Unstable Solid Electrolyte Interphase Layer Research Articles

Ultrathin amorphous TiO2 nanofilm-coated graphene with superior electrochemical performance for lithium-ion batteries

As a novel anode material for lithium-ion batteries, graphene has a higher theoretical capacity than graphite. However, the practical application of graphene suffers from a low coulombic efficiency and poor cycle stability due to the unstable solid electrolyte interphase layer. In this work, ultrathin amorphous TiO2 nanofilm-coated graphene is fabricated by controlled hydrolysis of tetrabutyl titanate on the surface of graphene, and shows superior cycle and rate performance compared to uncoated graphene because amorphous TiO2 nanofilms can prevent the detachment of graphene nanosheets and avoids the formation of thick and unstable solid-electrolyte interphase layers during cycling. Moreover, the electrochemical performance of TiO2-coated graphene can be further improved by controllable calcination of TiO2-coated graphene under a N2 atmosphere. The produced porosity of TiO2 nanofilms after calcination can improve both Li+ and electron transport, leading to the further improved lithium storage performance of graphene. This novel synthesis strategy may be employed in other graphene-based anode materials for high-performance lithium-ion batteries and other electrochemical devices.

Superior Stable and Long Life Sodium Metal Anodes Achieved by Atomic Layer Deposition.

Na-metal batteries are considered as the promising alternative candidate for Li-ion battery beneficial from the wide availability and low cost of sodium, high theoretical specific capacity, and high energy density based on the plating/stripping processes and lowest electrochemical potential. For Na-metal batteries, the crucial problem on metallic Na is one of the biggest challenges. Mossy or dendritic growth of Na occurs in the repetitive Na stripping/plating process with an unstable solid electrolyte interphase layer of nonuniform ionic flux, which can not only lead to the low Coulombic efficiency, but also can create short circuit risks, resulting in possible burning or explosion. In this communication, the atomic layer deposition of Al2 O3 coating is first demonstrated for the protection of metallic Na anode for Na-metal batteries. By protecting Na foil with ultrathin Al2 O3 layer, the dendrites and mossy Na formation have been effectively suppressed and lifetime has been significantly improved. Furthermore, the thickness of protective layer has been further optimized with 25 cycles of Al2 O3 layer presenting the best performance over 500 cycles. The novel design of atomic layer deposition protected metal Na anode may bring in new opportunities to the realization of the next-generation high energy-density Na metal batteries.

Thermal Induced Strain Relaxation of 1D Iron Oxide for Solid Electrolyte Interphase Control and Lithium Storage Improvement

High energy lithium ion battery based on multi‐electron redox reaction is often accompanied by inherent large volume expansions, sluggish kinetics, and unstable solid electrolyte interphase layer, leading to capacity failure. Here, thermal induced strain relaxation is proposed to realize the solid electrolyte interphase control. It is demonstrated that through thermal treatment, lattice strain is well released and defect density is well reduced, facilitating the charge transfer, improving the interparticle contacts and the contacts at the interface of electrode to withstand the huge volume expansion/contraction during cycling. In this way, the as‐prepared α‐Fe2O3 electrode at 800 °C with no protective shell shows an outstanding reversible capacity of 1200 mA h g−1 at 100 mA g−1 and an excellent high‐rate cyclability with a capacity fading of 0.056% per cycle for 1200 cycles at 5 A g−1. It is expected that such findings facilitate the applications of high capacity anode and cathode material systems that undergo large volume expansion.

