Description:FIELD OF THE DISCLOSURE
Various embodiments of the disclosure relate generally to energy storage devices. More specifically, various embodiments of the disclosure relate to a formation process for lithium-based energy storage devices or lithium-ion batteries, and batteries formed using the formation process.

DESCRIPTION OF THE RELATED ART
Lithium-ion batteries (LIBs) are the most widely used power storage and generation devices due to their comprehensive superiority in power density, energy density, cost, and safety. To meet the ever-growing demand for present and future applications, efforts are being made to further improve the characteristics of LIBs.
A LIB is composed of a cathode, an anode, and an electrolyte. In a typical LIB, the cathode and anode are formed as sheets by coating a slurry, comprising electrode active material, on current collectors and often may include a separator placed between the two electrodes. The assembly of an LIB 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 may start to react with the electrode materials, causing corrosion. Typically, the LIBs are pre-charged to a very low voltage (approximately 100mV) to prevent their corrosion.
After assembly, the LIB is subjected to “formation” or “activation” process to guarantee high performance, longer life-span, and safety throughout its term. It is estimated that the formation process of the battery roughly entails more than a third of the overall cost of battery manufacturing and is one of the most expensive processes in battery manufacturing. The formation step is time-consuming and hence reducing time and cost is essential.
Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.

SUMMARY OF INVENTION

In an embodiment of the present disclosure, formation process for a lithium-ion battery is provided. The lithium-ion battery includes an anode, a cathode and an electrolyte. The method comprises performing a first charging cycle of the battery, wherein the first charging cycle comprises charging at a constant current to attain a maximum safe voltage and switching to charging at a constant voltage corresponding to the maximum safe voltage to obtain a first charged battery. The method further comprises maintaining the first charged battery at a temperature in the range of 25℃ to 95 ℃ for a time in the range of 2 to 8 hours to form a rested battery. The method further comprises performing a second charging cycle of the rested battery after bringing it to an ambient temperature to an upper voltage limit of the battery to obtain a second charged battery, wherein the second charging is performed in multiple steps with incremental voltage increase. The method further comprises performing a final charging cycle of the second charged battery to a maximum voltage limit to form the lithium-ion battery.
In another embodiment, a lithium-ion battery formed by a method, according to an exemplary embodiment of the disclosure, is provided. The method comprises performing a first charging cycle of the battery, wherein the first charging cycle comprises charging at a constant current to attain a maximum safe voltage and switching to charging at a constant voltage corresponding to the maximum safe voltage to obtain a first charged battery. The method further comprises maintaining the first charged battery at a temperature in the range of 25℃ to 95 ℃ for a time in the range of 2 to 8 hours to form a rested battery. The method further comprises performing a second charging cycle of the rested battery after bringing it to an ambient temperature to an upper voltage limit of the battery to obtain a second charged battery, wherein the second charging is performed in multiple steps with incremental voltage increase. The method further comprises performing a final charging cycle of the second charged battery to a maximum voltage limit to form the lithium-ion battery.
These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of an energy storage device, in accordance with an exemplary embodiment of the present disclosure;
FIG. 2 is a flow chart of a method for formation of the lithium-ion battery (LIB) of FIG.1, in accordance with an exemplary embodiment of the present disclosure;
FIG. 3 is a plot of voltage (V) versus time (t) during formation of the LIB of FIG.1, in accordance with an exemplary embodiment of the present disclosure;
FIG. 4 includes a plot of battery capacity versus number of cycles (cycle number) and a plot of capacity retention versus C-Rate of LIBs, namely, a LIB formed using a conventional method and the LIB formed using the method as illustrated in FIG.2; and
FIG. 5 is a plot of efficiency versus voltage of the LIB formed using the method illustrated in FIG.2.
Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS
The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or method steps.
All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
As used herein, the term “lithium-ion based energy storage device” or “lithium-ion battery” (LIB) may refer to any conventional lithium-ion battery that has an anode, a cathode, a separator, electrolyte, and two current collectors. Example of LIBs may further include anode-free LIBs, lithium-ion polymer batteries, batteries with liquid electrolytes, and solid-state batteries.
As used herein, the terms “cathode” and “anode” may refer to the electrodes of a battery (interchangeably referred to as an energy storage device). During a charge cycle in a Li-ion battery, Li ions migrate from the cathode towards the anode through an electrolyte, while the electrons migrate from the cathode towards the anode via an external circuit. During a discharge cycle in the Li-ion battery, Li ions migrate from the anode towards the cathode through the electrolyte, while the electrons migrate from the anode towards the cathode via the external circuit.
