Positive Interface of All-solid-state Lithium-ion Battery

All solid state lithium-ion batteries

This article will discuss the chemical stability, electrochemical stability, mechanical stability, and thermal stability of the cathode interface of the all-solid-state lithium-ion battery, and summarize and discuss different influencing factors and optimization methods. Provide a reference for battery development and application.

The organic liquid electrolyte of conventional lithium-ion batteries is very easy to catch fire at high temperatures, causing thermal runaway of the battery, which poses a greater safety risk; at the same time, because the metal lithium negative electrode is very easy to produce dendrites in the electrolyte, piercing the diaphragm and causing short circuits in the battery. Therefore, traditional lithium-ion batteries based on organic electrolytes cannot use metallic lithium as a negative electrode, which limits the further improvement of battery energy density.

All-solid-state lithium-ion batteries have better safety than traditional lithium-ion batteries because they use high-temperature-resistant solid electrolytes instead of conventional organic liquid electrolytes. At the same time, since the mechanical properties of the solid electrolyte are far better than the electrolyte, it can theoretically effectively block the dendrites generated during the charge and discharge of the metal lithium negative electrode so that the all-solid lithium-ion battery can use the metal negative electrode to further improve the battery energy density.

However, the intrinsic electrochemical performance of the solid electrolyte and the stability of its interface with the positive and negative electrodes have limited the practical application of all-solid-state batteries. Especially in the positive electrode structure, the stability of the solid-solid interface between different components including active materials, conductive agents, and solid electrolytes limits the capacity and cycle life of the battery and is the main bottleneck hindering the improvement of battery performance. To

Among them, the poor chemical and electrochemical stability of the solid-solid interface leads to continuous chemical and electrochemical reactions at the solid-solid interface of the positive electrode material, causing the gradual consumption of lithium ions in the reaction process, causing the battery capacity to decline; its poor mechanical Stability leads to the peeling of the solid-solid interface of the positive electrode material, which reduces the contact area of ​​the positive electrode active material with the conductive agent and the current collector, greatly increases the battery impedance, and reduces the battery capacity and cycle life; poor interface thermal stability results Cathode materials and solid electrolytes are prone to decomposition and element penetration at high temperatures, causing the electrodes and electrolytes to undergo phase changes at high temperatures and thus fail, which limits the universality of the battery assembly process. Therefore, improving the stability of the solid-solid interface of all-solid-state lithium-ion battery cathode materials is the key to improving the electrochemical performance of all-solid-state lithium-ion batteries.

However, the unclear understanding of the fundamental scientific issues of the solid-solid interface of all-solid-state lithium-ion battery cathode materials limits its performance. This article will discuss the chemical stability, electrochemical stability, mechanical stability, and thermal stability of the cathode interface of all-solid-state lithium-ion batteries, and summarize and discuss different influencing factors and optimization methods. Provide a reference for battery development and application.

Chemical stability

The extremely large impedance of the solid-solid interface of the cathode material is the main factor that causes the poor electrochemical performance of the all-solid-state lithium-ion battery at room temperature, and the important reason for the excessive solid-solid interface impedance of the cathode material is the chemical stability and electrical stability of the interface. Poor chemical stability. Among them, the chemical stability of the interface refers to the ability of the interface to maintain the original physical and chemical properties in the absence of electric or magnetic field forces. In the solid-solid interface of the positive electrode material, the two manifestations of poor chemical stability are the interdiffusion of elements between the positive electrode materials and the formation of a space charge layer.

The interdiffusion of elements between cathode materials usually occurs at the interface between the oxide ceramic solid electrolyte and the oxide cathode material. Kim et al. used TEM and linear EDS to find that there is a 50-100nm element diffusion layer at the interface between LLZO and LCO at room temperature. As shown in Figure 1a, its main component is La2CoO4. However, due to the extremely slow inter-diffusion of elements between cathode materials at room temperature, it is difficult to characterize the products generated by the diffusion process.

