Composite lithium metal anode by melt infusion of lithium into a 3D conducting scaffold with lithiophilic coating

Lithium metal-based battery is considered one of the best energy storage systems due to its high theoretical capacity and lowest anode potential of all. However, dendritic growth and virtually relative infinity volume change during long-term cycling often lead to severe safety hazards and catastrophic failure. Here, a stable lithium–scaffold composite electrode is developed by lithium melt infusion into a 3D porous carbon matrix with “lithiophilic” coating. Lithium is uniformly entrapped on the matrix surface and in the 3D structure. The resulting composite electrode possesses a high conductive surface area and excellent structural stability upon galvanostatic cycling. We showed stable cycling of this composite electrode with small Li plating/stripping overpotential (<90 mV) at a high current density of 3 mA/cm² over 80 cycles.Nowadays the increasing demand for portable electronic devices as well as electric vehicles raises an urgent need for high energy density batteries. Lithium (Li) metal anode has long been regarded as the “Holy Grail” of battery technologies, due to its light weight (0.53 g/cm³) (1), lowest anode potential (−3.04 V vs. the standard hydrogen electrode) (1), and high specific capacity (3,860 mAh/g vs. 372 mAh/g for conventional graphite anode) (1). It possesses an even higher theoretical capacity than the recently intensely researched anodes such as Ge, Sn, and Si (2–10). In addition, the demand for copper current collectors (9 g/cm³) in conventional batteries with graphite anodes can be eliminated by employment of Li metal anodes, hence reducing the total cell weight dramatically. Therefore, Li metal could be a favorable candidate to be used in highly promising, next-generation energy storage systems such as Li−sulfur battery and Li−air battery.The safety hazard associated with Li metal batteries, originating from the uncontrolled dendrite formation, has become a hurdle against the practical realization of Li metal-based batteries (11, 12). The sharp Li filaments can pierce through the separator with increasing cycle time, thus provoking internal short-circuiting (12). Most previous academic research to settle this bottleneck focuses on solid electrolyte interphase (SEI) stabilization/modification by introducing various additives (13–17). These electrolyte additives interact with Li quickly and create a protective layer on the Li metal surface, which helps reinforce the SEI (13–17). Furthermore, recent study in our group has also shown the employment of interconnected hollow carbon spheres (18) and hexagonal boron nitride (19) as mechanically and chemically stable artificial SEI which effectively block Li dendrite growth.In addition to the notorious Li dendrite formation, another significant factor that contributes considerably to the battery short-circuiting is the volume change of Li metal during electrochemical cycling, which is usually overlooked (20, 21). During battery cycling, Li metal is deposited/stripped without a host material. Thus, the whole electrode suffers from a virtually infinite volume change (ratio of Li metal volume at completely charged state versus at the completely discharged state is infinite) compared with the finite volume expansion of several common anodes for lithium ion batteries such as Si (∼400%) (6) and graphite (∼10%) (19). As a result, the mechanical instability induced by the virtually infinite volumetric change would cause a floating electrode/separator interface as well as an internal stress fluctuation (21). However, little attention has been paid to the volume fluctuation problem of the “hostless” Li. We propose that a host scaffold to trap Li metal inside can effectively reduce the volume change of the whole electrode and therefore maintain the electrode surface.Herein, we report a newly designed Li–scaffold composite anode and its effectiveness on addressing the safety issue of traditional hostless Li metal electrode. The preexisting scaffold serves as a rigid host with Li uniformly confined inside to accommodate the electrode-level virtually infinite volume change of Li metal during cycling. To create the composite electrode as we designed in Fig. 1A, we need to find a suitable porous material to host the Li metal. An ideal scaffold for Li encapsulation should have the following attributes: (i) mechanical and chemical stability toward electrochemical cycling; (ii) low gravimetric density to achieve high-energy density of the composite anode; (iii) good electrical and ionic conductivity to provide unblocked electron/ion pathway, enabling fast electron/ion transport; and (iv) relatively large surface area for Li deposition, lowering the effective electrode current density and the possibility of dendrite formation. By considering these aspects, we choose carbon-based porous materials to provide the required features. Specifically, an electrospun carbon fiber network (11) was used as an example to illustrate the capability of this composite anode to sustain the volume fluctuation and shape change during each electrochemical cycle.Open in a separate windowFig. 1.Schematic and optical images of Li encapsulation by melt infusion. (A) Schematic illustration of the design of a Li–scaffold composite. (B) Li wetting property of various porous materials with and without the Si coating. (C) Time-lapse images of Li melt-infusion process for lithiophilic and lithiophobic materials. See the Supporting Information for full movies.How to encapsulate Li metal inside the porous carbon scaffold presents a major challenge. Compared with many of the battery electrode materials which can be fabricated via various synthetic processes, manufacturing of Li metal-based μm- and nanostructures are very difficult due to the high reactivity of Li (1, 12). Previously, studies on Li encapsulation aimed to entrap Li through electrochemical deposition. However, the lack of spatial control of the deposition and unsmooth Li surface due to dendritic Li formation impeded such progress (13, 22). Therefore, development of versatile and simple approaches for encapsulating Li inside porous carbon or other scaffold to create Li-based composite electrodes is highly desired.Li metal possesses a low melting point of 180 °C; it would liquefy into molten Li under anaerobic atmosphere when heated to its melting point. Inspired by the fact that water could be absorbed into a hydrophilic porous structure, we develop a new strategy: melt infusion of molten Li into a “lithiophilic” matrix, which has low contact angle with liquid Li. A porous material with a thin layer of lithiophilic coating has excellent wettability with liquefied Li and thus could function as the host scaffold for Li entrapment. In this study, the aforementioned electrospun carbon fiber network modified with lithiophilic coating–silicon (Si), was used as the scaffold for Li encapsulation. Li easily and quickly flows into the fiber layer region and occupies the empty spaces between each single fiber. The resulting composite structure, denoted as Li/C, remains both mechanically and chemically stable under galvanostatic cycling; moreover, it provides a stable electrode/electrolyte interface. The effective anode current density could also be reduced due to an enlarged surface area for Li nucleation process, which in turn causes superior electrochemical performances under the same test conditions. To summarize, in contrary to the hostless Li metal, the as-proposed Li/C composite anode is able to accommodate the volume variation and therefore mitigate the potential safety hazard; moreover, the reduced current density, rooted to larger surface area, also triggers a greatly improved electrochemical performance, with stable cycling of over 2,000 mAh/g for more than 80 cycles at a high current density of 3 mA/cm².