Description:FORMATION OF LITHIUM-ION BATTERY CELLS
Technical Field
[001] This disclosure relates generally to lithium-ion batteries, and more particularly to the formation of lithium-ion battery cells.
Background
[002] Today, batteries are indispensable as electronic devices, and have become an important part of everyday human lives. In particular, the demand for rechargeable batteries such as lithium-ion batteries has increased exponentially due to their comprehensive superiority in power density, energy density, cost, and safety. The high performance and low cost enable lithium-ion batteries to be used not only in portable electronic devices such as smartphones, tablets, laptops but also find applications in electric vehicles.
[003] A lithium-ion battery cell (or a lithium-ion cell) is composed of a cathode, an anode, and an electrolyte. The cathode and anode are usually formed by disposing of respective electrode materials on current collectors and may include a separator placed between the cathode and the anode. The assembly of the lithium-ion cell is complete when the electrolyte is filled in the space between the anode and the cathode. As the electrolyte is an organic liquid containing chemical additives, it reacts with the electrode materials. This causes corrosion of the electrodes (i.e., the cathode and the anode) and degradation of the electrolyte.
[004] A newly assembled lithium-ion cell is usually subjected to “formation", which is an important step in manufacturing lithium-ion batteries. Formation is the process of performing an initial charge-discharge operation on the cell to guarantee high performance, longer lifespan, and safety throughout its life. During the formation process, a Solid Electrolyte Interface (SEI) film is formed on one or both the electrodes of the lithium-ion cell. The SEI film helps in preventing the corrosion of the electrodes and the degradation of the electrolyte. However, achieving a stable SEI film has been a challenge.
SUMMARY
[005] In an embodiment, a process may include performing a formation process on a lithium-ion cell to obtain a formed lithium-ion cell. To perform the formation process, the process may include performing a first formation cycle on the lithium-ion cell at a first C-rate to obtain a first formed cell. The first C-rate may vary from about C/20 to about C/40. To perform the formation process, the process may further include performing a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell. The second C-rate may vary from about 1C to about 3C.
[006] Another embodiment discloses a process that may include performing a first formation cycle on a lithium-ion cell at a first C-rate to obtain a first formed cell and performing a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell. To perform the first formation cycle, the process may include charging the lithium-ion cell at the first C-rate to attain a first cut-off voltage. The first C-rate may vary from about C/20 to about C/40. The first cut-off voltage is in a range from about 2.5 V to about 3.5 V. To perform the first formation cycle, the process may further include, upon attaining the first cut-off voltage, charging the lithium-ion cell at the first cut-off voltage with a first limiting current. The first limiting current may be less than the first C-rate. After charging the lithium-ion cell at the first cut-off voltage, the process may include discharging the lithium-ion cell at the first C-rate to attain a second cut-off voltage. To perform the second formation cycle, the process may further include charging the first formed cell at the second C-rate to attain a third cut-off voltage. The second C-rate may vary from about 1C to about 3C. The third cut-off voltage may be between about 2.5 V to about 3.5 V. The third cut-off voltage may be less than the first cut-off voltage. To perform the second formation cycle, the process may further include, upon attaining the third cut-off voltage, charging the first formed cell at the third cut-off voltage with a second limiting current. The second limiting current may be less than the second C-rate and more than the first limiting current. After charging the first formed cell at the third cut-off voltage, the process may include discharging the first formed cell at the second C-rate to attain a fourth cut-off voltage.
[007] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[008] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles.
[009] FIG. 1 illustrates a schematic of an exemplary lithium-ion cell where embodiments of the present disclosure may be implemented.
[010] FIG. 2 illustrates a flow diagram of a process involving performing a formation process on a lithium-ion cell, in accordance with some embodiments of the present disclosure.
[011] FIG. 3 illustrates a flow diagram of a process involving performing a formation process on a lithium-ion cell, in accordance with some embodiments of the present disclosure.
[012] FIG. 4 is a graph representing Solid Electrolyte Interface (SEI) film resistance for various exemplary formed lithium-ion cells over a number of charge-discharge cycles of an aging process.
[013] FIG. 5 is a graph representing Nyquist plots of various exemplary formed lithium-ion cells at the 20th charge cycle of an aging process.
DETAILED DESCRIPTION
[014] Exemplary embodiments are described with reference to the accompanying drawings. Wherever convenient, the same reference numbers are used throughout the drawings to refer to the same or like parts. While examples and features of disclosed principles are described herein, modifications, adaptations, and other implementations are possible without departing from the spirit and scope of the disclosed embodiments. It is intended that the following detailed description be considered exemplary only, with the true scope and spirit being indicated by the following claims.
[015] As used herein, the term “lithium-ion battery” may refer to a rechargeable battery that includes one or more electrochemical cells (generally referred to as battery cells or lithium-ion cells) that use reversible reduction of lithium ions to store energy. Examples of lithium-ion batteries may include anode-free lithium-ion batteries, lithium-ion polymer batteries, batteries with liquid electrolytes, and solid-state batteries. The term “lithium-ion battery” or “lithium-ion cell”, as used herein, implies a single lithium-ion cell, unless specified otherwise.
[016] As used herein, the term “positive electrode” or “cathode” refers to an electrode of a lithium-ion cell at which reduction occurs and that supplies electrons during the charging of the lithium-ion cell. As used herein, the term “negative electrode” or “anode” refers to an electrode of the lithium-ion cell at which oxidation occurs and that accepts electrons during the charging of the lithium-ion cell. As used herein, the term “electrolyte” refers to a material that allows ions, for example, Li ions, to migrate therethrough, but does not allow electrons to conduct therethrough. As used herein, “current collector” refers to a bridging component that collects electrical current generated at the electrodes and connects with external circuits.
[017] As used herein, the term “C-rate” refers to a measure of a rate at which a cell is charged or discharged relative to a capacity of the cell, unless specified otherwise.
[018] As used herein, the term “cut-off voltage” refers to a predefined upper threshold value of voltage at which a cell is charged at a constant current or a lower threshold value of voltage at which a cell is discharged at a constant current, unless specified otherwise.
[019] As used herein, the term “limiting current” refers to a predefined upper threshold value of current at which a cell is charged at a constant voltage or a lower threshold value of current at which a cell is discharged at a constant voltage, unless specified otherwise.
[020] As used herein, the term “about” refers to ±20%, more preferably, ±10%, or still more preferably, ±5% of a value, unless specified otherwise.
[021] Referring now to FIG. 1, a schematic of an exemplary lithium-ion cell 100 where embodiments of the present disclosure may be implemented is illustrated. The lithium-ion cell 100 may include a positive electrode 106, a negative electrode 108, an electrolyte 110, and a separator 112. The lithium-ion cell 100 may further include a housing (not shown in FIG. 1) to enclose the positive electrode 106, the negative electrode 108, the electrolyte 110, and the separator 112. The lithium-ion cell 100 may be in a prismatic battery form, a pouch battery form, a cylindrical battery form, or any other shape that may consistently implement the arrangement of the positive electrode 106, the negative electrode 108, the electrolyte 110, and the separator 112 as shown in FIG. 1. Size and shape of the lithium-ion cell 100 may vary based on a specific application for which the lithium-ion cell 100 may be designed. For example, the size, capacity, and power-output specifications of the lithium-ion cell 100 for use in EVs may be different from those used for powering hand-held consumer electronic devices.
[022] The positive electrode 106 may include a positive electrode material 105a disposed on a first current collector 102 and the negative electrode 108 may include a negative electrode material 105b disposed on a second current collector 104. The separator 112 is an insulating layer to prevent a short circuit between the positive electrode 106 and the negative electrode 108.
[023] The first current collector 102 and the second current collector 104 may be made of a metal, an alloy of the metal, or a carbon-based material. Examples of the metal include, but are not limited to, aluminum, nickel, titanium, stainless steel, or copper. The first current collector 102 and the second current collector 104 can be in the form of foil, mesh, or foam. It is preferred that the first current collector 102 includes aluminum or an aluminum alloy and the second current collector 104 includes the carbon-based material.
[024] The positive electrode material 105a may include, but is not limited to, lithium metal oxides such as LMO (lithium manganese oxide), Li-NCA (lithium nickel cobalt aluminum oxide), Li-NMCO (lithium nickel manganese cobalt oxide), LCO (lithium cobalt oxide), LNO (lithium nickel oxide), LNMO (lithium nickel manganese oxide), LNCM (lithium nickel cobalt manganese oxide), similar such lithium-based metal oxides, or any combinations thereof. The negative electrode material 105b may include, but is not limited to, graphite, SiC nanocomposites, lithium titanium oxides, Sn particulates, Si particulates, or any combinations thereof. In a specific embodiment, the negative electrode material 105b includes lithium titanium oxide (LTO).
