Experimental and first-principles DFT study on the electrochemical reactivity of garnet-type solid electrolytes with carbon

The operating stability of the solid electrolyte component is one of paramount importance in the design of all-solid-state Li ion batteries. In this work, we investigated the origin of capacity fading during the charge process of an air-isolated Li ion battery with a garnet Li6.625La3Zr1.625Ta0.375O12 (LLZrTaO) solid electrolyte, a Li metal anode, and a LiFePO4 + carbon (LFP + C) active cathode material. Cyclic voltammetry measurements of the fabricated Li/LLZrTaO/(LLZrTaO + C) and Li/LLZrTaO/Al cells revealed a rise and an absence, respectively, of oxidation current which points to the garnet oxide electrolyte being decomposed via a reaction with carbon. XRD patterns of the solid electrolyte at post-charging showed no detectable impurity phases, suggesting that the decomposition product(s) is (are) likely made up of light elements. Based on first-principles calculations, the decomposition route may involve the formation of defective garnet by Li removal, oxygen release, and formation of products such as Li2CO3 and possibly of CO2; formation of the latter product is facilitated at elevated operating temperatures. Among the evaluated base garnet compounds (Li5+xLa3M2O12, with M: Nb Ta for x = 0 and M: Zr, Ti, Hf for x = 2), Li7La3Hf2O12 is predicted to be the most stable against Li2CO3 formation. A strong correlation has been determined between stability at charging and the electronegativity of the M cation, that is, the smaller the M electronegativity, the more stable is the garnet compound against carbon reactions.