Annealing determined β-phase polypropylene crystal texture, separator porous channels after biaxial stretching, and lithium-ion battery performances

Uneven porous channels tend to undergo structure-determined chemical deterioration as lithium-ion battery (LIB) operates, which may restrict lithium-ion migration behaviors within the separator and deteriorate cell performances. This research precisely regulates β-phase polypropylene crystal texture and porous channels after biaxial stretching based on the systematic annealing process to explore annealing decided separator porous channels and subsequent electrochemical performances of LIB. Suitable annealing temperature maximally concentrates lamellae layer dispersion and enhances thermal stability, which homogenizes biaxial tensile deformation and subsequent porous channels. Identical crystal and porous channel optimizations are also achieved by synchronously rising annealing temperature and shortening annealing time, especially annealing at 155 ℃ for only 10 s, which lowers the Li+ migration barrier and strengthens cell C-rate and cycling stability. This desirable improvement does not require the sacrifice of time cost to achieve, which verifies separator structure-chemical affected LIB performance and the application feasibility of annealing in the actual production of the dry double-drawn separator.


Introduction
Nowadays, continuously deteriorated environments and irreversible petrochemical materials consumption improve the energy-storage facility.Lithium-ion batteries (LIBs) feature desirable safety capability, high power energy density, and long calendar life, which have been attracting numerous attentions and are widely used in fields such as electronics, electric cars, and aerospace technologies [1][2][3].Lithium Highlights • Precisely regulating β-PP crystal texture and separator porous channel by systematic annealing.• Verifying application feasibility of annealing in actual separator production without sacrificing time cost.• Building reliable relationships between separator channel design and practical LIB applications.
355 Page 2 of 16 ions (Li + ) shuttle between the cathode and anode as LIBs operate.Electrochemical inert separator locates between two electrodes and physically functions in terms of impacting battery performances, which blocks two electrodes contact, conducts Li + within the connecting pores, and averts LIB thermal runaway by clogging pore canals [4,5].Separator electronic insulation signifies irrelevant to redox reactions of the LIB, but porous channels determine ion transport paths and electrode/separator interface process, further affecting or limiting comprehensive LIB performances [6,7].
Ideal separators ought to have soft character and sufficient mechanical to guarantee isolation functionality [8][9][10].Based on the consideration of performance and production cost, polyolefin such as polyethylene (PE) or polypropylene (PP) become the main material for the current mainstream separators [11][12][13][14].Two manufacturing techniques (dry process and wet process) have been developed, which both contain extrusion to prepare prefab film and stretching to generate porous channels [15][16][17].The dry process can be divided into uniaxial and biaxial techniques based on stretching modes.For the dry process containing biaxial tensile, PP is mixed with β-crystal nucleation catalyst and extruded into prefab films containing high purity β-phase PP (β-PP), directly followed by the biaxial stretching along the mechanical direction (MD, longitudinal tensile) and subsequent transverse direction (TD, transverse tensile) [18,19].Hence, this process owns advantages of continuous production, relatively simple equipment, low production cost, and environmental friendliness, which attracts numerous considerations in recent years.However, biaxial stretching causes dispersive porous sizes for separators and restricts its application in the high-tech fields due to the unique pore-forming way of stretched β-PP.[19][20][21] β-PP consists of loosely arranged lamellae, which can split directly under tensile to construct porous channels.Isotropic β-PP prefab film contains β-lamellae arranged in all directions and inevitably generates multiple β-lamellae deformation modes.During the longitudinal tensile along MD, lamellae vertical to tensile axis separate from each other and cause luxuriant fissures and pores, lamellae along the tensile axis slip and fit together to form compact α-phase PP (α-PP) firstly.In the next transverse tensile, fissures and pores continue to expand to final pore channels, while the compressed lamellae transform into coarse fibrils, ultimately presenting porous construction with wide size distribution [22,23].Besides, the polydisperse β-lamellae leads to dispersive weak inter-lamellae interfaces, which also deteriorates β-lamellae deformation and widens pore size distribution [24][25][26].To conquer this restriction of the dry double-drawn separators, many studies concentrate on optimizing pore channels by regulating supramolecular structure [27,28], preparing oriented β-lamellae [29], doping nano-particles or organic principle [30], and adjusting drawing conditions [31,32].
For the dry process with uniaxial stretching, prefab films after preparation first undergo annealing process to improve internal crystal structure, homogenize weak interface between shish-kebab lamellae, and finally obtain even microporous structure after stretching [33][34][35].Inspired by this step, the dry double-drawn prefab film can also be annealed to thicken lamellae, centralize lamellae layer dispersion, and provide optimized crystal structure foundation for subsequent pore formation during the biaxial stretching.However, the annealing process inevitably increases the time cost of the actual separator manufacturing process.The influence of various annealing conditions on the crystal structure of β-PP prefab film and the subsequent porous channels also remains unclear.Therefore, it is crucial to clarify the effect of annealing on separator porous channels and the new contradictions such as time cost increase in the actual production process.
In this research, β-PP crystal texture and porous channels after biaxial stretching were adjusted precisely based on the systematic annealing process to explore annealing decided separator porous channels and subsequent electrochemical performances of LIB.Refined crystalline texture characterizations, porous channel analyses, and battery performance tests uncovered that appropriate annealing temperature (140 ℃, ceiling temperature of α-β phase transition) concentrated lamellae layer dispersion and enhanced lamellae thermos-stability.Hence, separator pore size dispersion was centralized, which also homogenized current density and enhanced the cycle performance of LIB.Identical crystal and porous channel optimizations were achieved by synchronously increasing the annealing temperature and shortening the annealing time.Particularly, annealing at 155 ℃ for only 10 s could maximally improve the prefab film crystal structure and separator porous channels, which lowered the Li + migration barrier and strengthened the battery long-term performance.This desirable improvement does not sacrifice time costs, which verifies the application feasibility of annealing in the actual production of the dry double-drawn separator, clarified separators structure-chemical affected LIB performance, and established the reliable relationship between separator porous channel regulation and actual battery application.

