Description:BACKGROUND
[0001] Lithium-ion (Li-ion) cells (hereinafter referred to as (“cells”) have emerged as the dominant energy storage technology across a broad range of applications, including portable electronics, electric vehicles (EVs), and stationary grid storage systems. The widespread adoption of the Li-ion cells is attributed to favorable characteristics such as high energy density, long-cycle life, and relatively low self-discharge rates. In a typical Li-ion cell, energy storage and release are facilitated by the reversible movement of lithium ions between cathode and anode during charge and discharge cycles. However, as the demand for higher capacity and faster charging cells continues to grow, ways to enhance the performance of lithium-ion cells are required.
BRIEF DESCRIPTION OF FIGURES
[0002] Systems and/or methods, in accordance with examples of the present subject matter are described and with reference to the accompanying figures, in which:
[0003] FIG. 1 illustrates a block diagram of a charging and discharging system, in accordance with an example of the present subject matter;
[0004] FIG. 2 illustrates a graphical representation of voltage variations over time at various stages of charging and discharging of cell during cell formation process, in accordance with an example of the present subject matter;
[0005] FIG. 3 illustrates a graphical representation depicting charge retention in the cell during charging and discharging of secondary cells during formation cycle, in accordance with an example of the present subject matter;
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[0006] FIG. 4 illustrates a method for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter;
[0007] FIG. 5 illustrates another method 500 for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter;
[0008] FIG. 6 illustrates another method for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter; and
[0009] FIG. 7 illustrates a graphical representation depicting a comparative analysis of effectiveness of charging and discharging methods of the present subject matter, in accordance with an example of the present subject matter.
[0010] It may be noted that throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.
DETAILED DESCRIPTION
[0011] Traditionally, graphite has served as the anode material of choice due to its structural stability and consistent electrochemical performance. However, its theoretical specific capacity is limited to approximately 372 Milliampere-hour per gram (mAh/g), thereby constraining the energy density of Li-ion cells employing graphite anodes. This limitation is particularly critical in applications where high energy and power densities are required.
[0012] In recent years, silicon and silicon/graphite composite anodes have garnered significant attention as potential replacement for graphite anodes, owing to silicon’s considerably higher theoretical specific capacity,
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which is up to 3579 mAh/g. These anodes offer the advantage of tunable capacity, which can be engineered to meet specific performance requirements. Moreover, they are widely used cathode chemistries, such as nickel manganese cobalt oxide (NMC), and nickel cobalt aluminum oxide (NCA), enhancing the sustainability for next-generation high-performance batteries.
[0013] However, the practical implementation of silicon and silicon-based composite anodes remains challenged by several material and electrochemical limitations. A formation cycle, which is the initial charge-discharge cycle of a newly manufactured lithium-ion battery, plays a role in establishing a solid electrolyte interphase (SEI) layer and determining the long-term performance of the battery. However, there is a substantial consumption of lithium during the initial formation cycles. This over-consumption of lithium results in lithium loss. Lithium loss arises due to irreversible reactions between lithium and silicon, as well as the formation of a solid electrolyte interphase (SEI) layer. The irreversible nature of these reactions results in a significantly reduced initial coulombic efficiency (ICE), which may be as low as 60%, thereby directly impacting the discharge capacity and overall energy density of the cell.
[0014] For silicon-containing anodes, the formation cycle can be particularly challenging due to the dynamic nature of the silicon surface during lithiation and delithiation. The expansion and contraction of silicon particles can lead to continuous exposure of fresh surfaces, resulting in ongoing SEI formation and lithium consumption. This phenomenon can contribute to lower ICE values compared to traditional graphite anodes.
[0015] The lithium loss observed during the formation cycle may be classified into reversible and irreversible components. The reversible loss is associated with the alloying and dealloying of lithium with silicon during charge and discharge cycles. In contrast, the irreversible loss is primarily due to lithium consumed in non-reversible reactions, including irregular SEI
5
layer formation and other parasitic side reactions. This irreversible consumption reduces the amount of active lithium available for subsequent cycling, adversely affecting long-term battery performance.
[0016] Example approaches for providing improved formation methodologies for batteries are described. The present subject matter relates to approaches for mitigating irreversible lithium losses and enhance the initial coulombic efficiency of silicon-based anodes by extracting the residual capacity from silicon/graphite blend anodes.
[0017] For example, a method for charging and discharging a secondary cell during a formation cycle of the secondary cell (hereinafter referred to as a cell) is described. The cell may be a lithium-ion (Li-ion) cell having a positive electrode and a negative electrode. In an example, the negative electrode of the cell comprises up to 25% silicon by weight, i.e., 25% of a total weight of the negative electrode may comprise of silicon. In an aspect, the positive electrode of the cell may comprise a material selected from the group consisting of nickel manganese cobalt oxide (NMC), nickel cobalt aluminium oxide (NCA), and combinations thereof.
[0018] The method is performed during the formation cycle of the cell. The cell undergoes a formation cycle to activate one or more electrochemical components of the cell, such as lithium-ion, nickel-cadmium, or lead-acid batteries. The method for charging and discharging the cell during the formation cycle includes charging the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value. In the predefined number of charging cycles, each successive cycle has a voltage value greater than the voltage value of a previous cycle. The method further includes, upon charging the cell to a first voltage value, discharging the cell to a second voltage value, wherein the second voltage value is less than the first voltage value, and wherein the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a
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range of 3.6 V to 4.24 V. Continuing further, the method includes causing to apply a pulse charge cycle to the cell at a C-rate between 0.1C and 1C, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
[0019] In another example, a system for charging and discharging a secondary cell during a formation cycle of the secondary cell is described. The system may include a control engine to perform charging and discharging during the formation cycle of the cell. The system may be configured to perform the method for charging and discharging the secondary cell during formation cycle of the cell.
