Sulfur cathode

Abstract

A sulfur cathode comprising a cellulose composition containing a plurality of anion-functionalized cellulose nanofibers is described. The anion-functionalized cellulose nanofibers are highly charged and have a low aspect ratio. The sulfur cathode has a low porosity, high surface smoothness, and promotes the transport of Li ions while blocking the transport of polysulfide anions. A battery using the sulfur cathode has a high weight density and volume density.

Description

Technical Field

[0001] The present disclosure relates to a sulfur cathode and a lithium-sulfur battery incorporating the sulfur cathode. The sulfur cathode contains anionic-functionalized cellulose nanofibers that exhibit a high surface charge.

Background Art

[0002] Compared to lithium-ion (Li-ion) battery technology, lithium-sulfur (Li-S) batteries offer many potential advantages, including improved gravimetric energy density, reduced raw material costs due to the low cost of sulfur compared to transition metals used in Li-ion systems, and reduced environmental impact of cell materials.

[0003] However, the production of a viable Li-S battery is hindered by many problems, including the intrinsic insulating properties of sulfur, the so-called polysulfide "shuttle effect" where polysulfide dissolution in the electrolyte leads to loss of sulfur from the cathode, and the volume expansion of the sulfur cathode during operation.

[0004] Therefore, there remains a need to provide a sulfur cathode that addresses one or more of these problems. The present disclosure relates to these needs.

[0005] Any reference to prior art herein does not admit or imply that such prior art forms part of the common general knowledge in any jurisdiction, or that such prior art is understood by a person skilled in the art, regarded as relevant, and / or reasonably expected to be combined with other parts of the prior art.

Summary of the Invention

[0006] In one aspect, the present disclosure is a sulfur cathode comprising a) a cellulose composition comprising a plurality of anionic-functionalized cellulose nanofibers, and b) one or more sulfur-containing materials, and c) Provide a sulfur cathode comprising one or more conductive materials.

[0007] In embodiments, the sulfur cathode a) A cellulose composition of about 2 wt% to about 20 wt% comprising a plurality of anion-functionalized cellulose nanofibers, b) About 60 wt% to about 80 wt% of one or more sulfur-containing materials, c) About 10 wt% to about 30 wt% of one or more conductive materials.

[0008] In embodiments, the sulfur cathode a) A cellulose composition of about 5 wt% to about 15 wt% comprising a plurality of anion-functionalized cellulose nanofibers, b) About 65 wt% to about 75 wt% of one or more sulfur-containing materials, c) About 15 wt% to about 25 wt% of one or more conductive materials.

[0009] In embodiments, the plurality of anion-functionalized cellulose nanofibers have one or more of the following characteristics: a) A fibril diameter of about 0.1 nm to about 20 nm, b) A fibril length of about 20 nm to about 1000 nm, c) A fibril aspect ratio of about 10 to about 1000, and d) A zeta potential more negative than about -60 mV.

[0010] In embodiments, the one or more sulfur-containing materials comprise one or more of elemental sulfur, Li2S, and MoS2.

[0011] In embodiments, the one or more conductive materials comprise one or more of carbon black, graphite, graphene, activated carbon, carbon nanotubes, and carbon fibers.

[0012] In embodiments, the cathode has a porosity of less than about 50% or less than about 40%.

[0013] In an embodiment, the cathode has an arithmetic surface roughness of less than about 25 μm.

[0014] In an embodiment, the transport of polysulfide anions within the cathode is blocked relative to a sulfur cathode lacking the cellulose compositions disclosed herein.

[0015] In an embodiment, the transport of lithium ions within the cathode is facilitated relative to a sulfur cathode lacking the cellulose compositions disclosed herein.

[0016] In another aspect, the present disclosure provides a lithium-sulfur battery including a lithium anode, a separator, a cathode according to any one of the embodiments disclosed herein, and an electrolyte disposed between the anode and the cathode.

[0017] In an embodiment, the ratio of electrolyte volume to sulfur weight in the lithium-sulfur battery is less than about 6.0 μl / mg.

[0018] In an embodiment, the ratio of electrolyte volume to sulfur weight in the lithium-sulfur battery is from about 3.5 to about 5.0 μl / mg.

[0019] During charging or discharging, the sulfur cathode can facilitate the transport of lithium ions through the cathode.

[0020] During charging or discharging, the sulfur cathode can block the transport of polysulfide ions through the cathode.

[0021] In an embodiment, the plurality of anion-functionalized cellulose nanofibers have a fibril diameter of from about 0.5 nm to about 10 nm, or from about 1 nm to about 6 nm, or from about 2 nm to about 4 nm.

[0022] In an embodiment, the plurality of anion-functionalized cellulose nanofibers have a fibril length of from about 40 nm to about 700 nm, or from about 70 nm to about 500 nm, or from about 100 nm to about 400 nm.

[0023] In embodiments, the plurality of anion-functionalized cellulose nanofibers have a fibril aspect ratio of from about 15 to about 750, or from about 20 to about 500, or from about 25 to about 200.

[0024] In embodiments, the plurality of anion-functionalized cellulose nanofibers have a zeta potential of from about -60 mV to about -90 mV.

[0025] In embodiments, the plurality of anion-functionalized cellulose nanofibers are functionalized with one or more of carboxyl, phosphonate, sulfonate, sulfate, hydroxyl, nitrate, and carbonate.

[0026] In embodiments, the plurality of anion-functionalized cellulose nanofibers contain more than 1 mmol of anion groups per gram of nanofibers, or more than 1.5 mmol of anion groups per gram of nanofibers.

[0027] In embodiments, the plurality of anion-functionalized cellulose nanofibers contain from about 1 to about 4 mmol of anion groups per gram of nanofibers, or from about 1.5 to about 4 mmol of anion groups per gram of nanofibers, or from about 2 to about 4 mmol of anion groups per gram of nanofibers.

[0028] In embodiments, the plurality of anion-functionalized cellulose nanofibers are functionalized with carboxyl groups.

[0029] In another aspect, the present disclosure provides a) a cellulose composition comprising a plurality of anion-functionalized cellulose nanofibers; b) one or more sulfur-containing materials; c) one or more conductive materials; d) water, and provides a cathode slurry composition.

[0030] In embodiments, the ratio of water to the total weight of components a), b) and c) is from about 1 ml / g to about 40 ml / g, or from about 3 ml / g to about 30 ml / g.

[0031] In an embodiment, 0.01 seconds -1 At a shear rate, the viscosity of the 3 ml / g slurry is greater than about 1000 Pa·s when measured in the temperature range of 20 to 25 °C.

[0032] In another aspect, the present disclosure provides a method for preparing a cathode slurry composition according to any one of the embodiments disclosed herein, the method comprising mixing a cellulose composition comprising a plurality of anion-functionalized cellulose nanofibers, one or more sulfur-containing materials, one or more conductive materials, and water.

[0033] In an embodiment of the method, the cellulose composition, one or more sulfur-containing materials, and one or more conductive materials are dry mixed before being mixed with water.

