Si/Mnooh Composite with Superior Lithium Storage Property

Lithium-ion batteries (LiBs) have been successfully utilized as power sources in various applications ubiquitous in our daily lives, ranging from portable electronic devices to electric vehicles. To meet the increasing energy demand, however, development of LiBs with a higher energy density is inevitable. Using silicon (Si) as an anode is promising due to its high theoretical capacity (approximately 4200 mAhg-1), appropriate operating voltage (~0.4 V vs. Li/Li+), abundance, and environmentally benign nature.1 Despite the aforementioned advantages, several challenges remain before the commercial utilization of Si anodes, namely the low conductivity of its readily-formed surface oxide (SiO2), a large volume change (up to 400 %) during lithiation/delithiation, and the instability of Si/electrolyte interface (SiEI).2-3 As the large volume change leads to fracture and continuous evolution of SiEI, achieving a stable SiEI is a key to developing successful LiBs with Si anodes. Water, which is known to react with LiPF6 salt in carbonate-based electrolytes to generate corrosive hydrofluoric acid (HF),4 is a source for the electrode/electrolyte interface destabilization. Thus, it is essential to understand how the presence of even trace amounts of water in the system affects the interfacial chemistry of the LiB electrodes. While the performance of LiBs with graphite or lithium titanate negative electrodes have been previously studied with excess water in the electrolyte,5-6 information on the effect of water concentration in the electrolyte on Si anodes, and the SiEI in particular, is limited. In this work, we evaluated the electrochemical performance of Si anodes in a standard electrolyte (1.2 M LiPF6 in EC:EMC (3:7 wt%)) with varying concentration of water (10 - 1,000 ppm). Subsequent analyses on the cycled Si anodes were performed utilizing a variety of surface and bulk characterization techniques. The surface chemistry (e.g., composition and evolution) of SiEI was analyzed using Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Surface morphologies of cycled Si anodes were measured with tapping-mode atomic force microscopy (AFM), and the surface resistivities were mapped in contact-mode with scanning spreading resistance microscopy (SSRM). The information obtained is utilized to understand water concentration effects on the properties, reactivity, and evolution of the SiEI and the electrochemical cell performance. References Feng, K.; Li, M.; Liu, W.; Kashkooli, A. G.; Xiao, X.; Cai, M.; Chen, Z., Silicon-Based Anodes for Lithium-Ion Batteries: From Fundamentals to Practical Applications. Small 2018, 14 (8), 1702737.Chae, S.; Ko, M.; Kim, K.; Ahn, K.; Cho, J., Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries. Joule 2017, 1 (1), 47-60.Philippe, B.; Dedryvere, R.; Allouche, J.; Lindgren, F.; Gorgoi, M.; Rensmo, H.; Gonbeau, D.; Edstrom, K., Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-ray Photoelectron Spectroscopy. Chem. Mater. 2012, 24 (6), 1107-1115.Xu, K., Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104 (10), 4303-4417.Burns, J. C.; Sinha, N. N.; Jain, G.; Ye, H.; VanElzen, C. M.; Scott, E.; Xiao, A.; Lamanna, W. M.; Dahn, J. R., The impact of intentionally added water to the electrolyte of Li-ion cells. I. Cells with graphite negative electrodes. J. Electrochem. Soc. 2013, 160 (11), A2281-A2287.Burns, J. C.; Sinha, N. N.; Jain, G.; Ye, H.; Van Elzen, C. M.; Scott, E.; Xiao, A.; Lamanna, W. M.; Dahn, J. R., The impact of intentionally added water to the electrolyte of Li-ion cells: II. Cells with lithium titanate negative electrodes. J. Electrochem. Soc. 2014, 161 (3), A247-A255.

