%0 Journal Article
%A Cao, Chuntian
%A Shyam, Badri
%A Wang, Jiajun
%A Toney, Michael F.
%A Steinrück, Hans-Georg
%D 2019
%T Shedding X‑ray Light on the Interfacial Electrochemistry
of Silicon Anodes for Li-Ion Batteries
%U https://acs.figshare.com/articles/journal_contribution/Shedding_X_ray_Light_on_the_Interfacial_Electrochemistry_of_Silicon_Anodes_for_Li-Ion_Batteries/9764609
%R 10.1021/acs.accounts.9b00233.s001
%2 https://acs.figshare.com/ndownloader/files/17493104
%K LIB
%K Li-Ion Batteries ConspectusElectrochemical alloying reactions
%K model battery anode
%K Li x Si
%K group IV elements
%K battery operation
%K surface-normal density profile
%K half cell configuration
%K Li x Si lithiation product
%K SEI layers
%K next-generation anode materials
%K electrolyte decomposition product
%K XRR electrochemical cell
%X ConspectusElectrochemical alloying reactions of group IV elements, such as
Si, Ge, or Sn, with lithium provide a promising route to next-generation
anode materials for lithium-ion batteries (LIBs) due to their high
volumetric and gravimetric capacities. However, commercialization
of these anodes is still sparse owing to quick capacity fading and
limited Coulombic efficiency, which arise from large volume expansion
leading to particle cracking and subsequent electrochemical inactivity.
As a result, the solid electrolyte interphase (SEI), originating in
the decomposition of the electrolyte upon battery operation outside
the electrolyte’s thermodynamic stability window, grows uncontrollably.
While a large number of mitigation strategies have been developed,
an improved nanometer level fundamental understanding of the (de)lithiation
process and SEI formation, growth, and evolution is necessary to overcome
these challenges. Toward this end, many experimental and theoretical
approaches have been utilized but still provide an incomplete picture.
This is due to the difficulty of investigating buried interfaces and
interphases of lithiation products and thin SEI layers (nanometer-scale) in situ and with the desired nanometer accuracy.In
this Account, we illustrate the utilization of in situ X-ray reflectivity (XRR) to provide nanometer-scale insights on
the SEI nucleation, growth, and evolution, and well as the (de)lithiation
process of Si electrodes. XRR is a nondestructive and surface- and
interface-sensitive technique that allows for in situ investigations during battery operation under realistic electrochemical
conditions. Insight into the system is provided via the surface-normal
density profile, which is interpreted in terms of thickness, density,
and roughness of individual surface layers, allowing monitoring of
the interfacial morphology and chemistry evolution, through which
the SEI growth and Si (de)lithiation process can be resolved.We utilized a model battery anode consisting of a native oxide
terminated single crystalline Si wafer in half cell configuration
with standard electrolyte in a specifically designed in situ XRR electrochemical cell. We have resolved the nucleation and formation
process of the inner inorganic SEI and have observed two well-defined
inorganic SEI layers on Si anodes: a bottom-SEI layer (adjacent to
the electrode) formed via the lithiation of the native oxide and a
top-SEI layer mainly consisting of the electrolyte decomposition product,
LiF. This SEI layer grows during lithiation and contracts during delithiation.
Further, our results show that the lithiation of crystalline Si (c-Si)
is a layer-by-layer, reaction-limited, two-phase process with a well-defined
phase boundary between LixSi lithiation
product and c-Si; in contrast, the delithiation of LixSi and the lithiation of amorphous Si (a-Si) are
reaction-limited, single-phase processes. Moreover, we resolved the
influences of current density and the Si crystallographic orientation
of the reaction interface on the (de)lithiation process. The implications
of our findings are discussed with regard to battery performance.
%I ACS Publications