Anode-free Batteries Research Articles

Research Progress of Anode-Free Lithium Metal Batteries

Lithium-metal batteries (LMBs) are regarded as the most promising candidate for practical applications in portable electronic devices and electric vehicles because of their high capacity and energy density. However, the uncontrollable growth of lithium dendrite reduces its cycling ability and even causes a severe safety concern, which impedes the development of the technology. Although great efforts have been devoted to solving the lithium dendrite issue in recent years, the contradiction between the high cost of thin Li foil and the severe safety hazard of excess Li still exists. This is precisely the factor that inspired the development of anode-free lithium-metal batteries (AFLMBs). Compared to lithium-metal batteries, AFLMBs with a zero-excess Li anode possess an incredible, conceivable, and specific energy. Additionally, because the use of metal lithium is limited, the battery manufacturing will be safer and simpler, leading to a significant decrease in cost. However, comprehensive reviews on anode-free batteries are rare. Therefore, in this review, we aim to explain the essential development factors influencing the cycle life, energy density, cost, and working mechanism of anode-free batteries. We summarize different strategies to improve the cycling stability of AFLMBs, and we discuss the application of anode-free electrodes in other electrochemical energy storage systems. Moreover, it is believed that the combination of modification techniques, including electrolytes and current collectors, and the application protocols will be the most important solution for future anode-free batteries.

Quantifying the apparent electron transfer number of electrolyte decomposition reactions in anode-free batteries

<h2>Summary</h2> Solid electrolyte interphase (SEI) is pivotal in dictating the stability of anodes in non-aqueous batteries. However, electrolyte decomposition mechanism as an indispensable piece of the puzzle to construct a stable SEI is with few quantitative understandings. Herein, as a quantitative descriptor, the apparent electron transfer number (ETN) is acquired by a facile yet precise methodology in working lithium metal batteries with lithium bis(fluorosulfonyl)imide (LiFSI)-dimethyl carbonate (DMC)-based localized high-concentration electrolyte. Through accurate measurements of the electrolyte evolution and concurrently accumulated inactive Li by electrolyte quantitative nuclear magnetic resonance (ely-qNMR) and titration-qNMR, respectively, the decomposition rates of different electrolyte components and ETNs that define the fate of electrolyte can all be acquired in a "one-stop" fashion. The recognition of ETNs (1.0 for DMC and 5.1 for LiFSI) provides pioneering insights into the electrolyte decomposition mechanism and affords new visions for electrolyte design to promote the continuous rise of non-aqueous rechargeable batteries.

Carbon Nanotube Current Collector for Anode-free Battery

Carbon Nanotube Current Collector for Anode-free Battery

Development of quasi-solid-state anode-free high-energy lithium sulfide-based batteries

Anode-free lithium batteries without lithium metal excess are a practical option to maximize the energy content beyond the conventional design of Li-ion and Li metal batteries. However, their performance and reliability are still limited by using low-capacity oxygen-releasing intercalation cathodes and flammable liquid electrolytes. Herein, we propose quasi-solid-state anode-free batteries containing lithium sulfide-based cathodes and non-flammable polymeric gel electrolytes. Such batteries exhibit an energy density of 1323 Wh L−1 at the pouch cell level. Moreover, the lithium sulfide-based anode-free cell chemistry endows intrinsic safety thanks to a lack of uncontrolled exothermic reactions of reactive oxygen and excess Li inventory. Furthermore, the non-flammable gel electrolyte, developed from MXene-doped fluorinated polymer, inhibits polysulfide shuttling, hinders Li dendrite formation and further secures cell safety. Finally, we demonstrate the improved cell safety against mechanical, electrical and thermal abuses.