Due to their high capacity, Silicon (Si) anodes are very promising for the next generation of lithium-ion batteries. However, Si materials suffer from a high volume expansion, which cause cracking and accumulated growth of the solid electrolyte interphase (SEI), resulting in low coulombic efficiencies and lifetime of the Si anodes. Applying additives in the electrolyte is a facile and straightforward approach to improve the composition of the SEI and reduce cracking. For Si-containing anodes, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives. The salt lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite when applied as additive, but has been poorly investigated for Si anodes. In our study, we combine VC, FEC, and a variation of LiDFP concentrations to investigate the effect on Si-dominant anodes in detail. We conducted detailed electrochemical testing in half coin cells and Si/NCM pouch cells, including long-term cycling, rate capability tests, potential monitoring of the anode and electrochemical impedance spectroscopy. For the latter two, we applied a novel perforated reference electrode concept, which allows for broad and precise measurement in pouch cells, while avoiding a blocking effect of the lithium ions. Furthermore, post-mortem analysis was conducted to gain detailed insights into the SEI formation and microstructure influenced by the different additive concentrations.

Gum arabic grafted poly(acrylic acid) (GA‐g‐PAA) is synthesized as a mechanically robust water‐soluble binder for silicon (Si) anodes in lithium‐ion batteries by graft polymerization of acrylic acid onto GA backbone via a free radical reaction. 1H NMR and Fourier transform infrared spectroscopies and thermogravimetric analysis are systematically conducted to confirm the grafting polymerization process. GA‐g‐PAA with different grafting length of PAA shows enhanced adhesion strength and excellent flexibility after grafting. Optimal Si‐GA‐g‐8PAA electrode displays better cyclic stability, higher Coulombic efficiency and superior rate properties compared with a Si electrode with linear PAA binder. The Si‐GA‐g‐8PAA electrode exhibits a high electrical conductivity, low interfacial/charge transfer resistance and high lithium‐ion diffusion coefficient. GA‐g‐8PAA binder with grafted structure not only can maintain the mechanical and electrical integrity of the electrode, facilitating favorable electrochemical kinetics, but also assists in preserving a stable solid electrolyte interphase on Si surface upon long‐term cycling. Such a facile strategy for designing a novel grafted binder shows potential for practical application on high‐capacity anode materials with large volume change. © 2021 Society of Industrial Chemistry.

To response the dramatically increased demand in portable electronics, hybrid electric vehicles and large scale energy storage over the last decade, Li-ion batteries (LIBs) with higher energy density and power capability have been strongly desired. With respect to the anode if LIB, silicon have received a great deal of interest as fully lithiated (ca. Li22Si5) alloy has the highest theoretical capacity of any known material at 4140 mAh/g (3579 mAh/g for Li15Si4 at room temperature). Although it is about 10 times higher than that of conventional graphite anode, Si-based electrode has yet to find use in commercial application because of its low coulombic efficiency (C.E.) and poor capacity retention. Si is typically known to undergo over 300% of volume expansion on Li insertion, which leads to cracking and crumbling of active particles and the resultant loss of electrical contact between electro-active materials causes rapid capacity fading. Although extensive researches have been reported to alleviate the problems, most of approaches are commercially unviable as they usually involve complicated and prohibitively high cost processes. Hence, a Si anode with sufficient performances and low cost production is highly desired. At first, this study presents a novel cost-saving Si-base anode material with high coulombic efficiency and prolonged cyclability of excellent electrochemical performances using a low-cost synthesis of attractive route. As a part of our strategy, we attempted to employ a by-product derived from a granular poly-Si mass production system for the feedstock of solar cell application. In such a way, a certain amount of amorphous Si particles (ASP), the by-product, is derived homogeneously from the nucleation of gas dust in silane-based gas phase reactor. Our pioneering attempt is advantageously characterized by follows: 1) obtained sub-micron sized Si particle is superior to nano-sized Si in battery performances and business value and 2) the production cost is expected to be significantly lower than commercially available nano-Si by utilizing the by-product from an already matured mass production system. Further improvement in consideration of commercial feasibility and processing cost is achieved by employing ASP into spherical Si/graphite composite systems. In the optimized system, the mechanical stress of Si particles induced by huge volume expansion on Li insertion is further alleviated and the direct contact between Si and electrolyte is prevented, by which irreversible reaction leading to deterioration of coulombic efficiency is reduced. In this context, we have performed a systematic study on the influence of choice and application method of carbon materials which is a critical element in the composites. In such a way, the achieved advantageous characteristics are summarized as follows: 1) well-distributed Si particles in the composite system preventing agglomeration and 2) a strong bonding between Si and graphite maintaining electrical contact under repeated charge-discharge, 3) a ductile matrix to buffer the huge stress caused by the volume expansions and 4) buffer spaces inside the composite to accommodate the volume swings. It is worth noting that such an extraordinary ASP/graphite composite is manufactured in inexpensive way as the process is consists of only a few steps. The electrochemical characterization demonstrates remarkable performances as the coulombic efficiency at the first cycle reaches up to 91% and the capacity retention after 100 cycles is over 95% at 620 mAh/g. Material characterization is performed through several different analytical methods including HRTEM, in-situ solid state lithiation, in-situ dilatometer to elucidate and discuss the unique property of ASP and the superior performance of our spherical Si/graphite composite systems.