As used herein, the term “electrolyte” may refer to a material that allows ions, for example, Li ions, to migrate therethrough, but does not allow electrons to conduct therethrough.
As used herein, the term “current collector” may refer to a bridging component that collects electrical current generated at the electrodes and connects with external circuits.
As used herein, the term “energy density” can be defined as the amount of energy stored for a given unit weight of a battery material. It is expressed as watt hours per kilogram (Wh/kg).
The term “specific capacity” corresponds to the amount of electric charge (milliampere hours (mAh)) the material can deliver per gram of material. It is used to describe the performance of an electrode and is expressed as mAh per gram (mAh/g).
As used herein, the term “coulombic efficiency” is defined as the ratio of discharge capacity after full charge of the battery to the charging capacity of the same cycle. It describes the released battery capacity and is usually expressed as a fraction, i.e., it has a value of 1 or less than 1.
As used herein, the term “cycle life” refers to number of discharge-charge cycles (cycle number or number of cycles) the battery can undergo before it fails to meet the specific performance criteria. Cycle life is typically estimated under specific charge and discharge conditions.
As used herein, the term “at constant voltage (CV)” refers to a charging mode that allows current to flow into the battery from a charger until it reaches its pre-set voltage. The current will then taper down to a minimum value once that voltage level is reached.
As used herein, the term “at constant current (CC)” refers to a charging mode with the current level set at a fraction of the maximum battery capacity. In this charging mode, there is a possibility that the battery may overheat if it is over-charged, leading to premature battery replacement. The battery must be disconnected, or a timer function used once charged.
The term “at constant voltage / constant current (CVCC)” is a combination of the CV and CC charging modes. The charger limits the amount of current to a pre-set level until the battery reaches a pre-set voltage level. The current then reduces as the battery becomes fully charged. A regulated current raises the terminal voltage until the upper charge voltage limit is reached, at which point the current drops due to saturation.
As used herein, the term “at constant current / constant voltage (CCCV)” is a combination of the CC and CV charging modes. The battery is charged at a constant current until the battery reaches a pre-set voltage level typically at a C-rate of C/3 to C/5 followed by charging at constant voltage until the current decays off to a value of C/20 to C/50 C-rates.
As used herein, the term “battery capacity" is a measure of the charge stored by the battery and is determined by the mass of active material contained in the battery. It is expressed in ampere hours (Ah).
As used herein, the term “charge rate” or “C-rate” refers to the charge and/or discharge rate of the LIB. It is defined by the charge and discharge current relative to the battery capacity. A fully charged battery having a capacity of 10Ah, ideally, can charge or discharge 10A current for 1 hour and has a C-rate of 1C. The same battery, ideally, can discharge or charge 5A current for two hours which is expressed as having a C-rate of C/2 (0.5C). At a C-rate of 2C, the battery capacity of 10Ah can ideally charge or discharge 20A in 30 minutes. However, at higher C-rate there is loss due to heat generation and hence the battery capacity is lowered.
The formation of a LIB is the process of performing the initial charging cycle or charge/discharge operation on the battery. The term “formation” is also otherwise termed as “activation” of the LIB. During this process, solid electrolyte interphase (SEI) is formed at the electrode, mainly on an anode. Similarly, an electrolyte interphase may be formed at the cathode, which is otherwise known as cathode electrolyte interphase (CEI) layer. The SEI or CEI layers are mainly formed during the formation process and they grow continuously over the lifetime of the LIB. The formation of stable SEI and CEI layers is beneficial as they can act as a protective film around the respective electrodes, thus preventing electrode corrosion and electrolyte decomposition. The SEI and CEI layers are sensitive to many different factors and can significantly impact the performance of LIB over its lifetime. The LIB aging behavior is mainly influenced by the initial SEI and CEI formation and their growth over the lifetime of the LIB. The interphase layers, namely SEI and CEI, if any, need to form in a controlled manner through reactions with the electrolyte and additives, such that the outermost active electrode material portions passivate and do not further react with the electrolyte.
FIG. 1 is a schematic diagram that illustrates an energy storage device 100, in accordance with an embodiment of the present disclosure. In an embodiment of the present disclosure, the energy storage device 100 may be a lithium-based energy storage device, such as, for example, a lithium-ion battery (LIB), a lithium-ion polymer battery, or the like.
