Multiscale Researches on Lithium Ion and Lithium Metal Batteries

Abstract

Research and development of Lithium Ion Battery (LIB) have been extensively performed based on material science and cell technology. In order to accelerate the development of LIB, a multi-scale researches have to be conducted under one roof. Here, several researches from nm scale to m scale on LIB were introduced to discuss an importance of multi-scale research. Then, the new platform for LIB development was proposed based on our several researches. (1) Interfacial analysis between cathode and electrolyte in LIB by using in-situ Fourier Transform Infrared method, (2) Preparation of LiFePO₄ (LFP) with carbon coating, (3) Interfacial analysis on lithium metal anode for solid electrolyte interphase (SEI), (4) Preparation of 3 dimensionally ordered macroporous separator and its application to lithium metal battery, (5) Single particle measurement for evaluation of composite electrodes, (6) Failure mode analysis of LFP/Graphite cell.

1. Introduction

Rechargeable batteries have been extensively studied during last 30 years after a commercialization of Lithium Ion Battery (LIB) by SONY. LIBs have enabled various portable applications, such as laptop personal computer and smartphone. Recently, LIBs have been also utilized in electric vehicles and energy storage systems, in order to reduce carbon dioxide from vehicles and electric power generation systems by using natural energy, such as solar power and wind power. Lead acid battery and nickel metal hydride battery have been also used in electric power generation systems from natural energy. Recently, LIB became major battery system used in natural energy power generation system.^1,2 This trend is due to a larger energy density of LIB than those of traditional batteries. The performance of LIB has been improved during last 30 years. For example, the energy density of LIB was 200 Wh L⁻¹ in 1992. The energy density of LIB in 2022 was increased to 700 Wh L⁻¹. The energy density of rechargeable battery does not only depend on active materials used in cathode and anode, but also on cell structure and other components, such as electrolyte, current collector, binder and so on. The improvement of LIB during last 30 years is mainly due to an improvement of cell manufacturing technology. A part of improvement is due to new active materials. The energy densities of LIB calculated based on only cathode and anode materials are 1000 Wh L⁻¹ and 400 Wh kg⁻¹ which are higher than real energy density of commercialized LIB. Another important performance of LIB is a power density. LIB have to satisfy both demanded energy density and power density depending on applications. The power density of LIB depends on electrochemical performance of active materials, cell structure and other materials used in LIB. As shown in Fig. 1, LIB structure is seemed to be simple, just cathode layer, separator and anode layer are sandwiched to form cell. However, cathode layer, separator and anode layer have a porous structure including liquid electrolyte. Both cathode and anode are a composite electrode consisting of active material, polymer binder, and acetylene black. The separator is a porous thin film with specific porous structure depending on a kind of polymer. In this way, real LIB is a complicated electrochemical system. Therefore, an evaluation of energy density and power density is not so easy. These characteristics strongly depend on structure of electrodes, active materials used in electrodes, separator, current collector, and electrochemical properties of electrolyte, and so on. In order to estimate (or predict) the precise energy density and power density of LIB, a multi-scale development process is needed. Another important point is the interface between active materials and electrolyte. Especially, the solid electrolyte interphase (SEI) for anode side and the cathode electrolyte interphase (CEI) have a significant impact to cycle life of cell and safety of cell.³ SEI and CEI sometimes suppress an electrolyte decomposition during charging process of cell, leading to reversible discharge and charge behavior of LIB. Moreover, the life of cell is influenced by a current distribution in cell. Non-uniform current distribution provides lower cycle life of LIB and lower safety. Therefore, the uniform cathode and anode layers must be prepared and utilized in cell. In addition, a separator is an important material influencing current distribution in cell.⁴ Larger current distribution may provide lower cycle performance of LIB and sometimes lead to explosion of LIB.

Figure 1.

Schematic illustration of sandwich structure for LIB.

Thus, a development of LIB is not so easy. The materials and phenomena taking place in nm scale ∼ m scale should be well understood to develop rechargeable batteries including LIB and next generation ones. This is a multi-scale research and development for rechargeable battery. In this report, the multi-scale research and development for LIB are introduced based on our previous research, as described below.