To response the multiply increased demand in portable electronics, hybrid electric vehicles and large scale energy storage over the last decade, Li-ion batteries (LIBs) with higher energy density and power capability have been desired. With respect to the anode of LIB, Silicon have received a great deal of interest as the fully lithiated (ca. Li22Si5) alloy has the highest theoretical capacity of any known material at 4140 mAh/g (3579 mAh/g for Li15Si4at room temperature). Although it is about 10 times higher than that of a conventional graphite anode, a Si-based electrode has yet to find use in commercial application because of its low coulombic efficiency (C.E.) and poor capacity retention. Si is typically known to undergo over 300% of volume expansion on Li insertion, which leads to cracking and crumbling of active particles and the resultant loss of electrical contact between electro-active materials causes rapid capacity fading. Although extensive researches have been reported to alleviate the problems, most of approaches are commercially unviable as they usually involve complicated and prohibitively high cost processes. Hence, a Si anode with sufficient performances and low cost production is highly desired. This study presents a novel cost-saving Si-based anode material with high coulombic efficiency and prolonged cyclability of excellent electrochemical performances using a low-cost synthesis though the attractive route. As a part of our strategy, we attempted to employ a by-product derived from a granular poly-Si mass production system for the feedstock of solar cell application. In such a way, a certain amount of amorphous silicon particles (ASP), the by-product, is derived homogeneously from the nucleation of gas dust in a silane-based gas phase reactor. (Fig. 1 shows HRTEM image and SAD pattern for ASP.) Our pioneering attempt is advantageously characterized by follows: 1) obtained sub-micron sized Si particles are superior to nano-sized Si particles in the battery performance and business value and 2) the production cost is expected to be many times lower than nano-sized Si by utilizing the by-product from an already matured mass production. Further improvement in consideration of commercial feasibility and processing cost is achieved by employing ASP into spherical Si/graphite/carbon composite systems. In Fig. 2, it is seen that spherical ASP/graphite/carbon composite is obtained with the particle size around 7-11 μm. Representative cross-section view demonstrates carbon coated ASP are enclosed by graphite layers; though vacant areas exist sparsely. In the optimized system, the mechanical stress on Si particles induced by huge volume expansion on Li insertion is alleviated and the direct contact between Si and electrolyte is prevented, by which irreversible reaction leading to deterioration of coulombic efficiency is reduced. In this context, we have performed a systematic study on the influence of choice and application method of carbon material which is a critical element in the composites. In such a way, ASP are carbon coated before or during the fabrication process of spherical composites and the achieved advantageous characteristics are summarized as follows: 1) separation of each silicon particles, preventing agglomeration and 2) a strong bonding between Si and graphite for maintaining the electrical contact on charge-discharge, 3) a ductile matrix to buffer the huge stress caused by volume expansion and 4) vacant space inside the composite to accommodate the huge volume changes. It is worth noting that such extraordinary ASP/graphite/carbon composites are manufactured in an inexpensive way as the process consists of only a few steps. The electrochemical characterization demonstrates remarkable performances as the coulombic efficiency at the first cycle reaches up to 90% and the capacity retention after 100cycle is over 92% at 620 mAh/g. Material characterization is performed through several different analytical methods including HRTEM, in-situ solid state lithiation, in-situ dilatometer to support and discuss the unique properties of ASP and the superior performances of our spherical Si/graphite/carbon composite systems.

