Sebastian Altus1,2,Innes McClelland1,3,2,Denis Cumming4,2,Edmund Cussen1,2
University of Sheffield1,The Faraday Institution2,ISIS Neutron and Muon Source3,The University of Sheffield4
Sebastian Altus1,2,Innes McClelland1,3,2,Denis Cumming4,2,Edmund Cussen1,2
University of Sheffield1,The Faraday Institution2,ISIS Neutron and Muon Source3,The University of Sheffield4
Increasing interest in the commercialization of solid-state batteries as an energy source has emphasised the inherent problem of Li dendritic growth.<sup>1</sup> This fundamental mechanism hinders solid state electrolytes from operating at required critical current densities for electric vehicles.<sup>2,3</sup> In a concerted effort to identify the source of Li dendrite growth researchers have identified high electronic conductivity as the origin for Li dendrite formation in solid electrolytes.<sup>4</sup><br/>Here we investigated the isovalent doping of Garnet type Li<sub>6.4</sub>La<sub>3</sub>Zr<sub>1.4</sub>Ta<sub>0.6</sub>O<sub>12</sub> with the Mn<sup>4+</sup> cation to enable mixed ionic electronic conduction (MIEC). The ability to introduce small amounts of electronic conductivity would allow for controlled investigations into electrochemical performance and degradation as well as characterize the nucleation and propagation of Li dendrites in a variable electronic conductor. Diffraction analysis following solid state synthesis of Li<sub>6.4</sub>La<sub>3</sub>Ta<sub>0.6</sub>Zr<sub>1.4-x</sub>Mn<sub>x</sub>O<sub>12</sub> (x = 0.035, 0.07, 0.105) revealed successful stabilisation of the target cubic phase . However, the presence of Ruddlesden popper impurity compound LiLa<sub>4</sub>MnO<sub>8</sub> was also identified. To further investigate the affect of grain boundary located compound on the electrochemical properties, AC impedance and DC polarisation analysis were undertaken. Subsequent measurements revealed only a negligible change in electronic and ionic conductivity with only x=0.105 displaying a significant decrease in its ionic conductivity. To elucidate the stability of cycling Li through the solid state electrolyte composite, galvanostatic cycling was performed in a Li|Li symmetrical cell. Here it was found that increasing the dopant level reduced the resistance, thus increasing the critical current density. This was observed up to x=0.07 whereupon the secondary compound counteracted the effect and the critical current density decreased. Impedance spectroscopy analysis of Li in contact with the electrolyte revealed the formation of an interphase contribution. This may also be visually seen by a discolouration of the pellet. This work shows that the creation of a composite, where the secondary compound, even at low dopant levels, holding different electrochemical properties to the parent compound may affect important electrochemical attributes of the parent compound.<br/><br/><b><u>References</u></b><br/>1. Hatzell, K. B. <i>et al.</i> Challenges in Lithium Metal Anodes for Solid-State Batteries. <i>ACS Energy Lett.</i> <b>5</b>, 922–934 (2020).<br/>2. Kasemchainan, J. <i>et al.</i> Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells. <i>Nat. Mater. 2019 1810</i> <b>18</b>, 1105–1111 (2019).<br/>3. Flatscher, F., Philipp, M., Ganschow, S., Wilkening, H. M. R. & Rettenwander, D. The natural critical current density limit for Li7La3Zr2O12 garnets. <i>J. Mater. Chem. A</i> <b>8</b>, 15782–15788 (2020).<br/>4. Han, F. <i>et al.</i> High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes. <i>Nat. Energy</i> <b>4</b>, 187–196 (2019).