[0020] The system and method for charging and discharging the secondary cell during the formation cycle of the cell, of the present subject matter offers several advantages over existing approaches by improving solid electrolyte interphase (SEI) uniformity on the electrode surfaces. The controlled voltage steps and pulse charging cycles allow for consistent SEI formation, thereby reducing unwanted side reactions between the electrolyte and electrode materials. In this manner, the aforementioned approaches for charging and discharging the cell during formation cycle may enhance the cycle life of secondary cells by up to 5% due to the optimized SEI layer formed during the initial cycles.
[0021] Further, the tailored voltage profiles and current rates in the present approaches may lead to an increase in initial coulombic efficiency (ICE) of the secondary cell by 0.5% to 2%. This enhanced efficiency in the first cycle improves the overall cell performance. Additionally, the controlled charging and discharging also enhances electrode stability by managing voltage levels and charge/discharge rates, thereby mitigating structural changes in the electrode materials during initial cycling. This improved stability may lead to consistent performance over the cell's lifetime.
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[0022] The above aspects are further described in conjunction with the figures, and in the associated description below. It should be noted that the description and figures merely illustrate principles of the present subject matter. Therefore, various assemblies that encompass the principles of the present subject matter, although not explicitly described or shown herein, may be devised from the description, and are included within its scope.
[0023] FIG. 1 illustrates a block diagram of a charging and discharging system 100 (hereinafter referred to as system 100), in accordance with an example of the present subject matter. The system 100 may be coupled with one or more secondary cells (not shown in the figure) to perform the formation process of the cell. The formation process may refer to the initial charging and discharging of the cell after assembly of the cell. Upon assembly, every cell undergoes the formation process to activate the chemical material of the cell to allow the cell to become rechargeable. In an example, the system 100 may enable charging and discharging cycles to be performed during the formation process of the cell.
[0024] As depicted in FIG. 1, the system 100 includes a control engine 102. The Control engine 102, amongst other capabilities, may be configured to fetch and execute computer-readable instructions to perform charging and discharging of one or more secondary cells. In an example, the control engine 102 may be implemented as one or more processing units, microprocessors, microcomputers, microcontrollers, digital signal processors, central processing units, state machines, logic circuitries, and/or any devices that manipulate signals based on operational instructions. The functions of the various elements shown in the figure may be provided through the use of dedicated hardware as well as hardware capable of executing machine readable instructions. Although depicted in the form of functional module, the control engine 102 may be implemented in the form of electronic circuitry, which amongst other capabilities, may
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specifically be configured to charge and discharge one or more secondary cells.
[0025] The control engines 102, amongst other things, includes routines, programs, objects, components, and data structures, which perform particular tasks or implement particular abstract data types. The control engine 102 may also be implemented as, signal processor(s), state machine(s), logic circuitries, and/or any other device or component that manipulates signals based on operational instructions. Further, the control engine 102 can be implemented by hardware, by computer-readable instructions executed by a processing unit, or by a combination thereof. In one example, the control engine 102 may include programs or coded instructions that supplement the applications or functions performed by the system 100.
[0026] The data 104, on the other hand, includes data that is either stored or generated due to functionalities implemented by the control engine 102 or the system 100. It may be further noted that information stored by the control engine 102 for performing various functions by the system 100. In an example, the data 104 may include voltage data 106, current profile data 108, and other data 110.
[0027] In accordance with the present subject matter, the control engine 102 may collect data from various sensors (not depicted in the figure). In some aspects, the control engine may interface with analog-to-digital converters (ADCs) to measure voltage, current, and temperature at high sampling rates (for example, 1kHz to 100 kHz). The control engine may also interface with hall effect sensors or shunt resistors coupled with amplifier and ADCs to measure charging and discharging currents.
[0028] The collected data may be stored in as the data 104. In some examples, the control engine 102 may also implement digital filtering techniques to reduce noise in collected data.
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[0029] Further, the control engine 102 serves as the core component responsible for managing the charging and discharging process of cells. The control engine 102 may use proportional-integral-derivative (PID) control loops to regulate charging current and voltage. The control engine 102 may generate precise timing signals to control power electronics switches. The control engine 102 may also implement adaptive algorithms that adjust charging parameters based on battery response.
[0030] The control engine 102 executes a charging and discharging method which may include multiple stages such as constant-current (CC), constant voltage (CV), and pulse charging. The various stages of charging and discharging method are described in detail along with Table 1 and FIG. 2.
[0031] Referring back to the figure, the control engine 102 monitors the cell’s voltage, current, and temperature, and makes dynamic adjustment to the charging parameters. For instance, the control engine 102 modulates the charging current based on the cell’s voltage response. The control engine 102 further implements precise timing control for various charging and discharging cycles.
[0032] Continuing further, the data 104 may organize and store operational data. For example, the voltage data 106 may include time-stamped voltage measurements, statistical summaries (e.g., mean, variance), and voltage thresholds for different stages of the formation process. In some examples, the control engine 102 may store collected data pertinent to voltage as the voltage data 106. Additionally, voltage values pertinent to various stages of the charging and discharging cycles may also be stored as the voltage data 106.
[0033] The current profile data 108 may store predefined current waveforms for pulse charging, current measurements, and integrated current values for capacity calculations. In an example, the current profile data 108 includes predefined current rates (c-rates) profiles that are
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essential for implementing the charging/discharging cycles. These profiles may specify the exact current rates to be used at different stages of the charging and discharging cycles.
[0034] In another example, the current profile data 108 may also include real-time current measurements collected during the charging and discharging cycles. The real-time measurements allow the system to monitor the actual current flow and compare it against the intended C-rate profiles, enabling precise control and safety monitoring.
[0035] The various components of the system 100 may communicate operably to perform the charging and discharging cycles during the formation process of the cell.
[0036] The operation of the system 100 is further described in conjunction with FIG. 2 and Table 1.
[0037] As depicted in Table 1, a plurality of stages of the charging and discharging cycles (also referred to as “charging/discharging cycle”) performed during formation process of the cell are described.
Formation cycle
Serial No.
