Lithium-ion battery cathode and anode lithium supplement technology
Lithium supplement of the electrode material by pre-lithiation counteracts the irreversible lithium loss caused by the formation of SEI film.
Table of Contents
During the first charge of a lithium-ion battery, the organic electrolyte will reduce and decompose on the surface of anode such as graphite to form a solid electrolyte phase interface film, permanently consuming a large amount of lithium from the cathode materials, resulting in a low coulombic efficiency on the first cycle and reducing the capacity and energy density of the lithium-ion battery. To solve this problem, pre-lithiation technology has been investigated.
Lithium supplement of the electrode material by pre-lithiation counteracts the irreversible lithium loss caused by the formation of SEI film to increase the total capacity and energy density of the battery.
Anode lithium supplement technology
The common pre-lithiation method is anode lithium supplement, such as lithium supplement by lithium foil, lithium supplement by lithium powder, etc., are the pre-lithiation processes that are currently focused on development. In addition, there are also technologies that use lithium silicide powder and electrolytic lithium brine solution to perform pre-lithiation.
Lithium supplement via lithium foil
Lithium supplement by lithium foil is a technique of lithium supplement using self-discharge mechanism. The potential of lithium metal is the lowest among all electrode materials, and due to the potential difference, when the lithium ion battery anode material is in contact with the lithium metal foil, electrons move spontaneously toward the anode, accompanied by the embedding of Li+ in the anode.
The anode of silicon nanowires grown on a stainless steel substrate was subjected to lithium supplement by adding electrolyte dropwise and then directly contacting it with the lithium foil. Half-cell tests were performed on the lithium supplemented anode and found that: the open-circuit voltage without lithium supplement was 1.55V, and the first 0.1C discharge at The lithium supplemented anode had an open-circuit voltage of 1.55 V and an embedded lithium capacity of 3800 mAh/g at 0.01-1.00 V for the first 0.1 C discharge; the lithium supplemented silicon nanowire had an open-circuit voltage of 0.25 V and an embedded lithium capacity of 1600 mAh/g for the first time.
The tin-carbon anode was directly contacted with the electrolyte-impregnated lithium foil for 180 min, and lithium supplement was performed. Lithium supplement was tested with a half-cell, and the irreversible specific capacity of tin-carbon was reduced from 680 mAh/g to 65 mAh/g after lithium supplement. the anode was constituted as a full-cell, and the 1.0C multiplicity was tested at 3.1 to 4.8 The ICE tested at V is close to 100% , and the cycling is stable and the multiplicity performance is good.
Although anode pre-lithiation can be achieved by direct contact with lithium foil, the degree of pre-lithiation is not easily and precisely controlled. Insufficient lithiation does not sufficiently improve ICE; while excessive lithium supplement may lead to the formation of metallic lithium plating on the anode surface.
Researchers et al. improved the safety of lithium supplement via lithium foil, and designed a three-layer structure anode of active material/polymer/lithium metal that can be stable in ambient air for 30-60 min, which is sufficient for anode to be processed. The three layers are: a lithium metal layer electrochemically deposited on a copper foil, a protective layer of polymethylmethacrylate coated on the lithium layer, and a layer of active material.
Stabilized lithium metal powder
Lithium supplement by lithium powder is proposed by FMC, which developed SLMP with a specific capacity up to 3600mAh/g and a thin layer of lithium carbonate of 2% to 5% covered on the surface, which can be used in a dry environment. There are two main ways to apply SLMP to anode pre-lithiumization: adding it during the pulping process, or adding it directly to the surface of anode sheets.
Conventional anode slurry uses PVDF/NMP or SBR+CMC/deionized water system, but SLMP is incompatible with polar solvents and can only be dispersed in hexane, toluene and other non-polar solvents, so it cannot be added directly in the conventional slurry process. With SBR-PVDF/toluene system, SLMP can be directly mixed in the graphite electrode slurry. After pre-lithiation of anode by SLMP, the ICE of the cell increased from 90.6% to 96.2% at 0.01 to 1.00V and 0.05C.
It is simpler and easier to load SLMP directly onto the dried anode surface than adding it during the pulping process. SLMP was used to pre-lithiate the silica-carbon nanotube anode by dropping a 3% mass fraction SLMP/toluene solution onto the surface of the silica-carbon nanotube anode, and then pressing and activating it after the toluene solvent evaporated. After pre-lithiation, the first irreversible capacity of anode was reduced by 20% to 40% .
