Lithium-ion cell

Abstract

Lithium-ion cell, characterized in that - the positive electrode comprises a partially lithiated material as the active material, and - the negative electrode comprises a partially lithiated material as the active material, - wherein the partially lithiated material of the positive electrode is the active starting material of the positive electrode, - wherein the partially lithiated material of the negative electrode is the active starting material of the negative electrode, and - wherein the starting material of the positive electrode is a mixture of the fluoride of a metal, the elemental metal and lithium fluoride, characterized in that - the proportion of the fluoride of the metal in the starting material of the positive electrode is at least 50 mol%.

Description

[0001] The invention relates to a lithium-ion cell and a method for producing a lithium-ion cell.

[0002] According to the state of the art, see for example T. Wöhrle in Chapter 9, R. Korthauser, Handbook of Lithium-Ion Batteries, Springer Verlag Berlin Heidelberg 2013 on (secondary) lithium-ion cells or see for example the document EP0017400A1, lithium-ion cells (LIZ) are manufactured with a fully delithiated anode (typically a graphite anode without lithium content) and a 100% lithiated cathode (e.g. a so-called lithium cobalt oxide (LCO) cathode with maximum lithium content in the lattice structure as LiCoO2).

[0003] After the housing of a lithium-ion battery is closed during production, it is initially in an uncharged state, meaning the graphite of the negative electrode (also called the anode) is initially uncharged with lithium (Li). The cyclable lithium is initially present entirely in the positive electrode (also called the cathode).

[0004] To prepare the LIZ for use in an application, it must be charged for the first time, a process known as formation. This involves charging the LIZ for the first time up to the permitted end-of-charge voltage and then optionally discharging it for the first time up to the permitted end-of-discharge voltage. During the initial charge, a protective layer forms around the graphite surface of the negative electrode, known as the electrode-electrolyte interface (SEI). The SEI is a lithium-ion conductor and contains permanent, Li-containing compounds, meaning that this bound amount of Li is subsequently permanently unavailable for cycling. These compounds, which reflect the loss of Li, are often Li2CO3, which is typically bound in the SEI as an electrolyte decomposition product and no longer participates in further electrochemical reactions.

[0005] For example, according to T. Wöhrle in R. Korthauser, Handbook of Lithium-Ion Batteries, Springer Verlag Berlin Heidelberg 2013, pouch cells containing LiCoO2 and graphite are predominantly used as lithium-ion cells in smartphones. The initial charge is up to 4.2 V, whereby under these boundary conditions approximately 50 mol% of the Li ions are deintercalated from the LiCoO2. During the initial intercalation of these Li ions into the graphite anode, the electrolyte on the anode decomposes, with decomposition products such as Li2CO3 containing lithium ions, which are therefore not incorporated into the anode. The formation of the electrolyte decomposition products is irreversible, and the lithium bound in this way is no longer available for further electrochemical reactions or cycling. Accordingly, fewer Li ions are available for cycling during the initial discharge and subsequent charge / discharge processes.The initial discharge capacity is therefore smaller than the initial charge capacity. This loss is also referred to as formation loss. Quantitatively, it depends primarily on the anode type and electrolyte used.

[0006] During formation the following reaction occurs [Cathode: 2.4 LiCoO2] + [Anode: C6 (graphite)] → [Cathode: 2.4 Li 0,5 CoO2] + [Anode: LiC6] + 0.1 Li2CO3 (SEI).

