Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries

In the recent years, rechargeable lithium-ion batteries (LIBs) have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of LIBs. Especially for high energy and power densities, the electrode materials must possess a high specific storage capacity and a high coulombic efficiency. However, state-of-the-art anode and cathode electrode materials, e.g. graphite and LiFePO4 exhibit a high coulombic efficiency but a rather low theoretical storage capacity (372 and 170 mAh⋅g-1, respectively). In the last decade silicon has become a promising anode material due to its high theoretical specific capacity of 3579 mAh⋅g-1at ambient temperature. However, this high specific storage capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 300 % during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nano-sized active material or carbon coated silicon composite material. In addition to the materials science the electrode structure is of particular importance for the electrochemical performance. Electrode composition, binding mechanism due to the use of suitable binder polymers, particle size distribution of the active material or the modification of the SEI are some exemplary parameters to stabilize the electrode structure and to handle such high mechanical stress during lithiation/delithiation. Finally, the developed silicon anode must be implemented into a full cell by combining the anode with a suitable cathode. Because in a commercial LIB only the cell voltage is controllable, the electrode balancing of the full cell, that means the capacity ratio of the negative to positive electrode (N/P ratio) and the electrochemical voltage window in which the full cell is operated, is practically important to achieve a long cycle life and a high coulombic efficiency for the battery. For example, it can be ensured by adjusting the electrochemical potential window and choosing a well-balanced electrode design (normally N/P>1) that irreversible capacity losses can be prevented, which are correlated to lithium plating on the anodes surface during the charging process. Here we present electrochemical investigations of high capacity and high efficiency graphene coated silicon nanocomposite based electrodes prepared by using a water based wet chemical doctor blade manufacturing process. The commercially available Si/C-composite is mixed with graphite to obtain a Si/C-anode that provides a capacity of 1000 mAh⋅g-1 with an average coulombic efficiency >99 % over more than 500 cycles in half cells. The developed Si/C-anode is combined with a LiFePO4-cathode to build high-energy full cells. Investigations focus on the influence of the N/P ratio and the voltage window on the electrochemical performance of Si/C-LiFePO4 full cells. It will be shown that the developed Si/C-anode improves the energy density of the full cell regarding to a comparable C6-LiFePO4 full cell over more than 200 cycles.