[0034] In another aspect, the present disclosure provides a cellulose composition comprising a plurality of anion-functionalized cellulose nanofibers, the nanofibers having one or more of the following characteristics: a) A fibril diameter of about 0.1 nm to about 20 nm, b) A fibril length of about 20 nm to about 1000 nm, c) A fibril aspect ratio of about 10 to about 1000, and d) A zeta potential of less than about -60 mV.

[0035] In another aspect, the present disclosure provides a method for preparing a cellulose composition according to any one of the embodiments disclosed herein, the method comprising: a) Treating a cellulose source with one or more agents capable of anion-functionalizing cellulose nanofibers; and b) Subjecting the anion-functionalized cellulose nanofibers to shear conditions so as to reduce one or more fibril dimensions.

[0036] In an embodiment, the shear conditions include stirring an aqueous mixture of the anion-functionalized cellulose nanofibers.

[0037] In an embodiment, the shearing condition includes one or both of ultrasonic homogenization and high-pressure homogenization of an aqueous mixture of anion-functionalized cellulose nanofibers.

[0038] Advantages of the present disclosure of the sulfur cathode and the battery including the sulfur cathode are as follows: ● The sulfur cathode has high ionic conductivity and electrical conductivity, ● The sulfur cathode has relatively low porosity, ● The sulfur cathode has a relatively smooth surface, ● The sulfur cathode has an advantageous transport effect, promoting Li-ion transport while blocking polysulfide ion transport, ● The sulfur cathode slurry has rheological properties that facilitate processing, ● The battery using the sulfur cathode exhibits one or more of high weight and volumetric energy densities.

[0039] Any embodiment herein shall, unless otherwise specified, be modified as appropriate and applied to any other embodiment.

[0040] The present disclosure should not be limited in scope by the specific embodiments described herein for illustrative purposes only. Functionally equivalent products, compositions, and processes are clearly within the scope of the present disclosure as described herein.

[0041] Further aspects of the present disclosure and further embodiments of the aspects described in the previous paragraphs will become apparent from the following description, by way of example and with reference to the accompanying drawings.

Brief Description of the Drawings

[0042]

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Mode for Carrying Out the Invention

[0043] The disclosure described and defined herein is understood to cover all combinations of two or more alternative features mentioned or apparent from the text or drawings. All of these different combinations constitute various alternative aspects of the present disclosure.

[0044] Definitions For the purpose of interpreting this specification, terms used in the singular include the plural and vice versa.

[0045] As used herein, unless the context requires otherwise, the term "comprise", and variations such as "comprising", "comprises" and "comprised" are not intended to exclude further additives, components, elements or steps.

[0046] When referring to measurable values such as amounts, lengths of time, etc., "about" as used herein means a variation of ±20% or ±10% from the specified value, in some instances ±5%, in some instances ±1%, and in some instances ±0.1%, since such variations are appropriate for carrying out the disclosed methods.

[0047] Scope: Throughout the present disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is for merely convenience and brevity and should not be construed as a rigid limitation on the scope of the invention. Thus, the description of a range should be considered to specifically disclose all possible sub-ranges within that range as well as individual numerical values. For example, a description of a range such as 1 to 6 should be considered to specifically disclose sub-ranges such as 1 to 3, 1 to 4, 1 to 5, 2 to 4, 2 to 6, 3 to 6, as well as individual numbers within that range, such as 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the width of the range.

[0048] The present disclosure relates to a new sulfur cathode for a lithium-sulfur battery. The cathode contains anion-functionalized cellulose nanofibers having a high surface charge. A lithium-sulfur battery incorporating the new sulfur cathode has many advantageous properties.

[0049] Sulfur cathode A sulfur cathode according to the present disclosure includes a mixture of a cellulose composition containing a plurality of anion-functionalized cellulose nanofibers, one or more sulfur-containing materials, and one or more conductive materials.

[0050] In embodiments, the sulfur cathode of the present disclosure a) a cellulose composition containing a plurality of anion-functionalized cellulose nanofibers, from about 2 wt% to about 20 wt%, and b) one or more sulfur-containing materials, from about 60 wt% to about 80 wt%, and c) one or more conductive materials, from about 10 wt% to about 30 wt%.

[0051] In embodiments, the sulfur cathode of the present disclosure a) a cellulose composition containing a plurality of anion-functionalized cellulose nanofibers, from about 5 wt% to about 15 wt%, and b) one or more sulfur-containing materials, from about 65 wt% to about 75 wt%, and c) from about 15 wt% to about 25 wt% of one or more conductive materials.

[0052] In embodiments, the sulfur cathode of the present disclosure comprises from about 2 wt% to about 20 wt% of a cellulose composition, or from about 3 wt% to about 20 wt%, or from about 4 wt% to about 20 wt%, or from about 5 wt% to about 20 wt%, or from about 5 wt% to about 19 wt%, or from about 5 wt% to about 18 wt%, or from about 5 wt% to about 17 wt%, or from about 5 wt% to about 16 wt%, or from about 5 wt% to about 15 wt% of a cellulose composition.

[0053] The one or more sulfur-containing materials include one or more of elemental sulfur, Li2S, and MoS2. Other sulfur-containing materials typically utilized in the construction of sulfur cathodes are contemplated.

[0054] In embodiments, the sulfur cathode of the present disclosure comprises from about 60 wt% to about 80 wt% of one or more sulfur-containing materials, or from about 61 wt% to about 80 wt%, or from about 62 wt% to about 80 wt%, or from about 63 wt% to about 80 wt%, or from about 64 wt% to about 80 wt%, or from about 65 wt% to about 80 wt%, or from about 60 wt% to about 79 wt%, or from about 60 wt% to about 78 wt%, or from about 60 wt% to about 77 wt%, or from about 60 wt% to about 76 wt%, or from about 60 wt% to about 75 wt% of one or more sulfur-containing materials.

[0055] The one or more conductive materials include one or more of carbon black, graphite, graphene, activated carbon, carbon nanotubes, and carbon fibers. Other conductive materials typically utilized in the construction of sulfur cathodes are contemplated.

[0056] In an embodiment, the sulfur cathode of the present disclosure comprises one or more conductive materials in an amount of about 10 wt% to about 30 wt%, or about 11 wt% to about 30 wt%, or about 12 wt% to about 30 wt%, or about 13 wt% to about 30 wt%, or about 14 wt% to about 30 wt%, or about 15 wt% to about 30 wt%, or about 15 wt% to about 29 wt%, or about 15 wt% to about 28 wt%, or about 15 wt% to about 27 wt%, or about 15 wt% to about 26 wt%, or about 15 wt% to about 25 wt%.

[0057] Anion-functionalized cellulose nanofibers Cellulose nanofibers are an essential component of delignified wood. They are characterized by an aligned one-dimensional hierarchical structure rich in oxygen-containing polar functional groups (mainly hydroxyls) in the form of repeating anhydroglucose units that make up the cellulose molecular chain.

[0058] Cellulose nanofibers can be functionalized with various anion functional groups. Examples of anion functional groups include carboxyl, phosphonate, sulfonate, sulfate, hydroxyl, nitrate, and carbonate.