Lithium silicide powder
The small size of lithium silicide nanopowder is more favorable for dispersion in the anode. In addition, it is already in a swollen state and the volume change during cycling does not affect the structure of the whole electrode. At present, there are few studies on the additives used in lithium supplement way of lithium silicide powder, and only a few researchers have studied the lithium supplement performance and stability improvement of lithium silicide powder.
The half-cell system was charged and discharged at 0.01 to 1.00 V at 0.05 C. The ICE of silica anode increased from 76% to 94% with the addition of 15% lithium silicide powder; the ICE of intermediate phase carbon microspheres increased from 75% to 99% with the addition of 9% lithium silicide powder; and the ICE of graphite anode increased from 87% to 99% with the addition of 7% lithium silicide powder.
Lithium supplement by electrolysis of aqueous lithium brine solution
Whether lithium supplement is performed using lithium foil, SLMP or lithium silicide powder, the use of lithium metal is involved. Lithium metal is expensive, reactive, difficult to handle, and requires high costs for storage and transportation for protection. If lithium supplement process does not involve lithium metal, it can save cost and improve safety performance. Silicon can be lithium supplemented by electrolyzing an aqueous solution of Li2SO4 in an electrolytic cell, with the sacrificial electrode being a copper wire immersed in Li2SO4.
Cathode lithium supplement technology
A typical cathode lithium supplement is a small amount of high-capacity material added to the cathode synthesis process, and during the charging process, Li+ is removed from the high-capacity material to supplement the irreversible capacity loss during the first charge and discharge. Currently, the main materials used as cathode lithium supplement additives are: lithium-rich compounds, nanocomposites based on conversion reactions, and binary lithium compounds.
Lithium-rich compounds
Li-rich material Li1+xNi0.5Mn1.5O4 was used to compensate for the irreversible capacity loss of the Si-C|LiNi0.5Mn1.5O4 full cell. The capacity retention of cells using hybrid cathode with 0.33C for 100 cycles at 3.00 to 4.78 V is 75% , while that of cells using pure LiNi0.5Mn1.5O4 cathode is only 51%. Li2NiO2 can also be used as an additive to cathode lithium supplement, but is less stable in air. Li2NiO2 can be modified using isopropanolic aluminum, and Li2NiO2 material coated with alumina that is stable in air was synthesized with excellent lithium supplement results.
Nanocomposites based on conversion reactions
Despite the effectiveness of lithium-rich compounds as lithium supplement additive, the first lithium supplement effect is still limited by the lower specific capacity. Nanocomposites based on conversion reaction can contribute a large amount of lithium during the first charge of the battery due to the presence of a large charge/discharge voltage hysteresis, while the lithium-embedded reaction cannot occur during the discharge process.
By the synthesized nano-Co/lithium oxide composites cycled at 50mA/g at 4.1 to 2.5V, the specific capacity of the first charge reached 619mAh/g and the discharge specific capacity was only 10mAh/g; after 8h exposure to ambient air, the de-lithium specific capacity was only 51mAh/g smaller than the initial value, and after 2d placement, the de-lithium specific capacity was still 418mAh/g, which has good Environmental stability, compatible with the production process of commercial batteries.
Lithium fluoride is a potential cathode lithium supplement material because of its high lithium content and good stability. The M/LiF nanomaterials constructed by the conversion reaction can overcome the problems of low LiF conductivity and ionic conductivity, high electrochemical decomposition potential and harmful decomposition products, making lithium fluoride an excellent cathode lithium supplement additive. The theoretical capacity of lithium sulfide reaches 1166 mAh/g, but there are still many problems to be solved when used as lithium supplement additive, such as compatibility with electrolyte, insulation, and poor environmental stability.
Despite the higher lithium supplement capacity than lithium-rich compounds, nanocomposites based on conversion reactions can have residual inactive metal oxides, fluorides, and sulfides after the first lithium supplement, reducing the energy density of the battery. For more related knowledge, please refer to the fluoride ion battery article.