Electrochemical capacitors have attracted increasing attention not only for portable electric devices as smart phone and tablet PC, but also for large power sources in electric vehicles. The advantages of electric double layer capacitors (EDLCs) are high-rate capability and long cycle life, which quite fit the recent needs for smart grid and energy harvesting. The good performances of EDLCs are attributed to the fast charge/discharge mechanism by using electric double layer regardless of the Faradaic reaction with electrodes. However, the main drawback of EDLCs is a small energy density compared with the other energy storage devices, i.e. lithium ion batteries (LIBs), nickel-hydrogen batteries, etc [1]. To overcome this weak point, recently lithium ion capacitors (LICs) using Li pre-doped graphite anodes have been developed and partially released as commercial products. Fig. 1 shows the schematic models for the principles of EDLC and LIC. For the EDLC using two active carbon (AC) electrodes as both cathode and anode, the cell voltage was linearly changed by charging and discharging, which limits the energy density of EDLC to a low level. On the other hand, for the LIC system, the cell voltage becomes much higher by using Li pre-doped graphite anode with a low redox potential and the energy density also increased up to about 3 times larger than that of EDLC. In this study, to further enhance the energy density, we tried to fabricate a new LIC system using Li pre-doped silicon (Si) anode, i.e. Si-CAP (Fig. 2), and investigated the better conditions to prepare the Li pre-doped Si anode for the Si-CAP.The Si anode was prepared by coating slurry on Cu foil as a current collector. The slurry was prepared by mixing 83.3 wt% Si nano-powder (Alfa Aesar®, Φ = 50 nm), 5.6 wt% Ketjen Black (KB) as a conductive agent and 11.1 wt% carboxymethyl cellulose sodium (NaCMC) salt as a binder. For optimization, two types of thickness for the Si anode were prepared; one was ca. 20 μm and the other was ca. 12 μm. Li pre-doping of the Si anode was carried out by making a contact with Li foil and immersing in 1 M LiBF4 dissolved in propylene carbonate (Kishida Chemical Co, Ltd.), where the treatment time was set to 1, 2 and 3 hours. Coin-type cells with the Li pre-doped Si anodes, AC cathode and 1 M LiBF4/PC electrolyte were fabricated and the electrochemical properties were evaluated by charge/discharge tests in constant current (CC) mode at 30oC.Fig. 3 shows a typical charge/discharge curve of the Si-CAP with a Li pre-doped Si(thick, 2 h) anode. The cell voltage successfully increased up to 4.2 V vs. Li/Li+ and the charge/discharge curve was bent upward (not linear). These indicates that the Li pre-doped Si anode worked well as low potential redox electrode and increased the cell voltage of Si-CAP. The plots of discharged energy densities vs. cycle number for the Si-CAPs with various Li pre-doped Si anodes were shown in Fig. 4. For the EDLC using the same electrolyte (1 M LiBF4/PC), the energy density was ca. 40 mAh g-1, which was very stable during 50 cycles. On the other hand, all the Si-CAPs exhibited higher energy densities although the energy densities decreased with the charge/ discharge cycling. In comparison of the Si-CAPs with Si(thick) anodes, the energy density increased with an increase in the Li pre-doping time. However, the decrease in energy density was also more rapid and significant. This implies that the extension of Li pre-doping time caused to large volume change of Si nano-powders and gave a bigger damage. Therefore, to improve the method of Li pre-doping we used a thinner Si anode, which was easy to conduct the Li pre-doping in short time (1 h) and made the treatment more homogeneous. As a result, the Si-CAP with Si(thin, 1 h) anode exhibited the highest energy density of over 400 mAh g-1(Si), and 292 mAh g-1(Si) was remained even after 50 cycles, which was more than 7 times larger of that of EDLC. Therefore, the Si-CAP was quite attractive as one of the next-generation energy storage system. Further optimization of Li pre-doped Si anode and the cell performances will be discussed in the meeting.This study was supported by JST “A Tenure-track Program” from MEXT, Japan.[1] K. Naoi et al., Energy & Envirn. Sci., 5, 9363 (2012).

Further enhancements regarding the energy density and specific power of lithium ion batteries are absolutely necessary in order to satisfy the increasing requirements for automotive applications e.g. extended driving ranges. One way to realize such improvements, depicts the replacement of commonly used carbon-based anode materials with high capacity anodes.1 In this regard silicon (Si) containing composites are considered promising candidates for the replacement of carbonaceous anode materials due to the significantly higher specific capacity of Si. However, the use of Si is still hindered by several challenges that have to be overcome for a successful application. One major issue of Si-based anode materials is the strong capacity decay due to the huge volume changes (~ 300%) of Si during the lithiation/de-lithiation process, leading to an recurring solid electrolyte interphase (SEI) reformation, which results in the ongoing consumption of active lithium from the cathode.2 One concept to alleviate the detrimental effects previously mentioned and to boost the performance of such anode materials, comprises the combination of Si with a matrix material. The general idea behind this approach is to combine Si with a second phase that enhances the mechanical stability and can buffer the volumetric changes of Si and thus, enables the formation of a stable SEI.3,4 In this study, we present a silicon iron (Fe) composite that contains two phases, a crystalline Si phase and a second, intermetallic FexSiy-phase. The applied synthesis route yields materials that combine a porous structure with the aforementioned matrix approach. This composite design is believed to have a beneficial effect on the capacity retention during cycling since it may buffers the volumetric changes of the Si and simultaneously provides extra space for the volume changes. The applied synthesis route contains a ball-milling and washing step, and subsequently the addition of a thin carbon coating. The composites, synthesized this way, are investigated via scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and thermogravimetric analysis in order to characterize their structure, morphology and composition. Moreover, electrochemical studies on the long-term cycling and rate performance with regard to the application as anodes in lithium ion batteries are conducted. Thereby, this work focuses on the influence of a high temperature treatment, as well as the influence of the Fe to Si ratio on the electrochemical performance. Furthermore, the role of the stabilizing FexSiy matrix phase in the composite is investigated regarding the question whether this phase is inactive towards lithiation or if it contributes to the lithiation/de-lithiation capacity of the composite. References 1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964. 2 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. 3 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90. 4 Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-alloy based anode materials for Li secondary batteries. Chemical Society reviews 2010, 39, 3115–3141.

