Method of enhancing electrochemical cell performance

A method of enhancing performance of an electrochemical cell having a first electrode and a second electrode and electrolyte between the first and second electrodes. The first and second electrodes define a current flow path and the method comprises providing a changing magnetic field through the cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application represents the United States National Stage of International Application No. PCT/EP2020/050145, filed Jan. 6, 2020, which relates to and claims priority to British Patent Application Serial No. GB 1900171.8, filed Jan. 7, 2019, both of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the performance of electrochemical cells and particularly, although not exclusively, to increasing the speed of charge and discharge and the capacity of electrochemical cells.

BACKGROUND

Electrochemical cells are vital to many electrical systems, in particular portable devices such as mobile phones and laptops and, increasingly, electric vehicles.

The portability of electronic devices/vehicles are dependent on the performance of their cells. In general it is desirable to have a cell that has a large capacity and a short charging time to increase the ratio of time that the devices/vehicles can operate independently of an external power source to time that they must be connected to an external power source for charging.

Various types and configurations of electrochemical cells can be selected based on size, shape, voltage, current and other requirements. Examples of common shapes of cells are pouch cells, cylindrical cells, Swagelok cells and coin cells. The cells may also be connected in a battery to provide the appropriate voltage and/or current for the application.

If a cell is charged too quickly, many undesirable operation conditions can occur; such as: dendrite formation, metallic plating and current hot-spots, each of which can potentially increase the likelihood of a short circuit and damage to the cell.

A common method of quickly charging a cell while avoiding over-charging is to supply a constant current in a first charging phase until a predetermined voltage is reached in the cell and then, in a second charging phase, holding the voltage constant while the current decays to ensure the cell reaches capacity. The first charging phase quickly increases the charge in the cell and the second charging phase is slower.

A C-rate is a measure of the rate of charge or discharge of a cell and is the current divided by the capacity and with units of per hour.

The present invention has been devised in light of the above considerations.

SUMMARY OF THE INVENTION

The present invention provides a method of enhancing performance of an electrochemical cell having a first electrode and a second electrode and electrolyte between the first and second electrodes, the first and second electrodes defining a current flow path, the method comprising providing a changing magnetic field through the cell.

The changing magnetic field aids ion transport in the electrochemical cell thereby meaning that performance of the battery is enhanced. Optionally, the cell may include a separator between the electrodes. The transport of ions can be improved in the electrolyte and/or in the electrodes and/or in the separator. The changing magnetic field through the cell means that the magnetic flux in the cell varies over time in magnitude and/or direction and/or distribution.

One characteristic of the cell which can be enhanced using the above method is the speed of charging of the cell. Ion transport in cells is often the rate determining process during charging and so aiding the transport of ions speeds up charging of the cell. Another characteristic of the cell which can be enhanced using the above method is the speed of discharging as ion transport is improved in an analogous way to during charging. Another characteristic of the cell which can be enhanced using the above method is the capacity of the cell. This may be achieved by performing the method above on the cell during formation of the cell or during operation.

The changing magnetic field may be a rotating magnetic field and/or an oscillating magnetic field and/or a pulsing magnetic field.

The changing magnetic field may have a direction with a component perpendicular to the current flow path. The changing magnetic field may have a direction with a component parallel to the current flow path.

Rotation of the magnetic field may be around an axis having a component perpendicular to a direction of the magnetic field. The rotation of the magnetic field may be around an axis having a component parallel to a direction of the magnetic field.

Rotation of the magnetic field may be around an axis having a component perpendicular to a direction of the current flow path. The rotation of the magnetic field may be around an axis having a component parallel to a direction of the current flow path.

The rotating magnetic field may be provided by a spinning permanent magnet, or a temporary magnet, or electromagnet or may be provided by an array of electromagnets which are sequentially activated to effectively rotate the magnetic field.

The electrochemical cell may be a battery. The battery may be a coin cell, a cylindrical cell, a prismatic cell or a pouch cell.