Binary lithium compounds
The theoretical specific capacities of binary lithium compounds are much higher. Li2O2, Li2O and Li3N have theoretical specific capacities of 1168 mAh/g, 1797 mAh/g and 2309 mAh/g, respectively, and similar lithium supplement effects can be achieved with only small additions. Theoretically, the residues of these materials after lithium supplement are O2, N2, etc., which can be expelled during the formation of SEI film in the battery.
Commercially available Li3N was ground into powder with particle size of 1-5 μm and used as lithium supplemental. the first charge specific capacities of LiCoO2 electrodes with 1% and 2% Li3N added at 0.1C at 3.0-4.2V were 167.6 mAh/g and 178.4 mAh/g, respectively, under the half-cell system, which were up from pure LiCoO2 18.0 mAh/g and 28.7 mAh/g.
The commercial Li2O2 is mixed with NCM to compensate for the lithium loss during the first charge of the graphite anode. The NCM in the hybrid electrode plays the dual role of active material and catalyst. To catalyze the decomposition of Li2O2 efficiently, NCM obtained by adding 1% ball milling for 6 h to the cathode. full cell was charged and discharged from 2.75 to 4.60 V with a 0.3C reversible specific capacity of 165.4 mAh/g, which is 20.5% higher than the graphite|NCM full cell.
Tests show that the oxygen released from the decomposition of Li2O2 consumes the limited Li+ in the full cell, resulting in a significant capacity degradation of the Li2O2 added full cell, but the capacity can be recovered after the gas is expelled. The first charge of the battery in the actual production process is carried out in an open system, and the gases from the formation of SEI film and some side reactions are expelled before sealing, thus reducing the impact of O2 release.
Summary
Comparing the two lithium supplement methods, the lithium supplement reagents (lithium foil, lithium powder and lithium silicide powder) used in anode lithium supplement have high capacity, but the operation is complicated and requires high environmental requirements; by adding lithium supplement additive to the cathode in the cathode lithium supplement is safe and stable and compatible with the existing battery production process.
Future research on anode lithium supplement technology should focus on improving its stability in the battery manufacturing process, developing technical solutions compatible with industrial production and simple processes; cathode lithium supplement should focus on the development of lithium supplement capacity, the use of small amount, lithium supplement residue after the small amount of Additive system.
Lithium-ion battery cathode and anode lithium supplement technology
Lithium supplement of the electrode material by pre-lithiation counteracts the irreversible lithium loss caused by the formation of SEI film.
During the first charge of a lithium-ion battery, the organic electrolyte will reduce and decompose on the surface of anode such as graphite to form a solid electrolyte phase interface film, permanently consuming a large amount of lithium from the cathode materials, resulting in a low coulombic efficiency on the first cycle and reducing the capacity and energy density of the lithium-ion battery. To solve this problem, pre-lithiation technology has been investigated.
Lithium supplement of the electrode material by pre-lithiation counteracts the irreversible lithium loss caused by the formation of SEI film to increase the total capacity and energy density of the battery.
Anode lithium supplement technology
The common pre-lithiation method is anode lithium supplement, such as lithium supplement by lithium foil, lithium supplement by lithium powder, etc., are the pre-lithiation processes that are currently focused on development. In addition, there are also technologies that use lithium silicide powder and electrolytic lithium brine solution to perform pre-lithiation.
Lithium supplement via lithium foil
Lithium supplement by lithium foil is a technique of lithium supplement using self-discharge mechanism. The potential of lithium metal is the lowest among all electrode materials, and due to the potential difference, when the lithium ion battery anode material is in contact with the lithium metal foil, electrons move spontaneously toward the anode, accompanied by the embedding of Li+ in the anode.
The anode of silicon nanowires grown on a stainless steel substrate was subjected to lithium supplement by adding electrolyte dropwise and then directly contacting it with the lithium foil. Half-cell tests were performed on the lithium supplemented anode and found that: the open-circuit voltage without lithium supplement was 1.55V, and the first 0.1C discharge at The lithium supplemented anode had an open-circuit voltage of 1.55 V and an embedded lithium capacity of 3800 mAh/g at 0.01-1.00 V for the first 0.1 C discharge; the lithium supplemented silicon nanowire had an open-circuit voltage of 0.25 V and an embedded lithium capacity of 1600 mAh/g for the first time.
