N-Methyl-2-Pyrrolidone (NMP) Solvent for Lithium Battery: Its Applications
-Methyl-2-pyrrolidone which is also commonly known as NMP is a very strong solvent, aprotic in nature which means it is a liquid when present at room temperature. It is rich in both, properties and applications as well.
The characteristics and properties that are exhibited by the NMPs are proof of their capabilities and that is the reason because of which its applications are so vast too. These applications are said to be found in almost all the fields but the most important and prominent ones are found in the working of lithium-ion batteries. Lithium-ion batteries are one of the most used batteries in the industries and need to be efficient so in that regard, NMPs are now being used to enhance the efficiency of lithium-ion batteries specifically other than all the other areas where they are being used.
Introduction
N-Methyl-2-pyrrolidone (NMP) C5H9NO, is a strongly polar aprotic solvent. At room temperature, N-Methyl-2-pyrrolidone (NMP) C5H9NO, is a liquid. NMP is utilized as a solvent very frequently in various industrial applications because it has a remarkable combination of physicochemical characteristics. In petrochemical processing, the comparatively low affinity and volatility for aromatic hydrocarbons are misused as an extraction medium. Commonly, it is utilized for plastics, resins, and textiles’ surface treatment.
NMP in pharmacy
In pharmacy, NMP acts as a complexing agent. It also improves the drug's permeability and solubility when it simultaneously functions as a co-solvent. NMP is a significant solvent as it is used for crystallization and extraction of the drug too.
The recent rise in NMP
NMP has apparently gained attention recently as there is an increase in the usage of NMP because of NMP being utilized as a solvent for nanomaterials processing. When it comes to maintaining nanomaterial dispersion, NMP is remarkably useful. Such nanomaterials include single-walled carbon nanotubes (SWCNTs), graphene, and various other layered materialsafter they have been energetically separated from one another. When charge negatively, NMP dissolves these species, and when the contact is made, true solutions are formed without any outside agitation.
Efforts to understand NMPs
For understanding the success of NMP in these processes, it has been compared to the other solvents. Researchers are also observing NMP's capability of dissolving both neutral and iconic species. Consideration has been made on different chemical and physical factors, including the chance of degradation of solvent, especially under ultra-sonication. When it comes to uncharged nanomaterials, a major role is expected by the entropic contribution to the free energy to perform in defining solubility.
Properties
Volumetric properties
A mean of two determinations is represented by each density value. Cation's alkyl chain length and ion's structure and nature virtually determine the physicochemical characteristics of RTILs. When there is an increase in the cation’s alkyl chain length, there is a decrease in the neat XAN’s density value. However, the density is increased when the methoxy group is added to the smaller alkyl chain. The reason for this behavior is their improved capability for giving polar-polar attractive interactions and the difference in their molecular weight. All of this refers to the fact that neat XAN's compaction is inhibited by the long alkyl chain, whereas people have seen opposite effects after polar group's addition in a short chain, for instance, greater compaction of the neat ionic liquids is promoted. Generally, each XAN +NMP system's density trend is monotonic with a downward concavity.
In the presence of molecular solvents
With molecular solvents, RTILs are miscible, with high- to medium-dielectric constants, for instance, solvents that are immiscible with molecular solvent and contain polar groups. According to this study, NMP is an extremely polar solvent with a reasonably high relative dielectric constant (At 298.15 K temperature, e=32.2), that's why in NMP, all XAN are miscible over the whole mole fraction range. Processing density data were used for calculating excess molar volumes (VE), and molar volumes (Vm). Each curve in the falsely linear Vm trend has an upward concavity as every curve is fitted by a 2nd-degree polynomial equation. The upward concavity is magnified and can be seen easily. For each NMP + XAN system, there are negative VE values.
In comparison to attractive interactions
As compared to the attractive interactions in the neat compounds, the attractive interactions between the constituents are so much stronger that their presence is indicated by the reasonably negative VE values of EAN/PAN/BAN + NMP systems. In the rich region of NMP, the VE curves of every individual XAN + NMP system overlaps and displays a comparable trend. The length of the alkyl chain doesn't affect the VE curves because all the systems are almost the same. As compared to other XAN + NMP systems, the volume’s reduction is less for the MEOEAN + NMP system, representing that the mixture’s compaction can be prevented by a polar group’s addition, for instance, the methoxy one.