[025] Further, the electrolyte 110 is disposed between the positive electrode 106 and the negative electrode 108 The electrolyte 110 facilitates/allows the migration of lithium ions between the first current collector 102 and the second current collector 104. The electrolyte 110 may be a solid electrolyte or a liquid electrolyte. In an embodiment, the electrolyte 110 is an organic liquid containing chemical additives. The electrolyte 110 may include a lithium salt in an aqueous state. As will be appreciated, the lithium salt may dissociate into free ions in the aqueous state. Non-limiting examples of lithium salts may include LiPF6, LiAsF6, LiBOB, LiClO4, LiBF4, and the like.
[026] The positive electrode 106 and the negative electrode 108 are in ionic communication through the separator 112. As used herein, the phrase “ionic communication” refers to the traversal of the ions between the positive electrode 106 and the negative electrode 108 through the separator 112. The separator 112 may be ionically conducting while electrically insulating. In some embodiments, the separator 112 is an ionically conducting solid separator. In certain embodiments, the separator 112 is capable of transporting lithium ions between the positive electrode 106 and the negative electrode 108. Suitable materials for the separator 112 may include, but are not limited to, beta’-alumina, beta”-alumina, beta’-gallate, beta”-gallate, zeolite, lithium superionic conductor compounds, a polymer membrane, or combinations thereof.
[027] When the lithium-ion cell 100 is charged, the positive electrode 106 may oxidize the positive electrode material 105a to generate lithium-ions. The generated lithium-ions may be released in the electrolyte 110. The negative electrode 108 may receive the lithium-ions generated by the positive electrode 106 via the electrolyte 110. Further, when the lithium-ion cell 100 is discharged, the negative electrode 108 may oxidize the negative electrode material 105b to generate free electrons and lithium-ions. The positive electrode 106 may reduce the lithium-ions received through the electrolyte 110 to generate the positive electrode material 105a.
[028] While lithium ions are exchanged between the positive electrode 106 and the negative electrode 108 via the electrolyte 110, the free electrons may be exchanged via an external circuit between the first current collector 102 and the second current collector 104. Thus, when the lithium-ion cell 100 discharges, the free electrons may flow from the negative electrode 108 to the positive electrode 106. Consequently, the generated free electrons may generate an electric current that flows through the external circuit. The lithium-ion cell 100 may be connected in series and/or parallel with one or more similar lithium-ion cells to produce a greater voltage output and power density based on the desired application.
[029] Typically, when a lithium-ion cell is assembled and an electrolyte is filled between the electrodes the electrolyte starts reacting with the electrode materials of one or both the electrodes, causing corrosion of the electrodes and degradation of the electrolyte due to dissolution of the electrode materials. This corrosion and degradation of the electrolyte negatively impacts the life of the lithium-ion cell. To overcome this problem, in the present state of art, a newly assembled lithium-ion cell is usually subjected to a formation process that involves performing multiple charge-discharge cycles on the lithium-ion cell. During the formation process, a Solid Electrolyte Interface (SEI) film is formed on one or both the electrodes of the lithium-ion cell. The SEI film(s) helps in reducing or preventing the electrolyte from reacting with the electrode materials and/or the dissolution of the electrode material(s), thereby reducing the corrosion of the electrodes and the degradation of the electrolyte, which ensures continued electrochemical reactions in the lithium-ion cell.
[030] However, in order to achieve high performance, long life, good cycling ability, safety, and stability of the lithium-ion cell, a stable SEI film with a high lithium-ion conductivity (that may be referred to as ionic conductivity) is desirable. The ionic conductivity of the SEI film may, in part, depend on the thickness and porosity of the SEI film, and may be enhanced by tuning the porosity and thickness of the SEI film.
[031] As used herein and thereafter, the term “stable SEI film” refers to a SEI film that does not get modified or gets modified slightly during the use of a lithium-ion cell i.e., over a number of charge-discharge cycles at high C rates. In an embodiment, the SEI film gets modified slightly when a thickness, porosity, ionic conductivity, or a combination of any two or more thereof of the SEI film may change up to 10 % during the use of the lithium-ion cell. In another embodiment, the SEI film does not get modified when none of the thickness, porosity, and ionic conductivity of the SEI film changes during the use of the lithium-ion cell.
[032] Generally, the formation process is carried out at a low C-rate and/or involves multiple charge-discharge cycles in order to form a stable SEI film. Such formation process often takes a long time and leads to an SEI film having a low ionic conductivity (i.e., high resistance e.g., > 5 milliohms) when a number of charge-discharge cycles is high. Such SEI film may be detrimental to the cycle life of the lithium-ion cell as it may cause a high impedance, temperature rise, and poor power density. On the other hand, a smaller number of charge-discharge cycles during the formation process may produce an unstable SEI film that may cause degradation of the electrolyte more easily.
[033] Moreover, conventional formation processes may not be suitable for lithium titanate as a negative electrode in a lithium-ion cell due to a higher lithium intercalation potential of 1.55 V of lithium ions to titanate compared to that of lithium ions to graphite. Consequently, the lithium-ion cell is prone to gassing, reduction in capacity, and cycle life attenuation during long use (especially under high C-rate conditions and high-temperature environment). In these cases, the formation process for such lithium-ion cells may be performed at elevated temperatures and extended charge/discharge cut-off voltages. However, such conditions have a detrimental effect on the life of the lithium-ion cell.
[001] Embodiments of the present disclosure provide a formation process of a lithium-ion cell, where the lithium-ion cell is a newly assembled lithium-ion cell. The formation process, as disclosed herein, forms a stable and thin SEI film on the electrode(s) of the lithium-ion cell in a short period of time (e.g., less than 100 hours). Referring now to FIG. 2, a process 200 is depicted via a flow chart, in accordance with some embodiments of the present disclosure. The process 200 involves performing a formation process on a lithium-ion cell (such as the lithium-ion cell 100 of FIG. 1) to obtain a formed lithium-ion cell. In the embodiments, the lithium-ion cell is a newly assembled lithium-ion cell. The term “formed lithium-ion cell”, as used herein and thereafter, refers to a lithium-ion cell that has been obtained after performing a formation process on a newly assembled lithium-ion cell.
[034] The process 200 may include, at step 202, soaking the lithium-ion cell in an electrolyte at a temperature in a range from about 40°C to about 60°C for about 40-72 hours prior to performing the formation process, in some instances.
[035] The process 200 may include performing the formation process on the lithium-ion cell, at step 204. The formation process may be performed at a temperature in a range from about 40°C to about 55°C.
[036] At step 204, the formation process may include performing a first formation cycle on the lithium-ion cell at a first C-rate to obtain a first formed cell, at step 206. It may be noted that the first C-rate may vary from about C/20 to about C/40. In some embodiments, the formation process may include repeating the first formation cycle one or more times, at step 206.
[037] Further, at step 204, the formation process may include performing a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell, at step 208. It may be noted that the second C-rate may vary from about 1C to about 3C. In some embodiments, the formation process may include repeating the second formation cycle one or more times, at step 208.
[038] In some embodiments, the process 200 may include holding the first formed cell at an open circuit for a resting period of a predefined time duration (e.g., 10 minutes) after completing the first formation cycle, at step 206 and prior to performing the second formation cycle, at step 208.
[039] Further, at step 204, the formation process may include performing a third formation cycle on the second formed cell at a third C-rate to obtain a third formed cell, at step 210, in some embodiments. The third C-rate may vary from about 1C to about 3C. It may be noted that the third C-rate is higher than the second C-rate. In some embodiments, the formation process may include repeating the third formation cycle one or more times, at step 210.
[040] In some embodiments, the process 200 may include holding the second formed cell at an open circuit for a resting period of a predefined time duration (e.g., 10 minutes) after completing the second formation cycle, at step 208, and prior to performing the third formation cycle at step 210.
[041] Upon completion of the formation process, as described above with the help of flow chart 200 of FIG. 2, a SEI film may be formed on one or both a positive electrode or a negative electrode of the lithium-ion cell (e.g., the lithium-ion cell 100 of FIG. 1), which may be referred to as the formed lithium-ion cell throughout the present application. The SEI film so formed is stable, thin, porous, compact, heterogeneous, insulating to electrons, and ionically conductive (i.e., conductive for lithium ions). In one embodiment, the thickness of the SEI film so formed on the negative electrode of the formed lithium-ion cell may be in a range from about 5 nanometers to about 20 nanometers. In addition, the SEI film may have a suitable porosity and a high ionic conductivity. The high conductivity of the SEI film may be measured in terms of its resistance. In an embodiment, the resistance of the SEI film is less than 2 milliohms. In some embodiments, the resistance of the SEI film may range from about 1 milliohm to about 1.8 milliohms.
[042] In some embodiments where performing the formation process includes performing the first formation cycle and the second formation cycle, the second formed cell obtained after the completion of the second formation cycle at step 208 is the formed lithium-ion cell. In some other embodiments where performing the formation process includes performing the first formation cycle, the second formation cycle, and the third formation cycle, the third formed cell obtained after the completion of the third formation cycle at step 210 is the formed lithium-ion cell.
