Silicon Anodes In Lithium-ion Batteries Research Articles

Ternary Cross-Linked Multi-Functional Blended Polymers for High-Performance Silicon Anodes in Lithium-Ion Batteries

Silicon (Si) based anode material is highly attractive for next-generation lithium‐ion batteries (LIBs) due to its unparalleledtheoretical capacity and abundance. However, a severe problem of Si is the significant volume change associated with thelithiation processes, resulting in reduced capacity retention. Researchers have traditionally considered the roles ofinteractive binders and conductive additives as separate entities. These two components often lead to remarkablydecreased mass ratio of Si to nonactive material, which inevitably limits the electrode capacity. To achieve an enhanceutilization efficiency, herein, we have developed a multifunctional nanocomposite binder for high capacity nano-size Sibasedmaterial through a cross-linked polymer of carboxymethylcellulose (CMC), polyacrylic acid (PAA) spine withgraphenized polyacrylonitrile (PAN) through a gradual carbonizing route. This nanocomposite strongly interacts with Simaterial, providing a robust nanoarchitecture with abundant conductive pathways for Li-ion transport. CMC and PAA with alarge number of carboxyl groups provide binding ability by sturdy three-dimensional (3D) cross-linked network with relativelya thin SiO2 mechanical binding film on the surface of Si nanoparticles onto a highly porous carbon and forming a stablesolid electrolyte interface (SEI) layer. Meanwhile, graphenized PAN provides a highly interconnected conductivenanoarchitecture of a nitrogen-doped graphenized-like structure (NG), which provides enhanced pathways for lithium iondiffusion. Without any conductive additives, this nanocomposite material not only shows a high superior 1st dischargecapacity of 3473 mAh g-1, high initial Coulombic efficiency of 89%, excellent rate capability, and remarkable cycling life formore 600 cycles when cycled at high current density of 3000 mA g-1, but also maintaining good cyclability with a constanthigh areal capacity of ~ 2.7 mAh cm-2. Together with the ease of fabrication, this provides a promising avenue forcommercial LIBs. Figure 1

A robust network binder via localized linking by small molecules for high-areal-capacity silicon anodes in lithium-ion batteries

Silicon is known for high-capacity yet large-volume-change properties when used as lithium storage material. Polymeric binders substantially contribute to improving the cycling life and increasing areal capacity of silicon anode. Herein, a robust network binder (PG-c-ECH) via localized linking by small molecules is proposed to achieve stable cycling performance of silicon anode with high areal capacity. Peach gum (PG) is rich in abundant hydroxyl and carboxyl groups that conventionally ensure a strong binding affinity between the PG and Si. The epichlorohydrin (ECH) introduced as the short-chain chemical crosslinker further enhances the localized linking for the polymer chains. The resultant PG-c-ECH owns durable interfacial adhesion and outstanding mechanical properties to stabilize the structural integrity of Si anode. Herein, a robust network binder (PG-c-ECH) via localized linking by small molecules is proposed for high-areal-capacity Si anode. Because of its strong interfacial adhesion and excellent mechanical properties, the PG-c-ECH binder could tolerate the huge volume changes of Si active materials during repeated cycling. The Si electrodes using PG-c-ECH as the binder exhibit stable cycling performances at practical-level areal capacities. • A robust network binder (PG-c-ECH) via localized linking by small molecules is proposed. • The Si@PG-c-ECH electrode can be cycled over 10 mAh cm −2 for 35 cycles. • The Si@PG-c-ECH electrode achieves an ultrahigh discharge areal capacity of 60.00 mAh cm −2 . • The full-cell using the Si@PG-c-ECH anode shows a relatively stable cycling performance.