Electrochemical performance and interfacial investigation on Si composite anode for lithium ion batteries in full cell

The solid electrolyte interphase (SEI) formed by the conventional constant-current process is often uneven and fragile, leading to low initial Coulombic efficiency, which induces rapid capacity degradation of silicon (Si) anodes. To obtain stable and uniform SEI, a pulse voltage method is proposed to construct a lithium fluoride(LiF) rich bilayer SEI on the Si anode surface, which selectively involves more anions in the SEI formation reaction by applying pulse voltage perturbations without altering the composition of the electrolyte. In-depth research has found that the pulse voltage protocol establishes an oscillation screening mechanism for anions, facilitating the arrival and decomposition of more anions on the surface of the Si anode to form a LiF-rich bilayer SEI. Such SEI exhibits high mechanical stability and rapid lithium-ion transport kinetics, achieving the initial Coulombic efficiency (ICE) of the Si anode improved to 90.8% and an enhanced capacity retention rate.

Silicon (Si) anodes, which are among the most promising candidates for high-energy lithium-ion batteries (LIBs), have attracted considerable attention from both academic and industrial communities. However, Si anodes usually suffer from an inherently low conductivity and extremely large volume change during the lithiation and delithiation processes, and consequently exhibit an inferior rate capability and poor cycle life. In this paper, we report new porous polymer-derived carbon coated Si nanoparticles (NPs) as the next generation anodes for LIBs to overcome these serious problems. Specifically, a porous covalent triazine framework (CTF) polymer shell was synthesized by in situ trimerization of p-benzenedinitrile in molten ZnCl2. Then, core-shell structured Si/nitrogen-doped porous carbon (Si@NPC) spheres were easily produced after high-temperature annealing. As an anode for LIBs, Si@NPC delivers a high capacity of 1390 mA h g-1 at 0.5 A g-1, stable cycle performance (107% capacity retention at 1 A g-1 for 200 cycles), and excellent rate capability of up to approximately 420 mA h g-1 at 16 A g-1. Such an exciting performance can be attributed to the ultra-stable, highly conductive, N-doped, and porous carbon shell. This work not only offers a new solution to the large volume change of Si-based anodes, but also enables the synthesis of porous polymer-based core-shell structures for energy storage and conversion.

