Developing Strong Texture in Lithium Deposits Helps Designing Future Anode Materials for Li-Metal Batteries

Lithium–sulfur and lithium–air batteries have been used as high-energy storage systems for electric vehicles. Poor cycling efficiency and safety are the major limitations of Li-metal batteries and they arise due to the uncontrollable Li deposition process. Researchers at Department of Materials Science and Engineering and Stanford Nano Shared Facilities, Stanford University, USA and Stanford Institute for Materials and Energy Sciences, Stanford Linear Accelerator Center National Accelerator Laboratory, USA have performed some fundamental studies on electrodeposition of Li and characterized the resultant films using X-ray diffraction (XRD), morphological characteristics and Pole-figure analysis to establish a correlation between the crystallographic texture with the morphology of Li deposits. The fundamental understanding of electrocrystallization of Li helps to rationalize the use of suitable additives or inhibitors in the Li battery electrolytes and provides an insight on the design of future lithium anode materials for high-energy-density batteries.

The morphological features and texture of electrodeposited Li on lithium anodes of Li–S and Li–O₂ full-cell batteries using carbonate and ether based electrolytes with and without additives (inhibitors) viz., lithium polysulfides and LiNO₃ were investigated. Whisker-shaped elongated Li deposits are obtained using ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol/vol), 1 M LiPF₆ (Figs. 1(a) and 1(b)). In spite of a decrease in size with an increase in current density from 0.1 to 5 mA/cm², the characteristic shape of the Li deposits does not change, suggesting that current density has a less-pronounced role on the morphology of Li deposits. Li deposits obtained using 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 vol/vol), 1 M LiTFSI, 1% LiNO₃ possess a characteristic rounded shape (Figs. 1(c) and 1(d)) and a similar effect of current density on the size and morphology of Li deposits is also observed in this electrolyte as that of the carbonate electrolyte. Li deposits obtained using 5 M poly-sulfide as catholyte in Li–S full battery are round-shaped and addition of polysulfide in DOL/DME with 1% LiNO₃ leads to the formation of Li deposits that are uniform and round-shaped, even at higher current density and on the edge of the current collector (Figs. 1(e) ad 1(f)). Polysulfide and LiNO₃ exert synergetic effects to prevent the growth of whisker-shaped deposits. Li deposits obtained using ether-based electrolytes in a Li–O₂ full battery also exhibits round particle morphologies without any dendrite formation (Figs. 1(g) and 1(h)). The morphological features reveal that beyond current density and electrolyte solvent identity, the specific additives (e.g., LiNO₃) or the cross-over molecules (O₂, polysulfide) from the cathode side play a major role in determining the morphology of Li deposits.

Fig. 1 Morphology of Li deposits obtained using various electrolyte systems at 0.1 mA/cm², 1 mAh/cm²: (a and b) EC/DEC 1 M LiPF6; (c and d) DOL/DME 1 M LiTFSI, 1% LiNO₃; (e and f) Sulfur catholyte 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃; (g and h) TEGDME 1 M LiTFSI with Li₂O₂ as cathode (Scale bars: a, c, and g, 5 μm; e, 20 μm; d and f, 2 μm; b and h, 1 μm.)

XRD pattern indicates that Li (110) and Li (200) are the two major peaks of Li. Hence, pole figures were collected at 2θ angles of 36.19° and 51.97° corresponding to the locations of (110) and (200) Bragg peaks, respectively. The pole figures for plain Li metal foil show [100] out-of-plane preferred orientation (Fig. 2(a)). The (110) and (200) pole figures for Li electrodeposits obtained using various electrolytes are shown in Figs. 2(b)-2(d). The (110) pole figure of Li deposits obtained using EC/DEC electrolyte shows a disk-shaped, radially uniform diffraction intensity distribution, which indicates that the film’s texture is not clearly pronounced (Fig. 2(b)). It is evident that random orientations of the whiskers result in a rather broad distribution of crystallographic grain orientations. The (110) pole figures of Li deposits exhibit a sharp intensity concentration around ψ = 0°, indicating that the round shaped Li deposits are mostly textured with (110) planes parallel to the electrode substrate (Figs. 2(c) and 2(d)). The more pronounced [110] texture is due to the strong adsorption of the LiNO₃ and polysulfide additives during the crystal growth. It is difficult to establish a correlation between the morphology and texture of lithium deposits with the current density as well as with their SEI layer. Nevertheless, adsorption of inhibitor molecules (additives) in the electrolyte seems to be the dominant factor that leads to texturing of electrochemically deposited Li.