[0007] In the following discharge reaction, excess delithiated cathode material remains in the form of Li 0,5 CoO2 is a loss material that is carried in excess in the LIZ without participating in cycling processes. For a negative electrode made of natural graphite combined with a positive LCO electrode, the loss material amounts to approximately 10% of the active LCO, meaning that the LCO material must be provided in a 10% excess of the nominal capacity during production. DE 11 2004 001 344 T5 teaches electrodes containing a host material that is pre-lithiated or undergoes lithiation upon introduction of the electrolyte into a battery. Lithiation of the host material can be achieved by moving lithium metal and a host material, moving a lithium metal powder and a host material at a temperature above room temperature, applying pressure to a mixture of lithium metal and host material, contacting the host material with molten lithium metal, lamination of lithium foil or lithium mesh to an electrode containing the host material, or lamination of lithium metal or mesh to an electrode at elevated temperatures. DE 10 2010 047 572 A1 concerns aluminum-containing hydride anodes of the general formula (M 1 ) m (M 2 ) 3-m AlH6, where M 1 and M 2an alkali element which is independently selected from Li, Na and K, m is a number between 1 and 3 and n is a number = 3. Also described are galvanic elements, such as lithium batteries, which contain these aluminum-containing hydride anodes as anodes. A process for producing galvanic elements with aluminum-containing hydride anodes is also described. DE 10 2013 225 596 A1 relates to a non-aqueous electrolyte of a galvanic cell containing a redox-active additive, in which the additive has a structure of the general formulas A or B shown below: where X = O, S or Se; Y = N or C; Z = O, S or NR (R = alkyl with 1-8 C atoms); R 1 , R 2 , R 3 = independently of one another NCS, NCO, alkyl (1 to 8 C atoms), perfluoroalkyl (C n F 2n+1 with n = 1-8), aryl, F or Cl, CN, OR (R = alkyl with 1 to 8 C atoms), NR 2(R = alkyl with 1-8 carbon atoms) R4 = H, NCS, NCO, alkyl (1 to 8 carbon atoms), perfluoroalkyl (C n F 2n+1 with n = 1-8), aryl, F or Cl, CN, OR (R = alkyl with 1 to 8 C atoms), NR 2 (R = alkyl with 1-8 carbon atoms) R 5 = only for Y = C; H, NCS, NCO, alkyl (1 to 8 C atoms), perfluoroalkyl (C n F 2n+1 with n = 1-8), aryl, F or Cl, CN, OR (R = alkyl with 1 to 8 C atoms), NR 2(R = alkyl with 1-8 C atoms) and it has at least one isocyanate and / or isothiocyanate and / or isoselenocyanate group. A galvanic cell containing these redox-active additives is also described. US 2012 / 0315538 A1 describes porous electrodes in which the porosity has low tortuosity. The porous electrodes can be designed to be filled with electrolyte and used in batteries, and can have low tortuosity in the main direction of ion transport during charging and discharging of the battery.

[0008] It is an object of the invention to describe an improved lithium-ion cell and an improved method for producing a lithium-ion cell.

[0009] This object is achieved by a lithium-ion cell according to claim 1 and the method for producing a lithium-ion cell according to claim 5. Advantageous embodiments and developments of the invention emerge from the dependent claims.

[0010] In this invention, the terms lithium cell, lithium battery and conversion cell are not differentiated unless otherwise stated and are uniformly referred to as lithium-ion cell (LIC).

[0011] According to the invention, the positive electrode of the lithium-ion cell (LIC) comprises a partially lithiated material as active material and the negative electrode of the LIC comprises a partially lithiated material as active material.

[0012] In this document, "partially lithiated" means that the material contains at least partially active lithium, which is involved in the redox reactions when the LIZ is cycled. The active material is understood to be the reactive material that contributes to the electrochemical discharge and charge reactions of the LIZ to release or store electrochemically bound electrical energy.

[0013] Here, the partially lithiated material of the positive electrode is the active starting material of the positive electrode and the partially lithiated material of the negative electrode is the active starting material of the negative electrode. In other words, when the electrodes are incorporated into an LIZ, i.e., when they are incorporated into a housing of the subsequent LIZ during the manufacturing process, a material that is lithiated on both electrode types is introduced as the starting material for the active material. This improvement has a positive effect on the lifetime and energy density, as well as the specific energy of the LIZ.