Calorimetric properties
The mixing heat's calorimetric determination provides complementary information. For the XAN + NMP mixtures, the smoothed curves and the experimental points of partial molar enthalpies of the constituent, HE, excess molar enthalpies, and HEk are plotted as a mole fraction’s function of XAN, x1. The HE curves have a moderately asymmetric shape. According to Margules coefficient, for each system, the values of the minimum at x1 = 0.47: for EAN + NMP system= 3509 J mol1, for PAN + NMP system= 3319 J mol1, for BAN + NMP system= 3062 J mol1, and for MEOEAN + NMP system= 2689 J mol1. There are strong attractive interactions present between the mixture’s constituents, and this is suggested by every system’s extremely high negative excess enthalpies. The endothermic effect generally characterizes the mixtures between two different organic compounds whereas when attractive interactions are brought up by two components, the mixing is exothermic.
In the light of observations
There are observations of exothermic mixing effects of NMP with chloro-alkenes, water, and chloro-alkanes. In mixtures of NMP + 1,1,2,2- tetrachloroethane, the highest exceptionally value of 4750 J mol1 was observed. HE k’s non-specular peculiar shape explains the HE curve’s slight asymmetry. As compared to HE;1 2, excess partial molar enthalpies of XAN, HE;1 1, have higher absolute values at infinite dilution. A test was performed in which interactions were observed. When NMP solvent was used to dissolute XAN, the interactions were stronger than when the ionic liquids were used to dissolute NMP, referring that the NMP played stronger attractive interactions in the dissolution of the solute XAN, but in the other process, the roles were exchanged between the same components.
Notable point
The point that should be noted that there is a decrease in HE;1 k and HE x¼0:5, along with an increase in the difference between HE;1 k value, when there is an increase in the cation’s alkyl chain length, indication towards the fact that when the cation’s alkyl chain is smaller, the mixing process is more exothermic. The lowest HE x¼0:5 absolute value is displayed by the MEOEAN + NMP system, and it indicates that the interaction between XAN and NMP can be inhibited by the addition of the polar methoxy group.
Lithium-ion batteries
To fulfill the increasing requirements of hybrid electric vehicles, portable electronic devices, and electrical machines, etc. the most significant power source is the Lithium-ion rechargeable batteries. Recently, a huge amount of interest is seen in lithium-ion batteries’ commercialization and research activities as it can store energy on a large scale and on-board in the plug-in hybrid electric vehicles and electric vehicles. As the cobalt-oxide-based cathodes are being used in the commercialized lithium-ion batteries, a huge amount of efforts is being made to find alternatives to the expensive and toxic cobalt-oxide-based cathodes for potential usage in large scale applications, for instance, hybrid electric vehicles.
Major challenge
The major challenge is the creation of high-safety, high-performance, and low-cost Li-ion batteries for vehicle applications. Padhi et al. reported the electrochemical activities of Li-iron phosphate (LiFePO4), and it is a subject right now for being used as a cathode material for lithium-ion batteries of the new generation. LiFePO4 has many advantages, the main ones are its comparatively high theoretical specific capacity of 170 mAhg1, excellent thermal stability, low toxicity, cheap, abundant raw material resources, and flat voltage profile, whereas LiFePO4 has some disadvantages too, like its low ionic diffusivity and poor electronic conductivity. Impurities and defects easily block the 1-dimensional channels even though according to the theoretical calculation, the intrinsic ionic diffusion coefficient is as high as 108 cm2s1 for LiFePO4 to 107 cm2s1 for FePO4.
