Research status of pre-lithiation of lithium batteries and related patent technologies for lithium supplementation in China

During the first charge of a lithium-ion battery, the organic electrolyte will decompose and decompose on the surface of the negative electrode such as graphite, forming a solid electrolyte phase interface (SEI) film, which permanently consumes a large amount of lithium from the positive electrode, causing the Coulombic efficiency (ICE) deviation of the first cycle. Low, reducing the capacity and energy density of lithium-ion batteries.

Existing graphite materials have the first irreversible lithium loss of 5% to 10%, and for high-capacity anode materials, the first lithium loss is even higher (the irreversible capacity loss of silicon reaches 15% to 35%). In order to solve this problem, people have studied prelithiation technology. The electrode material is recharged with lithium through prelithiation to offset the irreversible lithium loss caused by the formation of the SEI film, so as to increase the total capacity and energy density of the battery.

FIRSTEK summarized the research progress of pre-lithiation technology in recent years from the two directions of lithium supplementation on the negative electrode and lithium supplementation on the positive electrode.

Anode lithium supplement technology

Common pre-lithiation methods are negative electrode lithium supplementation, such as lithium foil supplementation, lithium powder supplementation, etc., which are currently the key development pre-lithiation processes. In addition, there are also technologies for pre-lithiation using lithium silicide powder and electrolytic lithium salt aqueous solution.

1. Lithium foil replenishes lithium

Lithium foil replenishment is a technology that uses self-discharge mechanism to replenish lithium. The potential of metallic lithium is -3.05V (vs. SHE, standard hydrogen electrode), which is the lowest among all electrode materials. Due to the existence of the potential difference, when the negative electrode material is in contact with the metal lithium foil, electrons spontaneously move to the negative electrode, accompanied by the insertion of Li+ in the negative electrode.

N. Liu et al. dripped electrolyte on the negative electrode of silicon nanowires (SiNWs) grown on a stainless steel substrate, and then directly contacted the lithium metal foil to replenish lithium. A half-cell test was performed on the negative electrode after lithium supplementation. It was found that the open circuit voltage (OCV) of SiNWs without supplementation of lithium was 1.55V, and the specific capacity of lithium insertion during the first 0.1 C discharge at 0.01～1.00 V was 3800 mAh/g; after lithium supplementation The OCV of SiNWs is 0.25 V, and the specific capacity of lithium insertion for the first time is 1600mAh/g. The change of OCV and the specific capacity of lithium intercalation shows that after the replenishment of lithium, Si has partially reacted with Li.

J. Hassoun et al. directly contacted the tin-carbon (Sn-C) negative electrode with the lithium foil soaked in the electrolyte for 180 minutes to supplement lithium. Tested with a half-cell at 80 mA/g in 0.01 ~ 2. 00V, the irreversible specific capacity of Sn-C after replenishing lithium is reduced from 680 mAh/g (63%) to 65 mAh/g (14%). The negative electrode and LiNi0. 45Co0. 1Mn1. 45O4 constitute a full battery, and the ICE tested at a rate of 1. 0 C at 3.1 to 4.8 V is close to 100%, and the cycle is stable, and the rate performance is good; 5.0 C The specific discharge capacity reaches 110 mAh/g, which is only 14% lower than the discharge capacity of 0.2 C.

Although the negative electrode can be pre-lithiated in direct contact with the lithium foil, the degree of pre-lithiation is not easy to accurately control. Insufficient lithiation cannot sufficiently improve ICE; and excessive lithium supplementation may form a metal lithium plating layer on the surface of the negative electrode.

H. J. Kim et al. used an external short circuit to supplement lithium on the silicon oxide negative electrode (c-SiOx) through a lithium foil. The comparative experiment shows that when the resistance in the external short circuit is 100 Ω and the short circuit time is 30 minutes, the ICE can be maximized. The half-cell test of c-SiOx before and after the lithium supplementation was carried out, and the first 5 cycles of 0.07 C at 0.01 ~ 1.50 V, the coulombic efficiency of the electrode before lithium supplementation was 73.6% and 94.7. %, 96.6%, 97.5% and 98.0%; The coulombic efficiency of the electrode after lithium supplementation is 94.9%, 95.7%, 97.2%, 97.9% and 98.3% . A full battery is composed of c-SiOx and LiNi0. 8 Co0. 15 Al0. 05 O2, tested at 2.5 ~ 4.2 V with a current of 10 mA/g, and the specific discharge capacity of the battery after replenishing lithium is changed from that before replenishing lithium. 106. 33 mAh /g increased to 165.09mAh /g, ICE increased from 58. 85% to 85. 34%.

