New battery solid electrolyte breaking the cost performance limits
In order to realize the commercialization of all-solid-state lithium battery industry, solid-state electrolytes not only need to demonstrate excellent performance, but also need to have a strong enough cost competitiveness.
In this regard, this article will introduce a solid electrolyte that can meet the above requirements at the same time, which plays an important role in promoting the commercialization of all-solid-state batteries.
Table of Contents
Characteristics of the current solid electrolytes
From the point of view of performance, under ideal conditions, solid lithium ion battery electrolyte needs to have advantages in ionic conductivity, oxidation stability, reduction stability and moisture stability at the same time. The reported inorganic solid electrolytes can be roughly divided into three categories: oxides, sulfides and halides.
As a brittle material, oxides cannot meet the requirement of deformability. In contrast, sulfides and halides are both able to deform under specific pressures and are also relatively easy to achieve high ionic conductivity. However, Li2S, the raw material used to synthesize sulfide, is quite expensive, reaching $654.18/kg.
Considering that the mass ratio of Li2S in the raw material of sulfide solid electrolyte is generally more than 30%, the raw material cost will not be less than $196.25/kg. Halides can only achieve high ionic conductivity (> 1 mS cm-1) by using rare earth or indium-based chloride and other expensive raw materials for synthesis, so the raw material cost is also quite high, mostly above $190/kg.
Therefore, all these three types of materials can not meet the requirements of ionic conductivity, deformability and cost at the same time. The only exception is the solid electrolyte lithium zirconium chloride reported by Professor Ma Cheng’s research group at the University of Science and Technology of China in 2021.
Since it does not contain rare earth elements or indium, the raw material cost is less than $50/kg. However, the ionic conductivity of the material is low, only about 0.5 mS cm-1, which can not meet the requirements of ion transport efficiency.
In general, the current oxide, sulfide and halide solid electrolytes are not able to meet the application requirements in ionic conductivity, deformability and cost competitiveness at the same time.
However, many of these properties can be made up in other ways, for example, even if the oxidation or reduction stability of the solid electrolyte is not good, as long as the electrode active material is constructed on the appropriate coating material, then the battery can still play a good performance.
If such properties are excluded, the solid electrolyte still needs both good ionic conductivity (higher than 1 mS cm-1 at room temperature) and deformability (higher than 90% relative density at 250-350 MPa). However, current solid-state electrolytes cannot provide both performance advantages while being sufficiently cost-competitive (below $50/kg).
The new battery solid electrolytes
Professor Ma Cheng’s research group at the University of Science and Technology of China has designed a new oxychloride solid electrolyte Li1.75ZrCl4.75O0.5, which can meet the requirements of solid state battery in the above three aspects.
The room temperature ionic conductivity of Li1.75ZrCl4.75O0.5 reaches 2.42 mS cm-1, which meets the application requirements above 1 mS cm-1, and is not inferior to sulfide and rare earth/indium-based halides.
Moreover, lithium zirconium oxychloride also has good deformability, and the relative density after cold pressing at 300 MPa is as high as 94.2%, which exceeds that of solid electrolytes known for good deformability such as Li3InCl6 and Li10GeP2S12 (the density is lower than 90% under the same pressure).
Due to the above characteristics, the all-solid-state battery composed of Li1.75ZrCl4.75O0.5 shows excellent performance. The all-solid-state battery based on single crystal LiNi0.8Mn0.1Co0.1O2 can still reach the discharge capacity of 70.2mAh g-1 after 2082 cycles under the high current density of 1000 mA g-1. Close to the battery-like performance of Li2In1/3Sc1/3Cl4 solid electrolyte recently reported in Nature Energy (540 mA g-1, 3000 cycles, about 70 mAh g-1 final discharge capacity).
However, the cost of Li1.75ZrCl4.75O0.5 is much lower than that of Li2In1/3Sc1/3Cl4 ($11.60/kg vs. $4418.10/kg, which is less than 0.3% of the latter) and well below the $50/kg threshold mentioned above. This solid electrolyte material, which has strong competitiveness in cost and performance, paves the way for the commercialization of all-solid-state batteries.
Research process of the new solid electrolyte
The researchers first tried to synthesize a series of Li2+xZrCl6-xOx solid electrolytes by high-energy ball milling method. The chemical formula can also be expressed as (1-a)Li2ZrCl6-aLi4ZrCl4O2(a = x/2). X-ray diffraction shows that components with x≤0.25 exhibit P-3m1 structure. With the further increase of x, a phase with the structure of C2/m appears in the material and coexists with the P-3m1 phase.