[0014] In this document, the terms "starting material" and "active starting material" refer to the material that is / is incorporated into the LIZ during manufacturing and that, after the LIZ is manufactured, serves as the active material during operation of the LIZ as a secondary electrochemical storage device. This means that the LIZ is / is manufactured partially charged.

[0015] According to the invention, the starting material of the positive electrode is a mixture of the fluoride of a metal, the elemental metal and lithium fluoride (LiF).

[0016] An exemplary mixture may include iron fluoride (FeF3), iron (Fe) and lithium fluoride (LiF).

[0017] Advantageously, the starting material of the negative electrode is a mixture of an oxide of the metal, the elemental metal and lithium oxide (Li2O).

[0018] An exemplary mixture may include iron oxide (Fe2O3), iron (Fe) and lithium oxide (Li2O).

[0019] In addition, the proportion of the fluoride of the metal in the starting material of the positive electrode is at least 50 mol% and optionally or additionally the proportion of the oxide of the metal in the starting material of the negative electrode is at most 50 mol%.

[0020] With the stated composition, the LIZ is already 50% charged from the moment it is filled with electrolyte. Thus, once the housing is closed, the assembled LIZ is in a state that is advantageous for further handling of the LIZ – without the need to connect an external circuit. There is neither a high level of self-discharge as with even higher states of charge, nor is the occurrence of deep discharge likely. To minimize other damage mechanisms, such as decomposition of the electrolyte on the metal particles in the cathode, the state of charge should be at least 50%, preferably above 50%, and the metal oxide content in the anode should be at most 50%, preferably below 50% by exactly the amount that corresponds to the formation loss. The LIZ is then optimally balanced with regard to the loading of active material.

[0021] It is advantageous if the metal is iron, nickel or copper.

[0022] Partially lithiated electrodes, through which the active material is introduced into the LIZ, have the advantage of extremely low losses during the initial full charging of the LIZ (also referred to as formation), particularly lower losses than with state-of-the-art LIZs. Low losses during formation are associated with a higher energy density of the LIZ (in Wh / L) and a higher specific energy (in Wh / kg). The low losses during formation result from the reactivity of the starting materials. This reactivity is described using three exemplary embodiments (see below).

[0023] Due to the low losses during formation, the starting material can be used almost entirely without excess, i.e., without reserves for the formation, to achieve a specific nominal capacity of the LIZ. This allows for a higher lithium content in the LIZ than with the same material input using conventional, state-of-the-art electrodes with a fully lithiated cathode (such as LiCoO2). Since cost-intensive LiCoO2 or LiNiO2, in particular, is used to compensate for formation losses, a process using only lithium is more cost-effective.

[0024] The increased lithium content in the LIZ brings several immediate advantages, including higher energy density and higher specific energy. Furthermore, the power consumption during the first charge of the respective cell is reduced due to the partial lithiation of the anode, which is fully lithiated at full charge, thus reducing manufacturing process costs.

[0025] Furthermore, due to the partial lithiation of both electrodes during the production of a LIZ, it is possible to perform the initial cycling at a later time, e.g., after assembly of the LIZ or at the end user's site. This advantage is based on the fact that a cell equipped with partially lithiated electrodes can be used immediately after installation.

[0026] In addition, due to the avoidance of the “dead material” by the inventive pre-lithiation of the two electrodes during the production of the LIZ, the cost of particularly nickel- or cobalt-containing active material is reduced.

[0027] Furthermore, from today's perspective, the use of partially lithiated electrodes is compatible with future developments in lithium-ion battery technology. For example, partially lithiated electrodes are expected to be particularly economically promising in lithium systems with silicon-containing anodes or cathode conversion materials.

[0028] The method according to the invention for producing a lithium-ion cell comprises the steps - Mixing a first partially lithiated starting material for the positive electrode, - Mixing a second partially lithiated starting material for the negative electrode, - Production of the positive electrode with the first partially lithiated starting material as active material, - Production of the negative electrode with the second partially lithiated starting material as active material, - Installation of the positive electrode and the negative electrode in a cell housing, and - Closure of the cell housing.

