass spectrometry and the change in transport properties of the electrolyte is provided. In order to probe the intrinsic effect of the exchange on the lithium kinetics within the garnet structure, isolated from secondary corrosion product contributions, controlled-atmosphere processing was first used to produce proton-free Li6.55Ga0.15La3Zr2O12 (Ga0.15-LLZO), followed by degradation steps in a H2O bath at 100 °C, leading to the removal of LiOH secondary phases at the surface. The proton-exchanged region was analyzed by focused ion beam secondary ion mass spectrometry (FIB-SIMS) and found to extend as far as 1.35 μm into the Ga0.15-LLZO garnet pellet after 30 min in H2O. Impedance analysis in symmetrical cells with Li metal electrodes indicated a greater reactivity in grain boundaries than in grains and a significantly detrimental effect on the Li transfer kinetics in the Li metal/garnet interface correlated to a 3-fold decrease in the Li mobility in the protonated garnet. This result indicates that the deterioration of Li charge transfer and diffusion kinetics in proton-containing garnet electrolytes have fundamental implications for the optimization and integration of these systems in commercial battery devices.

Journal article

Zekoll S, Marriner-Edwards C, Hekselman AKO, Kasemchainan J, Kuss C, Armstrong DEJ, Cai D, Wallace RJ, Richter FH, Thijssen JHJ, Bruce PGet al., 2018, Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries, Energy & Environmental Science, Vol: 11, Pages: 185-201, ISSN: 1754-5692

Journal article

Hekselman, Kalita M, Plewa-Marczewska A, Żukowska GZ, Sasim E, Wieczorek W, Siekierski Met al., 2010, Effect of calix[6]pyrrole anion receptor addition on properties of PEO-based solid polymer electrolytes doped with LiTf and LiTfSI salts, Electrochimica Acta, Vol: 55, Pages: 1298-1307, ISSN: 0013-4686

Journal article

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