Composition dependent phase evolution in Li2+xZrCl6-xOx
When x≥1.0, the material exhibits only the C2/m phase. In the two-phase coexistence region between 0.25<x<1.0, the crystal structure of the material is particularly vulnerable to high-energy ball milling damage, and the crystallinity is less than 20%. Since Zr-based chloride solid electrolytes typically rely on amorphous phases for efficient ion transport, this could mean that materials with 0.25<x<1.0 coexistence of these double crystalline phases have higher ionic conductivity.
The electrochemical impedance spectroscopy (EIS) test shows that the components with double crystalline phase coexistence do have higher ionic conductivity. In agreement with the expected results, the room temperature ionic conductivity of the two-phase components with lower crystallinity is generally higher than that of the single-phase components.
At the component point x=0.5 (chemical formula:Li2.5ZrCl5.5O0.5), the ionic conductivity at room temperature reaches 1.17 mS cm-1, which is not bad even when compared with rare earth or indium-based halide solid electrolytes.
Li-ion transport behavior of Li2+xZrCl6-xOx
Although the ionic conductivity of the above materials has exceeded 1 mS cm-1, it can still be further improved. According to the trend of ionic conductivity with composition, the researchers found that when the composition of the two-phase region in the phase diagram is close to its phase boundary with the single-phase region, the ionic conductivity will be improved.
In order to accurately control the composition and make it close to the phase boundary, the researchers introduced a third component LiZrCl5 on the basis of the above component Li2.5ZrCl5.5O0.5 (that is, 75%Li2ZrCl6-25%Li4ZrCl4O2) with the highest ionic conductivity.
This results in a series of components (75%-y)Li2ZrCl6-25%Li4ZrCl4O2-yLiZrCl5 or Li2.5-yZrCl5.5-yO0.5. According to the X-ray diffraction results, with the increase of y, the diffraction peak intensity of P-3m1 phase in Li2.5-yZrCl5.5-yO0.5（y≤0.75） gradually increases, while that of C2/m phase gradually decreases.
When y=0.75, although the P-3m1 phase and the C2/m phase still coexist, the characteristic peak of the latter becomes extremely weak, indicating that the component is quite close to the phase boundary between the two-phase region and the single-phase region in the phase diagram.
As expected, the room temperature ionic conductivity of Li2.5-yZrCl5.5-yO0.5 increases significantly with the increase of y (that is, the composition continues to approach the phase boundary between the single-phase region and the two-phase region in the phase diagram).
For the two-phase region composition y=0.75 (chemical formula: Li1.75ZrCl4.75O0.5), the ionic conductivity of the material at 25°C reaches 2.42 mS cm-1, surpassing solid electrolytes such as Li3InCl6 and Li2In1/3Sc1/3Cl4, which are based on expensive raw materials.
Stuructures and ionic conductivities of Li2.5-yZrCl5.5-yO0.5
In addition to ionic conductivity, the deformability of Li1.75ZrCl4.75O0.5 is also quite excellent. This property can be evaluated by the relative density that the material can achieve under certain pressures. The better the deformability, the higher the relative density the material can achieve at a particular pressure.
Experimental tests show that the relative density of inorganic solid electrolytes such as Li6PS5Cl, Li10GeP2S12, Li3InCl6 and Li2ZrCl6, which are known for their good deformability, is lower than 90% at 300 MPa. In contrast, Li1.75ZrCl4.75O0.5 has a relative density of 94.2% at 300 MPa, so its deformability exceeds that of all the solid electrolytes mentioned above.
Compressibility of LZCO
Excellent ionic conductivity and good deformability allow all-solid-state batteries composed of Li1.75ZrCl4.75O0.5 solid-state electrolytes to demonstrate excellent performance.
An all-solid-state battery using uncoated LiCoO2 (LCO) as the positive electrode, Li-In alloy as lithium ion battery anode, Li1.75ZrCl4.75O0.5 as the solid electrolyte, and Li6PS5Cl as the buffer layer between Li1.75ZrCl4.75O0.5 and the negative electrode, the first cycle coulomb efficiency of up to 98.28% at 25 °C, 14 mA g−1. It is better than the same type of all-solid-state battery reported in the literature.
In addition, after 150 cycles of the LCO-based solid state battery at a high current density of 25 °C and 700 mA g−1, the capacity is basically not attenuated, and the discharge capacity of 102 mAh g−1 can still be achieved. A similar battery composed of Li2ZrCl6 has a similar discharge capacity (114 mAh g−1) after 100 cycles at a current density of only 1/10 of the above values ( 70 mA g−1).
