Effect of comb polymer dispersants with different molecular structures on the performance of LiFePO4 suspensions

Abstract A series of comb polymers poly(2-(dimethylamino)ethyl methacrylate (DMAEMA)-co-methacrylic acid (MAA)-co-methoxy polyethylene glycol methacrylate (MPEGMA)) (poly(DMAEMA-MAA-MPEGMA, DMM) were synthesized and used as N-methyl-2-pyrolidinone (NMP)-based lithium iron phosphate (LFP) suspension dispersants. The effects of the grafting density of the carboxyl group as the anchoring group and the chain length of the side chain of PEG, which plays the role of spatial site resistance, on the rheological properties and suspension stability of the slurry were systematically investigated. By investigating the adsorption amount and thickness of DMM on the LFP surface, combined with calculations based on the scalar law and Flory theory, the molecular structure of the comb polymer dispersant was revealed to influence the adsorption and dispersion performance. The dispersion of LiFePO4 was due to the synergistic effects of adsorption and steric hindrance effect, which resulted that dispersants with medium carboxyl density and PEG side chain length can improve the dispersion performance and stability. Graphical abstract


Introduction
Lithium iron phosphate batteries are widely used in new energy vehicles and energy storage because of their high temperature performance, long life, high safety and low cost. [1]However, the low electronic conductivity and diffusion coefficient of LFP affect the electrical performance of the battery. [2][5] However, this increases the difficulties in the process of the cathode slurry, resulting in poor dispersion of LFP particles and conductive agents, prone to agglomeration, high slurry viscosity, and low solids loading, which not only affects the electrical performance of the battery, but also rises the manufacturing cost of the battery. [6,7]10] The dispersant is adsorbed to the particle surface to improve the particle dispersion performance by transforming the particle/particle and particle/liquid medium interactions.Among all kinds of dispersants commonly used, comb polymer is widely used in cement, dye, ink and coal water slurry [11][12][13][14] because of its superior dispersion stability.A typical comb polymer consists of a backbone with anchoring groups and side chains grafted to the backbone at frequent intervals.The anchoring group on the skeleton mainly plays the driving role of adsorption on the surface of inorganic particles, while the side chain mainly plays the role of spatial repulsive force to dominate the dispersion behavior of inorganic particles in suspension system. [15,16]Many studies have revealed the influence of comb polymer on dispersion performance. [17,18]21] Although the inorganic particles such as carbon black, LiFePO 4 are well recognized as the important battery materials used in organic composite.Developing comb polymeric dispersants for non-aqueous systems is extremely valuable.
The objective of this study was to investigate the dispersion of NMP-based LFP slurries by comb polymers and to completely comprehend the effect of their molecular structure on the adsorption and dispersion behavior.For this purpose, a series of DMM comb polymer dispersants with different side chain lengths and carboxyl densities were synthesized.The effects of side chain length and carboxyl density of comb copolymers on adsorption, rheology and suspension stability were systematically investigated.In addition, the relationship between molecular structure and LFP slurry dispersion was explained using a computational approach based on the scalar law and Flory theory.The information generated helps to elucidate the relationship between structure and properties and to explain the mechanism of comb polymer dispersants.

Synthesis of dispersants
A series of comb polymers with different carboxyl densities and PEG branched chain (m ¼ 9, 13, 20) lengths were synthesized by free radical polymerization (herein DMM-A, DMM-B, DMM-C and DMM-400, DMM-650, DMM-1000).DMM-A, DMM-B and DMM-C had the same length of branched chains but different carboxyl densities.DMM-400, DMM-650 and DMM-1000 had the same carboxyl densities and different branched chain lengths.The synthetic route of the polymeric dispersant is shown in Figure S1.The design parameters of the molecule are shown in Table 1.
Representative example of DMM-B comb polymer: Firstly, MPEGMA monomer (42.8 g, 0.065 mol, 52%) was dissolved in NMP (98.4 ml) in a four-necked flask.DMAEMA (25.5 g, 0.162 mol, 31%) was dissolved in NMP (36.9 ml) to prepare the Solution A. MAA (13.9 g, 0.162 mol, 17%) was dissolved in NMP (36.9 ml) to prepare the Solution B. A mixture of AIBN (1.234 g, 1.5% of the total mass of the monomer), NDM (2.051 g, 2.5% of the total mass of the monomer) and NMP (73.8 ml) were prepared as Solution C. The temperature of the reaction system was controlled at 8062 � C: Solutions A, B and C were dropped into MPEGMA monomer solution, and the drop acceleration was controlled so that the reaction ended simultaneously after 1.5 h.After the dropping completed, the temperature of the reaction system and the stirring speed were maintained for 4.5 h.The polymerization was quenched by rapid cooling upon immersing the flask in an ice bath.The solvent NMP was removed using reduced-pressure distillation.The resulting copolymers were purified by triple precipitation in toluene and dried under vacuum, which was then collected as a yellow viscous substance.

