Stability of Interfaces in All-Solid-State Lithium Batteries
Authors: Barcha Cédric , Seznec Vincent , Delpuech Nathalie , Dedryvere Remi , Madec Lénaïc , Lethien Christophe , Hallot Maxime , Masquelier Christian 
Published: July 2021

High energy density batteries are needed with the rapid growth of electric vehicles market and the increasing demand for energy storage worldwide. Today, conventional lithium-ion batteries have some issues related to the use of flammable liquid electrolytes (safety problems). In addition to thermal stability issues, liquid electrolytes do not presently allow the use of lithium metal as a negative electrode due to the growth of uncontrollable dendrites during cycling. However, increasing the energy density in batteries requires to use metallic lithium which has a great specific capacity (≈ 3800 mAh / g). The substitution of liquid electrolytes by solid electrolytes within electrochemical energy storage devices is one of the most widely promising solutions today. Sulfide-based solid electrolytes are promising candidates for all solid-state batteries’ technology due to their high ionic conductivity, availability of different synthesis routes and low temperature processability, which are critical requirements for scalable fabrication of all-solid-state batteries (ASSB).

Many recent studies have demonstrated the reactivity at the interfaces of solid electrolytes with lithium metal during electrochemical cycling, leading to bad electrochemical properties. Indeed, when argyrodite solid electrolyte (Li6PS5Cl) is used in contact with metallic lithium, reduction into Li2S, LiCl and Li3P (5-7) occurs leading to an SEI formation not properly controlled and understood. There is hence a strong need to master the solid electrolyte / lithium metal interface. Moreover, argyrodite is also oxidized by positive active materials during electrochemical cycling, into P2Sx (x ≥ 5), polysulfides, elemental sulfur and phosphates (7-9).

The aim of our study is to master the interface between argyrodite and lithium metal, in order to reduce the reactivity, and increase the stability and life cycling. The use of selected protective layers, stable versus lithium and argyrodite will be presented. In our case we decided to work with lithium chloride (LiCl) and lithium sulfide (Li2S), which are relatively stable against lithium metal and argyrodite, as they come from the decomposition products of argyrodite. Dip-coating was used at the beginning, but surface coverage was not good enough, that’s why we developed a new way of coating called slow-evaporation process, which allows to increase the surface coverage of lithium electrode. We also believe that coated directly argyrodite pellet by Li3PO4 and Al2O3 could also decrease the reactivity at the interface. To do that we use atomic layer deposition (ALD), which allow to control the thickness of the coating at nanometer scale, and have a good surface coverage. The impact of temperature on surface coverage, and the coating thickness on reactivity and electrochemistry will be discussed. All these coatings were characterized by SEM, XRD, RAMAN, and XPS to analyzed interfaces chemical reactivity before and after cycling. In addition to interface stability, mechanical stability is also an important issue for ASSBs, leading to dendrites formation during cycling (10)(11). We will also present some strategies to increase the mechanical stability of argyrodite by using selected polymers as PTFE, PEO, or PVdF. Additionally, in order to reduce the oxidation of argyrodite at the positive electrode, a coating of Li4Ti5O12 on NMC111 by solution and spray-dryer will be also discussed.

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