Nucleation of Electrodeposited Lithium Metal: Dendritic Growth and the Effect of Co-Deposited Sodium

268

275

A comparative study of the electrode/electrolyte interface was carried out for lithium and sodium metal anodes in electrolytes consisting in 1 M LiPF 6 in EC 0.5 :DMC 0.5 (LP30) and 1 M NaPF 6 in both EC 0.5 :DMC 0.5 and EC 0.45 PC 0.45 DMC 0.1 . Symmetric Li/Li cells exhibited low polarization and smooth charge discharge curves with current densities of 0.1 and 1 mA/cm 2 . In contrast, large overpotentials were observed even at 0.1 mA/cm 2 for Na/Na cells. Such differences cannot be related to ionic conductivity of the electrolytes but are rather due to an enhanced interfacial resistance (R ct + R SEI ) as deduced from impedance measurements. The composition of the SEI layer was investigated by FTIR and found to be stable for Li electrodes but to evolve upon cycling for Na electrodes which is also in agreement with differences in surface morphology detected by SEM. A lower stability (partial solubility) of the SEI would also enable to understand the differences in the impedance of identical hard carbon (HC) electrodes in cells with either Li or Na counterelectrodes. These results cast some concerns on the reliability of the so termed half cell characterization and call for caution when interpreting the results of potential electrode materials for sodium ion batteries. The use of metallic anodes has attracted large attention in the battery field due to their high capacity and low operation potential. Nonetheless, almost all commercialized technologies are either primary cell (e.g. alkaline Zn/MnO 2 ), operating through chemical reaction mechanisms (e.g. Ni/Cd in which Cd oxidizes to Cd(OH) 2 , using liquid metal anodes (Na/S cells) or polymer 1 electrolytes, thus avoiding many of the problems of metal reversible plating/stripping in liquid electrolytes. Indeed, the formation of metal dendrites upon cycling was early identified as bottleneck for the development of safe Li metal anode batteries which lead to the commercialization of Li-ion batteries (LIB) to the expense of a severe penalty in energy density. Even so, lithium metal anodes are widely used as counter (or also reference) electrodes in the so termed half cell tests, performed mostly at laboratory scale within the battery research community. These are mostly intended to assess the electrochemical performance of a given compound as either positive or negative electrode material for LIB in a simple way, avoiding the assembly of full cells in which electrode balancing can severely affect performance. Their outcomes are typically the electrochemical capacity and operation potential values at different rates. The reliability of extrapolating such half cell testing results to potential performance in full cells relies on the stability of the Solid Electrolyte Interphase (SEI) typically formed on the surface of lithium metal anodes as a result of electrolyte degradation reactions. 2The development of a room temperature sodium (solid) metal anode battery technology, using conventional liquid electrolytes does not seem to have been seriously considered in the pa...

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confidence: 99%

On the Comparative Stability of Li and Na Metal Anode Interfaces in Conventional Alkyl Carbonate Electrolytes

Iermakova

Dugas

Palacín

et al. 2015

268

275

“…[4][5][6][7][8][9][10] Dendrite formation is also often a concern for fast charging and low temperature operation in current Li-ion battery technology. Li dendrite formation and growth can be controlled by many factors.…”

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confidence: 99%

Interfacial Study on Solid Electrolyte Interphase at Li Metal Anode: Implication for Li Dendrite Growth

Liu

Lin

et al. 2016

206

141

The Solid electrolyte interphase (SEI), either naturally formed or artificially designed, plays a critical role in the stability and durability of Li-ion batteries (LIBs). It is even more important for high energy density electrodes such as Li metal anodes, which is subjected to large volumetric and interfacial variations due to Li deposition/stripping cycles during operation. Currently, there is a lack of understanding of the role of SEI/Li interfaces and their mechanical and electrochemical properties. In this paper, we present an interfacial study to evaluate the two major SEI components, LiF and Li 2 CO 3 , based on density functional theory (DFT) calculations. The calculated interfacial energy results show that the Li 2 CO 3 /Li interface has higher interfacial mechanical strength. To achieve the practical application of reversible high energy density Li metal electrode (3860 mAh g −1 ) for future "beyond Li-ion batteries", 1-3 one of the most significant challenges is to mitigate irreversible Li-dendrite formation.4-10 Dendrite formation is also often a concern for fast charging and low temperature operation in current Li-ion battery technology. Li dendrite formation and growth can be controlled by many factors. Intrinsically, dendrite morphology is determined by material properties, such as surface energy and growth anisotropy. However, it may also be influenced by other factors such as local current distribution 11,12 (due to electrode surface roughness and compositional inhomogeneity, etc.), operation voltage and charging rate, 13,14 as well as electrolyte composition. 7,15The dendrite growth process can also be strongly affected by the properties of solid electrolyte interphase (SEI), the passivation thin layer between electrolyte and electrode. Electrolyte solvents, such as ethylene carbonate (EC) and dimethyl carbonate (DMC) will naturally reduce and decompose at the low potential Li metal surface and spontaneously form an SEI layer. 16 First proposed by Peled in 1970s, 17 SEI remains "the most important but least understood" 18,19 in rechargeable LIBs due to its complicated content that is highly dependent on numerous factors such as electrolytes and additives, 20 electrode surface 21,22 and operating conditions. 23 Many recent efforts have been focused on using different electrolytes to alter the property of SEI or developing artificial coatings on the Li surface to mitigate dendrite growth. Therefore, it is important to understand how the SEI and coating properties may impact Li dendrite formation.After decades of debates, 4,17,24,25 there are some general agreements on the role of SEI in Li dendrites formation mechanisms ( Figure 1). Since Li metal oxidizes instantly when in contact with electrolyte, the only two probable Li plating sites are inside SEI and at Li anode surface, which is further determined by two processes: electron tunneling from anode to SEI and Li ion diffusion from electrolyte to anode. In most LIBs, where the well formed SEI is insulating enough to block electron tunneling f...

