Polymer-coated electrode active material, its manufacturing and usage

The invention relates to polymer-coated electrode active materials, especially polymer- coated cathode active materials of type NCM, being used for improving the cycling stability of the electrode active material within electronic devices as, for example, batteries. The invention also relates to the manufacturing of the polymer-coated electrode active material and ist usage. The preferred usage of the polymer-coated electrode active material is its application as electrode active material within lithium ion batteries (lithium ion electrochemical cells), most preferably within solid-state lithium ion batteries (also called solid-state lithium ion electrochemical cells).

Description

Background of the technology

Thiophosphate-based solid-state batteries (SSBs), especially solid-state lithium ion batteries, resp. solid-state lithium ion electrochemical cells, with high-nickel ternary cathode materials LiNii- _x -yCoxMn _y O2 (NCMs) represent a promising next-generation energy storage technology due to their expected high specific discharge capacity and improved safety. However, a significant problem that needs to be overcome on the way to large-scale high energy-density applications is rapid capacity fading during cycling of the cell. Typically, interphase and crack formation during cycling of NCM cause contact loss and length increase of the lithium diffusion pathway. Herein, a uniform polymer-coating on NCM is obtained from a spray-drying process. This coating, which may be - for example - 4 nm thick, stabilizes the interface between electrode active material, e.g. NCM, and the solid electrolyte (SE), e.g. LiePSsCI. Electrochemical evaluations exemplarily provided herein confirm a significant improvement in long-term cycling performance and active mass utilization compared to uncoated NCM. Thereby, the coating effectively suppresses degradation of the NCM/SE interface, in particular the formation of oxygenated species. Furthermore, the polymer-coating (for example by usage of poly((4-vinyl benzyl)trimethylammonium bis(trifluoromethanesulfonylimide)), PVBTA-TFSI) reduces the extent of particle cracking. Overall, the experimental results provide by way of example a scalable process with a new polycation coating on NCMs for next-generation SSBs.

Polymer coating of electrode materials is known within the state of the art. But known polymer coatings are based upon neutral polymers, as is - for example described in US 20220069337 A1 and WO 2021/254215 A1 (e. g. PVP, polyvinylpyrrolidone). The same polymer (PVP) as is revealed in US 20220069337 A1 and in WO 2021/254215 A1 is used herein as exemplary reference material according to the state of the art, emphasizing the unexpected and surprising improvement in cycling stability of the invention revealed herein below (cf. Fig. 13).

The coating revealed in US 20220069337 A1 furthermore requires photocuring in order to form the final coating, whereat the inventive coating revealed herein below does not require photocuring, thus providing a great reduction of complexity in manufacturing of the coating beyond the state of the art.

WO 2021/254215 A1 is mentioning ionic compounds only as being comprised by the binder polymer, thus stating that they are only suitable for optimising the binder polymer, which itself is only a supportive compound for optimising the mechanical characteristics of the coating revealed in WO 2021/254215 A1. There is no hint in WO 2021/254215 A1 that the binder polymer is exerting a technical effect beyond modification of mechanical properties, whereat the polyelectrolytic polymers according to the invention revealed herein, do indeed have a technical effect in optimising cycle-stability as is shown below. The polyelectrolytic polymers according to the invention revealed herein are the material which bears the technical effect, instead of being just sort or part of a supportive binder polymer. Thus, WO 2021/254215 A1 provides no hint towards the subject matter of the invention revealed herein below.

Solid-state lithium batteries (SSBs) use solid electrolytes (SE) instead of organic liquid electrolytes, which are expected to fundamentally solve battery safety problems and are ideal chemical power sources for electric vehicles and large-scale energy storage. In order to achieve higher energy density, intercalation-type cathode active materials and thiophosphate-based SEs have drawn much attention. High-nickel ternary cathode material LiNii- _x -yCoxMn _y O2 (NCM) is considered a promising lithium battery cathode material due to its high energy density and low cost. However, severe capacity fading caused by interfacial degradation is observed when NCM and thiophosphate-based SEs are employed. In general, the interfacial degradation reactions between NCM and SE give rise to the adverse interfacial layer interfering with lithium ion and electron transport. Even at 0 state of charge, chemical reactions happen at the NCM/SE interface, causing detrimental capacity fading.

During the charging process in the first cycle, the electrochemical degradation of the interface with the solid electrolyte slows lithium transport due to the low electrochemical stability window of thiophosphate-based SEs. However, the discharge process often reaches 2.6 V vs. Li ⁺ /Li, which is not low enough for triggering reduction. Hence, oxidation of the SE is much slower in subsequent cycles. Since the thiophosphate-based SE directly contacts the NCM, the oxygen and lithium atoms in the surface region of the NCM undergo side reactions with the electrolyte, bringing about structural degradation on the surface of NCM and the passivation layer between NCM and SE. Oxygenated species like SOx ¹¹ ' and POx ¹¹ - can be detected by time-of-flight secondary ion mass spectrometry (ToF-SIMS) at the NCM/SE interface, and SO2 can be detected by differential electrochemical mass spectrometry during cycling. Oxygen loss causes mechanical cracking and plays an important role in interface degradation. On the other hand, the lattice contraction and expansion of NCMs contribute to capacity fading, however, SSBs have even more adverse effects than lithium-ion batteries (LIBs) due to the contact loss between SE and cathode materials. Consequently, research efforts are devoted to improving the stability of the electrode-electrolyte interface, but heretofore without success.

To enhance the interfacial stability between NCM and thiophosphate-based SE, surface modification by coating on NCM has been studied considerably. In general, wet coating and dry coating processes can hardly have a thin and homogeneous coating layer compared to atomic layer deposition. As a result, atomic layer deposition such as HfC>2 coated- NCM has also been studied. Many research efforts focuse on inorganic oxides as coating materials, such as LiNbOs, LieZnNb^u, LiAIC>2, Li2ZrOs, Li4Ti50i2, and U3B11O18. Such inorganic coatings are hard and brittle, so that they are prone to breaking upon being exposed to mechanical stress due to - for example - cyclic charging and discharging processes of batteries. Therefore, it is impossible to provide an inorganic coating that is reliably completely covering the surface of an active electrode material which is enduring mechanical stress. Compared with inorganic coatings, polymer coatings are relatively soft and can cover the substrate evenly if there is a force between the functional group and NCM; for instance, polyvinylpyrrolidone (PVP) can act as a surfactant modifying the surface of metal oxide. Although poly(3,4-ethylenedioxythiophene) modification on NCM and carbon additives in SSBs via molecular layer deposition has been studied, polymer-coated NCMs are mostly only adopted in Li-ion batteries with liquid electrolytes up to date. Heretofore it was unknown and unexpected that polymer coatings of NCMs can be combined with solid electrolytes, especially with thiophosphate-based solid electrolytes. The heretofore unadressed and unsolved problem of polymer coating of NCM with polyelectrolyte polymers is that the coating of NCM with polyelectrolyte polymers requires solvent stability with NCM, meaning that the solvent applied within the coating process for solving the polyelectrolyte polymer must not dissolve (extract) the Li ⁺ -ions out of NCM.

Polyelectrolyte polymers, resp. polyelectrolyte organic polymers, according to the invention revealed herein comprise polycationic polymers and polyanionic polymers (synonymously also called polycationic organic polymer or polyanionic organic polymer, resp. cationic polymer, cationic organic polymer, anionic polymer, anionic organic polymer). All types of polyelectrolyte polymers comprise polymers which are composed of monomers having one or more charged functional group (plus counter ion), either being involved in the polymerisation reaction or not being involved in the polymerisation reaction, i. e. the polymer molecules of the polyelectrolyte polymers do receive their polyelectrolytic structure (polycationic or polyanionic) by way of polymerising monomers having at least one charged functional group per monomer moiety. As is known within state of the art, being a polyelectrolytic polymer (polycationic or polyanionic) does not require that the monomer is already containing more than one charged functional group - the polyelectrolytic structure is achieved via polymerisation of monomers, no matter how many and what type of charged functional group the monomers have.

Furthermore, it is not necessary that all monomers, which are composing a polyelectrolyte polymer according to the invention, have at least one charged functional group (plus counter ion). A polyelectrolyte polymer according to the invention is also a copolymer, being composed of one or more types of monomers having at least one charged functional group and one or more types of polymers having no charged functional group, the polyelectrolytic polymer being formed by way of copolymerisation of at least one type of monomer, bearing at least one charged functional group with at least one type of monomer having no charged functional group.

According to usage within the state of the art, polyelectrolyte polymers (polycationic polymers, polyanionic polymers) are herein synonymously also called simply cationic organic polymers resp. anionic organic polymers, for it is - due to polymerisation - impossible for a polymer to have just one single charged functional group per polymer molecule, even if the monomer or a type of monomer (in case of a copolymer) bears only one charged functional group. Thus, polycationic polymer/cationic polymer and polyanionic polymer/anionic polymer are well known pairs of synonyms. Especially contact with water must be excluded in handling NCM. However, cationic (resp. polycationic) polymers like PVBTA-CL and LiTFSI can only be dissolved in water or other polar organic solvents, which are not stable with NCM. On the other hand, after ion exchange of PVBTA-CI to form PVBTA-LiTFSI, PVBTA-TFSI surprisingly can be dissolved in acetone instead of water. The solution of PVBTA-TFSI in acetone is stable with NCM and easy to manipulate by the spray drying process. Through this novel and heretofore unusual approach we have managed to coat the NCM with the polymer without affecting NCM stability. It is obvious to any person of ordinary skill in the art that these results, acquired herein by example of PVBTA-TFSI in combination with NCM, can also be applied to other polycationic polymers in combination with other cathode active materials (e.g. NCA) without leaving the scope of the invention.