Silicon(lithiated)–sulfur full cells with porous silicon anode shielded by Nafion against polysulfides to achieve high capacity and energy density

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.

Lithium ion batteries are considered to be the best choice for future hybrid electric vehicles (HEV) and full electric vehicles (FEV). HEV and FEC require batteries with higher specific energy, energy density, and longer cycle life than those used in consumer electronics. The available lithium ion batteries materials cannot meet the requirements of batteries to be used in FEV and HEV. The commonly used anode material is graphite. The theoretical specific capacity of graphite is 372 mAh g-1 and the performance of commercial graphite materials almost reaches their limit. Silicon anode material has attracted more attention of late due to its high capacity for lithium. Si can form an alloy with lithium (Li4.4Si) with a theoretical capacity of 4200 mAh g-1. In the past several decades, although silicon has been considered for use in lithium ion batteries, capacity decay with cycling is still a barrier to its application. It is believed that the large volume changes of Si (～300%) during lithiation and delithiation leads to rapid decay of capacity. Repeated cycling results in pulverization of silicon, which causes structural change of the Si electrode, and newly exposed surface area, which accommodates side reactions [1]. In this presentation, we researched the side reaction rate and investigated the capacity fading mechanism of Si anodes coupled with a lithium iron phosphate (LFP) counter electrode. A composite electrode of Si (obtained from Umicore) was prepared by our previously reported method [2]. 2325 coin cells were assembled for electrochemical measurements. LiPF6-EC/DEC (1.2M, 3/7 wt)+30 wt% FEC was used as electrolyte. Cycle performance of Si/LFP full cell is shown in Fig.1 a. Capacity fading happened continuously with cycling. Although the average voltage on charge and discharge drops, the difference remains relatively constant, which means capacity fading is not due to resistance rise. From 3-electrode cell data (in Fig. 1 b), the charge process was limited by anode side, initially, and then switched to the cathode side with cycling. Lithium in the cathode was consumed gradually with cycling. We concluded that the lithium imbalance is one of the factors that lead to the capacity fading. For Si anode, consumed lithium may take part in SEI formation or be trapped in Si. Electron energy loss spectroscopy (EELS) results show that lithium accumulated in Si particles with cycling. Since there is no impedance rise with cycling, lithium accumulation is mainly from lithium trapping inside Si particles. Particle isolation is another factor in Si anodes to cause capacity fading. From charge and discharge end point data, we calculated capacity fading rate caused by particle isolation and side reactions. The rate of side reactions is higher than that of particle isolation. C-rate transition from C/10 to C/5 charge and C/3 discharge results in catastrophic particle isolation followed by high rates of side reactions. Eventually, the rate of side reactions drops to their initial rate and particle isolation drops to a low level but this is much past end of life. Reference: [1] C. Chan, H. Peng, Y. Cui et al, Nature Nanotechnology 2008, 3, 31-35. [2] N. Yuca, H. Zhao, X. Song, M. Dogdu, W. Yuan, Y. Fu, V. S. Battaglia, X. Xiao, G. Liu, ACS Applied Materials & Interfaces. 2014, 6, 17111-17118. Fig.1. Electrochemical performance of LFP/Si (a. cycle performance, b. voltage profile). Figure 1

Highly aligned copper zinc tin sulfide nanorods electrophoretically deposited directly on the current collector are tested for suitability as Li-ion battery anodes in both half-cell (HC) and full-cell (FC) configurations. This facile fabrication process offers several advantages for high-performance nanostructured battery electrodes, notably the formation of a dense, conductive carbon and binder-free film maximizing active material content. High initial capacities of 1611 and 1369 mA h g–1 are achieved for the HC and FC, respectively. The capacity trends and degradation mechanisms for this combined alloying and conversion material are analyzed in detail using differential capacity plots and electrochemical impedance spectroscopy, and it is determined that an evolution in the electrode resistance (instead of typical material pulverization/delamination) is the major driver of an initial capacity fade followed by a dramatic capacity recovery. Differences in capacity retention trends between HCs and FCs are h...