[043] Further, the process 200 may include performing an aging process of the formed lithium-ion cell at step 212, in some embodiments. As used herein, the term “aging process” (also referred to as “aging cycle” or “aging”) includes performing a plurality of charge-discharge cycles at the same or different C-rates following a formation process of a battery cell such as a lithium-ion cell. The aging process may include a period of rest time after each of the plurality of charge-discharge cycles. During the aging process, several quality-related parameters are monitored to characterize the battery cell. Additionally, the aging process may enhance the stability of the battery cell. In an embodiment, a SEI film formed on the electrode(s) of the battery cell during the formation process may get modified slightly (e.g., between 1% -5%) during the aging process.
[044] The aging process, at step 212, may be performed at a temperature in a range from about 25°C to about 35°C. The aging process includes performing a predefined number of charge-discharge cycles on the formed lithium-ion cell at a C-rate in a range from about 1C to about 3C. Each of the charge-discharge cycles of the aging process may include charging and discharging the formed lithium-ion cell at the C-rate in a range from about 1C to about 3C.
[045] The SEI film formed on the electrode(s) of the lithium-ion cell during the formation process of the process 200 of FIG. 2 may show high stability during the aging process. During the aging process, the thickness of the SEI film may slightly increase during one or more initial charge-discharge cycles of the aging process and then become stable during the rest of the charge-discharge cycles of the aging process. In other words, the SEI film so formed achieves stability in a smaller number of charge-discharge cycles of the aging process as compared to those formed by conventional processes. Thus, the aging process may be completed in less time and thereby the whole process 200 including the formation process and the aging process may be completed in much less time (e.g., < 100 hours) as compared to that of the conventional processes. In one embodiment, the process 200 may be completed in a duration of time ranging from about 80 hours to about 90 hours.
[046] Referring now to FIG. 3, a detailed process 300 involving performing a formation process on a lithium-ion cell (such as the lithium-ion cell 100 of FIG. 1) is depicted via a flow chart, in accordance with some embodiments of the present disclosure. The lithium-ion cell is a newly assembled lithium-ion cell. The formation process of the process 300 includes performing a first formation cycle, at step 302, and performing a second formation cycle, at step 312. In some embodiments, the formation process may further include performing a third formation cycle, at step 321. Upon completion of performing the formation process on the lithium-ion cell, a formed lithium-ion cell is obtained.
[047] The process 300 may include soaking the lithium-ion cell in an electrolyte at a temperature in a range from about 40 °C to about 60 °C for about 40-72 hours prior to performing the formation process, in some instances.
[048] The process 300 may include performing the first formation cycle on the lithium-ion cell at a first C-rate to obtain a first formed cell, at step 302. It may be noted that the first C-rate may vary from about C/20 to about C/40. To perform the first formation cycle, the process 300 may include operating one or more charge-discharge cycles of the lithium-ion cell. During a charge-discharge cycle of the first formation cycle, the process 300 may first include charging the lithium-ion cell at the first C-rate to attain a first cut-off voltage, at step 304. The first cut-off voltage may be in a range from about 2.5 volts (V) to about 3.5 V. Further, upon attaining the first cut-off voltage, to perform the first formation cycle, the process 300 may include charging the lithium-ion cell at the first cut-off voltage with a first limiting current, at step 306. It should be noted that the first limiting current is less than the first C-rate. Further, after charging the lithium-ion cell at the first cut-off voltage, to perform the charge-discharge cycle of the first formation cycle, the process 300 may include discharging the lithium-ion cell at the first C-rate to attain a second cut-off voltage, at step 308. The second cut-off voltage may be in a range of about 1.4 to about 1.5 V. Further, upon attaining the second cut-off voltage, to perform the first formation cycle, the process 300 may include discharging the lithium-ion cell at the second cut-off voltage with the first limiting current, at step 310. In some embodiments, the process 300 may include repeating the charge-discharge cycle of the first formation cycle one or more times (e.g., two times).
[049] In some embodiments, the process 300 may include holding the first formed cell at an open circuit for a resting period of a predefined time duration (e.g., 10 minutes) after completing the first formation cycle at step 302 and prior to performing the second formation cycle at step 312.
[050] The process 300 may include performing the second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell, at step 312. It may be noted that the second C-rate may vary from about 1C to about 3C. To perform the second formation cycle, the process 300 may include operating one or more charge-discharge cycles of the first formed cell. During a charge-discharge cycle of the second formation cycle, the process 300 may first include charging the first formed cell at a second C-rate to attain a third cut-off voltage, at step 314. The third cut-off voltage may be in a range from about 2.5 V to about 3.5 V. It should be noted that the third cut-off voltage is less than the first cut-off voltage. Further, upon attaining the third cut-off voltage, to perform the second formation cycle, the process 300 may include charging the first formed cell at the third cut-off voltage with a second limiting current, at step 316. It should be noted that the second limiting current is less than the second C-rate and more than the first limiting current. Further, after charging the first formed cell at the third cut-off voltage, to perform the second formation cycle, the process 300 may include discharging the first formed cell at the second C-rate until a fourth cut-off voltage is attained, at step 318. The fourth cut-off voltage may be in a range from about 1.4V to about 1.5V. It should be noted that the fourth cut-off voltage is more than the second cut-off voltage. Further, upon attaining the fourth cut-off voltage, to perform the second formation cycle, the process 300 may include discharging the first formed cell at the fourth cut-off voltage with the second limiting current, at step 320. In some embodiments, the process 300 may include repeating the charge-discharge cycle of the second formation cycle one or more times (e.g., two times).
[051] In some embodiments, the process 300 may include holding the second formed cell at an open circuit for a resting period of a predefined time duration (e.g., 10 minutes) after completing the second formation cycle at step 312 and prior to performing an aging process at step 321, or the third formation cycle, at step 322.
[052] In some embodiments where the process 300 includes performing the first formation cycle at step 302 and the second formation cycle at step 312 to perform the formation process, the second formed cell is the formed lithium-ion cell. In these embodiments, the process 300 may include, at step 321, performing the aging process on the second formed cell. The process 300 may include performing the aging process at a temperature in a range from about 25°C to about 35°C. The aging process may include a predefined number of charge-discharge cycles where each of the charge-discharge cycles may include charging and discharging the second formed cell at a C-rate in a range from about 1C to about 3C.
[053] In some embodiments, the process 300 may include performing the third formation cycle on the second formed cell at a third C-rate to obtain a third formed cell, at step 322. The third C-rate may vary from about 1C to about 3C. It may be noted that the third C-rate is higher than the second C-rate. To perform the third formation cycle, the process 300 may include operating one or more charge-discharge cycles of the second formed cell. During a charge-discharge cycle of the third formation cycle, the process 300 may first include charging the second formed cell at the third C-rate to attain a fifth cut-off voltage, at step 324. The fifth cut-off voltage may be in a range from about 2.5 V to about 3.5 V. It should be noted that the fifth cut-off voltage is less than the first cut-off voltage. Further, upon attaining the fifth cut-off voltage, to perform the third formation cycle, the process 300 may include charging the second formed cell at the fifth cut-off voltage with a third limiting current, at step 326. It should be noted that the third limiting current is less than the third C-rate and more than the second limiting current. Further, after charging the second formed cell at the fifth cut-off voltage, to perform the third formation cycle, the process 300 may include discharging the second formed cell at the third C-rate until a sixth cut-off voltage is attained, at step 328. The sixth cut-off voltage may be in a range from about 1.4 V to about 1.5 V. It should be noted that the sixth cut-off voltage is more than the second cut-off voltage. Further, upon attaining the sixth cut-off voltage, to perform the third formation cycle, the process 300 may include discharging the second formed cell at the sixth cut-off voltage with the third limiting current, at step 330. In some embodiments, the process 300 may include repeating the charge-discharge cycle of the third formation cycle one or more times (e.g., two times). Upon completion of the third formation cycle at step 322, the third formed cell is the formed lithium-ion cell.
[054] In some embodiments, the process 300 may include holding the third formed cell at an open circuit for a resting period of a predefined time duration (e.g., 10 minutes) after completing the third formation cycle at step 322 and prior to performing an aging process, at step 332.
[055] The process 300 may further include, at step 332, performing the aging process on the third formed cell. The process 300 may include performing the aging process at a temperature in a range from about 25°C to about 35°C. The aging process may include a predefined number of charge-discharge cycles where each of the charge-discharge cycles may include charging and discharging the third formed cell at a C-rate in a range from about 1C to about 3C.