In comparison to the blockages
As compared to the blockages in 3D and 2D paths, the blockages in 1D paths are different as in them, there can be a movement of Li-ions around the blocked sites. The slow kinetics of discharging and charging processes are caused by the 1-dimensional channel's failure. The LiFePO4 particles should be constructed with particular morphologies for avoiding the long channels from getting blocked. Great electrochemical characteristics are exhibited by the nanosize pristine LiFePO4. The second component’s introduction. For instance, supervalent cations doping with carbon nanotubes (CNRs)/ conductive carbon/ graphene coating, and a non-carbon second phase coating (polymers, metal oxides) was used for surface modifications.
If you want to obtain more information about Lithium-Ion battery,
you can read our other blog post.
Applications
The pristine hydro-/solvo-thermal LiFePO4 nanoparticles
According to the XRD patterns, orthorhombic olivine LiFePO4, pure phase makes up both the solvo- and hydro-thermal products. The product’s high purity was assured as there was no detection of any distinct diffraction peaks of the other impurities. 4.5m2g1 was hydro-thermal LiFePO4 NPs BET surface area, and solvothermal LiFePO4's BET surface area was 11.8 m2g1. As compared to the hydrothermal product, a broadening characteristic was exhibited by the solvothermal product’s X-ray diffraction peaks, telling that NMP's introduction caused an increase in the production of smaller-sized LiFePO4 NPs. Their small size is the main feature of high-performance electrode material. In NMPs' absence, some hydrothermal product particles were made which were mostly made up of irregular rhombic plates or quasi-hexagonal plates whereas those that were made in the presence of NMPs, those thermosolvent products are made up of comparatively uniform NPs. LiFePO4 particles have 0.1-0.2 mm thickness, 0.15-0.7 mm width, and 0.3–1.0 mm length. As compared to the hydrothermal products, solvothermal LiFePO4 NPs are smaller.
According to XRD results
According to the XRD results, smaller particles of LiFePO4 were formed after solvent NMP was introduced. During the solvothermal reaction, the growth mechanism of the LiFePO4 crystal consists of total crystallization, partial dissolution, aggregation and growth, and dissolution of the precursor and nucleation. Purified NMP was boiled with O2 and H2O to obtain the activated NMP which is a strong polar organic solvent. This method of achieving it is an efficient reducing agent. This method enables metal ion’s rapid reduction. During LiFePO4’s solvothermal growth, the weak reducing agent is the polar NMP molecules, which inhibits Fe2+ oxidation to Fe3+ during LiFePO4’s growth. Polar NMP molecules also help in assuring the product's purity. As compared to the viscosity of common solvents like ethanol and water, the viscosity of NMP is much higher. Higher viscosity prevents the growth of larger particles by slowing down the ion diffusion rate. Thus, NMP has two functions; one as a LiFePO4 crystal's growth inhibitor and another as a solvent.
NMP as a solvent
When NMP was being utilized as the solvent, the ferrite nanocrystals and Ag nanoparticles were available. During the process of charge-discharge, a short diffusion length is expected of the nano-sized LiFePO4 particles for Li+ diffusion. The rate performance confirms all of this well. At 0.2 C current rate, hydrothermal LiFePO4’s reversible capacity is only 75 mAh g1 whereas the reversible capacity of the solvothermal LiFePO4 particles is 82 mAh g1. There was an enhancement seen in the crystallization of the LiFePO4 samples along with the decrease in the defect density after annealing at 700 C, making the diffusion of the Lithium-ion through the 1-dimensional channel, easy. After annealing, 110 and 94 mAhg1 are the capacities of solvo- and hydro-thermal samples. Large LiFePO4 particles are attained by hydrothermal synthesis but their discharge capacity is less than the discharge capacity of the smaller NPs of solvothermal LiFePO4. The advanced electrode materials are the nanosized particles with which have a short Li+ diffusion length. Although, carbon-free LiFePO4's capacity is still low. The low capacity can be because of 2 reasons, its limited cathode material's utilization and its poor electrical conductivity.
Carbon coating of the LiFePO4 nanoparticles
Applying a coating of carbon on the surface of LiFePO4 particles is the most efficient method of improving conductivity. A conductive network is formed by the carbon particles in the cathode coating for enhancing the LiFePO4 particles surface’s electronic conductivity and decreasing the electrode’s polarization resistance, therefore aiding in attaining the best rate performance. Small particles are formed by the additional carbon source. A thick carbon coating of few nanometers covers those small particles.
