Method for adapting the cell chemistry of a lithium-ion battery and electrolyte for a lithium-ion battery

Abstract

In various embodiments, a method for adapting the cell chemistry of a lithium-ion battery is described, comprising adding an additive to a liquid electrolyte for the lithium-ion battery, wherein the additive comprises an anionically fluorinated phosphate with at least one fluorocarbon residue. Furthermore, an electrolyte for a lithium-ion battery is provided.

Description

[0001] The present invention relates to a method for adapting the cell chemistry of a lithium-ion battery. Furthermore, an electrolyte for a lithium-ion battery is provided.

[0002] Thermal runaway (TR), also known as thermal runaway, is a critical phenomenon that can occur in lithium-ion batteries (LIBs), particularly those used in electric vehicles. It is a chain reaction in which overheating of an affected battery cell leads to further thermal events, ultimately resulting in an uncontrolled release of energy. This can lead to complete battery failure, fire, and even an explosion due to the gas generated within the affected battery cell.

[0003] Thermal runaway often begins with an internal or external trigger. Possible triggers include mechanical damage, electrical overload, overheating, or internal short circuits. These events lead to an increase in the temperature of the affected battery cell. Above a certain temperature, the electrolyte in the battery cell begins to decompose, resulting in further heat release. If this process continues as part of a chain reaction between adjacent battery cells, it is called thermal propagation, which can lead to the complete destruction of the entire battery.

[0004] To reduce the risk of thermal runaway, various measures are being taken: development of advanced battery management systems (BMS), implementation of mechanical protection regulations, and advanced cooling systems. Research is also being conducted on new battery materials that are less susceptible to thermal events.

[0005] Developing a new, more temperature-stable chemical cell system (cell chemistry) is time-consuming and expensive. Since the legal framework has changed and stricter requirements must be met, it would be advantageous to retain the existing, well-researched cell system while modifying it appropriately.

[0006] The aim of the present invention is to provide a way to optimize the currently widely used cell system of lithium-ion batteries with regard to the problem of thermal runaway.

[0007] In various embodiments, a method for adapting the cell chemistry of a lithium-ion battery is provided. The method involves adding an additive to a liquid electrolyte for the lithium-ion battery, wherein the additive comprises an anionic fluorinated phosphate with at least one fluorocarbon residue. The method can advantageously be carried out before the introduction of the adapted electrolyte into the battery cells. The provided method is also a method for providing a cell chemistry for a lithium-ion battery.

[0008] The process according to the invention can be carried out in conjunction with all electrolytes used in current lithium-ion cells that contain a lithium salt dissolved in an organic solvent, such as ethyl carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or a suitable mixture of at least two of these. The additive should be soluble in the electrolyte used, which is the case with the aforementioned electrolytes. Furthermore, the solvent of the electrolyte used should possess two main properties: dissolving capacity for lithium salts and ionic conductivity. Both properties apply to all currently common electrolytes in lithium-ion batteries.

[0009] The liquid electrolyte for the lithium-ion battery can be a conventional, state-of-the-art electrolyte used in LIBs. Common examples include salts such as lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), or lithium bis(oxalato)borate (LiBOB) dissolved in an aprotic solvent, as well as polymers of polyvinylidene fluoride (PVDF), lithium phosphate nitride (Li3PO4N), and lithium titanium thiophosphate (LiTi2(PS4)3), to name a few.

[0010] The present invention is therefore based on the fundamental idea of ​​adding an additional additive to a liquid electrolyte. The addition of the additive leads to polymerization, and the previously liquid electrolyte becomes a gel electrolyte based on a coordination polymer. The additive belongs to the phosphate molecular class and comprises a mono- or polyfluorinated phosphate with at least one fluorocarbon residue. In two

[0011] Fluorocarbon residues can be of the same or different lengths.

[0012] An exemplary additive can be lithium bis(2,2,2-trifluoroethyl)phosphate (Li[O₂P(OCH₂CF₃)₂] (LiBFEP). Further examples include Li[O₂P{OC(H)(CF₃)₂]₂. 2}2] or Li[O2P(OCH2CH2CF3)2)] can be used. The additive is essentially a mono- or diester of phosphoric acid, in which the residues are fluorinated hydrocarbons of varying or identical lengths. The number and position (i.e., bond position in the molecule) of the fluorine atoms are variable without negatively affecting the fundamental property of the additive with regard to its inventive use.

[0013] According to further embodiments of the process according to the invention, the additive can comprise a fluorinated phosphate, e.g., a fully fluorinated phosphate, with two fluorocarbon residues. The fluorocarbon residues can have the same length or different lengths.

[0014] According to further embodiments of the process according to the invention, at least one residue of the alkyl phosphate can have a mono- or polyfluorinated alkyl residue.

[0015] According to further embodiments of the method according to the invention, the additive can be added in a concentration between 0.2 M and 1 M. Such a selected amount of additive forms a viscous gel that also possesses good conductivity.

[0016] In further embodiments of the invention, an electrolyte for a lithium-ion battery is provided, wherein the electrolyte comprises an additive that includes a mono- or polyfluorinated phosphate with at least one fluorocarbon residue. The electrolyte according to the invention can be produced, for example, using the method described above. The additive is a film-forming additive that is mixed with a conventional electrolyte suitable for lithium-ion batteries.

[0017] According to further embodiments of the electrolyte according to the invention, it can comprise a gel electrolyte based on a coordination polymer.