Critical thickness of phenolic resin-based carbon interfacial layer for improving long cycling stability of silicon nanoparticle anodes

Silicon has a high theoretical capacity, still limits its application on Si-based anodes due to the problems of low electric conductivity, large volume change, continuous formation of unstable solid electrolyte interphase layer, and easy fracture during lithiation and delithiation process. Despite various carbon coating approaches are developed to fabricate carbon coated silicon core-shell and yolk-shell nanocomposites with improved electrochemical performance, the challenges including poor long-term cyclability, low Si mass ratio, and scalability remains. To overcome these challenges, we design an interfacial microporous carbon coating strategy on silicon nanoparticles to form homogeneous coaxial core-shell nanostructures. This synthesis sol-gel approach is simple, easy to scale up, and direct growth phenolic resins on the surface with uniform and controllable thickness. Additionally, the fabricated carbon layers form the microporous structures and phenolic resin frameworks, thus enabling the fast lithium ion transport and formation of stable solid electrolyte interphase film. By finely controlling the thickness of this phenolic resin-based carbon of 10nm, excellent protection of silicon nanoparticles as well as high electrochemical performance are achieved, delivering a high capacity of 1006mAhg−1 and Coulombic efficiency of >99.5% after 500 times at a current density of 500mAg−1.

The Significant Influence of the Conductive Carbon Additive on the Performance of High Areal Capacity Silicon Electrodes

The constant demand for lithium-ion batteries with higher energy density requires finding new electrode materials. Silicon-based electrodes are particularly attractive due to the higher gravimetric capacity of Si (3579 mAh g-1) compared to conventionally used graphite (372 mAh g-1). However, during the process of lithiation/delithiation, the silicon material suffers from a huge volume change, leading to the fracturing of the silicon particles, an unstable solid electrolyte interphase layer (SEI) and the disconnection of inter-particle contacts, which all have a negative repercussion on the electrode cycle life. Another serious challenge for commercializing silicon electrodes is to reach a high areal capacity of more than 6 mAh cm-2, in order to achieve an energy density improvement over the use of conventional graphite-based anodes. Silicon electrodes with such high areal capacity require very careful design of their formulation at different scales. In particular, a special attention must be paid to creating durable intimate contacts between the active material particles and the conductive additive network, so that sufficient electron transfer could be achieved throughout the electrode from the copper current collector while good mechanical stability of the electrode coating is still maintained.1 Our group has recently shown that high performance silicon-based anodes can be achieved by combining (i) the use of high-energy ball-milling as a cheap and easy process to produce nanostructured silicon powder, (ii) the processing of the electrode with carboxymethylcellulose (CMC) binder at pH 3 condition, which promotes the covalent grafting of the CMC to the Si particles; (iii) the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI.2 In the present work, silicon-based electrodes of various areal capacities were prepared by using either carbon black, vapor grown carbon nanofibers, or graphite nanoplatelets as conductive additive.3 It was observed that the electrical conductivity, capacity retention, and coulombic efficiency of the silicon electrode are significantly affected by the morphological characteristics of the used carbon additives. Spherical-shaped carbon black particles have a strong tendency to agglomerate, and thus fail in creating a conductive network resilient to the silicon particle volume change. In contrast, vapor grown carbon nanofibers maintain more durable contacts with silicon particles, compared to carbon black, by forming a more resilient conductive network due to their wire-like structure.4 Graphite nanoplatelets also create a continuous conductive network, which limits the mechanical degradation of the electrode coating, likely by playing the role of electrically conducting lubricant.5These results demonstrate that the choice of the conductive additive is of crucial importance for the optimization of silicon negative electrodes with commercially relevant areal capacities. References (1) Mazouzi, D.; Karkar, Z.; Reale Hernandez, C.; Jimenez Manero, P.; Guyomard, D.; Roué, L.; Lestriez, B. J. Power Sources 2015, 280, 533–549. (2) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L. Energy Environ. Sci. 2013, 6(7), 2145. (3) Karkar, Z.; Mazouzi, D.; Reale Hernandez, C.; Guyomard, D.; Roué, L.; Lestriez, B., Submitted (4) Lestriez, B.; Desaever, S.; Danet, J.; Moreau, P.; Plée, D.; Guyomard, D. Electrochem. Solid-State Lett. 2009, 12(4), A76. (5) Nguyen, B. P. N.; Gaubicher, J.; Lestriez, B. Electrochim. Acta 2014, 120, 319–326. Figure 1