[0029] The steps follow in indirect or direct order as shown, whereby in the case of indirect order other manufacturing processes can take place between the steps shown and in the case of direct order the steps take place directly one after the other without any other intermediate steps.

[0030] Here, the first partially lithiated starting material is a mixture of the fluoride of a metal, the elemental metal and lithium fluoride (LiF).

[0031] An exemplary mixture may include iron fluoride (FeF3), iron (Fe), and lithium fluoride (LiF). Another exemplary mixture may include Li2S and S8.

[0032] Furthermore, it is useful if the second partially lithiated starting material in the process is a mixture of an oxide of the metal, the elemental metal and lithium oxide (Li2O).

[0033] An exemplary mixture may include iron oxide (Fe2O3), iron (Fe) and lithium oxide (Li2O).

[0034] In addition, in the process, the proportion of the fluoride of the metal in the first partially lithiated starting material is at least 50 mol% and optionally or additionally the proportion of the oxide of the metal in the second partially lithiated starting material is at most 50 mol%.

[0035] With the stated composition, the LIZ is already charged to 50% relative to its nominal capacity from the moment it is filled with electrolyte. Thus, once the housing is closed, the assembled LIZ is in a state that is advantageous for further handling of the LIZ – without the need to connect an external circuit. There is neither a high self-discharge as with even higher charge levels, nor is the occurrence of deep discharge likely. To minimize other damage mechanisms, such as decomposition of the electrolyte on the metal particles in the cathode, the charge level should be at least 50%, preferably above 50%, and the metal oxide content in the anode should be at most 50%, preferably below 50% by the value corresponding to the formation loss. Furthermore, the electrical hazard in the event of accidents during processing of the LIZ (e.g.in the event of an external short circuit) is lower at a lower state of charge than at a higher state of charge.

[0036] It is advantageous for the process if the metal is iron, nickel or copper.

[0037] The manufacturing process is particularly advantageous. The installation of partially lithiated electrodes is easily manageable on an industrial scale. Furthermore, due to the partial lithiation of both electrodes during the production of an LIZ, it is possible to perform the initial cycling at a later time, for example, after assembly of the LIZ or at the end user's site. This advantage is based on the fact that a cell equipped with partially lithiated electrodes can be used immediately after installation.

[0038] The invention is based on the following considerations: Current lithium-ion batteries (LIBs) consist of an anode, a cathode, and a separator impregnated with electrolyte. In this arrangement, the cathode material contains all the lithium ions (Li-ions) that are stored and released into the anode and cathode over the battery's lifetime, see, for example, document EP0017400A1. When the LIB is first cycled, i.e., when the battery is first charged, a layer called a solid electrolyte interface (SEI) forms on the anode, which leads to the irreversible consumption of Li-ions in undesirable side reactions. Furthermore, other reactions occur during the lifetime of a battery that irreversibly remove Li-ions from the system.This reduces the total capacity of the battery and, from the start of cycling, part of the active materials is inactive because there are no longer sufficient Li-ions available in the system to lithiate all parts of the active material.

[0039] The formation losses during initial cycling must be taken into account during LIB production in order to achieve a specific target capacity after formation. This can result in considerable additional costs for the Li source, i.e., the generally cost-intensive cathode material. In addition, the excess cathode material used reduces the specific energy of the LIB. The "dead material" present after formation, which only supplies Li ions once, namely during formation, results in a reduction in the energy density and the specific energy. This is generally the case for all lithium-ion technologies in which only one of the two electrodes supplies Li ions. The energy density and the specific energy decrease even further with further cycling, as the side reactions continuously consume lithium. Such side reactions include, for example, the re-formation of the SEI on the anode in the event of damage (e.g.,due to external influences), decomposition of the electrolyte due to overcharging or storage with a high state of charge, penetration of moisture into the cell and, as a result, undesirable formation of hydrogen fluoride (HF).