Materials and preparation
Commercial polypropylene (T38F) with a weight-average molecular weight of 3.8 × 10 with a melt flow rate of 2.9 g/min (230 °C, 2.16 kg) was offered by the Lanzhou Petrochemical Company, China.The weight-average molecular weight (M w ) is 3.8 × 10 5 g/mol, the M w /M n (Mn is the number-average molecular weight) is 4.7.β-crystal polypropylene nucleating agent (NAB83) was purchased from GCH Technology Co, Ltd.Electrolyte (LB-006, 1 M LiPF 6 dissolved in diethyl carbonate (DEC) and ethylene carbonate (EC), 1:1, wt%) was purchased from DodoChem Company.LiCoO 2 cathode (mass loading: 14 mg/cm 2 , active substance fraction: 96wt%), stainless steel (SS), and lithium sheet were offered by Kejing Material Technology Co, Ltd. 0.3wt% β-crystal polypropylene nucleating agent (based on the commercial manufacture) was mixed into polypropylene uniformly and then underwent a twin-screw extruder (screw speed: 80 rpm, extrusion temperature: 230 ℃) equipped with cooling rolls (roll temperature: 120 ℃) to prepare prefab films (Fig. 1).Prefab films were then annealed at various conditions within a program temperature-controlled oven.Films were assigned as P-temperature-time (for instance, P-140-600 means prefab film was annealed at 140 ℃ for 600 s, P-25 implies un-annealed prefab film).Prefab films after annealing then underwent biaxial drawing, which stretched lengthways for 300% at 100 ℃ and transversely for 300% at 130 ℃ (drawing rate: 5%/s) to fabricate porous separators.

Scanning electron microscope (SEM)
Separators were sprayed with gold for 30 s and then observed by FEI Inspect F (FET, USA) SEM at accelerating voltage of 5 kV.To wipe out the amorphous region and observe crystal texture accurately, prefab films were immersed in a configured etching solution (1.5wt% potassium permanganate dissolved in concentrated phosphoric acid: condensed sulfuric acid = 1:2, bulk factor).Then the prefab films were sprayed with gold and then supervised by SEM at 5 kV.