LiFePO4/C particle’s morphologies and structures
During the process of carbon coating, there is a dispersion of LiFePO4 agglomerates into the separated particles after the heat treatment and solvo-/hydro-thermal reaction for 5 hours at 700 C in which glucose is used as the carbon source. After the carbon coating and annealing, the LiFePO4 structure was preserved well. Scientists also observed small pores in it which are supposedly useful and good for electrolyte penetration. The size of the LiFePO4 particles that were made from the solvothermal process is smaller than those LiFePO4 particles which were obtained from the hydrothermal process.
Solvothermal NMPs
A 100-200 nm length was possessed by the solvothermal LiFePO4 NPs. There was a carbon layer of 2.3-2.5 nm thickness on the two sample's surfaces. In TEM images of high resolution, homogeneous lattice fringes were displayed by the LiFePO4 NPs which were obtained from the solvo- or hydrothermal processes. The lattice fringes had a spacing of 0.296 or 0.299 nm, they were given to orthorhombic LiFePO4’s crystal plane. 24.9 and 10.5 m2g1 was the solvo- and hydro-thermal LiFePO4/C’s BET surface area.
Electrochemical performance of LiFePO4 from solvothermal route
2025-type coin half-cells were used at both, 60 and 25 C to validate LiFePO4's electrochemical characteristics and its samples for discovering its potential applications in high power lithium-ion batteries. With a 2.5-4.0 operative voltage range, CV was performed for both solvo- and hydro-thermal samples. At 0.1 mVs1 scanning rate, the first three cycles of both sample’s CV profiles are showed. Both samples' electrodes display a couple of cathodic and anodic peaks in the 3.26-3.60 V range. Redox peaks' position corresponds to Lithium+ extraction and insertion.
Anodic peak
The anodic peak during hydrothermal NP electrodes' first cycle is 3.7 V whereas the cathodic peak which corresponds to it is at 3.3 V. The anodic peaks in the 2nd and 3rd cycles are at 3.58 V whereas, in the same cycles, the cathodic peak that corresponds to is not changed.
Cathodic peak
For solvothermal electrodes, the cathodic peak in the first cycle is at 3.33 V whereas the anodic peak is at 3.58 V. The intensities of both the cathodic and anodic peak increase in the subsequent two cycles.
In comparison to hydrothermal sample
As compared to the hydrothermal sample, the solvothermal LiFePO4/C cathode material which has a smaller size distribution has better extraction/insertion reversibility because it affords almost symmetrical peaks.
At 0.2 C, the solvo/hydrothermal LiFePO4/C samples’ first and second charge-discharge curves are shown. 118 mAhg1 is the discharge capacity of hydrothermal LiFePO4/C composite whereas 147 mAhg1 is the solvothermal LiFePO4/C composite’s discharge capacity. Also, a flat plateau of around 3.4 V is displayed by the hydrothermal and solvothermal samples, while a longer voltage plateau and lower degree of polarization are displayed by the voltage plateau for solvothermal LiFePO4/C NPs. Due to remarkable electronic conductivity, the uniform carbon coating, and the LiFePO4 nanoparticle's ionic diffusivity, LiFePO4 has a superior Li storage performance. Thermal stability and rate performance are the two main factors for Li-ion battery cathode materials for electric vehicle applications.
At room temperature
At room temperature, at 0.2-5.0 C charge-discharge current, a reversible discharge capacity of 122-79 mAhg1 is displayed by the hydrothermal process-based LiFePO4/C. However, at the rate of 5 C, 0.5 C, and 0.2 C, the solvothermal process-based LiFePO4/C has comparatively low reversible capacities of 106, 140, and 150 mAh g1. When there is an increase in the current rate, there is a gradual decrease in the specific capacity although a high capacity stays at 5 C, meaning that high rate discharge and charge can be endured by the solvothermal LiFePO4/C.