[0059] The degree of anion functionality can be expressed as mmol of anion functional groups per gram of functionalized cellulose nanofibers. In an embodiment, the plurality of anion-functionalized cellulose nanofibers comprises more than 1 mmol of anion groups per gram of nanofibers, or more than 1.25 mmol of anion groups per gram of nanofibers, or more than 1.5 mmol of anion groups, or more than 1.75 mmol of anion groups, or more than 2.0 mmol of anion groups, or more than 2.25 mmol of anion groups, or more than 2.75 mmol of anion groups, or more than 3.0 mmol of anion groups, or more than 3.25 mmol of anion groups, or more than 3.5 mmol of anion groups.

[0060] In embodiments, the plurality of anion-functionalized cellulose nanofibers contain from about 1 to about 4 mmol of anionic groups per gram of nanofibers, or from about 1.5 to about 4 mmol of anionic groups per gram of nanofibers, or from about 2.0 to about 4 mmol of anionic groups, or from about 2.5 to about 4 mmol of anionic groups.

[0061] In embodiments, the plurality of anion-functionalized cellulose nanofibers may be characterized by having fibrils with advantageous lengths, diameters, and aspect ratios, as well as high surface charge.

[0062] In embodiments, the plurality of anion-functionalized cellulose nanofibers have a fibril diameter of from about 0.1 nm to about 20 nm, or from about 0.5 nm to about 10 nm, or from about 1 nm to about 6 nm, or from about 2 nm to about 4 nm.

[0063] In embodiments, the plurality of anion-functionalized cellulose nanofibers have a fibril length of from about 20 nm to about 1000 nm, or from about 40 nm to about 700 nm, or from about 70 nm to about 500 nm, or from about 100 nm to about 400 nm.

[0064] In embodiments, the plurality of anion-functionalized cellulose nanofibers have a fibril aspect ratio of from about 10 to about 1000, or from about 15 to about 750, or from about 20 to about 500, or from about 25 to about 200.

[0065] The anion-functionalized cellulose nanofibers according to the present disclosure may have a high surface charge. In embodiments, the zeta potential of the anion-functionalized cellulose nanofibers may be more negative than about -60 mV, or more negative than about -65 mV, or more negative than about -70 mV, or more negative than about -75 mV, or more negative than about -80 mV.

[0066] In embodiments, the zeta potential of the anion-functionalized cellulose nanofibers may be from about -60 mV to about -90 mV, or from about -60 mV to about -90 mV, or from about -60 mV to about -80 mV, or from about -70 mV to about -90 mV, or from about -70 mV to about -80 mV.

[0067] Although not desiring to be bound by theory, due to the resulting negatively charged microenvironment, the anion groups of the functionalized cellulose nanofibers can promote the transport of Li ions when used as components of the sulfur cathode, but are assumed to enable the electrostatic repulsion of polysulfides having anion characteristics.

[0068] Preparation of Anion-Functionalized Cellulose Nanofibers Natural cellulose nanofibers can be treated with appropriate agents to introduce anion functional groups into the cellulose molecular chains. Several methods for introducing such functional groups are known in the art.

[0069] For example, a useful method for introducing carboxyl functional groups is described in Mendoza, D. J., Browne, C., Raghuwanshi, V. S., Simon, G. P. & Garnier, G. One-shot TEMPO-periodate oxidation of native cellulose. Carbohydrate polymers 226, 115292 (2019).

[0070] Following the anion functionalization of the cellulose nanofibers, they can be subjected to high shear conditions to reduce one or more fibril dimensions. Suitable high shear conditions include, for example, one or more of high-pressure homogenization, ultrasonic homogenization, grinding, and cryogenic milling.

[0071] Sulfur Cathode Characteristics The sulfur cathodes of the present disclosure are characterized by their relatively low porosity. This is advantageous for minimizing the pore volume of the cathode and thus the electrolyte volume.

[0072] The porosity of the sulfur cathode can be less than about 50%, or less than about 45%, or less than about 40%, or less than about 35%.

[0073] Another advantageous feature of the sulfur cathodes of the present disclosure is their relatively low surface roughness. In embodiments, the cathode has an arithmetic surface roughness of less than about 25 μm, or less than about 20 μm, or less than about 15 μm, or less than about 10 μm. In embodiments, the arithmetic surface roughness can be from about 5 μm to about 25 μm, or from about 5 μm to about 20 μm, or from about 5 μm to about 15 μm.

[0074] Without being bound by theory, the observed low surface roughness may be due to the self-relaxation and orientation ability of the semi-crystalline anion-functionalized cellulose nanofibers. The sulfur cathode may contain fewer aggregated particles due to the repulsive forces generated by the high apparent surface charge of the cellulose nanofibers. Considering the cathode within a lithium metal battery system, all the tips of the rough surface have a high electric field, which attracts more Li ion flux and promotes the formation of dendrites. From this perspective, a useful cathode should desirably feature a relatively smooth surface, especially for large-scale applications, to minimize the duplication and amplification of surface defects.

[0075] Preparation of Sulfur Cathodes The sulfur cathodes of the present disclosure can be prepared by first combining a cellulose composition comprising a plurality of anion-functionalized cellulose nanofibers with one or more sulfur-containing materials, one or more conductive materials, and water to form a cathode slurry.

[0076] In some embodiments, the solid components are dry mixed before being mixed with water.

[0077] An advantageous property of the cathode slurries of the present disclosure is their viscosity. In embodiments, the viscosity of a 3 ml water / g total solids slurry at a shear rate of 0.01 second -1 is greater than about 1000 Pa·s when measured in the temperature range of 20 - 25 °C.

[0078] In embodiments, 0.01 second -1The viscosity of the 3 ml / g slurry at a shear rate of, when measured in the temperature range of 20 to 25 °C, is from about 1000 Pa·s to about 3000 Pa·s.

[0079] Useful cathode slurries can be prepared with a water to total solids ratio of from about 3 ml / g to about 30 ml / g.

[0080] A sulfur cathode can be prepared by coating a cathode slurry on an aluminum foil and drying it. Optionally, the coated foil can be subjected to calendering.

[0081] Lithium-sulfur battery The present disclosure provides a lithium-sulfur battery including a lithium anode, a separator, a cathode according to any one of the embodiments disclosed herein, and an electrolyte disposed between the anode and the cathode.

[0082] In an embodiment, the ratio of the electrolyte volume to the sulfur weight in the lithium-sulfur battery is less than about 6.0 μl / mg.

[0083] In an embodiment, the ratio of the electrolyte volume to the sulfur weight in the lithium-sulfur battery is less than about 6.0 μl / mg, or less than about 5.5 μl / mg, or less than about 5.0 μl / mg, or less than about 4.5 μl / mg, or less than about 4.0 μl / mg, or less than about 3.5 μl / mg.

[0084] In an embodiment, the ratio of the electrolyte volume to the sulfur weight in the lithium-sulfur battery is from about 3.5 to about 5.0 μl / mg.

[0085] Typical separators known in the art of lithium-sulfur batteries can be used.

[0086] A coin cell prepared with a cathode containing 70 wt% sulfur, 20 wt% carbon, and 10 wt% carboxylated cellulose nanofibers (about 2 mmol carboxyl groups per gram of nanofibers), 16.6 mg cm-2 with a sulfur loading and delivering areal capacities of 25 mAh cm -2 equal to that of -2 , which achieves a Coulombic efficiency of over 98% while delivering a specific capacity of 1500 mAh g -1 and corresponding to 89% sulfur utilization.