EXAMPLES
[056] The following examples are included to describe one or more particular embodiments of the present disclosure. It should be noted that the processes disclosed in the examples that follow merely represent exemplary embodiments of the present disclosure. However, as will be appreciated by those of skill in the art, in light of the present disclosure, changes can be made in the specific embodiments described and still a like or similar result can be obtained without departing from the spirit and scope of the present disclosure.
[057] In the following examples, three lithium-ion cells (Cell 1, Cell 2, and Cell 3) were formed. The positive electrode material of each of the three lithium-ion cells was a lithium-based metal oxide, and the negative electrode material of each of the three lithium-ion cells was LTO. The electrolyte included 1M LiPF6, a solution of ethylene carbonate and ethyl methyl carbonate in a ratio of 3:7, and 2 vol% vinylene carbonate solution. Further, the three lithium-ion cells were kept for soaking at 40 °C over a duration of 40 hours. After the soaking, the three lithium-ion cells were transferred into a temperature-controlled environment chamber set at a temperature of 45 °C. Then, the three lithium-ion cells were separately subjected to three different formation processes that are explained in the following three examples. Each formation process starts with a first formation cycle that includes a slow C-rate formation cycle and ends with a second formation cycle or a third formation cycle that includes a high C-rate formation cycle.

Example 1
[058] Cell 1 was first subjected to a first formation cycle. The first formation cycle corresponds to a slow C-rate formation cycle that includes one or more charge-discharge cycles at a low C-rate (e.g., lower than C/5). During a charge-discharge cycle of the first formation cycle, Cell 1 was first charged at a C-rate of C/20 till a cut-off voltage of 3.05 V was attained. Further, when the cut-off voltage of 3.05 V was attained, Cell 1 was charged at a constant voltage (i.e., 3.05 V) with a limiting current of C/40. After that, Cell 1 was discharged at a C-rate of C/20 till a cut-off voltage of 1.4 V was attained. Further, when the cut-off voltage of 1.4 V was attained, Cell 1 was discharged at a constant voltage (i.e., 1.4 V) with a limiting current of C/40. After completing the charge-discharge cycle, Cell 1 was again subjected to the same charge-discharge cycle of the first formation cycle one more time (i.e., Cell 1 was subjected to two repetitions of the charge-discharge cycles of the first formation cycle). Then, Cell 1 was kept on hold at an open circuit for resting period 1 of about 10 minutes. Upon completion of the first formation cycle and resting period 1, a first formed Cell 1 was obtained.
[059] Further, the first formed Cell 1 is subjected to a second formation cycle. The second formation cycle corresponds to a high C-rate formation cycle that includes one or more charge-discharge cycles at a high C-rate (e.g., > C/2). During a charge-discharge cycle of the second formation cycle, the first formed Cell 1 was charged at a C-rate of 1C till a cut-off voltage of 2.9 V was attained. Further, when the cut-off voltage of 2.9 V was attained, the first formed Cell 1 was charged at a constant voltage (i.e., 2.9 V) with a limiting current of C/10. Further, the first formed Cell 1 was discharged at a C-rate of 1C till a cut-off voltage of 1.5 V was attained. Further, when the cut-off voltage of 1.5 V was attained, the first formed Cell 1 was discharged at a constant voltage (i.e., 1.5 V) with a limiting current of C/10. After completing this charge-discharge cycle, the first formed Cell 1 was again subjected to the same charge-discharge cycle of the second formation cycle one more time (i.e., the first formed Cell 1 was subjected to two repetitions of the charge-discharge cycles of the second formation cycle). Then, the first formed Cell 1 was kept on hold at an open circuit for resting period 2 of about 10 minutes after performing the second formation cycle. Upon completion of the second formation cycle and resting period 2, a formed Cell 1 was obtained.