Characterization of poly(DMAEMA-MAA-MPEGMA)
FT-IR (Nicolet, is5) was carried out to study the functional groups and structure of DMM.Samples were measured at 25 � C in the range of 4000-400 cm À 1 using ATR with a resolution of 0.5 cm À 1 . 1 H NMR spectra of DMM were measured on a 600 MHz spectrometer (Bruker Ascend 600 MHz NMR spectrometer, Switzerland) using TMS (i.e., tetramethylsilane) as an internal standard and deuterated chloroform as a solvent, with the probe temperature was kept at 298 K.The molecular weight of DMM was measured by GPC (Waters 1515) using a standard polystyrene calibrated column system.NMP solution of 0.01 mol/L at 50 � C at a rate of 0.8 ml/min was used as the mobile phase.

Viscometry measurements
In order to carry out the flow tests, a dynamic shear rheometer was used.The measurements were performed at 25 � C using an Anton Paar MCR 92 rheometer with a parallel plate geometry having a diameter of 50 mm and 1 mm gap between the two plates.First, the dispersant is completely dissolved in NMP solvent at ambient temperature.Then, the ultrafine powder LiFePO 4 /C were added to the above solution and mixed thoroughly.The suspension was mixed by a planetary stirring defoamer (MAZERUSTAR KK-400WE) at room temperature for 1 h.LiFePO 4 slurry with a high solid content of 69 wt % was prepared.After homogenization, the suspension was transferred directly into the measuring fixture.The suspension viscosity was measured in controlled shear rate mode, consisting of the following steps: (i) a pre-shear rate of 100 s À 1 for 30 s; (ii) a linear increase in shear rate from 0 to 300 s À 1 over 40 s, followed by a return in another 40 s.

Stability measurement
The prepared suspension was rested in a 20 ml test tube, and 2 ml of the upper layer of the test tube at different resting times were aspirated every 12 h and placed on a rheometer plate.The distance of the vertebral plate was adjusted and pre-sheared for 30 s at 10 s À 1 , then 10 viscosity values were measured at 10 s À 1 and the average value was taken.The stability of the suspension was characterized by settling rate R (Eq. ( 1)).
where V 0 is the initial viscosity of the suspension on the test tube, and V 1 is the viscosity of the slurry on the test tube after resting for 48 h.The adsorption of DMM on LFP particles were determined by total organic carbon (TOC) measurement using a TOC analyzer (German element varioTOC select), and the samples were acidified to remove inorganic carbon before testing, and each sample was measured three times to take the average value.An X-ray photoelectron spectrometer (Thermo Scientific K-Alpha) was used for XPS analysis.A nonlinear multi-peak Gaussian curve fit was performed for the fine spectra of each element.The photoelectron intensity of Fe 2p decays after passing through the DMM adsorption layer because the LFP contains Fe 2p elements, while the polyacrylate dispersant does not contain Fe 2p.The thickness of the adsorbed layer will be obtained after integrating the area of the Fe 2p photoelectron peak.The experimental procedures and calculations are in accordance with the literature. [22]he thickness of the adsorbed layer of DMM on the surface of LFP particles is calculated by the following equation (2)：

Adsorption measurements
where I 0 is the initial photoelectron intensity; I b ð Þ is the photoelectron intensity after passing through the adsorption layer of thickness b; b is the thickness of the adsorption layer nm; k E k ð Þ is the average escape depth of photoelectrons nm, which is derived from the empirical equation (3): where E k is the kinetic energy of the photoelectron eV; a is the thickness of the single atomic layer nm, obtained from Eq. (4): where M is the relative atomic weight or relative molecular mass; m is the number of atom in the molecule; N is Avogadro's constant; q is the material density kg�m À 3 .