The battery may be a positive ion battery and the current flow path may be the direction of travel of positive ions. The battery may be a lithium-ion battery. Alternatively, the battery may be a negative ion battery and the current flow path may be the direction of travel of negative ions.

The cell may be for powering an electric vehicle, a mobile phone, a laptop computer, tablet or other portable or stationary device. The cell may be a fuel cell.

The electrochemical cell may be part of an array of two or more cells.

The magnetic field may be provided by a permanent magnet or temporary magnet or an electromagnet.

The electrolyte may be a solid, liquid or a gel. In particular, the electrolyte may be non-aqueous including organic electrolytes.

A magnetic field generator may be provided for generating the changing magnetic field. The magnetic field generator may be within the cell or external to the cell.

In another aspect, the present invention provides a charge accelerator for enhancing performance of an electrochemical cell, the device being configured to carry out a method of enhancing performance of an electrochemical cell described above.

In another aspect, the present invention provides a method of charging an electrochemical cell comprising the steps of providing a current or voltage to the cell from a power source whilst performing a method of enhancing performance of an electrochemical cell described above. In this disclosure, charging a cell includes moving ions in a cell whether imposed by a current or voltage or by spontaneous movement of the ions.

In another aspect, the present invention provides a method of discharging an electrochemical cell comprising the steps of extracting a current or voltage from the cell whilst performing a method of enhancing performance of an electrochemical cell described above.

In another aspect, the present invention provides a method of enhancing capacity of an electrochemical cell comprising the steps of forming or during operation of an electrochemical cell whilst performing a method of enhancing performance of an electrochemical cell described above.

DETAILED DESCRIPTION OF THE INVENTION

The arrangement of equipment shown inFIG.1can be used to enhance and monitor the performance of a cell1while charging or discharging. The cell1is located on top of a rotating magnetic field generator2. The cell1is connected to the potentiostat and computer3via terminals4. The potentiostat controls the potential across the cell1and may charge or discharge the cell1. The computer monitors the current, and/or capacity and/or voltage of the cell1. The rotating magnetic field generator2provides a rotating magnetic field through the cell1.

This arrangement can be used for testing the cell, but when monitoring of the cell1is not required, the potentiostat, computer3and terminals4can be removed and optionally replaced by a power source or drain for charging or discharging the cell.

In the arrangement ofFIG.1, the cell1is located on top of the rotating magnetic field generator2, but in other embodiments of the invention, the cell1and rotating magnetic field generator2may be oriented differently as long as the rotating magnetic field generator2can produce a magnetic field through the cell1.

The rotation of the magnetic field may be around an axis substantially parallel to the direction of the magnetic field produced. For example, the direction of the magnetic field produced in the cell1may be substantially parallel to a direction between the magnetic field generator2and the cell1and the rotation of the field may be around an axis parallel to a direction between the magnetic field generator2and the cell1as shown inFIGS.2A and2B.

Alternatively, the rotation magnetic field may be around an axis substantially perpendicular to the direction of the magnetic field produced. For example, the direction of the magnetic field produced in the cell1may be substantially perpendicular to a direction between the magnetic field generator2and the cell1and the rotation of the field may be around an axis parallel to a direction between the magnetic field generator2and the cell1.

The rotating magnetic field generator2in the arrangement ofFIG.1, may be replaced with a changing magnetic field generator that produces a changing magnetic field that varies in another way. For example, a changing magnetic field generator may be used in to produce a magnetic field that rotates and/or oscillates and/or pulses.

The following examples of apparatuses and methods show the effect of changing magnetic fields on the charging of several commercially available batteries. All of the examples show reduced time to charge the cells in the presence of a changing magnetic field. The cells are of various geometries and chemical make ups and are described in more detail below.

In this disclosure, the term “under field conditions” refers to the presence of a changing magnetic field. In the examples explained below, results for charging the cells in the presence of a changing magnetic field are shown along with comparative examples of the cells in the absence of a changing magnetic field. The arrangements used in the examples and in the comparative examples differ only in that the magnetic field provided in the comparative examples is constant in direction and magnitude, whereas the magnetic field provided in the examples is changing.