Sodium alginate reinforced 3D lithiophilic Ag–Cu framework for ultrastable Li deposition

The implementation of lithium (Li) anode has been greatly hindered by the low columbic efficiency (CE) and safety concerns, and these are also the main issues in anode-free batteries. Using three-dimensional (3D) anode, such as copper foam (CF), can lower the areal current density and reduce the overpotential, but the larger interface will induce more SEI issues, and additionally, its lithiophobic and naturally curving surface leads to difficult Li nucleation and even more uneven Li deposition. In this paper, sandwich-structured SA@Ag@CF electrode is fabricated in which CF substrate is uniformly covered by nano-sized Ag particle and an extra sodium alginate (SA) layer is further loaded on it. Synergistic effect has been brought by thus-designed electrode, in which Ag much increases the Li-affinity, on the other hand, the tightly-absorbed SA reduces electrolyte contact, alleviates the volume variation and regulates the Li+ diffusion. Ultra-stable Li deposition on 3D substrate has been first realized by the SA@Ag@CF electrode in corrosive carbonate electrolyte. With the SA@Ag@CF anode, the full cells demonstrate much improved long-term stability and rate performance.

Selection and Surface Modifications of Current Collectors for Anode-Free Polymer-Based Solid-State Batteries.

Anode-free batteries (AFB) have attracted increasing interest in recent times because they allow the elimination of the conventional anode from the cell, exploiting lithium inventory from a lithiated cathode. This implies a much simpler, cost-effective, and sustainable approach. The AFB configuration with liquid electrolytes is being explored widely in research but rarely using solid electrolytes. One of the main issues of AFB is the poor reversibility of the lithium-plating/striping process at the anode side. Therefore, in this work, different metal foils have been tested as anode current collectors (CC), and copper foil has been selected as the most promising one. Surface modifications of the selected copper foil have been achieved by its coating using composite layers made of carbon and different metal nanoparticles—such as Ag, Sn, or Zn—in different proportions and with different amounts of a binder. The impact of such coatings and their thickness on the electrochemical performance of single-layer solid-state anode-free pouch cells, based on a PEO electrolyte and a LiFePO4 cathode has been systematically studied. Consequently, a post-mortem analysis of the investigated solid-state AFB is also presented, trying to identify and elucidate possible failure mechanisms to enhance the electrochemical performance of solid-state AFB in the future.

Anode-Free Sodium Ion Batteries: Effect of Pressure on Sodium Plating on Copper

Sodium ion anode-free battery technology puts extra emphasis on high plating and stripping efficiency of sodium metal at the anode surface, due to the lack of an excess of the transporting ion. Recent studies have shown the importance of strict control of water content in enabling efficient plating and stripping of sodium on copper in the 1M NaPF6 in diglyme electrolyte[1]. We have achieved promising results at coin cell level in this configuration: 99.9% coulombic efficiency over 400 cycles, low nucleation overpotential of 40mV and 10mV hysteresis at 0.2mA cm-2, without dendrite formation. To demonstrate the scalability of the anode-free concept, we attempted plating and stripping of sodium metal on copper foils with areas that are 10-fold higher (>10 cm2) than those previously achievable in coin cells (~1cm2) using a split pouch cell configuration. Furthermore, we investigate the effect of pressure at this scale by using two methods. The initial method involves increasing spacer thickness, which acts to compress a wave spring within the split pouch cell. These experiments demonstrate that the plating and stripping voltage hysteresis can be decreased from 125mV -> 40mV -> 10mV, with increasing spacer thickness on the order 3mm -> 3.5mm -> 4mm. The hysteresis observed in coin cells is then matched when 4mm of spacers is used in split pouch cells, indicating a similar pressure applied in these two cell formats. Voltage instabilities are also reduced by increasing the spacer thickness, indicating the formation an increasingly stable solid electrolyte interface (SEI). Secondly, the optimal pressure for the Na||Cu half-cell is investigated by using a pressure calibration unit to accurately measure the applied force within a compressive jig.