Structure of DMM dispersant
The structure of DMM was investigated by FT-IR spectroscopy, as shown in Figure 1.The peak of 2917 cm À 1 is the methyl C-H vibrational absorption peak; 2868 cm À 1 is the C-H vibrational absorption peak on methylene; 1727 cm À 1 is the ester group C ¼ O absorption peak; 1455 cm À 1 is the C-H bending vibration peak.3525 cm À 1 is the O-H stretching vibration absorption peak introduced by MAA; 1249 cm À 1 is the C-N stretching vibration absorption peak on tertiary amine introduced by DMAEMA; 1097 cm À 1 is the C-O-C absorption peak in polyoxyethylene structure introduced by MPEG500MA.The characteristic peaks of DMAEMA, MAA and MPEG500MA all appear clearly at the corresponding positions, while the C ¼ C vibrational absorption peak near 1630 cm À 1 disappears.This indicates that the copolymerization of mentioned above occurred and the target product-DMM comb polymer was obtained.
Figure 2 shows a typical 1 H NMR spectrum of the DMM polymer.7.26 ppm peak is characteristic of the solvent deuterium chloroform.3.6 ppm peaks are attributed to the H of polyoxyethylene [-(CH 2 CH 2 O) 13 -].The peak at 3.3 ppm is the possible overlapping of the spectral peaks of the methyl group at the end of the PEG branched chain (peak at 3.36 ppm (-CH 3 )) and NMP (peak at 3.29 ppm (-CH 2 -)), which was solvent for free radical polymerization.[25] The peaks at 3.9-4.5 ppm is the overlapping of the spectral peaks of DMAEMA (peak at 4.25 ppm [-OCH 2 CH 2 -]) and MPEGMA (peak at 4.3 ppm [-(CH 2 CH 2 O) 13 -]).2.6 ppm peaks are attributed to the H of [-OCH 2 CH 2 -] in DMAEMA, and 2.2-2.5 ppm peaks indicate the H of methyl (-CH 3 ) in the terminal amine group, which overlaps with the peak at 2.36 ppm of NMP.0.7-0.8ppm peaks are the H of methyl (-CH 3 ) in the main chain.0.9-1.3ppm peaks are the H of (-CH 2 -) in the main chain.The peak at 1.26 ppm is attributed to the peak of NDM.The molecular properties of the comb polymer dispersants measured by GPC are listed in Table 1.The GPC spectrum is shown in Figure S2, which shows that the peaks of the dispersant molecules are narrow and uniform, indicating uniform molecular weight distribution.The conversion of MPEGMA monomers was analyzed by GPC.The MPEGMA monomer conversion is calculated according to Eq. (5): the results are shown in Table S1, higher yields of synthesized polymers and higher monomer conversions.
where C is the conversion of MPEGMA monomer, %; m 0 is the mass of MPEGMA monomer put into the reaction system, g; M is the mass of all the monomers put into the reaction system, g; a 0 is the percentage of MPEGMA to the total mass of the input monomer, which is calculated by the integral area of the GPC spectra, %.
As the proportion of MAA monomer increases, the molecular weight in the polymer becomes higher and the amount of carboxyl groups increases.With the same carboxyl density of dispersant, the molecular weight of the polymer increases as the length of PEG branched chains grows, while the amount of carboxyl groups decreases.