FIG.2shows an arrangement used to enhance performance of a pouch cell11. The magnetic field generator12produces a magnetic field having a direction parallel to the direction shown by the arrow inFIG.2A. The magnetic field produced rotates in the direction shown by the arrow inFIG.2B. The magnetic field passes through pouch cell11.

The magnetic field be offset from the axis of rotation to ensure that the magnetic flux in the cell changes over time.

The magnetic field generator12is an electromagnet powered by power supply15. Potentiostat13is connected to the pouch cell11and controls the potential over the cell and can be used to charge or discharge the cell.

The pouch cell11is formed of a first electrode and a second electrode separated from one another by an electrolyte. The electrodes are substantially parallel and extend across a length and width of the cell. The pouch cell11has contacts for each of the electrodes which may be connected to a potentiostat as shown inFIG.2.

The pouch cell11is oriented so that the direction of the magnetic field passes through the first electrode of the cell, through the electrolyte and through the second electrode. The direction of the magnetic field is parallel to the direction of a current path between the electrodes. The rotation plane of the magnetic field is parallel to the planes of the electrodes.

FIG.3shows the capacity of pouch cell11when charged in the presence of a magnetic field produced by the magnetic field generator12when the field is (i) rotating (shown by the dashed line) and (ii) not rotating (shown by the solid line). The pouch cell11was charged in two phases. In the first phase, the cell was charged from 3.4V to 4.2V at 0.841 A until 4.2V was reached and then in the second phase, the cell was held at constant voltage while the current decayed and capacity was reached.

The first phase where a constant current is applied to the cell can be seen by the horizontal line portions of the current graph inFIG.4. The second phase where voltage is held constant and current decays while full capacity is reached can be seen where the current changes. The straight horizontal lines to t=220 and 300 seconds for no changing field and changing field respectively, represent the constant current portion of the charging and t onwards representing the constant voltage portion of the charging.

A magnetic field was produced by an electromagnet in the magnetic field generator12. During the cycle shown by the dashed line, the electromagnet was spun at 1160 rpm. The results show that the time taken to charge the cell was reduced by 68% by the presence of the rotating magnetic field.

FIG.4shows the current and voltage during the charging of the pouch cell11in the conditions described in relation toFIG.3. Again, the dashed line shows the charge when the magnetic field is rotating and the solid line shows charging when the magnetic field is not rotating (it is constant). The graphs show that, in the presence of a rotating magnetic field, the time taken to reach the voltage of 4.2V is increased so that the first phase of charging where current is applied is maintained for a longer period. As the first phase of charging increases the charge held by the cell more quickly than the second phase, this means that charging overall is quicker when the rotating magnetic field is present through the cell.

FIG.5shows the capacity of the pouch cell11when charged in the presence of a magnetic field produced by magnetic field generator12when the field is (i) rotating (shown by the dashed line) and (ii) not rotating (shown by the solid line). The pouch cell11was charged in two phases. In the first phase, the cell was charged from 3.4V to 4.2V at 0.841 A until 4.2V was reached and then in the second phase, the cell was held at constant voltage while the current decayed and capacity was reached.

As can be seen inFIG.5, in both cycles the cells were charged to 4.2V, but in the cycle where the magnetic field was rotating, the capacity of the cell was increased by 5%. The speed of charge was also increased by the rotation of the magnetic field.

FIG.6shows results of testing a pouch cell21. The pouch cell21has a 400 mAh capacity and has dimensions of 5 cm by 2 cm by 0.5 cm. The pouch cell21is commercially available via the part information: +PL-402248-2C, 3.7V 400 mAh −PO 7006 20140726.

The rate of charge of the pouch cell21is shown inFIG.6for nine charge cycles. Cycles 1 to 3, 7 and 8 were in the presence of a magnetic field rotating at 1170 rpm and cycle 9 was in the presence of the magnetic field rotating at 1000 rpm. Cycles 4 to 6 were in the presence of a static magnetic field.

As can be seen inFIG.6, the rate of charging was consistently increased by the rotation of the magnetic field by around 15%.