A silicon (Si) anode is a high-capacity alternative for carbonaceous anodes in lithium ion batteries. However, a large volume change during cycling and continuous side reactions with the electrolyte significantly limit its applications. We designed a localized high-concentration electrolyte using 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether (OTE) as a diluent with the desired molecular structure tailored for a Si anode operating over a wide temperature range. This electrolyte exhibits an optimized ion solvation structure and enabled better cycling stability of Si anodes over a wide temperature range. Full cells with Si/graphite composite anodes and LiNi0.5Mn0.3Co0.2O2 cathodes retained 80% of their capacity after 500 cycles in 1.74 M lithium bis(fluorosulfonyl)imide in dimethyl carbonate with OTE (0.51:0.84:0.84) with enhanced thermal stability. A Si anode cycled in the new electrolyte exhibits a much thinner solid electrolyte interphase and a well-preserved dense structure. The electrolyte design principle developed in this work can be used to extend the cycle life of other high-capacity electrode materials with large volume changes.

Silicon (Si) is a promising anode material for high-energy-density anodes in rechargeable lithium-ion batteries (LIBs), given its high specific capacity (>3500 mA h g⁻1) and low lithiation potential (~0.4 V vs. Li/Li⁺).However, its commercial application is hindered by a substantial volume expansion of up to 300% during cyclingand intrinsically low electronic conductivity. Although polymer-based binders have been extensively explored to alleviate Si particle expansion, their non-conductive properties restrict efficient electron transfer throughout the Si anodes. Consequently, Si anodes necessitate large amounts of conductive additives, reducing the proportion of active material within the electrode and consequently resulting in energy density losses.Thus, overcoming these limitations is crucial for enabling the practical application of Si anodes in high-energy-density LIBs.Super P (SP) carbon black, a widely used commercial conductive agent, consists of small spherical particles that form conductive networks within the electrode based on a point-to-point conduction model.In this model, electrons must traverse numerous contact points, with each point introducing resistance and thereby diminishing the overall conduction efficiency. Consequently, SP alone is insufficient for achieving optimal conductivity in Si anodes. On the other hand, carbon nanotubes (CNTs) feature an elongated strand-like structure that provides continuous electron transport paths, promoting uninterrupted and efficient electron flow across the electrode. The large surface area of CNTs further increases contact with active materials, boosting conductivity even with minimal CNT content. However, incorporating CNTs in Si electrodes faces challenges due to the hydrophobicity of CNT surfaces and pronounced van der Waals interactions arising from their large surface area, which result in aggregation in polar solvent-based Si electrode slurries. As the non-uniform dispersion of CNTs can disrupt ion and electron flow, ensuring the homogeneous distribution of CNTs is imperative to enhance the electrochemical performance of electrodes.Several methods have been reported to improve the dispersibility of CNTs. Avilés et al. introduced oxygen-containing functional groups onto CNT surfaces through acid treatment, increasing the hydrophilicity of CNTs and promoting stronger interactions with polar solvents, thereby reducing aggregation.However, the use of strong acids can damage the sp² hybridization of CNTs, adversely affecting electron transport. Zhou et al. synthesized Si-CNT composites by growing CNTs on the Si surface via chemical vapor deposition at 850 °C, achieving stable cycling performance for up to 1200 cycles.Nevertheless, the high operational temperatures (700–1000 °C) required for synthesis limit its commercial feasibility. Alternatively, polymer grafting has emerged as an effective strategy, enabling surface functionalization at moderate temperatures (60–100 °C) with minimal impact on CNT structural stability.Herein, we synthesized polyacrylamide-grafted CNTs (PAM-g-CNTs) by grafting acrylamide (AAm) onto the CNT surface. The introduction of amide groups offers a strong affinity for water molecules, and hydrogen bonding interactions between aqueous solvent and PAM-g-CNT promote uniform dispersion in water-based slurries. This homogeneous arrangement of CNT ultimately facilitates an efficient network to transport lithium ions (Li+) and electrons.Also, PAM-g-CNT establishes a robust binding system through a hydrogen bond between polar functional groups and slurry components. Consequently, the PAM-g-CNT electrode could maintain a stable electrode structure beyond consecutive volume change based on stronger mechanical strength and adhesion of the electrode compared to CNT. The high diffusivity of ions and electrons and structural integrity made a synergetic effect on the electrochemical performance of the battery utilizing PAM-g-CNT. Notably, in electrodes with an elevated active material content of 80% and reduced binder and conductive agent ratios, PAM-g-CNT demonstrated stable cycling (72.1%) after 100 cycles. In contrast, commercial conductive agents exhibited substantial capacity fading (20.1%). PAM-g-CNT can efficiently interact with solvent and slurry components through hydrogen bonds, enhancing water affinity and improving the resistance of Si electrodes against swelling. These characteristics of conductive agents realized advanced electrochemical properties, rate performance, long-term cell performance, and electrodes with high energy density. Figure 1

