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 (Jlimiting) 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% Jlimiting), 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, Jlimiting. (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
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