[0087] Ah-level Li-S pouch cells fabricated with a similar cathode composition deliver an initial capacity exceeding 1200 mAh g -1 and an areal capacity of approximately 15 mAh cm -2 resulting in a high gravimetric energy of up to 330 Wh kg -1 and a volumetric energy density of 367 Wh L -1 .

Examples

[0088] Materials Elemental sulfur was purchased from Sigma-Aldrich. Conductive carbon powder as CABOT black pearl 2000 was purchased from Shandong Gelon LIB Co., LTD, China. Cellulose nanofibers (CNF) were supplied by The University of Maine, USA, or BioPRIA, Monash University, Australia. The carbon-coated glass fiber interlayer was composed of carbon (ASAC30, Adven Industries Inc., Canada), Gum Arabic (Hawkins Watts), and glass fiber (BG03013 separator, Hollingsworth & Vose, USA). Lithium bis(trifluoromethane)sulfonamide salt and lithium nitrate were purchased from Sigma-Aldrich and used directly without further purification. 1,2-Dimethoxyethane (DME) and dioxolane (DOL) solvents were purchased from Sigma-Aldrich. Li2S was purchased from Alfa Aesar for lithium polysulfide synthesis. Battery-grade aluminum foil was purchased from Japan Capacitor Industrial Co. Battery-grade copper foil was purchased from Shandong Gelon LIB Co., LTD, China. Celgard 2730 separator was purchased from Celgard Inc., USA. CNT was purchased from Nano Fibers, UK. Lithium chips (16 × 0.2 mm) were purchased from Shandong Gelon LIB Co., LTD, China.

[0089] Preparation of Carboxylated Cellulose Nanofibers Carboxylated cellulose nanofibers were synthesized via one-shot 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-periodate oxidation of bleached eucalyptus kraft pulp (BEK) of native cellulose nanofibers (CNF) as described in Mendoza, D. J., Browne, C., Raghuwanshi, V. S., Simon, G. P. & Garnier, G. One-shot TEMPO-periodate oxidation of native cellulose. Carbohydrate polymers 226, 115292 (2019). High-pressure or ultrasonic homogenization of the resulting oxidized BEK fibers resulted in the formation of highly charged cellulose nanofibers. In one example, a suspension of oxidized fibers (0.01 wt%) was sonicated for 2 minutes using an ultrasonic homogenizer at 19.5 kHz and 70% amplitude (ON / OFF, 5 seconds).

[0090] An aqueous native cellulose nanofiber dispersion was imaged by polarized microscopy using an LC-PolScope microscope. As shown in Figure 1a, highly birefringent regions with an alignment domain length of approximately 100 - 200 μm and a width of approximately 20 - 50 μm were observed. In contrast, carboxylated CNF had significantly smaller dimensions (Figure 1b, an alignment domain length of approximately 30 - 50 μm and a width of approximately 10 - 20 μm) compared to native CNF.

[0091] Effect of surface charge and aspect ratio of cellulose nanofibers Studies were conducted on various carboxylated CNFs with different carboxylate group contents. Generally, the fibers were degraded, the surface charge increased, and the aspect ratio decreased as more carboxylate groups were introduced.

[0092] Samples labeled as CNF2, CNF1.5, CNF1.4, and CNF1.2 were prepared to contain nominally 2, 1.5, 1.4, and 1.2 mmol of carboxyl groups per gram of solids.

[0093] Figure 2a shows the relationship between the carboxylate content of the CNF samples and their corresponding surface charge when measured by zeta potential. Carboxylate content levels of approximately 1.3 mmol / g to 1.9 mmol / g were examined. High levels of carboxylate groups resulted in a high and significant surface charge of -80 mV.

[0094] In addition, the lithium polysulfide adsorption test provided evidence of polysulfide confinement in highly charged carboxylated solids. Polysulfide adsorption was less compared to carboxymethyl cellulose (CMC), a cellulose binder commonly used in electrode manufacturing and having a relatively low surface charge (-20 to 30 mV, see Figure 2a).

[0095] Carboxylated CNF solids with different surface charges were dispersed in water and subjected to rheology tests. The steady shear rheology depicted in Figure 2b shows that all carboxylated CNF samples have similar degrees of shear-thinning characteristics. However, a steady increase in the zero-shear viscosity of the carboxylated CNF samples with surface charge was observed. This may be due to a higher level of fibrillation promoted by the repulsion between negatively charged carboxylated CNFs, resulting in a high nanofiber content and a strong network structure.

[0096] The electrolyte wetting of the carboxylated CNF film is represented by the contact angle between the film and the electrolyte droplet, as shown in Figure 2c. The contact angle of the sample decreases within 10 seconds after the electrolyte droplet lands on the surface of the carboxylated CNF film. From the plot in Figure 2c, it can be clearly observed that an increase in surface charge decreases the contact angle. An increase in the carboxylate group concentration may weaken the hydrogen bonds between the hydroxyl groups in the carboxylated CNF film and help the ester solvent present in the electrolyte to be absorbed more rapidly.

[0097] To perform a relative comparison of the ionic conductivity between carboxylated CNF samples, a 0.5 wt% carboxylated CNF solution-impregnated film was used in a symmetric cell. As depicted in Fig. 2d, an increase in surface charge can be seen to increase the ionic conductivity, indicating that carboxylate groups are more favorable for ion migration than hydroxyl groups.

[0098] Preparation of Sulfur Cathode Using a magnetic stir bar (600 rpm, room temperature, and dry environment), the cathode slurry was prepared by dry mixing all the components in the following order. Sulfur (0.7 g) and conductive carbon powder (0.2 g) were mixed for 24 h, followed by the addition of different carboxylated CNF powders (0.1 g) to the mixture, and the dry mixing of all three components was continued for another 24 h. Then, 3 mL of deionized (DI) water was added to 1 g of the well-mixed components. All the components were mixed in water for 12 h using a magnetic stir bar (600 rpm, room temperature, and air environment) to produce a homogeneous slurry. All the sulfur cathode slurries were coated on battery-grade Al foil using a laboratory-scale doctor blade and dried at room temperature for 6 h, followed by drying at 80 °C for 12 h under vacuum to remove all traces of the solvent. Calendaring was not performed on the cathode prior to both coin and pouch cell assembly.

[0099] The cathode slurry was prepared by mixing 10 wt% carboxylated CNF, 70 wt% sulfur particles, and 20 wt% carbon particles. The structure of the cathode slurry was also examined by polarized light microscopy. As shown in Fig. 1c, when natural CNF was used, no traces of any birefringent structure were present.

[0100] In contrast, carboxylated CNFs are characterized by a high surface charge and a low aspect ratio. At the same solid content, slurries with carboxylated CNFs contain a higher number density of fine fibers that can form domains. As depicted in Figure 1d, the carboxylated CNF slurry forms liquid crystal domains, which are evident from the high retardation patches that exceed the size of individual fibers. These long-range ordered structures indicate the possibility of forming layer-by-layer structures in 3D films.

