Engineering Alloying and Conversion Interlayers for Anode‐Less Solid‐State Batteries

Lithium (Li) metal is an attractive material for the use as the negative electrode of next-generation batteries such as Li-air and Li-sulfur batteries due to its high theoretical capacity (3860 mAh g-1) and the lowest electrochemical potential (-3.040 V vs. SHE). The use of a solid electrolyte is a potential solution to these issues inherent in Li metal. Garnet-type cubic Li7La3Zr2O12 (LLZ) is promising as a solid electrolyte due to various advantages, including high Li-ion conductivity, high chemical stability against Li metal and high stiffness. However, it is known that the resistance of the Li/LLZ interface ( R int) is high, which interferes with the operation of a LLZ-based solid-state battery at a practical rate. To date, several attempts have been made to reduce R int; application of high external pressure and temperature, tuning the chemical composition of LLZ, modification of the surface morphology of LLZ by optimization of the particle and grain sizes, and the insertion of a lithiophilic layer between Li and LLZ. Although these studies have provided potential strategies to reduce R int toward the successful operation of all-solid-state batteries, there is still limited information regarding how R int dynamically changes during repetitive Li deposition/dissolution cycles.To address this issue, AC impedance spectroscopy with a three-electrode setup, in which the interface between a working electrode and electrolyte can be examined independently from the other interface between a counter electrode and the electrolyte, is necessary. In the present work, we attempted to individually trace the dynamic change in R int at a Li/LLZ interface during Li deposition and dissolution reactions through the use of the three-electrode AC impedance technique. As a result, we clarified that the trace the dynamic changes in the charge transfer resistance at the Li/LLZ interface during Li dissolution and deposition. R int increased and decreased during Li dissolution and deposition, respectively, and the increase during dissolution was not completely offset during the subsequent deposition process.Figure 1a show the time courses of the W.E. potential and R int, respectively. The overpotential increased during Li dissolution, whereas it continued to decrease during Li deposition. Rint increased and decreased during Li dissolution and deposition, respectively, which is in good agreement with the trends for the overpotential. However, Rint did not return to the initial value after one cycle of dissolution/deposition, indicating that the change in Rint during dissolution is larger than that during deposition. The degree of R int change during Li dissolution and deposition grew larger with cycle number and the cell short-circuited in less than 10 cycles.Time courses of the overpotential and R int shown in Figure 1a can be well explained by following model (Figure 1b). First, let us consider the time courses during Li dissolution. As Li dissolution progresses, the size and/or the density of the voids formed at the Li/LLZ interface is inevitably increased. Therefore, the physical contact area at the Li and LLZ interfaces decreases, which in turn increases the effective current density and thus the overpotential during galvanostatic Li dissolution. Besides, R int also increased because R int is inversely proportional to the contact area between Li and LLZ. On the other hand, when the polarization was switched, the voids formed during the Li dissolution can only be partially filled via the subsequent Li deposition process. The contact area of the Li/LLZ interface is smaller in the beginning; therefore, a larger overpotential is required for Li deposition to proceed. However, with the progress of Li deposition, the contact area is gradually increased and thereby R int can be smaller. Therefore, the overpotential is decreased in association with the decrease in R int. Figure 1a shows that R int did not return to the initial value after one cycle of dissolution/deposition, which suggests that voids were gradually accumulated at the interface as the cycle proceeded. This is a problem from the viewpoint of obtaining stable cycle characteristics because the voids inevitably induce non-uniformity of the flux of Li+ ions at the interface, which leads to the growth of Li dendrites.Based on the results obtained through the present work, we strongly encourage the development of a strategy to prevent the formation of voids at the Li/LLZ interface, particularly during Li dissolution, toward the real application of a Li metal anode in all-solid-state secondary batteries. Figure 1