Differential scanning calorimetry (DSC)
Heating curves of prefab films were recorded by DSC3 + calorimeter (Mettler Toledo, Switzerland) from 30 ℃ to 200 ℃ at a heating rate of 20 ℃/min.Crystallinity degree (X C-DSC ) was obtained by calculating the ratio of the enthalpy value of prefab films (ΔH m ) to that of the 100% crystalline PP ( ΔH 0 m , 177 J/g for α-PP and 168.5 J/g for β-PP) [36]: β-crystal relative content (K β-DSC ) was the rate of the β-crystal crystallinity (X C-β ) to the overall crystallinity of α-PP (X C-α ) and β-PP (X C-β ) [37]:
The grazing incident small angle X-ray scattering (GISAXS) pattern was carried out on the EUSS 2.0 SAXS/ WA (Xenocs, France).The 1D scattering plots were integrated based on the GISAXS pattern by [40]: where q is the scattering vector module, θ is half of the scattering angle, and λ is the X-ray wavelength.

Mechanical properties
Prefab films and separators were cut into 80 mm × 10 mm and then executed tensile testing at 100 mm/min.Separator was cut into a circle (diameter: 6 cm) and the puncture strength was then measured on the puncture model (puncture needle diameter: 0.5 mm) at 300 mm/min.

Porosity and Gurley value
The separator porosity was acquired by [41]: where V s is the separator volume, V PP is the volume occupied by PP that acquired by dividing the separator mass by the density of PP.Gurley value was obtained by 4110N Gurley tester (Gurley Precision Instruments Inc., USA)

Electrochemical performances
Linear sweep voltammetry (LSV, 5 mV/s, 2-6 V) scans of Li-SS cells were obtained on Reference 3000 electrochemical workstation (Gamry Instruments, America, the same below) to measure the electrochemical stability window of separators.Bulk resistance (R b ) of the separator could be acquired based on the electrochemical impedance spectroscope (EIS) of the SS-SS cell (frequency range: 100 k-0.01 Hz, amplitude: 10 mV).Ionic conductivity (σ) was determined by [42]: where S and δ are the effective area and thickness of separators.Interface resistance (R i ) between the separator and Li plate was assessed based on the EIS of Li-Li cell under the frequency range of 100 k-0.01 Hz and amplitude of 10 mV.
Li + transference number (t + ) was obtained by the EIS and chronoamperometry of Li-Li cell [43]: where R c is the interface resistance of Li-Li cell after chronoamperometry, I i and I c are initial and stable currents on chronoamperometry plots, ΔV (10 mV) is the step potential.

Cell performances
The C-rate and cycling ability of LiCoO 2 -Li cell with prepared separator (electrolyte: 40μL) were measured on the CT3001A battery estimating device (Wuhan Blue Electric Co., Ltd., China, 2.75-4.2V for each cycle) at the constant temperature of 25 ℃ [44].The C-rate capability was assessed at the current density range of 0.2-8C.Cycling durability was carried out at the identical charge and discharge rate of 0.5C.

Properties of original prefab film
Figure 2 collects crystal structure analyses and mechanical properties of the original prefab film (P-25).P-25 possesses an average thickness of 89.5 μm (Fig. 2d), which is similar to that of the prefab film used in industrial production.SEM crystal image (Fig. 2a) shows typical β-phase construction that loosely arrayed β-lamellae constitute bundle-like supramolecular structure while compact α-lamellae cannot be detected.Solely β-phase (300) and (301) planes diffraction rings or peaks in GIWAXD pattern (Fig. 2b) and XRD spectrum (Fig. 2g), without any detectable of α-phase (110), (040), and (130) planes diffraction signals at 14.1°, 16.9°, and 18.6°, also signify high-pure β-phase for P-25.Further calculation uncovers that P-25 owns crystalline (X C-XRD ) of 56.4% and extremely high β-phase relative content (K β-XRD ) of 99.3%.Even diffraction rings and f lat azimuth integral of the (300) plane (Fig. 2e) indicate the isotropy, which can also be proved by the circularly distributed GISAXS pattern (Fig. 2c).Meanwhile, the dispersed GISAXS mode states broad lamellae layer dispersion.The 1D scattering plot presents a scattering extreme value at q m-max about 0.289 nm −1 , matching an average long period (L AC-B ) of 21.82 nm by Bragg's equation (L AC-B = 2π/q m-max ).The 1D correlation function of lamellae stacks electron distribution suggested by Strobl and Schneider is further gained based on: )dqm to exactly confirm the fine crystal structure parameters [45,46].Relevant long period (L AC-K ), lamellae layer (L C-K ), and amorphous thickness (L A-K ) are defined as the embedded illustration in  width at half max (FWHM) of the β-phase melting peak, which declares lamellae layer dispersion, reaches 9.2 ℃.This wide FWHM determines scattered β-lamellae layer dispersion and causes heterogeneous tensile deformation, reflected by the yield stress (σ y ) of 30.5 MPa and excessive softening stress drop (Δσ) of 8.2 MPa (Fig. 2i).
As tensile strain expands, a prominent secondary yield peak appears.Our previous studies [19,35] reported that concentrated β-lamellae layer dispersion implies more uniform deformation and greater secondary yield strain (ε 2 ).The ε 2 only appears at 391.2% due to the dispersive β-lamellae layer dispersion.Finally, P-25 fractures as deformation strain reaches the elongation at break (ε b ) of 514.5%.45%, β 2 : 25% and α: 30%, Fig. 4d-f).FWHM of the β-phase endothermic peak also maintains about 9 ℃ (Fig. 4c).All signs indicate that the crystal structure of prefab film is independent of t a as annealed below 100℃ due to insufficient thermal stimulation, which can be further proved by the bimodal XRD spectra that only β-phase (300) and (301) diffraction peaks appear while the α-phase (110), (040), and (130) planes diffraction signals are undetectable (Fig. 5a), together with the unchanged X C-XRD and K β-XRD as annealed at 100 ℃ for various time.