[0101] In further tests, it was shown that this self-organizing ability of the slurry is concentration-related and is likely driven by the enhanced osmotic pressure resulting from an increase in counterion charge. It is noted that at low concentration slurries (3.3 mg / ml), carboxylated CNFs exhibit a light retardation dispersed in an isotropic medium. The isotropic medium is a carbon slurry that has no optical retardation (Figure 1e). At a higher concentration of 33.3 mg / ml (Figure 1f), as is clear from the image, the proportion of crystalline carboxylated CNFs in the slurry increased. Finally, the 333.3 mg / ml slurry (Figure 1g) shows high retardation patches. As the fraction of nanofibers in the dispersion increases, the fibers become driven by a decrease in orientational entropy to an oriented order. This loss in orientational entropy is compensated by increasing the translational entropy, and therefore, as they become oriented, more space is freed up for new fibers. These results suggest that even in the presence of sulfur and carbon particles in the free volume within the carboxylated CNF domains, the cathode can still form an ordered structure driven by the osmotic pressure during drying.

[0102] Morphology and Electrical Conductivity of Cellulose Nanofibers Containing Cathodes The morphology of the cathode was examined using a profilometer. In the cathode having CMC as a binder (Figure 3a), the surface was uneven and had a height difference of approximately 276 μm according to the roughness profile. Scanning electron microscope (SEM) images supported this observation. Pits could be clearly identified on the surface of the cathode prepared with CMC (Figure 3b). As shown in Figure 3c, the microstructure of the cathode was examined under high magnification SEM, and the image depicts severe aggregation of the particles. In contrast, the cathode prepared with carboxylated CNF (CNF2) exhibits a relatively low surface roughness with a height difference of approximately 165 μm (Figure 3d). The SEM image of the carboxylated CNF cathode provides further evidence of a smooth surface (Figure 3e). Furthermore, the uniformly distributed particles of the carboxylated CNF cathode are clearly observed in Figure 3f.

[0103] The texture of the carboxylated CNF is represented in detail under high magnification top-view SEM (Figures 3g - 3i). Figure 3g illustrates the connection of the carboxylated CNF in the sulfur cathode and a mechanically robust microstructure. Another region of the carboxylated CNF is shown in Figure 3h and highlighted in Figure 3i. This region of the carboxylated CNF exhibits a very porous structure. As illustrated in Figure 3j, the carboxylated CNF functions as the skeleton of the cathode and supports a three-dimensional architecture throughout the cross-section of the cathode. Cross-section SEM images are shown in Figures 3k - 3m, and the structure separated by each layer is consistent throughout the carboxylated CNF cathode. This property is likely to be favorable for ion transport, resulting from excellent electrolyte retention properties and shortening of the ion transport path. Also, these mesoporous pores accommodate the volume expansion of sulfur particles during charging and discharging, maintaining the integrity of the cathode.

[0104] The porosity of the lithium-sulfur cathode can be estimated based on the SEM cross-section and the thickness measured from the formula,

Number

[0105] The calculated number is 33.6% porosity with respect to the cathode, which is significantly lower than the previously reported sulfur cathode. The highly negatively charged carboxylated CNF has a significant impact on the cathode architecture. This helps to retain the porous structure but also serves to compress the cathode with repulsive forces. Positron annihilation lifetime spectroscopy (PALS) tests showed the relationship between the CNF surface charge and the pore size. The pore size decreased from 0.428 nm (CNF1.2) to 0.411 nm (CNF2) as the surface charge increased. The synergistic effect of uniformly distributed pores and a relatively high compact cathode is favorable for the evaluation of the volumetric energy density.

[0106] Examination of the electron conductivity and ion conductivity of the CNF thick cathode Electron beam absorption current (EBAC) measurements were carried out at 14 mg cm -2It was carried out to evaluate the electrical properties of CMC and carboxylated CNF thick cathodes equal to the sulfur mass loading. As illustrated in Fig. 4a, the movable current source was provided by a scanning electron beam in the SEM. The external circuit measured the local absorption current flowing between the electron probe incident position and the cathode substrate. The absorption current profiles perpendicular to the cathode thickness direction from each cathode (the boxed regions in Figs. 4e and 4f) were captured by an EBAC amplifier and plotted in Fig. 4b. Clearly visible in this plot is that the average EBAC current from the carboxylated CNF cathode is greater than the average EBAC current from the CMC cathode. EBAC measurements are sensitive to sample surface roughness and porosity due to variations in secondary and backscattered electron emission, but can be used as an indicator technique for measuring electrical conductivity provided that the two samples have the same composition and similar porosity. In addition, the current was averaged perpendicular to the profiling direction within each box to minimize the effect of surface roughness in different samples. The in-plane conductivities of the CMC cathode and the carboxylated CNF cathode were also tested by measuring the resistance between the top and between the cathodes. The resistances of the CMC cathode and the carboxylated CNF cathode are 61.1 ohms and 34.5 ohms, respectively, suggesting better electrical / electronic conductivity of the carboxylated CNF cathode compared to the CMC cathode.

[0107] The EBAC current profile was compared with that of the CMC cathode (standard deviation 6.48×10 -11 ) and that of the carboxylated CNF cathode (standard deviation 5.48×10 -11) It is also noted that the variation occurs within a smaller range in , which suggests a more uniformly distributed electron conductivity of the carboxylated CNF cathode even at a larger thickness. Further details can be obtained from the cross-sectional SEM on the CMC cathode (Figs. 4c and 4d), as well as the in-situ EBAC mapping (Figs. 4e and 4f). It is worth noting that there is a distinct phase separation between the top and bottom of the CMC cathode in Fig. 4c. As a result of this phase separation, a much lower EBAC current is shown at the bottom in Fig. 4e. Based on the EDX line scan profile and the corresponding elemental mapping on the cross-section of the CMC cathode, the concentration of sulfur element is relatively high and the concentration of carbon element is low in the bottom region. This is due to larger particle size and higher density crystalline sulfur that settles at the bottom of the cathode during the coating and drying processes, resulting from the low viscoelastic slurry and unstable cathode structure. In comparison, there is no distinct phase separation in the carboxylated CNF cathode (Figs. 4d and 4f), which suggests good rheological properties of the carboxylated CNF cathode slurry and provides evidence of a more homogeneous distribution of sulfur particles and the conductive agent (carbon).

[0108] The ionic conductivity was examined using cyclic voltammetry (CV). According to the Randles-Sevick equation, a series of cyclic voltammograms with different scan rates were used for the calculation. The values of the lithium ion diffusion coefficient were 1.05×10 -6 cm 2 per second -1 ~2.92×10 -7 cm 2 per second -1 for the CMC cathode, and 3.25×10 -6 cm 2 per second -1 ~1.21×10 -6 cm 2 per second -1 for the carboxylated CNF cathode. The increase in the lithium ion diffusion coefficient for the carboxylated CNF cathode confirms the enhanced lithiation / delithiation kinetics and ionic conductivity of the sulfur cathode with the carboxylated CNF system.