Li metal-based all-solid-state batteries (ASSBs) can potentially combine the high energy of Li metal anodes and the safety of ASSBs. Among Li metal-based ASSBs, lithium-free or anodeless ASSBs are considered optimal battery configurations because of their higher energy density and economic advantages attributed to the absence of Li metal during the battery assembly process. Despite the extensive interest in Li-free ASSBs, they continue to suffer from low Coulombic efficiency and poor cycle performance. One reason for this inferior performance is the unstable interface between the current collector and solid electrolyte (SE), which can eventually lead to inhomogeneous Li deposition, dendritic Li growth, and internal short circuits. Various approaches including 3D porous anodes have been proposed to control the Li deposition behavior and improve the reversibility of anodeless ASSBs; however, there is no clarity on the mechanism and conditions for determining the Li deposition behavior in this emerging system.In this study, we systematically investigate the Li deposition behavior depending on the pore size of 3D anode and successfully demonstrate the strategy to obtain a highly reversible 3D porous anode for Li-free ASSBs. We found that more Li deposits could be accommodated within the pores of the anode with a smaller pore size using stacked Ni particles as the Li-hosting porous anode; this implies that the Li movement into the anode occurs via diffusional Coble creep. We proposed the modification of the Ni surface with carbon coating and Ag nanoparticle decoration (Ni_C_Ag particles) to further improve the Li storage capacity of the Ni-based 3D anode and, thereby, secure the interfacial contact between the 3D Ni anode and SE. The resulting Ni_C_Ag 3D anode successfully accommodated the entire Li deposit of 2 mAh cm−2 within the porous architecture without the separation of the anode/SE interface. We clarified the improved Li storage capacity of the Ni_C_Ag anode as follows.(1) C and especially Ag electrochemically react with Li ions above 0 V; thus, Li ions can be transported to 3D Ni_C_Ag porous anode before Li deposition at the SE/anode interface at < 0 V. Further, the high Li ion diffusion coefficient of lithiated carbon and Li-Ag alloy can further reduce Li ions within the pores of the 3D anode; therefore, Li deposition can occur within the porous 3D Ni anode.(2) Lithiophilic C and Ag facilitated the movement of Li via diffusional Coble creep. In particular, Ag with solid solubility in Li (Li(Ag)) can significantly enhance Li adatom mobility because of the identical structure of Li(Ag) with pure Li.(3) Li(Ag) is widely known to lower the energy barrier for Li nucleation.With the significantly reduced nucleation overpotential and interfacial resistance, the Ni_C_Ag anode showed high reversibility in Li deposition and stripping. The Ni_C_Ag anode could be cycled for more than 60 and 100 cycles with Li3PS4 and Li6PS5Cl0.5Br0.5 SE in half cells with a capacity limit of 2 mAh cm−2 and a current density of 0.5 mA cm− 2maintaining the CE of 97.9% and 96.9 %, respectively. Further, the synergistic effects of the stable anode/SE interface and reduced nucleation energy barrier enable stable NCM full-cell cycling at a room temperature of 30 °C. The NCM811 cathode/Ni_C_Ag anode full cell in the Li-free configuration showed an initial areal discharge capacity of 2 mAh cm−2, and it operated stably with a CE of 99.47% for 100 cycles. Figure 1