1. (1)    Interfacial analysis between cathode and electrolyte in LIB by using in-situ FTIR method.^5–14
2. (2)    Preparation of LiFePO₄ (LFP) with carbon coating as CEI^15–26
3. (3)    Interfacial analysis on lithium metal anode for SEI^27–46
4. (4)    Preparation of 3 Dimensionally Ordered Macroporous (3DOM) separator and its application to lithium metal battery^47–54
5. (5)    Single particle measurement and evaluation of composite electrodes^55–61
6. (6)    Failure mode analysis of LFP/Graphite cell⁶²

These researches were originally conducted independently. Here, these researches are introduced to discuss a multi-scale research and development of Li based rechargeable batteries.

2. Cell Performance and Cell Structure

Figure 2 shows a schematic illustration of LIB consisting of LiCoO₂ cathode, graphite anode, non-aqueous electrolyte, separator. The cathode and anode layers are porous composite electrodes consisting of active material particles, polymer binder and carbon black based conductive additive. The separator is a porous polymer film prepared from poly-olefin material.³ The electrolyte is non-aqueous organic solvent with lithium salt, such as a mixed solvent of ethylene carbonate and ethyl methyl carbonate containing 1.2 mol dm⁻³ LiPF₆. The electrolyte is maintained in pores of electrode layers and separator. In the electrolyte, Li⁺ ion and anion transfer between cathode and anode according to electrochemical reactions of cathode and anode. In a discharge process of LIB, Li⁺ ions are inserted into cathode active particles and extracted from anode active particles. In a charge process of LIB, Li⁺ ions are extracted from cathode active particles and inserted into anode active particles. These reactions are maintained by Li⁺ ion transfer between anode and cathode. Simultaneously, electrons transfer between cathode and anode through an external circuit. The electrons also move in conductive additives and active materials. These electrons are collected by Al and Cu collectors at cathode and anode, respectively. A lot of elemental processes are involved in the discharge and charge processes of LIB, as described below.

1. (1)    Charge transfer process at the interface between particles of active materials and electrolyte.
2. (2)    Li⁺ ion migration in electrolyte involved in pores of separator and electrode layers.
3. (3)    Li⁺ ion diffusion in particles of active materials.
4. (4)    Electron transfer between cathode and anode through external circuit.
5. (5)    Structural changes of active materials.
6. (6)    Degradation of active materials.
7. (7)    Electrochemical oxidation of electrolyte on particles of active materials and carbon black conducting additive in cathode layer.
8. (8)    Electrochemical reduction of electrolyte on particles of active materials and carbon black conducting additive in anode layer.
9. (9)    Formation of SEI and CEI on particles of active materials and conducting additives.
10. (10)    Volume change in particles of active materials and conducting additives.
11. (11)    Macroscopic volume change of electrode layers.
12. (12)    Degradation of electrode layers, mainly formation of crack inside electrode layers, leading to loss of electronic conductivity of electrode layers.
13. (13)    Corrosion of current collectors.
14. (14)    Exfoliation of electrode layers from current collectors.
15. (15)    Safety issue, internal short circuit, over-charge, over-discharge

All phenomena have to be understood to predict a performance of LIB and other rechargeable batteries.

Figure 2.

Schematic illustration of LIB consisting of LiCoO₂ cathode, graphite anode, non-aqueous electrolyte, separator and so on.