Additionally (as is obvious to the person of ordinary skill in the art), the invention is not only applicable with solid electrolytes, but also with liquid electrolytes and polymer electrolytes. In all cases, mechanical degradation of the NCM causes particle fracture during cycling and our coating helps mitigating this degradation mechanism. Therefore, our invention can be applied advantageously to all batteries using any type of electrolyte without leaving the scope of the invention.

Content of the invention

An objective of the present invention is to provide polymer-coated electrode active materials which can be prepared easily and with low cost, and which exhibit extraordinary high cycling stability in electronic devices like batteries.

The high cycling stability is - for example - preferably proven and defined by the reduced formation of phosphates (PO _X ~~ ) and/or sulfates/sulfites (SO _X ^— ) as known species of the decomposition of thiophosphate-based solid electrolytes in solid-state lithium ion electrochemical cells. The high cycling stability is also proven by the finding that pristine NCM cells show more fracturing/cracking within, resp. between, the (sintered) NCM- particles than cells, being prepared with coated NCM particles.

Electrode active material, as is understood herein, comprises cathode active material as well as anode active material. Cathode active material is the preferred electrode active material and amongst cathode active material high-nickel ternary cathode materials LiNii- _x -yCoxMn _y O2 (NCM) are the most preferred electrode active materials.

Thiophosphate based solid electrolytes according to the invention are thiophosphates of argyrodite type, for example LiePSsCI (LPSCI) and thiophosphates of type Li7+o- _P M ^lv _o M ^v i- oCh ₆ -pX _p , with M ^IV = Si, Ge, Sn; M ^v = P, Sb; Ch = O, S, Se; X = Cl, Br, I, BH ₄ and 0 < o < 1 ; 0 < p < 2. Besides thiophosphate based solid electrolytes also other solid electrolytes can be used without leaving the scope of the invention, for example such ones of NaSICON type, like for example Lii+xAl _x Ti2-x(PO4)3 (LATP), garnet type, like for example Li7La ₃ Zr ₂ 0i2 (LLZO), perovskite type, like for example Li3xLa2/3- _x TiO3(LLTO), LGPS-type, like for example LiioGeP2Si2.

Surprisingly, it was found that the polymer-coating of the active electrode material according to the invention is efficiently reducing the formation of oxygenated species like for example SOx ¹¹ ' and POx ¹¹ ', in - for example - thiophosphate based solid electrolytes, resp. on the surface of - for example - NCM, and by doing so is effectively enhancing cycling stability of the active electrode material to a great extent.

Therefore, in order to achieve the above objective, the present invention provides a polymer coating for active electrode materials and active electrode materials bearing this polymer coating as well as their usage in electronic devices, e.g. batteries, especially solid-state lithium ion batteries, resp. solid-state lithium ion electrochemical cells.

The polymer coating according to the invention is comprising either a thermoplastic polymer or a non-thermoplastic polymer or a combination of both types of polymers.

Besides using polycationic organic polymers for coating NCM, the scope of the invention also comprises the usage of polyanionic organic polymers, for example poly(acrylic acid), graphene oxide, polysaccharide gum, such as xanthan gum, poly(vinyl alcohol), polydopamine, chitosan, sulfonated polymers, such as sulfonated tetrafluoroethylene based fluoropolymer-copolymers, poly(styrene sulfonate), poly(3,4- ethylenedioxythiophene)/polystyrene sulfonate, sulfonated polyphenylene sulfone, copolymers with sulfonated polyphenylene sulfone, sulfonated poly(2,6-dimethyl-1 ,4- phenylene oxide), poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide], poly[(4- styrenesulfonyl)(trifluoromethyl(S-trifluooromethylsulfonyli mino)sulfonyl)imide], sulfonated poly(ether ether ketone), poly(3-sulfopropyl methacrylate), and poly[(4- styrenesulfonyl)(fluorosulfonyl)imide], phosphonated polymers, such as poly(vinylphosphonic acid), polyphosphonate, and poly([2-(methacryloyloxy)ethyl] phosphate). This list of polyanionic organic polymers is not meant to be limiting the scope of the invention. Suitable counter ions for the polyanionic organic polamers include, but are not limited to Aluminium(lll), Barium(ll), Beryllium(ll), Calcium(ll), Chromium(lll), Copper(l), Copper(ll), Gold(l), Gold(lll), Hydrogen(l), Iron(ll), Iron(lll), Lead(ll), Lead(IV), Lithium(l), Magnesium(ll), Manganese(ll), Manganese(lll), Manganese(IV), Mercury(ll), Potassium(l), Silver(l), Sodium(l), Strontium(ll), Tin(ll), Tin(IV), Zinc(ll), polyatomic cations, e.g. ammonium, hydronium, Mercury(l), Li ⁺ Na ⁺ K ⁺ Ca ²⁺ Mg ²⁺ Al ³⁺ polyatomic cations, NF , ammonium derivatives, NR3I , NF^F , NRHs ⁺ . R is independently selected from the list comprising alkyl, methyl, ethyl, propyl, iso-propyl, butyl, sec-butyl, iso-butyl, tert-butyl, pentyl, iso-pentyl, this list also not being understood to be confining the scope of the invention.

Similar positive effects upon long time stability of NCM have also been found by combining each type of polyelectrolyte polymer with one or more neutral polymers, forming a diluted polyelectrolyte polymer (i. e. polyelectrolyte polymer combined with a neutral polymer). Combining each type of polyelectrolyte polymer with at least one neutral polymer herein means either mixing of the different polymers mechanically, e.g. via compounding, cosolution etc. or chemically binding them together, for example via co-polymerisation. Thus, copolymers consisting of polycationic organic polymers and neutral organic polymers or polyanionic organic polymers with neutral organic polymers are also polycationic organic polymers resp. polyanionic organgic polymers according to the invention. Suitable neutral polymers are neutral polymers which contain at least one amide group per molecular moiety of the neutral polymer, the at least one amide group being either located within the backbone of the polymer chain or within a side chain of the polymer chain of the neutral polymer. Examples of suitable neutral polymers are polyvinylpyrrolidone, poly(N- vinylformamide), polyamide. This list of examples is not to be understood as being confining the scope of the invention.

The polymer coating revealed herein is especially useful for coating NCM which is to be applied together with thiophosphate-based solid electrolytes in solid-state lithium ion batteries I solid-state lithium ion electrochemical cells. In other embodiments the polymer- coating revealed herein is also useful for coating - for example - NCM which is to be applied together with a liquid electrolyte or a polymer electrolyte in lithium ion batteries. In conjunction with liquid electrolytes, unlike inorganic coatings, the polymer coating can show swelling with liquid electrolyte, reducing the battery's resistance and increasing the storage capacity. Some polymers, which have specific functional groups, for example polyethylene carbonate, can even increase the lithium transport number. Equally, polymer electrolytes are more compatible with polymer coating than inorganic coating.

The polyelectrolyte polymers according to the invention may also be produced via oxidation, reduction, protonation, and/or ion-exchange of the uncharged base polymer to form either polycationic or polyanionic polymers through these reactions.

In orderto have a homogeneous coating, the invention revealed herein teaches to use spray drying of a polyelectrolyte solution for coating an active electrode material, especially NCM materials. Spray drying can be classified as the category of wet coating methods applied in industry. Additionally, as polyelectrolyte can easily absorb on metal oxide due to electrostatic interactions, poly((4-vinyl benzyl)trimethylammonium bis(trifluoromethanesulfonylimide)) (PVBTA-TFSI) is adopted as an example of a coating material for NCM according to the invention herein, this example being understood not to confine the scope of the invention. PVBTA-TFSI is homogeneously coated on the surface of NCM and significantly improves the cycling performance of PVBTA-TFSI-coated NCM battery cells compared to pristine NCM cells. Without being bound to a certain theory, herein the key physical phenomena, such as contact loss, electrochemically inactive interfacial layer and lithium diffusion pathway are also described in detail by discussing the exemplarily applied cathode composite material and the electrochemical performance results.