Introduction All-Solid-State Lithium Battery (ASSLiB) is expected as one of the next-generation batteries to achieve high power and energy density and high safety. Applying the Li metal negative electrode with a high mass capacity and low standard electrode potential to the ASSB system is desirable; however, there is a critical problem of the Li dendritic growth which induces internal short-circuits. The Li dendritic growth is known to proceed even in the voids and cracks of solid electrolytes (SEs). Moreover, because the Li metal electrode is sandwiched between a current collector and SE, high mechanical stress due to volume expansion is applied to the electrode/SE interface during charging. For conventional planer current collectors, Li metal easily penetrates into SE through voids for the relaxation of mechanical stress (Fig. 1a). In contrast, for porous current collectors, the mechanical stress will be reduced because the deposited Li-metal can fill pores. In this study, we investigated the usefulness of an Au-coated membrane filter with micro-size pores as a porous current collector for suppressing the Li dendritic growth (Fig. 1b).1 Experimental A porous current collector was prepared by cutting a commercial membrane filter (Advantec, 16080004) with 8 µm-diameter pores and 7 µm-thickness into a circular shape. Gold or nickel was coated with a sputter coater to give electric conductivity. The gold-coated porous current collector is named as Au-MF, and nickel-coated one is named as Ni-MF.A laboratory-made cell was assembled in an Ar-filled grove box. Li3PS4-glass electrolyte as SE was prepared by mechanical-milling according to ref. 1, and pelletized in a polycarbonate tube. Gold was coated on the negative electrode side of the SE pellet. The SE was put between the porous current collector and Li-In alloy as the counter electrode, followed by pressing to obtain an ASSB cell.All charge/discharge measurements were conducted by galvanostatic technique at 30 ℃. The charge/discharge current density and deposition capacity for evaluating coulombic efficiency were 0.1 mA cm-2 and 0.1 mAh cm-2, respectively. Li deposition behavior in the porous current collectors was investigated by scanning electron microscope (SEM). Results and Discussion Fig. 2(a) shows an SEM image of the Au-MF as prepared. Circular pores with 8 µm in diameter are vertically and sparsely located. After charging at 0.010 mA cm-2 for 10 h, the surface morphology of Au-MF changed like moss (Fig. 2(b)), which means Li was deposited on the surface of Au-MF. When the Ni-MF, in which Ni layer does not form any alloys with Li, was used to charge at the current density of 0.010 mA cm-2 for 10 h, Li deposits were observed only in the pores of Ni-MF and surface morphology did not change (Fig. 2(c)). These results suggest that the Li deposition on Au-MF is due to the Li diffusion in the Au layer on the MF, and the mechanical stress on the Au-MF/SE interface is relaxed by the Li deposition in pores.Fig. 3 shows Coulombic efficiencies of Au-MF, Ni-MF, planer Ni foil and Au-coated Ni-foil as a function of cycle number. The Coulombic efficiency reached to the maximum at the 4th cycle in each case. The Au-MF showed the highest coulombic efficiency of 82 %, while other electrodes showed lower than 80 % values. The charge/discharge curve for the Ni foil and Ni-MF showed the short-circuit at the 8th cycle; however, the Au-MF and the Au-coated Ni foil did not make a short-circuit for 20 cycles. These results indicate Au-MF is superior in coulombic efficiency and the suppression of Li dendritic growth to the other current collectors.We assume the synergetic effect of Au-coating and pores on the current collector. During charging, the electrochemically prepared Li metal diffuses to the pores through the Au layer and fills the pores. This effective filling of the pores with Li metal is essential for suppressing the Li dendritic growth. The Coulombic efficiency depends on the amount of the dead Li, which is formed by interruption of conduction during the dissolution of lithium deposited in dendrite form. Preventing the dendritic growth of Li metal also leads to the increase in the Coulombic efficiency.In conclusion, the Au-coated porous current collector can effectively work for inhibiting the Li dendritic growth. Acknowledgement This work was partially supported by JST ALCA-SPRING. References Shinzo et. al., ACS Appl. Mater. Interfaces (2020), 12, 20, 22798 Figure 1

Strengthened s-p orbital hybridization on selenization interphase for stable lean lithium metal batteries.