Example 2
[060] Cell 2 was first subjected to a first formation cycle. The first formation cycle corresponds to a slow C-rate formation cycle that includes one or more charge-discharge cycles at a low C-rate (e.g., lower than C/5). During a charge-discharge cycle of the first formation cycle, Cell 2 was first charged at a C-rate of C/20 till a cut-off voltage of 3.05 V was attained. Further, when the cut-off voltage of 3.05 V was attained, Cell 2 was charged at a constant voltage (i.e., 3.05 V) with a limiting current of C/40. After that, Cell 2 was discharged at a C-rate of C/20 till a cut-off voltage of 1.4 V was attained. Further, when the cut-off voltage of 1.4 V was attained, Cell 2 was discharged at a constant voltage (i.e., 1.4 V) with a limiting current of C/40. After completing the charge-discharge cycle, Cell 2 was again subjected to the same charge-discharge cycle of the first formation cycle one more time (i.e., Cell 2 was subjected to two repetitions of the charge-discharge cycles of the first formation cycle). Then, Cell 2 was kept on hold at an open circuit for resting period 1 of about 10 minutes. Upon completion of the first formation cycle and resting period 1, a first formed Cell 2 was obtained.
[061] Further, the first formed Cell 2 was subjected to a second formation cycle. The second formation cycle corresponds to a high-rate formation cycle that includes one or more charge-discharge cycles at a high C-rate (e.g., > C/2). During a charge-discharge cycle of the second formation cycle, the first formed Cell 2 was charged at a C-rate of 1C till a cut-off voltage of 2.9 V was attained. Further, when the cut-off voltage of 2.9 V was attained, the first formed Cell 2 was charged at a constant voltage (i.e., 2.9 V) with a limiting current of C/10. Further, the first formed Cell 2 was discharged at a C-rate of 1C till a cut-off voltage of 1.5 V was attained. Further, when the cut-off voltage of 1.5 V was attained, the first formed Cell 2 was discharged at a constant voltage (i.e., 1.5 V) with a limiting current of C/10. After completing this charge-discharge cycle, the first formed Cell 2 was again subjected to the same charge-discharge cycle of the second formation cycle one more time (i.e., the first formed Cell 2 was subjected to two repetitions of the charge-discharge cycles of the second formation cycle). Then, the first formed Cell 2 was kept on hold at an open circuit for resting period 2 of about 10 minutes after performing the second formation cycle. Upon completion of the second formation cycle and resting period 2, a second formed Cell 2 was obtained.
[062] Further, a third formation cycle was performed on the second formed Cell 2. The third formation cycle corresponds to another high-rate formation cycle that includes one or more charge-discharge cycles at a high C-rate (e.g., > C/2). During a charge-discharge cycle of the third formation cycle, the second formed Cell 2 was charged at a C-rate of 3C till a cut-off voltage of 2.9 V was attained. Further, when the cut-off voltage of 2.9 V was attained, the second formed Cell 2 was charged at a constant voltage (i.e., 2.9 V) with a limiting current of C/3. Further, the second formed Cell 2 was discharged at a C-rate of 3C till a cut-off voltage of 1.5 V was attained. Further, when the cut-off voltage of 1.5 V was attained, the second formed Cell 2 was discharged at a constant voltage (i.e., 1.5 V) with a limiting current of C/3. After completing this charge-discharge cycle, the second formed Cell 2 was again subjected to the same charge-discharge cycle of the third formation cycle one more time (i.e., the second formed Cell 2 was subjected to two repetitions of the charge-discharge cycles of the third formation cycle). Then, the second formed Cell 2 was kept on hold at an open circuit for resting period 3 of about 10 minutes after performing the third formation cycle. Upon completion of the third formation cycle and resting period 3, a formed Cell 2 was obtained.

