Bio-based anode material production for lithium-ion batteries through catalytic graphitization of biochar: the deployment of hybrid catalysts.

Producing sustainable anode materials for lithium-ion batteries (LIBs) through catalytic graphitization of renewable biomass has gained significant attention. However, the technology is in its early stages due to the bio-graphite's comparatively low electrochemical performance in LIBs. This study aims to develop a process for producing LIB anode materials using a hybrid catalyst to enhance battery performance, along with readily available market biochar as the raw material. Results indicate that a trimetallic hybrid catalyst (Ni, Fe, and Mn in a 1:1:1 ratio) is superior to single or bimetallic catalysts in converting biochar to bio-graphite. The bio-graphite produced under this catalyst exhibits an 89.28% degree of graphitization and a 73.95% conversion rate. High-resolution transmission electron microscopy (HRTEM) reveals the dissolution-precipitation mechanism involved in catalytic graphitization. Electrochemical performance evaluation showed that the trimetallic hybrid catalyst yielded bio-graphite with better electrochemical performances than those obtained through single or bimetallic hybrid catalysts, including a good reversible capacity of about 293 mAh g-1 at a current density of 20 mA/g and a stable cycle performance with a capacity retention of over 98% after 100 cycles. This study proves the synergistic efficacy of different metals in catalytic graphitization, impacting both graphite crystalline structure and electrochemical performance.

Fundamental Understanding and Quantification of Capacity Losses Involving the Negative Electrode in Sodium-Ion Batteries.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (1 H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF6 in ethylene carboante mixed with diethylene carbonate electrolyte (i.e., 5 µAh h-1/2 pause or 2.78 mAh g·h-1/2 pause ) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAh h-1/2 pause or 0.72 mAh g·h-1/2 pause ).

The role of ethylene carbonate (EC) and tetramethylene sulfone (SL) in the dissolution of transition metals from lithium-ion cathodes.

Transition metal (TM) dissolution is a direct consequence of cathode-electrolyte interaction, having implications not only for the loss of redox-active material from the cathode but also for the alteration of solid electrolyte interphase (SEI) composition and stability at the counter electrode. It has widely been reported that the limited anodic stability of typical carbonate-based electrolytes, specifically ethylene carbonate (EC)-based electrolytes, makes high-voltage cathode performance problematic. Hence, the more anodically stable tetramethylene sulfone (SL) has herein been utilized as a co-solvent and a substitute for EC in combination with diethyl carbonate (DEC) to investigate the TM dissolution behavior of LiN0.8C0.17Al0.03 (NCA) and LiMn2O4 (LMO). EC|DEC and SL|DEC solvents in combination with either LiPF6 or LiBOB salts have been evaluated, with LFP as a counter electrode to eliminate the influence of low potential anodes. Oxidative degradation of EC is shown to propagate HF generation, which is conversely reflected by an increased TM dissolution. Therefore, TM dissolution is accelerated by the acidification of the electrolyte. Although replacing EC with the anodically stable SL reduces HF generation and effectively mitigates TM dissolution, SL containing electrolytes are demonstrated to be less capable of supporting Li-ion transport and thus show lower cycling stability.

On the compatibility of high mass loading bismuth anodes for full-cell sodium-ion batteries.

Metallic bismuth is here studied as an anode material for sodium-ion batteries. The details of electrochemical redox reactions, rate performance and cycled life were investigated using relatively high mass loading electrodes in two- and three-electrode full-cells. It demonstrated that the rate capability of bismuth anodes with high mass loading are not as good as indicated in previous literatures where low mass loading electrodes were used. It also indicated that the resistances causing a faltering rate performance may be connected to a loss in particle contact during desodiation. Efforts were also made to study the different electrochemical processes that occur during early cycles. Less advantageous characteristics of bismuth electrodes are also discussed. For example, several different electrolyte solutions were tested for compatibility with the bismuth system, where only glyme-based solutions seemed to facilitate robust cycling.