Introduction All-Solid-State Battery (ASSB) is expected as one of the next-generation batteries to achieve high power and energy density and high safety. A Li metal shows high mass capacity and low standard electrode potential, so it is a promising negative electrode active material. However, the Li metal has a critical problem such as the Li dendritic growth during charging, which induces internal short-circuits. The ASSB was expected to prevent the Li dendritic growth; however, currently it is known to proceed even in the voids and cracks of solid electrolytes (SEs). Moreover, because the Li metal electrode is sandwiched between a current collector and SE, there aren’t any spaces for Li deposition in the cell. This cell configuration enhances the mechanical pressure in Li and accelerates the Li dendritic growth in charging. We previously demonstrated that the current collector with micro-sized pores has the ability for absorbing the volumetric expansion of Li and relaxation of mechanical pressure in Li1. However, the suppression effect for the internal short-circuiting was limited because of low aperture ratio of the current collector.In this study, we used a mesh-like current collector with high aperture ratio named as a porous current collector (PCC) to introduce regularly arranged small rooms for Li metal deposition in a cell. Li metal was preferentially deposited in the pores of the PCC. We also investigated the effect of Au-coating on the PCC on the Li metal plating/stripping performance. The improved ASSB cell achieved a record-high value of critical current density (at which a short-circuit occurs) over 6 mA cm-2. Moreover, the Coulombic efficiency was 98.7 % for 300 cycles, which demonstrated highly reversible Li deposition/dissolution cycles. Experimental The PCC was prepared by cutting the commercial electroformed sieve (ASONE, S11H30) into a circular shape. Au-coating on the PCC was performed with a sputter coater.An electrochemical cell was assembled in an Ar-filled glove box. Li3PS4-glass (LPS) or 54Li3PS4-46LiI-glass (LPSLI) electrolytes were prepared by mechanical-milling, and pressed into a pellet at 360 MPa in a polycarbonate cylinder. Au thin film was coated on the negative electrode side surface of the SE pellet with a sputter coater. Then, The SE pellet was put between the PCC and Li-In alloy as the positive electrode, followed by pressing to obtain an ASSB cell.Li deposition/dissolution performances were evaluated by galvanostatic cycle tests at 30 °C. The critical current density was evaluated by increasing the current density by 0.1 mA cm-2 per cycle until the short-circuit occurs. The deposition capacity for each charge process was kept at 0.1 mAh cm-2. The charge/discharge current density and deposition capacity for evaluating coulombic efficiency were 0.1 mA cm-2 and 0.1 mAh cm-2, respectively. Results and Discussion Fig. 1(a) shows the SEM images of a pristine PCC. Square pores with a diameter of 11 µm are vertically and densely aligned. After charging at 0.064 mA cm-2, pores were filled with deposited Li (Fig. 1(b)). After peeling off the PCC from the SE, cubic shaped Li metal remained on the SE (Fig. 1(c)). After discharging at 0.064 mA cm-2, cubic shaped Li metal disappeared (Fig. 1(d)). These results suggest that the Li deposits in pores of the PCC reversibly dissolved.Fig. 2 shows the critical current density for different cell configurations. Increased critical current density means increased resistance to the short-circuit. The PCC exhibited higher critical current density than the planer Ni foil. Moreover, the LPSLI, which showed higher tolerance to the reductive decomposition with the Li metal than LPS2, also improved the critical current density. The Au-coating on the PCC greatly increased the critical current density up to 6.0 mA cm-2. Our previous result about the other PCC exhibited that the Li deposition behavior depended on the coating material, and the Li diffusion in the Au thin layer on the PCC would cause the suppression of the Li dendritic growth1.Fig. 3 shows Coulombic efficiencies for different current collectors with LPSLI as a function of cycle number. The drop of the Coulombic efficiency for the Ni foil suggests the short-circuit. The cycle number at which short-circuiting occurred increased for the PCC and Au-coated PCC. Notably, the Au-coated PCC achieved high Coulombic efficiency of 98.7 % even at 300th cycle. Such the significant improvement clearly demonstrates that the Au-coated PCC is effective for reversible deposition/dissolution of Li. Acknowledgement This work was partially supported by JST ALCA-SPRING. Reference Shinzo et al., ACS Appl. Mater. Interfaces (2020), 12, 20, 22798Suyama et al., Electrochimica Acta, 286, 158 (2018). Figure 1