Overall, the comparison of oxidative decomposition products by ToF-SIMS confirms that polymer coating, for example using PVBTA-TFSI, significantly improves the interfacial stability at the CAM/SSE interface, which is in good agreement with the electrochemical data shown herein. PVBTA-TFSI has been synthesized as an exemplary polymer according to the invention by free radical polymerization and used for NCM coating by spray drying, which is a scalable and controllable process. TEM and Tof-SIMS show that 1 wt% polymer has a homogeneous coating around 4 nm thickness. In terms of electrochemical analysis, the exemplary PVBTA-TFSI coating on the surface of NCM improves the cycling stability of thiophosphate-based SSBs. Correspondingly, EIS and active mass calculations clarify that the polymer coating reduces interfacial degradation. In addition, Tof-SIMS confirms electrochemical data by showing less oxygenated species formed on the interface of polymer-coated NCM. On the other hand, FIB-SEM corroborates the GITT and lithium diffusion length calculation that the particle cracking within NCM has been mitigated by the polymer coating. Overall, polymer coatings, e.g. PVBTA-TFSI, increase cycle stability by reducing mechanical degradation and thereby acts as a protective layer to reduce interface degradation. Lithium-based anionic polyelectrolyte-coated NCMs are also a pivotal solution in addressing capacity loss at the NCM/SE interface, thus enhancing the overall performance of NCMs in SSBs. Polyvinylpyrrolidone (PVP) can form a robust interfacial bond with the cathode material and lithium-based anionic polyelectrolyte by electrostatic force. Hence, we combined the abovementioned ideas to introduce the first polymer complex-coated NCM for SSBs in this research. We utilize a blend of PVP and lithiated polyphenylene sulfone sulfonate (LiPPSS) as the coating material for the LiNi0.9Mn0.05Co0.05O2 (NCM90), noted as UPPSS/PVP-NCM90. Spray drying is employed to achieve a uniform and thin layer of the polymer coating on the NCM90, leveraging electrostatic interactions between LiPPSS, PVP, and NCM90. The employment of this thin LiPPSS/PVP polyelectrolyte complex, resp. diluted polyelectrolyte polymer coating on NCM90 not only minimizes contact loss but also optimizes cycling performance. Furthermore, we delve into the degradation at the interface and the length of the lithium diffusion pathway within the cathode composite, providing further evidence for the enhanced electrochemical performance.

Characterization of anionic polymer-coated NCM90:

Regarding the study of usage of polyanionic polymers, we employ a similar methodology as was applied for our experiments done for polycationic polymers by utilizing a spray coating method to apply an anionic polymer coating onto NCM90. Our previous studies determined that a 1 wt% polymer (relative to the weight of NCM90) can provide an optimized coating thickness, yielding superior electrochemical performance and the most even coating layer. As a result, we use a 1 wt% polymer (relative to the weight of NCM90) as a standard of comparison using the spray coating method described herein below. However, it is observed that water and alcohol solvents in the coating process caused the leaching of Li ⁺ from NCM, necessitating the avoidance of these solvents. To address this issue, lithiated polyphenylene sulfone sulfonate (LiPPSS) is utilized initially to coat NCM90 as it can be easily dissolved in dimethylformamide (DMF). Furthermore, sulfonated poly(phenylene sulfone)s with highly electron-deficient aromatic rings exhibit exceptional thermal and thermo oxidative stability compared to other sulfonated poly(arylene)s. However, the energy selective backscattered (ESB) detector coupled with the scanning electron microscope (SEM) reveals a non-uniform coating characterized by the detachment of certain polymer particles from the NCM90 surface, as shown in Fig. 19. This outcome suggests that the electrostatic force between the LiPPSS and NCM90 is inadequate for achieving a uniform coating. Further exploration with infrared spectroscopy shows that PVP manifests a robust electrostatic attraction with LiPPSS, confirmed by the blue shift in C=O stretching from 1669 to 1656 cm ^-1 , as shown in Fig. 20. Unlike polyanions such as polyacrylic acid (PAA), which tends to precipitate immediately when mixed with PVP, when dissolving PVP and LiPPSS, the two polymers form a stable, transparent solution without separating complex polymer particles. This attribute is advantageous for the spray-drying process. Subsequently, we established a weight ratio between PVP and LiPPSS of 1 :1 for coating on NCM90, henceforth named UPPSS/PVP-NCM90. ESB and SEM demonstrate a marked reduction in surface polymer particles for LiPPSS/PVP-NCM90, as shown in Fig. 19. This observation demonstrates a uniform and homogeneous coating on the NCM90 using the LiPPSS/PVP complex.

Evaluation of rate capability

The rate capability of the coated NCM90, including polyvinylpyrrolidone 1 wt% coated NCM90 (PVP-NCM90), UPPSS/PVP-NCM90, poly((4-vinylbenzyl)trimethylammonium bis(trifluoromethanesulfonylimide)) 1 wt% coated NCM90 (PVBTATFSI-NCM90), and polymethyl methacrylate 1 wt% coated NCM90 (PMMA-NCM90), under diverse conditions at rates of 0.1C, 0.25C, 0.5C, 1 C, and reverting to 0.1 C are compared to the performance of pristine NCM90 (pNCM90), as shown in Fig. 21 . The evaluation utilizes pellet-type SSBs, NCM/Li ₆ PS ₅ CI/VGCF || Li ₆ PS ₅ CI || In-Li.

In Fig. 21 , the PVP-NCM90 SSB only has 138 mAh/g of discharge in the first 0.1C cycle and 70% of Coulomb efficiency, which is much lower than pNCM90 SSB (183 mAh/g and 77% of Coulomb efficiency), PVBTATFSI-NCM90 SSB (170 mAh/g and 79 of Coulomb efficiency), and UPPSS/PVP-NCM90 SSB (188 mAh/g and 80% of Coulomb efficiency). In addition, PVP-NCM90 SSB exhibited a poorer rate capability at all C-rates than UPPSS/PVP-NCM90 and pNCM90 SSBs. This could be - without being bound to a certain theory - attributed to the PVP coating layers without charge and ionic conductivity, which might obstruct Li-ion transport. Nevertheless, the comparatively lower capacity of PVP- NCM90 SSB implies that not all NCM90 surfaces are blocked entirely. This discrepancy might stem from the cell preparation process, including cathode composite mixing and cell pressing, which could cause the PVP coating to detach or damage, consequently exposing the NCM90 surface. A similar effect can also be observed in PMMA-NCM90, which has a much lower capacity than pNCM90, PVBTATFSI-NCM90, and LiPPSS/PVP-NCM90 for the first cycles (156 mAh/g and 74% of Coulomb efficiency). In the first cycle depicted in Fig. 21 , LiPPSS/PVP-NCM90 SSB demonstrated a lower charge capacity (234 mAh/g) at a rate of 0.1 C compared to pNCM90 SSB (237 mAh/g). However, its discharge capacity (188 mAh/g) and Coulomb efficiency (80% of Coulomb efficiency) at a rate of 0.1 C surpassed that of pNCM90 SSB (183 mAh/g and 77% of Coulomb efficiency) during the first cycle discharge process. The enhancement of UPPSS/PVP-NCM90 SSB in discharge capacity and Coulomb efficiency can be attributed to the reduced side reactions, which the LiPPSS/PVP complex coating could facilitate.

Fig. 22 shows that the LiPPSS/PVP complex coating reduces the electrochemical side reaction of LiePSsCI in the initial charging process below 3 V. Additionally, the area under the dQ/dE plot above 3.48 V (vs. Li ⁺ /Li-ln), or around 4.1 V (vs. Li ^+/ Li), for both LiPPSS/PVP- NCM90 SSB and the pNCM90 SSB are comparable in the first cycle charging step, indicating that the coating does not hinder the H2+H3 phase transition processes. Furthermore, when the UPPSS/PVP-NCM90 SSB is evaluated at different discharge rates, specifically 0.25C, 0.5C, and 1C, its rate performance demonstrates a noticeable improvement compared to pNCM90 and other coated NCM90 SSBs. This enhancement may be attributed to the complex coating layer of LiPPSS/PVP being more pronounced and effective at higher C-rates. Additionally, at the 25 ^th cycle, the UPPSS/PVP-NCM90 SSB shows better reversibility than pNCM90 SSB, as shown in Fig. 23. In conclusion, the UPPSS/PVP-NCM90 SSB consistently outperforms the pNCM90 and other coated NCM90 SSBs across all C-rates. The superior performance of the LiPPSS/PVP-NCM90 SSB is primarily attributed to the reduced side reaction without sacrificing H2+H3 phase transition processes through the implementation of the LiPPSS/PVP complex coating.

Cycling performance and active mass calculation

To conduct a more thorough investigation into 0.1C cycling performance, we assembled UPPSS/PVP-NCM90 and pNCM90 SSBs consisting of NCM/Li ₆ PS ₅ CI/VGCF || LiePSsCI || In-Li, allowing for a direct comparison.

Fig. 24 compares the 0.1C cycling performance between LiPPSS/PVP-NCM90 and pNCM90 cells. Initially, LiPPSS/PVP-NCM90 displays a discharge capacity of 190 mAh/g, while pNCM90 demonstrates a lower value of 180 mAh/g. Following 100 cycles of 0.1 C cycling, UPPSS/PVP-NCM90 retains the most capacity at 78.3%, while pNCM90's capacity retention is around 69%. After 152 ^nd cycle, UPPSS/PVP-NCM90 has a capacity retention of 71.6%, whereas pNCM90 lags is at approximately 59.6%. The better capacity retention for UPPSS/PVP-NCM90 demonstrates that 1 wt% of the LiPPSS/PVP complex coating layer enhances the NCM's long-term cycling stability in SSBs.