Investigation of Water-Soluble Binders for LiNi0.5 Mn1.5 O4 -Based Full Cells.

Two water-soluble binders of carboxymethyl cellulose (CMC) and sodium alginate (SA) have been studied in comparison with N-methylpyrrolidone-soluble poly(vinylidene difluoride-co-hexafluoropropylene) (PVdF-HFP) to understand their effect on the electrochemical performance of a high-voltage lithium nickel manganese oxide (LNMO) cathode. The electrochemical performance has been investigated in full cells using a Li4 Ti5 O12 (LTO) anode. At room temperature, LNMO cathodes prepared with aqueous binders provided a similar electrochemical performance as those prepared with PVdF-HFP. However, at 55 °C, the full cells containing LNMO with the aqueous binders showed higher cycling stability. The results are supported by intermittent current interruption resistance measurements, wherein the electrodes with SA showed lower resistance. The surface layer formed on the electrodes after cycling has been characterized by X-ray photoelectron spectroscopy. The amount of transition metal dissolutions was comparable for all three cells. However, the amount of hydrogen fluoride (HF) content in the electrolyte cycled at 55 °C is lower in the cell with the SA binder. These results suggest that use of water-soluble binders could provide a practical and more sustainable alternative to PVdF-based binders for the fabrication of LNMO electrodes.

Dissecting the Solid Polymer Electrolyte-Electrode Interface in the Vicinity of Electrochemical Stability Limits.

Proper understanding of solid polymer electrolyte-electrode interfacial layer formation and its implications on cell performance is a vital step toward realizing practical solid-state lithium-ion batteries. At the same time, probing these solid-solid interfaces is extremely challenging as they are buried within the electrochemical system, thereby efficiently evading exposure to surface-sensitive spectroscopic methods. Still, the probing of interfacial degradation layers is essential to render an accurate picture of the behavior of these materials in the vicinity of their electrochemical stability limits and to complement the incomplete picture gained from electrochemical assessments. In this work, we address this issue in conjunction with presenting a thorough evaluation of the electrochemical stability window of the solid polymer electrolyte poly(ε-caprolactone):lithium bis(trifluoromethanesulfonyl)imide (PCL:LiTFSI). According to staircase voltammetry, the electrochemical stability window of the polyester-based electrolyte was found to span from 1.5 to 4 V vs Li+/Li. Subsequent decomposition of PCL:LiTFSI outside of the stability window led to a buildup of carbonaceous, lithium oxide and salt-derived species at the electrode-electrolyte interface, identified using postmortem spectroscopic analysis. These species formed highly resistive interphase layers, acting as major bottlenecks in the SPE system. Resistance and thickness values of these layers at different potentials were then estimated based on the impedance response between a lithium iron phosphate reference electrode and carbon-coated working electrodes. Importantly, it is only through the combination of electrochemistry and photoelectron spectroscopy that the full extent of the electrochemical performance at the limits of electrochemical stability can be reliably and accurately determined.

Concentrated LiFSI-Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes.

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi0.33Mn0.33Co0.33O2 (NMC111), and LiNi0.5Mn1.5O4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal-Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111-Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (∼4.7 V vs Li+/Li).

Na-ion mobility in P2-type Na0.5MgxNi0.17-xMn0.83O2 (0 ≤ x ≤ 0.07) from electrochemical and muon spin relaxation studies.