FIG.7shows the time taken to charge pouch cell21in eight charge cycles with a rotating field at 1170 rpm, a rotating field at 1000 rpm, and a static magnetic field (labelled ‘no field’ in the figure). As can be seen inFIG.7, the time taken to charge the cell is consistently decreased by around 15% in the presence of a rotating magnetic field.

FIG.8shows results of testing a pouch cell31. The pouch cell31has a 200 mAh capacity and has dimensions of 2.5 cm by 1.7 cm by 0.5 cm. The pouch cell31is commercially available via the part information: −PL-651628-2C, 3.7V 210 mAh +PO 7994.

The rate of charge of the pouch cell31is shown inFIG.8for two charge cycles. The first cycle was in the presence of a magnetic field rotating at 1160 rpm and the second cycle was in the presence of a static magnetic field.

As can be seen inFIG.8, the rate of charging was dramatically increased by the rotation of the magnetic field.

FIG.9shows the time taken to charge pouch cell31in a charge cycle with a rotating field at 1160 rpm, and a static magnetic field (labelled ‘no field’ in the figure). As can be seen inFIG.9, the time taken to charge the cell at 4C is decreased by 58% in the presence of a rotating magnetic field.

FIG.10shows an arrangement used to enhance performance of a Swagelok-type cell41. The magnetic field generator42produces a magnetic field having the direction parallel to the direction shown by the arrow inFIG.10A. The magnetic field produced rotates in the direction shown by the arrow inFIG.10B. The magnetic field passes through Swagelok cell41.

The magnetic field be offset from the axis of rotation to ensure that the magnetic flux in the cell changes over time.

The magnetic field generator42is an electromagnet powered by power supply45. Potentiostat43is connected to the Swagelok cell41and controls the potential over the cell and can be used to charge the cell.

The Swagelok cell41is formed of a first electrode and a second electrode separated from one another by an electrolyte and separator material. The electrodes are substantially parallel and extend across a length and width of the cell. The Swagelok cell41has contacts for each of the electrodes which may be connected to a potentiostat as shown inFIG.10.

The Swagelok cell41is oriented so that the direction of the magnetic field passes through the cell, perpendicularly to the direction of a current path between the electrodes. The rotation plane of the magnetic field is perpendicular to the planes of the electrodes.

FIG.11shows results of testing Swagelok cell51. The Swagelok cell51has dimensions of 5 cm by 2.5 cm. The Swagelok cell51is commercially available as a LMO/Graphite Swagelok cell.

The rate of charge of the Swagelok cell51is shown inFIG.11for charge cycles at 1C, 2C and 3C, each in the presence of a rotating magnetic field and in the presence of a static magnetic field (labelled as no field in the figure). The rotating magnetic field had a speed of 1100 rpm.

As can be seen inFIG.11, the rate of charging was consistently increased by the rotation of the magnetic field.

FIG.12shows the time taken to charge Swagelok cell51in charge cycles at 1C, 2C and 3C, each in the presence of a rotating magnetic field and in the presence of a static magnetic field (labelled as no field in the figure). As can be seen inFIG.12, the time taken to charge the cell51is consistently decreased in the presence of a rotating magnetic field.

FIG.13shows an arrangement used to enhance performance of a cylindrical cell61. The magnetic field generator62produces a magnetic field having the direction parallel to the direction shown by the arrow inFIG.13B. The magnetic field produced rotates in the direction shown by the arrow inFIG.13C. The magnetic field passes through cylindrical cell61.

The cylindrical cell61is formed of a first electrode and a second electrode separated from one another by an electrolyte. The electrodes are rolled into a spiral configuration and extend across a length of the cell. The cylindrical cell61has contacts for each of the electrodes which may be connected to a potentiostat as shown inFIG.13.

The cylindrical cell61is oriented inFIG.13so that the direction of the magnetic field passes through the cross-section of the cylindrical shape, perpendicularly to a direction between the flat ends of the cylindrical shape. The rotation plane of the magnetic field is perpendicular to the planes of the end faces of the cylindrical cell.