Fig. 25 demonstrates a comprehensive comparison of the active mass calculation results, based on the reference (as shown in Fig. 26). In this study, active mass refers to the quantity of active material involved in the reaction.

UPPSS/PVP-NCM90 displays active mass retention of 83.7% after the 152 ^nd cycle, while pNCM demonstrates a lower retention value of 78.8%. The degradation of active mass is primarily attributed to contact loss, which has a dominant influence. This means that LiPPSS/PVP complex coating layer on NCM in SSB can significantly enhance the overall performance by mitigating the detrimental contact loss. The term "contact loss" is employed to encompass two fundamental surface degradation mechanisms that affect the performance of the battery: the formation of an electrochemically inactive interfacial layer due to unwanted side reactions (including chemical, electrochemical, and chemomechanical degradations) and the physical separation between the NCM and LiePSsCI electrodes. These mechanisms reduce the effective contact area between NCM and LiePSsCI during cycling, resulting in active mass loss and elevated interface resistance. While distinct, both contact loss mechanisms share comparable consequences on battery performance, making their experimental differentiation arduous.

Conversely, factors such as the passivation layer, which obstructs lithium and electron transport, along with particle cracking within the NCM, can extend the lithium diffusion pathway and interfere with efficient Li ⁺ transport during battery cycling. This increased lithium diffusion pathway length causes the disparity between the cycling performance and active mass retention values. An appropriately optimized coating can effectively counteract interface degradation and particle cracking. This creates a stable interface and ensures a shorter, unimpeded lithium diffusion pathway. By examples of detailed embodiments of the invention, we describe how LiPPSS/PVP complex coating fosters a stable interface and supports a shorter and unhindered lithium-ion diffusion pathway, thereby enhancing battery performance.

Stability window between PEDOT and LiePSsCI

To assess the electrochemical compatibility of poly(3,4-ethylenedioxythiophene) (PEDOT) and polyethylene glycol (PEO) with LiePSsCI, VGCFs are coated with either PEDOT or PEO utilizing the previously mentioned spray drying technique. These coated VGCFs are then used as the working electrode to identify potential side reactions. Electrochemical stability is examined through cyclic voltammetry, ranging from 0 V to 4 V (vs. Li ⁺ /Li-ln), using VGCF/ LiePSsCI as the working electrode and Li-ln-alloy as the reference and counter electrode. Comparing current densities and onset potentials between uncoated, PEDOT-coated or PEO-coated VGCFs, a noticeable peak current at around 3.7 V (vs. Li ⁺ /Li-ln) for both PEDOT and PEG was detected. This suggests that neither PEDOT nor PEO is electrochemically stable with LiePSsCI, rendering them inappropriate for coating applications (cf. Fig. 27).

Detailed embodiments of the invention

A person of ordinary skill in the art well knows that whenever a range of values (e.g. time, temperature, particle size, concentration, BET specific surface area etc.) is given, not only the explicitely named range is meant, but all other sub-ranges within the explicitely named range are also contemplated and are implicitely revealed within the present disclosure.

Exemplarily used Reagents and materials

(Vinylbenzyl)trimethylammonium chloride (VBTA-CI) monomer, reagent grade sodium persulfate (Na2S20s) initiator, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and vapor-grown carbon fibers (VGCF) are purchased from Sigma-Aldrich. Single crystal high nickel NCM83 (LiNi0.83Mn0.06Co0.nO2) is purchased from MSE Supplies (Particle size around 3-5 pm, and BET specific surface area around 0.5 - 0.9 m ² /g). LiePSsCI (LPSCI) is purchased from NEI Corporation. Indium foil is purchased from chemPUR GmbH with 100 pm thickness and punched into a circular electrode of 9 mm in diameter. Lithium foil is purchased from Albemarle Rockwood Lithium GmbH with 125 pm thickness and punched into a circular electrode of 6 mm in diameter.

Exemplary Synthesis of PVBTA-TFSI

Poly(vinylbenzyl)trimethylammonium bis(trifluoromethanesulfonyl)imide (PVBTA-TFSI) is synthesized through the route shown in Fig. 1. Firstly, free radical polymerization is conducted by mixing 5 g vinyl benzyl trimethylammonium chloride (VBTA-CI) and 0.3 ml of saturated Na2S20s solution in 20 ml of deionized water and then heated up and purged with N2 flow at 75 °C for 48 hours (molar ratio of VBTA-CI: Na2S20s= 1 :0.3). After polymerization, PVBTA-CI is purified by dialysis with a considerable amount of deionized water and further condensed by a rotary evaporator. The condensed polymer solution is then added dropwise to the LiTFSI solution (7.5 g LiTFSI dissolved in 20 g deionized water) and stirred overnight for ion exchange. Once the PVBTA-CI drops into the LiTFSI solution, the PVBTA-TFSI precipitates immediately. After the ion exchange reaction, PVBTA-TFSI is washed with deionized water by centrifuge three times to remove the residual LiTFSI and separate the PVBTA-TFSI solid. Afterwards, the PVBTA-TFSI is dried in a vacuum oven at 80 °C for 72 hours and then stored in the glove box. ¹ H NMR (400 MHz, deuterated acetone): 5 1.36- 1.99 (H1 , H2), 5 2.05 (acetone-d6), 5 3.14 (H6), 5 4.4-4.9 (H5), 5 6.5-7.4 (H3, H4). FT-IR of PVBTA-TFSI (cm’ ¹ ): 973 (C-N stretching), 1346, 1326, 1176, 1132, 1050 (TFSh), 1612, 1480, 1422 (aromatic C=C stretching), 3043, 2922, 2852 (alkyl C-H stretching), 3400 (H2O). FT-IR of PVBTA-CI (cm’ ¹ ): 973 (C-N stretching), 1612, 1480, 1422 (aromatic C=C stretching), 3015, 2922, 2852 (alkyl C-H stretching), 3400 (H2O). FT-IR of LiTFSI (cm’ ¹ ): 1327 (SO2 asymmetric stretching), 1245 (CF3 symmetric stretching), 1204 (CF3 asymmetric stretching), 1147 (SO2 symmetric stretching), 1065 (asymmetric S2N stretching).

Exemplary preparation of PVBTA-TFSI coated NCM

Mini Spray Dryer B-290 from BUCHI is used to coat PVBTA-TFSI on NCM, as shown in Fig. 1. 0.1 g (5 wt% compared to NCM) or 0.02 g (1 wt% compared to NCM) of polymer is mixed with 2 g of NCM and 30 g acetone as the precursor. The mixing step is conducted by vigorous stirring and takes about 1 hour to make sure the particle aggregation is broken into a smaller size. The inlet temperature is 150 °C, the volume flow (i. e. suction of vacuum pump) is 37 m ³ /hour, the feed rate of polymer solution is 8 mL/min, and the N2 flow is 40 L/min. The spray drying condition has been optimized to get the highest productivity around 50 to 70 wt%. Besides, the 5 wt% polymer-coated NCM and 1 wt% polymer-coated NCM are noted as 5P-NCM and 1 P-NCM, respectively. In addition, the 5 wt% PVP-coated NCM follows the same steps above but with ethanol as a solvent.

X-ray diffraction (XRD)

XRD is used to characterize PVBTA-TFSI and check the chemical stability between PVBTA- TFSI or LiTFSI and LPSCI by using Panalytical Empyrean XRD with Cu Ka radiation. Diffraction patterns are collected in a 20 angular range from 10° to 85° with a step size of 0.026°, 0.04 rad. soller slits, and 1/2° anti-scatter slit. To check the chemical stability between PVBTA-TFSI or LiTFSI with LPSCI, PVBTA-TFSI or LiTFSI are mixed with LPSCI in a weight ratio of 1 :1 (around 500 mg in total) by grinding in the agate mortar and then pressed into pellets (8 mm in diameter). After that, the pellets are heated up and maintained at 80 °C for 24 hours, followed by a grinding process in the agate mortar to turn pellets into powder. Finally, the powder after heating is characterized by XRD.

Thermogravimetric analysis (TGA)

TGA measurements are conducted with around 20 mg of samples by STA 409 PC (Netzsch- Geratebau GmbH) at the temperature ranging from 25 °C to 1000 °C, under Air/O2 with a heating rate of 10 °C/sec.

1 H Nuclear magnetic resonance (1 H NMR)

Bruker Avance II records the ¹ H NMR spectra of PVBTA-TFSI at 400 MHz in deuterated acetone.

Fourier-transform infrared spectroscopy (FT-IR)

FT-IR spectra of PVBTA-TFSI, PVBTA-CI, and LiTFSI are recorded with a total number of 96 scans on an ATR-FTIR Thermo Fischer Scientific iD5 ATR spectrometer (550 to 4000 cm-1). To check the chemical stability between PVBTA-TFSI or LiTFSI with LPSCI, PVBTA- TFSI or LiTFSI is mixed with LPSCI in the agate mortar and then pressed pellets. Subsequently, the pellets are heated up and maintained at 80 °C for 24 hours. FT-IR spectra are measured in a pellet form before and after the heating process.

