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

Sn/SnOx-loaded Hollow Carbon Spheres on Graphene as an Anode for Lithium-ion Batteries

Lithium-ion batteries (LIBs), due to their ability to offer a high capacity and long cyclability have established themselves as a potential high-energy storage device. Numerous developments are constantly being made to overcome the current limitations of LIBs such as volume expansion, diffusion of Li⁺ ions, etc. Researchers at Graduate School of Convergence Science and Technology and Advanced Institutes of Convergence Technology, Seoul National University, Republic of Korea have synthesized Sn/SnO_x-loaded uniform-sized hollow carbon spheres on graphene nanosheets (Sn-UHCS/G) and demonstrated its utility as a lithium-ion battery anode to overcome the limitations in LIBs.

The graphene nanosheets (G) were prepared by thermal reduction of exfoliated graphene oxide in vacuum at 300 °C for 3 h. Uniform-sized carbon coated iron oxides on graphene sheets (C@Iron-oxides/G) were prepared by heating a mixture of Fe(acac)₃, oleic acid and graphene nanosheets at 600 °C for 5 h in Ar atmosphere. The C@Iron-oxides/G spheres were etched using 3M HCl for 24 h, rinsed with DI water followed by ethanol and heated at 800 °C for 5 h to obtain uniform-sized hollow carbon spheres/graphene composite (UHCS/G) with a higher conductivity. The UHCS/G was mixed with Sn powder in a weight ratio of 5:5 and heated at 250 °C for 5 h under Ar atmosphere, which enabled loading of Sn in the UHCS/G by melt diffusion to yield Sn-UHCS/G composites. The various steps involved in the synthesis of Sn-UHCS/G is represented in Fig. 1.

Fig. 1 Various steps involved in the synthesis of Sn-UHCS/G

The SEM and TEM images of Sn-UHCS/G indicate uniform-sized hollow carbon spheres (diameter: ~ 10 nm) anchored on graphene nanosheets and the absence of any agglomerated Sn (Fig. 2). The uniform-sized carbon spheres provide a closed structure for Sn that helps to mitigate its direct contact with the electrolyte.

Fig. 2 (a) SEM; and (b, c) TEM images of Sn-UHCS/G

The first and second voltage profile curves of Sn-UHCS/G in the voltage range of 0.01 V to 3.00 V at 0.1 C are shown in Fig. 3(a). Although  the specific capacity of Sn-UHCS/G is relatively lower during the 1^st cycle due to the formation of SEI on the surface of Sn particles, its capacity is increased during the 2^nd cycle. The coulombic efficiency of Sn-UHCS/G is increased from 61.8 % for the initial cycle to 91.2 % during the subsequent cycles. Sn-UHCS/G exhibits a relatively stable capacity retention in the current density range of  0.1 to 3.0 A/g when compared to Sn/G, and pristine Sn (Fig. 3(b)). The discharge capacities of Sn-UHCS/G at 2.0 and 3.0 A/g are ~342 mA h/g and ~275 mA h/g, respectively. Cycling tests performed at 1.0 A/g indicate a relatively stable performance of Sn-UHCS/G for 1000 cycles when compared to Sn/G, and pristine Sn (Fig. 3(c)). The ability of UHCS/G to provide good electronic conductivity through improved coverage of the Sn particles on conductive carbon helps to achieve an improved performance.

Fig. 3 (a) 1^st and 2^nd discharge/charge curves of Sn-UHCS/G electrode; and (b, c) cycling performance of Sn-UHCS/G, Sn/G, and pristine Sn electrodes.

Sn-UHCS/G exhibited a good rate performance (290 mA/g at 3.0 A/g) and excellent cycle stability (284.1 mA h/g after 1000 cycles at 1.0 A/g). The better electrochemical performance of Sn-UHCS/G is due to the ability of (i) the nanosized Sn/SnO_x powders to mitigate the volume expansion during continuous cycles; and (ii) UHCS/G with a high surface area, good electrical conductivity and uniform distribution of Sn/SnO_x, which improves diffusion of Li⁺ ions as well as electrons and promotes diffusion/penetration of electrolyte in the electrode.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jeongyeon Lee et al., Sn/SnO_x-loaded uniform-sized hollow carbon spheres on graphene nanosheets as an anode for lithium-ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.127

 

Hierarchical Three-layered TiO2@carbon@MoS2 Tubular Nanostructures as Anode Materials for Lithium Ion Batteries

Lithium-ion batteries (LIBs) have received considerable attention as the power source for portable electronic devices. The anode materials used in LIBs suffer from limitations such as poor intrinsic electronic conductivity, sluggish Li⁺ ion transport kinetics and the inevitable volume change that occurs during the lithium insertion/de-insertion process. To overcome these limitations, researchers at School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore have demonstrated a multi-step synthesis route to prepare hierarchical tubular nanostructures by sequentially coating nitrogen-doped carbon (NC) layer and ultrathin MoS₂ nanosheets on TiO₂ nanotubes (designated as TiO₂@NC@MoS₂).