As discussed further below, in other embodiments, the cylindrical cell may alternatively be oriented so that the direction of the magnetic field passes through the ends of the cylindrical cell.

The magnetic field be offset from the axis of rotation to ensure that the magnetic flux in the cell changes over time.

The magnetic field generator62is an electromagnet powered by power supply65. Potentiostat63is connected to the cylindrical cell61and controls the potential over the cell and can be used to charge the cell.

FIG.14shows results of testing cylindrical cell71. The cylindrical cell71has dimensions of 6.5 cm by 1.8 cm. The cylindrical cell51is commercially available as a cylindrical 2190 mAh LG 18650 cell.

The rate of charge of the cylindrical cell71is shown inFIG.14for six charge cycles. Cycles 1 and 2 were in the presence of a magnetic field rotating at 1170 rpm, cycle 5 was in the presence of the magnetic field rotating at 1500 rpm and cycle 6 was in the presence of a magnetic field rotating at 1200 rpm. Cycles 3 and 4 were in the presence of a static magnetic field. In cycles 5 and 6, the cylindrical cell was positioned so at an orientation 90 degrees from the position shown inFIG.13Aso that a flat end of the cell faces the magnetic field generator62.

As can be seen inFIG.14, the rate of charging was generally higher in the presence of a rotating magnetic field.

FIG.15shows the time taken to charge cylindrical cell71in the charge cycles ofFIG.14. As can be seen inFIG.15, the time taken to charge the cell71was generally lower in the presence of a rotating magnetic field.

The principle of improved transport of ions resulting in improved charging speed and/or increased capacity within an electrochemical cell exhibited by the examples above may be explained by a reduction in activation energy as explained below for an example of a positive ion cell.

The ion velocity, v, within a liquid electrolyte will increase until overcome by drag forces, FD. The electric field force, FE, which drives the motion of the ion can be described by:

FE=zi⁢q⁢dvdxeg.1zicharge number of the ionq fundamental charge of an electron (1.6×10−19C)dV voltage differentialdx spatial differential

The drag force can be approximated from Stoke's law as:
FD=6πμrveq. 2μ viscosity of the liquidr radius of the ionv velocity of the ion

Equating the electric and drag forces determines the terminal velocity of the ion, thus mobility, ui:

The mobility influences the conductivity through the equation:
σi(|zi|F)ciuieq. 5cimolar concertation

In a polymer electrolyte the Ohmic resistance, σ, can be described by:

The Maxwell-Faraday equation predicts that a time varying magnetic field, δB/δt, will always accompany a spatially-varying, non-conservative electric field, E(r, t), described by:

∇×E⁡(r,t)=-δ⁢B⁡(r,t)δ⁢teq.9∇ Curl operator (infinitesimal rotation of a 3-dimensional vector field)r positiont time

And the electric field, E, at a given point is defined as the vector electric field force, FE, for a given charge, q:
FE=qEeq. 10

Therefore, the magnetic field influences the activation energy as:
B∝E∝FE∝ui∝σi∝exp(−ΔGact)  eq. 11

Finally conductivity is related to cell resistivity, ρi, thus Ohmic potential losses, ηohmic, via:

So, in the presence of a magnetic field, a polymer membrane conducting positive ions will experience a reduced Ohmic potential losses through a net reduction in the activation energy related to the ionic mobility associated with proton transport.
B∝ηohmiceq. 13

The magnetic field can be produced using a permanent magnet or an electromagnet.

Using a permanent magnet, in the presence of a magnetic field, a polymer membrane conducting positive ions will experience a reduced Ohmic potential losses through a net reduction in the activation energy

F=B2⁢A2⁢μ0eq.14B magnetic inductionA cross-sectional area of plungerμ0permeability of space

Using an electromagnet, in the presence of a magnetic field, a polymer membrane conducting positive ions will experience a reduced Ohmic potential losses through a net reduction in the activation energy:

F=CAnlleq.15C proportionality constantA cross-sectional area of plungern number of turns in the solenoidI currentl length of the solenoid