“…This morphology is shown in Figure 5a where 1.3 C/cm 2 was passed over 5000 s. This co-deposit was studied in detail in a previous paper. 11 The coulombic efficiency for the deposition and re-oxidation of the lithium/sodium system was calculated to be 44%. While it is possible for the absence of dendrites to lead to an increase in the coulombic efficiency, the coulombic efficiency of sodium from the CV results was only 41%.…”

Section: Resultsmentioning

confidence: 99%

“…6 Recent in-situ studies on dendrite growth have shown that the needles can grow from the tip or extrude from the base, with both processes occurring in any given electrolyte. [7][8][9][10][11][12] Lithium dendrite growth has previously been mitigated by physically confining the lithium metal behind a solid electrolyte, 13 however, this could lead to loss of contact in larger batteries as the lithium metal at the interface is shuttled to the cathode, causing delamination between the lithium anode, solid electrolyte, and current collector. There are also several reports of dendrite-free deposits in heavily fluorinated electrolytes 5,14,15 and the LiAsF6/dioxolane electrolyte.…”

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confidence: 99%

See 1 more Smart Citation

Effect of Alkali and Alkaline Earth Metal Salts on Suppression of Lithium Dendrites

Goodman

Kohl

2014

Self Cite

Lithium dendrites are needle-like structures that form during the electrodeposition of lithium metal. These whiskers complicate the use of lithium metal as an anode in lithium batteries because they can puncture the separator and short circuit the battery. In addition, the large surface area and poor adhesion of the deposit contributes to loss of coulombic efficiency. The effect of alkali and alkaline earth metal ions on the morphology of electrodeposited lithium metal has been studied. Varying concentrations of alkali and alkaline earth metal ions were added to a 1 M lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) trimethylbutylammonium bis(trifluoromethanesulfonyl)imide (N 1114 -TFSI) electrolyte. Lithium metal was electrodeposited from each electrolyte and examined ex-situ by scanning electron microscopy (SEM). Alkali metal ions, with the exception of sodium, had little or no effect on the deposited lithium morphology. However, alkaline earth metal ions at 0.05 M concentration significantly reduced the occurrence of dendrites. When the concentration of the alkaline earth metal ions was increased to 0.1 M, dendrites were completely eliminated and lithium was deposited in a sphere-like morphology. Energy dispersive X-ray spectroscopy (EDX) showed that no alkaline earth metals were found in the sphere-like deposits, suggesting that dendrite mitigation occurred through an adsorption mechanism. Lithium-ion batteries based on graphite anodes have been commercialized and used in mobile devices due to their high power and energy density. The graphite anode operates close to its theoretical capacity of 329 mAh/g. Thus, to increase the energy density of the overall battery, a new anode material must be developed. Reducing lithium ions on a substrate, rather than intercalating them into graphite, raises the specific capacity of the anode to 3861 mAh/g and is compatible with existing cathodes and future high-voltage cathodes.Lithium-metal anodes suffer from low cycling efficiency for several reasons. First, the solid electrolyte interface (SEI) that forms on graphite anodes to protect the electrode from further reaction with the electrolyte, does not form as well on a metallic lithium surface. Second, the lithium metal anode undergoes extreme volume changes when going between the charged and discharged states, which can significantly disrupt the SEI during each cycle. Finally, when lithium metal is deposited on a substrate, such as during battery charging, lithium does not deposit as a dense, cohesive, planar layer, but rather deposits as needle-like structures, sometimes called dendrites, as shown in Figure 1.Adding SEI forming additives and restricting the battery to shallow discharge cycles can mitigate some of these problems, however, these restrictions are not desirable. Vinylene carbonate (VC) has been shown to be a valuable additive for lithium metal batteries, yielding higher efficiencies and increased cycle life.1,2 VC and other cyclic carbonates have been shown to form an SEI via ring-opening reactions tha...