Silicon electrodes in lithium-ion batteries undergo large volume changes during (de-)lithiation.[1] This leads to severe morphological changes over extended charge/discharge cycles, resulting in particle pulverization and surface area increase concomitant with a continuous growth of the solid-electrolyte-interphase (SEI).[2] To address this issue and to improve cycle-life, various mitigation strategies have been used in the literature, such as the use of nanostructured particles of silicon.[3] While this reduces particle pulverization, the high surface area of nanostructured silicon leads to high initial losses due to SEI formation. In this work, a concept presented by Jantke et al. is investigated using micro-scale silicon particles operated under capacity-limited conditions (i.e., partial utilization of its capacity) in order to diminish the above described phenomena.[4] Since crystalline silicon is irreversibly transformed into an amorphous phase upon lithiation, the purpose of the partial capacity utilization is to maintain a crystalline core and thus to prevent a full particle pulverization.In the present study, we investigate the amorphization process of micro-scale silicon particles in a silicon dominant anode containing 70 %wt silicon over extended charge/discharge cycling in half-cells with a lithium reference electrode, varying the lower cutoff potential of the Si electrode. While the capacity of the Si electrode after formation remains constant for cutoff potentials of ≥170 mV vs. Li+/Li (see green line in Fig. 1) of the lithiation step, the capacity continuously increases over cycling for cutoff potentials of <170 mV vs. Li+/Li (see purple line in Fig. 1), implying an ongoing amorphization of the crystalline core (cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20, C-rates a referenced to the areal capacity of 1.0 mAh/cm2).[5] Interestingly, relaxation experiments show that the open circuit voltage (OCV) of the lithiation- and therefore amorphization reaction for crystalline silicon during the formation cycle is also observed at 170 mV vs. Li+/Li. We further validate the change of the amorphous/crystalline phase fraction over cycling by in-situ XRD measurements of Si/Li pouch-cells with a lithium reference electrode. Generating a specific amount of amorphous silicon during formation is achieved by using a defined capacity for subsequent cycling, which prevents the potential of the Si anode from dropping below the critical potential, and thus maintains the crystalline core. Finally, the cycling stability in full-cells with Si anodes and NMC622 cathodes will be presented. Our results offer insights into the effect and the progress of the silicon amorphization process when Si anodes are operated under capacity-limited conditions in order to avoid silicon particle pulverization and to enhance the performance and lifetime of silicon-based anodes. N. Obrovac, V. L. Chevrier, Chemical Reviews, 114, 11444 (2014).Wetjen, S. Solchenbach, D. Pritzl, J. Hou, V. Tileli, H. A. Gasteiger, J. Electrochem Soc., 165, A1503 (2018).Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, Journal of Power Sources, 257, 421 (2014).Jantke, R. Bernhard, E. Hanelt, T. Buhrmester, J. Pfeiffer, S. Haufe, J. Electrochem Soc., 166, A3881 (2019).N. Obrovac, L. J. Krause, J. Electrochem Soc., 154, A103 (2007). Figure 1 Delithiation capacity of Si//Li pouch-cells (loading: 1.7 mgSi/cm2; utilized areal capacity: 1.0 mAh/cm2) with a lower cutoff limit for lithiation of either 140 mV vs. Li+/Li (purple) or 170 mV vs. Li+/Li (green). Formation cycles were conducted by cycling at C/10 (with constant-current (CC) lithiation and delithiation), while subsequent cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20 (C-rates are referenced to the utilized areal capacity). Experiments were conducted at 25°C with 1M LiPF6 in FEC/DEC (2:8 v:v) and with two glassfiber separators (VWR). Figure 1

The silicon (Si) solid electrolyte interphase (SEI) faces cyclical cracking and reconstruction due to the ~350% volume expansion. Understanding the SEI dynamic morphology and chemistry evolution from delithiated to lithiated states is thereby paramount to engineering a stable Si anode. Fluoroethylene carbonate (FEC) is a preferred additive with widely demonstrated enhancement of the Si cycling. Thus, insights into the dynamics of the FEC-SEI may provide hints toward engineering the Si interface. Herein, complementary ATR-FTIR, AFM, tip IR, and XPS probing reveal the presence of an elastomeric polycarbonate-like matrix in the FEC-generated SEI which is absent from the FEC-free SEI. Adding FEC to the baseline 1 M LiPF6 in EC:EMC (1:1) electrolyte promotes formation of a thinner and more conformal SEI, and subdues morphology and chemistry changes between consecutive half-cycles. From AFM, morphological stabilization of the FEC-SEI occurs earlier. Furthermore, conventional SEI biproducts such as Li2CO3 and LiEDC appear in reduced quantities in the FEC-SEI implying a reduced quantity of Li-consuming species. The thin polymeric FEC-SEI enables deeper (de)lithiation of silicon. In conclusion, the enhanced mechanical compliance, chemical invariance, and reduced Li inventory consumption of the FEC-SEI are logically the key features underlying the Si cycling enhancement by FEC.