Figure 1 TEM photograph of LLZO/LCO interface (top) and linear EDS spectrum (bottom) (a); LCO/LPS interface (top) and LCO/LNO/LPS interface (bottom) calculated by DFT in steady state lithium Schematic diagram of ion concentration distribution (b)
Figure 1 TEM photograph of LLZO/LCO interface (top) and linear EDS spectrum (bottom) (a); LCO/LPS interface (top) and LCO/LNO/LPS interface (bottom) calculated by DFT in steady-state lithium Schematic diagram of ion concentration distribution (b)

In an all-solid-state lithium-ion battery, when transition metal oxide is used as the positive electrode and sulfide is used as the electrolyte, since the potential of lithium ions in the oxide is higher than that in the sulfide, the lithium ions are driven from the sulfide by the electric field force. The electrolyte migrates into the oxide cathode material until the potential at both ends of the interface is balanced. However, when equilibrium is reached, a low lithium-ion concentration region similar to the PN junction in an electronic conductor will be formed at the interface between the sulfide electrolyte and the oxide cathode material. This region is called the space charge layer. Since the lithium-ion concentration of the space charge layer is low, the ion conductivity in this region is low, which results in a high migration barrier for ions in this region, which results in a sharp increase in the impedance of this region. As shown in Figure 1b, the formation of a space charge layer can be effectively suppressed by coating LNO on the surface of the LCO. Yamamoto et al. used electron holography to characterize the interface between LCO and LPS and confirmed that there is a low ion concentration region formed by the rearrangement of lithium ions on the side of the interface near the LPS, that is, a space charge layer. Although the researchers are aware of the existence of the space charge layer and have confirmed that the space charge layer is the main reason for the excessive impedance of all-solid-state lithium-ion batteries based on sulfide solid electrolytes, they still understand the mechanism of the chemical formation process of the space charge layer. Unclear. At the same time, due to the effect of the applied potential difference, the chemical behavior of the space charge layer at the interface is more complicated.

Electrochemical stability

Different from the chemical stability, the electrochemical stability of the solid-solid interface of the solid-state lithium-ion battery cathode material reflects the ability of the interface to maintain the original physical and chemical properties under the action of an electric field. The electrochemical behavior of the solid-solid interface of all-solid-state lithium-ion battery cathode materials is very complex. With the different types of solid electrolytes and cathode materials and different pretreatment methods, the solid-solid interface of the cathode materials exhibits different electrochemical stability. This section will introduce the electrochemical stability of the solid-solid interface of all-solid-state lithium-ion battery cathode materials according to the type of electrolyte.

In an all-solid-state lithium-ion battery based on a sulfide solid electrolyte, due to the large contact area between the sulfide solid electrolyte and the positive electrode material, the inter-diffusion phenomenon of elements at the interface between the electrolyte and the positive electrode material during the charge and discharge process is easily characterized. Co3O4 was locally produced at the interface between the LPS solid electrolyte and LCO during charging and discharging, and its position was not fixed, indicating that Co3O4 was a product of local overcharge at the interface. Auvergniot et al. used a scanning Auger electron microscope to characterize the interface between LPSC solid electrolyte and LMO positive electrode. As shown in Figure 2a, they found that S, LiC, P2Sx, and Li2Sn were formed on the surface of LPSC, indicating that LPSC was oxidized during charge and discharge. The product of high electronic conductivity is the main reason for the electrochemical reaction at the interface between the sulfide solid electrolyte and the positive electrode material during charge and discharge. Recent studies have found that the highest valence band of LPS is higher than that of LFP. Due to the charge compensation mechanism, electrochemical activity occurs at the interface between LPS and LFP during the charging and discharging process, which eventually turns the interface into a lithium-poor region, which forms a space charge. As the SS bond and the PS4 tetrahedron continue to polymerize, the space charge layer continues to grow. In addition, Sumita et al. found that as the charge-discharge process progresses, the S-S bond in the sulfide solid electrolyte undergoes a reversible formation and fracture process, as shown in Figure 2b. It shows that under high voltage, the sulfide solid electrolyte has a certain oxidation ability, resulting in poor electrochemical stability under high voltage.

Figure 2 LMO/LPSC/Li-In all-solid-state battery positive side SAM before (top) and after cycling (bottom) SAM map (a); LPS/LFP interface electronic layer state density (LDOS) contour map (using +U energy level calculation results (top), using HSE06 hybrid functional energy level calculation results (bottom)) (b)
Figure 2 LMO/LPSC/Li-In all-solid-state battery positive side SAM before (top) and after cycling (bottom) SAM map (a); LPS/LFP interface electronic layer state density (LDOS) contour map (using +U energy level calculation results (top), using HSE06 hybrid functional energy level calculation results (bottom)) (b)

Compared with sulfide solid electrolytes, there is no space charge layer effect at the solid-solid interface between oxide solid electrolytes and cathode materials, so the electrochemical reaction of solid-solid interface between oxide solid electrolytes and cathode materials is mainly reflected in the solid electrolyte and cathode The inter-diffusion phenomenon of elements between material interfaces during charge and discharge. Kim et al. proved that the interface between LCO and NASICON solid electrolyte and LiPON did not change during charging and discharging. However, studies have shown that the interface between LCO and LMO and Garnet solid electrolyte LGLZO will decompose at 3.0 and 3.8V, respectively, and the rate of formation of decomposition products is much higher than that of the chemical reaction products between the two and LGLZO. Therefore, LCO The driving force for the decomposition of the interface between LMO and LGLZO under high voltage is mainly electrochemical driving force, so the electrochemical stability of LCO and LMO positive electrode and LGLZO is not good.

