FR2789805A1 - Secondary cell charging process, especially for lithium batteries, comprises maximum current charging to an intermediate cell voltage and repeated charging at reduced current values to increased voltage values - Google Patents

Abstract

A secondary cell charging process comprises maximum current charging to an intermediate cell voltage (V1) and repeated charging at reduced current values to increased cell voltage values. A secondary cell charging process comprises increasing the charging current to a maximum value (Imax) which is maintained until the cell voltage reaches an intermediate value (V1), supplying a current of value (I1) below the maximum value (Imax) until the cell voltage reaches a value (V2) above the intermediate value (V1) but below the final value (Vfin) and repeating this last step until the cell voltage reaches the final value (Vfin). An Independent claim is also included for a secondary lithium cell charged by the above process.

Description

Method for charging an electrochemical generator The present invention

  relates to an electrochemical generator whose lifespan is longer than that of known generators thanks to better stability of the cycling capacity. It relates in particular to the cycling conditions of this generator, and mainly the method for charging it. They are more particularly electrochemical generators

  rechargeable lithium batteries which can possibly be combined as a battery.

  Rechargeable electrochemical cells, the negative electrode of which contains an electrochemically active material which is a carbonaceous material into which lithium has been inserted, has a capacity which decreases as cycling takes place. Cycle after cycle, a small proportion of lithium is deposited in metallic form on the negative electrode. The deposited lithium gradually becomes electrochemically inactive by reaction with the electrolyte, and causes a recurrent and irreversible loss of capacity. The cycling conditions, and more particularly the manner in which the charging is carried out, influence the importance and the quality of this deposit, and consequently the loss of capacity during cycling. Indeed, this deposit of lithium occurs when the negative electrode reaches a too negative potential, that is to say a potential lower than the equilibrium potential of the Limeto / Li + system which is itself lower than the normal potential of insertion of lithium into carbon. This potential is all the more easily reached when the polarization is important, that is to say that the current of

load is high ..

  The usual charging process has two main steps. In a first step, a constant intensity is imposed on the generator. The generator voltage then increases to the maximum voltage it is allowed to

  reach at the end of the charge and which is characteristic of the type of generator.

  Then in a second step, the voltage is maintained at its value at the end of the first step for the time necessary to obtain the desired charge rate, while the intensity decreases freely monotonically until reaching a very low value . Since it is not technically easy to

  maintain a constant voltage, we can alternatively proceed in a simpler and less costly way by imposing a constant value on the intensity, a value which is decreased in successive stages as the voltage increases5 to re-reach its end of charge value.

  This process has the drawback of applying to the generator simultaneously the maximum charging current and the maximum voltage at the end of charging during the

first stage.

  For each electrode, the variation of the overvoltage according to the imposed current is authorized until the potential difference between the two electrodes is equal to the maximum end of charge voltage. However, the zero current potential of the usual positive electrodes, containing lithium insertion materials such as LiCoO2, LiNiO2, LiMn2O4 or all their derivatives, increases as a function of the state of charge. Thus at the start of charging, the voltage is removed from the maximum authorized voltage. By way of example, the voltage at the start of charging of a LiC6 / LiNiO2 element is approximately 3.5V at low current, while the maximum voltage at the end of charging is generally 4.0V. The overvoltage can become significant without the imposed current falling. Consequently, the negative electrode can reach a sufficiently negative potential for an unnecessary and hardly soluble lithium deposit to occur, which thus leads to a loss of capacity.

important and irreversible.

  It is possible to remedy this drawback by reducing the intensity of the current imposed throughout the duration of the charge in order to reduce the overvoltages. However, the duration of the charge is increased and may

  become incompatible with the intended application.

  The present invention aims to provide a secondary electrochemical generator whose loss of cycling capacity is minimized and

extended service life.

  Another object of the invention is to propose a method for charging a secondary electrochemical generator which makes it possible to reduce the loss of

cycling capacity.

  The object of the present invention is a method for charging a secondary electrochemical generator, characterized in that it comprises the following steps: (a) a charging current is imposed on said generator which increases to a maximum value Imox , (b) maintaining said current at said maximum value Im until the voltage of said generator reaches a voltage value V1 less than the value Vfn at the end of charge, (c) imposing a current of value I1 less than said maximum value Imox until said voltage reaches a value V2 greater than the value V1 and less than the value Vf ,, of end of charge,

  (d) the previous step is repeated (c) as many times as necessary until the voltage reaches a value Vn equal to Vfi, and the charge is stopped.

  The rise in current constituting the first step of the charging method according to the invention has the advantage of not suddenly imposing a high current on the generator. This avoids overvoltages detrimental to the life of the generator. Advantageously, said charging current increases at a speed of between 0.01 Imox and 0.10 Imox per minute during the first step, preferably between 0.015 Imax and 0.070 Imr. The duration of the rise in current is preferably between 15 minutes and one hour. This allows the generator to gradually go into operation without increasing

excessively long charging time.

  The charging method according to the invention also has the advantage of linking the decrease in current to the increase in voltage so as to avoid the coexistence of the maximum values of these two parameters. The preponderant influence of the combination of these parameters on the loss of cycling capacity has been shown. The simultaneous presence of high values of these, especially

  at the start of charging, has an accentuated effect on the aging of the generator.

  Preferably said maximum value I ^ of the charging current is between 0.1 Ic and lc, where lc is the theoretical current required to discharge said generator in one hour, that is to say the current necessary for

  discharge the nominal capacity C of the generator in one hour.

  The intensities of the different constant current load levels, as well as the voltage limits of each phase must be optimized according to the

  specificities of the battery (technology, nominal capacity, etc.). and the application made of it (duration of the charge, charger parameters, etc.).