Example 3
[063] Cell 3 was first subjected to a first formation cycle. The first formation cycle corresponds to a slow C-rate formation cycle that includes one or more charge-discharge cycles at a low C-rate (e.g., lower than C/5). During a charge-discharge cycle of the first formation cycle, Cell 3 was first charged at a C-rate of C/20 till a cut-off voltage of 3.05 V was attained. Further, when the cut-off voltage of 3.05 V was attained, Cell 3 was charged at a constant voltage (i.e., 3.05 V) with a limiting current of C/40. After that, the Cell 3 was discharged at a C-rate of C/20 till a cut-off voltage of 1.4 V was attained. Further, when the cut-off voltage of 1.4 V was attained, Cell 3 was discharged at a constant voltage (i.e., 1.4 V) with a limiting current of C/40. After completing the charge-discharge cycle, Cell 3 was again subjected to the same charge-discharge cycle of the first formation cycle one more time (i.e., Cell 3 was subjected to two repetitions of the charge-discharge cycles of the first formation cycle). Then, Cell 3 was kept on hold at an open circuit for resting period 1 of about 10 minutes. Upon completion of the first formation cycle and resting period 1, a first formed Cell 3 was obtained.
[064] Further, the first formed Cell 3 was subjected to a second formation cycle. The second formation cycle corresponds to a higher rate formation cycle that includes one or more charge-discharge cycles at a high C-rate (e.g., > C/2). During a charge-discharge cycle of the second formation cycle, the first formed Cell 3 was charged at a C-rate of 3C till a charge cut-off voltage of 2.9 V was attained. Further, when the cut-off voltage of 2.9 V was attained, the first formed Cell 3 was charged at a constant voltage (i.e., 2.9 V) with a limiting current of C/3. Further, the first formed Cell 3 was discharged at a C-rate of 3C till a cut-off voltage of 1.5 V was attained. Further, when the cut-off voltage of 1.5 V was attained, the first formed Cell 3 was discharged at a constant voltage (i.e., 1.5 V) with a limiting current of C/3. After completing this charge-discharge cycle, the first formed Cell 3 was again subjected to the same charge-discharge cycle of the second formation cycle one more time (i.e., the first formed Cell 3 was subjected to two repetitions of the charge-discharge cycles of the second formation cycle). Then, the first formed Cell 3 was kept on hold at an open circuit for resting period 2 of about 10 minutes after performing the second formation cycle. Upon completion of the second formation cycle and resting period 2, a formed Cell 3 was obtained.
[065] After performing three different formation processes on Cell 1, Cell 2, and Cell 3 as described in Example 1, Example 2, and Example 3, three respective formed cells – formed Cell 1, formed Cell 2 and formed Cell 3 were obtained. Each of formed Cell 1, formed Cell 2, and formed Cell 3 was subjected to an aging process, separately. The aging process was performed at an ambient temperature of 28°C. During the aging process, each of formed Cell 1, formed Cell 2, and formed Cell 3 was subjected to 100 charge-discharge cycles. During the aging process, a resistance (SEI film resistance) of each of the SEI films on the negative electrodes of formed Cell 1, formed Cell2, and formed Cell 3 was monitored by an AC impedance analysis at every 10-cycle interval at 50% Depth of Discharge (DoD). Frequency in a range from 1000 to 0.1 Hz was used for an AC impedance analyzer with a fixed 0.050 RMS amplitude current.
[066] Referring now to FIG. 4, a graph 400 represents SEI film resistance of SEI films formed on the respective negative electrodes of formed Cell 1, formed Cell 2, and formed Cell 3 over the charge-discharge cycles of the aging process.
[067] The graph 400 is a scatter plot with an x-axis representing a number of charge-discharge cycles of the aging process and a y-axis representing the SEI film resistance corresponding to formed Cell 1, formed Cell 2, and formed Cell 3. The SEI film resistance was calculated in a frequency region between about 1000 Hz and about 11 Hz for each of formed Cell 1, formed Cell 2, and formed Cell 3 corresponding to Example 1, Example 2, and Example 3 respectively. Higher values of SEI film resistance were observed in the graph 400 for formed Cell 2 (Example 2) and formed Cell 3 (Example 3) that were subjected to the formation processes including the last formation cycle of a high C-rate (i.e., 3C) in Example 2 and Example 3, respectively, as compared to that of formation cell 1 (Example 1) that was subjected to the formation process including the last formation cycle at a lower C-rate (i.e., 1C) in Example 1. The higher values of SEI resistance for Example 2 and Example 3 indicate the development of SEI films in formed Cell 2 and formed Cell 3.
[068] Stabilization of the SEI film corresponding to each of formed Cell 1, formed Cell 2, and formed Cell 3 during the respective aging processes was further examined. Each SEI film was studied using non-destructive Electrochemical Impedance Spectroscopy (EIS). During the aging processes, characteristics such as a thickness of the SEI film were qualitatively predicted by Nyquist plot. A Nyquist plot is a graph that plots the impedance values of a battery measured at multiple frequencies. The Nyquist plot is generally used to evaluate the formation of an SEI film on the electrodes of the battery. In the Nyquist plot, an imaginary component of an impedance of the battery at the y-axis is plotted against a real component of the impedance of the battery.
[069] Referring now to FIG. 5, a graph 500 represents Nyquist plots of formed Cell 1, formed Cell 2, and formed Cell 3, corresponding to Example 1, Example 2, and Example 3, respectively, at the 20th charge-discharge cycle (or 20th cycle) of respective aging processes. In graph 500, the x-axis represents a real part (Re (Z)) of an impedance (Z) and the y-axis represents an imaginary part (Im (Z)) of the impedance.
[070] As shown in graph 500, a depressed semicircle corresponding to formed Cell 1 of Example 1 was observed at the 20th cycle of the aging process, indicating negligible formation of an SEI film corresponding to formed Cell 1. A semicircle (not shown in Figures) was intensively detected corresponding to formed Cell 1 at the 50th cycle of the aging process. In contrast, a semicircle (a sharp semicircular bend near zero resistance) corresponding to each of formed Cell 2 of Example 2 and formed Cell 3 of Example 3 in graph 500 was observed, which indicated the presence of a stable SEI film corresponding to formed Cell 2 and formed Cell 3 at the 20th cycle of the aging process. It was further observed that there was no further increase in the SEI film resistance corresponding to each of formed Cell 2 of Example 2 and formed Cell 3 of Example 3 after the 20th cycle over the remaining cycles of the aging process. No further increase in the SEI resistance after the 20th cycle implied that a stable SEI film was obtained for each of formed Cell 2 and formed Cell 3 at the 20th cycle of the aging process, and there was no need to perform more charge-discharge cycles of the aging process. Thus, it can be concluded from graph 500 that the aging process accelerates at a much faster rate for the formation processes described in Example 2 and Example 3 (where the last formation cycle was a charge-discharge cycle at a 3C rate), compared to the formation process described in formed Cell 1 of Example 1 (where the last formation cycle is a 1C charge-discharge cycle) and hence takes a shorter time.
[071] Thus, embodiments of the present application help in overcoming the technical problem of obtaining a stable Solid Electrolyte Interface (SEI) film on the electrode(s) e.g., a negative electrode of a lithium-ion cell over a long period of time. The disclosed formation process enables the formation of a stable SEI film in a short period of time which sustains various operating temperatures and charge-discharge cycles at high C-rates throughout the life of a lithium-ion cell. Performing the formation process at a high temperature and at a combination of low C-rate and high C- rate charge-discharge cycles, as disclosed in the present application, drastically reduces the duration of the formation process and the aging process. In summary, the disclosed process enables achieving a stable SEI film with high ionic conductivity in a short period of time (<100 hours). As the whole process takes less time, it helps in decreasing the amount of electric energy consumption and an area accommodation of the lithium-ion cells during the process.
[072] As will be appreciated by those skilled in the art, the techniques described in the various embodiments discussed above are not routine, conventional, or well-understood in the art. The techniques discussed above provide for processes of formation of lithium-ion cells. The techniques may first perform a formation process on a lithium-ion cell to obtain a formed lithium-ion cell. Performing the formation process may include performing a first formation cycle on the lithium-ion cell at a first C-rate to obtain a first formed cell. The first C-rate may vary from about C/20 to about C/40. Performing the formation process may further include performing a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell. The second C-rate may vary from about 1C to about 2C.
[073] In light of the above-mentioned advantages and the technical advancements provided by the disclosed processes, the claimed steps as discussed above are not routine, conventional, or well understood in the art, as the claimed steps enable the following solutions to the existing problems in conventional technologies. Further, the claimed steps clearly bring an improvement in the functioning of the device itself as the claimed steps provide a technical solution to a technical problem.
[074] The specification has described the formation of lithium-ion cells. The illustrated steps are set out to explain the exemplary embodiments shown, and it should be anticipated that ongoing technological development will change the manner in which particular functions are performed. These examples are presented herein for purposes of illustration, and not limitation. Further, the boundaries of the functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternative boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the disclosed embodiments.
[075] It is intended that the disclosure and examples be considered exemplary only, with a true scope and spirit of disclosed embodiments being indicated by the following claims.
, Claims:CLAIMS
WE CLAIM:
1. A process (200) comprising:
performing (204) a formation process on a lithium-ion cell (100) to obtain a formed lithium-ion cell, wherein performing (204) the formation process comprises:
performing (206) a first formation cycle on the lithium-ion cell (100) at a first C-rate to obtain a first formed cell, wherein the first C-rate varies from about C/20 to about C/40; and
performing (208) a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell, wherein the second C-rate varies from about 1C to about 3C.