Dilute dual-salt electrolyte for successful passivation of in-situ deposited Li anode and permit effective cycling of high voltage anode free batteries

An investigation of the cycling stability of anode-free batteries (AFBs) using dilute dual-salt electrolytes is conducted in a mixture of ether and carbonate solvents. Using the AFB configuration, various compositions of the dual-salt electrolyte were optimized and it was found that 0.9M-LiTFSI+0.3M-LiDFOB in FEC/TTE (2:3, v/v) is the best. An in-depth investigation of the electrochemical performance of AFB was conducted in this optimized dual-salt composition in comparison to 1.2 M LiTFSI in the same solvent ratio. Accordingly, the performance of Cu||NMC cell in the dual-salt electrolyte surpassed that of the single-salt. The relative better performance of the AFB in the dual-salt electrolyte is attributed to the co-existence of dual-ion (TFSI−&DFOB−) in the electrolyte, which enhanced conductivity and introduces entirely new interphases via preferential decomposition mechanisms. The newly formed interface is stable, ionically conductive, and able to intercept parasitic side reactions between the electrolyte solvent and the deposited Li by blocking electron and solvent flow towards the deposit. As a result of the unique interfacial chemistry brought by this dual-salt system, this electrolyte supports a unique CE (98.6%) and capacity retention (63%) in 4.5V Cu||NMC cell and exhibited >27% improvement over the single salt electrolyte after 50 cycles.

An Anode-Free Zn-Graphite Battery.

The anode-free battery concept isproposed to pursue the aspiration of energy-dense, rechargeable metal batteries, but this has not been achieved with dual-ion batteries. Herein, the first anode-free Zn-graphite battery enabled by efficient Zn plating-stripping onto a silver-coated Cu substrate is demonstrated. The silver coating guides uniform Zn deposition without dendrite formation or side reaction over a wide range of electrolyte concentrations, enabling the construction of anode-free Zn cells. In addition, the graphite cathode operates efficiently under reversible bis(trifluoromethanesulfonyl)imide anion (TFSI- ) intercalation without anodic corrosion. An extra high-potential TFSI- intercalation plateau is recognized at 2.75V, contributing to the high capacity of graphite cathode. Thanks to efficient Zn plating-stripping and TFSI- intercalation-deintercalation, an anode-free Zn-graphite dual-ion battery that exhibits impressive cycling stability with 82% capacity retention after 1000 cycles is constructed. At the same time, a specific energy of 79Whkg-1 based on the mass of cathode and electrolyte is achieved, which is over two times higher than conventional Zn-graphite batteries (<30Whkg-1 ).

Co-Solvent Electrolyte Engineering for Stable Anode-Free Zinc Metal Batteries.

Anode-free metal batteries can in principle offer higher energy density, but this requires them to have extraordinary Coulombic efficiency (>99.7%). Although Zn-based metal batteries are promising for stationary storage, the parasitic side reactions make anode-free batteries difficult to achieve in practice. In this work, a salting-in-effect-induced hybrid electrolyte is proposed as an effective strategy that enables both a highly reversible Zn anode and good stability and compatibility toward various cathodes. The as-prepared electrolyte can also work well under a wide temperature range (i.e., from -20 to 50 °C). It is demonstrated that in the presence of propylene carbonate, triflate anions are involved in the Zn2+ solvation sheath structure, even at a low salt concentration (2.14 M). The unique solvation structure results in the reduction of anions, thus forming a hydrophobic solid electrolyte interphase. The waterproof interphase along with the decreased water activity in the hybrid electrolyte effectively prevents side reactions, thus ensuring a stable Zn anode with an unprecedented Coulombic efficiency (99.93% over 500 cycles at 1 mA cm-2). More importantly, we design an anode-free Zn metal battery that exhibits excellent cycling stability (80% capacity retention after 275 cycles at 0.5 mA cm-2). This work provides a universal strategy to design co-solvent electrolytes for anode-free Zn metal batteries.