Rheological behaviors
In this paper, the effect of comb polymer dispersants with different carboxyl densities and side chain lengths on the rheological properties of LFP suspensions was investigated while keeping the experimental conditions of temperature and pH consistent.Figure 3(a) shows the rheological curves of the suspensions with different additions of DMM-B.When the amount of DMM-B is 0.1%, the apparent viscosity of the slurry decreases with increasing shear rate, and exhibit shear thinning behavior and a significant shear plateau, typical of non-Newtonian fluids. [26]When the inter-particle gravitational force was greater than the hydrodynamic force, the LFP particles in the suspension would agglomerate to form flocs.As the shear rate increases, the hydrodynamic action improves and the LFP particle agglomerates are redispersed to form an ordered laminar structure and the viscosity of the suspension decreases.However, as the amount of DMM dispersant increases, the suspension changes from a shear thinning to a shear thickening phenomenon.This is due to the increasing diffusion of nonadsorbed dispersant into the solution.29] The viscosity of the suspension was lowest at a dispersant addition of 0.4% of LFP particles, and the relationship between dispersant structure and suspension dispersion performance was studied at this addition level.The effects of carboxyl density and side chain length on the viscosity of the cathode slurry with 69 wt% solids in the presence of 0.4 wt% DMM are shown in Figure 3(b) and 3(c).The comb polymer DMM-B with the most moderate carboxyl density and side chain length showed the lowest apparent viscosity for the cathode slurry at the same dose, and the parameters of the rheological parameters are shown in Table S2.
According to the fitting constant R 2 , the Herschel-Bulkley equation (s¼s 0 þkc n ) best describes the rheological behavior of the cathode slurry.For different types of slurries, the values of the flow characteristic index n show some differences.The rheological model parameter n for LFP suspensions without dispersant is less than 1 and the slurry is a pseudoplastic fluid.Suspensions containing dispersants with n > 1 are swelling dilatant fluids. [30]It can be observed that the dispersion properties deteriorate when the number of carboxyl groups of the anchoring group is too excessive, or the PEO side chain is too long.In order to explore the relationship between molecular structure and dispersion mechanism, the amount of dispersant adsorbed on the surface of LFP particles and the adsorption thickness were measured.

Suspension stability
The stability of DMM comb polymer as dispersant for LFP suspensions was further evaluated by testing the variation in viscosity of the upper layer of the resting suspension over 2 days.The effect of different structures of DMM comb polymer on the stability of suspensions is shown in Figure 4(a).It can be expressly seen that the stability of the suspension with DMM-B as dispersant is much better than the others.This is owing to the strong repulsive force provided by the PEG side chain of the comb polymer.Figure 4(b) shows the settling rate of the slurry after resting for 48 h, the slurry dispersed by DMM-B has the least number of particles that settle due to reagglomeration by desorption and the best slurry stability, which is consistent with the results of apparent viscosity.According to the literature, [31] the anchoring group in the backbone makes the polymer easily adsorbed to the particle surface.In addition, the PEG side chains extend into the solution to provide spatial repulsion.It is reasonable to infer that different adsorption can modify electrostatic interactions and spatial site resistance.However, it was observed in our experiments that DMM-1000 with long side chains is not as well dispersed as short chain DMM-650 (i.e.DMM-B) and is prone to flocculation and less stable.This is caused by the interparticle bridging due to the long side chains. [32]

Adsorption isotherms
Dispersants are adsorbed on the surface of inorganic particles and exhibit dispersity by preventing flocculation and agglomeration due to electrostatic repulsion or spatial site resistance. [33,34]Therefore, the adsorption of DMM comb polymer dispersants with different carboxyl densities and side chain lengths were investigated, as shown in Figure 5 respectively.For all DMM comb polymers, the adsorption curves have the same trend over the studied range, with two distinct plateaus in the isotherm.[37][38][39] At low DMM concentrations, the DMM comb polymer forms a monolayer coverage on the LFP surface by chemically bonding with Fe 2þ of LFP through amine and carboxyl groups until all adsorption sites are saturated. [8,40]By further increasing the DMM concentration, a second saturated adsorption platform was observed, which exhibited LS-type double-platform adsorption properties.Due to the unique structure of the comb copolymer, the second layer is generated by hydrogen bonding and/or electrostatic interactions between the lone electron pair on the amine and carbonyl groups of the DMM macromolecule and -OH [41,42] on the carboxyl group.
By varying the carboxyl density and side chain length, the comb polymer with medium carboxyl density and side chains length has the highest adsorption capacity as shown in Figure 5(a) and 5(b).Considering the unique structure of LFP/C particles, their surface consists of a low-polarity carbon-coated portion and a polar LFP bare-leakage surface.Figure 6(a) shows a TEM photograph of the LFP/C.Similarly, the literature mentions surface coverage of carbon on LFP is incomplete. [43]Accordingly, we proposed the adsorption conformation of the dispersant on the particle surface, as shown in Figure 6(b).For DMM-A, DMM-B and DMM-C, the bonding ability with polar LFP and the adsorption amount increases with the increase of carboxyl group content, the adsorption is more easier to occur.And when it increases further, the area occupied on the particle surface becomes larger, leading to a decrease in the number of adsorbed molecules and a decrease in the total adsorption. [44]For DMM-400, DMM-650 and DMM-1000 with different PEG lengths, the number of -COOH groups in the molecule decreases with increasing chain length, but the decrease is not significant.Also considering the effect of molecular weight, the adsorption of DMM-400 with short chains was lower than that of DMM-650.The long side chain of DMM-1000 shields the carboxyl groups and inhibits the adsorption, resulting in low adsorption.Moreover, the area occupied by individual molecules on the surface of LFP is large and the number of adsorbed molecules is low, results in the low total adsorption amount. [17,45]