Properties of annealed prefab film
Identical DSC heating curves when annealed at 120 ℃ (Fig. 3b) less than the 60 s also present similar X C-DSC , K β-DSC , and FWHM to that of the pristine P-25.However, an inconspicuous endothermic peak appears and migrates to the high-temperature as t a flies (indicated by red arrows in Fig. 3b), which derives from the melting of ordered structures generated within the amorphous regions.Therefore, the crystallinity of prefab films, both X C-DSC (Fig. 4a) and X C-XRD (Fig. 5h), increases slightly.β 1 melting peak fraction (F β1 ) also rises to about 60% markedly (Fig. 4d).While K β-DSC (Fig. 4b) and K β-XRD (Fig. 5i) still hold steady at about 75% and 99%, respectively.Besides, the FWHM shrinks gradually and reaches 7.7 ℃ as t a reaches 2400 s at 120 ℃ (Fig. 4c), indicating that annealing at 120 ℃ for a suitable time can partly optimize the β-lamellae layer dispersion.
Annealing at 130 ℃ within the 60 s does not alter DSC melting signals (Fig. 3c).The major difference with previous annealing lies in that longer t a varies β-phase melting peak shape visibly, where low-temperature F β1 decreases gradually (Fig. 4d), accompanied by highlighted high-temperature β 2 peak (F β2 , Fig. 4e).This peak permutation continues as t a increases, where F β2 increases rapidly from 16.6% to 59.9% and states the improved β-lamellae thermal stability.This can also be confirmed by the enhanced K β-DSC (increases from the initial 75.2% to 83.5% for 2400 s, Fig. 4b).However, the K β-XRD (Fig. 5i) presents a slight downward trend inversely, decreasing to 97.4% for 2400 s.Since there is no crystal structure damage during the XRD test, the measured K β-XRD indicates that tiny α-phase forms after annealing at 130 ℃ for 2400 s.While β-phase with higher thermal stability is harder to shift into α-phase as DSC heating scans, causing higher K β-DSC even if β-phase content does weaken slightly.The shrinkage of the β 1 peak also shortens the FWHM of the β-phase endothermic peak, ultimately decreasing to 5.8 ℃ for 2400 s, accompanied by the raised X C-DSC (44.9%) and X C-XRD (62.4%), which states the thicker and concentrated β-lamellae layer dispersion.
As T a reaches 140 ℃, fractions of two β-phase melting peaks continue to permute.When t a reaches 1200 s, the low-temperature β 1 melting peak disappears completely and β-phase endothermic signals transform into a unimodal high-temperature β 2 melting peak.Obviously, annealing at 140 ℃ maximally improves inferior β-lamellae since 140 ℃ slightly exceeds the β-phase endothermic signal starting point, which transforms into uniform and thicker β-lamellae.FWHM thus drops markedly to 2.8 ℃ as t a exceeds 1200 s Fraction of each endothermic peak in DSC scans: (d) low-temperature β 1 melting peak, (e) high-temperature β 2 melting peak, and (f) α-phase melting peak (Fig. 4c).Meanwhile, X C-DSC and X C-XRD increase to 46.9% and 63.8%, respectively.The maximal K β-DSC of 87.3% also signifies the most stable β-lamellae (Fig. 4b).However, long-time annealing at 140℃ promotes the formation of α-phase, diffraction peaks of α-phase (110), (040), and (130) planes emerge gradually on XRD spectra (Fig. 5d).K β-XRD lowers to 92.3% at t a of 2400 s but still remains above 90% (Fig. 5i).Clearly, 140 ℃ is a proper annealing temperature to centralize β-lamellae layer dispersion and improve β-phase thermal stability, but the cost is to pay enough annealing time, which inevitably increases the time cost and is against continuous separator production mode.
Clearly, the deficiency in annealing time can be effectively alleviated by suitably increasing the annealing temperature.This is especially true in the annealing process at 155 ℃ where single-peaked β-phase melting signals appear only  3f), together with high β-phase fraction (K β-DSC : 84.4%, K β-XRD : 93.2%) and narrow FWHM of 3.7 ℃.While 155 ℃ obviously exceeds the β-phase melting interval, accelerating the formation of α-phase.Ultimately, β-phase signals in the DSC scan completely disappear at t a of 2400 s, and only the endothermic peak of α-phase remains, β-phase (300) and (301) diffraction peaks are also hardly detected in the XRD spectra (K β-XRD : 8.7%, Fig. 5f), indicating that immoderate annealing deteriorates β-phase fraction and impacts microporous structure after tensile.Notably, the β-phase fraction rapidly lowers when annealed at 160 ℃ for only 10 s (K β-DSC : 46.7%, K β-XRD : 26.7%), causing the meaningless FWHM of 4.2 ℃.As t a exceeds 60 s, β-phase signals on both DSC and XRD plots vanish entirely, followed by the only remaining α-phase melting behavior on DSC scans (Fig. 3g) and diffraction peaks on XRD spectra (Fig. 5g), indicating that the annealing temperature of 160 ℃ is too high to make up for the disadvantage of the overlong annealing time.