ZY Cao and others have improved the safety of lithium supplementation with lithium foil. The designed active material/polymer/lithium metal three-layer structure negative electrode can be stable in ambient air (relative humidity 10%-30%) for 30-60 minutes, Enough negative electrode for processing. The three-layer structure is: a metal lithium layer electrochemically deposited on a copper foil, the lithium layer is coated with a polymethyl methacrylate (PMMA) protective layer and an active material layer. Changing the thickness of the lithium layer can control the degree of lithium replenishment. After the electrolyte is injected into the battery to dissolve the PMMA, the lithium layer and the active material are in direct contact to complete the prelithiation. Tested with 0.1C at 0.01 ～ 1. 00 V, using the graphite with the three-layer structure to replenish lithium, the ICE increased from 92.0% to 99.7%; after the pure silicon negative electrode was recharged with lithium, the first charge and discharge almost There is no loss of capacity. Although the use of lithium foil for lithium supplementation has a good effect, the lithium supplementation process needs to be completed in a temporary battery or electrochemical device, and it is difficult to scale up.

 2. Stabilized lithium metal powder (SLMP)

Lithium powder supplement lithium was proposed by Formica. The developed SLMP has a specific capacity of 3600 mAh/g, and the surface is covered with a thin layer of 2% to 5% lithium carbonate (Li2CO3), which can be used in a dry environment. There are two main ways to apply SLMP to the prelithiation of the negative electrode: adding it during the mixing process or directly adding it to the surface of the negative electrode.

The conventional negative electrode compound slurry uses polyvinylidene fluoride (PVDF) / methyl pyrrolidone (NMP) or styrene butadiene rubber (SBR) + carboxymethyl cellulose (CMC) / deionized water system, but SLMP is not compatible with polar solvents It is compatible and can only be dispersed in non-polar solvents such as hexane and toluene, so it cannot be added directly in the conventional mixing process. L. Wang et al. used the SBR-PVDF/toluene system to directly mix SLMP in the graphite electrode slurry. Firstly, graphite and PVDF are mixed in NMP solvent and dried to form PVDF-coated graphite; then SLMP, PVDF-coated graphite and conductive carbon black are mixed in toluene; finally, SBR is added. After the pre-lithiation of the negative electrode by SLMP, under the conditions of 0.01 to 1. 00 V and 0.05 C, the battery ICE increased from 90. 6% to 96.2%.

Compared with adding it during the mixing process, it is easier to load SLMP directly on the surface of the dry negative electrode. MW Forney et al. used SLMP to pre-lithiation of the silicon (Si)-carbon nanotube (CNT) negative electrode, and dropped the SLMP/toluene solution with a mass fraction of 3% on the surface of the Si-CNT negative electrode. Film, activate. After prelithiation, the first irreversible capacity of the negative electrode is reduced by 20% to 40%.

G. Ai et al. dispersed SLMP in a xylene solution containing 1% SBR/polystyrene to form a stable SLMP slurry. The SLMP slurry is coated on the surface of the dry negative electrode to realize the prelithiation of the negative electrode such as graphite and SiO. After pre-lithiation, graphite|Nickel-Cobalt-Manganese ternary material (NCM) full battery is tested at 0.1 C at 3.0～4.2 V, and the ICE is increased from 82. 35% to 87. 80%; SiO | NCM The ICE of the full battery has increased from 56.78% unlithiated to 88.12% after pre-lithiation.