Sodium transition metal oxides with a layered structure are one of the most widely studied cathode materials for Na+-ion batteries. Since the mobility of Na+ in such cathode materials is a key factor that governs the performance of material, electrochemical and muon spin rotation and relaxation techniques are here used to reveal the Na+-ion mobility in a P2-type Na0.5MgxNi0.17-xMn0.83O2 (x = 0, 0.02, 0.05 and 0.07) cathode material. Combining electrochemical techniques such as galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique with µ+SR, we have successfully extracted both self-diffusion and chemical-diffusion under a potential gradient, which are essential to understand the electrode material from an atomic-scale viewpoint. The results indicate that a small amount of Mg substitution has strong effects on the cycling performance and the Na+ mobility. Amongst the tested cathode systems, it was found that the composition with a Mg content of x = 0.02 resulted in the best cycling stability and highest Na+ mobility based on electrochemical and µ+SR results. The current study clearly shows that for developing a new generation of sustainable energy-storage devices, it is crucial to study and understand both the structure as well as dynamics of ions in the material on an atomic level.

Non-flammable liquid electrolytes for safe batteries.

With continual increments in energy density gradually boosting the performance of rechargeable alkali metal ion (e.g. Li+, Na+, K+) batteries, their safe operation is of growing importance and needs to be considered during their development. This is essential, given the high-profile incidents involving battery fires as portrayed by the media. Such hazardous events result from exothermic chemical reactions occurring between the flammable electrolyte and the electrode material under abusive operating conditions. Some classes of non-flammable organic liquid electrolytes have shown potential towards safer batteries with minimal detrimental effect on cycling and, in some cases, even enhanced performance. This article reviews the state-of-the-art in non-flammable liquid electrolytes for Li-, Na- and K-ion batteries. It provides the reader with an overview of carbonate, ether and phosphate-based organic electrolytes, co-solvated electrolytes and electrolytes with flame-retardant additives as well as highly concentrated and locally highly concentrated electrolytes, ionic liquids and inorganic electrolytes. Furthermore, the functionality and purpose of the components present in typical non-flammable mixtures are discussed. Moreover, many non-flammable liquid electrolytes are shown to offer improved cycling stability and rate capability compared to conventional flammable liquid electrolytes.

Strategies for Mitigating Dissolution of Solid Electrolyte Interphases in Sodium-Ion Batteries.

The interfacial reactions in sodium-ion batteries (SIBs) are not well understood yet. The formation of a stable solid electrolyte interphase (SEI) in SIBs is still challenging due to the higher solubility of the SEI components compared to lithium analogues. This study therefore aims to shed light on the dissolution of SEI influenced by the electrolyte chemistry. By conducting electrochemical tests with extended open circuit pauses, and using surface spectroscopy, we determine the extent of self-discharge due to SEI dissolution. Instead of using a conventional separator, ß-alumina was used as sodium-conductive membrane to avoid crosstalk between the working and sodium-metal counter electrode. The relative capacity loss after a pause of 50 hours in the tested electrolyte systems ranges up to 30 %. The solubility of typical inorganic SEI species like NaF and Na2 CO3 was determined. The electrolytes were then saturated by those SEI species in order to oppose ageing due to the dissolution of the SEI.

Understanding the redox process upon electrochemical cycling of the P2-Na0.78Co1/2Mn1/3Ni1/6O2 electrode material for sodium-ion batteries.

Rechargeable sodium-ion batteries have recently attracted renewed interest as an alternative to Li-ion batteries for electric energy storage applications, because of the low cost and wide availability of sodium resources. Thus, the electrochemical energy storage community has been devoting increased attention to designing new cathode materials for sodium-ion batteries. Here we investigate P2- Na0.78Co1/2Mn1/3Ni1/6O2 as a cathode material for sodium ion batteries. The main focus is to understand the mechanism of the electrochemical performance of this material, especially differences observed in redox reactions at high potentials. Between 4.2 V and 4.5 V, the material delivers a reversible capacity which is studied in detail using advanced analytical techniques. In situ X-ray diffraction reveals the reversibility of the P2-type structure of the material. Combined soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering demonstrates that Na deintercalation at high voltages is charge compensated by formation of localized electron holes on oxygen atoms.

Interfacial Reaction Mechanisms on Graphite Anodes for K-Ion Batteries.

