First-Principles Studies on Cation Dopants and Electrolyte|Cathode
Interphases for Lithium Garnets
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Abstract
Lithium garnet with the formula Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO) has many
properties of an ideal electrolyte
in all-solid state lithium batteries. However, internal resistance
in batteries utilizing these electrolytes remains high. For widespread
adoption, the LLZO’s internal resistance must be lowered by
increasing its bulk conductivity, reducing grain boundary resistance,
and/or pairing it with an appropriate cathode to minimize interfacial
resistance. Cation doping has been shown to be crucial in LLZO to
stabilize the higher conductivity cubic structure, yet there is still
little understanding about which cations have high solubility in LLZO.
In this work, we apply density functional theory (DFT) to calculate
the defect energies and site preference of all possible dopants in
these materials. Our findings suggest several novel dopants such as
Zn<sup>2+</sup> and Mg<sup>2+</sup> predicted to be stable on the
Li- and Zr-sites, respectively. To understand the source of interfacial
resistance between the electrolyte and the cathode, we investigate
the thermodynamic stability of the electrolyte|cathode interphase,
calculating the reaction energy for LLMO (M = Zr, Ta) against LiCoO<sub>2</sub>, LiMnO<sub>2</sub>, and LiFePO<sub>4</sub> (LCO, LMO, and
LFP, respectively) cathodes over the voltage range seen in lithium-ion
battery operation. Our results suggest that, for LLZO, the LLZO|LCO
is the most stable, showing only a low driving force for decomposition
in the charged state into La<sub>2</sub>O<sub>3</sub>, La<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>, and Li<sub>2</sub>CoO<sub>3</sub>, while
the LLZO|LFP appears to be the most reactive, forming Li<sub>3</sub>PO<sub>4</sub>, La<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>, LaFeO<sub>3</sub>, and Fe<sub>2</sub>O<sub>3</sub>. These results provide a
reference for use by researchers interested in bonding these electrolytes
to cathodes