Li plating on alloy with superior electro-mechanical stability for high energy density anode-free batteries

Anode-free batteries possess high energy density and avoid the use of reactive Li during battery fabrication, and thus are highly desirable for high energy density batteries. However, they encounter fast failure due to the inferior electrochemical reversibility. One main reason is the fast inactive Li accumulation caused by the coupling of side reactions and mechanics during complicated Li plating/stripping processes, among which, mechanical stability of plated Li on current collector is crucial but often ignored. To achieve good electro-mechanical stability, we construct a stable lithium-metal alloy interphase for Li plating, which sharps the Li deposition morphology and enhances the connection with deposited Li. The Li-metal alloy possesses reduced Li nucleation energy barrier and strong bonding with metallic Li, which enables dense and chunky Li deposits with firm connection with the substrate in sharp contrast the fully peeling off of metallic Li for bare Cu after 50 Li plating/stripping cycles. A Li-Sn alloy-modified Cu anode displays stable Li plating/stripping cycling with average Coulombic efficiency of 94.1% for 400 cycles at 1 mA cm–2 and 1 mAh cm–2, significantly outperforming the bare Cu electrode. As a demonstration, in an anode-free cell with Li rich Mn-based cathode, 16.7% increase in capacity and 14.1% increase in capacity retention for 20 cycles are achieved using alloy-modified anode. This sheds new insights on the improvement of dynamic mechanical stability between Li deposits and current collector by regulating Li plating and its connection with the current collector and guides the design of high-performance initial-Li-free anode.

Surface-roughened current collectors for anode-free all-solid-state batteries

Anode-free all-solid-state batteries (AFASSBs), composed of a fully lithiated cathode and a bare current collector (CC) that eliminates excess lithium, can maximize the energy density (because of a compact cell configuration) and improve the safety of solid-state systems. Although significant progress has been made by modifying CCs in liquid-based anode-free batteries, the role of CCs and the mechanism of Li formation on CCs in AFASSBs are still unexplored. Here, we systematically investigate the effect of the surface roughness of the CCs on the Li plating/stripping behavior in AFASSBs. The results show that the moderately roughened CC substantially improves the Coulombic efficiency and cycle stability of AFASSBs owing to the increased contact points between the solid electrolyte and the roughened CC. In contrast, the excessively roughened CC deteriorates the performance owing to the contact loss. Moreover, an ex situ interface analysis reveals that the roughened surface of the CC could suppress the interfacial degradation during the Li ion extraction from a sulfide solid electrolyte to a CC. This provides an indication to the origin that hinders the electrochemical performance of AFASSBs. These findings show the potential for the application of surface-engineered CCs in AFASSBs and provide guidelines for designing advanced CCs.

2D PdTe2 Thin-Film-Coated Current Collectors for Long-Cycling Anode-Free Rechargeable Batteries.

The practical implementation of anode-free batteries is limited by factors such as lithium dendrite growth and low cycling Coulombic efficiency (CE). In this study, the improvement in the electrochemical performance of anode-free rechargeable lithium batteries bearing a Cu current collector (CC) coated with PdTe2 thin films is reported. The optimized thickness and sputtering heating conditions of the PdTe2 layer are 15 nm and 473.15 K, respectively. Upon deposition on a CC, PdTe2 works as a seed layer that considerably improves the CE in half-cells, owing to its unique 2D structure that reduces the nucleation overpotential. A further contribution to the high performance is brought about by a CuTe interphase between the coating layer and Cu CC formed during heating. Such an interphase contributes to the high CE by improving the uniformity of the current density distribution on the CC that suppresses lithium dendrite growth. A low nucleation overpotential and uniform current density distribution, in turn, result in a smooth morphology of the plated Li. The full cell obtained with the PdTe2-coated CC exhibits a capacity retention of 70.7% after the 100th cycle, with an average CE of 99.65% at a 0.2C rate─an outstanding result in view of the rapid development of lithium-ion batteries.

