Patent Application
20230035720
The present invention is generally related to techniques for mass production of large-dimension, flexible, ultrathin, high ionic conductivity polymer-ceramic composite solid-state electrolyte membranes which are used in electrochemical devices such as all solid-state lithium-ion electrochemical cells and batteries.
Non-aqueous lithium electrochemical cells typically include an anode, a lithium electrolyte prepared from a lithium salt dissolved in organic solvents, and a cathode of an electrochemically active material. Organic solvents are added to solvate the lithium salt which provides mobile ions. During the electrochemical discharge process lithium-ions are transported through the electrolyte from the anode to the cathode. As lithium-ions are taken up by the cathode, there is a simultaneous release of electrical energy.
Solid-state electrolytes (SSE) can replace conventional organic liquid electrolytes, which are generally flammable and toxic. Conventional electrode materials and lithium metal anodes can be employed with a SSE. Lithium anodes have high inherent high capacities (C) which increase the cell voltage (V) and thereby improves the energy density of the battery (E=VC). There are two critical challenges to achieving high performance batteries using SSE: (1) low ionic conductivities of many SSE, and (2) the low mechanical strengths of electrolyte materials do not adequately prevent Li dendrite growth. SSE which are being explored are typically inorganic-based (depending on the lattice structure, they are garnet, perovskite, glass-ceramics etc.) and polymer-based. Solid polymer electrolytes can be manufactured by relatively simple, inexpensive techniques whereas fabricating solid inorganic electrolytes with well-defined compositions or lattice structures requires high temperature processes. Due to the high degree of coordination between Li ions and the polymer chain in sold-state polymer electrolytes, the chain-assisted Li+ transport mechanism is less robust at room temperature or below the melting temperature of the polymer. The attendant poor ionic conductivity is attributed to interference with Li ion transport. The addition of plasticizers to the solid-state polymer-based electrolyte improves the polymer chain mobility at room temperature which results in an increase in the ionic conductivity, but the plasticizers also reduce the mechanical strength of the solid-state polymer-based electrolyte.
So-called all-solid-state batteries contain exclusively solid materials, and in particular solid-state electrolytes, in contrast to conventional batteries containing liquid electrolytes. One of the main concerns of current all solid-state lithium-ion batteries (ASSLiB) is the poor contact between solid electrolyte and electrodes. The high contact resistance causes low-rate capability and poor cycling stability. With solid polymer electrolytes, lithium dendrites, which develop as an electrochemical cell undergoes charging and discharging cycles, can penetrate through the ‘plasticizer-softened’ polymer electrolytes to short circuit the cell.
The present invention is based, in part, on the development of a high ionic conductivity composite solid-state electrolyte membrane that is formed by infusing a porous polymer substrate (or separator) with an electrolyte mixture and curing the mixture. In a preferred embodiment, the porous polymer substrate comprises a large sheet or web that is continuously infused and cured with the electrolyte mixture. The porous polymer substrate serves as a framework to hold and support the electrolyte mixture during post-treatment.
In one aspect, the invention is directed to a roll-to-roll system for fabricating a composite solid-state electrolyte (SSE) membrane that comprises: a continuous source of a sheet of porous substrate which moves in a machine direction; a first coater that is configured to apply a first coat of a first solid electrolyte precursor mixture onto a first surface of the sheet of porous substrate; and a first module, located downstream of the first coater, comprising a first source of ultra-violet radiation and a first source of convection heat. The process produces a flexible, ultrathin, high ionic conductivity composite SSE membrane that can be used in all solid-state electrochemical pouch cells.
In preferred embodiments, the roll-to-roll system includes slot-die coating modules that are configured to coat SSE precursor solutions on both sides of a porous separator membrane substrate. Alternatively, the roll-to-roll system includes doctor blade coating systems that are configured to coat SSE precursor solutions onto a porous separator membrane.
In a preferred embodiment, the roll-to-roll system includes multifunctional modules that are configured to post-treat the precursor solutions after being coated onto the porous separator membrane. The module can effectuate UV crosslinking and solvent evaporating or thermal crosslinking. The solid electrolyte precursor mixture preferably includes: (i) a polymer matrix or precursors thereof, (ii) ceramic nanoparticles with diameters that range from 10 to 2000 nm, (iii) a lithium salt, (iv) a plasticizer.
In further aspect, the invention is directed to a composite solid-state electrolyte (SSE) membrane that includes a porous substrate that is a polymer network, ceramic nanoparticles with diameters that range from 10 to 2000 nm, lithium salt, and plasticizer distributed throughout the porous substrate. The porous substrate is preferably made of a polymer that is different from that of the polymer network. The SSE membrane is hybrid, ceramic-polymer nanocomposite material that typically exhibits high ionic conductivity over a wide temperature window (−40° C. to 90° C.) and excellent chemical/electrochemical stability with respect to the electrodes. The SSE has an amorphous structure and large dielectric constant environment that is favorable for lithium-ion dissociation and polymer chain-assisted ion conduction. It can be used as an electrolyte layer or integrated or added into an electrode layer to form a composite electrode. No liquid organic solvent is required with the composite electrode.