We, for the first time, successfully grafted well-aligned binary lithium-reactive zinc phosphide (Zn3P2) nanowire arrays on carbon fabric cloth by a facile CVD method. When applied as a novel self-supported binder-free anode for lithium ion batteries (LIBs), the hierarchical three-dimensional (3D) integrated anode shows excellent electrochemical performances: a highly reversible initial lithium storage capacity of ca. 1200 mA h g(-1) with a coulombic efficiency of up to 88%, a long lifespan of over 200 cycles without obvious decay, and a high rate capability of ca. 400 mA h g(-1) capacity retention at an ultrahigh rate of 15 A g(-1). More interestingly, a flexible LIB full cell is assembled based on the as-synthesized integrated anode and the commercial LiFePO4 cathode, and shows striking lithium storage performances very close to the half cells: a large reversible capacity over 1000 mA h g(-1), a long cycle life of over 200 cycles without obvious decay, and an ultrahigh rate performance of ca. 300 mA h g(-1) even at 20 A g(-1). Considering the excellent lithium storage performances of coin-type half cells as well as flexible full cells, the as-prepared carbon cloth grafted well-aligned Zn3P2 nanowire arrays would be a promising integrated anode for flexible LIB full cell devices.

The emerging electrical applications and transportation electrification call for safe and energy-dense batteries with long lifespans and considerable power outputs. For the current graphite-majority LIBs, the anode takes up more than 30% of the mass ratio and around 50% of the volume ratio within the cell unit because of the low specific capacity of the graphite (theoretical capacity ~372 mAh g-1), leading to a low energy density around 200 Wh kg-1. Replacing the insertion-type graphite anode with the conversion-type Si anode (theoretical capacity ~3579 mAh g-1) [1] or the Li-metal anode (theoretical capacity ~3860 mAh g-1) could help reduce the mass and volume portion to less than 5% and 10%, respectively, which is thus of great significance for further promotion of the energy density (>350 Wh kg-1) at the cell level. Aside from the performance-related issues, the safety concerns of LIBs should also be well addressed. Therefore, solid state electrolytes have triggered worldwide interest in the past few years for its incomparable safety of anti-combustion and no leakage. Besides, the solid-state electrolytes could also boost the promotion of battery energy density.However, most of the current research about solid-states electrolytes adopt Li-metal as the anode. Though solid-state Li-metal batteries (SSLMBs) are acclaimed to have the highest gravimetric specific energy density theoretically, it is challenged by several critical issues. First, the Li-metal anode could not well address the dendrite issue, which limits the critical current density (< 0.5 mA cm-2) and areal capacities (< 0.5 mA cm-2) of the battery [2]. Secondly, most of the reported SSLMBs are carried out at high-temperature (60℃ or above) or with incredibly huge pressure (fabrication at > 200 MPa; operation at > 50 MPa) [3][4], which is not realistic in the common scenarios. Besides, nearly all the reported SSLMBs are tested with large Li-excess (low reversibility of Li-plaiting and stripping) and limited cathode loadings (inefficient conductivity), leading to unacceptable energy densities of the cells.Herein, we present that the Si-anode, which is not as widely as Li-metal studied in solid-state batteries, is a well-qualified candidate for the Li-metal for practical solid batteries. The pure Si anode (~2.1 mAh cm-2) with the solid electrolyte can be stably cycled at room temperature without external pressure for more than 150 cycles with no sharp capacity fade and a remained capacity over 1800 mAh g-1. The Si||LiFePO4 full cell adopting this electrolyte could achieves 120 stable cycles with high Coulombic efficiencies (>99.9%). This study shows that the in-situ polymerized solid electrolyte could be well matched with Si-anode for practical high energy density battery configuration. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrbative Region, China (Project No. R6005–20). Reference [1] M.S. Kang, I. Heo, S. Kim, J. Yang, J. Kim, S.-J. Min, J. Chae, W.C. Yoo, High-areal-capacity of micron-sized silicon anodes in lithium-ion batteries by using wrinkled-multilayered-graphenes, Energy Storage Mater. 50 (2022) 234–242.[2] Q. Zhao, S. Stalin, C.Z. Zhao, L.A. Archer, Designing solid-state electrolytes for safe, energy-dense batteries, Nat. Rev. Mater. 5, (2020) 229–252.[3] D.H.S. Tan, Y.-T. Chen, H. Yang, W. Bao, B. Sreenarayanan, J. Doux, W. Li, B. Lu, S. Ham, B. Sayahpour, J. Scharf, E.A. Wu, G. Deysher, H.E. Han, H.J. Hah, H. Jeong, J.B. Lee, Z. Chen, Y.S. Meng, Carbon-free high-loading silicon anodes enabled by sulfide solid electrolytes, Science 373 (2021) 1494–1499.[4] Y. Ren, Z. Cui, A. Bhargav, J. He, A. Manthiram, A Self-Healable Sulfide/Polymer Composite Electrolyte for Long-Life, Low-Lithium-Excess Lithium-Metal Batteries, Adv. Funct. Mater. 2106680 (2021) 1–10. Figure 1