Practical High‐Voltage Lithium Metal Batteries Enabled by Tuning the Solvation Structure in Weakly Solvating Electrolyte

Li metal batteries (LMBs) are ideal candidates for future high-energy-density battery systems. To date, high-voltage LMBs suffer severe limitations because of electrolytes unstable against Li anodes and high-voltage cathodes. Although ether-based electrolytes exhibit good stability with Li metal, compared to carbonate-based electrolytes, they have been used only in ≤4.0 V LMBs because of their limited oxidation stability. Here, a high concentration electrolyte (HCE) comprising lithium bis(fluorosulfonyl)imide (LiFSI) and a weakly solvating solvent (1,2-diethoxyethane, DEE) is designed, which can regulate unique solvation structures with only associated complexes at relatively lower concentration compared to the reported HCEs. This effectively suppresses dendrites on the anode side, and preserves the structural integrity of the cathode side under high voltages by the formation of stable interfacial layers on a Li metal anode and LiNi0.8 Mn0.1 Co0.1 O2 (NMC811) cathode. Consequently, a 3.5 m LiFSI-DEE plays an important role in enhancing the stability of the Li||NMC811 cell with a capacity retention of ≈94% after 200 cycles under a high current density of 2.5 mA cm-2 . In addition, the 3.5 m LiFSI-DEE electrolyte exhibits good performance with anode-free batteries. This study offers a promising approach to enable ether-based electrolytes for high-voltage LMBs applications.

Materials, electrodes and electrolytes advances for next-generation lithium-based anode-free batteries

Abstract The high volumetric stack energy density (∼750 Wh L−1) is a must for grasping the practical application of electric vehicles with more than 100 km per day driving range. Such achievement requires significant advances in state-of-the-art battery technologies. The anode-free, derived from the metal-battery concept, germinates as one of the future potential battery configurations due to the highest, nearly theoretical gravimetric and volumetric energy density. Thus, moving from the graphite-based anode, where lithium is stored as ions, to anode-free cells, wherein lithium is plated as metal, can change the scenario of the electrochemical energy storing devices both in terms of energy density and fundamental mechanism. Although an anode-free battery theoretically provides higher stack energy density than a Li-ion battery, current developments are still underoptimized as these can barely hold for several cycles at room temperature due to the absence of an active lithium reservoir and still severe losses at the anode side. Hence, electrolyte engineering with suitable electrode material choice is highly desirable and extremely challenging in realizing next-generation anode-free batteries. Herein, we summarize the current developments and achievements in the direction of anode-free batteries. Central emphasis is set on electrolyte chemistries that should on one hand allow for high-efficiency initial nucleation, followed by subsequent electrodeposition and electrodissolution of lithium metal, while also forming stable anodic interphases with the high energy cathode materials currently in use. We also prospect for better batteries with higher energy density beyond the present status.

Operando Synchrotron Studies of Inhomogeneity during Anode-Free Plating of Li Metal in Pouch Cell Batteries

Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly characterize Li metal in standard batteries due to the extremely weak scattering from Li atoms. In this work, it is demonstrated the stripping and plating of Li metal can be effectively quantified during battery cycling in appropriately designed synchrotron XRD experiments that utilize an anode-free battery configuration in which a Li-containing cathode material of LiNi0.6Mn0.2Co0.2O2 (NMC622) is paired with a bare anode current collector consisting of either Cu metal (Cu/NMC) or Mo metal (Mo/NMC). In this configuration, it is possible to probe local variations in the deposition and stripping of Li metal with sufficient spatial sensitivity to map the inhomogeneity in pouch cells and to follow these processes with sufficient time resolution to track state-of-charge-dependent variations in the rate of Li usage at a single point. For the Cu/NMC and Mo/NMC batteries, it was observed that the initial plating of Li occurred in a very homogeneous manner but that severe macroscopic inhomogeneity arose on a mm-scale during the subsequent stripping of Li, contrasting with the conventional wisdom that the greatest challenges in Li metal batteries are associated with Li deposition.

