This disclosure relates to an electrode for a lithium ion battery having an electrically activated matrix formed from a functionalized polymer material, and a process for electrical activation of the matrix.
Hybrid vehicles (HEV) and electric vehicles (EV) use chargeable-dischargeable energy storages. Secondary batteries such as lithium-ion batteries are typical energy storages for HEV and EV vehicles. Lithium-ion secondary batteries typically use carbon, such as graphite, as the anode electrode. Graphite materials are very stable and exhibit good cycle-life and durability. However, graphite material suffers from a low theoretical lithium storage capacity of only about 372 mAh/g. This low storage capacity results in poor energy density of the lithium-ion battery and low electric mileage per charge.
To increase the theoretical lithium storage capacity, silicon has been added to active materials. However, silicon active materials suffer from rapid capacity fade, poor cycle life and poor durability. One primary cause of this rapid capacity fade is the massive volume expansion of silicon (typically up to 300%) upon lithium insertion. Volume expansion of silicon causes particle cracking and pulverization. This deteriorative phenomenon escalates to the electrode level, leading to electrode delamination, loss of porosity, electrical isolation of the active material, increase in electrode thickness, rapid capacity fade and ultimate cell failure.
Disclosed herein are electrodes that incorporate an electrically activated matrix into which active material is provided. The active material includes alloying particles, which, as used herein, are active catalyst particles that have a high lithium storage capacity resulting in large volume expansions during lithiation. The electrically activated matrix is activated during charging and discharging of the battery, and when activated, maintains the electrode structure and stability by expanding and contracting with the volume expansion and contraction of the alloying particles during lithiation and delithiation, respectively. The electrically activated matrix also reduces cracking and pulverization of the alloying particles, maintaining electrical conductivity between active materials, thereby maintaining battery energy density through the life of the battery.
Also disclosed are lithium ion batteries having the electrodes taught herein. One example of a lithium ion battery has an anode comprising a current collector, a separator, and an electrically activated matrix formed from a polymer material having a functional group capable of changing chain length upon electrical activation. The electrically activated matrix is positioned between the current collector and the separator. An active material layer comprises active particles that undergo volume expansion of greater than 50% during discharge of the battery and is deposited in the electrically activated matrix. During discharge of the battery, the active particles are in an expanded state and the electrically activated matrix is in a contracted state due to electrical activation, such that a force on the active particles from the electrically activated matrix in the contracted state forces expansion of the active particles in one planar direction. During charging of the battery, the active particles are in an unexpanded state and the electrically activated matrix is in an uncontracted state.
These and other aspects of the present disclosure are disclosed in the following detailed description of the embodiments, the appended claims and the accompanying figures.
The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:
Because the carbon material used in electrodes of conventional batteries, such as lithium ion batteries or sodium ion batteries, suffers from a low specific capacity, the conventional battery has poor energy density even though there is small polarization and good stability. Furthermore, batteries having electrodes of graphite or other carbon materials develop increased internal resistance over time, which decreases their ability to deliver current.
To address the poor energy density of carbon based electrodes, alternative active materials with higher energy densities are desired. Silicon, tin, germanium and their oxides and alloys are non-limiting examples of materials that may be added to an electrode active material layer to improve its energy density, among other benefits. One particular example is the use of silicon in lithium-ion batteries. Silicon based anode active materials have potential as a replacement for the carbon material of conventional lithium-ion battery anodes due to silicon's high theoretical lithium storage capacity of 3500 to 4400 mAh/g. Such a high theoretical storage capacity could significantly enhance the energy density of the lithium-ion batteries. However, silicon active materials suffer from rapid capacity fade, poor cycle life and poor durability. One primary cause of this rapid capacity fade is the massive volume expansion of silicon (typically up to 300%) upon lithium insertion. Volume expansion of silicon can cause particle cracking and pulverization when the silicon has no room to expand. This expansion can lead to electrode delamination, electrical isolation of the active material, capacity fade due to collapsed conductive pathways, and, like carbon based electrodes, increased internal resistance over time, which decreases their ability to deliver current.
Disclosed herein are electrodes that incorporate an electrically activated matrix into which active material is provided. The active material includes alloying particles, which, as used herein, are active catalyst particles that have a high lithium storage capacity resulting in large volume expansions during lithiation. The electrically activated matrix is activated during charging and discharging of the battery, and when activated, maintains the electrode structure and stability by expanding and contracting with the volume expansion and contraction of the alloying particles during lithiation and delithiation, respectively. The electrically activated matrix also reduces cracking and pulverization of the alloying particles, maintaining electrical conductivity between active materials, thereby maintaining battery energy density through the life of the battery.
As schematically illustrated in
As illustrated in
The electrically activated matrix 18 is formed from a functionalized polymer, a material that exhibits stimuli-responsive functions, thus achieving a desired output upon being subjected to a specific input. Polymeric materials exhibit a range of mechanical responses which depend on the chemical and physical structure of the polymer chains. At the microscopic level, the mobility of polymer chains in the presence of an external stimulus is dependent on the degree of cross-linking and entanglements present in the polymer, as well as the functional groups used along the polymer chain. There are several ways in which structures having functional chemical groups or chains of homopolymers or copolymers grafted onto a polymeric backbone can be generated, and are known to those skilled in the art.
The functionalized polymer used to form the matrix will be selected based on the operating temperature of the electrode, the required activation voltage of the material, the operational voltage of the electrode, the change in chain length desired and the direction of change in change length desired, as non-limiting examples.
As the alloying particles 16 expand, the electrically activated matrix 18 also expands, but in a controlled manner as described herein. As illustrated in
In another embodiment of an electrode 100, illustrated in
The electrode 100 of
Although the figures schematically illustrate one alloying particle 16 per matrix opening, more than one alloying particle 16 may be in one matrix opening. A carbon material 20 such as carbon black can fill the voids between the electrically activated matrix 18 and the active material 14. The active material 14 can include graphite and alloying particles 16 of silicon. The alloying particles 16 can also be tin, germanium and any other material known to those skilled in the art that has a high capacity for lithium.
The electrodes 10, 100 have a current collector 22 and a separator 24, as illustrated in
As shown in
As shown in
Also disclosed herein are lithium ion batteries including the electrodes described above as anodes.
The words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example’ or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X includes A or B” is intended to mean any of the natural inclusive permutations. That is, if X includes A or B, X can include A alone, X can include B alone or X can include both A and B. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form.
The above-described embodiments, implementations and aspects have been described in order to allow easy understanding of the present invention and do not limit the present invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structure as is permitted under the law.
Other embodiments or implementations may be within the scope of the following claims.