Voltage Hysteresis Model for Silicon Electrodes for Lithium Ion Batteries, Including Multi-Step Phase Transformations, Crystallization and Amorphization

Silicon has been an attractive alternative to graphite as an anode material in lithium ion batteries (LIBs). The development of better silicon electrodes and optimization of their operating conditions for longer cycle life require a quantitative understanding of the lithiation/delithiation mechanisms of silicon and how they are linked to the electrode behaviors. Herein we present a zero-dimensional mechanistic model of silicon anodes in LIBs. The model, for the first time, considers the multi-step phase transformations, crystallization and amorphization of different lithium-silicon phases during cycling while being able to capture the electrode behaviors under different lithiation depths. Based on the model, a linkage between the underlying reaction processes and electrochemical performance is established. In particular, the two sloping voltage plateaus at low lithiation depth are correlated with two electrochemical phase transformations and the emergence of the single broad plateau at high lithiation depth is correlated with the amorphization of c-Li15Si4. The model is then used to study the effects of crystallization rate and surface energy barriers, which clarifies the role of surface energy and particle size in determining the performance behaviors of silicon. The model is a necessary tool for future design and development of high-energy-density, longer-life silicon-based LIBs.

Highly Elastic Block Copolymer Binders for Silicon Anodes in Lithium-Ion Batteries.

Silicon (Si) is a promising anode material to replace the broadly adopted graphite due to its high capacity and abundant source. However, Si anodes suffer from severe problems of huge volume change (∼300%), and the commonly used binders like poly(vinylidene fluoride) (PVDF) cannot accommodate such changes. Here, we report a tough block copolymer PVDF-b-Teflon (PTFE) binder that can coalesce pulverized Si and thus enhance the stability of Si anodes. The suspension copolymerization of vinylidene fluoride and tetrafluoroethylene produces elastic PVDF-b-PTFE with large breaking elongations of >250% and high viscosity as well as high ionic conductivity and thermal stability. We show that 5 wt % of the binder forms elastic cobweb structures in the electrode matrix that can effectively coalesce Si particles and conductive agents together, enabling long cycling stability (>250 cycles) and high rate performance (1 C) for electrodes at a commercial-level Si loading of 1 mg·cm-2. The findings point out to a promising strategy for developing highly elastic and tenacious binders for electrodes with large volume changes during the electrochemical reactions.

Slime-inspired polyacrylic acid-borax crosslinked binder for high-capacity bulk silicon anodes in lithium-ion batteries

Binder plays a key role in maintaining the integrity of battery electrodes. In this work, we investigate the relationship between binder properties and performances of Si electrodes in a lithium-ion battery (LIB). We learn from nature and use borax as a crosslinker between strands of polyacrylic acid (PAA) to form a network in the binder. By adding different amounts of borax, we can control the degree of crosslinking and thus the mechanical and chemical properties of the binder. We find that increasing the amount of borax crosslinker increases the modulus of the polymer network and the stability of the corresponding Si electrode up to about 5 wt% borax. Even with Si microparticles with an electrode density of 1.25 g cm−3, we obtain a capacity of about 2470 mA h g−1 (3.1 mA h cm−2) and a capacity retention of 91.2% after 50 cycles. Cycle performance is extended beyond 500 cycles with a capacity limitation of 1000 mA h g−1. Though, excessive amount of borax consumes the functional groups of PAA and compromises electrode adhesion, resulting in a decrease in cycle stability. Striking a balance between mechanical strength and adhesion of the binder is important for further development of alloy-based anode materials for LIBs.

Surface SiO2 Thickness Controls Uniform-to-Localized Transition in Lithiation of Silicon Anodes for Lithium-Ion Batteries.