The high-energy lithium ion battery is an ideal power source for electric vehicles and grid-scale energy storage applications. Germanium is a promising anode material for lithium ion batteries due to its high specific capacity, but still suffers from poor cyclability. Here, we report a facile preparation of a germanium–graphene nanocomposite using a low-pressure thermal evaporation approach, by which crystalline germanium particles are uniformly deposited on graphene surfaces or embedded into graphene sheets. The nanocomposite exhibits a high Coulombic efficiency of 80.4% in the first cycle and a capacity retention of 84.9% after 400 full cycles in a half cell, along with high utilization of germanium in the composite and high rate capability. These outstanding properties are attributed to the monodisperse distribution of high-quality germanium particles in a flexible graphene framework. This preparation approach can be extended to other active elements that can be easily evaporated (e.g., sulfur, phosphorus) for the preparation of graphene-based composites for lithium ion battery applications.

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

Due to the high chemical reactivity of metallic lithium, safety and life-time of Lithium-ion cells are closely related to the aging phenomenon of lithium plating [1–5]. Lithium plating on graphite anodes can occur during charging at low temperatures, high C-rates, and high states-of-charge [1,3,4]. However, lithium deposition depends also on the combination of these parameters and can emerge in cases where it might not have been expected. For instance, lithium deposition can occur in commercial cells [3,4]. For safe and long-life operation of Lithium-ion batteries, lithium plating should therefore be prevented. Lithium plating is caused by negative anode potentials vs. Li/Li+ and can be determined in measurements with a reference electrode (see Figure 1). Furthermore, lithium plating is related to the activity of lithium ions in the electrolyte [5]. For example, it was recently found in our lab that the activity of lithium ions in the electrolyte is changing during the charging process [5]. This is in agreement with the change of the anode potential during charging (see Figure 1). In the present study, we present results from cells with and without lithium plating. The results are based on accelerated rate calorimetry (ARC) tests, measurements of lithium ion activity and of anode potentials in full cells vs. Li/Li+, as well as Post-Mortem results with a variety of physico-chemical analysis methods. A model is developed to explain the interplay between the main operational parameters influencing lithium plating. This model allows to prevent lithium deposition in commercial cells and therefore to increase safety and life-time significantly. [1] N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, M. Wohlfahrt-Mehrens, Detection of Li Deposition by Glow Discharge Optical Emission Spectroscopy in Post-Mortem Analysis, ECS Electrochem. Lett. 4 (2015) A100–A102. doi:10.1149/2.0041509eel. [2] M. Fleischhammer, T. Waldmann, G. Bisle, B.-I. Hogg, M. Wohlfahrt-Mehrens, Interaction of cyclic ageing at high-rate and low temperatures and safety in lithium-ion batteries, J. Power Sources. 274 (2015) 432–439. doi:10.1016/j.jpowsour.2014.08.135. [3] J.C. Burns, D.A. Stevens, J.R. Dahn, In-Situ Detection of Lithium Plating Using High Precision Coulometry, J. Electrochem. Soc. 162 (2015) A959–A964. doi:10.1149/2.0621506jes. [4] T. Waldmann, M. Kasper, M. Wohlfahrt-Mehrens, Optimization of Charging Strategy by Prevention of Lithium Deposition on Anodes in high-energy Lithium-ion Batteries – Electrochemical Experiments, Electrochimica Acta. 178 (2015) 525–532. doi:10.1016/j.electacta.2015.08.056. [5] B.-I. Hogg, M. Wohlfahrt-Mehrens, In Operando Li+-Activity Measurements in Lithium Ion Batteries-a Method to Develop and Optimize Safe Operating Strategies Even at Unfavourable Conditions, in: Meet. Abstr., The Electrochemical Society, 2015: pp. 343–343. Part of the research leading to these results has been performed within the MAT4BAT project (http://mat4bat.eu/) and received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n°608931. Figure 1 Measurement of anode potential vs. Li/Li+ for different temperatures in a full cell. Figure 1