Li metal is considered as one of optimal candidates for high-energy anode material because it has the highest theoretical specific capacity (3,860 mAh g-1) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode). However, safety concerns and low coulombic efficiency issues, which are caused by the inhomogeneous Li deposition/dissolution and continuous corrosion by electrolytes during battery cycling, have prohibited the use of metallic Li as anode in practical Li metal batteries. Lithium metal batteries (LMBs), which replace graphite anode with Li metal from conventional lithium ion batteries (LIBs), are considered as the most realistic approach to increase energy density of rechargeable battery. Furthermore, an anode-free LMB which features the only use of current collector in the anode compartment is the design to maximize the energy density of LMB. Because the current collector could affect the nucleation behaviors at the initial state of Li plating and the morphological development of the subsequently plated Li, it is a key component for achieving cycling stability of anode-free LMBs. Cu current collector is the most widely adopted current collector for the anode of LIBs and LMBs due to its high conductivity, mechanical property, and electrochemical stability, but is known to have a large Li nucleation overpotential in comparison with Au or Ag. That is because the binding energies of Li atom on bulk Li is much larger than that of Li atom on bulk Cu (which are around 24~30 and 2.5~2.7 kcal mol-1, respectively). Therefore, the surficial modulation of Cu current collector is needed to improve the affinity between Li metal and Cu and induce uniform initial Li deposition on Cu current collector. Cu collectors used for lithium ion battery, which are fabricated by electrodeposition method, have various crystalline facets on their surfaces. On the basis of the principle of nucleation and growth, the degree of crystalline misfit between adsorbent and substrate can vary depending on the surface facet of substrate. We believe that it can be a key principle for manipulating an initial Li nucleation mode on Cu substrate. In detail, we explore a facet selective Li nucleation and growth phenomenon on Cu and demonstrate that controlling the facet structure can improve the uniformity in Li deposition and the cycling stability. Preferential Li deposition on the Cu(100) plane is demonstrated by electrochemical analysis of the Cu single crystal surfaces and by EBSD analysis of the Li-deposited Cu surfaces. DFT calculations show that a difference in the Li adsorption energy during the initial Li deposition process among the Cu facets is responsible for the facet selectivity. A majorly (100) plane-orientated Cu foil fabricated with a simple annealing method has a more uniform Li nucleation with a 6-times higher nuclei density and a two-fold enhancement in the Li cycling stability compared with a conventional Cu foil with randomly oriented surface facets. The control of the surface facet provides a new design principle for the current collector of lithium metal batteries.

The anode-free lithium metal battery (AFLMB) is attractive for its ultimate high energy density. However, the poor cycling lifespan caused by the unstable anode interphase and the continuous Li consumption severely limits its practical application. Here, facile one-step heat treatment of the Cu foil current collectors before the cell assembly is proposed to improve the anode interphase during the cycling. After heat treatment of the Cu foil, homogeneous Li deposition is achieved during cycling because of the smoother surface morphology and enhanced lithiophilicity of the heat-treated Cu foil. In addition, Li2O-riched SEI is obtained after the Li deposition due to the generated Cu2O on the heat-treated Cu foil. The stable anode SEI can be successfully established and the Li consumption can be slowed down. Therefore, the cycling stability of the heat-treated Cu foil electrode is greatly improved in the Li|Cu half-cell and the symmetric cell. Moreover, the corresponding LFP|Cu anode-free full cell shows a much-improved capacity retention of 62% after 100 cycles, compared to that of 43% in the cell with the commercial Cu foil. This kind of facile but effective modification of current collectors can be directly applied in the anode-free batteries, which are assembled without Li pre-deposition on the anode.