3. Interface between Active Materials and Electrolyte, in nm Scale Behavior

3.1 Electrochemical stability at the interface between active material and electrolyte

The cycle life of LIB partly depends on the electrochemical stability of electrolytes at the interface between cathode and electrolyte. This is nano-scale behavior in LIB. The oxidation of non-aqueous electrolyte should not occur in the cell. However, the cathode materials used in LIB, such as LiCoO₂, LiAl_xNi_yCo_zO₂ (x + y + z = 1), LiNi_xMn_yCo_zO₂ (x + y + z = 1) and LiMn₂O₄, may have a catalytic activity for an oxidation of electrolyte. This catalytic activity leads to the electrochemical oxidation of electrolyte during charging process, resulting in lower coulombic efficiency of cell and increase of interfacial resistance between cathode material and electrolyte. In order to observe the electrochemical oxidation of non-aqueous electrolytes, an in-situ Fourier Transform Infrared (FTIR) measurement has been conducted by using the cell as shown in Fig. 3.⁹ The interfacial behavior between LiCoO₂ thin film electrode and electrolyte was observed to understand the oxidation process of electrolyte. Figure 4 shows the typical in-situ FTIR spectra. All spectra are Subtractively Normalized Interfacial FTIR ones (SNIFTIR).⁷ In these spectra, the downward and upward peaks were observed and corresponded to formation and disappearance of chemical species at the interface between LiCoO₂ and electrolyte, respectively. In general, non-aqueous organic electrolytes are thermodynamically stable at least up to 4.5 V vs. Li/Li⁺. However, the spectra in Fig. 4 indicates that the oxidation of electrolyte has occurred even below 4.5 V vs. Li/Li⁺. The oxidation of electrolyte is not so significant, but strongly influences a cycleability of cell. The detection of such very weak reaction is not so easy. The in-situ measurement is strongly required to detect such minor electrochemical and chemical reactions taking place on particles of cathode materials. From this in-situ detection, it can be said that the electrochemical oxidation strongly depends on a kind of electrode material. Transition metal oxide cathodes have a high catalytic reactivity for the oxidation of electrolytes. In order to suppress the undesirable oxidation of electrolyte, the surface coating on particles of cathode materials is useful. In fact, Al₂O₃, SiO₂, or ZrO₂ coating has been utilized to various transition metal oxide cathode materials to avoid the electrolyte oxidation.⁶³ Another possible electrolyte oxidation takes place on carbon materials as conductive additives. Some functional groups, such as –C–OH bond, –C=O, –COOH bonds, works as active reaction sites for electrolyte oxidation. In order to diminish the electrolyte oxidation, such functional groups should be reduced. For example, carbon nanotube and graphene materials are promising new conductive additives.⁶⁴

Figure 3.

Schematic illustration of electrochemical in-situ FTIR measurement cell. Reproduced with permission from Ref. 9. Copyright 2001, Elsevier

Figure 4.

SNIFTIR spectra for oxidation of electrolyte on LiCoO₂ thin film cathode in propylene carbonate with 1.0 mol dm⁻³ LiPF₆. Reproduced with permission from Ref. 7. Copyright 2005, Elsevier

The same investigation is required for all materials used in LIB to reduce undesirable reactions taking place on electrode materials and carbon materials. The in-situ FTIR is one of powerful method to observe the interfaces during charge and discharge of LIB. Recently, various other powerful analytical methods have been developed and applied to LIB and its materials.