[0109] Coin Cell Assembly and Electrochemical Tests A glass fiber interlayer (thickness 0.203 mm, diameter 16 mm, and maximum pore size 15.5 μm) was coated with an aqueous slurry mixture of 80 wt% carbon and 20 wt% Gum Arabic (8 mL of deionized (DI) water added to 1 g of well - mixed components) and acts as a conductive layer on the sulfur cathode. To cooperate with sulfur cathodes having different sulfur loadings, the mass of carbon content on the aforementioned carbon - coated glass fiber interlayer was 1 mg cm -2 for a sulfur cathode with a sulfur loading of 3 mg cm -2 , 1.5 mg cm -2 for a sulfur cathode with a sulfur loading of 6 mg cm -2 and 2 mg cm -2 for a sulfur cathode with a sulfur loading of 11 mg cm -2 . Thus, the total sulfur content including the sulfur cathode and the conductive interlayer was in the range of 56.7% - 62.1%. A Celgard separator (Celgard 2730, thickness 20 μm, diameter 16 mm, pore size 1 μm, and porosity 43%) was used as the separator. The electrolyte (<0.003% water content) was prepared by dissolving 1 M lithium bis(trifluoromethane)sulfonamide (LiTFSI) and 0.5 M lithium nitrate (LiNO3) in DOL and DME (1:1, v / v) in an argon - containing glove box (<0.1 ppm H2O and <0.1 ppm O2). The electrolyte - to - sulfur ratio was in the range of 8.6 - 22 μL mg -1 depending on the sulfur loading. For example, for a 3 mg cm -2 cathode, 15 μL of electrolyte was used to wet the cathode. 50 μL of electrolyte was used to wet the carbon - coated glass fiber and the Celgard separator. For a 6 mg cm -2 cathode, 20 μL of electrolyte was used to wet the cathode. 60 μL of electrolyte was used to wet the carbon - coated glass fiber and the Celgard separator. For an 11 mg cm-2 Regarding the cathode, 25 μL of electrolyte was used to wet the cathode. 70 μL of electrolyte was used to wet the carbon-coated glass fiber and Celgard separator. Typically, an increased amount of electrolyte was used for the increased sulfur loading of the cathode. 20 μL mg -1 An E / S ratio greater than 20 μL mg -1 An E / S ratio lower than 5 μL mg

[0110] For coin cell assembly, all steps were carried out inside an argon glove box, and electrochemical tests were performed by EC-lab (Bio-logic) in air and at room temperature. EIS measurements were carried out over a frequency range of 1 mHz to 1 MHz, with six data points per decade of frequency, a 10 mV rms alternating current (AC) voltage, and a constant potential signal with a 2.8 V vs. E ref direct current (DC) voltage.

[0111] Coin cell tests and electrochemical characteristics of CNF cathodes To verify the influence on the cycle performance of different carboxylated CNF cathodes, coin cells were constructed with cathodes consisting of sulfur, carbon, and carboxylated CNF with various surface charges. The coin cells were cycled at a 0.5C rate (0.5C) for 200 cycles. As plotted in Figure 5a, the results show that while all carboxylated CNF cathodes can establish a relatively stable cycle life at high charge-discharge rates, the capacity and capacity retention increase with the enhanced CNF surface charge. Similarly, as shown in Figure 5b, cathodes consisting of carboxylated CNF with higher surface charges were demonstrated to have improved rate capabilities from 0.1C to 1C. These results can be explained by the fact that carboxylated CNF with a higher surface ratio and a lower aspect ratio have better electrolyte wettability and more efficient Li-ion transport.

[0112] Considering the self-supporting architecture and the enhanced electron and ion conductivity between the carboxylated CNF thick cathode, the coin cell performance test was carried out using a cathode with a high sulfur loading. 7 mg cm -2 CNF2 and CNF1.2-based cathodes with a sulfur loading of -2 were cycled at 0.1 C. The comparison results (Figure 5c) demonstrated that the CNF2-based cathode had improved stability and a higher specific capacity. Furthermore, CNF2-based cathodes with ultra-high sulfur loadings of 11 mg cm -2 and 14 mg cm -2 (Figure 5d) also maintained a high reversible capacity with a Coulombic efficiency of approximately 98%. The tolerance of the cathode to sulfur loading was further investigated up to 16.6 mg cm -2 and the results are shown in Figure 5e. The areal capacity of the CNF2-based cathode is as high as 25 mAh cm -2 with good stability retention. The relationship between the specific capacity and the areal capacity versus sulfur loading is depicted in Figure 5f. It is noteworthy that the specific capacity of the CNF2-based cathode no longer decreases as the sulfur loading increases. The specific capacity varying between 1200 - 1500 mg cm -2 of the thick CNF2-based cathode is a sulfur loading-independent parameter that differentiates it from conventional lithium-sulfur batteries. The areal capacity of the battery continuously increases as the sulfur loading increases, which means that the thick cathode does not prevent the utilization of the active material.

[0113] Preparation of Pouch Cells The sulfur cathode of approximately 4 mg cm -2 was cut into 3 cm × 5 cm (cathode and Al substrate). The sulfur cathode of approximately 6.5 mg cm -2The sulfur cathode was cut into 6 cm × 5 cm (cathode and Al substrate). In the case of a double-sided cathode, sulfur slurry was coated on the back of the single-sided cathode to produce some sulfur loading on both sides. The Li foil (0.1 mm or 0.05 mm thick) was cut into the same size as the sulfur cathode (3 cm × 5 cm). The Al tab was welded to the prepared cathode, and the Ni tab was adhered onto the Li anode by a conductive Cu tap / 2 spot welder. Subsequently, a carbon-coated glass fiber interlayer or a carbon nanotube (CNT) paper interlayer was laminated on the Celgard separator, and then the cathode was laminated on top of the interlayer. Next, a piece of the Li anode was placed on the opposite side of the Celgard separator. 3.5 - 5 μL of electrolyte was injected into the laminate, and the package was sealed under vacuum. All cells were assembled in an Ar-containing glove box (<0.1 ppm H2O and <0.1 ppm O2).

[0114] Verification in an Ah-level Li-S pouch cell Considering the excellent coin cell performance of the CNF2-based cathode, the pouch cell was assembled with a double-sided cathode having dimensions of 3 cm × 4.5 cm and a sulfur loading of 450 mg. The 450 mg pouch cell was cycled at 0.05C. The cell had a capacity retention of 70% and a high Coulombic efficiency of >95% over 100 cycles and a specific capacity exceeding 900 mAh g -1 (Figure 6a).

[0115] Ah-level CNF2-based cathode pouch cells were also fabricated to verify a prototype with a high sulfur loading, a thin Li anode, and electrolyte-starved operation, as illustrated in the inset of Figure 6b. The parameters included a sulfur loading double-sided cathode of 12 mg cm -2 a negative capacity-to-positive capacity ratio (N / P ratio) of 1.35, and an electrolyte-to-sulfur ratio (E / S ratio) of 5 μl mg -1 As plotted in Figure 6b, the Ah-level pouch cell had a specific capacity of approximately 1200 mAh g at 0.05C (82 mA g -1 -1 ​equal to the current density), suggesting high sulfur utilization even under high sulfur loading and high current density operation. As a result, the cell delivers a practical specific energy of 330 Wh kg -1 The weight of each component for calculation is entered in the insert card of Fig. 5c. From the discharge plot depicted in Fig. 6c, the Ah-level pouch establishes a standard discharge profile and maintains a second plateau at 2.1 V from the first cycle to the 40th cycle.