Figure 6a and b exhibits SEM morphologies and GISAXS patterns of prefab film annealed at various conditions to accurately evaluate the crystal structure evolutions during annealing.The 1D-SAXS plots integrated from GISAXS patterns and 1D correlation functions of electron distribution within lamellae stacks are depicted in Fig. 6c and d films are listed in Fig. 6e.The L AC-K of all films suits L AC-B well, signifying the dependability of L AC-K obtained by the 1D correlation function.Scattering peak at 0.289 nm −1 states the L AC-B of 21.7 nm for P-25, together with the L AC-K of 21.35 nm, L C-K of 12.02 nm, and L A-K of 9.33 nm.Both P-140-2400 and P-155-10 own concentrated β-lamellae layer dispersion, therefore exhibiting visibly thicker β-lamellae in SEM images (Fig. 6a1 and a2).This can be proved by the convergent GISAXS patterns (Fig. 6b1 and  b2) and 1D-SAXS integral peaks (Fig. 6d).Besides, the L AC-K of P-140-2400 and P-155-10 increases to 23.92 nm and 23.21 nm, respectively.The L C-K of P-140-2400 and P-155-10 reaches 14.43 nm and 13.85 nm, the L A-K remains at 9.49 nm and 9.36 nm correspondingly, manifesting the thickened β-lamellae for two films, in which the larger mean lamellae layer and narrower lamellae layer distribution profit from the incrassation of inferior β-lamellae during the annealing process.While the drastic β-α phase transformation within P-155-600 lowers β-phase content and only remains little β-phase, which can be directly observed from SEM microscopic image (Fig. 6a3, the yellow area indicates the remaining β-phase).Therefore, 1D-SAXS peak of P-155-600 (Fig. 6d) broadens and shifts to lower q value of 0.213 nm −1 , corresponding to the L AC-B of 29.48 nm.L AC-K , L C-K , and L A-K also rise to 29.8 nm, 17.53 nm, and 12.27 nm due to the formation of α-phase.
Figure 7 presents tensile curves of prefab films annealed at various conditions, accompanied by σ y , Δσ, ε 2 , ε b , and necking width (ratio of the width of the sample stretched to a specific strain to the original width).Broadly distributed β-lamellae determines heterogeneous deformation for pristine P-25, causing a typical yield peak (σ y : 30.5 MPa, Δσ: 8.2 MPa) and general ε 2 of 391.2%.The necking width of P-25 also remains low condition and reaches 56.3% at ε = 400%.P-120-10 with ineffective annealing presents a similar sharp yield peak with σ y of 30.7 MPa and Δσ of 8.5 MPa, together with a necking width of 57.1% at ε = 400%, signifying the uneven deformation and causing alike ε 2 of 389.1% and ε b of 487.1%.The increasing t a at 120 ℃ improves the β-phase structure and alleviates deformation heterogeneity, which flattens the yield peak, and delays the secondary yield point.But even P-120-2400 still holds an obvious yield peak with Δσ of 6.1 MPa.P-140-10 also presents a high Δσ of 7.2 MPa and a small ε 2 of 394.9%, narrowing down to 57.8% of initial width at ε = 400%.The tensile curve becomes gentle as t a rises, especially for P-140-2400, reflected by the lowest Δσ of 2.8 MPa, markedly increased ε 2 of 495.9% and ε b of 537.2%, and the sluggish necking (64.8% at ε = 400%).Similarly, P-155-10 owns centralized β-lamellae dispersion profited from proper annealing condition, presenting the flat yield peak (σ y : 29.8 MPa, Δσ: 3.2 MPa) and high ε 2 of 485.6%.Both the uniform deformations of P-140-2400 and P-155-10 can facilitate homogeneous pore size after biaxial stretching.However, the β-crystal content of P-155-600 decreases significantly (K β-DSC : 34.6%, K β-XRD : 60.9%) and thus shows typical α-PP tensile characteristics on stress-strain curves.Since α-lamellae are composed of radial parent lamellae and tangential daughter lamellae developed epitaxially on parent lamellae, which is quite different from the loosely arranged β-lamellae, the interlocked α-lamellae enhance the α-phase intensity and deteriorate volatile deformation [47].Especially, the suddenly raised σ y of 33.6 MPa and Δσ of 9.2 MPa, disappeared secondary yield behavior, much lower ε b of 354.3%, and the fastest necking rate (51.1% at ε = 300%) of P-155-2400 with little β-phase signals states the heterogeneous deformation.