3.2 Interface between Li metal and electrolyte

Interfacial analysis on anode materials has been also conducted to understand SEI formation and electrolyte stability on carbon based anode, lithium metal anode and Si based anode. Here, the surface analysis on lithium metal anode is introduced. Before 1992, there were many researches on lithium metal. At 1992, LIB was commercialized by SONY. After 1992, most of researches on anode shifted from lithium metal anode to graphite anode (carbon based anode). Recently, next generation batteries have attracted much attention for new applications due to a requirement to higher energy density of rechargeable batteries.^1,2,65 Lithium metal anode will be used in these next generation batteries. Researches on lithium metal anode has been extensively conducted around the world.⁶⁵ Most of studies have focused on the surface analysis of lithium metal anode, which is related to the SEI formation in nonaqueous electrolytes. The surface reactions and surface modifications of lithium metal anode have been investigated by using X-ray Photoelectron spectroscopy (XPS), in-situ FTIR, Electrochemical Quartz Crystal Microbalance technique (EQCM) and Atomic Force Microscopy (AFM).^32,34 For example, XPS analysis on lithium metal surface and surface reaction of lithium metal was conducted to clarify the relationship between morphology of lithium metal deposits and SEI. Figure 5 shows the XPS spectra of O1s, C1s and Li1s for lithium metal foil surface used in primary lithium metal cells.⁴⁶ These peaks are assigned to Li, Li₂O (Li_xO), Li₂CO₃ and LiOH, indicating that lithium metal surface is originally covered by these lithium compounds. This is so-called “the native SEI” of lithium metal. According to depth profiles of spectra, the structure of the native SEI can be drawn, as shown in Fig. 6.⁴⁶ This native SEI is stable under dry atmosphere. However, the native SEI may change after an immersion of lithium metal in nonaqueous electrolytes. Figure 7 shows the XPS spectra of lithium metal after the immersion in propylene carbonate with LiClO₄ and LiPF₆ electrolytes. The SEI did not change significantly in propylene carbonate with LiClO₄ electrolyte. On the other hand, the SEI on lithium metal changed in propylene carbonate with LiPF₆. The main component of the surface was converted from Li₂O, Li₂CO₃ and LiOH to LiF and Li₂O. This result can be understood by a stability of anions in nonaqueous solvents. LiClO₄ is generally very stable. A small amount of HCl may be included in this electrolyte, as an impurity. LiPF₆ is slightly unstable when including H₂O in the electrolyte as impurity. H₂O and PF₆⁻ anion react to form HF (sometimes H₂O/HF adduct) in the electrolyte. HF reacts with Li₂O, Li₂CO₃ or LiOH to form LiF on the lithium metal surface. These chemical reactions take place during the immersion of lithium metal in propylene carbonate with LiPF₆. PF₆⁻ also slightly decomposed to form PF₅ and LiF. The thickness of newly formed SEI consisting of mainly LiF is much thinner than that of the native SEI. Figure 8 shows the scanning electron micrographs of lithium metal deposits in both electrolytes. The morphology of lithium metal deposited in propylene carbonate with LiPF₆ electrolyte is smooth and different from a dendrite shape. The dendrite formation of lithium metal has been discussed by many researchers to improve a cycleability of lithium metal anode.⁶⁵ From these results, it can be said that the SEI on lithium metal is one of important factors to suppress the lithium metal dendrite during the charging process. The SEI controls the electrochemical reaction of lithium metal in the range from nm scale to µm scale. However, the lithium metal dendrite is formed during many discharge and charge cycles. The SEI cannot suppress the dendrite formation completely. EQCM profiles during discharge and charge cycles was measured and compared with ideal curves corresponding to lithium metal deposition and dissolution.^32,34 The real behavior was far from an ideal one. This undesirable behavior is related to the physical property of the SEI on lithium metal. In-situ AFM measurement was conducted to observe the dynamic behavior of SEI on lithium metal surface. AFM images indicated the decomposition of SEI during lithium metal dissolution process. The SEI on lithium consisting of LiF is suitable for the smooth lithium metal deposition, but is not so stable mechanically (physically). The breakdown of LiF SEI leads to new chemical reactions of fresh lithium metal with electrolyte to produce Li₂CO₃, Li alkyl carbonate and so on. This undesirable phenomenon results in the decrease of homogeneity of SEI during deposition and dissolution cycles. So, the SEI could control the electrochemical reaction of lithium, when it was so stable. The SEI, however, is easily destroyed due to internal stress in the SEI. Therefore, the SEI has to be more stabilized to obtain reversible lithium metal deposition and dissolution. Probably, there are two methods for stabilizing of the SEI. One is a further development of SEI, but always takes care very much for a breakdown of the SEI. This is nm scale to µm scale achievement. Another way is a physical stabilization of lithium metal SEI by using separator. The separator has a strong effect on a current distribution near lithium metal surface. The separator directly contacts with lithium metal surface, providing a positive effect for electrochemical reaction of lithium metal. An important point is a pore structure of separator, which determines current distribution between separator and lithium metal surface. New type of separator have been developed by utilizing 3DOM structure.⁴⁹ Figure 9 shows the scanning electron micrographs of 3DOM separator consisting of polyimide which is a kind of engineering plastic and thermally stable up to 400 °C. The 3DOM structure provides a highly uniform current distribution at the interface between lithium metal and separator. The uniform current distribution is necessary to suppress the lithium metal dendrite formation. Another important effect of 3DOM separator is a mechanical stabilization of the SEI on lithium metal surface. Figure 10 shows the cell voltage change of Li/Li symmetric cell during the dissolution and deposition cycles. During more than 3000 cycles, the voltage profile was so stable, indicating a reversible dissolution and deposition of lithium metal. This excellent cycleability is due to less chemical reaction of lithium metal with electrolyte. In other words, the SEI was highly stabilized by the presence of 3DOM separator. This is an interfacial design from nm scale to several µm scale. By using 3DOM separator, lithium metal battery may be realized. Figure 11 shows the scanning electron micrographs of cross-sectional view of lithium metal anode used in the cell with LiNi_0.6Mn_0.2Co_0.2O₂ cathode, 3DOM separator and lithium metal anode. The thickness of lithium metal increased with increasing cycle number. The original thickness was 20 µm. After 400 cycles, thickness increased to 200 µm, which was ten times thicker than the original one. The thickness of lithium metal layer was totally different from that before cycling of cell. In the full cell, two different volume changes occur during the discharge and charge cycles. One is the large volume change of lithium metal and another is the small volume change of cathode. The volume changes in both anode and cathode are different each other, leading to poor contact between separator and lithium metal, specially at the discharge process. The positive effect of separator may be diminished by this phenomenon, leading to the dendrite formation by the chemical reactions with electrolytes. In order to keep a good contact between separator and lithium metal anode, an external pressure is useful. This is a technology in mm or cm scale. Thus, the multi-scale achievement should be done for the realization of lithium metal anode.