Lithium ion batteries (LIBs) has been the main energy storage medium for portable electronics since the commercialization by Sony Corporation since 1991. Ever growing demand for large-scale energy storage devices such as electrical vehicles requires the high energy density and long cycle life LIBs that could not be attained by the currently used battery. Graphite is the most common anode material in commercial LIBs, but its specific capacity is relatively low (372 mAh g-1) with a poor rate capability, which does not meet the high demand for large-scale applications. Silicon (Si) is one of the most promising anode candidates due to its high theoretical specific capacity (3579 mAh g-1 for Li15Si4), relatively low discharge potential (~0.4V vs Li+/Li), and abundance in nature. However, Si goes through a dramatic volume change during cycling and thus undergoes a drastic structural deformation, which induces loss of electrical contact between Si and current collector, breakdown of solid electrolyte interphase (SEI) layer, and Si aggregation during cycling, eventually resulting in failure of cyclic performance. Although design a various nanostructure forms or introduction of secondary phase layers has been realized as a method to achieve high-performance Si anode, these methods require complex reactions such as hydrothermal or template-assisted synthesis and usually involve hazardous reagents like SiH4 or HF, which eventually result in cost and barrier to scale up. From the practical view, use of commercial Si is the most promising, where the commercial production of Si has been benefitted from the development in semi-conductor industry. Conventionally, battery is prepared by mixing active materials and binder with conducting agent in organic solvent to produce viscous slurry and blading onto copper current collector. Recently, the selection of binder has turned out to play critical role in the battery performance as maintaining the structural integrity of electrode film. It has been revealed that carboxylic acid or hydroxyl functional groups terminated binders which is commercially available, such as polyacrylic acid (PAA), carboxymethyl cellulose (CMC or Na-CMC) or alginate (Alg or Na-Alg) binder, can result in enhanced cycling performance utilizing the hydrogen bonding interactions with the native oxide on Si, compared to that of conventional PVDF binder which has no functional group and bonds weekly with Si through a week Van der Waals interaction. Nonetheless, due to the linear chain characteristic of these binders, susceptible sliding upon the continual volume change of Si results in the contact loss between binder and Si, which eventually results in the irreversible capacity failure. Herein, we propose a novel concept that can achieve stable Si anode by transitioning the bonding nature between binder and Si from hydrogen bond to mechanically robust covalent bond, which can effectively restrain any large movement of Si, and eventually prevent the destruction of the electrical network during cycling. Inducing the esterification reaction between binder and Si can be an efficient method to form a covalent bond between those two materials. (eq. (1)) R-COOH+HO-R’ -> R-COO-R’+ H2O -------- (1) To induce an esterification reaction between binder and Si, three crucial points have to be addressed. First, as carboxylate (–COO-) functional group terminated binder such as Na-CMC can hardly react with the hydroxyl functional group (-OH), PAA binder which is terminated with the carboxylic acid functional group (-COOH) has been selected in our system. Second, while commercially purchased Si has a –OH functional group by a partially hydrolyzed SiO2 layer covering the Si particles, its functional group density is insufficient for an esterification reaction to take place. Therefore, terminating –OH functional group at the surface of Si is crucial factor to increase the site to react with PAA binder. Third, due to kinetically sluggish characteristic of esterification reaction, proper usage of esterification catalyst is required. Experimental procedure is as follows; firstly, as-received Si was pretreated in piranha solution to terminate –OH. Then, –OH terminated Si was prepared into slurry by mixing with Super-P and PAA binder, casted onto copper foil, dried overnight and punched for a coin-type cell. To induce the esterification reaction, punched cell was dipped into sodium hypophosphite catalyst solution and then annealed in 180 °C for 1 h. Three counter groups are denoted as HB-Cell (punched cell with no annealing process), PE-Cell (annealed cell without usage of sodium hypophosphite), and FE-Cell (annealed cell with addition of sodium hypophosphite.) The fabricated FE-Cell exhibits superior cycle stability (2200 mAh g-1 at 100 cycles) compare to HB-Cell or PE-Cell, demonstrating the effect of covalent bond on obtaining the high-performance Si anode. Figure 1