2. The process (200) as claimed in claim 1, wherein the performing (206) the first formation cycle comprises:
charging the lithium-ion cell (100) at the first C-rate to attain a first cut-off voltage, wherein the first cut-off voltage is in a range from about 2.5 V to about 3.5 V;
upon attaining the first cut-off voltage, charging the lithium-ion cell (100) at the first cut-off voltage with a first limiting current, wherein the first limiting current is less than the first C-rate;
after charging the lithium-ion cell (100) at the first cut-off voltage, discharging the lithium-ion cell (100) at the first C-rate to attain a second cut-off voltage; and
upon attaining the second cut-off voltage, discharging the lithium-ion cell (100) at the second cut-off voltage with the first limiting current.

3. The process (200) as claimed in claim 2, wherein the performing (208) the second formation cycle comprises:
charging the first formed cell at a second C-rate to attain a third cut-off voltage, wherein the third cut-off voltage is in a range from about 2.5 V to about 3.5 V, and wherein the third cut-off voltage is less than the first cut-off voltage;
upon attaining the third cut-off voltage, charging the first formed cell at the third cut-off voltage with a second limiting current, wherein the second limiting current is less than the second C-rate and more than the first limiting current;
after charging the cell at the third cut-off voltage, discharging the first formed cell at the second C-rate until a fourth cut-off voltage is obtained; and
upon attaining the fourth cut-off voltage, discharging the first formed cell at the fourth cut-off voltage with the second limiting current.

