Patent Application
20230103857
The invention relates generally to solid electrolyte interphase (SEI) preformed graphite, methods of forming SEI preformed graphite, apparatus for forming SEI preformed graphite, and electrochemical battery cells including an SEI preformed graphite electrode.
High energy density lithium-ion batteries (LIBs) are continuously pursued to fulfill the growing demands of the energy storage markets. However, the irreversible lithium loss due to the formation of solid electrolyte interphase (SEI) in the initial cycle on the conventional graphite anode greatly reduces the overall cell energy density of lithium-ion batteries, that is, the lost Li ions from forming SEI leads to the decrease of Li ions for the intercalation. For example, conventional commercial LIB battery cells typically are formed of a lithium transition metal oxide cathode (e.g., LiFePO4, LiCoO2, and LiMn2O4, etc.) and a graphite anode. Their energy density is dictated by the reversible number of the Li ions shuttled between the anode and cathode within per cell mass and cell voltage. However, during the initial charge process, there are 5-20% of the Li ions irreversibly consumed due to the formation of SEI layer on graphite surface. In the following cycles, there will be 5-20% of Li ions shortage for shuttling between the anode and cathode in the LIB cell operation. As the consequence, this initial lithium loss (ICL) greatly reduces the overall battery cell energy density. Some higher capacity anode materials, such as Sn and Si, make this situation even worse considering their much lower initial Coulombic efficiency (ICE).
To address this issue, methods of cathode prelithiation using additives have been widely explored to compensate this lithium loss. However, these additives with high lithium contents inevitably lower the loading of the cathode active materials. For example, cathode prelithiation with high Li content sacrificial reagents has been widely explored as a common strategy to compensate this lithium loss. The high lithium content of those reagents could greatly compensate the lithium loss and thus increase the full cell energy density. Nevertheless, the additives of these reagents inevitably lower the active cathode material loading and leave inactive residues which make it hard to fully unlock the potential of the full cell energy density. Moreover, those residues generated after the lithium compensation are typically not electrically conductive (e.g., S, metal oxide, Table 1) and consequently, destroy the spatial continuity of the whole electrode for electron transportation. The use of these additives may also be accompanied with the evolution of undesired gas products such as CO2, O2 and N2, raising the safety concerns of battery operation.
On the other hand, direct anode prelithiation techniques using chemical methods or stabilized lithium metal powder (SLMP) are effective approaches to elevate ICE but easily give rise to a further Li intercalation into these anode materials (e.g., graphite, SiOx). In this regard, an overcharged status is created once coupling them directly with most of the commercial Li-containing cathode materials (e.g., LiCoO2, LiFePO4). Tedious procedures such as redundant Li extraction, cell disassembly, electrode cleaning, and further full cell re-assembly are required for these prelithiated anode materials to ensure battery safety.
In contrast, an artificial SEI coating has been perceived as another way to protect anode material from potential side reaction. Certainly, the reduced SEI propagation with an artificial SEI layer typically leads to an elevated ICE and electrochemical performance. However, it is hard to control the loading of these artificial SEI coatings to a precise level, which is crucial to maximize the energy density of a battery cell with a minimum inactive material addition. Therefore, a complete solution to minimize the Li loss remains a persistent challenge.
Graphite is the main anode material choice for conventional commercial LIBs due to its superior reversibility and low cost. Unfortunately, the initial lithium loss caused by the SEI formation on the graphite anode typically equals approximately 20˜80 mAh g−1 of lithium specific capacity. Therefore, it would be desirable to directly preform a SEI layer on graphite with certain degree (e.g. >40 mAh g−1) before its application on LIBs in order to reduce or even eliminate this tendency of conventional LIBs.
According to one nonlimiting aspect, a method of making SEI preformed graphite is provided. The method includes disposing a suspension of graphite particles in an electrolyte between and in ionic contact with a cathode and an anode, wherein the anode comprises a lithium source, forming an SEI coating on the individual graphite particles in the suspension by generating a voltage between the cathode and the anode and across the suspension.
According to another nonlimiting aspect, an SEI preformed graphite is provided. The SEI preformed graphite includes a graphite powder having separate individual graphite particles, and a preformed SEI layer on each of a plurality of the graphite particles in powder form. The SEI layer is directly formed over and substantially completely covers the exterior surface of each of the plurality of graphite particle in the graphite powder.