Electrochemical performance of the Li-In,LPSCH-LZCO,LCO cell
When using single crystal LiNi0.8Mn0.1Co0.1O2（scNMC811） as the cathode materials, the all-solid-state battery still shows excellent cycle performance. The first cycle coulomb efficiency of the battery at 25 °C and 20 mA g−1 is 87.31%.
Even after 2082 cycles at a high current density of 1000 mA g−1, the discharge capacity can still reach 70.2 mAh g−1. Similar battery performance (540 mA g−1, 3000 cycles, about 70 mAh g−1 final discharge capacity) with Li2In1/3Sc1/3Cl4 solid electrolyte recently reported in Nature Energy.
However, since the synthesis of Li1.75ZrCl4.75O0.5 does not require the use of expensive compounds such as rare earth chloride and lithium sulfide, its raw material cost is only $11.60/kg, less than 0.3% of the raw material cost of Li2In1/3Sc1/3Cl4 ($4418.10/kg). It is also well below the $50/kg threshold mentioned above. Therefore, the Li1.75ZrCl4.75O0.5 is highly competitive in both cost and performance.
Electrochemical performance of the Li-In,LPSCH-LZCO, scNMCSII cell
Summary and prospect
Professor Ma Cheng’s research group designed and synthesized a new type of polycrystalline oxide chloride solid electrolyte Li1.75ZrCl4.75O0.5. In terms of performance, the material has more ionic conductivity than Li3InCl6, Li2In1/3Sc1/3Cl4 and other high-performance solid electrolytes, and is better than those easily deformed solid electrolytes such as Li6PS5Cl and Li10GeP2S12.
The discharge capacity of an all-solid-state battery composed of this material after 2082 cycles at a high current density of 1000 mA g−1 is close to that of a similar battery based on Li2In1/3Sc1/3Cl4 after 3000 cycles at 540 mA g−1.
In terms of cost, because Li1.75ZrCl4.75O0.5 can be synthesized from cheap compounds such as LiOH·H2O, LiCl, ZrCl4, its raw material cost is only $11.60/kg, which is not only lower than other solid electrolytes with similar properties (mostly around $200/kg or higher). Also lower than the $50/kg threshold required for commercialization.
Moreover, if synthesized from the cheaper ZrOCl2·8H2O, LiCl and ZrCl4, the cost of Li1.75ZrCl4.75O0.5 can be further reduced on the basis of $11.60/kg. The discovery of Li1.75ZrCl4.75O0.5 broke through the “cost performance” limit that solid electrolyte can achieve. This low-cost, high-performance solid-state electrolyte will provide a great boost to the commercialization of all-solid-state batteries.
New battery solid electrolyte breaking the cost performance limits
Characteristics of the current solid electrolytes
From the point of view of performance, under ideal conditions, solid lithium ion battery electrolyte needs to have advantages in ionic conductivity, oxidation stability, reduction stability and moisture stability at the same time. The reported inorganic solid electrolytes can be roughly divided into three categories: oxides, sulfides and halides.
As a brittle material, oxides cannot meet the requirement of deformability. In contrast, sulfides and halides are both able to deform under specific pressures and are also relatively easy to achieve high ionic conductivity. However, Li2S, the raw material used to synthesize sulfide, is quite expensive, reaching $654.18/kg.
Considering that the mass ratio of Li2S in the raw material of sulfide solid electrolyte is generally more than 30%, the raw material cost will not be less than $196.25/kg. Halides can only achieve high ionic conductivity (> 1 mS cm-1) by using rare earth or indium-based chloride and other expensive raw materials for synthesis, so the raw material cost is also quite high, mostly above $190/kg.
Therefore, all these three types of materials can not meet the requirements of ionic conductivity, deformability and cost at the same time. The only exception is the solid electrolyte lithium zirconium chloride reported by Professor Ma Cheng’s research group at the University of Science and Technology of China in 2021.
Since it does not contain rare earth elements or indium, the raw material cost is less than $50/kg. However, the ionic conductivity of the material is low, only about 0.5 mS cm-1, which can not meet the requirements of ion transport efficiency.
In general, the current oxide, sulfide and halide solid electrolytes are not able to meet the application requirements in ionic conductivity, deformability and cost competitiveness at the same time.
However, many of these properties can be made up in other ways, for example, even if the oxidation or reduction stability of the solid electrolyte is not good, as long as the electrode active material is constructed on the appropriate coating material, then the battery can still play a good performance.