3. Lithium silicide powder

Compared with the micron-sized SLMP, the size (100-200 nm) of nano-lithium silicide powder (LixSi) is smaller, which is more conducive to dispersion in the negative electrode. In addition, LixSi is already in an expanded state, and the volume change during the cycle will not affect the structure of the entire electrode. At present, there are few studies on LixSi lithium supplementation additives, and only J. Zhao et al. have studied the improvement of LixSi's lithium supplement performance and stability. In an argon atmosphere, the alloying reaction of silicon and metallic lithium at 200 ℃ is used to synthesize LixSi material coated with Li2O. The half-cell system was charged and discharged at 0.05 C at 0.01 ～1.00 V. After 15% LixSi was added, the ICE of the silicon negative electrode increased from 76% to 94%; the mesophase carbon microspheres (MCMB) with 9% LixSi added ) ICE increased from 75% to 99%; ICE of graphite anode with 7% LixSi increased from 87% to 99%. In the full battery system, the ICE of the graphite |LiFePO4 battery with 7% LixSi increased from 77. 6% to 90.8%, and it has a higher capacity in the subsequent cycle test.

The synthesized LixSi has good performance in replenishing lithium, but it can only maintain relative stability in dry air. After being exposed to dry air with a dew point of -50 ℃ for 5 days, the capacity decreases by 30% and is completely inactivated in the air environment. In order to improve the stability of LixSi, 1-fluorodecane can be used to reduce the particle surface to form a dense coating. After the coated LixSi is placed in dry air for 5 days, there is almost no attenuation. After being placed in air with a relative humidity of 10% for 6 hours, under the conditions of 0.01 to 1. 00 V and 0.02 C, the ratio The capacity is still as high as 1 604 mAh/g, and the capacity retention rate reaches 77%. Add 5% to the graphite negative electrode for lithium replenishment. Under the conditions of 0.005 to 1.500 V and 0.05 C, the ICE increases from 87.0% to 96.7%. In order to further improve the stability of LixSi, SiO and SiO2 can be used instead of Si as raw materials to synthesize LixSi-Li2O composite materials. After the composite material is placed in air with a relative humidity of 40% for 6 hours, the specific capacity is as high as 1 240 mAh/g under the conditions of 0.01 to 1. 00 V and 0.02 C. The LixSi-Li2O composite materials synthesized from the two raw materials all show excellent lithium replenishment performance.

4. Electrolytic lithium salt aqueous solution for lithium replenishment

Whether it is using lithium foil, SLMP or lithium silicide powder to supplement lithium, it involves the use of metallic lithium. Lithium metal is expensive, has high activity, and is difficult to operate. Storage and transportation require high costs for protection. If the lithium supplement process does not involve metallic lithium, it can save costs and improve safety performance. H. T. Zhou et al. replenish lithium for silicon by electrolyzing Li2SO4 aqueous solution in an electrolytic cell. The sacrificial electrode is copper wire immersed in Li2SO4. The lithium replenishment reaction is shown in formula (1):

The MnOx | Si full battery after electrolysis at a current of 1 A/g for 4.2 h, the lithium-supplemented MnOx | Si battery was tested at 0.5 ~ 3. 8 V, 0.5 C, 1.0 C, 2.0 C, 4 The specific capacities of 0 C and 8. 0 C are 160 mAh/g, 136 mAh/g, 122 mAh/g, 108 mAh/g and 92 mAh/g, respectively.

Positive Lithium Supplement Technology

Compared with the highly difficult and high-input negative electrode lithium supplementation, the positive electrode lithium supplementation is much simpler. A typical positive electrode lithium supplementation is to add a small amount of high-capacity material during the process of mixing the positive electrode. During the charging process, Li+ is removed from the high-capacity material to supplement the irreversible capacity loss of the first charge and discharge. At present, the materials used as positive electrode lithium supplement additives mainly include: lithium-rich compounds, nanocomposites based on conversion reactions, and binary lithium compounds.

1. Lithium-rich compound

G. Gabrielli et al. used lithium-rich materials Li1 + x Ni0. 5 Mn1. 5O4 to compensate for the irreversible capacity loss of the Si-C| LiNi0. 5Mn1. 5O4 full battery. The battery with a mixed positive electrode has a capacity retention rate of 75% at 0.33 C at 3. 00 to 4.78 V for 100 cycles, while the battery with a pure LiNi0.5 Mn1.5 O4 positive electrode is only 51%. In addition, the energy density of the Si-C | LiNi0. 5 Mn1. 5O4 battery using a mixed positive electrode is 25% higher than that of the graphite | LiNi0. 5Mn1. 5O4 battery.