Brunauer-Emmett-Teller analysis (BET)

Specific surface area calculations of polymer-coated and pristine NCMs are measured by the BET method. Before measurements, the samples are evacuated at 120 °C for 12 hours in standard glass tubes. BET measurements are then performed at an automated gas adsorption station (Autosorb-1-MP, Quantachrome Instruments) at 77 K maintained by liquid nitrogen in standard cryostats.

Scanning electron microscopy (SEM)

SEM (Merlin, Zeiss) at an accelerating voltage of 3 kV and accelerating current of 200 pA is adopted to characterize the morphology of NCMs. Back-scattered electron images and secondary-electron SEM images are taken by SEM. For sample preparation, polymer coatings and pristine NCMs are measured in a powder form, sticking tightly on the conductive carbon tape. Focused-ion beam scanning electron microscopy (FIB-SEM)

The cross-section of coated and pristine NCM powders and cathode composites pellets are analyzed using a XEIA Xe-plasma FIB (TESCAN). For sample preparation, FIB craters are milled in a low-angle condition with a 1 nA Xe-ion beam without any polished step under - 135 °C maintained by liquid nitrogen. After that, back-scattered electron images and secondary-electron SEM images are taken at an acceleration voltage of 3 kV and acceleration current of 200 pA.

Energy-dispersive X-ray spectroscopy (EDS)

After SEM or FIB-SEM measurement, energy-dispersive X-ray spectroscopy (X-Max- Extreme detector, Oxford Instruments) is adopted to characterize the coating layer on NCM. However, to get more precise signals from the EDS, the accelerating voltage and current are raised to 5 kV and 2 nA, respectively. Besides, the working distance is controlled at 5.5- 5.6 mm. Analyzed elements are carbon and sulfur.

Transmission electron microscopy (TEM)

The TEM images are bright field images in order to provide maximum contrast between inorganic core of the electrode active material and organic coating. The way how to make such images is well known within the state of the art, so that it need not be described herein. The TEM equipment used is a TVIPS TEMCam XF416FS camera, mounted on a JEOL JEM-3010 microscope, and run at 300 kV acceleration voltage.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS)

The technology of ToF-SIMS is well known within the state of the art, so that it need not be described herein. The equipment used is a M6 Hybrid SIMS (IONTOF GmbH), in combination with a 30 kV Bi-cluster primary ion gun for analysis and a 5 kV Ar-gas cluster source (GCIB) for depth profiling (sputtering). The samples are prepared in a glovebox and transferred into the measurement-chamber by use of the LEICA EM VCT500 shuttle (Leica Microsystems). Evaluation tool for the ToF-SIMS data is the software package SurfaceLab 7.2 (IONTOF GmbH).

Electrode composite and cell assembly

All of the cell tests are performed with a pellet-type cell casing. Firstly, 60 mg of LPSCI are pressed into a pellet within the peek cylinder insulator. Next, the cathode composite is made by mixing 70 wt% of pristine or coated NCM, 30 wt% of LPSCI, and an additional 1 wt% of VGCF in an agate mortar for around 20 minutes. Then the 12 mg cathode composite is pressed on one side of the electrolyte. Finally, indium (100 pm thickness and 9 mm in diameter) and lithium foils (125 pm thickness and 6 mm in diameter) are pressed on the other side as the anode. After assembling the whole stack of the cell is pressed under 30 kN for 3 minutes, resulting in around 400 pm solid-state electrolyte with around 30 pm cathode composite. Before electrochemical analysis, the whole cell is analyzed under an external aluminum framework (~50 MPa).

Further exemplary details:

For cyclic voltammetry, asymmetric cells of the following setup were prepared: InLi | LiePSsCII LiePSsCI, carbon fibers. To prepare 100 mg of cathode composite, 9.09 mg of VGCF (9.1 = wt %) is added to 90.90 mg LiePSsCI and mortared for 15 minutes. To prepare the cell for analysis, 80 mg of LiePSsClare pressed into a pellet as a separator within the PEEK cylinder insulator. 30 mg of LiePSsCI-carbon fiber composite is pressed on one side of the electrolyte. Finally, indium (100 pm thickness and 9 mm in diameter) and lithium foils (125 pm thickness and 6 mm in diameter) are pressed on the other side as the anode. After cell assembly, the whole cell stack is pressed under 30 kN for 3 minutes. Before electrochemical analysis, the whole cell is placed in an external aluminum framework (~50 MPa).

Electrochemical analysis

Batteries are charged and discharged within the voltage window between 2.0 and 3.7 V (vs. Li ⁺ /Li-ln) at 25 °C for cycling stability, chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The cycling stability is performed at MACCOR electrochemical workstation. Besides, EIS and CA are conducted by VMP-300 (BioLogic) electrochemical workstation. The whole procedure is also predicted in Fig. 2. The cycling test adopts two different currents, 0.1C and 0.25C; however, the EIS measurements are conducted at 0.1 C. Initially, batteries are charged to 3.15 V (vs. Li ⁺ /Li-ln), and then CA is maintained at 3.15 V (vs. Li ⁺ /Li-ln) until the current drops to less than 1%. After that, EIS is measured right after CA at 3.15 V (vs. Li ⁺ /Li-ln) from 1 MHz to 100 pHz. The sinusoidal amplitudes of EIS are applied as 10 mV for 1 MHz to 10 mHz; 5 mV for 10 mHz to 1 mHz; and 3 mV for 1 mHz to 100 pHz. The 0.25C cycle stability runs 200 cycles, and the impedance is measured at the first, second, 53rd, 104th, 155th, and 206th cycles under 0.1 C at 3.15 V, as is exemplarily shown in Fig. 3. The 0.1C cycle stability runs for 100 cycles, and the impedance is measured at first to fifth, 10th, 30th, 50th, and 100th cycles under 0.1C at 3.15 V, as shown in Fig. 4 (a). The procedure of how to perform the fitting of the impedance is well known by any person of ordinary skill in the art. The low-frequency part is fitted by the finite-space Warburg behavior (Zf _S ) with the function described in the further section. The galvanostatic intermittent titration technique (GITT) coupled with EIS is measured at VMP-300 (BioLogic) electrochemical workstation to determine the diffusion coefficient after 0.1C over 100 cycles. Coated NCM cells and pristine NCM cells are charged or discharged every 20 minutes, followed by 2 hours of relaxation after every charging or discharging pulse to record open-circuit voltage (Voc EIS is applied after every relaxation in the range of frequency from 1 MHz to 1 mHz with a sinusoidal amplitude of 10 mV.

Further exemplary details:

Batteries are charged and discharged within the voltage window between 2.0 V and 3.7 V (vs. Li ⁺ /Li-ln) at 25 °C for 0.1C cycling performance. Before the measurement, the cells are maintained at open circuit voltage for 3 hours. The cyclic voltammetry measurements use a two-electrode configuration with the Li-ln alloy anode acting as the reference electrode. First, the voltage sweep is initiated from OCV to 4 V versus the reference electrode at a scan rate of 1 mV s ^-1 for the oxidative sweep. Then, the sweep is reversed until 0 V before returning to the starting potential.

Characterization of PVBTA-TFSI coated NCM

Polyelectrolytes, e.g. PVBTA-TFSI, have affinity to inorganic surfaces and exert electrostatic effect with metal-oxide surface, implying the potential of polyelectrolytes as a promising coating material for inorganic substrates. As a result, PVBTA-TFSI is used as an exemplary coating material within the subject matter of the invention, for it is surprisingly chemically stable and therefore compatible with thiophosphate-based solid electrolytes, like - for example - LPSCL. However, as the normal wet-coating method often gives a non- homogeneous coating on the surface of particles, the spray-coating method is introduced herein due to its possibility of obtaining a homogeneous coating on NCM. Fig. 1 exemplarily shows a schematic illustration from polymer synthesis and the spray coating process using NCM/PVBTA-TFSI/acetone as the precursor. The precursor is heated to a temperature (e.g. 150 °C) much higher than the boiling point of the solvent, e.g. acetone, and sprayed out the nozzle. The coating process is then carried out during the drying process in the drying chamber. Finally, the dried powder is collected in the collecting chamber by the vacuum pump. The spray-drying method can usually obtain two different kinds of coated products, depending on the particle size. First, if particles are smaller than the drops sprayed out, the particles can be enclosed in the droplets, forming a uniform coating. Second, if particles are bigger than the droplets, there will be many exposed parts on the surface of the particles after drying. By using Mini Spray Dryer B-290 from BLICHI, a drop of the solution is around 25 pm, much bigger than the NCM particles (around 3 pm to 5 pm). Thus, according to the invention a uniform coating is achieved and mostly preferred. Nevertheless a non uniform coating exerting local deviations from the average value of thickness is also comprised by the scope of the invention.