Fig. 2 Pole-figure [left side: (110) and right side: (200)] analysis of Li films: (a) Li metal foil; (b) Li deposit in EC/DEC 1 M LiPF₆; (c) Li deposit in DOL/DME 1 M LiTFSI, 1% LiNO₃; and (d) Li deposit in sulfur catholyte 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃.

A growth diagram of Li is proposed consolidating the effects of commonly used additives and current density on lithium morphology (Fig. 3). Li deposits grown over the limiting current densities (J_limiting) is ramified and dendritic, a common inference in cells with large electrode spacings (Fig. 3(a)). At current densities far below the limit (0.1% J_limiting), in the case of no/weak inhibition (EC/DEC and DOL/DME electrolyte without any additives), the deposits usually show a whisker-like shape (Figs. 3(b) and 3(c)). In presence of inhibitors (HF and CsPF₆ as additives), when the extent of inhibition is increased, a large number of elongated crystals grow perpendicular to the substrate, resulting in the formation of coherent Li deposits (Fig. 3(d)). In presence of strong inhibitors, such as LiNO₃ or polysulfides, field oriented texture-type deposits are emerged (Figs. 3(e) and 3(f)). Such strong texturing generates compact Li deposits with a reduced surface area, lesser SEI formation, lower electrolyte consumption, and less dead lithium, which consequently improves cycling efficiency.

Fig. 3 Growth of lithium electrodeposits as a function of current density J and additives in electrolyte with inhibition intensity increasing in the horizontal direction: (a) optical image of dendritic and ramified Li deposit in EC/DEC 1 M LiPF6, below 0.1% of diffusion-limited current density, J_limiting. (b-e) SEM images of lithium deposit in (b) DOL/DME 1 M LiTFSI; (c) EC/DEC 1 M LiPF₆; (d) EC/DEC 1 M LiPF6, 100 ppm H₂O; (e) DOL/DME 1 M LiTFSI, 1% LiNO₃; and (f) sulfur catholyte, 5 M S8 dissolved in DOL/DME 1 M LiTFSI, 1% LiNO₃. (Scale bar: A, 200 μm; B, E, and F, 2 μm; C and D, 1 μm.)

It has been established that the morphology of the electrodeposited Li film is intrinsically determined by its crystallographic texture. Strongly textured Li represents compact, well-aligned deposits, while weak/non-textured Li points out mossy and whisker-like structure. Additives in electrolytes and the cross-over molecules from the cathode (O₂, polysulfide) play a critical role in determining the crystallographic texture because they hinder the cathodic process and selectively adsorb on different crystal planes. A growth diagram is proposed to correlate the texture and morphology of Li deposits. The electrolytes with additives of lower exchange current density is likely to generate Li deposits with a stronger texture and a uniform morphology. The fundamental understanding obtained from the correlation of texture with morphology helps designing new types of additives to produce Li deposition with controllable texture, which will form the basis for development of future lithium metal batteries.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: F. Shi et al., Strong texturing of lithium metal in batteries, PNAS, 114 (46) (2017) 12138-12143, November 14, 2017, doi: 10.1073/pnas.1708224114

Achieving Improved Stability of Li Anode through Dendrite Free Li Deposition using Ag Nanoparticles

Lithium metal batteries due to their high energy densities assume significance in portable electronics and electric vehicles. Nevertheless, dendrite formation during deposition, instability of the Li metal interface, and huge volume change are the major limitations that need to be solved for their effective utilization. The uncontrolled dendrite-growth of Li is considered responsible for the low reversibility, short cycle life, internal short circuiting, and safety hazards. Researchers at Department of Materials Science and Engineering, University of Maryland at College Park, USA have demonstrated a rapid Joule heating method to anchor Ag nanoparticles (Ag NPs) on carbon nanofibers (CNFs) to guide seeded nucleation and growth of Li to obtain smooth Li metal anode without dendrite growth.