If such properties are excluded, the solid electrolyte still needs both good ionic conductivity (higher than 1 mS cm-1 at room temperature) and deformability (higher than 90% relative density at 250-350 MPa). However, current solid-state electrolytes cannot provide both performance advantages while being sufficiently cost-competitive (below $50/kg).
The new battery solid electrolytes
Professor Ma Cheng’s research group at the University of Science and Technology of China has designed a new oxychloride solid electrolyte Li1.75ZrCl4.75O0.5, which can meet the requirements of solid state battery in the above three aspects.
The room temperature ionic conductivity of Li1.75ZrCl4.75O0.5 reaches 2.42 mS cm-1, which meets the application requirements above 1 mS cm-1, and is not inferior to sulfide and rare earth/indium-based halides.
Moreover, lithium zirconium oxychloride also has good deformability, and the relative density after cold pressing at 300 MPa is as high as 94.2%, which exceeds that of solid electrolytes known for good deformability such as Li3InCl6 and Li10GeP2S12 (the density is lower than 90% under the same pressure).
Due to the above characteristics, the all-solid-state battery composed of Li1.75ZrCl4.75O0.5 shows excellent performance. The all-solid-state battery based on single crystal LiNi0.8Mn0.1Co0.1O2 can still reach the discharge capacity of 70.2mAh g-1 after 2082 cycles under the high current density of 1000 mA g-1. Close to the battery-like performance of Li2In1/3Sc1/3Cl4 solid electrolyte recently reported in Nature Energy (540 mA g-1, 3000 cycles, about 70 mAh g-1 final discharge capacity).
However, the cost of Li1.75ZrCl4.75O0.5 is much lower than that of Li2In1/3Sc1/3Cl4 ($11.60/kg vs. $4418.10/kg, which is less than 0.3% of the latter) and well below the $50/kg threshold mentioned above. This solid electrolyte material, which has strong competitiveness in cost and performance, paves the way for the commercialization of all-solid-state batteries.
Research process of the new solid electrolyte
The researchers first tried to synthesize a series of Li2+xZrCl6-xOx solid electrolytes by high-energy ball milling method. The chemical formula can also be expressed as (1-a)Li2ZrCl6-aLi4ZrCl4O2(a = x/2). X-ray diffraction shows that components with x≤0.25 exhibit P-3m1 structure. With the further increase of x, a phase with the structure of C2/m appears in the material and coexists with the P-3m1 phase.
When x≥1.0, the material exhibits only the C2/m phase. In the two-phase coexistence region between 0.25<x<1.0, the crystal structure of the material is particularly vulnerable to high-energy ball milling damage, and the crystallinity is less than 20%. Since Zr-based chloride solid electrolytes typically rely on amorphous phases for efficient ion transport, this could mean that materials with 0.25<x<1.0 coexistence of these double crystalline phases have higher ionic conductivity.
The electrochemical impedance spectroscopy (EIS) test shows that the components with double crystalline phase coexistence do have higher ionic conductivity. In agreement with the expected results, the room temperature ionic conductivity of the two-phase components with lower crystallinity is generally higher than that of the single-phase components.
At the component point x=0.5 (chemical formula:Li2.5ZrCl5.5O0.5), the ionic conductivity at room temperature reaches 1.17 mS cm-1, which is not bad even when compared with rare earth or indium-based halide solid electrolytes.
Although the ionic conductivity of the above materials has exceeded 1 mS cm-1, it can still be further improved. According to the trend of ionic conductivity with composition, the researchers found that when the composition of the two-phase region in the phase diagram is close to its phase boundary with the single-phase region, the ionic conductivity will be improved.
In order to accurately control the composition and make it close to the phase boundary, the researchers introduced a third component LiZrCl5 on the basis of the above component Li2.5ZrCl5.5O0.5 (that is, 75%Li2ZrCl6-25%Li4ZrCl4O2) with the highest ionic conductivity.
This results in a series of components (75%-y)Li2ZrCl6-25%Li4ZrCl4O2-yLiZrCl5 or Li2.5-yZrCl5.5-yO0.5. According to the X-ray diffraction results, with the increase of y, the diffraction peak intensity of P-3m1 phase in Li2.5-yZrCl5.5-yO0.5（y≤0.75） gradually increases, while that of C2/m phase gradually decreases.
When y=0.75, although the P-3m1 phase and the C2/m phase still coexist, the characteristic peak of the latter becomes extremely weak, indicating that the component is quite close to the phase boundary between the two-phase region and the single-phase region in the phase diagram.
As expected, the room temperature ionic conductivity of Li2.5-yZrCl5.5-yO0.5 increases significantly with the increase of y (that is, the composition continues to approach the phase boundary between the single-phase region and the two-phase region in the phase diagram).