Li2NiO2 can also be used as a positive electrode lithium supplement additive, but its stability in the air is poor. M. G. Kim et al. used aluminum isopropoxide to modify Li2NiO2 and synthesized Li2NiO2 material that is stable in the air and coated with alumina, which has an excellent effect of replenishing lithium. Unadded LiCoO2 | graphite full battery, the ICE under the conditions of 2.75 ～ 4.20V, 0.2 C is 92%, and the battery with 4% Li2NiO2 added has almost no loss of capacity, and the rate performance is not affected by additives influences.

X. Su et al. added Li5FeO4 (LFO) to the LiCoO2 positive electrode to compensate for the capacity loss of the hard carbon negative electrode during the first charge. Half-cell test shows: LiCoO2 positive electrode with 7% LFO added at 0.1 C. The specific capacity of the first charge and discharge (2.75 ~ 4.30V) is 233 mAh/g and 160 mAh/g respectively, and the irreversible capacity accounts for 31%, which is enough to compensate Hard carbon 22% first irreversible capacity loss. Full battery test (2.75 ~ 4.30 V, 0.05 C) The results show: LiCoO2 with 7% LFO added | The reversible capacity of hard carbon full battery increased by 14%, energy density increased by 10%, and cycle performance Improved, the specific capacity retention rate of the full battery after 50 cycles has increased from less than 90% to more than 95%. For LiCoO2 positive electrode with LFO, the process of mixing and coating needs to be carried out in an inert atmosphere, and the stability of LFO in the air environment needs to be improved.

2. Nanocomposites based on conversion reactions

Although lithium-rich compounds have achieved certain effects as lithium supplement additives, the first lithium supplement effect is still limited by a lower specific capacity. Nanocomposites based on the conversion reaction, due to the large charge/discharge voltage hysteresis, can contribute a large amount of lithium during the first charge of the battery, while the lithium intercalation reaction cannot occur during the discharge process.

Y. M. Sun et al. studied the performance of M/lithium oxide (Li2O), M/lithium fluoride (LiF), M/lithium sulfide (Li2S) (M = Co, Ni and Fe) as positive lithium supplement additives. M/Li2O is synthesized by mixing MxOy and molten lithium in an argon atmosphere. The synthesized nano-Co/nano-Li2O (N-Co/N-Li2O) composite material is cycled at 4. 1 ～ 2. 5 V at 50 mA/g, the first charging specific capacity reaches 619 mAh/g, and the discharge specific capacity is only 10 mAh /g; After N-Co /NLi2O is exposed to ambient air for 8 hours, the specific capacity of delithiation is only 51 mAh/g lower than the initial value. After 2 days, the specific capacity of delithiation is still 418 mAh/g. It shows that NCo /N-Li2O has good environmental stability and is compatible with the production process of commercial batteries. Similar to N-Co /N-Li2O, N-Ni /N-Li2O and N-Fe /N-Li2O also have higher specific charge capacity (506 mAh /g and 631 mAh /g, respectively) and very low discharge The specific capacity (respectively 11 mAh/g and 19 mAh/g), excellent lithium supplement performance.

LiF has high lithium content and good stability, and is a potential positive electrode lithium supplement material. 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 LiF an excellent positive electrode lithium supplement additive. LiF/Co has a specific capacity of 520 mAh/g for lithium removal at 4.2 to 2.5 V, and a specific capacity of only 4 mAh/g for lithium insertion, indicating that LiF/Co's lithium replenishment capacity can reach 516 mAh/g. LiF/Fe has a specific capacity of 532 mAh/g for lithium removal and a specific capacity of 26 mAh/g for lithium insertion at 4. 3 to 2.5 V, indicating that LiF/Fe has a lithium replenishing capacity of 506 mAh/g. The LiFePO4 | Li half-cell with 4.8% LiF/Co added, and the specific capacity of the first charge with 0.1 C at 2.5 ~ 4.2 V reached 197 mAh /g, which is higher than the 164 mAh of the battery without LiF/Co /g increased by 20.1%, the specific discharge capacity is similar, and the cycle stability is not affected by additives.