Cycle
Voltage
Charge rate (C-rate)
1
CC
2.5-2.8 V
0.025C - 0.05C
2
CC
3.4-3.8 V
0.05C - 0.2C
3
CC
3.8-4.05V
0.2C - 0.3C
4
CC
4.05-4.25V
0.075C - 0.1C
5
Discharge
4.05-4.249V
0.1C - 1C
6
CC
4.05-4.25V
0.1C - 1C
7
Discharge
4.05-4.249V
0.1C - 1C
11
8
CCCV
4.05-4.25V
0.1C (0.01C limiting current)
9
Discharge
2.5V
0.1C
Table-1
[0038] The first stage in the charging/discharging cycle includes an initial constant current (CC) step. In constant current (CC) step, the charging current is maintained at a constant level by adjusting an output voltage.
[0039] In an example, during this step, the cell is charged at a very low C-rate, between 0.025C and 0.05C to a voltage between 2.5 V and 2.8 V. This slow charging causes decomposition of electrolyte in the cell and helps in the initial formation of the solid electrolyte interphase (SEI) layer.
[0040] The second CC step increase both the voltage and current. For example, the cell is charged to a voltage value ranging between 3.4 V and 3.8 V at a higher C-rate of approximately 0.05C to 0.2C. This step continues the controlled lithiation of the electrode materials.
[0041] In the third CC step, the cell is charged to 3.8 V - 4.05 V at a higher C-rate of 0.2 C to 0.3 C. Further, in the fourth step, the C-rate is reduced slightly while charging the cell to a higher voltage 4.05 V - 4.25 V. In an example, the cell may be charged at a C-rate of approximately 0.075 C to 0.1 C. This slower rate at higher voltage helps to carefully complete the charging process.
[0042] At the fifth step, the cell is discharged from a high voltage value achieved during the fourth step to a slightly lower voltage value ranging between 4.05 V and 4.249 V at a rate between 0.1C and 1C. This discharge step helps to stabilize the electrode materials.
[0043] Further, the cell is charged again to the high voltage range at a rate between 0.1C and 1C. The seventh step involves discharging the cell from the high voltage range at a rate between 0.1C and 1C. The discharging step is repeated twice to further stabilize the cell structure.
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[0044] At the eighth step, a constant current constant voltage (CCCV) with a limiting current is applied. At the constant current constant voltage step, both the current and voltage of the charge being provided to the cell is maintained constant to prevent overcharging by allowing the current to decrease as the battery reaches full capacity, thus maintaining the voltage. In an example, during this step, the cell is charged at 0.1C to the high voltage range, then held at constant voltage until the current drops to the limiting current of 0.01C. This step ensures the cell is fully charged while avoiding overcharging of the cell.
[0045] Finally, the cell is fully discharged to 2.5 V at a rate of 0.1C. This complete discharge helps to measure the full capacity of the cell and prepare the cell for normal use.
[0046] The stages of charging and discharging cycles of the formation process of the cell, in accordance with the present subject matter, are plotted graphically to better appreciate the present subject matter. A voltage profile curve depicting the voltage variation during various stages of charging and discharging of cell during cell formation process is described in conjunction with FIG. 2.
[0047] FIG. 2 depicts a graphical representation 200 (hereinafter referred to as “the graph 200”) of voltage variations over time at various stages of charging and discharging of cell during cell formation process, in accordance with an example of the present subject matter.
[0048] As depicted in the graph 200, the y-axis depicts voltage (V) ranging from 0.5V to 4.5V. The x-axis represents the total time (in hours) of the formation cycle, spanning from 00:00:00 to 38:58:00.
[0049] In operation, the cell formation process begins with charging the cell up to an initial voltage between 2.5 V and 2.8 V, as depicted at curve 202. During this stage, the curve 202 has a steep rise, indicating a rapid increase in voltage, as the cell begins to accept charge.
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[0050] The curve 202 then gradually rises into a plateau segment to reach 204. During this segment, the cell is charged to approximately 3.4 V to 3.8 V. This gradual rise in voltage corresponds to an initial formation of the SEI layer and a controlled lithiation of anode.
[0051] Further, the cell is charged to a higher voltage value to reach a level between 3.8 V to 4.05 V at 206, to further rise to a peak voltage value between 4.05 V to 4.25 V, depicted with the curve peaking at 208. At this stage, the lithium ions begin to intercalate into the electrode.
[0052] Upon reaching the peak voltage between 4.05 V to 4.25 V, the cell is discharged. As shown at 210, a drop in voltage between 4.05 V to 4.249 V is experienced by the cell at this stage. This leads to the dissipation of surface charge and partial redistribution of lithium ions.
[0053] At this stage a pulse charging is applied to the cell. Accordingly, the cell is again charged to the voltage value between 4.05 V to 4.25 V, as depicted at 212. The SEI continues forming during this stage. Upon reaching the voltage value between 4.05 V to 4.25 V, the cell is again discharged to a value between 4.05 V to 4.249 V, as shown at 214. In an example, the voltage value at which the cell is charged between 4.05 V to 4.25 V is greater than the voltage value at which the cell is discharged between 4.05 V to 4.249 V.
[0054] As shown in the graph 200, the same charging and discharging cycle as described above, is performed between 216 to 220. Therefore, the details are not described again to maintain brevity.
[0055] Continuing further, upon reaching 220, the battery is completely discharged to a lower voltage value, approximately up to 2.5 V. Accordingly, the curve 222 indicates a steep voltage drop. The process concludes with a slight voltage increase, allowing the cell to recover due to electrochemical relaxation and redistribution of lithium ions as depicted at 224.
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[0056] The graph 200 provides a comprehensive visual representation of the complex formation cycle for a secondary cell. The voltage profiles represented in graph 200 lead to an increase in the ICE of the cell, thereby improving the overall cell performance. Further, the pulse charging process improves SEI formation on the electrode surface by enabling uniform and consistent SEI formation.