Figure 5.

XPS spectra of O1s, C1s and Li1s for lithium metal foil surface.

Figure 6.

Schematic illustration for structure of the native SEI on lithium metal.

Figure 7.

XPS spectra of Li 1s, C 1s, O 1s and F 1s for lithium metal surface after immersion in propylene carbonate with 1.0 mol dm⁻³ LiClO₄ and propylene carbonate with 1.0 mol dm⁻³ LiPF₆. Reproduced with permission from Ref.46. Copyright 1992, Elsevier

Figure 8.

Scanning electron micrographs of lithium deposited in propylene carbonate with 1.0 mol dm⁻³ LiClO₄ and propylene carbonate with 1.0 mol dm⁻³ LiPF₆. Reproduced with permission from Ref. 44. Copyright 1994 The Electrochemical Society Inc.

Figure 9.

Scanning electron micrograph of 3DOM separator consisting of polyimide.

Figure 10.

Potential change of Li/Li symmetrical cell at current: 15.89 mA for 30 min (7.945 mAh), DOD 25 % (Li metal electrode 31.8 mAh) in ethylene carbonate with 1 mol dm⁻³ LiPF₆.

Figure 11.

Scanning electron micrographs of cross-sectional view of lithium metal anode used in the cell with LiNi_0.6Mn_0.2Co_0.2O₂ cathode, 3DOM separator and lithium metal anode. Reproduced with permission from Ref. 53. Copyright 2019 The Electrochemical Society Inc.

3.3 Interfacial control of cathode and electrolyte

LiCoO₂ is a good cathode material for LIB. During last 30 years, it has been utilized in various LIBs for portable applications and large size devices. However, LIB with LiCoO₂ has a safety issue problem due to oxygen release from LiCoO₂ when LIB is overcharged under abnormal conditions. In order to solve this safety issue, new cathode materials have been developed and applied to LIB. The most popular safe cathode material is LFP. The chemical bond between P-O is more covalent which suppress an oxygen release from FePO₄ at high temperature. This behavior is very useful to keep a safety of LIB. However, LFP does not have adequate electronic conductivity and Li⁺ ion conductivity. The diffusion resistance of Li⁺ ion can be diminished by using small particle which has shorter diffusion pathway. On the other hand, the low electronic conductivity can be solved by doping of different ions in LFP. Another important technique is a surface coating of LFP with carbon materials. Figure 12 shows the transmission electron micrograph of LFP prepared by a hydrothermal synthesis.^23–26 FeSO₄/7H₂O, H₃PO₄ and Li₂SO₄ are used as starting materials. These materials were dissolved into pure water with a small amount of ascorbic acid which is carbon sources. Through the hydrothermal process, LFP with 100 nm particle size was prepared and simultaneously the surface of LFP was covered by reduction product of ascorbic acid. After this hydrothermal synthesis, LFP particles were heated at 600 °C under N₂ atmosphere to convert products from ascorbic acid to carbon with high electronic conductivity. As shown in Fig. 12, the particle had a carbon layer which thickness was a few nm. The coating layer has high electronic conductivity which enables the smooth discharge and charge of LIB with LFP cathode due to lower contact resistance between LFP particles. This is nm scale technology and a kind of surface modification. All of LFPs used in practical LIB have this coating layer. The CEI has been also designed by other methods for various kinds of cathode materials. The artificial control of CEI is very important for cathode performance, and also electrolyte stability at cathode materials.⁶³

Figure 12.

Transmission electron micrograph of LFP prepared by a hydrothermal synthesis. Reproduced with permission from Ref. 25. Copyright 2005 The Electrochemical Society Inc.