According to yet another nonlimiting aspect, an electrochemical battery cell is provided. the electrochemical battery cell includes a first electrode of the SEI preformed graphite, a second electrode, and an electrolyte operatively coupling the first electrode with the second electrode to allow ions to flow from the one of the electrodes to the other of the electrodes.
According to still another nonlimiting aspect, a flow cell apparatus for forming SEI preformed graphite is provided. The flow cell apparatus includes a cathode, an anode including lithium spaced apart from the cathode and forming a volume between the cathode and the anode, a battery separator disposed between and separating the volume between the cathode and the anode, a permeable interlayer disposed between the battery separator and the cathode and defining a flow space between the interlayer and the battery separator and within the volume, a mixer configured to mix graphite powder in an electrolyte, and a pump configured to provide a flow of mixed graphite powder and electrolyte from the mixer to the flow space to engage graphite particles of the graphite powder against the cathode.
In some arrangements, the methods, SEI preformed graphite, electrochemical battery cells, and/or flow cell apparatus of the present disclosure may improve the energy density of LIBs by mitigating initial lithium loss and/or providing products, systems, and/or methods for achieving such improvements. These and other aspects, arrangements, features, and/or technical effects will become apparent upon detailed inspection of the figures and the following description.
The intended purpose of the following detailed description of the invention and the phraseology and terminology employed therein is to describe what is shown in the drawings, which include the depiction of and/or relate to one or more nonlimiting embodiments of the invention, and to describe certain but not all aspects of the embodiment(s) to which the drawings relate. The following detailed description also describes certain investigations relating to the embodiment(s) depicted in the drawings, and identifies certain but not all alternatives of the embodiment(s). As nonlimiting examples, the invention encompasses additional or alternative embodiments in which one or more features or aspects shown and/or described as part of a particular embodiment could be eliminated, and also encompasses additional or alternative embodiments that combine two or more features or aspects shown and/or described as part of different embodiments. Therefore, the appended claims, and not the detailed description, are intended to particularly point out subject matter regarded to be aspects of the invention, including certain but not necessarily all of the aspects and alternatives described in the detailed description.
The present disclosure provides a novel approach to solve one or more of the challenges described above with conventional LIB anodes. The present disclosure provides a facile method and system for graphite prelithiation by pre-forming SEI layers on the surface of graphite powders to form SEI preformed graphite (also referred to herein as “Pre-SEI graphite”) utilizing a specially designed flow cell. With these techniques, the Li accommodation in the graphite anode can be controlled by the operating time and current density in the flow cell for the electrochemical SEI formation. As a result, we demonstrate a 10% initial Columbic efficiency increase of a LiFePO4 (lithium ferrophosphate, or “LFP”) electrode in a full electrochemical battery cell configuration using the Pre-SEI graphite, compared with the pristine graphite anode. The electrochemical preformation of SEI on the graphite powders may in some arrangements offer a complete solution to offset initial lithium loss without a sacrifice of active cathode loading.
According to some aspects of the disclosure, a new method to prepare SEI preformed graphite (Pre-SEI graphite) is provided in order to mitigate the initial lithium loss by reducing SEI formation process in battery cell, which is intrinsically different from conventional cathode or anode prelithiation techniques. According to other aspects of the disclosure, this pre-SEI graphite may be fabricated in a new flow cell that is configured to electrochemically mimic the formation of SEI within battery cells. This new concept of Pre-SEI graphite enables a direct Li compensation on the freestanding graphite powders before being fabricated into an electrode (e.g., a cathode or an anode) rather than on a sophisticated already-fabricated electrode. This feature is also beneficial for the further scale up in industrial applications. Owing to the preformation SEI on graphite surface, approximately ˜25 mAh g−1 of the initial lithium loss has been compensated based on graphite half cells. The techniques of the present disclosure may lead to pre-SEI graphite providing a complete solution to solve the ICL issue, for example with the development of the electrolyte modification and flow cell design to a higher degree of SEI preformation. In any event, the inventors believe that the systems and techniques for electrochemical preformation of SEI on graphite disclosed herein open a new avenue to improve LIBs energy density by mitigating initial lithium loss.