Performances of separators
SEM images, surface pore size and fibril width distributions, porosity, and Gurley value of specific separators after biaxial tensile are depicted in Fig. 8 to investigate the annealing process affected porous constructions.Dispersive β-lamellae layer dispersion of pristine P-25 determines heterogeneous lamellae deformation mode, generating uneven micropores with an average size of about 119 nm.Numerous rough fibrils (mean width: 277 nm) and serried non-porous regions can also be detected in Fig. 8a.Low porosity of 37.4% and high Gurley value of 368 s of P-25 state poor pores connected condition and inevitably affects ion migration within the separator.Similar porous structures are shown in Fig. 8b and d for P-120-10 and P-140-10 owing to the insufficient annealing, accompanied by the unaltered porosity (37.6% for P-120-10, 38.2% for P-140-10) and Gurley value (374 s for P-120-10, 356 s for P-140-10).Prolonging t a concentrates β-lamellae layer dispersion, and elevates β-phase thermal stability.Therefore, P-120-2400 after biaxial tensile shows the improved porous channels with a pore size of 109 nm and finer fibril width of 189 nm.P-140-2400 possesses even pores (mean size: 74 nm), delicate fibrils (mean width: 133 nm), higher porosity (41.9%), and depressed Gurley value (306 s), which can guarantee lower ion transport hindrance and alleviate uneven Li + flux.P-155-10 presents similar micropores (mean size: 78 nm) and fibrils (mean width: 142 nm) compared with P-140-2400.The porosity of 40.9% and Gurley value of 320 s also indicate the same optimization effect for β-lamellae and porous channels compared with P-140-2400.However, the drastic β-α phase transition induced by increased t a seriously restricts the formation of micropore structure, leading to much lower porosity of 19.6%, excessive Gurley value of 873 s, and un-statistical fibrils width for P-155-600.
Separator confronts the conditions such as traction, winding, and puncturing during the cell preparation and subsequent operations.Adequate strength is therefore required to insure that separators will not be mechanically damaged and avoid internal short-circuit.Tensile and puncture curves are shown in Fig. 9a-c to explore the effect of the annealing process on separator mechanical properties.Uneven pores with plenty of coarse fibrils of P-25 determine the mediocre tensile strength (99.2 MPa) and puncture strength (19.8 g/μm, since the separator thickness varies slightly, it makes more sense to calculate puncture strength per unit thickness).The mechanical strength of P-25 is sufficient to cope with various stresses like piled force, winding tensile, and habitual crash even without improvement.P-120-10 and P-140-10 also present similar tensile (97.9 MPa for P-120-10, 99.1 MPa for P-140-10) and puncture (19.7 g/μm for P-120-10, 18.7 g/ μm for P-140-10) plots.Annealing at 120 ℃ for 2400 s improves β-phase texture and subsequent separator porous channels, which elevates tensile and puncture strength to 106.9 MPa and 21.9 g/μm, respectively.Visibly uniform porous channels of P-140-2400 generate reinforced mechanical strength, reaching 112.7 MPa and 22.1 g/μm for tensile and puncture strength.The tensile strength of P-155-10 slightly lowers to 105.2 MPa, while puncture strength rises to 23.4 g/μm, which can resist various operating conditions and offer better security.P-155-600 with immoderate annealing worsens porous construction and causes meaningless porosity.Mechanical strength of P-155, even if excellent (tensile strength: 112.4 MPa, puncture strength: 28.9 g/μm), is no longer of practical significance.The separator electrochemical stability window, which states adaptability to high power and energy cells, is depicted in Fig. 9d.P-25 