4. Precise Evaluation of Electrochemical Parameters for Cell Design of LIB

LIB has been designed by using electrochemical and physicochemical parameters for active materials, electrolytes, and other components. In order to design cell precisely, an evaluation of electrochemical parameters of active materials, electrolytes, and other components are very important. The measurement results are sometimes different each other, due to different methods and assumptions. Especially, the parameters estimated by using composite electrodes include a lot of factors of porous nature and material composition of electrodes. Therefore, more reliable measurements are needed. In this study, the evaluation of electrochemical parameters by using single particle measurement is introduced.⁵⁸ Figure 2 shows the schematic illustration of composite electrodes consisting of active materials, binders and conductive additives. This is a porous electrode with about 40 % porosity. The electrochemical response of this electrode depends on a kind of electrolyte, porosity, binding materials and conductive materials, as mentioned above. The electrochemical response unfortunately includes many factors related to the components and porosity of this composite electrode, so that the obtained results depend on measurement system. For the evaluation of the precise electrochemical parameters, a single particle measurement system has been developed and applied to various kinds of cathode and anode materials. Figure 13 shows the schematic illustration of single particle measurement system. A micro current collector was applied to one particle of active material, as shown in Fig. 13. The discharge and charge of one particle can be performed by using this measurement system. The polarization behavior, I-V curve, can be obtained by changing discharge and charge currents. Figure 14 shows the measured polarization behavior for LiCoO₂ particle in a mixed solvent ethylene carbonate and propylene carbonate containing 1.0 mol dm⁻³ LiClO₄. From this result, the I-V curve can be draw as Tafel plot. Figure 15 shows the Tafel plot for LiCoO₂. An exchange current and diffusion coefficient of Li⁺ ion in LiCoO₂ were estimated to be 1.63 mA cm⁻² and 1.64 × 10⁻¹⁰ cm² s⁻¹ for discharge and 1.38 × 10⁻¹⁰ cm² s⁻¹ and for charge. The transfer coefficient of the charge transfer is estimated to be 0.525. These electrochemical parameters are not influenced by porous structure of composite electrode, polymer binder, conductive additive and diffusion process of Li⁺ ion in electrolyte. By using these parameters, the electrode performance can be predicted by using a chemical engineering calculation. For example, the simulation programs have been published to predict the electrode and cell performance of LIB.⁶⁶

Figure 13.

Schematic illustration of single particle measurement system.

Figure 14.

Measured polarization behavior for LiCoO₂ particle in a mixed solvent ethylene carbonate and propylene carbonate containing 1.0 mol dm⁻³ LiClO₄.

Figure 15.

Tafel plot for LiCoO₂ in a mixed solvent ethylene carbonate and propylene carbonate containing 1.0 mol dm⁻³ LiClO₄.

5. Prediction of Composite Electrode and Cell of LIB

A prediction of electrode and cell of LIB have been performed by many researchers to design LIB cell.⁶⁶ Figure 16 shows the simulation flow of LIB based on special measurements and chemical engineering calculation. In order to simulate the electrochemical performance, the information in Fig. 16 should be obtained by several measurement methods. The precise electrochemical parameters for active materials can be obtained from the single particle measurement. The ionic conductivities in composite electrode and separator can be obtained by using four probe measurement with the cell as shown in Fig. 17. Three dimensional porous structures for composite electrode and separator are also needed for the simulation of electrode and cell. The porous structure can be obtained by using FIB-SEM. Figure 18 shows an example of 3D structure of composite electrode obtained by FIB-SEM. These porous structures are introduced in computer to construct LIB with the composite electrodes, electrolyte, and separator. The precisely evaluated electrochemical parameters, such as charge transfer resistance, diffusion coefficient, ionic conductivity and so on, are introduced in the simulation program to predict the electrochemical performance of the composite electrode and LIB. In addition, a volume change of composite electrode can be estimated by using mechanical properties of active material, conductive additive, and binder, leading to an expectation of life cycle of LIB. The protocol as shown in Fig. 16 is useful for quick development of LIB. A similar protocol can be developed for other kinds of batteries, such as Li metal battery, Li air battery, Li sulfur battery, Mg metal battery, and all solid state battery. In fact, some protocols have been reported.⁶⁶ The study on protocol has to be conducted in nm scale to m scale. In future, a platform for rechargeable battery development may be established by a combination of various protocols in which various measurement methods and simulation programs.

Figure 16.

Simulation flow of LIB based on special measurements and scientific calculation.

Figure 17.

Schematic illustration of cell for measurement for ionic conductivities in composite electrode and separator by using four probe measurement using the cell.