Since the voltage window of the polymer solid electrolyte (SPE) is small, when it is matched with the positive electrode material with a higher voltage platform, such as LCO and LNMO, an electrochemical reaction will occur at the solid-solid interface of the positive electrode material, resulting in battery capacity Attenuation, the cycle performance is greatly reduced. After 10 weeks of LCO/SPE/Li all-solid-state battery cycle, the capacity decays by 42%. The impedance test of the battery under high potential through AC impedance shows that as the time of the battery at high potential gradually increases, the impedance on the positive side gradually increases. Large, but there is almost no change in the impedance of the electrolyte and the negative electrode side, and a high-impedance Co3O4 phase is generated at the interface of LCO and SPE after cycling. Therefore, the poor stability of the interface between the cathode material and the SPE under high voltage is the main reason for the excessive polarization of the all-solid-state battery.

Mechanical stability

In an all-solid-state lithium-ion battery, the poor mechanical stability of the electrode or solid electrolyte will cause the electrochemical performance of the battery to drop significantly. Among them, the poor mechanical stability of the solid-solid interface of the positive electrode material will cause the polarization of the all-solid-state battery to increase significantly. The main reason for this phenomenon is that the positive electrode material undergoes phase change or lattice expansion/shrinkage during the deintercalation of lithium ions, which causes the crystal lattice size of the positive electrode material to change during charging and discharging. This volume effect will cause the interface between the positive electrode material and the conductive agent to be continuously formed during the charging and discharging process—fragmentation, which consumes transferable lithium ions and reduces the battery capacity. At the same time, the volume change of the positive electrode material during charging and discharging will cause it to peel off from the conductive agent and the current collector, which greatly increases the impedance of the battery. Tian and Qi performed calculations and simulations on the one-dimensional Newman battery based on the Poisson contact mechanics theory and found that the peeling behavior of the electrode from the conductive agent and the current collector occurred after the cycle, which led to the attenuation of the battery capacity. Bucci et al. used the force accumulation model to simulate the behavior of cracks on the positive side of the all-solid-state battery due to the volume change of the positive electrode material. As shown in Figure 3, it was confirmed that only the positive electrode material with low fracture energy and high volume change can make the full The positive side of the solid-state battery generates cracks during charging and discharging and spreads on the positive side.

Figure 3 Schematic diagram of the geometry, discretization and boundary conditions of the finite element model of the cathode material. Electrode material particles are embedded in solid electrolyte and electronic conductive agent particles
Figure 3 Schematic diagram of the geometry, discretization, and boundary conditions of the finite element model of the cathode material. Electrode material particles are embedded in the solid electrolyte and electronic conductive agent particles

Applying pressure on the positive side of the all-solid-state battery can effectively suppress the crack propagation phenomenon caused by the volume change of the positive electrode material. Janek et al. proved that pressurization can effectively suppress the crack propagation phenomenon caused by the volume change caused by the deintercalation of lithium ions during the charging and discharging process of LCO, and effectively improve the mechanical stability of the solid-solid interface of the solid-state battery cathode material. Koerver et al. found that even if the zero-volume strain material Li4Ti5O12 is used as the cathode material for all-solid-state batteries, cracks still occur on the anode side. Therefore, in all-solid-state batteries, improving the mechanical stability of the interface and improving the compatibility between cathode materials are the focus of future research on the solid-solid interface of all-solid-state battery cathode materials.