[0057] FIG. 3 illustrates a graphical representation (hereinafter referred to as the graph 300) depicting charge retention in the cell during charging and discharging of secondary cells during formation cycle, in accordance with an example of the present subject matter.
[0058] For example, FIG. 3 illustrates an analysis of charge retention of a cell charged using the charging and discharging methods of the present subject matter. In some aspects, the graph 300 represents various electrochemical reactions occurring within the cell during charging and discharging cycles. In an example, the graph 300 represents redox reactions occurring within the cell during charging and discharging.
[0059] Particularly, the graph 300 represents a differential capacity in milli-ampere-hour per volt (represented by dQ/dV) by plotting a relationship between charge and voltage (represented by V) of the cell during charging and discharging cycles.
[0060] The x-axis of the graph 300 represents voltage range during charging and discharging cycles, ranging between 2.0 V and 4.4 V, covering a full charge/discharge cycle of the secondary cell.
[0061] The y-axis of the graph 300 represents the differential capacity of the cell. The differential capacity of the cell relates to the change in charge with respect to voltage.
[0062] The graph 300 shows asymmetric peaks, with both positive curve 302, and negative curve 304. The positive curve 302 represent the charging cycle and the negative curve 304 represent the discharging cycle of the cell.
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Particularly, the positive curve 302 represents reduction reaction of the cell and the negative curve 304 represents oxidation reaction the cell.
[0063] As may be seen, the beginning of the positive curve 302 is mostly flat, depicting a slow and gradual increase in the differential capacity as the voltage progresses from 2.5 V to 3.0 V. The gradual increase represents low differential capacity, i.e., for each small change in voltage, only a small change in the stored charge is experienced by the cell. In an aspect, the flat curve represents minimal electrochemical activity in the cell during this phase. In some examples, this phase represents the decomposition of electrolytes in the cell.
[0064] Further, gradual undulations begin to appear, representing small-scale reduction reactions as the voltage increases up to a range between 3.4 V to 3.8 V. These gradual undulations depict an electrochemical behaviour of the cell during pulse charging, as the cell experiences intermediate lithium staging. In an example, this phase indicates graphite-silicon intercalation within the cell.
[0065] Continuing further, as the voltage progresses up to 4.4, a distinct sharp peak and valleys may be seen. In an example, the peak point to distinct transitions or reactions in the electrode material. In an aspect, the peaks indicate transitions or reactions occurring in an electrode material. These peaks may also correspond to lithium intercalation and SEI formation.
[0066] Similar dips and rises may be seen in the negative curve 304. As may be seen, discharge cycle begins after the voltage in the cell reaches up to 2.5 V, with a gradual release of stored charges, represented by a gradual downward trend of the negative curve 304.
[0067] Upon slowly releasing charge as the voltage progresses to 3.4, some areas of small dips and rise may be seen in the negative curve 304. The small dips and rise in the negative curve 304 represent the initial SEI
16
growth. Further, as the voltage further progresses, a large negative peak is experienced in the differential capacity of the cell. In an example, the large dip demonstrates a major lithium intercalation reaction, indicating a large capacity drop per unit voltage.
[0068] The positive curve 302 and the negative curve 304 of the graph 300 represents the redox reactions occurring within the cell as the charging and discharging cycles are applied to the cell. The smooth peaks with no abnormal shoulder peaks depicts suppression of irreversible reactions during the formation process, therefore avoiding irregular SEI layer formation and other parasitic side reactions, ensuring consistent performance and enhanced cell life.
[0069] FIG. 4 illustrates a method for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter. The method 400 may be performed by the system for charging and discharging the secondary cell during the formation cycle of the secondary cell, such as the charging and discharging system 100.
[0070] In an example, at block 402, an initial charging phase of the secondary cell is described. For example, the method 400 may include charging the secondary cell in a predefined number of charging cycles at a constant current. The current rate at this step may be set between 0.025 C and 0.1 C, and the cell may be charged to a predefined voltage value.
[0071] In some aspects, each successive cycle has a voltage value greater than the voltage value of a previous cycle. The predefined number of charging cycles may include a plurality of step-charging cycles at a constant current up to a predefined voltage value.
[0072] In some aspects, the predefined number of cycles may include a plurality of step charging cycles. For example, with each cycle, the cell may
17
be charged to a higher voltage value than a voltage value of the previous cycle.
[0073] In an example, during the predefined number of charging cycles, the C-rates may also vary or may remains constant.
[0001] At block 404, the method 400 may include, upon charging the secondary cell to a first voltage value, discharging the secondary cell to a second voltage value.
[0002] In one example, the second voltage value is less than the first voltage value. For example, the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V.
[0003] At block 406, a pulse charge cycle may be applied to the cell. In some aspects, the pulse cycle may be applied at a C-rate between 0.1C and 1C.
[0004] The pulse charge cycle may include a predefined number of alternating cycles. The alternating cycles may alternate between charging to the first voltage value and discharging to the second voltage value.
[0005] In some aspects, the alternating cycles may include a plurality of cycles of charging and discharging the cell over time. In some examples, the alternating cycles may be set at a constant C-rate. In another example, the C-rate may vary along the alternating cycles.
[0006] In a possible example according to the above disclosure, the cell may be charged to a value of 4.15 V and discharged to 3.9 V.
[0074] FIG. 5 illustrates another method 500 for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter. The method 500 may be performed by the system for charging and discharging the secondary cell during the formation cycle of the secondary cell, such as the charging and discharging system 100.
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[0075] In an example, the method 500 may begin at block 502. At block 502, the method 500 includes charging the secondary cell to a voltage value selected from a range of 2.5V and 2.8V at a C-rate between 0.025C and 0.05C.
[0076] The low voltage range and slow charging rate may initiate the formation process of the secondary cell. In an example, possible voltage values within this range could be 2.6V, 2.7V, or 2.75V. The C-rate could be set at 0.03C, 0.04C, or 0.045C.
[0077] This slow, controlled initial charge may help establish a stable foundation for the SEI layer.