Thermal Stability of Sb and Cu2Sb Anodes in Lithium-Ion Batteries

The thermal stability and reactions of Sb and the intermetallic Cu2Sb in lithium-ion batteries has been investigated and analyzed relative to graphite electrodes under the same conditions. Bulk and surface layer products of the thermal reactions have been characterized by ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Differential scanning calorimetry (DSC) has been employed to investigate the relative heat generation with these electrodes and their potential to enhance lithium-ion battery safety by reducing the low-temperature exothermic driving force for thermal runaway. Sb and Cu2Sb are both found to undergo low-temperature reactions similar to graphite, with reactions at, respectively, ∼90 and 130°C related to solid-electrolyte interphase (SEI) layer breakdown. The heat generation per unit capacity for both Sb and Cu2Sb at < 150°C is ∼37% lower than that with graphite, which is a critical range for prevention of thermal runaway. Because neither Sb nor Cu are shown to be present in the SEI layer in appreciable quantities, this reduction in heat generation is largely attributed to the greater volumetric capacities of antimony and Cu2Sb that allow for greater lithium storage with less surface exposure for the formation of a thermally unstable SEI layer.

Improved Electrochemical Performances of Methylated Amorphous Si Electrode Studied By Tof-SIMS

Silicon is considered as a promising anode material for Li-ion batteries due to its ability to insert large amounts of lithium, delivering a very high theoretical specific capacity of 3579 mAh/g, which is almost 10 times higher than graphite electrode (372 mAh/g). Nevertheless, a high volume variation (280 % for Li15Si4) during lithiation leading to a morphological damage of electrode materials and a huge capacity loss (of about 30%) observed during the first charge are the major drawbacks for application of Si as anode material. These damages can be considerably decreased or avoided by using nanosized materials, such as Si nanowires (SiNW), allowing better accommodation of volume variation.[i],[ii] Another solution to limit the consequences of these damages is to use electrolyte additives like vinylene carbonate (VC) and monofluoroethylene carbonate (FEC), having polymerizable features and the possibility of forming a SEI layer with improved mechanical properties.[iii],[iv] A new way to improve the Si electrode performances is the modification of the chemical bulk composition of the Si electrode material. In this work, we present the improved electrochemical performance of methylated amorphous silicon (a-Si0.9(CH3)0.1:H), a new type of Si-based material containing methyl (CH3) groups. In order to focus on the material itself, we compare thin-film electrodes of this material[v] to similar electrodes of hydrogenated amorphous Si (a-Si:H),[vi] without using additives or nanosize shaping of the material. The chemical modifications of these two electrodes induced by the electrochemical lithiation process were studied by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) negative-ion depth profiles. Experiments were performed in propylene carbonate (PC, purity > 99.7 %, 30 ppm H2O, Sigma-Aldrich) containing 1 M LiClO4 (purity > 99.99%, battery grade, Sigma-Aldrich).As compared to a-Si:H, the modified chemical composition of a-Si0.9(CH3)0.1:H has an influence on the electrochemical behaviour by shifting the lithiation plateau from 200 mV to 160 mV. The most important difference between the two types of electrodes is the instantaneous formation of a thin, stable and homogenous Solid Electrolyte Interphase (SEI) layer on a-Si0.9(CH3)0.1:H electrode during the first cycle. On the contrary, a thick unstable SEI layer is formed on a-Si:H, with possible dissolution/oxidation and cracking of the layer formed during the first cycle and continuous uptake of electrolyte decomposition products during the following discharge/charge cycles. The ion-depth profiles obtained by ToF-SIMS also evidence significant volume variations and swelling of a-Si:H electrodes which can lead to large morphological modifications, electrode cracking. The a-Si0.9(CH3)0.1:H electrode also shows much lower irreversible capacity during the first 100 discharge/charge cycles, as compared to the a-Si:H electrode. Finally, the diffusion coefficient of Li ions (determined from Li- ToF-SIMS profile) is found to be one order of magnitude higher in a-Si0.9(CH3)0.1:H than in a-Si:H, confirming its improved electrochemical performance. References [i] C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zang, Y. Cui, Nat. Nanotechnol. 3, 31 (2008).[ii] B. Laïk, D. Ung, A. Caillard, C.-S. Cojocaru, D. Pribat, J.-P. Pereira-Ramos, J. Solid State Electrochem. 14, 1835 (2010).[iii] M. Ulldemolins, F. Le Cras, B. Pecquenard, V. P. Phan, L. Martin, H. Martinez, J. Power Sources 206, 245 (2012).[iv] V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, D. Aurbach, Langmuir 28, 965 (2012).[v] L. Touahir, A. Cheriet, D. Alves Dalla Corte, J.-N. Chazalviel, C. Henry de Villeneuve, F. Ozanam, I. Solomon, A. Keffous, N. Gabouze, M. Rosso, J. Power Sources 240, 551 (2013).[vi] C. Pereira-Nabais, J. Światowska, A. Chagnes, F. Ozanam, A. Gohier, P. Tran-Van, C.–S. Cojocaru, M. Cassir, P. Marcus, App. Surf. Sci. 266, 5 (2013).