[0040] It is therefore proposed to install both the anode and the cathode in the battery in partially lithiated form. This means that upon installation, i.e., before the LIB is cycled for the first time, both electrodes already contain some lithium ions. As a result, a higher lithium content can be achieved in the lithium-ion cell or in an LIB composed of several similar lithium-ion cells than with the conventional installation of a fully lithiated cathode according to the state of the art.

[0041] The increased lithium content in the LIB brings several immediate advantages, including higher energy density, higher specific energy, and increased cycle life. Furthermore, the power consumption during the first charge of the respective cell is reduced due to the partial lithiation of the anode, which is fully lithiated at full charge.

[0042] Furthermore, due to the partial lithiation of both electrodes during the production of a lithium-ion cell, it is possible to perform the initial cycling at a later time, e.g., after the assembly of a LIB made of lithium-ion cells or at the end user's site. This advantage is based on the fact that a cell equipped with partially lithiated electrodes can be used immediately after installation.

[0043] In addition, due to the avoidance of the “dead material” (see above) through the inventive pre-lithiation of the two electrodes during the production of the lithium-ion cell, the cost of active material is reduced.

[0044] Furthermore, from today's perspective, the use of partially lithiated electrodes is compatible with future developments in LIB technology. For example, partially lithiated electrodes are expected to be particularly economically promising in lithium systems with silicon-containing anodes or cathode conversion materials.

[0045] The invention is described below using a preferred embodiment. Further details, preferred embodiments, and further developments of the invention will become apparent from this.

[0046] According to a first embodiment, a lithium-ion cell is equipped with partially lithiated electrodes during production. The cathode is an iron(III) fluoride (FeF3)-based electrode, and the anode is an iron(III) oxide (Fe2O3)-based anode. The composition of the cathode is approximately 25 mol% FeF3 and 75 mol% Fe + 3 LiF, ie 78 wt% a combination of one stoichiometric part of elemental iron and three stoichiometric parts of lithium(I) fluoride. The composition of the anode is approximately 50 mol% Fe2O3 and 50 mol% 2Fe + 3Li2O, i.e., 55 wt%, a combination of two stoichiometric parts of elemental iron and three stoichiometric parts of lithium(IV) oxide. The anode is initially charged, i.e., during cell manufacture, to 50% (based on a 100% full charge state of the cell with maximum lithiation of the anode), and the cathode to 75% (based on a 0% discharge state of the cell with maximum lithiation of the cathode). Consequently, the cathode is installed with an excess of lithium. This is primarily due to the reaction process during the initial charging after the lithium-ion cell's manufacture, the forming process. This involves the one-time formation of a protective layer on the anode surface, namely the SEI (solid electrolyte interface): (3 Fe + 9 LiF + FeF3) + (Fe2O3 + 2 Fe + 3 Li2O) → 4 FeF3 + (4 Fe + 6 Li2O) + 1.5 Li2CO3 (SEI)

[0047] In this process, lithium is irreversibly bound without leading to the formation of unused active material, as the SEI formation reaction produces the cathodic active material FeF3 and the anodic active material 2 Fe + 3 Li2O, both of which are reactants in the discharge reaction of the lithium-ion cell. The excess lithium added in the form of LiF on the cathode side compensates for formation losses. The excess lithium can be precisely adjusted, thus ensuring a higher energy density of the cell.