The energy storage device 100 comprises a cathode current collector 102 and an anode current collector 104. The cathode current collector 102 is coated with an electroconductive slurry 105a comprising a cathode active material to form a cathode 106. An anode 108 is formed by coating an electroconductive slurry 105b comprising an anode active material on the anode current collector 104. Examples of current collectors include, but are not limited to, aluminum, nickel, titanium, stainless steel, carbonaceous material, or copper. The current collectors can be in the form of foil, mesh, or foam. It is preferred, for the anode 108, to use a carbon-based current collector and for the cathode 106 to use an aluminum current collector. The electroconductive slurry 105a and the electroconductive slurry 105b may be prepared according to methods known in the art.
Examples of the anode active material may include, but are not limited to, graphite, SiC nanocomposites, lithium titanium oxides such as LiTiO2, Li4Ti5O12, Sn particulates, and Si particulates. In one embodiment of the present disclosure, the anode 108 comprises lithium titanium oxide (LTO). Examples of the cathode active material may include, but are not limited to, lithium metal oxides such as LMO (lithium manganese oxide), Li-NCA (lithium nickel cobalt aluminum oxide), Li-NMC (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 energy storage device 100 includes an electrolyte 110 that allows lithium ions to migrate therethrough and a separator 112 to separate the cathode 106 and the anode 108. The electrolyte 110 may be a solid electrolyte or a liquid electrolyte. Non-limiting examples of the electrolyte 110 may include LiPF6, LiAsF6, LiBOB, LiClO4, LiBF4, and LiPF6.
Although the energy storage device 100 is shown to include the anode 108, in other embodiments, the energy storage device 100 may be an anode-free battery without deviating from the scope of the disclosure.
FIG. 2 is a flow chart 200 that illustrates a method for forming the lithium-ion battery 100 of FIG.1, in accordance with an embodiment of the present disclosure. Referring to FIG. 2, the flow chart 200 illustrates exemplary operations 202 through 208 for forming the LIB 100.
At step 202, a first charging cycle is performed. The term “charging cycle”, as used herein, refers to charging of a battery (as indicated by a rise in voltage in a voltage versus time plot) followed by discharging of the battery (as indicated by a drop in voltage in a voltage versus time plot) to complete a cycle. The first charging cycle comprises charging at a constant current to attain a maximum safe voltage and switching to charging at a constant voltage corresponding to the maximum safe voltage to obtain a first charged battery. In other words, the first charging cycle is performed in CCCV mode. The maximum safe voltage is arrived at based on the material comprising the anode 108 and the cathode 106. In one embodiment, where the anode 108 comprises LTO, the maximum safe voltage is in a range of 1.8V to 2.2 V. In one embodiment, the maximum safe voltage for the anode 108 comprising LTO is 2.0 V.
The first charging cycle at step 202 is performed at a C-rate in the range of C/4 to C/6. The optimization of C-rate and the amount of current to be passed at step 202, is decided based on the electrode material requirement for optimal interphase layer formation, namely SEI and CEI.
FIG. 3 is a plot 300 that illustrates the voltage (V) versus time (t) along the formation process of the LIB 100. Referring to FIG.3, a curve 302 corresponds to the first charging cycle, of step 202. The first charging cycle is performed at the constant current mode to attain the maximum safe voltage of 2.0 V, at which point the current is turned off and the charging is continued at constant voltage mode to attain the voltage of more than 2.15 V. The charging of the LIB is followed by discharging as shown in the curve 302. The first charging cycle is performed at a C-rate of C/5 in the plot 300.
At step 204, the first charged battery is maintained at a temperature in the range of 25℃ to 95℃ and for a defined time to obtain a rested battery. In one embodiment, where the anode 108 comprises LTO, the temperature is 80℃. The first charged battery is maintained at the temperature for the defined time in the range of 2 to 8 hours. In one embodiment, where the anode 108 comprises LTO, the first charged battery is maintained at 80℃ for a time of up to 6 hours. In one embodiment, the first charged battery is maintained in an oven or an environmental chamber. Any suitable ovens as known in the art may be utilized for this purpose.
The step 204 is a critical step in the formation process. During the step 204, the electrolyte 110 remains in intimate contact with the anode 108 and the cathode 106. As the step 204 is performed at elevated temperatures, it helps in reducing electrolyte contact angle and improves “wetting” of the electrodes (106 and 108) by the electrolyte 110 whereby the electrolyte 110 penetrates through pores of the electrodes (106 and 108). The step 204 helps in the complete formation of SEI and CEI layers.
At step 206, a second charging cycle is performed after bringing the rested battery to the ambient temperature to obtain a second charged battery. The second charging cycle is performed in CCCV mode. The ambient temperature corresponds to the room temperature, in one instance. In one embodiment, the ambient temperature is a temperature in the range of 25℃ to 35℃.