[0078] At block 504, the method 500 may include charging the cell to a voltage value selected from a range of 3.4 V and 3.8 V at a c-rate between 0.05C and 0.2C.
[0079] At block 504, both the voltage and C-rate are increased from the voltage range and c-rate described at block 502. In an example, the possible voltage value could be 3.5V, 3.6V, or 3.7V. The C-rate might be set at 0.1C, 0.15C, or 0.18C.
[0080] This intermediate charging step may allow further development of the SEI layer while gradually increasing state of charge of the cell.
[0081] Block 506 describes another charging step for the cell. For example, in this step, the method 500 may include charging the cell to a voltage value selected from a range of 3.8V and 4.05V at a C-rate between 0.2C and 0.3C.
[0082] In some aspects, the charging step described at block 506 may help condition the cell for a full operational voltage range while still maintaining a controlled charging environment.
[0083] In some examples, the potential voltage values may be approximately 3.9V, 4.0V, or 4.05V. The C-rate might be set at 0.22C, 0.25C, or 0.28C.
19
[0084] Further, at block 508, the method 500 may include charging the cell to a first voltage value selected from a range of 4.05V and 4.25V at a current-rate between 0.025C and 0.1C.
[0085] In some aspects, at this step, the cell may be charged to a highest voltage value at a reduced charging rate. For example, the possible voltage values may be 4.1V, 4.15V, or 4.2V. The C-rate might be set at 0.05C, 0.075C, or 0.09C.
[0086] Continuing further, at block 510, the method 500 may include, discharging the cell to a second voltage value, upon charging to the first voltage value. As described, the first voltage value may be selected from the range of 4.05V and 4.25V.
[0087] In some aspects, the second voltage value may be less than the first voltage value. In an example, the second voltage value is selected from a range of 3.6V to 4.249V.
[0088] For example, the method 500 at block 510 may include charging the cell to 4.2V and discharging to 3.8V. In an alternative example, the method 500 may include charging the cell to 4.15V and discharging to 4.0V. This charge-discharge cycle may help stabilize an internal structure of the cell and further develop the SEI layer.
[0089] At block 512, a pulse charge cycle may be applied to the cell. In some aspects, the pulse cycle may be applied at a C-rate between 0.1C and 1C.
[0090] The pulse charge cycle may include a predefined number of alternating cycles. The alternating cycles may alternate between charging to the first voltage value and discharging to the second voltage value.
[0091] In some aspects, the alternating cycles may include a plurality of cycles of charging and discharging the cell over time. In some examples, the alternating cycles may be set at a constant C-rate. In another example, the C-rate may vary along the alternating cycles.
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[0092] In a possible example according to the above disclosure, the cell may be charged to a value of 4.15 V and discharged to 3.9 V. In some examples, a duration of charge and discharge during pulse charging may vary.
[0093] The pulse charging may help optimize the SEI layer formation, thereby improving the cell's long-term performance and cycle life. By precisely controlling voltage levels, charging rates, and cycling patterns throughout the formation process, the method 500 may optimize the cell's initial performance and long-term durability, particularly for advanced battery chemistries like those incorporating silicon in the anode.
[0094] FIG. 6 illustrates another method 600 for charging and discharging a secondary cell during a formation cycle of the secondary cell, in accordance with an example of the present subject matter. The method 600 may be performed by the system for charging and discharging the secondary cell during the formation cycle of the secondary cell, such as the charging and discharging system 100.
[0095] In an example, at block 602, an initial charging phase of the secondary cell is described. For example, in this step, the method 600 may include charging the secondary cell in a predefined number of charging cycles at a constant current. The current rate at this step may be set between 0.025 C and 0.1 C, and the cell may be charged to a predefined voltage value.
[0096] In some aspects, each successive cycle has a voltage value greater than the voltage value of a previous cycle. The predefined number of charging cycles may include a plurality of step-charging cycles at a constant current up to a predefined voltage value.
[0097] In some aspects, the predefined number of cycles may include a plurality of step charging cycles. For example, with each cycle, the cell may
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be charged to a higher voltage value than a voltage value of the previous cycle.
[0098] In an example, during the predefined number of charging cycles, the C-rates may also vary or may remain constant.
[0099] At block 604, the method 600 may include, upon charging the secondary cell to a first voltage value, discharging the secondary cell to a second voltage value.
[0100] In one example, the second voltage value is less than the first voltage value. For example, the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V.
[0007] Further, at block 606, a pulse charge cycle may be applied to the cell. In some aspects, the pulse cycle may be applied at a C-rate between 0.1C and 1C.
[0008] The pulse charge cycle may include a predefined number of alternating cycles. The alternating cycles may alternate between charging to the first voltage value and discharging to the second voltage value.
[0009] In some aspects, the alternating cycles may include a plurality of cycles of charging and discharging the cell over time. In some examples, the alternating cycles may be set at a constant C-rate. In another example, the C-rate may vary along the alternating cycles.
[0101] In a possible example according to the above disclosure, the cell may be charged to a value of 4.15 V and discharged to 3.9 V.
[0102] Continuing further, applying the pulse charge cycle may further include, as described at block 608, charging the cell with a constant current at a constant voltage equal to the first voltage.
[0103] This step is often referred to as constant current-constant voltage (CC-CV) charging, helps to fully charge the cell while preventing overcharging.
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[0104] In an example, the CC-CV charging may be performed with a limiting current of 0.01 C to prevent overcharging.
[0105] In an example, at this step, the cell may be charged at 0.1 C to a first voltage value of approximately 4.1 V. The voltage value is then held at 4.1 V until the C-rate drops to the limiting current at 0.01 C.
[0106] This step ensures the cell reaches its full capacity while maintaining safe operating conditions.
[0107] At final block 610, the method 600 includes discharging the cell to a voltage of 2.5V at a C-rate of 0.1C. This controlled discharge to a low voltage may help condition the cell and provide a consistent starting point for subsequent cycles. For example, in this step, the cell is discharged at 0.1C to 2.5V.