Silicon is a promising anode material for lithium-ion batteries because of its high capacity, but its widespread adoption has been hampered by a low cycle life arising from mechanical failure and the absence of a stable solid-electrolyte interphase (SEI). Understanding SEI formation and its impact on cycle life is made more complex by the oxidation of silicon materials in air or during synthesis, which leads to SiOx coatings of varying thicknesses that form the true surface of the electrode. In this paper, the lithiation of SiO2-coated Si is studied in a controlled manner using SiO2 coatings of different thicknesses grown on Si wafers via thermal oxidation. SiO2 thickness has a profound effect on lithiation: below 2 nm, SEI formation followed by uniform lithiation occurs at positive voltages versus Li/Li+. Si lithiation is reversible, and SiO2 lithiation is largely irreversible. Above 2 nm SiO2, voltammetric currents decrease exponentially with SiO2 thickness. For 2-3 nm SiO2, SEI formation above 0.1 V is suppressed, but a hold at low or negative voltages can initiate charge transfer whereupon SEI formation and uniform lithiation occur. Cycling of Si anodes with an SiO2 coating thinner than 3 nm occurs at high Coulombic efficiency (CE). If an SiO2 coating is thicker than 3-4 nm, the behavior is totally different: lithiation at positive voltages is strongly inhibited, and lithiation occurs at poor CE and is highly localized at pinholes which grow over time. As they grow, lithiation becomes more facile and the CE increases. Pinhole growth is proposed to occur via rapid transport of Li along the SiO2/Si interface radially outward from an existing pinhole, followed by the lithiation of SiO2 from the interface outward.

Effect of Phosphorus Doping on Conductivity, Diffusion, and High Rate Capability in Silicon Anode for Lithium-Ion Batteries

Poor long-term cycling stability and rate capability seriously restricted the commercial applications of silicon (Si) anodes, because of the huge volume change and the kinetic problems linked to th...

Improving the cyclability of silicon anodes for lithium-ion batteries using a simple pre-lithiation method

Despite intense studies into silicon anode materials, the large volume expansion of silicon remains a significant problem that reduces battery performance and shortens battery life. In this paper, a procedure to increase the cycle life of the silicon anode in full-cell configuration using a simple pre-lithiation method is proposed. By precisely controlling the thickness of the pre-deposited lithium through evaporation, the volume change of the silicon-containing anodes can be effectively reduced during discharge. A coin-cell fabricated with a high-capacity cathode and the pre-lithiated silicon–graphite anode shows improvements in cycling performance, as well as a higher discharge capacity. Electrochemical and microscopic analysis confirm that pre-lithiation successfully suppresses the volumetric changes. Thus, this work provides an effective method for the preparation of long-cycle-life Li-ion batteries containing silicon-based anodes.

Acrylic random copolymer and network binders for silicon anodes in lithium-ion batteries

The choice of a binder can greatly influence the electrochemical performance of lithium-ion batteries (LIBs), especially for silicon (Si) anodes. To systematically modulate the mechanical properties of binders and to investigate their impact on the electrochemical properties, we synthesize a series of random copolymers consisting of acrylic acid (AA) and n-butyl acrylate (n-BA) via reversible addition-fragmentation chain-transfer (RAFT) polymerization. The electrochemical properties of Si nanoparticle (SiNP) anodes prepared using copolymer binders exhibit a strong dependence on the relative composition of AA and n-BA. The incorporation of a moderate amount of n-BA (10–30 mol%) in the copolymer binder provides better cycling stability compared to a SiNP anode prepared using poly(acrylic acid) (PAA) binder. The binder containing 30 mol% of n-BA is further crosslinked by in-situ crosslinking using a variable amount of polyethylene glycol diglycidyl ether (PEGDE). The electrochemical properties (3050 mAh g−1 in the 1st cycle and 40% capacity retention in the 100th cycle at 0.5 A g−1) of a SiNP anode prepared using the crosslinked binder with 14 wt% of PEGDE show further improvement compared to those of a SiNP anode prepared using PAA binder (2310 mAh g−1 and 32% capacity retention in the 100th cycle at 0.5 A g−1).

Highly stable multi-layered silicon-intercalated graphene anodes for lithium-ion batteries

Abstract

Microporous Binder for the Silicon-Based Lithium-Ion Battery Anode with Exceptional Rate Capability and Improved Cyclic Performance.

