(352d) Mechanisms of Stable Cycling 3D Structured Anodes for High Energy Density Lithium Metal Batteries

Lithium (Li) metal is among the
most promising anode materials in next-generation high-energy-density
energy-storage-systems due to its ultrahigh theoretical specific capacity of
3860 mAh g⁻¹ and the lowest negative electrochemical potential
(−3.040 V vs. the standard hydrogen
electrode).^[1] However, Li dendrite growth, ¡°dead Li¡± formation and
unstable solid electrolyte interphase (SEI) have hindered its practical
applications.

3D Structured lithium metal anodes,
which possess customizable conductive framework for electron transfer and
designable pore structures for ion transfer, have been widely proposed to
settle these issues.^[2] We have
constructed several carbon material based structured anodes to investigate the
mechanisms in dendrite-free plating morphology, ¡°dead Li¡±-free stripping
morphology, and many other issues for achieving stable cycling 3D structured
anodes. Based on the unstacked graphene framework, we found that the ultralow
local current density induced by the high specific surface area of unstacked
graphene can make great progress for stable and high-performance lithium metal
anodes.^[3] Furthermore, with lithiophilic nitrogen-doped graphene
framework, metallic Li nucleation can be regulated, then even plating
morphology can be achieved.^[4] When apply the similar lithiophilic
surface to a structural stable skeleton like carbon fibers, both Li dendrite
growth and ¡°dead Li¡± formation can be inhibited, and ultimately a high Coulombic
efficiency can be achieved for high-capacity and high-rate Li metal batteries.^[5]

        However,
the mechanisms of stable cycling 3D structured anodes which can guide the
design of lithium metal anodes are heavily lacking due to the grand challenges
of current trial-and-error investigation based on complex materials innovation.
If a quantitative theoretical analysis can be proposed, reliable lithium metal
anodes with 3D host is highly expected. Thus, theoretical calculation such as phase
field models are also employed to quantitatively describe the lithium plating
and stripping process in various conductive structured lithium anodes.^[6]
We found that the structural areal surface area linearly determines the electroplating
reaction rate in the forepart kinetic process, which is limited by electron
transfer in the composite Li metal anode. Meanwhile, the structural
pore-volumetric surface area exhibits an inversely proportional relationship on
the electroplating reaction rate in later kinetic process, which is limited by
ion transfer in electrolyte. (Fig 1.) Structured lithium metal anodes with
larger areal surface area and smaller pore-volumetric surface area can be much
better for high rate and high capacity battery cycling.

Fig
1. Schematic illustration of the simulated structured lithium metal anode.^[6]

        Beyond
the design and adjustment of lithium metal anodes, further experiments and
simulations are required in revealing the mechanisms in lithium metal anodes,
such as Li plating and stripping process, dendrite growth, SEI formation and
its impact, etc., not only for 3D structured anodes. These mechanism
investigations are promising for high-energy-density lithium metal batteries
like Li¨CS and Li¨CO₂ batteries.

References:

[1]  X.-B. Cheng, R. Zhang, C.-Z. Zhao, Q.
Zhang, Chem. Rev. 2017, 117, 10403.

[2]   R. Zhang, N.-W. Li, X.-B. Cheng,
Y.-X. Yin, Q. Zhang, Y.-G. Guo, Adv. Sci. 2017,
4, 1600445.

[3]   R.
Zhang, X.-B. Cheng, C.-Z. Zhao, H.-J. Peng, J.-L. Shi, J.-Q. Huang, J. Wang, F.
Wei, Q. Zhang, Adv. Mater. 2016, 28, 2155.

[4]   R.
Zhang, X.-R. Chen, X. Chen, X.-B. Cheng, X.-Q. Zhang, C. Yan, Q. Zhang, Angew.
Chem. Int. Ed. 2017, 56, 7764.

[5]   R.
Zhang, X. Chen, X. Shen, X.-Q. Zhang, X.-R. Chen, X.-B. Cheng, C. Yan, C.-Z.
Zhao, Q. Zhang, Joule 2018, 2, 764.

[6]   R.
Zhang, X. Shen, X.-B. Cheng, Q. Zhang, Energy Storage Mater. 2019, normal">Accepted, DOI: 10.1016/j.ensm.2019.03.029