Potassium-ion (K-ion) batteries (KIBs) potentially offer numerous advantages over conventional lithium-ion batteries as a result of the high natural abundance of potassium and its lower positive charge density compared with lithium. This introduces the possibility of using K-ion in fast charging applications, in which cost effectiveness is also a major factor. Unlike in sodium-ion batteries, graphite can be used as an anode in K-ion cells, for which an extensive supply chain, electrode manufacturing infrastructure, and knowledge already exist. However, the performance of graphite anodes in K-ion cells does not meet expectations, with rapid capacity fading and poor first cycle irreversible capacities often reported. Here, we investigate the formation and composition of the solid electrolyte interphase (SEI) as well as K+ insertion in graphite anodes in KIBs. Through the use of energy-tuned synchrotron-based X-ray photoelectron spectroscopy, we make a detailed analysis at three probing depths up to ∼50 nm of graphite anodes cycled to various potentials on the first discharge-charge cycle. Extensive SEI formation from a KPF6/DEC/EC electrolyte system is found to occur at low potentials during the insertion of potassium ions into graphite. During the subsequent removal of potassium ions from the structure, the thick SEI is partially stripped from the electrode, demonstrating that the SEI layer is unstable and contributes to a significant proportion of the capacity upon both discharge and charge. With this in mind, further work is required to develop an electrolyte system with stable SEI layer formation on graphite in order to advance the KIB technology.

Unlocking high capacities of graphite anodes for potassium-ion batteries.

Graphite is considered a promising candidate as the anode for potassium-ion batteries (KIBs). Here, we demonstrate a significant improvement in performance through the ball-milling of graphite. Electrochemical techniques show reversible K-intercalation into graphitic layers, with 65% capacity retention after 100 cycles from initial capacities and extended cycling beyond 200 cycles. Such an affinity of the graphite towards storage of K-ions is explained by means of SEM and Raman analyses. Graphite ball-milling results in a gentle mechanical exfoliation of the graphene layers and simultaneous defect formation, leading to enhanced electrochemical performance.

Capacity fading mechanism of tin phosphide anodes in sodium-ion batteries.

Tin phosphide (Sn4P3) is here investigated as an anode material in half-cell, symmetrical, and full-cell sodium-ion batteries. Results from the half-cells using two different electrolyte salts of sodium bis(fluorosulfonyl)imide (NaFSI) or sodium hexafluorophosphate (NaPF6) show that NaFSI provides improved capacity retention but results from symmetrical cells disclose no advantage for either salt. The impact of high and low desodiation cut-off potentials is studied and the results show a drastic increase in capacity retention when using the desodiation cut-off potential of 1.2 V as compared to 2.5 V. This effect is clear for both NaFSI and NaPF6 salts in a 1 : 1 binary mixture of ethylene carbonate and diethylene carbonate with 10 vol% fluoroethylene carbonate. Hard X-ray photoelectron spectroscopy (HAXPES) results revealed that the thickness of the solid electrolyte interphase (SEI) changed during cycling and that SEI was stripped from tin particles when tin phosphide was charged to 2.5 V with NaPF6 based electrolyte.

On the Stability of NaO2 in Na-O2 Batteries.

Na-O2 batteries are regarded as promising candidates for energy storage. They have higher energy efficiency, rate capability, and chemical reversibility than Li-O2 batteries; in addition, sodium is cheaper and more abundant compared to lithium. However, inconsistent observations and instability of discharge products have inhibited the understanding of the working mechanism of this technology. In this work, we have investigated a number of factors that influence the stability of the discharge products. By means of in operando powder X-ray diffraction study, the influence of oxygen, sodium anode, salt, solvent, and carbon cathode were investigated. The Na metal anode and an ether-based solvent are the main factors that lead to the instability and decomposition of NaO2 in the cell environment. This fundamental insight brings new information on the working mechanism of Na-O2 batteries.