Silicon anodes have attracted much attention owing to their high theoretical capacity. Nonetheless, an inevitable and enormous volumetric expansion of silicon in the lithiated state restrained the development of the silicon anode for lithium-ion batteries. Fortunately, the utilization of the high-performance binder is a promising and effective way to overcome such obstacles. Herein, a polymer of intrinsic microporosity (PIM) is applied as the binder for the silicon anode, which is composed of a rigid polymer backbone, an intrinsic porous structure, and active carboxyl groups (PIM-COOH). Compared to the traditional binder, both the long-term stability and rate performance of the electrode using PIM-COOH as the binder are significantly improved. The mechanism responsible for the enhanced performance is investigated. The PIM-COOH binder provides stronger adhesion toward the current collector than the conventional binders. The unique rigid polymer backbone and porous structure of the PIM-COOH binder enable a good capability to withstand the volume change and external stress generated by the Si anode. The porous structure of the PIM-COOH binder enhances lithium-ion transportation compared to the SA binder, which improves rate performance of the silicon anode. This work provides a unique insight into design, synthesis, and utilization of the binders for lithium-ion batteries.

A conductive self-healing hydrogel binder for high-performance silicon anodes in lithium-ion batteries

Although silicon has been viewed as the ideal anode material due to its superior specific capacity, silicon materials easily break down because of its large volume changes during frequent charge-discharge cycles. Polymeric binders play key roles on maintaining the mechanical integrity of electrodes and contributing substantially to improving cycle lives. Here, we design and prepare that a novel conductive self-healing hydrogel (ESVCA) binder with 3D conductive network, excellent stretchablity and fast self-healing ability coating on the surface the Si particle by in-situ polymerization method. The stretchable 3D self-healable network structure could effectively restrain the huge volume change of Si particles during lithiation and delithiation cycles to maintain mechanical integrity of the Si electrodes while 3D continuous electron transport pathways insuring the high conductivities and better electric contacts among active Si particles. The Si-ESVCA electrode not only exhibits a reversible capacity of 1786 mA h g−1 at 500 mA g−1 with a capacity retention of 71.3% after 200 cycles at ambient temperature, but also maintains superior cycling stability with a capacity retention of 74.1% and remains a high reversible capacity of 1743 mA h g−1 after 200 cycles at current density of 2000 mA g−1 at elevated temperature.

Electrochemical Amorphization As a Method to Increase the Rate Capability of Crystalline Silicon Anodes for Lithium-Ion Batteries

Anodes manufactured from macroporous silicon have been studied. Galvanostatic cycling tests in half-cells with Li counterelectrode in a regime of charging capacity limited to Q1 = 1000 mAh/g showed that the inclusion of a modifying cycle with increased time of electrochemical lithiation into the testing schedule allowed the charge/discharge rate to be increased upon return to the initial regime. The results are interpreted in terms of a two-phase model with a sharp a-LixSi/c-Si interface. The insertion of a large amount of lithium during the modifying cycle leads to an increase in the thickness of an amorphous layer, within which the insertion and extraction of lithium in subsequent cycles proceed at a higher rate.

Preparation of an Amorphous Cross-Linked Binder for Silicon Anodes.

An amorphous cross-linked binder is prepared from abundant and low-cost sodium alginate and carboxymethyl cellulose by protonation and mixing and is used to improve the electrochemical performance of silicon anodes in lithium-ion batteries. The amorphous cross-linked structure, formed by intermolecular hydrogen bonding between the functional groups in the two polymers, effectively enhances the flexibility and strength of the binder, resulting in strong adhesion between the binder and other components in the silicon anodes. Furthermore, the binder tolerates large volume changes and reduces the pulverization of silicon during the charge-discharge process. The hydrogen bonding in the binder helps to maintain the anode integrity during the volume change, leading to an excellent cycling stability and superior rate capability with a capacity of 1863 mAh g-1 at 500 mA g-1 after 150 cycles.