[0108] The aforementioned methods of the present subject matter enable consistent and uniform SEI formation, thereby reducing unwanted side reactions between the electrolyte and electrode materials. Further, the tailored voltage profiles and current rates in the present approaches may lead to an increase in initial coulombic efficiency (ICE) of the secondary cell by 0.5% to 2%. This enhanced efficiency in the first cycle improves the overall cell performance.
[0109] Additionally, the controlled charging and discharging also enhances electrode stability by managing voltage levels and charge/discharge rates, thereby mitigating structural changes in the electrode materials during initial cycling. This improved stability leads to consistent performance and enhances the lifetime of the cell. In this manner, the aforementioned methods for charging and discharging the cell during formation cycle may enhance the cycle life of secondary cells by up to 5% due to the optimized SEI layer formed during the initial charging and discharging cycles.
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[0110] A comparative analysis of various methods of charging and discharging the secondary cells during the formation processes is depicted in Table 2.
S. No.
FP
Formation charging capacity (in Ampere-hours, Ah)
Formation discharging capacity (in Ampere-hours)
ICE %
Initial discharge capacity
(in Ah)
CE (%)
DCIR (mΩ)
1
FP1
31-33
29-29.5
89-89.5
28.62
99.573
3.41
2
FP2
31-33
29-29.51
89-89.5
28.661
99.410
3.37
3
FP3
31-33
29-29.5
89-89.5
28.39
99.64
3.62
4
Ref
31-33
28-28.9
87-88
27.97
99.61
3.70
Table 2
[0111] Table 2 depicts a comparative evaluation of various methods for charging and discharging the secondary cell during the formation process of the secondary cell against a conventional reference method (Ref) used during the formation process.
[0112] The table assesses the efficiency and electrochemical performance of each method by reporting key parameters such as formation capacity, initial columbic capacity, initial discharge capacity, cycle efficiency, and direct current internal resistance.
[0113] In conjunction with Table 2, S. No. stands for serial number indicative of a test analysis serial number, FP relates to Formation protocol
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and is indicative of different formation protocols used for obtaining the analysis, ICE stands for initial columbic efficiency of the cell, CE stands for the columbic efficiency of the cell after the initial cycles, and DCIR stands for Direct current internal resistance of the cell.
[0114] Formation protocols (FP1, FP2, and FP3) includes the methods for charging and discharging the secondary cell, as described in the present subject matter. However, the voltage values and C-rates used to perform FP1, FP2, and FP3 may vary slightly.
[0115] In the table, Formation protocol ‘Ref’ is indicative of the conventional charging and discharging cycle used in cell formation process.
[0116] The formation charging capacity indicates the charging capacity achieved during the formation process. As depicted in table 2, the formation charging capacity remains the same at 31 to 33 Ampere-hours (Ah), across the different formation protocols.
[0117] Further, the formation discharging capacity indicates the discharging capacity after charging during the formation process. As shown in table 2, FP1, FP2, and FP3 achieve similar discharge capacities ranging from 29-29.5 Ah, while the reference protocol (Ref) shows a lower discharge capacity of 28-28.9 Ah. This indicates that the formation protocols (FP1, FP2, FP3) efficiently retain capacity during the first discharge cycle compared to the reference method.
[0118] Continuing further, FP1, FP2, and FP3 demonstrate an ICE of 89-89.5%, while the reference protocol shows a lower ICE of 87-88%, suggesting that the new formation protocols are more efficient in their initial charge-discharge cycle, leading to better long-term performance.
[0119] Further, in the table 2, FP1 and FP2 demonstrate the highest values of initial discharge capacity at 28.62 Ah and 28.661 Ah respectively, followed closely by FP3 at 28.39 Ah. The reference protocol shows a
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notably lower initial discharge capacity of 27.97 Ah, indicating the lower retention of charge capacity over multiple cycles.
[0120] The CE % represents the Coulombic Efficiency after the initial cycles. FP1 shows the highest CE at 99.573%, followed closely by FP3 at 99.64% and the reference at 99.61%. FP2 shows a slightly lower CE at 99.410%.
[0121] Furthermore, in the DCIR column, the table 2 shows that FP1 and FP2 show the lowest DCIR values at 3.41 mΩ and 3.37 mΩ respectively, followed by FP3 at 3.62 mΩ. The reference protocol shows the highest DCIR at 3.70 mΩ, indicating that FP1, FP2, and FP3 lead to lower internal resistance and better power performance.
[0122] Therefore, table 2 shows that FP1, FP2, and FP3 enhance the capacity retention and efficiency of the cell formation process and further reduces the internal resistance within the cell, therefore providing improved approaches for charging and discharging the cell during the formation process. The efficiency of different formation processes is further studied and evaluated in conjunction with FIG. 7.
[0123] For example, FIG. 7 illustrates a graphical representation of a comparative analysis of effectiveness of charging and discharging method (s) of the present subject matter, in accordance with an example of the present subject matter.
[0124] In an example, the graphical representation (hereinafter referred to as the “graph 700”) obtains the comparison by measuring the capacity retention performance of the cell measured across different samples of cells having undergone different formation processes. The graph 700 represents the performance data of different secondary cells over time after experiencing a number of charge and discharge cycles.
[0125] The x-axis of the graph 700 represents the number of charging and discharging cycles with data extending to 100 cycles, while the y-axis
26
represents the capacity retention percentage of the cell, ranging from 86% to 100%.
[0126] The graph 700 displays three data curves 702, 704, and 706. The first curve 702 represents the capacity retention percentage of a cell formed using the conventional formation process. The second curve 704 represents the capacity retention percentage of a cell formed using pulse charging methods at a high voltage of approximately 3.75 V to 4.25 V. The third curve 706 represents the capacity retention percentage of a cell formed using the approaches for charging and discharging a cell during formation cycle as described in the present subject matter at the voltage ranges. As shown in the figure, each curve starts at 100% capacity retention and experiences a gradual decline over the course of several charge and discharge cycles.