owns the common stability of 4.79 V as potential rises.While the annealing process, whether sufficient or not, does not alter electrochemical windows, still maintains a constant upper limit of about 4.8 V, illustrating that the electrochemical window is insensitive to the homogeneity of normal porous channels.Figure 9e shows EIS plots of SS symmetric cell and corresponding σ, which are governed by the pore connection since PP features insulative.P-25 owns R b of 3.3Ω and σ of 0.41mS/cm due to the inferior porous channels.An inadequate annealing process cannot optimize porous channels, leaving alike σ of 0.42mS/cm and 0.44mS/cm for P-120-10 and P-140-10.P-120-2400, P-140-2400, and P-155-10 with even porous channels elevate σ to 0.53mS/ cm, 0.52mS/cm, and 0.50mS/cm, respectively.The interface resistance (R i ) of the Li-Li cell, which is indicated by the inside diameter of the EIS plot, declares the compatibility between the separator and lithium electrodes (Fig. 9f).P-25 and P-140-10 own Ri of 485Ω and 464Ω, respectively.P-140-2400 and P-155-10 with even pores exhibit the lower Ri of 364Ω and 371Ω.Besides, the Li + transport number (t + ) signifies the current proportion contributed by Li + transfer and is calculated based on the EIS and chronoamperometry (Fig. 9f-i).Four separators have similar t + of about 0.3 (0.309 for P-25, 0.296 for P-140-10, 0.313 for P-140-2400, and 0.314 for P-155-10) because separators are prepared by the same material.
The C-rate capability and discharge plots of LiCoO 2 -Li cells are presented in (Fig. 9j, k) to illustrate porous channels determined overall battery performances.Cell including P-25 possesses a discharge capacity of 140.7mAh/g at 0.2C.As current density increases, the discharge capacity of P-25 cell declines rapidly may be derived from ohmic polarization limiting ion transport [48].P-25 cell shows a low capacity of 49.3mAh/g at 1 st cycle at 8C and reduces to 43.7mAh/gonly cycles for 5 times.Cells with P-120-10 and P-140-10 also show the capacities of 141.5mAh/g and 141.7mAh/g at 0.2C, followed by the downhill capacity along with the increasing current density, lowing to 50.1mAh/g and 51.6mAh/g at 8C. P-120-2400, P-140-2400, and P-155-10 with uniform porous channels present alike capacity of 142.6mAh/g, 142.9mAh/g, and 142.2mAh/g at 0.2C but deferred descent capacity as current rises.Electrode-active substance governs cell capacity commonly.The separator also influences ion migration paths and thus varies cell dynamics.Even porous channels with superior connectivity endow separators with smoother Li + transport paths, which declines internal resistance and augments cell capacity [49][50][51].Especially, after cycling for 1 st at 8C, P-120-2400, P-140-2400, and P-155-10 show higher discharge capacity of 54.5mAh/g, 56.8mAh/g, and 56.2mAh/g.Figure 9l exhibits cycling stability and corresponding coulombic efficiency of cells cycled at 0.5C for 300 times to verify the effect of the even porous channels on the cell longterm properties.All cells show the alike capacity of about 135-138mAh/g at 1 st cycle.However, capacity differences between cells emphasize gradually with cycling.The discharge capacity of the P-25 cell reaches 117.4mAh/g at the 100 th cycle and ultimately lowers to 74.5mAh/g (capacity retention: 54.9%) at the 300 th cycle.P-120-10 and P-140-10 also keep low capacity retention of 53.4% (72.6mAh/g) and 58.7% (80.5mAh/g) as cycles to 300 th since inferior porous channels restrict Li + within cells from migrating to where they should transport [52].Optimized cycling stability can be detected for cells with P-120-2400, P-140-2400, and P-155-10, with remaining decayed discharge capacity of 88.2mAh/g (64.5%), 92.2mAh/g (67.1%), and 87.1mAh/g (63.1%) at 300th cycle.