CNFs prepared by electrospinning served as the host material. They were soaked in silver acetate and rapidly heated by Joule heating setup (Fig. 1(a)). When heated above the melting point of Ag (962 K) for only 0.1 s, the molten Ag gets self-assembled as Ag NPs. The high temperature promotes a strong bonding between Ag NPs and CNFs. The defects in the CNFs constrain the migration of Ag NPs. Rapid quenching of the CNFs seeded with Ag NPs below the melting point of Ag prevents agglomeration of Ag NPs. Fortunately, the CNFs could withstand the  thermal shock and preserved its graphitic structure.

Fig. 1 (a) Schematic of the Joule heating method for coating Ag NPs on CNFs (inset: morphology of CNFs prepared by electrospinning); (b) Digital image of the Joule heating set up. The sample was connected to Cu electrodes and heated by a current pulse in Ar-filled glove box.

The morphologies of Ag NPs on CNFs obtained by Joule heating for 0.05, 0.1, 0.5, and 4 s (Fig. 2) indicate that they are homogenous with an average size of 29–57 nm. The size of Ag NPs show a strong dependence on the thermal shock time; the shorter the time, the lesser the particle size (Fig. 2).

Fig. 2 SEM images of Ag NPs deposited on CNFs by Joule heating method  for (a) 0.05 s; (b) 0.5 s; and (c) 4 s.

The nucleation and growth of Li seeded by Ag NPs on CNFs is schematically represented in Fig. 3(a). Due to the zero nucleation overpotential, selective nucleation of Li occurs on AgNP/CNFs (Fig. 3(c)). Plating of Li is proceeded by alloying of Li with Ag NPs. The strong anchoring of Ag NPs on CNFs guides the formation of a smooth Li coating. During the growth stage, Li from AgNP/CNFs gradually fills the voids between the CNFs, resulting in the formation of an even Li metal anode without dendrite growth (Fig. 3(d)). The ability of the Ag NPs strongly bound on to CNFs to retain itself on the surface of the anode even after stripping of Li (Fig. 3(e)), could repeatedly guide the seeded nucleation of Li. Figs. 3 (f) and 3(g) show the inability of bare CNFs to promote uniform deposition of Li metal, due to the poor wettability of CNFs with Li, thus justifying the beneficial role of Ag NPs.

Fig. 3 (a) Schematic of Li nucleation and growth seeded by Ag NPs on CNFs; (b-g) SEM images: (b) pristine AgNP/CNFs without Li deposition; (c) initial Li nucleation on AgNP/CNFs; (d) Li deposited on CNFs guided by Ag NPs at 1 mA h/cm² of; (e) AgNP/CNFs after the first plating/stripping cycle: (f) bare CNFs without Ag nanoseeds; and (g) Li deposited on bare CNFs

The cycling performance of Li metal anodes using AgNP/CNFs as host (size of Ag NPs ≈40 nm) indicate an exceptional cycling stability at 0.5 mA/cm² for 500 h without short-circuiting with a high Coulombic efficiency of ≈98%. In contrast, self-nucleation of Li resulting in the formation of pillars and dendrites of Li metal dramatically decrease cycling stability of bare CNFs to 100 h. The discharge/charge profiles of Li anode seeded by AgNP/CNFs show a low overpotential (≈25 mV) with a negligible nucleation overpotential at 0.5 mA/cm², which is likely to promote a controlled growth of Li. In contrast, plating or stripping of Li on bare CNFs is accompanied with an initial nucleation overpotential.

The CNFs host modified by Ag NPs effectively regulates the deposition of Li, thus enabling the formation of a smooth Li anode without dendrites. The Li metal anodes developed using AgNP/CNFs exhibits a low voltage overpotential, an exceptional cycling stability and avoids problems due to short-circuiting.

T.S.N. Sankara Narayanan.

For more information, the reader may kindly refer: Chunpeng Yang et al., Ultrafine Silver Nanoparticles for Seeded Lithium Deposition toward Stable Lithium Metal Anode, Adv. Mater. 2017, 1702714, DOI: 10.1002/adma.201702714