Adsorption kinetics
The adsorption kinetics of DMM-B on LFP at a concentration of 20 mg/g was investigated, as shown in Figure 7.The DMM dispersant adsorbs rapidly on the particle surface and reaches a large value within 30 min for LFP.By 60 min, the adsorption saturation is basically reached, and then no further changes are observed.In the literature, different kinetic models are commonly used to evaluate the adsorption process. [46]In order to model the adsorption kinetics of DMM-B on LFP and to determine the kinetic parameters, such as the rate constant and the amount of adsorption at equilibrium, the adsorption kinetic curves were fitted by regression with pseudo-first order and pseudo-second order kinetic models.Kinetic model of pseudo first order, as expressed by Eq. ( 6): Kinetic model of pseudo second order, as expressed by Eq. (7): where k 1 and k 2 are the rate constants of the first-order and second-order kinetic models, respectively, q e and q t are the adsorption of dispersant on the particle surface at equilibrium and time t, respectively.The kinetic fitting equations are shown in Figure S3(a) pseudo-first order fitting equation and Figure S3(b) pseudo-second order fitting equation.The specific parameters are shown in Table S3.The correlation coefficient R 2 of the pseudo-second order kinetic equation is 0.997, indicating that the adsorption rate is controlled by the chemisorption mechanism, which involves electron sharing or transfer between the adsorbent and the adsorbate.

Adsorption thickness
The thickness of the adsorbed layer of the comb polymer on LFP was characterized by X-ray photoelectron spectroscopy.Figure 8(a) and 8(b) shows the XPS scanning spectra and N 1s spectra of LFP and LFP-B, respectively.It can be seen that the absorption peak of N1s appears after DMM adsorption, which proves that DMM is competently adsorbed on the surface of LFP particles.Fe 2p was chosen as a characteristic element to evaluate the thickness of the DMM adsorbed layer. [47]The thickness of the adsorption layer of the polymer on LFP calculated according to Eqs. ( 2)-( 4) is shown in Table 2.The adsorption layer of DMM-B has a moderate thickness compared to several other dispersants.
In addition, it can be observed that the position of the Fe 2p peak is shifted to a lower binding energy after the adsorption of DMM on LFP, which is due to the chelation of the DMM carboxyl group with the iron ion. [48]

The relationship between molecular structure and dispersion mechanism
It is commonly believed that comb polymers disperse inorganic particles by stretching long side chains in the solvent, which acts as a spatial site barrier effect. [49]The interparticle forces become repulsive when the layers of inorganic particles covered with comb polymers start to overlap. [50]ccording to the spatial stability theory, the spatial forces are expected to raise with the increase of the side chain length and density of the comb polymer, thus improving its dispersion.However, in our study, comb polymers with moderate side chain lengths and carboxyl densities showed the best dispersion.Thus, the structure of the comb polymer has a significant effect on particle dispersion, as discussed further below.First, the structure of comb polymers should be related to the conformation considering their special molecular geometry.The Gay and Raphael model [51] is commonly used to explain comb polymers within nanoscale pores based on the Flory free energy approach at the scalar law level.Based on their theory, comb polymers are defined as a collection of n chain segments, each containing N monomers and a P-monomer side chain.There are five different types of conformations: decorated chain   S4.Thus, the polymer is in the FBW state.For this type of conformation, the radius of a given nuclear R P is calculated according to Eq. ( 8) and the results are listed in Table S4.
Here, a was a constant for the comb polymers.When a comb polymer is adsorbed on the surface of an LFP, the polymer must reorganize so that the backbone approaches the surface and eventually becomes a hemispherical chain, and Flatt [52] developed the theory of Gay and Raphael to explain the conformation of adsorbed comb copolymer dispersants.The hemispherical radius and the surface occupied by each molecule can be calculated by the following equation (9).