Conclusion
In this study, the β-PP crystal texture and porous channels after biaxial stretching were regulated precisely based on the systematic annealing process to investigate annealing determined separator porous channels and subsequent electrochemical performances of LIB.Proper annealing temperature concentrated lamellae layer dispersion and enhanced β-lamellae thermal stability, which could homogenize biaxial tensile deformation and subsequent porous channels.However, this desirable improvement required the sacrifice of time cost to achieve.Therefore, the coupling relationship between annealing temperature and time is further explored to verify the application feasibility of annealing in the actual production of the dry double-drawn separator.Same crystal and porous channel improvements are also achieved by synchronously rising annealing temperature and shortening annealing time.Especially, annealing at 155 ℃ for only 10 s can maximally optimized crystal structure and following porous channels, which lowered the Li + migration barrier and strengthened cell C-rate and long-term performance.This ideal porous channel improvement does not require the sacrifice of time cost to achieve, which also clarified separators structure-chemical affected LIB performance, and established the reliable relationship between separator porous channel regulation and actual battery application.

Fig. 1
Fig. 1 Separator preparation process diagram Fig. 2f.L AC-K of P-25(21.35 nm)  suited with L AC-B states the dependability of L AC-K acquired from the 1D correlation function.P-25 possesses the L C-K of 12.02 nm and L A-K of 9.33 nm, respectively.DSC heating scan (Fig.2h) shows two evident endothermic signals about 140-150 ℃ and 160-170 ℃, corresponding in turn to the melting acts of β-phase and α-phase.Especially, the α-phase melting signal that appeared on the DSC plot, which is undetected in GIWAXD and XRD (K β-XRD > 99%), imputes α-β phase transition of the thermodynamically metastable β-crystal during the heating process, leading to relatively lower X C-DSC of 42.5% and K β-DSC of 74.8% than those obtained by diffraction tests.Besides, the β-phase melting section shows two peaks at β 1 at 143.9 ℃ and β 2 at 149.5 ℃.Therefore, the full

Figure 3 Fig. 3
Figure 3 presents specific DSC heating scans of prefab film annealed at different conditions to diagnose crystal structure evolution during the annealing process.Key parameters of prefab film (X c-DSC , K β-DSC , and FWHM) and fractions of each endothermic peak (calculated by Peak Fit) are depicted in Fig. 4. Corresponding XRD spectra, X C-XRD , and K β-XRD are shown in Fig. 5.When annealed at the specific annealing temperature (T a ) of 100 ℃ (Fig. 3a), melting signals remain virtually unchanged, holding typical β-crystal melting characteristics no matter how long the annealing time (t a ), maintaining the constant X C-DSC about 42%, K β-DSC about 75%, and hardly fluctuate endothermic peak fraction (β 1 :

Fig. 9
Fig. 9 Mechanical properties of separators: (a) tensile stress-strain curves and (b) puncture curves.(c) Tensile strength and puncture strength per unit thickness of separators.Electrochemical properties of separators: (d) electrochemical stability windows, (e) EIS scans of SS symmetrical cell, (f) EIS scans of Li symmetrical cell before 5g/mol was afforded by Lanzhou Petrochemical Company.Polypropylene (iPP, T38F)