[0018] Coordination polymers are characterized by one-, two-, or three-dimensional linkage via organic donor ligands. In principle, polymers of any length can be formed in this way. Shorter-chain hydrocarbons (HCCs) are generally more suitable for this purpose, as polymerization becomes increasingly difficult with increasing HCC length, since the oxygen atom is better protected or blocked from reaction attack by the HCC.

[0019] According to further embodiments of the electrolyte according to the invention, the additive can be a fluorinated phosphate with two fluorocarbons as residues, which may have different or the same length.

[0020] According to further embodiments of the electrolyte according to the invention, at least one residue of the alkyl phosphate can comprise a mono- or polyfluorinated alkyl residue.

[0021] According to further embodiments of the electrolyte according to the invention, the additive can be present in a concentration between 0.2M and 1M.

[0022] Generally speaking, the additive can be a mono- or dialkyl phosphate, wherein the alkyl group of the alkyl phosphate can have hydrocarbon chains of different lengths, and wherein at least one alkyl group of the alkyl phosphate is a mono- or polyfluorinated alkyl group.

[0023] The additive according to the invention, such as the aforementioned LiBFEP, polymerizes in a liquid electrolyte, thereby stabilizing the solid electrolyte interface (SEI) on the anode side. The anode can, for example, comprise graphite, silicon, silicon / graphite, SiOx, SiOx / graphite, silicon composite, or silicon composite / graphite. The electrolyte according to the invention simultaneously passivates the cathode, which may, for example, consist of NMC622 (lithium nickel manganese cobalt oxide with a nickel, manganese and cobalt ratio of 6:2:2), NMC811 (lithium nickel manganese cobalt oxide with a nickel, manganese and cobalt ratio of 8:1:1), Ni-rich cathode material, LFP (lithium ferrophosphate), or LMFP (lithium manganese ferrophosphate), and prevents or at least reduces the continuous oxidation of the electrolyte.The aforementioned effects contribute to the fact that, through the addition of the additive to the electrolyte according to the invention, the battery cell is more stable against thermal runaway at higher temperatures. This advantageous effect is due to the increase in the viscosity of the electrolyte and the formation of the cathode electrolyte interface (CEI) and the SEI. At the same time, the addition of the additive has no significant disadvantages for other properties of the battery cell, such as fast charging.

[0024] Cyclic voltammetry (CV) and linear sweep voltammetry combined with online electrochemical mass spectrometry (LSV-OEMS) on a conductive carbon electrode (C65 / PVDF composite) show that LiBFEP reduces electrolyte oxidation and LiPF6 decomposition at high temperatures. The incorporation of LiBFEP into LiPF6 in ethylene carbonate / ethylene methyl carbonate improves the Coulomb efficiency and capacity retention for LNMO / graphite cells. Ex-situ surface analysis of the electrodes reveals that the incorporation of LiBFEP leads to the formation of a cathode electrolyte oxidation (CEI) and a modification of the electrolyte electrolyte oxidation (SEI) at the anode. The formation of the CEI mitigates electrolyte oxidation and prevents LiPF6 decomposition, which in turn prevents RF-induced manganese dissolution from the cathode and SEI destabilization. Passivation of the cathode and stabilization of the SEI are responsible for the increased Coulomb efficiency and capacity maintenance.

[0025] The features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0026] Further advantages and embodiments of the invention will become apparent from the following description of exemplary embodiments and the accompanying drawings.

[0027] Fig. Figure 1 shows an exemplary additive which can be used within the scope of the present invention.

[0028] At the in Fig.The additive shown in Figure 1 is a di-esterified phosphate. At least one of the phosphate groups R1, R2 may be a fluorocarbon. If both phosphate groups R1, R2 contain a fluorocarbon, they may have the same or different structures, in particular, they may have the same or different lengths.

[0029] In a first example, one of the phosphate groups R1, R2 can have a hydrogen atom and the corresponding other phosphate group R2, R1 can have a trifluoroethyl atom.

[0030] In another example, both phosphate groups R1 and R2 can contain a trifluoroethyl group. The additive then corresponds to lithium bis(2,2,2-trifluoroethyl)phosphate (Li[O2P(OCH2CF3)2] (LiBFEP).

Claims

[1] Method for adapting a cell chemistry of a lithium-ion battery, comprising: Adding an additive to a liquid electrolyte for the lithium-ion battery, wherein the additive comprises an anionic fluorinated phosphate with at least one fluorocarbon residue. [2] Method according to claim 1, wherein the additive comprises a fluorinated phosphate with two fluorocarbons as residues. [3] Method according to claim 1 or 2, wherein at least one residue of the alkyl phosphate comprises a mono- or polyfluorinated alkyl residue. [4] Method according to any one of claims 1 to 3, wherein the additive is added in a concentration between 0.2M and 1M. [5] Electrolyte for a lithium-ion battery, wherein the electrolyte comprises an additive which has an anionic fluorinated phosphate with at least one fluorocarbon as a residue. [6] Electrolyte according to claim 5, wherein the electrolyte comprises a gel electrolyte based on a coordination polymer. [7] Electrolyte according to claim 5 or 6, wherein at least one residue of the alkyl phosphate comprises a mono- or polyfluorinated alkyl residue. [8] Electrolyte according to any one of claims 5 to 7, wherein the additive is present in a concentration between 0.2M and 1M.