[0116] The specific energy of the pouch cell was evaluated based on the following equation:

Equation

[0117] As illustrated in Fig. 6c, the specific energy decreased significantly at a low nominal voltage with the same specific capacity and total weight. For example, the calculated specific energy was 260 Wh kg at a nominal voltage of 1.8 V -1 but 300 Wh kg at a nominal voltage of 2.1 V -1 . The nominal voltage is governed by the reaction kinetics in the battery and requires good ion and electron conductivities in the cell. This Ah-level pouch maintains a nominal voltage of 2.1 V even with fewer electrolyte conditions (5 μl mg -1 ) and a thick sulfur-loaded cathode (12 mg cm -1 ), which may be due to the optimized architecture of the CNF2-based cathode. The dimensions of the pouch were measured, and the calculated volumetric energy density was 367 Wh L -1 .

[0118] The performance of this pouch cell was further examined by powering drones that require high weight energy density and high power (current density). The plot in Figure 6e shows the cycle profile of two 2.5 Ah pouches connected in series to provide the operating voltage of the drone. The inspection flight test of the drone demonstrated that the Li-S pouch pack was able to support a 10-minute hovering time before reaching the cut-off voltage. The capacity test of the after-service pouch pack showed that only one-third of the capacity was consumed during the 10-minute inspection flight, which means that the optimized voltage regulator and control substrate design can potentially triple the hovering time.

[0119] As shown in Figure 6f, the CNF2-based cathode pouch cell combines high specific energy, long cycle life, and high specific capacity. Of note are the ultra-high areal capacity of 15 mAh cm -2 and the standard nominal voltage. The pouch cells of the present disclosure outperform other reported Li-S pouch cells at the Ah level.

[0120] Overview of the main functions and comparison systems Sulfur cathodes prepared by mixing 70 wt% sulfur, 20 wt% carbon, and 10 wt% of different binder systems in deionized (DI) water were compared, and the results were collected in a table. Systems 1 and 2 used carboxymethyl cellulose (CMC), a binder commonly used in battery electrode fabrication, in both wet mixing (System 1) and dry mixing (System 2). Previous publications have demonstrated that CMC enables the formation of strong cross-linking bonds between sulfur and carbon particles through a method of dry mixing the solid components before adding water (see International Patent Application No. PCT / AU2019 / 051239). System 3 used the CMC glucose binder disclosed in Y. Huang et al, Nature Communications, (2021), 12:5375. System 4 is according to the present disclosure and utilized carboxylated CNF with a carboxyl group loading of approximately 2 mmol / g (CNF2).

Table 1

[0121] From the results, the system 4 according to the present disclosure has a desirable range of characteristics including a cathode slurry viscosity that enables ease of processing, a very low cathode porosity, a high areal capacity at a high sulfur loading, and a low electrolyte-to-sulfur volume-to-weight ratio.

[0122] Post-experimental study outside the Li-S pouch cell After the intense cycling, the pouch cell was disassembled, and the lithium metal anode and sulfur cathode were washed with DOL / DME and collected for SEM imaging. As illustrated in Fig. 7a, the surface of the lithium anode has two different morphologies. The first morphology (Fig. 7b) is a thick solid electrolyte interphase (SEI) layer film covered with a stone-shaped morphology that appeared for lithium plating. Another part of the lithium surface (Fig. 7c) is filled with a mature stone shape with a low surface area instead of the harmful dendritic growth that is often observed in previously reported systems. Such a desired lithium plating behavior maintains the integrity of the SEI layer, which may be due to the controlled lithium polysulfide access to the lithium anode. This regulatory ability of the present system is the synergistic effect of the highly charged CNFs in the cathode that blocks the transfer of negatively charged polysulfides from the cathode and the carbon-coated thin glass fiber interlayer, limiting the transport of lithium polysulfides.

[0123] As depicted by SEM of the cross-section (Fig. 7d) and top view (Fig. 7e) of the CNF-based cathode in the fully delithiated state, the cathode benefits from a separated microstructure to accommodate volume changes during cycling and does not develop large cracks after cycling. More structural details are provided by the higher magnification images (Fig. 7e). The CNF-based cathode demonstrates the preservation of the interparticle bonding, which indicates the high robustness of the carboxylated CNF backbone.

[0124] Cellulose nanofiber microscopic analysis A dilute suspension (about 0.001 wt%) of natural CNF and carboxylated CNF in water was sonicated with an ultrasonic probe at 70% amplitude for 2 minutes and analyzed by transmission electron microscopy. Figure 8 shows the images.

[0125] The sample was decomposed into nanofibers with a measured fibril diameter of less than 4 nm. The natural CNF sample (Figure 8(a)) showed long and aggregated nanofibers (fibril length of about 1000 nm), while the carboxylated CNF sample (Figure 8(b)) showed shorter individual nanofibers with improved separation (fibril length of 500 nm).

[0126] Analysis techniques Scanning electron microscopy imaging and EDX mapping. The newly prepared cathode sample was mounted on an aluminum stub with a conductive carbon tap and coated with iridium for front and cross-sectional imaging. A Nova 450 field emission scanning electron microscope (FESEM) and a Magellan 400 FESEM were used for secondary electron imaging and energy dispersive spectroscopy mapping (EDX). For the SEM study after external experiments, all cells were disassembled in an argon glove box after being terminated at full charge. The cycling electrodes (cathode and anode) were washed with 1 ml of DOL / DME (1:1, v / v), dried in vacuum for 12 hours, and then mounted on an aluminum stub with a conductive carbon tap in an argon glove box. The cycling electrodes were transferred from the argon glove box to a Merlin FESEM for analysis using a transfer vacuum module.

[0127] Transmission electron microscopy imaging Transmission electron microscopy (TEM) was performed using a FEI Tecnai F20. Dilute suspensions (about 0.001%) of natural CNF and carboxylated CNF were sonicated with a ultrasonic probe for 2 minutes at 70% amplitude and dried on a plasma-cleaned copper grid. The samples were then stained with 2% uranyl acetate, air-dried, and examined at 200 kV.

[0128] Rheology measurements Rheological measurements were carried out using a cone and plate geometry (diameter - 50 mm, cone angle - 2°) with a strain-controlled ARES G2 rheometer (TA instruments, USA). During the measurement, a constant gap of 0.045 mm and a temperature of 23.00 ± 0.01 °C were maintained. For steady-state measurements, the viscosity change as a function of shear rate in the range of 0.1 - 100 s -1 was recorded. Amplitude sweeps were performed at an angular frequency of 10 rad / s within the range of strain amplitudes from 0.1% to 100% to determine the linear viscoelastic (LVE) regime. Frequency sweeps were performed in the range of 0.1 - 100 rad / s.