S ¼ bP
Here, b was a constant for the comb polymers.The results obtained from the theory are listed in Table S4.It can be clearly seen that comb polymers with longer or denser side chains have larger hemispherical radii, which occupy slightly different areas on the surface of LFP particles.As shown in Figure 9(a)-9(c), the adsorption is not firm and the dispersion is poor when the side chains are denser.While more anchoring group comb polymer molecules have more binding sites to LFP and tend to lie flat on the particle surface, hence larger area.However, the side chain density is low, the spatial site resistance becomes few, and the dispersion becomes poor.Also as shown in Figure 9(d)-9(f), for comb polymers with short side chains, the spatial site resistance is weak and the dispersion is poor.On the other hand, the excessively long side chains of DMM-1000 bridged the particles and caused flocculation of the slurry, which also worsened the dispersibility.Thus, the dispersion is due to the synergistic effect of spatial repulsion and adsorption, which results in comb polymers with moderate side chain length and carboxyl density present the best dispersity.

Conclusions
55][56] Many studies [57][58][59][60] have been conducted by academics accordingly.Battery cathode slurry is a complex mixture of multiphase compounds.][63][64] Studies on the adsorption conformation and mechanism of interaction between dispersants and individual component LFP are rather poor. [65,66]Our study systematically illustrates the conformational relationship between the molecular structure of comb polymers and their performance from the perspective of the adsorption and dispersion mechanism of polymers on the particle surface, and also provides a reference for the development of novel dispersants.
A series of DMM comb polymers with different molecular structures were synthesized as dispersants for the LFP suspensions.The suspension adsorption experiments showed that the DMM polymers exhibited a two-platform adsorption on the LFP surface, controlled by chemisorption, with the maximum adsorption of comb polymers of medium side chain length and carboxyl density.Rheological experiments and stability tests showed that DMM was able to firmly adsorb and stretch the side chains, providing spatial repulsion and keeping the slurry stable.According to the calculations of scalar method and Flory's theory, the area of comb polymer molecules increases with the increase of carboxyl density and side chain length, which leads to the decrease of  adsorbed molecules.Comb polymer molecules with low carboxyl density or long side chains are unfavorable for adsorption and favorable for dispersion; high carboxyl density or short side chains are favorable for adsorption and unfavorable for dispersion.The dispersibility is due to the synergistic effect of spatial site resistance effect and adsorption behavior.Therefore, comb polymers with moderate side chain length and carboxyl density have the best dispersion.
series of DMM polymers were weighed, dissolved in NMP solvent, and mixed with LiFePO 4 /C for 60 min in a planetary stirring defoamer.A certain amount of the above prepared LiFePO 4 suspension was diluted, filtered, and washed several times with deionized water to remove the solvent NMP and the free DMM that did not play a role in dispersion.The collected filter cake was dried in an oven at 85 � C to produce DMM-A, DMM-B, DMM-C and DMM-400, DMM-650, DMM-1000 modified lithium iron phosphate powders (named LFP-A, LFP-B, LFP-C, and LFP-400, LFP-650, LFP-1000).

1H
NMR and FTIR spectroscopy clearly indicated the successful synthesis of the target polymer.

Figure 3 .
Figure 3. (a) Apparent viscosity-shear rate curves of cathode slurry with different dose of DMM-B at 69 wt% solids; (b) apparent viscosity-shear rate curves of cathode slurry with 69 wt% solids containing 0.4 wt% different carboxyl density DMMs; (c) apparent viscosity-shear rate curves of cathode slurry with 69 wt% solids containing 0.4 wt% different side chain length DMMs.

Figure 4 .
Figure 4. (a) Variation curve of viscosity of the upper layer of slurry with time; (b) settling rate after resting for 48h.

Figure 5 .
Figure 5. (a) Adsorption of different grafting density DMM on the surface of LFP; (b) adsorption of different side chain length DMM on the surface of LFP.

Figure 8 .
Figure 8.(a) XPS scan spectra of LFP and LFP-B; (b) N 1s spectra of LFP and LFP-B.

Table 1 .
Molecular characteristics of comb polymers.
m: polymerization degree of side chain; COOH (mmol/g): amount of COOH in one gram of polymer.

Table 2 .
DMM-B adsorption thickness on LFP surface.