  The total duration of the charge is between 1 hour and 10 hours. Compared to a load produced by one of the methods of the prior art by fixing the

  same maximum value Imu of the charging current and the same value Vf ,, of the end-of-charge voltage, the duration of the charge increases by less than 10% for a significantly extended generator life.

  The invention will be better understood and other advantages and features

  will appear on reading the description which follows, given under title

  limitative, accompanied by the appended drawing in which: FIG. 1 shows a charging method according to the prior art, the voltage V and the current 1, in amperes, are given on the ordinate, and on the abscissa the charging time T in hours - Figure 2 shows the charging method according to the present invention, the voltage V and the current I are given on the ordinate, and on the abscissa the time of

charge T in hours.

  It is understood that the various digital applications provided do not

  are only by way of nonlimiting examples.

  Electrochemical generators of the Lithium-lon type are manufactured comprising a negative electrode, the electrochemically active material of which is a carbonaceous material, and a positive electrode, the material of which

  electrochemically active is a lithiated oxide of a transition metal.

EXAMPLE 1

  A previously manufactured generator, whose nominal capacity Cn, is 45Ah and the voltage in the discharged state is 2.7V, is electrically cycled according to the prior art represented in FIG. 1 under the following conditions: constant current 1 ImX = 9A (i.e. 0.2 Ic, o Ic is the current required to discharge the nominal capacity Cn of said accumulator in one hour) until the generator reaches a voltage 2 equal to the end voltage load Vfjn = 4.0V, - load at a constant current 3 Il = 8A until the generator reaches the voltage 4 at the end of charge Vf1, = 4.0V, - load in successive stages at current 5 to 10 constant from 12 = 7A to 17 = 2A by lowering the current in steps of 1A as soon as the voltage 1 1 to 16 at the end of charge Vfin = 4.0V is reached, -last charge at a constant current 17 18 = 1A up to the generator reaches the voltage 18 at the end of the charge Vin = 4.0V,

- discharge at 22A up to 2.7V.

  After 120 cycles, the capacity loss per cycle is 0.16% of the

nominal capacity Cn.

EXAMPLE 2

  A generator previously manufactured, whose nominal capacity C. is 45Ah and the voltage in the discharged state is 2.7V, is electrically cycled according to the invention under the following conditions: (I) rise of the current from OA to 9A in 15 minutes, until the generator reaches a voltage of 3.7V, (2) loads at a constant current Imx = 9A (i.e. 0.2 Ic) until the generator reaches a voltage equal to one voltage V, = 3.7V, (3) charge at constant current 1 = 7A until the generator reaches the end of charge voltage V2 = 3.85V, (4) charge at constant current 12 = 5A until the generator reaches the end of charge voltage V3 = Vi, = 4.0V, (5) charge at a constant current 13 = 3A until the generator reaches the end of charge voltage V4 = Vfi, = 4,0V, (6) load at constant current 14 = 1A until the generator reaches the end of charge voltage V5 = V, = 4,0V, After 80 cycles, the loss of capacity per cycle is 0.05% of nominal capacity C ,, i.e. a capacity drop three times less than when

the known method is applied.

EXAMPLE 3

  The generators previously manufactured are assembled in a battery of six generators whose nominal capacity is Cn = 90Ah. This battery consists of three assemblies arranged in series and each comprising two generators arranged in parallel. It is subjected to an electrical cycling according to the invention, represented in FIG. 2, the charge of which comprises six stages: (1) raising of the current 20 from OA to 18A until at least one of the generators reaches a voltage 21 of 3.7V, i.e. a duration between 15mn and 60mn, 10 (2) charging at a constant current 22 Imax = 18A until at least one of the generators reaches a voltage 23 VI = 3.7V, ( 3) charge at a constant current 24 1I = 14A until at least one of the generators reaches a voltage 25 V2 = 3.85V, (4) charge at a constant current 26 12 = 10A until at least one of the generators reaches a voltage 27 V4 = Vin = 4.0V, (5) charges at a constant current 28 13 = 6A until at least one of the generators reaches a voltage 29 V5 = Vo = 4, 0V,

  (6) charge at a constant current 14 = 2A until at least one of the generators reaches a voltage 31 V6 = V6n = 4.0V.

  These last two additional steps are intended to ensure that the battery is fully charged.

(7) discharge at 45A up to 2.7V.

  A substantially linear growth in the battery voltage is obtained as shown in FIG. 2, which is a factor favorable to the stability of the capacity in cycling.

  After 250 cycles, the loss of capacity per cycle is 0.04% of the nominal capacity C ,.

Claims (5)

CLAIMS:   1. / A method of charging a secondary electrochemical generator, characterized in that it comprises the following stages: -on imposing on said generator a charging current which increases up to a maximum value Imax, -on maintaining said current at said maximum value lma, until the voltage of said generator reaches a voltage value V, less than the value Vfin at the end of charge, -on imposes a current of value 1, less than said maximum value Imox up to 0 this that said voltage reaches a value V2 greater than the value V1 and less than the value Vfn ,, at the end of charging, -the previous step is repeated as many times as necessary until the   voltage reaches a value Vn equal to Vfin, and the load is stopped.   2. / The method of claim 1, wherein said charge current increases at a speed between 0.015 Ir, and 0.070 Im <per minute during the first stage.   3. / Method according to one of claims 1 and 2, wherein said value   maximum Im.x of the load current is between 0.1 Ic and Ic, where lc is the   theoretical current required to discharge said generator in one hour.   4. / Method according to one of the preceding claims, in which the total duration   charge is between 1 hour and 10 hours.   5. / Secondary lithium electrochemical generator charged by the process according to   one of the preceding claims.