Thermal stability

After the solid electrolyte is mixed with the positive electrode material, the decomposition temperature will be much lower than its normal decomposition temperature. Studies have found that the thermal decomposition process usually begins at the solid-solid interface where the solid electrolyte is in contact with the cathode material, and then gradually spreads into the material. In the study of the thermal stability of the interface between the solid oxide electrolyte and the cathode material, Gellert et al. used XRD to characterize the thermal decomposition products of the solid-solid interface of the LATP and LMO cathode materials and found that the interface was decomposed at 500 ℃, and at the same time Lithium-free oxide is produced on the positive side of LMO, while lithium-containing phosphate such as Li3PO4 is produced on the electrolyte side of LATP. Since these lithium-containing phosphates have lower melting points, the decomposition temperature of the interface is further reduced. Inoue et al. found that the solid-solid interface of LLZTO, graphite, and Li0.47CoO2 decomposed at 480℃. Miara et al. used XRD and differential scanning calorimetry DSC to investigate the thermal stability of the interface between LLZO and LATP solid electrolytes and LCMO, LNMO, and LFMO spinel structure positive electrodes, as shown in Figure 4. As a result, the interface between LLZO and the spinel structure anode decomposes at 600°C, while the interface between LATP and the spinel structure anode begins to decompose at 700°C. As the lithium element diffuses from the solid electrolyte to the spinel cathode material, the interface between LLZO and the spinel cathode generates lithium-rich manganese oxide Li2MnO3 and various lithium-free oxides at a temperature higher than 600°C, while LATP and spinel The positive electrode interface generate Li3PO4, various lithium-free oxides and lithium-free phosphates at a temperature higher than 700°C.

Figure 4 Schematic diagram of the decomposition temperature of different spinel-type cathode materials, LATP and LLZO solid electrolytes and the decomposition temperature of the two-by-two mixed
Figure 4 Schematic diagram of the decomposition temperature of different spinel-type cathode materials, LATP and LLZO solid electrolytes, and the decomposition temperature of the two-by-two mixed

There are few studies on the thermal stability of the solid-solid interface between the sulfide solid electrolyte and the cathode material. Tsukasaki et al. used TEM and DSC to characterize the interface between the 75Li2S-25P2S5 system amorphous sulfide solid electrolyte and the NCM111 cathode material and found that it produced an unknown crystal phase at 200°C. Similar to the oxide solid electrolyte, when the solid-solid interface of the polymer solid electrolyte and the positive electrode material is heated to a certain temperature, the lithium salt in the polymer solid electrolyte reacts with the positive electrode material and the polymer matrix to produce products such as lithium carbonate. Thermal failure of the interface occurs. Xia et al. studied the thermal stability of the interface between PEO+LiTFSI polymer solid electrolyte and LiCoO2, LiNiO2, LiMn2O4, V2O5, V6O13, and LixMnO2 positive electrode through XRD and DSC tests, and found that the interface between polymer solid electrolyte and different positive electrode materials was at 210~ Decomposition occurred at 340℃, and the decomposition products were mainly Li2CO3, Li2O, LiF and other lithium-containing compounds, metal oxides and gases with unknown components. At the same time, the decomposition temperature of the interface between the positive electrode and the polymer solid electrolyte in the charged state is higher than the decomposition temperature of the interface in the discharged state.

Introduction to interface optimization methods

Since the working environment temperature of all-solid-state batteries is close to room temperature, compared to improving the chemical stability and thermal stability of the solid-solid interface of all-solid-state lithium-ion battery cathode materials, the electrochemical and mechanical stability of the interface are improved, and avoid The chemical reaction between the positive electrode and the solid electrolyte during the charging and discharging process prevents the crushing of the positive electrode particles during the charging and discharging process, which is the key to improving the electrochemical performance of the all-solid-state battery. Avoiding chemical reactions between the positive electrode material and the solid electrolyte during the charge and discharge process can effectively avoid the continuous decomposition-generation process at the interface, reduce the lithium ions consumed in the process, and improve the coulombic efficiency and cycle life of the all-solid battery. Suppressing the crushing of positive electrode particles during charging and discharging can avoid poor contact and interface damage caused by particle crushing, and improve the capacity and cycle life of all-solid-state batteries. In view of these two problems, effective methods for optimizing the solid-solid interface of the positive electrode of all-solid-state batteries mainly include the surface coating of positive electrode particles, preparation of three-dimensional porous solid electrolyte, and optimization and modification of low-melting ion conductors.

Cathode particle surface coating is the most commonly used method for optimizing the solid-solid interface of all-solid-state lithium-ion battery cathode materials. This method is to coat the surface of the cathode material with a layer of lithium that is stable under high voltage, has high ionic conductivity and electronic insulation. The ionic conductor achieves the purpose of isolating the positive electrode from the solid electrolyte and avoiding its reaction during charge and discharge. At the same time, this layer of the lithium-ion conductor can effectively prevent the positive electrode particles from being broken due to volume changes during charge and discharge. Common positive electrode surface coatings include Li3PO4, LiNbO3, and various lithium-ion conductors. Common processing methods are the sol-gel method, spraying method, screen printing method, spin coating method, pulse laser deposition (PLD), atomic layer deposition (ALD), etc. However, the surface coating of the positive electrode particles cannot solve the problem of too small a contact area between the positive electrode material and the solid electrolyte, so it cannot be used for all-solid-state batteries with an oxide solid electrolyte system. In addition, in addition to sputtering methods such as PLD and ALD, the products obtained by other coating methods have the problem of the uneven coating. The main reason is that other coating methods are all mechanical mixing. However, the preparation cost of sputtering methods such as PLD and ALD is relatively high. Therefore, how to use lower-cost means to coat the surface of the cathode material is the key to future research in this direction.