리튬 이차 전지용 실리콘 나노입자 음극의 고온 열안정성에 대한 Lithium bis(oxalato)borate첨가제의 효과

리튬2차전지용 음극활물질로 연구되고 있는 실리콘은 흑연에 비하여 높은 이론용량 (4200mAh/g for <TEX>$Li_{4.4}Si$</TEX>)을 가지기 때문에 고용량 음극소재로 각광받고 있다. 이러한 실리콘 음극은 반복적인 충방전에 의해 활물질 입자의 심각한 부피변화와 균열에 의한 새로운 표면이 전해액에 계속적으로 노출되는 문제로 인하여 두껍고 불안정한 피막생성을 유도한다. 불안정한 구조의 피막은 실리콘 음극의 전기화학적 성능뿐만 아니라 고온 열안정성을 저해할 수 있기 때문에 본 연구에서는 실리콘의 열안정성 향상을 위해 전해액 첨가제를 도입하여 피막구조를 변경하고자 한다. 전해액 첨가제인 lithium bis(oxalato)borate (LiBOB)가 실리콘 음극표면에 피막을 효과적으로 형성하였으며, 만충전 상태의 실리콘 음극의 <TEX>$60^{\circ}C$</TEX> 저장시 용량유지 특성을 개선하였으며 고온에서의 열안정성 크게 향상시켰다. Silicon (Si) has been investigated as promising negative-electrode (anode) materials because its theoretical specific capacity of 4200 mAh/g for <TEX>$Li_{4.4}Si$</TEX> is far higher than that of carbonaceous anodes in current commercial products. However, in practice, the application of Si to Li-ion batteries is still quite challenging because Si suffers from severe volume expansion and contraction and lead to a continuous solid electrolyte interphase (SEI)-filming process by cracking of Si. This process consumes the limited <TEX>$Li^+$</TEX> source, builds up thick and unstable SEI layer on the Si active materials, and will eventually disable the cell. Since unstable SEI reduces electrochemical performance and thermal stability of the Si anode, the surface chemistry of the anode should be modified by using a functional additive. It is found that lithium bis(oxalato)borate (LiBOB) as an additive effectively protected the Si anode surface, improved capacity retention when stored at <TEX>$60^{\circ}C$</TEX>, and alleviated exothermic thermal reactions of fully lithiated Si anode.