Solid electrolytes (SEs) that incorporate lithium (Li) metal anodes provide a promising pathway to improve battery energy density and safety. However, Li metal anodes still suffer from multiple challenges, including 1) uniform Li deposition and dissolution, 2) continual formation of a solid-electrolyte interphase (SEI), and 3) limited rate capability. To control the interfaces, predefined interlayers such as gold, ZnO, carbon, and other coatings have been used tune the Li metal and SE interface. However, there is a need for improved fundamental understanding of the mechanistic role of these interlayers on performance and stability. In this work, we explore the application of an amorphous carbon interlayer in an argyrodite SE system, which is used for in situ Li metal formation in an “anode-free” configuration.The incorporation of a carbon interlayer has been previously shown to act as a physical barrier between the SE and in situ formed Li metal, improve the uniform contact and current distribution across the SE/anode interface, and potentially suppress dendrite propagation [1]. However, the mechanism of this improved performance is not fully understood. Therefore, to study the influence of the carbon interlayer on interfacial performance, a detailed electrochemical analysis was performed.First, the transition in reaction pathway from carbon lithiation to Li nucleation and growth was observed to change as a function of charging rates. To probe the lithiation effects of the carbon interlayer on subsequent Li deposition, incremental electrochemical impedance spectroscopy (EIS) and current-interrupt analysis was performed to study the dynamic changes of the carbon interlayer during the charging process prior to Li nucleation. To confirm the mode of metallic Li nucleation and growth, operando video microscopy and post-mortem focused ion-beam/scanning electron microscopy (FIB-SEM) was utilized. The preferential location of Li deposition and growth at the current collector and carbon interface confirms the transport of Li-ion through the lithiated carbon interlayer. Moreover, the Li metal deposition behavior was observed to depend on the electrochemical properties of the carbon interlayer during the lithiation process. These fundamental electrochemical studies will further our understanding of the design requirements for interlayers that can enable Li metal anodes in solid-state battery systems.[1] Y.G. Lee, S. Fujiki, C. Jung, N. Suzuki, N. Yashiro, R. Omoda, D. S. Ko, T. Shiratsuchi, T. Sugimoto, S. Ryu, J. H. Ku, T. Watanabe, Y. Park, Y. Aihara, D. Im, I. T. Han, Nat. Ener. 5, 299-308 (2020)

Lithium (Li) has been considered as the promising negative electrode in rechargeable batteries, owing to the negative redox potential (-3.04 V vs. SHE), low density (0.534 g/cm3), and high theoretical specific capacity (3,860 mAh/g). However, the non-uniform stripping/plating process of the metallic Li is still challenging.[1] As one of the solutions, three-dimensional (3-D) metallic scaffolds were introduced to the negative electrode. The uniform electric field applied in the porous scaffolds could suppress the dendritic growth of the Li during the plating process. In addition, a decrease in the effective current density prolonged Sand’s time.[2] However, such a Li deposition mainly proceeded on the top surface and led to rapid surface clogging due to the strong surface electric field. In this presentation, we show the preferred Li deposition inside the 3-D copper (Cu) scaffold using gold nanoparticles (Au NPs). The lower Li nucleation energy barrier on the Au surface than that on the typical Cu current collector was investigated intensively.[3] However, the role of Au for spatially promoting Li deposition was not studied in depth. We added the Au NPs with an average particle size of ~7.5 nm to the bottom of 3-D Cu. The Au NPs with a total ~28 nmol reduced the charge-transfer resistance, which suggested the fast Li nucleation on the Au. In addition, we coated aluminum oxide (Al2O3) layer with ~ 26 nm of thickness on the top surface of 3-D Cu scaffold through the atomic layer deposition (ALD) process. This insulating layer further suppressed the Li deposition on the surface. As a result, apparent Li deposition towards the bottom of 3-D Cu was clearly demonstrated. During Li plating/stripping cycles, the surface clogging of the 3-D Cu/Au by an accumulation of solid electrolyte interphase (SEI) was also dramatically alleviated even in the absence of Al2O3 layer. It demonstrated the continuous Li deposition towards the bottom of the 3-D Cu/Au scaffold. Besides, symmetric 3-D Cu/Au cells exhibited over 95 % of Coulombic efficiency (CE) for ~120 cycles. The full cells comprised of the 3-D Cu/Au and LiFePO4 (LFP) electrodes delivered stable 300 cycles with a capacity of ~140 mAh/g at 1 C-rate. I will discuss the detailed role of the Au NPs and the Al2O3 layer, and comparative cell performances in the presentation.