[0127] As may be seen, the first data curve 702 shows a significant decline in capacity retention. The first data curve 702 begins to drop sharply after around 40-50 cycles. By the 100th cycle, the capacity drops to around 88%.
[0128] In an aspect, this fast degradation implies that the cell formed using conventional formation cycle are less resistant to long-term cycling, due to less stable electrode materials, weaker electrolyte compatibility, and faster SEI breakdown.
[0129] Further, the second data curve 704 follows a downward trend, starting near 100% and maintaining above 92% capacity at the end of 100 cycles. As may be seen, the rate of capacity loss degrades after approximately 60 cycles.
[0130] As may be noted, the third data curve 706 demonstrates an improved performance among the three samples. The third data curve 706 exhibits a slow and consistent decline as cycling progresses. By the 100th cycle, the cell is able to retain over 94% of the original capacity.
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[0131] Therefore, the gradual decline of the third data curve 706 shows a slower capacity fading compared to the first data curve 702, and the second data curve 704.
[0132] The aforementioned approaches for charging and discharging the secondary cell during the formation process of the cell play a crucial role in optimizing the formation of the solid electrolyte interphase (SEI) layer on the electrode surfaces. By employing controlled voltage steps and pulse charging cycles, these approaches promote consistent SEI development, which helps minimize undesirable side reactions between the electrolyte and electrode materials.
[0133] This optimized SEI formation not only enhances the stability of the electrodes by regulating voltage levels and charge/discharge rates but also mitigates structural changes during initial cycling. As a result, the cell exhibits improved electrode integrity and more consistent performance throughout its lifetime.
[0134] Moreover, the tailored voltage profiles and current rates used in these methods contribute to an increase in the initial coulombic efficiency (ICE) of the secondary cell, typically by 0.5% to 2%. This improvement in first-cycle efficiency directly boosts overall cell performance.
[0135] Collectively, these controlled charging and discharging strategies during the formation cycle may extend the cycle life of secondary cells by up to 5%, thanks to the enhanced SEI layer and improved initial efficiency.
[0136] Although aspects and other examples have been described in a language specific to structural features and/or methods, the present subject matter is not necessarily limited to such specific features or elements as described. Rather, the specific features are disclosed as examples and should not be construed to limit the scope of the present subject matter.
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I/We Claim:
1. A method (400, 500, 600) for charging and discharging a secondary cell during a formation cycle of the secondary cell, the method (400, 500, 600) comprising:
charging (402, 602) the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value, wherein each successive cycle has a voltage value greater than the voltage value of a previous cycle;
upon charging the secondary cell to a first voltage value, discharging (404, 510, 604) the secondary cell to a second voltage value, wherein the second voltage value is less than the first voltage value, and wherein the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V;
causing (406, 512, 606) to apply a pulse charge cycle at a C-rate between 0.1C and 1C, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
2. The method (400, 500, 600) as claimed in claim 1, wherein charging the secondary cell in the predefined number of charging cycles comprises:
charging (502) the secondary cell to a voltage value selected from a range of 2.5V and 2.8 V at a C-rate between 0.025C and 0.05 C;
charging (504) the secondary cell to a voltage value selected from a range of 3.4V and 3.8 V at a C-rate between 0.05C and 0.2C; and
charging (506) the secondary cell to a voltage value selected from a range of 3.8 V and 4.05 V at a C-rate between 0.2C and 0.3C.
3. The method (400, 500, 600) as claimed in claim 1, wherein after causing to apply the pulse charge cycle, the method (400, 500, 600) comprises charging (608) the secondary cell with a constant current at a constant voltage equal to the first voltage.
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4. The method (400, 500, 600) as claimed in claim 3, wherein charging the secondary cell with the constant current at the constant voltage is performed at a C-rate of 0.1C with a limiting current of 0.01C.
5. The method (400, 500, 600) as claimed in claim 4, wherein the method (400, 500, 600) further comprises discharging (610) the cell to a voltage of 2.5 V.
6. The method (400, 500, 600) as claimed in claim 5, wherein the discharging the cell is performed at a C-rate of 0.1C.
7. The method (400, 500, 600) as claimed in claim 1, wherein the secondary cell includes a negative electrode and a positive electrode.
8. The method (400, 500, 600) as claimed in claim 7, wherein the negative electrode of the secondary cell comprises up to 25% silicon by weight.
9. The method (400, 500, 600) as claimed in claim 7, wherein the positive electrode comprises a material selected from the group consisting of nickel manganese cobalt oxide (NMC), nickel cobalt aluminium oxide (NCA), and combinations thereof.
10. A system (100) for charging and discharging a secondary cell during a formation cycle of the secondary cell, comprising:
a control engine (102) to:
charge the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value, wherein each successive cycle has a voltage value greater than the voltage value of a previous cycle;
upon charging the secondary cell to a first voltage value, discharge the secondary cell to a second voltage value, wherein the second voltage value is less than the first voltage value, and wherein the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V;
30
cause to apply a pulse charge cycle at a C-rate between 0.1C and 1C, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
11. The system (100) as claimed in claim 10, wherein to charge the secondary cell in a predefined number of charging cycles, the control engine (102) is to:
charge the secondary cell to a voltage value selected from a range of 2.5V and 2.8 V at a C-rate between 0.025C and 0.05 C;
charge the secondary cell to a voltage value selected from a range of 3.4V and 3.8 V at a C-rate between 0.05C and 0.2C; and
charge the secondary cell to a voltage value selected from a range of 3.8 V and 4.05 V at a C-rate between 0.2C and 0.3C.
12. The system (100) as claimed in claim 10, wherein after causing to apply the pulse charge cycle, the control engine (102) is to charge the secondary cell with a constant current at a constant voltage equal to the first voltage.
13. The system (100) as claimed in claim 12, wherein the control engine (102) is to charge the secondary cell with the constant current at the constant voltage at a C-rate of 0.1C with a limiting current of 0.01C.