The multistep process involved in the synthesis of TiO₂@NC@MoS₂ tubular nanostructures is schematically represented in Fig. 1. MnO₂ nanowires (average diameter ≈40 nm) with a high-aspect ratio synthesized by hydrothermal method served as the starting template. A TiO₂ layer was deposited on the MnO₂ nanowires to develop core shell MnO₂@TiO₂ nanowires (step I). A layer of polydopamine (PDA) (thickness: 10 nm) was deposited over the MnO₂@TiO₂ nanowires to produce coaxial MnO₂@TiO₂@PDA nanowires (step II). Subsequently, the MnO₂@TiO₂@PDA nanowires were carbonized at 500 °C for 3 h under N₂ atmosphere followed by acid etching to remove the MnO₂ template (step III). In the meantime, the outer PDA layer is converted into NC shell for the core–shell TiO₂@NC nanotubes (step III). Finally, a layer of ultrathin MoS₂ nanosheets was grown on the surface of TiO₂@NC nanotubes by a hydrothermal reaction, which upon subsequent annealing (H₂/Ar atmosphere at 700 °C for 2 h) yields three-layered hierarchical TiO₂@NC@MoS₂ tubular nanostructures (step IV).

Fig. 1 Schematic of the multi-step synthesis process of TiO₂@NC@ MoS₂ tubular nanostructures: (I) TiO₂ coating; (II) PDA coating; (III) carbonizing and acid etching; and (IV) deposition of MoS₂ nanosheets and annealing.

The morphological features of TiO₂@NC@MoS₂ tubular nanotubes indicate that the hierarchical MoS₂ shell is composed of randomly assembled ultrathin nanosheets (Figs. 2(a) and 2(b)) while the TEM image (Fig. 2(c)) reveals its hollow structure.

Fig. 2 (a, b) FE-SEM; and (c) TEM images of TiO₂@NC@MoS₂ nanotubes

Galvanostatic charge/discharge voltage profiles indicate that the TiO₂@NC@MoS₂ electrode delivers high initial discharge and charge capacities of 1410 and 838 mAh/g, respectively, with a Coulombic efficiency (CE) of 59.4%. Pre-lithiation of TiO₂@NC@MoS₂ electrode is a viable option to bring the initial CE to ~100%. In spite of the low CE, the capacity quickly stabilizes after the 1^st cycle. The coincidence of the discharge–charge curves points out that the electrochemical reactions are highly stable and reversible after the first cycle (Fig. 3(a)). The average specific discharge capacity is decreased from  ≈925 to 612 mAh/g with an increase in current density from 0.1 to 2.0 A/g. However, the capacity of the electrode reverts back to 955 mAh/g when the current density is decreased from 2.0 to 0.1 A/g, thus confirming its good reversibility (Fig. 3(b)). The cycling performance of the TiO₂@NC@MoS₂ electrode indicate that it can retain a high reversible capacity of 590 mAh/g after 200 cycles.

The improved performance of the TiO₂@NC@MoS₂ nanotube electrode is due to synergetic effect of the three functional layers. In the sandwich-like structural arrangement, the inner layer of TiO₂ nanotubes serves as a skeleton of the hybrids, buffers the large volume variation of the electrode for stable cycling performance and shortens the diffusion distance of Li⁺ ions to achieve high rate capacities. The highly conductive N-doped C layer in the middle facilitates electron transfer within the hybrid, protects the overall 1D hollow structure, and prevents the MoS₂ nanosheets from restacking. The outer layer of ultrathin MoS₂ nanosheets with high surface area provides sufficient electrode/electrolyte contact area and reduces the diffusion length for the transfer of electrons and Li⁺ ions to realize a high specific capacity.

Fig. 3 Electrochemical performance of TiO₂@NC@MoS₂ tubular nanotubes for lithium storage: (a) Discharge/charge voltage profiles for the first 5 cycles at 0.2 A/g; and (b) Rate performance at various current densities.

The TiO₂@NC@MoS₂ tubular nanostructures exhibit enhanced lithium storage in terms of high capacity, long cycle life, and good rate performance and hence it can be considered as an effective electrode material for LIBs.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Sibo Wang et al., Rational Design of Three-Layered TiO₂@Carbon@MoS₂ Hierarchical Nanotubes for Enhanced Lithium Storage, Adv. Mater. 2017, 1702724, DOI: 10.1002/adma.201702724