The three-dimensional porous solid electrolyte can load the positive electrode in the pores of the porous solid electrolyte, so that the positive electrode material is in full contact with the solid electrolyte, and can also prevent the positive electrode material from being broken during charging and discharging. The preparation of three-dimensional porous solid electrolyte mainly includes the casting method and the template method. Compared with the more complicated process and higher cost casting method, the template method has a simpler process and lower cost. Zhang et al. used the template method to prepare a three-dimensional porous structure LAGP solid electrolyte and loaded the high nickel ternary cathode material NCM811 in the pores, as shown in Figure 5. Compared with ordinary LAGP ceramics, the three-dimensional porous structure LAGP can make all-solid-state batteries have Higher capacity, higher capacity to play, and better cycle performance. Three-dimensional porous solid electrolyte optimization is mostly used in oxide solid electrolyte and polymer solid electrolyte systems. However, the three-dimensional porous solid electrolyte cannot effectively improve the electrochemical stability of the solid-solid interface between the positive electrode material and the solid electrolyte, and coat it with the surface of the positive electrode particles. The combination is the focus of future research of this method.

Figure 5 Schematic diagram of a three-dimensional LAGP all-solid-state lithium-ion battery with high NCM811 cathode material loading
Figure 5 Schematic diagram of a three-dimensional LAGP all-solid-state lithium-ion battery with high NCM811 cathode material loading

Mixing a low melting point ion conductor into the positive electrode material, melt the ion conductor and cool it by applying a temperature higher than the melting point of the ion conductor, and evenly distribute it between the positive electrode material and the solid electrolyte. This method can not only prevent the positive electrode material from contacting the solid electrolyte. reaction. Moreover, the contact area between the positive electrode material and the solid electrolyte can be increased, and the problem of poor contact caused by the crushing of the positive electrode particles during charging and discharging can also be improved. Han et al. added a low melting point LCBO at the interface between LCO and LLZO, as shown in Figure 6. LCBO increases the contact area between the LCO and the solid electrolyte, and at the same time reduces the impact of particle breakage caused by the volume change of the LCO on the contact performance between the positive electrode material and the conductive agent during the charge and discharge process. In addition, the use of LCBO to isolate LCO and LLZO prevents the reaction between the two during charge and discharge. The three aspects work together to improve the electrochemical performance of the LCO/LLZO/Li all-solid-state lithium-ion battery. However, the preparation process of this method is very complicated and the cost is high.

Figure 6 Schematic diagram of all-ceramic cathode-solid electrolyte interface modification
Figure 6 Schematic diagram of all-ceramic cathode-solid electrolyte interface modification

The room temperature cycling performance, rate performance, and low Coulomb efficiency of all-solid-state lithium-ion batteries limit their practical applications. Poor solid-solid interface stability of cathode materials is the main reason for the poor room temperature performance of all-solid-state lithium-ion batteries. At present, significant results have been achieved in the optimization of the solid-solid interface, but there are still many key issues that need to be resolved: ①The microscopic mechanism of the wettability of the cathode material and the solid electrolyte interface is unclear; ②The active material in the cathode material The ratio in the middle is low; ③The selection of the composition and structure of the interface layer and the research on the interface compatibility of the interface layer with the positive electrode and the electrolyte are lacking. Solving the above problems is an important task of solid-solid interface research on all-solid-state lithium-ion battery cathode materials.

In addition, the characterization methods of the phase and morphology changes of the solid-solid interface of the all-solid lithium-ion battery cathode material during the charge and discharge process also limit the optimization of the solid-solid interface of the cathode material. Limited by the test accuracy, the most commonly used XRD method for phase analysis cannot be used to characterize the phase change of the solid-solid interface of the positive electrode during the charge and discharge process, which increases the research difficulty. At present, there are only a few commonly used methods for characterizing changes in the solid-solid interface of the positive electrode, such as SEM, TEM, XPS, and nuclear magnetic resonance (NMR), and the characterization effect is not good, and in-situ characterization methods are more scarce. Therefore, the development of new solid-solid interface characterization technology for all-solid-state lithium-ion battery cathode materials, especially combining various in-situ characterization methods, is an important direction for future solid-solid interface research on all-solid-state lithium-ion battery cathode materials.

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