[0048] Both electrodes are conversion electrodes, i.e. the active material of both electrodes participates in the charging and discharging reaction. The discharge reaction of the cell leads to an (exothermic) delithiation of the anode and lithiation of the cathode according to: [Cathode: 2 FeF3] +[Anode: (2 Fe + 6 Li2O)] → [Cathode: 2 (Fe + 3 LiF)] + [Anode: Fe2O3]

[0049] The usual charging reaction (i.e. charging other than the SEI formation reaction) therefore leads to (endothermic) lithiation of the anode and delithiation of the cathode according to: [Cathode: 2 (Fe + 3 LiF)] + [Anode: Fe2O3] → [Cathode: 2 FeF3] + [Anode: (2 Fe + 6 Li2O)]

[0050] According to a second embodiment, nickel(II) fluoride (NiF2) is used on the cathode side instead of the conversion material iron(III) fluoride (FeF3). In this embodiment, the composition of the cathode, analogous to the first embodiment, is approximately 55 mol% Ni + 2LiF and 45 mol% NiF2. The anode composition is approximately 66 mol% 2Fe + 3Li2O and 33 mol% Fe2O3. The formation reaction is given as: (5Ni + 10 LiF + 4 NiF2) + (Fe2O3 + 4 Fe + 6 Li2O) → 9 NiF2 + (6 Fe + 9 Li2O) + 2 Li2CO3(SEI).

[0051] The discharge reaction of the cell leads to delithiation of the anode and lithiation of the cathode according to: 9 NiF2 + (6 Fe + 9Li2O) → (9 Ni + 18 LiF) + 3 Fe2O3

[0052] The usual charging reaction (i.e., charging other than the SEI formation reaction) leads to lithiation of the anode and delithiation of the cathode according to: (9 Ni + 18 LiF) + 3 Fe2O3 → 9 NiF2 + (6 Fe + 9 Li2O)

[0053] According to a third embodiment, the compound copper(II) fluoride (CuF2) is used instead of the compound nickel(II) fluoride (NiF2). The reaction equations of the second embodiment apply analogously to the third embodiment, replacing the element Ni with the element Cu.

[0054] The advantages of the first embodiment are equally present in the second and third embodiments. The specific differences between, for example, CuF2 and FeF3, such as the higher potential of CuF2, do not adversely affect the advantages of the invention.

Claims

[1] Lithium-ion cell, characterized by , that - the positive electrode comprises a partially lithiated material as active material, and - the negative electrode comprises a partially lithiated material as active material, - wherein the partially lithiated positive electrode material is the active starting material of the positive electrode, - wherein the partially lithiated negative electrode material is the active starting material of the negative electrode, and - wherein the starting material of the positive electrode is a mixture of the fluoride of a metal, the elemental metal and lithium fluoride, characterized by , that - the proportion of metal fluoride in the starting material of the positive electrode is at least 50 mol%. [2] Lithium-ion cell according to claim 1, characterized by , that - the starting material of the negative electrode is a mixture of an oxide of the metal, the elemental metal and lithium oxide. [3] Lithium-ion cell according to claim 1 or 2, characterized by , that - the proportion of the oxide of the metal in the starting material of the negative electrode is not more than 50 mol%. [4] Lithium-ion cell according to one of claims 1 to 3, characterized by , that - the metal is iron, nickel or copper. [5] A method for producing a lithium-ion cell, comprising the steps - Mixing a first partially lithiated starting material for the positive electrode, wherein the first partially lithiated starting material is a mixture of the fluoride of a metal, the elemental metal and lithium fluoride, - Mixing a second partially lithiated starting material for the negative electrode, - Production of the positive electrode with the first partially lithiated starting material as active material, - Production of the negative electrode with the second partially lithiated starting material as active material, - Installation of the positive electrode and the negative electrode in a cell housing, and - Closure of the cell housing, characterized by , that - the proportion of metal fluoride in the first partially lithiated starting material is at least 50 mol%. [6] The process of claim 5, wherein the second partially lithiated starting material is a mixture of an oxide of the metal, the elemental metal and lithium oxide. [7] A process according to claim 6, wherein the proportion of the oxide of the metal in the second partially lithiated starting material is at most 50 mol%. [8] A method according to any one of claims 5 to 7, wherein the metal is iron, nickel or copper.