Turning now to the nonlimiting embodiments represented in the drawings,
As best seen in
In order to form the SEI preformed graphite 12 using the flow cell apparatus 10, volume between the cathode 26 and the anode 28 is filled with enough of the suspension 24 to allow the suspension to contact both the cathode and anode and allow for ion transfer therebetween in any suitable manner. Thereafter, an SEI coating 40 is formed on the individual graphite particles 14 by generating a voltage across the suspension 24 between the cathode 26 and the anode 28. Preferably, the flow path 44 and the interlayer 32 are configured so as to ensure that suspension 24 contacts and flows across and/or against the surface of the cathode 26 so as to ensure that individual graphite particles 14 in the electrolyte 22 contact the surface of the cathode while the voltage is realized. In this example, the flow space 36 thus also forms a reaction space where the graphite particles 14 can react with lithium ions in the electrolyte 22 to form the SEI coating 40. The resulting SEI preformed graphite 12 is formed of a graphite particle 14 having the SEI coating 40 coating substantially the entire outer surface of individual graphite particles 14.
After subsequent process steps, such as drying, the SEI preformed graphite 12 can be accumulated again into a powder form, in which some or all of the individual graphite particles are individually coated with the SEI coating 40 around their entire exterior surfaces. The resulting SEI preformed graphite 12 may subsequently be used to form other products, such as an electrode in an electrochemical battery cell. For example, an anode of the battery cell can be formed of the SEI preformed graphite 12 bound to electrically conductive carbon black by a binder and shaped into a desired form for the form factor of the battery. Other uses of the SEI preformed graphite 12 are also possible.
Further details regarding nonlimiting examples of the flow cell apparatus 10, SEI preformed graphite 12, process for forming the SEI graphite 12, and a battery cell formed from the SEI preformed graphite related to various experimental procedures leading the present invention(s) are described in more detail hereinafter.
SEI preformation in flow system: The preforming SEI processes were conducted in a specially designed semi-flow cell 16, in which 50 mL 2 M LiTFSI/0.4 M LiNO3 in DOL was used as the electrolyte 22. It is noted that typical ethylene carbonate (EC) and ethyl methyl carbonate (EMC) based electrolytes (e.g. 1.2 M LiPF6 in EC and EMC) may lead to severe graphite agglomeration and hinder the electrolyte flow. 150 mg graphite powers were dispersed into this electrolyte 22 before the operation of the flow cell system 10. In this flow cell 16, as best seen in
Electrolyte and electrode preparation: The electrolyte used in coin cells (electrochemical battery cells) was prepared by dissolving 1.2 M LiPF6 in the EC-EMC mixture solvent (3:7 by mass) with 2 wt. % additives of FEC. The (non-SEI) graphite electrode and the pre-SEI graphite electrode were prepared by slurry coating on Cu foil, which comprised 80 wt. % active materials (graphite or Pre-SEI graphite 12, respectively), 10 wt. % Super P, and 10 wt. % polyvinylidene difluoride (PVDF) as the binder. Similarly, LFP electrodes were followed the same mass ratio of 8:1:1 for LFP, Super P, and PVDF on Al foil. The areal capacity loading was about 1.6 mg cm−2, and 4.5 mg cm−2 of anode materials and cathode materials, respectively.
Materials: Graphite powders (MescoCarbon MicrBeads, MCMB), Li foil (99.9%), LiFePO4 (LFP) and Super P conductive carbon black were obtained from MTI Corporation (USA). Battery-grade LiPF6, ethylene carbonate (EC), and ethyl carbonate (EMC) were obtained from Novolyte Technologies (China). 1,3-dioxolane (DOL, 99%), LiNO3 (99%), and Fluoroethylene carbonate (FEC, 99%) were purcfigure hased from Sigma-Aldrich (USA). Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, >98%) was obtained from TCI chemicals (Japan).