Figure 18.

Example of 3D structure of composite electrode and separator measured by FIB-SEM.

6. Failure Mode Analysis of LFP/Graphite Cell, m Scale Analysis

The cycle life of battery is now very important to reduce CO₂ emission from battery manufacturing process. Twice longer cycle life leads to a half of CO₂ emission. In addition, the cost of battery can be reduced. The improvement of life cycle has to be done. However, the failure mode of battery is very complicated. So far, many researches have been published to propose the equation for life cycle of LIB. In our group, the life cycle of LIB with LFP cathode and graphite anode was investigated to establish the prediction equation for the cycle life of LIB. Figure 19 shows the discharge capacity change during discharge and charge cycles.⁶² The discharge capacity decreases with increasing cycle number depending on test conditions. In order to explain this behavior of LIB, the prediction equation is suggested based on the assumed failure mode of cell.

Figure 19.

Capacity loss of lithium ion battery (LFP cathode and graphite anode) at ○: 25 °C 1 C, ●: 25 °C 4 C, △: 45 °C 1 C, and ▲: 45 °C 4 C for measured values, and : 25 °C 1 C, : 25 °C 4 C, : 45 °C 1 C, and : 45 °C 4 C for simulated values. Reproduced with permission from Ref. 62. Copyright Authors, 2021. CC BY 4.0

In this study, the operating voltage, V(SOC, I) (SOC: State of Charge) was calculated by combining several empirical and theoretical formulas below including the Open Circuit Voltage (OCV) of LFP battery, ohmic overvoltage, reaction overvoltage and concentration overvoltage that were presented on the previous papers. On the other hand, for calculation of the battery deterioration rate, a mathematical model from the predicted phenomenon has been constructed. The concentration of Li ion and deterioration-causing substance that change complicatedly with the battery voltage, have been calculated by our own simulation program. The adopted Eq. 1 is as follow.   

\begin{align} V(\mathit{SOC},I) & = U_{0} + k_{1}\ln (\mathit{SOC}) + k_{2}\ln (1 - \mathit{SOC}) \\ & \quad- \frac{k_{3}}{\mathit{SOC}} - k_{4}\mathit{SOC}\\ & \quad - \text{I}\left\{r_{e} + \frac{1}{A_{\textit{ion}}}\exp\left(\frac{\text{B}}{T - T_{0}}\right)\right\}\\ & \quad - \frac{RT}{\alpha zF}\log \Biggl\{\frac{1}{2}\Biggl(\exp \left(\frac{\Delta G}{RT}\right)\frac{I}{zFA_{0}} \\ & \quad- \sqrt{\exp \left(-\frac{2\Delta G}{RT}\right)\left(\frac{I}{zFA_{0}}\right)^{2} {}+ 4}\Biggr)\Biggr\}\\ & \quad - \frac{RT}{zF}\ln\left\{1 + \frac{\exp (E/RT)\delta I}{zFDc_{l}}\right\} \end{align} (1)

The mechanism was predicted as follows. Equations 2 and 3 show examples of reaction equations for SEI layer formation, and Eqs. 4 and 5 show examples of sequential reaction.   

\begin{equation} \textit{Li$_{\textit{grp}}^{+}$} + e^{-} + S \rightleftarrows \textit{Li}_{\textit{ads}} \end{equation} (2)

  

\begin{equation} 2\textit{Li}_{\textit{ads}} + \textit{CO$_{3}^{2-}$} \rightleftarrows \textit{Li$_{2}$CO$_{3}$} + 2S + 2e^{-} \end{equation} (3)

  

\begin{equation} \text{X} \rightleftarrows Y + Z \end{equation} (4)

  

\begin{equation} \textit{Li$^{+}$} + e^{-} + \text{Z} \to \textit{LiZ} \end{equation} (5)

