Understanding the structural evolution of Li2S upon operation of lithium‐sulfur (Li‐S) batteries is inadequate and a complete decomposition of Li2S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li2S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li2S is also unclear. Herein, an in situ transmission electron microscopy (TEM) technique implemented with a microelectromechanical systems (MEMS) heating device is used to study the precipitation and decomposition of Li2S at high temperatures. It is revealed that Li2S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li2S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li2S that is difficult to achieve at RT becomes facile with increased Li+ ion conduction at high temperatures. These results indicate that Li+ ion diffusion in Li2S dominates its reversibility in the solid‐state Li‐S batteries. This work not only demonstrates the powerful capabilities of combining in situ TEM with a MEMS heating device to explore the basic science in energy storage materials at high temperatures but also introduces the factor of temperature to boost battery performance.
The
two biggest promises of solid-state lithium (Li) metal batteries
(SSLMBs) are the suppression of Li dendrites by solid-state electrolyte
(SSE) and the realization of a high-energy-density Li anode. However,
LMBs have not met their expectations due to Li dendrite growth causing
short-circuiting. In fact, Li dendrites grow even more easily in SSE
than in liquid electrolyte, but the reason for this remains unclear.
Here we report in situ transmission electron microscopy
observations of Li dendrite penetration through SSE and “dead”
Li formation dynamics in SSLMBs. We show direct evidence that large
electrochemomechanical stress generates cracks in the SSE and drives
Li through the SSE directly. We revealed that fresh Li nucleation
sites emerged in every discharge cycle, creating new “dead”
Li in the following charging cycle and becoming the dominant Coulombic
efficiency decay mechanism in SSLMBs. These results indicate that
engineering flaw size and reducing electronic conductivity in SSEs
are essential to improve the performance of SSLMBs.
Akin
to Li, Na deposits in a dendritic form to cause a short circuit
in Na metal batteries. However, the growth mechanisms and related
mechanical properties of Na dendrites remain largely unknown. Here
we report real-time characterizations of Na dendrite growth with concurrent
mechanical property measurements using an environmental transmission
electron microscopy–atomic force microscopy (ETEM-AFM) platform. In situ electrochemical plating produces Na deposits stabilized
with a thin Na2CO3 surface layer (referred to
as Na dendrites). These Na dendrites have characteristic dimensions
of a few hundred nanometers and exhibit different morphologies, including
nanorods, polyhedral nanocrystals, and nanospheres. In situ mechanical measurements show that the compressive and tensile strengths
of Na dendrites with a Na2CO3 surface layer
vary from 36 to >203 MPa, which are much larger than those of bulk
Na. In situ growth of Na dendrites under the combined
overpotential and mechanical confinement can generate high stress
in these Na deposits. These results provide new baseline data on the
electrochemical and mechanical behavior of Na dendrites, which have
implications for the development of Na metal batteries toward practical
energy-storage applications.
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