Electrochemical and materials characterization: Electrochemical tests were performed using CR2032 type coin cell. For half-cells, Li foils were used as the counter/reference electrode, and as-prepared electrodes (graphite, Pre-SEI graphite, LFP) were used as working electrodes (d=15 mm) with one piece of polypropylene separator (d=19 mm) (Celgard 2400) (USA). 75 μL of as-prepared EC-based electrolytes were used in each cell. The cut-off voltage was controlled at 0.005-2.000 V and 2.400-4.000 V (vs. Li+/Li) for graphite and LFP, respectively. For full cells (electrochemical battery cells), the mass ratio of the LFP and graphite (or Pre-SEI graphite 12) was controlled to 2.8:1, which corresponds to a N/P ratio of ˜1.02 according to the half-cell data. The cut-off voltage was controlled at 2.400-3.800 V (vs. Li+/Li) for these full cells. The cyclic voltammetry (CV) analyses of graphite and Pre-SEI graphite were conducted using a Solartron potentiostat (Ametek, UK) with a range of 0.005-2.000 V (Li+/Li) at a scan rate of 0.5 mV s−1. The electrochemical impedance spectra (EIS) were measured using a Solartron potentiostat with an AC voltage of 5 mV in the frequency range from 1000 kHz and 0.01 Hz (Model SI 1287). Galvanostatic charge-discharge of all of the half-cells and full cells were collected on a LAND-CT2001 battery testing instrument (Land Corp, China) at a determined C rate. The current density and specific capacity were based on the mass of the active materials loading. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were acquired from a JEOL 7800F and JEOL 2100F, respectively (JEOL, Japan). XPS data of surface chemistry was acquired using PHI 5000 VersaProbe II system (Physical Electronics, USA) that attached to an argon-atmosphere glovebox to avoid any contamination of moisture and air. The high-resolution spectra were obtained under the following conditions: 100 μm beam (25 W) with Al Kα radiation (hv)=1486.6 eV), Ar+-ion and electron beam sample neutralization, fixed analyzer transmission mode, and pass energy of 23.50 eV. The Shirly background data were subtracted from all spectra. The spectra were fitted to multiple Gaussian peaks by using the software package (XPSPEAK). The reported atomic concentrations were calculated from survey scans. The structure of graphite and Pre-SEI graphite were examined by X-ray powder diffraction (XRD) tests (Bruker D8 Discover, Bruker USA).
Design of Pre-SEI graphite: To evaluate the concepts of the Pre-SEI graphite 12 disclosed herein for mitigating the initial lithium loss, full cell energy density calculations based on LiNi0.8Co0.1Mn0.1O2 (NCM)-graphite full cell was conducted and compared with those typical cathode prelithiation additives, as shown in Table 1 below.
After the initial charge process, the high lithium content of those additives could greatly compensate the lithium loss and thus increase the cathode specific capacity and full cell energy density in the following cycles, as shown in
Because of this, we developed the “Pre-SEI graphite” 12 that is, a preformed SEI layer 40 on graphite powder 14 without using a battery cell disclosed herein. In this configuration, a SEI layer 40 is directly formed over the surface of graphite particles 14 in the anode and no inactive residues are produced, which means the full cell energy density could be elevated to a maximum extent compared those cathode additives in
Inspired by semi-solid flow batteries, a specially designed flow cell 16 was designed to prepare the pre-SEI graphite powder 12 of the present disclosure. For practical operation, the pre-SEI graphite 12 was prepared by mixing graphite powder with electrolyte 22 of lithium-ion batteries and followed by an electrochemical discharge/charge process (0.005-2.0 V vs. Li+/Li) in the flow cell 16 to mimic the formation of SEI in normal coin-type battery cells, as shown in
It has been widely accepted that small current is favorable for the SEI formation process. Different current densities were applied to explore the SEI formation process in the flow cell 16. At a quite large current density ( 1/100 C), there was limited SEI 40 formed (<2 mAh g−1). Such low Li compensation makes it hard to be transformed to practical lithium-ion batteries. However, a further reduction of formation current to 1/600 C led to a considerable SEI formation (˜27.6 mAh g−1). The contact between current collector (e.g., cathode 26) and dispersed graphite particles 14 in the suspension 24 appears to be important to ensure the preformed SEI propagation. A greatly reduced SEI preformation was found when the flow rate was elevated from 10 mL/mins to 20 mL/mins (˜5.5 mAh g−1). Moreover, there is very limited lithium intercalation because the formation of the insulating SEI layer 40 stops the further reaction between Li ions and the graphite particles 14 covered with the SEI layer 40. Such conclusion was further verified using X-ray powder diffraction (XRD), which presents a typical graphite layered structure patterns. After the formation of SEI on graphite, there is a 0.07° shift of the 2θ value (002 peak) to a higher degree. Typically, the graphite (022) peak shifts to a lower degree value along with the intercalation of lithium ions. The slight difference may come from the XRD measurement process (e.g., sample height difference). Accordingly, an SEI capacity as high as 27.6 mAh g−1 (7.4% of theoretical capacity of graphite) was formed in the pre-SEI graphite 12 of the present invention at a low current density condition.