A ureido-pyrimidinone (UPy)-functionalized poly(acrylic acid) grafted with poly(ethylene glycol)(PEG), designated PAU-g-PEG, was developed as a high performance polymer binder for Si anodes in lithium-ion batteries. By introducing both a ureido-pyrimidinone (UPy) unit, which is capable of self-healing through dynamic hydrogen bonding within molecules as well as with Si, and an ion-conducting PEG onto the side chain of the poly(acrylic acid), this water-based self-healable and conductive polymer binder can effectively accommodate the volume changes of Si, while maintaining electronic integrity, in an electrode during repeated charge/discharge cycles. The Si@PAU-g-PEG electrode retained a high capacity of 1,450.2 mAh g−1 and a Coulombic efficiency of 99.4% even after 350 cycles under a C-rate of 0.5 C. Under a high C-rate of 3 C, an outstanding capacity of 2,500 mAh g−1 was also achieved, thus demonstrating its potential for improving the electrochemical performance of Si anodes.

Cycle performance of silicon nano-flake powder (LeafPower® Si) anodes was improved by coating of perfluorosulfonic acid (PFSA) polymers (Nafion® and Aquivion®) as an artificial solid electrolyte interphase (SEI). The Si electrodes were coated with PFSA ionomer by addition of PFSA ionomer solution to aqueous slurry of the Si composite electrode. H+-type Nafion®-coated Si electrodes demonstrated improved capacity retentions and Coulombic efficiencies owing to the formation of a LiF-rich SEI composed of reductive decomposition products of Nafion®. The Nafion® coating suppressed the increase in polarization and the electrode swelling upon cycling because the LiF-rich SEI effectively suppressed electrolyte decomposition. Li+ and Cs+-type Nafion® and H+-type Aquivion®, which has a shorter side chain than Nafion®, provided similar positive effects on cycle performance of the Si electrodes. Furthermore, the Si electrodes achieved good cycleability up to 500 cycles owing to synergy effects of the Nafion® coating and an SEI-forming additive of fluoroethylene carbonate (FEC).

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

Fluoroethylene carbonate (FEC) has been commonly used as an electrolyte additive to improve the stability of the solid electrolyte interphase (SEI) at silicon and graphite-based anodes. Although the impact of FEC is well known, there is still lack of understanding how FEC has influence on interfacial chemistry and mechanics at the anode/electrolyte interface. In addition, relatively little attention has been paid to understanding of the effects of FEC on the cathode side. In this work, we symmetrically investigated the effects of FEC on the graphite/electrolyte and LiMn2O4/electrolyte interfaces at room and elevated temperatures and revealed associated interfacial chemistry and mechanics at the interfaces, using various electrochemical and material characterizations. We demonstrated that FEC has different effects on the interfacial behavior of the anode and the cathode. In the anode side, the SEI layer derived from FEC exhibited lower interfacial resistance and excellent thermal stability, showing excellent rate capability and improved cycle retention of cells. While the chemical and mechanical instability of the SEI layer were overcome by using FEC additive in the electrolyte, adverse effects of FEC on the cathode side were observed, especially at elevated temperatures. Poor cycling retention and a rapid increase in the interfacial resistance of the cathode were observed at elevated temperature. The poorer performance of the cathode in the FEC-containing cell at elevated temperature was attributed to the formation of a thicker surface layer and to increased Mn dissolution catalyzed by hydrofluoric acid (HF), which resulted from FEC dehydrofluorination initiated or accelerated by elevated temperature. Accordingly, it is suggested that the amount of FEC in a full cell must be optimized to minimize the adverse effects of FEC on cathode. Based on this work, the necessity of evaluating the effects of the electrolyte additive used on the cathode side should be emphasized in order to further improve the thermal stability of Li-ion batteries.