During the second charging cycle, at step 206, the rested battery is brought to an upper voltage limit of the battery. The upper voltage limit of an LIB is specific to the LIB and depends on the material and make of the LIB. The upper voltage limit can be derived from a theoretical potential difference between the cathode 106 and the anode 108. In one embodiment, for an anode 108 comprising LTO, the upper voltage limit is in the range of 2V to 3V. In one embodiment, the upper voltage limit is 2.3V for a LIB comprising LTO as the anode 108.
The second charging cycle, at step 206, is performed in multiple steps with incremental voltage increase. The incremental voltage includes voltage increments in the range of 0.1V to 0.5V. In one embodiment, the second charging cycle, at step 206, is performed in multiple steps with a voltage increment of 0.2V. The voltage increments of 0.2V are achieved by increasing the voltage by 0.2V from the previous voltage value.
Referring to FIG. 3, a curve 304 corresponds to the step 206. In the plot 300, the voltage increment is 0.2V i.e., the voltage is increased from the previous voltage value by 0.2V. As shown in FIG.3, the voltage is raised to above 2.35V in the first step, to about 2.55V in the second step, to about 2.75V in the third step, and in the final step, the voltage reaches to about 2.95V.
In one embodiment, the multiple steps involved in the second charging cycle (of step 206) correspond to number of steps in the range of 3 to 5. Each step of the multiple steps of the second charging cycle is performed at a C-rate in the range of C/2 to C/4. In a preferred embodiment, the second charging cycle, at step 206, is performed at a C-rate of C/3.
At step 208, a final charging cycle of the second charged battery is performed to a maximum voltage limit to form the LIB 100. The final charging cycle is performed in CCCV mode. The step 208 helps in complete charging of the LIB 100. The “maximum voltage limit”, as used herein, refers to the maximum voltage rating of a given LIB. The maximum voltage limit, as the name suggests is the maximum voltage achievable by a LIB. In practice, the maximum voltage limit is arrived at by conducting several charge cycles at CCCV mode on a particular LIB by which the maximum voltage attainable by the LIB is realized. In one embodiment, the maximum voltage limit is in the range of 2.8V to 3.4V for an anode 108 comprising LTO.
Referring to FIG.3, a curve 306 corresponds to the step 208. In the plot 300, the voltage is increased to the maximum voltage limit, in this instance to about 3V, at a C-rate of C/5. The lower C-rate value corresponds to low current and the step 208 is performed over an extended period of time.
In one embodiment, the step 208, is performed at a C-rate in the range of C/4 to C/6. In one embodiment, where the anode 108 comprises LTO, the step 208, is performed at a C-rate of C/5.
The formation process, as illustrated by the flow chart 200, for performing operations 202 through 208 takes about 40 hours. In one embodiment, the time for performing the method for formation is less than about 48 hours, more preferably less than 40 hours and most preferably less than 30 hours.
The formation process of the LIB 100, as illustrated by flow chart 200 takes less time as compared to conventional formation processes. In one example conventional method, the battery is subjected to multiple charging cycles to a specific voltage at a C-rate of C/20. For comparison, a LIB was formed using the above-mentioned formation process and the formation process took about 200 hours.
The LIB 100 formed was found to have enhanced electrochemical stability in terms of cycle life when compared to the LIB formed by following the conventional formation process. Referring to FIG. 4A, a plot 400 of battery capacity versus number of cycles (cycle number) is shown. A plot 402 corresponds to the LIB formed using the conventional formation process and a plot 404 refers to the LIB formed according to the method as illustrated by flow chart 200. The capacity of the battery (plot 404) remains more or less unchanged from the first cycle to even up to 120 cycles. This confirms the superiority of the formation method or formation protocol of the present disclosure as compared to conventional formation process.
The LIB 100 formed according to the method as illustrated in FIG.2 has better rate capability when compared to LIB formed using the conventional formation process. FIG. 4B includes a plot 410 of capacity retention versus C-Rate that indicates the rate capability of the LIBs. A LIB with high rate capability is able to generate a considerable amount of power, that is, it suffers very little voltage loss even at high current loads and is considered to be superior in performance. A dotted plot 412 shows the rate capability of LIB formed according to the conventional formation process. Dotted plot 414 depicts the rate capability of the LIB 100 formed according to the method as illustrated in FIG.2. The capacity retention of the LIB 100 was found to be better than LIB formed by conventional formation process. The method, as illustrated by flow chart 200, for the formation of the LIB 100 shows higher efficiency when compared to conventional methods for formation. FIG. 5 is a plot 500 of coulombic efficiency (CE) versus voltage of the LIB 100. A curve 502 of the plot 500 indicates the rate at which the CE of the LIB 100 was achieved. The faster rate at which the CE was achieved is indicative of the superior performance of the LIB 100 formed according to embodiments of the present disclosure.
It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims.