SEI structures of Pre-SEI graphite: Turning now to
It has been widely accepted that the SEI structures dictate the electrochemical performance of battery materials. However, up till now, the SEI formation process was typically limited within the battery cell, which means that the electrolyte additives for a stable SEI formation have to be compatible with anode and cathode materials. In this regard, the flow cell system 10 of the present invention may also provide a way to artificially pre-design SEI structures outside the battery cell by simply modifying electrolyte components without the limitation of battery materials compatibility.
Electrochemical performance of Pre-SEI graphite: The possibility of using the SEI preformed graphite 12 to offset initial lithium loss in an electrochemical battery cell was also experimentally investigated.
An EIS experiment was carried out to study the interfacial properties of the SEI preformed (pre-SEI) graphite 12. The wetting degree of the electrolyte 22 with the electrode could be measured by the high frequency resistance (HFR) of the cell. It was found that the electrode formed with pre-SEI graphite 12 of the present invention demonstrates a slightly smaller HFR (˜2.9 Ω) compared with pristine graphite (˜3.0 Ω), suggesting a higher wetting degree due to the preformation of the SEI layer 40.
The electrochemical performance of the pre-SEI graphite 12 was evaluated in full electrochemical cells by incorporating the pre-SEI graphite 12 with LiFePO4 (LFP) cathode materials. The mass ratio between LFP and pre-SEI graphite 12 (or graphite) was carefully controlled to 2.8:1, which was corresponding to a N/P ratio of ˜1.02. The full cell composed of LFP and Pre-SEI graphite cell exhibited an ICE value of 50.5%, which is 10.1% higher than those in pristine graphite anode cells (40.4%). Accordingly, the reversible cathode specific capacity with pre-SEI graphite of the present invention is 23.7% higher compared with the conventional graphite anode cells (18.6 mAh g−1) after 100 cycles at 0.1 C rate. The increased capacity retention is benefited from the reduced initial lithium loss with preformed SEI graphite 12 on the graphite anode.
Conclusions: A new pre-SEI graphite concept is disclosed that is believed to compensate the ICL of lithium-ion batteries, which is intrinsically different from previously known cathode or anode prelithiation strategies. Owing to the preformation of SEI 40 on graphite particles 14, the improvement of the CE has been confirmed in both half-cells and LFP based full cells. A higher ICE of the pre-SEI graphite 12 could be further obtained by tuning SEI formation electrolyte components in flow cells. Thus, more explorations based on the pre-SEI concepts disclosed herein, such as specific flow cell designs and current control, may provide a complete solution to fully address the ICL issue of the conventional technology. The direct Li compensation of freestanding graphite powders makes it easy to meet the requirements for industrial scale-up. Moreover, the SEI preformed graphite concepts disclosed herein may further allow the pre-design of specific SEI structure outside the battery cells with the modification of the electrolyte components, and may be a transformative strategy to next-generation high-capacity anode materials with low ICE such as Si and Sn, which opens a new avenue to fully unlock the energy density potential of practical Li-ion batteries.
As previously noted above, though the foregoing detailed description describes certain aspects of one or more particular embodiments of the invention, alternatives could be adopted by one skilled in the art. For example, the methods, apparatus, products, and their components, could differ in appearance and construction from the embodiments described herein and shown in the drawings, functions of certain components of the methods, apparatus, products, and their components, could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, and various materials could be used in the fabrication of the apparatus, products, and/or their components. As such, and again as was previously noted, it should be understood that the invention is not necessarily limited to any particular embodiment described herein or illustrated in the drawings.
This application claims the benefit of U.S. Provisional Application No. 63/251,064 filed Oct. 1, 2021, the contents of which are incorporated herein by reference.
This invention was made with government support under contract number DE-ACO2-06CH11357 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63251064 | Oct 2021 | US |