$\textit{Li}_{\textit{grp}}^{ + }$, Li_ads, and S indicate Li-ion in graphite, Li adsorbed on the surface of the anode, and adsorption sites on the anode surface. X is considered to be a material such as electrolyte or electrode, and Y and Z are its decomposition products. According to this decomposition reaction, Li and the decomposition product irreversibly react, leading to the Li battery capacity failure. In this study, simulations were performed assuming that X is $PF_{6}^{ - }$, Y and Z are $PF_{5}^{ - }$ and F*, but there are many candidates for deterioration reaction, and the reaction formula is likely to be more complicated. These reactions proceed simultaneously, and each reaction rate changes depending on the electrode potential. This model is able to fit to the complicated degrading behavior of the lithium-ion battery, as shown in Fig. 19. In particular, the reversible reaction model of SEI layer formation can explain the phenomenon that the capacity gradually recovers when the battery is held at a high SOC and then changed to a low SOC. The proposed prediction for the battery failure is applied to a real system to predict the cycle performance of battery life. The prediction is reasonably performed and agrees with real calendar life characteristics. However, the cycle life is not yet fully matched and needs further improvement in the future. If the simulation formula can accurately reproduce the deterioration of the LIB, it is possible to suggest the operating mode of the battery to improve the life of the LIB. By using this equation, the operation mode of battery can be suggested to improve cycle life of LIB. This equation is useful for LIB used in this study, but it cannot be applied to other batteries. The prediction equation was obtained after long cycle test. It takes so long time. In future, the prediction can be conducted by using the protocol described above to reduce time for development of new LIB and other batteries. Thus, m scale researches usually need a much longer time. When the prediction method (protocols) from nm scale to m scale is developed, the development of battery is accelerated. As a result, the cost and CO₂ production for battery development and manufacturing can be reduced very much.

7. Structural Control of Active Materials, Electrolytes and Other Parts

The structural control of active materials, electrolytes, and other parts have been performed by try and error method. A lot of time is spent for development of materials used in LIB. Recently, many analytical tools have been utilized in material researches for the battery development. The development of materials is now accelerated. Moreover, the simulation method from battery (cell) to material is very useful to reduce the development time for battery. The battery performance can be predicted by using the above protocol. Based on this prediction, the direction for material development including the structural and compositional control of materials can be decided. For example, suitable particle size, porosity, shape, and electronic or ionic conductivity to active material used in batteries for different applications can be determined. In this case, the study in the range from nm scale to m scale has to be performed.

8. Summary

Various researches have been performed for LIB to improve its performance, such as energy density, power density, life cycle, safety and so on. Most of researches have focused on only interface, bulk property, cell performance, or other. These studies are very important, but it takes a lot of time for battery development. On the battery development platform, these studies will be done in future. By using the platform, all of researches will be more meaningful. Our researches have been introduced in this report. These researches are included into the platform (protocols) to predict material, cell, and battery performance.

CRediT Authorship Contribution Statement

Kiyoshi Kanamura: Writing – original draft (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

This paper summarizes the research results of “Multiscale Creation of Battery and Fuel Cell Materials Based on Structural and Interface Design”, which was the subjects of ECSJ Society Award (Takei Award).

K. Kanamura: ECSJ Fellow

References

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List of Symbols

• A₀ (−) frequency factor
• A_ion (−) factor of ion transport
• B (K) constant
• c_l (mol m⁻³) concentration in liquid
• D (m² s⁻¹) diffusion coefficient of Li⁺
• E (J mol⁻¹) activation energy for Diffusion coefficient
• F (C mol⁻¹) Faraday constant
• ΔG (J mol⁻¹) Gibbs free energy
• k₁ ∼ k₄ (−) parameters to fit the measured OCV
• I (A) battery current
• R (J K⁻¹ mol⁻¹) universal gas constant
• r_e (Ω) electric resistance
• T (K) absolute temperature
• T₀ (K) temperature correction value
• U₀ (V) battery voltage at SOC = 0
• z (−) number of electrons
• a (−) transfer coefficient
• δ (m) diffusion thickness

Biographies

Kiyoshi Kanamura (Professor, Graduate School of Urban Enviromental Sciences, Tokyo Metropolitan University)

Kiyoshi Kanamura was born in 1957. He graduated from Faculty of Engineering, Kyoto University in March 1982, and earned Doctor of Engineering in 1987. He worked in Kyoto University as Research Instructor and then Asociate Profressor in 1984–1998. He moved to Tokyo Metropolitan University as Associate Professor in 1998 and promoted Professor in 2002. He was awarded Young Researcher Award (Sano prize) from Electrochemical Society of Japan in 1992, Research Award from Energy Technology Division Electrochemical Society Inc. in 2005, and Society Award (Takei Prize) from Electrochemical Society of Japanin 2022. His research interests are electrochemical energy conversion systems and matreials. Hobby: Golf, Sake.