[0129] Electron beam absorption current (EBAC) measurements The EBAC measurements were carried out on a FEI Nova NanoSEM 450 FEG SEM equipped with a DEBEN GW Type 31 amplifier. The cross-sectional battery electrode samples were mounted on thin slide glasses to provide electrical insulation from the SEM stage. Subsequently, the cross-sectional sample stubs were attached in an edge-on orientation to the electron beam. Through a vacuum feedthrough, the electrode copper substrate was connected to the EBIC amplifier, which converted the current signal into a voltage output within the range of 0 - 1V and then digitized it into 16-bit grayscale values by the SEM system. All EBAC mappings were acquired using an electron beam with 5 kV, spot size 5 and an objective lens of 50 μm, resulting in an incident beam current of approximately 5 nA. Due to the limited bandwidth of the amplifier in the quantitative mode, the scanning dwell time was set to a conservative 8.2 ms. For each mapping, the dark current was acquired by blanking the electron beam in the first few lines of the scan and then subtracted in post-processing.

[0130] Surface roughness measurement The profilometry measurement of the electrodes was carried out using an Olympus LEXT OLS5000 laser confocal microscope. Each scan took approximately 5 minutes. The instrument automatically calculated the arithmetic surface roughness R a and.

[0131] Zeta potential The carboxylated and homogenized CNFs were centrifuged at 12,000 g for 5 minutes to separate any non-fibrillated fibers. The resulting supernatant was analyzed by dynamic light scattering and zeta potential using a particle size and zeta potential analyzer (Brookhaven Nanobrook Omni). The measurements were carried out 5 times for each sample.

Claims

1. It is a sulfur cathode, A cellulose composition containing multiple anion-functionalized cellulose nanofibers, One or more sulfur-containing materials, A sulfur cathode comprising one or more conductive materials.

2. A cellulose composition comprising 2 to 20% by weight of a plurality of anion-functionalized cellulose nanofibers, One or more sulfur-containing materials in an amount of 60-80% by weight, A sulfur cathode according to claim 1, comprising 10 to 30% by weight of one or more conductive materials.

3. A cellulose composition containing 5 to 15% by weight of a plurality of anion-functionalized cellulose nanofibers, One or more sulfur-containing materials in an amount of 65-75% by weight, A sulfur cathode according to claim 1, comprising 15 to 25% by weight of one or more conductive materials.

4. The aforementioned plurality of anion-functionalized cellulose nanofibers have the following characteristics: Fibril diameter of 0.1 nm to 20 nm, Fibril length of 20 nm to 1000 nm, Fibril aspect ratios of 10 to 1000, and A zeta potential more negative than -60mV, A sulfur cathode according to any one of claims 1 to 3, having one or more of the following:

5. The one or more sulfur-containing materials mentioned above are elemental sulfur, Li ２ S, and MoS ２ A sulfur cathode according to claim 1, comprising one or more of the following.

6. The sulfur cathode according to claim 1, wherein the one or more conductive materials include one or more of carbon black, graphite, graphene, activated carbon, carbon nanotubes, and carbon fibers.

7. The sulfur cathode according to claim 1, wherein the cathode has a porosity of less than 50% or less than 40%.

8. The sulfur cathode according to claim 1, wherein the cathode has an arithmetic surface roughness of less than 25 μm.

9. The sulfur cathode according to claim 1, wherein the transport of polysulfide anions within the cathode is inhibited compared to a sulfur cathode lacking the cellulose composition.

10. The sulfur cathode according to claim 1, wherein the transport of lithium ions within the cathode is promoted compared to a sulfur cathode lacking the cellulose composition.

11. A lithium-sulfur battery comprising a lithium anode, a separator, a sulfur cathode, and an electrolyte disposed between the anode and the cathode, The sulfur cathode is, A cellulose composition containing multiple anion-functionalized cellulose nanofibers, One or more sulfur-containing materials, A lithium sulfur battery comprising one or more conductive materials.

12. The lithium sulfur battery according to claim 11, wherein the ratio of electrolyte volume to sulfur weight is less than approximately 6.0 μl / mg.

13. The lithium sulfur battery according to claim 11, wherein the ratio of electrolyte volume to sulfur weight is approximately 3.5 to approximately 5.0 μl / mg.

14. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers have fibril diameters of about 0.5 nm to about 10 nm, or about 1 nm to about 6 nm, or about 2 nm to about 4 nm.

15. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers have fibril lengths of about 40 nm to about 700 nm, or about 70 nm to about 500 nm, or about 100 nm to about 400 nm.

16. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers have fibril aspect ratios of about 15 to about 750, or about 20 to about 500, or about 25 to about 200.

17. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers have a zeta potential of about -60 mV to about -90 mV.

18. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers are functionalized with one or more of carboxyl, phosphonate, sulfonate, sulfate, hydroxyl, nitrate, and carbonate.

19. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers contain more than 1 mmol of anionic groups per gram of nanofiber, or more than 1.5 mmol of anionic groups per gram of nanofiber.

20. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers contain about 1 to about 4 mmol of anionic groups per gram of nanofiber, or about 1.5 to about 4 mmol of anionic groups per gram of nanofiber, or about 2 to about 4 mmol of anionic groups per gram of nanofiber.

21. The lithium sulfur battery according to claim 11, wherein the plurality of anion-functionalized cellulose nanofibers are functionalized with carboxyl groups.

22. The lithium-sulfur battery according to claim 11, wherein the sulfur cathode facilitates the transport of lithium ions through the cathode during charging or discharging.

23. The lithium sulfur battery according to claim 11, wherein during charging or discharging, the sulfur cathode prevents the transport of polysulfide ions through the cathode.

24. Cathode slurry composition, A cellulose composition containing multiple anion-functionalized cellulose nanofibers, One or more sulfur-containing materials, One or more conductive materials, A cathode slurry composition containing water.

25. The cathode slurry composition according to claim 24, wherein the ratio of the total weight of water to the cellulose composition, the sulfur-containing material, and the conductive material component is 1 ml / g to 40 ml / g.

26. 0.01 seconds －１ The cathode slurry composition according to claim 24, wherein the viscosity of a 3 ml / g slurry at a shear rate is greater than 1000 Pa.s when measured in a temperature range of 20 to 25°C.

27. A method for preparing the cathode slurry composition according to claim 24, comprising the step of mixing a cellulose composition comprising a plurality of anion-functionalized cellulose nanofibers, one or more sulfur-containing materials, one or more conductive materials, and water.

28. The method according to claim 27, wherein the cellulose composition, one or more sulfur-containing materials, and one or more conductive materials are dry-mixed before being mixed with water.

29. A cellulose composition comprising multiple anion-functionalized cellulose nanofibers, wherein the nanofibers have the following characteristics: Fibril diameter of 0.1 nm to 20 nm, Fibril length of 20 nm to 1000 nm, Fibril aspect ratios of 10 to 1000, and A zeta potential more negative than -60mV, A cellulose composition having one or more of the following.

30. a) A step of treating a cellulose source with one or more agents capable of anionic functionalizing cellulose nanofibers, b) The step of subjecting the anion-functionalized cellulose nanofibers to shear conditions in order to reduce the size of one or more fibrils, A method for preparing the cellulose composition according to claim 29, comprising:

31. The method according to claim 30, wherein the agent is an oxidizing agent.

32. The method according to claim 30, wherein the shear condition in step b) includes stirring the aqueous mixture of the anion-functionalized cellulose nanofibers.

33. The method according to claim 30, wherein the shearing conditions in step b) include either or both ultrasonic homogenization and high-pressure homogenization of the aqueous mixture of anion-functionalized cellulose nanofibers.

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