The invention is directed a roll-to-roll manufacturing system for large-size, flexible and ultra-thin composite SSE membranes which are particularly suited for fabricating all solid-state lithium-ion electrochemical cells and batteries.
A roll 4 of the porous plastic material is unwound by an unwinder 5 and supplies a continuous sheet 6 that is advanced in the machine direction (MD) by a plurality of rollers 34, 36. The sheet 6 is coated on the top (first) surface with a layer of polymeric-ceramic electrolyte slurry by coater 8 to produce coated sheet 10. The amount of slurry applied is sufficient to infiltrate into approximately halfway into the sheet 6. A dual-functional module 12 exposes the coated sheet 10 to convection heat to remove excess solvent from the electrolyte slurry. Where necessary, the module 12 directs ultra-violet radiation into the electrolyte slurry to cross-link polymers therein to form a polymer network that is uniformly distributed throughout the pores of the porous plastic material Sheet 14 which comprises a polymeric-ceramic composite solid electrolyte formed within the porous plastic material passes through the dual rollers of a calendar 16 to produce smooth solid electrolyte with a uniform thickness along the width in the cross direction, which is perpendicular to the MD.
Turning rollers 18, 20 maneuver sheet 14 so that the second (uncoated) side of the porous polymer material is on top. A layer of polymeric-ceramic electrolyte slurry is applied thereon by coater 22 to produce coated sheet 24 which is passed through a dual-functional module 26 which exposes the coated sheet 24 to convection heat and where necessary, to ultra-violet radiation. Sheet 30 which comprises a polymeric-ceramic composite solid electrolyte distributed throughout pores of the porous plastic material passes through the dual rollers of a calendar 30 to yield a flexible composite SSE membrane which includes a porous substrate that is a polymer network, ceramic nanoparticles, lithium salt, and plasticizer distributed throughout the porous substrate. The composite SSE membrane preferably has a thin electrolyte upper and lower electrolyte layer, with each layer comprising a polymer network, ceramic nanoparticles, lithium salt, and plasticizer but without the substrate. That is, the thin electrolyte layers protrude from the planar surfaces of the composite SSE membrane. A rewinder 31 takes up the composite SSE membrane to form roll 32. The total thickness of the flexible SSE membrane is typically 30 μm to 300 μm and preferably 40 μm to 180 μm and when present the upper and lower electrolyte layers each is typically 10 μm to 120 μm and preferably 15 μm to 80 μm in thickness as part of the total thickness. The porous plastic substrate typically comprises 30% to 70% and preferably 45% to 60% by weight of the entire composite SSE membrane with the remainder being consisting of the solid-state electrolyte.
The polymeric-ceramic electrolyte slurry comprises polymer precursors, lithium salt, ceramic nanoparticles and plasticizers. Preferred polymer precursors include, for example, ethylene oxide, ethylene glycol diacrylate, and acrylonitrile. These polymer precursors from poly (ethylene oxide) (PEO), poly (ethylene glycol diacrylate) (PEGDA), poly(acrylonitrile) (PAN), poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP), respectively. The polymer precursors typically comprise 10 to 50 wt % and polymeric matrix, which is derived from the polymer precursors, typically comprises 30 to 95 wt % of the subsequent solid electrolyte formed within the porous plastic substrate. A preferred polymer matrix is PEGDA which is derived by in-situ UV-polymerization using phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, or IRGACURE 819, as the initiator. Another preferred polymer matrix is P(VDF-HFP), which is generally used for high-voltage (˜5V) battery cell.
The lithium salt is any lithium salt that is suitable for use in a non-aqueous solid-state electrolyte. Preferred lithium salts include, for example, LiC2F6NO4S2 (LiTFSI), LiClO4, and LiPF6. The lithium salt preferably comprises 20 to 60 wt % of the polymeric-ceramic electrolyte slurry. A preferred lithium salt comprises a mixture of lithium salts that includes lithium bis(oxalato)borate or LiB(C2O4)2 (LiBoB), which serves as a lithium salt enhancer, to improve ion transport within solid electrolyte that is formed within the porous plastic substrate. Due to low solubility or miscibility of LiBoB, only a small amount of LiBoB should be added into the polymer-ceramic electrolyte slurry. When employed, the weight ratio of LiBoB to solid electrolyte formed within the porous plastic substrate is about 0.4-0.6 wt %.
The ceramic nanoparticles are preferably AlxLi7−xLa3Zr1.75Ta0.25O12 wherein x ranges from 0 to 0.85 (LLZO) and have diameters that range from 10 nm to 2000 nm. The LLZO preferably comprises 5 to 70 wt % of the polymeric-ceramic electrolyte slurry and of the solid electrolyte that is formed within the porous plastic substrate. Incorporating LLZO into a polymer solid electrolyte produces a solid electrolyte with enhanced structure integrity and high ionic conductivity.