14. The system (100) as claimed in claim 13, wherein the control engine (102) is further configured to discharge the cell to a voltage of 2.5 V.
15. The system (100) as claimed in claim 14, wherein the control engine (102) is to discharge the cell at a C-rate of 0.1C.
16. The system (100) as claimed in claim 10, wherein the secondary cell includes a negative electrode and a positive electrode.
17. The system (100) as claimed in claim 16, wherein the negative electrode of the secondary cell comprises up to 25% silicon by weight.
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18. The system (100) as claimed in claim 16, wherein the positive electrode comprises a material selected from the group consisting of nickel manganese cobalt oxide (NMC), nickel cobalt aluminium oxide (NCA), and combinations thereof.
32
ABSTRACT
CHARGING AND DISCHARGING CYCLE FOR FORMATION OF LITHIUM-ION CELLS
The present subject matter discloses a method and system (100) for charging and discharging a secondary cell during a formation cycle. For example, the method includes charging the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value. Upon charging the secondary cell to a first voltage value, the cell is discharged to a second voltage value, wherein the second voltage value is less than the first voltage value. Further, a pulse charge cycle is applied to the cell, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
To be published with Fig. 2 , Claims:I/We Claim:
1. A method (400, 500, 600) for charging and discharging a secondary cell during a formation cycle of the secondary cell, the method (400, 500, 600) comprising:
charging (402, 602) the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value, wherein each successive cycle has a voltage value greater than the voltage value of a previous cycle;
upon charging the secondary cell to a first voltage value, discharging (404, 510, 604) the secondary cell to a second voltage value, wherein the second voltage value is less than the first voltage value, and wherein the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V;
causing (406, 512, 606) to apply a pulse charge cycle at a C-rate between 0.1C and 1C, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
2. The method (400, 500, 600) as claimed in claim 1, wherein charging the secondary cell in the predefined number of charging cycles comprises:
charging (502) the secondary cell to a voltage value selected from a range of 2.5V and 2.8 V at a C-rate between 0.025C and 0.05 C;
charging (504) the secondary cell to a voltage value selected from a range of 3.4V and 3.8 V at a C-rate between 0.05C and 0.2C; and
charging (506) the secondary cell to a voltage value selected from a range of 3.8 V and 4.05 V at a C-rate between 0.2C and 0.3C.
3. The method (400, 500, 600) as claimed in claim 1, wherein after causing to apply the pulse charge cycle, the method (400, 500, 600) comprises charging (608) the secondary cell with a constant current at a constant voltage equal to the first voltage.
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4. The method (400, 500, 600) as claimed in claim 3, wherein charging the secondary cell with the constant current at the constant voltage is performed at a C-rate of 0.1C with a limiting current of 0.01C.
5. The method (400, 500, 600) as claimed in claim 4, wherein the method (400, 500, 600) further comprises discharging (610) the cell to a voltage of 2.5 V.
6. The method (400, 500, 600) as claimed in claim 5, wherein the discharging the cell is performed at a C-rate of 0.1C.
7. The method (400, 500, 600) as claimed in claim 1, wherein the secondary cell includes a negative electrode and a positive electrode.
8. The method (400, 500, 600) as claimed in claim 7, wherein the negative electrode of the secondary cell comprises up to 25% silicon by weight.
9. The method (400, 500, 600) as claimed in claim 7, wherein the positive electrode comprises a material selected from the group consisting of nickel manganese cobalt oxide (NMC), nickel cobalt aluminium oxide (NCA), and combinations thereof.
10. A system (100) for charging and discharging a secondary cell during a formation cycle of the secondary cell, comprising:
a control engine (102) to:
charge the secondary cell in a predefined number of charging cycles at a constant current with a current-rate (C-rate) between 0.025C and 0.1C to a predefined voltage value, wherein each successive cycle has a voltage value greater than the voltage value of a previous cycle;
upon charging the secondary cell to a first voltage value, discharge the secondary cell to a second voltage value, wherein the second voltage value is less than the first voltage value, and wherein the first voltage value is selected from a range of 4.05 V to 4.25 V and the second voltage value is selected from a range of 3.6 V to 4.249 V;
30
cause to apply a pulse charge cycle at a C-rate between 0.1C and 1C, wherein the pulse charge cycle comprises a predefined number of alternating cycles of charging to the first voltage value and discharging to the second voltage value.
11. The system (100) as claimed in claim 10, wherein to charge the secondary cell in a predefined number of charging cycles, the control engine (102) is to:
charge the secondary cell to a voltage value selected from a range of 2.5V and 2.8 V at a C-rate between 0.025C and 0.05 C;
charge the secondary cell to a voltage value selected from a range of 3.4V and 3.8 V at a C-rate between 0.05C and 0.2C; and
charge the secondary cell to a voltage value selected from a range of 3.8 V and 4.05 V at a C-rate between 0.2C and 0.3C.
12. The system (100) as claimed in claim 10, wherein after causing to apply the pulse charge cycle, the control engine (102) is to charge the secondary cell with a constant current at a constant voltage equal to the first voltage.
13. The system (100) as claimed in claim 12, wherein the control engine (102) is to charge the secondary cell with the constant current at the constant voltage at a C-rate of 0.1C with a limiting current of 0.01C.
14. The system (100) as claimed in claim 13, wherein the control engine (102) is further configured to discharge the cell to a voltage of 2.5 V.
15. The system (100) as claimed in claim 14, wherein the control engine (102) is to discharge the cell at a C-rate of 0.1C.
16. The system (100) as claimed in claim 10, wherein the secondary cell includes a negative electrode and a positive electrode.
17. The system (100) as claimed in claim 16, wherein the negative electrode of the secondary cell comprises up to 25% silicon by weight.
31
18. The system (100) as claimed in claim 16, wherein the positive electrode comprises a material selected from the group consisting of nickel manganese cobalt oxide (NMC), nickel cobalt aluminium oxide (NCA), and combinations thereof.