The structure of PVBTA-TFSI is confirmed by 1 H NMR spectroscopy and thoroughly matches the ¹ H NMR spectra of the literature, as is shown in Fig. 5. After the polymerization and ion-exchange process, the chemical shift of the double bonds ( ¹ H NMR: 5= 6.71 ppm, 5.82 ppm, and 5.26 ppm in the ¹ H NMR spectra) completely disappears in the PVBTA-TFSI spectra; instead, the new proton absorption peak appears at 5 of 1.5 ppm, indicating that the polymerization has completed. Moreover, the aromatic protons (H3,4), benzylic protons (H5), and the protons of trimethylammonium cations (H6) still exist in the PVBTA-TFSI spectra, indicating that the polymer backbone structure is stable after anion exchange. Fig. 6 shows the IR spectra of PVBTA-CI, and PVBTA-TFSI after anion exchange. Compared to PVBTA-CI, PVBTA-TFSI shows new characteristic bands at 1346 cm ^-1 , 1176 cm ^-1 , 1132 cm ^-1 , and 1050 cm ^-1 , corresponding to asymmetric SO2 stretching, CF3 asymmetric stretching, symmetric SO2 stretching, and asymmetric S-N-S stretching, respectively. These new peaks in the PVBTA-TFSI spectrum are attributed to TFSI anionic groups, demonstrating successful anion exchange. XRD is utilized to measure if there is any crystalline phase from LiTFSI, as shown in Fig. 7. It can be seen that PVBTA-TFSI only shows a broad, amorphous peak at around 20°, indicating that the PVBTA-TFSI is an amorphous polymer without any LiTFSI separated out. The chemical stability between NCM and LPSCI is investigated by heating LPSCI and PVBTATFSI at 80 °C for 24 h, and then the samples are examined by XRD and FT-IR measurements before and after heating, as shown in Fig. 8A (FTIR) and Fig. 8B (XRD). The result confirms that PVBTA-TFSI is chemically stable with LPSCI. Furthermore, the thermal stability of PVBTA-TFSI is measured by TGA, as shown in Fig. 9. The decomposition temperature of PVBTA-TFSI is around 365 °C, and most of the decomposition happens around 400 °C to 600 °C, which is much higher than the temperature (150 °C) applied for coating.

The specific surface area of coated and pristine NCM are evaluated using the Brunauer- Emmett-Teller (BET) model. Pristine NCM, 5P-NCM, and 1 P-NCM have a surface area of, for example, 0.593 m ² /g, 0.291 m ² /g, and 0.412 m ² /g, respectively. Although SEM can hardly tell the difference in particle aggregation between pristine and polymer-coated NCM, BET measurement demonstrates that the surface area decreases when the coating weight percent becomes higher. This may be because the polymer makes the agglomeration severe. TEM is recorded in bright field TEM image to enhance contrast between the coating layer and the NCM particles. Regarding one example of the invention (presented in Fig. 10), it is confirmed that all observed particles are uniformly covered by a homogenous coating layer with only small deviations in thickness, as shown in Fig. 10 ((a) - pristine NCM, (b) - 1 P-NCM, (c) - 5P-NCM). The 1 P-NCM has a uniform coating of around 4 nm, while the 5P- NCM has a much thicker coating layer of around 10 nm.

In order to characterize the coating composition and distribution even more precisely, ToF- SIMS is carried out as a surface sensitive technique harnessing its high lateral resolution (below 50 nm). To chemically identify the coating on the NCM particle structure, pressed PVBTA-TFSI powder is measured as a reference material. Because of the collision cascade initiated by the highly energetic analysis beam charged fragments such as CH2OF-, SNO- and CF3- are formed during the ToF-SIMS measurement with a decent signal intensity. These fragments are also formed at the coated samples, confirming that the PVBTA-TFSI coating is present on the NCM particles.

Overall, taking the SEM, TEM and ToF-SIMS results into account, the PVBTA-TFSI coating microstructure can be defined as follows: The coating is uniformly distributed and completely covering the NCM particles. While the 1 wt% coating is about 4 nm thick, the 5 wt% coating is thicker (about 10 nm). In certain embodiments the coating has local polymer aggregation with a thickness of, for example, 40 to 100 nm on the surface (observed at the example of the 5 wt%-coating with 10 nm thickness; any person of ordinary knowledge in the art well knows that the local polymer aggregations will vary with the value of average thickness and be of corresponding size). Based on these analytical results, both, 1 wt% and 5 wt% coatings, are effectively preventing the direct physical contact between CAM and SSE and are consequently stabilizing the interface.

Electrochemical Characterization

Rate capability test

The performance of coated cathode is defined and evaluated by rate capability and cycling stability in comparison to pristine NCM. Different C-rate performances are investigated to elucidate the rate capability, as shown in Fig. 11 (a). Initially, PVBTA-TFSI coated NCM do not perform better than pristine NCM. However, at 0.5C and 1C rates, 1 wt% coated NCM shows better performance and the 5 wt% coated NCM cell has the same capacity as the pristine NCM cell. Without being bound by a certain theory, the better rate capability of 1 wt% coated NCM at 0.5C and 1C rates may be due to the protection of polymer coating after several cycles. Besides, at the 25 ^th cycle, the coated NCM shows better reversibility in the discharge process, as shown in Fig. 12 (b). Fig. 11 (b) magnifies the galvanostatic charging and discharging profile from 2 V to 3 V (vs. Li ⁺ /Li-ln) in differential form for the first charge cycle. The current decreases as the polymer content increases. This demonstrates that the polymer is protecting the electrode from the electrochemical degradation of the unstable interface between electrode and electrolyte, and this effect majorly occurs in the first cycle. However, the protective layer gives rise to a larger overpotential at around 3 V (vs. Li ⁺ /Li-ln) for the coated cells compared to the pristine cell in the first cycle, as shown in Fig. 12 (a).

In order to observe the effect of the PVBTA-TFSI coating layer on battery performance, 5P- NCM, 1 P-NCM, and pristine NCM cells are galvanostatically cycled at 0.1 C (Fig. 4 (a)) and 0.25C (Fig. 4 (d)), respectively. During the cycling, the EIS test is performed at 3.15 V (vs. Li ⁺ /Li-ln) under the current of 0.1 C at specific cycles. The potential of 3.15 V (vs. Li ⁺ /Li-ln) is chosen to ensure a sufficiently high lithium diffusion coefficient and lowest charge transfer resistance (R _c t) while avoiding significant degradation during the measurement. Moreover, the impedance of the layered oxide cathode material depends on the electrode's state of charge, so it is necessary to measure the impedance at a fixed potential. To ensure that the impedance can be measured at a sufficiently steady state, the CA is used right before the EIS measurement. The cell voltage is stabilized at 3.15 V (vs. Li ⁺ /Li-ln) until the current drops to less than 1 %. This allows Li ⁺ diffusing into the CAM particles to balance lithium concentration within the NCM particles. Besides, 2 hours of relaxation time is used to record Voc after every charging and discharging process to calculate the active mass.

For the 0.1C cycling performance (Fig. 4 (a)), it is shown that the capacity retention of 1 P- NCM is the highest at about 86%, while that of 5P-NCM is about 75.3%. However, the capacity retention of pristine NCM is only around 70.4%. These results match 0.25C cycling performance (Fig. 4 (d)) and confirm that 1 P-NCM significantly improves the long-term cycling stability of NCM in SSBs. However, not all polymers can be used as a protective layer while maintaining a high capacity around 180 mAh/g. Without being confined to a certain theory, probably, the intermolecular interactions and interactions with the cathode material need to be strong. Also, the counterion TFSI- to the polycation likely helps with lithium ion conduction through the thin coating layer, which does not intrinsically contain lithium, but may obtain a small amount of lithium ions for lithium ion conduction from either the electrolyte or the cathode material. Therefore, it is comprised by the scope of the invention that the polyelectrolyte polymer is a polycationic polymer, a polyanionic polymer, a zwitterionic polymer, a polymer being a polycation being complexed with a polyanion or a mixture of any two or more of the named polyelectrolyte polymers. The range of lithium ionic conductivity being usable in the polymer coating according to the invention is very broad, for the thickness of the polymer coating is very small, preferably below 100 nm, so that even a low ionic conductivity is not a disadvantage.

Without being bound to a certain theory, improved cycling performance of SSB, being caused by the invention, can be due to multiple reasons. Firstly, the mitigation of the contact loss and the electrochemically inactive interfacial layer between the active material and the electrolyte can better utilize the active mass (actual amount of active materials that have been utilized) of the cathode material. Interface decomposition between the active material and the electrolyte includes the chemical, electrochemical, and chemo-mechanical degradation. The Interface decomposition may form an electrochemically inactive surface or a high resistance layer. Besides, the electrochemically inactive surface layers and the chemo-mechanical contraction can cause a loss of contact between the NCM and the solid electrolyte, leading to increased R _c t in the impedance spectrum. The other reason for the improvement being achieved by the invention could be alleviating the increased length of the lithium diffusion pathway. Lithium diffusion pathway length may increase due to particle cracking caused by the volume change, contact loss and interface decomposition listed above.