Homogeneous Li deposition guided by ultra-thin lithiophilic layer for highly stable anode-free batteries

Lithium metal is the most promising anode material for high‐energy‐density batteries due to its high specific capacity of 3860 mAh g−1 and low reduction potential of −3.04 V versus standard hydrogen electrode. However, huge volume change, safety concerns, and low efficiency impede the practical applications of Li metal anodes. Herein, it is shown that the nitrogen‐doped graphene modified 3D porous Cu (3DCu@NG) current collector can mitigate the above problems. The N‐doped graphene, coating on the surface of 3D current collector, not only contributes to a uniform Li+ flux, but also leads to a scattered distribution of electrons throughout the surface, finally contributing to a uniform Li deposition and the improved electrochemical performance. In addition, the continuously porous structure of 3DCu@NG provides a space for the metallic Li deposition and could effectually accommodate the volume expansion during cycling. As a result, the Li‐3DCu@NG anode exhibits a high areal capacity of 4 mAh cm−2, a high Li utilization of ≈98%, and an ultralow voltage hysteresis of ≈19 mV. The multifunctional N‐doped graphene modified 3D porous current collector promisingly provides a strategy for safe and high‐energy lithium metal anodes.

Shielded electric field-boosted lithiophilic Sites: A Janus interface toward stable lithium metal anodes

Electrification of the transportation sector has been an important development in the global renewable energy transition. However, electric vehicle adoption has been hindered by concerns about the range from a full charge, as well as opposition to hours-long charging times. Thus, more energy-dense batteries that can withstand aggressive fast charging conditions are essential to the widespread adoption of electric vehicles. While lithium-ion batteries have been the staple for rechargeable products, including electric vehicles, they are approaching their theoretical energy density limits. Lithium metal batteries (LMB) present a solution to concerns about vehicle range since Li metal plates directly onto the Cu current collector, drastically improving the energy density.Commercialization of the LMB has been limited due to a variety of degradation mechanisms, including the formation of high surface area filamentary lithium deposits, known as dendrites, at the anode and capacity loss due to repairing the fractured solid electrolyte interphase (SEI). Under fast charging conditions, the formation of dendrites is exacerbated because 1. high current densities pin the anode to high potentials, supporting the formation of small, high surface area lithium deposits and 2. entering the Sand’s time regime (when Li+ concentrations at the electrode drop to zero) confines Li plating to the tips of Li deposits. Additionally, fast charging conditions exacerbate the breakdown of the SEI by intensifying ion concentrations at low impedance fractures in the SEI.In our work, we aim to make LMBs viable for fast charging conditions by growing resistive Al2O3 thin films via atomic layer deposition (ALD) on the Cu current collector using TMA and water. The deposited thin films, although resistive, contain defects of low resistance which serve as the sites of Li nucleation. These nucleation sites then act like ultramicroelectrodes and encourage radial diffusion of Li+,which in turn leads to the formation of low surface area, dense, planar Li deposits. Dense and low surface area Li morphology is known to be favorable as it reduces Li corrosion, as well as SEI fracturing due to volume expansion. ALD ensures the thickness is precisely varied to identify the thin film resistance at which favorable Li deposition morphology and SEI quality are best conserved under fast charging conditions. Scanning electron microscopy (SEM) results show that despite high current densities pinning the anode to high potentials, low surface area Li deposition morphology is preserved at current densities up to 20 mA/cm2 . Additionally, the resistive films slow down electron transfer to delay the onset of Sand’s time. X-ray photoelectron spectroscopy (XPS) also shows that the Al2O3 thin films promote higher incorporation of anionic species into the SEI than the bare Cu current collector control under fast charging conditions, a difference that we attribute to the presence of acidic sites on the Al2O3. Anion-derived SEIs are more homogeneous, strong, and robust. Collectively, these improvements in the SEI and Li properties lead to cells made with the Al2O3-coated current collectors having enhanced performance, including Coulombic efficiencies of >80% for three times as many charge/discharge cycles as the control at 5 mA/cm2.