The tin-carbon anode was directly contacted with the electrolyte-impregnated lithium foil for 180 min, and lithium supplement was performed. Lithium supplement was tested with a half-cell, and the irreversible specific capacity of tin-carbon was reduced from 680 mAh/g to 65 mAh/g after lithium supplement. the anode was constituted as a full-cell, and the 1.0C multiplicity was tested at 3.1 to 4.8 The ICE tested at V is close to 100% , and the cycling is stable and the multiplicity performance is good.
Although anode pre-lithiation can be achieved by direct contact with lithium foil, the degree of pre-lithiation is not easily and precisely controlled. Insufficient lithiation does not sufficiently improve ICE; while excessive lithium supplement may lead to the formation of metallic lithium plating on the anode surface.
Researchers et al. improved the safety of lithium supplement via lithium foil, and designed a three-layer structure anode of active material/polymer/lithium metal that can be stable in ambient air for 30-60 min, which is sufficient for anode to be processed. The three layers are: a lithium metal layer electrochemically deposited on a copper foil, a protective layer of polymethylmethacrylate coated on the lithium layer, and a layer of active material.
Stabilized lithium metal powder
Lithium supplement by lithium powder is proposed by FMC, which developed SLMP with a specific capacity up to 3600mAh/g and a thin layer of lithium carbonate of 2% to 5% covered on the surface, which can be used in a dry environment. There are two main ways to apply SLMP to anode pre-lithiumization: adding it during the pulping process, or adding it directly to the surface of anode sheets.
Conventional anode slurry uses PVDF/NMP or SBR+CMC/deionized water system, but SLMP is incompatible with polar solvents and can only be dispersed in hexane, toluene and other non-polar solvents, so it cannot be added directly in the conventional slurry process. With SBR-PVDF/toluene system, SLMP can be directly mixed in the graphite electrode slurry. After pre-lithiation of anode by SLMP, the ICE of the cell increased from 90.6% to 96.2% at 0.01 to 1.00V and 0.05C.
It is simpler and easier to load SLMP directly onto the dried anode surface than adding it during the pulping process. SLMP was used to pre-lithiate the silica-carbon nanotube anode by dropping a 3% mass fraction SLMP/toluene solution onto the surface of the silica-carbon nanotube anode, and then pressing and activating it after the toluene solvent evaporated. After pre-lithiation, the first irreversible capacity of anode was reduced by 20% to 40% .
Lithium silicide powder
The small size of lithium silicide nanopowder is more favorable for dispersion in the anode. In addition, it is already in a swollen state and the volume change during cycling does not affect the structure of the whole electrode. At present, there are few studies on the additives used in lithium supplement way of lithium silicide powder, and only a few researchers have studied the lithium supplement performance and stability improvement of lithium silicide powder.
The half-cell system was charged and discharged at 0.01 to 1.00 V at 0.05 C. The ICE of silica anode increased from 76% to 94% with the addition of 15% lithium silicide powder; the ICE of intermediate phase carbon microspheres increased from 75% to 99% with the addition of 9% lithium silicide powder; and the ICE of graphite anode increased from 87% to 99% with the addition of 7% lithium silicide powder.
Lithium supplement by electrolysis of aqueous lithium brine solution
Whether lithium supplement is performed using lithium foil, SLMP or lithium silicide powder, the use of lithium metal is involved. Lithium metal is expensive, reactive, difficult to handle, and requires high costs for storage and transportation for protection. If lithium supplement process does not involve lithium metal, it can save cost and improve safety performance. Silicon can be lithium supplemented by electrolyzing an aqueous solution of Li2SO4 in an electrolytic cell, with the sacrificial electrode being a copper wire immersed in Li2SO4.
Cathode lithium supplement technology
A typical cathode lithium supplement is a small amount of high-capacity material added to the cathode synthesis process, and during the charging process, Li+ is removed from the high-capacity material to supplement the irreversible capacity loss during the first charge and discharge. Currently, the main materials used as cathode lithium supplement additives are: lithium-rich compounds, nanocomposites based on conversion reactions, and binary lithium compounds.
Lithium-rich compounds
Li-rich material Li1+xNi0.5Mn1.5O4 was used to compensate for the irreversible capacity loss of the Si-C|LiNi0.5Mn1.5O4 full cell. The capacity retention of cells using hybrid cathode with 0.33C for 100 cycles at 3.00 to 4.78 V is 75% , while that of cells using pure LiNi0.5Mn1.5O4 cathode is only 51%. Li2NiO2 can also be used as an additive to cathode lithium supplement, but is less stable in air. Li2NiO2 can be modified using isopropanolic aluminum, and Li2NiO2 material coated with alumina that is stable in air was synthesized with excellent lithium supplement results.