Contact loss and electrochemically inactive interfacial layer

In order to quantify and analyze the contact loss and electrochemically inactive cathode/electrolyte interfacial layer, active mass in operating cells is measured by using a method which is well known to any person of ordinary skill in the art. Experimental results are depicted in Fig. 4(b), which is depicting results of exemplary measurements under 0.1 C, and Fig. 15, which is depicting results of exemplary measurements under 0.25 C. Briefly described: A cell composed of an NCM cathode and an In/InLi anode with a fixed potential has equilibrium open circuit potential (Voc). The Voc follows a well-defined function vs. state of charge representing lithium content (Li_X) of the NCM. Using the reference data of the Voc function with Li_X (Fig. 14), the actual specific capacity (Q _ac t) can be determined by the difference in state of charge after the charging and discharging process. Then the active mass (m _ac t) can be calculated by the measured discharge capacity (Qmeas) and Q _ac t in the following equation [1],

Qmeas m _act = - [1]

Qact

The m _ac t of polymer-coated NCM cells and pristine NCM cells of 0.1C cycling test are compared in Fig. 4 (b). Initially, 1 P-NCM, 5P-NCM, and pristine NCM cells have m _ac t around 7.6 mg, 7.4 mg, and 7.8 mg, respectively. Active mass for polymer coating is slightly less than pristine since some parts of polymer coating on NCM are too thick, making NCM becomes insulated. However, after 0.1C rate for 100 cycles, the retention of m _ac t of pristine, 1 P-NCM, and 5P-NCM cells are around 81.1%, 92%, and 85.5%, respectively. The better m _ac t retentions for polymer-coated NCM cells during cycling can - without being bound by a certain theory - be explained as the alleviation of electrochemically inactive interfacial layer. Nevertheless, 1 P-NCM shows better m _ac t retention than 5P-NCM as the polymer coating of 5P-NCM is hindering ion transport or charge transport to some degree because of higher thickness. The exemplarily provided data shows best results so far with a thickness of 4 nm to 5 nm, but these results are not to be understood as being confining the scope of the range of thickness. Depending on the ion conductivity of the polyelectrolyte (e.g. polycationic polymer) the thickness oft he coating can vary between 0.1 nm and 1000 nm. Similar results can also be observed in the 0.25C cycling test (Fig. 15).

EIS measurements of 0.1 C are carried out during cycling, as shown for example in Fig. 4 (c), and exemplary results are shown in Fig. 16. The interface between the cathode and the solid electrolyte is an essential factor affecting the R _c t, providing information on contact loss and interface degradation. In the first cycle at 3.15 V (vs. Li ⁺ /Li-ln), thicker polymer coating layers give rise to higher R _c t (pristine NCM: 19.24 Q, 1 P-NCM: 21.9 Q, 5P-NCM: 28.5 Q), matching the first cycle m _ac t results. Nonetheless, the R _c tof the pristine NCM cell increases more significantly after 100 cycles than that of 1 P-NCM and 5P-NCM cells (pristine NCM: 173 Q, 1 P-NCM: 115.1 Q, 5P-NCM: 106.9 Q). The significant increase in Rct of the pristine NCM cell may - without being bound to a certain theory - be caused by severe contact loss and interface degradation. EIS measurements show that the polymer coating acts as a protective layer at the NCM/LPSCI interface, matching the results of m _ac t calculation and the dQ/dV discussion. Additionally, 1 P-NCM and 5P-NCM cells have similar R _c t after 100 cycles, indicating that 1 wt% of the polymer coating layer is sufficient to act as a protective layer for NCM. The EIS measurements for 0.25C cycling cells (cf. , e.g., Fig. 3) also show the same conclusion as the 0.1C results.

Lithium diffusion pathways within the cathode composite

The lithium diffusion pathway length within the cathode composite is expected to increase during cycling due to contact loss, interface degradation, and NCM particle cracking. Warburg behavior in EIS is fitted by the particle size distribution (EIS-PSD) model - as is known to any person of ordinary skill in the art - to determine the length of the lithium diffusion pathway within the cathode composite. In general, the ideal finite-space Warburg behavior describes the diffusion throughout the sample volume, including the ion blocking boundary at the current collector and the innermost center of the NCM. If the frequency is low enough to reach the blocking boundary, the impedance will have capacitive- 1 ike behavior, which has a continuous transition from 45° to 90° in the Nyquist-plot. Consequently, to get the finite-space Warburg behavior in EIS, the lower cutoff frequency is applied to 100 pHz. Finite-space Warburg impedance element of cylindrical particles ( ) is used to describe the complex geometry of Li ⁺ diffusion in the NCM, and the thickness of the cylindrical particle (Ldin) can be taken as the lithium diffusion pathway length. Therefore, if the lithium diffusion pathway length increases, the finite-space Warburg behavior will behave as a bigger particle. The Zf _s ^yl,lld " ^cal has a function of characteristic time constant of lithium diffusion T, (Eq. [3]) and the volume fraction of particle contribution AQ; (Eq. 4).

Eq. 4 is used to fit the finite space diffusion tail of EIS coupled with the transition line model (TLM), as is well known in the state of the art. C _diff is the total differential capacity of the entire electrode (C^ff = dQ/dE ) which is assumed as 340 mAh V" ¹ g ^-1 (3.15 V (vs. Li ⁺ /Li- In), and * = 0.6 ) and calculated from the reference data (Fig. 14). D _Li of NCM is assumed as 10' ¹¹ cm ² S' ¹ at 25 °C, cited from Literature well known within the state of the art and obtained by the fitting of semi-finite part of diffusion tail in EIS. Fitting of impedance yields the £ _diff values and the volume fractions (represented in a culmination form) are shown in Fig. 17 (a). It can be seen that L _diff of pristine NCM increased considerably (around 1.1 pm to 4.3 pm) compared to 1 wt% coated NCM cell (around 1.0 pm to 2.2 pm) and 5 wt% coated NCM cell (around 1.0 m to 2.7 pm). The results of CA draw the same conclusion as EIS-PSD results mentioned above. CA is applied to have the voltage maintained at 3.15 V (vs. Li ⁺ /Li-ln) until the current was reduced to 1%, allowing the lithium-ion concentration within the NCM particles to reach the equilibrium state. The time it took was positively related to lithium diffusion path length, i.e., the longer the time, the longer the lithium diffusion path. Fig. 17 (b) shows that at the 100 ^th cycle of 0.1 C cycling, pristine NCM cell needs 599.8 minutes to reach the equilibrium state. However, 5 wt% coated and 1 wt% coated NCM cells need 459.8 and 347.1 minutes, respectively. The results from CA and EIS-PSD calculation show that the pristine NCM cell has a much longer lithium diffusion path length than the coated NCM cells after cycling. The significantly increased lithium diffusion path length of pristine NCM cell provides the evidence of the interface degradation and the particle cracking within the particles. Additionally, CA results for 0.25C cycling give the same results as for 0.1C

In order to compare the lithium transport within the NCM particles after cycling test, GITT coupled with EIS are adopted to determine the diffusion coefficient D _Li after 100 cycles of 0.1 C test. Assuming negligible changes in Co happen during polarization steps, the open circuit potential Voc is assumed to follow the equation [5] below in the case of semi-infinite diffusion.

Vo is the open circuit potential Voc of the previous cycle. Co is the equilibrium concentration, i.e., lithium concentration before the polarization step. Z _w is the Warburg coefficient determined from ZR ₆ versus data by EIS after every relaxation process. W is the enhancement factor taking the form as / dln(q) , meaning that the activity gradient concerns the concentration gradient if only one ionic and electronic species are considered. Additionally, n, F, A, are the number of transferred electrons, Faraday's constant, and the electrochemically active electrode surface, respectively. As NCM may lose contact from electrolyte during cycling, the electrochemically active electrode surface A after cycling was estimated: A= (BET area of NCM) x (active mass after cycling)/(active mass before cycling). W can be obtained through GITT experiment by formula [5] as supportive proof. Therefore, Du (Fig. 18 (b)) can be determined by formula [6] with the Z _w gotten from EIS (Fig. 18 (a)). Moreover, since the surface area of active materials might change due to grinding and mixing during the preparation of cathode composite, apparent diffusion coefficient D“ ^pp is used herein. It is clear from Fig. 18 (b) that fi ^app of pristine NCM cell is much lower than the coated NCM cells after 100 cycles. However, 5P-NCM cell have the highest , and 1 P-NCM cell’s D°f ^p is a bit lower than that of the 5P-NCM cell. The reason for lowest of pristine NCM cell may - without being bound to a certain theory - be due to the more severe cracking inside the pristine NCM particle compared to the coated NCM. However, increasing polymer content for coating can hinder the cracking, giving rise to a highest coated NCM cell.

Morphology and interfacial stability after cycling

Low-angle FIB-SEM is employed to observe the morphological change in the cathode composite after cycling. Pristine NCM cell shows more fracture within the particles than the coated NCM cells, and 5P-NCM cell has the least cracking. Without being bound to a certain theory, one of the reasons for alleviating cracking may be due to the less lithium extracted from the NCM, especially for the H2+H3 phase transition during the charging step, which has the most severe volume change. However, the capacity and the area under the dQ/dV plot above 4.1 V (vs. Li ⁺ /Li) is similar for both polymer-coated NCM cells and the pristine NCM cell, indicating that all the NCM cells have similar H2+H3 phase transition process, as shown in Fig. 12 (a). As a result, the alleviation of the cracking in coated NCM is because the polymer mitigates the volume expansion and contraction of the NCM cathode material during cycling. For optimising this effect, the thickness of the coating is to be increased. Because electrical conductivity is reduced with increasing thickness of the coating, there is an optimum thickness of the coating, ranging from 0.1 nm to 1000 nm, the optimum thickness for the coating being exemplarily tested so far ranging from about 4nm to about 5 nm.