For the two-phase region composition y=0.75 (chemical formula: Li1.75ZrCl4.75O0.5), the ionic conductivity of the material at 25°C reaches 2.42 mS cm-1, surpassing solid electrolytes such as Li3InCl6 and Li2In1/3Sc1/3Cl4, which are based on expensive raw materials.
In addition to ionic conductivity, the deformability of Li1.75ZrCl4.75O0.5 is also quite excellent. This property can be evaluated by the relative density that the material can achieve under certain pressures. The better the deformability, the higher the relative density the material can achieve at a particular pressure.
Experimental tests show that the relative density of inorganic solid electrolytes such as Li6PS5Cl, Li10GeP2S12, Li3InCl6 and Li2ZrCl6, which are known for their good deformability, is lower than 90% at 300 MPa. In contrast, Li1.75ZrCl4.75O0.5 has a relative density of 94.2% at 300 MPa, so its deformability exceeds that of all the solid electrolytes mentioned above.
Excellent ionic conductivity and good deformability allow all-solid-state batteries composed of Li1.75ZrCl4.75O0.5 solid-state electrolytes to demonstrate excellent performance.
An all-solid-state battery using uncoated LiCoO2 (LCO) as the positive electrode, Li-In alloy as lithium ion battery anode, Li1.75ZrCl4.75O0.5 as the solid electrolyte, and Li6PS5Cl as the buffer layer between Li1.75ZrCl4.75O0.5 and the negative electrode, the first cycle coulomb efficiency of up to 98.28% at 25 °C, 14 mA g−1. It is better than the same type of all-solid-state battery reported in the literature.
In addition, after 150 cycles of the LCO-based solid state battery at a high current density of 25 °C and 700 mA g−1, the capacity is basically not attenuated, and the discharge capacity of 102 mAh g−1 can still be achieved. A similar battery composed of Li2ZrCl6 has a similar discharge capacity (114 mAh g−1) after 100 cycles at a current density of only 1/10 of the above values ( 70 mA g−1).
When using single crystal LiNi0.8Mn0.1Co0.1O2（scNMC811） as the cathode materials, the all-solid-state battery still shows excellent cycle performance. The first cycle coulomb efficiency of the battery at 25 °C and 20 mA g−1 is 87.31%.
Even after 2082 cycles at a high current density of 1000 mA g−1, the discharge capacity can still reach 70.2 mAh g−1. Similar battery performance (540 mA g−1, 3000 cycles, about 70 mAh g−1 final discharge capacity) with Li2In1/3Sc1/3Cl4 solid electrolyte recently reported in Nature Energy.
However, since the synthesis of Li1.75ZrCl4.75O0.5 does not require the use of expensive compounds such as rare earth chloride and lithium sulfide, its raw material cost is only $11.60/kg, less than 0.3% of the raw material cost of Li2In1/3Sc1/3Cl4 ($4418.10/kg). It is also well below the $50/kg threshold mentioned above. Therefore, the Li1.75ZrCl4.75O0.5 is highly competitive in both cost and performance.
Summary and prospect
Professor Ma Cheng’s research group designed and synthesized a new type of polycrystalline oxide chloride solid electrolyte Li1.75ZrCl4.75O0.5. In terms of performance, the material has more ionic conductivity than Li3InCl6, Li2In1/3Sc1/3Cl4 and other high-performance solid electrolytes, and is better than those easily deformed solid electrolytes such as Li6PS5Cl and Li10GeP2S12.
The discharge capacity of an all-solid-state battery composed of this material after 2082 cycles at a high current density of 1000 mA g−1 is close to that of a similar battery based on Li2In1/3Sc1/3Cl4 after 3000 cycles at 540 mA g−1.
In terms of cost, because Li1.75ZrCl4.75O0.5 can be synthesized from cheap compounds such as LiOH·H2O, LiCl, ZrCl4, its raw material cost is only $11.60/kg, which is not only lower than other solid electrolytes with similar properties (mostly around $200/kg or higher). Also lower than the $50/kg threshold required for commercialization.
Moreover, if synthesized from the cheaper ZrOCl2·8H2O, LiCl and ZrCl4, the cost of Li1.75ZrCl4.75O0.5 can be further reduced on the basis of $11.60/kg. The discovery of Li1.75ZrCl4.75O0.5 broke through the “cost performance” limit that solid electrolyte can achieve. This low-cost, high-performance solid-state electrolyte will provide a great boost to the commercialization of all-solid-state batteries.