The LLZO is synthesized by mixing stoichiometric amounts of starting powders including LiOH H2O, La2O3, ZrO2, Al2O3 and Ta2O5 and milling the mixture via high energy ball milling in ethanol media for 8-12 hrs. Zirconia balls (average diameters of 5 mm) balls at a ball-to-powder weight ratio of about 20:1 and about 360 rpm milling speed. After milling, the collected slurry is dried (80° C., 2-3 hrs), crushed, and sieved (through a 200 mesh), and calcined at about 900° C. for 6 hours to fully decompose LiOH. The as-calcined powders are then ball-milled again in ethanol for 6-12 hrs. Planetary ball mill was used, followed by drying process. The dried powders were pressed into pellets with diameters of about 9.5 mm at about 300 MPa, and then sintered with a temperature range from 800° C. to 1150° C. for about 4 hrs to obtain particles with size from 100 nm to 2000 nm. Both calcination and sintering processes are carried out with samples in alumina crucibles covered by alumina lids, and the pellets are embedded in the prepared powder in order to mitigate losses of volatile components and accidental contamination. As is apparent, when synthesizing LLZO of the formula Li7La3Zr1.75Ta0.25O12, that is when x is 0, no Al2O3 is used.
A feature of the invention is that the size of LLZO nanoparticles can be tuned by controlling temperature of synthesis. The calcine temperature determines the particle sizes of LLZO. Generally, high calcined temperature and long calcined time produce larger size LLZO particles. It has been demonstrated that a calcine temperature of about: (i) 950° C., (ii) 1000° C., and (iii) 1050° C. yields LLZO nanoparticles with diameters of about 100 to 600 nm, 1000 to 1200 nm, and 1 to 2 μm, respectively.
The plasticizer is an aprotic compound that serves as a liquid medium in which the polymer precursors are polymerized to form a polymer matrix. The plasticizer comprises dimethyl sulfoxide (DMSO), succinonitrile (SCN), glutaronitrile (GN), ethylene carbonate (EC), propylene carbonate (PC), sulfolane (SL) and mixtures thereof. In particular, the solid electrolyte can contain essentially a single plasticizer. The plasticizer preferably comprises 10 to 60 wt % of the polymeric-ceramic electrolyte slurry and that solid electrolyte formed within the porous plastic substrate.
The flow chart in
The flow chart in
The composite SSE membrane of the present invention exhibits ionic conductivity of greater than 1×10−5 S/cm (at room temperature of 20° C.) and has a large electrochemical window of up to 5.8V (at room temperature). In addition, it has a wide use temperature with a thermally stable temperature of up to 150° C. and a glass transition temperature of less than −60° C. Finally, the composite SSE membrane shows low interfacial resistance, good compatibility with both lithium metal and cathode materials, and enhanced mechanical strength with a Young's modulus that exceeds 50 MPa.
During the fabrication process of large-scale electrolyte, certain types of monomer or polymer precursors, such as PEGDA, will be polymerized or crosslinked, which could establish polymer network and have strong interaction between Li salt and ceramic nanoparticles, as presented in
The polymer-ceramic composite SSE membrane is particularly suited for use in all solid-state electrochemical cells in the form of pouch cell and battery. A “pouch cell” type battery as shown in
After the cathode, the electrolyte and the anode are stacked and the cell is assembled, the entire structure is calendared to ensure intimate contact between the layers High temperature is also applied during calendaring process as the polymer becomes “soft” at high temperatures so that it becomes easier and more effective to compress the structure and create compact contacts between polymer and electrode. The temperature is selected based on the formulation of the solid electrolyte which ranges from 60° C. to 180° C.
The pouch cell as shown in
Once the electrolyte, cathode, and anode layers are made, cathode sheets, solid electrolyte and anode sheets are stacked layer-by-layer by Z-folding stacking format. Then, the cell core is welded and sealed in laminated aluminum case. Finally, the pouch cell is pressed by pneumatic hot press machine, where the pouch cell pressed with a pressure of 400 psi and temperature of 150° C. for 10 minutes.
In the exemplary pouch cell, SSE membrane positioned between anode and cathode comprises 12.5 wt % polymer matrix that is derived from PEGDA, 37.5 wt % EC, 33 wt % LiTFSI, 17 wt % Al0.15Li0.85La3Zr1.75Ta0.25O12 with diameters that range from 10 to 2000 nm. The SSE membrane is fabricated by using roll-to-roll manufacture system presented in
The optimized mass energy density and volume energy density of all solid-state pouch cell prototype could reach over 200 Wh/kg and 400 Wh/L.
The foregoing has described the principles, preferred embodiment and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.