SEI Formation on Single Crystal Si Electrodes in Organic Carbonate Electrolytes

Silicon, with a theoretical capacity of 4200 mAh, is an attractive alternative to conventional anode materials for lithium ion batteries[ 1 ]. However, silicon like other intermetallic electrodes exhibits poor passivating behavior in organic electrolytes. Moreover, it suffers from a significant volume variation (300%) upon cycling in Li-ion cells, which repeatedly damages the solid-electrolyte interphase (SEI) layer and contributes to the interfacial instability of Si[ 2 ]. The organic electrolyte composition[ 3 ] and surface orientation of silicon crystal[ 4 ] have been reported to play a major role in the SEI formation process, which is critical for battery electrochemical performance and lifetime[ 5 ].This work focuses on characterization of interfacial phenomena during the SEI formation on Si single crystal electrodes in organic carbonate electrolytes. The SEI formation on the facets (100), (110) and (111) was controlled electrochemically during the first potentiostatic charge in a three electrode cell from 2.5 vs. Li/Li+ to 500 mV (before Si-Li alloying) and 10 mV. The effect of four different electrolytes was evaluated: 1 M LiPF6, EC/DMC (1:1), 1 M LiTFSI in EC/DMC (1:1), 0.5 M LiBOB in EC/DMC (1:1), 1 M LiPF6/2% LiBOB in EC/DMC (1:1). Composition, structure and morphology of the SEI layer were investigated by a combination of ex situ FTIR, LIBS, AFM and SEM-EDX.The impact of the electrolyte composition on the kinetics of the SEI formation, its chemical composition and morphology was clearly identified. In LiPF6- and LiBOB-containing electrolytes, a thick SEI layer is formed with respectively LiF and LiC2O4 as main component whereas in LiTFSI-based electrolyte, the formation of a thin unstable SEI layer rich in organic carbonates occurs at lower decomposition potential. This results in remarkable difference during alloying. An extensive fissuration is observed at the silicon surface with LiTFSI while this damage is drastically attenuated with LiPF6 and LiBOB. Interestingly LiBOB as additive in LiPF6-based electrolyte leads to an earlier formation of a passivating surface film at the silicon surface.Upon polarization, Si single crystal with (100), (110) and (111) orientation displays quantitative and qualitative differences in the electrochemical spectroscopic response. Chemical composition of the SEI layer varied strongly on the different Silicon crystal facet silicon surfaces at potentials >0.5 V and included EC and DEC solvents decomposition products. The SEI on the Si (111) crystal facet is a mixture of organic carbonates whereas the SEI on Si (100) consists mainly of LiF.At potentials below 0.5 V the SEI undergoes reformation due to Si cracking and structural transition from crystalline to amorphous silicon upon alloying with lithium. This results in similar SEI chemical composition for the three different crystal orientations. The correlation between surface activity, composition and stability of the SEI layer, and crystal orientation will be discussed. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Electrochemical properties and interfacial reactions of LiNi0.5Mn1.5O4−δ nanorods

LiNi0.5Mn1.5O4−δ which possesses a high voltage of 4.7V vs. Li+/Li and stable structure has been considered as a promising cathode material for high energy Li-ion batteries. In this study, well-crystallized LiNi0.5Mn1.5O4−δ nanorods with diameter of ∼50nm and length of ∼500nm were synthesized by a hydrothermal-solid state process followed by a lithiation reaction. It delivers ∼120mAhg−1 discharge capacity under 30mAg−1 at 3.5–4.9V, and 90mAhg−1 at 150mAg−1. An unstable solid electrolyte interphase (SEI) film with thickness 2–10nm was observed by transmission electron microscopy. The thick and unstable SEI layers are responsible for its low rate performance and low coulombic efficiency during cycling.

In situ AFM studies of SEI formation at a Sn electrode

Early stages of the solid electrolyte interphase (SEI) formation at a tin foil electrode in an ethylene carbonate (EC) based electrolyte were investigated by in situ AFM and cyclic voltammetry (CV) at potentials >0.7 V, i.e., above the potential of Sn–Li alloying. We detected and observed initial steps of the surface film formation at ∼2.8 V vs. Li/Li + followed by gradual film morphology changes at potentials 0.7 < U < 2.5 V. The SEI layer undergoes continuous reformation during the following CV cycles between 0.7 and 2.5 V. The surface film on Sn does not effectively prevent the electrolyte reduction and a large fraction of the reaction products dissolve in the electrolyte. The unstable SEI layer on Sn in EC-based electrolytes may compromise the use of tin-based anodes in Li-ion battery systems unless the interfacial chemistry of the electrode and/or electrolyte is modified.