Design of networked solid-state polymer as artificial interlayer and solid polymer electrolyte for lithium metal batteries

Introduction Among all the possible secondary battery systems, Li metal-based batteries have attracted increasing interest in recent decades due to the unapproachable theoretical specific capacity (3860 mAh g-1) and low redox potential (-3.04 V vs. NHE) of Li. However, the growth of lithium dendrites will cause security problems which restrict the application of lithium metal. According to previous work, the current collector could play an important role in the protection of the lithium metal. In this work, Here we demonstrate that a hollow carbon nanofiber with proper interior to exterior radius ratio can enable Li-ions to deposit on the inner surface of the channels selectively due to the drifting effect from the structural stresses. Based on this principle, a lotus-root like structure is further designed to realize a dendrite-free hybrid Li anode with a high Li loading capability. The lotus-root like carbon nanofiber (LCNF) anode, with being coated by a lithiated Nafion (LNafion) layer as artificial solid electrolyte interface (SEI), achieves a capacity of >3600 mAh gcarbon-1 for Li deposition/stripping along with a greatly improved CE. Result and discussion Fig. 1 shows the schematic diagrams of LCNF and the solid carbon nanofiber (SCNF, the sample prepared via the same route as LCNF but without PS addition in the precursor solution) before and after Li deposition. The LCNF was prepared by a simple electrostatic spinning route followed by high-temperature carbonization process. For the carbonizing process of PAN, it is widely accepted that PAN undergoes a cyclization reaction and forms a highly conjugated structure, which rending the resulting material both insoluble and infusible. At the PAN/PS ratio of 1:0.5, the LCNT with more than 10 parallel channels (~50 nm) was obtained. The transmission electron microscopy (TEM image (Fig. 1F) clearly demonstrate that the longitudinal straight multi-channels are homogeneously distributed in carbon nanofibers parallelly. Carbon nanofibers with uniform size (~500 nm) are intertwined into the freestanding LCNF paper, and the external surface of the fibers is smooth and clean (Fig. 1J). Compared with LCNF, SCNF has no channels inside (Fig. 1E) and appears predisposed to fracture (Fig. 1I). Fig. 2G and 2H show the TEM images of Li-LCNF and Li-SCNF (Li deposition state, after 30 cycles at a current density of 1 mA cm-2 for 8 mAh cm-2 (it amounts to 3640 mAh gcarbon -1 or 0.94 g Li infilled 1.0 g carbon calculated based on the areal density of LCNF of 2.2 mg cm-2) . It can be clearly seen that a large amount of Li uniformly deposits within the inner channels while the outer surface of the nanofibers is relatively clean (Fig. 1L). The diameter of the Li-filled LCNF keeps almost the same as that of the original LCNF before Li deposition. As expected, Li ions are preferentially deposited on the inner surface of the channels, and thus the outer surface of the Li-LCNF anode is free of Li dendrites. In sharp contrast, for the Li-SCNF sample with the same amount of Li deposition, Li ions only deposits on the outer surface (Fig. 1G), and lots of mossy deposits can be observed (Fig. 1K). In summary, a new strategy to control Li deposition via matrix geometry design is proposed. We demonstrate that a hollow carbon nanofiber with proper interior to exterior radius ratio can control Li deposition via the drifting effect from the structural stresses. The lotus-root like carbon nanofiber matrix we further developed enables Li preferentially to deposit on the inner surface successfully and therefore the stable carbon shell provides a strong barrier to suppress Li dendrite. Furthermore, the multichannel structure also endows the LCNF anode with a high Li loading capability due to its abundant inner hollow space. With the aid of the Nafion SEI, the Li-LCNF anode realizes a high specific capacity of >3600 mAh gcarbon -1 for Li deposition/stripping along with a greatly improved CE. The CE reaches over 99% and maintains in the following long-term cycles. Figure 1