Influence of Crystallinity of Lithium Thiophosphate Solid Electrolytes on the Performance of Solid‐State Batteries

Abstract

achieve high safety and competitive energy and power density. [1,2] The solid electrolyte (SE) is the key component in ASSBs and has been extensively explored for several decades. [3,4] Among the different types of solid electrolytes, lithium thiophosphates have attracted ever-increasing attention for ASSBs due to their very high ionic conductivity and facile processing at room temperature. [5,6] Recent theoretical simulations suggest that the ionic conductivity of SEs in ASSB cathodes needs to reach at least 10 -2 S cm -1 in order to obtain comparable performance with commercial lithium-ion batteries with liquid electrolytes-a target that may only be achieved with thiophosphate electrolytes. [7] Therefore, significant research effort is spent on improving the ionic conductivity of thiophosphate SEs. [8,9] Glasses in the quasi-binary system xLi 2 S•yP 2 S 5 prepared by mechanical ball milling exhibit a conductivity of up to 10 -4 S cm -1 (e.g., 75Li 2 S•25P 2 S 5 glass [7525-glass], 70Li 2 S•30P 2 S 5 glass [7030-glass]) at room temperature. [10] The conductivity is enhanced by the precipitation of metastable phases upon heating, forming glass-ceramic dispersions: crystalline phases in an amorphous matrix, in this work denoted as "gc". [11] Although very high ionic conductivities above 10 mS cm -1 have been achieved in some Solid electrolytes (SEs) largely define the properties of all-solid-state batteries (ASSBs) and are expected to improve their safety, stability, and performance. Their ionic conductivity has much improved in recent years, enabling higher power and energy density. However, more subtle parameters, such as crystallinity, may also influence the electrochemical performance of cells. In this work, the correlation between the performance of ASSBs and thiophosphate SEs having the same stoichiometry, but different crystallinity is investigated. In In/InLi | SE | LiCoO 2 @ LiNb 0.5 Ta 0.5 O 3 model cells, better cycling and rate performance is achieved when using glass/glass-ceramic SEs (e.g., 75Li 2 S•25P 2 S 5 glass, 70Li 2 S•30P 2 S 5 glass, and Li 6 PS 5 Cl glass-ceramic). This can be mostly attributed to the mitigation of contact loss by the glass/glass-ceramic SEs compared to their crystalline SE counterparts. Furthermore, the SE decomposition at typical cathode potentials is investigated by using SE and carbon composites as cathodes. Larger volume changes and more severe decomposition are observed with crystalline SEs in the SE/carbon composite cathode after cycling. The crystalline SEs show higher electronic partial conductivity which results in more degradation in the composite cathode. This work sheds light on optimized composite cathode design for ASSB by carefully choosing solid electrolytes with appropriate mechanical and (electro-)chemical properties.

Ionic Conductivity versus Particle Size of Ball‐Milled Sulfide‐Based Solid Electrolytes: Strategy Towards Optimized Composite Cathode Performance in All‐Solid‐State Batteries

“…Furthermore, our results indicate that the annealing transformation of gc‐LPSC+ to μc‐LPSC+ does not lead to any advantage for ASSB applications, if low‐energy wet milling (LWM) is carried out subsequently. A recent study of the Janek group shows that crystalline SEs suffer from additional, rather unexpected, disadvantages compared to their glass‐ceramic counterparts of the same stoichiometry [26] . Both the higher electronic conductivity and the stronger tendency to contact loss upon cycling lead, in sum, to a worse long‐term performance of microcrystalline SEs in ASSBs.…”

Solution Processing via Dynamic Sulfide Radical Anions for Sulfide Solid Electrolytes

“…The decomposition reaction kinetics is affected by the electronic conductivity of the SEs because insertion of lithium requires a supply of both ion and electron to the reaction field. [46][47][48] From the DC polarization measurements, 5PS-Li 7 P 3 S 11 -270 SEs exhibit a lower electronic conductivity (1.3 Â 10 À10 S cm À1 at room temperature) than CL-Li 7 P 3 S 11 SEs (4.7 Â 10 À10 S cm À1 at room temperature), which very likely results in the difference in electrochemical stability between CL-Li 7 P 3 S 11 and 5PS-Li 7 P 3 S 11 -270 SEs.…”

Recent progress of sulfide electrolytes for all-solid-state lithium batteries

“…The issue of volume expansion shrinking is especially severe in Li-S ASSLBs due to the violent volume difference between S particles and Li 2 S. After long cycles, the accumulated side reactions induce the pulverization of cathode particles. For this reason, the complete crystal structure of the cathode is broken, which irreversibly decreases the capacity of ASSLBs [174] .…”