The comparative ToF-SIMS experimts of pristine NCM and coated NCM revealed herein provide proof for the reduction of decomposition products of electrolyte and thus the positive influence being exerted by the invention upon the electrochemical cycling stability of electrode active materials, especially cathode active materials, in combination with electrolytes. It is well known to the person of ordinary skill in the art how to perform such comparative ToF-SIMS measurements, e.g. how to set the following experimental parameters:

Number of cycles of an electrochemical cell to be run before being disassembled and investigated (e.g. up to several hundred cycles);

Choice of decomposition processes to be considered (e. g. processes between current collector and solid electrolyte and/or between carbon additive and solid electrolyte and/or between CAM (i.e. coating according to the invention) and solid electrolyte);

Choosing indicator molecules for indicating oxidative decomposition, e.g. phosphates (PO _X ~~ );

Choice of suitable parameters for providing sufficient statistical reliability of the experimental results (e.g. measuring up to 10 or even more spectra per sample).

The experimental findings are:

The amount of PO _X - (especially PO-, PO2- and PO3-), is significantly reduced for the coated samples, proving that the exemplarily tested 1 wt% and 5 wt% PVBTA- TFSI coatings effectively reduce the formation of phosphates. Thus, the decomposition of the thiophosphate-based solid electrolyte is inhibited by the exemplarily tested PVBTA-TFSI coating. There are more PO2- and PO3- fragments detectable with the 1 wt% coating than with the 5 wt% coating, proving that thicker coatings are more effective in suppressing the decomposition of the solid electrolyte than thinner coatings.

Description of the drawings

Fig. 1 : Exemplary Schematic illustration of the PVBTA-TFSI synthesis and process for spray coating of NCM particles with PVBTA-TFSI.

Fig. 2: Exemplary schematic depiction of Electrochemical analysis. The charging processes are shown by way of continue-lined arrows, and the discharging processes are shown by way of dash-lined arrows. ASSBs are galvanostatically charged to 3.15 V (vs. Li ⁺ /Li-ln) and then maintained the potential at 3.15 V (vs. Li ⁺ /Li-ln) until the current drop to less than 1%. After that, EIS is measured from 1 MHz to 100 pHz. Subsequently, The ASSBs are galvanostatically charged to 3.7 V (vs. Li ⁺ /Li-ln), followed by a 2 hours relaxation process. Finally, The ASSBs are discharged to 3.7 V (vs. LF/Li-ln), followed by a 2 hours relaxation process as well. For purpose of clarity it is pointed out that the inserted graphs of the first (main) part of Fig. 2 are duplicated and depicted again enlarged, following the first part of Fig. 2. The correlation of the inserted graphs to the enlarged copies of the inserted graphs is easily recognizable by the progression of the depicted values resp. curves.

Fig. 3: Exemplary depiction of EIS of 1 P-NCM, 5P-NCM, and pristine NCM cells, each measured at 0.25 C after chronoamperometry. Fig. 4: Exemplary depiction of: (a) Long-term cycling capability at 0.1 C, (b) corresponding active mass evolution at 0.1 C, (c) Nyquist plot for 1 ^st and 100 ^th cycle (measured at 0.1 C), (d) long-term cycling capability at 0.25 C

Fig. 5: Exemplarily depicts ¹ H NMR spectrum of the PVBTATFSI (400 MHz, deuterated acetone): 5 1.36-1.99 (H1, H2), 52.05 (acetone-d6), 53.14 (H6), 54.4-4.9 (H5), 5 6.5-7.4 (H3, H4).

Fig. 6: Exemplary depiction of FT-IR spectra of PVBTA-TFSI, PBVTA-CI, and LiTFSI.

FT-IR of PVBTA-TFSI (cm’ ¹ ): 973 (C-N stretching), 1346, 1326, 1176, 1132, 1050 (TFSI-), 1612, 1480, 1422 (aromatic C=C stretching), 3043, 2922, 2852 (alkyl C-H stretching), 3400 (H2O). FT-IR of PVBTA-CI (cm’ ¹ ): 973 (C-N stretching), 1612, 1480, 1422 (aromatic C=C stretching), 3015, 2922, 2852 (alkyl C-H stretching), 3400 (H2O). FT-IR of LiTFSI (cm’ ¹ ): 1327 (SO ₂ asymmetric stretching), 1245 (CF3 symmetric stretching), 1204 (CF3 asymmetric stretching), 1147 (SO ₂ symmetric stretching), 1065 (asymmetric S ₂ N stretching).

Fig. 7: Exemplary depiction of XRD measurement. The XRD of LiTFSI and LiTFSI prove that PVBTA-TFSI is an amorphous polymer.

Fig. 8A: Exemplary depiction of IR-spectra. FT-IR of PVBTATFSI, mixed with LPSCL shows no difference before and after heating.

Fig. 8B: Exemplary depiction of XRD patterns of PVBTATFSI mixed with LPSCL after heating also match the peak before heating completely.

Fig. 9: TGA analysis of PVBTA-TFSI, demonstrating an example of the upper limit temperature for spray drying. (Air/O2 with a heating rate of 10 °C/sec).

Fig. 10: Exemplary TEM images of (a) pristine NCM, (b) 1 P-NCM, and (c) 5P-NCM. TEM (and ToF-SIMS evaluation, data not shown here) confirm a completely covering and homogenous PVBTA-TFSI coating on the NCM CAM (i.e. NCM particles, exemplarily coated).

Fig. 11 : Exemplary comparison of (a) rate capability test, (b) the differential capacity plot of 1 ^st cycle at 0.1C with enlargement from 2~3 V (vs. Li ⁺ /Li-ln). Inset figure shows capacity and coulombic efficiency of different C-rates for coated and pristine NCM cells for 25 ^th cycle, cycled at 0.1C. The inset figure within part (b) is depicted additionally outside of part (b) enlargened, for sake of readability.

Fig. 12: Exemplary comparison of differential capacity plot of (a) 0.1C 1 ^st cycle and (b) 0.1C 25 ^th cycle of polymer coated and pristine NCM cells. Fig. 13: Exemplary comparison of 0.1 C cycle stability of pristine NCM, 5P-NCM, and 5wt% PVP coated NCM (the first one, pristine NCM, and the latter one, PVP coated NCM, being the comparative examples according to the state of the art). 5P-NCM and 5wt% PVP coated NCM have similar capacity retention but different initial capacity. As is well known within the state of the art, capacity retention is extremely dependent from initial capacity. With increasing initial capacity, capacity retention is rapidly decreasing. Thus, the similar values of capacity retention despite different initial retention of 5P-NCM (according to invention) and 5 wt% PVP coated NCM (state of the art), is an unexpected and extraordinary effect of improvement of capacity retention.

Fig. 14: Exemplary relationship between x in LixNio.83Mno o6Coo.11O2 and Voc is obtained by galvanostatically charging and discharging a liquid Lithium- ion battery at 0.02C. After every 20 minutes of charging or discharging, it relaxes for two hours to obtain Voc. Besides, x in LixNio.83Mno.06Coo.11O2 can be calculated by the current and time.

Fig. 15: Exemplary comparison of corresponding active mass evolution under 0.25C cycling test.

Fig. 16: Exemplary data of EIS measurements

Fig. 17: Exemplary evolution of the lithium diffusion pathway length Lditr by (a) EIS-PSD model and (b) CA measurement.

Fig. 18: Exemplary depiction of (a) Warburg coefficient (Z _w ), determined by EIS, and (b) Diffusion coefficient “i ^P , measured by GITT method.

Fig. 19: ESB and SEM pictures, exemplarily demonstrating a marked reduction in surface polymer particles for LiPPSS/PVP-NCM90 coating.

Fig. 20: Infrared spectroscopy exemplarily shows that PVP manifests a robust electrostatic attraction with LiPPSS.

Fig. 21 : Exemplary comparison of rate capabilities of differently coated NCM.

Fig. 22: Exemplary data showing that the LiPPSS/PVP complex coating, resp. LiPPSS/PVP-diluted coating, reduces the electrochemical side reaction of LiePSsCI in the initial charging process below 3 V.

Fig. 23: Exemplarily selected data showing the better reversibility of LiPPSS/PVP- NCM90 SSB than the one of pNCM90 SSB.

Fig. 24: Exemplary comparison of the 0.1C cycling performance between LiPPSS/PVP- NCM90 and pNCM90 cells. Fig. 25: Exemplary comprehensive comparison of the active mass calculation results.

Fig. 26: Exemplary reference data for the VOC function related to index x in LixNi0.9Mn0.05Co0.05O2.

Fig. 27: Exemplarily showing cyclic voltammetry of PEDOT and LiePSsCI