Electrochemical performance and interfacial properties of Li-metal in lithium bis(fluorosulfonyl)imide based electrolytes.

Successful usage of lithium metal as the negative electrode or anode in rechargeable batteries can be an important step to increase the energy density of lithium batteries. Performance of lithium metal in a relatively promising electrolyte solution composed of lithium bis(fluorosulfonyl)imide (LiN(SO2F)2; LiFSI) salt dissolved in 1,2-dimethoxyethane (DME) is here studied. The influence of the concentration of the electrolyte salt -1 M or 4 M LiFSI- is investigated by varying important electrochemical parameters such as applied current density and plating capacity. X-ray photoelectron spectroscopy analysis as a surface sensitive technique is here used to analyze that how the composition of the solid electrolyte interphase varies with the salt concentration and with the number of cycles.

Towards an Understanding of Li2 O2 Evolution in Li-O2 Batteries: An In†Operando Synchrotron X-ray Diffraction Study.

One of the major challenges in developing high-performance Li-O2 batteries is to understand the Li2 O2 formation and decomposition during battery cycling. In this study, this issue was investigated by synchrotron radiation powder X-ray diffraction. The evolution of Li2 O2 morphology and structure was observed under actual electrochemical conditions of battery operation. By quantitatively tracking Li2 O2 during discharge and charge, a two-step process was suggested for both growth and oxidation of Li2 O2 owing to different mechanisms during two stages of both oxygen reduction reaction and oxygen evolution reaction. From an observation of the anisotropic broadening of Li2 O2 in XRD patterns, it was inferred that disc-like Li2 O2 grains are formed rapidly in the first step of discharge. These grains can stack together so that they facilitate the nucleation and growth of toroidal Li2 O2 particles with a LiO2 -like surface, which could cause parasitic reactions and hinder the formation of Li2 O2 . During the charge process, Li2 O2 is firstly oxidized from the surface, followed by a delithiation process with a faster oxidation of the bulk by stripping the interlayer Li atoms to form an off-stoichiometric intermediate. This fundamental insight brings new information on the working mechanism of Li-O2 batteries.

SEI Formation and Interfacial Stability of a Si Electrode in a LiTDI-Salt Based Electrolyte with FEC and VC Additives for Li-Ion Batteries.

An electrolyte based on the new salt, lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), is evaluated in combination with nano-Si composite electrodes for potential use in Li-ion batteries. The additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are also added to the electrolyte to enable an efficient SEI formation. By employing hard X-ray photoelectron spectroscopy (HAXPES), the SEI formation and the development of the active material is probed during the first 100 cycles. With this electrolyte formulation, the Si electrode can cycle at 1200 mAh g(-1) for more than 100 cycles at a coulombic efficiency of 99%. With extended cycling, a decrease in Si particle size is observed as well as an increase in silicon oxide amount. As opposed to LiPF6 based electrolytes, this electrolyte or its decomposition products has no side reactions with the active Si material. The present results further acknowledge the positive effects of SEI forming additives. It is suggested that polycarbonates and a high LiF content are favorable components in the SEI over other kinds of carbonates formed by ethylene carbonate (EC) and dimethyl carbonate (DMC) decomposition. This work thus confirms that LiTDI in combination with the investigated additives is a promising salt for Si electrodes in future Li-ion batteries.

Charge Localization in the Lithium Iron Phosphate Li3Fe2(PO4)3 at High Voltages in Lithium-Ion Batteries.

Possible changes in the oxidation state of the oxygen ion in the lithium iron phosphate Li3Fe2(PO4)3 at high voltages in lithium-ion (Li-ion) batteries are studied using experimental and computational analysis. Results obtained from synchrotron-based hard X-ray photoelectron spectroscopy and density functional theory (DFT) show that the oxidation state of O(2-) ions is altered to higher oxidation states (O(δ-), δ<2) upon charging Li3Fe2(PO4)3 to 4.7 V.