Nanocomposites based on conversion reactions
Despite the effectiveness of lithium-rich compounds as lithium supplement additive, the first lithium supplement effect is still limited by the lower specific capacity. Nanocomposites based on conversion reaction can contribute a large amount of lithium during the first charge of the battery due to the presence of a large charge/discharge voltage hysteresis, while the lithium-embedded reaction cannot occur during the discharge process.
By the synthesized nano-Co/lithium oxide composites cycled at 50mA/g at 4.1 to 2.5V, the specific capacity of the first charge reached 619mAh/g and the discharge specific capacity was only 10mAh/g; after 8h exposure to ambient air, the de-lithium specific capacity was only 51mAh/g smaller than the initial value, and after 2d placement, the de-lithium specific capacity was still 418mAh/g, which has good Environmental stability, compatible with the production process of commercial batteries.
Lithium fluoride is a potential cathode lithium supplement material because of its high lithium content and good stability. The M/LiF nanomaterials constructed by the conversion reaction can overcome the problems of low LiF conductivity and ionic conductivity, high electrochemical decomposition potential and harmful decomposition products, making lithium fluoride an excellent cathode lithium supplement additive. The theoretical capacity of lithium sulfide reaches 1166 mAh/g, but there are still many problems to be solved when used as lithium supplement additive, such as compatibility with electrolyte, insulation, and poor environmental stability.
Despite the higher lithium supplement capacity than lithium-rich compounds, nanocomposites based on conversion reactions can have residual inactive metal oxides, fluorides, and sulfides after the first lithium supplement, reducing the energy density of the battery. For more related knowledge, please refer to the fluoride ion battery article.
Binary lithium compounds
The theoretical specific capacities of binary lithium compounds are much higher. Li2O2, Li2O and Li3N have theoretical specific capacities of 1168 mAh/g, 1797 mAh/g and 2309 mAh/g, respectively, and similar lithium supplement effects can be achieved with only small additions. Theoretically, the residues of these materials after lithium supplement are O2, N2, etc., which can be expelled during the formation of SEI film in the battery.
Commercially available Li3N was ground into powder with particle size of 1-5 μm and used as lithium supplemental. the first charge specific capacities of LiCoO2 electrodes with 1% and 2% Li3N added at 0.1C at 3.0-4.2V were 167.6 mAh/g and 178.4 mAh/g, respectively, under the half-cell system, which were up from pure LiCoO2 18.0 mAh/g and 28.7 mAh/g.
The commercial Li2O2 is mixed with NCM to compensate for the lithium loss during the first charge of the graphite anode. The NCM in the hybrid electrode plays the dual role of active material and catalyst. To catalyze the decomposition of Li2O2 efficiently, NCM obtained by adding 1% ball milling for 6 h to the cathode. full cell was charged and discharged from 2.75 to 4.60 V with a 0.3C reversible specific capacity of 165.4 mAh/g, which is 20.5% higher than the graphite|NCM full cell.
Tests show that the oxygen released from the decomposition of Li2O2 consumes the limited Li+ in the full cell, resulting in a significant capacity degradation of the Li2O2 added full cell, but the capacity can be recovered after the gas is expelled. The first charge of the battery in the actual production process is carried out in an open system, and the gases from the formation of SEI film and some side reactions are expelled before sealing, thus reducing the impact of O2 release.
Summary
Comparing the two lithium supplement methods, the lithium supplement reagents (lithium foil, lithium powder and lithium silicide powder) used in anode lithium supplement have high capacity, but the operation is complicated and requires high environmental requirements; by adding lithium supplement additive to the cathode in the cathode lithium supplement is safe and stable and compatible with the existing battery production process.
Future research on anode lithium supplement technology should focus on improving its stability in the battery manufacturing process, developing technical solutions compatible with industrial production and simple processes; cathode lithium supplement should focus on the development of lithium supplement capacity, the use of small amount, lithium supplement residue after the small amount of Additive system.
If you want to know more relevant information, you can refer to the top 5 lithium supplement manufacturers in China that I compiled before.