 
, Claims:CLAIMS
We claim,

1. A method for formation of a lithium-ion battery comprising an anode, a cathode and an electrolyte, the method comprising:
performing a first charging cycle of the battery, wherein the first charging cycle comprises charging at a constant current to attain a maximum safe voltage and subsequently charging at a constant voltage corresponding to the maximum safe voltage to obtain a first charged battery;
maintaining the first charged battery at a temperature in the range of 25℃ to 95 ℃ for a time in the range of 2 to 8 hours to form a rested battery;
performing a second charging cycle of the rested battery after bringing it to an ambient temperature to an upper voltage limit of the battery to obtain a second charged battery, wherein the second charging is performed in multiple steps with incremental voltage increase; and
performing a final charging cycle of the second charged battery to a maximum voltage limit to form the lithium-ion battery.

2. The method as claimed in claim 1, wherein the first charging cycle is performed at a C-rate in the range of C/4 to C/6.

3. The method as claimed in claim 1, wherein maintaining the first charged battery comprises maintaining the first charged battery in an oven.

4. The method as claimed in claim 1, wherein the incremental voltage comprises voltage increments in the range of 0.1V to 0.5V.

5. The method as claimed in claim 1, wherein the ambient temperature comprises a temperature in the range of 25℃ to 35℃.

6. The method as claimed in claim 1, wherein the upper voltage limit is in the range of 2V to 3V.

7. The method as claimed in claim 1, wherein the multiple steps correspond to number of steps in the range of 3 to 5.

8. The method as claimed in claim 1, wherein the second charging cycle is performed in multiple steps where each step is performed at a C-rate in the range of C/2 to C/4.

9. The method as claimed in claim 1, wherein the maximum voltage limit is in the range of 2.8V to 3.4V.

10. The method as claimed in claim 1, wherein the final charging cycle is performed at a C-rate in the range of C/4 to C/6.

11. A lithium-ion battery comprising an anode, a cathode and an electrolyte, wherein the lithium-ion battery is formed by a method comprising:
performing a first charging cycle of the battery, wherein the first charging cycle comprises charging at a constant current to attain a maximum safe voltage and subsequently charging at a constant voltage corresponding to the maximum safe voltage to obtain a first charged battery;
maintaining the first charged battery at a temperature in the range of 25℃ to 95 ℃ for a time in the range of 2 to 8 hours to form a rested battery;
performing a second charging cycle of the rested battery after bringing it to an ambient temperature to an upper voltage limit of the battery to obtain a second charged battery, wherein the second charging is performed in multiple steps with incremental voltage increase; and
performing a final charging cycle of the second charged battery to a maximum voltage limit to form the lithium-ion battery.

12. The lithium-ion battery as claimed in claim 11, wherein the anode comprises graphite, SiC nanocomposites, lithium titanium oxides, LiTiO2, Li4Ti5O12, Sn particulates, Si particulates or any combinations thereof.

13. The lithium-ion battery as claimed in claim 11, wherein the cathode comprises lithium metal oxides, LMO (lithium manganese oxide), Li-NCA (lithium nickel cobalt aluminum oxide), Li-NMC (lithium nickel manganese cobalt oxide), LCO (lithium cobalt oxide), LNO (lithium nickel oxide), LNMO (lithium nickel manganese oxide), LNCM (lithium nickel cobalt manganese oxide), or any combinations thereof.

14. The lithium-ion battery as claimed in claim 11, wherein the electrolyte comprises LiPF6, LiAsF6, LiBOB, LiClO4, LiBF4, and LiPF6.

15. The lithium-ion battery as claimed in claim 11, wherein the second charging cycle is performed in multiple steps where each step is performed at a C-rate in the range of C/2 to C/4 and wherein the multiple steps correspond to number of steps in the range of 3 to 5.