Lithium ion batteries (LIBs) are considered as key technology for stationary energy storage systems and especially as power supply for electric vehicles (EVs). Even though LIBs are already used in EVs, there is a need of further improvements of the LIBs to achieve driving ranges of more than 500 km, what is considered to be a value for greater consumer acceptance of EVs. To reach this goal, the specific energy and energy density of LIBs need to be increased to approximately 350 Wh kg-1 and 750 Wh L-1 at cell level, respectively. The use of alternative anode materials to replace the state-of-the-art graphite anode, is considered as an efficient strategy to increase the energy density of LIBs. Silicon (Si) turns out to be the most promising material for advanced anodes in LIBs, as it offers a nearly 10 times higher specific capacity compared to graphite. However, the implementation of electrode materials containing contents of more than 5-10% Si in commercial LIBs is still hampered by huge volume changes leading to a continuous solid electrolyte interphase (SEI) re-formation, loss of active lithium and, therefore, to a poor capacity retention.[1] The application of nano-sized Si materials like nanoparticles, nanowires or thin films is reported to significantly improve the performance of Si based anodes, due to better accommodation of the huge volume changes upon lithiation/de-lithiation. Additionally, Si thin film electrodes exhibit improved specific capacities in comparison to typical composite-based electrodes, because of the absence of inactive components like binder and conductive additives.[2] An additional approach to improve the performance of Si based electrodes is the addition of additives to the electrolyte. Electrolyte additives like fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are commonly known to enhance the capacity retention of Si electrodes by forming a more stable SEI, thus, preventing ongoing electrolyte decomposition and continuous active lithium loss.[3] Isocyanate compounds are able to undergo reductive polymerization and, therefore, may be considered as effective electrolyte additives for Si anodes. Actually, several isocyanates were reported to function as effective film-forming additives for graphite-based negative electrodes.[4] Within this work, magnetron sputtering was utilized for the preparation of thin film Si anodes, which contain neither a binder nor a conductive agent. Therefore, the effect of the electrolyte additive can be directly related to the Si active material. Since it was recently reported, that lithium consumption related to SEI reformation is the main failure mechanism of lithium ion full cells containing a Si anode[5], the electrochemical performance of these Si thin film electrodes was investigated in Si/NMC-111 full cells using different electrolyte formulations. DFT calculations (HOMO/LUMO energies) were performed prior to electrochemical investigations for reductive and oxidative stability predictions of the electrolyte solvent and additive molecules. The addition of the pentafluorophenyl isocyanate electrolyte additive leads to an increased Coulombic efficiency and a significantly enhanced capacity retention of the LIB full cells during prolonged cycling, in comparison to the baseline electrolyte. Post-mortem investigations of the negative Si electrodes by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were performed to study the SEI layer, formed in the different electrolyte formulations. The enhanced cycling performance of the full cells can be correlated to an improved SEI formation. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] S. Dalavi, P. Guduru, B.L. Lucht, Performance Enhancing Electrolyte Additives for Lithium Ion Batteries with Silicon Anodes, Journal of The Electrochemical Society, 159 (2012) A642-A646. [4] C. Korepp, W. Kern, E.A. Lanzer, P.R. Raimann, J.O. Besenhard, M.H. Yang, K.C. Möller, D.T. Shieh, M. Winter, Isocyanate compounds as electrolyte additives for lithium-ion batteries, Journal of Power Sources, 174 (2007) 387-393. [5] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887.