Cross-linked hyperbranched polyethylenimine as an efficient multidimensional binder for silicon anodes in lithium-ion batteries

We report the use of cross-linked hyperbranched polyethylenimine (PEI) as a commercially available, eco-friendly, water-soluble, efficient polymer binder for silicon anode of lithium ion battery for the first time. Compared with traditional one-dimensional linear binders, the PEI with 3-dimensional topological hyperbranched structure and high density of amino groups can facilitate stronger multidimensional hydrogen-bonding interactions with silicon nanoparticles which may more effectively buffer large volume change of silicon particles during lithiation/delithiation process. In addition, the cross-linked 3-dimensional network of PEI limits movement of silicon nanoparticles and maintains integrity of the anode. Through optimizing the amount of cross-linking agent, 1,4-butanediol diglycidyl ether, to balance bonding reversibility and mechanical strength of PEI, the cross-linked Si/PEI anode shows significantly improved cycling performance with a specific capacity of 2180 mAh g−1 at a current density of 500 mA g−1 after 100 cycles.

Tuning PAA-Based Binders Towards Optimized Performance for Silicon Anode in Lithium-Ion Batteries

Poly(acrylic acid) (PAA) based binders have been widely used for silicon anodes in lithium-ion batteries to mitigate the issues caused by reversible volumetric expansion of silicon nanoparticles during lithiation/delithiation process. However, the performance of silicon anodes using PAA-based binders is still far from meeting the requirement of practical applications and the factors affecting the performance of these binders are not fully understood. Neutralization of PAA binders using LiOH is a common strategy for processing silicon anodes in lithium-ion batteries. In our previous studies, lithiated PAA solutions showed significant increased viscosity compared to straight PAA ones, which could benefit the large-scale lamination process and enable the fabrication of mechanically robust electrodes. However, lithiated PAA binders suffer more capacity loss in cycling tests due to the weakened adhesion and more severe side reaction of silicon particles that are mostly related to the increase of pH value of the binder solutions. In this work, non-lithium bases (e.g., NBu4OH, NH4OH, and Et3N) were used to neutralize and tune the viscosity of PAA solution. The resulting modified PAA binder solutions achieved desired viscosity with less increase of pH value. Notably, electrodes made with PAA-NH4 binder showed improved cell performance than electrodes made with PAA-Li binder. Despite the pH difference between PAA and PAA-Li binder solutions, the solvation behavior of the binders could also affect cycling performance. Therefore, a series of PAA solutions were prepared and investigated by small-angle X-ray scattering. The radius of gyration (Rg) of the binders were found to be significantly different depends on lithiation and the choice of solvent. Electrodes made with these binder solutions were evaluated in half cells to explore the relationship between solvation and performance of binders.

Use of Electrochemical Impedance Spectroscopy to Reveal Ageing Mechanisms in Silicon Anodes for Lithium-Ion Batteries