The theoretical capacity of Li2S reaches 1166 mAh/g, but as a lithium supplement additive, there are still many problems that need to be solved, such as compatibility with electrolyte, insulation, and poor environmental stability. Studies have found that introducing metals into Li2 S to form L2 S/M composite materials can solve these problems. Li2S/Co, which is a combination of CoS2 and metallic Li, has a lithium replenishment capacity of 670 mAh/g. The LFP electrode with 4.8% Li2 S /Co added, at 2.5 to 4.2 V, the specific capacity of the first charge with 0.1 C is 204 mAh/g, which is 42 mAh/g higher than the electrode without addition g. The Li2 S /Fe synthesized by FeS2 and metal Li has a lithium replenishment capacity of 480 mAh/g. Although the lithium replenishment capacity is lower than Li2 S /Co, the raw material FeS2 is rich in resources and low in price, so Li2 S /Fe has a better commercial application prospect. Although more lithium-rich compounds have higher lithium replenishment capacity, nanocomposites based on conversion reactions will leave inactive metal oxides, fluorides and sulfides after the first replenishment of lithium, reducing the energy density of the battery.

3. Binary lithium compound

Compared with lithium-rich oxides (about 300 mAh/g) and conversion reaction composite materials (500-700 mAh/g), the theoretical specific capacity of binary lithium compounds is much higher. The theoretical specific capacities of Li2O2, Li2O and Li3N reach 1168 mAh/g, 1797 mAh/g and 2309 mAh/g respectively, and only a small amount of addition is needed to achieve a similar lithium supplement effect. Theoretically, the residue of these materials after replenishing lithium is O2, N2, etc., which can be exhausted during the formation of the SEI film of the battery.

K. Park et al. ground commercial Li3N into powder with a particle size of 1 to 5 μm, which is used as a lithium supplement additive. In the half-cell system, with 1% and 2% Li3N LiCoO2 electrodes added, the first charge specific capacity with 0.1 C at 3.0 to 4.2 V is 167.6 mAh /g and 178.4 mAh / g, the purer LiCoO2 increased by 18. 0 mAh /g and 28.7 mAh /g. After adding 2% Li3N, the discharge capacity of LiCoO2 | SiOx /C@ Si battery at 0.5 C at 1.75 to 4. 15 V is 11% higher than that of the battery without additives. In order to solve the problem of the conductivity of the mixed electrode, Li3N is deposited on the surface of the LiCoO2 electrode to reduce the impact on the electrode conductivity. The full battery with 5% additives deposited on the surface of the positive electrode has a discharge specific capacity of 126.3 mAh/g, which is 14.6 mAh/g higher than the battery without additives, and the rate performance is similar to the cycle performance. In addition, loading Li3N on the surface of a dry electrode can avoid the incompatibility of Li3N with the slurry solvent (such as methylpyrrolidone).

Y. J. Bie et al. mixed commercialized Li2O2 with LiNi0. 33 Co0. 33 Mn0. 33 O2 (NCM) to compensate for the lithium loss during the first charge of the graphite negative electrode. The NCM in the hybrid electrode plays a dual role of active material and catalyst. In order to efficiently catalyze the decomposition of Li2O2, 1% NCM-6 h (NCM obtained by ball milling for 6 h) was added to the positive electrode. Graphite | NCM/NCM-6 h /2% Li2O2 full battery is charged and discharged at 2.75 ～ 4.60 V, and the first reversible specific capacity of 0.1 C is 207.1mAh/g, which is 7 higher than graphite | NCM full battery 8%; 0.3 C reversible specific capacity is 165.4 mAh/g, which is 20. 5% higher than graphite | NCM full battery. Tests have shown that the oxygen released by the decomposition of Li2O2 will consume the limited Li+ in the full battery, resulting in a significant capacity decline in the full battery with Li2O2, but the capacity can be restored after the gas is discharged. The first charging of the battery in the actual production process is carried out in an open system. The gas formed by the formation of SEI film and some side reactions will be discharged before sealing, so the influence of O2 release can be reduced.

A. Abouimrane et al. studied the effect of micron-sized Li2O as a positive electrode lithium supplement additive. 20% Li2O-added SiO-SnCoC| Li1. 2Ni0. 15Mn0. 55Co0. 1O2 full battery, with 10 mA/g at 2.0 ~ 4. 5 V cycle, the first discharge specific capacity increased from 176 mAh /g to 254 mAh /g. The experimental results show that the lithium-rich material Li1. 2Ni0. 15Mn0. 55Co0. 1O2 plays the dual role of active material and catalyst.