A Sodium–Antimony–Telluride Intermetallic Allows Sodium‐Metal Cycling at 100% Depth of Discharge and as an Anode‐Free Metal Battery (Adv. Mater. 1/2022)

Anode-Free Batteries In article number 2106005, Hui Dong, David Mitlin, and co-workers report an intermetallic for anode-free batteries. The winter scene shows a birthday cake with lit candles that are melting: the cake represents the complex structure of a cycled sodium-metal anode with the protruding candles being the dendrites. A slice, obtained by cryogenic focused ion beam (FIB) microscopy, further reveals the underlying porosity that develops during repeated plating and stripping of the metal. The thick non-uniform red frosting on the cake represents the unstable solid electrolyte interphase (SEI) that drives the dendrite growth.

Ternary-salt gel polymer electrolyte for anode-free lithium metal batteries with an untreated Cu substrate

A gel electrolyte, which comprises polymers to regulate Li+transport and ternary salts to reinforce the interface layer, enables Li+to reversibly deposit on plain Cu foil and the resulting anode-free batteries to work with excellent stability.

Gradient nano-recipes to guide lithium deposition in a tunable reservoir for anode-free batteries

Anode-free batteries (AFBs) have the potential of ultra-high energy density, but lithium dendrite has largely hindered their practical applications. In this work, an in-situ grown gradient solid electrolyte interface (SEI) layer on pre-designed micro-hole-grid (MHG) Cu reservoir is proposed to guide uniform lithium deposition and propagation, by which the electrode interface is stabilized and the surface stress is considerably decreased endowing the AFBs with outstanding areal capacity and cycling performance. The gradient SEI is confirmed by cryogenic-transmission electron microscopy (Cryo-TEM) and XPS to be composed of (1) an elastic organic top layer to hinder non-Li species migration and withstand volume fluctuation, and (2) a lithophilic LiCl-rich bottom layer to render the rapid lithium supply. Operando electron paramagnetic resonance (EPR) shows a dynamic Li deposition, unambiguously demonstrating a dendrites-free behavior. As a result, the full cells of a gradient SEI modified Cu reservoir paired with a LiFePO4 cathode exhibit high capacity of 95 mAh g–1 and Coulombic efficiency (CE) of 99.5% after 100 cycles, much better than the control cells with planar current collectors (1.6 mAh g–1, 2.6%). These findings are enlightening in engineering better interphases for high energy and safe rechargeable lithium metal batteries.

A Sodium-Antimony-Telluride Intermetallic Allows Sodium-Metal Cycling at 100% Depth of Discharge and as an Anode-Free Metal Battery.

Repeated cold rolling and folding is employed to fabricate a metallurgical composite of sodium-antimony-telluride Na2 (Sb2/6 Te3/6 Vac1/6 ) dispersed in electrochemically active sodium metal, termed "NST-Na." This new intermetallic has a vacancy-rich thermodynamically stable face-centered-cubic structure and enables state-of-the-art electrochemical performance in widely employed carbonate and ether electrolytes. NST-Na achieves 100% depth-of-discharge (DOD) in 1 m NaPF6 in G2, with 15 mAh cm-2 at 1mA cm-2 and Coulombic efficiency (CE) of 99.4%, for 1000 hof plating/stripping. Sodium-metal batteries (SMBs) with NST-Na and Na3 V2 (PO4 )3 (NVP) or sulfur cathodes give significantly improved energy, cycling, and CE (>99%). An anode-free battery with NST collector and NVP obtains 0.23% capacity decay per cycle. Imaging and tomography using conventional and cryogenic microscopy (Cryo-EM) indicate that the sodium metal fills the open space inside the self-supporting sodiophilic NST skeleton, resulting in dense (pore-free and solid electrolyte interphase (SEI)-free) metal deposits with flat surfaces. The baseline Na deposit consists of filament-like dendrites and "dead metal", intermixed with pores and SEI. Density functional theory calculations show that the uniqueness of NST lies in the thermodynamic stability of the Na atoms (rather than clusters) on its surface that leads to planar wetting, and in its own stability that prevents decomposition during cycling.