The key for the exploitation of renewable energy and thus the replacement of fossil fuels and traditional energy sources is the achievement of effective energy storage strategies. In this regard, rechargeable Lithium-ion batteries (LIBs) play a significant role due to the high gravimetric and volumetric energy, high power density, long cycle life and low self-discharge property. However, the new applications of LIBs need higher energy densities than the present LIB technology can offer. Replacing the commercially used carbon anodes with Silicon anodes can increase the capacity more than ten times. Unfortunately, implementing Silicon anodes is still challenging due to the poor cycling stability caused by the severe volume changes of Silicon upon lithiation and delithiation during charge and discharge respectively. To improve the cycling stability a fundamental understanding of the complex processes and the degradation mechanisms is necessary. The processes in a Silicon anode during cycling have been studied by using electrochemical impedance spectroscopy (EIS) as it is a powerful, noninvasive technique to resolve the various processes in an electrode. For the investigation, a Silicon electrode using 1 M LiPF6 in EC/DEC electrolyte was cycled between the voltage potential limits of 50 mV and 1 V at a constant current. EIS measurements have been conducted at different state of charge during 200 discharge/charge cycles. The experimentally obtained impedance data were analyzed by a semi-mathematical model based on a porous electrode model. The model simulates the impedance response of a porous Silicon electrode, composed of spherical intercalation particles. The radii of the particles are dynamically changing because of expansion and contraction due to lithiation and delithiation, respectively. The impedance of a single particle is calculated considering diffusion of the Lithium cations (Li+ ions) in the particles, the solid electrolyte interphase (SEI) film and interfacial process. The particle size distribution of the Silicon powder and porous electrode theory is used to simulate the total impedance of the Silicon anode. All complex spectra obtained from experiments and simulation showed the same elements reported in literature for Silicon anodes before[1,2]. The mathematical model was used to identify important parameters to analyze the experimental data on a fundamental basis. Evaluating the experimental data revealed the large impact of the porosity reduction on the impedance during cycling. It was shown that after an expected increase in active surface area with lithiation due to the expansion of the Silicon particles, the active surface area suddenly drops. This drop in active surface area was found to correlate to a critical porosity of 0.1. It was concluded that the decrease in porosity leads to a diffusion limitation of the ions in the electrode bulk and therefore to a severe decrease in accessible surface area. Increased surface area and reduced porosity after the same lithiation time with progressive cycling indicates that the expansion of the Silicon particle is not fully reversed with delithiation. This indicates an ageing mechanism of trapped Lithium in the electrode[3]. Discontinuous diffusion behavior were correlated to cracking of the Si particle which leads to new ions diffusion pathways. Furthermore, the results indicate a formation of new SEI with lithiation and a partially redissolution with delithiation, which was also reported by Philippe et al.[4]. The attached figure shows the ratio of the experimentally obtained active surface area Aexp and the modelled active surface area Amod (based on expansion due to lithiation) and the diffusion coefficient of Li+ ions in the electrode. The results are shown for cycle 12 during the lithiation from 1 V to 50 mV (a) and for the cycle 1 to 200 after a lithiation to 50 mV (b).

Chitosan-g-Poly(acrylic acid) Copolymer and Its Sodium Salt as Stabilized Aqueous Binders for Silicon Anodes in Lithium-Ion Batteries

Silicon (Si) anodes display high specific capacity but mostly suffer from poor cycling ability owing to their dramatic volume expansion during the discharge/charge process. An effort was devoted to...

Structure design and mechanism analysis of silicon anode for lithium-ion batteries

Silicon-based material is one of the most promising substitutes of widely used graphite anodes for the next generation Li-ion batteries due to its high theoretical capacity, low working potential, environmental friendliness, and abundant natural resource. However, the huge volume expansion and serious interfacial side reactions during lithiation and delithiation progresses of the silicon anode are the key issues which impede their further practical applications. Rational designs of silicon nanostructures are effective ways to address these problems. In this progress report, we firstly highlight the fundamental scientific problems, and then focus on recent progresses in design, preparation, in - situ characterization methods and failure mechanism of nanostructured silicon anode for high capacity lithium battery. We also summarize the key lessons from the successes so far and offer perspectives and future challenges to promote the applications of silicon anode in practical lithium batteries.

A novel high-performance 3D polymer binder for silicon anode in lithium-ion batteries

For high-capacity silicon (Si) anodes, the design of new binder is a feasible way to overcome the rapid capacity decay attributed to the large volume change of silicon (Si) anode in the repeated charging/discharging process. Here a newly designed binder with 3D structure was developed using CMC as the backbone, and acrylamide (AM), acrylic acid (AA) as the branch. The molecular structure was characterized by Fourier transform infrared (FTIR), and ethanol washing was applied for getting rid of unreacted monomers. The multifunctional binder with 3D structure and rich polar groups was prepared by cross-linking grafting. Polyacrylamide provides a strong adhesion and contributes to the formation of the solid electrolyte intermediate phase (SEI) layers on the surface of electrodes. The results show that CMC and polyacrylic acid with carboxyl groups not only strengthened the bonding force between the current collectors and the silicon nanoparticles (SiNPs), also improved the linkage among SiNPs. Therefore, the loading weight of commercial Si was about 0.75 mg cm−2, even after 150 deep cycles, and a high capacity of 1210.7 mAh g−1 was resulted in the Si anode. The prepared novel high-performance binder shows a potential application on the silicon anode in lithium-ion batteries.