4. The process (200) as claimed in claim 1, further comprising soaking (202) the lithium-ion cell (100) in an electrolyte at a temperature in a range from about 40 °C to about 60 °C prior to performing the formation process.

5. The process (200) as claimed in claim 1, wherein the formation process is performed at a temperature in a range from about 40 °C to about 55 °C.

6. The process (200) as claimed in claim 1, wherein the performing (204) the formation process further comprises performing (210) a third formation cycle on the second formed cell at a third C-rate to obtain a third formed cell, wherein the third C-rate is higher than the second C-rate.

7. The process (200) as claimed in claim 6, wherein the third C-rate varies from about 1C to about 3C.

8. The process (200) as claimed in claim 3, wherein the performing (204) the formation process further comprises performing (210) a third formation cycle on the second formed cell at a third C-rate to obtain a third formed cell, wherein the third C-rate is higher than the second C-rate and the performing (210) the third formation cycle comprises:
charging the second formed cell at the third C-rate to attain a fifth cut-off voltage, wherein the fifth cut-off voltage is in a range from about 2.5 V to about 3.5 V, and wherein the fifth cut-off voltage is less than the first cut-off voltage;
upon attaining the fifth cut-off voltage, charging the second formed cell at the fifth cut-off voltage with a third limiting current, wherein the third limiting current is less than the third C-rate and more than the second limiting current;
after charging the second formed cell at the fifth cut-off voltage, discharging the second formed cell at the third C-rate until a sixth cut-off voltage is obtained; and
upon attaining the sixth cut-off voltage, discharging the second formed cell at the sixth cut-off voltage with the third limiting current.

9. The process (200) as claimed in claim 1, further comprising performing (212) an aging process on the formed lithium-ion cell.

10. A process (300), comprising:
performing (302) a first formation cycle on the lithium-ion cell (100) at a first C-rate to obtain a first formed cell, wherein the first C-rate varies from about C/20 to about C/40 and wherein the performing the first formation cycle comprises:
charging (304) the lithium-ion cell (100) at the first C-rate to attain a first cut-off voltage, wherein the first cut-off voltage is in a range from about 2.5 V to about 3.5 V; and
upon attaining the first cut-off voltage, charging (306) the lithium-ion cell (100) at the first cut-off voltage with a first limiting current, wherein the first limiting current is less than the first C-rate;
after charging the lithium-ion cell (100) at the first cut-off voltage, discharging (308) the lithium-ion cell (100) at the first C-rate to attain a second cut-off voltage; and
performing (312) a second formation cycle on the first formed cell at a second C-rate to obtain a second formed cell, wherein the second C-rate varies from about 1C to about 3C and wherein the performing the second formation cycle comprises:
charging (314) the first formed cell at the second C-rate to attain a third cut-off voltage, wherein the third cut-off voltage is in a range from about 2.5 V to about 3.5 V, and the third cut-off voltage is less than the first cut-off voltage; and
upon attaining the third cut-off voltage, charging (316) the first formed cell at the third cut-off voltage with a second limiting current, wherein the second limiting current is less than the second C-rate and more than the first limiting current; and
after charging the first formed cell at the third cut-off voltage, discharging (318) the first formed cell at the second C-rate to attain a fourth cut-off voltage.

11. The process (300) as claimed in claim 10, further comprising performing (321) an aging process on the second formed cell, wherein performing the aging process comprises performing a predefined number of charge-discharge cycles at a C-rate in a range from about 1C to about 3C.

12. The process (300) as claimed in claim 10, further comprising performing (322) a third formation cycle on the second formed cell at a third C-rate to obtain a third formed cell, wherein the third C-rate varies from about 1C to about 3C and is higher than the second C-rate, and wherein the performing the third formation cycle comprises:
charging (324) the second formed cell at the third C-rate to attain a fifth cut-off voltage, wherein the fifth cut-off voltage is between about 2.5 V to about 3.5 V, and wherein the fifth cut-off voltage is less than the first cut-off voltage;
upon attaining the fifth cut-off voltage, charging (326) the second formed cell at the fifth cut-off voltage with a third limiting current, wherein the third limiting current is less than the third C-rate and more than the second limiting current; and
after charging the second formed cell at the fifth cut-off voltage, discharging (328) the second formed cell at the third C-rate to attain a sixth cut-off voltage.

13. The process (300) as claimed in claim 12, further comprising performing (332) an aging process on the third formed cell, wherein performing the aging process comprises performing a predefined number of charge-discharge cycles at a C-rate in a range from about 1C to about 3C.